Remember your cell phone, laptop computer, tablet, and other mobile electronic devices? Most of these devices employ “lithium-ion batteries (LIBs)” which allow for the significant size reduction of batteries due to the high energy-density per unit volume – in other words, there is a high density of electric carries that can be used in charging/discharging of batteries. In addition to these mobile devices, LIBs can be now applied to the power sources used in electric/hybrid vehicles. As storage batteries are incorporated to wind-power, solar-power, and other such natural power systems that do not contribute to greenhouse gas-emission, LIBs can also play a role in overcoming global environmental issues.
Nevertheless, for further developments of LIBs, we still have big challenges. One of the technical problems is of safety in conventional LIBs since they use a flammable liquid electrolyte; as a precaution, used batteries should be collected in order to avoid fire accidents. In addition, both durability and period of charging are issues that need further improvements. Although some other types of batteries (e.g., solid-state batteries made of only non-flammable solid components) can solve the flammability problem, we still have significant discussions about the mechanisms underlying the charging and discharging phenomena.
Electron microscopy for lithium detection can be a key for the further progress in science and engineering of LIBs, as it provides essential information for understanding the basic mechanisms of lithium-ion motion during charging and discharging. For example, when we can determine the specific sites or positions in the positive-electrode crystal from which lithium ions can be retracted during charging, the results provide highly useful information for avoiding the undesired deterioration of crystallinity (i.e., formation of structural defects in the electrode crystal) induced by charging/discharging cycles. The deterioration in crystallinity actually causes unwanted resistance for the lithium transportation. Another contribution of electron microscopy is a direct observation of the electric field produced in a LIB—i.e., revealing the electrostatic potential lines within a LIB during charging/discharging, in addition to the conventional crystal structure analysis using an electron probe. Actually, researchers are highly interested in the examination of a significant drop in the electrostatic potential, referred to as an “electric double layer”, which can be generated in the interface between electrode and electrolyte. The observations provide conceptual insights not only for the basic science of batteries but also for new classes of device engineering, for example, new types of field emitter transistors, using ionic liquid and other useful electrolytes.
“Electron microscopy for lithium detection can be a key for the further progress in science and engineering of LIBs”
From a viewpoint of the methodology of electron microscopy, however, lithium detection is a challenge since lithium is a light element (heavier than just hydrogen and helium in the periodic table) which allows only weak interaction in our electron probe. To tackle this problem, researchers use special methods of imaging, including annular bright-field scanning transmission electron microscopy (abbreviated by ABF-STEM) that can be highly sensitive to the light elements. Electron energy-loss spectroscopy (EELS), which examines the energy-loss in probe electrons that have traversed a battery specimen, provides another route for determining the positions of lithium ions within batteries. Another useful tool is electron holography, which is a type of interferometry using electron probes. This method can visualize the electrostatic potential variations that may be induced by electrode potential changes due to the lithium insertion/extraction reaction. We should notice an essential role of the advanced method of specimen preparation on the achievement of electron microscopy studies. For example, a sophisticated technique of the specimen preparation allows for “operando observation” which represents a direct observation of the charging and/or discharging phenomena of LIBs using electron microscopy.
Using new tools with ultrahigh resolution and/or sensitivity is vitally important for progress in science. For example, with reference to the issues of astrophysics, detection of the extremely weak signal due to gravitational waves (predicted by Albert Einstein a hundred years ago) could be achieved thanks to advanced technologies to reduce the noise and any other uncertainties in observations. Regarding the science and technology related to LIBs, which also require sophisticated experiments for detection of the light element lithium, advanced electron microscopy methods shed further light on understanding the physics, chemistry, and engineering underlying the charging and discharging phenomenon.
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