Ni-rich layered oxides are potential candidates for cathodes of high energy density, and meanwhile low cost and environmental benign by reducing the usage of Cobalt.  To develop Ni-rich layered oxides as durable high-energy cathodes in lithium-ion batteries, construction of a solid solution in the transition metal layers by adding a second or even third cation into the LiNiO₂ framework is becoming an effective strategy for stabilizing the structure and/or tailoring electrochemical properties. It has been reported that electrochemical performance is greatly affected by even a small amount of Ni-ion mixture in the Li-ion layers, and this kind of structural disorder can be controlled by improved synthetic protocols. In order to improve the electrochemical properties of the material it is critical to understand its structure chemistry during the high temperature synthesis process.  Herein, we report on developing new high-capacity Ni-rich cathodes with enhanced cyclability via tuning the stoichiometry and structure of the materials. Systemic investigation is made to the phase evolution of Ni-rich LiNi_1-x-yMn_xCo_yO₂ (x+y ≤ 0.3) under different annealing temperatures by in situ synchrotron X-ray diffraction (XRD) measurements, combined with neutron diffraction and transmission electron microscopy/spectroscopy. Structural characterization indicates intriguing phase transitions during the formation of LiNi_1-x-yMn_xCo_yO₂ in the high temperature annealing process, which are closely related to cation mixture. Structure related order parameters can be used to describe these phase transitions and to determine all of the phases in such a complicated Ni-Mn-Co composition space, which, however, is challenging due to their strong dependence on the Mn and Co stoichiometry (i.e. x and y values). The in-situ XRD studies gain us access to the phase diagram in the Ni-rich region of the Ni-Mn-Co space (x+y ≤ 0.3), thereby enabling the design of synthetic protocols for preparing high-capacity LiNi_1-x-yMn_xCo_yO₂ cathodes with stabilized structure and reasonable cycling stability.

The work is supported by DOE-EERE under the Advanced Battery Materials Research (BMR) program, under Contract No. DE-SC0012704.