As governments across the world outline plans to prohibit the sale of combustion vehicles within the next 10–20 years, the transition from engine to motor is going to be fast-tracked. The UK government estimates that CO₂ emissions from standard gasoline family cars ranges between 179–207 g/km driven. It is therefore unsurprising that road travel accounts for 15% of global CO₂ emissions.

The need for electricity-fuelled vehicles to curtail harmful emissions is evident, but there remains one key hurdle which is slowing progression – batteries. More specifically, how can battery technology be optimised to create a sustainable, cost-effective, safe and high-performance product? These are the principal factors being pursued to further increase the commercial appeal and appetite for EVs.

The Li-ion battery
Latest estimates from the US Energy Information Administration (EIA) suggest that EVs account for approximately 0.7% of the total light-duty vehicles worldwide. The overwhelming majority of these vehicles use lithium-ion (Li-ion) batteries to store their energy. Despite being a relatively new technology, only becoming commercialised in 1991, the Li-ion battery has become ubiquitous in EV application, brushing aside its inferior predecessor, the lead-acid battery.

Li-ion batteries entered the automotive industry promising high efficiency, performance and lifespan, traits which continue to make it the standardised battery option for most portable electrical applications today. Continual research and development into this technology has incrementally taken it to the point where it can compete on par with the combustion engine. No longer are EVs purely a method of decarbonisation, they are now fast becoming the combustion engine’s superior as a transportation method, both in terms of performance and economics, according to the International Renewable Energy Agency (IRENA).

Future innovation within this field will see battery performance steadily improve, highlighted by DNV’s expectation that lithium-cobalt batteries will be able to achieve energy densities of 300 Wh/kg by 2030. One example of this development is Tesla’s ‘tab-less’ battery, which is not a change of chemistry, but instead a clever change in structural design. The tab-less configuration reduces the distance electrons need to travel, minimising ohmic resistance and improving thermal management and energy density.

All well and good then? Perhaps not…

Downsides of Li-ion batteries
There is no doubt that Li-ion batteries are currently the prevailing battery technology on the market, but they are subject to a handful of fundamental drawbacks.

The first of these are safety concerns. Li-ion batteries require a considerable amount of thermal management, with the primary danger coming from a failure referred to as thermal runaway – a process whereby an internal short-circuit within the battery causes it to rapidly discharge. This discharge of energy produces excessive amounts of heat, kick-starting a chemical chain reaction and igniting the flammable liquid electrolyte, potentially resulting in an explosion. Reasons for this malfunction can begin with improper operation, interior or external damage, or exposure to temperatures outside the rated operating range.

If this danger seems exaggerated, one merely has to cast one’s mind back to when a now infamous mobile phone model started exploding.

Another concern over the Li-ion battery regards the materials it is composed of, specifically lithium and cobalt, two metals that enable Li-ion batteries to have the performance capabilities that they do. The problem with these metals is that they are rare and thus cause a host of problems along the supply chain. The geographical concentration of these materials means that supply reliability is subject to volatility. They are also predominantly mined in developing countries where human rights abuses of the workforce and neighbouring communities also become a concern.

These problems, adding to the cost of the metals and exacerbated by their rarity, will undoubtedly see the price continue to rise as EV production accelerates. To give some perspective, the company Statista approximates that the entire cobalt supply on the planet equates to 7.6mn tonnes, with the International Energy Agency (IEA) estimating that annual demand in 2020 was 140,000 tonnes, with a prediction of this rising to 661,000 tonnes by 2040 if current demand trends continue.

Despite the best efforts of lithium-based battery innovation, its Achilles’ heel will always remain the sustainability of lithium itself… Lithium is a rare earth metal with a finite resource on this planet.

Recent commercial breakthroughs in creating a nickel-manganese cathode chemistry have seen the emergence of cobalt-free batteries. Not only is this a positive for long-term sustainability, the cost of cobalt in a Li-ion battery is, on average, higher than all the other metals within it combined (namely lithium, nickel, aluminium, iron and manganese). Cobalt-free Li-ion batteries are the logical next step in the development process, but with cobalt-free currently being a trade-off for energy density (~32% Wh/kg less says Emerging Technology News), there is still a lot left to be desired.

Positive Li-ion developments
One promising evolution of the Li-ion battery is the solid-state battery (lithium-metal). The significant design change here is that the liquid electrolyte within Li-ion batteries is replaced with a solid electrolyte. The primary advantages in having a solid-state electrolyte are that it has inherently superior performance capabilities and will greatly improve upon previous safety standards.

