Synthesis and characterization of lithium-conducting composite polymer-ceramic membranes for use in nonaqueous redox flow batteries

Redox flow batteries (RFBs) are a burgeoning electrochemical platform for long-duration energy storage, but present embodiments are too expensive for broad adoption. Nonaqueous redox flow batteries (NAqRFBs) seek to reduce system costs by leveraging the large electrochemical stability window of organic solvents (> 3 V) to operate at high cell voltages and to facilitate the use of redox couples that are incompatible with aqueous electrolytes. However, a key challenge for emerging nonaqueous chemistries is the lack of membranes/separators with suitable combinations of selectivity, conductivity, and stability. Single-ion conducting ceramics, integrated with polymeric fillers to make flexible composites, may offer a pathway to the performance attributes needed for competitive nonaqueous systems. Here, we explore composite polymer-inorganic binder-filler membranes for lithium-based NAqRFBs, investigating two different ceramic compounds with NASICON-type (NASICON: sodium (Na) Super Ionic CONductor) crystal structure, Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li1.4Al0.4Ge0.2Ti1.4(PO4)3 (LAGTP), blended with a polyvinylidene fluoride (PVDF) polymeric matrix. We characterize the physicochemical and electrochemical properties of the synthesized membranes as a function of processing conditions and formulation using a range of microscopic, spectroscopic, and electrochemical techniques. We then integrate select composite membranes into a single electrolyte flow cell configuration and perform polarization measurements with different redox electrolyte compositions. We find that mechanically robust, chemically stable LATP/PVDF composites can support > 40 mA cm−2 at 400 mV cell overpotential, but further improvements are needed in selectivity. The insights gained through this work begin to establish the foundational knowledge needed to advance composite polymer-inorganic membranes/separators for NAqRFBs.