Role of Temperature on Electrochemical Li⁺ Insertion into Wadsley-Roth Niobium Tungsten Oxides

The demand for energy storage materials for space-limited and low temperature applications motivates the search for electrode materials that have high volumetric capacities and fast kinetics. One strategy to achieve this is through the use of insertion-type electrodes that can undergo at least 1 e - transfer per formula unit (for high volumetric capacity) with little structural change (for fast kinetics). Such behavior is possible during electrochemical Li + insertion into niobium tungsten oxide Wadsley-Roth phases. The Wadsley-Roth phases contain orthogonal crystallographic shear planes, which are planes of edge-sharing MO 6 octahedra, that form a m x n block structure around a corner-sharing network. In this study, we directly compared three different Wadsley-Roth compounds: Nb 12 WO 33 (3 x 4), Nb 14 W 3 O 44 (4 x 4), Nb 16 W 5 O 55 (4 x 5). We hypothesized that the presence of crystallographic shear planes will increase electrochemical ion insertion kinetics because their presence would decrease the lattice flexibility of the material by decreasing the degrees of freedom available for structural deformation. To test this hypothesis, we performed galvanostatic charge discharge (GCD), galvanostatic intermittent titration technique (GITT), and potentiostatic electrochemical impedance spectroscopy (pEIS) in a non-aqueous electrolyte at 25°C and at 0°C to study the kinetics of electrochemical Li + insertion into different block size Wadsley-Roth compounds. At 25°C, all three Wadsley-Roth compounds showed capacities of > 1 e - ,Li + per transition metal (T. M.) with the largest block size Nb 16 W 5 O 55 (4 x 5) inserting 1.2 Li + / T.M. (volumetric capacity of 1200 mAh/cm 3 ). Rate capability measurements at 25°C show increased capacity for larger block sizes at slow (dis)charge rates and improved rate capability and maximum capacity in Nb 14 W 3 O 44 which contains a symmetric 4 x 4 block size. Rate capability measurements taken at 0°C show that all three Wadsley-Roth compounds possess a lower maximum capacity compared to those at 25°C and that larger block size compounds possess improved rate capabilities independent of block symmetry. Diffusivity measurements conducted using GITT show that all three compounds possess Li + diffusion rates (D Li+ ) between 10 -10 cm 2 /s to 10 -9 cm 2 /s with little dependence on block size. We used variable temperature powder X-ray diffraction to investigate the temperature dependent structural dynamics of these Wadsley-Roth compounds. Upon heating, the materials show negative thermal expansion (NTE), with Nb 16 W 5 O 55 exhibiting a volumetric contraction of 1.6% when heated to 600°C. The relationships between the atypical NTE behavior of these Wadsley-Roth oxides and their electrochemical cycling behavior are discussed here for the first time.