Abstract:

When LiFePO₄ (LFP) is used as a cathode material, the lithium-ion battery has high safety and long cycle life. It is currently the most widely used cathode material, but its battery rate performance is poor. One of the effective means to improve the rate performance of lithium-ion battery is to use LFP material of nanometer size. However, the mechanism of the LFP particle of nano size affecting the eletrochemical process during the charge and discharge of lithium-ion battery still remain unclear. In this work, a quasi-two-dimensional model of lithium-ion battery was established to simulate the discharge process. The influence of LFP particle size on the rate performance of lithium-ion battery was quantitatively studied. The diffusion rate and electrochemical reaction rate at solid-liquid interface were quantitatively analyzed. The results indicate that the resistance in solid phase is the key factor limiting the performance of lithium-ion battery. In the case of small particle LFP as electrode material, the diffusion path of lithium metal within the particles was shortened and the interface between electrode material and electrolyte increased, thus the electrochemical reaction rate was faster, presenting better rate performance. While in the case of large particle LFP as electrode material, the low solid phase diffusion rate of LFP material resulted in low electrochemical reaction rate and thus deteriorate rate performance of the lithium-ion battery. Size reduction of LFP could effectively shorten the migration path of the metal lithium in the electrode material and reduce solid phase diffusion resistance, therefore enhance the rate performance of the lithium-ion battery.

Key words: lithium-ion battery, mathematical modeling, lithium iron phosphate, particle size, rate performance

摘要：

LiFePO₄（LFP）作为正极材料时，锂离子电池安全性高且循环寿命长，是目前应用最广泛的正极材料，但其电池倍率性能较差。提升倍率性能的有效手段之一是将LFP材料颗粒纳米化，但材料纳米化过程中颗粒粒径减小对于锂离子电池充放电过程中锂在固液相的扩散及表面电化学反应的影响机制仍缺乏清晰的认识。采用锂离子电池的准二维模型，模拟锂离子电池的放电过程，定量研究了正极材料颗粒粒径对锂离子电池倍率性能的影响，分析了固液相扩散速率与电化学反应速率受LFP材料颗粒粒径的影响程度。研究结果表明：电极材料中固相扩散阻力是锂离子电池电化学性能的主要限制因素。小粒径的LFP作为正极材料时，电极材料内的金属锂的迁移路径较短，同时颗粒与电解液的接触面积增加，界面的电化学反应速率较快，放电倍率对于锂离子电池性能影响较小；大粒径的LFP作为正极材料时，电极材料内的金属锂扩散路径的增加和较高的固相扩散阻力限制了界面的电化学反应速率，导致锂离子电池的倍率性能显著降低。LFP材料的纳米化可以有效减小固相扩散阻力，提升锂离子电池的倍率性能。

关键词: 锂离子电池, 数学模拟, 磷酸铁锂, 颗粒粒径, 倍率性能