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One of the most quickly evolving areas in energy science is batteries. Batteries play an important part in reducing the effects of climate change, as they can be applied in electric vehicles and used for energy storage in the electric grid.

Lithium metal batteries are among the most promising types of batteries in today's energy market. These anodes have a far greater energy storage capacity compared to graphite anodes but are susceptible to reduced performance after many charge-discharge cycles.

Now (2023), researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory wanted to get a better understanding of battery functioning and used an advanced X-ray technique to have a look at the movements of components inside an operating battery cell. This was the first time that scientists were able to observe these movements in detail while the battery charged and discharged.

The research was aimed at unraveling two problems often encountered in lithium-metal batteries: battery swelling and shrinking during charging and discharging as well as the build-up of detrimental materials on the surface of the anode. Therefore, the scientists had a close look at the movements of the anode, the cathode (positive electrode) and separator inside an operating battery cell.

When a lithium metal battery cell charges, lithium ions move from the cathode through the electrolyte to the anode. The anode expands as lithium atoms are deposited on its surface. During discharging, the anode contracts as lithium is stripped away. This back-and-forth process is normal.

During battery charging, the anode expands as a result of lithium atom deposition on its surface. When the battery is discharging, the lithium is stripped away and the anode contracts, which stipulates a constant back-and-forth process. In an ideal battery, a uniform layer of lithium would be reversibly deposited over a thousand cycles. As lithium is a highly reactive material, the metal quickly reacts with the electrolyte interphase once deposited on the anode. This forms a layer of material over the lithium called the solid electrolyte interphase. This interphase separates the electrolyte and lithium metal so they do not continuously react, as such reactions could eventually consume the entire electrolyte and inhibit the proper functioning of the cell.

However, the interphase material can pose a problem if it gets too thick and the deposited lithium starts more reactions, which leads to more interphase layers. These layers can grow into one another and form little pores. These pores add to the volume of the interphase.

To get greater insight, energy dispersive X-ray diffraction was used at Argonne’s Advanced Photon Source. An X-ray beam was directed into a lithium-ion battery cell as it was repeatedly charged and discharged. The cell used was a coin cell with an anode made of lithium metal and a cathode made of a metal oxide material. The changing patterns of the scattered X-ray beams were measured which enabled the scientists to analyse changes in the atom arrangements in the materials of the cell. Based on these arrangement changes, the movements of the cell’s components could be quantified and analysed.

Scientists have long tried to improve the efficiency of lithium-metal batteries. In 2021, the qualities of Sb as interfacial layer were analysed. The material was found to fulfill this function very well as it allowed for superior wetting of Li onto a LLZO surface, resulting in a remarkably low Li/LLZO interfacial resistance of 4.1(1) Ω cm². Using soft and hard X-ray photoelectron spectroscopy and focused ion beam time-of-flight secondary ion mass spectrometry they gained an atomistic insight into Sb-coated LLZO interface as well as the formation of a Li-Sb alloy as an interlayer. It was also found that the Li/Sb-coated LLZO/Li symmetrical cells had a high critical current density of up to 0.64 mA cm⁻² and low overpotentials of 40–50 mV at a current density of 0.2 mA cm⁻² without applying external pressure. The electrochemical performance of Sb coated-LLZO pellets was also assessed with an intercalation-type V2O5 cathode. Li/Sb-coated-LLZO/V₂O₅ full cells were found to provide stable capacities of around 0.45 mAh cm⁻², with a peak current density of 0.3 mA cm⁻².

Image: a) Voltage profiles of Li/LLZO/Li symmetrical cells comprising Sb-coated and uncoated LLZO pellets at current densities of 0.05 and 0.1 mA cm⁻² (first 2 cycles) and 0.2 mA cm⁻² (from 3 rd cycle onward). The measurements were performed at room temperature, without the employment of external pressure, and with a capacity limitation of 0.1 mAh cm −2 per half-cycle. b) Cross-section SEM images of a pristine and cycled symmetrical cell composed of a heat-treated LLZO pellet without Sb coating. c) Cross-section SEM micrographs of a pristine and cycled sample symmetrical cell comprising Sb-coated LLZO pellet



Source: Romain J.-C. Dubey, Jordi Sastre, Claudia Cancellieri/ Building a Better Li‐Garnet Solid Electrolyte/Metallic Li Interface with Antimony/ Advanced Energy Materials11(39):2102086, September 2021/ DOI:10.1002/aenm.202102086/ Open Access This is an Open Access article is distributed under the terms of the Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

In 2022, scientists achieved a highly stable lithium-metal battery (LMB) employing garnet-type oxide electrolyte by introducing a carbon-based interlayer. They demonstrated through theoretical calculations and experiments that the design effectively regulated Li deposition away from the solid electrolyte, thus preventing dendrite penetration. They also showed how delicate the interface condition between the interlayer and solid electrolyte was and worked out an effective strategy to achieve an optimal interface. The garnet-type oxide-based LMB exhibited a high energy density of ~ 680 Wh/L for over 800 cycles at room temperature without using external pressure.

Image: (a, b) Two scenarios of Li deposition in LMBs with interlayer. (a) LLZTO side plating, (b) current collector side plating. (c) Atomic model of LLZTO/Li interface



Source: Ju-Sik Kim, Gabin Yoon, Sewon Kim, Shoichi Sugata/ Room-temperature–low-pressure-operating high-energy lithium metal batteries employing garnet-type solid electrolytes and anode interlayers/ Samsung Advanced Institute of Technology, April 2022/ DOI:10.21203/rs.3.rs-1551346/v1/ 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)

Analysing the mechanisms inside a lithium-metal battery has an important bearing on improving the technology underlying these batteries: In this study, the researchers demonstrated that energy dispersive X-ray diffraction could be used to quantify the amount of reversible lithium plating and stripping in an operating battery cell as well as the amount of lithium metal deposited on the interphase. This is extremely valuable as it can also be used to screen for the most effective advanced electrolytes. Moreover, the X-ray technique can make out problems concerning the separator compression caused by expanding battery components. This compression can cause the pores of the separator to close, which can slow down or even stop the movement of lithium ions between the electrodes. It can also be used to determine lithium concentration gradients in the cathode.

The next research step is to use the technique to evaluate how next-generation electrolytes can optimise lithium deposition and the interphase layer. The scientists also want to research how different anode surfaces — like copper — can alter and improve these processes.

By the Editorial Board