Fundamental Insight into Zr Modification of Li- and Mn-Rich Cathodes: Combined Transmission Electron Microscopy and Electrochemical Impedance Spectroscopy Study

While zirconium-based coatings are known to improve the cycling stability of a number of lithium ion battery cathodes, the microstructural origin of this enhancement remains uncertain. Here we combine advanced transmission electron microscopy (high-resolution transmission electron microscopy, high-angle annular dark field, electron energy loss spectroscopy, and energy-dispersive X-ray spectroscopy) with electrochemical impedance analysis to provide new insight into the dramatic role of Zr surface modification on the electrochemical performance of Li- and Mn-rich (LMR) cathodes (Li[Li_0.2Ni_0.13Co_0.13Mn_0.54]O₂). It is demonstrated that a Zr-based rock-salt structure layer with a thickness of 1-2 nm is formed on the surface of the LMR. This layer is effective in suppressing the deleterious phase transformation of LMR from initial layered composite combining Li₂MO₃ and LiMO₂ to the disordered rock-salt phase, leading to an enhanced long-term cycling performance and rate capability. Electrochemical impedance spectroscopy analysis demonstrates that the Zr coating does not affect the cathode electrolyte interface (CEI), with the surface film impedance (R_sf) being virtually identical in both cases after 100 cycles, at 45.1 versus 45.6 Ω. Conversely, the Zr coating tremendously stabilizes the cathode interfacial structure. The charge-transfer impedance (R_ct) in the baseline unmodified LMR increases from 34.2 Ω at cycle 3 to 729.2 Ω at cycle 100. For the Zr-modified specimen, R_ct increases dramatically less, from 19.7 to 76.9 Ω. The key finding of this study is then that Zr is actively incorporated into the structure of the cathode but does not affect CEI stability. This fundamental result should guide future surface modification strategies for a range of cathode materials.