I recently read two articles about ARPA-E (the Advanced Research Projects Agency's Energy division) having developed the "Holy Grail" of batteries. Both articles made it sound like a done deal, but neither provided any concrete information. As it turns out, ARPA-E's director, Ellen Williams, said that the agency has "reached some holy grails in batteries – just in the sense of demonstrating that we can create a totally new approach to battery technology, make it work, make it commercially viable, and get it out there to let it do its thing,” Note the phrases "some holy grails" and "we can create…" Somehow, that was interpreted as ARPA-E having beaten out Elon Musk (founder and CEO of Tesla Motors) in the search for the perfect battery.

Imagine this conversation between Elon Musk and ARPA-E:

That's NOT how it happened.

I contacted ARPA-E and chatted with Department of Energy spokesman Andrew Gumbiner, who gave me some information related to ongoing projects that the agency is helping to fund.

First, we need to recognize that the "AR" in ARPA stands for Advanced Research.The projects that ARPA sponsors are not going to give us products in the next year or two; it's more like a five to fifteen-year time scale. ARPA-E focuses on high-risk, high-reward projects. That's the nature of R&D. Some things eventually pan out while others don't. As Albert Einstein quipped, "If we knew the answers in advance, it wouldn't be called research." With that said, let's take a look at some of the more promising battery technology that's being supported by ARPA-E.

Battery Chemistry

While Li-ion is the current battery of choice for EVs and many grid-level storage applications, it has a few weaknesses, including cost and the need for overcharge and undercharge protection circuitry. Li-ion is adequate, but hardly the Holy Grail of batteries. I tell my engineering students that if they're interested in chemistry, they should consider working in the rechargeable battery industry. Here's a sample of what today's battery chemists are creating.

Zinc-Air Batteries

An internal combustion engine doesn't run on gasoline, it runs on a gasoline-oxygen mixture. With oxygen readily available in the atmosphere, a large portion of the fuel doesn't need to be carried in the vehicle. Likewise, a battery that uses air as one of its chemicals has a higher energy density (energy per unit of weight) than one whose chemicals are all packed inside the cell. Zinc-air batteries operate on this principle, but because dendrites form on the zinc electrodes while recharging, they've been relegated to the disposable battery market.

Chemical engineers at Arizona State University and Fluidic Energy have been working on a rechargeable metal-air battery for grid-level storage and backup power stations. Through a combination of ionic salts enhanced with additives, coupled with nanostructured zinc electrodes, these zinc-air batteries can withstand numerous charging cycles without dendrite formation. The goal is to create a battery that provides several kW of power over a 4 - 72 hour period at a cost that's comparable to diesel generators. ARPA-E provided about $8M in seed money to start the project; since then, Fluidic Energy has secured over $150M from private sector investors. The project spawned eight patents, with another one pending at the time of this writing.

Organic Flow Batteries

Unlike Li-ion and zinc-air batteries, flow batteries have a pair of tanks to hold the electrolytes. Since the storage capacity is limited by the electrolyte volume, flow batteries can be scaled up simply by using larger (or additional) tanks. Their use is obviously limited to stationary applications like grid-level storage.

Image courtesy of US Department of Energy

Harvard University and Sustainable Innovations LLC are designing an inexpensive flow battery based on organic molecules - many of which can be found in common plants like rhubarb - in a water-based electrolyte. Using computer simulations, the research team evaluated millions of chemical combinations and found several thousand with promising characteristics. These batteries can be made from non-toxic materials and don't require precious metals. The group expects to have a commercially-available storage system ready for pilot testing by the end of 2017.

Battery Electrodes

Delivering high levels of electrical power requires a battery with low internal resistance. Quite often, the electrodes are the weak link in that chain. In addition, a battery's electrodes tend to corrode over time, reducing its capacity and power delivery capabilities, and eventually rendering the battery useless. Primus Power is working on advanced electrodes that can deliver exceptional performance and last up to 50 years.

