Unraveling the Complex Delithiation Mechanisms of Olivine-Type Cathode Materials, LiFe_xCo_1-xPO₄

The delithiation mechanisms occurring within the olivine-type class of cathode materials for Li-ion batteries have received considerable attention because of the good capacity retention at high rates for LiFePO₄. A comprehensive mechanistic study of the (de)lithiation reactions that occur when the substituted olivine-type cathode materials LiFe_xCo_1-xPO₄ (x = 0, 0.05, 0.125, 0.25, 0.5, 0.75, 0.875, 0.95, 1) are electrochemically cycled is reported here using in situ X-ray diffraction (XRD) data and supporting ex situ ³¹P NMR spectra. On the first charge, two intermediate phases are observed and identified: Li_1-x(Fe³⁺)_x(Co²⁺)_1-xPO₄ for 0 < x < 1 (i.e., after oxidation of Fe²⁺ to Fe³⁺) and Li_2/3Fe_xCo_1-xPO₄ for 0 ≤ x ≤ 0.5 (i.e., the Co-majority materials). For the Fe-rich materials, we study how nonequilibrium, single-phase mechanisms that occur discretely in single particles, as observed for LiFePO₄ at high rates, are affected by Co substitution. In the Co-majority materials, a two-phase mechanism with a coherent interface is observed, as was seen in LiCoPO₄, and we discuss how it is manifested in the XRD patterns. We then compare the nonequilibrium, single-phase mechanism with the bulk single-phase and coherent interface two-phase mechanisms. Despite the apparent differences between these mechanisms, we discuss how they are related and interconverted as a function of Fe/Co substitution and the potential implications for the electrochemistry of this system.