Sony brought Lithium Ion batteries to market in 1991. Today, the same underlying battery tech powers everything from mobile phones to electric vehicles (EV). However, Lithium Ion batteries are far from perfect. Charging times are slow, energy density is average, and on the odd occasion, they spontaneously combust. Remember why the Samsung Note 7 was banned on flights in 2016? How do we design better, safer batteries? The general consensus is by changing the electrolyte from liquid to solid. This would make the battery compact, light, energy dense, and less prone to bursting into flames. However, this seemingly simple solution has been really difficult to execute. This article looks into why solid state battery development is taking forever and whether it is likely to have as big of an impact as some think.
How Lithium Ion batteries work
Lithium Ion batteries work through an ‘electrochemical reaction’, which is a process that is accompanied by the passage of current. It involves transfer of electrons between two substances. In a typical Lithium Ion battery, these substances are a cathode made of Lithium and an anode made of Graphite.
When charging a Lithium Ion battery, electrons from the cathode move towards the anode through the external power source (the charger). At the same time, Lithium ions flow from the cathode to the anode through the liquid electrolyte that separates them. When a charged battery is connected to a device, the opposite happens. Electrons move from the anode to the cathode through the connected device, while Lithium Ions flow from the anode to the cathode through the liquid electrolyte. The battery dies when all of the electrons and ions make their way back to the cathode. To be used again, the battery has to be charged. Pretty straightforward, right?
Why Lithium is a good choice for batteries
A quick look at its position in the periodic table can answer why. Lithium is the lightest of all metals. It is also the metal that gives up its electrons most easily to produce positive ions. Both these factors have tremendous implications in final applications. From a user’s perspective, batteries that add to the weight of their equipment or take too long to charge are strict no-no’s!
The case for solid state batteries
Though Lithium is a great choice for battery cathodes on paper, in reality, it is not quite the case. Over time, Lithium Ion deposits can accumulate on the face of the anode forming spikes called dendrites. Left unchecked, these dendrites branch out all the way to the cathode, causing a short circuit. This is especially dangerous as the liquid electrolytes used in Lithium Ion batteries are inflammable. Dendrites breaking through to the cathode exposes highly reactive Lithium to the liquid electrolyte, resulting in swollen batteries or worse, explosions.
Not only does solid electrolytes make batteries safer, they also make them compact. Listed below are the three main reasons for the excitement surrounding solid state batteries when compared with liquid electrolyte batteries:
- Higher energy density – up to three times higher
- Faster recharge times – up to 6 times faster
- Much simpler battery management systems
However, despite a lot of effort since the 1970s, solid state batteries have only found use in tiny devices like pacemakers.
Why is it difficult to make solid state batteries?
When Lithium touches an electrolyte, it transfers electrons at the interface. This creates a layer at the interface (interphase) with properties that depend on the two contacting substances. Some interphases let electrons through, others ions. But the most common type of interphase lets through a mix of electrons and ions. The ideal interphase for solid Lithium batteries should meet these criteria – be thermodynamically stable, allow reversible Lithium Ion movement, and prevent side reactions. As you may have guessed, such an interphase is not easy to develop.
To make matters worse, most materials used to develop all-solid-state batteries do not go well with water. Not only is it extremely difficult to ensure a water/moisture free environment in a commercial battery production facility, battery damage, such as EV collisions and prolonged rough terrain driving can add to safety concerns.
Further, batteries that perform well in a lab do not always work well in real EVs. Each bump on the road jars the battery separating the electrolyte from the electrodes momentarily, which may trigger dendrites or voids.
Important breakthrough
While the first patent for solid state batteries was granted in 1976, progress has been slow. One important breakthrough was made by Toyota in 2011 when they announced an electrolyte made of solid sulfides. This was the first time ever that a solid electrolyte performed as well as a liquid electrolyte. Further studies showed that sulfides may not be the way to go though. Turns out these batteries release a toxic gas when damaged. So while the sulfide approach finally proved solid electrolytes can perform as well as liquid electrolytes, toxic gas made it impractical.
