2,2,6,6-Tetramethylpiperidine-1-oxyl-Oxidized Cellulose Nanofiber-Embedded Polymer Electrolytes for All-Solid-State Lithium Metal Batteries

Due to the rapidly increasing demands of lithium-ion batteries, it is imperative to develop separators and even solid-state electrolytes with high biocompatibility/biodegradability to increase their sustainable economic values. In this context, we develop a series of 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofiber (TOCN)-embedded solid polymer electrolytes (SPEs) through an environmentally friendly one-pot, organic solvent-free manufacturing process, for which the natural TOCN acts as a rigid 3D skeleton and polyethylene glycol (PEG3350) acts as the soft matrix. Unlike typical inorganic fillers (e.g., Al2O3), the addition of a small amount of TOCN into PEG allows the formation of free-standing SPE films. In addition to greatly improving the mechanical strength, the introduced TOCN preserves sufficient ion transport channels. The optimized free-standing TP28 SPE membrane possesses a decent ionic conductivity of 4.89 × 10–4 S cm–1 associated with a tLi+ value of 0.31 at 80 °C, a high electrochemical stability window of >4.0 V, and a high tensile strength of 1.10 MPa. It also exhibits effective functionality and compatibility with the Li-metal (Li-symmetric cell cycling over 900 h at 80 °C) anode and the LiFePO4 cathode. The assembled all-solid-state LiFePO4|TP28|Li battery delivers great rate capability and cycling performance (96.5% after 100 cycles) with an initial capacity of 151 mAh g–1 at 0.1 C at 60 °C. Our results demonstrate the promising potential of lignocellulosic biomass for fabricating biocompatible SPEs for Li-metal batteries that can largely increase their sustainable economic values.