Extreme Defect Tolerance for Electrochemical Intercalation in Wadsley–Roth Structures Demonstrated by Metastable NaNb7O18

Careful control over the defect chemistry of crystalline compounds is typically critical to transport phenomena (ion, electron, phonon, etc.). In battery materials, particularly in one-dimensional (1D) ion conductors, even small concentrations of defects in the diffusion pathway can be pernicious. Wadsley–Roth block phases are interesting as 1D ion conductors that possess high (multi-electron) redox capacity and amongst the highest ionic diffusion coefficients of any electrode materials. The origin of their high-rate transport has been heavily studied and attributed, in part, to the existence of parallel one-dimensional long-range diffusion pathways that ions can interchange between with low activation barriers. These parallel channels could impart defect tolerance to this 1D conductor, but no direct evidence for this behavior has been reported. 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) within Wadsley–Roth-like blocks of octahedra. Despite the high point defect concentration, NaNb7O18 can reversibly lithiate to Li7NaNb7O18 and cycle 200 mAh g–1. There is still 100 mAh g–1 accessible capacity in 3 mins in large 4–23 µm (D10–D90) particles. Operando synchrotron diffraction shows an asymmetric lithiation/delithiation process with two first-order phase transitions including one with nearly zero volume change. LixNaNb7O18 also exhibits evidence for reversible Nb–Nb bond formation. From the same operando diffraction measurements, the Nb–Nb distances of niobium atoms in edge-sharing octahedra at the block peripheries vary from ca. 3.4 Å to 2.8 Å back to 3.4 Å over one lithium insertion/extraction cycle, which is in quantitative agreement with the Nb–Nb bond formation charge storage mechanism recently proposed from the computational lithiation of several different Wadsley–Roth compounds. Experimental determination of the defect chemistry of NaNb7O18 and its (i) tolerance to tunnel-blocking defects, (ii) two-phase structure evolution, and (iii) reversible Nb–Nb bond formation during intercalation are presented. These observations not only serve to advance our understanding of ion transport and charge storage in the large family of high-rate Wadsley–Roth compounds but provide new insights into the design criteria for charge transport and charge storage in high-performance mixed ionic–electronic conductors.