The Lithium-Ion Battery and Electric Cars
Last year, in 2019, John B. Goodenough, M. Stanley Whittingham, and Akira Yoshino shared the Nobel Prize in Chemistry for the development of the lithium-ion battery. Indeed, the lithium-ion battery is currently at the forefront of commercial battery technology and represents the most rapidly increasing type of rechargeable battery in the world. Lithium-ion batteries first entered the market in 1991 and have since transformed society as these lightweight and effective batteries power everything from our smartphones and laptops to a growing share of our cars.
Indeed, the most common type of battery in electric cars is the lithium-ion battery. Lithium is an exceptional material for making batteries as it is a lightweight metal that readily forms ions while offering a relatively high nominal cell voltage of 3.6 V. Electric cars are, in many ways, fundamentally better vehicles relative to their fossil fuel-consuming counterparts. Relative to internal combustion motors, the induction motors of electric cars offer rapid acceleration and make very little noise. Moreover, there are significantly fewer moving parts in an electric car, thus they are cheaper to maintain. Electric cars are also cheaper to charge when compared to the cost of an equivalent amount of gasoline.
So what is holding back the electric car revolution? In 2018, less than half of 1% of all passenger cars in the United States were plug-in electric vehicles. Despite the lower operating cost of an electric car, the upfront cost is significantly higher relative to a comparable gasoline-powered vehicle. This means it takes a long time to offset the higher upfront cost by the savings on maintenance and gasoline. The batteries of electric cars are responsible for the higher price tags, which ultimately makes electric cars too expensive for most of us. For example, the battery accounts for about 25% of the total $35,000 for the most affordable option from Tesla, the Model 3. Notably, this is significantly cheaper compared to just 5 or 10 years ago. The average global cost for lithium-ion batteries was ~$1,200 per kWh in 2010, which has come down to ~$175 per kWh in 2018, based on a report from Bloomberg New Energy Finance. When the battery cost drops below $125 per kWh, the US Department of Energy predicts that electric cars will be cheaper to own and operate in most parts of the world than gas-powered cars. The range, or miles per single charge, of electric vehicles has also been a concern; however, analogous to the steady increase in affordability of electric cars, their range has been gradually extended as the technology develops. The typical range for today’s electric cars is around 250 miles.
Lithium-ion batteries are becoming more affordable as the scale of production increases. However, new advancements are needed to obtain more energy-dense, more powerful, and safer batteries at a lower cost. Lithium-ion batteries have the same basic components as any other battery: a cathode, an anode, a separator, and an electrolyte. While the anode of lithium-ion batteries is typically made of graphite, the cathode material often depends on what the battery will be used for as not all lithium-ion batteries are created equal. The electrode materials play a large part in dictating the performance metrics of a lithium-ion battery, and both the anode and cathode continue to be focal points of ongoing research and development efforts.
In a charged lithium-ion battery, lithium atoms begin sandwiched between layers of graphite, a carbon-based material. Upon discharge, each lithium atom loses one electron which travels through an external wire to the cathode, powering a load (e.g. lightbulb, electronic device, car) in the process. Meanwhile the lithium ions (Li+) generated at the graphite anode follow the negatively charged electrons by traveling through the electrolyte, passing through the separator, to recombine with the electrons at the cathode, which is often made of nickel, manganese, and cobalt. Once all of the lithium is consumed in the anode, the battery is spent and can be recharged by running through process is reverse with an external source of electrical energy.
One strategy that may decrease the cost of batteries is to increase their energy density, which would mean smaller and/or fewer batteries are needed for the same amount of energy. This can be done by modifying either electrode, and while improvements to the cathode have been made over the years to increase their energy density, the anode is ripe for innovation. Graphite serves as a relatively dense anode that is cheap and reliable. However, its energy density pales in comparison to the theoretical density of silicon, which can absorb lithium ions significantly better. The problem is that silicon swells to four times its initial volume, in contrast to graphite whose volume remains largely unchanged during charging and discharging. The expansion of silicon results in damage to the solid electrolyte interphase, which leads to poor battery stability.
Another area of development in lithium-ion batteries concerns the electrolyte. Flammable liquids are currently used for the electrolyte that allows facile transport of ions, but at the risk of catastrophic fire. Solid electrolytes based on ceramics that can operate at room temperature or high-temperature polymers are potential replacements for the liquid electrolyte that avoid the flammability problem. However, it is difficult to have complete contact between solid electrolytes and the nooks and crannies of high-surface area electrodes. Polymers can have liquid-like properties at elevated temperatures (above 105 °C or 220 °F), but this is not conducive to use in smartphones and many other applications. As a result, ion transport is often limited and solid-state lithium-ion batteries have been restricted to low-power uses.
Lithium-ion batteries have revolutionized the world that we live in and have the potential to do so much more as this technology continues to advance.
Jonah W. Jurss, Ph.D. is a chemistry professor at the University of Mississippi who does research in the field of renewable energy. He joined the Department of Chemistry and Biochemistry in 2014.