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

  • Huanrui Zhang
  • Jianjun Zhang
  • Jun Ma
  • Gaojie Xu
  • Tiantian Dong
  • Guanglei CuiEmail author
Review Article
  • 91 Downloads

Abstract

Layered transition metal oxides such as LiNixMnyCo1−xyO2 and LiNixCoyAl1−xyO2 (NCA) (referred to as ternary cathode material, TCM) are widely recognized to be promising candidates for lithium batteries (LBs) due to superior reversible capacities, high operating voltages and low production costs. However, despite recent progress toward practical application, commercial TCM-based lithium ion batteries (LIBs) suffer from severe issues such as the use of flammable and hazardous electrolytes, with one high profile example being the ignition of NCA-based LIBs used in Tesla Model S vehicles after accidents, which jeopardizes the future development of TCM-based LBs. Here, the need for TCM and flammable liquid electrolytes in TCM-based LBs is a major obstacle that needs to be overcome, in which conflicting requirements for energy density and safety in practical application need to be resolved. To address this, polymer electrolytes have been demonstrated to be a promising solution and thus far, many polymer electrolytes have been developed for high-performance TCM-based LBs. However, comprehensive performances, especially long-term cycling capabilities, are still insufficient to meet market demands for electric vehicles, and moreover, comprehensive reviews into polymer electrolytes for TCM-based LBs are rare. Therefore, this review will comprehensively summarize the ideal requirements, intrinsic advantages and research progress of polymer electrolytes for TCM-based LBs. In addition, perspectives and challenges of polymer electrolytes for advanced TCM-based LBs are provided to guide the development of TCM-based power batteries.

Graphical Abstract

Keywords

Lithium batteries Ternary cathode material All-solid-state polymer electrolyte Gel polymer electrolyte 

1 Introduction

Great efforts have been made in the utilization of renewable energies (e.g., solar, wind, tidal, and geothermal energy) and development of efficient energy storage systems to cope with increasing energy demands. And as efficient and commercialized secondary energy storage devices, lithium batteries (LBs) possess desirable properties (e.g., high discharge capacity, high operating voltage, excellent cycling stability, environmental friendliness and so on) and are considered to be promising energy storage systems [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]; and thus far, rechargeable lithium ion batteries (LIBs) have gained wide acceptance in the development of consumer electronics, electric vehicles as well as stationary storage systems, in which global sales of light passenger vehicles using LIBs as the energy source exceeded 1 million units in 2017 [14]. However, because of the fast-growing consumer market for LIB powered products, the development of LBs technologies has been forced to focus on higher energy densities [15, 16, 17, 18, 19, 20, 21], leading to the development of many high energy density cathode active materials such as layered oxides (LiNixM1−xO2, M = Co, Mn and Al), lithium-rich layered oxides (Li1+xM1−xO2, M = Co, Mn, Ni, etc.), spinel oxides (e.g., LiMn1.5Ni0.5O4), and polyanionic compounds (e.g., LiFexMn1−xPO4) [22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]. And among these, LiNixMnyCo1−xyO2 (NMC) and LiNixCoyAl1−xyO2 (NCA) (referred to as ternary cathode material, TCM) have been widely recognized as some of the most promising candidates for high energy density LBs due to advantages such as large reversible capacities and operating voltages as well as cost-effective characteristics (Fig. 1) [33], and have been successfully applied in commercial electric vehicles. For example, Tesla Modal S and X vehicles currently possess the highest energy density (~ 170 Wh kg−1) of all electric vehicles available commercially as a result of the utilization of NCA-type cathodes. Moreover, NCM-based LIBs have been employed to power GM Chevy Bolt vehicles, delivering an energy density of ~ 160 Wh kg−1.
Fig. 1

Approximate range of average discharge potentials and specific capacities of common intercalation-type cathodes obtained through experimental results. The acronyms for the cathode active materials are: lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium cobalt phosphate (LCP), lithium iron phosphate (LFP), lithium iron fluorosulfate (LFSF), and lithium titanium sulfide (LTS) [33].

Copyright 2015, Elsevier

Despite the progress toward practical application, however, commercial TCM-based LIBs still encounter severe problems. For example, TCM easily undergoes cation mixing and phase transformation, especially at high working potentials and elevated temperatures, which are detrimental to the electrochemical properties of TCM-based LIBs. Moreover, there are trade-offs between high energy density and high stability in TCM-based LIBs based on different TCM electrochemical configurations arising from separated ratios of transitional metal cations.
  1. (1)
    Cation mixing: TCM mainly exhibits O1 and O3 crystal structures as well as a hybrid crystal phase of the two which is referred to as H1-3, which forms during partial lithium deintercalation (Fig. 2a–c) [34, 35, 36]. Here, all crystal textures show a close and well-organized oxygen pattern along with interstitial sites among alternating layers occupied by lithium ions and transition metal (M = Ni, Co, Mn and Al) cations, in which a formula of LixMO2 can be used to represent the crystal structures of the TCM. However, Ni2+ ions can easily occupy 3b lithium sites due to a similar ionic radius to Li+ [37], and other transition metal ions such as Mn2+ can also appear in the cation mixing layer. This cation mixing can occur during the synthesis process of TCM as well as electrochemical operations and is detrimental to the electrochemical performances of TCM-based LBs.
    Fig. 2

    Related crystal structures for TCM. Blue octahedra represent MO6 units and green octahedra/tetrahedra represent Li sites [22].

