Crosslinked ionogels containing inorganic additives for safer lithium metal batteries

Lithium-ion batteries (LIBs) are a well-exploited technology for electrochemical energy storage since the 90s. LIBs are used in a wide range of devices and new applications, like electric vehicles and micro-mobility systems, are emerging in the last decade. Unfortunately, LIBs are expected to reach in next future theoretical limits in terms of gravimetric and volumetric density. Solid electrolytes (SEs) have been investigated as a replacement for liquid ones to increase energy density and cell safety of lithium metal batteries. Gel polymer electrolytes (GPEs), consisting of a polymer matrix, a lithium salt and a plasticizer, are promising electrolytes for LIBs with better conductivities than SEs. Conventional plasticizers utilized are organic solvents that are flammable with limited thermal stability, toxic and unstable at high potential, therefore limiting the use of low-potential cathodes. Ionic liquids (ILs) are low melting point salts that have been used as plasticizers in polymer matrices to form ionogels (IGs). The advantages of ILs compared to conventional plasticizers are their negligible vapor pressure, non-flammability, high thermal and chemical stability, wide electrochemical stability (ESW) and good ionic conductivity. However, Li+ mobility and rate performance are still a challenge due to high viscosities and ionic conductivities not comparable to conventional electrolytes. Active and passive ceramic filler can be added to polymer electrolytes to increase conductivity and mechanical properties. Al2O3, SiO2 and ZrO2 were used as passive fillers to decrease polymeric crystallinity and increase robustness of membranes. In this framework, a GPE for lithium metal batteries was developed with the idea of incorporating during free radical polymerization the IL as a plasticizer in a one-pot preparation method, without further preparation steps. The polymer backbone was prepared by the thermal reticulation of butyl methacrylate (BMA) with polyethylene glycol diacrylate (PEGDA) as a crosslinking agent. The addition of ILs during the polymerization enabled the preparation of a self-standing membrane with interesting ionic conductivity (10-3-10-4 S cm-1 at room temperature) and a wide electrochemical stability window (5-5.5 V vs Li+/Li). ZrO2 nanoparticles were added as filler to increase polymer electrolyte tenacity and favor the mobility of Li+, that in IGs tends to form ionic clusters tremendously decreasing the lithium transport number. Testing cells were assembled with lithium metal anode and LFP cathode to evaluate electrochemical performances during galvanostatic cycling under different C-rates. Developed membranes enabled to reach a specific capacity of 110-120 mAh g-1 at room temperature.