Touted as the holy grail of next generation batteries, solid-state technology is still years from being commercialised. But its not stopped the next cab off the battery rank exciting the industry when it comes to replacing traditional lithium-ion cells, as Paul Crompton finds out.
In the midst of the world’s fascination with lithium-ion lays an offshoot of the technology that is garnering excitement (albeit, reservedly) within the battery industry. The reason solid-state batteries are exciting is two-fold: energy density and safety.
Arguably, lithium-ion’s most inconvenient truth is the technology is prone to thermal runaway, with the ubiquitous technology making headlines around the world when personal devices to utility-scale energy storage systems catch alight.
For example, there have been around 20 fires in Tesla cars since 2013, although, to be fair to the US company the vast majority have been the result of crashes.
In 2017 Japanese multinational Sony recalled its VAIO laptops because the ‘lithium-ion battery packs can overheat, posing burn and fire hazards’. The same year Samsung was forced to recall its Note 7 mobile phones for similar reasons.
In 2018, there were 23 fires at battery energy storage installations in South Korea. Its government and a national standards committee put the blame on everything but the chemistry. Nevertheless, they were energy storage systems fires. Last year the US’ energy storage safety record was marred by a fire at the McKicken facility in Arizona, owned by utility Arizona Public Service.
Dendritic growth in batteries
A team at the Department of Energy’s Pacific Northwest National Laboratory (PNNL) has suggested dendrites– a main cause of cell failure– begin when lithium-ions start to clump, or ‘nucleate’, on the surface of the anode, forming a particle that signifies the birth of a dendrite.
Scientists from PNNL reported in Nature Nanotechnology journal the origin of dendrites in a lithium metal battery lies in the “SEI” or solid-electrolyte interphase, a film where the solid lithium surface of the anode meets the liquid electrolyte. The researchers pinpointed an ethylene carbonate, a solvent added to electrolyte to enhance battery performance, as a culprit.
Battery researchers know this and are battling to commercialise the solid-state battery, because one of the most important benefits of the technology is safety, says research fellow Xiaoen Wang and Fangfang Chen from Institute for Frontier Materials at Deakin University in Australia.
However, to think solid-state batteries are 100% safe is a fallacy. They suffer from dendrites at least as much as conventional lithium-ion batteries— and in fact David Greenwood, professor of Advanced Propulsion Systems at WMG the University of Warwick, says his research facility is discovering some new degradation modes unique to the technology. Although he isn’t publishing just what those new degradations are yet.
“The main safety benefit is from removing the VoC electrolyte which can act as an accelerant if a cell goes into thermal runaway,” Greenwood says. “The remaining cell materials (the anode and cathode) are just as flammable as before though, and there is plenty of stored energy in the cell to act as a source of ignition.”
“Solid-state batteries are not very different from lithium-ion, really; the chemistry is quite similar,” says Denis Pasero, production commercialisation manager at UK firm Ilika.
“I think what is particularly interesting for the electric vehicle market is the safety aspect. The liquid electrolyte in lithium-ion is known to create gasses and potentially explode. So EV manufacturers know that’s not good; it’s toxic and explosive in a battery,” he says.
The other reason solid-state technology has the potential to disrupt the battery industry comes from its theoretical power.
Some industry experts believe the search for higher energy density is the main driver for pushing the technology to commercialisation, with safety benefits (if indeed they prove to be significant) secondary.
In mobile applications this could mean higher mileage per charge, or smaller packs delivering the same range. A number of electric vehicles (EVs) have passed the 200-mile range. Tesla’s Model S Long Range 3 reportedly has a 375‑mile range— using the European Union-based World Harmonised Light Vehicle Test Procedure (WLTP) test. Vehicle OEMs are hoping to continue this trend moving forward.
Unquestionably higher energy densities and advanced drive units will be needed to push EV adoption. Solid-state batteries could be the answer.
The Holy Grail
“Solid-state lithium-ion is a good technology, and is potentially the most attractive. It could be the holy grail of batteries because it offers higher energy density and is potentially safer than traditional lithium-ion batteries,” says Dr Allan Paterson, head of Programme Management at the Faraday Institute.
