【作者】 刘浩；

【导师】 郭孝东； 但勇；

【作者基本信息】 四川大学 ， 化学工程（专业学位）， 2021， 硕士

【摘要】 富锂锰基层状氧化物因其高能量密度和低成本优势成为下一代理想的锂离子电池正极材料。然而,富锂材料存在的许多问题阻碍了其商业化应用。首先,在高电压下氧和锂的不可逆损失会导致其首圈库伦效率较低;其次,电极材料易与电解液发生副反应,形成较厚的电极材料电解质界面层（CEI）,降低材料的离子和电子电导率,导致较差的倍率性能;然后,材料晶体结构在高电压下因为膨胀、收缩所产生的结构应力会造成二次颗粒破裂,导致不理想的循环性能;最后,过渡金属离子的迁移和溶解会诱导材料晶体结构出现层状相到尖晶石相的不可逆相变,导致长周期循环过程中严重的电压滞后及电压衰减。目前针对上述问题主要的改性策略包括表面涂层、表面/体相离子掺杂、形貌调控、构建浓度梯度材料结构等。虽然这些改性策略都能一定程度上解决富锂材料所存在的问题,改善其电化学性能,但是对于均匀异质相涂层协同离子掺杂的改性策略仍然缺乏相关研究。因此,设计一种巧妙且深入的异质相涂层掺杂表面整合策略应用于富锂材料,对于加快其商业化进程具有重要意义。本论文以无钴富锂锰基层状氧化物Li_1.2Ni_0.2Mn_0.6O₂正极材料作为研究对象,进行了不同种类异质相涂层掺杂改性富锂材料的研究,探讨了其对材料的结构稳定性以及电化学性能的影响。本论文主要研究内容如下:尖晶石相钴酸锂涂层和钴离子表面浓度梯度掺杂改性富锂正极材料。采用湿化学包覆和二次高温煅烧制备钴离子改性的富锂材料,并通过优化涂层含量提升材料的电化学性能。在600℃的煅烧温度下,硝酸锂和乙酸钴在富锂材料表面生成立方晶系的尖晶石相钴酸锂涂层;同时,钴离子在材料的表面区域发生扩散、迁移,呈现出钴离子梯度分布。晶格相容的尖晶石相钴酸锂涂层隔绝了电极材料的界面副反应,材料表面区域引入的钴离子提升了材料的阳离子有序和电子电导率,两者协同作用抑制了富锂材料晶体结构的不可逆相变（层状相到尖晶石相）,从而提升其电化学性能。其中1wt%的LMR-1富锂正极在0.1 C电流下可释放263.46 m Ah g^-1的放电比容量,对应的首圈库伦效率为85.67%,在0.5 C和1 C电流下循环200周容量保持率分别为94.56%和91.41%,在10 C大电流下可释放127.06 m Ah g^-1的放电比容量。四价锗离子晶格掺杂和锗酸锂涂层改性富锂正极材料。采用Ni_0.25Mn_0.75CO₃作为前驱体,在前驱体和锂盐高温煅烧时加入Ge O₂制备锗离子改性的富锂材料,并通过优化掺杂量提升材料的循环性能。在高温煅烧过程中,锗离子完全扩散至富锂材料晶格内部,实现均匀的锗离子晶格掺杂;同时,部分Ge O₂与表面碳酸锂反应原位生成厚度约为5 nm的锗酸锂涂层。晶格中引入的锗离子会诱导Ni²⁺发生氧化,从而降低富锂材料的阳离子混排,其次,纳米级的锗酸锂涂层不仅有效地隔绝了界面副反应,而且加快了锂离子传输。锗离子改性的富锂材料表现出优异的循环稳定性。其中2 mol%的G2富锂正极材料在1 C电流下循环300周和2 C电流下循环500周的容量保持率分别为92.26%和75.41%。更多还原

