Ion-Doping-Site-Variation-Induced Composite Cathode Adjustment: A Case Study of Layer–Tunnel Na_0.6MnO₂ with Mg²⁺ Doping at Na/Mn Site

Abstract

Composite cathodes have attracted great attention due to the integrated advantages of each pure structure. Also, the component ratio deserves a careful modulation to further improve the corresponding electrochemical performance. Mn-based layer–tunnel hybrid composite became a focus in sodium-ion batteries due to the superiority in terms of high performance, low cost, and nontoxicity. In the previous reports, the structure modulation was carried out via changing the synthesis condition, varying the transition-metal-element ratio, and different ion doping. Also, it is still challenging to explore a more feasible method to simplify the adjustment process. Herein, we introduced Mg2+ into Na sites or transition-metal sites in Na0.6MnO2 and first discovered the doping-site-variation-induced structural adjustment phenomenon. Specifically, Mg doping in transition-metal sites could be beneficial for the growth of the P2-type structure, while layer/tunnel component ratio decreased when locating Mg2+ in Na sites. The P2–O2 phase transformations could be effectively suppressed by locating Mg2+ in both sites in high-voltage regions and thus improve the cycling performance. The designed material, Na0.6Mn0.99Mg0.01O2, could attain a decent capacity of 100 mA h g–1 at 1000 mA g–1 and a satisfied retention of 76.6% after 500 cycles. Additionally, ex situ X-ray diffraction analysis experiments verify the excellent structural stability of Mg-substituted samples during charge–discharge processes. Moreover, the Na0.6Mn0.99Mg0.01O2 also displays superior sodium-ion full-cell properties when merged with hard carbon anode. Thus, this research may indicate a proper novel thread for designing high-performance composite electrodes.

Simultaneously Enhancing Structural Stability and Cationic Redox in Na0.67Mn0.75Fe0.25O2 through a Synergy of Multisite Substitution

“…The XRD peaks of all samples are consistent with the P2-type triangular prism structure with the P6 3 /mmc space group (No 194). ,, As presented in Figure b, the (002) peaks shift toward the lower angle as a result of multielement substitution. The left shift of the (002) peak indicates the increase of the cell parameter c value, which is conducive to the enlargement of Na layer spacing. ,, The atom occupancy of bare-MF, MFCa, MFMg, and MFCaMg from Rietveld refinement has been shown in Tables S1–S4. The doped atoms have been introduced into the designed lattice sites successfully. In addition, the XRD refinement results have been applied to reconstruct the crystal structure of all four samples, as provided in Figure a–d.…”

“…The left shift of the (002) peak indicates the increase of the cell parameter c value, which is conducive to the enlargement of Na layer spacing. 41,48,50 The atom occupancy of bare-MF, MFCa, MFMg, and MFCaMg from Rietveld refinement has been shown in Tables S1−S4. The doped atoms have been introduced into the designed lattice sites successfully.…”

“…It is well accepted that the Jahn−Teller effect resulting from the component of Mn 3+ can lead to structure distortion, which is harmful to the stability of the layered structure and further causes rapid capacity fading. 27,50 Therefore, the decrease of Mn 3+ benefits the layered structural stability and cycling performance of the SIBs. Figure 5b demonstrates that the ratios of Mn 3+ and Mn 4+ were 1.0231, 0.6279, 0.6383, and 0.7733 for bare-MF, MFCaMg, MFCa, and MFMg, respectively.…”

Optimizing the Mg Doping Concentration of Na3V2–xMgx(PO4)2F3/C for Enhanced Sodiation/Desodiation Properties

“…This phase transition was observed for the undoped oxide from a series of ex situ X-ray diffraction (XRD) patterns upon charging, while the Mg-doped sample still preserved the P2 structure. It was further confirmed that regardless of the occupancies of Mg 2+ at the transition-metal sites or Na sites, the undesirable P2–O2 phase transition can be clearly suppressed, leading to superior cycling stability . Nevertheless, the effects of Mg 2+ doping on the material and electrochemical properties of NVPF have not yet been explored.…”

“…It was further confirmed that regardless of the occupancies of Mg 2+ at the transition-metal sites or Na sites, the undesirable P2−O2 phase transition can be clearly suppressed, leading to superior cycling stability. 29 Nevertheless, the effects of Mg 2+ doping on the material and electrochemical properties of NVPF have not yet been explored. This study will thus address this issue.…”