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.

With the ever‐increasing requirement for high‐energy density lithium‐ion batteries (LIBs) to drive pure/hybrid electric vehicles (EVs), considerable attention has been paid to the development of cathode materials with high energy densities because they ultimately determine the energy density of LIBs. Notably, the cost of cathode materials is still the main obstacle hindering the extensive application of EVs, with the cost accounting for 40% of the total cost of fabricating LIBs. Therefore, enhancing the energy density and simultaneously decreasing the cost of LIBs are essential for the success of EV/hybrid EV industries. Among the existing commercial cathodes, Ni‐rich layered cathodes are widely employed because of their high energy density, relatively good rate capability, and reasonable cycling performance. Ni‐rich layered cathodes containing Co are now being reconsidered due to the increasing price of Co, which is much higher than that of Ni and Mn. In this report, the recent developments and strategies in the improvement of the stabilities of the bulk and surface for Co‐less Ni‐rich layered cathode materials are reviewed.

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Nano-scale Al2O3 coating was effective at resolving the degradation pathways of the cathode surface in sodium-ion batteries.

Potassium-ion batteries have emerged as an alternative to lithium-ion batteries as energy storage systems. In particular, K x MnO2 has attracted considerable attention as a cathode material because of its high theoretical capacity and low cost. In this study, partial substitution of Mn in P3-type K0.5MnO2 with divalent Ni is performed, resulting in a first discharge capacity of approximately 121 mAh (g-oxide)−1 with 82% retention for 100 cycles. Operando synchrotron X-ray diffraction analysis reveals the occurrence of phase transition from P3 to O3 on charge and O3–P3–P′3 transition on discharge at the first cycle, where P′3 is a new distorted form of the P3 phase, accompanied by reversible Mn4+/3+ and Ni3+/2+ redox pairs, as evidenced by X-ray absorption spectroscopy. The reduced variation in the lattice parameters during de/potassiation for P3-K0.5[Ni0.1Mn0.9]­O2 relative to P3-K0.5MnO2 is suggested as a possible reason for the enhanced electrochemical performance of K0.5[Ni0.1Mn0.9]­O2. These results open the possibility of using inexpensive and high-capacity Mn-based cathode active materials for potassium-ion batteries.

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.

1901912 (1 of 10) Herein, Ti 4+ in P′2-Na 0.67 [(Mn 0.78 Fe 0.22 ) 0.9 Ti 0.1 ]O 2 is proposed as a new strategy for optimization of Mn-based cathode materials for sodium-ion batteries, which enables a single phase reaction during de-/sodiation. The approach is to utilize the stronger Ti-O bond in the transition metal layers that can suppress the movements of Mn-O and Fe-O by sharing the oxygen with Ti by the sequence of Mn-O-Ti-O-Fe.It delivers a discharge capacity of ≈180 mAh g −1 over 200 cycles (86% retention), with S-shaped smooth charge-discharge curves associated with a small volume change during cycling. The single phase reaction with a small volume change is further confirmed by operando synchrotron X-ray diffraction. The low activation barrier energy of ≈541 meV for Na + diffusion is predicted using first-principles calculations. As a result, Na 0.67 [(Mn 0.78 Fe 0.22 ) 0.9 Ti 0.1 ]O 2 can deliver a high reversible capacity of ≈153 mAh g −1 even at 5C (1.3 A g −1 ), which corresponds to ≈85% of the capacity at 0.1C (26 mA g −1 ). The nature of the sodium storage mechanism governing the ultrahigh electrode performance in a full cell with a hard carbon anode is elucidated, revealing the excellent cyclability and good retention (≈80%) for 500 cycles (111 mAh g −1 ) at 5C (1.3 A g −1 ).reactions. [3][4][5] Although SIBs have intrinsic drawbacks, such as their low operation voltages relative to those of LIBs and the difficulty of ready insertion of sodium ions because of the larger size of Na + ions (1.02 Å) compared with Li + ions (0.76 Å), these difficulties can be mitigated with a high capacity to compensate for the low operation voltage. [5] Layered cathode material for SIBs (Na x MeO 2 ) has received particular attention owing to their relatively high capacity and structural stability. Layered Na x MeO 2 (x = 0.5-1 and Me; transition metal) consist of MeO 2 layers sharing edges with MeO 6 octahedra were classified into two groups based on structure: trigonal prismatic (P type: P2, P3, and P′2) and octahedral (O type: O3). [6] The differences in these structures are attributed to sodium ions being respectively located at the district trigonal prismatic or octahedral crystallographic sites sandwiched between the MeO 2 sheets. Among them, many works have introduced Mn-based cathode materials, mainly P2-type materials, in which sodium ions are located at prismatic sites with an AABB oxygen stacking sequence, because of their low cost, good performance, and nontoxicity. [7,8] Recently, many works about P2-type materials have been investigated such as Na x MnO 2 , [7,9] Na x CoO 2 , [10] Na x VO 2 , [11] Na x [Ni,Mn] O 2 , [12] Na x [Fe,Mn]O 2 , [13] Na x [Ni,Fe,Mn]O 2 , [14] Na x [Mg,Mn] O 2 , [15] Na x [Ni,Mg,Mn]O 2 . [16] P2-type materials crystallize in a hexagonal structure; however, the transition metal layers can be distorted at higher temperature with stabilization in an orthorhombic structure (represented as P′2) albeit with the same chemical composition. [17,18] Many works have introduced Mn-based c...