Investigation of the possible incorporation of protons into oxide cathodes during chemical delithiation

Tuning Oxygen Redox Reaction through the Inductive Effect with Proton Insertion in Li-Rich Oxides

As a parent compound of Li-rich electrodes, Li₂MnO₃ exhibits high capacity during the initial charge; however, it suffers notoriously low Coulombic efficiency due to oxygen and surface activities. Here, we successfully optimize the oxygen activities toward reversible oxygen redox reactions by intentionally introducing protons into lithium octahedral vacancies in the Li₂MnO₃ system with its original structural integrity maintained. Combining structural probes, theoretical calculations, and resonant inelastic X-ray scattering results, a moderate coupling between the introduced protons and lattice oxygen at the oxidized state is revealed, which stabilizes the oxygen activities during charging. Such a coupling leads to an unprecedented initial Coulombic efficiency (99.2%) with a greatly improved discharge capacity of 302 mAh g^-1 in the protonated Li₂MnO₃ electrodes. These findings directly demonstrate an effective concept for controlling oxygen activities in Li-rich systems, which is critical for developing high-energy cathodes in batteries.

Mechanism of Mg extraction from MgMn2O4 during acid digestion

The Mg extraction process of MgMn₂O₄ was studied by chemical digestion in HNO₃ at different concentrations and reaction times. Mg extraction from MgMn₂O₄ is a two-step two-phase reaction, via the intermediate Mg_0.5Mn₂O₄ to the fully oxidised Mn₂O₄.

Anionic Redox in Rechargeable Lithium Batteries

The extraordinarily high capacities delivered by lithium-rich oxide cathodes, compared with conventional layered oxide electrodes, are a result of contributions from both cationic and anionic redox processes. This phenomenon has invoked a lot of research exploring new kinds of lithium-rich oxides with multiple-electron redox processes. Though proposed many years ago, anionic redox is now regarded to be crucial in further developing high-capacity electrodes. A basic overview of the previous work on anionic redox is given, and issues related to electronic and geometric structures are discussed, including the principles of activation, reversibility, and the energy barrier of anionic redox. Anionic redox also leads to capacity loss and structural degradation, as well as voltage hysteresis, which shows the importance of controlling anionic redox reactions. Finally, the techniques used for characterizing anionic redox processes are reviewed to aid the rational choice of techniques in future studies. Important perspectives are highlighted, which should instruct future work concerning anionic redox processes.

An in-situ gas chromatography investigation into the suppression of oxygen gas evolution by coated amorphous cobalt-phosphate nanoparticles on oxide electrode

The real time detection of quantitative oxygen release from the cathode is performed by in-situ Gas Chromatography as a tool to not only determine the amount of oxygen release from a lithium-ion cell but also to address the safety concerns. This in-situ gas chromatography technique monitoring the gas evolution during electrochemical reaction presents opportunities to clearly understand the effect of surface modification and predict on the cathode stability. The oxide cathode, 0.5Li₂MnO₃∙0.5LiNi_0.4Co_0.2Mn_0.4O₂, surface modified by amorphous cobalt-phosphate nanoparticles (a-CoPO₄) is prepared by a simple co-precipitation reaction followed by a mild heat treatment. The presence of a 40 nm thick a-CoPO₄ coating layer wrapping the oxide powders is confirmed by electron microscopy. The electrochemical measurements reveal that the a-CoPO₄ coated overlithiated layered oxide cathode shows better performances than the pristine counterpart. The enhanced performance of the surface modified oxide is attributed to the uniformly coated Co-P-O layer facilitating the suppression of O₂ evolution and offering potential lithium host sites. Further, the formation of a stable SEI layer protecting electrolyte decomposition also contributes to enhanced stabilities with lesser voltage decay. The in-situ gas chromatography technique to study electrode safety offers opportunities to investigate the safety issues of a variety of nanostructured electrodes.

Demonstrating Oxygen Loss and Associated Structural Reorganization in the Lithium Battery Cathode Li[Ni0.2Li0.2Mn0.6]O2

The cathode in rechargeable lithium-ion batteries operates by conventional intercalation; Li+ is extracted from LiCoO2 on charging accompanied by oxidation of Co3+ to Co4+; the process is reversed on discharge. In contrast, Li+ may be extracted from Mn4+-based solids, e.g., Li2MnO3, without oxidation of Mn4+. A mechanism involving simultaneous Li and O removal is often proposed. Here, we demonstrate directly, by in situ differential electrochemical mass spectrometry (DEMS), that O2 is evolved from such Mn4+ -containing compounds, Li[Ni(0.2)Li(0.2)Mn(0.6)]O2, on charging and using powder neutron diffraction show that O loss from the surface is accompanied by diffusion of transition metal ions from surface to bulk where they occupy vacancies created by Li removal. The composition of the compound moves toward MO(2). Understanding such unconventional Li extraction is important because Li-Mn-Ni-O compounds, irrespective of whether they contain Co, can, after O loss, store 200 mAhg(-1) of charge compared with 140 mAhg(-1) for LiCoO(2).