Thermally Driven Interfacial Degradation between Li₇La₃Zr₂O₁₂ Electrolyte and LiNi_0.6Mn_0.2Co_0.2O₂ Cathode

Abstract

Solid-state batteries offer higher energy density and enhanced safety compared to the present lithium-ion batteries using liquid electrolytes. A challenge to implement them is the high resistances, especially at the solid electrolyte interface with the cathode. Sintering at elevated temperature is needed in order to get good contact between the ceramic solid electrolyte and oxide cathodes and thus to reduce contact resistances. Many solid electrolyte and cathode materials react to form secondary phases. It is necessary to find out which phases arise as a result of interface sintering and evaluate their effect on electrochemical properties. In this work, we assessed the interfacial reactions between LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) and Li 7 La 3 Zr 2 O 12 (LLZO) as a function of temperature in air. We prepared model systems by depositing thin-film NMC622 cathode layers on LLZO pellets. The thin-film cathode approach enabled us to use interface-sensitive techniques such as X-ray absorption spectroscopy in the near-edge as well as the extended regimes and identify the onset of detrimental reactions. We found that the Ni and Co chemical environments change already at moderate temperatures, on-setting from 500 °C and becoming especially prominent at 700 °C. By analyzing spectroscopy results along with X-ray diffraction, we identified Li 2 CO 3 , La 2 Zr 2 O 7 , and La(Ni,Co)O 3 as the secondary phases that formed at 700 °C. The interfacial resistance for Li transfer, measured by electrochemical impedance spectroscopy, increases significantly upon the onset and evolution of the detected interface chemistry. Our findings suggest that limiting the bonding temperature and avoiding CO 2 in the sintering environment can help to remedy the interfacial degradation.

Thermally-driven reactivity of Li0.35La0.55TiO3solid electrolyte with LiCoO2cathode

“…Moreover, practical consideration of the environment, such as temperature and gas species involved in the synthesis, is still difficult to simulate and can lead to divergences between computational predictions and experimental observations. For example, in our recent work on LLZO-NMC and LLZO-LCO interface, 16,17 we experimentally found that decomposition products deviate from earlier computationally predicted secondary phases. [35][36][37] Hence, experimental probing of these interfaces can not only help to elucidate fundamental mechanisms but also detect decomposition onset conditions which are important for validating and benchmarking computational predictions.…”

“…The approach for modelling the interfacial feature was adopted from previous studies. 16,53,54 Finally, the tail was modelled by using another constant phase element (CPE tail ) in series with the above circuit elements, as reported elsewhere to model Li-blocking Au electrode. 51,52 Fig.…”

“…The temperature effect is important because sintering at elevated temperatures can provide good interfacial contact between the ceramic cathode and ceramic electrolyte materials to reduce the interface contact resistances. However, as we see from earlier work, 16,17,38 this can lead to thermally-driven interactions between the solid electrolyte and cathode and insulating secondary phases at the interface. The challenge of probing the interface was addressed by using a thin-lm cathode layer deposited on top of a dense LLTO electrolyte pellet.…”

“…In this respect, our previous works have probed the onset of thermally-driven degradation of the garnet structured Al-doped Li 7 La 3 Zr 2 O 12 solid electrolyte with layered transition metal oxide cathode interfaces, with implications to electrochemical properties. 16,17 In particular, we have found that carbon dioxide present in the environment has a detrimental effect on interface charge transfer kinetics due to the formation of lithium carbonate and other delithiated insulating secondary phases at the interface. At the same time, the local change in the structure and chemistry at the interface begins at temperatures as low as 500 C, well-below typical ceramic sintering temperatures needed for bonding at the interface.…”

“…We already employed a similar setup using a thin-lm cathode in our previous recent work on the garnet structured Li 7 La 3 Zr 2 O 12 solid electrolyte. 16,17 We performed synchrotron X-ray absorption near-edge spectroscopy (XAS) to probe the structural and chemical change of LCOjLLTO interface and determine the onset of reaction. Grazing incidence X-ray diffraction (GIXD) was utilized to detect the crystalline decomposition phases at the interface.…”