By Shagun Maheshwari & Roby Gauthier
Contents:
The Kardashev scale suggests that civilization is only as advanced as the amount of energy it can use at a given time. In order to advance civilization, an obvious endeavour is to transition from finite sources of energy (gas, coal, fossil fuels) to abundant sources (renewable energy: wind, solar).
Now you might wonder why this hasn’t happened yet, especially with solar energy being cheaper to generate energy than fossil fuels. The culprit is our lack of an efficient renewable energy storage system that has a high energy density, is safe, cost-effective and reliable to store renewable energy over a prolonged period of time.
Fortunately, many research groups and companies around the world are working toward solving this problem. One of the most promising technologies being worked on is batteries. From powering electric transportation to decarbonising the grid, the ripple effects of batteries will be monumental if we can improve them. We need safer, more powerful, and energy-dense batteries to be made cheaply in order to achieve a sustainable future.
Since the invention of the voltaic pile by Alessandro Volta in 1799, the first non-rechargeable battery, innovations have turned batteries into a game-changer in many technology fields. This resulted in the growth of the portable electronic, electric vehicle, and grid energy storage market.
Today, due to its high volumetric energy density, its good capacity retention, and great overall performance, lithium-ion cells dominate the electric vehicle market, and the recent construction of the Nevada Gigafactory by Tesla is a great reminder of that.
According to a 2013 review by the Martin Winter research group, Li-ion batteries have specific energy 2x as big as NiMH (nickel-metal hydride) ******batteries, 3x as big as NiCd (nickel-cadmium) batteries, and 6x as big as Lead-acid batteries at a C-rate of 1C which is defined as a discharge rate that results in the battery being fully discharged after 1 hour. This explains the remarkable advantage of lithium-ion batteries over older battery technologies.
On the grid energy storage side, the lithium-ion battery also dominates currently. This could change however as the redox-flow battery market grows. Invented in 1976 by NASA, redox-flow batteries are unique in the way they are built, as they contain two separate tanks, a catholyte and an anolyte tank, in addition to the electrodes (see figure below). The tanks are respectively filled with a catholyte electrolyte and an anolyte electrolyte. First, the catholyte is pumped from its tank to the positive electrode of the battery, then a redox reaction occurs at the electrode and as a result, the liquid goes back to its tank. Similarly, the anolyte is pumped from its tank to the negative electrode, a redox reaction occurs at the electrode and then the liquid goes back to its tank.
In the ideal case, due to the presence of a membrane, the anolyte and the catholyte shouldn't mix, but protons from the solvent are allowed to cross the membrane to achieve charge neutrality. A big advantage of redox flow batteries is that they have a low self-discharge and the energy and power of these batteries are decoupled. This means that if you want more energy, you just need to increase the size of the tanks and add more electrolytes. If you need more power, you just need to increase the size of the flow battery electrodes instead of the tanks. These advantages, among others, make that technology very attractive.
However, the redox flow battery has some issues like reduced efficiency due to the pumps, possible leaks of the catholyte or anolyte in the event of an accident, energy loss through hydrogen gas formation, and other losses such as shunt currents.