集流体塑性变形对锂离子电池双层电极中锂扩散和应力的影响

Effects of plastic deformation in current collector on lithium diffusion and stress in bilayer lithium-ion battery electrode

针对锂离子电池双层电极结构,建立了综合考虑锂扩散、应力、浓度影响的材料属性及集流体弹塑性变形的理论模型.基于所建立的模型,主要研究了在充电过程中集流体可能发生的塑性变形对电极中锂扩散及应力的影响.数值结果表明集流体的塑性变形会减弱其对活性层的约束,这不仅使得集流体和活性层中的应力得到明显缓解,而且还促进了锂在活性层中的扩散,提高了活性层的有效容量.与此同时,研究了集流体的屈服强度和塑性模量这两个参数的影响,结果表明,较小的屈服强度和较小的塑性模量能进一步弱化约束,松弛电极活性层中的应力,并增加其有效充电容量.研究结果为分层电极的结构设计和性能优化提供了一定的参考.

Lithium-ion batteries（LIBs） have already become indispensable energy storage devices, as they can meet urgent requirements for higher energy and power density in the applications ranging from portable electronics to electric vehicles.However, in the process of charging and discharging of LIB, the diffusion-induced stress associated with inhomogeneous Li concentration in the electrode may cause the electrode material to damage, and then further degrade storage capacity and cycling performance of LIB. Therefore, it is important to quantitatively understand the mechanism relating to the stress evolution in electrode during electrochemical cycling, which will be conducive to developing effective methods of relieving the diffusion induced stress. In this work, a bilayer electrode model is proposed by taking into account Li diffusion, built-in stress, concentration-dependent material properties and elastoplastic deformation of current collector.Based on the established model, the influences of the possible plastic deformation in the current collector on the lithium diffusion and stress evolution of bilayer electrode during charging are investigated. The numerical results show that the plastic deformation of current collector can weaken the constraint between current collector and active layer, which leads to a smaller electrode curvature and more homogeneous lithium concentration in the active layer. The relaxation effect of the plastic deformation not only significantly relieves the stresses at the bottom and top surface of active layer,but also promotes the diffusion of lithium into active layer, which can improve the structural reliability of the electrode and increase the effective capacity of the active layer. Furthermore, the influences of the yield strength and plastic modulus of the current collector are discussed. The results indicate that the constraint between the current collector and active layer becomes weaker with reducing yield strength and plastic modulus of current collector, respectively. In other words, the further stress relaxation in the electrode indicates that the capacity can be enhanced upon reducing the yield strength and plastic modulus of current collector, respectively. Considering our results, it is expected that a bilayer electrode composed of the current collector with smaller mechanical strength enjoys simultaneous improvement in battery usable capacity and structural reliability. Consequently, the results of this paper provide a route to improving the cycle performance of bilayer lithium-ion battery electrode.