Computational analysis of chemomechanical behaviors of composite electrodes in Li-ion batteries
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Mechanical reliability is a critical issue in all forms of energy conversion, storage, and harvesting. In Li-ion batteries, mechanical degradation caused by the repetitive swelling and shrinking of electrodes upon lithiation cycles is now well recognized; however, the impact of mechanical stresses on Li transport and hence the capacity of batteries is less obvious and underestimated. In particular, the stress field within the heterogeneous electrodes is complex, making the characterization of the chemomechanical behaviors of electrodes a challenging task. We develop a finite element program that computes the coupled Li diffusion and stresses in three-dimensional composite electrodes. We employ the reconstructed models of both cathode and anode materials to investigate the mechanical interactions of the constituents and their influence on the accessible capacity. The state of charge in the percolated particles is highly inhomogeneous regulated by the stress field. An ample space of design is open for the optimization of the capacity and mechanical performance of electrodes by tuning the size, shape, and pattern of active particles, as well as the properties of the inactive matrix.
I. INTRODUCTION
Li-ion batteries are the major power source in portable electronics and electric vehicles. 1,2 Innovation in the battery technology has been driven by the imperative demand of materials of light weight, high energy density, fast charging, long lifespan, and low cost.3–5 Mechanical degradation of batteries caused by the repetitive swelling and shrinking of electrodes in the lithiation cycles is now well recognized.6,7 The stress induced structural disintegration impedes electron conduction and causes persistent loss of capacity of batteries in the long-term cycles. In particular, the mechanical failure has become the bottleneck in the commercialization of high-capacity electrodes because of the massive volumetric deformation associated with the electrochemical processes. Figure 1 gives a brief survey of the volumetric strain of different types of cathode and anode materials. It is evident that drastic deformation is inherent to the high-capacity electrodes, such as S cathode as well as conversion and insertion type anodes.8–12 The repetitive volumetric change in the active materials generates a complex field of stresses in the electrodes and leads to various sources of mechanical degradation including fracture,13–17 plasticity,18–20 and cavitation.21,22
Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2016.302
While mechanical failure of batteries is frequently observed, less understood is the impact of stresses on the electrochemical processes, that is, how the locally generated stresses modify the energy landscape and kinetics of Li transport, interfacial reactions, and hence the capacity and potential of batteries.23–26 Developing continuum models and numerical methods for the coupled Li diff
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