Image courtesy of US Department of Energy

Primus replaced the traditional carbon-based electrodes with a combination of metals that provide better electrical conductivity and help facilitate the chemical reactions between the catalysts and the electrolyte. Using the new electrodes in a zinc-bromine flow battery resulted in a fivefold increase in power density. They also increased the number of charging cycles by more than a factor of eight, from around 2000 to 17000. ARPA-E's $2M grant sparked the research, and since then Primus has raised over $60M in private capital. A Primus battery system is currently under test at a Marine Corps microgrid in Miramar, CA. Coupled with a 230 kW photovoltaic array, the Primus battery helps the base reduce demand charges and power mission-critical systems during grid outages.

Better Li-ion Batteries

Even with the research into alternative battery chemistries, Li-ion will continue to be a major player in the EV, home storage, and grid storage markets for at least another decade - probably longer. Battery maker 24M, working with researchers from MIT and Rutgers University, have simplified the Li-ion battery manufacturing process, making them less expensive to produce. And as a nice side-effect of their innovation, the resulting batteries actually perform better than today's state-of-the-art Li-ion batteries.

Image courtesy of US Department of Energy

In the above image, the battery on the right is a multi-cell Li-ion stack. Notice the many layers and the separators between them - these are inactive materials that don't produce any electricity. In effect, they're dead weight, reducing the energy density of the battery. The cell on the left is a single battery cell with thicker electrodes made of a semi-solid slurry of chemicals: a graphite anode and a lithium iron phosphate cathode. The elimination of extra layers not only reduces the cost of manufacturing, it also decreases the battery weight.

The team has developed a prototype cell that withstands more than a thousand recharge cycles, delivers 85% round-trip efficiency, and allows rapid charging and discharging. The long-term goal is to design a battery that costs about $100/kWh (that's roughly one-fourth of the cost of today's Li-ion batteries) and can survive 5000 charging cycles.

Four US patents resulted from this project, which was funded in part by a $6M ARPA-E grant. Batteries made with this technology can be used for grid-level storage as well as EVs.   

Battery Management Systems

Batteries don't require a lot of human intervention, but in some respects they're still in the "high-maintenance" category - it's just that the maintenance is largely automated. A battery management system (BMS) controls the charging process, ensuring that the optimal voltage and current levels are applied according to the battery's state of charge (SoC). a BMS also regulates the discharging to prevent the load from draining the battery too deeply. A good BMS provides optimal performance and long battery life.

A BMS requires sensors to monitor temperature, voltage, and current. State of charge can't be measured directly, so a microcontroller runs a sophisticated algorithm to estimate SoC based on voltage, current (in and out), and temperature levels. A battery consists of multiple cells, and the cells have minor differences due to tolerances in the manufacturing process; ideally the BMS will constantly adjust the charging so the cells are all at equal levels.

Sensors are typically installed inside the battery and connected to the microcontroller with wires. The electrical connection is bulky and susceptible to electromagnetic interference. To reduce weight and improve functionality, a team of engineers from the Palo Alto Research Center (PARC) and LG Chem Power developed an array of optical sensors with a fiber optic (FO) connection. The optical interface allows many sensors to communicate over the same FO cable, which not only saves space but allows more sensors to be used within the BMS.

Here's a short video demonstration:

Image and video courtesy of the US Department of Energy

An early prototype achieved a five percent weight reduction and a ten percent cost saving compared to a battery with today's sensors. Eventually, the fiber optic sensor array could reduce system cost by up to 15%. Since the battery pack is one of the more expensive features of an electric vehicle, reducing the battery cost is a significant step towards electrified transportation.

Battery Testing

When engineers design a new type of battery, how do they predict its lifespan? Two primary factors that affect the life of a battery are wear and age. These parameters are evaluated through a combination of accelerated testing and data extrapolation. The problem is that today's testing procedures take a long time, due to the limitations of the voltage and current sensors. For a battery that's designed to last ten years, it could take up to three years just to verify its performance and lifespan. Shortening the time between prototype and commercial product is crucial to the development of new energy storage technology.

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