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Review  |  Open Access  |  22 May 2024

Polymer-based electrolytes for high-voltage solid-state lithium batteries

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Zixuan Wang#
, 
Jianxiong Chen#
, ... 
Xin Guo*
Energy Mater 2024;4:400050.
10.20517/energymater.2023.130 |  © The Author(s) 2024.
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Abstract

Increasing the charging cut-off voltage of lithium batteries is a feasible method to enhance the energy density. However, when batteries operate at high voltages (> 4.3 V), the degradation of liquid organic carbonate electrolyte is accelerated and may cause safety hazards. Polymer-based electrolytes with inherently high safety and good electrochemical stability can prevent the electrolyte degradation in high-voltage solid-state lithium batteries. This paper provides a comprehensive and in-depth review of the design strategies, recent developments, and scientific challenges associated with polymer-based electrolytes for high-voltage applications. Emphases are placed on the interfacial compatibility between electrolytes and cathodes, such as mechanical contacts and interface chemical stability, which are critical to the lifespan of high-voltage lithium batteries. Moreover, guidelines for the future development of high-voltage solid-state lithium batteries are also discussed.

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Keywords

Polymer-based electrolyte, interfacial compatibility, high-voltage stability, high energy density, lithium battery
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INTRODUCTION

The current development of electric vehicles is constrained by various factors such as limited driving range, short battery lifespan, and slow charging speed, as outlined in previous studies[1]. The predominant focus for addressing these challenges has been on enhancing the high-voltage tolerance of batteries. Polymer-based electrolytes stand out among various electrolytes due to their excellent processability, simple preparation procedures, and high safety[2,3]. However, these electrolytes in batteries are not universally applicable, as the strong oxidative/reductive nature of electrodes often challenges the realization of long-term, high energy density batteries[4,5]. Therefore, it is crucial to develop polymer-based electrolytes[6] with a wide electrochemical window and achieve compatibility between polymer electrolytes (PEs) and electrodes[7,8].

PEs can be classified into three categories: solid (SPEs), gel (GPEs) and composite PEs (CPEs) [Table 1]. In the 1880s, Wright and Fenton et al. initiated research on SPEs. The initial SPE matrix was polyethylene oxide (PEO), which could form ion-conductive polymers with electrical conductivity varying with temperature[9,10]. Lately, Vashisht and Armand applied polymers such as PEO and polyphenylene oxide (PPO) in solid-state batteries, introducing a new class of electrolytes, SPEs, which garnered attention[11]. However, SPEs exhibit a notable and critical flaw: notoriously low ionic conductivity that fails to meet practical application requirements[8,12]. Therefore, in 1975, Feuillade et al. added plasticizers to the polymer-salt system, in which the liquid content exceeded 20-30 wt.%, thereby introducing a new polymer system-GPEs[13-15]. GPEs combine the advantages of liquid and solid electrolytes. The liquid component exhibits excellent ion transport capacity and promotes good interface contact. It wets the gaps between electrode particles and between the electrode and the electrolyte, facilitating the formation of effective interfaces[16-18]. However, in order to adapt to the high-voltage cathode to achieve high-voltage lithium batteries, its electrochemical stability needs to be improved. In 1982, fillers were added to PEs for the first time to improve the mechanical properties and interface stability of the electrolytes[19]. For instance, Croce et al. employed nanoceramic powders as fillers to enhance the ionic conductivity of PEO-based PEs[20]. Subsequently, research on CPEs commenced. However, challenges such as the high cost of nanomaterials and difficulties in their dispersion in polymer matrices raised questions about the feasibility of industrial-scale application.

Table 1

Comparison of various types of polymer electrolytes

Polymer electrolytesAdvantagesDisadvantages
Solid polymer electrolytes (SPEs)Good mechanical propertiesLow ionic conductivity
Poor interfacial compatibility
Composite polymer electrolytes (CPEs)Good ionic conductivityPoor interfacial compatibility
Difficult to disperse fillers
Gel polymer electrolytes (GPEs)Good ionic conductivity
Good interface contact		Poor mechanical strength

In recent years, researchers have conducted extensive studies on PEs, particularly focusing on the high-voltage performance, which is crucial for achieving high energy density in lithium batteries. The application of high-voltage PEs in lithium batteries must meet the following two criteria: (1) wide electrochemical window: > 4.3 V (vs. Li/Li+), to match the high-voltage cathode material, so that it is not easily oxidized and decomposed at high potentials; and (2) Good interface compatibility: forming stable interfaces with the cathode and anode to ensure the normal operation of batteries and improve their performances.

This paper reviews the latest progress of polymer-based electrolytes for high-voltage lithium batteries [Table 2]. Combined with the selection of polymer functional monomers and electrolyte preparation strategies, the properties of PEs are customized. Importantly, this review not only provides insights into the high-voltage design of PEs but also emphasizes the significance of addressing the polymer electrolyte-electrode interfaces. It discusses effective approaches targeting the polymer electrolyte-electrode interfaces, combining our research progress in the field of polymer batteries with updated developments, focusing on the industrial scalability, cycling stability, and safety issues of polymer batteries.

