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It is clear that we are in the middle of an electric vehicle revolution. Despite making only around half a million cars a year, the well-known electric car manufacturer Tesla became the world’s most valuable car company in 2020. Major companies such as Volvo and Jaguar, keen not to be left behind, are pledging to sell only electric vehicles by the end of the decade.
However, a major obstacle to the widespread adoption of electrical vehicles has been providing the necessary energy storage in a form which is compact, lightweight and capable of providing consistent, high power output across repeated charging cycles. It is only recently that battery technology has advanced sufficiently such that electric cars are becoming a viable replacement for conventional internal combustion engine cars. The huge amount of innovation in this field can clearly be shown by the number of new patent filings in this area. The European Patent Office recently reported an average increase of 14% a year since 2000 in patent families being filed relating to electricity storage, compared with 3.5% across other technologies.
Despite the high number of technological advances, the basic design of most batteries works on the same principles as the first batteries created in the early 19th century, and contains three components:
When the anode and the cathode are connected via an electrical circuit, electrons flow from the anode to the cathode via the circuit thereby providing electrical energy which may be used, for instance to power a car’s electrical motor. Simultaneously, ions flow between the anode and cathode via the electrolyte which is impermeable to electrons, balancing the charge created by this flow of electrons.
In seeking to provide batteries which can meet the demanding requirements of electrical vehicles, each of the above components has been the subject of extensive research and innovation. The leading technology is currently lithium ion batteries, which are used in almost all electric vehicles and consumer electronics. The first lithium ion battery was patented in 1976, and since then this technology has boomed to account for around 38% of the patent family filings directed to batteries.
Lithium (Li) is particularly advantageous because, as well as being one of the smallest and lightest elements on the periodic table, it has an extremely high electrochemical potential (i.e. provides a high amount of energy per atom). Modern lithium ion batteries contain a lithium compound as the cathode, and an anode which is typically a carbon based material such as graphite. As lithium ions are extremely small, they are capable of flowing from the lithium compound to position between the carbon layers of the anode when the battery is charged. These batteries provide an extremely high energy density (energy per unit battery volume) and specific energy (energy per unit battery mass) compared with previous chemistries.
Much of the recent innovation in Li ion batteries has been directed to optimising the cathode material. Early Li ion batteries, and those used in most consumer electronics, use lithium cobalt oxide (LCO), or lithium manganese oxide (LMO). However, the need to provide lightweight batteries, critical for electric vehicles, has led to new materials which allow for lighter batteries, in particular lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate (LFP), and lithium nickel cobalt aluminium (NCA).
Despite the large amount of progress already made, electric cars remain significantly heavier than their conventional counterparts, which naturally reduces their efficiency. This extra weight can almost entirely be attributed to the large battery packs required which can weigh in excess of 500 kg. This weight is less problematic than in conventional cars, as the energy used to accelerate the additional mass can be partially recovered with “regenerative breaking” where energy lost in deceleration is used to recharge the batteries. However, further improvements in the energy density of batteries will be needed if larger electric vehicles are to become standard.
In addition to reducing weight, challenges remain in providing batteries which are rapidly chargeable, safe, compact, and able retain their capacity across hundreds of recharge cycles, all whilst remaining cost effective.
Significant improvements in all of these areas are already being made by optimising the various components of known lithium ion batteries. However, in order to maintain the current rate of progress more substantial redesigns may be required.
Solid state batteries
With an average increase in patents filed of around 25% a year since 2010, solid state batteries provide an interesting new technology which may address some of the shortcomings with present Li ion batteries.
Solid state batteries work in the same way as conventional lithium ion batteries, but use a solid electrolyte (to separate the cathode and anode) rather than a conventional liquid or gel electrolyte. A variety of sophisticated organic, inorganic and composite materials are currently being developed as materials for the solid electrolyte. These batteries have the potential to provide significantly improved energy density, faster charging and improved safety compared with current Li ion batteries. Further, the improved efficiency of these batteries would reduce the need for the large and complex cooling systems needed for the battery packs in current electric vehicles.
Solid state batteries are currently too expensive and not developed enough for widespread commercial use. However, this remains an area of development of great interest to electric vehicle manufacturers, with significant breakthroughs expected within the next decade.
Redox flow batteries
Redox flow batteries are another area of technology which has attracted interest. Rather than using a solid anode and cathode, these batteries use positive and negative liquids containing redox-active species. These liquids are stored in separate tanks and are circulated to the electrodes, which are separated by an ion exchange membrane.
This design has the advantage that once the liquids are discharged new, charged liquids can be pumped into the tanks to replace the discharged liquid, rather than recharging the fuel source directly. This would avoid the need for lengthy charging times, which remains a drawback to electric vehicles. However, at present, flow cell batteries provide around an order of magnitude lower charge density than Li ion batteries, so this technology does not currently provide a credible alternative to current batteries.
Due to the substantial amount of innovation already made in battery technology, electric vehicles seem set to challenge combustion engine vehicles as the major source of transportation in the near future. However, in order to live up to expectations, continued improvements in the battery technologies needed to power these vehicles will need to keep pace with the onerous requirements for such vehicles. It will be interesting to see how technology in this area progresses in the coming years.
Article by: Charlie O'Neill | 21 June 2021