Germanium Nanowire Battery Electrodes with Engineered Surface-Binder Interactions Exhibit Improved Cycle Life and High Energy Density without Fluorinated Additives

Nanostructured Group-IV materials hold great promise as high-capacity alloying electrodes for electrochemical energy storage. However, in order to stabilize their cycling performance, it has been necessary to incorporate fluorinated electrolyte additives, such as fluoroethylene carbonate (FEC), to mitigate recurring irreversible reactions that result in an unstable solid–electrolyte interphase (SEI) layer at the electrode/electrolyte junction. This highlights the critical importance of interfacial chemistry for producing batteries with high capacity retention. Yet, the surface chemistry of the active material is often neglected prior to electrode fabrication. Here we investigate the electrochemical cycling of germanium nanowire (Ge NW) composite alloying electrodes with controlled surface chemistry, operating in the presence and in the absence of fluorine-containing electrolytes and binders. We demonstrate that controlled electrode surface modification, even when paired with completely fluorine-free binders and electrolyte components, significantly improves the capacity retention and longevity of Ge NW-based electrodes. Moreover, for certain surface chemistry/binder pairings, the inclusion of fluorinated additives actually appears to degrade electrode performance, while the inclusion of nonfluorinated additives (such as vinylene carbonate, VC) yielded optimal device performance. These observations highlight that the active material surface chemistry can influence the components of the SEI layer that affect cycle life in addition to the reductive decomposition of electrolyte solvents and additives. Additionally, we systematically investigate the often-overlooked impact of different electrode slurry preparation techniques on the performance of Ge NW composite electrodes fabricated with different polymeric binders. These results present significant chemical and processing guidelines to consider in the production of next-generation, high-capacity batteries.