    Copyright 2017, Wiley

     
  2. (2)

    Phase transformation: O3 crystal frameworks in TCM can irreversibly transform from O3 to spinel or disordered rock-salt motifs during operation (Fig. 2d–e). This transformation is thermodynamically favorable during the delithiation process [37] and is caused by the migration of metal ions (~ 25%) from the M cation layer to the particular octahedral sites in the lithium ion layer as well as the subsequent densification of the layered MO2 into a rock-salt MO along with the release of oxygen [33, 34]. Moreover, these irreversible phase transformations can generate thin coatings onto TCM surfaces and kinetically limit Li diffusion across the reconstruction phase, causing detrimental impacts on the long-term cycling and C-rate ability of TCM-based LBs [22].

     
  3. (3)

    Trade-offs between high energy density and high stability: Varying electrochemical configurations of TCM based on varying ratios of transitional metal cations tend to exhibit varying electrochemical and thermal properties. For example, the increasing ratios of Ni cations in NMCs can enhance working voltages and specific capacities but lead to inferior cycling stabilities and rate performances as a result of severe Ni/Li cation mixing. In addition, Ni cation content is inversely proportional to thermal stability in NCMs [38, 39, 40]. In another example, increasing Mn cation content leads to increased operational safety but at the expense of the formation of more spinel phases. As for Co, although Co cations can effectively mitigate disordered crystal textures resulting from Ni/Li cation mixing and gain good cycling and rate performances, higher Co content leads to decreased cell capacity [23]. Furthermore, in the comparison between NMC and NCA, NCA can provide higher rate capabilities than NMC due to high electron and ion-conductive characteristics; and similar to Mn cations in NMC, Al cations in NCA can demonstrate inactive electrochemical properties and enable NCA to possess better cycling stabilities. However, despite the presence of synergistic effects of transitional metal cations in TCM, both NMC and NCA deliver relatively inferior cyclabilities due to irreversible side reactions and mechanical degradation on the electrode surface at high working potentials and elevated temperatures. In addition, higher energy density TCM tends to exhibit inferior structure stability and electrochemical performance.

     
Overall, the issues in TCM-based cathodes demand high requirements from corresponding electrolyte systems, which cause serious issues in electric vehicle applications such as Tesla Model S vehicles and major challenges to the future development of TCM-based LIBs using liquid electrolytes. In addition, aside from potential safety hazards, TCM-based LIBs using liquid electrolytes also suffer from parasitic side reactions between cathodes and electrolytes, unquenchable lithium dendrite formations, and severe cathode/anode interactions.
  1. (1)
    Safety hazards: Uncontrollable side and exothermic reactions between liquid electrolytes and oxygen species released from structurally unstable cathode active materials during cycling and especially at elevated temperatures or high voltages [26] in TCM-based LIBs can cause extreme reactions as seen in accidents involving Tesla electric vehicles. In addition, volatile and flammable liquid electrolytes can greatly contribute to thermal runaway, causing small ignitions to grow significantly, in which chain reactions during thermal runaway of LIBs have been extensively studied from qualitative interpretations (Fig. 3) [41]. And as a result, safety hazards caused by traditional organic electrolytes are a major challenge in the commercialization of TCM-based LIBs.
    Fig. 3

    Qualitative interpretation of the chain reactions during thermal runaway [41].

    Copyright 2018, Elsevier

     
  2. (2)

    Parasitic side reactions between cathodes and electrolytes: The parasitic and irreversible side reactions between cathodes and liquid electrolytes, as well as the continuous oxidative decomposition of liquid electrolytes on cathode surfaces are intrinsic flaws in which the dissolution of transition metal ions from TCM is thought to be one of the dominant factors and is responsible for the poor cycling lifespan and capacity decay in TCM-based LIBs. Here, acidic species from liquid electrolytes such as PF5 and HF generated from side reactions of LiPF6 can severely corrode TCM and facilitate the significant dissolution of transition metal cations, especially at elevated temperatures or high operating voltages [42, 43, 44, 45], increasing the impedance of the cathode and electrolyte interface with the accumulation of dissolution side products causing capacity decay. In addition, carbonates in liquid electrolytes are highly reactive toward oxygen species generated from cathodes and acidic compounds such as HF evolving from LiPF6 to produce complex side products including polycarbonates, semi-carbonates and so on [46, 47], impeding lithium ion transfer and deteriorating cell performances. Furthermore, transition metal ions can also act as Lewis acids to activate these side reactions, leading to severe decomposition of organic solvents, which is again accelerated under high temperatures or highly delithiated states.

     
  3. (3)

    Unquenchable lithium dendrite formation: Continuous lithium dendrite formation restricts the application of liquid electrolytes in lithium metal batteries in which lithium dendrite growth in liquid electrolytes can penetrate separators and cause short circuiting and subsequent safety issues [48]. In addition, liquid electrolytes lack mechanical strength and cannot impede lithium dendrite formation.

