Researchers at the Paul Scherrer Institute PSI have achieved a breakthrough on the path to practical application of lithium metal all-solid-state batteries – the next generation of batteries that can store more energy, are safer to operate, and charge faster than conventional lithium-ion batteries.

All-solid-state batteries are considered a promising solution for electromobility, mobile electronics, and stationary energy storage – in part because they do not require flammable liquid electrolytes and therefore are inherently safer than conventional lithium-ion batteries.

Two key problems, however, stand in the way of market readiness: On the one hand, the formation of lithium dendrites at the anode remains a critical point. These are tiny, needle-like metal structures that can penetrate the solid electrolyte conducting lithium ions between the electrodes, propagate toward the cathode, and ultimately cause internal short circuits. On the other hand, an electrochemical instability – at the interface between the lithium metal anode and the solid electrolyte – can impair the battery’s long-term performance and reliability.

To overcome these two obstacles, the team led by Mario El Kazzi, head of the Battery Materials and Diagnostics group at the Paul Scherrer Institute PSI, developed a new production process: “We combined two approaches that, together, both densify the electrolyte and stabilise the interface with the lithium,” the scientist explains. The team has reported these results in the journal Advanced Science.

The problem with densification

Central to the PSI study is the argyrodite type Li₆PS₅Cl (LPSCl), a sulphide-based solid electrolyte made of lithium, phosphorus, and sulphur. The mineral exhibits high lithium-ion conductivity, enabling rapid ion transport within the battery – a crucial prerequisite for high performance and efficient charging processes. This makes argyrodite-based electrolytes promising candidates for solid-state batteries. Up to now, however, implementation has been hampered by the difficulty of densifying the material sufficiently to prevent the formation of voids that lithium dendrites could penetrate.

To densify the solid electrolyte, research groups have relied on one of two approaches: applying very high pressure to compress the material at room temperature or employing processes that combine pressure with temperatures exceeding 400 degrees Celsius. In the latter approach, known as classical sintering, the application of heat and pressure causes the particles to fuse into a denser structure.

Both methods, however, can lead to undesirable side-effects: Compression at room temperature is insufficient because it results in a porous microstructure and excessive grain growth. Processing at very high temperatures, on the other hand, carries the risk of breaking down the solid electrolyte. Therefore the PSI researchers had to pursue a new approach to obtain a robust electrolyte and a stable interface.

The temperature trick

To densify argyrodite into a homogeneous electrolyte, El Kazzi and his team did incorporate the temperature factor, but in a more careful way: Instead of the classic sintering process, they chose a gentler approach in which the mineral was compressed under moderate pressure and at a moderate temperature of only about 80 degrees Celsius. This gentle sintering proved successful: The moderate heat and pressure ensured that the particles arranged themselves as desired without altering the material’s chemical stability. The particles in the mineral formed close bonds with each other, porous areas became more compact, and small cavities closed. The result is a compact, dense microstructure resistant to the penetration of lithium dendrites. Already, in this form, the solid electrolyte is ideally suited for rapid lithium-ion transport.

However, gentle sintering alone was not enough. To ensure reliable operation even at high current densities, such as those encountered during rapid charging and discharging, the all-solid-state cell required further modification. For this purpose, a coating of lithium fluoride (LiF), only 65 nanometres thick, was evaporated under vacuum and applied uniformly to the lithium surface – serving as a ultra-thin passivation layer at the interface between the anode and the solid electrolyte.

This intermediate layer fulfils a dual function: On the one hand, it prevents the electrochemical decomposition of the solid electrolyte upon contact with the lithium, thus suppressing the formation of “dead,” inactive lithium. On the other hand, it acts as a physical barrier, preventing the penetration of lithium dendrites into the solid electrolyte.

Best results after 1,500 cycles

In laboratory tests with button cells, the battery demonstrated extraordinary performance under demanding conditions. “Its cycle stability at high voltage was remarkable,” says doctoral candidate Jinsong Zhang, lead author of the study. After 1,500 charge and discharge cycles, the cell still retained approximately 75 percent of its original capacity. This means that three-quarters of the lithium ions were still migrating from the cathode to the anode. “An outstanding result. These values ​​are among the best reported to date.” Zhang therefore sees a good chance that all-solid-state batteries could soon surpass conventional lithium-ion batteries with liquid electrolyte in terms of energy density and durability.

Thus El Kazzi and his team have demonstrated for the first time that the combination of solid electrolyte mild sintering and a thin passivation layer on lithium anode effectively suppresses both dendrite formation and interfacial instability – two of the most persistent challenges in all-solid-state batteries. This combined solution marks an important advance for all-solid-state battery research – not least because it offers ecological and economic advantages: Due to the low temperatures, the process saves energy and therefore costs. “Our approach is a practical solution for the industrial production of argyrodite-based all-solid-state batteries,” says El Kazzi. “A few more adjustments – and we could get started.”