Potassium-ion batteries (PIBs) are one of the promising alternatives to lithium-ion batteries (LIBs). Layered potassium manganese oxides are more attractive as cathodes for PIBs due to their high capacity, low cost, and simple synthesis method but suffer from the Jahn−Teller effect of Mn 3+ in material synthesis. Here, a layered P3-type K 0.67 Mn 0.83 Ni 0.17 O 2 material with a suppressed Jahn−Teller effect was successfully synthesized. K 0.67 Mn 0.83 Ni 0.17 O 2 delivers a specific capacity of 122 mAh g −1 at 20 mA g −1 in the first discharge, superior rate performance, and good cycling stability (75% capacity retention cycled at a high rate of 500 mA g −1 after 200 cycles). Besides, the K ion diffusion coefficient of the K 0.67 Mn 0.83 Ni 0.17 O 2 electrode can reach 10 −11 cm 2 s −1 , which are larger than the Ni-free electrode. The X-ray diffraction and electron diffraction analyses demonstrate that appropriate nickel could suppress the Jahn−Teller effect and reduce the structural deterioration, resulting in more migration pathways for K ions, thus enhancing the rate capability and cycling performance. These results provide a strategy to develop highperformance cathode materials for PIBs and deepen the understanding of structural deterioration in layered manganese-based oxides.

thus designing high-performance cathodes is essential for the commercial application of SIBs. [7][8][9] So far, layered transition metal oxides, polyanions, and organic compounds are the main several kinds of cathodes to be studied for SIBs. [10][11][12] Among these materials, the layered oxides is one of the most promising candidates for their large theoretical capacities and simple analogies with LiTMO 2 . [13] Among the layered oxides, layered Fe-based oxides have attracted great interest due to their low cost and abundant resources. O3-type NaFeO 2 (NFO) has been extensively studied, [14][15][16] however, the reversible capacity is limited about 80 mA h g −1 at the narrow potential region, far below the theoretical capacity. One of the major hurdles is the Fe-ion migration to the Na layers when charged to high potential, thereby blocking the Na-ion diffusion path. [17] Doping with 3d transition-metal ions [18] in the slabs of O3-type NFO such as Ti, [19] Co, [20,21] and Ni [22] is an usual way, which indeed enhances the electrochemical performance. Through active Ni doping, O3-NaFe 0.3 Ni 0.7 O 2 could deliver a large capacity of 135 mA h g −1 , however, the capacity still rapidly drops within 30 cycles. Thus, to improve the structural stability remains a great challenge.Herein, we report a strategy of Ru substitution in NFO, which successfully suppresses the Fe-ion migration during cycling. We employed the advanced atomic-scale characterization to monitor the migration of Fe-ion.Moreover, the particle cracks is first discovered in cycling NFO which may be another major reason for the poor electrochemical performance. [23][24][25] Fe-ion migration has a direct correlation with the particle cracks generation. Results show that huge and intense internal strain which is the driving force for the particle cracks, arises in the distorted structure coming from Fe-ion migration. Besides, Fe-ion migration is part of the atomic mechanism at the initial stage of the particle cracks growth. [26] Correspondingly, Ru doping could also reduce the generation of particle cracks effectively.Comparatively, Ru doping NFO demonstrates the Fe-migration-free phenomena and accordingly suppresses the resulted side effect. Ru doping NFO could deliver a large Sodium-ion batteries have huge potential in large-scale energy storage applications. Layered Fe-based oxides are one of the desirable cathode materials due to abundance in the earth crust and high activity in electrochemical processes. However, Fe-ion migration to Na layers is one of the major hurdles leading to irreversible structural degradation. Herein, it is revealed that distinct Fe-ion migration in cycling NaFeO 2 (NFO) should be mainly responsible for the strong local lattice strain and resulting particle cracks, all of which results in the deterioration of electrochemical performance. More importantly, a strategy of Ru doping could effectively suppress the Fe-ion migration and then reduce the local lattice strain and the particle cracks, finally to greatly enhance the sod...

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