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• Five types of Na x MnO 2+ z are prepared by the rheological phase reaction method. • Na x MnO 2+ z materials with different sodium contents exhibit rod-shaped, rod-sheet mixed, and sheet-like morphology. • Na 0.44 MnO 2 exhibits excellent cyclic stability and high rate capability. • Na 0.44 MnO 2 exhibits a high contribution rate of pseudo capacitance. • In situ XRD testing showed the reversible phase transition of NMO-0.44 material during charge and discharge processes. Sodium-ion batteries (SIBs) experience a re-emerging academic interest due to their low cost and high abundance as compared with lithium-ion batteries (LIBs) which have considerably increased the worldwide cost due to high actual demand. At present, a representative amount of cathode/anode SIBs materials are developed; facilitating the Na+ insertion mechanism understanding and the respective influence on structure change, which in most cases it is directly related to sodium concentration. In this work, the rheological phase reaction method is used to synthesize five types of Na x MnO 2+ z materials with different sodium content and use them as the cathode materials for SIBs. The physical properties and electrochemical performances of Na x MnO 2+ z materials with different concentrations are investigated by physical characterization means, electrochemical tests, and a series of computational methods. Inductively coupled plasma optical emission spectrometer (ICP-OES) test results indicate that the right materials have been successfully synthesized, while scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements show how the changes in sodium concentration significantly impact the morphology of the formed particles. In the results of electrochemical investigations, NMO-0.67 material has a high initial specific discharge capacity, but NMO-0.44 material has a reasonable cycling stability, keeping its discharge specific capacity of 83.06 mAh/g after 1000 cycles at a high current density of 360 mA g−1. Galvanostatic intermittent titration technique (GITT) testing and pseudo-capacitance analyses are also used to disclose the dynamic properties of NMO-0.44 material. In conclusion, the rheological phase reaction approach may be used to successfully create Na x MnO 2+ z materials, and variations in sodium content have a big impact on their structure and electrochemical performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Na 2 THBQ were combined with CDs as cathode materials in SIBs for the first time. • o -Na 2 THBQ/CDs-2 performed good cycling stability and rate performance. • The mechanism of o -Na 2 THBQ/CDs-2 was explored by ex-situ tests. • The sodium ion diffusion coefficient of the materials was tested by GITT. Organic compounds are considered auspicious due to their widespread presence in nature, low cost, and sustainability. However, the inherent drawbacks of these organics' compounds, like low-conductivity and high-solubility, restrict their large-scale application. To solve these problems, we innovatively combined carbon quantum dots (CDs) with active substances (tetrahydroxybenzoquinone ortho -disodium salt (o -Na 2 THBQ)) at different mass ratios. The optimized o -Na 2 THBQ/CDs-2 (the mass ratio of o -Na 2 THBQ to CDs is 10:2) exhibited excellent electrochemical performance, the first-cycle discharge specific capacity was 410 mAh g−1 and maintained 182 mAh g−1 after 500 cycles, and the Coulombic efficiency was nearly 100 %. After 1300 cycles, the o -Na 2 THBQ/CDs-2 composite demonstrated a high specific capacity of 63 mAh g−1 at 500 mA g−1. This work provides a reference to design an optimal composite ratio of carbon dots (CDs) and organics to fabricate cathode materials for high-performance sodium-ion batteries. [ABSTRACT FROM AUTHOR]