Correlations between lithium local structure and electrochemistry of layered LiCo1−2xNixMnxO2 oxides: 7Li MAS NMR and EPR studies

Abstract

Advanced ⁷Li MAS NMR technologies and high frequency EPR are combined to identify structural motifs and their relation to electrochemical properties of layered lithium–cobalt–nickel–manganese oxides LiCo_1−2xNi_xMn_xO₂ (0 < x ≤ 0.5) used as cathode materials in lithium ion batteries. Structural–chemical shift regularities were established by systematic variation of the ratio of diamagnetic Co³⁺ to paramagnetic Ni/Mn ions with variable valences. While EPR allows identifying the oxidation state of transition metal ions inside the layers, ⁷Li NMR probes the local structure of Li with respect to transition metal ions located in two adjacent layers. For assignment of the lithium chemical shifts, we examine first magnetically diluted LiCo_1−2xNi_xMn_xO₂ with x = 0.02, where paramagnetic ions are stabilized only in Mn⁴⁺ and Ni³⁺ form. Then the studies are extended towards the intermediate compositions with x = 0.10 and 0.33, containing simultaneously paramagnetic Mn⁴⁺, Ni³⁺ and Ni²⁺ ions and diamagnetic Co³⁺ ions. The benefit of using NMR with ultrafast spinning rates is demonstrated for the end composition LiNi_0.5Mn_0.5O₂ having only paramagnetic Ni²⁺ and Mn²⁺ ions. The local structure of Li is quantified in respect of the number of Ni²⁺ and Mn⁴⁺ neighbors. It has been demonstrated that Ni²⁺ and Mn⁴⁺ are non-randomly distributed around Li and their distribution depends on the method of synthesis. The extent of local cationic order and its effect on the electrochemical properties of LiNi_0.5Mn_0.5O₂ are discussed.