The zinc-ion battery (ZIB) is a 2 century-old technology but has recently attracted renewed interest owing to the possibility of switching from primary to rechargeable ZIBs. Nowadays, ZIBs employing a mild aqueous electrolyte are considered one of the most promising candidates for emerging energy storage systems (ESS) and portable electronics applications due to their environmental friendliness, safety, low cost, and acceptable energy density. However, there are many drawbacks associated with these batteries that have not yet been resolved. In this Review, we present the challenges and recent developments related to rechargeable ZIB research. Recent research trends and directions on electrode materials that can store Zn 2+ and electrolytes that can improve the battery performance are comprehensively discussed.

Surface stabilization of cathode materials is urgent for guaranteeing longterm cyclability, and is important in Na cells where a corrosive Na-based electrolyte is used. The surface of P2-type layered Na 2/3 [Ni 1/3 Mn 2/3 ]O 2 is modified with ionic, conducting sodium phosphate (NaPO 3 ) nanolayers, ≈10 nm in thickness, via melt-impregnation at 300 °C; the nanolayers are autogenously formed from the reaction of NH 4 H 2 PO 4 with surface sodium residues. Although the material suffers from a large anisotropic change in the c-axis due to transformation from the P2 to O2 phase above 4 V versus Na + /Na, the NaPO 3 -coated Na 2/3 [Ni 1/3 Mn 2/3 ]O 2 /hard carbon full cell exhibits excellent capacity retention for 300 cycles, with 73% retention. The surface NaPO 3 nanolayers positively impact the cell performance by scavenging HF and H 2 O in the electrolyte, leading to less formation of byproducts on the surface of the cathodes, which lowers the cell resistance, as evidenced by X-ray photoelectron spectroscopy and time-of-flight secondary-ion mass spectroscopy. Time-resolved in situ high-temperature X-ray diffraction study reveals that the NaPO 3 coating layer is delayed for decomposition to Mn 3 O 4 , thereby suppressing oxygen release in the highly desodiated state, enabling delay of exothermic decomposition. The findings presented herein are applicable to the development of high-voltage cathode materials for sodium batteries.

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SIBs), for which the reaction chemistries are similar to those of LIBs. [4][5][6][7][8] To reach similar energy densities as LIBs, promising cathode materials for SIBs must possess high capacity to compensate for their intrinsically low operation voltages. As the capacities of cathode materials can reach their limit when using transition metal redox, it is anticipated that redox of oxygen in the crystal structure can contribute additional capacity and boost the resulting energy density. [9,10] Representative works were performed in the early 2000s, specifically, on Li 2 MnO 3 (Li[Li 1/3 Mn 2/3 ]O 2 ) layered material, [11,12] which has the same crystal structure as typical LiTMO 2 (TM = transition metal). Li 2 MnO 3 is electrochemically inactive because Mn 4+ /Mn 5+ redox is not available within the normal cutoff voltage window. However, the material delivered a capacity beyond the theoretical limit attributed to the transition metal redox (300 mAh g −1 ). [12] Earlier works suggested that the delivered capacity could be attributed to oxygen loss from the oxide lattice; [12] however, state-of-the-art characterization later proved that the main contributor to the capacity was associated with the oxygen redox, [13] which triggered the intensive study of oxygen redox. Recently, there are some arguments to verify the chemical state of lattice oxygen during electrochemical reaction. Earlier work by Tarascon et al. [9] demonstrated the oxygen activity using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and electron paramagnetic resonance (EPR) in Li 2 Ru 1-y Sn y O 3 compound. In situ or operando Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) become important tools in identifying the formation of peroxolike species (O 2 n− ). [14,15] Yang and Devereaux [16] highlighted the importance of using resonant inelastic X-ray scattering (RIXS) to identify the activity of lattice oxygen in oxide materials. From the above facts, it is considered that combination of the above-mentioned characterization tools with theoretical thermodynamic prediction may provide more reliable results to understand the oxygen redox chemistry.The oxygen redox reaction has also been extensively investigated in SIBs to achieve additional capacity. [17][18][19] For SIBs, Na 2 MnO 3 (Na[Na 1/3 Mn 2/3 ]O 2 ), which has the same crystal structure as Li 2 MnO 3 , has also been considered despite the large difference in the ionic size between sodium and manganese. For Recently, anionic-redox-based materials have shown promising electrochemical performance as cathode materials for sodium-ion batteries. However, one of the limiting factors in the development of oxygen-redoxbased electrodes is their low operating voltage. In this study, the operating voltage of oxygen-redox-based electrodes is raised by incorporating nickel into P2-type Na 2/3 [Zn 0.3 Mn 0.7 ]O 2 in such a way that the zinc is partially substituted by nickel. As designed, the resulting P2-type Na 2/3 [(Ni 0.5 Zn 0.5 ) 0.3 Mn 0.7 ]O 2 electrode ...