     
  4. (4)

    Severe cathode/anode interactions: The liquidity of electrolytes can increase interactions between cathodes and anodes and has a harmful effect on anode intercalation/deintercalation chemistry [45]. For example, transition metal ions (i.e., Mn2+) dissolved from the cathode can diffuse across the electrolyte and the separator and deposit onto the anode. Furthermore, the ion-exchange process between dissolved Mn species in the electrolyte and lithium ions deposited on the anode can further accelerate the erosion of cathode lattices [49, 50]. Additionally, chemical crossover of oxygen released from the cathode can cause thermal runaway without internal short circuiting [41].

     

Given these internal drawbacks of electrolytes, many strategies such as surface cathode modification and additive introduction have been employed to enhance the electrochemical performance and safety of traditional TCM-based LIBs [51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62]. In addition, flame-retardant electrolytes are also being investigated, such as flame-retardant solvents [63, 64, 65, 66, 67, 68] and highly concentrated electrolytes [69, 70, 71, 72, 73, 74, 75, 76]. However, the use of these low flammable electrolytes in LBs again presents compromises between electrochemical performance, costs and safety. Therefore, the balance between energy density and safety is a major issue in the practical application of TCM-based batteries in which polymer electrolytes have been widely considered to be an alternative solution [77, 78, 79, 80, 81, 82, 83, 84, 85]. This is because compared with liquid electrolytes, polymer electrolytes exhibit advantages such as high safety, transition metal ion dissolution suppression from cathodes, lithium dendrite suppression close to anodes and interaction reduction between cathodes and anodes [86, 87, 88, 89]. In addition, due to the higher safety tolerance of lithium metal anodes in comparison with liquid electrolytes [90], polymer electrolytes can couple with lithium metal anodes to provide higher energy density and lower cost TCM-based LBs [80].

And as a result, many types of polymer electrolytes have been explored for high-performance TCM-based LBs. However, although polymer electrolytes possess good battery performances, comprehensive performances such as long-term cycling stability are still insufficient to meet electric vehicle demands. In addition, reviews on polymer electrolytes specifically for TCM-based LBs are lacking. Because of this, this review will present the ideal requirements and natural strengths of polymer electrolytes for TCM-based LBs and provide detailed discussions into the advantages of various polymer electrolytes (e.g., polymers containing ethoxylated (EO) motifs, polyacrylates, polycarbonates, vinylidene fluoride (VDF)-based polymers, polyacrylonitriles (PAN), as well as aromatic polymers). In addition, perspectives and potential challenges of polymer electrolytes for practical application in TCM-based LBs are presented to guide future development of polymer electrolyte-based LBs using TCM, especially for power batteries.

2 Ideal Requirements, Inherent Advantages and Recent Progress of Polymer Electrolytes for TCM-Based LBs

Polymer electrolytes originated from the seminal study by Wright et al. [91] in 1973 have been widely explored in rechargeable LB application [92]. And based on the use of liquid plasticizers, polymer electrolytes can be divided into two types: the all-solid-state polymer electrolyte (APE) and the gel polymer electrolyte (GPE). Here, several recent reviews have summarized the properties and development of polymer electrolytes [3, 80, 93, 94, 95]; however, these reviews do not discuss the polymer electrolyte requirements of TCM-based LBs. And because of this, the essential requirements for ideal polymer electrolyte systems toward high energy density TCM-based LBs are presented as follows:
  1. (1)

    High ionic conductivities of more than 10−4 S cm−1 for APEs and more than 10−3 S cm−1 for GPEs at room temperature: Ionic conductivity is the dominant parameter of electrolytes, strongly influencing electrochemical behavior, and is tied to the rate capability of battery cells.

     
  2. (2)

    A relatively wide electrochemical stability window (> 4.4 V): This factor can endow polymer electrolytes with high anti-oxidation abilities if used with high-voltage TCM-based cathodes.

     
  3. (3)

    Superior mechanical strength: As reported by Monroe and Newman [96], polymer electrolyte films with a shear modulus of 6 GPa can restrain lithium dendrite generation and growth. Based on this, polymer electrolytes with robust mechanical strength can significantly restrain lithium dendrite formation in LBs.

     
  4. (4)

    Tight interfacial contact and good compatibility between electrodes and electrolyte systems: This factor can enhance cycling stabilities and provide high capacity retention during operation.

     
  5. (5)

    Thermal stability and chemical inertness of cell components during operation: Thermal stability enables operation at wide temperature ranges, and chemical inertness ensures high electrochemical performance.

     
  6. (6)

    A high lithium ion transference number (t Li + ) close to 1: High t Li + can lead to superior power densities by suppressing the concentration polarization of polymer electrolytes.

     
Great efforts have been devoted to the development of polymer electrolyte-based LBs using TCM as the cathode active material due to advantages over liquid electrolytes (Fig. 4). These advantages include high safety, suppression of transition metal ion dissolution from cathodes, suppression of lithium dendrite formation close to anodes and reduction of cathode/anode interactions, endowing polymer electrolyte-based cells with superior safety characteristics and electrochemical performances.
Fig. 4

Schematic illustration of the inherent advantages of a polymer electrolyte-based LBs using TCM as the cathode active material as compared with b corresponding LBs using liquid electrolytes

  1. (1)

    High safety: The superior safety characteristic of polymer electrolytes can be ascribed to the lack of electrolyte leakage which is present with liquid electrolytes, and is an inherent characteristic.

