The significance of fillers in composite polymer electrolytes for optimizing lithium battery

Abstract

Solid polymer electrolytes constructed from polymers have high safety, outstanding thermal stability, and minimal flammability as their merits. Many researchers have been working on creating high-performance Li-based batteries composed of solid polymers. Composite polymer electrolyte (CPEs) electrochemical characteristics, which might include conductivity of ions, ion transfer numbers, and electrochemical durability, play an essential role when assessing the performance of energy conservation and conversion systems. Inorganic additions may enhance ionic conductivity by producing an ion transport percolation network. Additionally, the most effective filler composition can improve CPE electrochemical process stability, diminishing adverse responses and deterioration throughout gadget operations. In this article, we talk about active filler-based composites of polymers that make excellent solid electrolytes for the large-scale production of solid-state battery packs. We review the analysis and performance of active filler-based composite polymer electrolytes and look into the design of high-performance composite electrolytes.

Introduction

The exploration of alternative polymer-composite substances for electrolytes or separators for lithium-ion and lithium-based batteries has increased exponentially in the twenty-first century [1]. In recent times, due to their exceptional characteristics, including a high density of energy [2], lightweight [3], extended cycle life [4], flexible morphologies, and minimal leakage, polymeric lithium-ion batteries, as well as all micro-batteries, are considered to be the most prospective recharging chemical source of power [5, 6]. An electrolyte is an essential component in the battery that permits ions to travel between two electrodes, allowing electrical current to flow over the battery to generate and store energy. Rechargeable batteries are composed of electrodes (a cathode and an anode) separated by an electrolyte. Between the charging and discharging cycles, the electrolyte is in a state of charge, transmitting ions between the two electrodes. The terminology “polymer” comes derived from the Greek poly, indicating “many,” while Mer indicates “part.” The electrolyte is often the solution of acid, base, or salt. Compared to the liquid electrolyte, polymer electrolyte is much more flexible and lightweight. There are a few various electrochemical windows. Its limited conductivity at low temperatures is the primary drawback for polymer electrolytes [7]. Since the previous two decades, polymer electrolytes (PEs) have sparked substantial attention due to their potential usage in solid-state batteries, fuel cells, sensors, super-capacitors, etc. [8,9,10,11]. Li-based rechargeable battery polymer electrolytes could generally be split into three primary classifications: composite polymer electrolytes (CPEs), gel polymer electrolytes (GPEs), and solid polymer electrolytes (SPEs) [12] (Fig. 1).

Fig. 1

The properties of liquid and different kinds of polymer electrolytes

Despite the encouraging breakthroughs in the field of solid-state polymer electrolytes, considerably, it still has to be accomplished to eliminate the three significant challenges standing in the way of large-scale commercial use of such a stable solid-state electrolyte. Optimizing it is (a) physical and thermal stability, (b) lithium-ion conductivity, and (c) electrode-electrolyte interfacial interaction which is the most important and demanding properties. A polymer-based electrolyte finds it more challenging to get more excellent ionic conductivity beyond 25°C to 30°C, which is considered just under the processing temperature of the battery. However, while operating at high temperatures, polymer-based batteries’ thermal stability, mechanical properties, and charging/discharging cycle stability suffer [2, 3, 12,13,14,15]. A few strategies are employed to optimize the polymer electrolyte system; notably, interpenetration, cross-linking blending, and co-polymerization are frequently used to coordinate polymer with several other polymers, but they would not considerably improve the electrolyte’s mechanical properties. Ionic liquid, fast-ion conductive ceramics, lithium salts, and inert ceramic fillers were among the composites incorporated into the polymer [16]. Utilizing polymeric materials including poly(methyl methacrylate), poly(styrene), poly(ethylene oxide), poly(acrylonitrile), poly(methyl methacrylate), poly(vinylidene fluoride), (polyvinyl chloride) poly(vinylidene chloride-co-acrylonitrile), and poly(methyl methacrylate-co-acrylonitrile), an excellent polymer electrolyte has been produced in lithium-based rechargeable battery.

Composite polymer electrolyte

CPEs primarily exclusively incorporates polymer matrices and Li salts (LiF, LiNO₃, LiTFSI, and LiPF₆) together with ceramic fillers. It is possible to categorize further the fillers used into active (LLTO, LLZTO, LATP, LGP, etc.) and inactive fillers (TiO₂, SiO₂, Al₂O₃, CeO₂, ZrO₂, SnO₂...etc.) (Fig. 2) [5]. The CPEs’ effectiveness can be affected by the essential properties, size, shape, and content of these fillers. The composition of the fillers, in particular, has an essential effect on the efficiency of the CPEs and ion transfer paths. Li-ions are primarily transported by way of the matrix of polymer phases in the composite polymer electrolyte with a small filler content, the permeating network created by the structure of the matrix phase, and the particles of filler in the CPEs with average filler content, and the ongoing ceramic particle phases in the CPEs with elevated filler content [17,18,19,20]. Composite electrolytes containing discrete ceramic fillers communicate substantially reduced ionic conductivity compared to disinfected polymer electrolytes at higher ceramic loadings (>30 vol%). A recent study discovered an elevated interface, a barrier for ion transport across the polymer-ceramic interaction [21]. High temperatures are capable of speeding the breaking down of polymer electrolytes. The chains of polymers could break down at extreme temperatures, leading to decreasing mechanical durability and enhanced ionic conductivity. Thermodynamics can be challenging in applications demanding high-temperature stability, such as fuel cells or batteries. Polymer electrolytes’ ionic conductivity can be decreased at low temperatures. The movement of ions throughout the polymer matrix minimizes as temperature decreases, resulting in lowered ion transport, which may impact how they work with devices used in temperatures below freezing [22, 23].

Fig. 2

The classification of composite polymer electrolyte

High humidity levels could trigger the polymer electrolyte to take in water. This might prove advantageous for applications where water needs to be present for proton transport, such as proton-exchange membrane fuel cells. Excessive humidity could result in the plasticization or expansion of the polymer, weakening its mechanical integrity and possibly lowering its performance. Low humidity levels could have the opposite direction effect, causing the polymer electrolyte to dehydrate. This may contribute to decreased ionic conductivity because the required water content for ion transport is not maintained. The mechanical characteristics of polymer electrolytes can be altered through high pressure. It could result in changes in the crystallinity or amorphous structure of the polymer, impacting its mechanical properties and performance in general [24,25,26,27].

Why polymer electrolyte is required with active fillers

SPEs are in high demand due to the promise of higher energy density, safety, and flexibility with next-generation lithium-ion batteries (LIBs). The major setback of SPEs is their low ionic conductivities, generally ranging from 10⁻⁸ to 10⁻⁷ Scm⁻¹ at ambient temperature, and exhibit good mechanical characteristics [5]. The excellent conductivity of ions for (GPEs) gel polymer electrolytes at ambient temperature is a benefit (10⁻⁴ to 10⁻³ Scm⁻¹). This is comparable to the manufacturing electrolyte. Unfortunately, the trouble concerning membranes is that they generally exhibit poor mechanical characteristics since liquid electrolyte absorption has relaxed their physical structure. It plays a vital role in efficient implementations since it can cause problems with winding tension and internal short circuits during cell development and operation [28,29,30].

Researchers have observed that the involvement of fillers minimizes the polymer’s tendency to crystallize or makes a vital impact pathway for ion transport at the interaction among fillers and polymeric, improving the solid polymer electrolyte’s ionic conductivity. TiO₂, SiO₂, and Al₂O₃ are inert ceramic fillers that do not contribute to Li-ion conduction [4, 31]. To compare inactive organic filler polymer electrolytes, active inorganic filler-based polymer electrolytes are among the most widely deployed solid electrolytes throughout all batteries. Active filler–based solid polymer electrolytes, including like as perovskite (LLTO) Li_0.33La_0.557TiO₃, garnet (LLZO)Li₇La₃Zr₂O₁₂, NASICON (LATP) Li_1.3Al_0.3Ti_1.7(PO₄)₃, and sulfide (LGPS) Li₁₀GeP₂S₁₂, have high lithium-ion conductivities (1×10⁻³ S cm⁻¹ambient temperature), good thermal stability (ceramics can withstand temperatures >1000°C), and electrochemical window (>5 V) [3, 32]. The addition of active fillers to the mixture changes the molecular makeup of the polymer phases as well as the interaction state of lithium-ion molecules as they communicate with other molecules, including anions and chains of polymers, which influences the motion of lithium ions all through the polymer phase, and this is usually advantageous. Several variables help contribute to more excellent ion conductivity:

• (1) The addition of inorganic filler substances reduces the degree of crystallization and glass transition temperature (Tg) of the polymer phase, increasing amorphous conducive areas and accelerating segmental motion.

