Introduction

Poly(ethylene oxide) (PEO)–based solid-state polymer electrolyte has been identified as one of the most potential candidates for the next generation of solid-state lithium-ion batteries benefiting from its excellent machinability, low cost, and acceptable interfacial stability [1,2,3]. However, the inherent low oxidatively decomposed voltage (~ 3.9 V vs. Li/Li+) of PEO restricts its further application to match with commercial LiCoO2 (LCO) and derivatives at high voltage [4, 5]. It is found that LCO can accelerate the PEO degradation and release gas above 4.2 V (vs. Li+/Li) [6]. In addition, the dehydrogenation of PEO produces H+, which is capable to react with lithium bis(trifluoromethane)sulfonimide (LiN(SO2CF3)2, LiTFSI) and forms bis(trifluoromethane)sulfonimide (HN(SO2CF3)2, HTFSI) in the LCO/PEO-LiTFSI/Li system [7]. As a strong acid, HTFSI is highly corrosive to both LCO and PEO, causing unstable interfaces between PEO and LCO and resulting in the deterioration of battery cycle performance. Therefore, it is very significant to develop effective strategies to improve the high-voltage stability of PEO-based solid-state electrolytes.

Construction of an artificial barrier layer (mainly coating with Li+-conductive matrix) between LCO and PEO-based electrolyte is one of the efficient strategies to suppress the cathode/electrolyte direct contact and thus decrease the accelerated degradation of PEO induced by Co [8, 9]. As such, the electrochemical window can be increased. For example, Liang et al. applied a thin coating of lithium tantalate (LTO) on high-voltage LCO by atomic layer deposition, making PEO-based solid electrolyte and LCO cathode match well [10]. However, it is hard for a uniform coating to completely inhibit the cathode/electrolyte contact. In addition, the coating layer may increase the interfacial resistance.

In the liquid electrolytes, some lithium-salt additives, such as lithium bis(oxalato)borate (LiB(C2O4)2, LiBOB) [11], lithium difluoro(oxalate)borate (LiBC2O4F2, LiODFB) [12], and lithium difluorophosphate (LiPO2F2) [13, 14], are able to widen the electrochemical window of the electrolyte via pre-oxidation. In this way, a uniform and stable cathodic electrolyte interface (CEI) layer on the surface of the cathode side can be readily formed [15, 16]. This phenomenon provides ideas for the high-voltage application of such kind additives in the PEO-based solid electrolytes. Recently, Zhao et al. designed a three-salt system of LiBOB, LiNO3, and LiTFSI, which enabled the oxidation potential of PEO up to 4.6 V [17]. However, the reduction and deposition of LiNO3 at the anode side cause the irreversible variation of composition and phase. Therefore, the stability of such a system is still not satisfactory. LiPO2F2 as a successfully applied additive in the liquid electrolyte can form a dense protective film on the cathode surface and shows excellent resistance to temperature [18, 19]. Moreover, LiPO2F2 shows good stability on the anode side. However, as far as we know, the effects of LiPO2F2 on the electrochemical window stability of PEO-based solid-state polymer electrolyte (SPE) are not studied before, let alone applied in high-voltage systems.

Herein, aiming at the incompatibility between PEO and LiCoO2 at high voltage, LiPO2F2 was employed as the high voltage additive to ameliorate PEO-based solid electrolyte. The influences of LiPO2F2 addition on basic electrochemical properties such as ionic conductivity, lithium-ion migration number, electrochemical window, mechanical properties, and thermodynamic properties were comprehensively demonstrated. Moreover, the interface compatibility of PEO-based polymer electrolytes with positive and negative electrodes was revealed as well. When coupled with commercial LCO, the LCO/PEO-LiTFSI-LiPO2F2/Li battery performed stably at 4.2 V and exhibited an initial specific capacity of 123.4 mAh g−1 with excellent cycling stability.

Experimental section

Chemicals

The PEO (Sigma, Mw = 4,000,000, 99.9%), LiTFSI (Sigma, 99.95%), and LiPO2F2 (Sigma, 99.9%) were purchased from the commercial appliers and dried at 60 °C in a vacuum oven for 48 h before they were used.

