The demand for the increase in energy density of lithium batteries is steadily growing. The use of three-electron redox of transition metals is expected to be an important strategy to further increase the energy density of positive electrode materials with less transition metal ions. Recently, our group has reported that Mo³⁺ ions are diluted in lithium-excess oxides, Li₂Ti⁴⁺O₃, Li₃Nb⁵⁺O₄, and Li₄Mo⁶⁺O₅, which have higher lithium contents, and proposed as a new series of high-capacity electrode materials.[1] For instance, Li_9/7Nb_2/7Mo_3/7O₂ delivers a large reversible capacity of ca. 280 mAh g^-1, which nearly corresponds to the theoretical capacity based on Mo³⁺/Mo⁶⁺.

In this study, this concept is extended into other lithium-excess compounds, and the use of Li₃PO₄ is targeted. A new binary system with Mo³⁺ is prepared according to the chemical formula of x Li₃PO₄ – (1 – x) LiMoO₂. Since the molar mass of P is much smaller than that of Nb, the P-based samples are expected to deliver higher reversible capacity. Crystal structures and electrode performance in this binary system are systematically examined. A mixture of Li₃PO₄ and LiMoO₂ was mechanically ball milled at room temperature with a ZrO₂ container and balls. The samples of x = 0.4 (Li_9/7P_2/7Mo_3/7O₂) and 0.6 (Li_11/8P_3/8Mo_1/4O₂) show a relatively small reversible capacities in Li cells (Figure 1). In contrast, the sample of x = 0.2 (Li_7/6P_1/6Mo_2/3O₂) delivers the highest reversible capacity of ca. 330 mAh g^-1with relatively higher operating voltage compared with the samples without P ions.[1]

From these results, we will discuss the possibility of a new series of high-capacity positive electrode materials with Mo³⁺ions for rechargeable lithium batteries.

Reference

[1] S. Hoshino, et al., and N. Yabuuchi, ACS Energy Letters, 2, 733 (2017).