The origin of the irreversible capacity of O3type NaFeO 2 charged to high voltage is investigated by analyzing the oxidation state of Fe and phase transition of layered NaFeO 2 cathodes for sodium-ion batteries during the charging process. In-situ X-ray absorption spectroscopy results revealed that charge compensation does not occur through the Fe 3+ /Fe 4+ redox reaction during sodium extraction as no significant shift to high energy was observed in the Fe K-edge. These results were reinforced with ex-situ near-edge X-ray absorption spectroscopy, which suggests that oxygen redox activity is responsible for charge compensation. Formation of Fe 3 O 4 product occurs because of oxygen release at high voltage when more than 0.5 Na is extracted from the structure; this is observed by transmission electron microscopy. NaFeO 2 irreversibility is due to the formation of Fe 3 O 4 with oxygen release, which inhibits Na insertion into the structure.

The surface of olivine NaFePO4 was modified with polythiophene (PTh) to develop a high-performance cathode material for use in Na-ion batteries. The Rietveld refinement results of the prepared material reveal that PTh-coated NaFePO4 belongs to a space group of Pnma with lattice parameters of a = 10.40656 Å, b = 6.22821 Å, and c = 4.94971 Å. Uncoated NaFePO4 delivers a discharge capacity of 108 mAh g(-1) at a current density of 10 mA g(-1) within a voltage range of 2.2-4.0 V. Conversely, the PTh-coated NaFePO4 electrode exhibits significantly improved electrochemical performance, where it exhibits a discharge capacity of 142 mAh g(-1) and a stable cycle life over 100 cycles, with a capacity retention of 94%. The NaFePO4/PTh electrode also exhibits satisfactory performance at high current densities, and reversible capacities of 70 mAh g(-1) at 150 mA g(-1) and 42 mAh g(-1) at 300 mA g(-1) are obtained compared with negligible capacities without coating. The related electrochemical reaction mechanism has been investigated using in situ X-ray absorption spectroscopy (XAS), which revealed a systematic change of Fe valence and reversible contraction/expansion of Fe-O octahedra upon desodiation/sodiation. The ex situ X-ray diffraction (XRD) results suggest that the deintercalation in NaFePO4/PTh electrodes proceeds through a stable intermediate phase and the lattice parameters show a reversible contraction/expansion of unit cell during cycling.

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Ni-based cathode materials have received significant attentions as the advanced electrode materials for NIBs. However, they suffered from the rapid capacity fading due to the side reactions mainly occurring at cathode-electrolyte interphase (CEI).

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The electronic structures of bare and ZrO 2 -coated Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 electrode systems were investigated using a combination of time-resolved X-ray diffraction and soft X-ray absorption spectroscopy (XAS) techniques. The ZrO 2 coating on the surface of Li(Ni 0.8 Co 0.15 Al 0.05 )O 2 was effective in elevating the onset temperature of the dissociation of charged Li 0.33 (Ni 0.8 Co 0.15 Al 0.05 )O 2 , which will enhance the safety of Li-ion cells. Soft XAS spectra of the Ni L II,III -edge in the partial electron yield mode were obtained, which showed that the enhanced electrochemical properties and thermal stability of the cathode materials by ZrO 2 coating can be attributed to the suppression of unwanted Ni oxidation state changes at the surface.

Sodium-rich metallic Na x+z has received significant attention as a low-cost alternative to the conventional electrode materials used in Li-ion batteries. However, the poor cyclability of Na x Cl remains a major challenge to its practical application. Here, a simple method is developed for improving the electrochemical performance of Na x Cl by controlling the upper limit of cut-off voltage. It is demonstrated that additional Na-vacancy defects can be introduced in the NaCl structure during the high-voltage activation process at 4.5 V. The structure then accommodates more sodium ions during the next discharge, resulting in increased capacity. At the same time, Cl-ions released by NaCl decomposition are oxidized to form Cl-based organic species at the active material interfaces. This plays a crucial role in protecting the NaCl electrode from undesired side reactions at high voltage. In short, this control of the charging protocol helps to induce more vacancies in the NaCl structure, as well as form stable interphases on the electrode surface, contributing to the increased capacity and enhanced cycle stability. This study will help in exploring a new approach for developing low-cost and high-capacity electrode material, which can potentially be applied in future energy-storage systems.