EDL Charges of Solid Electrolytes Quantified
The electric double layer (EDL) effect at solid electrolyte/electrode interfaces has been a key topic in all-solid-state batteries. However, its characterization remains difficult. In this study, we show how to evaluate the EDL effect with solid electrolytes by using diamond-based transistors.
Solid electrolyte-based electrochemical devices have attracted attention for use in a wide range of applications from all-solid-state lithium ion batteries(ASS-LIBs) to memristors. While various bulk properties of solid electrolytes have been intensively investigated to identify ways of improving the performance of such devices with a view to their practical use, recent reports indicated that not only the bulk properties but also the electric double layer (EDL) (or space charge layer) have an effect, which is due to ionic carrier density modulation in the vicinity of electrode/solid electrolyte playing a crucially important role in the device operation. For example, EDL formation accompanied by Li+ depletion has been discussed in relation to high interfacial resistance of ASS-LIBs.
While, until the present, the EDL effect in such solid state ionic devices has been one of the key issues, one additional important question needs to be asked; Does the EDL effect occur even in solid electrolyte systems with rigid frameworks? While the importance of the clarification is widely understood, direct observation of the EDL in the vicinity of solid/solid electrolytes interface has been difficult. It is because the EDL for solid electrolytes is expected to be extremely thin and extent of ion concentration variation is small. Furthermore, the excess charge of ions in the EDL of solid electrolytes is expected to be more readily neutralized by the additional electronic carrier doped in the solid electrolytes and the resultant redox state modulation of the component ions. Such electrochemical charge neutralization (compensation) inside solid electrolytes can potentially deteriorate EDL capacitance greatly. However, the evaluation of the EDL capacitance in solid electrolytes is not straightforward because, in conventional electrochemical analyses, based on passing electric currents through electrolytes, such an electrochemical charge neutralization (i.e., chemical capacitance) can, in general, be observed with a large capacitance, which in turn makes the evaluation of EDL capacitance difficult. Therefore, a novel approach is required to clarify EDL behavior in solid electrolyte systems.
Herein, we use a novel method to show that the EDL effect, and its suppression at solid electrolyte/electronic material interfaces, can be characterized on the basis of the electric conduction characteristics of hydrogenated diamond(H-diamond)-based EDL transistors (EDLTs). Whereas H-diamond-based EDLT with a Li-Si-Zr-O Li+ solid electrolyte showed EDL-induced hole density modulation over a range of up to three orders of magnitude, EDLT with a Li-La-Ti-O (LLTO) Li+ solid electrolyte showed negligible enhancement, which indicates strong suppression of the EDL effect. Such suppression is attributed to charge neutralization in the LLTO, which is due to variation in the valence state of the Ti ions present. The method described is useful for quantitatively evaluating the EDL effect in various solid electrolytes.