TiNb₂O₇ and VNb₉O₂₅ of ReO₃ Type in Hybrid Mg–Li Batteries: Electrochemical and Interfacial Insights

Abstract

As one of the beyond-lithium battery concepts, hybrid metal-ion batteries have aroused growing interest. Here, TiNb2O7 (TNO) and VNb9O25 (VNO) materials were prepared using a high-temperature solid-state synthesis and, for the first time, comprehensively examined in hybrid Mg–Li batteries. Both materials adopt ReO3-related structures differing in the interconnection of oxygen polyhedra and the resulting guest ion diffusion paths. We show applicability of the compounds in hybrid cells providing capacities comparable to those reached in Li-ion batteries (LIBs) at room temperature (220 mAh g–1 for TNO and 150 mAh g–1 for VNO, both at 0.1 C), their operability in the temperature range between −10 and 60 °C, and even better capacity retention than in pure LIBs, rendering this hybrid technology superior for long-term application. Post mortem X-ray photoelectron spectroscopy reveals a cathode–electrolyte interface as a key ingredient for providing excellent electrochemical stability of the hybrid battery. A significant contribution of the intercalation pseudocapacitance to charge storage was observed for both materials in Li- and Mg–Li batteries. However, the pseudocapacitive part is higher for TNO than for VNO, which correlates with structural distinctions, providing better accessibility of diffusion pathways for guest cations in TNO and, as a consequence, a higher ionic transport within the crystal structure.

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

“…As it has been already mentioned, our previous work on microscale TNO in hybrid Mg-Li batteries revealed a significant capacitive-like contribution of about 53 % at 0.01 mV s -1 to the charge storage mechanism in TNO, 19 similar to 50.9 % at 0.05 mV s -1 as reported for TNO with a particle size of about 40 nm in Li-batteries. 18 Generally, the capacity of an insertion electrode material always implies contributions from a non-Faradaic double-layer capacitance and a Faradaic pseudocapacitance, which reflects a redox process on the surface or surface-near region of the electrode without any structural changes in the bulk and without any limitations of the solid-state diffusion.…”

“…As we recognized previosly, 19 at a low current density, the long-term cycling of TNO in Mg-Li batteries results in a more stable behavior than in Li-batteries. Under these conditions, a smooth Li-deposition on the Li-foil without noticeable "dead" Li-dendrites must be expected for the Li-only control experiment.…”

“…Note, no formation cycles with a lower rate were performed prior to the main measurements. As a result, for both kind of batteries, a slight increasing of the capacity was observed for the first ten cycles, corresponding to the formation of a cathodeelectrolyte-interface (CEI) 19 , see Fig. 8a.…”

“…If the storage mechanism would be dominated by redox pseudocapacitance, then such a clear gradual redox of all involved metals could not be observed. Therefore, the capacitive contributions, obtained from sweep-dependent CV in both, Mg-Li and Li-cells on microscale TNO in previous work, 19 must be clearly considered as an intercalation pseudocapacitance, according to the definition of Augustyn et al 40 In the literature, a detailed structural analysis of Li-intercalated TNO including cation and anion sites, interatomic distances and occupancies of possible Li-positions is reported solely for two compositions Li2.67TiNb2O7 and Li3.33TiNb2O7, using ex situ neutron powder diffraction studies and first-principles calculations. 34 Other structural investigations were performed operando or ex situ using laboratory or synchrotron XRD experiments.…”

Mg-Li Hybrid Batteries: The Combination of Fast Kinetics and Reduced Overpotential

“…Wadsley-Roth phase oxide electrode materials for LIBs, such as Nb 2 O 5 , exhibit a larger pseudocapacitive contribution, thereby allowing rapid charging and discharging. Maletti et al evaluated the ability of TiNb 2 O 7 (TNO) and VNb 9 O 25 (VNO) in Mg-Li hybrid batteries [46]. Both the materials have ReO 3 -like structure with shear planes.…”