The development of highly reversible multielectron reaction per redox center in sodium super ionic conductor-structured cathode materials is desired to improve the energy density of sodium-ion batteries. Here, we investigated more than one-electron storage of Na in NaVCr(PO). Combining a series of advanced characterization techniques such as ex situ V solid-state nuclear magnetic resonance, X-ray absorption near-edge structure, and in situ X-ray diffraction, we reveal that V/V and V/V redox couples in the materials can be accessed, leading to a 1.5-electron reaction. It is also found that a light change on the local electronic and structural states or phase change could be observed after the first cycle, resulting in the fast capacity fade at room temperature. We also showed that the irreversibility of the phase changes could be largely suppressed at low temperature, thus leading to a much improved electrochemical performance.

electric vehicles. [1] The next-generation LIBs must meet a multitude of stringent requirements, including excellent cycle stability, high capacity, high energy and power densities, high safety, and low cost. [2] As an alternative to the graphite anode for commercial LIBs, Si has attracted considerable attention, due to its high theoretical capacity of 3579 mA h g −1 , low operating potential, abundance, and low cost. However, the practical applications of Si-based anode materials are hindered by low Li + diffusivity and electrical conductivity and especially by drastic volume changes (≈300%) during lithiation/delithiation. The latter problem can cause Si pulverization, loss of electrical contact, and consumption of active Li associated with the unstable evolution of solid electrolyte interphases (SEIs). As a result, Si-based anodes generally exhibit low Coulombic efficiency, poor cycle stability, and rate capability. [3] To address these problems, one effective approach is to use Si nanoparticles (NPs) for the facile accommodation of large volume changes. But the high specific surface area associated with nanomaterials causes aggregation of Si NPs during cycling and provides plenty of active surface sites to form SEIs. Moreover, repeated expansion and contraction of Si NPs induce fracture and uncontrolled formation of Si/C composites represent one promising class of anode materials for next-generation lithium-ion batteries. To achieve high performances ofSi-based anodes, it is critical to control the surface oxide of Si particles, so as to harness the chemomechanical confinement effect of surface oxide on the large volume changes of Si particles during lithiation/delithiation. Here a systematic study of Si@SiO x /C nanocomposite electrodes consisting of Si nanoparticles covered by a thin layer of surface oxide with a tunable thickness in the range of 1-10 nm is reported. It is shown that the oxidation temperature and time not only control the thickness of the surface oxide, but also change the structure and valence state of Si in the surface oxide. These factors can have a strong influence on the lithiation/delithiation behavior of Si nanoparticles, leading to different electrochemical performances. By combining experimental and modeling studies, an optimal thickness of about 5 nm for the surface oxide layer of Si nanoparticles is identified, which enables a combination of high capacity and long cycle stability of the Si@SiO x /C nanocomposite anodes. This work provides an in-depth understanding of the effects of surface oxide on the Si/C nanocomposite electrodes. Insights gained are important for the design of high-performance Si/C composite electrodes.

Structural and interfacial instability of the LiCoO 2 cathode under a voltage exceeding 4.5 V (vs Li/Li +) severely hinders its practical applications for high-energy-density lithium batteries. Herein, a modified electrolyte with nitriles (suberonitrile or 1,3,6-hexanetricarbonitrile) and fluoroethylene carbonate (FEC) coadditives is demonstrated to form an ultrathin and uniform interface layer on LiCoO 2 cathode under a synergetic effect. As such, LiCoO 2 / Li cells display excellent cyclability at a cutoff voltage of 4.6 V with a capacity retention over 72% after 300 cycles and 60% after 200 cycles at 30 and 55 °C, respectively, even achieving operation at a high current rate (10 C) upon 500 cycles as compared to the controls with fast-falling capacity to zero. Furthermore, an adsorption-coordination mechanism between nitriles and cobalt and synergetic effect of coadditives are explored by the alliance of spectroscopic analysis and theoretical calculations. The contributed lone-pairs on the N 2p orbital of nitriles in coordination lowers the real oxidation state of Co 3+/4+ so that it decreases its catalysis on electrolytes, and the synergy from nitrile-derived species regulates FEC to form an LiF-containing electroninsulated interface layer. This work shares a new insight to nitriles with the synergy of coadditives and paves a way to refine (ultra)high-voltage LiCoO 2 cathode for high-energy-density energy storages.

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