There is a constant drive to improve the specific energy and energy density of the state-of-the-art lithium-ion battery. High energy can be achieved by either choosing a cathode material that operates at a higher potential or has a higher specific capacity. Compared with the 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, resulting in a specific energy of ~800 Wh kg^-1, which is about 25% higher than those of the conventional LFP and LMO lithium-ion batteries. Although Co is more expensive than the other transition metals, the energy cost of LCP is expected to be cheaper than all commercialized lithium-ion batteries on the market [1] due to the improved energy density (FIG.1).

However, LCP suffers from severe capacity fade due to the low intrinsic electronic/ionic conductivity, structure deterioration and electrolyte decomposition [2]. In order to improve the cyclability and reduce the cost of LCP, Co ions were partially substituted by cheaper elements such as Fe, Cr and Si etc [3]. Meanwhile, a carbon-coating was used to improve the electronic conductivity of LCP. Electrochemical tests showed that both carbon-coating and substitution greatly improved the cycling performance of LCP, which suggests that the substituted LCP is a very promising cathode candidate for high energy lithium-ion battery.

References

[1] S. Brutti, S. Panero, ACS Symposium Series, 1140, Chapter 4, 69 (2013).

[2] K. Tadanaga, F. Mizuno, A. Hayashi, T. Minami, M. Tatsumisago, Electrochemistry 71, 1192 (2003).

[3] J. L. Allen, J. L. Allen, T. Thompson, S. A. Delp, J. Wolfenstine, T. Richard Jow, J. Power Sources, 327, 229 (2016).

FIG. 1. Energy density and energy cost of LCP compared with other cathode materials currently used in lithium-ion batteries.