It’s hard to imagine modern life without batteries. They are everywhere. From cellphones and laptops, to cars and medical devices, to the birthday card that plays “Happy Birthday” when you open it. The multitude of sizes, types and applications of batteries is mindboggling, but at their basic they rely on one principle: take a chemical reaction, and run it one way to release the energy, or reverse the reaction to store it. The rest is just details. But, as the saying goes, the devil is in the details. It is the particularities of each battery that make the energy storage game so interesting and full of possibilities.
For much of history, the battery reaction stored the energy on or in the solid electrodes. For example, one of the first batteries built in 1799 by Alessandro Volta (forever immortalized in the measurement unit of Volt) relied on dissolution of zinc in sulfuric acid to make the energy. Meanwhile, the modern lithium ion battery shuffles lithium ions between the ceramic cathode and the graphite anode, where each side soaks up lithium ions like a sponge. However, a dissolved metal doesn’t always deposit back in the same shape, and the “breathing” of electrodes in a lithium ion battery, as they soak in or release lithium, creates mechanical stresses that over time destroy the battery. Such issues (along with a few others) get in the way of long life of rechargeable batteries, and require some advanced engineering to solve. Of course, there is no shortage of good engineers, and if my laptop battery life is any indication, lithium ion batteries have come a long way in solving their problems. This engineering comes with a cost, but the high value added battery products such as cellphones, computers and cars can afford the premium.
But true or hybrid, flow batteries can be fully charged and discharged on the daily basis, to follow the daily fluctuations of power generation and demand, with little fade in capacity.
The world is getting away from fossil fuels (which themselves are millennia-old energy stored by ancient life forms and conveniently buried), and towards renewables, such as wind and solar. The energy from these sources doesn’t always match up to the demand, and to be able to use renewable power effectively (as Hawaii aims to have renewables meet 100% of its electricity demand), this energy has to be stored somewhere. Such applications require batteries that can store a lot of energy safely and, most importantly, cheaply.
An approach to storing lots of energy cheaply has been put forth by NASA in the 70s, and had since been picked up by major research groups including Pacific Northwest National Laboratory and about 20 or so startups. This approach is instead of storing the energy on the electrodes, is to store it in the liquid electrolyte. This design, called a redox flow battery (RFB), consists of recirculating electrolyte, which is charged and discharged in the reactor cell. The reactor cell, in a nutshell, is two electrodes with some space for the electrolyte to flow and a membrane separating the positive and negative electrolytes. Stacking several of such cells back to back creates a battery, with more cells creating higher voltage and yielding more power. More energy meanwhile, can be added by increasing the amount of electrolyte. This flexibility is one of the key features of RFBs.
There is a wealth of chemistries explored for these batteries 1, but generally these could be divided into two categories: true flow batteries and hybrid. True flow batteries charge the electrolyte on both sides of the cell, and energy and power are completely divorced from each other, as mentioned earlier. An example of this is one of the leading commercial designs, an all-vanadium flow battery. It uses different oxidation states of vanadium on both sides, V2+ and V3+ on the negative side and V4+ and V5+ (as VO2+ and VO2+ respectively) on the positive. A particular advantage of this chemistry is that using vanadium on both sides eliminates concerns of electrolyte mixing. For this reason, with some occasional rebalancing, this electrolyte can serve virtually forever.
Hybrid flow batteries, such as a zinc-bromine system, rely on plating the metal on one of the electrodes. The other side of the cell is charged like in a normal flow battery. As mentioned before, the plating is never perfectly even, and these batteries require an occasional “stripping run”, which completely discharges the battery, removing all of the metal from the electrode. Additionally, there can be issues, such as growth of dendrite in case of zinc plating. While this issue had been resolved for the zinc-bromine batteries, it has doomed efforts of Plurion to commercialize a zinc-cerium system a few years back. Naturally, plating the metal places an upper limit on the energy that a battery can store before the electrolyte flow is blocked. But the tradeoff of this is a higher energy density, which comes from both the positive and negative ions in the solution participating in the energy storage reaction.
But true or hybrid, flow batteries can be fully charged and discharged on the daily basis, to follow the daily fluctuations of power generation and demand, with little fade in capacity. In theory, these batteries should last over 10,000 cycles 2, which amounts to over 25 years of daily use. In practice, however, their lifetimes are significantly less. A major issue, that has been only partially addressed, is the durability of the electrodes. While they do not store the energy, the aggressive chemistry of charging and discharging tends to corrode even the chemically resistant carbon (typical material of choice, given its low cost and high resilience). The result is flow batteries that perform short of their potential, leading to increased maintenance costs and slower adoption rates in the energy storage market.
A downside of flow batteries in the commercial space is that they are not a fully developed technology. That is also a tremendous advantage, as that means that there is a wide playing field for improvement. The electrode degradation problem, as well as some others, is not without a solution, and many research groups and companies are staking their future in improving the RFB design. For example, eChemion is tackling this exact problem, developing a chemical coating that will prevent electrode degradation while preserving the energy-harvesting capability of the carbon electrodes. Another shortcoming of RFBs, their relatively low energy density, while of little consequence in stationary applications, presently precludes them from mobile applications, such as portable electronics or vehicles. But this direction is also not without attention. PNNL has recently developed a zinc polyiodide which has the potential to rival lithium ion in energy density (theoretical 322 Wh/l for polyiodide vs. 223 Wh/l for LiFePO4 cathodes 3.
Meanwhile professor Yet-Ming Chiang’s group at MIT has developed a flow battery based on lithium ion chemistry 4. In it pulverized cathode and anode materials are suspended in a slurry, which is charged and discharges like an RFB electrolyte. The result is lithium battery energy density with the versatility of an RFB. This so called “Cambridge Crude” is now being commercialized by 24M technologies with $50M investor funding. Both of these technologies would allow RFBs to make headway into the automotive market, but they might not be the first, as nanoFlowcell is claiming a flow battery powered car called Quant. While the amount of money spent on the lavish marketing of Quant seems to have left little for R&D, they might surprise us yet.
That said, redox flow batteries remain sufficiently strong in the arena of stationary energy storage and renewable power integration. They are already sufficiently advanced to rival alternatives, and continue to improve year upon year.
1. Puiki Leung, Xiaohong Li, Carlos Ponce de León, Leonard Berlouis, C. T. John Low, Frank C. Walsh, “Progress in redox flow batteries, remaining challenges and their applications in energy storage”, RCS Advances, 2012, 2, 10125-10156; 2. Bruce Dunn, Haresh Kamath, Jean-Marie Tarascon, “Electrical Energy Storage for the Grid: A Battery of Choices”, Science, 2011, 334, 928-935; 3. Bin Li, Zimin Nie, M. Vijayakumar, Guosheng Li, Jun Liu, Vincent Sprenkle, Wei Wang, “Ambipolar zinc-polyiodide electrolyte for a high-energy density aqueous redox flow battery”, Nature Communications, 2015, 6:6303 | DOI: 10.1038/ncomms7303; 4. Mihai Duduta, Bryan Ho, Vanessa C. Wood, Pimpa Limthongkul, Victor E. Brunini, W. Craig Carter, Yet-Ming Chiang, “Semi-solid lithium rechargeable flow battery”, Advanced Energy Materials, 2011, 1, 511-516