【作者】 王杰；

【导师】 邓远富； 覃旭松；

【作者基本信息】 华南理工大学 ， 化学工程（专业学位）， 2021， 硕士

【摘要】 面对当今巨大的能源需求和日益严重的环境问题,锂离子电池自成功开发并商用以来,越来越扮演着举足轻重的角色。正极材料作为锂离子电池的关键组分,成为当前制约锂离子电池性能提升的关键。橄榄石型的磷酸锰铁锂(Li Fe_xMn_1-xPO₄,缩写为LFMP)正极材料因其具有高的理论比容量(>170 m Ah·g^-1),原料成本低廉,与商用电解液兼容且安全性良好等优点,有望成为下一代商用正极材料。然而,橄榄石正极材料固有的离子和电子导电率差的缺点限制了LFMP的实际应用。研究人员通过颗粒纳米化、碳包覆、离子掺杂和形貌调控等方法,对一系列不同铁锰比例的LFMP材料进行了性能改善相关研究并取得了良好的成果,然而,LFMP至今还未像三元正极材料那样,找到最佳的Fe/Mn比例。目前,获得广泛认可的是,当LFMP中锰的含量超过0.8时,将会导致局部Jahn-Teller畸变过大,使高锰含量的LFMP不适宜用作理想的正极材料。此外,研究者在对橄榄石型的磷酸铁锂（LFP）正极材料的研究中发现了反位缺陷效应,并通过非化学计量比的方式成功将其抑制,进而改善了材料的电化学性能。基于非化学计量方法在二元体系的LFMP材料的相关研究中鲜有报道的情况,本论文主要通过非化学计量比的设计对LFMP材料进行相关改性研究,具体的工作如下:（1）通过保持阳离子化合价总数不变的策略,采用固相法成功制备了一系列不同Fe/Mn比例的非化学计量Li_1.05Fe_xMn_0.975-xPO₄/C正极材料（x=0.2、0.3、0.4和0.5）,并通过XRD、Raman、FTIR、SEM和TEM对其进行了表征以及利用充放电测试对其比容量、倍率性能和循环性能进行了研究,结果表明,Li_1.05Fe_0.5Mn_0.475PO₄/C具有最优的放电比容量和能量密度,在0.1 C倍率下,该材料具有146.7 m Ah·g^-1的放电比容量和518.2Wh/kg的能量密度,在5 C倍率下具有96.9 m Ah·g^-1的放电比容量和330.5 Wh/kg的能量密度;同时,该材料具有良好的循环性能,在1 C倍率下循环150圈后,其容量保持率达到99.8%。进一步研究表明,由于水洗可去除材料中存在的部分Li₃PO₄和可能的其它杂质,导致Li_1.05Fe_0.5Mn_0.475PO₄/C-Water材料的比容量和首圈库伦效率明显提高,在0.1 C倍率下,其比容量和首圈库伦效率分别达到151.7 m Ah·g^-1和98.1%。（2）在此基础上,精细调变Li_1.05Fe_0.5Mn_0.475PO₄/C材料的铁锰比例,制备了Li_1.05Fe_0.5Mn_0.5PO₄/C和Li_1.05Fe_0.475Mn_0.5PO₄/C两个材料,进一步探讨不同非化学计量方式对Li_1.05Fe_0.5Mn_0.475PO₄/C材料结构和性能的影响。XRD表征结果显示,Li_1.05Fe_0.5Mn_0.5PO₄/C、Li_1.05Fe_0.475Mn_0.5PO₄/C和Li_1.05Fe_0.5Mn_0.475PO₄/C材料中Li₃PO₄的含量逐渐减少;SEM表征结果发现,相较于Li_1.05Fe_0.5Mn_0.475PO₄/C材料,Li_1.05Fe_0.5Mn_0.5PO₄/C和Li_1.05Fe_0.475Mn_0.5PO₄/C材料的二次颗粒明显变大;氮气吸附-脱附实验结果表明,Li_1.05Fe_0.5Mn_0.475PO₄/C材料具有最大的比表面积(S_BET=30.45 m²·g^-1),而Li_1.05Fe_0.475Mn_0.5PO₄/C材料的比表面积最小(27.69 m²·g^-1)。以上综合因素导致了Li_1.05Fe_0.5Mn_0.5PO₄/C和Li_1.05Fe_0.475Mn_0.5PO₄/C的电化学性能下降。循环伏安与电化学阻抗测试和分析结果表明,Li_1.05Fe_0.5Mn_0.475PO₄/C材料具有最大的锂离子扩散系数和最小的电荷转移阻抗。其中,铁对应充放电平台锂离子扩散系数为5.203×10^-11～7.949×10^-11cm²/s,锰对应充放电平台锂离子扩散系数为1.942×10^-11～2.966×10^-11 cm²/s。更多还原

