Conversion reaction mechanisms of cubic bismuth phosphate Bi_13.1PO_δ as cathode in lithium-ion batteries

Compounds containing bismuth ions (Bi³⁺) as cathodes used in lithium-ion batteries based on chemical conversion reaction were supposed to be the most promising cathodes. They were considered to break through the storage capacity because a Bi atom could react with three Li⁺ ions and simultaneously transfer three electrons at a time during conversion reaction. In the paper, the cubic phase bismuth phosphate Bi_13.1PO_δ as cathode used in lithium-ion batteries was proposed. The results showed that the crystalline Bi_13.1PO_δ with sizes of 2–5 μm synthesized via hydrothermal method possessed excellent capacities and poor cycling stability. The initial discharge and charge capacities reached to 706 and 419 mAh g⁻¹ at current density of 39.06 mA g⁻¹. To improve the cycling stability of the cathode material, the chemical conversion process was focused on investigation. The elemental analyses indicated that the ratio of Bi:P ratio was decreased from 12.98 to 4.00, which suggests that about 8 Bi atoms participated in the reaction. Therefore, the capacities of the material were very high. The reduced Bi transformed from the beginning isolated state to ribbon or plate one, and coated on the surface of the active materials, inhibiting further conversion reaction. Consequently, the cycling stability of the cathode material was less than the capacity performance.