【作者】 陈振宇；

【导师】 戴长松；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2013， 博士

【摘要】 为了应对能源危机和环境污染的挑战，世界各国都对发展用于电动汽车的动力锂离子电池正极材料给予了极高的重视。已研发的正极材料中，磷酸铁锂电池由于环境友好、安全性好、循环寿命长等优点而成为动力电池的希望。但是磷酸铁锂也存在大电流充放电性能差和低温放电容量低等问题，为此本文对Li₃V₂（PO₄）₃材料和xLiFePO₄-yLi₃V₂（PO₄）₃复合材料的制备和电化学性能进行研究，并在实验室研究的基础上，开展中试和规模化制备Li₃V₂（PO₄）₃材料的工艺研究。对Li₃V₂（PO₄）₃材料进行了Cu离子和Mg离子掺杂改性的研究。XRD结构精修结果表明，通过对结构中Li-O键的改变，Cu掺杂改性改变了Li₃V₂（PO₄）₃材料的放电行为，在4.05V出现了一个新的放电平台，这对研究提高Li₃V₂（PO₄）₃材料在3.0⁴.8V范围的循环性能，提供了一个新的窗口。与未掺杂的磷酸钒锂相比，Mg离子掺杂使Li₃V₂（PO₄）₃材料的晶胞缩小，但是却使结构中Li1和Li2离子的Li-O键的键长变长，使结构对锂离子的束缚减弱，更有利于锂离子的可逆嵌入脱出；XANES研究表明，Mg掺杂改善了Li₃V₂（PO₄）₃材料中VO6八面体的对称性。在3.0⁴.3V的范围进行10C倍率充放电测试，Li₃(V_0.9Mg_0.1)₂（PO₄）3材料的放电比容量达到100mAh/g，高于未掺杂改性的Li₃V₂（PO₄）₃材料，使Li₃V₂（PO₄）₃材料更适合动力电池领域的应用。对碳热还原法制备7LiFePO₄-Li₃V₂（PO₄）₃复合材料的烧结温度与合成时间进行了优化实验，优化的合成工艺为在770℃烧结12h。在完成合成工艺优化以后，对7LiFePO₄-Li₃V₂（PO₄）₃复合材料的物理性质进行了研究。XRD研究结果表明，复合材料中LiFePO₄和Li₃V₂（PO₄）₃材料的晶胞体积均有收缩。XANES研究表明，复合材料中的LiFePO₄材料中Fe离子的价态略有升高，Li₃V₂（PO₄）₃材料中V离子的价态略有降低，而且VO6八面体的对称性得到提高，结构稳定性得到改善，表明复合材料中发生了Fe对Li₃V₂（PO₄）₃材料中V的取代，和V对LiFePO₄材料中Fe的取代。复合材料具有更好的大电流充放电性能，在1500mA/g电流密度下进行充放电时，其放电比容量为89mAh/g，显著高于单纯的LiFePO₄材料的70mAh/g，而且过电势也有明显的减少。进而对7LiFePO₄-Li₃V₂（PO₄）₃复合材料进行了Mg掺杂改性研究，改性后的复合材料比没有掺杂改性的复合材料放电比容量更高，不同扫描速度下进行的循环伏安测试表明，Mg掺杂提高了复合材料中LiFePO₄材料的Li离子扩散系数。对Li₃V₂（PO₄）₃材料和7LiFePO₄-Li₃V₂（PO₄）₃复合材料进行了低温放电行为研究。Mg掺杂改性可以显著提高Li₃V₂（PO₄）₃和7LiFePO₄-Li₃V₂（PO₄）₃复合材料在低温下的放电容量。Mg掺杂7LiFePO₄-Li₃V₂（PO₄）₃复合材料在-30℃条件下的放电比容量达到89mAh/g，比LiFePO₄材料的放电比容量50mAh/g高出39mAh/g，比未掺杂改性复合材料的72mAh/g高出17mAh/g。通过对称电池的研究，发现Mg掺杂改性可以有效的降低Li₃V₂（PO₄）₃材料和7LiFePO₄-Li₃V₂（PO₄）₃复合材料在低温条件下的电荷传递电阻Rct，提高交换电流密度，进而改善材料的电化学反应活性。对Li₃V₂（PO₄）₃/C材料进行了中试和规模化制备工艺的研究。首先发现尽管采用水作为球磨介质会改变Li₃V₂（PO₄）₃/C材料合成的反应历程，但仍然可以合成出纯相的Li₃V₂（PO₄）₃/C材料。为了解决中试研究得到的产品颗粒粗大且呈现空心球形、电化学性能不理想的问题，对合成Li₃V₂（PO₄）₃/C材料的原料进行两次球磨处理后，使合成的Li₃V₂（PO₄）₃/C材料呈现出多孔的实心球结构，更有利于电解液的浸润和振实密度的提升，从而提高了材料的电化学性能。在中试研究的基础上开展Li₃V₂（PO₄）₃/C材料规模化制备研究，合成出了具有良好性能的Li₃V₂（PO₄）₃/C材料，在10C倍率充放电条件下，材料的放电容量达到了79mAh/g。良好的电化学性能表明，规模化制备获得了具有应用价值的Li₃V₂（PO₄）₃/C材料。更多还原

