Ionic Liquid/Poly(ionic liquid)-based Semi-solid State Electrolytes for Lithium-ion Batteries

Ionic liquids (ILs) have appeared as the most promising electrolytes for lithium-ion batteries, owing to their unique high ionic conductivity, chemical stability and thermal stability properties. Poly(ionic liquid)s (PILs) with both IL-like characteristic and polymer structure are emerging as an alternative of traditional electrolyte. In this review, recent progresses on the applications of IL/PIL-based semi-solid state electrolytes, including gel electrolytes, ionic plastic crystal electrolytes, hybrid electrolytes and single-ion conducting electrolytes for lithium-ion batteries are discussed.


INTRODUCTION
With the continuous advancement of science and technology, the accompanying wearable and portable devices provide convenience for people′s daily lives. Therefore, high energy density and environmentally friendly energy storage devices are highly desirable. [1,2] Lithium-ion batteries have been regarded as promising candidates for energy storage and conversion due to their lightweight, high energy density, and good cycling stability. However, traditional lithium-ion batteries based on organic liquid electrolytes usually suffer from potential fire and explosion hazards, [3−6] mainly due to the leakage of organic liquid electrolyte. Solid state electrolytes have been regarded as the alternatives for batteries. [7−9] Compared with liquid electrolytes, solid state electrolytes could overcome the leakage of liquid electrolyte, but they often exhibit relatively lower ionic conductivity and poor electrolyte/electrode interfacial contact. [10−13] Therefore, preparation of ionic liquid (IL) and poly(ionic liquid) (PIL)-based semi-solid state electrolytes for lithium-ion batteries has been extensively studied. [14−18] Ionic liquids (ILs) are molten organic salts with negligible vapor pressures, high ionic conductivity, thermal stabilities, and wide electrochemical window. [19] These properties enable their applications in the energy field, especially lithium-ion batteries. [20−22] On the other hand, poly(ionic liquid)s (PILs), which combine both the properties of ILs and mechanical durability of polymers, have also been widely used as semi-solid state electrolytes for energy devices. [23−26] This review focuses on IL/PIL-based semi-solid state electrolytes, including gel electrolytes, ionic plastic crystal electrolytes, hybrid electrolytes and single-ion conducting electrolytes for lithium-ion batteries.

IL-based Gel Electrolytes
The IL-based gels have been widely investigated as semi-solid electrolytes because of their relatively high ionic conductivity and low interfacial resistance. [27−29] For example, IL-based clickionogels have been recently synthesized. [30] The prepared ILbased click-ionogels exhibit high mechanical properties and resilience, high ionic conductivity, transparency, and nonflammability performance over a wide temperature range (−75 °C to 340 °C). In addition, the IL-based gel exhibits extraordinarily high elasticity and flexibility. The promising features of click-ionogels promote innovative practical applications in flexible, ultra-thinning and safe devices. [30,31] Guo and co-workers reported the preparation of IL-based polymer gel electrolyte using core-shell structured SiO 2 nanoparticles as functional fillers. [32] The shell layers of SiO 2 -PAA@Li increased both lithium transference number and ionic conductivity of the electrolyte. The silica core structure improved thermal stability and increased compatibility between electrolyte and lithium electrode. The lithium dendrites growth during cycling could be inhibited by such a stable solid state electrolyte. The gel polymer electrolytes exhibited high ionic conductivity of 0.74 × 10 −3 S·cm −1 at ambient temperature. The fabricated cell showed an initial specific capacity of 138 mA·h·g −1 at 0.05 C and only 13% capacity loss after 100 cycles.
Poly(vinylidenefluoride-co-hexafluoropropene) (PVDF-HFP) with excellent electrochemical stability, easy thin film forming ability, and high thermal stability has been studied as potential matrices for gel electrolytes. Zhu et al. reported the preparation of a flexible thin-film IL-based gel electrolyte, composed of PVDF-HFP and a room temperature IL, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) (Fig. 1). [33] The prepared gel electrolyte exhibited good thermal stability without weight loss up to 370 °C and relatively high ionic conductivity. The fabricated battery showed perfect flexibility and stable energy delivery capability. Singh et al. also prepared an IL-based gel electrolyte containing PVDF-HFP and imidazolium-type ILs. [34] The prepared electrolyte showed good electrolyte/lithium electrode interfacial contact and high coulomb efficiency. Zhang et al. prepared ILdoped gel polymer electrolytes for flexible lithium-ion polymer battery. [35] The fabricated cells exhibited perfect flexibility by bending easily without breaking and good discharge capacity.

