Leveraging Operando Characterization Methods to Reveal Failure and Optimization Mechanisms of Group IV Semiconductor Battery Anodes

Abstract

Expanding the application space of lithium battery technology will require electrode materials that exhibit performance metrics which outperform their modern counterparts. The availability of prospective next-generation electrode materials to meet this demand is promising, as the group IV semiconductors Si and Ge exhibit an order of magnitude higher capacity for lithium storage than traditional electrode materials. However, there exist several issues related to the compatibility between these candidate electrode materials and components that presently make up a battery, problems that can be mitigated by the impractical use of expensive and toxic fluorinated compounds. This thesis focuses on the molecular nature of stability and instability mechanisms between group IV semiconductor composite electrodes and their surrounding solid and liquid matrices, with specific attention on the electrode interface. First, we distinguish cycling behavior of Si anodes by the interfacial chemistries that develop as a function of the Si lithiation state and fluorine content using vibrational spectroscopy. Next, we probe the cycling performance of germanium nanowire anodes in the solid-state and learn that electrochemical accessibility to crystalline Li15Ge4 and amorphous LixGey phases determine their cycle life. We further observe that surface functionalization of Ge nanowires eliminates the need for fluorinated compounds in the battery, highlighting a strategy to circumvent the barriers described earlier. Because the dynamics of electrochemical phenomena are strongly affected by the high local fields at the electrode interface, we end with a novel quantitative analysis of the electric fields present at the electrode/electrolyte junction with the use of a systematic calibration of electrolyte solvents to electric fields. We expect these results to lend guidelines that better inform the design rules for batteries utilizing next-generation, high capacity active materials.