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New Spectroscopic Technique Reveals the Dynamics of Operating Battery Electrodes Print
Wednesday, 29 January 2014 00:00

Developing high-performance batteries relies on material breakthroughs. During the past few years, various in situ characterization tools have been developed and have become indispensable in studying and the eventual optimization of battery materials. However, soft x-ray spectroscopy, one of the most sensitive probes of electronic states, has been mainly limited to ex situ experiments for battery research.

New Approach Yields New Insights

Real-world battery electrodes are integrated systems, including active materials, binder, electrolyte, and additives. Detecting the ion and electron dynamics in such a matrix has been a critical but technical challenge. Employing a high-precision laser carving system and commercial polymer electrolyte, researchers developed battery cells for operando soft x-ray spectroscopic experiments. By virtue of the high sensitivity of soft x-rays to the chemical and physical status of materials, they found distinct ion and electron dynamics in different battery cathodes. While some electrodes, e.g. Li(NiMnCo)O2, show uniform and fast charge dynamics during battery operation, others, e.g. LiFePO4, display strong nonuniformity and relaxation effects. The observation of phase nucleation close to the current collector contradicts the conventional understanding that the rate-limiting factor of battery operation is ion diffusion from the electrolyte/separator side.

The approach is already yielding insights into how lithium-­ion battery electrodes charge. Researchers have used it to study a lithium-­ion cathode made of nickel, manganese, and cobalt and found that its “electrochemical state of charge” is uniformly distributed across the electrodes as it charges. They also studied a different lithium-­ion cathode made of iron and phosphate​​—and found some surprises. The phase transformation starts near the current collectors, and then it takes hours of relaxation time before the electrodes’ “electrochemical state of charge” is uniformly distributed. This counters conventional wisdom about the charging process of lithium-­ion battery electrodes, which indicates that theoretical simulations and further experiments are needed to shed light on the phenomenon.

Recent ALS work could change this trend. Researchers have developed a new technique based on soft x-­ray spectroscopy that could help scientists better understand and improve the materials required for high-performance lithium-ion batteries. The technique measures something never seen before: the migration of ions and electrons in an integrated, operating battery electrode.

Over the past few years, scientists have developed several ways to study the changes in a working electrode. These include techniques based on hard x-rays, electron microscopy, neutron scattering, and nuclear magnetic resonance imaging. But most of these methods track structural changes. They don’t track electron and ion dynamics directly, which is very important in the push to understand and optimize battery performance.

Realization of operando soft x-ray spectroscopy of Li-ion batteries and the revealed charge dynamics in a LiFePO4 cathode during the charging process. Red and blue color in the oval indicate charged and discharged phases that can be detected by soft x-ray spectroscopy with position sensitivity.

Improving energy storage technology has become a critical and formidable challenge for modern sustainable energy applications, especially for electric vehicles. Lithium-ion battery technology provides a high-efficiency solution for energy storage. However, significant improvements in cost, safety, capacity, and power density are needed for current lithium-ion batteries to meet requirements for transportation applications. A practical lithium-ion battery electrode is a complex system consisting of active materials, electrolyte, binder, additives, and current collectors. Lithium-ion batteries operate with ion and electron migration through such an integrated matrix, leading to evolving chemical and physical states in electrodes throughout electrochemical cycles. Although ex situ techniques have provided much valuable information for understanding individual components in equilibrium states, the dynamics of lithium-ion batteries, as one integrated multi-component system, can only be characterized through in situ experiments. Tremendous efforts have been made to develop various in situ techniques based on hard x-ray, electron microscopy, neutron scattering, and nuclear magnetic resonance techniques for battery research. In particular, because of their penetration depth, hard x-ray techniques such as diffraction and absorption spectroscopy have received early and wide use for in situ studies of batteries.

This soft x-ray spectroscopy image reveals the chemical and physical changes in the electrode material during battery operation. The sensitive probe technique could clarify the charge and discharge mechanism of battery electrodes and map the migration of the electrons and ions.

Compared with hard x-ray and other techniques, soft x-ray spectroscopy is a more direct and efficient experimental probe of the electronic states near the Fermi level. In battery materials, these key electronic states in the vicinity of Fermi level fundamentally regulate the properties pertaining to battery performance, such as electron conductivity, ion diffusion, open-circuit voltage, safety, structural stability, and phase transformation. In particular, for transition-metal (TM) oxide-based cathodes, TM-3d and anion-p states can be directly detected using soft x-ray absorption spectroscopy (sXAS) through dipole-allowed transitions. However, the short penetration depth of soft x-rays requires ultrahigh-vacuum or high-vacuum environments. Several recent in situ studies using soft x-ray absorption, photoemission spectroscopies, and microscopy have generated important results on specially designed model electrochemical cells; however, in situ sXAS of a real lithium-ion battery cell remains challenging because of the complexity of lithium ion batteries.

Researchers Gao Liu (left) and Wanli Yang at ALS Beamline 8.0.1.

Scientists at the ALS and the Environmental Energy Technology Division (EETD) have developed an in situ system for sXAS studies of lithium-ion batteries. Two different electrode systems were deliberately selected and compared. The surface sensitivity of soft x-ray techniques was utilized for position-dependent studies of charge transportation during battery operation. Combining battery fabrication, morphology characterization, and in situ and ex situ sXAS techniques, the researchers were able to unveil the charge dynamics in battery electrodes that are regulated by charge transport, mesoscale morphology, and phase transformation. Additionally, the contrast between different electrode systems and the comparison between in situ and ex situ results provide soft x-ray fingerprints of metastable phases during the electrochemical process.

 

 


 

Research conducted by: X.S. Liu (ALS), D.D. Wang (EETD), G. Liu (EETD), V. Srinivasan (EETD), Z. Liu (ALS), Z. Hussain (ALS), and W.L. Yang (ALS)

Research funding: This work is supported by the Director Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences. Research was funded in part by the Energy Department’s Office of Energy Efficiency and Renewable Energy and Berkeley Lab’s Laboratory Directed Research and Development Program.

Publication about this research: X.S. Liu, D.D. Wang, G. Liu, V. Srinivasan, Z. Liu, Z. Hussain, and W.L. Yang,  Nature Communications 4, 2568 (2013).

Work was performed at Lawrence Berkeley National Laboratory, ALS Beamline 8.0.1.

ALS Science Highlight #282

 

ALSNews Vol. 349