     
  2. (2)

    Suppression of transition metal ion dissolution from cathodes: Polymer electrolytes can act as robust barriers between TCM cathodes and liquid electrolytes and can significantly suppress interfacial side and exothermic reactions including the dissolution of transition metal ions. This characteristic enhances electrochemical performances and safety properties.

     
  3. (3)

    Suppression of lithium dendrite formation: Polymer electrolytes possess high mechanical strength and enhanced uniformity in the lithiation/delithiation process, suppressing dendrite formation.

     
  4. (4)

    Reduction of cathode/anode interaction: Polymer electrolytes as a solid-state form can restrain chemical shuttling between electrodes, especially for transitional metal cations, which are closely tied to the Lewis base sites in the polymer framework.

     

Overall, the inherent advantages of polymer electrolytes indicate great potential in TCM-based LB applications as compared with liquid electrolytes. And according to the different structural characteristics of polymer matrixes, advances in polymer electrolyte-based LBs using TCM can be classified into six types: polymers containing EO motifs, polyacrylates, polycarbonates, VDF-based polymers, PAN, and aromatic polymers.

2.1 Polymers Containing Ethoxylated Motifs

Polymers with EO motifs can favor the coordination and dissociation of lithium salts. This type of polymer can also provide a flexible backbone and good mechanical strength, allowing state-of-art polymer electrolytes based on these polymers to be widely coupled with TCM-based cathodes (Table 1).
Table 1

Reported EO-containing polymer electrolytes containing ethoxylated motifs and their electrochemical properties

Polymer matrix

Organic solvents/lithium salt/other additives

Ionic conductivity (S cm−1)

ESW versus Li (V)

Cell configuration

Voltage region versus Li (V)

Year

Ref.

PEO

LiBOB/MgO

4.5

LiNi1/3Mn1/3Co1/3O2/lithium metal

3.0–4.2

2011

[97]

SiO2–PEO

LiTFSI

4.2

LiNi0.6Mn0.2Co0.2O2/lithium metal

3.0–4.3

2018

[98]

P(EO/MEEGE)

LiTFSI

LiNi1/3Mn1/3Co1/3O2/graphite

2.5–4.2

2017

[99]

Wheat flour/PEO

LiTFSI

2.62 × 10−5 at 25 °C

4.2

Li(Ni0.8Co0.1Mn0.1)O2/lithium metal

2.0–4.2

2017

[100]

P(P(EO–PO))

EC–GBL/LiBF4

3.2 × 10−3 at 25 °C

4.5

LiNi0.8Co0.15Al0.05O2/graphite

3.0–4.3

2004

[102]

(P(POAGA))

EC–PC–EMC–DEC/LiPF6

6.2 × 10−3 at 25 °C

5.0

LiNi1/3Mn1/3Co1/3O2/graphite

2.8–4.4

2006

[103]

P(POAGA)

EC–PC–EMC–DEC/LiPF6

6.2 × 10−3 at 25 °C

LiNi1/3Mn1/3Co1/3O2/graphite

2.8–4.4

2006

[104]

P(PEODMA)

BMP-TFSI-VC/LiTFSI

1 × 10−3 at 25 °C

4.6

LiNi1/3Mn1/3Co1/3O2/lithium metal

3.0–4.3

2013

[105]

P(DPHA-PEGMEM)

EC–DEC/LiPF6

LiNi1/3Mn1/3Co1/3O2/graphite

3.0–4.2

2016

[107]

P(EO/MEEGE)

LiBF4

LiNi1/3Mn1/3Co1/3O2/lithium metal

2.5–4.2 under 60 °C

2013

[108]

P(EO/MEEGE)

LiBF4

LiNi1/3Mn1/3Co1/3O2/lithium metal

3.0–4.2 under 50 °C

2017

[109]

P(EO/EH)

LiTFSI

1.8 × 10−5 at 60 °C

LiNi0.5Mn0.2Co0.3O2/lithium foil

3.0–4.3

2014

[110]

P(EO/MEEGE/AGE)

LiTFSI

10−4 at 60 °C

LiNi1/3Mn1/3Co1/3O2/graphite

4.2

2014

[111]

P(MA475–DMA550)

P14FSI/LiTFSI

4 × 10−4 at 25 °C

4.8

LiNi1/3Mn1/3Co1/3O2/lithium metal

3.0–4.2

2016

[112]

Open image in new window

EC–EMC–FEC/LiPF6

LiNi0.5Mn0.2Co0.3O2/lithium metal

2.5–4.3

2017

[113]

ESW, electrochemical stability window; EO, ethylene oxide; EC, ethylene carbonate; GBL, gamma-butyrolactone; PC, propylene carbonate; EMC, ethyl methyl carbonate; DEC, diethyl carbonate; FEC, fluoroethylene carbonate; MEEGE, 2-(2-methoxyethoxy) ethyl glycidyl ether; BMP-TFSI, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide; VC, vinylene carbonate; PEODMA, poly(ethyleneglycol) dimethacrylate; MA475, poly(ethylene glycol) methyl ether methacrylate (Mn = 475); DMA550, poly(ethylene glycol) dimethacrylate (Mn = 550); P14FSI, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide; AGE, allyl glycidyl ether

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 LiNi1/3Co1/3Mn1/3O2/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 LiNi0.6Co0.2Mn0.2O2/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 LiNi1/3Co1/3Mn1/3O2/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 LiNi1/3Co1/3Mn1/3O2 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 tLi+ of 0.51. In addition, the researchers reported that their synthesized cells based on high-voltage LiNi0.8Co0.1Mn0.1O2 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 LiNi1/3Co1/3Mn1/3O2 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 LiNi1/3Mn1/3Co1/3O2 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.
Fig. 5

Schematic diagram of the double surface modification on both electrodes and cycling performance of a properly designed cell. Operation conditions: 4.2/2.5 V, (48 cycles (C/4)/2 cycles (C/8)) × n loops. Operation temperature: 50 °C [109].