• (2) The lithium salts are simple to dissociate and release a more significant number of lithium ions owing to the Lewis acid-base influence triggered by inorganic fillers.

The many lingering problems of low-energy activation active filler made ion hopping achievable. Active fillers could disperse a significant quantity of lithium ions, increasing the free lithium ions accessible within the active filler-polymer interaction while supporting more swift ion transport [8, 33, 34].

Effect of active fillers on polymer electrolyte

Active fillers are put forward to the enhanced efficiency of polymer electrolytes, which are used in many electrochemical appliances comprising lithium-ion rechargeable batteries and fuel cells. Fillers like these usually consist of inorganic substances applied to the matrix of polymers to improve the electrolyte’s conductivity of ions, mechanical strength, electrochemical stability…etc.

Ionic conductivity

Polymers have been employed for embedding active fillers. Active fillers consistently have increased ion conductivity (over 10⁻⁴ S cm⁻¹). This may be because of several variables, such as the numerous continuous flaws in low activation energy active fillers that enable relatively easy ion hopping. In addition, active fillers may supply an enormous amount of lithium ions, boosting the concentration of free lithium ions at the active filler-polymer interaction. As a consequence, the overall ionic conductivity increases. Perovskite, garnet…etc., and other active fillers were instances [35].

Li⁺ transference number

Researchers subsequently examine an attribute associated with ion conduction on the inside of a polymer matrix. The charge transmits and, consequently, the current of an identified ion were directly represented using the transference number. The lithium-ion transference ratio (Li⁺) shows the current transported by cations (Li⁺) in the electrolytes. It is more beneficial than the lithium content being a high (Li⁺) substance to enhance the kinetics of interactions among electrodes and to reduce the gradient of concentration through the battery, permitting the internal voltage drop from the battery to be decreased and the current at its output to be raised. The Lewis acids (the hydrogen molecules of the acid ceramics surfaces) determine a hydrogen connection between the lithium salt anions and the ether’s oxygen atoms (Lewis’ base). This improves the dissociation of salt and minimizes crystallinity. Consequently, the transference number Li⁺ developed, whereas the number of sites containing Lewis’ acids decreased, resulting in a higher boost in Li⁺ [36].

Mechanical strength

Compared to the polymer matrix, inorganic fillers’ insertion enhances tensile strength despite reducing elongation-at-break. Employing a polymer with exceptional viscoelasticity, self-healing, and film-forming capability may result in an active filler’s electrolyte with high mechanical stability. The quantity of inorganic fillers present additionally impacts the mechanical properties of electrolytes. Ceramic-in-polymer electrolytes are becoming very adaptable [37].

Electrochemical window

The electrochemical window (ESW) is the voltage that is the difference that exists between the electrolyte’s reduction and oxidation potential or the difference of energy between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO). An ESW is essential for assessing the battery’s performance because it demonstrates the limit of electrolytes’ resistance to oxidation and reduction [38]. The antioxidant potential of composite polymer electrolytes could be boosted by applying active fillers. This becomes most apparent in how it impacts the polymer’s electron-hopping energy levels. On the other hand, grafting optimizes inorganic fillers having polar groups (–OH, –COOH, etc.), which can also help strengthen polymer matrices. On the other side, inorganic filler elemental doping causes imperfections in the surface. These inadequacies can stabilize the lithium salt while optimizing the electrochemical attributes of the electrolyte [35].

Thermal stability

Several processes have been studied and explored to enhance their thermal stability, including cross-linking, creating block copolymers, adding plasticizers, and inserting ceramic fillers. The distribution of ceramic filler materials in a matrix of polymers (relationship between the polymers and ceramics fillers) for the production of CPEs has captured the attention of researchers in the vast majority of these efforts because it has the possibility of helping boost the thermal durability of polymeric electrolytes [39].

Chemical stability

The chemical property of the active filler electrolyte determines storage and manufacturing facilities’ situations, and it also impacts the selection of suitable electrode materials. Polymer electrolytes typically exhibit robust chemical stability in the air; however, they must be maintained in an environment with little moisture to prevent H₂O adsorption. Fillers made of inorganic materials can collect contaminants of H₂O or solvents at filler surfaces, preventing H₂O or solvent interaction with the Li anode. It is essential to point out that a polymeric element of the active filler’s electrolyte may function as a layer of buffering to prevent direct contact and severe sensitivity of inorganic electrolyte (including Ti-containing oxides, which are LATP and LLTO) with the Li-metal anode [37].

Mechanisms of ion conduction for polymer electrolytes

The ion transfer mechanism is the essential characteristic of ionic conductivity. The motion of vacant positions or interstitial ions in the phase change of bulk composite polymer electrolytes results in fast ion conduction. In contrast, the movement of ions in a matrix of polymers according to an electric field is related to the breaks/formations of bonds of coordination throughout the local segment’s motions of chains of polymers, which mostly happens in the amorphous sections. Most solid polymer electrolytes have a maximum ionic conductivity below 10⁻³ S cm⁻¹ at ambient temperature [40,41,42] (Fig. 3).

Fig. 3

A The relationship between polymer stiffness and ionic conductivity, and (B) the behavior of polymer segments above and below the glass transition temperature, which promotes ion conduction

The amount of moving Li⁺ is impacted by the rate of dissolution and dissociation in the lithium salts. The polymers’ molecules should work jointly with Li⁺ to increase the dissolution degrees within the lithium salts, corresponding to the Lewis acid-base properties. As a result, polar chemicals such as –O–, C=O, –N–, –S–, and C=N are routinely integrated into polymer matrix architectures.

CPEs have an ionic conductivity given by

$$\sigma =\sum n\;q\;\mu$$

Here, n is the number of carriers, q is the ionic charge, and μ is the carrier mobility [43, 44].

The requirement, as mentioned earlier, could potentially be encountered by commonly employed active fillers such as LLTO (perovskite), LLZO (garnet), LAGP (NASICON), and LGPS (sulfide), and the ion conductivity of the majority of composite polymer electrolytes is well achieved 10⁻³ S cm⁻¹ at ambient temperature. Active filler reinforcing the mentioned ion-conducting routes occurs in composite polymer electrolytes, and incredible beneficial effects are accomplished. This ceramic layer acts as a plasticizer, diminishing polymer crystallinity and increasing the proportion of amorphous structure, enhancing lithium-ion mobility. Additionally, the acidic compounds on the surface of ceramics possess a powerful attraction for anions, facilitating the dissociation of lithium salts, which gives rise to a more significant amount of free Li-ion. Similarly, numerous vacant spaces occur on the surfaces of ceramics, enabling lithium ions to hop among the two and consequently offering a more manageable way compared to polymeric electrolytes [20, 45,46,47]. Some ion transmission models have been mentioned, notably Vogel-Tamman-Fulcher (VTF), Arrhenius, Williams–Landel–Ferry (WLF), and free-volume models (Fig. 4).

Fig. 4

The strategies for improving the ion conductivity of polymer electrolytes

Vogel-Tamman-Fulcher

The VTF equation is also fulfilled by the polymeric segments’ mobility and the conductivity of the ion relationship.

$$\sigma ={AT}^{0.5\ \textrm{e}-\textrm{Ea}/k\left(T-{T}_0\right)}$$

A is the pre-exponential factor, Ea is the apparent activation energy, K is a constant, T is the ambient temperature, and T₀ is the ideal glass transition temperature. It is generally considered that Tg is 50 K lower than the glass transition temperature (Tg) [48].

The VTF equation indicates that Li⁺ transportation is linked to the polymer chain segmentation movement. Li⁺ transfer in SPEs frequently happens mainly in the amorphous space above Tg. Polymers with a lower Tg exhibit greater segment mobility, which at first may promote the movement of ions and consequently boost the conductivity of ions. The VTF model additionally indicates that minimizing activation energy may increase ionic conductivity; consequently, eliminating a high amount of lithium salts has advantages for swift ion transport [49].

