The discharge capacity of LiCoPO4, prepared using carbon-containing precursors, heated under different oxygen partial pressures (high-purity argon, air, pure oxygen) was investigated. LiCoPO4 heat-treated under high-purity argon showed a higher discharge capacity over the entire cycle life compared to under air or pure oxygen. No difference in the discharge capacity of LiCoPO4 heat-treated under air or pure oxygen was exhibited. The enhanced discharge capacity of LiCoPO4 heat-treated under argon compared to air/oxygen was a result of its higher electrical conductivity and smaller particle size. The higher electrical conductivity and smaller particle of LiCoPO4 heat-treated under argon is likely do the presence of cobalt phosphides and increased carbon content.