【Abstract】 Lithium-rich manganese-based oxides have become ideal cathode materials for the next generation of lithium-ion batteries because of its high energy density and low cost.However,many problems existing in lithium-rich cathode materials also impede its commercial application.Firstly,the low initial coulomb efficiency in the initial cycle attributed the loss of oxygen and lithium at high potential（4.8 V）;Secondly,the electrode material is prone to side reaction with electrolyte,forming a thick cathode electrolyte interphase（CEI）layer,which induces reduces the ionic and electronic conductivity of the material,leading to poor rate capability;At the same time,the structural stress caused by lattice expansion/contraction would lead to particle crack,resulting in unsatisfying cycling performance;Finally,the migration and dissolution of transition metal ions will lead to the phase transformation from layer to spinel structure,resulting in voltage hysteresis and voltage decay during prolonged cycling.To address these problems,many modification methods are proposed like surface coating,surface/bulk ions doping,full concentration gradient materials.Although these modification methods can solve the existing problems to a certain extent and improve its electrochemical performance,there are still lack of researches on the modification methods such as homogeneous heterostructures phase coating and doping.Therefore,it is of great significance to design an ingenious and in-depth heterostructures phase coating and doping surface integration method for the application of lithium-rich manganese-based oxides and accelerate its commercialization.In this paper,based cobalt-free lithium-rich manganese-based oxide Li_1.2Ni_0.2Mn_0.6O₂,different kinds of heterostructures phase coating and doping modified lithium-rich cathode materials were studied,and the effects on the structure stability and electrochemical performance are studied.The research contents are as follows:Fd-3m LiCoO₂ coating layer and surface concentration gradient cobalt doping modified lithium-rich cathode material.The cobalt modified lithium-rich materials were prepared by wet chemical coating method and subsequent high temperature calcination,and the electrochemical performance of materials were improved by optimizing the coating content.At the calcination temperature of 600℃,Li NO₃ and Co（CH₃COO）₂·4H₂O will generate cubic crystal Fd-3m LiCoO₂ layer on the surface of the lithium-rich material.Meanwhile,cobalt ions will gradually diffuse and migrate in the surface region of the material,and the gradient distribution of cobalt ions will be presented.Lattice-compatible Fd-3m LiCoO₂ layer isolate the side effects of the electrode interface,and the surface cobalt doping promote orderly of cation and electronic conductivity of the cathode material,the synergistic effect of coating and doping inhibits the irreversible phase transformation of layer to spinel structure,thus improve the electrochemical performance of lithium-rich cathode materials.The 1 wt%LMR-1 cathode exhibits a high discharge capacity of 263.46 m Ah g^-1 at0.1 C,corresponding to the initial cycle of coulomb efficiency is 85.67%.After 200 cycles,the discharge capacity retention is 94.56%at 0.5 C and 91.41%at 1 C.At the same time,the modified cathode exhibits s a discharge capacity of 127.06 m Ah g^-1 at 10 C.Lattice tetravalent germanium doping and Li₂Ge O₃ coating layer modified lithium-rich cathode material.Ni_0.25Mn_0.75CO₃ carbonate was as the cathode material precursor,Ge O₂ was added during the precursor and lithium source calcination process to synthesis germanium modified lithium-rich materials,and the cycling performance was improved by optimizing the doping content.During the calcination process at high temperature（850℃）,germanium will completely be diffused into the lithium-rich material to achieve uniform lattice ions doping.At the same time,the part of Ge O₂ will react with surface Li₂CO₃ to generate in-situ Li₂Ge O₃ coating layer with a thickness of 5 nm.The lattice germanium ions doping will induce the oxidation of Ni²⁺to reduce the cation mixing of lithium-rich materials,and the surface uniform coating layer not only isolate the side reaction between the electrode material and electrolyte,but also stabilize the surface crystal structure and accelerate the lithium-ions diffusion.The modified cathode by germanium ions shows satisfied cycle life.The discharge capacity retention of 2 mol%G2 cathode is 92.26%at 1 C for 300 cycles and 75.41%at 2 C for 500 cycles,respectively.更多还原