Being non-flammable and non-volatile to air exposure means that events like thermal runaway become much lower risk with solid-state electrolytes. This suggests that this new generation battery requires fewer safety systems, reducing both the cost of production and the volume of the battery pack. It is calculated that lithium-metal batteries will offer more than double the energy density of the Li-ion equivalent (460–500 Wh/kg according to Emerging Technology News). In addition to this, 4–6 times faster recharging rates can be expected, along with an extended battery lifespan. While it may seem too good to be true, lithium-metal battery technology is just around the corner, with working prototypes currently being tested for operation in EVs.

It is clear that there is still much development left for lithium-based battery technology. As new manufacturing techniques and composite materials become available (such as sulphur-carbon nanofibre, lithium ferrophosphate, and silicon-carbon nanotubes), the safety, sustainability, cost and performance of Li-ion batteries will incrementally improve.

Despite the best efforts of lithium-based battery innovation, its Achilles’ heel will always remain the sustainability of lithium itself. As previously mentioned, lithium is a rare earth metal with a finite resource on this planet. If the supply becomes strained amidst the compounding necessity for energy storage, the increasing cost of the battery system will drastically worsen its cost competitiveness.

This risk could be mitigated if a circular economy lifecycle model could be more effectively introduced. Recycling of Li-ion batteries is becoming increasingly more accessible, but the challenging process still remains inefficient and expensive. This is due to difficulties in estimating the state of battery health, challenges in physical and chemical battery separation, and requirements for energy-intensive recycling processes.

Sodium-ion batteries
One technology beginning to show potential as a commercial option for EVs is the sodium-ion battery. Earlier this year, CATL (one of the largest lithium battery manufacturers in the industry) unveiled a sodium-ion battery product which the company plans to introduce into EV battery packs, alongside Li-ion cells. While this doesn’t necessarily spell the end of the Li-ion battery, it does suggest that sodium-ion batteries are competitive with lithium technology and that their unique advantages could make them preferable or complementary for certain applications.

The main benefit of the sodium-ion battery is the cost factor. Sodium is more geographically available than lithium and vastly more abundant, meaning that the cost of sodium for battery applications will remain inexpensive and have lower price volatility. Furthermore, sodium-based battery technology is also inherently non-flammable and able to operate within a wider temperature range, improving on current safety performance.

Sodium-ion batteries share a very similar design to that of Li-ion batteries, with the primary functional difference being that sodium-ions shuttle between the cathode and anode instead of Li-ions. These comparable designs mean that a mature supply chain is already in place to support a seamless transition of technologies.

Despite high hopes, however, the reality check for sodium-based batteries is that their energy density (~160 Wh/kg) falls significantly short of the latest EV models (250+ Wh/kg). Research predicts that the sodium-ion battery will be able to bridge this gap and produce energy densities over 200 Wh/kg with future design optimisation.

Lithium is the current king of battery materials and there’s a high chance that status will never change. It is the lowest density metal and also boasts the highest electro-chemical potential.

Organic radical batteries
Looking into the future of battery innovation, one concept being touted as a future option for EV application is the organic radical battery (ORB). This form of battery is unique in the sense that it does not include metal-based materials; instead, it utilises organic radical polymers (flexible plastics) to transfer electrons. The absence of rare materials in this design solves the sustainability issues confronting conventional batteries.

However, despite successful prototypes being developed, commercialisation of ORBs is still a long way off. These initial trials have been able to provide evidence that organic radical batteries are capable of achieving comparable (in some cases superior) charging times and power densities to Li-ion batteries, without the environmental or safety drawbacks. Furthermore, these batteries have the potential to be produced and recycled at significantly lower costs than traditional batteries, due to the materials involved.

Despite indications of promise, it remains difficult to pass accurate judgement on the practical capabilities of organic radical batteries within these early stages of development. Only with continued research and innovation into this field will we find better understanding on whether these batteries could possess the performance capabilities required for EV use.

The future of EV battery technology
The Li-ion battery is an impressive product and it will not be easy to knock it off its pedestal. Lithium is the current king of battery materials and there’s a high chance that status will never change. It is the lowest density metal and also boasts the highest electro-chemical potential, making it the ideal candidate. The next 5–10 years will see Li-ion technology further optimised, squeezing out every last drop of its capability.

Solid-state lithium batteries are predicted to evolve from this, offering improved performance, sustainability and safety standards. The significant drawback that still remains is their reliance on lithium metal, with the uncertain supply causing concern over long-term commercial viability.

Other technologies are beginning to emerge for commercial use, but instead of replacing lithium-based options they should be used to supplement them. Solid-state lithium-metal battery cells, in combination with sodium-based battery cells, would create the best complementary solution – achieving improved safety, better sustainability, greater cost competitiveness and a more than sufficient performance package. As outlined at the beginning of this article, those are the key factors being targeted in the pursuit of developing improved energy storage solutions for EVs – assisting on the journey towards a more sustainable future.