Ionic Plastic Crystals Electrolytes
Ionic plastic crystals are compounds composed of organic molecules exhibiting disorder while reserving high diffusivity and plasticity. [36] The intrinsic solid-solid phase transitions below their melting point endow organic plastic crystals with good mechanical flexibility and plastic properties. [37] Therefore, they can be easily deformed without fracture under an applied force.
The pioneering application of ionic plastic crystals as solid state electrolytes was reported by Angell in 1986. [38] The prepared electrolyte exhibited a small entropy of fusion at −90 °C and high conductivity at 75 °C. In 2011, MacFarlane et al. prepared pyrrolidinium salt-based ionic plastic crystal electrolyte for rechargeable lithium-ion batteries. [39] The prepared cell exhibited good electrolyte/lithium electrode interfacial contact, and high performance even after 50 cycles (about 110 mA·h·g −1 at 0.2 C). An pyrazolium type ionic plastic crystal, N,N-diethyl-3-methylpyrazolium bis-(trifluoromethanesulfonyl)imide (DEMPyr 123 ), was prepared by Armand et al. [40] When 10 mol% lithium bis(trifluoromethylsulfonyl)imide (LiT-FSI) salt was added to DEMPyr 123 , the electrolyte showed an ionic conductivity of 1.7 × 10 −3 S·cm −1 at 20 °C and a wide electrochemical stability window of 5.5 V. Ionic plastic crystals based on N,N-dimethylpyrrolidinium cation have also been reported. [41,42] However, relatively narrow electrochemical window (−3.0 V to +2.3 V) hindered their future applications in lithium-ion batteries. Howlett and co-workers reported a novel ionic plastic crystal electrolyte consisting of polyethylene separator and 4 mol% lithium bis(fluorosulfonyl)imide (LiFSI) doped triisobutylmethyl phosphonium bis(fluorosulfonyl)imide (P 1444 FSI), which exhibited a high electrochemical window (~6 V). [43] Two phase transition temperatures (from 8 °C to 24 °C and from 24 °C to 36 °C) indicate they are typical plastic crystals (Fig. 2). The fabricated electrolyte showed a high conductivity (0.26 × 10 −3 S·cm −1 ) and good discharge capacity (160 mA·h·g −1 ) at 22 °C. Recently, Pringle et al. reported an organic ionic plastic crystal electrolyte containing 90 mol% lithium bis(fluorosulfonyl)imide and 10 mol% pyrrolidinium bis(fluorosulfonyl)imide. [44] The resultant quasi-solid state electrolyte exhibited a conductivity of 0.24 × 10 −3 S·cm −1 at 30 °C, and a lithium ion transference number of 0.68. This result showed a new approach for designing safer quasi-solid state electrolytes with good electrochemical and transport properties.

