On-demand capacity extraction through thermo-electrochemical hysteresis in metastable Li-Cu alloy

Metastable states provide new opportunities to access functional materials inaccessible through conventional equilibrium pathways by employing path-dependency. In lithium (Li) batteries, irreversible loss of Li at the anode caused by its high reactivity is a crucial factor of capacity decay. While prelithiation can compensate for initial losses, safe storage at later cycles and supplementation upon demand during the life of batteries is still limited. Herein, we introduce a strategy of storing Li as Li-Cu alloy, a metastable phase with characteristic thermo-electrochemical hysteresis. Synthesize by a thermal reaction at 400 o C, Li-Cu current collector (CC) demonstrates capacity extraction of lithium ions up to 2 mAh cm -2 areal capacity on demand at any stage for extended cycle life in both LMBs and LIBs. This functionality is based on two key features - 1) the phase is only accessible through thermal process, inhibiting electrochemical re-alloying with Cu during cycling; and 2) the high extraction potential (1.2V versus Li/Li + ) allows reservoir to remain inactive within conventional operating window until intentionally activated by a designed protocol. We demonstrate that accessing desirable metastable phase through hysteretic pathways transforms CCs into Li reservoir with on demand capability, offering new design platform for high energy density batteries.