Recent developments suggest that the race to power electric vehicles (EV) with solid-state batteries (SSB) has gained momentum. In January 2024, Toyota Motor Corporation (Toyota City, Japan) confirmed its previously stated plans to start producing SSB EV in the 2027–2028 timeframe
[1]. In May 2024, it emerged that the Chinese government plans to invest more than six billion CNY (830 million USD) in projects intended to speed up SSB development
[2]. In June 2024, the automaker Nio (Shanghai, China) began supplying customers with EVs containing “semi-solid-state” batteries—a hybrid technology that could serve as a stepping stone to fully solid versions
[3]. In September 2024, SAIC Motor (Shanghai, China), China’s largest automobile manufacturer, announced that it would deliver its first SSB-powered vehicles in 2025
[4].
SSB employ a solid electrolyte, ditching the liquid electrolytes used in the lithium-ion batteries found in virtually all the EVs now sold around the world. This swap credibly promises benefits including improved safety, faster charging, and longer driving ranges. Small devices such as pacemakers already use SSB, and deploying larger versions to improve the performance of EVs could assist the transition away from combustion engine vehicles, a key strategy for combating climate change. But the companies developing these batteries face a range of challenges, from the vagaries of battery chemistry to the exacting demands of mass manufacturing, and no consensus has yet emerged about the best solutions
[5]. “It is not simple to mass produce them, or to find the right balance between lifetime, safety, energy density, and costs,” said Thomas Schmaltz, senior scientist at the Fraunhofer Institute for Systems and Innovation Research in Karlsruhe, Germany, and a co-author of roadmap reports for SSB development over the coming decade
[6],
[7].
Conventional EV batteries shuttle lithium ions between two electrodes during charging or discharging. The lithium travels through a liquid electrolyte that typically contains lithium salts dissolved in an organic carbonate. By tweaking the composition of electrodes and other components, companies have steadily improved lithium-ion batteries to give them higher energy density—the amount of energy they can store in a given mass or volume—along with faster charging and other features
[8]. Yet many consumers remain anxious about the range of their EV
[9], and there may not be much scope for further significant gains. “We are reaching a very optimized stage, where the improvements will get smaller,” said Schmaltz.
The liquid electrolytes in lithium-ion batteries also pose a fire hazard. After many cycles of charging and discharging, spikes of lithium called dendrites can grow into the electrolyte and cause short circuits that ignite the liquid
[10]. “One of the most important reasons why we want SSB is actually safety,” said Helena Braga, an associate professor of engineering physics at the University of Porto, Portugal, whose research has focused on SSB. “These accidents do not happen all the time, but, when they do, they can cause a lot of damage.”
SSB contain a solid electrolyte—typically oxides, sulfides, or polymers—which avoids this fire hazard, while still allowing lithium ions to migrate between the electrodes. That enables SSB to safely use electrodes with a much higher charge capacity
[11]. Whereas lithium-ion batteries generally use a graphite anode, SSB can use anodes of silicon, or even lithium metal. By avoiding the dead weight of graphite, they can store 2–10 times as much energy as a lithium-ion battery of equivalent size
[5].
Three years ago, many companies were focused on lithium-metal anodes, but that proved to be a significant bottleneck for SSB development, said Rory McNulty, product director of new technology at London, UK-based Benchmark Mineral Intelligence, a market research firm focused on the battery and EV supply chain. A key problem is that both lithium-metal and silicon anodes expand and contract significantly during operation, which opens gaps at the interface with the solid electrolyte and worsens performance
[12]. “Anywhere the electrode does not touch the electrolyte, that part of your battery cannot be used anymore,” said McNulty.
One solution is to put the battery layers under pressure. This helps to keep the interfaces together and suppresses dendrite formation, Schmaltz said. But while researchers have seen good results from this approach in the lab, it has been difficult to achieve similar benefits at the lower pressures that would be practical for working commercial batteries
[7].
To keep costs down and avoid carrying extra unused weight, it is important to ensure that no lithium is wasted in SSB. To enable that, a lithium-metal anode must be composed of thin foils that are mere microns thick. Traditional foil manufacturing processes such as extrusion and rolling were not up to the task, so companies have had to develop alternative manufacturing processes, said McNulty.
Some battery makers in China have instead pivoted to using more traditional graphite-based anodes, along with cathodes built from materials that have already proved their worth in lithium-ion batteries, such as nickel manganese cobalt (NMC) oxides
[13]. However, using graphite anodes means that these SSB are likely to offer only marginal benefits compared to conventional lithium-ion batteries, said Schmaltz.
