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• The K 0.4 Mn 0.7 Ti 0.1 Ni 0.1 Cu 0.1 O 2 exhibits superior rate and long cycling performance. • DFT calculations indicate that the electronic energy level has been changed. • In-situ XRD analysis reveals the K+ storage mechanism in K 0.4 Mn 0.7 Ti 0.1 Ni 0.1 Cu 0.1 O 2. Mn-based layered oxides have become one of the most promising cathode materials in potassium-ion batteries (PIBs) due to their high theoretical specific capacity. Nonetheless, the Jahn-Teller effect of Mn3+ leads to lattice distortion and a high K+ migration barrier, resulting in structural instability during the charge/discharge processes. To effectively overcome these problems, this work provides a method to change electronic energy levels to suppress the Jahn-Teller effect through the synergistic regulation of multiple specific element doping. The X-ray diffraction pattern of K 0.4 Mn 0.7 Ti 0.1 Ni 0.1 Cu 0.1 O 2 shows its P3-type structure. The electrochemical test results demonstrate that K 0.4 Mn 0.7 Ti 0.1 Ni 0.1 Cu 0.1 O 2 exhibits superior rate capacity, higher discharge specific capacity, and significant cycling stability (with a capacity retention rate of 80 % after 860 charge/discharge cycles). The in-situ X-ray diffraction pattern demonstrates the high reversibility of the electrochemical process of K+ insertion/extraction. First-principles calculations have confirmed that the electronic energy levels of K 0.4 Mn 0.7 Ti 0.1 Ni 0.1 Cu 0.1 O 2 have better dispersion, the K+ migration channel has been effectively optimized, and the K+ migration barrier has been reduced. This study provides a significant and effective reference for the design of advanced PIBs with high performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • NFMPP could deliver a higher average working potential of 3.27 V with a theoretical capacity of 120 mAh g−1. • In-situ XRD and ex-situ XPS revealed NFMPP presented good electrochemical reversibility. • Carbon coating NFMPP exhibits excellent electrochemical performance. • The NFMPP-HC full cell shows a maximum power density of 260 W kg−1 with a maximum specifc energy of 130 Wh kg−1. Iron-based phosphates as a low cost and high structural stability cathode materials for sodium ion batteries (SIBs) have been widely studied. However, the working potential basing on Fe3+/Fe2+ redox is very low (less than 3.05 V vs. Na/Na+), which has obviously affect on the energy/power density. In this work, we choose the non-precious metal manganese (Mn) to improve the working potential of Na 3 Fe 2 (PO 4)(P 2 O 7) (NFFPP). A series of mixed-phosphate Na 3 Fe 2-x Mn x (PO 4)(P 2 O 7) (x = 0.0, 0.5, 1.0, 1.5 and 2.0) compounds are firstly proposed as cathode materials for SIBs. After optimization, the as-prepared Na 3 FeMn(PO 4)(P 2 O 7) (NFMPP) could deliver a higher average working potential of 3.27 V (vs. Na/Na+) with a theoretical capacity of 120 mAh g−1. Furthermore, the carbon coating NFMPP cathode shows a reversible initial specific capacity of 105 mAh g−1 at 0.1C with a corresponding coulombic efficiency of 99%. The in-situ XRD and ex-situ XPS results indicate that this cathode material goes through the reversible redox Fe3+/Fe2+ and Mn3+/Mn2+ with a reversible two Na-ion insertion/deinsertion process. [ABSTRACT FROM AUTHOR]