Self-recovery in Li-metal hybrid lithium-ion batteries via WO3 reduction

Abstract

It has been a challenge to use transition metal oxides as anode materials in Li-ion batteries due to their low electronic conductivity, poor rate capability and large volume change during charge/discharge processes. Here, we present the first demonstration of a unique self-recovery of capacity in transition metal oxide anodes. This was achieved by reducing tungsten trioxide (WO₃) via the incorporation of urea, followed by annealing in a nitrogen environment. The reduced WO₃ successfully self-retained the Li-ion cell capacity after undergoing a sharp decrease upon cycling. Significantly, the reduced WO₃ also exhibited excellent rate capability. The reduced WO₃ exhibited an interesting cycling phenomenon where the capacity was significantly self-recovered after an initial sharp decrease. The quick self-recoveries of 193.21%, 179.19% and 166.38% for the reduced WO₃ were observed at the 15^th (521.59/457.41 mA h g⁻¹), 36^th (538.49/536.61 mA h g⁻¹) and 45^th (555.39/555.39 mA h g⁻¹) cycles respectively compared to their respective preceding discharge capacity. This unique self-recovery phenomenon can be attributed to the lithium plating and conversion reaction which might be due to the activation of oxygen vacancies that act as defects which make the WO₃ electrode more electrochemically reactive with cycling. The reduced WO₃ exhibited a superior electrochemical performance with 959.1/638.9 mA h g⁻¹ (1^st cycle) and 558.68/550.23 mA h g⁻¹ (100^th cycle) vs. pristine WO₃ with 670.16/403.79 mA h g⁻¹ (1^st cycle) and 236.53/234.39 mA h g⁻¹ (100^th cycle) at a current density of 100 mA g⁻¹.