Utilizing the capacity below 0 V to maximize lithium storage of hard carbon anodes

Compared with conventional graphite anode, hard carbons have the potential to make reversible lithium storage below 0 V accessible due to the formation of dendrites is slow. However, under certain conditions of high currents and lithiation depths, the irreversible plated lithium occurs and then results in the capacity losses. Herein, we systematically explore the true reversibility of hard carbon anodes below 0 V. We identify the lithiation boundary parameters that control the reversible capacity of hard carbon anodes. When the boundary capacity is controlled below 400 mAh g−1 with current density below 50 mA g−1, no lithium dendrites are observed during the lithiation process. Compared with the discharge cut-off voltage to 0 V, this boundary provides a nearly twice reversible capacity with the capacity retention of 80% after 172 cycles. The results of characterization and finite element model reveal that the large reversible capacity below 0 V of hard carbon anodes is mainly benefited from the dual effect of lithium intercalation and reversible lithium film. After the lithium intercalation, the over-lithiation induces the quick growth of lithium dendrites, worsening the electrochemical irreversibility. This work enables insights of the potentially low-voltage performance of hard carbons in lithium-ion batteries.