纳米颗粒的表面效应和电极颗粒间挤压作用对锂离子电池电压迟滞的影响

硅作为锂离子电池电极材料之一,其应力效应尤为突出,进而将影响电池性能.本文建立了电化学反应-扩散-应力全耦合模型,并研究了恒压充放电条件下扩散诱导应力、表面效应和颗粒间挤压作用对电压迟滞的影响.结果发现,应力及其导致的电压迟滞程度与颗粒尺寸相关.在大颗粒(颗粒半径r 100 nm)中,扩散诱导应力是导致电势迟滞效应的主要因素,这将导致电池能量耗散.对于纳米级小颗粒(r 100 nm)而言,表面效应占据主导,表面效应虽然能缓解电压迟滞,同时却会使驱动电化学反应部分的过电势回线下移,造成锂化容量衰减.本文综合考虑了扩散诱导应力和表面效应,得出:半径为10 nm的颗粒将会使电极具备较好的综合性能.此外,对于硅电极而言,颗粒间挤压作用会使应力回线向压应力状态演化,进而导致锂化容量的衰减.计算结果表明,在电极设计中,对孔隙率设定下限值有助于提升电极性能.

Influences of nanoscale particles and interparticle compression in electrodes on voltage hysteresis of lithium ion batteries

As one of high capacity electrode materials of lithium ion battery, silicon suffers significant stress effects, which further affects the voltage performance of battery. In this paper, a reaction-diffusion-stress coupled model is established, and the stress induced voltage hysteresis with consideration of diffusion induced stress, surface effects and interparticle compression under potentiostatic operation are investigated. It is found that stress and stress induced voltage hysteresis are dependent on particle size. For big particles, the diffusion induced stress is dominant and further aggravates the hysteresis of both stress and the overpotential consumed by it, indicating that more energy dissipates due to the stress effects. For small particles, especially ones with radius of a few nanometers, surface effects play a more prominent role than diffusion induced stress and the stress evolves into the state of compressive stress on the whole, leading the hysteresis of overpotential to be consumed by stress shrink and making the hysteresis plot of overpotential used to drive electrochemical reaction move downward. The electrode potential first reaches a cutoff voltage and finally the capacity of lithium ion battery decays. Therefore, too large or too small particle size in the electrode can both have a negative effect on the performance of lithium ion batteries, which indicates that an optimal size of the electrode particles must be designed in terms of electrode structure. Based on the calculation, particles with around 9 nm in radius are an appropriate option for electrode design in consideration of both diffusion induced stress and surface effect. In addition, for silicon electrodes, the silicon particles inevitably squeeze each other in a charge and discharge cycle. Therefore, interparticle compression is considered in this case. In detail, interparticle compression pushes the plot of stress hysteresis to the compressive state and leads to lower lithiation capacity, which makes the overpotential plot consumed by stress move downward and accordingly the overpotential plot used to drive the electrochemical reaction move upward. Denser electrode would strengthen this effect due to higher particle compression. It is indicated that for electrode design, the minimum of porosity ratio of electrodes should be adopted because higher interparticle compressive stress would reduce the battery capacity. Our results reveal that the voltage hysteresis of lithium ion batteries is related to the active particle size and the porosity ratio of the electrode, which is of great significance for guiding one in designing the lithium ion batteries.