PROF M. Goosey
Over the last twenty years or so there has been a major transition in the technology used in secondary batteries. The once ubiquitous nickel-cadmium designs were first supplanted by nickel-metal hydride technology, only themselves to be replaced by cells employing lithium-ion chemistry. It is true to say that electric vehicles have really only become viable as a result of the improvements in energy density and reduced costs that have been possible in recent years with lithium-ion technology; the cost of lithium-ion batteries has dropped by almost an order of magnitude in the last twelve years. However, it is also true that, despite incremental improvements in recent years, battery technology and cost are still the key factors impacting the growth of the electric vehicle market. They are considered to be too expensive, as a proportion of an overall vehicle’s cost and they are not able to store sufficient energy within the ideal volume/mass. Current lithium-ion vehicle batteries store around 250 to 300 kW per kilogramme, but this really needs to double and, ideally, to be closer to 1,000 kW per kilogramme. In short, compared to conventional petrochemical based fuels, lithium-ion batteries still have some way to go!
Although current electric vehicles are typically powered by lithium-ion batteries, there has been, and continues to be, much work undertaken to both reduce their costs and to increase their overall efficiencies. Key target areas for improvement include the total amount of energy that can be stored, the rate at which the energy can be delivered, the number of charge-discharge cycles that can be achieved (lifetime and performance loss) and their overall safety. Therefore, in order for electric vehicles to become the preferred choice over conventionally fuelled cars, a number of improvements still have to be made. For example, there is talk in the industry of a vehicle battery target of 1 million miles with only a 10 % loss in capacity. Some experts are banking on achieving this via continuous improvements in the lithium-ion technology, while others are working on completely new types of batteries. Fortunately, there is no shortage of improvement ideas for lithium-ion batteries, as it is believed there is significant scope for energy density improvements. For example, there is much interest in replacing the standard graphite anode materials with silicon. This approach can, theoretically, offer major energy density improvements, but there are reliability challenges to be addressed in terms of the mechanical integrity of silicon anodes when they are required to contain such large numbers of lithium ions. Silicon is not the only electrode material being investigated, with lithium iron phosphate (LFP) and lithium titanate oxide (LTO), being investigated as cathode and anode materials respectively. Even lithium itself is being considered as an anode material. If electrode performance issues can be overcome, significantly improved lithium-ion batteries could become available.
Further out, other commercially viable improved battery technologies may well emerge, but these are not expected to come on stream for another ten years or so. Again, there are numerous alternative technologies being developed, with key ones being based on lithium-sulphur, sodium-ion and various solid-state approaches, especially as the latter avoid the use of the flammable materials associated with some notable battery fires in recent years. There is thus a considerable interest in solid state batteries. However, even if such new battery technologies are successfully demonstrated, they may well have difficulty in usurping lithium-ion, given the huge global investment that is currently taking place to install lithium-ion manufacturing capacity at so-called ‘Gigafactories’ around the world. Lithium-ion therefore seems set to be the dominant battery technology for the immediate future.
However, other researchers are working on radically new approaches that offer the potential for a paradigm shift in cost and performance. In the longer term, there may even be a move from batteries to hydrogen fuel cells, but that is another story. There is, of course, no single answer yet to what type of technology will ultimately dominate in the electric vehicle market, but it is interesting to consider the current state of the art and to see what some of the new approaches might offer. For now, making incremental improvements to lithium-ion is the preferred route, especially as it is clear that considerable progress has already been made with the lithium-ion battery technology; current offerings are far superior to those of just a few years ago and many people believe that further incremental improvements will continue to be made.
In summary then, it is clear that there are numerous opportunities for battery technology to develop further and it seems certain that cell technology will enable vehicle performance to improve, while also reducing costs. Batteries are predicted to achieve parity with conventional fuels in just a few years and, given the large investment being made globally to develop new cell designs and to achieve massive production scale up, these targets are likely to be met. What is not yet so clear is just what specific battery technology will enable this to happen.