Table 2

Progress of high-voltage polymer electrolytes

PEsPolymer matrixLiquid electrolytePreparation methodESW
(V)		σ
(S cm-1)		t+Positive
electrode		Voltage
range (V)		Cycling lifespan
(rate)		Ref.
SPEC5-POE-FLiTFSI/DMFSolution casting51 × 10-4-NCM8113.6-4.35200 (0.15 C)[28]
SPEPVC/POEA/ANLiODFBSolution casting4.64.47 × 10-5 (60 °C)0.32 (60 °C)LCO3-4.3150
(0.5 C, 60 °C)		[32]
SPEPHMOLiTFSISolution casting4.61.26 × 10-40.52NCM8113.0-4.3250 (0.2 C)[140]
SPEPDADMAFSILiFSI/PYR13FSISolution casting5.48 × 10-40.44NCM8113-4.6120 (0.1 mA cm-2)[141]
SPEPVDF-HFP/PAN/UiO-66-SO3LiLiTFSI/EMIMTFSISolution casting57.5 × 10-40.65NCM8113-4.3150 (0.2 C)[142]
SPEPME/LiPVFMLiTFSI/SNSolution casting53.57 × 10-40.62LCO3-4.45225 (0.5 C)[143]
CPESiO2/VECLiTFSISolution casting51.35 × 10-30.55LCO3-4.5200 (0.1 C)[48]
CPEPEGDA/LLZOLiDFOB/EC/DMCSolution casting5.131.25 × 10-3 (55 °C)0.57LCO3-4.3200 (0.2 C)[57]
CPEPEO-PVDF-HFP/LATPLiTFSI/ACNSolution casting5.211.67 × 10-40.49NCM8112.5-4.2400 (0.5 C)[144]
CPEPVDF-HFP/Ce-NCATPLiTFSI/DMFSolution casting52.16 × 10-30.88NCM8113-4.3200 (0.2 C)[145]
CPEPVDF/g-C3N4LiFSI/DMFSolution casting4.76.9 × 10-40.49NCM8112.5-4.31,700 (1 C)[146]
CPEPDOL/YSZLiPF6/LiTFSISolution casting4.92.75 × 10-40.65NCM6222.8-4.3800 (0.5 C)[49]
GPEPVDF/HECLiTFSI/PEGDMESolution casting5.257.8 × 10-4-LNO3.5-5200 (0.2 C)[147]
GPEPAN@LAGP/PEGDALiTFSI/DMFSolution casting53.7 × 10-4-NCM8112.8-4.3270 (0.5 C)[148]
SPEUFF/PEO/PANLiTFSIElectrospinning4.96.8 × 10-50.5NCM8112.8-4.3100 (0.35 mA cm-2)[149]
CPEγ-Al2O3-PPOLiBOB-LiTFSI/NMP/SNElectrospinning5.63.38 × 10-40.7NCM6222.8-4.3120 (0.3 C)[150]
GPEPVDF-HFP/BaTiO3LiPF6/EC/DMCElectrospinning5.15.2 × 10-30.7NCM5322.5-4.5200 (5 C)[151]
SPECA-PEGMEALiFSI/SN/FECThermal crosslinking4.91.9 × 10-30.56NCM6222.8-4.3250 (0.1 C)[152]
SPEUPyMA/PEGDALiTFSI/NMLThermal crosslinking5.23.42 × 10-4 (20 °C)0.66LCO3.0-4.61,000 (0.5 C)[153]
SPECA-PEG/LLZTOLiTFSI/Li
DFOB		Thermal crosslinking4.91.63 × 10-3 (30 °C)0.4LCO3-4.5100 (0.2 C)[31]
SPEPETEALiPF6/FEC/FEMC/TFTFEThermal crosslinking5.66.4 × 10-40.48LLO2-4.8200 (0.5 C)[154]
SPEP(VEC-co-TFEMA)LiTFSIThermal crosslinking5.75.02 × 10-50.44NCM8113.0-4.5300 (0.1 C)[155]
SPEPS/Sn(Oct)2LiTFSIThermal initiation51.5 × 10-40.78LLO2-4.6100 (0.1 C)[156]
GPEMMA/TEGDMA/ETPTALiDFOB/FECThermal crosslinking5.652.24 × 10-40.48NCA3-4.5200 (0.2 C)[64]
GPEUPyMA/TEGDALiFSI/FEC/EMC/EGMEAThermal crosslinking5.92.2 × 10-30.75NCM8112.8-4.7500 (0.5 C)[157]
GPELiNO3/DMAALiPF6/EC/DEC/FECThermal crosslinking--0.57NCM6223-4.3400 (1 C)[158]
GPEDOL-PEELiTFSI/LiDFOB/FECThermal crosslinking4.52.36 × 10-30.6NCM622-300 (0.5 C)[159]
GPETFEMA-PETALiTFSI/LiDFOB/FECThermal crosslinking4.891.88 × 10-30.61LCO3-4.5500 (1 C)[160]
GPEPEGDE-PEALiPF6/DMC/FECThermal crosslinking4.87 × 10-40.47LCO3-4.35150 (0.2 C)[161]
GPETFPO-PEELiDFOB/LiTFSIThermal crosslinking5.13.98 × 10-30.59NCM6224.5300 (0.5 C)[162]
GPEETPTA/SNLiTFSI-LiDFOB/FECThermal crosslinking5.41.08 × 10-30.64LCO3.0-4.3300 (0.5 C)[163]
GPEPEGDA/PETA/PPOLiPF6/EMC/FECThermal crosslinking4.58.676 × 10-40.65NCM8112.8-4.3400 (1 C)[164]
GPEPMBALiTFSI-LiBOB/EC/EMCThermal crosslinking5.641.385 × 10-30.63NCM6223.0-4.3700 (1 C)[65]
GPEDAP-PETEALiPF6/FEC/FEMC/HTEThermal crosslinking5.52 × 10-30.43LLO2-5200 (0.5 C)[165]
SPEPVC-TF3LiTFSI/NMPThermal initiation4.93 × 10-4 (60 °C)0.76NCM8112.5-4.3150 (0.1 C)[166]
SPEB-PEGMA/VC/ANLiTFSIThermal initiation5.19.24 × 10-4 (25 °C)0.86NCM8112.8-4.3100 (0.5 C)[167]
GPEPCUMALiDFOB/EC/DMCThermal initiation51.27 × 10-30.44LCO3-4.45200 (0.5 C)[168]
CPEP(MVE-MA)LiDFOB/PCThermal initiation5.21.34 × 10-30.54LCO2.75-4.45700
(1 C, 60 °C)		[169]
CPELATP/PEO/PANLiTFSI/DMFThermal initiation4.56.26 × 10-4 (60 °C)0.82NCM6222.8-4.3120 (0.5 C)[170]
SPEIL/OFHDODA/VECLiTFSIUV5.081.37 × 10-30.4NCM5232.8-4.5200 (0.5 C)[30]
GPEP(AMPSLi-MPEGA-AN)LiPF6/EC/PCUV5.062.5 × 10-3-LCO3-4.4120 (0.2 C)[66]
GPESNLiTFSI/LiDFOBEutectic gels4.81.87 × 10-30.64LCO3-4.45200 (1 C)[171]

ESW: Electrochemical stability window; σ: ionic conductivity; t+: ionic transference number.

HIGH-VOLTAGE POLYMER ELECTROLYTES

High-voltage solid polymer electrolytes

SPEs comprise lithium salts and polymer matrices, devoid of liquid solvents, exhibiting high safety, excellent mechanical properties, and good processability[2,3,21]. Among them, PEO stands out as the earliest studied and most widely applied SPE matrix of PEs. However, its poor oxidation stability limits its matching with the high-voltage cathode, mainly due to the unstable lone pair electrons of the ether oxygen atoms on the PEO chains[22-24]. Introducing antioxidant groups into polymer chains has proven effective in enhancing the high-voltage stability of polymer-based electrolytes. For instance, Yang et al. improved the electrochemical stability window (ESW) from 4.05 to 4.3 V by replacing the terminal group -OH of polyethylene glycol with a more stable -OCH3 group[25]. This demonstrates that introducing strong antioxidant groups is an efficient strategy. To further demonstrate the impact of group substitution on ESW, Pandian et al. calculated the effect of substituent modification on PEO surfaces on ESW by density functional theory (DFT), as shown in Figure 1A[26]. CF3 and CN functional groups have higher oxidation stability and can provide a wider ESW. Fluorination and cyano functional groups have been shown to have antioxidant properties[27].

