X-Ray Detection of Manganese Contamination in Lithium-Ion Batteries
A smart phone, a Chevy Volt, and a Boeing 787 Dreamliner have at least one thing in common; they use lithium-ion batteries to store energy. The three main components of lithium-ion batteries are the anode, the cathode, and the electrolyte. Scientists and engineers are constantly looking for ways to improve the properties of these components to enhance battery performance, targeting a safer airplane, a less-expensive electric car, and a longer battery life for our smart phone. Sanketh Gowda and his coworkers at the Center for Electrical Energy Storage (CEES) have studied the synergy between these components in a manganese-based lithium-ion battery. They used X-ray absorption spectroscopy to understand the chemical state of manganese that is lost from the positive electrode (cathode), when it is deposited on the negative electrode (anode). Their results shed light into important parameters that need to be considered in dealing with manganese loss in these batteries and, therefore, designing a better and longer lasting battery.
Lithium ions shuttle through the electrolyte, from the anode to the cathode and vice versa during discharge and charge cycles, respectively. The battery capacity is defined as the amount of lithium ions that can be intercalated into each electrode. The cathode of a lithium-ion cell is typically an oxide compound using elements such as cobalt, nickel, or manganese. Lithium manganese oxide (LMO) cathodes are abundant and low cost. LMO's structure allows lithium ions to intercalate easily, making it suitable for applications requiring high power, such as hybrid electric vehicles; for example, the Chevy Volt. However, LMO cathodes suffer from irreversible manganese loss during battery operation resulting in a rapid capacity fade of the cells, especially at high temperatures.
Although the manganese loss mechanism is not fully understood, evidence shows that manganese dissolution from the cathode into the electrolyte occurs in the form of positive manganese ions (Mn+2). During the charge cycle, the Mn2+ ions migrate to the anode and contaminate it, a process that causes degradation in battery performance. It is unclear how the contamination happens, but the oxidation state of the manganese ions is believed to play an important role in the functioning of the anode. Gowda and his coworkers have established whether manganese incorporates at the anode surface as a positive ion or is reduced to metallic manganese (Mn0).
"To determine why the capacity of LMO cells fade, we must understand the battery chemistry," said Gowda. To do this, the team used hard X-ray absorption spectroscopy at the Advanced Photon Source. This approach has several advantages. First, the technique is very sensitive to the oxidation state of manganese. Second, the X-rays can penetrate through 300 micrometers of material (about a tenth of the thickness of human skin), which means a typical graphite anode with a thickness of 50 micrometers can be fully analyzed. Finally, trace amounts of manganese can be detected.
"Careful sample preparation is required for such measurements, because even a trace amount of oxygen exposure can change the oxidation state of manganese," said Gowda.
With this technique, Gowda and his coworkers confirmed the presence of Mn0 on the surface of graphite anode particles. During the first few charging cycles, the electrolyte decomposes on the anode surface and forms a solid layer called the solid electrolyte interphase (SEI). A thin and dense SEI layer is desirable because it protects the surface of the anode from further reactions with the electrolyte. However, the presence of Mn0 in the SEI layer of the graphite electrodes would increase the electrical conductivity of the SEI and thus accelerate the electrolyte decomposition. This continuous growth of SEI can result in the loss of active lithium in the cell and, consequently, capacity fade.
Currently, manufacturers have to make up for the capacity fade by making bigger, heavier batteries and by cooling the system to reduce the manganese dissolution. This increases the cost, mass, and volume to the battery significantly. Having a fundamental understanding of the degrading mechanisms will lead to electrodes with better capacity retention, resulting in longer-lasting batteries, and eventually a cheaper Chevy Volt.
This work was supported by the Center for Electrical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy Office of Science’s Office of Basic Energy Sciences (BES). SR Gowda was supported by a Director’s Postdoctoral Fellowship at Argonne National Laboratory. Use of sector 20 facilities at the Advanced Photon Source and microscopy facilities at the Center for Nanoscale Materials at Argonne, supported by BES, is acknowledged. Electrodes for cell fabrication were obtained from Argonne’s Cell Analysis, Modeling and Prototyping Facility.
Gowda SR, KG Gallagher, JR Croy, M Bettge, M Thackeray, and M Balasubramanian. 2014. “Oxidation State of Cross-Over Manganese Species on the Graphite Electrode of Lithium-Ion Cells,” Physical Chemistry Chemical Physics 16:6898-6902. DOI: 10.1039/C4CP00764F