Ni incorporation into Na2/3MnO2 suppresses the cooperative Jahn–Teller distortion and leads to a P2–O2 phase transition with about 13% of volume change.

resources. [1][2][3][4][5] However, because of the slightly higher standard electrode potential of sodium (−2.7 V vs the standard hydrogen electrode (SHE)) than lithium (−3.02 V vs SHE), high-capacity electrode materials are needed for SIBs to compensate for their lower operation voltage and to achieve energy densities comparable to those of LIBs. It can be assumed that the cost of SIBs can be lowered relative to that of LIBs because of the cost-effectiveness of sodium resources. [6] These factors encourage us to explore potential highenergy-density cathode materials for SIBs.Among the various crystal structures of Na-containing compounds, layered structures have been intensively investigated because of their high capacity delivered under moderate operation conditions. The related layered structures are classified as O3, P2, P3, etc., according to the stacking sequence of the transition metals and alkali ions. [7][8][9] Layered sodium transition metal oxides of the form Na x MO 2 (M = Mn, Fe, Co, Ni, Cr, etc.) are stable in the P2 structure for Na contents (x) in the range of 0.3-0.7. This structure enables sodium ion diffusion between the two face-sharing trigonal prismatic sites such that P2-type layered materials generally exhibit higher discharge capacities than other layered materials. [10][11][12][13][14][15][16][17][18] Na 0.67 MnO 2 is one of the most common compounds among P2-type materials. [19][20][21][22][23] It delivers a high discharge capacity of over 175 mAh g −1 but exhibits severe capacity fade during cycling. One of the main reasons for this capacity fade is the structural disintegration caused by the Jahn-Teller effect of Mn 3+ ions in the MnO 6 octahedra, with elongation of the Mn 3+ -O distance along one direction. This Jahn-Teller effect can be mitigated by increasing the overall Mn oxidation state by substituting Mn with other elements, such as Ni, Co, Fe, Mg, and Al. [24][25][26][27][28][29] For example, improved cyclability was achieved in Mg-substituted Na 0.67 Mn 1−x Mg x O 2 (0.0 ≤ x ≤ 0.2) by suppressing the electrochemical activity of the Jahn-Teller Mn 3+ ions; however, this improvement was attained at the expense of the specific capacity. [26] Recently, Yabuuchi et al. [27] introduced a high-capacity P2-type Na 2/3 [Mg 0.28 Mn 0.72 ]O 2 material that exhibited capacities of over 150 mAh g −1 on charge and 220 mAh g −1 on discharge by raising the upper voltage cut-off to 4.6 V. Note that the average oxidation state of Mn in the compound is ≈3.85+, meaning that it cannot exhibit such a high charge capacity. The A high-rate of oxygen redox assisted by cobalt in layered sodium-based compounds is achieved. The rationally designed Na 0.6 [Mg 0.2 Mn 0.6 Co 0.2 ]O 2 exhibits outstanding electrode performance, delivering a discharge capacity of 214 mAh g −1 (26 mA g −1 ) with capacity retention of 87% after 100 cycles. High rate performance is also achieved at 7C (1.82 A g −1 ) with a capacity of 107 mAh g −1 . Surprisingly, the Na 0.6 [Mg 0.2 Mn 0.6 Co 0.2 ]O 2 compound is able to deliver...