Interfacially stable and high-safety lithium batteries enabled by porosity engineering toward cellulose separators

Nanocellulose has been certified as a promising candidate to substitute the plastic-based materials applied in energy storage devices, such as lithium-ion batteries (LIBs) and lithium-sulfur batteries, due to its biodegradability, high heat resistance, strong mechanical robustness, and favorable wettability. However, the nanocellulose separators suffer from compact microstructures due to the hydrogen bonding and van der Waals force between cellulose fibrils, resulting in obstacles for electrolytes uptaking and Li + migrating. A facile approach named as porosity engineering that combines filtration and templating methods is proposed to fabricate cellulose separators with tunable pore size and porosity. Based on the elaborate structure design, the effect of pore size and porosity on separator and battery performances are investigated. The experimental and numerical results show that nanopores are conducive to the uniform evolution of the electrode-electrolyte interface (EEI). The physical and electrochemical measurements suggest that the separators with the proper porosity can retain high ionic conductivity and mechanical strength. Consequently, the nanoporous separator with the porosity of 61.5% (NCSP-2) is considered as the suitable option for LIBs, delivering stable Li depositing/stripping cycling for over 800 h in symmetrical Li/Li cells and high capacity retention of 91.3% after 400 cycles at 1C in full LiFePO 4 /Li cells. In this work, a facile approach, named porosity engineering or structure design, was demonstrated to fabricate cellulose separators with controllable pore size and porosity. Based on the investigations of the porous features, a separator with all-around performances, including biodegradability, high thermal stability, compatible interface, high ionic conductivity, and mechanical robustness, was obtained and correspondingly endowed lithium-ion batteries with interfacial stability, long lifespan and improved safety. Apart from experimental studies, the phase-field equation was employed to simulate the 2D morphological evolution of the electrode-electrolyte interphase influenced by the pore structure of separators. Accordingly, the critical role of nanosized pores in suppressing the lithium dendrites was directly clarified by the computational results concerning electrochemical kinetics and distribution of Li + concentration, which is more visual than the qualitative explanation. • Cellulose separators with controllable porosity and pore size were prepare. • The effects of porosity and pore size on separator performances were investigated. • The nanopores of separator were conducive to stablizing EEI evolution. • The as-designed separator delivered good cycling performance and rate capatibility.