Rechargeable Na metal batteries have gained great recognition as a promising candidate for nextgeneration battery systems, largely on the basis of the high theoretical specific capacity (1165 mAh g −1 ) and low redox potential (−2.71 V versus the standard hydrogen electrode) of Na metal, as well as the natural abundance of Na and the similarities between these batteries and lithium batteries. Much effort has been dedicated to improving the electrochemical performance of rechargeable Na batteries through the development of high-performance cathodes, anodes, and electrolytes. Nevertheless, the practical application of Na metal batteries is quite challenging because the high chemical and electrochemical reactivity of Na metal electrodes with organic liquid electrolytes leads to low Coulombic efficiencies and limited cycling performance. Severe electrolyte decomposition at the Na metal electrode results in the formation of a resistive and non-uniform surface film, leading to dendritic Na metal growth. To control the Na metal electrode-electrolyte interface for high performance Na metal batteries, considerable efforts have been made to find electrolyte systems that are stable at the Na metal electrode. Using fluoroethylene carbonate (FEC) as an electrolyte additive for in situ formation of an artificial solid electrolyte interphase (SEI) layer could stabilize the anodeelectrolyte interface. However, the FEC-derived SEI acted as a resistive layer, impeding the sodiation-desodiation process and reducing the reversible capacity of the anodes. Finding new electrolyte systems that are stable at the Na metal electrode and possess high oxidation durability at high-voltage cathodes is necessary for the development of high-performance Na metal batteries.Very recently, there are some papers which introduced significant breakthroughs in lithium battery electrolytes. It is reported that improving the cycling efficiency of lithium plating/stripping and suppressing the formation of dendritic lithium metal is possible by using highly concentrated electrolytes, even at high current densities. And it is also reported that highly concentrated electroltyes can inhibit the dissolution of transition metals out of the 5 V-class LiNi0.5Mn1.5O4 (LNMO) electrode material and the corrosion of the Al current collector at high voltage conditions. After reading these papers, I thought that applying this highly concentrated electrolyte system to sodium metal batteries could be the solution for improvements in the electrochemical performance of Na metal anodes coupled with high-voltage cathodes.In this study, an ultraconcentrated electrolyte composed of 5 M sodium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane will be introduced for Na metal anodes coupled with high-voltage cathodes.Using this electrolyte, a very high Coulombic efficiency of 99.3% at the 120 th cycle for Na plating/stripping is obtained in Na/stainless steel (SS) cells, with highly reduced corrosivity toward Na metal and high oxidation durability (over 4.9 V versus Na/Na ...

4,4′-Biphenyldicarboxylate sodium coordination compounds with different crystal structures are evaluated as anode materials for Na-ion batteries.

Various doped materials have been investigated to improve the structural stability of layered transition metal oxides for lithium‐ion batteries. Most doped materials are obtained through solid state methods, in which the doping of cations is not strictly site selective. This paper demonstrates, for the first time, an in situ electrochemical site‐selective doping process that selectively substitutes Li+ at Li sites in Mn‐rich layered oxides with Mg2+. Mg2+ cations are electrochemically intercalated into Li sites in delithiated Mn‐rich layered oxides, resulting in the formation of [Li1−xMgy][Mn1−zMz]O2 (M = Co and Ni). This Mg2+ intercalation is irreversible, leading to the favorable doping of Mg2+ at the Li sites. More interestingly, the amount of intercalated Mg2+ dopants increases with the increasing amount of Mn in Li1−x[Mn1−zMz]O2, which is attributed to the fact that the Mn‐to‐O electron transfer enhances the attractive interaction between Mg2+ dopants and electronegative Oδ− atoms. Moreover, Mg2+ at the Li sites in layered oxides suppresses cation mixing during cycling, resulting in markedly improved capacity retention over 200 cycles. The first‐principle calculations further clarify the role of Mg2+ in reduced cation mixing during cycling. The new concept of in situ electrochemical doping provides a new avenue for the development of various selectively doped materials.

The issue of the safety of Li‐ion batteries is becoming more critical with the increase in their size for applications in large energy storage devices, such as hybrid electric vehicles (HEVs), and energy storage systems (ESSs) for smart grids. The thermal runaway of Li‐ion batteries is considered to be caused by their flammable components, such as the volatile carbonate solvents of electrolytes. Room temperature ionic liquids (RTILs) have recently received much attention because of their characteristics of non‐flammability and non‐volatility. In addition, RTILs show high ionic conductivity and a wide electrochemical stability window. Therefore, RTIL‐based electrolytes are considered one of the most promising candidates to improve the safety of Na‐ion, as well as Li‐ion batteries; indeed, RTIL‐based electrolytes have shown excellent improvements in terms of thermal stability and electrochemical performance. This review provides a comprehensive overview of selected RTIL materials, including their physicochemical and electrochemical properties. Moreover, we discuss the failure mechanisms of certain RTIL‐based electrolytes with various electrodes to suggest alternative strategies for improving their electrochemical performance.

Mild‐acid Zn‐MnO2 batteries have been considered a promising alternative to Li‐ion batteries for large scale energy storage systems because of their high safety. There have been remarkable improvements in the electrochemical performance of Zn‐MnO2 batteries, although the reaction mechanism of the MnO2 cathode is not fully understood and still remains controversial. Herein, the reversible dissolution/deposition (Mn2+/Mn4+) mechanism of the MnO2 cathode through a 2e− reaction is directly evidenced using solution‐based analyses, including electron spin resonance spectroscopy and the designed electrochemical experiments. Solid MnO2 (Mn4+) is reduced into Mn2+ (aq) dissolved in the electrolyte during discharge. Mn2+ ions are then deposited on the cathode surface in the form of the mixture of the poorly crystalline Zn‐containing MnO2 compounds through two‐step reactions during charge. Moreover, the failure mechanism of mild‐acid Zn‐MnO2 batteries is elucidated in terms of the loss of electrochemically active Mn2+. In this regard, a porous carbon interlayer is introduced to entrap the dissolved Mn2+ ions. The carbon interlayer suppresses the loss of Mn2+ during cycling, resulting in the excellent electrochemical performance of pouch‐type Zn‐MnO2 cells, such as negligible capacity fading over 100 cycles. These findings provide fundamental insights into strategies to improve the electrochemical performance of aqueous Zn‐MnO2 batteries.