This review summarizes the use of ionic liquids in Na secondary batteries and discusses their electrochemical performance with various electrode materials.

A review to delineate the chemistry and physics for honeycomb layered oxides.

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Sodium manganese orthosilicate, Na 2 MnSiO 4 , was synthesized by a sol-gel method and investigated for use as a positive electrode material for Na secondary batteries using Na[FSA]-[C 3 C 1 pyrr][FSA] (FSA = bis(fluorosulfonyl)amide anion and C 3 C 1 pyrr = N-methyl-N-propylpyrrolidinium cation) ionic liquid electrolyte in the temperature range 298-363 K. Carbon coating and elevation of operational temperatures significantly improved the electrode performance. A reversible capacity of 125 mAh g-1 (90% of the theoretical value based on a one-electron transfer process) was found at a rate of C/10 (13.9 mA g-1) within 2.0-4.0 V at 363 K, with an acceptably high rate capability. Na 2 MnSiO 4 became amorphous upon the electrochemical removal of sodium, exhibiting a similar behavior as its lithium equivalent. Both Na 2 MnSiO 4 and its desodiated form (Na 0.8 MnSiO 4) possess remarkable thermal stability, suggesting their safety characteristic.

Halogen redox couples offer several advantages for energy storage such as low cost, high solubility in water, and high redox potential. However, the operational complexity of storing halogens at the oxidation state via liquid‐phase media hampers their widespread application in energy‐storage devices. Herein, an aqueous zinc–dual‐halogen battery system taking the advantages of redox flow batteries (inherent scalability) and intercalation chemistry (high capacity) is designed and fabricated. To enhance specific energy, the designed cell exploits both bromine and chlorine as the cathode redox couples that are present as halozinc complexes in a newly developed molten hydrate electrolyte, which is distinctive to the conventional zinc–bromine batteries. Benefiting from the reversible uptake of halogens at the graphite cathode, exclusive reliance on earth‐abundant elements, and membrane‐free and possible flow‐through configuration, the proposed battery can potentially realize high‐performance massive electric energy storage at a reasonable cost.

A comprehensive understanding of the charge/discharge behaviour of high-capacity anode active materials, e.g., Si and Li, is essential for the design and development of next-generation high-performance Li-based batteries. Here, we demonstrate the in situ scanning electron microscopy (in situ SEM) of Si anodes in a configuration analogous to actual lithium-ion batteries (LIBs) with an ionic liquid (IL) that is expected to be a functional LIB electrolyte in the future. We discovered that variations in the morphology of Si active materials during charge/discharge processes is strongly dependent on their size and shape. Even the diffusion of atomic Li into Si materials can be visualized using a back-scattering electron imaging technique. The electrode reactions were successfully recorded as video clips. This in situ SEM technique can simultaneously provide useful data on, for example, morphological variations and elemental distributions, as well as electrochemical data.

The electrochemical performance of a Na 2 FeP 2 O 7 positive electrode has been evaluated in an inorganic ionic liquid NaFSA-KFSA (FSA = bis(fluorosulfonyl)amide) at 363 K. The electrode delivers a reversible capacity of 91 mAh g-1 with excellent rate capability (59 mAh g-1 at 2000 mA g-1) and a capacity retention of 91% over 1000 cycles, which facilitates the development low-cost and high-safety sodium secondary batteries for large-scale energy storage applications. The average oxidation state of iron increases upon sodium extraction, as evidenced by the edge shift of an X-ray absorption spectroscopy analysis. According to an extended X-ray absorption fine structure analysis, the sodium extraction is accompanied by a shortening of Fe-O bonds.

The electrochemical properties of a Na 2 FeP 2 O 7 positive electrode were investigated in the ionic liquid sodium bis(fluorosulfonyl)amide (NaFSA)-N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (C 3 C 1 pyrrFSA). A stable charge-discharge behavior was obtained over the temperature range 253-363 K. The cell offered nearly a one-electron theoretical capacity of about 90 mAh g −1 at 298-363 K. The rate capability showed considerable enhancement with increasing temperature, delivering 50 mAh g −1 at a very high rate of 4000 mA g −1 (ca. 41 C) at 363 K. Furthermore, the Na 2 FeP 2 O 7 positive electrode demonstrated excellent cyclability, exceeding 300 cycles, in terms of both capacity retention and coulombic efficiency at 298-363 K, as only negligible capacity fade (<1%) was observed. This good performance over a wide temperature range will meet the requirements of the versatile applications expected for Na secondary batteries.promising platform for the broad spectrum of applications expected for Na secondary batteries. 4 5 6 7 8