Operation Mechanism in Hybrid Mg–Li Batteries with TiNb₂O₇ Allowing Stable High-Rate Cycling

We studied the structural evolution and cycling behavior of TiNb2O7 (TNO) as a cathode in a nonaqueous hybrid dual-salt Mg–Li battery. A very high fraction of pseudocapacitive contribution to the overall specific capacity makes the material suitable for ultrafast operation in a hybrid battery, composed of a Mg-metal anode, and a dual-salt APC–LiCl electrolyte with Li and Mg cations. Theoretical calculations show that Li intercalation is predominant over Mg intercalation into the TNO in a dual-salt electrolyte with Mg2+ and Li+, while experimentally up to 20% Mg cointercalation was observed after battery discharge. In hybrid Mg–Li batteries, TNO shows capacities which are about 40 mA h g–1 lower than in single-ion Li batteries at current densities of up to 1.2 A g–1. This is likely due to a partial Mg cointercalation or/and location of Li cations on alternative crystallographic sites in the TNO structure in comparison to the Li-intercalation process in Li batteries. Generally, hybrid Mg–Li cells show a markedly superior applicability for a very prolonged operation (above 1000 cycles) with 100% Coulombic efficiency and a capacity retention higher than 95% in comparison to conventional Li batteries with TNO after being cycled either under a low (7.75 mA g–1) or high (1.55 A g–1) current density. The better long-term behavior of the hybrid Mg–Li batteries with TNO is especially pronounced at 60 °C. The reasons for this are an appropriate cathode electrolyte interface containing MgCl2 species and a superior performance of the Mg anode in APC–LiCl electrolytes with a dendrite-free, fast Mg deposition/stripping. This stable interface stands in contrast to the anode electrolyte interface in Li batteries with a Li anode in conventional carbonate-containing electrolytes, which is prone to dendrite formation, thus leading to a battery shortcut.