Effects of Mechanical Stresses on Lithium Ion Batteries.

Abstract

Structural batteries are multifunctional materials that are expected to perform multiple roles, i.e. power generation and mechanical support. Properly realized, the concept can dramatically improve gravimetric properties by elimination of parasitic masses tolerated in conventional devices. However, the significant structural loads that such batteries would be expected to bear, given their multiple roles, would exceed those experienced by conventional batteries, and thus must be studied and understood in the context of device safety, lifetime and cost. Key sources of mechanical stresses were identified and quantified in this effort, including manufacturing-induced loads, thermal loads, kinetic loads and structural loads. It was found that stresses due to compression during electrode fabrication can be as large as 2000 MPa, thus having the potential to induce mechanical failure of constituent materials, while stresses induced by structural loads in an unmanned vehicle example studied, were less than 1 MPa. Thermal stresses were found in one embodiment, to exceed 100 MPa, largely due to mechanical confinement of batteries embedded for structural support. Supporting experiments were conceived in this work to study the effect of loads on electrochemical performance. A novel design of a structural battery was developed, to simultaneously conduct mechanical and electrochemical experiments. Diffusion coefficients were determined while various loadings were applied. Also, stresses exerted on the battery were observed while the battery was charged and discharged. Statistically meaningful correlation was not observed for diffusion coefficients with respect to stresses, but forces applied to carbon fiber significantly varied during charging/discharging of batteries, and the magnitude of variance of force was strongly correlated with the number of charges. Molecular dynamics simulations, finally, were performed to examine findings from experiments. Diffusion coefficients of lithium ions were measured while graphite lattices were stressed. Simulation results differed substantially from experimental findings, in that substantial alterations in diffusivities were predicted with application of load. Future work should focus on rationalizing the differences in these elements, by development of improved experiments, and improved accounting for disorder, in simulations.