硅酸盐学报

层状O3型-NaNi_0.6Co_0.2Mn_0.2O₂(NNCM622)材料因其比容量较高、成本低易合成，被认为是极具潜力的钠离子电池(NIBs)正极材料之一。本工作通过调节合成过程中的热处理温度，系统研究了不同煅烧温度(700、750、800℃)对层状O3型-NNCM622正极材料结构、形貌及电化学性能的影响。结果表明，在750℃、24 h热处理条件下合成样品的结晶度最好，样品呈现的是由1次颗粒聚集而成的2次规整的球形颗粒组成。对该材料在1.5～4.2 V电压范围内进行电化学性能测试，其在0.1 C倍率下的放电比容量为150.5 mA·h·g^–1，初始库仑效率为86.5%;5 C高电流密度下放电比容量为85.9 mA·h·g^–1，库仑效率为83.2%。

Introduction Sodium-ion batteries(NIBs) have attracted much attention as promising alternatives to lithium-ion batteries due to the natural abundance of sodium, low cost, and enhanced safety. Among various cathode materials, layered transition metal oxides, particularly O3-type structures, are considered highly attractive due to their high specific capacity, moderate operating voltage, and facile synthesis. High-nickel ternary oxides such as NaNi_0.6Co_0.2Mn_0.2O₂(NNCM622) exhibit superior capacity and economic viability, making them candidates for next-generation NIBs. However, the electrochemical performance of these materials is highly sensitive to synthesis conditions, especially the heat treatment process, significantly affecting the crystallinity, morphology, and structural stability. This work was to investigate the influence of calcination temperature on the structure, morphology, and electrochemical properties of O3-type NNCM622 cathode materials synthesized via a conventional solid-state reaction. Method In the preparation of O3-type NNCM622 cathode materials, stoichiometric amounts of Ni_0.6Co_0.2Mn_0.2(OH)₂ precursor and NaOH(with a 5% excess of sodium source) were thoroughly mixed and subsequently calcined in an oxygen atmosphere at 700, 750 ℃ and 800 ℃ for 24 hours, yielding the final products denoted as NNCM622-700, NNCM622-750, and NNCM622-800, respectively. Results and discussion The X-ray diffraction(XRD) patterns show that all the samples crystallize in an O3-type layered structure(space group R-3m). The clear splitting of the(006)/(012) and(018)/(110) diffraction peaks indicates a well-developed layered structure. The Rietveld refinement reveals that the sample calcined at 750 ℃(i.e., NNCM622-750) possesses the lowest refinement discrepancy factor(R_wp = 3.374%) and optimal crystallinity. In contrast, the sample NNCM622-700 has a poorer crystallinity due to incomplete reaction at a lower temperature, while the sample NNCM622-800 likely suffers from sodium volatilization and potential structural distortions at a higher temperature. The X-ray photoelectron spectra(XPS) on the sample NNCM622-750 indicate the coexistence of Ni²⁺/Ni³⁺, Co, and Mn in the material, which is consistent with the expected valence states for this composition. The inductively coupled plasma optical emission spectra(ICP-OES) show that the elemental compositions of all the samples are close to the theoretical stoichiometry. The images of scanning electron microscopy(SEM) reveal that all the samples consist of spherical secondary particles with the sizes of 2–10 μm formed due to the agglomeration of numerous primary particles. The sample NNCM622-700 has larger primary particles due to insufficient reaction, whereas the sample NNCM622-800 exhibits over-sintering and particle cracking. The images of transmission electron microscopy(TEM) and the mapping of energy dispersive X-ray spectroscopy(EDS) of the sample NNCM622-750 further confirm its well-crystallized layered structure with an interplanar spacing of 0.263 nm, corresponding to the(102) plane, and a homogeneous distribution of elements Na, Ni, Co, and Mn. The electrochemical performance is evaluated in half-cells versus sodium metal. The NNCM622-750 cathode has a high initial discharge capacity of 150.5 mA·h·g^–1 at 0.1 C(1 C = 150 mA·h·g^–1) within the voltage window of 1.5–4.2 V, with a high initial Coulombic efficiency of 86.5%. This cathode also demonstrates a superior rate capability, retaining discharge capacities of 106, 95 mA·h·g^–1 and 85.9 mA·h·g^–1 at 1 C, 2 C, and 5 C, respectively. After 100 cycles at 1 C, this cathode maintains 55.3% of its initial capacity, showing a better cycling stability than the other two samples. In contrast, the NNCM622-700 and NNCM622-800 cathodes deliver lower initial capacities(i.e., 119.8 mA·h·g^–1 and 97.8 mA·h·g^–1, respectively) and suffer from more rapid capacity decay. This performance degradation is attributed to incomplete sodiation(for NNCM622-700) and sodium loss/structural defects(for NNCM622-800), increasing the electrode impedance and hindering Na diffusion. The cyclic voltammetry(CV) curves of the NNCM622-750 cathode shows a pair of well-defined redox peaks around 2.3/2.7 V, corresponding to the Ni²⁺/N⁴⁺ redox reaction. The minimum peak potential separation(ΔV = 0.4 V) for NNCM622-750 indicates the lowest polarization and optimum reaction reversibility among the three samples. The electrochemical impedance spectra(EIS) reveal that the NNCM622-750 electrode has the minimum charge-transfer resistance(i.e., R_ct = 327.1 ?). Furthermore, based on the Warburg coefficient derived from the EIS data, the maximum Na⁺ diffusion coefficient(D_Na⁺) for NNCM622-750(i.e., 7.53×10^–15 cm²·s^–1) can be obtained, compared to that for NNCM622-700(i.e., 5.12×10^–15 cm²·s^–1) and NNCM622-800(i.e., 1.92×10^–17 cm²·s^–1). These results corroborate its superior rate performance. Conclusions The nickel-rich O3-type NNCM622 cathode materials were synthesized by a high-temperature solid-state reaction method, and their microstructures and electrochemical properties were investigated. The O3-type NNCM622 cathode material prepared under calcination at 750 ℃ for 24 h exhibited a superior crystallinity, a well-defined layered structure, and the minimum Na⁺ diffusion barrier due to its excellent structural and compositional synergy. As a result, the NNCM622-750 cathode delivered the maximum initial discharge specific capacity(i.e., 150.5 mA·h·g^–1 at 0.1 C with a coulombic efficiency of 86.5%), superior rate performance(i.e., 85.9 mA·h·g^–1 at 5 C), and relatively high cycling stability(i.e., capacity retention of 55.3% after 100 cycles at 1 C). An appropriate calcination temperature could enhance the reversibility and cycling performance of the cathode material, thereby improving the structural stability and electrochemical properties of nickel-rich O3-type NNCM622 cathodes. This work could present a simple and low-cost preparation process for synthesizing nickel-rich O3-type NNCM622 cathode materials with superior electrochemical performance, offering valuable insights for addressing the challenges associated with heat treatment in the development of next-generation layered cathode materials for sodium-ion batteries.