Alleviating Anisotropic Volume Variation at Comparable Li Utilization during Cycling of Ni-Rich, Co-Free Layered Oxide Cathode Materials

Abstract

Driven by demand for greater energy densities, Ni-rich cathode materials, such as lithium nickel cobalt manganese (NCM) and nickel cobalt aluminum (NCA) oxides, with compositions approaching the lithium nickel oxide (LiNiO2) end-member have been investigated intensively. While such compositions are targeted assuming the redox activity of nickel will lead to higher capacities, the role of even small amounts of Mn and Co in these systems is of great importance. To raise considerations about the role of Mn and Co, operando X-ray diffraction has been used to resolve the structure–electrochemistry relationships in a series of Ni-rich NMX (LiNi1–yMnyO2, y = 0.25, 0.17, 0.10, 0.05) cathode materials. To ensure a meaningful comparison, the upper cutoff potential was varied as a function of the Mn content in the material to ensure comparable states of delithiation and thereby provide a capacity-normalized comparison of the structural evolution. During the first cycle all materials deliver a specific charge capacity exceeding 230 mAh g–1, corresponding to a residual Li content of x(Li) ≈ 0.15, and exhibit a structural evolution free of any first-order phase transitions. Monitoring the structural parameters of the materials during cycling shows that Mn substitution substantially reduces the magnitude of expansion/contraction of lattice parameters even when comparable amounts of Li are removed from the structure and more significantly also reduces the anisotropy of the volume changes. Thus, these Co-free, Ni-rich materials hold promise as high-capacity cathodes with good structural and mechanical stability.