Layered Cathode Materials with Controlled Particle Assembly for High Energy Lithium-Ion Batteries

Stoichiometric layered lithium transition metal oxides (LiNi_1-x-yMn_xCo_yO₂) represent a family of cathode materials suitable for high-energy lithium ion batteries. However, their structural instability (e.g., surface reconstruction from the layered structure to a rock-salt/spinel mixed structure) when charged to high voltages impedes efforts to improve their energy densities. Here, we report on a study of LiNi_1-x-yMn_xCo_yO₂ materials prepared by a spray pyrolysis technique to provide improved cycling performance.

The as-prepared LiNi_1-x-yMn_xCo_yO₂ materials assemble into spherical structures that consist of nanosized primary particles. Revealed by transmission electron microscopy, these primary particles are “fused” together to form atomically disordered interfaces that may allow for lithium and electron diffusion. LiNi_1-x-yMn_xCo_yO₂ materials with different ratios of transition metals were synthesized, i.e., LiNi_1/3Mn_1/3Co_1/3O₂, LiNi_0.4Mn_0.4Co_0.2O₂, LiNi_0.5Mn_0.3Co_0.2O₂, LiNi_0.6Mn_0.2Co_0.2O₂, most of which show improved electrochemical performance (i.e., discharge capacity, capacity retention) compared to the counterparts synthesized by the conventional co-precipitation method, although different thermal treatment protocols are needed after the spray pyrolysis process. Surface reconstruction was quantified by soft X-ray absorption spectroscopy to investigate the surface phase transition of the LiNi_1-x-yMn_xCo_yO₂ materials after electrochemical cycling, and it is found that the materials prepared by spray pyrolysis have thinner surface passivation layers (a rock-salt/spinel mixed structure), which accounts for better capacity retention. This study suggests that morphological engineering by spray pyrolysis may offer an efficient pathway towards stable high-energy layered cathode materials.