Insertion compounds have been dominating the cathodes in commercial lithium‐ion batteries. In contrast to layered oxides and polyanion compounds, the development of spinel‐structured cathodes is a little behind. Owing to a series of advantageous properties, such as high operating voltage (≈4.7 V), high capacity (≈135 mAh g−1), low environmental impact, and low fabrication cost, the high‐voltage spinel LiNi0.5Mn1.5O4 represents a high‐power cathode for advancing high‐energy‐density Li+‐ion batteries. However, the wide application and commercialization of this cathode are hampered by its poor cycling performance. Recent progress in both the fundamental understanding of the degradation mechanism and the exploration of strategies to enhance the cycling stability of high‐voltage spinel cathodes have drawn continuous attention toward this promising insertion cathode. In this review article, the structure–property correlations and the failure mode of high‐voltage spinel cathodes are first discussed. Then, the recent advances in mitigating the cycling stability issue of high‐voltage spinel cathodes are summarized, including the various approaches of structural design, doping, surface coating, and electrolyte modification. Finally, future perspectives and research directions are put forward, aiming at providing insightful information for the development of practical high‐voltage spinel cathodes.

Two distinct advantages of nonaqueous redox flow batteries (RFBs) are the feasibility of building a high cell voltage (without a constraint of the water-splitting potential) and the operability at low temperatures (without a concern of freezing below 0 °C). However, electrochemically active organic redox couples are usually selectively soluble in specific nonaqueous solvents, and their solubility is relatively low (in contrast to that in aqueous solutions). The selective and low solubility of redox couples seriously constrict the practical energy density of nonaqueous RFBs. Herein, we present a hybrid nonaqueous RFB with a solid zinc anode and a liquid (2,2,6,6tetramethylpiperidin-1-yl)oxyl (TEMPO) cathode. Toward accessing a high solubility of the TEMPO cathode and to sufficiently accommodate the discharge products of a Zn anode, asymmetric electrolyte solvents, viz., propylene carbonate (PC) and acetonitrile (ACN), have, respectively, been employed at the cathode and anode. To prevent a mixing of the two asymmetric electrolyte solvents, a NASICON-type Na + -ion conductive solid-state electrolyte (SSE, Na 3 Zr 2 Si 2 PO 12 ) is employed to serve as a mediator-ion separator. The shuttling of Na + ions through the Na 3 Zr 2 Si 2 PO 12 SSE sustains the ionic charge balance between the two electrodes. The Zn-TEMPO nonaqueous cell with a stable energy density of ca. 12−18 Wh L −1 over 50 cycles was demonstrated.

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Among the diverse set of secondary batteries, redox flow batteries (RFBs) have long been considered as a highly promising, affordable system for grid-scale energy storage in facilitating electricity generation from naturally sustainable sources. [8] Traditionally, research and development of RFBs have been mainly focused on aqueous systems in which redox active electrode materials are dissolved in water. [9] However, there are two major limitations associated with the use of aqueous electrolytes: 1) the cell voltage of an aqueous RFB is strictly constrained by the electrochemical window of water electrolysis; and 2) aqueous RFBs are not able to work over a broad range of temperatures due to the limitation of the freezing point and boiling point of water. [10] Therefore, RFBs with nonaqueous electrolytes have drawn much interest in the electrochemical energy storage community. [11] In contrast to water-based systems, nonaqueous electrolytes can offer a relatively broader potential window for operating RFBs. [12] Moreover, nonaqueous electrolytes can usually offer a lower freezing point and a higher volatile temperature (in contrast to water), allowing the operation of nonaqueous RFBs below 0 °C in cold-weather regions and at high temperatures in hot-weather regions. [13] Most of the research on nonaqueous RFBs has been focused on the exploration of organic redox couples and the screening of suitable nonaqueous electrolytes. The relevant cell chemistries and systems have usually been tested/demonstrated with traditional porous polymer membranes to separate the positive and negative electrolytes. [14] However, the use of porous polymeric membranes usually poses undesirable crossover of chemical species between the anode and cathode. [15] Research into membranes that are especially for nonaqueous RFBs is rare. In this study, we present a mediator-ion approach for the advancement of nonaqueous RFBs by employing a Na +-ion solid electrolyte. The anode and cathode electrolytes are separated with a solid electrolyte, which can totally circumvent the cross-mixing of the two liquid electrode materials. Electrochemical reactions at the anode and cathode are ionically linked by the shuttling of Na +ions through the solid electrolyte membrane. The proposed mediator-ion RFB concept is validated and demonstrated with a chromium (III) acetylacetonate symmetric redox chemistry and nitrobenzene (NB)-bromine redox couple. Nonaqueous redox flow batteries (RFBs) provide significant advantages over aqueous RFBs because the use of nonaqueous electrolytes offers the possibilities of operating RFBs at higher voltages (above the water breakdown voltage) and low temperatures (below 0 °C, the freezing point of H 2 O). However, the advancement of RFBs has long been plagued by the chemicalcrossover problem due to a lack of reliable separators. Herein, a single-ion mediation strategy is presented for the advancement of nonaqueous RFBs by employing a Na +-ion solid electrolyte. The catholyte and anolyte in a single cell are physically and elect...