The Through-Flow Electrochemical Cell - a Breakthrough Technology

Abstract

The paper describes a new approach to energy-storage electrochemical cells that is “chemistry-agnostic” and promises to solve several problems of existing batteries. These problems include: Charging rates are too slowIntrinsic failure-mechanisms compromise safetyEnergy Density is inadequate for mobile applicationsLifetime is too short for large-scale permanent installations The key idea of the “Through-Flow Cell” is that the electrolyte is pumped continuously in a closed loop through a porous anode, porous separator, and porous cathode, in a direction aiding the active ionic flow. Although the idea of moving a liquid electrolyte is not new, there have not been any successful commercial products until now. It should not be confused with the “Flow-Battery”, an entirely different concept where two different fluids flow separately within anode and cathode. Previous attempts to develop a reliable Through-Flow Cell foundered on the difficulty of immobilizing active materials on the anode and cathode collectors while fluid flows freely through surrounding pore-spaces. I describe a new active-material encapsulation method to achieve that crucial objective, and a new more convenient cell configuration.Applications will include large grid-scale installations and could include high-power transportation vehicles such as trains and buses. The basic cell is configured as a stack-able, de-mountable, and repairable module. It is a completely self-contained package incorporating an adaptive electrolyte pumping system with optional heating and/or cooling to optimize efficiency in a wide range of environmental conditions. In grid-scale systems the ability to easily remove a low-performing cell from a stack, repair, and replace it is a very desirable feature that extends the life of a high-CAPEX installation almost indefinitely.The new cell design has a conservative capacity rating of 3000 Amp.hr (using 300 mA.hr/gm active materials) and a continuous current rating of 2000 Amps, with the cell voltage being determined by the chemistry employed. For the Lithium-ion chemistry with a nominal voltage of 3.7v it has 10 KW.hr energy capacity @ 90% discharge. A linear stack of 100 cells makes a battery storing 1 MW.hr on a rack 9 meters in length; easily accommodated in an enclosure the size of a shipping container that could contain up to eight such racks.For some people the idea of a battery with moving parts runs counter to notions of simplicity found in the standard sealed cylindrical cells. In practice, however, large assemblies of such cells require complex external liquid cooling and possibly fire-suppression systems. When individual cells fail, they are usually disconnected by blowing fuses, resulting in a steady degradation that is not easily repairable.In contrast to sealed cells, some unique advantages of the electrolyte circulation technique can be summarized as follows: Effective Mobility of active-species ions is dramatically boosted by the flow velocity, translating into reduced series resistance between electrodes. Peak charging or discharging currents can be ten times higher without overheating or loss of charge capacity when the flow velocity is only one centimeter per second.Flow direction is reversed when switching from charging to discharging, which reverses dendrite growth that is a potential source of catastrophic failure.The interior temperature of the cell can be directly controlled and excess heat removed from the electrolyte by an external heat-exchanger, or added in cold climates.The Anode and Cathode, being 3-Dimensional porous structures, incorporate greatly increased volumes of active material compared to typical 2-Dimensional foil electrodes.Electrolyte flowing across the surface of active material inside porous electrodes efficiently sweeps away ion-deficient liquid from within the Slipping-Plane near the Solid-Electrolyte Interface (SEI), reducing the parasitic Zeta-Potential. Results from a proof-of-principle laboratory cell using the Nickel Metal-Hydride chemistry will be briefly described, and a full-scale commercial cell design presented in more detail. The very important active-material encapsulation technique is also described along with its unique ability to tolerate large changes in volume of the active material without degradation. Current cylindrical “jelly-roll” format cells cannot tolerate significant volume change caused by ion-intercalation inside active materials without physical destruction.The new technology may enable complete replacement of Graphite by Silicon in the Anodes of Lithium-ion cells. Silicon has a charge capacity almost four times greater than Graphite, but expands up to 400%. In another example, Cathodes in the Lithium-Sulfur chemistry may tolerate far higher Sulfur loading with corresponding increased energy density.In summary, this new focus on the mechanics of energy-storage cells shows promise for a step-change increase in power-density, reliability, and safety. As described, the idea can be applied to any new improvement in chemistry that uses a liquid electrolyte as soon as it achieves maturity. Figure 1