High capacity conversion electrode materials – like silicon and germanium – that undergo a large volume change upon lithiation and delithiation, require the formation of a robust solid-electrolyte interphase (SEI) layer, which is critically important to mitigate deleterious irreversible reactions that occur at the electrode-electrolyte interface. Over time, these irreversible reactions at the electrode surface result in capacity fade by decreasing the number of mobile lithium ions within the cell and by creating a more hindered diffusion path for lithium ions as they travel toward the electrode. To combat the recurrence of these irreversible reactions in high-capacity conversion electrode systems, the addition of electrolyte additives, such as fluoroethylene carbonate (FEC), is typically deemed necessary to promote stable cycling and improve capacity retention. The observed improvements associated with the inclusion of these virtually universally employed additives have been previously attributed to fluorinated reductive decomposition products that enable the formation of a robust SEI layer.

Clearly, careful control over interfacial chemistry and surface reactions are critical for the design of robust electrochemical systems; however, the surface chemistry of the active material is typically ignored prior to electrode formulation. In most studies of high capacity conversion electrode materials, the surfaces remained unmodified, and the active material is incorporated irrespective of what uncontrolled – potentially oxidized – surfaces happen to be present in the native state. Herein, we systematically investigate the electrochemical cycling of germanium nanowire composite conversion electrodes with controlled surface chemistry, formulated with an array of polymeric binders, both in the presence and in the absence of fluorinated electrolyte additives. We observe that controlled modification of the nanowire electrode surface, when paired with certain binders, dramatically improved germanium nanowire-based electrode cycling retention and longevity. In fact, some of these pairings exhibit improved capacity retention in the absence of the fluorinated additives typically deemed necessary for stable cycling, whereas the addition of the fluorinated additive caused the cell capacity to fade more rapidly. Finally, through careful control of surface chemistry and strategic choice of binder material, we show that germanium nanowire-based conversion electrodes can maintain capacity retention in the complete absence of fluorinated additives, fluorinated electrolytes, and fluorinated binders.