Arrhenius model

As shown here, the equation developed by Arrhenius may be utilized to describe ion hopping.

$$\sigma =A/T{\textrm{e}}^{-\textrm{Ea}/ RT}$$

where σ is the ionic conductivity, A is the pre-exponential factor, T is the ambient temperature, Ea is the apparent activation energy, and R is the ideal constant.

The Arrhenius model formula demonstrates that raising the temperature and decreasing the activation energy can significantly improve the conductivity of ionic compounds. The polymer’s backbone and accessory chains’ movement frequency and amplitude should similarly increase. The coordination grows more precise as temperatures rise. The amorphous polymer’s segmental motion would become fully dissociated from the movement of the ions, and ions would have to use less energy to jump from one location to another [50, 51].

Free volume model

The vibration within chain molecules in polymeric electrolytes may influence the space around the molecular chains, resulting in vacancies and accessible spaces. The free volume of polymer electrolytes can be influenced by the molecular weight of their end groups, the temperature, and the lithium salt. The free volume model concept highlights that the free volume dimension determines the migration of ions and electrolytes’ conductivity of ions. Somebody has connected the free volume to polymer segment motion and discovered that boosted free volume might enhance polymer segment motion.

The inverse proportion of the free volume fraction can be roughly linearly connected to the logarithmic of ionic conductivity.

$$\textrm{In}\ \sigma ={\sigma}^{\ast }+\beta /{\textrm{f}}_{\textrm{V}}$$

where fv is the free volume fraction, σ* is the ionic conductivity at the highest free volume fraction, and β is the activation coefficient of the thermal free volume fraction.

As mentioned before, the free volume model formula above demonstrates that temperature enormously affects free volume. While temperatures grow, the polymeric segment’s vibrating ability improves to overcome potential barriers to movement between locations, and the process of expansion produces additional free volume that encourages the transportation of the polymeric sections, Li⁺, or solved molecule. Consequently, a boost in the conductivity of ions may additionally be related to enhancements in polymer segmentation movement and ions’ hopping capability [52].

Williams–Landel–Ferry model

The Williams-Landel-Ferry equation has been developed after improving the VTF equation to account for the consequences of polymer viscosity and chain segment relaxation.

$$\textrm{Log}\ \textrm{aT}=\left\{\left(-{\textrm{C}}_1\right)\left(\textrm{T}-{\textrm{T}}_{\textrm{S}}\right)/\left({\textrm{C}}_2\right)+\left(\textrm{T}-\textrm{Ts}\right)\right\}$$

where aT is the displacement factor, which represents the ratio of the relaxation time of the chain segments, and Ts is the reference temperature, usually represented by Tg. C₁ and C₂ are constants [30, 37].

The WLF formula additionally demonstrates, therefore, that the volume of freedom has influenced the movement of ions in polymer electrolytes and that the micro-Brownian movement of chain segments has an essential effect on ion transport in amorphous domains. Decreased Tg is one more advantageous approach to improving the mobility of polymer segments with excellent ionic conductivity. Ions that are charged could readily travel in the space provided by the polymer-free volume and migrate among sites of coordination along chains of polymers exceeding the Tg. The WLF designs also prioritize the significance of free volume and the surrounding temperature, and expanding the temperature may substantially alter the free volume size/fraction and crystallization; nevertheless, enhancing the operating temperature of solid-state lithium-ion batteries for fast ion transmission would not be practical [53, 54].

The interface between the electrode and electrolyte

The interface between the electrodes and the electrolyte plays a role in how a battery performs. This becomes more crucial when using composite polymer electrolytes. To achieve performance in lithium-based batteries, it is essential to establish an efficient interface between the composite polymer electrolyte and the electrodes [55].

However, it is worth noting that even though most composite polymer electrolytes handle the movement of lithium ions, we should recognize the conduction of ions at the point where the electrode and electrolyte meet. This ion conduction at the electrode interface differs from during most phases in composite polymer electrolytes. Additionally, establishing a connection between the electrolyte and electrode remains challenging for solid-state lithium batteries (SSLBs). Various factors, like how the electrolyte and electrode interact, as growth and disintegration in high-pressure areas, all impact interface stability. When it comes to the batteries made with CPEs, three types of interfaces are involved: the connection between the anode and CPEs, the interface between the cathode and CPEs, and finally, the interface between fillers and polymer materials [56,57,58]. In the years, there has been increasing attention given to composite polymer electrolytes consisting of both organic and inorganic components as a solution to tackle these problems (Fig. 5).

Fig. 5

The interface between electrode and electrolyte

Developments of the CPEs/cathode interfaces

The electrolytes generate numerous polarization regions because of interactions between molecules in an electrical field, decreasing electromechanical features [59]. High-voltage compatibility of the electrolyte indicates its ability to withstand oxidative breakdown thermodynamically. Every component of the electrolytes (polymers, lithium salts, additives, etc.) needs to possess a lower maximum occupied molecular orbital (HOMO) compared to the cathode. Utilizing Lewis acid and base correlations (vacancy, dipole-to-dipole correlations, and hydrogen bonding) with polymeric and lithium salts, inorganic fillers can enhance the electrochemical capacity of CPEs [60].

The cathode/electrolyte interaction necessitates solid electrolytes with excellent adaptability to offer low interaction resistance. As a result of comparing the high viscosity and flexibility of the two-component composite cathode and the electrolyte, no holes or voids were found in the interface regions, enabling the cathode’s close interaction with the CPEs and minimal interface impedance [61]. The transport of the ions and electrons inside a cathode is equally essential as the cathode/electrolyte interaction in pursuing the high energy density and expanded life span of the all-solid-state batteries. Establishing ion/electron conducting routes in both the cathode and interfaces is vital for reducing polarization while maintaining the battery’s total capacity [34, 62, 63].

Furthermore, compared to beneficial interaction and an established incorporated conducting network, the broad electrochemical window of CPEs needs to be present for developing a stable connection between cathode and CPEs, which is crucial for applying high-voltage cathode in high-energy-density systems [64, 65]. Overall, the electrochemical range of polymer electrolytes is less than 5 volts vs. Li/Li⁺; nevertheless, ceramics electrolytes can reach up to 9V or even higher. For CPEs, a ceramic filler could increase the electrochemical window above 5V, allowing most cathode materials with substantial operating voltage voltages to be supported. Generally, a higher ceramic concentration may enhance electrochemical stability [66, 67].

Developments of the CPEs/anode interfaces

On the contrary, the anode/electrolyte connection needed a solid electrolyte to survive through the penetration of metal lithium dendrites. The minimal interface resistance can be achieved due to the flexibility of the polymer electrolytes, regardless of the low mechanical characteristics that make it impossible for them to endure the absorption of the metallic lithium dendrite [68, 69]. The polymer host offers continuous ion-conducting networks and safeguards ceramic fragments from the electrode to minimize adverse effects; nonetheless, they also offer kind interaction with electrodes for reducing Li-ion transfer resistance, possibly establishing a uniform Li-ion movement and preventing lithium dendrite creation. Owing to the insufficient connection that exists between the lithium anode and the CPEs, too much impedance and heterogeneous lithium-ion deposition happen, leading to unsatisfactory rate stability and uncontrollable lithium dendrite growth possess excellent compatibility with Li metal and functions effectively in suppressing lithium dendrite development [70, 71].

Lithium dendrites have the potential to penetrate the electrolyte, establishing contact with both the cathodes and anodes. Recent research suggests incorporating inorganic additives can enhance the compatibility between the solid-state electrolyte and the anode [72,73,74]. These are some of the roles played by additives in reducing interfacial issues. Firstly, they help regulate ions’ movement in the electrolyte’s bulk phase, ensuring a more uniform distribution of lithium ions [75]. This regulation helps control and prevent the formation of lithium dendrites at their source. Secondly, these additives increase the strength of CPEs, which helps suppress lithium formation and dendrite growth [76, 77].

Interface between fillers and polymer materials

The interface challenges can develop among electrodes and CPEs and chains of polymers and ceramic fillers. Numerous investigations have already been performed to identify novel techniques for improving the interface between polymers and ceramics. Despite the interactions connecting ceramic nanoparticles and polymer chains in CPEs attracting fewer resources, it is of the utmost importance to supply a low-resistance connection between ceramic and polymer for outstanding performance in all-solid-state batteries [78]. To enhance the performance of all solid-state rechargeable batteries, it is possible that improving the interactions between fillers and polymer chains in CPEs could effectively address the underlying issues associated with CPEs [79]. The uniformly distributed dispersion of ceramic fillers certainly contributes to constructing continuous fast speeds and equal Li-ion migration channels because of the high ions conducting the interphase between ceramics and polymer. Ceramics particle dimension, form, structure, and three-dimensional ceramic framework in CPEs affect the interface characteristics between ceramic and polymer. Considering that interface investigation is at the beginning stages, multiple states of different chemical and physical addresses have been employed for creating an optimal ceramic-polymer interface, which is crucial for promoting the creation and application of all-solid-state lithium battery packs [80].

