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Li$_x$CoO$_2$ based batteries have serious capacity degradation and safety issues when cycling at high-delithiation states but full and consistent mechanisms are still poorly understood. Herein, a global neural network potential (GNNP) is developed to provide direct theoretical understandings by performing long-time and large-size atomic simulations. We propose a self-consistent picture as follows: (i) CoO$_2$ layers are easier to glide with longer distances at more highly delithiated states, resulting in structural transitions and structural inhomogeneity; (ii) at regions between different phases with different Li distributions due to gliding, local strains are induced and accumulate during cycling processes; (3) accumulated strains cause the rupture of Li diffusion channels and result in formation of oxygen dimers during cycling especially when Li has inhomogeneous distributions, leading to capacity degradations and safety issues. We find that large tensile strains combined with inhomogeneous distributions of Li ions play critical roles in the formation processes of blocked Li diffusion channels and the oxygen dimers at high-delithiation states, which could be the fundamental origins of capacity degradations and safety issues. Correspondingly, suppressing accumulations of strains by controlling charge and discharge conditions as well as suppressing the gliding will be helpful for improving the performance of lithium-ion batteries (LIBs)., Comment: 17 pages, 5 figures, 1 table

NiFe oxyhydroxide is one of the most promising oxygen evolution reaction (OER) catalysts for renewable hydrogen production, and deciphering the identity and reactivity of the oxygen intermediates on its surface is a key challenge but is critical to understanding the OER mechanism as well as designing water-splitting catalysts with higher efficiencies. Here, we screened and utilized in situ reactive probes that can selectively target specific oxygen intermediates with high rates to investigate the OER intermediates and pathway on NiFe oxyhydroxide. Most importantly, the oxygen atom transfer (OAT) probes (e.g. 4-(Diphenylphosphino) benzoic acid) could efficiently inhibit the OER kinetics by scavenging the OER intermediates, exhibiting lower OER currents, larger Tafel slopes and larger kinetic isotope effect values, while probes with other reactivities demonstrated much smaller effects. Combining the OAT reactivity with electrochemical kinetic and operando Raman spectroscopic techniques, we identified a resting Fe=O intermediate in the Ni-O scaffold and a rate-limiting O-O chemical coupling step between a Fe=O moiety and a vicinal bridging O. DFT calculation further revealed a longer Fe=O bond formed on the surface and a large kinetic energy barrier of the O-O chemical step, corroborating the experimental results. These results point to a new direction of liberating lattice O and expediting O-O coupling for optimizing NiFe-based OER electrocatalyst.