Preparation and electrochemical properties of cationic substitution Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) as cathode material for lithium-ion batteries

Abstract

Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) cathode material for lithium-ion batteries was synthesized by traditional solid-phase doping method used Li₂CO₃, MnCO₃, (C₂H₅O)₄Si, and MgC₂O₄·2H₂O (or NiC₂O₄·2H₂O or C₂O₃) as staring materials. The suitable calcination temperature (700 °C) was obtained by TG-DTA curve. The influence of different doping amount on the crystal structure, micromorphology, and electrochemical properties of Li₂MnSiO₄ was studied by XRD, SEM, and electrochemical performance measurement. The XRD patterns indicate that the Li₂MnSiO₄ crystallized in an orthorhombic structure with a space group of Pmn2₁. It can be seen from SEM images that Mg doping has no effect on the micromorphology, Cr doping can refine the powder, and the particle size is about 200–800 nm. The electrochemical performance measurement demonstrates that the Li₂Mn_0.98Cr_0.02SiO₄ shows the best electrochemical performance with an initial discharge capacity of 29.8 mAh g⁻¹ at 0.1 C, and the discharge capacity retention rate after 70 cycles is 20.1%.

Introduction

Lithium-ion batteries (LIBs) are widely used in a variety of portable electronic products and will be widely used in the electric vehicle and hybrid vehicle industry [1,2,3]. Padhi et al. [4] reported for the first time that LiFePO₄, a phosphate compound with olivine structure, has high capacity, good cycling performance, safety, low cost, and environmental friendliness. Recently, the Li₂MSiO₄ (M = Mn, Fe, Co, Ni) series, which is isomorphic with LiFePO₄, has also attracted people’s attention [5, 6]. Among them, the theoretical voltage platform of Li₂MnSiO₄ is about 4.1 V, which is equivalent to the voltage platform of LiCoO₂ (3.9–4.0 V). It is suitable for the current battery system and maybe a cheap green electrode material to replace LiCoO₂ in the future [7,8,9]. R. Dominiko et al. [10] synthesized Li₂MnSiO₄ by the improved Pechini method, but the material has the disadvantages of small specific capacity, poor cycle performance, and poor conductivity [5]. Therefore, material modification is necessary.

At present, the main methods of surface modification of Li₂MnSiO₄ are carbon coating [7, 11, 12] and metal doping. The theoretical capacity of Li₂MnSiO₄ can reach 333 mAh g⁻¹, and the first voltage platform is about 4.1 V [10]. Gong et al. [13] have synthesized the Li₂Mn_1−xMg_xSiO₄/C (x = 0–0.3) cathode by the sol-gel method in orthorhombic Pmn2₁ form. The Mg doping improved the specific capacity of Li₂MnSiO₄ up to 229 mAh g⁻¹. Deng H et al. [14] believe that the substitution of Ni has some effect on the preparation of pure phase, grain refinement, increase of discharge capacity, decrease of resistivity, and increase of diffusion coefficient of lithium ion. Cheng H M et al. [15] found that Li₂Mn_92.5%Cr_7.5%SiO₄/C significantly improved the cycle stability and discharge capacity. Its initial discharge capacity can reach 255 mAh g⁻¹. After 50 cycles, the discharge capacity is still around 60 mAh g⁻¹.

In this work, we have successfully prepared Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) by the solid-phase method to modify Li₂MnSiO₄ samples. The effect of metal (Mg, Ni, Cr) doping on Li₂MnSiO₄ was evaluated by XRD, SEM, and electrochemical performance measurement.

Experimental

Material preparation

Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) was synthesized by solid-phase doping, and its preparation process is illustrated in Fig. 1. Using deionized water as the dispersant, the raw materials Li₂CO₃, MnCO₃, (C₂H₅O)₄Si, and MgC₂O₄·2H₂O (or NiC₂O₄·2H₂O or C₂O₃) were weighed according to the amount ratio of certain substances, in which Li:Mn:M:Si = 2:0.98:0.02:1 (M = Mg, Ni, Cr). The raw materials were dissolved in deionized water to form a uniform solution and milled for 5 h can get the liquid mixture, which would be drying in a drying oven at 80 °C, then sieved it with 100 mesh screen mesh to obtain the mixed raw material. Put the mixed raw material in the tubular furnace and calcined at 400 °C for 8 h, then sieved with 100 mesh screen mesh, the precalcined material was obtained. The precalcined material was placed in the tubular furnace and calcined at 700 °C for 8 h, sieving with 200 mesh screen mesh to obtain the final calcined Li₂MnSiO₄ cathode material doped with different metals.

