Insights into the Positive Effect of Post-Annealing on the Electrochemical Performance of Al2O3-Coated Ni-Rich NCM Cathodes for Lithium-Ion Batteries

Ni-rich Li(Ni1–x–yCoxMny)O2-based cathodes still suffer from low cycling stability, which arises from capacity fading and impedance rise due to parasitic side reactions at the interface. Surface coatings have shown promising results in stabilizing the cathode surface and improving the cycling stability. However, a comprehensive understanding on the beneficial effect of the coating is still missing. In this paper, we used a solution-based technique to coat Ni-rich Li(Ni0.70Co0.15Mn0.15)O2 with a thin Al2O3 layer followed by post-annealing at 600 °C. Electrochemical characterization shows a drastic improvement of the cathode's cycling stability due to the coating. After post-annealing, the cycling stability is even further improved, accompanied with its C-rate performance. Structural characterization confirms that annealing results in the formation of an amorphous Al2O3/LiAlO2 coating layer, which exhibits increased lithium-ion conductivity compared to the Al2O3 coating. More importantly, temperature-dependent impedance measurements reveal that the coatings do not affect the activation energy of the charge transport, which guarantees a sufficient electronic conductivity between the secondary NCM particles in the cathode. Thus, the Al2O3/LiAlO2 layer not only inhibits direct contact between electrode and electrolyte, preventing side reactions and stabilizes the performance, but also facilitates conductive pathways for lithium ions while preserving the electronic connectivity between cathode's particles, leading to a low interfacial resistance and excellent rate capability. The results show the importance of providing a sufficiently high electrical conductivity accompanied with low activation energies in coating layers for both ions and electrons, which needs to be considered in design strategies for next-generation lithium-ion batteries.