Polymer-based electrolytes for high-voltage solid-state lithium batteries

Figure 1. SPEs with antioxidant functional groups. (A) Effect of various functional groups (substituted at C1 carbon) on the oxidation and reduction potentials (eV) of PEO (EO5) electrolyte. This figure is quoted with permission from Pandian et al.[26] Copyright 2018 American Chemical Society. (B) LSV curve of Li|P(IL-OFHDODA-VEC)|carbon. (C) ESW of PIL, POFHDODA, PVEC, P(IL-OFHDODA), P(IL-VEC), and P(OFHDODA-VEC). This figure is quoted with permission from Tang et al.[30] Copyright 2023 Royal Society of Chemistry. (D) LSV of PGL and CA-PGL at a scan rate of 0.5 mV s-1. (E) Electrostatic potential of poly(CA-PEG). This figure is quoted with permission from Lv et al.[31] Copyright 2021 Royal Society of Chemistry. (F) Preparation diagram and characterization of PVCN-SPE.[32] Copyright 2022, Elsevier.

Sun et al. introduced a strong electron-withdrawing trifluoroacetic acid group on poly-oxalate (POE), which enhanced the electronegativity of the highest occupied molecular orbital (HOMO), and the HOMO electrons were transferred to the oxalate center, thereby improving the oxidation stability[28]. Benefiting from this, the fluorine-terminated SPEs remained stable until voltages above 5 V, while the non-fluorine-terminated SPEs were oxidized at 4.8 V. Xie et al. further confirmed that the F atom transfers electrons to the middle of the polymer main chains, thereby preventing the decomposition and oxidation of the chains, and increasing the oxidation potential of the SPEs[29]. In addition to introducing fluorinated groups to improve the oxidation stability of the electrolytes, the formation of fluorinated crosslinked network structure not only improves the oxidation resistance of the electrolyte but also enhances the mechanical properties. Tang et al. also improved the oxidative stability of SPEs using a polyfluorinated crosslinking agent[30]. The SPEs exhibited good oxidation resistance through the multi-fluorinated segment induced electron-withdrawing effect and the constructed crosslinked network structure for electron effect transmission. The fluorinated SPEs can achieve a wide ESW of 5.08 V, as shown in Figure 1B. As demonstrated in Figure 1C, 2,2,3,3,4,4,5,5-diacrylate (OFHDODA) is the key to improving the electrochemical stability of SPE. The fluorine atoms with strong electron absorption properties can reduce the HOMOs and the lowest unoccupied molecular orbitals (LUMOs) of the fluorinated electrolytes, thus making the electrolytes show better oxidation resistance and reduction resistance.

Polymer-based electrolytes for high-voltage solid-state lithium batteries

Figure 2. Fillers for improving the oxidation potential of CPEs. (A) Schematic diagram of preparing the CPE membrane from VEC precursor with Li salt and nanoparticles. (B) Molecular electrostatic potential energy mappings of PVEC and TFSI-. And Intermolecular interaction in CPE by DFT calculation. (C) ESW of the as-involved electrolyte. This figure is quoted with permission from Wang et al.[48] Copyright 2022 Elevier. (D) Improving PDOL performance by YSZ. (E) Electrochemical floating analysis of cells assembled with NCM622 cathode for SE and CSE. This figure is quoted with permission from Yang et al.[49] Copyright 2022 Wiley. (F) Schematic illustration of in-situ fabricated LCO/electrolyte/Li cells. (G) Linear sweep voltammetry (LSV) curves of PGPE and A-PGCE. This figure is quoted with permission from Cai et al.[57] Copyright 2022 Elevier. (H) LSV curves of the PEO/LiTFSI and PEO/LSTZ electrolytes. This figure is quoted with permission from Xu et al.[59] Copyright 2019 National Academy of Sciences of the United States of America.

In addition to the fluorine group, the strong electron-absorbing -C≡N group can also improve the oxidation stability of the electrolyte. Lv et al. prepared cyano-enhanced high-voltage SPEs by in-situ copolymerization of ethyl 2-cyanoacrylate (CA) and polyethylene glycol methyl acrylate (PEG)[31]. The SPEs form a stable interface layer rich in -C≡N and LiF on the surface of the LiCoO2 (LCO) cathode, thereby improving the oxidation stability of ethylene oxide (EO) and inhibiting its decomposition. The wide ESW of the SPEs can reach 4.9 V [Figure 1D]. As shown in Figure 1E, the electrostatic potential of poly (CA-PEG) calculated by DFT shows that the negative electrostatic potential of -C≡N is much lower than that of EO segments, indicating strong interaction between the positive electrode and -C≡N, thus slowing the decomposition of EO segments. The team also synthesized rigid-flexible SPEs by crosslinking reaction of rigid poly (ethylene carbonate), flexible polyoxyethylene bis (amine) (POEA) and antioxidant aminopropionitrile (AN) without initiator. The preparation method and electrolyte characterization are shown in Figure 1F[32]. The polymer exhibits a wide ESW and excellent thermal stability. In summary, introducing antioxidant functional groups is an effective method to enhance the oxidative potential of SPEs. Therefore, to design high-voltage-resistant SPEs, attention should be paid to selecting antioxidant groups, molecular structure design, and structural charge distribution. However, despite the excellent high-voltage stability exhibited by SPEs, their interface contacts limit their development, which will be discussed in detail in the following sections.

High-voltage composite polymer electrolytes

CPEs are composed of polymers, lithium salts and inorganic fillers, which have good processability and flexibility[33,34]. Inorganic fillers will adsorb trace water and impurities in polymers, improving the oxidation resistance of CPEs and increasing the electrochemical window[35,36]. According to the ionic conductivity of inorganic fillers, they can be classified into inert and active fillers. Commonly used inert fillers are silicon dioxide (SiO2)[37-39], alumina (Al2O3)[40], titanium oxide (TiO2)[41,42], magnesium oxide (MgO)[43], and zinc oxide (ZnO)[44]; active fillers are garnet type, NASICON type, and perovskite type[45]. Next, we will discuss the influence of different fillers on realizing high-voltage CPEs.

The inert fillers cannot conduct ions. When they form a crosslinked structure with the polymer matrix through physical/chemical interactions, the crystallinity of the polymer will be reduced, which is conducive to the ion transport. In addition, some chemical groups on the surfaces of the inert fillers have Lewis acid-base interaction with the anions/cations of lithium salts, which will promote the dissociation of the lithium salts and increase the number of free lithium ions, and this interaction improves the electrochemical stability of CPEs[46,47]. Wang et al. prepared size-adjustable antioxidant CPEs using a solution casting method, as shown in Figure 2A[48]. DFT calculations revealed the reasons for the increase in the antioxidant capacity of CPEs, as shown in Figure 2B. The area around the O atom on the C=O group of poly(vinyl ethylene carbonate) (PVEC) and the O=S=O group of bis(trifluoromethylsulfonyl)imide (TFSI-) has a high electron density, indicating that the O position of these groups in CPEs can preferentially form hydrogen bonds. Through the H atoms on the SiO2 surfaces and the O atoms on the C=O and O=S=O groups in PVEC and TFSI, local intermolecular hydrogen bonding interactions can be formed. The local intermolecular interaction of hydrogen bonds in CPEs enhances the oxidation resistance, and its electrochemical window can reach 5 V [Figure 2C]. Inorganic fillers can also improve the ionic conductivity of PEs and enhance their electrochemical properties. Yang et al. introduced a Lewis acid filler yttrium stabilized zirconia (YSZ) and in-situ polymerization of 1,3-dioxolane (DOL) to increase the polymerization conversion of DOL monomers to 98.5%[49]. A large number of Lewis acid sites (oxygen vacancy, Zr4+ and Y3+) on the YSZ surface can not only improve the degree of polymerization of DOL monomers, but also reduce the formation of by-products, thus improving high-voltage stability. As shown in Figure 2D, the leakage current of the PE remains very low at 4.9 V. The resulting solid electrolyte interface (SEI) is rich in Li2ZrO3 and its high ionic conductivity can guide the uniform deposition of lithium ions and delay the growth of lithium dendrites from the source [Figure 2E].

