The Theoretical Modeling Of Failure Fracture Of Active Materials Of Lithium-ion Batteries During Lithiation

Active materials play a decisive role in the performance and cycle life of lithium-ion batteries. The commercial active materials of lithium-ion batteries at present is Li COO2(cathode) and graphite(anode), having a relatively lower capacities which could not be satisfied with the public’s demand for higher energy and longer cycle life. Therefore, it is of great urgency to seek new active materials with greater properties. However, active materials with greater properties(such as Si, Sn, Al, Li Fe PO4) have a common disadvantage that is huge volume change. Inner stresses would generate due to the great volume change(expand and contract) during lithiation/delithiation of the electrode materials. Stress developed and damage effect accumulated during recycling. The crack would produce when the damage effect reached a certain value, which would cause the degradation of cycle performance and the failure of electrode materials in the end.To solve these problems, according to the microcosmic mechanism of electrode materials’ failure, this paper proceeded from the relationships of the materials size, the state of charging and the fracture of materials, used the research methods of combining theoretical modeling and experimental verification, presented a series of model for different material structures, and found the critical failure sizes which were tested. The main contents and innovations of this paper are as follows:1. We presented the theoretical modeling of thin film failure and found the critical failure size of Si thin film and Sn thin film are 33 nm and 15 μm respectively, below which cracking does not occur, and above which surface cracking will take place, taking into the material parameters of Si and Sn.2. We presented the theoretical modeling of solid materials failure which divide into nanowire modeling and nanoparticle modeling, and found the critical failure size of Si nanowire and Si nanoparticle are 70 and 90 nm respectively, below which cracking does not occur, and above which surface cracking will take place, taking into the material parameters of Si.3. We presented the theoretical modeling of hollow materials failure which divide into hollow wires and hollow particles, and found the full use ratio of Si hollow wire and Si hollow particle are 0.15 and 0.10 respectively, the full use ratio of Sn hollow wire and Sn hollow particle are 0.18 and 0.12 respectively, below which the hollowspace of the material is full used, and above the hollow space of the material is available, taking into the material parameters of Si and Sn.4. We verified that the critical failure size of Sn thin film is 10 μm by the experiment of electroplating Sn thin films. By controlling the Plating time, we electroplate different thickness Sn thin films on Cu substrates(10 μm, 18 μm, 26 μm, 35 μm). We control the SOC of Sn thin films by means of controlling the charging time and found that the 10 μm Sn thin film fractured when SOC=1, the 18 μm and 26 μm Sn thin films fractured when SOC=0.72 and SOC=0.5 respectively, and the 35 μm Sn thin film fractured when SOC =0.36. These conclusions totally agree with the critical curve of the theoretical modeling of thin film.