In Li-ion batteries—the major power source in portable electronics and electric vehicles today—mechanical reliability is a critical issue when it comes to energy conversion and storage. Mechanical stresses caused by the repetitive swelling and shrinking of electrodes in Li-ion batteries during charge/discharge cycles have a big impact on the capacity of the batteries. Developing models that describe the mechanical degradation is crucial for optimized high energy-density battery materials and next-generation battery designs, with enhanced performance and longer lifetimes.

Up to now, much of the academic focus has been on two-dimensional electrode structures that often comprise an anode, a cathode, an electrolyte and active particles and ions. They also focus on idealized structures of electrodes composed of isolated particles. These active particles undergo electrochemical insertion that is an intrinsically simple and reversible host/guest redox reaction in which the active particles act as solid hosts for mobile guest Li-ions while an electro-chemical charge transfer takes place. As a result of the insertion and extraction of lithium, the active particles undergo large volume changes.

Kejie Zhao, professor of mechanical engineering at Purdue University, with his group has recently developed a finite element program for three-dimensional (3D) composite electrodes that are comprised of particles that interact with each other, in order to investigate the influence and evolution of the complex stress fields on the capacity of the battery. Along with Rong Xu and Luize Scalco de Vasconcelos, Zhao reports and describes the program in their recent article in the Journal of Materials Research.

Zhao says that this program is an important tool for designing battery materials to obtain optimum performance for capacity as well as mechanical stability, depending on the system. “Many think of batteries as a pure electrochemical element, but there is a lot of mechanics taking place inside and microstructure definitely matters,” Zhao says.

Conventional rechargeable Li-ion batteries have two electrodes with different chemical potentials. The electrodes are composed of active particles, an inactive matrix composed of polymer binders and additives, and pores filled with an electrolyte. While an external circuit connects the two electrodes, the electrolyte separates them physically, conducting Li-ions but not electrons.

During discharge of the battery, electrons flow from the anode to the active particles in the cathode, while Li-ions go through the electrolyte into the cathode to balance the potential difference and complete the redox reaction. Both the ionic and the electronic processes are reversed when the battery is charged and this reversibility is of importance for the lifetime of a battery.

The computer program developed by Zhao and his colleagues computes the Li transport coupled with mechanical stresses for three-dimensional electrodes. “Li-diffusion will cause deformation and stress to the electrodes. This stress will subsequently influence the diffusion of Li. It’s a kind of loop,” Zhao says.

The researchers reconstructed 3D models of an LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) cathode and a SnO anode, used in commercial batteries, based on extensive x-ray tomographic microscopy data, provided by the Wood group at ETH Zürich. The 3D models, which correspond to realistic reconstructions of the materials, are then implemented into a finite element program to study the co-evolution of stresses and Li-storage.

In finite element analysis, an initial complicated problem is subdivided into finite elements—in other words, smaller, simpler parts. The equations which model the finite elements are then assembled into a larger system of equations that models the entire problem. The uniqueness and novelty of this program, according to Zhao, is that it includes two parameters that are very important for real materials: plasticity and large deformation.

Gurpreet Singh, Associate Professor of Mechanical and Nuclear Engineering at Kansas State University, who was not involved in the study, believes that this program could allow selection of appropriate electrode materials and geometries to minimize capacity loss and improve cycling stability of the cell. He also adds that the model is more realistic in predicting chemomechanical behaviors compared to previous works based on single particle models. “This study has furthered our understanding of structure–chemical–mechanical property interactions in lithium-ion battery electrodes,” Singh says.

Zhao and his team are now working on validating the mechanics models by experiments. “Batteries are very complex systems; the physical, chemical, and thermo-processes are all coupled together and it is very hard to separate one from another,” Zhao says. “My group is focused on the mechanics of electrochemical systems—bridging the interdisciplinary fields can be truly rewarding toward the energy solutions.”

Read the abstract in the Journal of Materials Research.