The demand for rechargeable electrochemical energy storage with high gravimetric density (GED) and specific power (SP) is driven by drivetrain power and energy requirements in ground transportation, unmanned aerial vehicles (UAVs), electrification of avionics, and miniaturization of consumer-electronic gadgets. Contemporary battery materials impose a finite limit on GED/SP due to thermal and mechanical failure modes of battery electrodes and membrane separators. The shelf-life of stored charge in rechargeable devices does not scale linearly with maximum SP and has led to trends referred to as ‘range-anxiety’, ‘compulsive charging’, etc. In this context, we … compare the driving range of electric vehicles from a full recharge and note that the driving miles per recharge time (min) for commercially viable battery technology is ~0.1–0.4 (plugin EVs to Tesla’s 85 kWh battery pack).

We introduce this new metric—driving miles per recharge time (min), abbreviated as MPM miles per min)—that an average driver can use (akin to MPG) and relate the time spent at the charging station to an anticipated range. The scientific challenges in designing rechargeable batteries with high GED, SP and high MPMs can be understood from the mechanics of charge storage in electrode materials.

It should be noted that the projected technology roadmap for rechargeable lithium-ion batteries lacks a practical solution to design batteries with high GED, SP and 100 s of MPM (as is typical of internal combustion engine powered vehicles). We note that true mass-market adoption of greener and sustainable transportation will require technologies that do not compromise on MPM.

—Herya and Sundaresan

Comparison of commercially available electric vehicles driving range to recharge time (min) (for a full recharge), abbreviated as MPM (and for use akin to MPG). The driving miles per recharge time (min) for commercially viable battery technology is ~0.1–0.4 (plugin EVs to Tesla’s 85 kWh battery pack). Herya and Sundaresan (2016). Click to enlarge.

Redox flow batteries (RFB) overcome the deficiencies observed in the mechanics of charge storage in electrodes in Li-ion batteries by using electrolytes (anolytes and catholytes) to store ions. In RFBs, the reducing/oxidizing species are dissolved in aqueous or organic solvents and are pumped across an ion selective membrane.

Despite recent advances in energy and power density, however, RFBs still require pumps—which are parasitic components and require a minium scale of operation on the order of 100s of kW to a few MWs, the OSU researchers noted. This limits the current application of RFBs to backup in grid balancing; they are not being widely considered for mobile applications.

In this context, a membrane that can keep the redox species separated on either side and allow ion transport until it is needed will eliminate the need for pumps, preserve the high energy density recently demonstrated and broaden the application of liquid electrodes in energy storage. In addition, such a membrane with controlled ion transport will minimize equilibration of charges, one of the primary modes of self- discharge in supercapacitors, and enable the use of electrodes with higher capacity and subsequently supercapacitors with higher energy density. Towards this goal, we present for the first time in this article a membrane that can regulate transmembrane ion transport as a function of its redox state.

— Herya and Sundaresan

To create the smart membrane separator, the researchers doped polypyrrole doped with dodecylbenzenesulfonate (PPy(DBS)) and electropolymerized it on an Au-sputtered track-etched polycarbonate (PCTE) substrate. The PPy(DBS) membrane spans the pores of the substrate and forms a barrier for ion transport. The Au-layer between the PCTE substrate and PPy(DBS), and a counter electrode kept immersed in the electrolyte, enables the application of an electrical field to PPy(DBS) independent of electrodes kept on either sides of the membrane.

Applying an electrical potential to the conducting polymer membrane alters the redox state, allows it to conduct ions across the membrane. The team reported a maximum conductance of 30 µS cm^-1 and a current gain of 60x as the polymer switches between oxidized and reduced states.

The researchers, Travis Herya and Vishnu-Baba Sundaresan, call the membrane a “smart membrane separator” and define it as “a programmable ionic conductor that exhibits continuously varying ionic impedance due to an external stimulus and applied in an energy storage device (supercapacitor or RFB).”

Structure and function of a smart membrane separator. (A) Implementation in an energy storage device uses double layer electrodes (similar to supercapacitor) or liquid electrodes (similar to RFB) kept separated by the smart membrane separator. A control circuitry is connected to the membrane separator regulate the ionic impedance and ion transport between the electrodes. (B) PPy(DBS) is formed over a porous substrate spanning the pores. Electrical potential applied across the thickness of the PPy(DBS) using a counter electrode and the conductive layer between PPy(DBS) and porous substrate varies the redox state of PPy(DBS). (C) In the oxidized state, transmembrane impedance is high and there is minimal current across PPy(DBS). In the reduced state, cation ingress into PPy(DBS) enables ion transport across the membrane via hopping through dopant sites in PPy(DBS). Travis Herya and Sundaresan (2016). Click to enlarge.

In laboratory tests, the engineers found that their membrane reliably controlled charging and discharging in batteries powered by ions of lithium, sodium and potassium. They connected batteries to an LED light, programming the holes to open and close in precise patterns. The membrane allowed the batteries to function normally, but reduced charge loss to zero when the batteries were not in use.

In addition to its use in an RFB, the smart membrane separator can be used in a supercapacitor or a hybrid battery to provide additional control input to preserve the state of charge (SOC) and prolong the shelf-life.

We expect that a variety of other conducting polymers when formed across a porous substrate will enable the creation of ionic redox transistors and lead to a library of ionic devices that could extend beyond energy storage in chemical separation, bioengineering (DNA sequencing), desalination and drug delivery. By choosing an appropriate energy storage device architecture, such as a supercapacitor or RFB, the smart membrane separator will become an enabling component for realizing high gravimetric energy density and specific power.

—Herya and Sundaresan (2016)

The patent-pending technology was inspired by how living cell membranes transport proteins in the body. OSU will license the technology to industry for further development.

This research was funded by the National Science Foundation.

Resources

• Travis Herya and Vishnu-Baba Sundaresan (2016) “Ionic redox transistor from pore-spanning PPy(DBS) membranes” Energy Environ. Sci., 9, 2555-2562 doi: 10.1039/C6EE01448H