Extreme Defect Tolerance for Electrochemical Intercalation in Wadsley–Roth Structures Demonstrated by Metastable NaNb ₇ O ₁₈

Careful control over the defect chemistry of crystalline compounds is typically critical to transport phenomena (ion, electron, phonon). In one-dimensional (1D) ion conductors, even small concentrations of defects in the diffusion pathway can be pernicious. Wadsley-Roth block phases are 1D ion conductors with high redox capacity and among the highest diffusivity of any battery electrode materials. The origin of their high-rate transport has been attributed to parallel tunnels that could impart defect tolerance, but direct evidence is limited. Herein, a new lithium-ion battery negative electrode material, NaNb₇O₁₈, is described that exhibits extreme defect tolerance. Multimodal characterization combining neutron diffraction, ²³Na solid-state NMR spectroscopy, and DFT calculations reveals that more than half of the Na⁺ in NaNb₇O₁₈ is in diffusion tunnel-blocking cuboctahedral environments (similar to the perovskite A-site). Despite the high point-defect concentration, NaNb₇O₁₈ can reversibly lithiate to Li₇NaNb₇O₁₈ and cycle 200 mAh·g^-1 in 10 h and 100 mAh·g^-1 in 3 min in large 4-23 μm (D₁₀-D₉₀) particles. <i>Operando</i> synchrotron diffraction shows an anisotropic and asymmetric lithiation/delithiation process with two first-order phase transitions including one with nearly zero volume change. Li_<i>x</i>NaNb₇O₁₈ also exhibits evidence for reversible Nb-Nb bond formation. From the same <i>operando</i> diffraction measurements, Nb-Nb distances between edge-sharing octahedra at the block peripheries vary from <i>ca</i>. 3.4 to 2.8 and back to 3.4 Å over one lithium insertion/extraction cycle, in quantitative agreement with the Nb-Nb bond formation charge storage mechanism recently proposed from the computational lithiation of several different Wadsley-Roth compounds.