Copyright 2017, Elsevier

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 LiNi1/3Mn1/3Co1/3O2/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 LiNi1/3Mn1/3Co1/3O2 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 LiNi0.5Co0.2Mn0.3O2/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.
Fig. 6

ac Schematic illustration of the different interfacial chemistries of a bare Li metal and b Li metal with a grafted skin in a carbonate electrolyte. c Chemical structure of the polymer skin, composed of a cyclic ether group (pink) and a polycyclic main chain (blue) [113].

Copyright 2017, ACS

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 Polyacrylates

Polyacrylates are generally prepared through the radical polymerization of acrylate monomers and are cost-effective and possess good interfacial compatibilities. As a result, polyacrylates have been applied as polymer matrixes for TCM-based LBs and can be classified into three types: polymethyl methacrylate, polycyanoacrylate and cross-linked polyacrylates (Table 2).
Table 2

Reported polyacrylate-based electrolytes and corresponding electrochemical properties

Polymer matrix

Organic solvents/lithium salts/other additives

Ionic conductivity (S cm−1)

ESW versus Li (V)

Cell configuration

Voltage region versus Li (V)

Year

Ref.

PMMA/PVDF

EC–EMC–DMC/LiPF6

3.2 × 10−4 at 25 °C

4.5

LiNi1/3Mn1/3Co1/3O2/graphite

3.0–4.3

2007

[114]

Poly(ethyl α-cyanoacrylate)

EC–DMC/LiPF6

2.54 × 10−3 at 25 °C

4.7

LiNi0.5Co0.2Mn0.3O2/graphite

3.0–4.3

2015

[115]

PTAEP

EC–DMC/LiPF6

LiNi1/3Mn1/3Co1/3O2/lithium metal

2.8–4.6

2013

[116]

ipn-PEA

LiPF6

2.2 × 10−4 at 25 °C

4.5

LiNi0.5Co0.2Mn0.3O2/Li

2.5–4.2

2016

[117]

P(PETEA)

EC–DEC–EMC/LiPF6

8.46 × 10−3 at 25 °C

LiNi0.8Co0.15Al0.05O2/graphite or graphite–Si/C

2.75–4.2

2017

[118]

ESW, electrochemical stability window

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 LiNi1/3Co1/3Mn1/3O2 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 LiNi0.5Co0.2Mn0.3O2 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 LiNi0.8Co0.15Al0.05O2 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).
Fig. 7

Electrochemical performances of the four types of full batteries. ac Cycling performances at a 0.5 C/1 C at 25 °C, b 0.5 C/1 C at 45 °C and c 0.5 C/5 C at 25 °C. d Rate performances at 25 °C. e Gas generation of the batteries after 280 cycles at a temperature of 45 °C, and f after 200 cycles at a large discharge rate of 5 C. Here, NCA/liquid electrolyte/graphite, NCA/liquid electrolyte/(graphite–Si/C), NCA/PETEA-based GPE/graphite and NCA/PETEA-based GPE/(graphite–Si/C) are denoted as NLG, NLGS, NPG and NPGS, respectively [118].

Copyright 2017, RSC

Fig. 8

Schematic illustrations of LBs during cycling: a LiNi0.8Co0.15Al0.05O2 (NCA)/liquid electrolyte/graphite and b NCA/PETEA-based gel polymer electrolyte/graphite [118].

Copyright 2017, RSC

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 Polycarbonates

Polycarbonates containing abundant carbonate groups with relatively high dielectric constants can efficiently solvate lithium ions and suppress lithium ion aggregation. As a result, these polymers tend to possess high ionic conductivities and can be promising candidates for high-performance LBs with TCM cathodes, including poly(ethylene ether carbonate) (PEEC) and polycyclic carbonate (Table 3).
Table 3

Reported polycarbonate-based electrolytes and their electrochemical properties

Polymer matrix

Organic solvents/lithium salts/other additives

Ionic conductivity (S cm−1)

ESW versus Li (V)

Cell configuration

Voltage range versus Li (V)

Year

Ref.

