Controllable formation of lithium carbonate surface phase during synthesis of nickel-rich LiNi0.9Mn0.1O2 in air and its protection role in electrochemical reaction

It has been well recognized that Li2CO3 is inevitably formed at the surface of nickel rich layered compounds (NRLC) upon storage in air which is usually considered to be detrimental to the performance. Actually, its formation may have been triggered during synthesis of NRLC. However, little is known of its formation during synthesis of NRLC in air and its interaction behaviors with electrolyte during electrochemical process. Herein, we successfully tailor Li2CO3 surface phase simply by controlling calcination time during synthesis of structurally well ordered LiNi0.9Mn0.1O2 particles with great similarity in structure, morphology, size and crystallinity in air using a facile sol-gel method. The thickness of Li2CO3 layer is for the first time observed to decrease with increased calcination time, accompanied by the reduced Ni2+ at surface. More oxygen vacancies caused by more Ni2+ owing to the difficulty of oxidizing Ni2+ to Ni3+ at the surface are proposed here to be responsible for the formation of more Li2CO3 for shorter calcination times. Li2CO3 surface phase is evidenced to favor not only suppressing the H2/H3 phase transition but alleviating the interaction of LiNi0.9Mn0.1O2 with electrolyte because of the coating effect, which therefore improves the cycling stability of LiNi0.9Mn0.1O2 at low rates. Therefore, this study not only provides a new insight into the surface changes of LiNi0.9Mn0.1O2 with calcination time at high temperature during synthesis in air, but opens up a new avenue to tailoring Li2CO3 surface phase as desired by controlling the oxidization extent of Ni2+ to Ni3+ to modulate oxygen vacancies at surface through employing different oxidizing atmospheres or calcination times for performance improvement or exploration of novel preparation approaches of high performance NRLC.