The characterization of c-rate dependent hard carbon anode fracture induced by lithium intercalation

Abstract

This study sought to evaluate hard carbon's fracture characteristics under different cycling rates due to its lithium solid solubility and isotropic nature. In addition to the evaluation, an electrochemical shock map was modified from a previous study to try and predict what conditions of particle size and C-rate are necessary to cause brittle fracture events in hard carbon. Subsequently, hard carbon anodes were created using a formulation of hard carbon, carbon black, and Kureha binder and subjected to two or three cycles of C-rates varying from C/10 to 5 C. Data evaluation suggests that for every C increase approximately nine more percent of the particles in the system will develop cracks. Further analysis of the data shows that low C-rate anodes may have been affected by inhomogeneous mixtures, skewing the linear relationship to a higher than accurate value in the linear plot. Additionally, a C-rate limit that prevents any brittle fracture from occurring can be found at c/10 or lower. When comparing the anodes to the model, the model shows accuracy in C predicting failure conditions for the higher C-rate anodes. When applied to lower C-rates (below c/2), the 2 accuracy of the model begins to fall. Possible solutions to this problem include finding more accurate material properties for hard carbon or redefining the model to account for some unique value (i.e. - the hard carbon's geometry) associated with the hard carbon. Additionally, more anodes should be tested to create a larger sampling that can average cells that have inhomogeneous mixtures.