PIL-based Gel Electrolytes
PIL-based gel electrolytes have been extensively studied because of their manufactural feasibility, high conductivity, and thermal stability. [45,46] Generally, PILs exhibit good physicochemical compatibility with ILs and ensure the long-term stability of the ternary lithium salt, exhibiting improved electrochemical performances and would be of great interest for developing high performance lithium-ion batteries.
In 2009, PIL-based gel electrolytes containing poly(diallyldimethylammonium) backbone were reported by Mecerreyes et al. [47] and Appetecchi et al. [48] The prepared PIL showed high chemical stability after contact with lithium anode, as well as acceptable room temperature conductivity and wide electrochemical stability window (about 7.0 V). Preliminary battery tests showed that Li/LiFePO 4 solid-state cells could deliver about 140 mA·h·g −1 at 40 °C. Yang et al. prepared a class of solid state polymer electrolytes via dipping the PIL matrix with organic ionic plastic crystal, [49] N-ethyl-Nmethylpyrrolidinium bis(fluorosulfonyl)imide (P 12 FSI) (Fig. 3). The prepared PIL-P 12 FSI-LiTFSI electrolyte revealed flexible mechanical characters, broad operating temperature range (25-80 °C), as well as potential to suppress the lithium dendrite growth.
Other than cationic PILs, polyzwitterion and polyanion have also been employed as polymer matrices for gel polymer eleytrolytes. An imidazolium type dicationic PIL, poly(N,N,N-trimethyl-N-(1-vinlyimidazolium-3-ethyl)-ammonium bis(trifluoromethanesulfonyl)imide) was synthesized by Yin and co-workers. [25] The electrolyte showed low glass transition temperatures at around 54 °C, high thermal stability to about 330 °C, as well as high ion conductivity at around 10 −4 S·cm −1 at low medium temperatures. The excellent battery performance promotes the application of dicationic PIL-    based electrolytes for lithium metal batteries.
Zwitterionic compounds (cationic and anionic charges are immobilized on the same molecule) are extremely effective Li-ion dissociation enhancers. [50,51] Zheng et al. prepared a flexible polymer electrolyte film based on lithium-containing zwitterionic PIL. [52] The lithium ion mobility of the polymer electrolytes can be improved owing to the Li + dissociation generated by the zwitterionic effect. Peng et al. reported the preparation of a liquid-like polyanionic electrolytes. [53] The Li/LiFePO 4 cell equipped with anionic PIL electrolyte performed high capacity, outstanding cycling stability and rate capability. The excellent performances of batteries may be due to the formation of stable solid-electrolyte interface film at the lithium-electrolyte interface during cycling.
PIL-based hybrid electrolytes where ionic moieties are chemically bound to the inorganic particles/substrates have been recently studied. Zhang et al. reported a high-performance PIL/silica hybrid ionogel electrolytes with LiTFSI/IL as the ion conducting phase. [54] The electrolyte promoted a uniform Li electrodeposition and suppressed the growth and proliferation of lithium dendrites, which allowed the stable cycling of the Li symmetric cell for more than 500 h (0.1 mA·cm −2 ).
Similar work was recently reported by Ye and co-workers (Fig. 4). [55] The ion transport properties of PIL-based polymer electrolytes could be enhanced by the incorporation of 2D silica nanofillers, which provided continuous and interconnected ion transfer pathways. The hybrid electrolytes showed remarkably improved cycling performance and ionic conductivity (increased by 1130% at room temperature).
Moreover, PIL-based electrolyte could be used as interlayer for lithium-sulfur battery. PPy@PIL-PAN nanofibers prepared via electrospinning were used as interlayers for lithium-sulfur batteries. The PIL-based electrolyte exhibited good capacity of 844 mA·h·g −1 at 0.1 C and super absorbability of polysulfide.

Single-ion Conducting Electrolytes
When anion is bonded to the polymer backbone, the cation becomes the only free movable ion of electrolyte, and thus single-ion conducting electrolytes are formed. [57] Usually, lower glass transition temperature and higher polymer segment mobility can improve the ionic conductivity of single-ion conductors. [57] High ionic conductivity and lithium ion transference number of single-ion conducting electrolytes could suppress the growth of lithium dendrites on electrodes. [58] In 1984, Bannister et al. reported using anionic polymer to obtain single-ion conducting electrolytes. [59] The purpose was to restrict anion from combining with lithium ion and therefore accelerate the transfer of lithium ion. Mecerreyes et al. prepared a single-ion conducting triblock copolymer electrolyte, [60] consisting of PEO and poly(lithium 1-[3-(methacryloyloxy)propylsulfonyl]-1-(trifluoromethylsulfonyl)imide) blocks, poly(LiMTFSI). The fabricated triblock copolymer electrolyte exhibited low glass transition temperature (−55 °C to 7 °C) and high ionic conductivity (about 10 −4 S·cm −1 at 70 °C).
Recently, a novel PIL-based single-ion conducting electrolyte for solid-state lithium-ion batteries prepared by RAFT polymerization was reported by Shaplov and co-workers (Fig. 7a). [61] Single-ion conducting electrolytes could be used for solid-state lithium-ion batteries, exhibiting good conductivity of 2.3 × 10 -6 S·cm −1 at 25 °C and delivering high specific capacity to 130 mA·h·g -1 (Fig. 7b). Controlling the molecular structure by RAFT polymerization, PIL-based single-ion conducting electrolyte achieved high ionic conductivity and high electrochemical stability, suggesting promising candidates for semi-solid state lithium-ion batteries. [62] Zhou et al. presented a single-ion conducting electrolyte composed of LiPSsTFSI and poly(ethylene oxide), [63] which displayed low glass transition temperature of 44.3 °C and high Li-ion transference number, making it a promising semi-solid state electrolytes for lithium-ion batteries.

CONCLUSIONS
In summary, recent progresses on the application of IL/PILbased electrolytes, including gel electrolytes, ionic plastic crystal electrolytes, hybrid electrolytes and single-ion conducting electrolytes, for semi-solid state lithium batteries are reviewed. Since the liquid nature of ILs still limits their applications in battery due to leakage problems, growing attention has been paid to IL/PIL-based semi-solid state electrolytes. Although the conductivity of electrolyte and electrolyte/electrode interface resistance should be improved, the IL/PIL-based semi-solid state electrolytes certainly have potential applications in flexible lithium batteries and wearable electronics.