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Sodium-ion batteries (SIBs) are gaining much attentions due to their potentials to achieve large scale energy storage coupled with relatively low cost. However, typically used organic electrolytes with high flammability and poor thermal stability have impeded further developments of SIBs in large-scale energy storage. Ionic liquids (ILs), featuring excellent thermal stability, non-flammability and wide electrochemical window have been promising alternative electrolytes for SIBs. Herein, rechargeable Na/Na3V2(PO4)3cells with NaPF6-incorporated 1-butyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide BMITFSI IL electrolyte are demonstrated to reduce the flammability of the electrolyte and improve the safety of cells. Besides, the electrochemical performances of SIBs are enhanced by forming a stable solid-electrolyte interphase (SEI) layer on the cathode. The optimized Na/Na3V2(PO4)3cells with the IL electrolyte exhibit high initial discharge specific capacity of 107.2 mA h/g and good cycling performance, which is superior to that using carbonate organic electrolyte. Remarkably, the formation mechanism of the SEI layer on the Na3V2(PO4)3cathode in NaPF6/BMITFSI IL electrolyte for SIBs is put forward in this work according to density functional theory (DFT) calculations and confirmed by Raman, energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and fourier-transform infrared spectra (FTIR) tests, where NaOH, Na2SO4, Na2S2O7and NaF are assigned as the major inorganic components of SEI. These results show that NaPF6/BMITFSI is a promising electrolyte to improve the safety and electrochemical performances of SIBs.

Enhancing electrochemical activity and structural stability of transition metal sulfides (TMSs) are critical for improving the capacity output and retention of TMSs-based batteries. Here, we report a new paradigmatic approach for fabricating TMSs/C composites, adopting a metallothermic-sulfidation-carbonization strategy (2TM + CS2= 2TMS + C) based on zeolitic imidazolate frameworks (ZIFs) to synchronously construct a compact TMSs@Graphene/CNxtriple heterostructure. The obtained structure features crystalline TMSs nanoparticles wrapped by few-layer graphene and totally embedded within porous carbonized polyhedral frameworks. All three nanocomponents of TMSs@Graphene/CNxare connected via chemical bonding of S−C and TM−C, forming a chemical cross-linked nanostructure. Such structure design bears intrinsic advantages in improving the volumetric-efficiency for accommodating TMSs and electrical properties, enabling promising electrochemical performance in lithium- and sodium-ion storage. As a representative, the ZnS@Graphene/CNxelectrode exhibits a high capacity of 891.5 mAh g−1and an excellent retention of 80 % after 1000 cycles in lithium-ion batteries. More notably, this general metallothermic-sulfidation-carbonization mechanism can be applicable to all ZIFs, defining a new ZIFs-derived TMSs/C heterostructures.

Whether FeF3can take active part in electrochemical reaction is largely determined by its conductivity, which can be affected by the band gap and crystallite dimension. In this communication, the density of states (DOS) of FeF3and Ti-doped FeF3were calculated using a first principle density functional theory (DFT). Moreover, crystalline size was calculated according to Debye-Scherrer Equation. The results indicate that Ti-doping can reduce the band gap and impact the microcrystal growth of FeF3at the same time. Both effects work synergistically on capacity loss and cycling stability; while impact antagonistically on charge transfer resistance (Rct), Li+diffusion coefficient (DLi+) and specific capacity, leading to the excellent electrochemical performances of Fe(1−x)TixF3/C. The Fe0.99Ti0.01F3/C nanocomposite achieves an initial capacity of 219.8mAh/g and retains a discharge capacity of 173.6mAh/g after 30 cycles at room temperature in the voltage range of 2.0–4.5V. The hysteresis of discharge voltage plateau is significantly mitigated as well. In addition, the three-electron reaction of Fe0.99Ti0.01F3/C during 1.0–4.5V exhibits a high initial specific discharge capacity of 764.6mAh/g. This study suggests that not only the band gap, but also the microcrystalline structure can be changed by Ti-doping, both of which have remarkable effects on the electrochemical properties, providing a new perspective on the effect of cation dopant.

Advanced lithium-ion batteries for renewable energy storage applications have become a major research interest in recent years. Much better performance can be realized by improvements in the material surface design, especially for the cathode materials. Here, we present a new design for a surface protective layer formed via a facile aqueous solution process in which a nano-architectured layer of LiF/FeF3is epitaxially grown on bulk hierarchical Li-rich cathode Li[Li0.2Ni0.2Mn0.6]O2. Coin cell tests of this material in the voltage range of 2–4.8V indicated a high reversible capacity (260.1mAhg−1at 0.1C), superior rate performance (129.9mAhg−1at 20C), and excellent capacity retention. Differential scanning calorimetry showed good thermal stability. The enhanced capacity and cycling stability are attributed to the suppression of interfacial side reactions as well as the conversion reaction resulting from the introduction of LiF/FeF3as a surface protective layer.

The shuttle effect in lithium-sulfur (Li–S) batteries mainly originates from the diffusion of soluble polysulfides (LiPSs) and their depressed redox kinetics. Herein, we report a synergistic engineering layer composed of acorn shell porous carbon/Sn4P3nanodots electrocatalyst (AS PC-Sn4P3). The synergistic engineering layer can not only serve as a conductive interface but also provide a dual-adsorption barrier to retain active material and inhibit the LiPSs migrating. More importantly, electrocatalytic Sn4P3nanodots supported on acorn shell porous carbon (AS PC) within synergistic engineering layer effectively promote lithium ion diffusion, LiPSs conversion, Li2S2/Li2S deposition, and accelerate the electrochemical redox reaction and therefore curb the soluble LiPSs shutting behavior. As a result, enhanced Li–S battery performance is achieved with synergistic engineering layer, e.g., excellent cycling stability over 900 cycles at 1.0C with a low capacity decay of 0.046% per cycle, a good rate performance, and a high areal capacity of 8.7 mAh cm−2under lean electrolyte conditions.