Lithium-ion batteries have been a staple in device manufacturing for years, but the liquid electrolytes they rely on to function are quite unstable, leading to fire hazards and safety concerns. Now, researchers at Penn State are pursuing a reliable alternative energy storage solution for use in laptops, phones and electric vehicles: solid-state electrolytes (SSEs).
According to Hongtao Sun, assistant professor of industrial and manufacturing engineering, solid-state batteries - which use SSEs instead of liquid electrolytes - are a leading alternative to traditional lithium-ion batteries. He explained that although there are key differences, the batteries operate similarly at a fundamental level.
"Rechargeable batteries contain two internal electrodes: an anode on one side and a cathode on the other,"" Sun said. "Electrolytes serve as a bridge between these two electrodes, providing fast transport for conductivity. Lithium-ion batteries use liquid electrolytes, while solid-state batteries use SSEs."
Solid-state batteries offer improved stability and safety when compared to traditional lithium-ion batteries but face several manufacturing and conductivity challenges, Sun explained. For example, the high temperatures introduced in the fabrication process, especially with ceramic-based SSEs, can hinder their production and practical implementation.
To overcome this challenge, Sun and his team used a technique known as cold sintering - a process where powdered materials are heated, treated with a liquid solvent, and compressed into a denser form - to incorporate a highly conductive ceramic-polymer composite SSE known as LATP-PILG. The method is referred to as "cold" because it operates at significantly lower processing temperatures than traditional sintering, instead relying on applied pressure and a small amount of liquid solvent to complete the process. They published their approach in Materials Today Energy.
Traditional ceramic-based SSEs are typically composed of polycrystalline grains - materials made up of hundreds of tiny crystals - separated by grain boundaries. According to Sun, these grain boundaries are considered defects that hinder the transport of conductive ions. To reduce conduction loss in ceramic-based SSEs, Sun's team co-sintered a poly-ionic liquid gel (PILG) with LATP ceramics to form a polymer-in-ceramic composite SSE, an ideal material for use due to its stability and high conductivity.
The PILG acts as a highly conductive "grain boundary" in the SSE, facilitating ion transport across engineered boundaries rather than through defect-prone natural interfaces. Sun said the team initially attempted to use traditional high temperature sintering to develop their new SSEs, but they immediately ran into problems.
"One of the fabrication challenges of LATP-based composite SSEs is that the sintering temperature for ceramic is very high, to the point that traditional sintering would actually burn up any additives such as the polymer compound before the ceramic could be properly densified," Sun said. "This is why we had to implement cold sintering, to keep temperatures much lower."
