Revisiting polymeric single lithium-ion conductors as an organic route for all-solid-state lithium ion and metal batteries

Kihun Jeong , Sodam Park and Sang-Young Lee *
Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea. E-mail:

Received 17th September 2018 , Accepted 22nd November 2018

First published on 23rd November 2018

The current surge in demand for high-performance batteries has inspired the relentless pursuit of advanced battery materials and chemistry. Notably, all-solid-state lithium-ion batteries and lithium metal batteries that have recently come into the spotlight have stimulated our research interest in solid-state electrolytes as a promising alternative to conventional liquid electrolytes. Among the various solid-state electrolytes explored to date, polymeric single lithium-ion conductors (polymeric SLICs) have garnered considerable attention as an organic approach that is different from the widely investigated solid inorganic electrolytes. A salient feature of polymeric SLICs is the predominant contribution of Li+ ions to the ionic conductivity, thus enabling the Li+ ion transference number to reach almost unity. This exceptional single ion transport behavior of polymeric SLICs, in combination with their solid-state nature, flexibility and facile processability, brings remarkable benefits to the battery structure and performance, which lie far beyond those achievable with typical dual-ion conductive electrolytes. In this review, we describe the current status and challenges of polymeric SLICs in terms of chemical/structural design and synthesis strategies. Also, the development direction and future outlook of polymeric SLICs are presented with a focus on their potential for application in the newly emerging Li battery systems.

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Kihun Jeong

Kihun Jeong is a post-doc in the Department of Energy Engineering at Ulsan National Institute of Science and Technology (UNIST), Korea. He received his MS and PhD in Chemistry and Biochemistry from Kyushu University, Japan in 2013 and 2016, under the supervision of Prof. Seiji Ogo. His research interests are focused on organic materials for energy applications.

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Sodam Park

Sodam Park received her BS in Energy Engineering from UNIST in 2017. She is currently an MS–PhD integrated course student under the supervision of Prof. Sang-Young Lee. Her research interests lie in synthetic and mechanistic studies on solid polymer electrolytes for Li batteries.

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Sang-Young Lee

Sang-Young Lee is a professor and a head of the School of Energy and Chemical Engineering at UNIST. He received his BA in Chemical Engineering from Seoul National University in 1991, and MS and PhD in Chemical Engineering from KAIST in 1993 and 1997. He served as a post-doc fellow at Max-Planck Institute for Polymer Research from 2001 to 2002. Before joining UNIST, he worked at Batteries R&D, LG Chem as a principal research scientist. His research interests include solid polymer electrolytes, permselective separator membranes, cellulose-based paper batteries, printed power sources and flexible/wearable batteries.

1. Introduction

The forthcoming ubiquitous smart energy era, which will find widespread use of flexible/wearable portable electronics, the Internet of Things (IoT), grid-scale energy storage systems (ESSs) and electric vehicles (EVs), has spurred the relentless pursuit of advanced rechargeable power sources with reliable electrochemical performance and safety tolerance.1–3 Among the numerous rechargeable batteries, lithium-ion batteries (LIBs) have undoubtedly garnered a great deal of attention due to their high energy density and well-balanced electrochemical performance.4,5 Despite the successful commercialization and widespread popularity of LIBs in various applications, the never-ceasing demand for high energy density, long-term cyclability and safety has strongly pushed us to search for new battery materials and chemistry that lie far beyond those accessible with conventional LIB technology.

Taking into consideration that batteries are basically operated by the combined transport phenomena of electrons and ions, special attention should be paid to electrolytes among major battery components.6,7 Conventional LIB electrolytes consist of lithium (Li) salts and liquid-state solvents. Although liquid electrolytes are most widely used and show well-tailored attributes suitable for LIBs, they still have critical problems in terms of battery performance and safety, and thus remain a formidable obstacle to further advancement of LIBs.7,8 Specifically, liquid electrolytes are vigorously engaged in exothermic chemical reactions in almost all steps of cell fire or explosion, revealing that liquid electrolytes are a primary cause of the safety failure of LIBs. Also, liquid electrolytes often cause interfacial side reactions with electrode materials, thus generating unwanted byproducts, which eventually give rise to the performance degradation.

The aforementioned challenges of liquid electrolytes become more serious at Li metal anodes.8–10 Li metal has recently garnered considerable attention as a promising anode candidate due to its extremely high specific capacity (3860 mA h g−1) and the lowest redox potential (−3.04 V vs. standard hydrogen electrode (SHE)). However, most liquid electrolytes in contact with Li metal tend to cause unreliable and nonuniform Li plating/stripping, resulting in the growth of mossy and dendritic Li. As a consequence, serious deterioration of electrochemical performance (in particular, poor Coulombic efficiency, operating voltage drop and electrical contact between electrodes) is often encountered in Li metal batteries (LMBs). In addition, upon exposure to harsh conditions such as high temperature or short-circuit occurrence, the liquid electrolytes violently react with the Li metal, eventually giving rise to the more notorious safety failure.

These longstanding issues of the liquid electrolytes described above push us to pay more attention to solid-state electrolytes as a promising alternative. Note that solid-state electrolytes have recently drawn substantial attention due to the rapidly growing interest in all-solid-state Li batteries.11–15 In particular, driven by their remarkable progress in ionic conductivity, solid inorganic electrolytes, especially sulfide- and oxide-based ones, have been extensively investigated.15–18 However, solid inorganic electrolytes have suffered from many technical problems such as mechanical flexibility, processability, grain boundary resistance and electrode/electrolyte interfacial stability.12,19,20

In addition to the aforementioned solid inorganic electrolytes, a variety of solid-state electrolytes have been extensively investigated,11,21–25 including dual-ion conductive solid polymer electrolytes (SPEs) and polymeric single Li-ion conductors (SLICs). The major characteristics of these solid-state electrolytes are briefly summarized in terms of ionic conductivity, Li+ ion transference number (tLi+), electrochemical stability window, flexibility/processability and safety (Fig. 1a), revealing that polymeric SLICs can be suggested as an appealing candidate to outperform solid inorganic electrolytes although they have not yet fulfilled all the requirements of ideal electrolytes (in particular, their low ionic conductivity is a major concern).

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Fig. 1 (a) Comparison of various solid electrolytes in terms of their major characteristics. (b) Conceptual illustration of the structural uniqueness and ion transport behavior of polymeric SLICs with different chemical features.

A salient feature of polymeric SLICs, from the viewpoint of chemical structure, is the presence of immobilized anion groups that enable the dominant contribution of Li+ ions to the ionic conductivity.23–25 As a consequence, the tLi+ value of polymeric SLICs is close to unity, whereas those of dual-ion conductive electrolytes (i.e., liquid electrolytes and typical SPEs) remain at <0.5.26,27 This exceptional single ion transport behavior of polymeric SLICs brings remarkable benefits to the electrochemical performance of the resultant power sources, because Li+ ions are predominantly engaged in the redox reaction while anions remain relatively inactive. Some theoretical studies showed that SLICs can effectively suppress the occurrence of ionic concentration gradient, thus allowing stable Li plating/stripping on the Li metal surface28–30 and maximal utilization of electrode materials23,31 even at high current density conditions. Note that the significance of polymeric SLICs has recently been underlined by the sharply increasing number of research articles on their use in all-solid-state LIBs and LMBs.32–34 The above-mentioned facts encouraged us to revisit polymeric SLICs as a new organic route that is different from the currently popular approaches based on solid inorganic electrolytes.

Herein, we review the current status and challenges of polymeric SLICs in terms of chemical/structural design and synthesis strategies. Also, the development direction and future outlook of polymeric SLICs are discussed with a particular focus on their application in next-generation Li batteries such as all-solid-state LIBs and LMBs.

