Polymer Electrolytes for High Energy Density Ternary Cathode Material-Based Lithium Batteries

2.1.1 Polyoxyethylene

Polyoxyethylene (PEO)-based electrolytes have been applied in TCM-based LBs due to high lithium ion-solvation abilities, low glass transition temperatures and good interfacial compatibilities. However, these electrolytes suffer from low ionic conductivities at room temperature because conductivity is governed by the segmental motion of the polymer chains in the polymeric amorphous region. Here, the addition of nanosized inorganic fillers can provide facile transfer pathways for lithium ions and reduce the glass transition temperature of pristine PEO, thus endowing these polymer electrolytes with enhanced ionic conductivities. In addition, improved electrochemical properties such as anodic stability can also be achieved with the addition of inorganic fillers. For example, Lu et al. [97] reported that the use of PEO₂₀–LiBOB-nanosized MgO as the electrolyte produced a high anodic stability of 4.5 V and that the corresponding LiNi_1/3Co_1/3Mn_1/3O₂/Li cell delivered a superior 1st discharge capacity of 156.8 mAh g⁻¹ and a higher capacity retention (91% after 20 cycles at 0.2 C) at 80 °C than cells without inorganic fillers. The researchers in this study also reported the important role of LiBOB in the passivated and stable cathode/electrolyte interface (CEI). Recently, Archer et al. [98] reported a solid-state hybrid polymer electrolyte (SiO₂–PEO/LiTFSI) composed of silica nanoparticles covalently grafted with a PEO chain (5 kDa) and reported that this type of soft glassy solid polymer material endowed their assembled LiNi_0.6Co_0.2Mn_0.2O₂/Li cell with an initial discharge capacity of 178 mAh g⁻¹ and a capacity retention of over 97% after 25 cycles through a simple CEI-forming process by first wetting the cathode with a LiBOB-containing liquid electrolyte (0.4 M LiBOB, 0.6 M LiTFSI, 0.05 M LiPF₆ in EC/DMC). Here, the researchers also indicated that the SiO₂ nanocores with a uniform multilayered arrangement can passivate the Li anodes without limiting ionic access to the electrode.

Aside from the addition of inorganic fillers, inorganic coatings sandwiched between the cathode sheet and the electrolyte can also enhance the electrochemical performance of PEO-based APE LBs. For example, Seki [99] reported that LiNi_1/3Co_1/3Mn_1/3O₂/P(EO/MEEGE)–LiTFSI/graphite cells can produce a high initial discharge capacity (128 mAh g⁻¹) and acceptable capacity retention (78% over 20 cycles) at 30 °C by coating LiAlO₂ onto the LiNi_1/3Co_1/3Mn_1/3O₂ surface, clearly outperforming the pristine cell. In another study, Lin et al. [100] also reported that the blending of PEO polymers can effectively enhance performances through synergistic effects. Here, the researchers prepared a biocompatible and biodegradable APE through the mixing of natural wheat flour with PEO and LiTFSI and reported that the resulting APE produced an ionic conductivity of 2.62 × 10⁻⁵ S cm⁻¹ at 25 °C with a t_Li+ of 0.51. In addition, the researchers reported that their synthesized cells based on high-voltage LiNi_0.8Co_0.1Mn_0.1O₂ demonstrated excellent rate performances (a capacity of 132.3 mAh g⁻¹ at 10 C) at 100 °C, suggesting the potential application of natural materials in high-voltage energy storage systems.

Aside from low ionic conductivities at room temperature, all-solid-state polymer batteries using PEO as the polymer host often operate at higher temperatures than its melting temperature, resulting in polymer electrolytes with weak mechanical strength. In addition, narrow electrochemical stability windows restrict application in high-voltage TCM-based cells. And although these drawbacks can be mitigated through the introduction of inorganic fillers, inorganic coatings, and blending with other polymers, weaknesses caused by the oxidative decomposition of pristine PEO cannot be eradicated. Therefore, to resolve this issue, copolymerization approaches have been developed to optimize pristine PEO.

2.1.2 Polymers with a Cross-Linked Network

In general, most polymer electrolytes comprised of only polyethylene oxide (PEO) are electrochemically unstable above 4.0 V versus Li/Li⁺ and usually decompose around 3.6 V [101]. To address this, copolymerization with other monomers is an effective method to enhance the electrochemical stability windows and ionic conductivities of corresponding polymer electrolytes. Furthermore, GPEs with cross-linked network structures through copolymerization tend to possess desirable mechanical properties and large uptake of liquid electrolytes.

