Cr-substituted LiCoPO₄ core with a conductive carbon layer towards high-voltage lithium-ion batteries

Highlights: • Novel composite of LiCo_1-1.5xCr_xPO₄/C was prepared. • The composite and structure of LiCo_1-1.5xCr_xPO₄/C were investigated. • Cr-substituted into LiCoPO₄/C does not change its morphology. • Cr-substitution improved electrochemical performance of LiCoPO₄ significantly. • The relation between the amounts of Cr and performance of was LiCoPO₄ discussed. - Abstract: Electrical and ionic conductivity are two major limiting factors for LiCoPO₄ cathode material. To overcome these shortcomings, a Cr-substituted LiCoPO₄ core with a conductive carbon layer cathode material is synthesized using the sol-gel method. The physical chemistry properties of these materials are systematically investigated by using various characterization methods. For instance, the XRD and Rietveld refinement results reveal that Cr successfully substitutes the Co within the LiCoPO₄ core to form LiCo_1-1.5xCr_xPO₄/C (x = 0, 0.02, 0.04, 0.06) without changing the olivine structure but exhibits a decrease in the unit cell volume with increasing Cr substitution. SEM and TEM images indicate that Cr substitution does not lead to changes in the basic morphology of LiCo_1-1.5xCr_xPO₄/C (x = 0, 0.02, 0.04, 0.06) material, which is composed of agglomerated nanoparticles with an 8 nm carbon layer on the surface. The EDS and XPS results confirm that Cr is uniformly distributed on the surface and that the oxidation state of Cr is +3. FTIR spectra indicate that the antisite defect concentration decreases with increasing Cr substitution. Furthermore, Cr substitution significantly improves the electrochemical performances of LiCo_1-1.5xCr_xPO₄/C (x = 0.02, 0.04, 0.06) cathode. Notably, the LiCo_0.94Cr_0.04PO₄/C delivers an initial discharge capacity of 144 mA h g⁻¹ at 0.1 C and shows a capacity retention of 71% after 100 cycles between 3.0 and 5.0 V. The CV and EIS results indicate that the polarization is reduced and that the electronic and ionic conductivities are improved by Cr substitution. The good electrochemical performances for Cr-substituted LiCoPO₄/C electrodes are attributed to the lower antisite defect concentration, as the reduction of polarization, the improvement of electronic and ion conductivity and the uniform carbon layer. These features will accelerate the commercial application of LiCoPO₄ towards the start-art of the high voltage lithium-ion batteries.