Electrochemical lithium intercalation chemistry of condensed molybdenum metal cluster oxide: LiMo₄O₆

The electrochemical lithium-ion intercalation properties of molybdenum metal-cluster oxide Li_xMo₄O₆ (0.33 ≤ x ≤ 1.0) in an organic electrolyte of 1.0 M LiPF₆ in ethylene carbonate/dimethyl carbonate (1:2 v/v) were characterized for the first time. Li_0.66Mo₄O₆ (tetragonal, P4/mbm, a = 9.5914(3) Å, c = 2.8798(1) Å, V = 264.927(15) ų, Z = 2) was prepared via ion-exchange of indium and lithium ions from InMo₄O₆ (tetragonal, P4/mbm, a = 9.66610(4) Å, c = 2.86507(2) Å, V = 267.694(2) ų, Z = 2), which was first synthesized from a stoichiometric mixture of In, Mo, and MoO₃ via a solid-state reaction for 11 h at 1100 °C. Then, Li_0.33Mo₄O₆ was obtained via electrochemical charge of the electrode at 3.4 V vs. Li. The electrochemical lithium-ion insertion into Li_0.33Mo₄O₆ occurs stepwise: three separate peaks were observed in the cyclic voltammogram and three quasi-plateaus in the galvanostatic profile, indicating a complicated intercalation mechanism. However, examination of the structural evolution of Li_xMo₄O₆ during the electrochemical cycle indicated a reversible reaction over the measured voltage range (2.0–3.2 V) and x range (0.33 ≤ x ≤ 1.00). Despite the excellent electrochemical reversibility, Li_xMo₄O₆ showed poor rate performance with a low capacity of 36.3 mAh g⁻¹ at a rate of 0.05 C. Nonetheless, this work demonstrates a new structural class of lithium cathode materials with condensed metal clusters and 1D tunnels, and provides a host material candidate for multivalent-ion batteries.