When it comes to making computer circuits, silicon is king. Contenders for the throne include optical switches, DNA, proteins, germanium and graphene. Each has legitimate grounds to be considered but has struggled in development, while silicon-based computers have relentlessly improved, such that the performance gap between silicon and its potential usurpers has widened.
A similar widening gap has occurred in rechargeable batteries. If silicon is the king of electronic circuits, lithium is the queen of batteries. I find it surprising that one element so thoroughly dominates battery technology but, like king silicon for circuits, queen lithium has properties that make it superior to all the alternatives.
Batteries comprise three essential components: the negative terminal (also known as the anode), the positive terminal (the cathode) and the interior soup of ions called the electrolyte, usually a liquid or gel. When the negative and positive terminals are connected through an external circuit such as your flashlight, the battery discharges by driving electrons from the negative terminal to the positive terminal, providing the energy to generate light. Inside the battery, the circuit is completed by the flow of positive ions through the electrolyte. In a lithium-ion battery, these positive ions are lithium atoms that have been stripped of an electron.
Most of the billions of dollars spent each year to develop better lithium-ion batteries are invested in improving the materials for the terminals and the electrolyte. The negative terminal has to act like a sponge, absorbing and storing as many positively charged lithium ions as possible. Graphite is most commonly used, but variations such as graphene that can absorb even more lithium ions without swelling are actively being sought. The positive terminal is often made from lithium cobalt oxide, though there are many alternatives in production and in development. The electrolyte includes lithium salts such as lithium hexafluorophosphate.
What makes lithium special? For starters, it is the lightest of all metals and the third-lightest element, sitting in the periodic table immediately after hydrogen and helium. Further, of the metals commonly used for batteries, lithium has the highest ‘working voltage’ – the voltage difference between the negative terminal and the positive terminal.
This combination of a high working voltage (up to 3.6 volts) and light weight contributes to lithium batteries having the highest energy storage per kilogram, making them ideal for mobile applications. Thanks to lithium-ion (Li-ion) batteries, a Tesla car can get away with a battery weight of 600 kg, compared with 4,000 kg or more if it were to rely on conventional lead acid batteries.
Unlike lead acid batteries, in use for more than a century, lithium-ion batteries can be discharged down to about 10% of their rated capacity without failure, and do so thousands of times. They do not carry the curse of the memory effect that reduces the working lifetime in nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) batteries unless they are fully discharged before recharging. Further, lithium-ion batteries left sitting on the shelf will lose charge at a much slower rate than other battery chemistries.
Like any chemical at high concentrations, lithium is harmful to humans if ingested but is otherwise very low on the toxicity scale; in fact, it is so relatively harmless that for more than 50 years lithium carbonate salt has been routinely used as a medicine to treat bipolar disorder.
Wonderful as they are, lithium-ion batteries do have some drawbacks. For example, they lose peak capacity after a few years of operation. Further, because of safety concerns, battery packs must be made with complex protection circuits to limit overheating and maximum currents.
It is difficult to predict where the next big battery breakthrough will come from. The competition is intense and advances are announced daily. My money is on replacing the liquid electrolyte with a solid-state electrolyte that is a kind of glass. If successful, the solid-state electrolyte will allow faster charging, increased safety, up to three times the energy density and longer lifetimes.
Sound too good to be true? The latest announcement from Toyota Motor Corporation confidently claims it will introduce solid-state lithium-ion batteries in 2022. While it is early days, solid-state lithium-ion batteries can provide a step change in performance that will give us electric cars able to go 1,000 km and smartphones that can be used for several days between charges.
Alan Finkel is an electrical engineer, neuroscientist and Chief Scientist of Australia.
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