Research Laboratory news

Aenert. Research Laboratory news
Nowadays many major players as well as start-ups are actively involved in the electric vehicles industry and racing to develop safer and more efficient batteries. One type of battery which has received increased attention lately is solid-state batteries. Other than lithium-ion batteries, they use a solid electrolyte, which allows for leaving out a separator component used for keeping the positive electrode and the negative electrode apart.
Solid-state batteries are seen as safer alternative to lithium-ion batteries because they do not need a flammable liquid electrolyte required in traditional lithium-ion batteries. This greatly reduces the risk of fire. Moreover, they are lighter than traditional batteries, have a shorter charging time and are expected to allow EVs to run over longer distances. However, while solid-state batteries have proved to be a promising candidate in terms of performance, their lifespan may be shorter than conventional batteries’, as they can eventually form cracks and need replacing.

To tackle this problem, scientists at the Argonne National Laboratory have now (2023) launched a study focusing on a group of electrolytes called argyrodites, a class of solid-state electrolytes containing sulphur. Argyrodites have several advantages over other solid-state electrolytes. They are endowed with higher ionic conductivity and can quickly transport ions through a battery. This, in turn, can enable a faster charge rate for electric vehicles.  Argyrodites are also easier and cheaper to process into the pellets used in batteries.

Argyrodites, however, are difficult to manufacture. They are highly reactive with air and therefore handling them in a battery production plant can be difficult. Also, they easily react with other electrode materials such as lithium. When these reactions occur, they produce chemicals which lower the quality of the electrolyte/electrode interfaces. The reactions can also inhibit the movement of lithium ions, reduce battery performance and cause dendrites, needle-like lithium structures which decrease battery safety and duration.

Therefore, the researchers wanted to develop a new method which would enable to precisely design the chemistry of the argyrodite’s surface. They chose the atomic the layer deposition process commonly used in the chip production industry and adapted it to their purposes. This coating method uses chemical vapors that react with the surface of a solid material to form a thin film.

Through atomic layer deposition, the argyrodite electrolyte was coated in powder form. The powder was heated and exposed to water vapor and trimethyl aluminum, producing a thin coating of alumina (aluminum oxide) on all of the individual electrolyte particles. Also, a characterisation technique called X-ray absorption spectroscopy was employed to make sure that the coating did not disrupt the chemical structure of the underlying argyrodite. This involved shooting the material with synchrotron X-ray beams and measuring the transmission and absorption of the X-rays in the material.

Moreover, the researchers used two techniques to determine that the coatings adapted well to the contours of individual electrolyte particles. The first technique, called scanning transmission electron microscopy, created images of the material structure using a focused electron beam.

The second technique, known as energy-dispersive X-ray spectroscopy, evaluated the elements in the material by detecting X-rays emitted from the electrons used in the scanning transmission electron microscopy technique.  The researchers also found that the coatings dramatically reduced the powder’s reactivity with air.

In a next step, the researchers pressed the coated powders into pellets and installed the pellets in a laboratory-scale battery cell with an anode (negative electrode) made of lithium metal. They repeatedly charged and discharged this battery as well as another battery made with uncoated electrolytes, comparing their performance.

For many years, scientists have tried to improve the efficiency of solid-state batteries to make them a competitive player in the global markets. In 2023, scientists sought to find means to improve lithium ion transport pathways by means of a solid electrolyte. They used the significantly higher neutron attenuation coefficient of one of the most abundant stable isotopes of lithium, ⁶Li, to their advantage and performed neutron imaging on a purpose built all‐solid‐state lithium–sulfur battery. By using a higher ⁶Li content in the anode and employing natural lithium in the solid electrolyte separator and the cathode, the contrast was enhanced which helped discerm, during the initial discharge, which of the mobile lithium ions diffused through the cell from the anode and which were initially contained in the solid electrolyte. As neutrons are sensitive to the different lithium isotopes, operando neutron radiography revealed the lithium-ion diffusion through the cell while in situ neutron tomography was able to present the distribution of the trapped lithium ions inside the cell in charged and discharged states.

Image: a) Representation of the cell setup for neutron imaging, with cell composition displayed in the expanded regions. b) Radiography images showing neutron attenuation for a cell where the Li–In anode is enriched with ⁶Li (top) and one where all Li is the naturally occurring isotopic mix (bottom). The images are displayed within the same contrast range and show a cell in the pristine state. c) Plots displaying the cross‐section of the median neutron attenuation (Σmedian) across the images displayed in (b)

Source: Robert Bradbury, Nikolay Kardjilov, Georg F. Dewald, Alessandro Tengattini/ Visualizing Lithium Ion Transport in Solid‐State Li–S Batteries Using Li Contrast Enhanced Neutron Imaging/ Advanced Functional Materials 33(38), June 2023/ DOI:10.1002/adfm.202302619/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

Also in 2023, scientists used large-scale molecular dynamics simulations to study and reveal the atomistic pathways and energy barriers of lithium crystallisation at the solid interfaces. Lithium crystallisation was found to take multi-step pathways mediated by interfacial lithium atoms with disordered and random-closed-packed configurations as intermediate steps, which caused the energy barrier of crystallisation. This allowed the applicability of Ostwald’s step rule, i.e. the formation of polymorph structures, to be extended to interfacial atom states, and enabled a rational strategy for lower-barrier crystallisation by promoting favourable interfacial atom states as intermediate steps through interfacial engineering.

Image: A schematic of multiple-step pathways of Li crystallization The Li⁺ (orange, anion shown in red) in solid electrolytes (SE) goes through disordered-Li (cyan) and/or rHCP (random hexagonal close-packed)-Li (green) in the interfacial Li layer at the SE interface, and transforms into the crystalline BCC (body-centered cubic)-Li metal (blue)

Source: Menghao Yang, Yunsheng Liu, Yifei Mo/ Lithium crystallization at solid interfaces/ Nature Communications 14(1), May 2023/ DOI:10.1038/s41467-023-38757-2/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

Coating the electrolyte in solid-state batteries has several advantages: The team found that it  significantly decreased the reactivity of the electrolyte with the lithium anode. It also reduced the rate at which electrons leak out of the electrolyte. This is important because leaking electrons are believed to result in reactions which can form dendrites. Together, the coating’s benefits can significantly increase the number of times a solid-state battery can charge and discharge before its performance begins to degrade. The scientists believe that the coating enables the electrolyte to make better contact with the anode. They also observed an unexpected benefit of the coating: It could double the ionic conductivity of the electrolyte.

The successful completion of the study is a large contribution towards promoting solid-state battery technology. The coating technique can be used with different electrolytes and coatings, which could bring forth a wide range of different solid-state battery types.

By the Editorial Board