Fig. 1

Schematic diagram of Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) preparation process

Material characterization

TG-DTA is a common method to study phase diagram and phase transformation. We use TG and DTA to get the best calcination temperature [16]. Thermogravimetric analysis of the precursor was recorded by the thermogravimetry and differential thermal analysis (TG-DTA, HCT-2) at a heating speed of 10 °C min⁻¹ from 20 to 800 °C in argon. The X-ray diffraction analyzer used in this paper is DX-2500. The phase and crystal properties of the material are analyzed under the condition of 30 kV and 25 mA. The scanning electron microscope used in this paper is SSX-550. In order to increase the conductivity of the sample surface, the sample was sprayed with gold before the experiment to obtain the best observation effect.

Electrochemical measurements

The positive electrode was composed of 75% active substance, 17% conductive carbon black and 8% polyvinylidene fluoride (PVDF). N-methyl-2-pyrrolidinone (NMP) was added to make a slurry with a certain fluidity, which was evenly coated on the aluminum foil, dried at 80 °C, and then rolled on the roller press to make the electrode smooth, compact, and flat. Place the electrode flat in a vacuum drying oven at 80–100 °C and bear heavy pressure. The processed electrode was cut into disks with a diameter of 10 mm as a spare electrode. The electrolyte was mixed with 1 mol/L LiPF₆ EC:DMC (1:1) (Beijing Chemical Reagent Research Institute). The separator was Celgard 2400, and the metal lithium sheet was used as the negative electrode. The CR2032 button battery was packaged in the glove box filled with argon. Galvanostatic charge and discharge tests were performed using a battery test system (LAND, CT2001A made by China) between 1.5 and 4.8 V, the current density was 0.1 C, and the test temperature was room temperature. Cyclic voltammetry (CV) was performed on a CHI660C electrochemical analyzer (Chenhua, China) at a scanning rate of 0.05 mV s⁻¹.

Results and discussion

The preparation temperature has a great influence on the crystallization and structural properties of the material, so it is necessary to study the appropriate calcination temperature to prepare Li₂MnSiO₄, in order to make its electrochemical performance better. TG-DTA measurement is carried out to study the thermal stability of Li₂MnSiO₄ under air atmosphere [17,18,19]. The thermogravimetry of the precursor mixture is carried out in N₂ atmosphere; the TG-DTA curve is shown in Fig. 2. The heating rate is 10 °C min⁻¹, and the gas flow rate of N₂ is 30 mL min⁻¹.

Fig. 2

TG-DTA curve of the mixture of MnCO₃, Li₂CO₃ and (C₂H₅O)₄Si

The total reaction equation of Li₂MnSiO₄ powders synthesized with Li₂CO₃, MnCO₃ and (C₂H₅O)₄Si as precursors can be expressed as follows:

$$ {\mathrm{Li}}_2\mathrm{C}{\mathrm{O}}_3+\mathrm{MnC}{\mathrm{O}}_3+{\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\right)}_4\mathrm{Si}\overset{{\mathrm{N}}_2}{\to }{\mathrm{Li}}_2\mathrm{MnSi}{\mathrm{O}}_4+\mathrm{else} $$

(1)

According to the mass change before and after the reaction, the loss on ignition rate can be calculated as follows:

$$ \mathrm{Ignition}\ \mathrm{loss}=1-\mathrm{Productivily}=1-\frac{160.906}{73.8+114.95+208.33}\approx 60\% $$

(2)

Consistent with the weight loss in Fig. 2.