Influence of size, dimensionality, concentration, and alignment of filler in polymer electrolyte

Size

The overall size of particles of engraved materials like ceramics significantly impacts the conductivity of ions of SPEs. Particles that are smaller that have a greater specific area of surface and a more significant number of abundant active sites could make the host more crystalline of polymer substrates and help promote the disintegrating of the lithium salts with greater efficiency, generating numerous ion-conducting pathways transporting lithium charges. The formation of long-range lithium-ion channels with long ranges with consequently powerful ionic conductivity is made possible by the nano- and micro-meter scales. Nano-sized particles also accelerate the advancement of polymer electrolyte ion conductivity, and their more minor size results in higher ionic conductivity. Due to their high surface energy, ceramic material nanoparticles rely primarily on aggregating and separating phases in a polymer matrix, to guarantee high polymer electrolyte standardization and prevent nonuniform lithium-ion (Li⁺) transport [10, 80, 81].

Dimensionality

The ceramic electrolyte’s shape directly influences the path and dimension associated with the lithium-ion direction; they are the amount of supply. Nanomaterials have also been divided as zero-dimensional (0D) (which includes nanoparticles), one-dimensional (1D) (including variables such as nanotubes and nanorods), two-dimensional (2D) (such as graphene), and three-dimensional (3D) as well (such as nano prisms and nanoflowers). By reducing polymer crystallinity while providing more extraordinary Li⁺ diffusion pathways to CPEs, (0D) nanoparticles could enhance Li⁺ conductivity and mechanical strength. Despite significantly improving Li⁺ conductivity, soft CPEs with low percolating threshold values (30 wt%) cannot successfully prevent lithium dendrite development [78]. For CPEs, an interconnecting framework has been constructed for elaborately generated 2D LLZO nanosheets or 3D LLZO nanowires. Furthermore, it provides broadened ionic conduction of the channels [82] (Fig 6).

Fig. 6

The dimensionality. A Nanoparticles, B nanorod, C nanoplates, D nanofiber

Nevertheless, substantial agglomerate associated with this filler may persist, decreasing the composite electrolytes’ long-term effectiveness. A middle layer between stiff inorganic fillers and an organic polymer matrix must be constructed to tackle this interface compatibility problem and permit a consistent distribution of inorganic additives with high content [24]. The enhanced performance in the conductivity of ions could be attributed essentially to the achievement of a continuously 3D ion-conducting networking path that offers far-reaching rapid lithium-ion transmission pathways. As an outcome, especially in comparison with ceramics nanoparticles, the nanowires, fibers, and nanosheets in the polymer substrate can provide continuous Li-ion transportation pathways, contributing to the more effective conductivity of ions. In addition, ceramics fillers’ nanowires, fiber, and nanosheets could be used for the construction of 3D or 2D lithium-ion transport routes and networks for high-efficiency, far-reaching lithium-ion transmission [20, 80].

Alignment of filler

Inorganic substances in polymers enable one to utilize CPEs completely. Inorganic compounds are frequently spread randomly in polymers. The included number of inorganic fillers causes disruptions in polymer crystallization, increasing the conductivity of CPE ions. Nevertheless, filler substances will inevitably come into the polymer, prohibiting the formation of a percolating network. Subsequently, it is feasible to generate percolating networks by promoting the dispersion and transport of fillers in polymers. The inorganic fillers and polymers interact via Lewis’ acid-base interaction. The idea behind it is based on the presumption that fast-ion-conducting pathways could have been constructed on the surface of the filler. Ceramic filler materials have developed from solitary 0D particles, 1D nanowires, and 2D nanosheets to continuous three-dimensional frameworks and bulk components. There are (0D and 1D) inorganic fillers randomly distributed throughout the polymer chains, whose distribution is similarly irregularly disseminated. Li-ions that pass through channels in solid composite electrolytes appear disorganized since nanoparticles and nanowires are distributed randomly within the polymer matrix. Considering various ceramics with different structures, nanowires significantly boost the conductivity of ions in polymeric electrolytes [82].

Additionally, due to their structural characteristics, 2D fillers have immense significance. Small-sized, two-dimensional nanosheets are becoming increasingly common among researchers for practical applications. This is caused by the challenge of offering continuous transport of ion pathways in the large dimensions of 2D nanosheets of material. In addition, larger-size 2-dimensional nanosheets show very little ability to prevent polymeric matrix crystallization. Nevertheless, 2-dimensional fillers possess a high specific surface area, a fragile spherical structure, and a substantial aspect ratio. Whenever the size of the 2D nanosheet is small enough, a more significant contact area between it and the polymer matrix could be produced. Among the 2D nanosheet-polymer interactions, additional ion conductivity will be constructed, culminating in improved ionic conductivity. In the polymer matrix, the additive (3D) swiftly aggregates. A successful approach for addressing the dispersion mentioned above is to build a 3D framework by controlling the position of the filler space in the polymers. Moreover, the inorganic system has excellent mechanical strength, enabling it to prevent lithium dendrites from forming while boosting cycle stability [20, 83] (Fig. 7).

Fig. 7

The (A) alignment of nanoparticles (NPs), B alignment of random nanowires (NWs), C aligned nanowires (NWs) conducting channels involving the lithium ions transport in composite polymer electrolytes

Concentration of fillers

The filler concentration polymer electrolytes have vital functions. The functional structures on the surface of inorganic additives will additionally impact carrier concentration in composite polymer electrolytes and polymer chain movement. The intrinsic transport of ions reflects these variances in effectiveness. Furthermore, some filler quantities can improve the relationship between the electrode and the electrolyte by acting synergistically and lowering the ion transport barrier at the interface. Additionally, because of the many continuous inefficiencies in active fillers that have low energy of activation that enables facile ion hopping, active fillers may provide an enormous amount of lithium ions, thereby increasing the number of free ions of lithium at the point of contact between the active filler and the polymer itself [78]. As a consequence, the general conductivity of ions changes. perovskite, garnet, LISICON, and various other active fillers are examples. Whenever the active filler ratio is below 40%, CPEs could provide a significant amount of free lithium ions. Once the concentration of active filler exceeds a specific point, it forms a porous network. The way ions behave in transportation mechanisms is shifting at the moment. The specific quantity concentration of inorganic additives and polymers includes the complex relationship of the Lewis acid-base connection. This is based on the presumption that fast-ion-conducting communication could have been developed on the surface of the filler conductivity of ions of frequently employed active fillers for polymers. Active fillers were familiar with excellent ion conductivity (> 10⁻⁴ S cm⁻¹) [20].

Properties of lithium salt

In the case of lithium batteries, the electrolytes act like an interface over ion transfer between negative and positive electrodes. Lithium salt is the supplier of the lithium-ions in the polymeric electrolyte (not the single-ion polymer electrolyte), has become a vital component of the electrolyte, and has a significant impact on the battery’s functionality variables, including capability, beneficial temperatures, cycle capability, and security accomplishments. The nature and quantity of lithium salts present have an essential effect not merely on the polymer’s solid-state electrolyte’s film-forming capacity but additionally on the conductivity of ions and interfacial durability of the electrolyte’s solid state and the electrodes themselves (Table 1)

Table 1 The properties of lithium salt

Relationships of polymer, lithium salt, and inorganic fillers

The polymers, inorganic filler, and lithium salt simultaneously interplay in composite polymer electrolyte. This relationship is accomplished mainly through two distinct forms:

• The connection that occurs exists between the filler and the lithium salt. This is accomplished by altering the chemical environment of lithium-ion batteries.

• The relationship between the filler and the polymer is primarily seen in shifts in the conductivity of ions and Li⁺. Modifications in the polymer aggregated architecture have been involved. It is identified by the degree of crystallinity, glass transition temperature, and spherulites [35].