XPEEC-1

LiTFSI

1.6 × 10−5 at 25 °C

4.9

LiNi0.6Mn0.2Co0.2O2/Li

3.0–4.3

2017

[119]

P(OEGMA-CCMA)

EC–DMC/LiPF6

2.3 × 10−3 at 25 °C

5.5

LiNi1/3Mn1/3Co1/3O2/graphite

2.8–4.2

2014

[120]

PVCA

EC–DMC/LiDFOB

5.59 × 10−4 at 25 °C

4.8

LiNi0.6Mn0.2Co0.2O2/graphite

3.0–4.2

2017

[121]

ESW, electrochemical stability window

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/LiNi0.6Co0.2Mn0.2O2 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 LiNi1/3Co1/3Mn1/3O2/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 LiNi0.6Co0.2Mn0.2O2/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 Vinylidene Fluoride-Based Polymers

VDF-based polymers possess high dielectric constants (ɛ ≈ 8.4) due to the presence of polar C–F groups and can endow polymer-based GPEs with superior ionic conductivities as compared with other polymer matrix materials [122]. Furthermore, these polymers also possess excellent flame resistance, good chemical resistance and excellent thermal stability. As a result, these matrix materials have aroused much attention in the development of high energy density TCM-based LBs, and as early as 2007, poly(vinylidene fluoride) (PVDF) has been used as a polymer matrix for GPEs in LiNi1/3Mn1/3Co1/3O2-based LBs [123]. In addition, VDF-based copolymers possess more chain conformations of the ferroelectric β-phase as compared with PVDF and can endow polymer electrolytes with improved electrochemical properties. And as reported in TCM-based LBs, two types of VDF-based copolymers have been applied as polymer host materials, including poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) and poly(difluoroethylene-trifluoroethylene) (P(VDF-TrFE)) (Table 4).
Table 4

Reported VDF-based polymer-based electrolytes and their electrochemical properties

Polymer matrix

Organic solvents/lithium salts/other additives

Ionic conductivity (S cm−1)

ESW versus Li (V)

Cell configuration

Voltage range vs. Li (V)

Year

Ref.

PVDF

EC–PC/LiPF6

LiNi1/3Mn1/3Co1/3O2/Li4Ti5O12

3.0–4.6

2007

[123]

P(VDF-HFP)

EC–DMC/LiPF6/TiO2

0.98 × 10−3 at 20 °C

LiNi1/3Mn1/3Co1/3O2/graphite

2.8–4.2

2008

[124]

P(VDF-HFP)

BMP-TFSI/LiTFSI/VC

4.8

LiNi1/3Mn1/3Co1/3O2/Li

3.0–4.3

2011

[125]

P(VDF-HFP)

MPPyrr-TFSA-PC/Li-TFSA

1.23 × 10−3 at 20 °C

5.0

LiNi1/3Mn1/3Co1/3O2/graphite

3.0–4.2

2013

[126]

P(VDF-HFP)

EC–DEC/LiPF6/SiO2

1.7 × 10−3 at 25 °C

LiNi1/3Mn1/3Co1/3O2/carbon

3.0–4.5

2013

[127]

P(VDF-HFP)

EC–DMC/LiCF3SO3/Al2O3

7.1 × 10−3 at 25 °C

4.9

LiNi1/3Mn1/3Co1/3O2/Li

2.5–4.6

2013

[128]

P(VDF-HFP)

EC–DEC/LiPF6/silica fillers

3.78 × 10−3 at 30 °C

LiNi0.5Co0.2Mn0.3O2/Li

2.5–4.3

2014

[129]

P(VDF-HFP)

EC–DMC/LiPF6/Al2O3

4.1 × 10−3 at 25 °C

5.0

LiNi1/3Mn1/3Co1/3O2/Li4Ti5O12

3.5–4.2

2017

[130]

P(VDF-TrFE)

EC–DEC/LiPF6

Li1.2Mn0.54Ni0.13Co0.13O2/Li

2.0–4.6

2017

[131]

P(VDF-HFP)

EC–EMC/LiPF6/TiO2

LiMn0.8Ni0.1Co0.1O2/Li

2.8–4.2

2018

[132]

ESW, electrochemical stability window; BMP-TFSI, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide; Li-TFSA, lithium bis(trifluoromethylsulfonyl)azanide; MPPyrr-TFSA, 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl) azanide

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 LiNi1/3Mn1/3Co1/3O2/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 LiNi1/3Co1/3Mn1/3O2/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 LiNi0.8Mn0.1Co0.1O2/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 Li1.2Mn0.54Ni0.13Co0.13O2 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.

2.5 Polyacrylonitrile

Polyacrylonitrile (PAN)-based GPEs have proven to possess high anti-oxidative abilities (above 4.5 V vs. Li) and ionic conductivities as well as superior thermal stabilities. In addition, further performance enhancements can be obtained through the introduction of inorganic fillers or blending with other polymers. And because of these advantages, PAN have been explored as hosts for GPEs to couple with high-voltage TCM-based cathodes (Table 5). For example, Scrosati et al. [133] prepared a PAN-based GPE with the addition of Al2O3 in which the GPE was matched with high-voltage LiNi0.8Co0.16Al0.04O2 cathodes and reported that the resulting GPE produced high ionic conductivities (8 × 10−3 S cm−1) and high anti-oxidative abilities (> 5.5 V vs. Li). Despite these promising performances, however, inferior cycling stabilities and rate capabilities still need to be addressed. Based on this, Kim et al. [134] investigated a cross-linked composite GPE using P(MA-SiO2-TEGDA) as the host material in which the P(MA-SiO2-TEGDA) was prepared through the polymerization of mesoporous SiO2 nanoparticles containing tri(ethylene glycol) diacrylate (TEGDA) motifs as cross-linking sites on the fibrous PAN membrane. Here, the researchers reported that the resulting LiNi1/3Co1/3Mn1/3O2/graphite full cell delivered superior cyclabilities (88% capacity retention after 300 cycles at 0.5 C under room temperature) and rate abilities (a discharge capacity of 142.7 mAh g−1 at 5 C and 55 °C), outperforming samples with non-porous SiO2 or without SiO2. The researchers in this study also noted that the mesoporous SiO2 particles played an important role in reducing HF in the electrolyte and restrained the dissolution of transition metal ions from the NCM at elevated temperatures. In another study, Park et al. [135] attempted to address the severe capacity decay of Ni-rich layered oxide cathode-assembled LBs at elevated working voltages by developing a cross-linked fibrous composite separator through the thermal cross-linked polymerization of reactive silica nanoparticles and TEGDA on a fibrous polyacrylonitrile (PAN) membrane. Here, the corresponding LB using the three-dimensional and fully interconnected fibrous composite separator soaked with liquid electrolytes and a LiNi0.6Co0.6Mn0.2O2 positive electrode demonstrated significantly improved electrochemical performances including a high discharge capacity (172.5 mAh g−1 at 0.5 C), good capacity retention (94% after 200 cycles at 0.5 C), and high rate ability (about 86 mAh g−1 at 5 C) at room temperature and was superior to samples without SiO2.
Table 5

