The metal manganese could help to build rechargeable batteries that are more affordable and environmentally friendly than existing batteries. However, one of the major challenges is that they tend to degrade too quickly to be attractive for electric vehicles and other applications. Now researchers using the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, have looked in detail at the degradation mechanism and proposed a way to counteract it.
During charging and discharging, the manganese in a battery’s cathode tends to dissolve and then redeposit in ways that reduce the device’s capacity over time. In commercially available batteries, scientists have not been able to track the evolution of material on the surface of the electrodes during operation and have been limited to looking at the effects after the cycle was complete. To overcome that problem and observe the material changes as they happen, the team placed a lithium manganese oxide cathode built on carbon paper into an aqueous, lithium-based electrolyte. That electrolyte sped up the dissolution process, allowing the researchers to study it in operation over a short cycling period.
The research team saw that the rate of dissolution and redeposition of the manganese changed as the voltage running through the electrode increased, with different mechanisms dominating different voltage ranges. Below 1 V, against a silver/silver chloride reference electrode, the main activity was dissolution of the metal into the electrolyte, owing to an effect called Jahn-Teller distortion, in which the length of bonds between atoms changes. Between 1 and 1.2 V, surface reconstruction becomes dominant and facilitates the dissolution rate. Between 1.2 and 1.55 V, both factors collectively contribute to the dissolution process.
The researchers also wanted to know where the manganese went when it returned to the surface of the electrode. Was it deposited at random, or back to where it came from, or did it concentrate in certain areas? They found that the answer changed with progressive cycles. Early on, the metal was redeposited heterogeneously, and gradually built up into areas of high manganese concentration. After several hours of cycling, however, the distribution of the material became more uniform.
Having seen how the state of the electrode evolved during cycling, the team tested a method they thought might make the process run more smoothly and improve the battery’s performance. They added Nafion, a sulfonated polymer. Nafion has been widely used in fuel cells and catalysts as a coating to reduce unwanted reactions and slow down surface degradation. Their idea was that the polymer would act as a coating layer to insulate the cathode particles against protons in the electrolyte, which contributes to the dissolution process. Sure enough, simply adding Nafion when they were crafting the electrode reduced the electrode’s loss of capacity during cycling.
To watch the process in action, the researchers performed X-ray fluorescence microscopy (XFM) at beamline 2-ID-E at the APS. Because it’s a fluorescence technique, the photons emitted when the X-ray strikes a particular element have a particular energy level, allowing researchers to identify the manganese, in this case. They scanned the X-ray beam over the sample, allowing them to measure the distribution of the metal in two dimensions, using a beam size that provided them with the needed spatial resolution for their study.
The team also performed hard and soft X-ray absorption spectroscopy at DOE’s SLAC National Accelerator Laboratory, which told them about the oxidation state of the manganese during cycling. Additionally, they performed transmission electron microscopy at DOE’s Brookhaven National Laboratory, which provided a detailed picture of the atomic structure of the electrode, complementing the X-ray studies. – Neil Savage
See: Y. Zhang1, A. Hu1, D. Xia1, S. Hwang2, S. Sainio3, D. Nordlund3, F. M. Michel1, R. B. Moore1, L. Li4, F. Lin1, “Operando characterization and regulation of metal dissolution and redeposition dynamics near battery electrode surface,” Nat. Nanotechnol. 18, 790-797 (2023)
Author affiliations: 1Virginia Tech; 2Brookhaven National Laboratory; 3SLAC National Accelerator Laboratory; 4Argonne National Laboratory.
The work was supported by the National Science Foundation under CBET 1912885 (F.L.). This research used resources of the Advanced Photon Source, US Department of Energy (DOE), Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. The use of the Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory, was supported by the US DOE, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-76SF00515. This research used the electron microscopy resources of the Center for Functional Nanomaterials (CFN), US DOE, Office of Science User Facility, at Brookhaven National Laboratory under contract no. DE-SC0012704. Y.Z. and F.L. thank the beamline scientist R. Davis at SSRL for help with the hard XAS measurements.
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