Meanwhile, researchers and companies are still experimenting with various solid electrolytes (
Fig. 1). In 2016, electric buses produced by Blue Solutions in France started using SSB with lithium-metal anodes and a polymer electrolyte
[14]. However, according to McNulty, the polymer’s poor conductivity means that charging takes hours, and the electrolyte also requires pre-heating
[15].
Oxide-based electrolytes could offer better performance, but these materials have been brittle and difficult to manufacture, McNulty said. Alternatively, sulfide-based electrolytes generally offer the fastest charging, but because they are not as chemically stable as oxides it can be more difficult to create a reliable interface with the electrodes
[7].
Overall, it has proved difficult to combine high energy density, safety, and rapid charging in a single SSB. That is in part because the ideal materials or battery design to optimise one characteristic are often detrimental to the others, said Schmaltz. For example, although SSB with lithium-metal anodes have higher energy density, they are also harder to charge quickly, and lithium can plate onto the anode unevenly, significantly shortening the battery’s lifetime, Schmaltz said.
That is why Nio’s semi-SSB could act as a useful stepping stone. “It is a way of assessing the manufacturing process, and the ability to produce solid electrolytes, without necessarily having all the challenges of an all-solid-state system,” McNulty said.
Semi-SSB contain a gel or liquid component that sits between the electrolyte and the electrodes, offering a malleable interface that fills in gaps when the solid structures flex. Crucially, they can use many of the same manufacturing processes as traditional lithium-ion batteries, significantly easing their path to market
[3].
Nio’s semi-SSB produced by WeLion New Energy (Changzhou, China) employ an oxide-based electrolyte
[16]. The batteries have an energy density of 260 W·h·kg
−1, at least 50% higher than the lithium-iron-phosphate (LFP) batteries preferred by most EV makers in China
[3]. The Nio EV also boasts a 150 kW·h battery pack that dwarfs the typical 50–100 kW·h lithium-ion packs. With more energy on board, Nio’s vehicle has a driving range of more than 1000 km on a single charge
[3]. One report suggests that by May 2024 WeLion had made enough SSB for several thousand vehicles
[17].
“That improvement in range to 1000 km is quite impressive,” said McNulty. The cars are designed so that the entire battery pack can be swapped out in an automated station. This strategy is often used to avoid lengthy recharging times, but it will also enable Nio and WeLion to more easily study how the semi-SSB stand up to real-world use, so that they can be further optimized
[18].
Toyota’s effort still targets using fully-SSB with a sulfide solid electrolyte. The company says it has made unspecified technical breakthroughs that will enable the batteries to charge to 80% capacity in 10 min and offer a driving range of up to 1200 km
[1]. It also claims it can build the batteries just as fast as on a lithium-ion production line
[19].
However, Toyota has been promising the imminent arrival of its SSB for many years, and some industry watchers expect the company’s schedule could slip again
[20]. Still, a wide range of other battery makers, including Contemporary Amperex Technology Co., Ltd (CATL); Ningde, China, LG Chem (Seoul, Republic of Korea), and Samsung (Suwon-si, Republic of Korea), are all reporting advances that could see them commercially producing SSB for EVs by 2030
[21]. In January 2024, Volkswagen announced that it had successfully tested a lithium-metal-anode SSB from QuantumScape, a San Jose, CA, USA-based start-up
[13]. In the laboratory, the SSB achieved 1000 recharging cycles without losing more than 5% of its energy capacity, and QuantumScape says its batteries will reach an energy capacity of 400 W·h·kg
−1 [3]. Benchmark predicts that while China currently boasts the largest share of SSB manufacturing capacity, other regions are also likely to increase their output over the coming decade (
Fig. 2).
Despite these signs of progress, it remains to be seen whether SSB can be mass-produced quickly enough, and at a sufficiently low cost, to make them a serious rival to conventional lithium-ion batteries
[13]. “Can they run the SSB production lines as fast as lithium-ion battery lines, and still manage to get those perfect solid interfaces? That is the real question that still hangs over them,” McNulty said.
For now, SSB will likely undergo more years of development before they see a wider rollout. “Around 2030, we should start seeing entry into the premium applications: luxury sports vehicles and military applications, where performance and safety are primary and cost is a secondary consideration,” said McNulty. He thinks SSB might finally reach mass-market EV between 2033 and 2038. “There will still be a huge place for lithium-ion batteries for a long time to come.”
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