Fast Charging from Low Li-Ion Migration Barriers in Wadsley–Roth NaNb7O18 Anodes

While current electric vehicles are approaching internal combustion engine vehicles in terms of driving range, the relatively long charging time of batteries represents a fundamental challenge. Materials used as anodes show slow ion insertion, which is usually responsible for the inability of automotive batteries to charge rapidly. To address this challenge, research into the kinetics of solid-state ion insertion is needed. The essential properties of fast-charging electrodes include high electronic and ionic conductivities, mechanical and chemical stability, and a 3D framework with channels for ion transport, especially when the added cost of nanostructuring is not desirable. In recent years, there has been increasing recognition that Nb-based shear-structured oxides, many belonging to the Wadsley–Roth class of compounds, show fast insertion. We focus here on NaNb₇O₁₈, a member of this Wadsley–Roth family that has not been previously studied as an anode material for Li-ion batteries. Bulk NaNb₇O₁₈ is shown to demonstrate high cyclability, retaining over 90% capacity even after 1000 cycles at a relatively rapid 2C rate. Potentiometric entropy measurements support the presence of two-phase reaction mechanisms (which is usually contraindicated for fast charging) and point to the role of intralayer ion ordering. The energy barrier between Li sites is found to be low, which is likely to be an important contributor to the fast lithiation kinetics in this compound. The electrochemical analysis points to apparent diffusion coefficients in the range of 10^–12 cm² s^–1 and a low overpotential close to 130 mV. An analysis of the lithiation kinetics of related Wadsley–Roth compounds finds that fast intercalation/deintercalation is robust across this family of compounds, regardless of the details of the intercalation mechanism.