Hydro-Québec and the U.S. Army Research Laboratory (ARL) have used Cr, Fe and Si to improve the performance of olivine LiCoPO₄ in cathodes for lithium-ion batteries.

In a paper in the Journal of Power Sources, they report that 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.

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

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 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. 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. 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. In addition, electrolyte additives were also employed to improve the attractiveness of LCP. 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.

—Liu et al.

Based on earlier work, the team prepared 1kg/batch of substituted LCP of composition Li-Co_0.82Fe_0.0976Cr_0.0488Si_0.00976PO₄.

In addition to the elimination of capacity fade, the the substituted LCP delivered a reversible capacity of ∼120, 118, 113, 108 and 90 mAh/g at C/2, 1C, 3C, 5C and 10C, respectively.

The researchers attributed the high-rate capability to the Li vacancies and mobile hole polarons by Cr, Fe and Si substitutions, as well as a carbon-coating.

Combined with the merits of Fe, Cr dopants, strong covalent band of Si-O and uniform C-coating, it is reasonable to believe that defective Fe/Cr/Si-substituted LCP will be a promising next-generation high-voltage cathode for high-energy lithium-ion batteries.

—Liu et al.

Resources

• D. Liu, W. Zhu, C. Kim, M. Cho, A. Guerfi, S.A. Delp, J.L. Allen, T.R. Jow, K. Zaghib (2018) “High-energy lithium-ion battery using substituted LiCoPO₄: From coin type to 1 Ah cell,” Journal of Power Sources, Volume 388, Pages 52-56 doi: 10.1016/j.jpowsour.2018.03.077