基于碳酸酯基电解液的4.5V电池

4.5 V Li-ion battery with a carbonate ester-based electrolyte

尖晶石结构的LiNi0.5Mn1.5O4(镍锰酸锂)正极材料相对于金属锂的平台电压高达4.7 V,高电压镍锰酸锂电池单体工作电压比磷酸铁锂电池高1.3 V,比三元材料电池高0.9 V,具有比能量高和成本低的优势,可满足后补贴时代新能源汽车对动力电池长续航里程和经济性的需求。镍锰酸锂电压平台高是一个巨大的优势,同时也是其商业化的重大障碍,其充电截止电压相对于金属锂高达4.9 V,一直以来被认为现有碳酸酯基电解液不可能承受如此之高的工作电压,导致其商业应用止步不前。氟化碳酸酯、噻吩、离子液体等曾被引入取代目前商用锂离子电池电解液中的碳酸酯溶剂,但仍然未见到循环性足够好和库仑效率足够高的全电池数据。本文展示了基于碳酸乙烯酯/碳酸二甲酯溶剂的高电压正极全电池数据。通过对镍锰酸锂正极材料的界面进行改性,抑制电解液与界面的副反应,使全电池的效率和循环性能有了较大的提升,但性能离商业应用仍有略微的差距。在材料改性的基础上,通过使用含多种电解液添加剂的GDY电解液后,进一步抑制碳酸酯基电解液与界面的副反应,提高全电池的效率和循环性能,并与已报道的采用耐高电压溶剂的全电池数据进行对比,显示出了更为优异的电化学性能。全电池在25℃和1 C下循环1000周后容量保持率可达88.02%,充放电库仑效率为99.93%。55℃和1 C下循环300周后容量保持率高达93.88%,充放电库仑效率为99.80%,这些性能已经满足了动力电池的应用要求。此外通过全电池充放电曲线的中值极化电压和阻抗随循环的对比,显示了材料改性和电解液优化对电解液副反应的抑制作用。这表明具有4.5 V工作电压的LiNi0.5Mn1.5O4/石墨电池可以基于现有的碳酸酯基电解液实现商业化。

LiNi0.5Mn1.5O4(LNMO) spinel is a cathode material with a voltage plateau of 4.7 V versus Li. An LNMO cell shows an operation voltage that is 1.3 and 0.9 V higher than that of a LiFePO4 and NCM cell, respectively. The LNMO cell has the advantages of higher specific energy and lower cost and can meet the long range per charge and low cost requirements of electric vehicles in the postsubsidy era after 2020. The high operating voltage is undoubtedly a great advantage;however, the extremely high cut-off voltage(4.9 V vs. Li+) is also a huge obstacle in its commercialization. The widely used carbonate ester-based electrolytes are considered unsuitable owing to the intrinsic thermodynamic tendency of these carbonates to decompose at potentials well below the thermodynamic threshold required for the reversible reactions of these high-voltage systems. Alternatives such as fluoride carbonate, thiophene, and ionic liquids were introduced to replace the carbonate solvent used as the electrolyte in present commercial lithium-ion batteries. However, full battery data have not been reported with respect to optimum cycle performance and a sufficiently high coulombic efficiency for practical batteries. In this study, LiNi0.5Mn1.5O4/graphite full cells are demonstrated using an electrolyte that is based on carbonate esters. The side reactions between the electrolyte and interface are inhibited by modifying the interface of lithium nickel-manganate cathode materials to improve the efficiency and cycling performance of the full cells. However, there is still a gap between commercial applications.The side reactions are further inhibited using the GDY electrolyte to improve the efficiency and cycling performance of the full cells. The full cells show superior electrochemical performance compared to that of the reported full cells that use high-voltage-resistant solvents. The capacity retention rate of the full cells can reach 88.02% and the coulombic efficiency can reach 99.93% at 1 C and 25 ℃ after 1000 cycles. The capacity retention rate of the full cells can reach 93.88% and the coulombic efficiency is99.80% at 1 C and 55 ℃ after 300 cycles. This performance has already met the requirements of EV applications. In addition, the inhibition effect of material modification and electrolyte optimization on the side reactions between the electrolyte and interface is demonstrated by comparing the median polarization voltage and impedance during cycling. These results indicate that LiNi0.5Mn1.5O4/graphite full cells, which are based on carbonate ester electrolytes, with a working voltage of 4.5 V can be commercialized.