ion batteries (LIBs) by virtue of their similar charge storage mechanism, as well as the abundance and low cost of Na resources. [1][2][3][4] Compared with lithium ions, sodium ions have a larger radius and heavier mass (1.02 vs. 0.76 Å and ≈23 vs. ≈6.9 g mol −1 ), which makes SIBs compete unfavorably in terms of energy density with LIBs, thus limiting their applications in portable electronics and electric vehicles. However, SIBs show great promise in the applications where cost and sustainability are top priority, such as large-scale energy storage. [5][6][7][8][9] Like LIBs, cathode materials are also the main factor limiting the energy density and cost of SIBs. Finding suitable sodium intercalation hosts is pressing. Until now many types of materials have been explored, including layered transition metal oxides, polyanionic compounds, Prussian blue-based compounds, and organic compounds. [10][11][12] Among these candidates, layered transition metal oxides Na x TMO 2 (TM referring to transition metals) is extremely promising on account of its simple structure, high compositional diversity, easy synthesis and attractive electrochemical performance. According to the oxygen stacking sequence and the Na + coordination environment, Layered transition metal oxide P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 usually suffers from largevolume phase transitions and different Na-vacancy ordering during sodium (de)intercalation, incurring rapid capacity decline and poor rate capability. Herein, an effective strategy based on synergetic effect of selected multiple metal ions is designed for P2-type cathodes with improved performance. The role of tetravalent titanium provides high redox potential, inactive divalent magnesium stabilizes the structure, and the monovalent lithium smooths the electrochemical curves. The combined analysis of in operando X-ray diffraction, in operando X-ray absorption spectroscopy and density functional theory calculations demonstrates the contribution of multi-metal ions converts the unfavorable and large-volume P2 to O2 transition into a moderate "Z"-intergrowth structure by increasing the energy barrier of transition metal slab gliding. As a consequence, the resultant P2-Na 0.7 Li 0.03 Mg 0.03 Ni 0.27 Mn 0.6 Ti 0.07 O 2 electrode delivers a reversible capacity of 134 mAh g −1 , a working voltage of 3.57 V, excellent cycling stability (82% of capacity retention after 200 cycles), and superior rate performance (110 mAh g −1 at 4 C). Full cells fabricated with a hard carbon anode achieve an energy density of 296 Wh kg −1 . This study presents a route to rationally design cathode materials with this functionalization to improve the cell performance for sodium-ion batteries.

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Layered oxide cathodes usually exhibit high compositional diversity, thus providing controllable electrochemical performance for Na-ion batteries. These abundant components lead to complicated structural chemistry, closely affecting the stacking preference, phase transition and Na + kinetics. With this perspective, we explore the thermodynamically stable phase diagram of various P2/O3 composites based on a rational biphasic tailoring strategy. Then a specific P2/O3 composite is investigated and compared with its monophasic counterparts. A highly reversible structural evolution of P2/O3-P2/O3/P3-P2/P3-P2/Z/O3'-Z/O3' based on the Ni 2 + /Ni 3.5 + , Fe 3 + /Fe 4 + and Mn 3.8 + /Mn 4 + redox couples upon sequential Na extraction/insertion is revealed. The reduced structural strain at the phase boundary alleviates the phase transition and decreases the lattice mismatch during cycling, endowing the biphasic electrode a large reversible capacity of 144 mAh g À 1 with the energy density approaching 514 Wh kg À 1 .

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