Preparation of PEO-based electrolytes

The LiPO2F2-modified electrolyte was prepared by mixing PEO, LiTFSI, LiPO2F2, and acetonitrile. In detail, 0.2175 g LiTFSI and 0.0050 g LiPO2F2 were dissolved in 15 ml acetonitrile and stirred continuously for 4 h at room temperature until a homogeneous solution formed. After that, PEO was added into the above solution (EO:Li+ = 15:1 in mole) and finally gel-like slurry with a certain viscosity can be achieved. Then, the slurry was poured into the polytetrafluoroethylene gasket with an inner radius of 17 mm in the glove box (the contents of water and oxygen were below 0.1 ppm). The formed disks were dried at room temperature for 24 h, and further 60 °C for 6 h in the glove box to obtain the polymer solid electrolyte membrane. The basic electrolyte was made in the same way, except without LiPO2F2.

Preparation of solid-state battery

The commercial LCO was employed as electrode active materials and mixed with a binder (poly(vinylidene fluoride), PVDF) and conductive carbon black (Super P) in a weight ratio of 8:1:1. The homogenous slurry can be achieved after the addition of N-methyl pyrrolidone (NMP). And then, it was pasted onto aluminum foil and cut into small pieces with a diameter of 12 mm after drying at 80 °C for 4 h in a vacuum oven. Finally, 2025-type coin cells were assembled with the LCO cathode, PEO solid electrolyte, and Li-metal anode in an argon-filled glovebox.

Electrochemical measurements

Ionic conductivity

The ionic conductivity was measured by alternating current (AC) impedance with the disturbance voltage of 10 mV and scanning frequency range from 1 MHz to 1 Hz to acquire the ontology impedance (Rb) of PEO. The detailed value was calculated by the following equation [20].

$$\upsigma =\mathrm{l}/({\mathrm{SR}}_{\mathrm{b}})$$
(1)

where σ is ionic conductivity (S∙cm−1), l means the thickness of PEO (cm), Rb stands for the intrinsic impedance of PEO (Ω), and S corresponds to the apparent areal of the electrode (cm2).

Electrochemical oxidation potential window

The electrochemical oxidation potential window of PEO was determined by linear sweep voltammetry (LSV) tests in Li/PEO/stainless steel (SS) coin-cells. The steel sheet worked as a working electrode (WE), and Li-metal worked as the reference electrode (RE) and counter electrode (CE). The scanning rate was 10 mV s−1 in the range of 0–7.0 V and selected the useful data ranged from 2.75 to 6.0 V.

Transference number of lithium-ion (t+)

The lithium ion migration number of PEO-based solid electrolyte was measured by constant voltage polarization method and electrochemical impedance spectroscopy (EIS). The polarization curve was obtained under the conditions of a disturbance voltage of 10 mV and a scanning time of 1000 s. The EIS was performed under the conditions of a disturbance voltage of 10 mV and a test frequency of 1 MHz-1 Hz before and after polarization; therefore, the t+ value can be calculated according to Eq. (2) [21]

$${\mathrm{t}}^{+}=\frac{{\mathrm{I}}_{\mathrm{s}}\left(\Delta \mathrm{V}-{\mathrm{I}}_{0}{\mathrm{R}}_{0}\right)}{{\mathrm{I}}_{0}\left(\Delta \mathrm{V}-{\mathrm{I}}_{\mathrm{s}}{\mathrm{R}}_{\mathrm{s}}\right)}$$
(2)

where t+ is the number of lithium ion migration, I0 and Is are the initial and steady-state currents, R0 and Rs represent the initial and steady-state interface impedances, and ∆V means the constant voltage polarization voltage (10 mV).

Stability to lithium and charge–discharge test

The stability of electrolyte to lithium metal was analyzed by testing the lithium symmetric battery. In detail, Li/PEO/Li cell was used to test the polarization voltage at 60 °C at 0.1, 0.2, and 0.3 mA cm−2 to characterize the stability of the electrolyte/lithium interface. In addition, LCO/PEO/Li coin-cells were assembled and the cycle performance and rate capability were tested in the voltage range of 2.8–4.2 V at 60 °C.

Characterization

The micro-morphology information of the electrolyte was collected by scanning electron microscope (SEM, MIRA3LMH). X-ray diffractometer (XRD, Rigaku TTR-III) was applied to characterize the quantitative analysis of solid electrolytes with the copper targets. The thermal stability of PEO-based solid electrolyte was characterized by thermogravimetric-differential thermal analysis (TG-DSC, TA-Instrument’s SDT Q600). The melting transition temperature and crystallinity of PEO-based solid electrolytes were revealed by using differential scanning calorimetry. The crystallinity was calculated by Eq. (3):

$${\mathrm{X}}_{\mathrm{c}}=\frac{\Delta {\mathrm{H}}_{\mathrm{m}}}{\Delta {\mathrm{H}}_{\mathrm{m}}^{0}}\times 100\mathrm{\%}$$
(3)

where Xc demonstrates the crystallinity of SPE; \(\Delta {H}_{m}\) equals the melting heat of polymer solid electrolyte, corresponding to the melting-related endothermic peak area in the DSC curve; and \(\Delta {H}_{m}^{0}\) means the melting heat value of PEO with 100% crystallinity (− 213.7 J g−1).

