In theory, solid-state batteries offer two key advantages over conventional lithium-ion batteries: first, they contain no flammable liquid components, making them significantly safer to operate. Second, they promise higher energy densities thanks to a thin lithium-metal anode, which allows more energy to be stored in the battery. This, in turn, can enable greater ranges or lighter vehicles.
This is why the technology is considered pivotal for the future of electric vehicles (EVs). Volkswagen, for instance, invested in the US specialist QuantumScape as early as 2012 and plans to license its technology with the aim of producing solid-state cells through its battery subsidiary PowerCo. Meanwhile, US battery developer Factorial has secured Mercedes-Benz and Stellantis as strategic investors.
However, two key challenges have so far hindered the market readiness of solid-state batteries: first, the formation of lithium dendrites at the anode poses a critical issue. These are tiny, needle-like metal structures that can penetrate the lithium-ion-conducting solid electrolyte between the electrodes, spread towards the cathode, and cause short circuits within the battery. Second, there is electrochemical instability at the interface between the lithium-metal anode and the solid electrolyte, which affects the long-term performance and reliability of the battery.
Researchers at the Paul Scherrer Institute (PSI) in Switzerland have now addressed these two challenges and developed a new manufacturing process. Mario El Kazzi, Head of the Battery Materials and Diagnostics Group at PSI, explains: “We combined two approaches that, together, both densify the electrolyte and stabilise the interface with the lithium.”
At the heart of the PSI study is the argyrodite-type Li₆PS₅Cl (LPSCl), a sulphide-based solid electrolyte composed of lithium, phosphorus, and sulphur. This mineral exhibits high lithium-ion conductivity, enabling rapid ion transport within the battery—a key requirement for high performance and efficient charging. These properties make argyrodite-based electrolytes promising candidates for solid-state batteries.
Until now, two methods for densifying the solid electrolyte have been used, each with undesirable side effects: pressing at room temperature is insufficient, as it leads to a porous microstructure and excessive grain growth. Processing at very high temperatures above 400 degrees, on the other hand, risks decomposing the solid electrolyte.
The PSI researchers therefore devised a new approach: they compressed the material at moderate temperature and pressure. This results in fewer gaps in the material, making it harder for dendrites to grow through. Additionally, the researchers evaporated a mere 65-nanometre-thin coating of lithium fluoride (LiF) under vacuum and applied it uniformly as an ultra-thin film onto the lithium surface. This layer protects the interface and helps avoid chemical issues.
The new method performs exceptionally well: in laboratory tests, the battery cell retained around 75 per cent of its capacity after 1,500 charge and discharge cycles. PSI therefore sees strong potential for solid-state batteries to soon surpass conventional lithium-ion batteries with liquid electrolytes in terms of energy density and durability.
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