The dispersive endothermic peak of precursor mixture at 75–150 °C is the volatilization of organic matter in TEOS, which is lighter in weight and less in weight loss curve. It decomposes when the temperature is higher than the atmospheric temperature, and the decomposition speed increases with the increase of temperature. There is a large degree of weight loss near 300–650 °C, and there are different intensity endothermic peaks on the DTA curve. It can be seen from the DTA curve that there are obvious weight loss processes at 387 °C and 640 °C, corresponding to the decomposition of the mixed raw materials. After 690 °C, TG and DTA curves do not show obvious weight loss and heat absorption and exothermic peak (curve burr was a noise of testing instrument). This shows that when the temperature is higher than 700 °C, the decomposition of the precursor is completed, and Li₂MnSiO₄ is basically completed. Therefore, 700 °C is selected as the minimum calcination temperature.

Figure 3 shows the XRD patterns of Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr). From Fig. 3, we can see that the main peaks of Li₂MnSiO₄ and Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) samples correspond to the Li₂MnSiO₄ phase of orthogonal structure, the space group is Pmn2₁, and the five main diffraction peaks correspond to (010), (011), (111), (210), and (002) crystal faces, respectively, which indicates that the doped metal atoms have successfully dissolved into the Li₂MnSiO₄ lattice.

Fig. 3

XRD patterns of Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) samples

Compared with the JCPDS standard card (75-7861), the undoped Li₂MnSiO₄ sample has MnO impurity phase. With Mg and Ni doping into Li₂MnSiO₄ sample, the peak value of MnO diffraction increases. Different from Li₂Mn_0.98Ni_0.02SiO₄ sample, the main diffraction peak intensity of Li₂Mn_0.98Mg_0.02SiO₄ sample decreases, which shows that Mg doping has no effect on the enhancement of crystallinity, but it may also be related to Mg doping amount. However, in Li₂Mn_0.98Cr_0.02SiO₄ sample, the MnO impurity phase diffraction peak disappeared gradually, and the new diffraction peaks appeared near 2θ value of 18°, 44°, and 65°, respectively. According to the analysis, Li_1.13Mn₂O₄ is produced. Because Li_1.13Mn₂O₄ can be used as a positive electrode material of battery and has excellent electrochemical performance, the electrochemical performance of Li₂MnSiO₄ will be greatly improved [20].

Figure 4 is the SEM of Li₂MnSiO₄ and Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) samples. It can be seen from the figure that the crystal growth of undoped Li₂MnSiO₄ is complete, and the particles are relatively large. Most of the primary particles gather together and gather into the secondary particles of large particles. The particle morphology of Li₂Mn_0.98Mg_0.02SiO₄ sample (Fig. 4b) and Li₂MnSiO₄ sample (Fig. 4a) is similar, and the maximum particle size of the two samples can reach 3000 nm, which shows that Mg doping has no significant effect on the microstructure of Li₂MnSiO₄. From Fig. 4c, it can be seen that the crystal surface of Li₂Mn_0.98Ni_0.02SiO₄ sample appears tiny particles and gradually becomes larger, and the particle morphology tends to be regular, and the crystallinity gradually increases. Figure 4 d is the SEM of Li₂Mn_0.98Cr_0.02SiO₄ sample. We can see that many particles still maintain the primary particle shape, and the maximum size is basically 1100 nm, far less than Mg ion and Ni ion doping. This shows that the size of the particles is reduced and the diffusion property of the ions in Li₂MnSiO₄ crystal is improved. The larger the size of the crystal, the longer the distance of Li⁺ diffusion in the solid phase, and the more limited the electrical properties of the material are. Therefore, the reduction of particle size is helpful to the development of the specific capacity of materials.

Fig. 4

SEM images of a Li₂MnSiO₄ and b–d Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) samples were particle size summary figures (e–h)

Figure 5 shows the first charge and discharge curve of Li₂MnSiO₄ and Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) samples at 0.1 C. The first discharge capacity of Li₂MnSiO₄, Li₂Mn_0.98Mg_0.02SiO₄, Li₂Mn_0.98Ni_0.02SiO₄, and Li₂Cr_0.98M_0.02SiO₄ is 12.2 mAh g⁻¹, 9.1 mAh g⁻¹, 8.5 mAh g⁻¹, and 29.8 mAh g⁻¹, respectively. It can be seen from the figure that the specific capacity of Li₂Mn_0.98Mg_0.02SiO₄ modified by Mg²⁺ is not higher than that of the undoped sample. According to the research [21], Mg²⁺ has good solubility, which can improve the stability of pure P2₁/n structure. However, when the value of x is in the range of 0.0–1.0, the substitution of Mg has no effect on the conductivity of electrons and ions.

