18650型锂离子电池的循环容量衰减研究

研究了18650型锂离子电池常温循环性能和容量衰减机理。采用恒流-恒压制式对锂离子电池进行200次充放电循环测试,用交流阻抗技术对不同循环次数的电池进行分析,将不同循环次数的电池正负极与锂片分别组成半电池测试其容量,利用扫描电子显微镜法(SEM)、X射线衍射光谱法(XRD)、空气渗透仪等测试手段对不同循环次数后的锂离子电池正负极、隔膜的形貌和结构进行了表征。结果表明,电池在前200次循环过程中容量衰减率为15.6%;而正极和负极容量分别损失6.6%和4.3%。电池容量衰减主要来自于活性锂离子的损失以及电极活性材料的损失,活性锂离子的损失可能是由于在循环过程中电解液与正负极活性材料反应不断消耗活性锂离子造成的;正极活性材料层状结构规整度下降,离子混排度提高,负极活性材料上沉积钝化膜,石墨化程度降低,隔膜孔隙率下降,导致电池电荷传递阻抗增大,脱嵌锂能力下降,从而导致容量的损失。

Capacity fading of 18650 Li-ion cells with cycling

The cycle performance and capacity fading mechanism of 18650 Li-ion batteries cycled at room temperature were studied.The charge-discharge cycle of 18650 Li-ion batteries was tested for 200 cycles by constant current-constant voltage(CC-CV) protocol.The electrochemistry properties of batteries after different cycles were studied by EIS.The capacities of cathode and anode pieces cut from the disassembled cell after different cycles were studied against fresh lithium.The morphology and structure of cathode,anode and separator of Li-ion batteries after different cycles were characterized by SEM,XRD and air-permeability test.The results show that the cell loses 15.6% of its initial capacity after 200 cycles,and the cathode and anode loses 6.6% and 4.3% of their initial capacities after 200 cycles respectively.The capacity fading of batteries can be attributed primarily to the loss of active Li+ and the losses of cathode and anode active materials.The loss of active Li+ is caused by the side reactions of electrolyte species on the electrode.More cation mixing and less ordered layer structure are observed from the cathode material.The degree of graphitization of anode material decreases and the surface film is developed on the anode.The porosity of separator decreases.These cause the increase of charge transfer impedance and the losses of capacity.