Reported PAN-based electrolytes and their electrochemical properties

Polymer matrix

Organic solvents/lithium salts/other additives

Ionic conductivity (S cm−1)

ESW versus Li (V)

Cell configuration

Voltage range versus Li (V)

Year

Ref.

PAN

PC/LiPF6

8 × 10−3 at 25 °C

5.5

LiNi0.8Co0.16Al0.04O2/lithium metal

3.5–5.1

2002

[133]

P(MA-SiO2-TEGDA)/PAN

EC–EMC/LiPF6

1.8 × 10−3 at 25 °C

LiNi1/3Co1/3Mn1/3O2/graphite

3.0–4.5

2016

[134]

P(MA-SiO2-TEGDA)/PAN

EC–EMC–DEC/LiPF6/FEC

2.1 × 10−3 at 25 °C

LiNi0.6Co0.6Mn0.2O2/graphite

2.6–4.3

2017

[135]

ESW, electrochemical stability window

Overall, PAN-based polymer electrolytes possess several merits such as high oxidative resistances. However, these electrolytes suffer from inferior compatibility with lithium metal anodes and poor mechanical strength. Here, interfacial optimization close to lithium metal anodes can be achieved through the copolymerization or introduction of inorganic or organic additives as well as polymer blending.

2.6 Aromatic Polymers

Compared with traditional polymer matrixes, aromatic polymers are usually encapsulated onto positive cathode surfaces and can act as robust barriers between liquid electrolytes and cathodes. And compared with inorganic coatings, polymer encapsulating layers on positive cathodes possess many merits, including good solution processability and film formation ability, as well as being lightweight, mechanically tunable and possessing a strong affinity to cathode active materials. And integrated with liquid electrolytes, battery systems using aromatic polymer coatings on cathode surfaces enable versatility in battery shape and design, and exhibit enhanced safety and performance at high charging voltages. In addition, aromatic polymers can easily form highly continuous films with nanosized thicknesses that can effectively promote the cycling performance of high-voltage LBs using TCM cathodes. Overall, aromatic polymers in the operation of TCM cathodes can be classified into two types: polyimide (PI) and polypyrrole (PPy) (Table 6).
Table 6

Reported aromatic polymer-based electrolytes and their electrochemical properties

Polymer matrix

Organic solvents/lithium salts/other additives

Ionic conductivity (S cm−1)

ESW versus Li (V)

Cell configuration

Voltage range versus Li (V)

Year

Ref.

Open image in new window

P(PMDA-ODA)

EC–DMC/LiPF6

1.5 × 10−4

LiNi1/3Co1/3Mn1/3O2/graphite

2.9–4.8

2012

[136]

Open image in new window

P(BPADA-pPD)

EC–DMC/LiPF6/VC

5.0

LiNi0.6Co0.6Mn0.2O2/graphite

2.7–4.2

2017

[137]

PPy

EC–EMC/LiPF6

LiNi1/3Co1/3Mn1/3O2/Li metal

2.8–4.6

2011

[139]

PPy

EC–DMC–EMC/LiPF6

LiNi0.8Co0.1Mn0.1O2/Li metal

2.8–4.3

2014

[140]

PPy

EC–DMC/LiPF6

Li1.2Ni0.54Co0.13Mn0.13O2/Li metal

2.0–4.8

2013

[141]

Zirconium-doped PPy

EC–DMC–EMC/LiPF6

Li1.2Ni0.5Co0.2Mn0.3O2/Li metal

3.0-4.6

2016

[142]

ESW, electrochemical stability window

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 LiNi1/3Co1/3Mn1/3O2 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 Li1Ni0.8Co0.1Mn0.1O2 and the electrolyte. In another studies, PPy has also been used to coat high-voltage Li1.2Ni0.54Co0.13Mn0.13O2 and Li1Ni0.5Co0.2Mn0.3O2 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.