Compared to inert fillers, active fillers not only retain some characteristics of the inert fillers but also can directly participate in the transport of lithium ions, thereby further improving the electrochemical performance of the CPEs. Typical active fillers include oxide type (garnet type, perovskite type and NASICON type) and sulfide type fillers (Li10SnP2S12 (LSnPS), Li10GeP2S12 (LGPS), etc.)[50-52].

Garnet-type fillers are widely used in CPEs due to their high ionic conductivity, wide potential window (> 5 Vvs. Li/Li+) and good compatibility with lithium metal anodes[53-55]. Chio et al. added Li7La3Zr2O12(LLZO) fillers to PEO, and the electrochemical stability increased with rising LLZO content. Thanks to the synergistic effect between organic matrix and LLZO, the ionic conductivity of SPEs has been greatly improved[56]. However, the inherent rigidity of inorganic solid electrolytes leads to poor electrolyte/electrode interface contact, which slows down the dynamics of charge and discharge, thus affecting the battery performance. Cai et al. introduced a small amount of Li6.5La3Zr1.5Ta0.5O12 (LLZTO) nanoparticles into polyethylene glycol-diacrylate (PEGDA)-based GPE and prepared a novel asymmetric composite electrolyte with a LLZTO-rich layer by in-situ polymerization [Figure 2F][57]. The mechanically stable LLZTO layer induced uniform deposition of lithium and inhibited lithium dendrites, and its inherent high oxidation stability increased the decomposition potential of CPEs up to 5.13 V [Figure 2G].

Although the perovskite ceramic electrolyte has high ionic conductivity (10-3 S cm-1) and high-voltage stability, it has poor compatibility with metallic lithium[58]. Xu et al. introduced Li3/8Sr7/16Ta3/4Zr1/4O3 (LSTZ) perovskite that can also exist stably in a humid environment and used it as fillers to prepare a flexible PEO/lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)/LSTZ solid composite electrolyte[59]. Ta5+ in LSTZ formed a strong bond with F- in TFSI- anions, which promoted the dissociation of lithium ions and improved the ionic conductivity of the electrolyte. The introduction of LSTZ also broadened the electrochemical window of the electrolyte [Figure 2H].

High-voltage gel polymer electrolytes

GPEs generally consist of a polymer matrix, a plasticizer and a lithium salt. The polymer matrix presents a crosslinked spatial network structure to provide a supporting skeleton, the plasticizer fills the structural gap, and the lithium salt and the polymer are dissolved in the plasticizer. They are a semi-solid electrolyte that improves the ionic conductivity while preventing the leakage of organic liquids. Their preparation methods mainly include ex-situ preparation and in-situ preparation[60]. The ex-situ preparation approach is to obtain GPEs and then assemble batteries, which leads to insufficient interface contact between electrodes and GPEs, resulting in an increase in resistance and a decline in the electrochemical performance of batteries[61,62].

The in-situ preparation method is to uniformly dissolve the polymer monomer, lithium salt, plasticizer and initiator into a liquid electrolyte, and then inject it into a battery cell to induce monomer polymerization under external conditions to form a GPE[60,63]. Here, we will mainly discuss the influence of in-situ preparation method on realizing high-voltage GPEs.

The in-situ polymerization of GPEs can be classified into thermal polymerization and photopolymerization. Thermal polymerization is a free radical polymerization initiated by heat treatment, which is cured at a corresponding temperature according to different types of initiators. Sun et al. explored the conductive mechanism of liquid electrolyte and GPE[64]. Lithium difluoro (oxalate) borate (LiDFOB) was dissolved in a fluoroethylene carbonate (FEC) solvent as a liquid electrolyte (FLE). Despite its high dielectric constant, FLE exhibits low ionic conductivity and poor electrochemical performance. F-GPE was synthesized by in-situ polymerization of methacrylate (MMA), triethylene glycol dimethacrylate (TEGDMA) and ethoxylated trimethylolpropane triacrylate (ETPTA) in FLE at 60 °C using azobisisobutyronitrile (AIBN) as an initiator, as shown in Figure 3A. Through molecular dynamics simulation, it is proved that F-GPE produces an electron-rich delocalized interface layer in the liquid phase and polymer matrix, providing a path for the lithium-ion migration, so that GPEs and lithium ions have higher binding energy, and the ionic conductivity is also improved. The ESW of the prepared F-GPE can reach 5.65 V, much higher than that of the liquid electrolyte. Chen et al. added 3 wt.% N,N′-methylenebisacrylamide (MBA) as a crosslinking agent into an alkaline liquid electrolyte, and AIBN as an initiator to obtain a quasi-solid electrolyte after in-situ thermal polymerization at 60 °C for 4 h[65]. They calculated the molecular orbital energy levels of each electrolyte component using the DFT to evaluate their redox activity. The molecular orbital energy of PAM-GPE and characterization are shown in Figure 3B. Among them, poly (N,N′-methylenebisacrylamide) (PMBA) has the LUMO energy level, indicating that it will be preferentially reduced on the lithium metal anode surface. At the same time, it also has the lowest HOMO, showing high oxidation resistance. This indicates that it is promising to block the electrolyte in the crosslinked structure of PMBA to prevent it from being decomposed under high voltages. The electrolyte was tested by linear sweep voltammetry (LSV). The results showed that the prepared PAM-GPE had an electrochemical stable voltage of 5.64 V, higher than 5.28 V of the liquid electrolyte. The thermal polymerization is carried out after the battery assembly is completed. This process is conducive to the full penetration of the electrolyte to form good interface contacts.

Polymer-based electrolytes for high-voltage solid-state lithium batteries

Figure 3. In-situ preparation of GPE. (A) Preparation and characterization of F-GPE. This figure is quoted with permission from Sun et al.[64] Copyright 2023 Wiley. (B) Molecular orbital energies and characterizations of PAM-QSE. This figure is quoted with permission from Chen et al.[65] Copyright 2021 American Chemical Society. (C) Preparation and characterization of SGPE. This figure is quoted with permission from Gao et al.[67] Copyright 2022 American Chemical Society.

