Chemical and Mechanical Degradation of the Interface between Li7La3Zr2O12 Electrolyte and LiNi0.6Mn0.2Co0.2O2 Cathode as a Result of Electrochemistry
Authors: Kim Younggyu , Chandra Subhash , Waluyo Iradwikanari , Hunt Adrian , Yildiz Bilge 
Published: September 2023

All solid-state Li-ion cells promises high energy density and high safety standards over conventional Li-ion cells based on liquid electrolytes. The first principle calculations shows that the most of the potential candidates for solid electrolytes for Li-ion cells have small electrochemical stability window. The limited electrochemical stability window leads to electrochemical reactivity with the cathode when cycled beyond the voltage limit. The reactions products are often leads to increased overpotential, hence rapid capacity fade during continuous cell operation. In addition to the electrochemical reactions, mechanical incompatibility further makes the cathode-solid-electrolyte interfaces prone to cracking, as cathodes undergoes structural changes during lithiation/delithiation. Indeed many experimental works have also showed limited cyclability of all-solid-state Li-ion cells owing to interfacial side reactions and mechanical cracking. Therefore, electrochemistry induced mechanical failure and electrochemical side reactions at the interface remains the bottleneck for the development of all-soli-state Li-ion cells. This paramount challenge invites us to systematically investigate the aforementioned problem by probing non-destructively near the cathode-solid-electrolyte interface. We introduce a model system that utilizes very thin layer of LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode (60-600 nm) on top of bulk Al-doped-Li7La3Zr2O12 (Al-LLZO) solid-electrolyte pellet. This unique system allows us to non-destructively probe the electrochemical reaction by combined in-operando and ex-situ X-ray absorption spectroscopy (XAS), with probing depth beyond the cathode-solid-electrolyte interfaces.

In this work, we elucidate the mechanism of electrochemical failure of NMC622 cathode and Al-LLZO solid-electrolyte interface at different temperature, through combined ex-situ and in-operando characterization techniques. The study revealed that the electrochemical reactivity is dependent on cell operation temperature. For the ex-situ samples that was cycled at higher temperature (80 ⁰C), the reduction of transition metals in cathode, i.e., nickel and cobalt was evident by soft X-ray absorption spectroscopy (XAS) of their respective L-edges. The results match closely with DFT prediction for the interfaces at high voltage. On the other hand, the in-operando XAS experiment, where characterization was done at room temperature, the reduction of transition metals in cathode, and hence electrochemical reactivity, was not observed. These results remarkably outline that electrochemical reactivity of solid-electrolyte and cathode interfaces at high voltage is kinetically inhibited at low or near room temperature, and hence very sensitive to cell operating condition. Nonetheless, it was found that irrespective of operating conditions i.e., temperature, the cell capacity rapidly decays as number of cycles continue. The focused ion beam scanning electron microscopy (FIB-SEM) revealed that this can be mostly attributed to intergranular cracks in NMC622 cathode films as well as to delamination of the cathode-solid-electrolyte interfaces during cell operation. As these cracks, in cathode films and at the interfaces, grows, the overpotential/impedance increases due to limited contacts for effective Li+ migration.

The current work contributes to the field by showcasing interfacial degradation pathways in cell operation by novel design of experiments that allows for both in-operando and non-destructive characterization. The findings of this work will guide towards engineering cells of practical use.

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