Interacting between polymers and inorganic fillers

Since the inorganic fillers can frequently be very tiny sized and equally spread within the polymer matrix, there are additionally numerous insignificant interfacial fields in the whole composites’ polymer electrolyte. Adding to that, maintaining the condition of these interface locations is difficult due to the substantial differences in chemical and physical properties between the matrix of polymers and inorganic ceramics fillers. Surfaces of inorganic filling agents are defect-rich and highly receptive, helping particles to come into contact with different elements and create complicated interface regions. For example, composite electrolytes established from active organic fillers and a polymer substrate are one option. Compared to the mixture of the polymer matrix and lithium salts, active fillers are lithium-rich, and the state of lithium ions in the interfacial regions changes substantially from those that qualify the vast majority of the polymer matrix and active fillers. These modifications significantly alter the lithium-ion conducting pathways [33].

Interacting between polymers and lithium salts

The fundamental ion-dipole relationship among ions and polymers is the reason for the lithium salt disassociation. It is the essential factor responsible for liberated concentrations of ions and having a significant impact on ionic conductivity. In addition, the relationship between anion and anion receiver could boost the lithium ion’s transfer number, thus decreasing the polarization of concentration of lithium batteries. As a result of its strong polarity, plastic crystals might reduce the chemical interaction capability of Li⁺ by polymers and ionize lithium salt, which makes it favorable to the decomplexation of Li⁺ by polymers, which contributes to a boost in the conductivity of ions. The ion-dipole relationship possesses an essential impact on ionic conductivity likewise. The ionic conductivity was highest among polyethers at a salt level of 5 mol%. The conductivity was reduced, and the Tg value gradually enhanced with increasing lithium-salt concentrations to 5 mol%. Owing to the significant attraction among polyether and Li⁺, cross-linking formations have been formed, limiting segmental movements within local chains [91].

Interacting between fillers and lithium salt

The groups of chemicals occur upon the surfaces of inorganic fillers. A combination of the lithium salts, such fillers exhibit significant Lewis acid-base interaction. Bonds of hydrogen, holes, and dipole-dipole relationships are characteristics of this interaction [92]. The effective reactions between lithium salt anionic groups and ceramics filler groups of chemicals promote lithium salt dissociation and boost free Li⁺ concentration at the point of interaction [93]. The unique properties of chemicals on the outer surface of inorganic additives may interact with lithium ions to weaken the bonds between lithium-ion and polymer compounds, allowing lithium ions to travel considerably faster [52].

Type of active fillers

Active fillers are essential additions to composite polymer electrolytes. They have several benefits, notably increased thermal stability, mechanical strength, and electrochemical performance. By thoughtfully choosing and introducing suitable active fillers, investigators could modify the characteristics of CPEs according to the specific demands of different electromechanical electronic gadgets, offering the possibility of intriguing and efficient energy preservation and conversion technologies. Crucial additions. They have several benefits, notably increased electrochemical performance, thermal stability, and mechanical strength (Fig. 8).

Fig. 8

The types of active fillers

Garnet-type composite polymer electrolytes

The most recent identification of (LLZO) garnet-type material electrolytes has been viewed as one of those most appealing along with essential electrolytes for solid-state batteries, with potential advantages in substantial electrochemical stability (>6 V versus Li/Li⁺ from CV experiments), excellent thermal strength, energy density, safety, and high ionic conductivity (range 10⁻⁴ to 10⁻³ S/cm), which offer an outstanding possibility for battery technology [90]. The crystal structure-based garnet-type filler (LLZO) classified into two types: the first one is cubic (space group Ia-3d, cubic-LLZO) and the second is tetragonal (space group I41-acd, tetragonal-LLZO) crystal structures. The fundamental disadvantage of the tetragonal LLZO is its poor lithium-ion conductivity of 10⁻⁶ S cm⁻¹, whereas the cubic LLZO has a desirable high ionic conductivity of 10⁻⁴ S cm⁻¹. However, at room temperature, a cubic crystal shape is unsustainable [94].

To improve the room temperature stability of cubic LLZO, doped super-valent cations include Al³⁺ and Ga³⁺. Additionally, multiple elements like Nb⁵⁺ and Ti⁴⁺ have been incorporated into the cubic LLZO crystalline structure to enhance its conductivity massively. The above component-doped LLZO displays conductivity ions of up to 10⁻³ S/cm at average temperature and remarkable chemical strength across a broad range of temperatures [95,96,97]. In addition to the conceptual design, one of the most notable improvements in garnet-type solid electrolytes should generate future electrolyte growth. At ambient temperature, Ta-doped LLZO ceramics electrolyte (LLZTO) does have a Li⁺ ion conductivity of 10⁻⁴ to 10⁻³ Scm⁻¹, which would be considerably greater than LLZO and even comparable to conventional aqueous electrolytes. Ta-doped LLZTO-polymer composite electrolytes have also been researched by incorporating Ta-doped LLZTO particles into the polymeric matrix, which have characteristics including lightweight, good processability, and good electrode contact. Unfortunately, overall, Li⁺ transfer numbers are slightly lower than 0.8 owing to the inert matrix of polymer, and heating treatment above 150 °C continues to be needed to dissolve the polymers and produce composite electrolytes [89]. Ting Yang et al. demonstrate that by adding just 5% to a ceramics filler that includes untreated cubic-phase LLZO nanowires, as Al-doped and Ta-doped LLZO nanowires, composite polymer electrolytes ionic conductivities in the 10⁻⁴ S/cm range at 20°C [98]. To stabilize the cubic LLZO and generate Li-ion vacancy, super-valent cation substitution on the Zr sites (16b) was affected. Ta-enriched LLZO with only a factor of 0.5, Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLTZO), offers the greatest vacancy allowing Li-ion transmission [99].

Yi, Maoyi, et al. show that Ga-doped Li₇La₃Zr₂O₁₂ (Ga-LLZO) is a potential solid electrolyte with a more excellent Li-ion conductivity of 5×10⁻³ S/cm at 30°C and an activation energy of 0.28 eV [100]. Dopants are substances that have the potential to enhance the number of vacancies disordered within the Li sub-lattice, accelerate improved hopping pathways for Li⁺, and enhance conductivity. The replacement of 0.2–0.24 mol of aluminum (oxidation of Al³⁺) for lithium (Li) outcomes in a generation of 0.4–0.48 mol of lithium (Li⁺) vacancy each LLZO unit of formula and maintains the cubic phases at 298 K, the conductivity of ionic substances increases to 0.4 mS/cm throughout this phase of the reaction, and Ta⁵⁺ doping has been selected over Al-doped for the reason that Ta. At the same time, Al is not positioned on the Li sublattice, which consequently offers more substantial conductivity. As a matter of fact, at 298K, Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (0.5 lithium (Li) vacancy per formula unit) is blessed with a conductivity of ions within the proximity of 1 S/cm [101,102,103] (Table 2).

Table 2 The properties of Garnet-based composite-based polymer electrolytes (GCPEs)

NASICON-type composite polymer electrolyte

The solid electrolyte material NASICON-type (LATP) (Na-Super-Ionic Conductor) has been frequently utilized in CPEs for lithium (Li⁺)-based batteries because it exhibits a significant increase in conductivity when compared to other substances with equivalent crystal framework [116]. For describing the prospective structures of crystals of NASICON materials, three types of systems are frequently utilized:

• 1. NASICON: C2/c, ~ a 15.1Å, b~ 8.7Å, c~ 21.6Å, and 90.2°, which generally occurs at temperatures near 300 K.

• 2. NASICON: C2/c, a 15.1Å, b~ 8.7Å, c~ 8.8Å, and 124°, which occurs at temperatures ranging from 300 to 450 K.

• 3. NASICON: R3c, an~ 8.7Å, and c 21.9Å, the highest symmetry framework in the NASICON substances, could be accomplished above 450 K.