【Abstract】 To face the challenge of energy crisis and environmental pollution, differentcountries all over the world have paid great attention to the development of positivematerials of power lithium ion battery for electric vehicles. In current status,LiFePO₄material is the new hope for power battery, because of its good safety andlong cycle life, and it is friendly to environment. But its rate performance andlow-temperature performance are poor. Therefore the charge and dischargeperformance at high current density and low temperature of Li₃V₂（PO₄）₃materialand xLiFePO₄-yLi₃V₂（PO₄）₃composite material was studied. Moreover, based onthe study in laboratory, the key technology of pilot scale test and scale production ofLi₃V₂（PO₄）₃material were studied.The Cu-doping effects and Mg-doping effects on Li₃V₂（PO₄）₃material werestudied. XRD refinement study indicated that Cu doping changed the dischargebehavior of Li₃V₂（PO₄）₃material with the emergence of a new voltage plateau at4.05V via changing the bond length of Li-O. It opens a new window to find the wayto enhance the cycle performance of Li₃V₂（PO₄）₃material in the range of3.0–4.8V.Mg doping decreased the cell volume of Li₃V₂（PO₄）₃material, and lengthens theLi-O bond of Li1and Li2ion in the structure, which decreases the constraint of Liion in the structure, and is beneficial to the reversible intercalation/deintercalationof lithium ion. X-ray absorption near edge structure study revealed that Mg dopingimproved the symmetry of VO6octahedron. At10C rate, the discharge capacity ofLi₃(V_0.9Mg_0.1)₂（PO₄）3material reaches100mAh/g in the range of3.0-4.3V, which ishigher than the undoped Li₃V₂（PO₄）₃material. This indicates that the Mg dopingexerts the significant positive effect on the electrochemical performance ofLi₃V₂（PO₄）₃material, and makes the Li₃V₂（PO₄）₃material more suitable to thepractical application in the power battery field.The calcination temperature and time for carbothermal synthesis of7LiFePO₄-Li₃V₂（PO₄）₃composite material were optimized. The optimized synthesistemperature is770℃, and the optimized synthesis time is12h. After the synthesistechnique was optimized, the physical properties of7LiFePO₄-Li₃V₂（PO₄）₃ composite material was studied. The XRD study indicates that the cell volumes ofLiFePO₄and Li₃V₂（PO₄）₃in the composite are decreased. The XANES studyreveals that the valence state of Fe ion in LiFePO₄material is slightly increased,while the valence state of V ion in Li₃V₂（PO₄）₃material is slightly decreased. Inaddition, the symmetry of VO6octahedron is enhanced, and the stability of thestructure is improved. It reveals that in the composite, Fe substituts V inLi₃V₂（PO₄）₃material, V substituts Fe in the LiFePO₄material. The study onelectrochemical properties of the7LiFePO₄-Li₃V₂（PO₄）₃composite material revealsthat the charge and discharge performance of the composite at high current densitiesis much better than that of LiFePO₄material. When charged and discharged at thecurrent density of1500mA/g, the discharge capacity of the composite was89mAh/g, which is much higher than that of original LiFePO₄material （70mAh/g）,and the over potential of the composite was lower than that of original LiFePO₄material. The study on Mg doping of7LiFePO₄-Li₃V₂（PO₄）₃composite materialindicates that the electrochemical performance of the composite is enhanced. Thecharge and discharge test indicates that the discharge capacity of Mg-dopedcomposite material is higher than the undoped composite. The cyclicvoltammograms at different scan rates reveals that Mg doping increases thelithium-ion diffusion coefficient of the LiFePO₄in the composite.The low-temperature study of Li₃V₂（PO₄）₃material and7LiFePO₄-Li₃V₂（PO₄）₃composite material reveals that the main reason for the decrease of the dischargecapacity of Li₃V₂（PO₄）₃material at low temperatures is the increase of the chargetransfer resistance. Mg doping can significantly increase the discharge capacity ofLi₃V₂（PO₄）₃material and7LiFePO₄-Li₃V₂（PO₄）₃composite material at lowtemperatures. At-30℃, the discharge capacity of Mg-doped7LiFePO₄-Li₃V₂（PO₄）₃composite material is89mAh/g, which is39mAh/g higher than that of originalLiFePO₄material （50mAh/g）, and17mAh/g higher than that of the undopedcomposite （72mAh/g）. Based on the symmetry cell study, it is found that Mgdoping can decrease the charge transfer resistance Rctof Li₃V₂（PO₄）₃material and7LiFePO₄-Li₃V₂（PO₄）₃composite material, enhance the exchange current density,and improve the electrochemical reactivity.In this paper, key technology of the pilot scale test and scale production ofLi₃V₂（PO₄）₃/C material was studied. Using water as the ball-milling medium changed the reaction process of the synthesis of Li₃V₂（PO₄）₃/C material. However,pure Li₃V₂（PO₄）₃/C material was still obtained. The particles of the obtainedproduct in pilot scale test were hollow spheres with large size, and theelectrochemical performance was poor. To circumvent the hollow-sphere particleproblem, a twice ball-milling procedure was employed. The particles of theobtained Li₃V₂（PO₄）₃/C material were porous solid spheres，which were good forthe infiltration of the electrolyte and the increase of tap density. Thus, theelectrochemical performance of the material was enhanced. Based on the pilot scaletest, an industrial scale production study of Li₃V₂（PO₄）₃/C material was carried out,and the electrochemical performance of the as-prepared material was excellent. At10C charge and discharge rate, the discharge capacity of the material is79mAh/g.The excellent performance indicates that a well prepared Li₃V₂（PO₄）₃/C materialwith application potential is achieved in the scale production.更多还原