2. Design, synthesis and characteristics of polymeric SLICs

Ideal polymeric SLICs are required to possess the following electrolyte characteristics: ionic conductivity, tLi+ value, mechanical properties and chemical/electrochemical stabilities. A vast diversity of materials strategies has been undertaken to reach the abovementioned goal. Here, depending on their structural features, polymeric SLICs are classified as follows: polyanions, organic/inorganic hybrids and anion acceptor-containing polymers (Fig. 1b and Table 1). Each class of polymeric SLICs is systematically discussed in terms of their design, synthesis and electrolyte characteristics.
Table 1 Structural features and electrolyte characteristics of various polymeric SLICs
Classification Anion formula Structural feature Electrolyte preparation method Ionic conductivity (S cm−1) t Li+ Electrochemical window (V) T g (°C) Ref.
a Li+ ion transference number. b Glass transition temperature. c EMC: ethyl methyl carbonate. d From ref. 42. e 152 °C for the neat polymeric SLIC (from ref. 39). f 44.3 °C for the neat polymeric SLIC. g For the neat ionophilic regions. h For the neat hydrophobic regions. i 10 °C for the neat polymeric SLIC. j For the PEO regions. k For the polystyrene regions. l As the counter anion of the imidazolium cation.
Polyanions Sulfonates –SO3 Homopolymer PEO blend 3.0 × 10−8 (RT) 0.85 35
–SO3 Comb-branched copolymer Neat 2.0 × 10−7 (25 °C) −59 36
Soaked with PC/EMCc 1.47 × 10−4 (25 °C) 36
–CHFCF2SO3 Comb-branched copolymer Neat ∼10−5 (RT) −50 37
–SO3 Graft copolymer Soaked with EC/DMC 2.08 × 10−5 (30 °C) 0.93 4.4 38
Sulfonylimides –SO2N(−)SO2F Homopolymer PEO blend 1.43 × 10−5 (60 °C) 0.90 4.5 40
–SO2N(−)SO2CF3 Homopolymer PEO blend 8.42 × 10−6 (70 °C) 0.92d e 41
–SO2N(−)SO(=NSO2CF3)CF3 Homopolymer PEO blend 7.61 × 10−5 (70 °C) 0.91 ∼4 −15.1f 41
–SO2N(−)SO2CF3 Comb-branched copolymer Neat ∼10−4 (60 °C) 0.93 −47 42
–SO2N(−)SO2CF3 Linear triblock copolymer Neat 1.3 × 10−5 (60 °C) 0.85 ∼5 32
–SO2N(−)SO2CF3 Comb-branched copolymer Neat 1.2 × 10−5 (55 °C) 0.83 4.5 −61 33
–SO2N(−)SO2CF3 Linear triblock copolymer Neat ∼10−4 (70 °C) 0.91 4.3 −31.7 34
–SO2N(−)SO2CF3 Homopolymer Neat 1.02 × 10−4 (90 °C) 0.99 ∼5 −24 43
–SO2N(−)SO2CF3 Multi-block copolymer Soaked with EC ∼10−3 (30 °C) 1 ∼4.9 ∼150g, ∼220h 45
–SO2N(−)SO2CF3 Graft polymer PVdF-HFP blend, soaked with EC/DMC 2.16 × 10−4 (25 °C) 0.94 4.4 96
Tetrahedral borates –BO4 Bisphenolate A-linked polymer PEO blend 1.7 × 10−4 (60 °C) 0.92 5.5 46
–BO4 Crosslinked copolymer Soaked with GBL 1.47 × 10−3 (25 °C) 0.89 5.2 47
–BO4 Crosslinked copolymer Soaked with GBL 1.32 × 10−3 (25 °C) 0.92 6.0 48
–BO4 Tartrate-linked polymer PVdF-HFP blend, soaked with EC/DMC 5 × 10−4 (RT) 0.91 5.0 50
–BO4 Butanetetracarboxylate-linked polymer PVdF-HFP blend, soaked with EC/PC 2.4 × 10−4 (20 °C) 0.87 4.3 52
–(C6H4)4B Locally π-conjugated polymer Soaked with PC 3.6 × 10−5 (27 °C) 53
–(C6F4)4B Locally π-conjugated polymer Soaked with PC 2.7 × 10−4 (28 °C) 0.93 ∼3.7 53
–BO4 Anionic COF PVdF blend, soaked with PC 3.05 × 10−5 (RT) ∼0.8 ∼4 54
Organic/inorganic hybrids SiO2 hybrids –SO3 Core (SiO2)–shell (polyanion) PEGDME blend 2.2 × 10−7 (60 °C) 61
–SO3 Core (SiO2)–shell (polyanion/oligoether) PEGDME blend 1.3 × 10−6 (60 °C) 61
–N(−)SO2CF3 Core (SiO2)–shell (polyanion) PEGDME blend ∼10−5 (60 °C) 62
Al2O3 (or SiO2) hybrids –SO2N(−)SO2CF3 Core (Al2O3 or SiO2)–shell (anion/oligoether) PEO/PEGDME blend ∼10−4 (60 °C) 5 65
Mesoporous SiO2 hybrid –SO2N(−)SO2CF3 Anion/oligoether-impregnated SiO2 PEO blend ∼10−3 (25 °C) ∼0.9 −43 66
Polysiloxanes –N(−)SO2CF3 Anion/oligoether-cografted polysiloxane Neat 1.3 × 10−6 (25 °C) −67.1 68
–(CF2)2O(CF2)2SO3 Anion/oligoether-cografted polysiloxane Neat 2.5 × 10−6 (25 °C) −70 70
–SO2N(−)SO2C6H5 Anion-grafted polysiloxane PVdF-HFP blend, soaked with EC/PC 7.2 × 10−4 (RT) 0.89 4.1 −17i 71
Silsesquioxane hybrid –SO2N(−)SO2CF3 Polyanion-grafted silsesquioxane SEO blend 1.1 × 10−5 (90 °C) 0.98 ∼4 −10,j 107k 74
Anion acceptor-containing polymers Boroxine ring-based polymers Boroxine ring-linked PEO Li salt (LiCl, LiTf, LiTFSI, etc.) blend ∼10−7 to ∼10−5 (30 °C) 0.62–0.88 4.9 −56 to −40 76
Urea-modified calix[4]arenes Macrocycle Additive in LiI/PEO blend 3 × 10−5 (95 °C) 0.8–1.0 0 79
Calix[6]pyrrole Macrocycle Additive in LiTf/PEO blend ∼10−5 (60 °C) 0.74 82
Calix[2]-p-benzo[4]pyrrole Macrocycle Additive in LiTf/PEO blend ∼10−5 (60 °C) 0.78 85
Ionic liquid-modified PMMA N(−)(SO2CF3)2l Core (PMMA)–shell (ionic liquid) LiTFSI/PC blend 3.13 × 10−3 (RT) 0.96 5.18 86

2.1. Polyanions

A facile and straightforward approach for obtaining polymeric SLICs is the polymerization of monomeric Li salts (i.e., polymerizable precursors consisting of anion moieties and counter Li+ ions). However, the resultant polymeric SLICs (hereinafter also called polyanions) are strongly associated with Li+ ions, and thus have difficulty in providing reliable ionic conductivity. Also, they tend to be mechanically weak, making them unsuitable for application in solid-state electrolyte membranes. To address these issues, polyanions are often combined with flexible ethylene oxide chains that can solvate Li+ ions through blending with poly(ethylene oxide) (PEO) or introduction of copolymerizable oligoether units. A variety of chemical structures have been explored for application in polymeric SLICs (Fig. 2). Depending on their anion groups, polyanions can be classified as follows: sulfonates, sulfonylimides and tetrahedral borates. A pivotal point in the design of anion groups is the realization of a broad negative charge distribution that can weaken ionic association with Li+ ions, eventually facilitating Li+ ion transport.
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Fig. 2 Representative polyanions based on (a–d) sulfonates, (e–l) sulfonylimides and (m–q) tetrahedral borates.

