Flow batteries can provide scalable, low-cost energy storage for renewable energy sources (e.g. wind and solar).
Flow batteries are currently limited by their energy density. Introducing solid-state ion-insertion compounds in a mixed-conducting, flowable suspension increases the energy density. However, the increased viscosity of this suspension incurs two efficiency losses; the electroactive region extends the cell stack and non-uniformity of the flow field. These inefficiencies reduce the discharge energy and overall energetic efficiency. This technology models suspension flow batteries and determines the material properties and flow volume control needed to obtain both a discharge energy as a percentage of theoretical value and an energetic efficiency >95%.
A model of electrochemical kinetics and flow was developed to identify operating conditions and rheological behavior that maximizes electrochemical performance. This model was applied to three active materials, two solid state lithium-ion compounds (LiFePO4 and LiCoO2) and one redox solution (VO2+/VO2+). From this model, precisely tuned flow volumes, large yield stresses, large Navier slip coefficients, and two-phase-like active-materials were seen to produce the greatest electrochemical efficiencies. Ideally, plug-flow is achieved, which maximizes energetic efficiency and capacity over cycling because there is no residual charged material. However, since plug-flow in most cases is unobtainable, the following conditions were determined to maximize efficiency and capacity: (β - Navier Slip Coefficient, w - channel width, µp - plastic viscosity, τo - yield stress, and ῡ - mean axial velocity component) β > w/µp, τo > 100µpῡ/w, and an aliquot factor, pump volume compared to volume between the current collector and separator, between 0.5-0.75. Adhering to these a conditions, a discharge energy (as a percentage of the theoretical value) and an energetic efficiency >95% was observed.
Energetic efficiency that exceeds 95%
Discharge energy 95% of theoretical value
Applies to flow batteries with varying parameters