In one example, Oh and Amine [102] studied a cross-linked GPE based on a polymer (P(P(EO–PO))) containing ethylene oxide (EO) and propylene oxide (PO) motifs and reported an electrochemical stability window of up to 4.5 V, with the resulting cell exhibiting a high capacity of more than 150 mAh g⁻¹ at 0.2 C and decent rate performances (139 mAh g⁻¹ at 2 C). However, the researchers also reported that these batteries possessed inferior cycling stabilities; especially at 50 °C, due to resistances arising from the CEI. In another example, Kim et al. [103] developed a GPE containing a cross-linked network of polyoxyalkylene glycol acrylate (POAGA) which demonstrated enhanced electrochemical stabilities up to 5.0 V versus Li/Li⁺. Here, the researchers reported that the POAGA-based cells exhibited a capacity of 146.3 mAh g⁻¹ at 2 C rate under ambient temperatures and 109.7 mAh g⁻¹ at 0.5 C rate at − 20 °C, as well as good cycling stabilities with a retention of 89.7% of the initial capacity after the 300th cycle. In addition, overcharge tests were also conducted in this study and demonstrated that this polymer electrolyte system provided good safety as well. POAGA and its methylated derivative-based polymer electrolytes have also been investigated by other groups to match with LiNi_1/3Co_1/3Mn_1/3O₂ cathodes and have also been reported to demonstrate good cyclability and safety properties [104, 105].

Despite these advantages, however, inferior cycling capabilities, especially at elevated temperatures, which predominately result from cathode degradation, significantly hinder application in electric vehicles [106]. To solve this issue, Jung et al. [107] developed a GPE (P(DPHA-PEGMEM)) through the copolymerization of dipentaerythritol hexaacrylate (DPHA) with poly (ethylene glycol) methyl ether methacrylate (PEGMEM) and reported that assembled batteries based on this GPE demonstrated superior cyclability at 80 °C (89% capacity retention at 3 C after 50 cycles). Here, the researchers attributed the rate performance partly to the buffering effect of lithium ion dissolution being extracted from active materials under high rate operations by polymer electrolytes. Furthermore, the researchers also reported that the ionic conductivity of their GPE was less susceptible to temperature changes and possessed enhanced thermal stabilities (up to 210 °C).

In another study, Kobayashi et al. [108] attempted to suppress the irreversible oxidative reaction of polyethers in all-solid-state TCM-based LBs through cathode surface modifications and reported that the surface modification of NMC type cathodes by using sodium carboxymethyl cellulose (CMC) enabled the P(EO/MEEGE)-based APE to demonstrate good compatibility with 4 V class cathodes. Here, the resulting battery exhibited enhanced cycling performances with 50% capacity retention after 1500 cycles at 60 °C, benefiting from decreased oxidative decomposition of polyethers as a result of the CMC coating as compared with that of the bare cells. The researchers in this study also demonstrated that oxidation reactions took place in the main chain with branched polyethers in the P(EO/MEEGE) and that the dominant factor leading to the degradation of all-solid-state LBs after long-term cycling can be the degradation of TCM cathodes at high temperatures, which is the same as in battery systems using liquid electrolytes. Based on this, Kobayashi et al. [109] conducted surface modifications by coating lithium bis(oxalate)borate (LiBOB) and polyvinyl chloride onto the surface of a LiNi_1/3Mn_1/3Co_1/3O₂ cathode and a graphite anode, respectively (Fig. 5), and reported that the modifications not only suppressed the oxidative decomposition of polyether, but also reduced lithium consumption on the graphite anode, with the resultant cell using P(EO/MEEGE) as the polymer matrix exhibiting excellent long-term cycling stabilities (60% capacity retention after 5400 cycles at 50 °C (Fig. 5), indicating that the inherently safe and the long cycling lifespan of all-solid-state polymer LBs are promising for practical application.