3 Conclusion and Perspectives

The implementation of TCM-based LBs has increased concerns over safety, especially after the severe burning incidents in Tesla electronic vehicles with the use of liquid electrolytes. To address these safety concerns, polymer electrolytes are promising candidates in the development of commercial TCM-based LBs. However, insufficient cell performances, particularly cycling stability, are issues that need to be resolved in polymer electrolytes and TCM-based LBs. In this review, the essential requirements for ideal polymer electrolyte systems as well as the advantages of polymer electrolytes in comparison with liquid electrolytes for TCM-based LBs are discussed. And based on the structural differences of reported polymer matrixes, polymer electrolyte-based batteries containing TCM can be divided into six types as follows:
  1. (1)

    Polymers with EO motifs: Polymers with EO motifs have been mainly employed in APE LBs based on TCM due to their ability to coordinate and dissociate lithium salts as well as their flexible backbone and good mechanical strength. However, these EO motif polymers suffer from low ionic conductivities at room temperature, low t Li + and inferior anti-oxidative abilities, and further research should focus on bulk modifications through the introduction of functional groups with high oxidative resistances and strong coordination capabilities with transition metal ions, as well as blending with advanced inorganic electrolytes.

     
  2. (2)

    Polyacrylates: Polyacrylates-based GPEs are inexpensive, possess good interfacial compatibility, and are promising alternatives to replace conventional liquid electrolytes and resolve associated safety hazards in TCM-based LBs. This is especially true for cross-linked polyacrylates which possess attractive advantages such as the suppression of lithium dendrite formation and liquid electrolyte decomposition, and are promising in TCM-based LBs. As for the successful application of cross-linked polyacrylate-based electrolytes, future development should focus on the modification of lithium anode surfaces to enhance interfacial compatibility.

     
  3. (3)

    Polycarbonates: Polycarbonates-based GPEs are another type of polymer electrolytes in which polycyclic carbonate-based GPEs possess good mechanical properties and excellent immobilization of liquid electrolytes and deserve to be further developed for TCM-based LBs.

     
  4. (4)

    VDF-based copolymers: VDF-based copolymers possess desirable internal strengths such as outstanding nonflammability characteristics, making them good choices for high safety LBs based on TCM. The drawbacks of this type of polymer electrolyte, including insufficient stability with lithium anodes, can be resolved through the modification of lithium metal anode surfaces, the introduction of organic/inorganic additives, and the use of solvent-in-salt systems.

     
  5. (5)

    Polyacrylonitrile (PAN)-based GPEs: PAN-based GPEs possess superior anti-oxidative abilities, high ionic conductivities and thermal stability, and have great potential as hosts to match with high-voltage TCM-based cathodes. However, these polymers possess inferior interfacial compatibility with lithium metal anodes, which can be improved through copolymerization, introduction of inorganic or organic additives, and polymer blending.

     
  6. (6)

    Aromatic polymers: Compared with the above-mentioned conventional polymer hosts, aromatic polymers can build robust barriers between TCM cathodes and liquid electrolytes and suppress undesirable side reactions close to the cathode. These characteristics endow resultant cells with significantly improved electrochemical performances under high-voltage or high-temperature conditions. And of the various aromatic polymers, cells with PPy-coated cathodes cannot be considered as a GPE due to the low uptake of the liquid electrolyte and should be used together with other polymer matrixes to obtain excellent electrochemical properties in TCM-based LBs.

     
Overall, although progress has been made in polymer electrolyte-based LBs with TCM as the cathode active material, comprehensive performances such as long-term cycling stability are insufficient to meet the market demand of electric vehicles. However, with increasing efforts, commercial applications of high safety polymer electrolyte-based TCM LBs will ultimately be realized in the near future, and prospects concerning polymer electrolytes toward TCM-based LBs should focus on the following aspects:
  1. (1)

    Because organic solvents are unavoidable in GPEs, leading to potential safety risks, it is necessary to develop efficient flame-retardant GPEs to ensure application safety in LBs employing conventional GPEs. In addition, it is of great importance to evaluate the thermal safety of polymer electrolyte-based LBs based on TCM through Accelerating Rate Calorimeter.

     
  2. (2)

    More efforts need to be devoted to the investigation of interfacial properties between electrodes and polymer electrolytes by using in situ technologies, especially the evolution of CEI layers.

     
  3. (3)

    The development of intelligent polymer electrolytes with intelligent responsive properties (i.e., anti-thermal or shock-proof GPEs) is a promising developmental route.

     
  4. (4)

    Novel preparation methods (apart from solution-casting methods and in situ polymerization methods) of polymer electrolytes need to be developed.

     
  5. (5)

    Focuses should be placed on the development of flexible and stretchable polymer electrolytes as wearable and smart device technologies become more mature.

     
  6. (6)

    Because polymer matrixes containing interfacial film-forming motifs possess excellent compatibility with corresponding electrodes [143], this is an effective method to optimize interfacial properties between polymer electrolytes and electrodes.

     

Notes

Acknowledgements

This study was financially supported by the National Natural Science Fund for Distinguished Young Scholars (51625204), the Key Research and Development Plan of Shandong Province P. R. China (2017GGX40119), the National Natural Science Foundation of China (51703236 and 51803230), the Youth Innovation Promotion Association of CAS (2016193), and the National Key R&D Program of China (Grant No. 2018YFB0104300).

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© Shanghai University and Periodicals Agency of Shanghai University 2019

Authors and Affiliations

  1. 1.Qingdao Industrial Energy Storage Research Institute, Qingdao Institute of Bioenergy and Bioprocess TechnologyChinese Academy of SciencesQingdaoChina

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