A redox-active organic salt for safer Na-ion batteries

Overcharge abuse can trigger thermal runaway when a device is left unattended. Redox shuttles, as economic and efficient electrolyte additives, have been proven to provide reliable and reversible protection for state-of-the-art Li-ion batteries (LIBs) against overcharge. Here, a functional organic salt, trisaminocyclopropenium perchlorate (TAC•ClO₄), is developed and employed as a redox shuttle for overcharge protection in a Na-ion battery system. This type of novel redox shuttle molecule is reported for the first time. As a unique ionic compound with the smallest aromatic ring structure, TAC•ClO₄ exhibits distinctive attributes of fast diffusion, high solubility, and ultrahigh chemical/electrochemical stability in both redox states. With merely 0.1 M TAC•ClO₄ in electrolyte, Na₃V₂(PO₄)₃ cathode can carry overcharge current even up to 10C or 400% SOC. Na₃V₂(PO₄)₃/hard carbon cells demonstrated strong anti-overcharging ability of 176 cycles at 0.5C rate and 54 cycles at 1C rate with 100% overcharge. Moreover, TAC•ClO₄ addition has little impact on the electrochemical performance of Na-ion batteries, especially on the rate performance and the initial Columbic efficiency. Interestingly, a unique and reversible electrochromic behavior of TAC•ClO₄ electrolyte can promptly provide the device an overcharge alarm under a designed potential to further enhance the safety level.