Characterization of cathode-electrolyte interface in all-solid-state batteries using TOF-SIMS, XPS, and UPS/LEIPS

Abstract

In recent years, all-solid-state batteries (ASSBs) have been attracting attention as the next generation batteries for electric vehicles, energy storage systems, etc. Despite the growing interest, there are still many challenges faced in the commercial use of ASSBs. One of the biggest issues is the internal resistance, especially generated at the interface between solid electrolyte and electrode. The internal resistance at the interface limits the charge-discharge cycling performances. In order to solve this issue, it is necessary to examine the chemical and physical interactions at the interface. In this study, we have performed a detailed characterization of a LiPON/LiCoO2 interface using time-of-flight secondary ion mass spectrometry, x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and low-energy inverse photoelectron spectroscopy to obtain information on chemical species, chemical compositions, chemical states, and energy band diagrams. These powerful techniques have revealed that an interlayer between LiPON and LiCoO2 was formed due to the temperature rise during the manufacturing process. The temperature rise caused a change of the LiPON network structure and stimulated Co reduction in the LiCoO2 layer near the interface. Energy band diagram analysis suggests that the electron diffusion from LiPON to LiCoO2 may have triggered the reduction of Co. We concluded that the chemical changes that occur at the interface caused an increase in interfacial impedance. Preventing the chemical reduction of Co would be a key to minimize the internal resistance. In this article, the detailed chemical interactions between the LiPON and LiCoO2 layers will be discussed.

Electronic properties of lithium-ion conductive amorphous lithium phosphorus oxynitride

“…The n-type property of the LPN-0 is not unique and has been reported for other LiPON films and amorphous lithium phosphate thin films. 8,11 The most probable charged defects responsible for the n-type properties will be the interstitial Li or oxygen vacancies which function as electron donors. However, the LPN-0 exhibits the smallest Li/P ratio, and the composition corresponds to a Li-deficient state compared to the target (Li 3 PO 4 ).…”

“…3 The thin-film SSBs using LiPON are capable of charge-discharge cycles over tens of thousands of cycles. 4 To ensure satisfactory performance, studies have been directed at interface phenomena such as low-resistive interface formation, 5,6 side reaction mechanisms at the interface, 7,8 and stability of LiPON versus Li. 9,10 The LiPON may be formed with a wide compositional range, hence the LiPON prepared by various research groups typically has different compositions and chemical bonding states.…”

“…9,10 The LiPON may be formed with a wide compositional range, hence the LiPON prepared by various research groups typically has different compositions and chemical bonding states. Given that both the chemical potential and electronic properties of LiPON have been reported to correlate with reactivities at the electrode/LiPON interface, 7,8 these properties will need to be well documented if we are to understand fully the interface phenomena. Several studies have reported that LiPON (or amorphous lithium phosphate) has a wide band gap of over 5 eV, and where the E F is close to the CBM.…”

“…Several studies have reported that LiPON (or amorphous lithium phosphate) has a wide band gap of over 5 eV, and where the E F is close to the CBM. 8,11 The potential window of LiPON has been estimated by theoretical calculation assuming crystalline materials, 12,13 but there is no guarantee that the potential window of amorphous LiPON is transferrable to that of crystalline LiPON. This situation will also be true for other amorphous solid electrolytes used as coating layers on electrodes in sulphide-based SSBs.…”

“…Both the VBM and the CBM values were estimated by the linear approximation of the slopes shown by dotted lines in Fig.3a-d. Here, the UPS gave a measure of both the position of the vacuum and the ionization potential (VBM) versus the E F , while the LEIPS measured the electron affinity (CBM) versus vacuum 8. The absolute values of CBM, VBM, and E F versus vacuum are summarized in Fig.3e.…”