Electrochemical Lithium Insertion and Migration Energetics in Wadsley-Roth Molybdenum-Based Oxides

Decreasing the operating temperature of lithium-ion batteries would facilitate their application in low temperature environments. One way to do this is to utilize electrode materials that exhibit high power capability at room temperature. Many of the Wadsley-Roth (WR) family of transition metal oxides show high capacities with long cycling lifetimes and minimal structural deformation at room temperature, rendering them promising for low temperature operation. These oxides are typically formed from the solid-solution of Nb 2 O 5 with other highly oxidized metal oxides such as MoO 3 , WO 3 , and TiO 2 and contain periodic crystallographic shear defects which create m x n blocks of corner-connected octahedral units. Here, we investigated the energetics of electrochemical Li-ion insertion into two structurally related Wadsley-Roth phases, Nb 12 MoO 33 and Nb 14 Mo 3 O 44 , which have a 3 x 4 and 4 x 4 block structure, respectively. Galvanostatic charge-discharge experiments show that Nb 14 Mo 3 O 44 has a higher capacity (235 mAh/g or 1.14 Li+/transition metal) at slower (dis)charge rates (0.1 C) compared to Nb 12 MoO 33 (156 mAh/g or 0.79 Li+/transition metal). However, Nb 12 MoO 33 has better capacity retention at faster rates (2 C and 5 C). Similarly, upon extended cycling (500 cycles) at a rate of 1 C, Nb 12 MoO 33 retains over 80% of its initial capacity whereas Nb 14 Mo 3 O 44 only retains 62%. We further probed the energetics of electrochemical Li+ (de)intercalation with variable-temperature galvanostatic intermittent titration technique (VT-GITT). Nb 12 MoO 33 displays a larger activation energy for Li-insertion (0.033(2) eV) compared to Nb 14 Mo 3 O 44 (0.02(1) eV), suggesting more facile diffusion into the larger block size structure. Upon electrochemical Li+ de-insertion, both materials display similar activation energies of 0.020(4) eV and 0.02(1) eV, for Nb 12 MoO 33 and Nb 14 Mo 3 O 44 , respectively. The similar activation energies upon electrochemical Li+ insertion and de-insertion displayed by Nb 14 Mo 3 O 44 suggests no thermodynamic driving force resulting in a significantly decreased capacity at faster rates.