We need better energy storage solutions. There might be other ways to achieve this beyond lithium. Image credit: The Oxford Scientist.
In the 1980s, John Goodenough discovered that a specific class of materials—metal oxides—exhibit a unique layered structure with channels suitable to transport and store lithium at high potential. It turns out, energy can be stored and released by taking out and putting back lithium ions in these materials. Around the same time, researchers also discovered that graphite, a form of layered carbon, exhibited a similar mechanism for charge storage at low potential. By shuttling lithium ions from graphite to the metal oxide connected by a fluid medium, an electrolyte, a battery can be constructed. The lithium-ion battery (LiB) was born.
Prior to this discovery, batteries were bulky devices made for stationary applications. However, with the advent of LiBs, significantly more energy could be stored in lighter and smaller batteries due to the large potential difference of the electrodes. This enabled the emergence of portable technologies such as walk-mans, laptops, and eventually mobile phones. Over the course of 20 years, extensive resources were invested to optimise battery materials. As a result, we can now store significantly more energy in LiBs over many charging cycles at an unprecedented low cost.
Batteries for Electric Vehicles
With ongoing climate change, countries around the globe need to decarbonise to prevent a climate disaster. Around 15% of global CO2 emissions come from road transport, and electric vehicles (EVs) are one option to reduce the carbon footprint. This poses two important questions: can current LiBs meet all the requirements to replace an internal combustion engine (ICE), and can LiBs be produced at scale to electrify the whole automotive sector?
To answer the first one, it is necessary to define the key requirements of batteries in EVs. There is a consensus that LiBs need to fulfill five main criteria: range, charging speed, lifetime, safety, and price. So how good are LiBs on these metrics? You can buy a Tesla vehicle today with a range of 240 to 800 km per charging cycle depending on weather conditions and the model. Recent advancements in range have been achieved through optimisation of packaging and design, rather than major changes in the underlying chemistry. However, Tesla and other car manufacturers recently started to add silicon to negative electrodes. This will further improve the range and, more importantly, the charging speed of their cars.
With ongoing climate change, countries around the globe need to decarbonise to prevent a climate disaster.
Nowadays, it is possible to recharge a Tesla from 10-80% in under 30 minutes. With the implementation of silicon anodes, supercharging can be accelerated to below 10 minutes. Jeff Dahn, a pioneering developer of the LiB and Principal Cell Materials Engineer at Tesla, recently visited Oxford for a talk. He showcased commercially available batteries investigated in his lab that could cycle for over one million miles and potentially a century-long under the right conditions. He claims that LiBs degrade slower than the cell casing they are stored in. With such performance, Dahn envisions a second-life usage of EV batteries. This can be used for stationary grid storage to facilitate the implementation of renewable energy sources.
Cell safety, a crucial measure for the wide-range adoption of the LiB, has long been criticised as a pitfall. Such criticism was first raised by the emergence of viral videos of burning EVs that could not be treated by conventional fire extinguishers. Tesla has received top safety ratings over the last few years for their whole fleet from European and American agencies (EU NCAP, NHTSA). The company claims fire incidents are 11 times lower for their EVs compared to an average vehicle in the US.
However, there are also critical voices on cell safety. During a guest lecture at Oxford, Professor Ralph E. White condemned EV crash tests for not considering fully charged batteries, which are more reactive, and thereby overestimating their overall safety. The EU NCAP does not clarify in its official technical bulletin at what state-of-charge the cars are tested. Clarifying testing protocols is easy, and can help reduce uncertainties related to the safety standards of EVs. Responsible institutions should prioritise transparency to enhance trust and ensure the public’s confidence in EV safety.
Lastly, costs have been a major concern for the widespread integration of EVs, and batteries contribute a significant amount to the hefty price tag. Experts predict an average battery pack price of $100/kWh is required to achieve cost parity with ICE vehicles. In 2022, the global average was $151/kWh. To put these numbers into perspective, the base model of the Tesla Model Y starts at $45,000 and comprises a 75-kWh battery, which costs roughly a quarter of the total car. Even with government subsidies, this remains too expensive for the average citizen. However, Tesla intentionally focused on the luxury car market initially to establish itself and EVs in general before entering the mass market segment.
Matters of range, emissions and the right chemistries
Outside of North America and Europe, customers are less sensitive to the range. Car companies, particularly Chinese producers, are focusing more on small entry-level vehicles. For example, the Wuling Mini EV is equipped with a 14-kWh battery, which translates to a range of 170 km and is sold for less than $6,000 in China. With more than one-million-unit sales, it is the best-selling electric car in China. Smaller EVs also have the advantage of reaching the break-even point earlier, which is the time when EVs have emitted less CO2 over their lifetime compared to ICE vehicles.
