The capacity fluctuation phenomenon during cycling, which is closely related with solid electrolyte interphase and plays a key role for the design for advanced electrode, could be frequently observed in the titanium-based anode. However, the underlying reason for capacity fluctuation still remains unclear with rare related reports. Here, the origin of capacity fluctuation is verified with a long-life NaTiO anode. The reaction mechanism, structural evolution and reaction kinetics during the reported sodiation/desodiation processes were carefully investigated. The gradually enhanced diffusion controlled contribution resulted in the capacity increasing. And the capacity decay could be ascribed to the irreversible reaction of metallic titanium formation and the increasing potential polarization. It is worth noting that sodium ions seem to partially reduce NTO to metallic state, which is irreversible. The present study can provide more information for the design of advanced NaTiO anode.

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Voltage polarization during cycling, the charge potential increase of anode or discharge plateau decrease of cathode, is widely observed and would lower the output voltage. Conversely, an anomalous potential plateau negative migration phenomenon was observed in Cu x S anode of sodium-ion battery. In this study, the background mechanism was clarified from the switch of intercalation−conversion reactions and structure evolution. The dynamic cooperation between intercalation and conversion reactions may root the potential plateau negative migration during cycling. In the initial stage, the intercalation-type reaction with Na 3 Cu 4 S 4 and Na 4 Cu 2 S 3 products at 2.13 and 1.92 V would dominate the early migration process of potential plateaus. In the second stage, the conversion-type reaction dominated by Na 2 S and metallic copper formed at 1.85 and 1.53 V in the later period. The aforementioned results would provide new perspective on the electrochemical behavior of transition-metal sulfide anode and provide a clue for reducing voltage polarization.

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.

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