Sulfonate-based polymeric SLICs have been widely investigated due to their easy accessibility and moderate negative charge distribution. Sun et al. reported a blend electrolyte consisting of lithium poly(4-styrenesulfonate) (LiPSS; Fig. 2a) and PEO,35 which showed a room temperature (RT) ionic conductivity of 3.0 × 10−8 S cm−1 at an optimal Li+/ethylene oxide molar ratio of 1/8. A tLi+ value of 0.85 was obtained for the blend electrolyte, exhibiting single ion conduction behavior. The low RT ionic conductivity of the blend electrolyte is mostly due to the presence of crystalline PEO regions. With a rise in temperature, PEO blend electrolytes show an increase in ionic conductivity. However, they become viscous and lose their dimensional stability above the melting temperature of PEO.32

To resolve this problem, the copolymerization of monomeric Li salts and flexible oligoethers has been suggested to prevent crystallization and strengthen the mechanical properties of the resultant polymer electrolytes. For example, a comb-branched copolymer (Fig. 2b)36 showed a higher ionic conductivity (2.0 × 10−7 S cm−1 at 25 °C) than the LiPSS/PEO blend. A further increase in the RT ionic conductivity (∼10−5 S cm−1) was observed for a comb-branched copolymer containing a perfluoroalkyl sulfonate group (–CHFCF2SO3; Fig. 2c) that can delocalize negative charge and thus alleviate ionic association with Li+ ions.37 The combined effects of the copolymerization-driven suppression of crystallization and the strong electron-withdrawing group contributed to the significant improvement in the ionic conductivity.

Recently, Zhao et al. reported a new class of sulfonate-based copolymers, which were synthesized by radiation-initiated cografting of two monomers (i.e., 2,2,3,4,4-hexafluorobutyl methacrylate and 2-acrylamido-2-methylpropanesulfonic acid) onto poly(vinylidene fluoride) (PVdF) chains (Fig. 2d).38 Note that the grafting reaction is initiated by a high-energy gamma-ray without a chemical initiator, and thus yields the product with high purity and a well-defined chemical structure. The resultant graft copolymer swelled with an ethylene carbonate (EC)/dimethyl carbonate (DMC) (1/1 (v/v)) mixed solvent provided an ionic conductivity of 2.08 × 10−5 S cm−1 at 30 °C along with a high tLi+ value of 0.93.

Sulfonylimides, which contain multiple electron-withdrawing groups, have attracted much attention due to their broad charge distribution and plasticizing characteristics. Various homopolymers based on sulfonylimides have been reported.39–41 Zhou et al. investigated the charge distribution effect of sulfonylimides by comparing the electrolyte characteristics of the three following types of polymeric SLICs: LiPSS (Fig. 2a), lithium poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide] (LiPSTFSI; Fig. 2e) and lithium poly[(4-styrenesulfonyl)(trifluoromethyl(S-trifluoromethylsulfonylimino)sulfonyl)imide] (LiPSsTFSI; Fig. 2f and see also Fig. S1a).41 These polymeric SLICs were mixed with PEO to fabricate self-standing electrolyte membranes. Over a wide range of temperature, the highest ionic conductivity was observed for LiPSsTFSI/PEO (e.g., 7.61 × 10−5 S cm−1 at 70 °C), followed by LiPSTFSI/PEO (e.g., 8.42 × 10−6 S cm−1 at 70 °C) and then LiPSS/PEO (e.g., 8.35 × 10−8 S cm−1 at 70 °C) (Fig. S1b), indicating that the highly delocalized negative charge of the anion moiety facilitates ionic dissociation. These polymeric SLICs/PEO showed lower ionic conductivities than the conventional LiTFSI/PEO (TFSI = N(−) (SO2CF3)2), which is probably due to the large volume fraction of the nonconductive polystyrene backbone of the polymeric SLICs. Meanwhile, a high tLi+ value of 0.91 was obtained for LiPSsTFSI/PEO. Furthermore, significantly low glass transition temperatures (Tg) were observed for LiPSsTFSI (44.3 °C) and its PEO blend (−15.1 °C), exhibiting the weakened ionic association and low crystallinity of LiPSsTFSI (Fig. S1c).

Copolymers consisting of LiPSTFSI and flexible oligoether chains (Fig. 2g and h) were investigated to overcome the low ionic conductivity of the highly crystalline PEO blend electrolytes.32,42 These copolymers showed much higher ionic conductivities (e.g., ∼10−4 for the comb-branched copolymer (Fig. 2g)42 and 1.3 × 10−5 S cm−1 for the linear triblock copolymer (Fig. 2h)32 at 60 °C) as compared to that (vide supra) of the LiPSTFSI/PEO blend. Note that high tLi+ values were observed for both copolymers (0.94 for the comb-branched and 0.85 for the triblock copolymer), demonstrating their well-defined single Li+ ion transport behavior. Similarly, the copolymers modified with sulfonylimides via introduction of alkyl chains (Fig. 2i and j) also showed facile Li+ ion transport behavior and an amorphous characteristic (e.g., an ionic conductivity of 1.2 × 10−5 S cm−1 at 55 °C, a tLi+ value of 0.83 and a Tg value of −61 °C for the comb-branched diblock copolymer (Fig. 2i)33 and an ionic conductivity of ∼10−4 S cm−1 at 70 °C, a tLi+ value of 0.91 and a Tg value of −31.7 °C for the linear triblock copolymer (Fig. 2j)34). These results verify the advantageous effect of the copolymerization of monomeric Li salts and flexible oligoether chains on the suppression of crystallization.

Matyjaszewski et al. recently reported a homopolymer that contains both sulfonylimide and oligoether chains at a single branch (Fig. 2k).43 To synthesize the homopolymer, the sulfonylimide and oligoether precursors were coupled via triazole linkage based on the azide–alkyne cycloaddition (i.e., Click Chemistry),44 which was then ultraviolet (UV)-irradiated with an initiator to induce polymerization. Driven by the amorphous/conductive ethylene oxide phase, the resultant polymer showed an ionic conductivity of ∼10−4 S cm−1 at 90 °C, a tLi+ value of 0.99 and a Tg value of −24 °C. It also provided good electrochemical stability (∼5 V (vs. Li/Li+)).

Iojoiu et al. reported a gel-type polymeric SLIC based on an EC-swelled multi-block copolymer comprising an ionophilic block (perfluoroether sulfonylimide) and a hydrophobic block (perfluoroaromatics) (Fig. 2l).45 A noteworthy feature of the gel-type polymeric SLIC is the nanophase separation structure. The ionophilic blocks, driven by their preferential affinity for EC, provided a high ionic conductivity (∼10−3 S cm−1 at 30 °C). Meanwhile, the EC-free hydrophobic blocks contributed to the mechanical properties of the gel-type polymeric SLIC. Good anodic stability up to ∼4.9 V was observed, along with single Li+ ion conduction behavior.

Tetrahedral borates have been extensively investigated due to their design diversity. Zhang and Cheng et al. synthesized a polymeric borate linked by bisphenolate A (Fig. 2m).46 The ionic conductivity decay issue of PEO-based SPEs upon the increase of Li salts was functionally resolved by introducing the charge-delocalized polymeric borate. The highest ionic conductivity of 1.7 × 10−4 S cm−1 with a tLi+ value of 0.92 was obtained at 60 °C for the polymeric borate/PEO blend electrolyte prepared with an optimal molar ratio of Li+/ethylene oxide (1/8). In addition, a wide electrochemical window of 5.5 V was observed for the polymeric borate.

Tetrahedral borate-based gel-type polymeric SLICs have been investigated due to their high ionic conductivity, mechanical flexibility and good electrochemical stability (>5 V).47–52 Meng et al. reported a gel polymer membrane containing a three-dimensionally (3D) crosslinked polymeric borate (Fig. 2n) prepared through UV-initiated polymerization.48 The precursors dissolved in gamma-butyrolactone (GBL) were cast on an electrospun PVdF scaffold, which was then exposed to UV irradiation to produce the self-standing membrane (Fig. S2a). The resultant SLIC membrane with a 3D crosslinked network showed good mechanical strength (tensile strength of 7.2 MPa and elongation at break of 269%; Fig. S2b) and flexibility, despite the presence of a liquid component (i.e., GBL). It also provided an outstanding ionic conductivity of 1.32 × 10−3 S cm−1 with a tLi+ value of 0.92 at 25 °C and high electrochemical stability up to 6 V, which was possibly due to the broad charge distribution of the borate caused by the adjacent carbonyls. Liu and Cui et al. synthesized a polymeric borate linked by tartrate (Fig. 2o).49,51 The SLIC membrane composed of the polymeric borate and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), after being soaked in an EC/DMC (1/1 (v/v)) mixed solvent, showed an ionic conductivity of 5 × 10−4 S cm−1 with a tLi+ value of 0.91 at RT, and also good electrochemical stability (up to 5.0 V).

