(Invited) Operando Characterization of Energy Conversion and Storage Devices Using Scanning Probe Microscopy

Abstract

Energy conversion/storage devices, such as solar cells (SCs) and lithium ion batteries (LIBs), have been actively researched along with the growing interest in energy and environment issues. In order to further improve the performance of these devices, it is essential to have a deeper understanding of their working principles. To achieve this, the characterization of their physical and chemical properties under device operation conditions, so-called operando measurement, is thought to be highly effective. For example, in the case of SCs, important information related with photovoltaic conversion processes can be obtained by measuring the electrical potential, electronic states, and charge distribution in the vicinity of the excitation center under light irradiation conditions. In the case of LIBs, characterization of electrical potential and lithium ion distribution in the electrodes and electrode-electrolyte interfaces during charging and discharging processes is expected to provide strong insight into the origin of interface resistance and mechanism of cell degradation. Up to date, X-ray and electron spectroscopy techniques have led the field of operando measurement. However, the spatial resolution of those techniques is insufficient to characterize next generation devices that actively utilize nanostructure. Thus, in our group, as an operando measurement technique to evaluate the local properties at nanoscale and even down to the atomic scale, we have been working on developing scanning probe microscopy (SPM) systems that operate under various environments (under light irradiation, voltage application, inert atmospheric conditions). In this talk, we will introduce recent our works related to the characterization of perovskite SCs and all-solid-state LIBs by operando nano-scale profiling of electrical potential distribution using Kelvin probe force microscopy (KPFM). Perovskite solar cells (PSCs) have attracted great research interest owing to their advantages in high power conversion efficiency and low-cost fabrication. Although the power conversion efficiency of PSCs is very high (record is 22.7 %), the detailed photovoltaic conversion processes has not well understood yet compared with the conventional inorganic SCs. In this work, we used KPFM to measure internal electrical potential distribution of PSCs during device operation. We observed that the change of electrical potential induced by illumination occurred strongly at the interface between the perovskite film and charge transport layers, which suggests that charge separation mainly occurred at those interfaces, not in the perovskite film. Also, we found that the position of charge separation varied depending on the composition of perovskite materials and the device structure. These findings will be helpful for understanding the fundamental mechanism within PSCs and achieving high device efficiency. All-solid-state (ASS) LIBs are a promising candidate for next-generation energy storage devices. However, solid electrolytes for the current ASS-LIBs have various disadvantages, such as low power densities caused by high ionic resistivity at the interfaces between active materials and solid electrolytes. The high ionic resistivity has been attributed to the Li depleted layer or defects at the interfacial layer. For the fundamental understanding of the origin of interfacial resistivity, novel in situ techniques for measuring the distribution of the internal potential and/or Li ion concentration of LIB cells are strongly required. In this work, we combined Ar ion milling under non-atmospheric conditions with cross-sectional KPFM for direct imaging of the internal electrical potential distribution of the ASS-LIBs. We succeeded in the direct visualization of the change in the potential distribution of the cathode composite electrode arising from the battery charging (electrochemical reaction). The results obtained provided several insights into the battery operation, such as the behavior of Li ions and inhomogeneity of electrochemical reactions in the electrode.