【Abstract】 In face of the current huge energy demand and increasingly serious environmental problems,lithium-ion battery has been playing an important role since its successful development and commercialization.As an important component of lithium-ion batteries,the cathode materials have become the key to restricting the performance improvement and large-scale commercialization of lithium-ion batteries.Phospho-olivine lithium fermanganese phosphate cathode material(Li Fe_xMn_1-xPO₄,abbreviated LFMP)is expected to become the next generation of commercial cathode material,due to its high theoretical specific capacity(>170 m Ah·g^-1),low raw material cost,compatibility with commercial electrolyte and good safety.However,the inherent disadvantages of poor ion and electron conductivity of phospho-olivine cathode materials limit their practical applications.Through particle diffusion nanocrystallization,carbon coating,ion doping and morphology regulation,a series of LFMP materials with different Fe/Mn ratios have been investigated for performance improvement,and good results have been obtained.However,the optimal Fe/Mn ratio has not been found as for ternary cathode materials.At present,it is widely accepted that the local Jahn-Teller distortions will be excessive when the manganese content exceeds 0.8 and is not suitable for use as an ideal cathode material.In addition,some researchers have found the inverse defect effect in the study of Li Fe PO₄,and successfully suppressed it by non-stoichiometric ratio method,thus the electrochemical properties of the material have been improved.Based on the non-stoichiometric methods in binary system of LFMP materials are rarely reported in the relevant research,this paper mainly through the design of non-stoichiometric ratio to modify LFMP materials,the main research works are as follows:（1）By keeping the total number of cationic valence constant,a series of non-stoichiometric Li_1.05Fe_xMn_0.975-xPO₄/C cathode materials（x=0.2,0.3,0.4 and 0.5）with different Fe/Mn ratios are successfully prepared by solid phase method.The physical characterizations are performed by XRD,Raman,FTIR,SEM and TEM,and its specific capacity,rate performance and cycle performance are studied by charge-discharge tests.The experimental results demonstrate:Li_1.05Fe_0.5Mn_0.475PO₄/C has the best discharge specific capacity and energy density.It have the discharge specific capacity of 146.7 m Ah·g^-1 and the energy density of 518.2 Wh/kg,as well as 96.9 m Ah·g^-1 and 330.5 Wh/kg at 0.1 C and 5 C,respectively.Good cycle performance,99.8%capacity retention after 150 cycles at 1.0 C.Further study shows that the specific capacity and first cycle coulomb efficiency of Li_1.05F_0.5M_0.475PO₄/C-Water increase significantly,duing to the removing of a little Li₃PO₄ and other impurities by washing with water.Its first cycle specific capacity and coulomb efficiency are 151.7 m Ah·g^-1 and 98.15%at 0.1 C,respectively.（2）Based on this basis,Li_1.05Fe_0.5Mn_0.5PO₄/C and Li_1.05Fe_0.475Mn_0.5PO₄/C samples are prepared,and the effects of different non-stoichiometric methods on the structure and properties of Li_1.05Fe_0.5Mn_0.475PO₄/C are further investigated.Structural characterizations by XRD confirm that the content of Li₃PO₄ decreases gradually in Li_1.05Fe_0.5Mn_0.5PO₄/C,Li_1.05Fe_0.475Mn_0.5PO₄/C and Li_1.05Fe_0.5Mn_0.475PO₄/C.SEM characterization results show that the primary particles of Li_1.05Fe_0.5Mn_0.5PO₄/C and Li_1.05Fe_0.475Mn_0.5PO₄/C are significantly larger,compared with that of the Li_1.05Fe_0.5Mn_0.5PO₄/C sample.BET tests show that the Li_1.05Fe_0.5Mn_0.475PO₄/C sample has the largest specific surface area(S_BET=30.45 m²·g^-1),while the specific surface area of Li_1.05Fe_0.475Mn_0.5PO₄/C is smallest(S_BET=27.69 m²·g^-1).The above-mentioned comprehensive factors lead to the degradation of electrochemical performance of the Li_1.05Fe_0.5Mn_0.5PO₄/C and Li_1.05Fe_0.475Mn_0.5PO₄/C samples.The results of CV and EIS further show that Li_1.05Fe_0.5Mn_0.475PO₄/C has the largest lithium ion diffusion coefficient and the smallest charge transfer impedance.Its lithium ion diffusion coefficients are 5.203×10^-11～7.949×10^-11 cm²/s for iron and 1.942×10^-11～2.966×10^-11 cm²/s for manganese sites,respectively.更多还原