Solid-state cells could be a game changer
Battery technology is emerging as a key differentiator among electric vehicle projects. With most of the EV powertrain beyond the battery pack achieving efficiency figures in excess of 95%, improvements in battery tech offer the greatest potential for range and performance gains.
Incremental improvements have been made by adjusting the chemical makeup of a cell’s electrodes or electrolytes, or by optimizing the sizing, layout and structure of cells within a battery pack. However, none of these modest improvements compares to the giant performance leap promised by solid-state batteries.
Solid-state cells generally use the same lithium-ion-based chemical reaction to store and discharge energy as conventional cells. The difference lies in the electrolyte used to separate the anode and cathode and facilitate the movement of lithium ions. Conventional cells use a liquid-based electrolyte – commonly a lithium salt suspended in an organic solvent – while solid-state cells swap that out for a wafer-thin solid electrolyte, usually made from either ceramic, polymer or glass.
Removing the liquid electrolyte brings many potential benefits. Solid-state cells are lighter and more compact than their liquid-based counterparts, meaning pack weight can be reduced, or energy capacity increased. They should be more resistant to lithium dendrite formation, which will improve power discharge performance and raise potential charging speeds, along with extending the service life of the battery pack. In addition, once mass-manufacturing is achieved, they should be easier and quicker to make than conventional Li-ion cells thanks to the removal of solvents and having fewer production steps.
There are even more pronounced benefits offered by solid-state cells in the field of battery safety. Fires caused by defective or damaged lithium-ion cells have been widely publicized, including the current problems faced by the Chevrolet Bolt and its LG-sourced batteries. In many cases, these fires occur because internal failure or external damage has caused the volatile lithium electrolyte to become exposed to the outside air, causing it to ignite and setting off a chain reaction that can destroy the whole battery pack. Solid electrolytes sidestep these problems entirely and are highly resistant to fire and explosion – even in the event they are punctured or impacted.
While they offer many theoretical benefits, no company has yet demonstrated the ability to mass manufacture solid-state cells for light vehicles, with most still at the bench-test stage. There are still a number of issues left to solve including designing the solid electrolyte and electrodes in such a way that they interface evenly across their entire surface, because any warping can create gaps that limit cell efficiency. In addition, material stability has been shown to be a problem with the electrolyte’s brittleness leading to microscopic fractures that limit cell performance.
Blue Solutions, a subsidiary of France’s Bolloré, has won a contract to supply its solid-state cells for Daimler’s eCitaro G articulated urban bus – the first recorded commercial supply agreement for the technology. However, the optional solid state pack needs to be deliberately heated to a relatively high operating temperature of between 50C and 80C – sapping some range in the process and making it inappropriate for use in light vehicles with their unpredictable usage patterns.
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