Understanding the role of aluminium on determining the surface structure and electrochemical performance of layered cathode

The electrochemical properties of layered cathodes could be enhanced by doping aluminium. However, the underlying mechanism of Al-ions behaviour is deficient, which obstructs further application. Herein, by adjusting the aluminium content in the model LiNi0.85-xCo0.15AlxO2 (LNCA) materials, the samples with optimum aluminium content (x=0.05) exhibit an excellent electrochemical performance (98.6% capacity retention at 275mAg-1). While for the samples with excessive aluminium (x=0.15, 0.30), a fast decay of cycling stability could be observed. Meanwhile, the reversible capacity in initial cycles is also far inferior to the theoretical value. Such abnormal phenomena could be attributed to the structure cracking and the impedance of Li-ions migration in higher aluminium samples. According to the microstructure observation, an unexpected beneficial heterostructure is found to cover on the samples with optimum aluminium, while in higher aluminium samples, it is missing. Furthermore, with the activation barrier calculation confirmed, Al-ions is found to prefer to thermodynamically occupying the tetrahedral interstices instead of the octahedral sites in Li layers at a high delithiation state. Due to such selective occupancy, proper aluminium content could improve the stability of the layered cathode during cycling. However, the excessive aluminium content instead impedes the formation of beneficial surface heterogeneity during synthesis and deeply affects the Li-ions migration during cycling. Therefore, the electrochemical performance of higher aluminium samples could suffer severe decay. These results and discoveries significantly advance the microstructural design guidance for next-generation layered cathode materials.