The broad spectrum of attainable combinations within the fundamental formula is an exciting component of NASICON building construction. Na_xM₂(SiO₄)_z(PO₄)3z, whereby x may fluctuate bounded by 0 and 4 M, can be divalent, trivalent, tetravalent, and pentavalent transitional metal cations. By swapping the location of the (M) site, electrochemical and ion conduction attributes may be adjusted for specific applications [117]. The NASICON type has two phases. First, the conducting phase is LiTi₂(PO₄)₃ (LTP), while the insulating phase is (AlPO₄) (ALP). Its conducting phase LiTi₂(PO₄)₃ (LTP) is made up of voids in a 3-D system of conductivity routes, and interstitial Li⁺ ions migrate through the conduction routes via ultrafast and slower modes. At the room’s temperature, the overall conductivity is 10⁻³S/cm. On the other hand, LATP’s boundary grain conductivity is several orders of magnitude lower than its bulk value, decreasing the total operational effectiveness of a solid-state battery. Ordinary grain boundary conductivity has been caused by a combination of variables, including low density because of grain spacing, the existence of insulating additional phases such as AlPO₄ at the grain boundary, and decreased size of particles due to a diminished sintering capability. Secondary phases can be generated in modest quantities by adding the lattice with the trivalent elements Sc³⁺ and Y³⁺, with an ionic radius more remarkable than the equivalent host cations (Ti⁴⁺). The s preparation process causes second-phase separation at the grain boundaries [118, 119]. The conducting phase LTP structural unit is constructed from up of TiO₆ octahedrons and PO₄ tetrahedrons, which form a three-dimensional interconnected channel that is excellent for lithium-ion transfer. In the meantime, a partial substitution of trivalent cations (Al³⁺, Fe³⁺, and Y³⁺, for example) may significantly improve ionic conductivity. Following the investigation, partial replacement of Ti⁴⁺ in the LTP crystal structure with Al³⁺ ions to generate Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) could boost the conductivity of ions through improving concentrations of carriers and system configurations by preventing Ti⁴⁺ oxidization [120].

The particular gel polymer electrolyte (GPE) membranes exhibit insufficient mechanical strength, especially after electrolyte uptake. Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) has been distributed or embedded in the gel polymer matrix to enhance mechanical stability. Furthermore, the connection of inorganic particles using electrolytes made of polymers has been extensively researched [115]. As a consequence of their high lithium-ion conductivity and expansive electrochemical windows, NASICON-type (LATP) composite form substances Li_1+xAl_xM_2-x(PO₄)₃ (M = Zr, Ge, Ti, etc.) have been receiving a lot of interest as solid electrolyte substances, thereby providing a natural match for the electrolyte substance in extremely all-solid-state lithium-ion batteries having a high voltage and glass-ceramics with NASICON-type structured compound (LAGP) which offer better stability as well as elevated relative densities due to a substantially more solid microstructure compared to LAGP ceramics [121]. The NASICON-type structured (LATP) more effectively both air and water stability owing to strong P–O bond correlations within the LATP framework and having low relative density ~2.9 g/cm³ versus 5.1 g/cm³ for Li₇La₃Zr₂O₁₂ [122].

Several beneficial techniques, which include incorporating the sintering disabilities, enhancing the procedure for sintering, and doping, also known as elements via a solid electrolyte structure, are frequently employed to improve its capacity to transport lithium ions around within solid electrolytes as well as elements’ doping systems utilized which is a simple and effective way of improving both the physical and electrochemical characteristics of LATP solid electrolytes. Further investigations on Si⁴⁺, Y³⁺, Ge⁴⁺, Ga³⁺, and Nb⁴⁺ suggested that doped and inclusiveness within reliable electrolyte systems might enhance the lattice’s volumes and compression density within the system as a whole, which contributed to improved ion conduction of solids electrolyte [123,124,125]. Zhao, Erqing et al. show that PEO-LITFSI-LATP ionic conductivity values of solid electrolytes that have been modified containing LATP nanoparticles calcined as 750°C are 4.02×10⁻⁴ S/cm and 7.42×10⁻⁶ S/cm at 55°C and 25°C, correspondingly, as those of solid-state electrolytes reform with LATP nanoparticles heated at 900°C were 4.18×10⁻⁴ S/cm 4.93×10⁻⁶ S/cm and at 50°C and 25°C, respectively [126] (Table 3).

Table 3 The properties of NASICON-based composite-based polymer electrolytes (NCPEs)

Perovskite-type composite polymer electrolyte

Perovskite (ABO₃)-type (A and B are both metal cations) lithium-ion conductivity with structurally associated changes has been identified in the past few years. Several investigations have documented ranging the ionic conductivity measurements via switching out of the two s well as B sites by various ions, and with ionized the conductivity values accomplishing 10⁻³ Scm⁻¹ (x=0.11) following optimization determined suitable large quantities Li⁺ conductive properties’ information among different solid electrolytes. The majority of the widespread formulations with a solid-state perovskite-type lithium lanthanum titanate electrolyte were Li_3x La _(2/3)-xTiO₃ (LLTO), using values of x that varied between 0.07 and 0.13 and energies of activation fluctuating around 0.3 to 0.4 eV. The enhanced conductive properties of LLTO ions are attributed to a significant amount of A-site void positions, and researchers have discovered that cationic shortfalls at a location resulting from each of the four neighboring TiO₆ hexagonal shapes may facilitate Li⁺ cationic transfers throughout bottlenecks [139,140,141,142]. The cations a deficit in the A location reduced monovalent cation conductivity via ions hopping throughout the bottleneck that was generated by the four adjacent nations. BO₆ octahedra compared with LLTO was paired with metallic Li; the conductivity of ions enhanced substantially when compared with blocking electrodes like stainless steel. The ionic transportation boosts as the ambient temperature increases. At increasing temperatures, solid electrolytes showed 2- to 3-fold more substantial conductivity. Doping is also known as additionally enhanced Li-ion propagation by raising the volume of the bottleneck. To boost ionic conductivity, fluoride ion substitution (F for O-2) has further been investigated [143].

The absence of these structures enables monovalent cations that are present to make their way through the barriers generated by the four neighboring BO octahedra. Because the A- and the B-sites in the mentioned framework can tolerate an assortment of ions that have different pleasant states, many investigations upon the effect on the conductivity of ions of the A-site substitutions in Ln_0.5Li_0.5TiO₃ (Ln = La, Pr, Nd, Sm) and the B-site swapping in Ln_1/3Li_xNb_1-x, TiO₃ (Ln = La, Nd) are being carried out and released. The best possible conductivity at regular temperatures is 10⁻³ s/cm⁻¹ [144]. The ionic conductivity of La-doped (LLTO) is lowered; nonetheless, putting La with more significant Sr ions increases the ion conductivity. Consequently, it was recently reported that changing ions that are smaller with La improves A-site area decrease, which leads to reduced constriction sizes throughout lithium-ion movement [145].

Doping, more commonly referred to as the technique, has become increasingly prevalent to improve the general conductivity of LLTO electrolytic membranes. The enhancement in performance may have been attributable to improved bulk conductivity and boosted grain border conductivity or some combination of both. The overall conductivity of Ge-doped LLTO electrolyte membranes (1.2×10⁻⁵ S cm⁻¹) was a factor of one order of magnitude more substantial than that found in organic LLTO. The all-around conductivity of Zr-doped LLTO (5.84×10⁻⁵ S cm⁻¹) was approximately 1.9 times that of the undoped specimen. The sort mentioned above of transformation enhanced the overall conductivity alongside the boundary grain conductivity within the electrolyte films [64, 146]. Zhu, Pei, and co-workers investigate PEO/LiTFSI/LLTO solid composite electrolyte ionic conductivity values. The highest value acquired whenever 15 wt% LLTO was introduced to PEO/LiTFSI concrete composite electrolyte at a temperature at room temperature was 2.4×10⁻⁴ S cm⁻¹. PEO/LiTFSI, PEO/LiTFSI/LLTO (10 wt%), and PEO/LiTFSI/LLTO (20 wt%) composed of solid electrolytes possess ionic conductivity values of 1.75 ×10⁻⁵ S cm⁻¹, 6.2 ×10⁻⁵ S cm⁻¹, and 5.5 ×10⁻⁵ S cm⁻¹, respectfully [147] (Table 4).

Table 4 The properties of perovskite-based composite polymer electrolytes (PCPEs)

Sulfide-type composite polymer electrolyte

Solid sulfide electrolytes have been gaining prominence because of their high level of conductivity, which renders them analogous to liquid electrolytes. Solid sulfide electrolytes are produced by swapping oxygen-based ions in oxide solid electrolytes with sulfide ions. According to the lower electro-negativity, the amount of interaction involving sulfide and the lithium ions is less than that connecting oxygen and lithium-ion, which therefore could result in a more incredible amount of free-moving lithium ions. Additionally, a sulfide ion’s circumference is more significant than an oxygen ion. As a direct consequence, the sulfide solidified electrolyte could offer expanded migration tunnels overall lithium ions, which would prove advantageous for lithium-ion transportation. Due to being at an ambient temperature, solid sulfide electrolytes demonstrate high ionic conductivity values that fluctuate between 10⁻³ and 10⁻⁴ S cm⁻¹ [157,158,159].

