High energy density lithium cells are definitely a good fit for portable devices. Image: wikimedia user: kristoferb.
As renewable energy explodes worldwide and displaces legacy power generation systems, stationary energy storage will be implemented with increasing regularity to allow electrical systems to operate more efficiently with lower prices, fewer emissions and increased reliability. Because of this, the energy storage market is expected to grow from 172MW in 2014 to 12,147MW in 2024, according to Navigant Research. So it is only natural that companies across the globe are scrambling to get their piece of this rapidly growing pie. To date, the vast majority of the entries into the energy storage market have depended on lithium-based battery chemistry, but, the idea that lithium-ion is the technological and economic front-runner in the stationary storage space is a myth that is in dire need of de-bunking.
One size battery does not fit all
Manufacturers of lithium-ion batteries for EVs and handheld electronics would naturally like to apply their technology that was designed with only one application in mind – high energy density – to large-scale energy storage. But just because it is right for your phone, laptop, or hoverboard, it doesn’t mean lithium is the right chemistry for far more demanding, higher energy uses. Lithium-ion’s high energy density is useful for personal electronics where (smaller) size matters, but for stationary storage applications that need to have the ability to handle high power and/or long duration applications multiple times a day, a far more versatile, robust energy storage system is required.
Zinc-iron flow batteries utilise one native platform to perform both energy services (measured in kilowatt hours) which involve longer, steady discharge of the battery at lower power and power services (measured in kilowatts) which is a rapid discharge at higher power. To perform the same functions using lithium-based storage, you’d need two complete systems; one for power, one for energy. This is because one type of lithium cell is used for power applications and a different type of lithium cell is needed for energy services and a single storage system cannot accommodate both. Duration, cycle life, versatility, and overall battery life are areas where the chemistry and design of lithium-ion energy storage systems don’t stack up to zinc-iron battery stacks.
Battery manufacturers list capacity for energy and power, but manufacturers’ specifications generally state that lithium-ion should not be discharged below 20% state of charge (SOC). This means that the available power is actually only around 80% of the initial power rating. A redox flow battery, on the other hand, has access to 100% of its capacity at full state of charge for 20 years.
Cross section of zinc-iron flow battery. Image: ViZn Energy Systems.
The cost of the part does not equate to the whole
Gigafactory is a really cool word, but it will probably have a bigger impact on the English language than it will on the cost of lithium-ion batteries. Crystalline silicon PV modules can account for up to 60-80% of the cost of a PV system (more than the balance-of-system, inverter, racking, and installation costs combined), so reductions in the cost of mining and refining silicon are directly proportional to a drop in the price of solar PV panels.
Meanwhile, lithium cells account for less than 35% of the cost of stationary storage systems, so the same correlation between lithium cost reductions and stationary storage prices dropping simply does not exist. Lithium-ion module prices could be cut in half, but that would only result in a 17% reduction in the overall system price of a lithium-ion energy storage unit. What impact will a Gigafactory-produced lithium-ion battery have on the cost of cell equalisation software, containerisation, cooling, fire suppression, top level controls, siting, permitting, etc., all of which are all already commoditised? On the other hand, the zinc-iron batteries are close to 70% of the overall system price in a flow battery because it comes containerised and it doesn’t need a cooling system, making it much more analogous to the silicon-PV panel story.
The giant elephant in the room is named safety
Large-scale lithium-ion battery makers know how important safety is. That’s why they design very complicated cooling and fire suppression systems for their units to keep the system from entering thermal runaway. As distributed energy storage systems become more and more commonplace, the likelihood will grow that they need to be deployed in highly populated areas next to schools, hospitals, office buildings, etc., so even if all costs were equal lithium-ion begins to be a difficult choice.
Zinc-iron, on the other hand, is an inherently safe chemistry. Not only is it stable, non-explosive and non-flammable, it is also non-toxic and contains no dangerous gases or acids in its electrolyte. The water-based electrolyte doesn’t require fire suppression or parasitic cooling systems and there is no risk of explosion. This means that flow batteries are much more suitable for deployment in urban environments or close to where people actually live and work.
It's all about the revenue
Stationary energy storage systems must be able to withstand punishing duty cycles, sometimes requiring full charge and discharge cycles multiple times a day. While a redox flow battery can facilitate these requirements several times in a day if necessary, most lithium-ion based energy storage systems are capable of only one daily cycle and often have a mandated rest period. The fire suppression and cooling systems necessary for lithium-ion energy storage systems also add to the size and weight of the overall units and decrease efficiency, add complexity, and increase field reliability concerns. These limitations reduce the amount of revenue-grade energy the system can support every day, week, month and year. Furthermore, lithium-ion cells degrade continuously over time and they need to be replaced frequently. The expected lifespan of a lithium-ion battery is roughly nine to 10 years, but zinc-iron flow batteries can last 20 years or more, which means they support energy at a much, much lower price per kilowatt-hour over time than lithium-ion.
But lithium is so cheap and will only get cheaper…right?
For some, conventional thinking says that the cost of lithium-ion batteries will follow a downward trajectory similar to what the crystalline solar panel industry has experienced over the last 30 years because the cost of lithium had been dropping steadily starting in the 1990s, until about 10 years ago. But here’s the thing; lithium is not silicon and stationary energy storage is not a solar panel.
Silicon is the second most abundant element in the Earth’s crust (about 28% by mass) after oxygen, while lithium is the 33rd most abundant element (about 0.0002% by mass). Plus, additional elements such as cobalt that are needed to make lithium cells are far more difficult to find than widely-available silicon and can have devastating effects on the communities surrounding mines. Add to this that despite predictions, the price of lithium has increased dramatically from about US$7,000 per tonne in mid-2015 to over US$20,000 per tonne recently, impacting not only the stationary storage sector but also the EV and consumer electronics industries that compete for the same raw materials. Meanwhile, zinc and iron – used in zinc-iron redox flow batteries – are far more readily available. Iron is the fourth most abundant element and constitutes about 5% of the Earth’s crust and there is over three times as much zinc in the ground as there is lithium and it is generally produced as a by-product of mining other metals.
Image: ViZn Energy Systems.
Lithium-ion batteries certainly have their place in the electronics world as their energy density in a small form factor has facilitated the ever-decreasing size of all our mobile devices. But if people are waiting on the low-cost lithium magic bullet to make grid-scale stationary storage widely available, they should instead simply go with the flow.