Are solid-state batteries the solution to liquid electrolytes?

As we have discussed in previous editions of Inside IP, there has been a fundamental shift in focus in the automotive industry towards electric vehicles. Yet, even within the all-electric sector of the automotive industry there are already two major technologies, battery electric vehicles and fuel cell electric vehicles, competing to be the vehicle that drives us into the future.

Of these technologies in the all-electric sector of the automotive industry, perhaps the best known or most commonly commercially-available are battery powered electric vehicles. The batteries that power these vehicles are generally lithium-ion batteries, which use an aqueous electrolyte solution to allow the flow of current. Such batteries typically provide ranges of around 80 to 100 miles on a single charge, with some luxury models providing a range of up to 250-300 miles.

Although the improvements in liquid electrolyte lithium-ion battery technology in such a short space of time is nothing short of remarkable, there remain some issues that have not yet been fully addressed or still require improvement, especially to make the users’ experience more comparable to what they are used to.

For example, the range of most liquid electrolyte lithium-ion battery electric vehicles may be considered to be relatively short, and the typical recharging times, of from 30 minutes to several hours, may be considered to be too long, especially when compared to vehicles powered by petrol or diesel. Other areas for improvement include energy density and safety, i.e. reducing the risk of fires.

To be (aqueous), or not to be, is that a solution?

However, another type of battery is tipped to be able to address each of the drawbacks mentioned above. This type of battery is known as a solid-state battery.

An idea of the number of patent families filed by some of the companies working on solid state battery technology can be seen in the Figure above, which shows the number of patent families each company owns in this technical field.

It is claimed that the advancement in solid-state battery technology will result in batteries that will have a range of more than twice the distance of a conventional lithium ion battery vehicle under the same conditions. By some estimates, replacing the Tesla Roadster battery with a solid-state battery would result in a range of 620 miles from its 200 kilowatt-hour battery. It is also estimated that the recharging time could be cut to as little as 10 minutes, reducing recharging time by at least two-thirds. All this without the need to reduce the cabin space of the vehicle.

Even ardent critics of electric vehicles would have to admit that these figures are impressive. Perhaps impressive enough to convince even the most diehard ‘petrolhead’ that electric vehicles should be viewed as at least an equal alternative to fossil fuel powered vehicles.

To understand how solid-state batteries are able to provide these advantages, we need to understand the difference between liquid electrolyte lithium-ion batteries and solid-state batteries.

One of the main differences between solid-state batteries and conventional lithium-ion batteries is the electrolyte. The electrolyte of a battery is the medium that provides the ion transport mechanism between the cathode, the positive electrode, and anode, the negative electrode, of a cell.

When a battery cell is supplying electric power, an oxidisation reaction takes place at the anode releasing electrons into the circuit and a reduction reaction takes place at the cathode acquiring electrons from the circuit. The electrolyte facilitates the movement of charged ions from the anode to the cathode.

In most lithium-ion batteries, this electrolyte is typically an aqueous solution formed from: salts, which transport the ions; solvents, the liquid in which the salts are dissolved; and additives. A membrane is provided in the electrolyte between anode and cathode to separate the electrodes, and an impermeable casing is provided to prevent leakage of the electrolyte.

However, as the name suggests, in a solid-state battery, the electrolyte is a solid rather than an aqueous solution or gel. The chemistry of the solid electrolyte can be quite complex but is generally formed from ceramics, glass, or sulphides.

A solid improvement?

One immediate advantage of solid-state batteries over lithium-ion batteries is the removal of the flammable liquid electrolytes. The solid electrolytes used instead are more resistant to changes in temperature and physical damage, produce up to 80% less heat, and are able to handle more charge/discharge cycles before degradation makes them unusable. All this points towards a longer battery lifetime.

Other immediate advantages include the removal of the membrane and casing required for a liquid electrolyte. This reduces the weight and volume of each cell. The reduction in weight should lead to an increase in energy density of the battery. However, even if energy densities of lithium-ion batteries cannot be achieved by the first solid-state batteries, this can be compensated by adding more solid-state cells into the battery due to the reduced volume of the solid-state cells.

Solid-state electrolytes will also enable the integration of higher performance materials, such as lithium metal electrodes, in place of graphite anodes. The lithium electrodes are more energy dense which would increase the battery capacity, potentially doubling an electric vehicles, range, and can be coated to prevent dendrite formation.

High-voltage cathode materials could also be used to take advantage of the larger electrochemical window that the solid electrolytes can provide. Some experts believe that such advances can push the energy density of solid-state batteries over 1,000 Wh/L, compared to the 700 Wh/L currently achieved by high end lithium-ion batteries.

A solid grasp of the future?

All the statistics sound very promising. But, unless the numbers and estimates are translated into real world results, the technology cannot really be considered to be a success. The usual worries for new technologies are also concerns for solid-state batteries. That is, can solid-state battery manufactures scale up their prototype batteries into commercial battery packs that meet the performance requirements of electric vehicles, and can the battery packs be manufactured in a cost effective manner that makes them a financially realistic option for most consumers?

As a technology, the advantages of solid-state batteries (larger capacity, potentially higher energy density, increased safety) will be extremely attractive to electric vehicle manufacturers. It is clear that such batteries would represent the ‘game-changer’ that electric vehicle enthusiasts are talking about. And with the amount of investment from one of the biggest automotive companies in the world, it appears that the question is when we will see solid-state battery powered electric vehicles, not if.