The sulfide-based solid electrolytes are renowned for their helpful lithium-ion conductivity and broad electrochemical windows at ambient temperature. In the meantime, the compressible modulus of sulfide electrolytic is extremely low. Hence, its processing ability is outstanding, indicating that simple freezing may significantly decrease bulk resistivity at ambient temperature. Among the sulfide solid electrolytes, Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 offers the most effective lithium-ion conductivity towards the temperature of the room (25 S/cm), thereby rendering it an ideal application for solid-state Li-based batteries. Nevertheless, the chemical stability could be better and complicate industrial uses of Li_9.54Si_1.74P_1.44S_11.7Cl_0.3 [160]. A nonwoven product the framework has recently been employed for strengthening the sulfide electrolyte which generated thin (70–100 μm) composite material electrolytes alongside moderate ion conductivity (0.1 to 0.3 S cm⁻¹) as well as incorporating an excellent-conductivity sulfide with just a tiny amount of polymer compounds appears to offer a feasible technique for manufacturing thin composite electrolytes membranes that have elevated ionic conductivity; this is accepting that ion percolating is not hampered through the binder [161].

Elimination for crystallization within the polymeric encompassing the nanofiller, modifications in the Li⁺ polymer complexity, and improved transport through the surface are the methods used by the sulfide-type solid-state electrolyte for conductivity enhancement. It is particularly mandatory to emphasize that in these composite polymer structures containing different nanoparticles distributed in a polymer matrix, the ceramic’s fillers primarily enhance the conductivity within the polymer’s phases [162]. Nevertheless, making use of sulfide solid electrolytes continues to confront two significant obstacles: (1) chemical imbalance in the atmosphere caused by aqueous hydrolysis and (2) electromechanical instability using metal Li anode and oxide cathode components [163, 164]. The composite electrolytes encompassing LiSn₂(PO₄)₃ (LSP) wt. 30% PEO+LiClO₄ show a maximum ionic conductivity of 3.48 ×10⁻⁵ Scm⁻¹ at 27°C, resulting in maximization to 1.18×10⁻⁴ Scm⁻¹ at 60°C. The less activation energy of 0.34 eV results from greater lithium-ion mobility in a composite electrolyte [165] (Table 5).

Table 5 The properties of sulfide-based composite-based polymer electrolytes (SCPEs)

Inactive fillers

In general, inactive particle fillers fall into two groups: metal oxide ceramic particles (Al₂O₃, MgO₂, TiO₂, and SiO₂) and ferroelectric ceramic particles (BaTiO₃, PbTiO₃, and SrBi₄Ti₄O₁₅) [17]. The incorporation of nano-sized passive (or) inactive filler raised the Lewis-acid-based relationship that existed between the polarized function categories located on the filler’s surfaces and the ions in the charged particles, establishing an area for the transfer of charge translocation, which improved the mobility of ions and conductive properties. Due to their high dielectric value and separation of charges, ferroelectric ceramic fillers are used to enhance membrane polarization. The ion’s conductivity and Li⁺ interaction durability are improved by reinforcing polymeric amorphous areas [8, 176, 177]. Inactive additives incorporated into a PEO-based polymer matrix can change crystallization dynamics by adding local unstructured areas at the filler-polymer interaction. The density of entanglement in the amorphous space increases by constructing network nodes at the filler-polymer interface, resulting in improved mechanical properties of the materials. Li⁺ migration effectiveness is improved, leading to more excellent Li⁺ conduction [178].

Incorporating silicon dioxide, also known as SiO₂, in polymeric electrolytes was discovered to increase ionic conductivity, leading to greater interfacial, mechanical, and thermal distinctive features. The silica nanoparticles have a high surface-to-volume proportion, and Si–O–Si chains communicate with functional polymer groups, resulting in additional ion dissociation and improving ionic conductivity. Incorporating nonconducting silica nanoparticles with high concentrations could restrict the effective surface area between the electrode-electrolyte contact and affect the conductivity of ions [179]. However, a significant SiO₂ loading, on the other hand, may produce agglomerate and pulverization, which cause lagging kinetics. Likewise, using SiO₂ nanoparticles within a polymer solution could result in agglomeration and particle formation throughout the process, resulting in poor cycle efficiency [180].

Several include the more effective thermal, mechanical, and chemical capabilities, and also, because of its high surface area, titanium dioxide (TiO₂) is a suitable option for emerging technological innovations [181]. The PbTiO₃ (lead titanate) is a perovskite that exhibits a distorted tetragonal phase. Physical filler infiltration through the polymer’s hosting is commonly utilized to reduce crystallinity, promote physical durability, and increase electrolyte absorption in the solution [14]. The nanocomposite polymer electrolyte (NCPE) is primarily produced by combining metal oxides, including TiO₂, SiO₂, Al₂O₃, and ZrO₂, and the aluminates and titanates. Even small quantities of these fillers might have affected polymer electrolytes’ mechanical strength and ion conductivity. The enhanced ionic conductivity of NCPEs is influenced by various factors, including the function of ceramics filler as a kind of solid plasticizer [182,183,184,185] (Table 6).

Table 6 The properties of inactive fillers -based composite-based polymer electrolytes

Synthesis methods for composite polymer electrolyte preparation

The nanocomposites of polymers are produced through a wide range of methodologies, among them the solution cast technique, in situ polymerization technique, melt intercalation technique, spin coating technique, hot press technique, and dip coating technique.

Solution cast technique

Due to the convenience of manufacturing, it is the most common technique for manufacturing films from polymers of different thicknesses (50–300 μm). In the beginning, by continuous stirring, the necessary quantity of polymer host dissolves in the acceptable solvent. A desired amount of salt is subsequently added to the homogenous polymeric matrix and is stirred again until a homogeneous and homogeneous solution appears. For polymer nanocomposite production, a nanoparticle is initially mixed with the solvent before ultrasonication is employed to enhance the dispersion. The resulting solution is subsequently added to the polymer solution containing salt and stirred until the solution turns homogeneous. Subsequently, the viscous solutions are cast in a petri dish and kept at ambient temperature to allow the solvent to evaporate. Finally, the viscous solution is cast in a petri dish and kept at room temperature to allow the solvent to evaporate. The petri dish is subsequently placed in an oven under a vacuum to remove the remaining solvent, and the protective film gets peeled off [192,193,194,195] (Fig. 9).

Fig. 9

The preparation of the solution casting technique

Melt intercalation technique

This technology is fundamental because it helps protect the environment, is cost-effective, and does not require solvents. It is worth noting that optimizing the heat properties is necessary to improve how particles spread out, as extreme temperatures can change the surface properties of nanoparticles and cause them to break down. The process starts with heating the polymer matrix at a temperature. Then, add the nanoparticles while carefully mixing everything to ensure an even distribution. In addition, how the clay/nanofiller is modified on its surface, how it interacts with the polymers it is mixed, and various manufacturing factors all play a role, in how the nanoparticles are dispersed throughout [196,197,198,199] (Fig. 10).

Fig. 10

The preparation of the melt intercalation technique

Spin coating technique

The technique is identical to the solution cast method. The primary advantages of the spin coating method are its simplicity and relative ease of preparation, and it produces homogeneity thicknesses varying from a few nanometers to a few microns. A small solution is set on a substrate and then placed on the spin coater, which may revolve at the required speed. The spreads get spread on a substrate by centrifuge followed by heating that evaporates the solvent from the surface. To be coated, the substrate’s rotating axis must be perpendicular to the substrates. The overall thickness of the film is influenced by (i) the viscosity of the mixture, (ii) the level of concentration of the mixture, (iii) the rotational speed, and (iv) the spin. Fortunately, this technique is only beneficial for extremely viscous mixes rather than particularly high-viscosity mixtures. The spin coater rotation is unsuitable for distributing the mixture droplet to produce a thin film in a gel-like mixture [200,201,202] (Fig 11).

Fig. 11

The preparation of the spin coating technique

Hot press technique

The hot press technique is distinctive because it is low cost, is solvent-free, generates an excellent film with thick materials, and offers a quicker strategy. The setup contains a weighing cylinder, a heating chamber, a basement, and a temperature controller.