Current status of solid state batteries for EVs
Following Toyota’s solid sulfide battery breakthrough in 2011 there’s been a steep rise in patent activity. Tesla has made EVs cool, affordable, and practical (almost). The fact remains that it relies on a plug-in charge system that does impose some restrictions. For instance, charging the EV overnight at home is not an option for renters using on-street parking. Also, road trips planned with access to super-charger stations in mind may force one to alter routes and destinations. Thus, developing solid state lithium batteries that eliminate safety issues, reduce charging times, decrease weight, and increase driving range is probably the only way other auto manufacturers can one-up Tesla.
Toyota is at the forefront of solid state battery development with over a thousand patents in the field. In fact, the company has been testing solid state battery equipped vehicles in the real world for over a year. Based on their findings, service life is not up to the mark. But they believe the technology is viable and are on track to develop solid state batteries by 2025. Thus, it seems highly unlikely that Toyota’s upcoming bZ4x EV, stated for release in early 2022, will house solid state batteries. Given battery development is hyper competitive, specifics of the materials used are extremely well guarded. However, considering that auto materials makers in Japan are setting up facilities specifically to make solid electrolytes, it is likely that we are looking at years, not decades before manufacturers release fully solid state battery equipped vehicles.
Latest trends in EV battery charging
Companies like Contemporary Amperex Technology Co. Limited (CATL) and SEMCORP have become global leaders in lithium ion battery materials in a relatively short time. China has never been considered a global powerhouse when it comes to cars. This may be about to change thanks to their rapid progress in battery R&D. The Chinese manufacturers Nio targets release of solid state battery equipped vehicles in late 2022. Their batteries are claimed to have a capacity of 150 kWh, which is good for 600 miles on a single charge. While this in itself is impressive, where it truly stands out is in the ability to swap drained batteries for freshly charged ones in 3 minutes! As of today, the company operates 400 of such battery exchange stations.
Taiwan’s Gogoro is an EV battery-swapping network that has seen remarkable success lately. They have over 800,000 battery packs on the road today. Hero MotorCorp has partnered with Gogoro to bring this platform to India, which along with China are the two largest markets for electric scooters globally. Is this a better alternative to plugging your vehicle in and waiting for it to charge at superchargers or hyper charger networks?
Swapping drained batteries of my remote controlled cars for fresh ones and continuing to play while the first set of batteries charged was extremely satisfying for me as a kid. So I love the battery swapping concept, provided it is quick and free – or very reasonably priced.
Final thoughts
Range anxiety, long battery charging times, and fire risks are very real problems for EVs. Though improving battery performance is an obvious route to address these concerns, it is not the only way. Modular vehicle designs that allow swapping of drained batteries for fresh ones seem to address the first two issues surprisingly well. The fully automated battery swapping infrastructure developed by Nio gets this done in 3 minutes. The process is even faster for electronic scooters as the batteries are small enough to be swapped manually.
The issue of fire safety of current EV batteries is significant and we must focus on safety solutions both at the battery level as well as large-scale charging infrastructure. At the battery level, a fairly simple fix may be round the corner going by a recent research breakthrough from Nanyang Technological University, Singapore. The solution involves adding an ‘anti-short layer‘ between the electrodes of the battery, much like adding a layer of cheese between two slices of bread. This seems like a straightforward, inexpensive, easy to implement fix to make new and existing Lithium batteries fire proof.
This is not to say that solid state batteries have no role to play going forward. It is definitely better to have a lighter, compact, more energy dense battery than the state-of-the-art today. This will make EVs lighter which translates to improved driving range, and perhaps better driving dynamics. These batteries may also dramatically influence the battery swapping ecosystem. The swapping infrastructure at Nio requires a hi-tech automated system to swap batteries of it’s SUV. The smaller solid state batteries of the future may enable manual swapping of batteries for SUVs and trucks, much like electronic scooters of today.
But do EVs need solid state batteries? No. EVs will thrive with or without solid state batteries.