Viscoelastic inorganic glass electrolyte paves the way for pressure-free all-solid-state batteries

A groundbreaking discovery in the realm of solid-state batteries has been accomplished by a team of researchers led by Professor Hu Yongsheng at the Institute of Physics (IOP) within the Chinese Academy of Sciences (CAS). Their investigation into a novel viscoelastic inorganic glass (VIGLAS) electrolyte has recently been published in the prestigious journal Nature Energy.

Solid-state batteries have often been hailed as the next big revolution in battery technology, offering a promising solution to the safety issues associated with traditional liquid lithium-ion batteries, all while significantly increasing energy density. This technological achievement has the potential to transform key industries such as electric vehicles, energy storage, and mobile devices.

Yet, the practical application of solid-state batteries faces several hurdles, including interface stability and manufacturing costs. Organic polymer-based solid-state batteries, for instance, excel in mechanical stability at the interface but falter in terms of chemical stability. This limitation restricts their energy density due to compatibility problems with high-voltage cathodes.

On the other hand, commercially viable inorganic sulfide-based solid-state batteries, while promising, come with the downside of high production costs and the need for extremely high pressures, reaching several tens of atmospheres. This poses substantial obstacles to their commercialization. Therefore, the quest for a novel electrolyte material that can effectively overcome these issues has become paramount in advancing solid-state battery technology.

Two ion transport mechanisms in LACO. Credit: IOP

In this study, the researchers have achieved a remarkable transformation of delicate room-temperature molten salts, namely LiAlCl4 and NaAlCl4, into viscoelastic glasses known as LiAlCl2.5O0.75 (LACO) and NaAlCl2.5O0.75 (NACO) by introducing oxygen atoms in place of certain chlorine atoms.

What sets this material apart is its extraordinary ability to bend and fold with ease at room temperature, challenging the previous assumption that inorganic solid electrolytes couldn’t exhibit the mechanical flexibility associated with organic ones. This discovery opens up an entirely new frontier in the development of solid-state electrolytes.

The researchers have unveiled both the formation and ion conduction mechanisms of these inorganic glasses, known as VIGLAS materials. These materials exhibit a glass transition temperature (Tg) lower than room temperature, resulting in a viscoelastic behavior reminiscent of polymers at ambient conditions. This low Tg can be attributed to the balanced oxygen-chlorine ratio, where oxygen bridges play a crucial role in forming appropriately sized Al-O-Al networks. These networks, in turn, limit atomic rearrangement during condensation.

Furthermore, the presence of trace amounts of uncoordinated LixAlCl3+x, acting as “plasticizers,” contributes to the reduction in Tg. In terms of ion conductivity, the oxygen bridges in VIGLAS materials shorten the distance between Li-Li pairs, thereby facilitating Li+ ion hopping. Additionally, VIGLAS electrolytes exhibit chain segment motion similar to that observed in PEO polymer electrolytes. This motion promotes collective Li+ ion migration in the vicinity, thereby enhancing ionic conductivity.

Cycling performance of Li- and Na-based all-solid-state batteries without additional stacking pressure. Credit: IOP

Importantly, VIGLAS materials not only exhibit remarkable deformability comparable to organic polymers but also inherit the desirable traits of conventional inorganic electrolytes. These traits include a high voltage tolerance of up to 4.3 V and impressive ionic conductivity of more than 1 mS/cm.

This inherent advantage effectively solves the challenges of mechanical and chemical stability at the positive electrode interface in solid-state batteries. As a result, it enables the groundbreaking achievement of true ambient temperature operation in inorganic all-solid-state lithium and sodium batteries without the need for external pressure (maintained at < 0.1 MPa).

Currently, the commercialization of solid-state batteries faces significant challenges associated with high manufacturing costs and complex production processes. Fortunately, this study presents an ideal solution. First, the production cost of this innovative solid-state electrolyte material is exceptionally low, primarily because its core component, aluminum, is abundant in the Earth’s crust.

This results in a material cost of only $6.85 per kilogram for LACO and only $1.95 per kilogram for NACO. At 2% and 0.6%, respectively, these costs represent a fraction of the current mainstream Li6PS5Cl solid-state electrolyte, which has a high price tag of $319 per kilogram.

Another advantage is that these VIGLAS materials have a low melting point, below 160℃, allowing them to effectively infiltrate porous electrodes like a liquid under suitable heating conditions. This capability makes it possible to achieve commercial cathode loadings in excess of 20 mg/cm2.

Furthermore, these materials exhibit ductility similar to organic polymers, enabling the manufacture of large-area electrolyte films using techniques such as roll-to-roll processes. These remarkable characteristics make this novel solid-state electrolyte material highly competitive in terms of both material and manufacturing costs, making it an ideal choice to address the challenge of pressure-free operation in all-solid-state battery cathodes.

Source: Chinese Academy of Sciences

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