Operation Mechanism in Hybrid Mg–Li Batteries with TiNb<sub>2</sub>O<sub>7</sub> Allowing Stable High-Rate Cycling
Sebastian Maletti, Oleg Janson, Abraham Herzog‐Arbeitman, Ignacio Guillermo Gonzalez Martinez, Ronny Buckan, Johanna Fischer, Anatoliy Senyshyn, Alexander Missyul, Martin Etter, Daria Mikhailova
Abstract
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.