Structural and Electrochemical Properties of "Disordered, Spinel-like" Structures Derived from NCM111 Materials By Chemical Delithiation

Abstract

Li-ion batteries (LIBs) are being widely used in various consumer electronics applications and battery electric vehicles. In current and next-generation LIBs, particularly Ni-rich NCMs (e.g. Li1+x[Ni0.6Co0.2Mn0.2]1-xO2) are considered as a promising class of cathode active materials (CAMs) due to their high specific capacities, their high power and energy densities, and their good structural stability.[1] In order to meet cycle-life requirements and to maintain a safe state of operation, LIBs are only cycled within a specific potential range, significantly limiting the usage of the specific capacity achievable with these materials. A major problem for NCMs are over-charge and/or harsh cycling conditions, which lead to highly aggravated capacity fading and ultimately to cell-failure. One fundamental degradation mechanism for NCM materials is the deleterious oxygen evolution from the NCM surface at high degrees of delithiation (>80% state of charge, SOC),[1,2] which results in the formation of an oxygen-depleted spinel and/or rock salt phase on the NCM particle surface as well as in a concomitant oxidation of the electrolyte.[3,4] A thorough understanding of the exact structures of the O-depleted phases produced by lattice oxygen release from the NCM materials and of their effects on the cycling performance of LIBs are still missing, which hampers the development of countermeasures. In the literature, oxygen release from partially delithiated NCMs has been frequently studied by thermal treatment, showing that with increasing Ni content the temperature at which oxygen is released decreases while the amount of released oxygen increases.[3,5–7] A detailed study of chemically delithiated NCM111 (Li1+x[Ni1/3Co1/3Mn1/3]1-xO2) using electrochemical testing methods, thermogravimetric analysis coupled with mass spectrometry (TGA-MS), X-ray diffraction (XRD), scanning electron microscopy, and gas physisorption measurements is the main objective in this work. Chemical delithiation to various degrees was executed with the oxidation agent NO2BF4 in anhydrous acetonitrile. Electrochemical testing suggests minor modification of the NCM material upon chemical delithiation resulting in less specific reversible capacity and that an electrochemically inactive phase is formed after oxygen depletion, which increases polarization and significantly decreases the reversible specific capacity of the NCM material. TGA-MS analysis is used to characterize the oxygen release behavior of the chemically delithiated samples, while XRD analysis by Rietveld refinement leads to the observation and allows the quantification of a spinel phase after thermal treatment of the chemically delithiated materials. Disintegration of the secondary structure of the NCM material was observed during chemical and electrochemical delithiation, resulting in a surface area increase by a factor of »10 and »5, respectively. The obtained results help for a better understanding of the processes happening during the oxygen release of NCM materials at high SOC, while providing reference data of phases found electrochemically in over-charge and/or under harsh cycling conditions.