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电池的能量密度制约着新能源汽车的续航里程，而提升电解液的工作电压是改善锂离子电池的能量密度的有效途径之一。普通电解液的最高限制电压为4.35 V，当电压高于4.5V后电解质会变得极不稳定，与充电状态下高氧化态的正极发生反应，导致大量可燃气体生成，并且与电极间的副反应也加速了电池循环性能衰减和安全性的降低。因此研究高电压稳定性好的电解液可促进新能源汽车的发展，具有重要的应用价值，论文取得的研究成果如下：（1）采用1.0 mol/L LiPF6+ 0.1 mol/L LiDFOB -FEC/PC/DMC的电解液体系，其电化学窗口达到5.5 V（vs. Li/Li+）以上，并对铝箔有良好的钝化作用。LiNi0.5Mn1.5O4在该电解液中的放电比容量可达128.7 mAh/g，库伦效率大于99%。200次充放电循环后，比容量仍达108.2 mAh/g，容量保持率达到85.3%。CV和SEM分析结果表明电解液与LiNi0.5Mn1.5O4具有较好的相容性和脱嵌锂可逆性，在正极表面生成了保护膜，阻止了电解质的继续分解。 （2）研究了不同负极材料Li4Ti5O12、硅碳和石墨等在高压电解液中的相容性和循环性能。研究结果表明，不同负极在该电解液中均具有良好的循环特性。在1 C倍率下，钛酸锂放电比容量达157.2 mAh/g以上；室温时520次循环后，容量保持率约为74%，库伦效率超过99.5%，但其高温性能和大倍率性能较差；CV测试中，氧化峰与还原峰形成中心对称的封闭曲线，电极脱嵌锂可逆性较好。硅碳在上述高压电解液中以0.33 mAh/cm2测试，首次比容量可达707 mAh/g；经过100次充放电循环后，比容量降至550 mAh/g。传统石墨电极在电解液中的比容量、库伦效率和循环性均较好，在0.33 mAh/cm2的电流密度下，100次充放电循环后，放电比容量仍达323.3mAh/g，放电比容量未见衰减。;Energy density of lithium-ion batteries is one of most important factor which influences the mileage of electric vehicles. Enhance the voltage of electrolyte can effectively improve the energy density. In the conventional electrolytes, the highest cut-off voltage of is only 4.35 V due to the unstable carbonate sovent decomposed at 4.5 V, which lead to an oxidation reaction occurred in charge state with high oxidation state metal surface and cause the electrolyte decomposition. Furthermore, a large amount of combustible gas will be produced in this process which affecting the rate performance and the safety of lithium-ion battery. Research and development of high voltage electrolyte with stability is of great significance to accelerate the development of electric vehicles. The mainly innovative achievements of this research are as follows,(1) A high voltage electrolyte (1 mol/L LiPF6+0.1 mol/L LiDFOB – FEC / PC / DMC) with an electrochemical window of 5.5 V was obtained which can effective passivate aluminum foil. Battery test demonstrated that initial reversible discharge capacity of LiNi0.5Mn1.5O4 was up to 128.7 mAh/g with 99% coulombic efficiency, and a specific capacity of 108.2 mAh/g and 85.3% cyapacity retention is still remained in 200 cycles. The results from CV and SEM showed better compatibility and lithium ion reversibility between the electrolyte and LiNi0.5Mn1.5O4. The positive interfacial membrane is formed on the cathode surface, which inhibits the decomposition of the electrolyte and improves the cycle performances of the battery. (2) The systematical research on compatibility and cyclicity among Li4Ti5O12, silica and graphite in electrolyte was performed. The results from the semi-cell tests with three kinds of negative electrode materials showed that the electrolyte exhibit good cylce performance. When charging and discharging rate is less than 1 C, lithium titanate electrode can be released more than 157.2 mAh/g of specific capacity. The specific capacity can reach up to 74% with coulomb efficiency more than 99.5% after 520 charge discharge cycles at room temperature, yet its high temperature performance and large multiplier performance was unsatisfactory. In the CV test, the oxidation peak and the reduction peak form the center symmetric closed loop curve, and the electrode has good de-intercalation lithium reversibility. The initial discharge capacity of silicon/carbon composite is 707 mAh/g at a current density of 0.33 mAh/cm2. However, the cycle performance of the battery was not good, and the discharge capacity reduced to 550 mAh/g in 100 cycles. Graphite electrode has the very good specific capacity, coulomb efficiency and cycle stability. The discharge capacity was 323.3 mAh/g after 100 discharge cycles at a current density of 0.33 mAh/cm2. The battery capacity shows no obvious attenuation.
|张亮. 锂离子电池高电压电解液的研究[D]. 中国科学院研究生院,2017.|
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