A new manufacturing technique is helping to overcome some of the major hurdles in developing more reliable and durable all-solid-state batteries. Combining a controlled sintering step with precise passivation, researchers from Paul Scherrer Institute PSI (Villigen, Switzerland; www.psi.ch) focused on densifying the sulfide-based solid electrolyte argyrodite (Li₆PS₅Cl), which is a promising battery material due to its high lithium-ion conductivity. However, its use is limited because of challenges in sufficiently densifying the material to eliminate voids where lithium dendrites (tiny spike-shaped structures) can form, which can wreak havoc on battery efficiency and safety.
“Our method is highly effective because it simultaneously addresses the two main bottlenecks that limit the performance of sulfide-based lithium-metal all-solid-state batteries: electrolyte microstructure and anode-electrolyte interfacial stability. First, we introduce a controlled sintering step for the argyrodite solid-electrolyte separator. This process drastically improves separator quality by collapsing residual voids and promoting particle rearrangement, increasing the pellet density to above 95%. As a result, grain boundary density is reduced, interfacial resistance is lowered, and mechanical integrity is significantly enhanced, mitigating crack formation,” explains Mario El Kazzi, head of the Battery Materials and Diagnostics group at the Paul Scherrer Institute PSI. This results in a material that both supports ionic transport and is protected against dendrite penetration.
Following sintering, the next step is to apply an ultrathin passivation layer of lithium fluoride (LIF) on the lithium metal anode. “This layer plays a dual role: chemically and electrochemically, LiF is stable at low potentials and electronically insulating, which suppresses solid electrolyte decomposition and limits the formation of resistive byproducts and electrochemically inactive lithium that drive capacity fading. Mechanically, its high interfacial energy acts as an effective barrier to dendrite nucleation and propagation,” notes El Kazzi.
Typically, densification methods for sulfide electrolytes employ uniaxial pressure at room temperature, but this new work shows the importance of mild thermal activation in homogeneous densification. “While some studies have explored densification at very high pressures and elevated temperatures, these conditions often compromise the solid electrolyte chemical stability. To our knowledge, this is the first work to show that mild sintering, performed at moderate temperature and pressure, can significantly delay lithium dendrite propagation while preserving the chemical integrity of the argyrodite electrolyte,” he adds.
So far, the team has demonstrated this technology on coin-sized cell configurations, starting with 350-micron solid argyrodite electrolyte pellets, which are sintered at 80ºC under 50 MPa for 6 hours. In the coming months, a new project will demonstrate the technology in pouch cells, with eventual pilot-scale implementation on the horizon. Demonstration in pouch cells “represents a critical step toward industrially relevant cell architectures,” adds El Kazzi. The team also believes this gentle sintering approach could be beneficial to a broad range of solid electrolytes, including halides and polymer-based formulations.