The distance between the ion-conducting moieties plays a crucial role in the site-to-site ion movement. Several approaches have been reported to construct shorter pathways for Li+ ion conduction along polymeric borates that are characterized by ordered backbone structures.53,54 Long et al. synthesized locally π-conjugated tetraarylborate polymers (Fig. 2p) in order to build short-distance Li+ ion hopping networks.53 Note that the solvents used in this synthesis significantly affected the microscale morphology and ionic conductivity of the resultant borate polymers. Upon using dimethyl sulfoxide (DMSO), a borate polymer with a dense and smooth appearance (Fig. S3a, left) was synthesized. It showed a moderate ionic conductivity of 3.6 × 10−5 S cm−1 at 27 °C after being soaked in propylene carbonate (PC). In comparison, the borate polymer with a grainy appearance (Fig. S3a, right), which was synthesized from dimethylformamide (DMF), was found to be almost nonconductive (<10−8 S cm−1), although the two borate polymers were chemically identical. This result indicates that their bulk ionic conductivity depends largely on the formation of continuous microscale conducting networks. Meanwhile, the effect of negative charge distribution on the ionic conductivity was investigated with a perfluorinated borate polymer. Due to the weakened lithium–borate interaction, the perfluorinated borate polymer exhibited an improved ionic conductivity (2.7 × 10−4 S cm−1 at 28 °C; see also Fig. S3b) with a high tLi+ value of 0.93.

Stemming from traditional polymer-based approaches, solid electrolytes based on porous crystalline materials (e.g., covalent organic frameworks (COFs), metal–organic frameworks (MOFs), etc.) have recently been investigated to offer directional Li-ion conduction pathways through their ordered pore arrays.54–60 However, most of them needed to additionally incorporate Li salts, which thus allowed the undesirable transport of anions as well as Li+ ions. Zhang et al. reported that an anionic COF bearing borate linkages (Fig. 2q) can function as an SLIC.54 The structural order of the anionic COF was confirmed by its characteristic powder X-ray diffraction (PXRD) pattern that revealed multiple peaks in the 2 theta range of 2–35 degree (Fig. S4a). The anionic COF was blended with PVdF and then swelled with PC. The resultant membrane showed an ionic conductivity of 3.05 × 10−5 S cm−1 at RT with a tLi+ value of ∼0.8. In addition, its ionic conductivity showed linear proportionality with temperature (Fig. S4b;Ea = 0.24 eV atom−1), which is a typical behavior of solid inorganic electrolytes. A similar temperature dependence of ionic conductivity was observed for tetraarylborate polymers (Fig. S3b)53 due to the absence of the ethylene oxide group. The ionic conductivity of conventional SPEs (i.e., ethylene oxide groups are included) is known to show non-linearity with temperature (e.g., see Fig. S1b). One limitation of the anionic COF is the incorporation of the solvent (PC) to enable ion transport. Considering their unique ion transport behavior via well-defined directional ion channels, the porous crystalline solid electrolytes described above, although they are in the very early development stage, hold promise as a newly emerging alternative to conventional SPEs. Future studies will be devoted to diversifying their chemical structure to further improve ion transport and tailor other electrochemical attributes.

2.2. Organic/inorganic hybrids

Combination of SPEs with inorganic materials has been extensively investigated as a versatile/facile fabrication technique, which could thus move the resultant organic/inorganic hybrids closer to their practical application in Li batteries. As a representative approach, inorganic nanoparticles have been incorporated into SPEs in an effort to improve their ionic conductivity, immobilize anions and control their mechanical/physicochemical properties.

Liu et al. prepared silica (SiO2) nanoparticles functionalized with various polymeric SLICs (Fig. 3a).61,62 For example, sodium 4-styrenesulfonate was polymerized onto initiator-decorated SiO2 nanoparticles, and subsequently, Na+ ions were exchanged into Li+ ions.61 This hybrid electrolyte, after being mixed with poly(ethylene glycol) dimethyl ether (PEGDME), showed an ionic conductivity of 2.2 × 10−7 S cm−1 at 60 °C. A further increase in the ionic conductivity (1.3 × 10−6 S cm−1 at 60 °C) was observed for the hybrid electrolyte prepared through copolymerization of poly(ethylene glycol) methacrylate (PEGMA) and the monomeric salt onto SiO2 nanoparticles, which may be attributed to the enhanced affinity of the polyelectrolyte layer for the PEGDME phase. Note that the ionic conductivities of these hybrid electrolytes were much higher than those of the conventional SiO2 nanoparticles modified with monolayer anions,63,64 which may result from the higher Li+ ion concentration.

image file: c8ta09056d-f3.tif
Fig. 3 Representative organic/inorganic hybrid SLICs. (a) Polyanion-modified SiO2 hybrids. (b) Sulfonylimide/oligoether-cografted Al2O3 (or SiO2) hybrids. (c) Sulfonylimide/oligoether-incorporated mesoporous SiO2 hybrid. (d) Anion (sulfonylimide or perfluoroether sulfonate)/oligoether-cografted polysiloxanes. (e) Sulfonylimide-grafted polysiloxane. (f) Sulfonylimide-modified silsesquioxane hybrid.

Replacing the sulfonate by a sulfonylimide that possesses an electron-withdrawing trifluromethyl group (anion formula: –N(−)SO2CF3) promoted the dissociation of Li+ ions, thereby providing a higher ionic conductivity (∼10−5 S cm−1 at 60 °C).62 Similarly, Armand et al. reported alumina (Al2O3) and SiO2 nanoparticles modified with a more electron-deficient sulfonylimide (anion formula: –SO2N(−)SO2CF3; Fig. 3b),65 which showed further improvement in ionic conductivity (∼10−4 S cm−1 at 60 °C) and a wide electrochemical window (up to 5 V).

Recently, Choi et al. reported an organic/inorganic hybrid electrolyte featuring directional Li+ ion conducting nanochannels.66 To prepare the hybrid electrolyte, a sulfonylimide group (anion formula: –SO2N(−)SO2CF3) and oligoether chains were cografted into the inner pore wall of mesoporous SiO2 (Fig. 3c). The hybrid electrolyte featured two-dimensional (2D) hexagonal pore arrays (Fig. S5).67 Note that the hybrid electrolyte/PEO mixture showed a high ionic conductivity of ∼10−3 S cm−1 even at 25 °C, which was nearly two orders of magnitude higher than that observed for a control sample containing nonporous SiO2 hybrid SLICs (∼2 × 10−5 S cm−1). This is ascribed to the unique mesoporous structure with a large amount of Li charge carriers and ethylene oxide strands. In addition, a high tLi+ value of ∼0.9 was observed for the mixture. These results demonstrate the advantageous effect of the well-defined conducting/solvating nanochannels on the directional motion of Li+ ions.

Shriver et al. explored polysiloxanes bearing highly flexible [Si–O]n units as a backbone of polymeric SLICs (Fig. 3d).68–70 Their amorphous nature and low Tg were expected to beneficially affect ion conduction at low temperature. The polysiloxane SLICs were synthesized by copolymerizing monomeric trifluoromethylsulfonamide (anion formula: –N(−)SO2CF3) and short oligoethers on the polysiloxane backbone.68 The polysiloxane SLICs with a well-designed anion/oligoether ratio showed very low Tg values (<−60 °C). Benefiting from their low crystallinity, the polysiloxane SLICs provided a relatively high ionic conductivity of 1.3 × 10−6 S cm−1 at 25 °C, in comparison to conventional polymeric SLIC/PEO blends (in general, <10−6 S cm−1 at 25 °C).23–25 Notably, replacement of the anion moieties by a more flexible and electron-deficient perfluoroether sulfonate group (anion formula: –CF2CF2OCF2CF2SO3) further promoted the dissociation of Li+ ions, resulting in the enhancement of ionic conductivity (2.5 × 10−6 S cm−1 at 25 °C).69,70 Cheng et al. reported a polysiloxane SLIC bearing a styrenesulfonyl(phenylsulfonyl)imide group (anion formula: –SO2N(−)SO2C6H5; Fig. 3e).71 A membrane composed of the polysiloxane SLIC/PVdF-HFP (soaked in an EC/PC (1/1 (v/v)) mixed solvent) exhibited an ionic conductivity of 7.2 × 10−4 S cm−1 at RT, along with a tLi+ value of 0.89.