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In addition to the surface coating of cathodes, compositing cathodes with ion-conductive materials can also enhance the electrochemical capability of polymeric cells, and especially rate ability. For example, Nan et al. [110] studied a composite TCM cathode with ion-conductive 0.44LiBO₂·0.56LiF as well as electron-conductive In₂O₅Sn (ITO) and reported that the resulting all-solid-state polymer battery using the copolymer P(EO/EH) of epichlorohydrin and ethylene oxide as the matrix produced a specific discharge capacity of 136 mAh g⁻¹ with a surface capacity of up to 6 mAh cm⁻² at 0.1 C after 20 cycles. Here, the researchers attributed these enhanced performances to the enhanced ionic and electronic percolation of the composite electrodes as well as the improved interfacial contact.

2.1.3 Polymers Containing Ethoxylated Sidechains

Polymers containing ethoxylated sidechains typically possess sufficient flexibility at the EO-based segment and favor ionic conductivity. For example, Ghamouss et al. [112] synthesized a solvent-free gel polymer matrix (P(MA475-DMA550)) through the free radical copolymerization of methacrylate-based oligomers in the presence of LiTFSI and ionic liquid RTIL and reported that the resultant free-standing GPE provided an ionic conductivity close to 4 × 10⁻⁴ S cm⁻¹ at room temperature and that the corresponding LiNi_1/3Mn_1/3Co_1/3O₂/GPE/Li battery provided a good specific discharge capacity (118 mAh g⁻¹ at 0.1 C and 79 mAh g⁻¹ at 0.2 C). In another study, Shono et al. [111] conducted in-depth investigations into the capacity decay of polyether-based electrolytes by performing capacity fading analyses using pseudo-reference electrodes to measure the operation of a P(EO/MEEGE/AGE)-based solvent-free LIB with LiNi_1/3Mn_1/3Co_1/3O₂ as the cathode active material. Here, the researchers reported that the resultant cell achieved a capacity retention of 50% after 1000 charge/discharge cycles, and that the successive and irreversible loss of active lithium at the graphite anode is the main reason for capacity reduction, rather than the oxidation decomposition of the polyether-based electrolyte.

In another study, Wang et al. [113] investigated another polymer matrix containing cyclic ethoxylated sidechains and a polycyclic main chain (Fig. 6c) which was obtained through a ring-opening metathesis polymerization. Here, the researchers coated the polymer matrix onto a Li metal surface to regulate SEI structures as well as Li deposition/dissolution behaviors, and reported that the cyclic ether groups with a high affinity to Li metal surfaces can participate in the formation of SEI structures and compete with carbonate-based electrolyte systems to enable more compact and flexible SEI structures, whereas the polycyclic main chain can provide high mechanical strength to modulate Li deposition morphologies (Fig. 6a–b). And as a result, the corresponding LiNi_0.5Co_0.2Mn_0.3O₂/Li battery in this study delivered significantly improved Coulombic efficiencies and long cycle lifespans with a capacity retention of 90.0% after 400 cycles and steady voltage plateaus in contrast with the bare cell, providing a feasible strategy to design polymer matrixes coupled with TCM-based cathodes/Li full cells. However, despite the promising performances, this type of polymer matrix is cost-prohibitive, and unstable at highly delithiated states due to the presence of cyclic ketal motifs with inferior anti-oxidative capabilities in the polymer host.

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Polymers containing ethoxylated motifs have proven to be effective polymer matrixes for high energy density TCM-based LBs. However, insufficient ionic conductivity at room temperatures, low t _Li ⁺ and inferior anti-oxidative abilities hinder application in high-voltage TCM-based LBs. Despite this, these polymer electrolytes can be further optimized through the introduction of functional groups with high oxidative resistances and strong coordination capabilities with transition metal ions (e.g., cyano or sulfone groups) as well as blending with advanced inorganic electrolytes with high ionic conductivities.

2.2.1 Polymethyl Methacrylate

Polymethyl methacrylate (PMMA) exhibits an excellent affinity to liquid electrolytes and can accommodate large uptake of liquid electrolytes. In addition, GPEs based on PMMA possess relatively good compatibility with lithium anodes. However, this polymer inherently suffers from insufficient mechanical strength, and to address this, Holze et al. [114] blended in poly(vinylidene fluoride) (PVDF) to enhance the mechanical properties of PMMA-based polymer matrixes and reported that after the absorption of liquid electrolytes, the blended polymer-based electrolyte provided an electrochemical stability window of 4.5 V and an ionic conductivity of 3.2 × 10⁻⁴ S cm⁻¹ at 25 °C. Despite this, however, this GPE still suffered from inferior cycling stabilities, especially in the presence of lithium anodes.

