There is a constant drive to improve the specific energy and energy density of the state-of-the-art lithium-ion battery. High energy is achieved by either choosing a cathode material that operates at a higher potential or has a higher specific capacity. Compared with conventional 4-V cathodes such as LiCoO₂ (LCO), LiNi_xCo_yAl_zO₂ (NCA), LiMn₂O₄ (LMO) and 3.5-V LiFePO₄ (LFP), LiCoPO₄ (LCP) operates at 4.8 V (vs. Li/Li⁺) with a theoretic capacity of 167 mAh g^-1. The higher voltage of LCP results in specific energy of ~800 Wh kg^-1, which is about 25% higher than that of conventional cathodes in lithium-ion batteries. Although Co is more expensive than the other transition metals, the energy cost of LCP is expected to be less than other commercialized lithium-ion batteries on the market [1] due to the improved energy density. LCP suffers from severe capacity fade due to the low intrinsic electronic/ionic conductivity, structural deterioration and electrolyte decomposition [2, 3]. A diverse range of synthesis strategies, such as planetary milling, microwave heating and spray-pyrolysis et al, were explored to yield smaller particles and/or composites, but the above-mentioned shortcomings and the electrochemical performance remain unsatisfied [4-6]. Deposition of carbon coatings or precipitation of Co₂P under high-temperature annealing in inert atmosphere produced a significant increase of over 10⁵ in the electrical conductivity of LCP [7]. In addition, electrolyte additives were also employed to improve the attractiveness of LCP [8, 9]. These two approaches, however, do little to stabilize the cathode material itself, or only protect the electrolyte from decomposition during cycling. In this work, substitution of Cr, Fe and Si, as well as the use of a carbon-coating, improved the performance of LiCoPO₄. The structural analyses and electrochemical properties are discussed. Cr, Fe and Si were added to improve the performance of olivine LiCoPO₄ in cathodes for lithium-ion batteries. A substituted-LiCoPO₄ in a half cell delivered a reversible capacity of 125 mAh/g at C/3 rate, with no capacity loss after over 100 cycles at 25 °C. The well-known capacity fade of LiCoPO₄-based cathodes was almost completely eliminated by substituting Cr, Fe and Si. The electrochemical data of coin type battery and 1 Ah laminate cell will be shown.

Acknowledgments

Financial support from Hydro-Quebec and the US Army Research Lab is gratefully acknowledged.