Traditional PEO-based SPEs tend to show a trade-off between ionic conductivity and mechanical strength.72 As an intriguing approach to address this issue, microphase-separated polystyrene-b-poly(ethylene oxide) (SEO) comprising a rigid structural block (i.e., polystyrene) and a soft conducting block (i.e., PEO) was often used in SPEs as an alternative to typical PEO.72,73 Balsara et al. combined the SEO with a silsesquioxane hybrid SLIC (Fig. 3f) to obtain a high modulus SLIC membrane.74 The distribution of silsesquioxane hybrid SLICs dispersed in a microphase-separated SEO matrix was identified by scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) analyses (Fig. S6). The EDS mapping image showed that dark polystyrene-rich regions are dominated by carbon (C; blue), while the bright PEO-rich regions have high concentrations of fluorine (F; red), sulfur (S; yellow) and silicon (Si; green), indicating that the silsesquioxane hybrid SLICs are preferentially located in the PEO-rich regions of SEO. The resultant membrane with an optimal ratio of silsesquioxane hybrid SLIC to SEO exhibited a moderate ionic conductivity (1.1 × 10−5 S cm−1 at 90 °C) along with a tLi+ value that is close to unity (0.98).

2.3. Anion acceptor-containing polymers

Lewis acid-based anion acceptors have been introduced into SPEs with the aim of increasing tLi+ values. Mehta et al. investigated a series of boroxine ring (B3O3)-based polymers as anion acceptors, in which boron atoms function as the Lewis acid (Fig. 4a).75–78 Mixing the boroxine-based polymers with various Li salts (e.g., LiCl, LiTf (Tf = CF3SO3), LiTFSI, etc.) resulted in the immobilization of free anions through Lewis acid–base interaction, thereby providing high tLi+ values in the range of 0.62–0.88.76 Moreover, they showed an ionic conductivity of up to ∼10−5 S cm−1 at 30 °C, indicating that the boroxine rings promote the ionic dissociation of Li salts in addition to the anion trapping effect.
image file: c8ta09056d-f4.tif
Fig. 4 Representative anion acceptors used in SPEs. (a) Boroxine ring-containing polymers, (b–d) calixarene derivatives and (e) PMMA modified with an ionic liquid-type moiety. Purple colored areas indicate anion-trapping sites.

Another class of representative anion acceptors exploited in SPEs are calixarene derivatives (Fig. 4b–d). These supramolecular compounds have been widely used as additives to form complexes with free anions via multiple hydrogen bonds. Scrosati et al. showed that urea-modified calix[4]arene additives (Fig. 4b) provided high tLi+ values in the range of 0.8–1.0 for LiI/PEO electrolytes by successfully trapping free iodides.79 The calix[4]arene additives were also effective for LiTf/PEO electrolytes, suggesting their broad compatibility with various Li salts in SPEs.80 Meanwhile, the contribution to tLi+ values was negligible (tLi+ = 0.15–0.18) when the additives were used in SPEs with high PEO concentration, revealing that the anion trapping capability of calix[4]arenes tends to compete with other interactions (i.e., ion–ion and ion–matrix interactions) of SPEs. Despite the beneficial contribution to tLi+ values, the bulky calix[4]arene additive gives rise to steric hindrance, resulting in the use of an excessively large amount of calix[4]arene additive (100 mol% of additive per Li salt) and an impediment to the segmental motion of polymer matrices. To address these challenges, calix[6]pyrrole (C6P; Fig. 4c), which is less bulky but effective for trapping anions, was investigated.81–84 For example, LiBF4/PEO81 and LiTf/PEO82 blend electrolytes containing only 12.5 mol% of C6P per Li salt showed tLi+ values of 0.78 and 0.74, respectively. As another example, calix[2]-p-benzo[4]pyrrole (CBP; Fig. 4d) was introduced into the LiTf/PEO electrolyte and improved its tLi+ value from 0.23 to 0.78.85 Moreover, the effect of CBP additive on the interfacial stability between the LiTf/PEO electrolyte and the Li metal anode was investigated by monitoring the change in interfacial resistance of Li|Li symmetric cells. The CBP additive mitigated the growth of interfacial resistance (Fig. S7), indicating that its anion-trapping capability could hamper the formation of unwanted passive layers on the Li metal surface.

Ng et al. prepared poly(methyl methacrylate) (PMMA) grafted with an ionic liquid-type moiety (Fig. 4e).86 In this system, the hydroxyl (–OH) group on the PMMA particle surface effectively immobilized the free anions to restrict their mobility. Furthermore, the counter anion TFSI offered a weak binding toward the Li+ ion, allowing the Li+ ion to act exclusively as a mobile charged species. As a consequence, the blend electrolyte composed of the ionic liquid-modified PMMA/LiTFSI/PC provided a high RT ionic conductivity of 3.13 × 10−3 S cm−1 with a tLi+ value close to unity (0.96).

From the understanding of the previous studies described above, we conclude that size, geometry and anion selectivity of the anion acceptors play viable roles in determining the electrochemical performance of SPEs.

To enable practical application of polymeric SLICs, much attention should be paid to widening their electrochemical stability window, in addition to enhancing the Li+ ion conduction characteristics (i.e., ionic conductivity and tLi+ value). For example, as observed in sulfonylimides and tetrahedral borates with electron-withdrawing moieties,32,43,45–51,65,86 electron-deficient functional groups could be effective for enhancing the anodic stability of polymeric SLICs as well as improving the ionic dissociation described above. Notably, the development direction of Li batteries that use high-voltage cathodes and Li metal anodes for higher energy densities underlines the significance of the electrochemical stability of polymeric SLICs.

3. Application of polymeric SLICs in Li batteries

Driven by the remarkable progress in the development of state-of-the-art polymeric SLICs, their application in power sources has been investigated for the last decade. Here, our interest is mainly focused on application in all-solid-state Li batteries, quasi-solid-state Li batteries and Li metal protective layers. Table 2 briefly describes the application examples of polymeric SLICs in the Li battery systems mentioned above.
Table 2 Application examples of polymeric SLICs in Li batteries
Classification Electrolyte Cell configuration Initial capacity, mA h g−1 (current density) Cycle number (capacity retention, %) Operating temperature (°C) Ref.
a Examined at varied current densities.
All-solid-state Li batteries Triblock copolymer (neat) Li|LFP ∼160 (C/15) ∼90a (—) 60–80 32
Comb-branched copolymer (neat) Li|LFP 130 (C/15) 100a (—) 70 33
Triblock copolymer (neat) Li|LFP ∼100 (C/2) 300 (77.8) 70 34
Polymeric borate/PEO blend Li|LFP 147 (C/25) ∼90a (—) 50–80 46
Organic/inorganic (Al2O3) hybrid/PEO/PEGDME blend Li|LFP ∼130 (C/2) 130 (>90) 70 65
Organic/inorganic (silsesquioxane) hybrid/SEO blend Li|LFP 158 (C/15) 90 74
Quasi-solid-state Li batteries Gel polymer SLIC, in situ synthesized onto the cathode Li|LFP 126 (C/10) 100 (98) 25 89
UV-crosslinked gel polymeric borate Li|LFP 140 (1C) 380 (91) RT 48
Polymeric borate/PVdF-HFP/PC blend Li|LFP ∼168 (C/2) 60 (95.2) 80 49
Polymeric borate/PVdF-HFP/EC/DMC blend Li|LTO ∼150 (C/2) 60 (92.8) 20 50
LTO|LFP ∼135 (C/2) 100 (86.6) 20 50
Li|LMO 100 (C/2) 100 (76.9) 55 51
Multi-block copolymer/EC blend Li|NCM111 ∼100 (C/5) 230 (—) 40 45
Polymeric borate/PVdF-HFP/EC/PC blend Li|LFP 135 (C/10) 170a (—) 25–100 52
Polysiloxane SLIC/PVdF-HFP/EC/PC blend Li|LFP 141 (C/10) ∼150a (—) RT–80 71
Ionic liquid-modified PMMA/LiTFSI/PC blend Li|LTO 134 (1C) 500 (—) RT 86
UV-crosslinked gel polymeric sulfonate Li|S ∼1000 (C/2) 100 (92) RT 94
Lithiated Nafion/sulfolane/diglyme blend Li|S 920 (C/10) 100 (79) RT 95
Anion-modified PEMA/PVdF-HFP/EC/DMC blend Li|S ∼760 (1C) 1000 (—) 25 96
Protective layers for Li metal Microthick Nafion/1 M LiPF6 in EC/DEC blend layer Li|LCO ∼130 (C/5) 360 (82.6) RT 103
Nanothick Lithion layer Li|NCA ∼150 (C/2) 400 (94) RT 104