Results and discussion

It can be seen from Fig. 1a that the ionic conductivity reaches the highest when the mass ratio of LiPO2F2 is 1.0 wt%, implying a certain amount of LiPO2F2 is beneficial to the improvement of ionic conductivity. The reason might be assigned to that the addition of LiPO2F2 reduces the crystallinity of the PEO segment, resulting in an improved segment activity and thus making ion transport easier [22, 23]. However, the movement of segments might be limited, and thus the migrations of lithium ions become slow with the increase of LiPO2F2. In detail, the ionic conductivity of PEO after 1.0 wt% LiPO2F2 addition is calculated as 1.9 × 10−4 S cm−1 compared to 1.1 × 10−4 S cm−1 without LiPO2F2. It should be noted that the low conductivity of PEO/lithium polymer electrolyte is attributed to the high crystallinity of PEO which restricts the lithium-ion transfer [24]. The influence of temperature on ionic conductivity of PEO-electrolyte before and after LiPO2F2 addition is shown in Fig. 1b. The value of ionic conductivity varies from 6.2 × 10−6 S cm−1 at 25 °C to 1.9 × 10−4 S cm−1 at 60 °C, revealing the enhanced conductivities of PEO with the increased temperature. The detailed conductivity value is calculated by Eq. (1) according to EIS spectra (Fig. 1c-d) and shown in Table 1. It is known that the interaction between lithium-ion and PEO chain becomes weakened when the temperature is higher than the melting point of PEO and thus the ionic conductivity can also be improved [25, 26].

Fig. 1
figure 1

a Ionic conductivity with different contents of LiPO2F2, b ionic conductivity, c–d impedance spectra of PEO-LiTFSI and PEO-LiTFSI-LiPO2F2 electrolyte film at different temperatures

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Table 1 Ionic conductivity of polymer solid electrolytes at different temperatures (S cm−1)
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The LSV and the galvanostatic step test curves of SS/PEO-LiTFSI-LiPO2F2/Li and SS/PEO-LiTFSI/Li are shown in Fig. 2a-b. It can be seen from Fig. 2a that PEO-LiTFSI possesses two decomposition peaks. The one is around 3.2 V, which can be assigned to the decomposition of LiTFSI, and the other one (~ 3.8 V) is affiliated with the decomposition of PEO [27, 28]. As for the PEO-LiTFSI-LiPO2F2 system, three decomposition peaks can be found. Peaks around 3.2 and 4.0 V are the decompositions of LiTFSI and LiPO2F2, which is similar to PEO-LiTFSI [29, 30]. In addition, the peak around 4.8 V is attributed to the decomposition of PEO. It can be concluded from the above illustrations that the voltage window of PEO widens from 3.8 to 4.8 V after the LiPO2F2 addition. This is because LiPO2F2 decomposes earlier than PEO and prevents the decomposition of PEO. As illustrated in Fig. 2b, the constant potential steps are performed at 3.8, 4.0, and 4.2 V. The results show that the current of PEO-LiTFSI-LiPO2F2 was close to 0 at various voltages, indicating that PEO-LiTFSI-LiPO2F2 is more stable than PEO-LiTFSI.

Fig. 2
figure 2

Electrochemical parameters tests before and after LiPO2F2 addition. a LSV profiles, b constant potential step experiments of Li//SS battery with or without LiPO2F2, c polarization curves, and d cycling performance of Li//Li battery with or without LiPO2F2

