(a,b) The schematic depiction of experimental setup of the in situ cell for simultaneous cycling and X-ray spectroscopic measurement. An array of holes with 50 μm diameter is drilled with high-precision laser on the current collector. The incident soft X-ray beam and excited fluorescence photon pass through the array of holes on the current collector. (c) A photo of the in situ soft X-ray setup for studying LIBs. (d,e) SEM and energy dispersive X-ray spectroscopy images of the top surfaces of the in situ cells after the cathode layers formed with the slurry process for NMC (d) and LFP (e) electrodes. The scale bars are 100 μm in the left panels and 1 μm in the right panels, respectively. Images shown here are after one electrochemical cycle. NMC forms large secondary particles that are composed of agglomerated primary smaller particles. Electrolyte matrix forms microdomains in NMC (d), whereas it is superficially distributed on LFP nanoparticles (e). Liu et al. Click to enlarge.

Over the past several 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.

Improving energy storage technology has become a critical and formidable challenge for modern sustainable energy applications, especially for electric vehicles. Lithium-ion battery (LIB) technology provides a high-efficiency solution for energy storage. However, significant improvements in cost, safety, capacity and power density are needed for current LIBs to meet the requirements for transportation applications. A practical LIB electrode is a complex system consisting of active materials, electrolyte, binder, additives and current collectors. LIBs operate with ion and electron migration through such 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 LIB, as one integrated multicomponent 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 for battery research. In particular, because of its penetration depth, hard X-ray techniques such as diffraction, and absorption spectroscopy have received early and wide use for in situ studies of batteries.

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 this work, we have developed an in situ system for sXAS studies of LIBs. Two different electrode systems are deliberately selected and compared. The surface sensitivity of soft X-ray techniques is utilized for position-dependent studies of charge transportation during battery operations. Combining battery fabrication, morphology characterization, and in situ and ex situ sXAS techniques, we are 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.

—Liu et al.

The new technique relies on the Advanced Light Source, a Department of Energy national user facility located at Berkeley Lab that generates intense light for scientific research. The facility emits soft X-rays, which are very good at interacting with electrons. Because of this, soft X-ray spectroscopy is an ideal probe for studying battery characteristics such as electron and ion diffusion.

However, soft X-rays are so-named because they don’t penetrate very far, only about 100 nanometers, which isn’t far enough to study an electrode covered by a metal foil.

To overcome this, the spectroscopists at the Advanced Light Source worked with battery experts in the Environmental Energy Technologies Division. The team employed a laser system to etch a special detection window on the top foil layer. This allows soft X-rays to penetrate into an electrode for measurement.

This research was performed at beamline 8.0.1 at the Advanced Light Source, with battery cells assembled at the Environmental Energy Technologies Division. It 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.

The Advanced Light Source is a third-generation synchrotron light source producing light in the x-ray region of the spectrum that is a billion times brighter than the sun.

Resources

• Xiaosong Liu, Dongdong Wang, Gao Liu, Venkat Srinivasan, Zhi Liu, Zahid Hussain & Wanli Yang (2013) “Distinct charge dynamics in battery electrodes revealed by in situ and operando soft X-ray spectroscopy,” Nature Communications 4, Article number: 2568 doi: 10.1038/ncomms3568