The research objective of this award is to understand the fundamentals in coupled mechanics and electrochemistry in lithium (Li)-ion batteries. Silicon is an attractive anode material being closely scrutinized for use in Li-ion batteries because of its highest-known theoretical charge capacity of 4,200 mAh/g. However, the development of silicon anode Li-ion batteries has lagged behind because of their large mechanical deformation, i.e., up to 400 percent volumetric change, during electrochemical reactions, which results in fracture, pulverization and early capacity fading. In other words, this coupled mechanics (e.g., volumetric change) and electrochemistry problem is a bottleneck to the development of silicon anode Li-ion batteries. The objective of this project is to understand the fundamental mechanical properties of the lithiated silicon anodes using combined experimental and theoretical methods. Specific studies will focus on i) nanoindentation experiments to obtain the influence of lithiation on mechanical properties as a function of depth; coupled with ii) continuum modeling to extract the stress-strain curve of lithiated silicon anodes under different state of charges.
If successful, the results of this research will help facilitate the fundamental understanding of the coupled mechanics and electrochemistry of silicon anodes in Li-ion batteries, specifically the mechanical behavior of lithiated silicon. The theoretical and experimental methodology can be also applied to other electrodes and materials in Li-ion batteries, such as the cathode. The successful implementation of the proposed research will contribute to a new area of mechanics of materials, namely, coupled mechanics and electrochemistry, which will potentially lead to a range of transformative applications in battery and energy-related fields. The graduate students involved in the research will be trained in a multidisciplinary environment.
The development of high-energy storage devices is one of top most important research areas in recent years and rechargeable batteries are anticipated to be the primary sources of power for modern-day requirements. Lithium (Li) ion battery is one such rechargeable batteries that has been investigated because of their high energy density, no memory effect, reasonable life cycle, and one of the best energy-to-weight ratios and has applications in portable electronic devices, satellites, and potentially electric vehicles. Silicon is an attractive anode material being closely scrutinized for use in Li-ion batteries because of its highest-known theoretical charge capacity of 4,200 mAh/g. However, the development of Si-anode Li-ion batteries has lagged behind because of their large mechanical deformation, i.e., up to 400% volumetric change, during electrochemical reactions, which results in fracture, pulverization and early capacity fading. In other words, this coupled mechanics (e.g., volumetric change) and electrochemistry problem is the bottleneck on the development of Si anode Li-ion batteries. Therefore, a fundamental understanding of this coupled behavior of mechanics and electrochemistry will not only advance our knowledge on the failure of Si under lithiation, but also provide a basis to resolve this bottleneck in the development of the promising Si-anode Li-ion batteries. Intellectual merit: This project is to study (1) nanoindentation experiments to obtain the influence of Si on mechanical properties as a function of depth; and (2) multiscale modeling of lithiated silicon anodes under different state of charges. This research program has built the capability of extract stress-strain relations from experiments by developing theoretical models/numerical platform and the indentation experiments have measured the modulus and hardness of the lithiated Si. The specific results are: (1) We have developed a finite element framework to study fully coupled large deformation and mass diffusion problem in electrodes; (2) We have measured the modulus and hardness of the lithiated Si using nanoindentation and the results show that the lithiated Si has much lower modulus and hardness compared with unlithiated Si. This measurement is based on a continuous stiffness measurement (CSM) technique, which provides the advantage of an instantaneous measurement of modulus and hardness during indentation. Combined with the theoretical simulation, the stress-strain curve will be extracted. Moreover, we are developing a new approach to accurately measure the Li composition, rather than just using the in-direct method, such as cut-off voltage; thus more accurate material properties can be obtained by combined experiments and theoretical efforts. Broader impacts: The proposed research will facilitate our understanding of fundamentals in the coupled mechanics and electrochemistry in Si anodes in Li ion batteries. The theoretical and experimental methodology can be also applied to other electrodes in Li-ion batteries, such as cathode. The successful implementation of the proposed research will create a new area of mechanics, namely, coupled mechanics and electrochemistry, which will lead to a range of transformative and innovative applications in battery related fields and provide solutions to the "Grand Challenges for Engineering" in the 21st century outlined by the U.S. National Academy of Engineering. In addition to the research activities, the proposed program contains an integrated educational plan, aimed at the broader objective of training and educating graduate students on issues of application and development of mechanics to energy technology. Five graduate students have been partially involved in this project and one PhD student graduated in 2014 and is currently employed by Intel. Five papers have been published. The mechanics models have been implemented in the PI’s graduate level course "Elasticity" as several course projects.