Photopolymerization is initiated through free radical polymerization under ultraviolet (UV) or gamma-ray irradiation, with light acting as the initiator. Zeng et al. synthesized a novel lithiated interpenetrating network GPE by blending polyethylene glycol-acrylate (MPEGA), PEGDA, and AN in the presence of lithiated 2-acrylamido-2-methylpropanesulfonic acid, followed by two stages of UV irradiation polymerization[66]. This GPE has a wide electrochemical window of 5.06 V and good thermal stability (decomposition temperature of 400 °C). Nevertheless, the fabrication of the GPE involves two rounds of UV irradiation, leading to complex procedures and increased costs. Gao et al. realized a fast and efficient photocurable GPE using UV light to irradiate ETPTA and solvent-based ionic liquid for 30 s[67]. The preparation method and electrolyte characterization are shown in Figure 3C. The formed uniform silica gel-based GPE exhibits a wide electrochemical stable voltage of 5.2 V. Although photocuring can achieve rapid polymerization of materials, large-scale use of light radiation can cause harm to the human body and is costly.

In order to achieve high-voltage solid-state lithium batteries, it is not only necessary to focus on the high-voltage stability of the PEs, but also to consider the ionic conductivity, electrode and electrolyte compatibility, and the feasibility for achieving industrial development. Compared with SPEs and CPEs, the in-situ polymerization strategy of GPEs is more likely to be commercialized due to their good processability and flexibility and the high compatibility of the preparation process with the existing liquid battery technology. GPEs do not depend on conformational interfaces and, therefore, can adapt to the volume change of the electrodes during cycling. However, careful selection of the composition of GPEs is crucial, as the chemical composition of the interface ultimately determines the lifespan of batteries.

COMPATIBILITY OF POLYMER ELECTROLYTES WITH HIGH-VOLTAGE CATHODES

The design of high-voltage batteries also needs to consider the compatibility between PEs and electrodes, normally involving two main aspects: first, poor physical contacts at the cathode-electrolyte interface lead to solid-solid contact deterioration (SPEs and CPEs). Following repeated lithium insertion/extraction cycles, the cathode undergoes phase transformation and surface reconstruction, leading to lattice mismatches and non-conformal contacts that result in high interfacial resistance[68]. This problem can be addressed through strategies such as electrolyte supplementation, deposition of external coatings, integration of electrode/electrolyte structures, and in-situ polymerization[69]. Another crucial aspect is the side reactions between the electrode material and the electrolyte, such as the decomposition of the electrolyte and the unstable cathode-electrolyte interface (CEI)/SEI due to the incompatibility between the electrolyte and the electrode[70,71]. The limited oxidative capability of the electrolyte leads to interface instability between the PE and the high-voltage cathode, causing inevitable electrolyte oxidation, degradation, and rapid capacity loss[72-74].

Interfacial physical contacts

As a critical battery component, the solid composite cathode directly determines the battery's output capacity and energy density[75,76]. However, there are compatibility challenges between high-voltage cathodes and conductive PEs[77]. Typically, porous electrodes with poor contact with the electrolyte exhibit discrete ion transport, exposing transition metal regions, which causes the electrolyte preferential decomposition into non-conductive interfaces, resulting in rapid capacity decay. This is due to the unevenness of the state of charge (SOC) within active particles and the loss of structural integrity at the sub-particle level[78], ultimately leading to sub-particle-level deactivation[79]. Conversely, cathodes with uniform contacts exhibit uniform particle-level SOC and high oxidation stability. The local microstructure of a high-loaded cathode is closely related to ion transport discreteness and capacity decay. In-depth understanding of the local structure and charge state of the cathode and their interaction with interface evolution can help achieve compatibility between the electrolyte and the cathode.

The ion transport discreteness in the cathode is influenced by its porosity. PEs have difficulty penetrating porous cathodes[80], leading to non-uniform distribution of local electric fields at heterogeneous contact points, reducing the electrochemical stability of batteries. Zhu et al. developed a liquid polymer electrolyte (LPE) composed of brush-like polymers with a main chain of polyphosphonitrile and a side chain of oligomeric EO[81]. LPE addresses issues such as electrolyte oxidation, poor lithium plating/stripping performance, and interface instability on the cathode. The flowable viscous LPE fully wets or even infiltrates the electrode to maintain good contact at the interface. LiFePO4 and Ni0.8Co0.1Mn0.1O2 (NCM811) batteries using this LPE achieve long-term stable cycling in the temperature range of 60 to 120 °C, with a Coulombic efficiency of about 100%. Increasing the surface coverage of the electrolyte and improving the inherent stability of the electrolyte are expected to achieve a stable interface. An et al. designed a high-loaded composite cathode (12 and 28.6 mg cm-2) with a fully active surface through a solvent-free in-situ liquid-solid conversion strategy[82]. As shown in Figure 4A, the dual modifications through physical structure reshaping and chemical methods enable high-loaded NCM811 batteries to exhibit an excellent lifespan exceeding 10,000 h and over 150 cycles with a cathode loading of 28.6 mg cm-2.

Polymer-based electrolytes for high-voltage solid-state lithium batteries

Figure 4. The mechanical contact between polymer-based electrolytes and electrodes. (A) High state of charge in particle scale at local contact accelerates the decomposition of electrolyte. This figure is quoted with permission from An et al.[82]. Copyright 2023 Wiley. (B) Schematic representation and electrochemical performance of solid-state batteries. This figure is quoted with permission from Wang et al.[84] Copyright 2018 Wiley. (C) Interface optimization between SIP-derived coating and cathode, along with the enhanced cycling performance of NCM622||Li cell under high voltage conditions. This figure is quoted with permission from Chen et al.[87] Copyright 2023 Wiley.

Additionally, a cathode normally undergoes volume changes during the cycling process, while the deformability and wetting properties of the solid electrolyte are poor, which leads to interface gaps[83]. Therefore, achieving a flexible interface between the electrolyte and the electrode is crucial. Wang et al. utilized poly(acrylonitrile-butadiene) (PAB) polymer as a coating for LiNi0.6Mn0.2Co0.2O2 cathode material, as depicted in Figure 4B[84]. The PAB nano-soft layer can enhance the physical contact between the cathode and the solid electrolyte. Coupling the cathode with lithium metal and poly(ethylene-acrylic ester) (PEA) solid electrolyte, the solid-state battery exhibited excellent rate performance (99 mAh g-1 at 3 C) and good cycling stability (75% capacity retention after 400 cycles). Quasi-solid or gel polymer electrolytes (QSEs or GPEs) containing organic small molecule plasticizers/solvents are expected to improve interface contacts[74] and room temperature ionic conductivity[85,86]. Surface in-situ polymerization (SIP) with optimized interface interactions can enhance interface compatibility between various cathodes and PEs, offering the potential for enduring high-voltage tolerance. Chen et al. achieved durable and tunable SPEs on the cathode side through in-situ polymerization of a polymerizable ether-based electrolyte (SIP), as illustrated in Figure 4C[87]. SIP involvement in constructing a homogeneous polyether/LiTFSI coating can optimize the interface interactions between the PE and the LiNixCoyMnzO2 (NCM, x + y + z = 1, x ≥ 0.8) cathode. Additionally, its derived ultra-thin CEI-like coating can provide close/conformal contact between the cathode and the electrolyte. The assembled NCM811||Li full cell achieved super-stable cycling performance and high Coulombic efficiency.

