In battery technology parlance, solid-state batteries (SSBs) are sometimes called “forever batteries.” That’s because these batteries promise to far outlast traditionally technologies that rely on the electrochemical discharge of ionic solutions. Eventually those solutions lose charge capacity and need to charged more often. Charging itself puts strain on battery connections and heat sinks, as lithium (Li)-ion batteries “breathe” during charge and discharge cycles. The result is a battery in which capacity begins to drop and performance begins to degrade very quickly.
That is not so for SSBs, which continue to be the holy grail of battery technologies. If realized, SSBs could be completely disruptive to normal battery technologies and all the devices that have grown to rely on Li-ion.
SSB vs Li-ion technologies
The mechanics of generating electricity is similar for SSBs and Li-ion units: electric current is generated by the movement of ions between the cathode and anode through an ionic conducting medium called an electrolyte. The distinguishing difference between solid-state and conventional batteries is that Li-ion devices use liquid or polymer gel electrolytes whereas SSBs use solid electrolytes made from glass, ceramics, solid polymers or sulfides.
Another difference between the two battery technologies is that Li-ion systems have a separator between the cathode and anode. A separator is an electron insulator, a permeable membrane that ensures only ions can pass between the anode and cathode, preventing short-circuiting. A separator is not required with a solid electrolyte, but the materials used by Li-ion separators are being applied to solid electrolyte interfaces in SSBs to mitigate dendrite growth. Dendrites are small Li metal growths that can pierce battery interfaces and cause electrolyte failures and have been one of the most critical technical challenges in SSB research and development (R&D).
Energy density is the amount of energy a battery contains in proportion to its weight and size. Space saving with solid electrolytes means SSBs have more energy density, so more compact batteries can be designed that last longer and charge faster.
Battery technology is evolving and the composition of materials used in both types of batteries differs between manufacturers. Cathodes for both technologies usually use battery chemistries like nickel-cobalt-aluminum, nickel-manganese-cobalt, lithium-cobalt or lithium-iron-phosphate. Li-ion battery anodes are usually made from carbon or graphite (a critical component in electric vehicle (EV) battery technology for manufacturers who are sticking with conventional battery technology).
Because SSBs don’t use flammable materials, they can operate at higher temperatures than conventional batteries. This advantage also lends itself to faster charging because the batteries don’t overheat. In addition, a solid electrolyte doesn’t freeze up like a liquid electrolyte may, resulting in reduced battery performance.
SSBs can use up to 35% more Li than Li-ion batteries but Japanese researchers are working on Li-free solid-state EV batteries that use manganese ions instead.
The big challenges facing SSBs
Why haven’t SSBs been more prevalent? Well, there are a few engineering hurdles battery researchers must resolve first.
One of the biggest challenges has been the development of Li dendrites, which are asymmetrical Li growths from the anode into the cathode material. This creates conductive inefficiencies, potentially can crack the electrochemical materials and in some cases lead to thermal runaway. Researchers have attempted to solve this by eliminating the use of Li, experimenting with different separator types, operating SSBs at high temperature or toughening the cathode.
This of course, leads to high expenses in the manufacture of SSBs, which remain a novel and prototypical technology that has yet to reach mass production. SSBs also require vacuum deposition equipment, which is also slow and expensive.
There are also challenges along the anode and cathode interface. Electrochemical reactions can at times create byproducts that passivate surface area between the materials, increasing resistance and decreasing performance.
Finally, SSBs are prone to the same breathing required by Li-ion liquid electrolytes. First, this puts mechanical strain on electrodes and other connections, which if impaired affects battery performance and potentially safety. In Li-ion batteries, this is sometimes solved with a spring or foam material that provides consistent pressure against the connections. This has proved more difficult to solve for with solid-state materials, which require even higher pressures.
Big investments
None of these challenges are deemed insurmountable – at least when faced with prospects and riches of a new disruptive type of battery.
A company called QuantumScape claims to have ironed out most of the problems associated with building SSBs that, despite their potential to address the shortcomings of conventional Li-ion units, have held them back from being more widely adopted, particularly in the multi-billion dollar EV market. The company has a ballooning share price along with a $300 million VW investment. Unfortunately, the company has also been plagued by legal concerns over allegedly misleading claims about its SSB technology.
QuantumScape’s Forever Battery is not the first or only one of its kind. Numerous auto manufacturers – including Toyota, Nissan and Chevrolet – are doing their own low-key R&D into Forever Battery technology for the EV market. NASA too continues development of its SSB technologies. Others – like Dyson, Fisker and Tesla – have elected instead to focus on improving the performance of conventional Li-ion batteries.
But QuantumScape is not alone. If manufacturers get the formula right, SSBs will be smaller, charge faster, be safer to use, dramatically reduce emissions, be quicker to manufacture, last longer and have a greater range compared to Li-ion systems. However, there are numerous challenges scaling laboratory successes in new battery technologies into real-world, practical applications.
Applications for SSBs
SSBs are in a race to take over from Li-ion batteries, the battery technology currently favored for use in EVs and for industrial electrification.
SSBs for potential EV use are called Bulk SSBs because they can store a lot of energy Thin-film SSBs store less energy but can last a long time and are ideal to power relatively small-scale applications and processes, like internet of things devices, smart homes appliances, medical equipment like pacemakers, wearables, computer components, energy harvesting devices, and sensors in smart environments and manufacturing automation systems.
Here’s the rub: While there are numerous solid- or semi-solid-state batteries currently in the market or available for import from China that are suitable for these relatively small-scale applications, the mass production of cost-effective SSBs suitable for powering EVs is still a work in progress. A rare vehicular exception is the nickel- and cobalt-free, Li-ion polymer battery from Blue Solutions that powers the Bluebus.
The way forward
The broad adoption of SSBs to replace conventional batteries in EVs is still some way off – experts suggest the mass production of SSBs is unlikely to happen before 2030, maybe even later. If so, the last word must go to Donald Sadoway, Professor of Materials Chemistry at the Massachusetts Institute of Technology and cynic, who told Forbes that by that time, solid-state may well be superseded by more advanced technologies.
