In situ x-ray photoelectron spectroscopy study of lithium carbonate removal from garnet-type solid-state electrolyte using ultra high vacuum techniques

Abstract

Solid-state electrolytes (SSEs) are of significant interest for their promise as lithium ion conducting materials but are prone to degradation due to lithium carbonate formation on the surface upon exposure to atmosphere, adversely impacting Li ion conduction. In situ x-ray photoelectron spectroscopy monitored changes in the composition of the SSE Li garnet [Li6.5La3Zr1.5Ta0.5O12 (LLZTaO)] upon annealing in ultrahigh vacuum (UHV) and upon Ar+ ion sputtering. Trends in core level spectra demonstrate that binding energy (BE) calibration of the Li 1s at 56.4 eV yields a more consistent interpretation of results than the more commonly used standard of the adventitious C 1s at 284.8 eV. Annealing one ambient-exposed sample to >1000 K in UHV effectively reduced surface carbonate and oxygen, leaving significant amounts of carbon in lower oxidation states. A second ambient-exposed sample was subjected to 3 keV Ar+ ion sputtering at 500 K in UHV, which eliminated all surface carbon and reduced the O 1s intensity and BE. These methods present alternative approaches to lithium carbonate removal than heating or polishing in inert atmospheres and are compatible with fundamental surface science studies. In particular, the data show that sputtering at mildly elevated temperatures yields facile elimination of carbonate and other forms of surface carbon. This is in contrast to annealing in either UHV or noble gas environments, which result in carbonate reduction, but with significant remnant coverages of other forms of carbon.

Nickel and Cobalt Oxidation State Evolution at Ni-Rich NMC Cathode Surfaces during Treatment

“…However, these levels decreased substantially upon heating (Table S5), which suggests a transformation to sp 3 -type “adventitious” carbon, even at relatively mild temperatures. This is consistent with another report on UHV annealing of lithium-containing garnet surfaces at slightly higher temperatures …”

“…This is consistent with another report on UHV annealing of lithium-containing garnet surfaces at slightly higher temperatures. 29 The Li 1s spectra (Figure S6) provide insight into various surface species and the underlying NMC lattice. As in the C 1s region, a decline in carbonate content is observed with heating of the NMC 811 sample.…”

Toward Moisture-Stable and Dendrite-Free Garnet-Type Solid-State Electrolytes

“…Despite the manifold advantages offered by garnet-type SSEs, a few challenges, such as high interfacial resistance and dendrite formation even at lower current densities hinder their targeted applications. , The high interfacial resistance originates from the poor wettability of the garnet SSE with metallic Li at the interface. ,, This can be attributed to the unstable nature of the garnet surface against a moisture-containing environment where proton-lithium (H + /Li + ) exchange takes place. This interchange reaction leads to the formation of insulating lithium carbonate (Li 2 CO 3 ) through intermediates (as shown in eqs and ), which blocks the Li-ion transport across the interface. − Although efforts were made to remove the lithium carbonate layer from the garnet surface, , there are no studies that prevent the formation of lithium carbonate on the surface of the solid electrolyte altogether. The high electronic conductivity of the garnet-type SSE (10 –8 –10 –7 S cm –1 ) was found to be responsible for the easy propagation of Li dendrites . The mechanism of such a phenomenon was attributed to the leakage of a small amount of electrons into the electrolyte that combines with the Li ions to form metallic and dead Li inside the garnet-type SSE .…”

An All-Solid-State Battery with a Tailored Electrode–Electrolyte Interface Using Surface Chemistry and Interlayer-Based Approaches

“…5 A garnet-type SSE also exhibits a wide electrochemical stability window (6 V vs Li/Li + ). 6,8 Although a garnet-type SSE has multiple advantages, a few grand challenges hinder its targeted applications. At the anode-SSE interface, microscopic investigation of the interaction between a cubic garnet-type SSE and metallic Li revealed transformation of several atomic layers of the cubic phase to the tetragonal phase.…”

“…12,13 In addition, the relatively high electronic conductivity of the garnet-type SSE induces tunneling of electrons across the SSE, leading to lithium dendrite propagation. 14 Several approaches have been proposed to overcome these challenges, including the use of (i) Li alloy anodes, 15,16 (ii) hybrid electrolytes, 17,18 (iii) 3-D structured anodes, 19,20 (iv) improved surface morphology, 8,21 and (v) interface modifications. 22−24 Among the various methods used, improving surface morphology and interface modification were found to be the most effective.…”