P过量反应体系合成LiMn_0.8Fe_0.19Mg_0.01PO₄正极材料

【作者】 褚旭；

【导师】 方海升； 廖兴群；

【作者基本信息】 昆明理工大学 ， 冶金工程（专业学位）， 2021， 硕士

【摘要】 合成是材料研究和应用的基础,材料的合成影响材料的结构和性能。水热法已广泛应用于制备各种功能类材料,因为它可以在较低的温度下直接从溶液中制备晶体粉末,不需要任何的后处理,除此之外还可以通过控制水热合成的条件灵活地改善晶体粉末的结构和性质,如相组成、粒度和形貌等。由于上述的这些优点,水热法是制备锂离子电池正极材料的常用方法之一,已应用于三大锂离子电池正极材料之一的锂过渡金属磷酸盐（LiMPO₄,M=Fe,Mn）材料的制备。近20年来水热法是基于Li过量的反应体系。最近我们组提出了一个基于P过量反应体系的新策略,用来水热法合成材料LiMPO₄（M=Fe,Mn）。LiMPO₄（M=Fe,Mn）材料可以在P过量反应体系中合成,且合成的样品具有良好的电化学性能,但反应体系中P过量度对LiMPO₄（M=Fe,Mn）材料的水热合成和性能如何影响尚不清楚。故本文选取多组分LiMn_0.8Fe_0.19Mg_0.01PO₄材料为研究对象,在P过量反应体系中水热合成一系列LiMn_0.8Fe_0.19Mg_0.01PO₄材料,详细探究了P过量度对LiMn_0.8Fe_0.19Mg_0.01PO₄材料的影响。相关测试结果表明,随着P过量度的调节,水热反应悬浮液的初始pH值也随之改变,从而改变了Fe²⁺在水热反应悬浮液中的稳定性及水热产物颗粒的分散性。当P过量度固定时,水热反应悬浮液的初始pH值超过10时对水热合成LiMn_0.8Fe_0.19Mg_0.01PO₄材料是不利的,发生不良的Fe²⁺氧化和颗粒团聚现象增多,导致其性能较差。在水热反应悬浮液的初始pH值不变的情况下,增加P的过量度可以在一定程度上抑制水热合成过程中非预期的Fe²⁺氧化的发生,改善样品颗粒的分散性,从而显著地提高水热法合成LiMn_0.8Fe_0.19Mg_0.01PO₄/C复合材料的电化学性能。在所有合成的复合材料中,在P过量80%和水热反应悬浮液的初始pH值为9.39的条件下合成的LiMn_0.8Fe_0.19Mg_0.01PO₄/C复合材料在0.1 C倍率下的放电比容量为146.6 m Ah g^-1,其具有最小的电荷转移电阻和最大的Li⁺扩散速率。综上所述,适度的P过量可以抑制水热合成过程中不良氧化反应的发生和改善LiMn_0.8Fe_0.19Mg_0.01PO₄颗粒的分散性,因此制得的样品具有更好的电化学性能。更多还原

【Abstract】 Synthesis is of fundamental importance for materials research and application,and the structure and property of a material will depend on how it is synthesized Hydrothermal synthesis is known as a powerful process widely utilized to prepare various functional materials,because it can prepare crystalline powders directly from the solution at substantially lower temperatures without the need of post heat treatment,and moreover the powders structure and property such as phase composition,particle size and morphology can be flexibly tuned by controlling the hydrothermal synthesis conditions.Owing to these advantages,hydrothermal synthesis has become a common way to prepare cathode materials for lithium-ion batteries,especially the olivine lithium transition metal phosphates（LiMPO₄,M=Fe,Mn）which are one of three types of cathode materials in practical use.The strategy for hydrothermal synthesis was always based on a Li excess reaction system for nearly two decades.Recently our group communicated a new strategy based on a P excess reaction system to hydrothermally synthesize LiMPO₄（M=Fe,Mn）.We demonstrated that LiMPO₄（M=Fe,Mn）could be hydrothermally synthesized in the P excess reaction system and could give competitive electrochemical performance.In addition,the hydrothermal synthesis and property of LiMPO₄（M=Fe,Mn）was apparently affected by the degree of P excess in the reaction system,which deserves further detailed studies to better understand the chemistry of this new hydrothermal synthesis based on a P excess reaction system.Therefore,in this paper,a multi-component LiMn_0.8Fe_0.19Mg_0.01PO₄ is hydrothermally synthesized in the P excess reaction system as a case study and the effect of the degree of P excess was investigated in detail.In this paper,a series of LiMn_0.8Fe_0.19Mg_0.01PO₄ samples are hydrothermally synthesized from the P excess reaction system and the effect of P excess was studied in detail.Our results show that varying the degree of P excess accompanies changed pH value of the reaction suspension,which can alter the stability of Fe²⁺in the suspension and the particles dispersion of the hydrothermal product.When the degree of P excess is fixed,the pH values exceeding 10is negative to hydrothermal synthesis due to the occurrence of undesirable Fe²⁺oxidation and increased particles,which lead to poor performance.When the pH value remains constant,increasing P excess can suppress the occurrence of undesired Fe²⁺oxidation during hydrothermal synthesis and improve the particles dispersion,which can significantly improve the electrochemical performance of the hydrothermal samples.The electrochemical test demonstrates LiMn_0.8Fe_0.19Mg_0.01PO₄/C composite with 80 at.%P excess and pH=9.39 shows the superior specific discharge capacity of 146.6 m Ah g^-1 at0.1 C and also exhibits the smallest charge transfer impedance and the largest lithium-ion diffusion coefficient.In conclusion,a proper degree of P excess can improve the hydrothermal synthesis of LiMn_0.8Fe_0.19Mg_0.01PO₄,and much better electrochemical performance can be achieved for the obtained product.更多还原