In September, car manufacturer Toyota published a battery technology roadmap, outlining their research focus and their goals for producing a diverse, all-electric fleet in the next decade. This alone is not that surprising – the electrification of the automotive sector has been underway for some time. But the announcement marked a notable pivot for the company, which until fairly recently, had been publicly, and aggressively, resisting the transition to battery electric vehicles (EVs), focusing instead on hybrids and vehicles powered by hydrogen fuel cells.
The bulk of Toyota’s new roadmap focuses on achieving significant improvements in the energy density, cost competitiveness and charging speeds of liquid electrolyte batteries. Their goal is to produce three different battery types, with each one’s performance optimized for a particular EV design.
However, it was the mention of an unspecified “breakthrough in solid-state battery technology” that has caused it to shift its “development focus to mass production” that really got people talking.
What are solid-state batteries?
Mainstream batteries – such as the lithium-ion ones that power your smartphone and most of today’s electric cars, or the lithium-iron-phosphate (or LFP) batteries increasingly favoured for electric buses – all have one thing in common. They rely on a liquid electrolyte, which facilitates the flow of charged particles (ions) between the battery’s two electrodes, generating an electric current. Solid-state batteries replace this liquid with – you guessed it – another solid.
The idea is not a new one – Michael Faraday discovered two solid electrolytes in the 1800s. In more recent years, solid-state batteries have found widespread use in small electronic devices such as heart pacemakers and RFID tags. The obstacle to their use in larger-scale applications surrounds their manufacturing – for example, the active materials themselves are sensitive to oxygen and moisture, which complicates their assembly, greatly increasing the cost involved.
But the potential benefits of adopting solid-state batteries are significant. These battery materials can withstand low and high temperatures, making them very useful in extreme conditions. Their high resistance to heat also means they can be safely charged more quickly than lithium-ion batteries. And solid-state battery materials are likely to have higher energy densities too, which means that for the same weight, you can store more power.
Solid-state batteries also don’t have to be the same size or shape as liquid-electrolyte batteries. They are more structurally stable, and don’t contain any volatile liquids at risk of leaking. As such, they can be made smaller and thinner than today’s lithium-ion systems.
What is Toyota working on?
The specific details of this “technological breakthrough that overcomes the longstanding challenge of battery durability”, (first teased back in June) remain scant, at least by academic standards. We got some insight in November, when the company signed a new cooperation agreement with Idemitsu Kosan, Japan's second-largest oil refiner. Idemitsu’s main business involves extracting crude oil and turning it into a range of commercial chemicals, including lubricants and plastics. According to Toyota’s press announcement, they’ve also been turning by-products into various battery-related materials since 2001, and the two companies have been collaborating on materials development since 2013. This latest agreement is firmly focused on commercialization and mass production – specifically, of solid-state batteries that use sulfide electrolytes.
Toyota’s stated goal is for their solid-state batteries to ultimately have a range of >1,200km, and to go from 10 – 80% charge in 10 minutes or less. This compares to the Tesla Model Y, which currently has a range of 542 km, and fast-charges in 27 minutes.
How far are they from achieving it? From what I can tell, Idemitsu already has a small-scale pilot facility for producing electrolytes, and the two companies are constructing a larger facility that will integrate materials manufacture and battery assembly. Their roadmap points to having commercial batteries by 2027-28, followed by mass production at some point after that. At this stage, it’s hard to say whether that timeline is achievable – I always err on the side of cynical – but the company is making a lot of noise.
A major challenge for solid-state batteries is around maintaining good contact between the electrodes and the electrolyte sandwiched between them. In addition, repeated charge-discharge cycles can cause cracks to form between these components, significantly limiting the lifetime of the battery. During the press conference, Toyota President and CEO Koji Sato suggested that it’s here the breakthrough has been made, referring to “a highly flexible, adhesive, and crack-resistant solid electrolyte,” made by “combining the material technologies of both companies”. He continued, “we are now on the path toward achieving both performance and durability in solid-state batteries.”
In terms of specifics, that’s about as much as we know….and unfortunately, all we’re likely to know for a while. Given the potential commercial impact of the research, it’s perhaps unsurprising that the relevant developments are being published within patents, rather than through peer-reviewed papers. And it seems that their intellectual property officers have been busy protecting everything they possibly can. JustAuto recently reported that Toyota has registered a staggering 8,274 solid-state battery patents; many of which are jointly assigned to Idemitsu (here’s one example from 2016).
Who else is working on solid-state batteries?
The JustAuto patent search (linked above) shows that other automakers, including Hyundai, Kia and Honda, are actively researching their own solid-state batteries. But in terms of patent numbers, they trail far behind the Toyota-Idemitsu behemoth. Nissan and Honda have also made commitments to solid-state battery development, while others, including Volkswagen, Ford, BMW, and Mercedes-Benz are working with external battery manufacturers to take this step. To date, Tesla has not detailed any solid-state battery plans.
What are the implications?
Toyota is focused on cars, but if the manufacturing challenges are overcome, and the costs reduced, solid-state batteries could find use in e-bikes as well as larger commercial and transport vehicles. Trucks, buses and ships, which today, are dominated by fossil-fuels, could make the switch. These batteries are even being proposed for home energy storage systems.
NASA has a research program that’s looking to develop sulfur-selenium solid-state batteries for use in aircraft, but that application looks to be further from commercialization than those for use in road vehicles. Best not to hold your breath for electric planes.
An idea I’ve come across several times is that having small, light, efficient batteries might lead to a shift to smaller and lighter EVs. This would be a hugely positive move for many reasons – heavy vehicles do more damage to infrastructure, produce more particulate pollutants from their tyres, and pose greater threats to cyclists and pedestrians. Having fewer of them on the roads would benefit everyone. However, I fear that instead, we’ll see what we’ve seen in the internal combustion engine market – efficiency gains being outweighed (literally) by the continued bloat of vehicle size. Four of the five top-selling cars in the US this year were SUVs or pickup trucks (often known as ‘utes’). In New Zealand in 2022, all of the top five fitted into that class. Toyota’s own debut battery electric vehicle – the bZ4X – is an SUV.
So, whilst, as a materials scientist, I’m excited about the potential that solid-state batteries offer, I remain cautious about what they’ll mean for us in the future. They might reduce our reliance on certain critical minerals, transform transport, and lead to reduced emissions and better air quality in our cities. They might be yet another magical ‘silver bullet’ that delays and distracts from the urgent need to take society-level action on climate change. A seemingly ‘green’ technology that’s actually powered by the oil industry, and an excuse to keep buying private passenger vehicles. I’m really not sure, but I’ll be keeping an eye on it.