Synthesis and Characterization of Lithium-Conducting Composite Polymer–Ceramic Membranes for Use in Nonaqueous Redox Flow Batteries

Abstract

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.

A Complex Investigation of LATP Ceramic Stability and LATP+PVDF Composite Membrane Performance: The Effect of Solvent in Tape-Casting Fabrication

“…At the same time, one can see a decent number of pores on the membranes’ surface, especially in the cases of DMSO and NMP solvents ( Figure 6 b,c), which can be ascribed to a more severe polymer’s globularity ( Figure S10, Supplementary Materials ). Such a membrane’s morphology impedes its implementation in RFBs, because through pores can be a source of redox species crossover (i.e., membrane’s permeability) [ 16 , 34 , 35 , 36 ].…”

“…The composite membranes were fabricated using a simple tape-casting method ( Figure S2, Supplementary Materials ) close to the previously reported [ 35 ] but with several minor changes (casting temperature, heating bath, drying atmosphere etc.). The component ratios of LATP:PVDF and PVDF:solvent were optimized in our previous publications to achieve the suitable IC/permeability trade-off and for the ease of the casting procedure; the ratios equal 45:55 and 15:85, respectively [ 34 , 35 ]. The as-synthesized ceramic pellets were manually ground in the agate mortar, followed by the intense ball-milling (SPEX 8000, Metuchen, NJ, USA) for 90 min in an agate vial with one milling sphere to achieve the mean particle size of about 1 μm, according to our previous report [ 35 ].…”

“…From now on, we additionally expected the introduction of the inert matrix to protect the LATP filler from the ambient humidity and preserve its originally high IC. Then, we reported our first version of the tape-casting fabrication route of composite LATP+PVDF membranes [ 34 ] along with their general electrochemical properties and primary performance in organic flow and Li-hybrid cells [ 35 , 36 ]. We showed that the fabricated membranes demonstrated ultimate stability to metallic Li, high IC (3.4 ∙ 10 −4 S cm −1 ), good integrity, but relatively high permeability to (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)—a model redox active compound [ 35 ].…”

Towards durable Li-hybrid flow batteries: composite membrane development, cell performance, and perspective

“…Table S7 † summarizes the LATP+PVdF fabrication parameters, variation ranges, exact conditions, and the nal composite's characteristics optimized herein and in our previous studies. [31][32][33][34] Analyzing the literature data and following our explorations, we collected valuable experience of working with complex Li-HFB systems. Table 1 provides our recommendations to solve the main issues of the Li-HFB concept, which are categorized into membrane, catholyte, and design-related problems.…”

“…Then, we obtained flexible and robust LATP+PVdF composites with promising performance: 31 IC of 3.4 × 10 −4 S cm −1 and superior stability to metallic Li. However, we observed sufficient crossover of the catholyte's redox-active compounds, which led to the Li-HFB capacity decay.…”