Mitigating the Large‐Volume Phase Transition of P2‐Type Cathodes by Synergetic Effect of Multiple Ions for Improved Sodium‐Ion Batteries

Abstract

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.

Transition metal oxides as a cathode for indispensable Na-ion batteries

“…Finally this was concluded that strategy of doping the elements is the rationale route to improve the electrochemical performance of the cathode material. 89 …”

The Relationship between Initial Coulombic Efficiency and Transition Metal Ion Redox in P2-Na0.85[Cu0.1FexMn1–x]O2 Cathodes

“…The (004) diffraction peak also has a similar change trend, and (100) diffraction peak shows the opposite change trend. This reflected that the lattice parameter c increased and the lattice parameters a and b decreased during the Na + extraction process, which was mainly attributed to the increase of electrostatic repulsion between adjacent TM layers . After the initial charge–discharge process, the lattice parameter c is slightly larger than that of the pristine electrode.…”

“…This reflected that the lattice parameter c increased and the lattice parameters a and b decreased during the Na + extraction process, 55 which was mainly attributed to the increase of electrostatic repulsion between adjacent TM layers. 56 After the initial charge−discharge process, the lattice parameter c is slightly larger than that of the pristine electrode. This means that there is a small range of irreversible changes, which may lead to capacity fading during long-term cycling.…”

Structural degradation mechanisms and modulation technologies of layered oxide cathodes for sodium‐ion batteries

“…Cu 2+ doping at the TM site can stabilize the TM layer, and the Zn 2+ at the Na site can enhance the electrostatic cohesion between two adjacent TM layers, changing the phase transition from an irreversible P2−O2 phase to a reversible P2−Z phase. 97 Similarly, Cheng et al [ 103 ] designed a P2‐Na 0.7 Li 0.03 Mg 0.03 Ni 0.27 Mn 0.6 Ti 0.07 O 2 cathode to mitigate the large volume change via multiion cosubstitution (Li−Mg−Ti). The synergic effect of these three metal ions suppresses the unfavorable P2−O2 transition with large volume change ( V = 19.4%) and promotes the formation of a “Z”‐intergrowth structure with small volume change ( V = 2.5%) by increasing the energy barrier of TM slab gliding (Figure 8h–j).…”

“…Reproduced with permission: Copyright 2022, Wiley. [ 103 ] NMR, nuclear magnetic resonance; XRD, X‐ray diffraction.…”