But what are solid-state batteries? The term refers to the use of solid-state electrolytes in lithium-metal or lithium-ion batteries. The reason the technology is garnering interest is because the above points make it an obvious replacement in any application that uses lithium-ion today.
In research and development terms there is 20 years between lithium-ion and solid-state— but let’s not forget it took 30-odd years for lithium-ion to go from the laboratory to being used in one million US vehicles last year.
So, within the next decade, can we expect to see OEMs using solid-state batteries as business-as-usual? The simple answer is yes, but there are many obstacles to clear first.
Pasero adds the caveat that, although his company uses ceramic film for its full solid-state batteries, some firms are really making semi-solid batteries more like a gel or polymer, which he says are more devices in terms of development.
“Firms are talking about mass volumes of what they call solid-state, which is very possibly semi-solid, and some even predict product launches early this year with the goal of ramping up commercialisation possibly with semi-solid batteries next year, going to full solid maybe a couple of years later,” he says.
The disrupter in EV applications is that its potentially higher energy density could make the cells thinner and pack them with more energy than lithium-ion batteries. In practice that makes an EV battery that has either a longer range or a smaller battery for the same range.
But the challenges standing in the way of commercialisation are myriad and fall roughly into two categories: technology and commercialisation.
Questions hanging over the technology side are multiple: how to develop a suitable electrolyte; how to improve or optimise the ion conduction in cathodes and anodes; how to improve low temperature performance (relatively low ionic conductivities for solid polymer system); how to manipulate the electrode/electrolyte contact and composition of SEI. The list goes on. So why bother, you ask?
Firstly, exchanging the liquid has the advantage of replacing a bulky and flammable component with a much thinner and safer material performing the same purpose.
To do this there are two types of electrolyte used to replace the polymer separator and liquid electrolyte (generally a VOC with additives) with a solid interface– either a polymer film or a very thin layer of ceramic, garnet or perovskite.
The benefits of polymer membranes are they are scalable to mass production and easy to integrate into a battery factory. The downside is that the ionic conductivity of the polymer isn’t so great, and the polymer is thicker than the garnet/perovskite/ceramic alternative. It means the material struggles to give the power required for most applications, says Greenwood.
The other inorganic electrolyte materials (garnets, ceramics or perovskites) are much harder to mass manufacture but have a higher charge and discharge rate due to higher ionic conductivity. However, the downside is they are very slow to make and almost all of them are brittle, which will be a challenge when making thin and large area batteries (such as pouch cells) as the electrolyte could break easily.
“Both types work better at higher operating temperatures (to increase ionic conductivity), and both need the cell to be held under mechanical pressure,” Greenwood says. “These are solvable with good engineering, but reduce the net benefit of the technology as savings in battery cost and weight are offset by costs for thermal management systems or clamping systems.
“Once these issues are cracked, solid-state will come to market, but in the meantime there’s still lots of research needed.”
The second point is commercialisation, in so much as the challenges of moving a brand new technology from laboratory desk to mass production can be costly, never mind manufacturing costs before beneficial economies of scale have been reached (more of which later).
For example, while the polymer precursors and lithium salts are commercially available in Deakin University’s research, the price of the salt is still relatively high. “This is true for most of research at early stage. If the market is big enough and more investors join in, I think the price will go down,” Wang says.
When looking at the technology’s potential markets, Pasero believes they clearly fall into two distinct areas: large format and micro batteries.
The first market includes those where cost isn’t a factor: high performance cars, aerospace, EVs, drones and satellites are obvious growth areas in the near term, he says.
Today, solid-state batteries are coming to market at a price level comparative to the early days of lithium-ion batteries so the approach of companies is to look at niche markets, or markets where the technology is definitely superior, says Pasero. “So we are talking about racing cars, sports cars, high powered vehicles that perhaps OEMs will understand the difference of cost compared to performance.
“Aerospace is another that’s quite key when looking at drones, satellites or vertical take-off vehicles where cost doesn’t need to be low but performance is important.”
What Ilika is also considering is the development of batteries up to 2030, asking questions like ‘will our driving interests be the same in 10 years time’?
