【作者】 罗鑫；

【导师】 韦伟峰；

【作者基本信息】 中南大学 ， 材料学， 2023， 硕士

【摘要】 由于具备高的工作电压和理论容量,O′3-Na₃Ni₂Sb O₆和P2-Na_0.67Mn_0.67Ni_0.33O₂两类典型的镍基层状过渡金属氧化物正极材料受到广泛研究。然而由于钠离子扩散动力学较差以及高电压不可逆相转变、过渡金属离子溶出和界面副反应等,O′3-Na₃Ni₂Sb O₆蜂窝有序层状材料和P2-Na_0.67Mn_0.67Ni_0.33O₂层状材料的倍率性能和循环寿命有待进一步改善。本论文针对上述问题,利用材料微结构调控的策略,分别采用形貌调控和梯度结构设计的方法来提升其倍率性能和循环寿命等电化学性能,并深入研究了上述改性方法的作用机理,主要研究内容如下:（1）采用静电纺丝技术制备了蜂窝有序Na₃Ni₂Sb O₆层状正极材料,制备出的材料为由纳米尺寸颗粒组装成的空心纤维形貌。纳米尺寸颗粒能够有效缩短Na⁺的传输路径从而加快Na⁺的扩散动力学。空心纤维结构有利于电解液的充分渗透。纳米纤维的三维网络骨架结构能够缓冲Na⁺重复脱嵌于层状材料带来的形变应力,提升结构稳定性。因此,具有纳米颗粒组装空心纤维形貌的蜂窝有序Na₃Ni₂Sb O₆层状正极材料的Na⁺扩散动力学得到显著改善(扩散系数由3.66×10^-14 cm²s^-1提升至1.11×10^-12 cm² s^-1),0.1 C下循环50圈容量保持率由41.1%提升至56.6%。（2）采用高温固相法合成了兼具离子掺杂和表面包覆的P2/P3@MgO梯度结构,深入研究了Mg²⁺掺杂和MgO包覆层对P2-Na_0.67Mn_0.67Ni_0.33O₂层状材料晶体结构、形貌、电化学性能和相变演化过程的影响。其中,Mg²⁺掺杂促进了P3相的形成,而P3相更大的Na层间距为Na⁺的传输提供了更宽阔的通道。Mg²⁺有效抑制了高电压下层状材料P2-O2的相变过程,将其转化为体积变化更小的P2-Z转变。MgO包覆层有效隔绝了电解液中HF和H₂O对层状材料的腐蚀,抑制了界面副反应的发生和过渡金属离子的溶出。因此,P2/P3@MgO梯度结构在1.5-4.5 V电压区间内的电化学性能得到大幅提升,0.2 C循环100圈容量保持率由原始的42.6%提升至69.7%。图42幅,表11个,参考文献127篇更多还原

【Abstract】 Owing to high working voltage and theoretical capacity,O’3-Na₃Ni₂Sb O₆ and P2-Na_0.67Mn_0.67Ni_0.33O₂,two kinds of typical nickel-based layered cathode materials,have been widely studied.However,the rate performance and cycle life of O’3-Na₃Ni₂Sb O₆ and P2-Na_0.67Mn_0.67Ni_0.33O₂ are still hindered by unfavorable Na⁺diffusion kinetics and unreversible phase transition at high voltage,dissolution of transition metals and surface side reactions.Herein,to address above issues,microstructural regulation through morphology regulation and design of gradient structure were applied to improve electrochemical performance.The function mechanism of above modification methods was deeply studied.The main contents are as follows:（1）Honeycomb ordered Na₃Ni₂Sb O₆ layered cathode material was synthesized by electrospinning,which shows a hollow fiber morphology assembled by nanoscale particles.The nano-sized particles can effectively shorten the transport path of Na⁺and accelerate the diffusion kinetics of Na⁺.The hollow structure is beneficial to the full penetration of the electrolyte.The three-dimensional network skeleton structure of nanofibers can cushion the deformation stress caused by the repetitious sodiation/desodiation of layered materials,improving the structural stability.Therefore,the Na⁺diffusion kinetics of honeycomb ordered Na₃Ni₂Sb O₆ layered cathode material with hollow fiber morphology assembled by nanoparticles is significantly improved from 3.66×10^-14 cm²s^-1 to 1.11×10^-12 cm² s^-1,and the capacity retention is increased from 41.1%to 56.6%after 50 cycles at 0.1 C.（2）P2/P3@MgO gradient structure with both ion doping and surface coating was synthesized by high temperature solid state method.The effects of Mg²⁺doping and MgO coating on the crystal structure,morphology,electrochemical performance and phase transition evolution of P2-Na_0.67Mn_0.67Ni_0.33O₂ layered material were studied.Among them,Mg²⁺doping promotes the formation of P3 phase,while the larger Na layer spacing of P3 phase provides a wider channel for the diffusion of Na⁺.In addition,Mg²⁺effectively suppresses the P2-O2 phase transition process of layered material at high voltage,transforming it into P2-Z transition with smaller volume change.Besides,the MgO coating can effectively isolate the corrosion of layered materials by HF and H₂O in the electrolyte,and inhibit the interfacial side reactions and the dissolution of transition metal ions.Consequently,the electrochemical performance in the voltage range from 1.5-4.5 V are greatly improved,and the capacity retention of 0.2 C is increased from 42.6%to 69.7%after 100 cycles.更多还原