Fig. 5

The first charge and discharge curve of Li₂MnSiO₄ and Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) samples at 0.1 C

The specific capacity of Li₂Mn_0.98Ni_0.02SiO₄ sample modified by Ni²⁺ doping did not increase compared with the undoped sample. By referring to the literature [14, 22], we think that the reason for the poor electrochemical performance of Ni-doped samples is that almost all Ni²⁺ is separated from the silicate structure. In addition, there is an optimal balance between Ni and Ni²⁺, which has the best effect on improving the electrochemical performance. However, once the content of Ni exceeds or falls below a certain limit, the electrochemical performance will deteriorate. It can also be seen from the figure that the specific capacity of Li₂Mn_0.98Cr_0.02SiO₄ sample modified by Cr³⁺ doping is higher than that of the undoped sample, and the voltage difference between charge and discharge is also reduced, indicating that adding Cr is conducive to improving conductivity, reducing cell polarization, and improving charge and discharge dynamic characteristics of the material. Cr³⁺ is used to replace Mn²⁺ in Li₂MnSiO₄. The ion radius of Cr³⁺ (0.052 nm) is larger than that of Mn²⁺ (0.046 nm). In the process of doping Cr³⁺, with the increase of doping amount, the lattice gap is filled first, and then the conductivity will decrease. However, when it is increased to a certain extent, more and more lattice voids will be generated, which will provide better channels for the movement of electrons and ions, so as to improve the conductivity. Therefore, in the process of chromium doping, the electrochemical performance is the curve of decreasing and increasing cycle.

The electrochemical cycle performance of Li₂MnSiO₄ and Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) samples at 0.1 C is shown in Fig. 6. It can be seen from the figure that the discharge specific capacity of undoped samples and each doped sample decreases with the increase of cycle times.

Fig. 6

Electrochemical cycle curve of Li₂MnSiO₄ and Li₂Mn_0.98M_0.02SiO₄ (M = Mg, Ni, Cr) samples at 0.1 C

Excluding Cr-doped sample, the cycle performance of Mg-doped and Ni-doped samples are lower than that of the undoped sample. This may be due to the fact that Mg doping has little effect on the performance of Li₂MnSiO₄ [21], while Ni has a multivalent state, which will cause lattice shrinkage in the process of charging and discharging, so it has a great influence on the structural stability of the material, and may lead to poor cycle performance. The first discharge specific capacity of Li₂Mn_0.98Cr_0.02SiO₄ can reach 29.8 mAh g⁻¹; after 70 cycles, it is 6.0 mAh g⁻¹; and its discharge retention rate is 20.1%. Because the bond energy of Cr-O (1086 kJ mol⁻¹) is greater than that of Mn-O (402 kJ mol⁻¹), the stability of Li₂MnSiO₄ structure can be enhanced by substituting Cr for Mn, and the cyclic performance of the material can be improved.

Conclusions

Compared with Li₂MnSiO₄ sample, the electrochemical performance of Li₂Mn_0.98Cr_0.02SiO₄ synthesized by the solid-phase method is improved most obviously. Through XRD patterns, we can see that the second-phase Li_1.13Mn₂O₄ with good electrochemical performance is formed in the sample after Cr doping, which improves the electrochemical performance of the undoped sample. The primary particle size of Cr doped sample powder is smaller, which improves the diffusion ability of lithium-ion in Li₂MnSiO₄ crystal and contributes to the specific capacity of the material. In the process of Cr³⁺ doping, the defect chemical mechanism changes with the increase of doping amount. The conductivity decreases first and then increases, which affects the electrical properties. At the same time, Cr-O bond energy is greater than Mn-O bond energy, and a proper amount of Cr replacing Mn can enhance the stability of Li₂MnSiO₄ structure, thus improving the cyclic performance of the material. The improvement of Li₂Mn_0.8Cr_0.2SiO₄ is the most obvious, the first discharge capacity is 29.8 mAh g⁻¹, the discharge capacity after 70 cycles is 6 mAh g⁻¹, and the capacity retention rate is 20.1%.