Examination of Graphite Particle Cracking as a Failure Mode in Lithium-Ion Batteries: A Model-Experimental Study

Abstract

Capacity fade in lithium-ion batteries remains an area of active research, with failure of the graphite anode thought to be an important contributor. While the formation of the solid electrolyte interphase and the subsequent loss of cyclable lithium have been well studied, mechanical degradation remains an area where ambiguity remains. While there appears to be little experimental evidence that suggest that macroscopic particle cracking occurs, mathematical models have suggested that this phenomenon is likely. The goal of this paper is to clarify this ambiguity by combining experimental cycling, mathematical stress modeling, and post-mortem microscopy. We experimentally determine an average diffusion coefficient of lithium in graphite using a thin-layer electrode and use this information in a diffusion-induced stress model. Our results suggest that cracking is not likely during lithiation due to the proximity of equilibrium potential to the cutoff potential. On delithiation, at 25 • C, even at 30 C rate cracking is unlikely while at −10 • C, a rate of 10 C can lead to particle cracking. By extrapolating the results of the thin-layer electrode to a thick porous electrode, we found that graphite cracking is unlikely to occur during typical vehicle operations. In recent years, there have been numerous studies on lithium-ion batteries for vehicle applications. [1][2][3][4][5] Of particular interest are the various degradation mechanisms encountered when operating the battery under severe conditions (i.e., wide temperature ranges and high Crates) typical for vehicle applications.6 One likely degradation mechanism is the mechanical breakdown of the active materials. Lithium insertion (or extraction) into (or out of) the lattice of the active materials causes volume change (expansion and contraction), resulting in stress generation and possible fracture of the particles. For example, in graphite anodes, crack generation can expose new surfaces to the electrolyte, leading to further formation of the solid electrolyte interphase (SEI) 7,8 and/or particle isolation from the electronic network, leading to impedance growth, loss of cyclable lithium, and capacity fade.Mathematical models, based on diffusion-induced stress generation, have been used to investigate particle fracture and have suggested that graphite particle cracking is likely under certain conditions. [9][10][11][12][13] For the diffusion-induced stress model, diffusion coefficient in solid phase is a key parameter to determine the concentration gradient in the particle, which in turn affects the stress generated.9 However, there is a wide disparity in the measured diffusion coefficient of lithium in graphite, ranging from 10 −16 to 10 −11 m 2 /s. [14][15][16][17][18][19] This wide range can be attributed to different kinds of graphite used in these studies in addition to different techniques used to extract the diffusion coefficient. Model predictions would suggest that for the lower value of diffusion coefficient (1.0 × 10 −14 m 2 /s) particle cracking durin...

Mechanical Degradation of Graphite/PVDF Composite Electrodes: A Model-Experimental Study

“…These conditions describe the electrodes used in our previous work. 11 2. The mechanical influence of the SEI film is negligible.…”

“…Graphite is known to undergo a volume change of approximately 10% during cycling, 10 and previous modeling work 3,11 has explored the possibility of mechanical damage in isolated graphite particles due to such volume changes. However, in typical commercial graphite electrodes, active material particles are not isolated, but instead bound into a porous structure with polymer binder and conductive agent.…”

Statistical Physics-Based Model of Solid Electrolyte Interphase Growth in Lithium Ion Batteries

“…2 Battery aging is linked to the (electro-)chemical and mechanical degradation of the electrodes and the electrolyte. 2 Three main degradation mechanisms prevail at the particle level during the cycling of LIBs: (1) growth of the solid electrolyte interphase (SEI) at the negative electrode, 2,3 (2) formation of cracks in the SEI layer at the negative electrode, 4 and (3) dissolution and isolation of nanocrystalline particles at the positive electrode. 5 These processes lead to capacity fade and power loss.…”

Reduced Order Modeling of Mechanical Degradation Induced Performance Decay in Lithium-Ion Battery Porous Electrodes

“…36 The effect of stress generation inside electrode active particles has been incorporated within the "porous electrode theory" to study its impact on cell performance. 37,38 The "porous electrode theory" has also been extended to incorporate system-level parameters, such as, cost, life, and safety of the LIB. 39 Generation of mechanical stress within LIB active particles has been investigated thoroughly in the last decade.…”