Layered Li_xNi_yMn_yCo_1-2yO₂ Cathodes for Lithium Ion Batteries:  Understanding Local Structure via Magnetic Properties

The magnetic properties of layered LiNi_yMn_yCo_1-2yO₂ (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_0.5Mn_0.5O₂, 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 Ni²⁺ 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 Li₂MnO₃ at 36.5 K and in NaNi_0.5Mn_0.5O₂ at 55 K. The continuing increase of net magnetization in this temperature range indicates at least partial ferromagnetic interlayer ordering in LiNi_0.5Mn_0.5O₂ as opposed to Li₂MnO₃ and NaNi_0.5Mn_0.5O₂, which is caused by Ni²⁺ 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 Ni²⁺ in the transition metal layer decreases, the cluster ordering transitions disappear, and only the spin-glass freezing is observed in LiNi_0.4Mn_0.4Co_0.2O₂ and LiNi_1/3Mn_1/3Co_1/3O₂ 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_0.5Mn_0.5O₂ is studied. Oxidation of Ni²⁺ (S = 2) to Ni³⁺ (S = ¹/₂) to Ni⁴⁺ (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.