Perspectives in in situ transmission electron microscopy studies on lithium battery electrodes
Introduction
The high demands for rechargeable lithium ion batteries (LIBs) have significantly increased each year for mobile electronics, electric vehicles, and stationary energy storage over the past two decades as a convenient power source [1, 2, 3]. In order to fulfill the demands in these applications, tremendous research progress has been made at stabilizing new electrode materials with high performance and identifying the fundamental failure mechanisms that prevent a prolonged cycle life. Alloy and conversion materials with high specific capacity like silicon, sulfur, and conversion metal oxides have the most promise to increase the energy density of LIBs, but also have scientific challenges to retain their theoretical capacities upon repeated charge and discharge cycles [4, 5, 6, 7]. These new electrode materials have created a high interest for experimental techniques that are able to provide direct, atomic-scale, and real-time observation during the electrochemical processes. Such demands lead to extensive researches about in situ X-ray diffractography [8, 9], Raman [10, 11], Fourier transform infrared spectroscopy (FTIR) [12], and nuclear magnetic resonance (NMR) [13, 14], among others.
Among the direct characterization tools, an in situ transmission electron microscopy (TEM) offers exceptional opportunities for monitoring the dynamic processes of various electrode materials during electrochemical reaction at high spatial and temporal resolution [15••, 16••, 17••]. Depending on the experimental set up (open or closed cell configuration), the structural change of electrodes or the interface between a liquid electrolyte and a solid electrode can be observed upon battery charge/discharge cycles. This can bring new observations of reaction processes to light and provide a deep understanding for the fundamentals of a new electrode material's battery chemistry. The development of the in situ TEM characterization also benefits from the technology of TEM instrument combined with aberration corrected high-resolution imaging, electron energy-loss spectroscopy (EELS), and energy dispersive X-ray spectroscopy (EDS) which have enabled many discoveries in dynamic materials processes at the atomic level with reliable chemical information.
Section snippets
Open-cell configuration using a bias holder
In a typical battery system, a battery consists of a positive and negative electrode and an organic liquid electrolyte. To mimic the system inside TEM, the demonstration of a liquid electrolyte was initially a formidable challenge because the TEM column needs to maintain a high vacuum level of about 10−5 Pa. Since typical carbonate type electrolytes cannot be directly used due to a high vapor pressure, ionic liquid electrolytes with very low vapor pressures were used to enable a successful
An electrochemical liquid-cell configuration using silicon/silicon nitride membrane chips
Although the open-cell configuration has provided plenty of electrochemical process observations on the electrode materials, there are several shortcomings: (1) the ionic liquid and solid electrolytes are not common electrolytes for Li ion batteries such as organic carbonates; (2) the contact between an electrode and an solid electrolyte occurs only partially which gives inhomogeous reaction on the electrode; (3) the relatively large overpotential is accompanied to overcome the resistance of
Challenges and opportunities of in situ TEM studies
In situ TEM on battery electrodes is becoming a feasible characterization tool to understand the behavior inside electrodes using very direct structural and chemical evolution. With ongoing development of TEM technology and in situ holders, the future in situ TEM studies can be much more similar to real reaction conditions but still need to improve the challenges below. First, ionization effect of the electron beam should be considered especially to closed cell configuration. The organic
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
Y.C. acknowledges the support from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy. H.-W.L. acknowledges support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology under NRF-2012R1A6A3A03038593.
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