电动汽车锂离子电池碳负极扩散应力与微观结构失效机理研究

Study of the Mechanism of Diffusion Stress and Microstructure Failure of Carbon Anode for Lithium-ion Batteries

扩散应力易引起电极体积变化、电极颗粒破裂和脱落,导致电极材料失效,从而引起锂离子电池容量的衰减。为探究不同材料厚度和放电倍率下的扩散应力规律以及扩散应力与微观结构破坏间的关联性,采用试验与仿真相结合的方法进行相关研究。首先,制备3种不同厚度(25、36、48μm)的石墨负极,与三元正极组装成纽扣全电池;其次,以相同充电倍率(0.2C)、不同放电倍率(1C、2C、5C)在25℃下进行循环测试,为模型验证及微观测试提供样本;随后,根据电化学及扩散力学原理建立电化学-力耦合模型,并通过不同倍率放电工况对模型进行验证;进一步,利用控制变量法,基于所建模型研究不同材料厚度与放电倍率下扩散应力的规律;最后,基于电镜扫描和X射线衍射测试,对循环后的负极形貌及微观结构进行表征,结合模型仿真研究扩散应力与负极微观结构的破坏关联性。研究结果表明:随着放电倍率增大或材料厚度减小,扩散应力增大、负极损坏程度加深,可根据拉伸屈服强度将扩散应力与微观结构变化关系分为2个阶段;进一步,引入剥落指数定量描述微观结构失效,发现剥落指数与扩散应力之间存在幂函数关系。研究结果可为揭示扩散应力与容量损失之间的关联性提供思路。

The volume change,breakage,and peeling of electrode materials are easily caused by diffusion stress,resulting in the failure of electrode materials and degradation of battery capacity.The test and simulation joint methods were used to explore the law of diffusion stress and the correlation between diffusion stress and structural damage at the micro scale for different material thicknesses and discharge rates.Three types of graphite negative electrodes with thicknesses of 25,36,and 48 μm were prepared and then assembled with the ternary positive electrode as button full batteries.The cycling tests were implemented with the same charge rate(0.2 C) and different discharge rates(1 C,2 C,and 5 C) at 25 °C to provide samples for model verification and microstructure investigation.According to the principles of electrochemistry and diffusion mechanics,an electrochemical-force coupling model was established.The model was verified at different discharge rates.Based on this model,the law of diffusion stresses under different material thicknesses and discharge rates were studied using the control variable method.Scanning electron microscopy and X-ray diffraction tests were performed to characterize the morphology and microstructure of the negative electrode.Combined with a model simulation,the failure correlation between the diffusion stress and microstructure of the negative electrode was studied.The results show that with an increase in the discharge rate or a decrease in the material thickness,the diffusion stress and damage degree of the negative electrode increased and was manifested as an increase in the crystal plane spacing of the graphite particles.The relationship between the diffusion stress and microstructural change can be divided into two stages according to the tensile yield strength.Furthermore,an exfoliation index was introduced to quantitatively describe the microstructure failure.A power function relationship between the exfoliation index and diffusion stress was found.This research can provide inspiration for revealing the correlation between diffusion stress and capacity loss.