Among all candidates of positive electrode materials for lithium ion batteries, the layered oxides have gained growing research interest in recent years, especially the Ni rich layered oxides and the mixed transition metal (Ni,Co,Mn) layered oxides. They can reach more than 220mAh/g capacity when charged above 4.5V, making them one of the highest energy density positive electrode materials among all known intercalation compounds for cathode materials.  Being the source of major capacity improvement, the high voltage operation (>4.5V), on the other hand, also cause unexplained phenomena in these layered oxides. In classical layered oxides, the transition metal (TM) oxide host TMO2
should retain its host structure during initial discharging–charging, while lithium ions are intercalated–de-intercalated from the host. In contrast, recent studies suggested that after high voltage operation in layered oxides, certain amounts of transition metal ions migrate from the transition metal layers to lithium layers. Our previous work  has suggested that the migration may be assisted by oxygen vacancies generated in the late charging states (high voltages), but three critical questions remain unanswered: (1) how can we visualize and quantify the change in structure and chemical content at the atomic level (2) what is the oxygen activity upon high voltage and how does it assist transition metal migration and (3) what are the impact of these high voltage phenomena on rate capability and long-term cycling.
LiNi0.8Co0.15Al0.05O2 (NCA), LiNi1/3Co1/3Mn1/3O2 (NCM333) and LiNi0.5Co0.2Mn0.3O2(NCM523) are used as model compounds for the in-depth study. Figure 1 and Table 1 summarize the electrochemical properties of NCA and NCM333 and 532.
A suite of powerful experimental tools and computational tools has been deployed. To probe the bulk structural change, morphological and chemical changes, we combined X-ray diffraction (XRD), Neutron diffraction (ND), Pair Distribution Function (PDF) analysis, Electron Energy Loss Spectroscopy (EELS) and Transmission X-ray Microscopy (TXM), analytical Transmission Electron Microscopy (TEM). We will reveal and discuss the factors that determining the high voltage stability of oxides materials in alkaline ion batteries.
This work is supported by the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, with Award Number DE-SC0012583.
1) Z. H. Lu, D. D. MacNeil and J. R. Dahn, Electrochem. Solid- State Lett., 2001, 4, A191–A194
2) K. J. Carroll, D. Qian, C. Fell, S. Calvin, … and Y. S. Meng, Phys. Chem. Chem. Phys., 2013 15, 11128