(Oxalato)borate: The key ingredient for polyethylene oxide based composite electrolyte to achieve ultra-stable performance of high voltage solid-state LiNi_0.8Co_0.1Mn_0.1O₂/lithium metal battery

Searching novel polyethylene oxide (PEO) based composite electrolyte that can be compatible with high voltage cathodes (> 4 V) at room temperature is a long-standing challenge, but essential to achieve high-energy-density for solid-state lithium (Li) metal batteries. Herein, we develop an advanced dual-salts reinforced PEO/garnet solid-state composite electrolyte (SCE) with high ionic conductivity of 9 × 10⁻⁴ S cm⁻¹ and an electrochemical stability window up to ~ 4.7 V, where the lithium bis(oxalate)borate (LiBOB) is used as additive with LiClO₄. The dual-salts reinforced SCE can achieve excellent room temperature cycling performance of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811)/Li metal batteries. The multiplied polymer-ion interactions and formation of Li‒B‒O/C‒O species originated from the (oxalato)borate group of LiBOB effectively promotes the electrochemical stability of the PEO based composite electrolyte and its wetting properties with electrodes. Unwanted non-conductive decomposition products, such as Li₂O and Li₂CO₃, and NCM particles cracking during cycling are successfully suppressed. The NCM811 solid-state cells using the optimized dual-salts composite electrolyte delivers high specific capacity of 190 mA h g⁻¹ and exhibit stable cycling performance for more than 200 cycles at 25 °C.