Layered LixNiyMnyCo₁₋₂yO₂ Cathodes for Lithium Ion Batteries:  Understanding Local Structure via Magnetic Properties

The magnetic properties of layered LiNi y Mn y Co 1-2 y O 2 ( y = 0.5, 0.45, 0.4, and 1 / 3 ) compounds are studied in order to understand the transition metal ion distributions via their magnetic interactions. In LiNi 0.5 Mn 0.5 O 2, an increase of magnetization is found below 100 K with ac magnetic susceptibility revealing broad peaks at 96, 40, 13, and 7 K. The low-temperature neutron diffraction and heat capacity studies do not reveal long-range magnetic ordering; the magnetic component of heat capacity shows a broad peak at 10 K. This behavior is explained by assuming a nonrandom distribution of transition metals. The 96 K transition is attributed to the ordering of clusters of Ni 2+ spins in the transition metal and lithium layers, which are coupled by a 180° superexchange mechanism. The wide 40 K peak is explained by an increase of the cluster size due to intralayer Ni and Mn spin ordering, by analogy with antiferromagnetic ordering transitions in Li 2 MnO 3 at 36.5 K and in NaNi 0.5 Mn 0.5 O 2 at 55 K. The continuing increase of net magnetization in this temperature range indicates at least partial ferromagnetic interlayer ordering in LiNi 0.5 Mn 0.5 O 2 as opposed to Li 2 MnO 3 and NaNi 0.5 Mn 0.5 O 2, which is caused by Ni 2+ ions in the lithium layer. The 7−13 K anomalies are ascribed to the freezing of cluster magnetic moments. With increasing Co content, the amount of Ni 2+ in the transition metal layer decreases, the cluster ordering transitions disappear, and only the spin-glass freezing is observed in LiNi 0.4 Mn 0.4 Co 0.2 O 2 and LiNi 1/3 Mn 1/3 Co 1/3 O 2 at 10 and 7 K, respectively. This is consistent with the lack of long-range ordering of the transition metal ions in these compounds. The evolution of the magnetic properties upon electrochemical cycling of LiNi 0.5 Mn 0.5 O 2 is studied. Oxidation of Ni 2+ ( S = 2) to Ni 3+ ( S = 1 / 2 ) to Ni 4+ ( S = 0) is observed upon lithium removal as well as breakage of the partial magnetic ordering when 0.3 Li is removed. The latter is explained by the preferential oxidation of the Ni ions in the transition metal layers involved in the 180° magnetic exchange.