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

Yasser Ashraf Gandomi; Irina V Krasnikova; Nikita O Akhmetov; Nikolay A Ovsyannikov; Mariam A Pogosova; Nicholas J Matteucci; Christopher T Mallia; Bertrand J Neyhouse; Alexis M Fenton Jr; Fikile R Brushett; Keith J Stevenson

doi:10.1021/acsami.1c13759

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

ACS Appl Mater Interfaces. 2021 Nov 17;13(45):53746-53757. doi: 10.1021/acsami.1c13759. Epub 2021 Nov 4.

Affiliations

¹ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
² Center for Electrochemical Energy Storage, Skolkovo Institute of Science and Technology, Moscow 121205, Russian Federation.
³ Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.

PMID: 34734523
DOI: 10.1021/acsami.1c13759

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 into a flexible polymer matrix, may offer a pathway to attain performance attributes needed for enabling 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) superionic conductor) crystal structure, Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) and Li_1.4Al_0.4Ge_0.2Ti_1.4(PO₄)₃ (LAGTP), each 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 and electrochemical techniques. Importantly, the electrochemical stability window of the as-prepared membranes lies between 2.2-4.5 V vs Li/Li⁺. 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. Overall, the insights gained through this work begin to establish the foundational knowledge needed to advance composite polymer-inorganic membranes/separators for NAqRFBs.

Keywords: composite polymer−ceramic membranes; lithium superionic conductor; membrane; nonaqueous electrochemistry; redox flow batteries; separator.