2.2.2 Polycyanoacrylate

Polycyanoacrylate-based electrolytes can exhibit high anti-oxidative abilities and can be used as a polymer matrix to couple with high-voltage TCM-based cathodes. For example, our group [115] developed a composite electrolyte comprised of poly(ethyl α-cyanoacrylate) integrated with a poly(ethylene terephthalate) (PET) non-woven film by using a rigid-flexible coupling strategy to optimize the cycling ability of NCM-based LIBs at high operating voltages. Here, the resulting GPE provided a wide electrochemical stability window of 4.7 V and a good ionic conductivity (2.54 × 10⁻³ S cm⁻¹ at room temperature), with the corresponding NCM/graphite full cell demonstrating excellent cycling stabilities with a capacity retention of 91% after 200 cycles (113 mAh g⁻¹ at the 200th cycle) at 0.5 C. These results demonstrated that this type of GPEs can suppress the dissolution and diffusion of transition metal ions from the NCM cathode at high working potentials and as a result, polymer electrolytes using polycyanoacrylate as a matrix are potential candidates to match with TCM-based LBs at highly delithiated states. However, this polymer in TCM-based LBs still face challenges, such as poor stability in the presence of lithium anodes and insufficient mechanical properties.

2.2.3 Cross-Linked Polyacrylates

Cross-linked polymers can exhibit superior mechanical properties, and the mechanical strength of cross-linked polyacrylates can outperform linear polyacrylates. As a result, researchers have attempted to develop cross-linked polyacrylate-based polymer electrolytes for application in TCM-based LBs. For example, Lee et al. [116] synthesized a poly(tris(2-(acryloyloxy)ethyl) phosphate) (PTAEP)-based GPE using a direct UV-assisted coating method on an as-formed LiNi_1/3Co_1/3Mn_1/3O₂ cathode and reported that the synthesized GPE endowed the corresponding cell with superior capacity retention (84% vs. 73%) as compared with pristine cells at 1 C after 50 cycles with a charge cutoff voltage of 4.6 V. Here, the researchers reported that PTAEP can act as an ion-conductive protective film to suppress interfacial side reactions close to the cathode, and that the body of the phosphate groups in the polymer host can provide excellent safety properties as a flame retardant.

In another example, Guo et al. [117] developed a poly(ethoxylated trimethylolpropane triacrylate)/PEO (ipn-PEA)-based APE with a rigid-flexible network for room temperature LBs and reported an excellent ionic conductivity of 2.2 × 10⁻⁴ S cm⁻¹ with a high mechanical strength of approximately 12 GPa, and reported that the corresponding cell using LiNi_0.5Co_0.2Mn_0.3O₂ as the cathode active material at a charge cutoff voltage of 4.3 V produced a specific capacity of 117 mAh g⁻¹ after 200 cycles at 0.5 C. Here, the researchers reported that the interpenetrating polymer network with its robust mechanical strength not only suppressed the formation of lithium dendrites, but also improved interfacial stability during long-term cycling, providing significant insights into the relationship between interfacial compatibility and structural features of polymer matrixes in TCM-based LBs.

Cross-linked polyacrylate-based GPEs can also be assembled with NCA cathodes to replace liquid electrolytes and improve the safety of LBs. For example, Lin et al. [118] tested a cell composed of LiNi_0.8Co_0.15Al_0.05O₂ as the cathode active material and graphite–Si/C as the anode by preparing a polymer matrix with a three-dimensional framework through the in situ polymerization of pentaerythritol tetraacrylate (PETEA) and reported that the obtained cell exhibited markedly improved cyclability (84% capacity retention after 50 cycles), decreased gas production, and enhanced safety as compared with a corresponding cell using liquid electrolytes (73% capacity retention after 50 cycles) (Fig. 7). Here, the researchers suggested that the enhanced performances are due to the robust three-dimensional structure of the polymer matrix, which not only enabled superior compatibility between the electrodes and the GPE, but also reduced the decomposition of electrolytes and prevented unwanted electrochemical side and heat reactions between the electrodes and the electrolyte during cycling (Fig. 8).

