Synthesis and Characterization of the Lithium-Rich Core–Shell Cathodes with Low Irreversible Capacity and Mitigated Voltage Fade

Abstract

Lithium-rich layered Ni–Mn–Co oxide materials have been intensely studied in the past decade. Mn-rich materials have serious voltage fade issues, and the Ni-rich materials have poor thermal stability and readily oxidize the organic carbonate electrolyte. Core–shell (CS) strategies that use Ni-rich material as the core and Mn-rich materials as the shell can balance the pros and cons of these materials in a hybrid system. The lithium-rich CS materials introduced here show much improved overall electrochemical performance compared to the core-only and shell-only samples. Energy dispersive spectroscopy results show that there was diffusion of transition metals between the core and shell phases after sintering at 900 °C compared to the prepared hydroxide precursors. A Mn-rich shell was still maintained whereas the Co which was only in the shell in the precursor was approximately homogeneous throughout the particles. The CS samples with optimal lithium content showed low irreversible capacity (IRC), as well as high capacity and excellent capacity retention. Sample CS2-3 (the third sample in the 0.67Li1+x(Ni0.67Mn0.33)1–xO2·0.33Li1+y(Ni0.4Mn0.5Co0.1)1–yO2 CS2 series) had a reversible capacity of ∼218 mAh/g with 12.3% (∼30 mAh/g) irreversible capacity (IRC) and 98% capacity retention after 40 cycles to 4.6 V at 30 °C at a rate of ∼C/20. Differential capacity versus potential (dQ/dV versus V) analysis confirmed that cells of the CS samples had stable impedance as well as a very stable average voltage. Apparently, the Mn-rich shell can effectively protect the Ni-rich core from reactions with the electrolyte while the Ni-rich core renders a high and stable average voltage.