Composites of Layered M(HPO4)2 (M = Zr, Sn, and Ti) with Reduced Graphene Oxide as Anode Materials for Lithium Ion Batteries

Tetravalent metal phosphates (M(HPO₄)₂, M = Zr, Sn, and Ti) have robust layered structures with interlayer d spacings over 7.5 Å, but show poor electrical conductivity. On the other hand, single-atomic-layered reduced graphene oxide (rGO) sheets exhibit a high electrical conductivity. In this work, the combination of rGO and M(HPO₄)₂ is explored for their potential as anode materials for lithium ion batteries (LIBs). Specifically, rGO/M(HPO₄)₂ composites are prepared, and their electrochemical performances are investigated systematically. In comparison with bare M(HPO₄)₂, the rGO/M(HPO₄)₂ composites exhibit larger specific capacity, higher rate capability, better cyclic stability, lower voltage for lithium ion insertion and extraction, and improved first Coulombic efficiency. We propose that the superior electrochemical performances of the composites are primarily contributed to the large interlayer space of M(HPO₄)₂ and the rGO sheets cladded on the surfaces of the layered M(HPO₄)₂. The attached rGO sheets bridge the layers together forming a network that is beneficial for the electron and ion diffusion within the composites, thus enhancing the discharge/charge rate capability of the composites. In addition, the attached rGO sheets provide extra anchoring sites for Li⁺; the specific capacity of the composites as anode materials is thus enhanced.