Transient batteries play a pivotal role in the development of fully autonomous transient devices, which are designed to degrade after a period of stable operation. Here, a new transient separator-electrolyte pair is introduced for lithium ion batteries. Cellulose nanocrystals (CNCs) are selectively located onto the nanopores of polyvinyl alcohol membranes, providing mobile ions to interact with the liquid electrolyte. After lithiation of CNCs, membranes with electrolyte uptake of 510 wt%, ionic conductivities of 3.077 mS center dot cm(-1), electrochemical stability of 5.5 V versus Li/Li+, and high Li+ transport numbers are achieved. Using an organic electrolyte, the separators enable stable Li metal deposition with no dendrite growth, delivering 94 mAh center dot g(-1) in Li/LiFePO4 cells at 100 mA center dot g(-1) after 200 cycles. To make the separator-electrolyte pair transient and non-toxic, the organic electrolyte is replaced by a biocompatible ionic liquid. As a proof of concept, a fully transient Li/V2O5 cell is assembled, delivering 55 mAh center dot g(-1) after 200 cycles at 100 mA center dot g(-1). Thanks to the reversible Li plating/stripping, dendrite growth suppression, capacity retention, and degradability, these materials hold a bright future in the uptake of circular economy concepts applied to the energy storage field.The authors gratefully acknowledge financial support from ETH Zurich (ETH Research Grant ETH-45 18-1). The authors thank Medicell Membranes Ltd. for kindly providing Visking dialysis membranes. The authors acknowledge support from the Scientific Center for Optical and Electron Microscopy (ScopeM) of ETH Zurich. The authors also thank Dr. Dipan Kundu for helpful discussions on transport number