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Overall, polyacrylate-based electrolytes are promising candidates to replace conventional liquid electrolytes and resolve the safety concerns of TCM-based LBs. And among these, cross-linked polyacrylates demonstrate attractive properties such as the significant suppression of lithium dendrite formation and the reduction of liquid electrolyte decomposition, all of which can advance the future development of TCM-based LBs. However, side reactions between ester groups in polyacrylate polymer structures and lithium are prevalent, but may be optimized through the addition of film-forming additives on the anode such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC) or through the introduction of organic or inorganic coatings onto the lithium metal anode.

2.3.1 Poly(Ethylene Ether Carbonate)

PEEC-based electrolytes prepared through a ring-opening polymerization of ethylene carbonate can exhibit superior ionic conductivities and higher t _Li ⁺ than those of PEO-based electrolytes. For example, Kim et al. [119] synthesized an amorphous polymer matrix (XPEEC-1) through a photocross-linking reaction between PEEC and tetraethyleneglycol diacrylate (TEGDA) and reported that the cross-linked polymer electrolyte possessed a high ionic conductivity of 1.6 × 10⁻⁵ S cm⁻¹ at room temperature and a high t _Li ⁺ of 0.40. In addition, the corresponding solid-state Li/LiNi_0.6Co_0.2Mn_0.2O₂ cell in this study delivered an initial discharge capacity of 141.4 mAh g⁻¹ and good capacity retention (90.2% of the initial discharge capacity after 100 charge/discharge cycles at 0.1 C) at 25 °C, demonstrating potential application in all-solid-state TCM-based LBs under ambient temperatures.

2.3.2 Polycyclic Carbonate

Compared with linear carbonates, cyclic carbonates possess higher dielectric constants and therefore can dissociate lithium ions more efficiently. In addition, cyclic carbonate polymers can absorb large amounts of the liquid electrolyte and demonstrate higher mechanical strengths. In one study, Lex-Balducci et al. [120] synthesized a polymer matrix (P(OEGMA-CCMA)) by copolymerizing cyclic carbonate methacrylate (CCMA) with oligo (ethylene glycol) methyl ether methacrylate (OEGMA) and reported that the significant uptake of the liquid electrolyte in the resulting polymer host structure (550% of its own weight) resulted in a high ionic conductivity of 2.3 × 10⁻³ S cm⁻¹ at 25 °C and that the corresponding LiNi_1/3Co_1/3Mn_1/3O₂/graphite full cell exhibited stable cycling characteristics with high discharge capacities (> 91% capacity retention after 80 cycles with a capacity of 122 mAh g⁻¹) and Coulombic efficiencies (> 99.5%) at 0.5 C. In addition, the researchers also reported that the cyclic carbonate motif endowed the obtained polymer electrolyte with good mechanical stability. In another study, our group [121] developed a novel poly(vinylene carbonate) (PVCA)-based GPE through an in situ polymerization method that possessed a superior mechanical property of 2 GPa, outperforming most polymer electrolytes, and produced a high ionic conductivity of 5.59 × 10⁻⁴ S cm⁻¹ and a wide electrochemical stability window of above 4.8 V versus Li at 25 °C. In addition, the corresponding LiNi_0.6Co_0.2Mn_0.2O₂/PVCA-based GPE/graphite cell demonstrated excellent safety and interfacial compatibility, indicating that PVCA-based electrolytes are potential alternatives for TCM-based LBs.

Polycarbonate-based electrolytes exhibit relatively strong coordination abilities to lithium ions and usually possess high ionic conductivities and polycyclic carbonates that endow electrolytes with high mechanical properties and the uptake of liquid electrolytes should be further explored as polymer matrixes for TCM-based LBs. However, polycarbonate-based electrolytes suffer from inferior chemical stabilities in the presence of lithium or basic cathodes, but this defect can be resolved through the introduction of atomic layer coatings onto the lithium anode.

2.4.1 Poly(Vinylidene Fluoride-Hexafluoropropylene)

P(VDF-HFP) contains an amorphous phase (-HFP) which can immobilize large amounts of the liquid electrolyte as well as a crystalline phase (-VDF) which can enhance mechanical properties, and compared with PVDF, P(VDF-HFP) exhibits decreased crystallinity and glass transition temperatures. Researchers have also reported that ionic liquids as additives can further optimize P(VDF-HFP)-based electrolyte performances due to desirable properties such as low flammability, excellent thermal stability, high ionic conductivity and more [125, 126]. In one study, Kim et al. [125] reported that a cell comprised of the LiNi_1/3Mn_1/3Co_1/3O₂/GPE containing P(VDF-HFP) and ionic liquid/Li delivered better cyclability than cells based on GPEs containing commercial liquid electrolytes (1.0 M LiPF₆ in EC/DEC). In this study, it should be noted that additives such as VC, FEC and EC were also applied to stabilize the SEI layer and alleviate the reductive breakdown of ionic liquids, leading to good compatibility between the GPE and the lithium anode. In another study, Hofmann et al. [126] reported that ionic liquids play an important role in promoting the cycling capability of TCM-based cells.

