Improvement of Thermodynamic Phase Stability and High-Rate Capability of Li Layered Oxides.

The use of elemental doping in lithium cobalt oxide (LCO) cathode material at high cutoff voltage is a widely adopted technique in the field of rechargeable batteries to mitigate multiple unfavorable phase transitions. However, there is still a lack of fundamental understanding regarding the rationality of each doping element implemented in this method, specifically considering the various thermodynamic stability and phase transitions. Herein, we investigated the effect of Ti doping on an O2 phase LCO (LCTO) cathode material that exhibited enhanced rate performance. We suggest that the incorporation of Ti into an O2 phase LCO results in the mitigation of multiple-phase transitions and the improvement of phase stability, thereby yielding a high-rate-capable cathode material. Through a combination of experimental and computational calculations, we demonstrate that Ti doping improves the thermodynamic stability and kinetics of Li-ions during the cycling process.

Real-Time Visualizing Nucleation and Growth of Electrodes for Post-Lithium-Ion Batteries.

ConspectusUntil recently, most studies on nucleation and growth mechanisms have employed electrochemical transient measurements, and numerous models have been established on various metal electrode elements. Contrary to the conventional tip-induced nucleation and growth model, a base-induced nucleation and growth mode was discovered not so long ago, which highlighted the importance of direct real-time observations such as visualization. As analysis techniques developed, diverse in situ/operando imaging methods have spurred the fundamental understanding of complex and dynamic battery electrochemistry. Experimental observations of alkali Li and Na metals are limited and difficult because their high reactivity makes not only the fabrication but also the analysis itself challenging. Na metal has high reactivity to electrolytes. Accordingly, it is difficult to visualize the Na deposition in real-time due to gas evolution and resolution limitation. Only a few studies have examined the Na deposition and dissolution reactions in operando. It is generally believed that the Mg anode is free from the dendrite growth of Mg metal, and Mg deposition preferentially occurs along the surface direction. However, whether the Mg anode always follows the dendrite-free growth has currently become a controversial topic and is being discussed and redefined based on real-time imaging analyses. In addition, a variety of morphological evolutions in the metal anodes are required to be systematically distinguished by key parameters. Real-time imaging analysis can directly confirm the solid-liquid-solid multiphase conversion reactions of S and Se cathodes. S and Se elements belong to the same chalcogen group, but their crystal structures and morphological changes significantly differ in each electrode during deposition and dissolution reactions. Therefore, it is necessitated to discuss the nucleation and growth behaviors by examining intrinsic properties of each element in chalcogen cathodes. Considering that a mechanistic understanding of the Se cathode is in its infancy, its nucleation and growth behaviors must be further explored through fundamental studies. In this Account, we aim to discuss the nucleation and growth behaviors of metal (Li, Na, and Mg) anodes and chalcogen (S and Se) cathodes. To elucidate their nucleation and growth mechanisms, we overview the morphological evolutions on the electrode surface and interface by in situ/operando visualizations. Our recent studies covering Li, Na, Mg, S, and Se electrodes verified by operando X-ray imaging are used as critical resources in understanding their nucleation and growth behaviors. Overall, with validation of the complex and dynamic nucleation and growth behaviors of metal and chalcogen electrodes by in situ/operando visualization methods, we hope that this Account can contribute to supporting the fundamental knowledge for the development of high-energy-density metal and chalcogen electrodes.

Selective dealloying of chemically disordered Pt-Ni bimetallic nanoparticles for the oxygen reduction reaction.

Changes in electronic and compositional structures of Pt-Ni electrocatalysts with 44% of Ni fraction with repeated chemical dealloying have been studied. By comparing the Pt-enriched surfaces formed using hydroquinone and sulfuric acid as a leaching agent, we found that hydroquinone generated Pt-enriched surfaces exhibit the highest oxygen reduction reaction (ORR) activity after repeating the treatment twice. In particular, it was found that while sulfuric acid causes an uncontrollable dissolution of Ni clusters, the unique selectivity of hydroquinone allows the preferential dissolution of Ni atoms alloyed with Pt. Despite its wide usage in the field, the results show that traditional acid leaching is unsuitable for Pt-Ni alloys with a high Ni content and an incomplete alloying level. We finally proved that the unique and lasting selectivity of hydroquinone enables an incompletely alloyed Pt-Ni catalyst to obtain a highly ORR active Pt shell region without an extensive loss of Ni.