Atomic Scale Study of Oxygen Loss and Structural Degradation in Oxide Cathodes and a Mitigation strategy

This thesis presents the identification of nano-scale degradation mechanisms associated with the layered oxide cathode materials of Li-ion batteries (LIBs), and our novel approach in tackling some of the uncovered challenges. Layered oxide cathodes with the general formula of LiMO2 (M= Co, Ni, Mn) are the prime member of positive LIB electrodes, widely used in commercial applications. However certain challenges such as structural instability that leads to rapid capacity fade, voltage polarization and even thermal runaway and ignition of the batteries are not fully comprehended and are remaining to be addressed. Oxygen release from the oxygen-containing positive electrode materials is one of the major structural degradations resulting in rapid capacity/voltage fading of the battery and triggering the parasitic thermal runaway events. This thesis summarizes the recent progress in understanding the mechanisms of the oxygen release phenomena and corelative structural degradations observed in four major groups of cathode materials: layered, spinel, olivine and Li-rich cathodes. Utilizing in-situ scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) at high temperatures, we show that LixCoO2 cathode crystals undergo inhomogeneous oxygen loss that are localized at the surface of individual particles. Using atomic resolution imaging and electron diffraction analysis, we correlated the oxygen release from LixCoO2 to the surface-originated phase conversions such as layered to spinel and rock salt phases at elevated temperatures. Corroborating the experimental findings with ab-initio molecular dynamics (in collaboration with Prof. Balbuena’s group from Texas A&M University) we proposed the facet-dependent thermal instability of LiCoO2 cathodes for the first time. Next, we show that the surface-originated structural decomposition can be hindered by depositing a conformal coating of 2D reduced graphene oxide on individual LixCoO2 particles. By conducting various electrochemical tests, thermal analysis and in-situ heating transmission electron microscopy the effectiveness of the graphene-coating on the structural stability of LixCoO2 was evaluated. Through computational calculations and modeling (by Prof. Balbuena’s group from Texas A&M University) it was shown that the rGO layers could suppress O2 formation more effectively due to the strong C-Ocathode bond formation at the interface of rGO/LCO where low coordination oxygens exist. This systematic investigation uncovered a reliable approach for hindering the oxygen release reaction and mitigating the thermal instability of battery cathodes. Layered oxide cathodes that are the dominant positive electrodes in current commercial LIB technologies, are reaching to their intrinsic capacity limits, restraining the advancement of the batteries. Therefore, Li- and Mn-rich layered cathodes with capacities of ⁓300 mAhg-1 are considered as the next generation of LIB cathodes. Despite their high energy densities, Li- and Mn-rich, layered-layered, xLi2MnO3•(1-x)LiTMO2 (TM=Ni, Mn, Co) (LMR-NMC) cathodes require further development in order to overcome issues related to bulk and surface instabilities such as Mn dissolution, impedance rise, and voltage fade. One promising strategy to modify LMR-NMC properties has been the incorporation of spinel-type, local domains to create ‘layered-layered-spinel’ cathodes. However, precise control of local structure and composition, as well as subsequent characterization of such materials, is challenging and elucidating structure-property relationships is not trivial. Therefore, detailed studies of atomic structures within these materials are still critical to their development. Herein, aberration corrected scanning transmission electron microscopy (AC-STEM) is utilized to study atomic structures, prior and subsequent to electrochemical cycling, of LMR-NMC materials having integrated spinel-type components. The results demonstrate that strained grain boundaries with various atomic configurations, including spinel-type structures, can exist. These high energy boundaries appear to induce cracking and promote dissolution of Mn by increasing the contact surface to electrolyte as well as migration of Ni during cycling, thereby accelerating performance degradation. These results present insights into the important role that local structures can play in the macroscopic degradation of the cathode structures and reiterate the complexity of how synthesis and composition affect structure-electrochemical-property relationships of advanced cathode designs.