Interfacial chemical reactions

The working potential of the high-voltage cathode exceeds 4 V; the practical operation of lithium batteries is constrained by the oxidative nature of the cathode[88,89]. Intense oxidation of transition metals and irreversible loss of lattice oxygen at high charge states lead to electrolyte decomposition and reduced conductivity at the cathode-electrolyte interface[90]. Molecular structure design can enhance the antioxidant properties of PEs, aiding in constructing a stable CEI[91]. Copolymerization of multiple monomers can compensate for the limitations of individual polymers, widening the ESW. This is a critical requirement for the compatibility of PEs with high-voltage cathode materials. However, PEs containing easily oxidizable oxygen-containing groups typically exhibit narrow ESW, resulting in poor cycling stability under high potentials. Conversely, widening of the ESW up to 4.8 V can be achieved by ring-opening polymerization of oxidizable cyclic carbonates using organic metal catalysts[92]. Additionally, it is noteworthy that strong electron-withdrawing groups can further enhance the antioxidant properties of the polymer matrix[93], as previously discussed. Solid electrolytes possess stronger dendrite suppression capabilities than liquid electrolytes, offering potential for applications in high-capacity lithium metal anodes[94]. However, due to the accumulation of interface voids and volume changes during repeated lithium stripping processes, contact losses between solid electrolytes and electrodes result in large ion transport barriers, deteriorating the electrochemical performance of solid-state lithium metal batteries[95].

Constructing functional interfaces[92] and electrode modifications[96] can achieve compatibility between PEs and electrodes, where the interface components and structural characteristics directly influence the rapid transmission of lithium ions. Metal-organic frameworks (MOFs), due to their high surface area, structural stability, tunable porosity, and abundance of Lewis acid sites, have been utilized for electrode interface modification or as electrolyte fillers to enhance ion transport and charge transfer processes, thereby suppressing lithium dendrite formation[97]. Qian et al. reported a porous and robust MOF coating (MOF-199), where the sturdy MOF interfacial layer physically inhibits lithium dendrite growth[98]. Its highly polar structure promotes uniform Li+ concentration, thereby mitigating excessive SEI formation. Lithium metal batteries protected by MOF-coating achieved 97% Coulombic efficiency at a current density of 1.0 mA cm-2. Huo et al. employed cationic MOFs (CMOFs) to immobilize anions and guide uniform Li+ deposition, enabling dendrite-free solid-state batteries[99]. CMOFs, through electrostatic interactions with charge carriers and their high surface area, securely bind anions, enhancing the transference number of Li+ to 0.72[99]. Furthermore, customization with CMOFs grafted with -NH2+ groups can protect the polymer chain's ether oxygen via hydrogen bonding, widening the ESW to 4.97 V. Cui et al. achieved excellent lithium-ion transport and interface stability by utilizing a MOF layer and atomic layer deposition of Al2O3 at the modified polymer matrix interface[100]. The inert Al2O3 nano-coating enhances affinity with the lithium negative electrode and participates in forming the SEI film, reducing the diffusion barrier for lithium ions. After 500 cycles within a wide voltage range of 2.0-4.8 V, the capacity retention remained at 84.6%. Inorganic coatings can improve the electrochemical stability of the interface; however, they often involve intricate processing steps and high stiffness, making it challenging to accommodate the volume changes of the cathode during charge/discharge cycles. In-situ formation of a homogeneous electrode/electrolyte interface layer via electrochemical methods, using highly flexible polymers, can simultaneously mitigate cathode-electrolyte side reactions and enhance contacts at the solid-solid interface during the cycling process. Ma et al. performed UV-curing of FEC and triethyl phosphate (TEP) carbonate-based precursors separately on two electrodes, developing a dual-function polymer electrolyte (BDFPE) with low interfacial resistance [Figure 5A][101]. The functional additives FEC and TEP have low LUMO and high HOMO, aiding in interface formation[46]. Regulation of the electrolyte molecular structure and additive composition can achieve compatibility between the electrolyte and high-voltage cathodes, such as fluorination and nitrogenation[102]. The solid electrolyte adheres closely to the lithium metal anode and the nickel-rich cathode, facilitating continuous lithium flux and avoiding large interface transfer barriers between the electrode and electrolyte. Benefiting from the BDFPE design, Li||Li symmetric cells achieved smooth and dendrite-free lithium deposition after an extended 1,800 h of cycling at 1 mA cm-2. Qi et al. reported fluorinated and nitrided polymer electrolytes (F&NPE) composed of 2,2,3,4,4,4-hexafluorobutyl acrylate (HFBA) and N,N′-methylenebisacrylamide (MBAM), as depicted in Figure 5B[103]. Based on molecular-level design of the PE, robust CEI and SEI were simultaneously constructed. The lithium-affinitive N-(C)3 in the SEI guided uniform distribution of Li+, promoting the transport of Li+ through LiF and Li3N for uniform Li+ deposition and stripping. Additionally, antioxidative fluorination and nitrogenation moieties in the CEI suppressed parasitic reactions between the cathode and electrolyte and structural degradation of the cathode. Therefore, NCM622/F&NPE/Li cells exhibited an 85.0% capacity retention after 500 cycles at 4.5 V voltage and 0.5 C.

Polymer-based electrolytes for high-voltage solid-state lithium batteries

Figure 5. Interface chemistry between polymer electrolytes and electrodes. (A) Preparation of bidirectional functional polymer electrolytes and their assembled battery performance. This figure is quoted with permission from Ma et al.[101]. Copyright 2023 Wiley. (B) Stable interface design for NCM622/F&NPE/Li batteries and cycling performance of NCM622//Li batteries. This figure is quoted with permission from Qi et al.[103] Copyright 2023 Wiley. (C) Solid electrolyte interface formed on lithium metal electrodes and electrochemical performance of batteries. This figure is quoted with permission from Li et al.[107]. Copyright 2023 Springer Nature.

In-situ polymerization with liquid monomers, similar to conventional liquid electrolytes, exhibits excellent wetting properties. Therefore, it can permeate electrodes to achieve sufficient contact with the active material, enabling close interaction between electrodes and PEs[3,104]. Lithium batteries face challenges in cold climates due to insufficient dynamics in the electrolyte itself and at the electrode/electrolyte interfaces. At low temperatures, inadequate ion and charge transport dynamics in the electrolyte and at the electrode/electrolyte interface can lead to structural changes at the SEI[105,106]. Li et al. reported a PE, polyoxymethylene (POM), prepared by in-situ polymerization using the precursor 1,3,5-trioxane[107]. POM has a wide bandgap and can stabilize the NCM811 cathode at high voltages (e.g., > 4.4 V)[108]. PEs can form a dual-layer SEI on the lithium metal anode [Figure 5C], consisting of an amorphous LixBOyFz outer layer and an inner layer rich in LiF. LixBOyFz and LiF are excellent electron insulators with a large electrochemical window, suppressing electrolyte decomposition and dendrite formation[109]. The amorphous LixBOyFz is highly plastic, mechanically adapting to the volume changes of the electrode[110].

