Understanding the Lifetime of Battery Cells Based on Solid-State Li₆PS₅Cl Electrolyte Paired with Lithium Metal Electrode

Abstract

All-solid-state batteries with solid electrolytes having ionic conductivities in the range of those of liquid electrolytes have gained much interest as safety is still a major issue for applications. Meanwhile, lithium metal seems to be the anode material of choice to face the demand for higher capacities. Still, the main challenges that come with the use of a lithium metal anode, i.e., formation and growth of lithium dendrites, are still not understood very well. This work focuses on the reasons of the lifetime behavior of lithium symmetric cells with the solid electrolyte Li 6 PS 5 Cl and lithium electrode. In particular, the voltage increases during the application of a constant current density are investigated. The interface between the lithium metal electrode and the solid electrolyte is analyzed by X-ray photoelectron spectroscopy, and the resistance changes of each electrode during stripping and plating are investigated by impedance spectroscopy on a three-electrode cell. A main factor for the lifetime influenced by lithium dendrite formation and growth is the buildup of a lithium vacancy gradient, leading to voids which decrease the interface area and therefore increase the local current density. Additionally, those lithium vacancies in lithium metal represent a limitation for conductivity rather than migration in solid electrolyte. Further experiments indicate that the seedlike plating behavior of lithium also plays a key role in increased local current density and therefore decreased lifetime. Plating of only a small amount of lithium leads to small areas of well-connected interfaces, resulting in high local current density. A medium amount of plated lithium leads to larger areas of interface between lithium and electrolyte, balancing the current density distribution. In contrast, a high amount of repeatedly deposited lithium leads to lithium seed plating on top of already plated lithium. Those seed spots grown on top represent a better interface connection, which again leads to higher local current densities at those spots and therefore results in shorter lifetimes due to short circuits caused by lithium dendrites.

A critical discussion on the analysis of buried interfaces in Li solid-state batteries. Ex situ and in situ/operando studies

“…XPS is the most widely used method to study the chemical evolution of interfaces in SSBs. Paradoxically; almost all the reported experiments rely on the analysis of the surface of the materials obtained after the manual separation 23,24,26,[114][115][116][117] or manual etching 31,47 of the electrode/solid electrolyte interface. These operations can be easier for the Li/solid electrolyte interface, but in all cases, the procedures lack reproducibility and raise concerns about if interphases are being actually probed.…”

“…Indeed, a survey of the literature on SSBs shows plenty of examples on spectroscopic and microscopic characterization of solid/solid interfaces, but almost no report mention how these buried interfaces could be analysed (see for instance refs. [18][19][20][21][22] ) and a few of them describe procedures that depend on the manual skills of a researcher to separate mechanically two connected phases (see for instance refs [23][24][25][26] ). Obviously, these approaches are questionable for sound reasons.…”

“…The surface of the two coatings can be then probed; however, the spatial interpretation of the results is challenging as the exact depth position of the analysed region in the full thickness of the original interphase is unknown and the chemical composition of the interphase has been showed to be not homogeneous. In this context, the most widespread approach 24,[28][29][30] to tackle this problem consists in performing depth profiling studies in which sequential steps of material sputtering via ionic bombardment and spectroscopy/spectrometric analysis are combined to reconstruct the chemical composition of the interphase through its thickness (see section 5 for more details). In this perspective, we provide experimental results warning about this strategy.…”

Engineered Three-Electrode Cells for Improving Solid State Batteries

“…No EIS in 3 electrodes configuration was reported with such cells and it is probable that the asymmetric cell geometry or the position of the reference might bring artefact loops in the impedance spectra. On the other hand, EIS could be performed in 3 electrodes configuration using gold plated tungsten wire located between anode and cathode as reference 9 , which may not provide a stable voltage upon cycling 10 . More recently, a reference electrode made of a Ni mesh coated with lithium titanate allowed to follow the voltage of both electrodes, and also properly record their impedances 11 .…”

Issue of Lithium-indium Anode in High Energy and Power All-Solid-State Lithium Batteries

“…Lithium metal is recognized as the most attractive choice for anode material to achieve high energy density due to the low electrochemical potential (-3.04 V vs. the standard hydrogen electrode) and high theoretical specific capacity (3860 mAh g -1 ). However, pure Li metal is unsatisfying in sulfide-based ASSLBs because of the severe interfacial side reactions 6,7,8 and the growth of Li dendrites 9,10,11 . Lithium alloys provide an attractive alternative to construct a stable SSEs-electrode interface that enables the long-term cycling for ASSLBs 12 .…”