New Metal Phosphide with Topotatic Reaction As an Anode Material for Lithium Ion Rechargeable Battery

The current level of performances of Lithium Ion Batteries (LIBs) is not enough to meet commercial demands for new applications(xEV, ESS), in particular, with respective to energy density. Graphite and hard carbons are commonly used as a negative electrode material for LIBs, but higher-capacity alternatives are being consistently sought due to their limited capacity as an anode material of next generation LIBs for large-scale power applications. Recently, various Si, and Sn based compound including transition metal oxide, multiphase alloy, and intermetallic compounds have been extensively studied as alternatives to the existing carbon based anode material. These materials show much higher capacities than those of carbonaceous material. However, they are suffering from huge lattice volume change (4.4Li+Si ® Li_4.4Si, ΔV>300%) through conversion reaction mechanism during Li insertion, which results in poor cycle life of LIBs.

In this respect, metal phosphides have been alternatively suggested as a promising anode material for their reversibility, and large amount of lithium uptake at relatively low potential. It is not clarified yet until now, but the electrochemical reaction in metal phosphides with lithium might be classified into two reaction mechanisms, depending on electronegativity difference between metal and phosphorous atoms. Obviously, the large difference in electronegativity between constituting elements inevitably brings about decomposition of host composition (structure) with lithium intercalation, which can be readily understood by considering the Gibbs formation free energy. Even if being composed of elements with the very small difference in electronegativity, however, the reaction mechanisms are different with chemical stoichiometry of metal phosphides (MP_x). In case of SnP_x (Electronegativity : Sn=1.96, P=2.19) intermetallic compounds, the SnP_0.75 equilibrium phase shows typical conversion reaction mechanism, on the other hand, SnP_0.94metastable phase has been observed to be on intercalation reaction mechanism during lithiation process [1,2].

In this work, we have investigated the reaction mechanism of MoP_x(Electronegativity : Mo=2.16, P=2.19) with various stoichiometry, relating with electrochemical properties. We finally propose newly designed metal phosphide with ternary composition showing topotatic reation.

References

[1] Young-Ugk Kim, Churl Kyung Lee, Hun-Joon Sohn, and Tak Kang, Journal of The Electrochemical Society, 151 (2004) A933-A937

[2] Youngsik Kim, Haesuk Hwang, Chong S. Yoon, Min G. Kim, and Jaephil Cho, Adv. Mater.19 (2007) 92–96