Aside from ionic liquids, the addition of inorganic fillers can also enhance the cell performance of P(VDF-HFP)-based GPEs. For example, Wu et al. [124] synthesized a macroporous nanocomposite polymer membrane comprised of P(VDF-HFP) and TiO₂ through an in situ hydrolysis of Ti(OC₄H₉) and reported that the resulting GPE demonstrated enhanced ionic conductivities (9.8 × 10⁻⁴ vs. 1.4 × 10⁻⁴ S cm⁻¹ at 20 °C) and entrapment of liquid electrolytes (125 vs. 56 wt%), as well as decreased crystallinity (a melting point of 146.6 vs. 142.2 °C) and better rate capabilities (> 10 mAh g⁻¹ higher at 0.5–5 C) than the GPE without TiO₂. In a further study, Kim et al. [127] prepared another composite GPE with P(VDF-HFP) and core–shell SiO₂(Li⁺) nanoparticles which also presented a high ionic conductivity of 1.7 × 10⁻³ S cm⁻¹. Here, the composite membrane-assembled LiNi_1/3Co_1/3Mn_1/3O₂/carbon cell demonstrated a superior capacity retention of 95% after 100 cycles at 0.5 C and a better rate capability of 167 mAh g⁻¹ at 5 C rate as compared with that of a pristine P(VDF-HFP)-based membrane. Furthermore, other inorganic fillers including mesoporous modified-silica fillers and Al₂O₃ have also been blended with P(VDF-HFP), and the obtained composite films have also been reported to exhibit superior mechanical and electrochemical properties as compared with TCM cathodes [128, 129, 130]. In addition, Wang et al. [132] recently studied a Li-eliminating composite GPE to avoid electrical short circuiting in which the Li-eliminating GPE was prepared by coating a porous P(VDF-HFP) polymer matrix embedded with TiO₂ nanoparticles onto the surface of a conventional Celgard separator which is facing the cathode. Here, the researchers reported that the TiO₂-containing composite system can react with lithium dendrites penetrating through the separator during cycling and significantly decrease the short circuiting of the resulting Cu/Li-eliminating composite GPE/Li cell as compared with a pristine cell using a pristine polypropylene (PP) separator, allowing the corresponding LiNi_0.8Mn_0.1Co_0.1O₂/Li LB to demonstrate better capacity retentions (> 90% vs. 63% after 155 cycles at 1 C) and rate capabilities (approximately 115 vs. 49 mAh g⁻¹ at 11 C).

2.4.2 Poly(Difluoroethylene-Trifluoroethylene)

Kong et al. [131] reported that poly(difluoroethylene-trifluoroethylene (P(VDF-TrFE)) possesses considerable electronic and ionic conductivities and can be coated onto Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode surfaces to stabilize the cathode active material and mitigate unwanted interfacial side reactions close to the cathode. In their study, the researchers reported that their β-P(VDF-TrFE)-coated sample provided a high initial capacity of 262.8 mAh g⁻¹ at 0.1 C, a superior capacity retention of 90.8% after 100 cycles at 0.1 C and a high rate capability of 116.8 mAh g⁻¹ at 5 C, which exceeded the performance of samples with no coating, demonstrating that conductive P(VDF-TrFE) coatings have potential for practical application in TCM-based LBs.

The addition of ionic liquids or inorganic fillers into PVDF-HFP matrixes can improve the performance of resulting cells, and compared with ionic liquids, inorganic filler-based composite gel electrolytes demonstrate better mechanical and electrochemical properties and higher rate performances. Here, VDF-based copolymers as gel polymer matrixes are promising in TCM-based cell applications due to inherent advantages such as outstanding nonflammability. Despite these properties, however, these polymers suffer from high production costs and insufficient stability with lithium.