To achieve widespread adoption of EVs in the Western market, it is crucial to ensure the availability of smaller and more affordable EV models. This is becoming more and more important as the EU’s recent policy aims to ban all CO2-emitting cars after 2035. If local automotive industries fail to transition towards producing cost-effective EVs, it could lead to significant geopolitical dependencies with unforeseen consequences in the pursuit of climate neutrality.
Considering all these factors, it appears that LiBs have the potential to replace combustion engines. However, the main challenge lies in the availability of key battery elements such as cobalt, nickel, and lithium. The automotive industry has made significant progress in reducing the amount of cobalt used in positive electrodes, but lithium demand remains a concern. While there is enough of the element on the planet, it is not readily accessible for battery production. Extraction, processing, and refining of raw ores through heavy industrial processes are necessary before the cell components can be produced.
Establishing new mining sites to meet the demand for lithium can take over a decade, leading to a supply-demand gap. Consequently, there was a surge in pack battery prices in 2022, breaking the trend of a continuous price reduction seen over the past 20 years. This shortage in lithium has sparked considerable interest in alternative battery chemistries. This year, the world’s biggest battery producer, the Chinese company CATL, announced the commencement of mass production of sodium-ion batteries (SiB) for use in EVs. The mass production of a non-lithium-based battery marks a milestone for the field and soon more companies are going to follow.
While SiB cannot provide the same energy density as LiB, sodium is 1000 times more abundant than lithium and therefore cheaper. Dual pack solutions (SiB + LiB) will be an enabler for the electrification of the automotive sector, combining the high energy density of LiB and the abundance of SiB. In April 2023, Chinese producer BYD launched “Seagull”, the first EV featuring a SiB in a dual-pack battery, with retail prices below the equivalent of $11,000 and a range of over 300 km.
The Future of Batteries
So, what does the future hold for battery technologies? Numerous post-lithium technologies are being investigated and developed in academia and start-ups. However, commercialising any new battery chemistry is a serious challenge because current LiBs already do their job so well. Any new cell chemistry would need to significantly outperform LiB in most metrics described above and be scalable in production. Let us take solid-state batteries (SSB) as an example. In SSB, the liquid electrolyte is substituted with a solid compound which improves cell safety and enables anode-less design. Instead of storing lithium ions into an electrode, they can be directly deposited onto the current collector. This can enable a step increase in energy density and faster charging. Start-ups like QuantumScape from the US have demonstrated prototypes that appear to outperform current LiBs in almost every metric. However, the manufacturing of SSBs differs fundamentally from liquid LiBs. As a result, substantial investments are required to optimise processing and enable a production at scale.
Are companies willing to invest in extending their EV range from 600 to 1000 kilometres and improving cell safety, or should they allocate resources to other features of the EV? It’s an opportunity cost that companies like Toyota and VW are willing to pay. Tesla on the other hand has kept its focus on scaling up the manufacturing of current LiBs without any publicly known information about their investment in SSBs.
Progress in battery chemistry is essential in our mission to meet the 2050 ‘net zero goal’.
The implementation of a new cell chemistry becomes very likely if there is at least one metric where current LiBs are insufficient, such as the scarcity of lithium, i.e. the price that enabled SiBs. This can also be the case for other applications, such as short-range electric aircraft. To electrify the aviation sector, even higher energy densities and safer chemistries are required. Despite the continuous improvement in recent years, LiBs still face one fundamental challenge: a limited amount of lithium ions can be stored in the layered structure of metal oxide and graphite. For example, lithium-cobalt-oxide, the original compound discovered by Goodenough in the 80s, can insert and remove reversibly 0.5 lithium ions per cobalt metal centre. If more lithium is extracted, the layered structure collapses, and the battery fails. Consequently, LiBs are fundamentally limited in how much energy can be stored within the battery, without increasing its size or weight.
Conversion cathodes are the most prominent class of materials to address this limitation. In contrast to the limited storage sites in layered cathodes, conversion compounds undergo a chemical transformation during cell dis- and recharge. This enables the storage of multiple lithium ions per metal centre. Theoretically conversion cathodes can provide up to a five-fold increase in energy density. However, they fall short on other performance metrics such as cycle life and charging-discharging rates. Like the three-decade-long development of LiB, enormous resources will be needed to address those shortcomings. While they might not compete against LiB in the aviation sector, other green technologies such as fuel cells or synthetic fuels, which have an intrinsically higher energy density, need to be considered. Or, more fundamentally, is it better to invest in, e.g. carbon capture technologies to compensate for CO2 emissions in the aviation sector?
Progress in battery chemistry is essential in our mission to meet the 2050 ‘net zero goal’. We could achieve this through the electrification of road and aviation, as well as grid-scale storage for renewables. However, the future of batteries can only be evaluated with a holistic consideration of opportunity costs and emerging technologies beyond their own existence.