In Situ Investigation of Lithium Metal–Solid Electrolyte Anode Interfaces with ToF‐SIMS

Abstract

Solid‐state batteries with a lithium metal anode (LMA) are promising candidates for the next generation of energy storage systems with high energy and power density. However, successful implementation of the LMA requires deeper insight into the lithium metal–solid electrolyte (Li|SE) interface. Since lithium is highly reactive, reaction products form when it comes into contact with most solid electrolytes (SEs) and the resulting interphase can have detrimental effects on cell performance. To better understand the formation of interphases, Li|SE interfaces are studied with time‐of‐flight secondary‐ion mass spectrometry (ToF‐SIMS), which provides chemical information with high sensitivity in 2D as well as 3D and is a valuable complement to commonly used techniques. To investigate the interphase, lithium is deposited in situ on SE pellets either through lithium vapor deposition or electrochemical lithium plating. Subsequent depth profiling provides information about the stability of the Li|SE interface and about the microstructure of the formed interphase. At the Li|Li6PS5Cl interface of lithium metal with argyrodite‐type Li6PS5Cl, an apparently covering Li2S‐rich layer is found as major part of the interphase. Independent of the deposition method, a combination of ToF‐SIMS and atomic force microscopy indicates a thickness of about 250 nm for the Li2S‐rich interlayer.

Instability of the Li7SiPS8 Solid Electrolyte at the Lithium Metal Anode and Interphase Formation

“…Also, signals that appear only after a certain sputter time, like the SiS 3 − signal, were identified, which indicates the formation of an SEI. 32 The LiSi − signal shows a maximum in signal intensity before the mentioned SiS 3 − signal appears, which may indicate the presence of Li/Si compounds above other interphase reaction products. To learn more about the evolution of the interface with time, the sample with 3 μm of lithium on top of the SE pellet was stored under ultrahigh vacuum (UHV) conditions and remeasured after defined periods of storage time.…”

“…ToF-SIMS was used to further investigate the Li|t-Li 7 SiPS 8 interface and to study the nature of the formed interphase. 32 For this purpose, 1−3 μm thick lithium layers were deposited on the surface of an SE pellet and the resulting bilayers were investigated by ToF-SIMS depth profiling. In Figure 4a, the depth profile through a 3 μm thick lithium layer is shown.…”

“…This example emphasizes the need for time-dependent acquisition of ToF-SIMS depth profiles for the Li|SE samples as discussed elsewhere. 32 The product formation of the reaction between t-Li 7 SiPS 8 and the lithium metal was characterized by solid state MAS NMR spectroscopy. In Figure 5a, the 7 Li NMR spectrum of the mixture is shown.…”

“…17,28,30,31 Otto et al recently reported on another technique to characterize the interphase and distinguish between the SEI and the MCI. 32 In this method, lithium is deposited on top of the SE and the formed interphase is investigated with time-of-flight secondaryion mass spectrometry (ToF-SIMS). Depending on the nature of the interphase, depth profiles through the deposited lithium layer and across the lithium|SE interface show distinct features and allow the differentiation between the SEI and the MCI.…”

“…Depending on the nature of the interphase, depth profiles through the deposited lithium layer and across the lithium|SE interface show distinct features and allow the differentiation between the SEI and the MCI. 32 Since neither the practical electrochemical stability window of silicon-containing SEs nor the general mechanism in contact with lithium are known, the applicability of highly conductive t-Li 7 SiPS 8 is unclear. As the conductivity of pure Li 4 SiS 4 is approximately five orders of magnitude lower than that of t-Li 7 SiPS 8 , Li 4 SiS 4 is not a suitable SE.…”