Super Soft All-Ethylene Oxide Polymer Electrolyte for Safe All-Solid Lithium Batteries
Here we demonstrate that by regulating the mobility of classic −EO− based backbones, an innovative polymer electrolyte system can be architectured. This polymer electrolyte allows the construction of all solid lithium-based polymer cells having outstanding cycling behaviour in terms of rate capability and stability over a wide range of operating temperatures. Polymer electrolytes are obtained by UV-induced (co)polymerization, which promotes an effective interlinking between the polyethylene
... e (PEO) chains plasticized by tetraglyme at various lithium salt concentrations. The polymer networks exhibit sterling mechanical robustness, high flexibility, homogeneous and highly amorphous characteristics. Ambient temperature ionic conductivity values exceeding 0.1 mS cm −1 are obtained, along with a wide electrochemical stability window (>5 V vs. Li/Li + ), excellent lithium ion transference number (>0.6) as well as interfacial stability. Moreover, the efficacious resistance to lithium dendrite nucleation and growth postulates the implementation of these polymer electrolytes in next generation of all-solid Li-metal batteries working at ambient conditions. Present energy storage and production devices are based on combustible organic solvents that carry the risks of leakage and related fire hazards. This forces the manufacturer to enclose battery components in heavier and peculiar packaging structures to meet the stringent safety requisites. Such heavy protective packaging reduces the overall amount of ready-to-use energy (energy density). An all-solid construction will certainly enhance the overall performance of any energy storage and conversion devices that can be thought of 1 . An ideal ion conducting polymer electrolyte for ambient temperature energy storage has been the dream pursued by many researchers 2-4 . An enhanced safety may be achieved by complete replacement of organic carbonate-based liquid electrolytes, thus leading to facile and leak-free fabrication, flexibility, compactness, reduced weight and laminated structures 5 . Scientific community has been focusing on polymer systems containing -EO-moieties for an all-solid cell construction, and other polymers (e.g., PMMA, PVdF) in mostly hybrid/ gel configurations 6,7 . It is well known that polyethylene oxide (PEO) crystallizes at about 55 °C 8,9 and ionic as well as segmental mobility are limited to its molten (amorphous) state at elevated temperatures. This impedes its widespread application into the global market 10 . However, at elevated temperatures such polymers lose their dimensional stability being in molten state, which leads to non-homogeneity inside the cell 11 . This facilitates the diffusion of Li + ions through localized favourable paths, increasing both concentration gradients and defects that may create temperature deflections and short-circuits. Attempts have been also made to reduce the crystallinity to retain the amorphicity by incorporating various ceramics/metal oxides (e.g., Al 2 O 3 , CeO 2 , ZrO 2 , TiO 2 ) 12 , but the target remains unachieved so far. Another problem is the solvent-based preparation procedure, where complete and "concrete" solvent removal is a herculean task; indeed, in most cases the unavoidable traces of solvent persisting in the polymer matrix create various interfacial stability issues, even enabling thermal runaway reactions. Four decades after the discovery of ionic conduction in polymer electrolytes by Fenton et al. 13 and Armand et al. 14 , the topic remains red hot. Moreover, the all-solid-state Li-metal battery concept is still appealing due to assured high energy density 15 . A polymer electrolyte that is less reactive towards Li-metal and highly resistant to dendrite formation/penetration would potentially open up the possibility of Li-metal based accumulators to entry in the market. After decades of efforts, PEO matrix still struggles to meet the requirements of global market due to its low ambient temperature conductivity and inferior safety arising from non-uniform plating/stripping of Li + ions, which often results in hazardous dendrite formation. A compromise between the characteristics of an all-solid state 16 and a gel-like 17 polymer electrolyte might represent the vital knot that researchers need to accomplish the goal. In this direction, remarkable results are reported in literature with the addition of room temperature ionic liquids 18 into thermoplastic 19 or thermoset 20 polymer matrix. However, the problems such as restricted Li + ion diffusion, cost and rate capabilities become the hurdle towards the commercialisation of such materials. Thus, slowly and steadily the scientific community is moving towards the addition of noncarbonated high boiling, thermally stable organic solvents such as organic nitriles 21 , glymes 22 , etc. Moreover, in the case of thermoplastic polymers the separator size deformation with varying temperature is a tough issue, while thermosets are limited by unsuitable ionic conductivity and brittleness of the polymer matrix. Glymes (various lengths) are well known for complexing with metal-ions through their multiple ether-like oxygen atoms 23 . When lithium salt is dissolved in glyme-based solvents, they show promising ionic conductivity and Li + ion transport properties 24, 25 . Due to the excellent properties imparted by glymes in the liquid electrolyte, recently they have received plenty of attention for next-gen systems beyond Li-ion, such as lithium sulphur 2-4 and lithium air rechargeable batteries    . Free radical photopolymerisation is a low cost, solvent-free and energy saving technique very well established for many applications in an easily implemented and versatile fashion 29-31 . Photopolymerisation can be suitably adapted to the preparation of polymer electrolytes due to its eco-friendliness, which is a key aspect that influences the fate of large-scale polymer electrolyte manufacturing 32 . Moreover, UV-induced reaction on multifunctional monomers permits rapid in situ generation of intimate electrode/electrolyte interfaces, which currently represent a major striking point to be fixed in the field of electrochemical devices 33 . Thus, the whole electrolyte preparation can be carried out in the absence of solvent. Most of the systems referring to glyme-based electrolytes are either blended with thermoplastic materials or directly used in their liquid form. Little work 34, 35 has been devoted to study their possible implementation in a self-standing, softly cross-linked thermoplastic polymer matrix. In the present work, we use a system based on PEO and tetraglyme, and we directly cross-link it in one-pot along with the supporting lithium salt under UV irradiation to retain the solid-like nature and dimensional stability. By concurrent exploitation of photo-induced cross-linking and in situ functionalization procedures, kinetically driven inhibition of the PEO chains crystallization is readily achievable at ambient conditions, leading to polymer electrolytes that possess solid-like properties without hampering ionic mobility. They are prepared by mixing PEO as the polymer matrix, bis[2-(2-methoxyethoxy)ethyl]ether (tetraglyme, TEGDME) as the active plasticizer, lithium bistrifluoromethane sulfonimide (LiTFSI) as the source of Li + ions and 4-methyl benzophenone (MBP) as the light-induced hydrogen abstraction mediator (photoinitiator). Under UV excitation, MBP abstracts an acidic proton from a methylene group and generates a free radical chain 36,37 . This free radical can combine with another free radical belonging to the same chain or other -EO-chains to interlink themselves. Tetraglyme also possesses methylene groups that can undergo hydrogen abstraction and following inter-radical reactions to form oligomers, or bond to the adjacent PEO chains. The final interlinked solid polymer electrolyte (ISPE) films are mechanically robust, highly flexible, homogeneous and largely amorphous. They also exhibit excellent properties in terms of compatibility with the lithium metal electrode and suppression of hazardous dendrite growth. The sum of these characteristics enlighten the striking prospects of the newly developed ISPE as electrolyte separators in both Li-ion and Li metal batteries conceived for high energy and/or power demanding applications, including hybrid vehicles and smart grid storage systems.