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Na3V(PO4)2 was reported recently as a novel cathode material with high theoretical energy density for Sodium-ion batteries (SIBs). However, whether V3+/V4+/V5+ multielectron reactions can be realized during the charging process is still an open question. In this work, Na3V(PO4)2 is synthesized by using a solid-state method. Its atomic composition and crystal structure are verified by X-ray diffraction (XRD) and neutron diffraction (ND) joint refinement. The electrochemical performance of Na3V(PO4)2 is evaluated in two different voltage windows, namely 2.5–3.8 and 2.5–4.3 V. 51V solid-state NMR (ssNMR) results disclose the presence of V5+ in Na2−xV(PO4)2 when charging Na3V(PO4)2 to 4.3 V, confirming Na3V(PO4)2 is a potential high energy density cathode through realization of V3+/V4+/V5+ multielectron reactions. [ABSTRACT FROM AUTHOR]

Realization of multi-electron reactions per transition metal is an effective strategy to increase the energy density of cathode materials, and comprehensively understanding of their underlying mechanism is necessary to further improve the electrochemical performance of the sodium-ion batteries. Here, we investigate and realize multi-electron reactions in NaVOPO 4 , which could deliver very high specific capacity. In addition, we explore its electrochemical mechanism in great detail by a series of advanced techniques, including XANES, in situ SXRD, 23Na, 31P, and 51V solid state NMR. The results demonstrate that V3−δ/V4+/V5+ redox couples can be realized in NaVOPO 4 , and a biphasic transition followed by a monophasic reaction process occurred during discharge process. Finally, the causes of battery failure are analyzed and corresponding strategies to improve electrochemical performance of NaVOPO 4 are discussed. [ABSTRACT FROM AUTHOR]

One of the big challenges for enhancing the energy density of lithium ion batteries (LIBs) to meet increasing demands for portable electronic devices is to develop the high voltage lithium cobalt oxide materials (HV-LCO, >4.5V vs graphite). In this review, we examine the historical developments of lithium cobalt oxide (LCO) based cathode materials in the last 40 years. According to the research topics at different periods, three research and developing stages of LCO are classified and elaborated. At the first stage, the researches on the basic properties of LCO are comprehensively summarized, which includes crystal/electronic structure, phase changes, electrochemical process, and failure mechanism. At the second stage, the elegant development of modification methods, which aim at improving the stability of the bulk structure, thermal and interface, are introduced in detail. The third stage started from 2018, high capacity (>190 mAh g−1), highly stable and high-rate capability of LCO is being developed and expected to be commercialized in the next few years. Finally, some perspectives of the remaining challenges, as well as the promising applications of HV-LCO, are also discussed. Image 1 • The R&D of LCO cathodes in the last 40 years have been reviewed. • Three developing stages based on the application voltage of LCO are overviewed. • LCO modifications and underlying mechanisms are classified and discussed. [ABSTRACT FROM AUTHOR]

Abstract Ni-rich cathode materials can deliver higher energy density with a lower cost compared with other conventional layered oxide materials. However, the cycling performance of Ni-rich materials needs further improvement, and the irreversible phase transformation related to the cell failure is not fully understood yet. Although an H1 H2 H3 structural change process is revealed, a more specific explanation about the difference of these hexagonal phases is needed for deeper comprehension and further targeted modification of Ni-rich materials. In this work, the different phase transformation mechanisms of LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) and LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) are investigated by C X-ray diffraction (XRD) measurement, and the relationship between structural change and capacity degradation is discussed, showing that H3 phase can be harmful for cycling. In addition, 6Li solid state nuclear magnetic resonance (ss-NMR) experiments are carried out for detecting the Li chemical environment at different state of charge, and the data can be associated with structural change obtained from in operando XRD results. From the analysis mentioned above, a new explanation of H1/H2/H3 is given, and a relationship between long range and local structural change has been put forward for the first time. Graphical abstract Image 1 Highlights • The phase changes of NCM materials are studied by in operando XRD. • NMR reveals Li environment at different states of charge and discharge. • A more specific explanation of H1/H2/H3 phase has been put forward. • The relationship between long range and local structure has been built up. [ABSTRACT FROM AUTHOR]