COMPATIBILITY OF POLYMER-BASED ELECTROLYTES WITH OTHER NEGATIVE ELECTRODES

The formation of a stable SEI layer on negative electrodes is crucial for extending the lifespan and maintaining high capacity in lithium-ion batteries (LIBs)[111]. The ideal physical thickness of a SEI layer is on the order of a few angstroms (Å), with high mechanical strength to accommodate volume expansion and contraction during charge and discharge processes[112]. Graphite is a common negative electrode material for LIBs, and the formation of its surface SEI layer is influenced by graphite type, electrolyte composition, electrochemical conditions, and temperature. The slow electrochemical processes on the graphite surface limit the fast charging performance of LIBs, including lithium desolvation in the SEI and lithium transport in the SEI[113]. Additionally, graphite's low equilibrium potential (~0.1 V vs. Li/Li+) results in a low overpotential capacity limit, forming metallic lithium when the anode potential drops below 0 V (vs. Li/Li+), leading to poor cycle stability and even safety issues. SEI composition and structure are crucial in modulating the lithium desolvation structure at the negative electrode interface and accelerating the rapid charging process of LIBs. Electrolyte additives, high-concentration electrolytes, and graphite surface coatings contribute to forming a stable SEI, preventing further electrolyte decomposition[114]. The electronic resistance increases the potential of the graphite surface and shifts the negative electrode surface potential in the stable window of the electrolyte. The electronic insulation and compact structure of the SEI prevent continuous reduction of the anode surface by the electrolyte and the intrusion of solvents into the SEI, embedding further into bulk graphite.

The specific capacity of silicon (Si) negative electrodes is ten times that of traditional graphite anodes (372 mAh g-1)[115]. However, the alloying/dealloying reactions between Si and Li cause significant volume changes, leading to severe anisotropic stress and particle fragmentation[116]. Furthermore, volume changes result in continuous variation of the SEI interface, rapidly depleting electrolyte and lithium ions. Unique nanostructures (nanowires, nanotubes, core/shell structures, nanoporous materials, etc.) and composites using electrochemically inert/low-active materials (such as carbon, conductive polymers, etc.) can reduce internal stresses and leverage their supporting structure, large specific surface area, and short lithium diffusion length to enhance their cycling lifespan. Nevertheless, limitations in volumetric energy density and reduced mass loading on electrodes will restrict industrial applications. It is noteworthy that silicon and graphite can be processed using the same commercial production lines, enabling high manufacturability and low investment. Si/graphite/C composite material designs contribute to enhancing negative electrode capacity and structural stability[117], thereby reducing battery pack and manufacturing costs.

GAS GENERATION IN HIGH-VOLTAGE LITHIUM BATTERIES

The highly lithiated surfaces of the cathode/anode during charging/discharging processes in high-voltage lithium batteries lead to gas generation upon oxidation/reduction. Gas evolution in the battery due to swelling results in capacity decay, increased impedance, loss of cell integrity, and poses new safety risks[118]. Therefore, controlling gas generation in the cathode, anode, and electrolyte is crucial for the safe and high-performance operation of batteries.

High-voltage cathode materials are key to achieving high energy density in lithium batteries; however, the cathode side is often a significant source of battery gases. For instance, layered materials [Figure 6A], such as LixMO2, LixNiO2 (LNO), and LixMnO3 (LMO) (x < 1), are metastable kinetically and undergo oxygen activation at high voltages, leading to oxygen release[119]. Similarly, LCO undergoes irreversible phase transition to CoO2 via spinel and rock-salt structures during over-oxidation, releasing O2. Subsequently, CoO2 undergoes further oxidation, resulting in oxygen loss. Over-delithiation of the cathode generates heat and releases O2, which, in turn, accelerates electrolyte decomposition[120,121]. For instance, layered transition metal oxide cathodes can react with carbonate-based solvents [e.g., propylene carbonate (PC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dimethyl carbonate (DMC)] in QSEs to produce CO2[122,123]. Ring-structured solvents are prone to ring-opening reactions and decomposition; thus, selecting chain-structured carbonates in the GPE reduces gas evolution.

Polymer-based electrolytes for high-voltage solid-state lithium batteries

Figure 6. Gas evolution in lithium-ion batteries (LIBs). (A) Release of O2 and CO2 from layered transition metal oxide cathodes[119]. Copyright 2021, Elsevier. (B) Structural evolution, gas generation, and interface reactions during high-voltage charging of lithium-rich and manganese-rich layered oxide cathodes[126]. Copyright 2022, Elsevier. (C) Schematic description of the proposed electrochemical and chemical electrolyte oxidation pathways (exemplarily shown for EC)[37]. Copyright 2017, IOP Publishing. (D) Suppression of gas generation and particle fracture with carbon-coated silicon nanoparticles[138]. Copyright 2018, American Chemical Society.

Chemical reactions at the interface between the cathode and electrolyte are the primary sources of gas generation in batteries. For instance, when charging voltage exceeds 4.5 V, the escape of 1O2 from LiNixMnyCozO2 (NMC) lattices leads to further oxygen release. Additionally, cathode contaminants, such as lithium carbonate (Li2CO3), often appear on the cathode surface during NMC or lithium-rich manganese-based layered cathode material preparation and storage[124]. Under a relatively low voltage of 3.8 V, the decomposition reaction 2Li2CO3 → 4Li+ + 4e- + 2CO2 + 1O2 occurs, and at high voltages, 1O2 reacts vigorously with the electrolyte, further producing gases such as CO2 and CO[125]. Two typical ternary nickel-rich layered cathode materials, NCM and LiNixCoyAlzO2 (NCA, x + y + z = 1, x ≥ 0.8), generate CO and CO2 gases at their surfaces through (i) electrochemical oxidation; and (ii) chemical oxidation[126]. As illustrated in Figure 6B, electrochemical oxidation at the cathode surface occurs at voltages higher than electrolyte decomposition[126]. For example, EC oxidation at the positive electrode interface produces CO, CO2, and protons (R-H+), leading to electrolyte degradation; subsequently, proton reduction at the negative electrode produces H2. Conversely, phase transitions in layered NCM structures and the release of highly active singlet oxygen (1O2) lead to chemical oxidation of EC, resulting in CO2, CO, and H2O. Higher nickel content facilitates lattice oxygen oxidation, with oxygen being released after a decrease in oxygen binding strength[127]. Additionally, surface restructuring of nickel metal ions (e.g., LiMO2 → MO + ½O2) contributes to increased oxygen release[128].

