(Invited) “Decrepitation” and Mechanical Degradation: In Situ Investigation to Understand Chemo-Mechanical Stability in Batteries

Mechanical degradation and fracture have long been recognized as challenges that have impeded development of large-volume-change electrode materials that exhibit stable cycling behavior. Bob Huggins (along with Bill Nix) was among the first to quantify mechanical fracture and “decrepitation” or fracture processes in alloying anode materials, predicting that fracture during electrochemical reaction should be dependent on particle size, among other variables [1]. In recent years, our understanding of how reaction processes relate to mechanical degradation in battery materials has advanced significantly, and in situ / operando experimental techniques have been key for this progress. Here, I discuss my group’s recent efforts in understanding chemo-mechanical degradation in battery materials. First, in situ transmission electron microscopy (TEM) experiments are presented in which individual nanocrystal electrode materials are observed to react with various alkali ions (Li + , Na + , and K + ). For FeS 2 nanocrystals, the reaction with all three ions involves conversion to form a mixture of phases, along with volume expansion. Despite larger volume changes during reaction with Na + and K + , only reaction with Li + was observed to cause fracture. This surprising result was found to be due to the shape of the reaction front as it evolves during reaction, with sharp corners during lithiation resulting in stress concentration and fracture. In addition to this work, other in situ TEM investigations of reaction processes in metal alloying anode nanocrystals have revealed the importance of the mechanical properties of the native oxide in determining transformation pathways during electrochemical cycling. Finally, our research on chemo-mechanical degradation at interfaces within solid-state batteries will be briefly discussed. This work points to the necessity of understanding and controlling chemo-mechanical phenomena in high-capacity battery materials for developing next-generation battery materials. [1] R. A. Huggins, W. D. Nix “Decrepitation Model for Capacity Loss During Cycling of Alloys In Rechargeable Electrochemical Systems” Ionics, 2000, 6, 57.