Abstract
LiFePO4 (LFP) as a Li cathode is known for its safety and good rate capability but its adoption has been limited by its low voltage of only 3.5V vs. Li+/Li. Replacement of Fe by Co leads to a 4.8V cathode and up to 40% higher energy density. Can we couple the promise of a safe phosphate system with a high energy density? The path towards using LiCoPO4 (LCP) as cathode is challenging. There is an inherent capacity fade observed for unsubstituted LCP cycling in conventional electrolyte owing to both cathode and electrolyte deterioration. The mechanical properties of LCP were studied and are similar to LFP and are thus not responsible for its capacity fade. LCP’s electrical conductivity is several orders of magnitude lower than LFP. In order to realize the promise of the LCP system we have addressed structural stability and electronic conductivity through substitutional chemistry, electrolyte reformulation and surface modification. Even a low level of Fe substitution into LCP increases the electrical and ionic conductivity and stabilizes the de-lithiated structure through change in the electronic structure which shift the redox activity toward the transition metal and away from oxygen. Substitution of Cr for Co further increases the redox activity of Co leading to higher discharge capacity than for Fe-only substituted LCP. This stabilization mechanism is confirmed through spectroscopic measurements, through DFT calculation and is evidenced by extremely reduced discharge capacity fade during electrochemical cycling. Addition of Si to the cathode reduces the reactivity with the electrolyte as evidenced by an increase in the coulombic efficiency. Electrolyte stability has been further addressed through sacrificial additives, changes in solvent composition and surface modification of LCP. This paper will discuss the most recent progress in the path towards use of LCP as cathode material in lithium batteries and the path forward.
Published Version
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