Structural and electrochemical insights into novel Wadsley Roth Nb₇Ti_1.5Mo_1.5O₂₅ and Ta₇Ti_1.5Mo_1.5O₂₅ anodes for Li-ion battery application

Niobium based anodes are gaining increasing popularity for application in high-power lithium-ion batteries, due to their high theoretical capacities, inherent safety at high current densities, and long-term stability. Here, we report the discovery and characterisation of a new Wadsley Roth niobate system, Nb₇Ti_1.5Mo_1.5O₂₅, showing that it is isostructural with known systems: Nb₉PO₂₅ and Nb₉VO₂₅. To evaluate the material's electrochemical performance, including performance at high current densities (for potential high power applications), and long-term stability, Li half-coin cells were prepared. The material showed an initial capacity of 268(9) mA h g^-1 at 0.01 A g^-1 (voltage range of 2.5-1.0 V). However, in subsequent cycles, some of this initial capacity is lost, which is attributed to Li trapping associated with the presence of reducible MoO₄ units, similar to the situation observed for isostructural Nb₉VO₂₅. After this initial irreversible capacity loss, the material showed good performance at high current density rates, such that at 2 A g^-1 and 4 A g^-1 respective capacities of 132(10) mA h g^-1 and 115(14) mA g^-1 were delivered. Moreover, the material showed respectable capacity retention (97%) after being cycled for 100 cycles at 0.2 A g^-1. In order to identify the different Nb, Ti, Mo redox couples involved in this system, a Ta analogue was also synthesized (Ta₇Ti_1.5Mo_1.5O₂₅) and the electrochemical performance for this phase is also reported. This phase shows a lower initial capacity at 0.01 A g^-1 (140(3) mA h g^-1) than the Nb analogue in the same voltage range, which can be increased (225 mA h g^-1) if a lower cutoff voltage (0.5 V) is applied. The capacity retention for this Ta system after 100 cycles at 0.2 A g^-1 is similar to the Nb analogue (97%). Further work has explored whether the Nb-Ti-Mo contents could be varied, and these results showed that single phase Nb_10-2xTi_xMo_xO₂₅ samples could be prepared for 1.5 ≤ x ≤ 1.75, and electrochemical testing results for the x = 1.75 endmember are also reported. Overall, this research highlights the synthesis and electrochemical characterisation of two new Wadsley Roth phases, and further highlights the challenges associated with the presence of reducible cations in tetrahedral sites in such structures with respect to minimising initial irreversible capacity loss.