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The polymer solid electrolyte is assembled into Li/PEO/Li symmetrical battery, and the polarization voltages of the battery at different current densities are measured. The interface stability between the polymer solid electrolyte and lithium metal anode is characterized by comparing the variation trend of polarization voltage. As shown in Fig. 2c, the polarization voltage fluctuates with the current density. In detail, the polarization voltages of Li/PEO-LiTFSI-LiPO2F2/Li increase to 32, 61, and 85 mV when the current densities are 0.1, 0.2, and 0.3 mA cm−2. As a contrast, the polarization voltages of Li/PEO-LiTFSI/Li are 33, 66, and 106 mV which is much higher than that of the Li/PEO-LiTFSI-LiPO2F2/Li system. The higher polarization voltage indicates the higher lithium-ion reaction impedance at the electrode/electrolyte interface [31]. The results manifest that lithium-ion migration ability becomes stronger in Li/PEO-LiTFSI-LiPO2F2/Li batteries. Moreover, the long-term cycling performance of different electrolyte systems at 0.1 mA cm−2 is tested, as shown in Fig. 2d. The Li/PEO-LiTFSI-LiPO2F2/Li transfers a much smaller polarization value than Li/PEO-LiTFSI/Li (23 mV vs. 34 mV) even after 120 h cycles, revealing the improved cycling performance by the addition of LiPO2F2. The smaller electrochemical polarization of LCO/PEO-LiTFSI-LiPO2F2/Li cells can be attributed to the increased ionic conductivity of the polymer electrolyte after the addition of LiPO2F2 which results in a more compatible interface interfaces, reduces the electrochemical polarization and concentration polarization. Taking the above illustrations into conclusion, the introduction of LiPO2F2 enhances the ionic conductivity and reduces polarization which are benefits to the improvement of full-cell performance.

It can be concluded from the thermal stability of PEO in Fig. 3a that PEO keeps stable even when the temperature increases to 800 °C. The melting transition temperature (Tm) and crystallinity (Xc) are characterized by the DSC curve, as shown in Fig. 3b. The results show that the melting transition temperatures of PEO, PEO-LiTFSI, and PEO-LiTFSI-LiPO2F2 polymer electrolyte membranes are 81.2, 62.2, and 62.7 °C, respectively. It should be noted that the melting transition temperature is related to the ionic conductivity of polymer solid electrolytes [32, 33]. When the temperature is higher than the melting transition temperature, the amorphous areas of PEO increase and the movement of the PEO chain segment enhances, resulting in improved lithium-ion transport efficiency [34, 35]. Therefore, the low melting transition temperature is beneficial for the improvement of ionic conductivity. The detailed thermodynamic properties are calculated by Eq. (3) and exhibited in Table 2. The above results indicate that the melting transition temperature, endothermic peak area, and crystallinity of polymer solid electrolyte decrease after the addition of LiPO2F2 and lithium salt, which can be further confirmed by XRD results (Fig. 3c). The lithium-ion migration number is a vital parameter to evaluate the lithium-ion conductivity of solid electrolytes. The lithium-ion migration numbers of PEO-LiTFSI and PEO-LiTFSI-LiPO2F2 are calculated according to Eq. (2) and the values are 0.22 and 0.28 by the EIS test results (Fig. 3d-f). The increased lithium-ion migration number makes the formation of Li-ion easier.

Fig. 3
figure 3

Thermodynamic and structural properties before and after LiPO2F2 addition. a–c TG, DSC, and XRD tests of PEO, PEO-LiTFSI, and PEO-LiTFSI-LiPO2F2 membranes, d constant voltage polarization profiles, e–f AC impedance spectra at different polarization states of Li//Li battery with or without LiPO2F2

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Table 2 Thermodynamic properties of polymer solid electrolyte membranes
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Figure 4a-b shows the AC impedance spectra of the LCO/SPE/Li cell before and after cycles. After fitting, the interface impedance (Rs) and volume impedance (Rf) of the LCO/PEO-LiTFSI-LiPO2F2/Li battery before cycling are 14.8 and 99.3 Ω, which both are lower than the LCO/PEO-LiTFSI/Li battery (16.8 and 327.5 Ω). After 100 cycles, the resistance value increases (Table 3) while the interfacial impedance of LCO/PEO-LiTFSI-LiPO2F2/Li cells is still lower than LCO/PEO-LiTFSI/Li cells. This is consistent with the aforementioned ion conductivity tests and lithium-ion migration number results. The rate performances of LCO/PEO-LiTFSI/Li and LCO/PEO-LiTFSI-LiPO2F2/Li cells are characterized and shown in Fig. 4c. After activated 5 cycles at 0.1 C, the specific discharge capacity of the battery increased. However, the capacity of LCO/PEO-LiTFSI/Li is almost zero when the current density increased to 2–3 C. When the current density returns to 0.1 C, the capacity increases after multiple activations. The cycling performance of LCO/SPE/Li battery with different electrolyte systems is tested and shown in Fig. 4d. After 100 cycles, the specific capacity of LCO/PEO-LiTFSI-LiPO2F2/Li battery decays from 123.4 mAh g−1 to 103.2 mAh g−1 while the discharge specific capacity of LCO/PEO-LiTFSI/Li decreases from 76.5 mAh g−1 to 4.8 mAh g−1, which reveals a serious capacity attenuation after cycles.