3.1. All-solid-state Li batteries

All-solid-state Li batteries have attracted sharply increasing attention due to the exclusion of volatile/flammable organic liquid constituents (Fig. 5a), which eventually enables safety reinforcement, bipolar cell configuration (i.e., multiple cell stacking), simple packaging and shape versatility.
image file: c8ta09056d-f5.tif
Fig. 5 Application of polymeric SLICs in all-solid-state Li batteries (insets: chemical structure of the corresponding polymeric SLICs). (a) Cell configuration of an all-solid-state Li battery containing a polymeric SLIC. (b) Discharge profiles and (c) cycling performance of Li|LFP cells containing a triblock copolymer. Reprinted with permission from ref. 32. Copyright 2013 Springer Nature. (d) Cycling performance and (e) discharge profiles of the Li|LFP cells containing a polymeric borate. Reprinted with permission from ref. 46. Copyright 2016 Elsevier. (f) Cycling performance of a Li|LFP cell containing an Al2O3-based hybrid SLIC (inset: charge/discharge profiles). Reprinted with permission from ref. 65. Copyright 2015 Wiley-VCH. (g) Discharge profiles of a Li|LFP cell containing silsesquioxane-based hybrid SLIC/SEO (solid lines) and LiTFSI/SEO (dashed lines) blend electrolytes. Reprinted with permission from ref. 74. Copyright 2017 American Chemical Society.

In comparison to solid inorganic electrolytes that have been extensively investigated in all-solid-state batteries, a limited number of studies have been reported for polymeric SLICs. Armand et al. presented the first example using the triblock copolymer (Fig. 2h).32 An Li|LiFePO4 (LFP) cell employing the block copolymer electrolyte was fabricated, in which the LFP cathode was composed of carbon-coated LFP/block copolymer/carbon black (60/32/8 (w/w/w)). The discharge profiles (Fig. 5b) showed well-defined potential plateaus up to a current density of C/2 at 80 °C along with a polarization proportional to the applied current density, whereas transport limitations, probably due to a high porosity (45%) of the prepared LFP cathode, were observed at higher current densities. The cell operated with a stable capacity retention (>80 cycles) over a wide range of operating temperatures (60, 70 and 80 °C) (Fig. 5c). Stimulated by this study, several approaches have been employed to apply structurally similar polymeric SLICs (bearing TFSI analogues; Fig. 2i and j) to all-solid-state batteries.33,34

Cheng et al. examined the borate-based polymeric SLIC (Fig. 2m)/PEO blend electrolyte for an all-solid-state Li|LFP cell,46 in which the cathode consisted of LFP/PVdF/acetylene black/lithium bis(4-carboxyphenylsulfonyl)imide (75/10/10/5 (w/w/w/w)). Cycling stability for over 90 cycles was observed under various operating conditions, in which the current densities varied from C/25 to C/2.5 at 50–80 °C (Fig. 5d). In particular, a discharge capacity of 147 mA h g−1 (current density = C/25) was achieved at 80 °C (Fig. 5e), and ∼100 mA h g−1 was delivered even at a relatively low temperature of 50 °C. This decent cell performance was attributed to the high ionic conductivity and electrochemical stability (vide supra: ∼10−4 S cm−1 and up to 5.5 V, respectively) of the polymeric borate/PEO blend electrolyte.

In addition to the polymeric SLICs described above, organic/inorganic hybrid SLICs were applied in all-solid-state Li batteries. Armand et al. designed an Li|LFP cell containing the sulfonylimide/oligoether-cografted Al2O3 hybrid (Fig. 3b) with PEO/PEGDME.65 Note that the cathode was composed of carbon-coated LFP/carbon black/LiPSTFSI (Fig. 2e)/PEO (63/7/12/18 (w/w/w/w)). The Li|LFP cell showed good cycling performance at a current density of C/2 at 70 °C (discharge capacity = ∼120 mA h g−1 after 130 cycles) (Fig. 5f), demonstrating the superior electrochemical performance of the hybrid SLIC compared to conventional dual-ion conducting composite SPEs.87 Balsara et al. compared the electrochemical performances of silsesquioxane-based hybrid SLIC/SEO (Fig. 3f) with a conventional LiTFSI/SEO in Li|LFP cells.74 At very low current densities (Fig. 5g, left), the two battery systems showed similar discharge capacities at 90 °C (e.g., 158 for the hybrid SLIC/SEO (solid lines) and 155 mA h g−1 for the LiTFSI/SEO (dashed lines) at a discharge current density of C/15). When the current density was increased (Fig. 5g, right), the decrease in discharge capacity was more pronounced for the hybrid SLIC/SEO cell, mostly due to the intrinsically lower ionic conductivity of the hybrid SLIC/SEO electrolyte (i.e., 1.1 × 10−5 for the hybrid SLIC/SEO and 5.6 × 10−4 S cm−1 for the LiTFSI/SEO at 90 °C). Meanwhile, much sharper knees were revealed in the discharge curves of the hybrid SLIC/SEO cell as compared to the results of the LiTFSI/SEO cell. This result is probably attributed to the single Li-ion transport character of the hybrid electrolyte, while detailed theoretical modelling88 is required to better understand this difference in the discharge curves.

Note that previously reported all-solid-state batteries containing polymeric SLICs have been investigated at elevated temperatures (>50 °C) and low current densities, which could be attributed to the low RT ionic conductivity of polymeric SLICs. To achieve a practically meaningful level of charge/discharge rate capability, the polymeric SLICs need to provide an ionic conductivity of at least 10−4 S cm−1 at ambient temperature.22 Hence, future studies should be devoted to improving the performance of the polymeric SLICs, with particular attention to the ionic conductivity and also electrochemical stability.

3.2. Quasi-solid-state Li batteries

Gel-type polymeric SLICs, which contain liquid plasticizers in addition to the polymeric SLIC matrix, have been used in quasi-solid-state electrochemical systems (Fig. 6a). Although the gel polymer approaches may not be completely free from liquid electrolyte-triggered safety issues, some meaningful performance advancements including high ionic conductivity, good electrode/electrolyte interfacial contact and improved processability have been achieved.
image file: c8ta09056d-f6.tif
Fig. 6 Application of gel-type polymeric SLICs in quasi-solid-state Li batteries. (a) Cell configuration of a quasi-solid-state Li battery containing a gel-type polymeric SLIC membrane. (b) Schematic illustration depicting the preparation of the gel polymer SLIC-embedded cathode. (c) Cycling performance of the Li|LFP cell containing the gel polymer SLIC-embedded cathode. Reprinted with permission from ref. 89. Copyright 2016 American Chemical Society. (d) Cycling performance of the Li|LFP cell containing a UV-crosslinked gel polymeric borate (inset: charge/discharge profiles). (e) Photographs of the flexible cell lighting up an LED lamp in flat (left) and bent (right) states. Reprinted with permission from ref. 48. Copyright 2018 Wiley-VCH.

Gerbaldi et al. prepared an electrolyte-embedded LFP cathode by in situ copolymerization of gel polymer SLIC precursors, which were directly cast onto a preformed cathode (LFP/carbon black/PVdF = 80/10/10 (w/w/w)) (Fig. 6b).89 The Li|(gel polymer SLIC-embedded) LFP cell showed a good cycling performance (capacity retention = 98% after 100 cycles) at a current density of C/10 (Fig. 6c). The cell operating at 25 °C was able to deliver a high specific capacity of 126 mA h g−1, which is far beyond those achievable with traditional all-solid-state polymeric SLIC batteries.