PEs can widen the ESW of the electrolyte by crosslinking structures or grafting high-voltage-resistant groups, effectively suppressing electrochemical oxidation of the electrolyte and reducing gas evolution from layered transition metal oxide cathodes under high voltages.

The escape of negative electrode gases at low potentials is crucial for achieving high performance in LIBs[129]. Due to the absence of a stable SEI on the negative electrode surface during the initial cycles of battery operation[130], electrolyte components on the negative electrode side are prone to generating highly flammable gases (hydrogen, ethylene [C2H4), and propylene], leading to thermal runaway of the battery. In QSEs, under high voltage, EC undergoes electron transfer at the negative electrode, resulting in the electrochemical reduction and generation of lithium ethylene dicarbonate (LEDC), C2H4, and CO[131]. As shown in Figure 6C, C2H4 production on graphite surfaces can be attributed to electrolyte reduction at 0.8 V[132]: 2EC + 2Li+ + 2e- → C2H4 + (CH2OCO2Li)2 and EC + 2Li+ + 2e- → C2H4 + Li2CO3. Other solvents, such as EMC, contribute to CO production, while PC-based electrolytes facilitate the production of CO2, CO, and C3H8/C3H6[133]. DMC solvent promotes CH4 generation. Additionally, the choice of salt significantly influences gas generation; for example, lithium bis(oxalato)borate (LiBOB)-based electrolytes undergo decomposition reactions under overcharge conditions, reacting with highly active oxygen released from the positive electrode material to produce CO2. PEs can suppress interfacial reactions between the electrolyte and electrode and gas generation[134].

Electrode surface coatings and artificial SEI layers can passivate the electrode surface. For instance, carbon [Figure 6D], ceramic[135], and atomic layer deposition coatings[136] inhibit gas generation while maintaining high ionic conductivity[137]. Using antioxidant polymers at the positive electrode and anti-reductive polymers at the negative electrode can mitigate secondary reactions at the battery interfaces, ensuring high-voltage operation of the battery. Furthermore, the design of PE compositions facilitates the formation of stable SEI layers on electrode surfaces, suppressing parasitic reactions and addressing gas generation issues[138]. To avoid continuous formation and dissolution of SEI layers, electrolyte components used for SEI formation should prioritize reduction or oxidation (with low LUMO and high HOMO). Electrolyte structure design is often employed to suppress interface reactions (including high-concentration electrolytes) to broaden the ESW and inhibit gas generation in high-concentration or locally concentrated electrolytes[139].

CONCLUSIONS AND OUTLOOK

This article reviews the research progress and challenges of PEs for high-voltage solid-state batteries. PEs need to simultaneously have broad ESW and compatibility with electrode interfaces. The high-voltage compatibility can be achieved by designing layered composite structures or interface passivation. Developing GPEs is an effective approach to addressing challenges electrolytes face under high and low temperature conditions (such as low electrochemical stability, interface degradation, electrolyte decomposition, etc.). Nonetheless, there are many aspects to be improved for high-voltage PEs, with potential solutions being detailed below:

(1) Intrinsic stability of polymer-based solid electrolytes. Intrinsic stability refers to the minimal occurrence of electrolyte decomposition reactions between polymer-based solid electrolytes and electrodes under high voltages. In polymer systems, matrices with electron-withdrawing groups generally exhibit higher stability. Therefore, strategies such as designing the polymer molecular structure, grafting electron-withdrawing groups, and increasing the degree of polymer crosslinking hold potential for stabilizing electrolytes under high-voltage conditions.

(2) Non-intrinsic stability of polymer-based solid electrolytes. For polymer-based solid electrolytes inherently unstable under high voltages, non-intrinsic high-voltage stability can be achieved by designing interface passivation layers and implementing composite electrolyte strategies. (1) The interface passivation layers can generate an ideal interfacial phase with high ionic conductivity and low electronic conductivity, dynamically suppressing electrochemical decomposition and facilitating stable operation of batteries under high voltages; and (2) constructing composite electrolytes regulates local molecular interactions, thereby delaying the high-voltage decomposition of solid electrolytes.

(3) Advanced characterization techniques and theoretical computations. Currently, solid electrolytes and the electrode/electrolyte interfaces in solid-state systems remain shrouded in mystery. Combining in-situ observations and multiscale calculations makes it possible to elucidate the interactions among polymer chains, small molecules, and ions in the system, guiding the design of novel polymer-based solid electrolytes. Additionally, advanced techniques are employed to track interface evolution and characterize interfaces under high voltages. This enables a more detailed investigation of interface reactions, ion transport across multiphase interfaces, and other complex interface behaviors.

(4) Developments of polymer-based electrolytes with high anodic stability are highly desired for enhancing the energy density of lithium batteries. The engineering parameters, such as thin electrolytes, high-loading cathodes, and suitable Negative/Positive capacity (N/P) ratios, are very important to achieve high energy density; the electrochemical performance of high-voltage batteries with the combination of these parameters is suggested to be further explored.

Therefore, the following research proposals are made for PE: For polymers such as PAN and PVDF that are compatible with 5 V-level high-voltage cathode materials, improving their negative electrode stability and room-temperature ionic conductivity are key for future research. For SPEs, reducing electrolyte thickness while ensuring mechanical performance can compensate for the low ionic conductivity, thereby enhancing rate capability and reducing the operating temperature of solid-state lithium batteries. Additionally, ceramic particle dispersing, plasticizer addition, and liquid solvent incorporation all contribute to enhancing their room temperature performance.

GPEs simultaneously address issues of low ionic conductivity and electrode-electrolyte interface contacts. Future research on GPEs will primarily focus on improving safety and reducing liquid content. For instance, polymer grafting or copolymerization with halogen and phosphate-based monomers can eliminate radicals during battery ignition, thereby achieving flame-retardant effects. Importantly, GPEs can be adapted to existing liquid battery production lines, significantly reducing production complexity and cost. Transitioning from quasi-solid-state to all-solid-state polymer batteries will gradually eliminate flammable liquid batteries, enhancing the battery safety.

DECLARATIONS

Authors’ contributions

Proposed the topic of this review: Fu J

Prepared the manuscript: Wang Z, Chen J

Collectively discussed and revised the manuscript: Wang Z, Chen J, Fu J, Li Z, Guo X

Availability of data and materials

Not applicable.

Financial support and sponsorship

This work was supported by the Natural Science Foundation of Hubei Province, China (Grant No. 2022CFA031) and Dongguan Innovative Research Team Program (2020607101007).

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

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Polymer-based electrolytes for high-voltage solid-state lithium batteries
Zixuan Wang, ... Xin Guo

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Wang, Z.; Chen, J.; Fu, J.; Li, Z.; Guo, X. Polymer-based electrolytes for high-voltage solid-state lithium batteries. Energy Mater. 2024, 4, 400050. http://dx.doi.org/10.20517/energymater.2023.130

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