Fig. 4
figure 4

Electrochemical performance of LCO//Li battery with or without LiPO2F2 as solid-state electrolyte. a–b AC impedance spectra before and after cycles, c rates capability, d cycling stability test at 0.2 C, e–f dQ/dV curves, g–h charge–discharge profiles (voltage range: 2.8–4.2 V, 1 C = 145 mA g−1)

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Table 3 Impedance values before and after cycles of LCO/PEO-LiTFSI/Li and LCO/PEO-LiTFSI-LiPO2F2/Li cells
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The dQ/dV curves of the LCO/PEO/Li battery before and after cycling are shown in Fig. 4e-f. At the beginning of the cycle, LCO/PEO-LiTFSI-LiPO2F2/Li cells possess lower potential difference (0.03 V) than LCO/PEO-LiTFSI/Li cells (0.08 V). After 100 cycles, the dQ/dV peak of the redox reaction cannot be found in the LCO/PEO-LiTFSI/Li cells, indicating LCO/PEO-LiTFSI-LiPO2F2/Li cells transfer higher reversibility. In other words, the interface of the LCO/PEO-LiTFSI/Li battery is unstable and the addition of LiPO2F2 inhibits the side reaction between the electrode and electrolyte interface during cycles. As shown in Fig. 4g-h, the discharge specific capacity of LCO/PEO-LiTFSI-LiPO2F2/Li battery at 1, 10, 30, 50, 70, and 100 cycles are 123.4, 110.5, 114.7, 111.5, 109.4, and 103.4 mAh g−1, which are also better than LCO/PEO-LiTFSI/Li (76.5, 31.9, 21.4, 14.2, 8.7, 4.8 mAh g−1). It can make a conclusion that the cycle stability of PEO-based electrolytes can be effectively improved when matched with high-voltage LCO by the addition of the LiPO2F2 additive.

SEM is applied to characterize the morphology transformation of LCO/PEO-LiTFSI/Li and LCO/PEO-LiTFSI-LiPO2F2/Li cells after 100 cycles, as shown in Fig. 5a and d. After 100 cycles, a layer of membrane covers on the surface can be only found in LCO/PEO-LiTFSI-LiPO2F2/Li. It is speculated that the film is derived from the LiPO2F2 decomposition during cycles. To verify the conjecture, the cells after cycles are disassembled and the LCO/PEO-LiTFSI/Li and LCO/PEO-LiTFSI-LiPO2F2/Li three-layer structure are scanned by EPMA to test the element distribution. In specifically, the Co element exists in the anode (Fig. 5b and e). When it comes to P element distribution, it can be only detected in LCO/PEO-LiTFSI-LiPO2F2/Li cells (Fig. 5c and f), which certifies the previous conjecture holds. Taking the above results into account, the LiPO2F2 decomposes before PEO during cycles and protects it from failure. In this way, the stability of PEO is improved and thus can match with LCO at high voltage.

Fig. 5
figure 5

SEM images and element analysis of LCO/PEO-LiTFSI-LiPO2F2/Li cell (a–c) and LCO/PEO-LiTFSI/Li cell (d–f) after 100 cycles

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Conclusions

In this work, LiPO2F2 was successfully applied as a high-voltage additive to enlarge the narrow electrochemical window of PEO-based solid electrolytes. On one side, the addition of LiPO2F2 can improve the ion conductivity by decreasing the crystallinity of PEO, and helped to enhance the Li+ transportation. More importantly, the LiPO2F2 decomposed before PEO during cycles and formed a dense and stable CEI on the cathode surface which can protect PEO from decomposition under high voltage. As a result, the electrochemical performance of PEO at high voltage was largely improved with 1.0 wt% LiPO2F2 addition. When assembled with commercial LCO and Li metal into a solid-state battery, a high specific capacity of 123.4 mAh g−1 and excellent cycling stability of 83.8% after 100 cycles were achieved. This work provides an effective method to improve the stability of PEO solid electrolytes at high voltage and thus it can match with the commercial cathode. In this way, the practical application of solid-state batteries was largely put forward.