Applications of lithium ion batteries in grid storage and electric vehicle require long lifetime. Surface coating of active material is an effective way to improve the long-term cycling of lithium ion batteries.¹ Most of the coating methods reported so far are wet-coating method, which produces waste liquid and requires additional drying process. In this study, LiNi_0.6Mn_0.2Co_0.2O₂ was coated with nano-alumina using a convenient dry-coating method and characterized in Li cells.

Commercial LiNi_0.6Mn_0.2Co_0.2O₂ powders (obtained from Umicore) were mixed with 3% Al₂O₃ (13 nm, from Sigma-Aldrich) using a dry-coating method. For comparison, LiNi_0.6Mn_0.2Co_0.2O₂ powders were also mixed with 3% Al₂O₃ by hand-grinding. Electrodes were made with active material, PVDF binder, carbon black at a ratio of 86:7:7 and dried under vacuum at 120 ℃ overnight. 1M LiPF₆ in a solution of EC and DEC (volume ratio 1:2) was used as electrolyte and Li foil was used as counter/reference electrode.

Figure 1(a) shows the SEM image of LiNi_0.6Mn_0.2Co_0.2O₂ powders. Secondary particles with spherical morphology were observed. The average particle size is ~1 μm for primary particle and ~10 μm for secondary particle. Figure 1(b) shows the SEM image of LiNi_0.6Mn_0.2Co_0.2O₂ powders coated with 3% Al₂O₃ using the dry coating method. Figure 1(c) shows the SEM image of LiNi_0.6Mn_0.2Co_0.2O₂ powders mixed with 3% Al₂O₃ by hand-grinding. The dry coating method produced a more homogeneous and dense coating, compared to materials produced by hand-grinding.

Figure 2 shows the cycling performance at C/2 for all the materials cycled between 3 V – 4.4 V. Compared to pristine NMC 622, the materials processed with 3% Al₂O₃ displayed lower reversible capacity. However, the cycling performance was improved for both materials. Notably, materials dry-coated with 3% Al₂O₃ showed little capacity fade in 50 cycles. This study gives an example of using dry coating method for improving the cycling performance of positive electrode materials for lithium ion batteries.

References

1. Z. Chen et al., J. Mater. Chem., 20, 7606-7612 (2010)