Researchers at the Pacific Northwest National Laboratory have led a multi-institutional effort to develop a transmission electron microscopy (TEM) technique that can provide insights into the structural and chemical evolution of electrode materials of real batteries in real time.
In a paper published in the ACS journal Nano Letters, the authors note that TEM studies of lithium-ion batteries over the past few years have used an open-cell configuration in which the electrolyte is either solid lithium oxide or an ionic liquid, which is point-contacted with the electrode. This cell design is inherently different from a real battery, in which liquid electrolyte forms conformal contact with electrode materials. As a result, the knowledge gleaned from open cells can deviate significantly from that from a real battery; the new operando TEM electrochemical liquid cell is designed to address this issue.
One of the fundamental challenges for the battery research is the direct observation of the structural and chemical evolution of the components of the battery and how this directly correlates with the battery properties. Traditionally, observation of the structural and chemical evolution of the battery has mostly relied on ex-situ or post-mortem studies, which in many cases can provide key insights with respect to the structural changes of the battery materials but is lacking in dynamic information. Over the past few years, tremendous progress has been made toward developing methodologies for in situ direct observation of structural and chemical evolution of electrodes used for lithium ion batteries. Most notably, the development of an in situ TEM cell that is based on an open-cell configuration using a single nanowire and either an ionic liquid or Li2O as the electrolyte.
… However, three typical deficiencies are associated with the open-cell configuration. First, for the open cell, the electrolyte is only in point contact with the electrode, which may inadvertently modify the diffusion pattern of Li ions in the electrode, and therefore, what has been obtained is not necessarily representative of the case where the electrode is fully immersed in the liquid electrolyte in a real battery. Second, for the case of using Li2O as the electrolyte, a large overpotential is normally applied to drive the Li ions into the electrode, which may change the kinetics and phase behaviors of solid-state electrode lithiation. Third, the use of the ionic liquid or Li2O electrolyte excludes some of the fundamental processes which only occur in real electrolytes and the battery operating conditions, such as the interaction between electrolyte and electrode and the SEI layer formation. To address the shortcomings of the open cell, in this work we describe a liquid-cell based approach for in situ or more precisely operando TEM studies of lithium ion batteries using a battery relevant liquid electrolyte and a lithium metal counter electrode.—Gu et al.
To demonstrate the new technique, they studied the lithiation/delithiation behavior of single Si nanowires. While some of lithiation/delithation behaviors of Si obtained using the liquid cell are consistent with the results from the open-cell studies, the team also discovered new insights different from the open cell configuration. These included the dynamics of the electrolyte and, potentially, a future quantitative characterization of the solid electrolyte interphase (SEI) layer formation and structural and chemical evolution.
The liquid cell gave us global information about how the electrodes behave in a battery environment, and it will help us find the solid electrolyte layer. It has been hard to directly visualize in sufficient detail.—Chongmin Wang, PNNL
Wang and his colleagues at the Joint Center for Energy Storage Research (JCESR) developed a wet battery cell in a transmission electron microscope at the Environmental Molecular Sciences Laboratory (EMSL), the DOE’s Environmental Molecular Sciences Laboratory on the PNNL campus. The battery had one silicon electrode and one lithium metal electrode, both contained in a bath of electrolyte.
When the team charged the battery, they saw the silicon electrode swell, as expected. However, under dry conditions, the electrode is attached at one end to the lithium source—and swelling starts at just one end as the ions push their way in, creating a leading edge. In this study’s liquid cell, lithium could enter the silicon anywhere along the electrode’s length. The team watched as the electrode swelled all along its length at the same time.
The total amount the electrode swelled was about the same, though, whether the researchers set up a dry or wet battery cell. That suggests researchers can use either condition to study certain aspects of battery materials.
We have been studying battery materials with the dry, open cell for the last five years. We are glad to discover that the open cell provides accurate information with respect to how electrodes behave chemically. It is much easier to do, so we will continue to use them.—Chongmin Wang
The researchers were unable to see the SEI layer in this initial experiment. In future experiments, they will try to reduce the thickness of the wet layer by at least half to increase the resolution, which might provide enough detail to observe the solid electrolyte interphase layer.
This liquid-cell nanobattery approach provides indispensable complementary information to the widely applied open-cell approach. The complete electrochemical process can be more fully understood by combining open-cell and liquid-cell battery TEM techniques. The open-cell approach provides important information regarding the composition, phase transformation, and atomic resolution structural changes of the electrode itself, allowing high-resolution microscopy to be obtained. On the other hand, the liquid cell allows the usage of any form of liquid electrolyte that is relevant to real battery and full emersion of the electrodes. Therefore, the liquid cell has tremendous potential for the study of the electrolyte−electrode interaction: the SEI formation and growth kinetics.
To successfully and quantitatively measure the SEI layer using this liquid-cell battery approach, optimized cell development and electron dose calibrations are underway. Our results using this in situ liquid electrochemical cell open a new avenue for the study of the SEI formation in all of the electrochemical reactions inside a working battery cell.—Gu et al.
This work was supported by the Department of Energy’s Offices of Science and of Energy Efficiency and Renewable Energy.
Meng Gu, Lucas R. Parent, B. Layla Mehdi, Raymond R. Unocic, Matthew T. McDowell, Robert L. Sacci, Wu Xu, Justin Grant Connell, Pinghong Xu, Patricia Abellan, Xilin Chen, Yaohui Zhang, Daniel E. Perea, James E. Evans, Lincoln J. Lauhon, Ji-Guang Zhang, Jun Liu, Nigel D. Browning, Yi Cui, Ilke Arslan, and Chong-Min Wang ( 2013) “Demonstration of an Electrochemical Liquid Cell for Operando Transmission Electron Microscopy Observation of the Lithiation/Delithiation Behavior of Si Nanowire Battery Anodes,” Nano Letters doi: 10.1021/nl403402q