“We are thinking there will be shared ownership of cars, which has already started to happen,” says Pasero. “The idea is people use their cars for an hour a day and in that scenario the cars are used a bit more during the course of a day. So the batteries will not need to be long range, like they are when used by one person, but will require rapid charging and cycle life. This is important factor in designing batteries of the future.”
For land and marine transport applications, lithium-ion is expected to dominate for the next 8-10 years, say industry experts, beyond that, sodium-ion and solid-state look ‘very promising’. However, neither technology is ready for mass production yet, with the former being tested in aircraft (by, among others, UK firm Oxis) and the latter expected to be launched in a prototype car by Toyota next year at the Tokyo Olympics. The company says it expects full production will be around five years later.
This is optimistic of the Japanese firm, according to people within the industry. Such vehicles are very much prototypes at this stage, which is very different to large-scale manufacturing models and device integration.
Meanwhile, megawatt-scale batteries that store renewable energy could be a good application. But when it comes to stationary storage, the less obvious market, at least in the short term, is residential-scale systems (a few kilowatt hours) like Tesla’s Powerwall, where cost is a more important factor to the end users.
“In the long term we’re hoping solid-state technology will be manufactured at high volumes to bring costs down,” Pasero says.
The second market is micro batteries, which is the area Ilika was looking at before they began research on EVs following a working partnership with Toyota. Solid-state batteries for internet-of-things (IoT) applications such as medical implants are an interesting market, but it’s an incumbent technology, says Parero.
At WMG, the challenge its researchers have is the solid electrolytes aren’t as conductive as they’d like, and some are very hard to make in large quantities.
“For automotive and aerospace applications we need high power (especially in charging) and very large production volumes,” says Greenwood. “Given current progress I would estimate at least 8-10 years before solid-state batteries make it into these applications.”
Greenwood suggests that if solid-state batteries can deliver against their promises then motive markets would be a great application. “They offer some, but not quite as big a benefit to aerospace as the gains are more in volumetric energy density (the amount of energy per unit volume) than gravimetric energy density (the amount of energy per unit mass).”
While Toyota should be lauded for being at the forefront of solid-state development, many within the industry believe the OEM’s prototype, due to be seen on Tokyo’s streets next year, won’t be a commercial vehicle.
Going from prototype to full commercialisation will certainly not be until the second half of the 2020s in our view, says Pasero of the Toyota vehicle.
Toyota uses a sulfide electrolyte chemistry that delivers a very high performance but suffers the same issues of traditional lithium-ion, namely safety. Its concerns mainly focus around the creation of a toxic gas called H2S.
Ilika has been working on a commercially funded, thin film solid-state materials synthesis and characterisation technology project with Toyota Research Institute in the US. Since 2017 it has been testing candidate materials using its high throughput platform.
The south-England firm is working on a road map that it hopes will take its technology to the 500Wh/kg mark, although this could still be up to five years away.
“More realistically, in the short term, we are starting at 300Wh/kg and moving to 350Wh/kg later this year,” says Pasero. “We see limitations, our modeling tells us we could go to 500Wh/kg with our future developments.”
He cites cycle life as needing more work, with Ilika aiming for thousands of cycles, although the company isn’t releasing the data on what it can achieve right now. The company anticipates 6C (10 minutes) for a full charge, and is developing cells with partner BMS and pack designers and OEMs to put their cells in modules and packs.
“What’s important for solid-state in terms of performance is the ability to achieve higher power and that’s why manufacturers of drones, satellites and sports cars are interested in this,” Pasero says.
In Australia, Wang, Chen and professor Maria Forsyth at Deakin University, in collaboration with the POLYMAT, CIC Energigune scientists in Spain, reported in the journal Joule of using a commercially available polymer as solid electrolyte system to replace the liquid solvents typically used in lithium-ion batteries.
It was found, through molecular simulations, the reported polymer electrolyte was weakly bonded to the lithium ion that the sufficient lithium conductivity could be achieved to support battery operation.
To date, they have conducted tests on a coin cell battery at 50 cycles at low charging rate (10 hours to full charge state), and will run them at temperatures above 60°C in the future.
Wang said: “Overall, our electrolyte material shows promising performance compared with other polymer systems, but there is still a lot of work to do to optimise the performance. The room temperature conductivity is not ideal compared with a liquid system.”