Initially, the polymer, the salt, and the nanoparticle are crushed for the desired amount of time in the agate mortar. The manufactured mixture is subsequently heated (to close to the polymer melting point), and the slurry that emerges is then sandwiched between the stainless-steel (SS) blocks. The mixture is followed by pressing pressure-controlled equipment, and the resulting film is utilized for further characterization [203, 204] (Fig. 12).

Fig. 12

The preparation of the hot press technique

Dip coating

The unique characteristic of dip coating is that it makes it possible for us to get excellent quality film on both sides of the substrates despite becoming a low-cost procedure. In the beginning, the chosen substrates are immersed in the solvent. In a three-step process, solvent evaporation is followed by immersion, deposition, and drainage. In immersion, the substrate is submerged in solution at a pace that allows sufficient time for coating. In the phases that follow phase, deposition, and drainage, the substrate is immersed in the solvent for a sufficient amount of time to allow the substrate to interact with the solvent. The substrate is now progressively drawn out at an even rate, which causes the creation of a thin coating on the substrate. In the final phase of evaporation, the solution of the solvent has evaporated, and the substrate may be warmed to eliminate any remaining solvent. The speed of elimination and the density of the resulting solution can be utilized to modify the thickness and quality of the film [205,206,207,208], (Fig 13).

Fig. 13

The preparation of the dip coating technique

Strategies and outlooks of the performance of composite polymer electrolytes

CPEs have been receiving ample attention in recent years as potential possibilities to improve further the performance of lithium-ion rechargeable batteries (LIBs) and lithium-metal batteries (LMBs)…etc. These substances incorporate the beneficial properties of solid electrolytes having polymers, intending to boost the security, stability, and general effectiveness of lithium-based battery packs. Several techniques and opportunities could enhance composite polymer electrolytes using lithium-based batteries. Here are some strategies and outlooks to consider for improving their performance

Polymer matrix selection

Identify a polymer matrix with excellent conductivity of ions, mechanical strength, and thermal stability. As a result of the ability, they have to communicate with the ions of lithium while providing mechanical solid properties; polymers that react, such as polyethylene oxide (PEO), polypropylene oxide (PPO), polyacrylonitrile (PAN), and the corresponding derivatives are often utilized.

Enhancing ionic conductivity

Ceramic filler incorporation

Including ceramic additives that include oxides or sulfide nanomaterials through the matrix of polymers could enhance the conductivity of ions within the composite electrolyte. These additives provide pathways for lithium-ion transportation, reducing the overall resistance.

Nanostructured materials

Utilizing nanoscale substances, nano-sized particles, or nanofibers, nanowires can increase surface area and accelerate lithium-ion transport, raising a whole conductivity.

Improved electrochemical stability

Electrolyte additives

Incorporating additives that produce stable passivation layers on the electrode surfaces may be beneficial in slowing down the degradation of the two components of the composite electrolyte and the electrodes themselves. These modifications boost a battery’s long-term reliability.

Cross-linking polymers

The cross-linking chains of polymers may enhance the electrolyte’s mechanical and chemical stability. This minimizes the possibility of dendritic development, circumvents leakage, and improves cycle endurance in lithium-ion batteries.

Mechanical properties

Flexibility and stretchability

Constructing composite polymer electrolytes with improved mechanical properties permits them to maintain their stability when they are challenged by mechanical strain or deformations. This feature is especially essential for flexible and wearable battery storage devices.

Electrode-electrolyte interface

The relationship between the solid electrolyte and an electrode is essential in ensuring the general operation of the batteries. Composite polymer electrolytes can be generated to enhance wetting and interaction with the electrode components. This may increase charging and discharging accelerates by minimizing interface impedance.

Safety

Nonflammability

Although polymeric electrolytes are generally not flammable, presenting flame-resistant substances may enhance the protection of composite electrolyte polymers.

Suppression of dendrite formation

For the prevention of shorts in electrical circuits and thermal runaway in lithium-metal battery packs, it is essential to avoid the formation of lithium dendrites; the purpose of preventing dendrite formation, composite electrolytes may supply support by mechanical means.

Synthesis and manufacturing

Scalability

Subsequently, it is of the utmost importance for the creation of methods for synthesis that can be sustainable using massive operations manufacturing methods to go from laboratory-scale research to manufacturing for commercial use. Developing sustainable and inexpensive methods for depositing the composite electrolyte upon materials for electrodes, which might include solution casting, electrospinning, or inkjet printers, will improve manufacturing possible production.

Solvent-free fabrication

Solvent-free technologies, including solid-state blending or melting processing, may mitigate solvent-related obstacles while simplifying manufacture.

Conclusion

CPEs help boost electrochemical systems’ functioning, including batteries and fuel cell systems. Finally, the subsequent essential characteristics of composite polymer electrolytes could be summarized as follows.

Ionic conductivity

CPEs demonstrate more excellent conductivity of ions than conventional electrolytic polymers. The room-temperature (RT) ionic conductivity of most active fillers stays in the 10⁻⁴ Scm⁻¹ order of magnitude, with just a few achieving values close to 10⁻³ Scm⁻¹. At room temperature, passive fillers have a low ionic conductivity of 10⁻⁶ to 10⁻⁵ S cm⁻¹ or even much lower. In practice, however, the ionic conductivity of solid-state electrolytes has been calculated to be 10⁻³ S cm⁻¹. The mechanism of ionic transportation in composite polymer electrolytes is still being investigated thoroughly, which is the most significant impediment to achieving the requisite degree of ionic conductivity. Consequently, it becomes essential to comprehend the transportation mechanism of Li-ions in CPEs along with the enhancement process of inorganic additives in ion conductivity.

Mechanical stability

Using inorganic additions enhances the tensile strength and endurance of CPEs, decreasing problems such as bloating and leakage that can occur with pure polymer electrolytes. This advancement is consistent with the safe and reliable operation of electrochemical equipment. To maximize the effectiveness of processing and physical, chemical, and electromechanical compatibility with both the anode and cathode electrodes, the mechanical strength and thermal stability of CPEs must be significantly expanded since their performance is closely dependent upon the safety of batteries.

Wide electrochemical stability window

The ESW is the voltage difference between the oxidation and reduction potentials of the electrolyte. CPE electrochemical stability could be substantially upgraded. CPEs’ electrochemical stability window determines which could be connected with high-voltage electrode materials. On the other hand, the interaction between the electrochemical stability of the CPEs and electrode components has an essential influence on battery cycling stability. The required electromechanical stability window for polymer electrolytes of 5 to 5.5 V (versus Li/Li⁺) is suitable for practical application.

Interface stability

Incorporating inorganic filler substances in CPE composite electrolytes of polymers may enhance the interface stability between the electrode and electrolyte surfaces, minimizing side reactions and expanding general device efficiency and lifespan. Interface stability in CPEs significantly impacts the performance and long-term viability of various electrochemical appliances, such as fuel cells, rechargeable batteries, and supercapacitors. To enhance ions’ conductivity and physical properties, active fillers such as nanoparticles or similar conducting components can be established in polymeric electrolytes. It is essential to maintain the overall performance and durability of the CPEs by ensuring the interface stability between their polymer matrix and these active fillers.

Li⁺ transfer number

The transferred ratio is quite close to 1 for lithium-ion batteries. This suggests that practically all of the lithium ions in the battery play a role in the electrochemical events. Active filler-based composite polymer electrolytes have a significantly higher lithium transference number (Li⁺) (0.9 vs. 0.5) than passive filler-based composite polymer electrolytes. One of the most significant advantages of lithium-ion batteries is their high transfer number, which allows for fast charging and draining.

Future challenges

The relevance of the apparent benefits of maximizing the performance of CPEs remains challenging. Among them are preserving ion conductivity while preserving mechanical stiffness, reducing interface resistance, and addressing filler dispersion and aggregation difficulties. More investigation must be conducted to tackle the obstacles to ultimately fulfilling the CPE’s promise.

Composite polymer electrolytes have many opportunities for creative ideas for polymers for electrochemical devices since they have boosted ion conductivity, mechanical durability, electromechanical stability, interfacial stability, and adaptability. Continuous advancement and study improvements must continue to solve remaining problems and enhance the effectiveness of CPEs employed to provide valuable recommendations. The concept is to continuously determine the combinations and interactions of SPEs and inorganic fillers. Overall, CPEs indicate promising potential for real-world applications in the foreseeable future.