Graphite Particle Cracking as a Failure Mode during Li-Ion Battery Cycling

In recent years, there has been aggressive research on lithium ion batteries for vehicle applications. For better understanding of the battery lifetime, the degradation mechanisms have attracted much interest because vehicle batteries are generally operated under more severe conditions than consumer electronics applications, i.e. wider temperature ranges and higher rates.  One likely degradation mechanism is the mechanical breakdown of the particles.  To investigate this potential degradation mechanism, numerical predictions, which are based on the diffusion-induced stress, are proposed ^1-5.

For example, in graphite anodes, the crack generation at the particle surface may accelerate self-discharge due to the exposure of the new surface to electrolyte solvents, leading to further formation of the solid electrolyte interphase (SEI). In previous papers^1,3,4, numerical predictions have been used to estimate tendencies of graphite particles to crack, suggesting that this phenomenon is likely at reasonable rates.  However, these papers use a value of lithium diffusion coefficient in graphite that is approximately two to three orders of magnitude lower than that reported experimentally by Dokko et al.⁶, where the diffusion coefficient was measured with a single graphite particle. In order to reconcile this difference, a closed loop system combining the determination of diffusion coefficients from experimental data and model predictions based on the determined value is required to consistently understand the particle crack generation criteria.   

In this study, we focus on stresses in a graphite particle and identify propensities of the particle crack generation criteria at high cycling rates, which is particularly important for vehicle batteries. To understand the criteria, we will propose the following approach that combines experimental tests and mathematical model predictions:

1. Determination of transport property (diffusion coefficient) in the graphite particle from experimental data
2. Mathematical predictions of the stresses in the particle using the above transport  property
3. Durability tests of the graphite electrode
4. Identification of the propensities for the crack generation by model-experimental comparisons

      The experiments were performed using 2325-typed coin cells. To conduct durability tests at high cycling rates, a thin graphite electrode (ca. 20 um) was prepared. The graphite electrode, polypropylene (Celgard 2400), and lithium foil were used as a working electrode, a separator, and a counter electrode, respectively. A few drops of 1 M LiPF₆in EC:DEC (1:2) were added as an electrolyte solvent. In the durability tests, the graphite-lithium half-cells were cycled at different C-rates and temperatures. Then, the particle surface was observed with a scanning electron microscope.

The mathematical model to predict the stresses in the particle was constructed by the coupling of the diffusion equation, graphite particle volume change, and mechanical equations. All the equations were numerically solved using finite-element package COMSOL Multiphysics.

The presentation will discuss the experimental results, the diffusion coefficient that should be used, the predictions of the model, and the agreement of the model with experimental observations on durability of graphite.    

References

1. J. Christensen and  J. Newman, J. Solid State Electrochem., 10, 293 (2006)
2. X. Zhang, W. Shyy, and A. Sastry, J. Electrochem. Soc., 154, A910 (2007)
3. S. Renganathan, G. Sikha, S. Santhanagopalan, and R. White, J. Electrochem. Soc., 157, A155 (2010)
4. P. Barai and P. Mukherjee, J. Electrochem. Soc., 160, A955 (2013)
5. Y. Dai, L. Cai, and R. White, J. Power. Sources, 247, 365 (2014)
6. K. Dokko, N. Nakata, Y. Suzuki, and K. Kanamura, J. Phys. Chem. C, 114, 8646 (2010)