Pseudocapacitive Responses for Systems with Two Transition Metals—M₃Nb₂O₁₄ (M = Mo, W) Bronze Phase Materials

Recently, bronze phase materials have gained attention as high-rate lithium-ion battery anode materials. Their large tunnels and open framework are attractive for lithium-ion diffusion and high-rate performance, while the presence of two transition metals, such as W and Nb, provide a means to achieve high energy density from multi-electron redox. In this paper we report on the structure and electrochemical properties of two different bronze phase compositions having the same stoichiometry: W₃Nb₂O₁₄ and Mo₃Nb₂O₁₄. It is significant to note that both bronze phase materials exhibit pseudocapacitive characteristics. This includes a box-like characteristic in cyclic voltammetry (CV) experiments and nearly linear voltage-capacity curves from galvanostatic (GV) experiments. The CV results for W₃Nb₂O₁₄ were used to determine the b-value from the relationship between current (I) and sweep rate (v):= ab^v<br/>The peak located around 2.1V versus Li/Li+ shows a value of 0.85 and the peak around 1.7V vs Li/Li+ shows value of 0.95, which suggests a surface-controlled redox reaction. XPS measurements indicate that both transition metals are undergoing redox reactions. The lithium capacity for the W₃Nb₂O₁₄ bronze phase at C/5 is the theoretical value of 139 mAh/g corresponding to a 5 electrons redox process. In contrast to the stable behavior of W₃Nb₂O₁₄ bronze, the Mo₃Nb₂O₁₄ bronze phase undergoes an irreversible structural rearrangement upon cycling. Although the CV is initially box-like, the CV permanently becomes a broad peak shape when the scan rate is above 0.5mV/s. Mo₃Nb₂O₁₄ exhibits 220 mAh/g at C/5 and thus achieves multi-electron redox (1 Li per transition metal = 192mAh/g), but the capacity retention is worse than that of W₃Nb₂O₁₄. The change in CV shape is attributed to an irreversible structural rearrangement. XPS measurements indicate that although the Nb returns to the 5+ state on oxidation, the Mo oxidation state does not return to 6+, suggesting that lithium is trapped in the structure and leads to increased overpotential during cycling. Our results indicate that pseudocapacitive behavior is not limited to systems with a single transition metal ion. The results for both W₃Nb₂O₁₄ and Mo₃Nb₂O₁₄ bronzes show that pseudocapacitive responses can be achieved in systems where two transition metal ions undergo redox reactions.