Abstract:
Lithium-ion batteries are attractive candidates as energy storage devices for solar power, wind power, and electric vehicle applications; however, their capacities and energy densities are still not enough to meet the industry standards. Although replacing the existing electrode materials with higher performance materials could address the capacity issue to some extent, it poses other challenges. These new anode materials (such as Si, Sn, Al etc.) undergo large volume changes (~100-270%) upon reacting with lithium which induces significant amount of stresses in the electrodes during battery operation. These stresses cause fracture and mechanical damage, which also promotes chemical degradation. Both of these processes lead to rapid capacity fade. Although cathode materials do not undergo significant volume changes, mechanical degradation, due to stresses, is still a major issue. Hence, there is a need to understand the mechanical behavior of electrode materials and coupling between electrochemistry and mechanics to be able to design efficient, durable, and higher-energy density batteries.
Stress evolution in Si and Al thin films during electrochemical cycling was measured by monitoring substrate curvature using the multi beam optical sensor method. Strain rate sensitivity, fracture properties, and biaxial modulus of Si were also measured as a function of state of charge. After reacting with lithium, Si becomes ductile and undergoes plastic deformation. In contrast to graphite, elastic modulus of Si decreases with Li concentration. Further, a continuum mechanics model was developed to simulate the mechanical response of the Si electrodes during electrochemical cycling. Model predictions agreed very well with the experimental data. Finally, stress measurements were conducted on composite anode (based on graphite) and cathode (based on Li1.2Ni0.15Mn0.55Co0.1O2). The stress data from these composite electrodes was then used in an approximate semi-analytical method to estimate the pressure that the casing of a commercial (jelly roll configuration) Li-ion battery will undergo as a function of state of charge.
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