锂离子电池5V正极材料的制备与电化学性能研究

【作者】 张真；

【导师】 刘兴泉；

【作者基本信息】 电子科技大学 ， 材料科学与工程， 2011， 硕士

【摘要】 LiNi_0.5Mn_1.5O₄正极材料不仅具有高达4.7V的充放电平台,而且具有环境友好、成本低、安全性能好、能量密度高、功率密度高和循环性能好等优点,因此有望成为动力电池的候选正极材料之一。本论文主要围绕减少LiNi_0.5Mn_1.5O₄正极材料的4V放电平台,以研究制备电化学性能优异的高电位LiNi_0.5Mn_1.5O₄正极材料的方法为主要任务,主要采用了恒电流充放电、循环伏安和交流阻抗等测试手段,以及XRD、SEM等表征手段对LiNi_0.5Mn_1.5O₄正极材料进行了较详细的研究。主要内容包括:（1）采用溶胶凝胶-自蔓延燃烧法合成了LiNi_0.5Mn_1.5O₄正极材料,详细考察了原料的加入顺序、初始pH、原料的选择、柠檬酸的量、焙烧温度、退火时间、镍和锂的过量程度对合成最终样品性能的影响。最佳条件合成的正极材料LiNi_0.5Mn_1.5O₄首次放电比容量为121.7mAh/g,第20次循环的放电比容量为119.5mAh/g,放电比容量保持率为98.2%,循环性能较好,消除了4V放电平台。（2）为了进一步改进LiNi_0.5Mn_1.5O₄正极材料的电化学性能,对其进行了掺杂Ga和In二种元素。通过掺杂并没有提高正极材料在常温和0.2C充放电倍率下的放电比容量,但是掺杂Ga和In都提高了正极材料的充放电倍率性能。XRD测试结果表明,Ga³⁺已经进入到尖晶石的晶格,并占据了Ni²⁺位;SEM测试结果表明, Ga的掺杂没有改变最终合成样品的颗粒大小和形貌;充放电性能测试表明,LiGa_δNi_0.5-δMn_1.5O₄（δ=0.02）具有最高的放电比容量和循环稳定性;循环伏安测试表明,LiGa_δNi_0.5-δMn_1.5O₄（δ=0.02）的两次循环伏安曲线的氧化还原峰位基本重合,说明该样品的循环稳定性较为优异;交流阻抗测试表明,LiGa_δNi_0.5-δMn_1.5O₄（δ=0.02）具有最小的电荷转移阻抗,也证实了该样品具有优异的循环稳定性。（3）对材料LiNi_0.5Mn_1.5O₄进行了包覆Li₄Ti₅O₁₂。从放电比容量和高电位平台相对容量来考虑,包覆后的前驱体的最适宜的热处理温度为600℃。包覆Li₄Ti₅O₁₂后,明显提高了正极材料的放电比容量保持率,其中,包覆2%的样品在倍率性能测试中的容量保持率为99.3%。循环伏安和交流阻抗测试也都表明,包覆Li₄Ti₅O₁₂后,正极材料的循环稳定性得到了提高,其中,包覆2%的样品的电荷转移阻抗最小,将最有利于锂离子的嵌入与脱出,极大改善了循环稳定性。更多还原

【Abstract】 LiNi_0.5Mn_1.5O₄ cathode material, which has the advantage of a potential plateau at about 4.7V, as well as environmentally friendly, low cost, good safety performance, good cycle performance, high power and high energy density, is expected to be one of the best cathode materials for the power battery.In this paper, how to reduce the 4V plateau of LiNi_0.5Mn_1.5O₄ cathode material was concerned, in order to synthesize high voltage cathode material with high specific capacity and excellent cycle stability. Constant current charge-discharge test, Cycle Voltammagram（CV）, AC impedance, X-ray Diffraction（XRD） and Scan electron microscope（SEM） were used to characterize the performance of the cathode material. The main contents include:（1） LiNi_0.5Mn_1.5O₄ cathode material was synthesized by Sol-Gel-SCR （Self-Combustion Reaction）, and many influencing factors were considered, such as, the order of addition of raw materials, initial pH, selection of raw materials, the relative amounts of citric acid, sintering temperature, annealing time, the best level of nickel and lithium excess. LiNi_0.5Mn_1.5O₄ cathode material, synthesized with optimum conditions, showed excellent electrochemical performance. The initial discharge capacity was 121.7mAh/g, and the discharge capacity of 20th cycle was 119.5mAh/g, so the discharge capacity retention ratio rate was 98.2%, which showed well cycle performance. In the charge-discharge performance curve, the 4V discharge platform was disappeared.（2） To further improve the electrochemical properties of the cathode material, the Ga-doping and In-doping LiMδNi_0.5-δMn_1.5O₄（M=Ga, In） were prepared.The discharge capacity of the cathode materials was not raised by doping at room temperature, charge and discharge rate of 0.2C, but greatly enhanced the rate property. XRD results showed that, Ga entered into the spinel lattice and occupied the nickel position. SEM results showed that Ga-doping did not change the grain size and morphology. Charge and discharge performance tests showed that LiGa_δNi_0.5-δMn_1.5O₄（δ=0.02） showed the highest discharge capacity and cycle stability. The CV curves indicated thatδ=0.02 sample showed well cycling performance. The AC impedance curves also indicated thatδ=0.02 sample showed more excellent cycle stability, due to the lowest electrochemical impedance.（3） Li₄Ti₅O₁₂-coated LiNi_0.5Mn_1.5O₄ cathode material was prepared. Experimental results showed that the best heat treatment temperature of coating precursor is 600℃. The discharge capacity retention rate of Li₄Ti₅O₁₂-coated LiNi_0.5Mn_1.5O₄ cathode materials was much higher than that of the uncoating material, which of the 2wt.% Li₄Ti₅O₁₂-coated LiNi_0.5Mn_1.5O₄ samples was 99.3%. CV and AC impedance curves showed that the cycle performance of cathode materials was improved after Li₄Ti₅O₁₂-coating. The 2wt.% Li₄Ti₅O₁₂-coatig, due to the lowest electrochemical impedance, would be most beneficial to the Lithium-ion diffusion, and greatly improved the cycle stability.更多还原