Real-time Measurement Of Overall Deformation For LiCoO₂ Battery At Charge/Discharge Conditions And Analysis Of Tensile Properties Of Copper Foil

Lithium ion batteries with high energy density, high rated voltage, green environmental protection and many other advantages, have been widely used in electronics and renewable energy. However, different service conditions and load states during the process of electrochemical cycle play an important role in the electrochemical and safety performance of lithium ion batteries. Therefore, it is necessary to real-time measure the deformation evolution of the lithium ion battery during charge-discharge processes. In this paper, we have developed a set of experimental equipment with three-dimensional digital image correlation technique to analyze the surface field of commercial pouch type LiCoO₂ batteries under different chargedischarge processes with 1 C and 0.8 C rate. Furthermore, the mechanical properties of cathode copper foil collector were estimated by uniaxial tensile tests. The main research contents are as follows,Firstly, our developed experimental apparatus was used to in-situ measure the surface displacement field and strain field evolution of commercial pouch type LiCoO₂ battery under different service conditions, which can provide a good experimental method and reliable experimental equipment for studying the process of electrochemical cycle.Secondly, the results show that the maximum displacement along the x-direction is 23 μm and 10 μm after 50 cycles at 1 C rate and 0.8 C rate, respectively. Similarly, the maximum displacement in the y-direction is 35 μm and 29 μm, respectively. The maximum displacement along the z-direction is about 194.8 μm and 174.6 μm, respectively. The surface strain field test results show that after the 50 th full charge, strain on the cell surface in the x-direction, y-direction and major principal strain are, respectively, 0.185% and 0.132%, 0.275% and 0.146%, 0.244% and 0.185%, 0.497% and 0.224% after many cycles at 1 C rate and 0.8 C rate.Thirdly, fracture strength, elastic modulus and elongation of copper foil were tested by uniaxial tensile after many cycles with 1 C, 0.8 C and 0.5 C rate. It is found that the fracture strength, elastic modulus and elongation of copper foil decrease with the increase of cycles and circulation rates. These properties are estimated as about 256.5 MPa, 19.2 GPa and 4.14% for as-received samples, respectively. However, after 55, 150, 200 cycle times at 1 C rate, they gradually down up to 218.9 MPa, 12.9 GPa and 2.79%, 159.9 MPa, 9.9 GPa and 2.15%, 134.5 MPa, 8.8 GPa and 1.61%, respectively. Moreover, after 55, 150, 200 cycle times at 0.8 C rate, they decreases to 231.2 MPa, 13.9 GPa and 3.06%, 169.7 MPa, 11.7 GPa and 2.22%, 148.5 MPa, 9.4 GPa and 1.74%, respectively. After 55, 150, 200 cycle times at 0.5 C rate, they display as 235.1 MPa, 14.6 GPa and 3.40%, 173.5 MPa, 12.2 GPa and 2.83%, 155.6 MPa, 9.7 GPa and 1.99%, respectively.