Lithium-ion batteries have revolutionised our lives, with the average home now full of rechargeable devices harbouring the technology. The importance of this was recognised recently when John Goodenough, M Stanley Whittingham and Akira Yoshino were awarded the 2019 Nobel Prize in Chemistry for their work in the early development of lithium-ion energy storage devices.
One reason for the hype is that lithium-ion technology could help reduce emissions and clean up our cities. The WHO estimates that road transport accounts for around 17% of global CO2 emissions. A world in which fossil fuel-powered vehicles are replaced by rechargeable alternatives is appealing to anyone interested in reducing the devastating impact of climate change, or improving local pollution levels in our cities.
An important area of development is providing batteries with high capacity and high voltage, a combination which results in the delivery of more energy. The capacity of the cathode is the limiting factor for lithium-ion batteries. Cathode material development has therefore been a crucial area of research.
Lithium cobalt oxide (LCO), discovered by Goodenough in the early 1980s, has been the most commercially successful cathode material, still being found in many lithium-ion batteries used today. It has the advantage of providing a large specific capacity of 274 milliamp hours per gram of material (mAh/g). Despite this, LCO suffers from reduced capacity at high discharge rates, which is a major concern when developing batteries for EVs. Small electronic devices operate on a trickle of current from the battery – a low discharge rate. But the capacity of a battery will often be lower when a battery is discharged more quickly. A heavy EV needs a lot of power over a small space of time when accelerating, so another area of development has concentrated on finding alternatives to LCO, which maintain high capacity even at higher discharge rates. On top of this, the high cost of elemental cobalt renders LCO too expensive for widespread commercial use.
Based on the low cost and low environmental impact of manganese relative to cobalt, compounds such as lithium manganese oxide (LMO) have been investigated as potential cathode materials, but a major stumbling block for such manganese-containing materials is capacity fade. Lithium-ion batteries lose some capacity with every charge. For a mobile phone this isn’t a huge problem, but for an EV this means range would reduce for the vehicle over time.
LCO and LMO fall within the broader category of transition metal oxide materials and there has been much research into other materials within this category. But some scientists have taken their research into entirely new categories. For example, lithium iron phosphate (LFP) is a so-called “polyanion” material, which offers thermal stability and high power, but its low voltage means it probably won’t make it into the EVs of the future.