Meng et al. introduced a gel polymer membrane based on a UV-crosslinked polymeric borate (Fig. 2n and S2a) in an Li|LFP cell.48 The resultant cell showed a stable cycling performance for 380 cycles (Fig. 6d, red dots; capacity retention = 91%) even at a relatively high current rate of 1C, making it superior to that containing a conventional liquid electrolyte (blue dots; capacity retention = 74%). This excellent cell performance of the SLIC membrane was due to the high ionic conductivity (vide supra: >10−3 S cm−1) and good interfacial contact with the electrodes. Driven by its quasi-solid characteristic, the SLIC membrane was successfully applied to a flexible battery. The resultant flexible battery was able to light up a light emitting diode (LED) lamp in flat (Fig. 6e, left) and bent (180°) states (right), and maintained a voltage of 3.40 V for both states.

High-voltage quasi-solid-state Li batteries have been developed to achieve high-energy density. Cui et al. reported a high-voltage Li|LiMnO2 (LMO) cell employing a gel-type polymeric borate SLIC membrane (Fig. 2o).51 LMO has been considered as one of the promising cathode materials due to its cost-effectiveness and facile production. However, LMO-based batteries suffer from poor cyclability at elevated temperatures (>55 °C), which is known to arise from Mn dissolution triggered by unwanted side products of LiPF6-dissolved liquid electrolytes. In the Li|LMO cells, the polymeric borate SLIC membrane provided superior cyclability (at 55 °C) compared to a conventional liquid electrolyte (1 M LiPF6 in EC/DMC) (Fig. 7a and b). After the cycling test, the Li metal anode assembled with the SLIC membrane showed a smooth surface morphology (Fig. 7c), while a large amount of randomly formed Li dendrites was observed on the Li metal anode in contact with the liquid electrolyte (inset). Similar results were reported in other studies.48,49 This improvement was attributed to the immobilized anion groups of the SLIC membrane that could allow uniform Li+ ion flux toward the Li metal anode. Iojoiu et al. reported an Li|LiNi1/3Co1/3Mn1/3O2 (NCM111) cell employing a multi-block copolymer (Fig. 2l)-based gel-type SLIC membrane.45 The resultant cell showed a decent cycling performance for over 230 cycles at a moderate current density of C/5 at 40 °C (Fig. 7d and e), which was ascribed to the good electrochemical stability (∼4.9 V) of the SLIC membrane.

image file: c8ta09056d-f7.tif
Fig. 7 Application of gel-type polymeric SLICs in high-voltage quasi-solid-state Li batteries (insets: chemical structure of the corresponding polymeric SLICs). (a) Cycling performance of Li|LMO cells containing a gel-type polymeric borate SLIC membrane (red dots) or a liquid electrolyte (black dots). (b) Charge/discharge profiles of a Li|LMO cell containing a SLIC membrane at the 1st and 100th cycles. (c) Scanning electron microscope (SEM) image showing the surface morphology (after the cycling test) of the Li metal anode assembled with the SLIC membrane. The inset is the result of the Li metal anode in contact with the liquid electrolyte. Reprinted with permission from ref. 51. Copyright 2014 Elsevier. (d) Cycling performance and (e) charge/discharge profiles of a Li|NCM111 cell containing a gel-type multi-block copolymer SLIC membrane. Reprinted with permission from ref. 45, the Royal Society of Chemistry.

To date, most of the all-/quasi-solid-state Li batteries containing polymeric SLICs have been investigated with relatively low-voltage cathode materials (e.g., LFP and Li4Ti5O12 (LTO); see Table 2). Therefore, polymeric SLICs possessing a wide electrochemical window32,43,45–51,65,86 are highly preferred for high-voltage cathode materials (e.g., NCM, LiCoO2 (LCO) and LiNi0.5Mn1.5O4 (LNMO)) and Li metal anodes to develop high-energy solid-state batteries.

Recently, polymeric SLICs have been used for high-energy lithium–sulfur (Li–S) batteries. A formidable challenge facing Li–S batteries is the polysulfide shuttle effect which eventually worsens the cycling performance.90,91 Polymeric SLICs, due to their ion selectivity (anion group-driven repelling of polysulfides, i.e., Donnan exclusion),92,93 have been explored as a promising electrolyte membrane to prevent the shuttle effect.94–96

Archer et al. designed an Li–S cell employing an SLIC membrane based on a UV-crosslinked polymeric sulfonate (Fig. 8a).94 To investigate the polysulfide repelling ability of the SLIC membrane, H-shaped liquid cells were prepared with the following configuration: half of the cell containing a reddish polysulfide solution (solvent: 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME)) and the other half containing a colorless blank solvent were connected through either a routine Celgard separator or the SLIC membrane (Fig. 8b). The polysulfides in contact with the Celgard separator migrated to the opposing side, which was confirmed by the obvious color change in the blank solvent from colorless to red. On the other hand, the blank solvent separated by the SLIC membrane remained colorless for at least 18 h, indicating the suppression of polysulfide diffusion across the SLIC membrane. The Li–S cell containing the SLIC membrane showed a high specific capacity of ∼1000 mA h g−1 at a current density of C/2 and operated for 100 cycles (Fig. 8c, red dots; capacity retention = 92%). Due to the efficient polysulfide repelling ability, a Coulombic efficiency of >98% was observed during the cycling, which is a significant improvement compared to the cells fabricated with the pristine Celgard separator (green dots) or a non-sulfonated poly(ethylene glycol)dimethacrylate (PEGDMA)-based membrane (black dots).

image file: c8ta09056d-f8.tif
Fig. 8 Application of gel-type polymeric SLICs in quasi-solid-state Li–S batteries. (a) Schematic illustration depicting the preparation and the ion selectivity of the SLIC membrane based on the UV-crosslinked gel polymeric sulfonate. (b) Photographs of the H-shaped liquid cells connected through a Celgard separator (top) or the SLIC membrane (bottom), in which the polysulfide is asymmetrically contained. (c) Cycling performance of the Li–S cells containing a non-sulfonated PEGDMA membrane (black dots), a Celgard separator (green dots) or the SLIC membrane (red dots). Reprinted with permission from ref. 94. Copyright 2016 American Chemical Society. (d) Schematic illustration depicting the ion selectivity of the SLIC membrane based on lithiated Nafion. Charge/discharge profiles of the Li–S cells containing (e) a liquid electrolyte or (f) the SLIC membrane at the 1st, 10th and 30th cycles. (g) Bipolar stacked Li–S cell configuration based on the SLIC membrane. (h) Charge/discharge profiles of the single (black lines) and the bipolar (red lines) Li–S cells. (i) Cycling performance of the bipolar Li–S cell. Reprinted with permission from ref. 95. Copyright 2017 American Chemical Society.

Similarly, Kim et al. designed an Li–S cell using the lithiated Nafion that was swelled with a sulfolane/diglyme (1/1 (v/v)) mixed solvent (Fig. 8d).95 The electrochemical performance of the Li–S cell was compared with that of a control cell containing a 1 M LiTFSI in sulfolane/diglyme (1/1 (v/v)) electrolyte with a polyethylene (PE) separator. The control cell showed a serious capacity fading and an overcharging behavior during the cycling (Fig. 8e), probably due to a significant polysulfide loss from the sulfur cathode. On the other hand, the cell with the SLIC membrane showed a stable capacity retention during the cycling (Fig. 8f), indicating that the polysulfides can be effectively confined in the sulfur cathode. Furthermore, the SLIC membrane was applied to an Li–S battery with a bipolar configuration (Fig. 8g). The bipolar cell configuration is known to enable the minimal use of electrochemically inert components such as metallic foil current collectors and packaging substances, thereby contributing to a higher volumetric energy density beyond those achievable with a simple electrical connection of individual cells.97 The resultant bipolar Li–S cell showed a voltage of 4.1 V (Fig. 8h), which was exactly two times higher than the single cell voltage. In addition, the discharge capacity of the bipolar cell (940 mA h g−1) was nearly the same as that of the single cell (920 mA h g−1), showing that the sulfur cathode performance was not impaired at the bipolar structure. Note that the bipolar cell showed a stable cycling performance for at least 80 cycles (Fig. 8i).

