Rutooj D. Deshpande

Effects of Concentration-Dependent Elastic Modulus on Diffusion-Induced Stresses for Battery Applications

Most lithium-ion battery electrodes experience large volume changes associated with Li concentration changes within the host particles during charging and discharging. Electrode failure, in the form of fracture or decrepitation, can occur as a result of repeated volume changes. It has been found recently that many electrode materials, such as graphite, Si, and LiFePO 4 , change their elastic properties upon lithiation. However, previous diffusion-induced stress (DIS) models have not considered this relationship. In this paper, we developed a mathematical model, with the assumption of a homogeneous isotropic cylindrical electrode particle, to describe the effect of concentration-dependent Youngs modulus on DIS in battery electrodes. The DIS model considers both increasing and decreasing Youngs modulus with concentration. The model shows that the concentration dependence of Youngs modulus has a significant effect on peak stress and stress evolution in the electrodes. Insertion and deinsertion are not symmetric in stress profiles. We conclude that Li stiffening is beneficial to avoid surface cracking during delithiation, and moderate Li softening is beneficial to avoid particle cracking from the center during lithiation.

Diffusion Induced Stresses and Strain Energy in a Phase-Transforming Spherical Electrode Particle

Lithium insertion and removal in lithium ion battery electrodes can result in diffusion induced stresses (DISs) which may cause fracture and decrepitation of electrodes. Many lithium ion electrode materials undergo formation of two or more phases during lithium insertion or removal. In this work, we mathematically investigate the DISs in phase transforming electrodes using a coreshell structural model. We examine the concentration jumps at phase boundaries that result in stress discontinuities, which in turn can cause cracking. The influence of the mechanical properties of the two phases on stress evolution, stress discontinuity, and strain energy are clarified. The trends obtained with the model may be used to help tune electrode materials with appropriate interfacial and bulk properties so as to increase the durability of battery electrodes.

Mesopores inside electrode particles can change the Li-ion transport mechanism and diffusion-induced stress

Following earlier work of Huggins and Nix [ Ionics 6, 57 (2000)], several recent theoretical studies have used the shrinking core model to predict intraparticle Li concentration profiles and associated stress fields. A goal of such efforts is to understand and predict particle fracture, which is sometimes observed in degraded electrodes. In this paper we present experimental data on LiCoO 2 and graphite active particles, consistent with previously published data, showing the presence of numerous internal pores or cracks in both positive and negative active electrode particles. New calculations presented here show that the presence of free surfaces, from even small internal cracks or pores, both quantitatively and qualitatively alters the internal stress distributions such that particles are prone to internal cracking rather than to the surface cracking that had been predicted previously. Thus, the fracture strength of particles depends largely on the internal microstructure of particles, about which little is known, rather than on the intrinsic mechanical properties of the particle materials. The validity of the shrinking core model for explaining either stress maps or transport is questioned for particles with internal structure, which includes most, if not all, secondary electrode particles.

Understanding Diffusion-Induced-Stresses in Lithium Ion Battery Electrodes

Most lithium ion battery electrodes experience large volume expansion and contraction during lithiation and delithiation, respectively. Electrode failure, in the form of fracture and decrepitation, can occur as a result of repeated volume changes. In this paper, we provide an overview of our recent work on modeling the evolution of concentration, stress, and strain energy within a spherical- or cylindrical-electrode element under various charging-discharging conditions. Based on the analytic results, we propose tensile stress and strain energy based criteria for the initiation and propagation of cracks within the electrodes. We will also discuss “size effects” on stresses and fracture of electrodes. These results may help guide the development of new materials for lithium ion batteries with enhanced durability and performance.