Oxygen vacancies enhance the electrochemical performance of carbon-coated TiP2O7-y anode in aqueous lithium ion batteries

Nanocrystalline Iron Pyrophosphate-Regulated Amorphous Phosphate Overlayer for Enhancing Solar Water Oxidation

A rational regulation of the solar water splitting reaction pathway by adjusting the surface composition and phase structure of catalysts is a substantial approach to ameliorate the sluggish reaction kinetics and improve the energy conversion efficiency. In this study, we demonstrate a nanocrystalline iron pyrophosphate (Fe₄(P₂O₇)₃, FePy)-regulated hybrid overlayer with amorphous iron phosphate (FePO₄, FePi) on the surface of metal oxide nanostructure with boosted photoelectrochemical (PEC) water oxidation. By manipulating the facile electrochemical surface treatment followed by the phosphating process, nanocrystalline FePy is localized in the FePi amorphous overlayer to form a heterogeneous hybrid structure. The FePy-regulated hybrid overlayer (FePy@FePi) results in significantly enhanced PEC performance with long-term durability. Compared with the homogeneous FePi amorphous overlayer, FePy@FePi can improve the charge transfer efficiency more significantly, from 60% of FePi to 79% of FePy@FePi. Our density-functional theory calculations reveal that the coexistence of FePi and FePy phases on the surface of metal oxide results in much better oxygen evolution reaction kinetics, where the FePi was found to have a typical down-hill reaction for the conversion from OH* to O₂, while FePy has a low free energy for the formation of OH*.

Similarity between the redox potentials of 3d transition-metal ions in polyanionic insertion materials and aqueous solutions

The transition-metal ions in a solid matrix are oxidised and reduced via a solid-state redox reaction during the charge/discharge process of lithium insertion materials, which are commonly used as positive and negative electrodes in lithium-ion batteries. Therefore, the electrode potentials of lithium insertion materials should be different from the redox potentials of transition-metal ions in aqueous solution (i.e., the standard electrode potential). In this study, the solid-state redox potentials of the transition-metal ions in polyanionic materials with three distinct structures (i.e., olivine, NASICON-type, and MOXO₄-type structures, where M = 3d transition-metal ion, and X = P or S) were surveyed to understand the electrode potentials of lithium insertion materials. The redox potentials of the transition-metal ions in polyanionic materials were very similar to those in aqueous solution despite the differences between the environments of these ions in the MO₆ octahedron in polyanionic materials and the aqua complexes of [M(H₂O)₆]ⁿ⁺ in aqueous solutions. The high coefficient of determination (R² ≈ 0.990) of these two potentials indicated that the solid-state redox potential for the lithium insertion reaction in polyanionic materials can be estimated using the standard electrode potential of the corresponding transition-metal ion in aqueous solution. Finally, the similarity between the redox potentials of the transition-metal ions in polyanionic materials and those in aqua complexes is discussed from the thermodynamic perspective. The present findings on the similarity of the redox potentials of transition-metal ions in different media could provide useful insights into the design of novel insertion materials for rechargeable batteries based on lithium, sodium, potassium, and magnesium, among other metals.

The oxygen vacancy in Li-ion battery cathode materials

The substantial capacity gap between available anode and cathode materials for commercial Li-ion batteries (LiBs) remains, as of today, an unsolved problem. Oxygen vacancies (OVs) can promote Li-ion diffusion, reduce the charge transfer resistance, and improve the capacity and rate performance of LiBs. However, OVs can also lead to accelerated degradation of the cathode material structure, and from there, of the battery performance. Understanding the role of OVs for the performance of layered lithium transition metal oxides holds great promise and potential for the development of next generation cathode materials. This review summarises some of the most recent and exciting progress made on the understanding and control of OVs in cathode materials for Li-ion battery, focusing primarily on Li-rich layered oxides. Recent successes and residual unsolved challenges are presented and discussed to stimulate further interest and research in harnessing OVs towards next generation oxide-based cathode materials.