The team also has to perform long cycling tests, having only conducted 50 cycles at low charging rate.
The team led by Alfred Deakin professor Maria Forsyth is also working on a variety of solid electrolytes, and developing and prototyping high energy and safe battery systems for next generation devices.
They’ve made a solid organic ionic plastic crystal–polymer composite electrolyte that can be used at a high voltage (4.6V) and supports a long cycling life at a fast charging-discharging rate (1,300 cycles, one hour to full charge state).
Talga Technologies’, a subsidiary of Australian firm Talga Resources’, is part of a UK government funded consortium working toward developing a graphite-based anode to allow solid-state batteries to meet the performance demands required by EVs.
Consortium members also include: UK-based chemicals company Johnson Matthey and battery research and UK development institute Sheffield University.
Talga believes solid-state batteries are theoretically capable of very high performance parameters, but in practice can suffer a range of technical and commercial issues that have hindered development, particularly for larger-scale applications such as EVs.
The company believes the anode is a major bottleneck in the development of the technology, whereby the use of metallic lithium can cause a range of issues leading to slower charge/discharge characteristics, safety issues both within the battery and in mass production, as well as higher production costs.
In terms of materials, Ilika could be going away from cobalt-rich materials as it is toxic and the impact of mining it in the Democratic Republic of Congo is well documented.
“The move for solid-state materials is going toward nickel-rich materials because they offer better performance and use less cobalt, but they are quite unstable in traditional lithium-ion batteries. Nickel-rich materials will be more stable in solid-state because the chemical reaction is not happening in a liquid electrolyte,” says Pasero.
“The negative of this is the supply chain. Most of the cathode and anodes are not very different than those used in lithium-ion. The big difference is in creating a supply chain for ceramic materials and that needs to be established. Some of those materials are not produced at high volume and low cost, although economies of scale will help with this.”
Recyclability, which is becoming more important the farther down the de-carbonisation road the world travels, is one means of securing a supply chain that can meet demand.
There’s some reason for stating the technology will be easier to recycle and require less greenhouse gasses, a factor being higher density batteries will need less of the same materials to deliver the same service.
Modelling has been done on solid-state technology, and there have been reports published but at the moment no one is really sure of its credentials. Pasero says he’s seen a report that solid-state could provide a better impact on the environment, but quantifies that it’s about 25-65% better than traditional lithium-ion.
It’s been suggested solid-state batteries are at the same price point as lithium-ion was a decade ago, but what are the potential costs of manufacturing this new technology, a major factor when commercialising a product, so it’s prudent to look at equipment as well the materials.
A reason most lithium-ion companies are developing solid-state is because the process for making the cells is similar, so there’s no need to replace installed plant with expensive special equipment.
Ilika is examining printing techniques that have the potential to be cheaper, so in terms of cathode and anode manufacturing the cost is not going to be very different, says Pasero.
“The cost of manufacturing the electrolyte requires another ink-based process that requires higher temperatures rather than filing a pouch with liquid.
“This could bring an overall reduction in cost in terms of packaging. Conventional lithium-ion is susceptible to the issue of ingress such as moisture. So at the end of the day the probable costs would be much lower in terms of pouch manufacturing, but in the early days it might be solid-state that potentially has a higher manufacturing cost.”
Greenwood says there are challenges in finding materials with the right properties for the electrolyte, and manufacturing processes to make them at very high rate.
“A typical battery factory coats electrode foil at about 120m2/minute— many of the current solid-state materials take hours to coat an electrode the size of a paperback book,” he says.
“Beyond that, there will be challenges in understanding and preventing degradation of the electrolyte layer, and maintaining the interfaces between it and the anode and cathode materials— especially as these expand at differential rates as the battery is charged, discharged, heated or cooled.”
In a slew of battery technologies being discussed, solid-state lithium-ion has the potential to be the biggest EV market disrupter— at least in the foreseeable future. However, it’s clear the chemistry is still being fine-tuned, but then nothing in any of the industrial revolutions has been commercialised in a perfect configuration straight away; even Apple is on the eleventh generation of arguably the world’s best phone. As always, early adopters will be the guinea pigs for tomorrow’s technology.