Storage characteristics of LiNi0.8Co0.1+xMn0.1-xO2 (x=0, 0.03, and 0.06) cathode materials for lithium batteries

Abstract

LiNi0.8 Co0.1+x Mn0.1-x O2 cathodes with x=0, 0.03, and 0.06 were prepared by firing a mixture of stoichiometric amounts of LiOH H2 O and coprecipitated Ni0.8 Co0.1+x Mn0.1-x (OH) 2 powders at 800°C for 15 h. Using these powders, their storage characteristics upon exposure to air and electrolytes at 90°C were compared before charging and after charging to 4.3 V with a variation of the storage time. The discharge capacities of the LiNi0.8 Co0.1+x Mn0.1-x O2 cathodes with x=0, 0.03, and 0.06 were 192, 186, and 184 mAh/g, respectively, while their irreversible capacity ratios were 13, 12, and 8%, respectively. As the Co content (x) increased in the cathode, both the Ni2+ content in the lithium 3a sites, and the contents of the LiOH and Li2 CO3 impurity phases decreased. In particular, changes in the oxidation state of the Ni and Mn ions after 4.3 V charging upon storage at 90°C were monitored using X-ray absorption near-edge spectra, and Ni4+ was found to reduce to Ni3+ while the oxidation state of the predominant Mn4+ did not change. However, residual Mn3+ ions in the cathodes dissolved into the electrolytes. Moreover, the cathodes stored at 90°C for 7 days were transformed into a spinel phase (Fd3m), regardless of the Co content. In an effort to resolve this dissolution problem, Al2 O3 and Co3 (PO4) 2 nanoparticles were coated onto the cathode (LiNi0.8 Co0.1 Mn0.1 O2) with the highest amounts of metal dissolution at 90°C. The results showed that the Co3 (PO4) 2 -coated cathode exhibited a greatly decreased metal dissolution and decreased its irreversible capacity by 5%, compared with a bare and Al2 O3 -coated cathode.