Article

In-situ XPS: Investigating Stable Interfaces for Improved Solid-State Battery Performance

Surface Analysis Spotlight: XPS

by Sarah Zaccarine XPS Scientist

The development of solid-state batteries (SSBs) has gained significant attention due to their potential for enhanced safety and energy density compared to traditional lithium-ion batteries (LIBs). SSB performance is greatly affected by the stability of interfaces throughout the battery cell, which vary depending on the materials chosen for the cathode, electrolyte/separator, and anode. Lithium metal anodes can achieve excellent performance but are expensive and highly reactive, leading to unwanted reactions at the solid electrolyte (SE)/electrode interface that can impact long-term performance. It is therefore critical to design electrolytes with high ionic conductivity/Li⁺ transport, a wide electrochemical operating window, and stable interfaces.

Composite solid electrolytes (SEs) comprised of polymer matrices, inorganic fillers, and Li salts offer better performance compared to single-solid-state electrolytes, but incomplete understanding of external/internal interface stability has hindered their practical use in SSBs. In these materials, Li⁺ transport is governed by organic/inorganic interfaces within the SE and at the interface formed between the SE and neighboring electrode, which also governs long-term performance. Understanding interfacial stability is vital to designing SEs with improved internal and SE/electrode interfaces to drive higher performance in SSBs, but it has been challenging to characterize. A recent study by Huo et. al¹ utilizes XPS to investigate the use of thiophosphate-based composite SEs in preventing dendrite formation on lithium metal anodes, therefore improving interface stability and overall battery performance.

In this work, composite electrodes composed of Li₆PS₅Cl (LPSCl) and polyethylene glycol dimethyl ether (PEGDME) were analyzed with a PHI 5000 VersaProbe II XPS instrument. The XPS results were used to directly assess the critical parameter of interfacial stability by comparing composition of the composite SE to the individual starting materials (PEG polymer and LPSCl). The S 2p and P 2p XPS results show similar species detected in the composite (Figure 1a, b) and individual LPSCl material (Figure 1c, d), indicating stability of the PEGDME/LPSCl interface. This improved stability was confirmed by ionic conductivities measurements by electrochemical impedance spectroscopy (EIS), which showed that the PEGDME/LPSCl interface retained its ionic conductivity better over time, due to its improved interfacial stability. These combined results highlight the ability of XPS to track key parameters governing battery performance.

Figure 1. (a) S 2p and (b) P 2p XPS spectra collected for the PEGDME/LPSCl interface within the composite SE,

and (c) S 2p and (d) P 2p XPS spectra collected for the LPSCl individual material.

The PEGDME/LPSCl composite was then modified by adding LiFSI to inhibit dendrite growth and LiTFSI to improve Li+ mobility (referred to as PEGDMEL@mix). The formation of the solid electrolyte interphase (SEI) at the SE/anode interface was monitored via XPS equipped with an in-situ lithium deposition system (Figure 2) designed based on previous work by the group, where the monatomic Ar+ ion gun is used to deposit thin metal films on the sample surface directly within the chamber.2 Here, Li metal was deposited on the PEGDMEL@mix surface, and XPS spectra were collected immediately after each step of deposition of the Li metal anode layer, resulting in measurements of the SEI species as the layer formed in-situ (Figure 3).

Figure 2. Schematic illustration of in-situ Li deposition setup in XPS analysis chamber.

The initial species present from the PEGDME/LPSCl materials changed significantly with sequential Li deposition steps; after only 5 min, the decomposition of LiFSI led to the growth of LiF (Figure 3a, F 1s) and Li_xS_yO_z (Figure 3b, S 2p) species and the -SO₂F (F 1s, S 2p) species decreased/disappeared with increased Li deposition time. The decomposition of LiTFSI likely also contributed to the LiF and corresponding decrease in -CF₃ (F 1s) and -SO₂CF₃ (S 2p) species. The reduction of polysulfide species and Li₂S₂O₄ (S 2p) results in the formation of Li₂S (S 2p) and Li₂O (Figure 3c, O 1s) species starting after 5min of Li deposition. With increased Li deposition time, the LiF, Li₂S, and Li₂O species become dominant, indicating that they are the primary species in the SEI layer that forms at the SE/electrode interface in the PEGDMEL@mix sample.

Figure 3. In-situ XPS spectra of a) F 1s, b) S 2p, and c) O 1s after multiple lithium deposition times.

Further measurements on Li/PEGDMEL@mix/Li symmetrical cells suggested that compared to the control, Li/LPSCl/Li cells, the LiF-rich Li/PEGDMEL@mix interface was more stable and less prone to degradation and dendrite growth over time, ultimately leading to much better long-term cycling stability tested in full SSB cells. This was attributed to the presence of a thin LiF-rich SEI layer, demonstrated by the XPS results above, that prevented interfacial degradation and Li dendrite growth.

In conclusion, the PEGDMEL@mix composite SE shows great promise to achieve performance goals. The use of surface analysis, particularly in-situ XPS results revealing the formation of a uniform LiF-rich SEI, was vital to interpret the electrochemical performance and degradation results, allowing the authors to guide cell design for improved performance and stability.

For more information on the capabilities of PHI XPS instruments for in-situ and ex-situ chemical analysis, please attend Dr. Kateryna Artyushkova’s upcoming Invited Talk on “Advances in XPS Analysis of Battery Materials” at the 244^th ECS Meeting in Gothenburg, Sweden this October.

¹ H. Huo; M. Jiang; B. Mogwitz; J. Sann; Y. Yusim; T.-T. Zuo; Y. Moryson; P. Minnmann; F.H. Richter; C.V. Singh; J. Janek. “Interface Design Enabling Stable Polymer/Thiophosphate Electrolyte Separators for Dendrite-Free Lithium Metal Batteries.” Angewandte Chemie International, 2023, 62, e202218044.

² S. Wenzel; T. Leichtweiss; D. Krüger; J. Sann; J. Janek. “Interphase formation on lithium solid electrolytes – An in situ approach to study interfacial reactions by photoelectron spectroscopy.” Solid State Ionics, 2015, 271, 98-105