2.6.1 Polyimide

PI polymers can generally be prepared through the thermal imidization of diamine and dianhydride monomers and possess rigid polymer backbones and high electrochemical stability windows. In addition, these polymers can effectively prevent close contact between TCM cathodes and liquid electrolytes, and therefore suppress CEI irreversible side reactions. Lee et al. [136] were the first to propose a surface modification strategy based on P(PMDA-ODA)-directed nanoscale (about 10 nm) skins which can significantly improve the performance and thermal stability of high-voltage TCM-based LIBs. In their study, the researchers wrapped a LiNi_1/3Co_1/3Mn_1/3O₂ surface with a nanoscale layer of polyimide and reported good cycling performances (66% capacity retention after the 50th cycle) at high operating voltages (2.8–4.8 V vs. Li). However, the researchers also reported that the ionic conductivity of the PI film with the uptake of the liquid electrolyte was relatively low (1.5 × 10⁻⁴ S cm⁻¹) and resulted in poor LIB rate performances with the PI coating. In another study, l’Abee et al. [137] reported that their PI-based separator (P(BPADA-pPD)) exhibited superior total porosity (up to 71%), excellent anti-oxidative ability (5 V vs. Li), high electrolyte uptake (> 300 wt%), and high dimensional stability (up to 220 °C), with the corresponding NMC/graphite pouch cell using the highly porous separator demonstrating enhanced capacity retention (89.3% vs. 81.9%) and slightly lower Coulombic efficiency (99.77% vs. 99.91%) as compared with their polyolefin reference after 1000 (1 C/2 C) cycles. Overall, although these results suggest that polyimides are promising for TCM-based high-voltage LBs, P(BPADA-pPD) solid films suffer from long soaking times (up to 21 days) and unsatisfactory rate performances, which can be optimized through the introduction of F or CF₃ in the polymer host or the addition of highly polar additives such as ionic liquids in terms of the liquid electrolyte. In addition, the synthesis of PI is complex and expensive and is unsuitable for large-scale industrial production applications.

2.6.2 Polypyrrole

Different from PI-based protective layers, PPy coatings possess superior electronic conductivities (on the order of 10² S cm⁻¹ [138]) due to an inherently conjugated and relatively coplanar backbone. In addition, these coatings are cost-effective, easily prepared and are electrochemically active for lithium ion deposition and stripping (theoretical capacity of 72 mAh g⁻¹) at 2.0–4.5 V versus Li, allowing PPy coatings to act as both a conductive film and an organic cathode material. A PPy-coated TCM cathode was first prepared by Ren et al. [139] in 2011 in which the researchers reported that the obtained cell using lithium metal as the anode produced a specific discharge capacity of 182 mAh g⁻¹ after 50 cycles at 0.1 C and voltage limits of 2.8–4.6 V, outperforming that of bare cathode-based LBs (only 134 mAh g⁻¹). Wang et al. [140] also reported that PPy-coated cathodes possessed notably enhanced cycling stabilities (69.8% vs 44.5% capacity retention after 100 cycles at 2 C rate and 60 °C) and rate capabilities (132.4 vs 115.2 mAh g⁻¹ at 10 C under ambient temperature) as compared with pristine materials. Here, the researchers suggested that these enhanced battery performances can be attributed to the beneficial characteristics of the PPy coating, including the increased electron conductivity and the suppression of unwanted side reactions between the highly delithiated Li₁Ni_0.8Co_0.1Mn_0.1O₂ and the electrolyte. In another studies, PPy has also been used to coat high-voltage Li_1.2Ni_0.54Co_0.13Mn_0.13O₂ and Li₁Ni_0.5Co_0.2Mn_0.3O₂ and has been reported to enhance cell cycling performances and rate capabilities [141, 142]. These results indicate that PPy-based surface modifications on TCM cathodes can enhance cell performances such as rate performances, and are promising for practical application.

Overall, aromatic polymers can prevent contact between TCM cathodes and liquid electrolytes, and significantly optimize cell performances under high voltages or temperatures. As for PI coating-based cell systems, they usually require complex and expensive processing steps and possess unsatisfactory rate capabilities. Alternatively, PPy systems possess advantageous characteristics such as easy preparation, low costs and high electronic conductivity, allowing corresponding cells to possess superior rate performances. In general, cells with PPy-coated cathodes are not considered to be GPE cells due to the low uptake of liquid electrolytes by PPy. However, this disadvantage can be overcome through the blending with other polymers or through structural modifications to enhance liquid electrolyte immobilization.