3.3. Protective layers for Li metal anodes

Another important application field of polymeric SLICs is the Li metal protective layers (Fig. 9a). The thin, anion-rich polymeric SLIC layer is expected to act as a kind of permselective membrane: i.e., to allow the uniform and reliable flow of Li+ ions toward the Li metal electrode with selectively excluding anion transport, eventually enabling stable Li plating/stripping without concerns of Li dendrite growth. Meanwhile, solid inorganic electrolytes have been investigated as protective layers to prevent Li dendrite growth on Li metal.98–102 However, fabricating thin and dense inorganic electrolyte layers without voids or defects has remained a formidable challenge, along with securing electrochemical stability at Li metal anodes.
image file: c8ta09056d-f9.tif
Fig. 9 Application of polymeric SLICs as Li metal protective layers. (a) Cell configuration of an LMB with an SLIC protective layer. (b) Galvanostatic cycling performance of the Li|Li symmetric cells with/without a Nafion-based protective layer. SEM images of (c) protected (inset: cross-section SEM image) and (d) pristine Li metal after the cycling tests. Reprinted with permission from ref. 103. (e) Snapshot images of Li plating behavior. (Top: pristine Li metal, middle and bottom: Li metal coated with 200 nm and 8.8 μm thick Lithion-based protective layers, respectively). (f) Cycling performance of the Li|NCA cell, in which the Li metal electrode is coated with a 200 nm thick protective layer (inset: charge/discharge profiles of the 10th, 100th and 200th cycles). Reprinted with permission from ref. 104. Copyright 2017 Elsevier.

Kim et al. reported a microthick protective layer based on a Nafion/liquid electrolyte (1 M LiPF6 in EC/diethyl carbonate (DEC) (1/1)) blend.103 The Li plating/stripping behavior of an Li|Li symmetric cell employing the protective layer was investigated. The symmetric cell substantially improved the cycling performance (over 2000 h at a high current density of 10 mA cm−2) as compared to a control cell (without the protective layer), although larger overpotentials were observed in the initial cycles (Fig. 9b). Note that the flat and uniform protective layer on the Li metal electrode was stably preserved even after the cycling test (Fig. 9c), whereas a mossy and dendritic Li deposition was observed over the entire surface of the pristine Li metal (Fig. 9d), demonstrating the advantageous effect of the protective layer on the uniform Li plating/stripping behavior.

Archer et al. reported a Lithion-based protective layer with reduced thickness (∼200 nm).104 The Li plating behavior at a current density of 4 mA cm−2 was examined using a customized optical visualization cell that was mounted on the sampling platform of an optical microscope to monitor the Li metal interface in real time (Fig. 9e). The snapshot images of the electrodeposition process revealed that unstable Li plating started to occur at a low capacity (∼0.1 mA h cm−2) for pristine Li metal, and eventually, mossy/dendritic Li deposition proliferated over the electrode surface. On the other hand, the flat and uniform surface morphology was maintained for the Li metal electrode with a ∼200 nm thick protective layer even at the high deposition capacity. Meanwhile, the use of a much thicker (8.8 μm) protective layer resulted in a local and severe dendritic Li deposition. It was plausibly explained that the thick coating layer increased the overall interfacial resistance, thereby causing the Li deposition to preferentially occur on the defects or poor coverage areas. This result demonstrates the importance of uniformity and optimal thickness of the protective layers in the Li electrodeposition. In addition, the electrochemical performance of an Li|LiNi0.8Co0.15Al0.05O2 (NCA; areal mass loading = 19.9 mg cm−2) cell containing the protective layer was examined, aiming at sustainable full-cell construction. The resultant cell showed a stable cycling performance for 400 cycles (capacity retention = ∼94%) at a current density of C/2 without an appreciable increase of overpotential (Fig. 9f), which is due to the good interfacial stability and Li+ ion-rich condition between the protective layer and the Li electrode.

4. Summary and outlook

The ever-increasing demand for advanced power sources including all-solid-state LIBs and LMBs has spurred us to revisit polymeric SLICs, which hold promise as an appealing alternative to the currently widespread liquid electrolytes. A salient feature of the polymeric SLICs is the predominant contribution of Li+ ions to the ionic conductivity, thus enabling tLi+ to reach almost unity. This exceptional single ion transport behavior of polymeric SLICs, in combination with their solid-state nature, flexibility and facile processability, brings remarkable benefits to the battery structure and performance (Fig. 10a), which lie far beyond those achievable with typical dual-ion conductive electrolytes.
image file: c8ta09056d-f10.tif
Fig. 10 (a) Summary of polymeric SLICs in terms of their advantageous characteristics and potential applications. (b) Schematic representation of the challenges and development direction of polymeric SLICs as a function of materials design and battery technology (HOMO: highest occupied molecular orbital, LUMO: lowest unoccupied molecular orbital).

Ideal polymeric SLICs should fulfill the following electrolyte requirements: ionic conductivity, tLi+ value, mechanical properties and chemical/electrochemical stabilities. Depending on their structural features, polymeric SLICs are classified as follows: polyanions, organic/inorganic hybrids and anion acceptor-containing polymers. The design, synthesis and major characteristics of these polymeric SLICs were systematically described in this review, along with their influence on ion transport phenomena and battery performance.

As a prerequisite to facilitate Li+ ion transport, the ionic association between polyanions and Li+ ions should be weakened, which can be promoted by broadening the negative charge distribution of anion moieties. The introduction of electron-withdrawing groups (e.g., carbonyl, sulfonyl, perfluoroalkyl and perfluoroaryl groups) can efficiently delocalize the negative charge of the anion moieties. The incorporation of anion acceptors into SPEs is known as an effective way to improve tLi+ values. As another approach for fast Li+ ion conduction, the polymeric backbone design has been suggested to provide short-distance and/or directional Li+ ion transport networks. Meanwhile, the well-dispersed inorganic particles in SPEs improve the mechanical properties and amorphous characteristics of the resultant SPEs. The structural diversity (e.g., mesoporous structure) of the inorganic particles contributed to improving Li+ ion transport and other electrochemical attributes.

Driven by these advantageous effects of the polymeric SLICs described above, their application to batteries has been investigated with a particular focus on electrolyte membranes for all-/quasi-solid-state Li batteries or Li metal protective layers. Gel-type polymeric SLICs tend to provide high ionic conductivity, good electrode/electrolyte interfacial contact and facile processability. As a consequence, quasi-solid-state Li batteries assembled with gel-type polymeric SLICs have shown a satisfactory level of electrochemical performance in terms of RT battery operation, cyclability and facile adaptability with Li metal anodes. However, the safety concerns and form-factor limitations of quasi-solid-state batteries, which are mainly due to the presence of liquid electrolytes in gel-type polymeric SLICs, have not yet been completely resolved. Meanwhile, all-solid-state Li batteries containing solvent-free polymeric SLICs have not yet reached a practically meaningful level of electrochemical performance particularly at RT conditions, despite their promising potential in terms of safety tolerance and mechanical robustness. Furthermore, most of the previously reported all-solid-state batteries employed electrode materials with relatively low voltage plateaus. To address these performance challenges, future studies should be devoted to improving the electrolyte properties of polymeric SLICs, with special attention to “RT” ionic conductivity and electrochemical stability for facile combination with Li metal anodes and high-voltage cathodes.

Along with addressing the aforementioned materials issues, the development direction of polymeric SLICs should be focused on elaborate combination with battery systems, including electrode design compatible with polymeric SLICs, engineering of electrode/electrolyte interfaces and full-cell fabrication aiming at higher volumetric/gravimetric energy densities. The challenges and future prospects of the polymeric SLICs described above are schematically illustrated in Fig. 10b, in terms of materials design and battery system technology. Benefiting from their unique electrochemical/physicochemical features, which are difficult to achieve with conventional liquid and solid inorganic electrolytes, polymeric SLICs are anticipated to play a viable role as a new electrolyte strategy for next-generation Li battery systems that can expedite the advent of the so-called “Battery of Things (BoT)” era. In addition, the comprehensive information and discussion on the polymeric SLICs reviewed herein provide useful guidance and insights for the development of new solid-state electrolytes based on non-lithium as well as lithium electrochemistry.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the Basic Science Research Program (2017M1A2A2087810, 2018R1A2A1A05019733, 2018M3D1A1058624 and 2017R1D1A1B03033699) and Wearable Platform Materials Technology Center (2016R1A5A1009926) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education and the Ministry of Science, ICT and Future Planning.


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Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta09056d
These authors contributed equally to this work.

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