Structure, Chemistry, and Charge Transfer Resistance of the Interface between Li₇La₃Zr₂O₁₂ Electrolyte and LiCoO₂ Cathode

Abstract

All-solid-state batteries promise significant safety and energy density advantages over liquid-electrolyte batteries. The interface between the cathode and the solid electrolyte is an important contributor to charge transfer resistance. Strong bonding of solid oxide electrolytes and cathodes requires sintering at elevated temperatures. Knowledge of the temperature dependence of the composition and charge transfer properties of this interface is important for determining the ideal sintering conditions. To understand the interfacial decomposition processes and their onset temperatures, model systems of LiCoO2 (LCO) thin films deposited on cubic Al-doped Li7La3Zr2O12 (LLZO) pellets were studied as a function of temperature using interface-sensitive techniques. X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and energy-dispersive X-ray spectroscopy (EDS) data indicated significant cation interdiffusion and structural changes starting at temperatures as low as 300⁰C. La2Zr2O7 and Li2CO3 were identified as 2 decomposition products after annealing at 500°C by synchrotron X-ray diffraction (XRD). X-ray absorption spectroscopy (XAS) results indicate the presence of also LaCoO3, in addition to La2Zr2O7 and Li2CO3. Based on electrochemical impedance spectroscopy, and depth profiling of the Li distribution upon potentiostatic hold experiments on symmetric LCO|LLZO|LCO cells, the interfaces exhibited significantly increased impedance, up to 8 times that of the as-deposited samples after annealing at 500 o C. Our results indicate that lower-temperature processing conditions, shorter annealing time scales, and CO2-free environments are desirable for obtaining ceramic cathode-electrolyte interfaces that enable fast Li transfer and high capacity.

Dopant‐Dependent Stability of Garnet Solid Electrolyte Interfaces with Lithium Metal

“…EIS spectra exhibit two semicircles and a low-frequency tail/semicircle, and these spectra were fit using a L − R 1 − (R 2 Q) − W equivalent electrical circuit model, which takes into account chemical diffusion at the interface (see the Experimental Section for details). [16] To verify this assignment, a separate EIS measurement was carried out from 7 MHz to 250 mHz using symmetric Au||LLZO||Au cells ( Figure S9, Supporting Information), which further revealed the bulk impedance contribution and yielded grain boundary impedance response similar to that observed in Li-Li symmetric cells. This model indicates an incomplete, first semicircle with a peak at ≈300 kHz that corresponds to grain boundary conduction in the LLZO pellets.…”

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materials are a particularly promising class of solid electrolytes for all-solidstate lithium metal batteries, as they are predicted to have a wide electrochemical stability window, [5,6] can be synthesized with very high density (>97%) [7,8] and, through aliovalent doping, can achieve room temperature Li-ion conductivities as high as ≈1.0 mS cm −1 with negligible electronic conductivity. [9] However, significant fundamental issues remain unresolved for garnet-based all-solid-state batteries, including low accessible current densities, [10] the persistence of Li dendrite formation, [11,12] and perhaps most importantly, ambiguities as to whether the interfaces between LLZO and both Li metal [13,14] and high voltage oxide cathodes [15,16] are stable over extended cycling. Indeed, developing deep understanding of the intrinsic reactivity between solid electrolytes and relevant electrode materials is crucial to developing high voltage solidstate batteries with long lifetimes, as the presence of any significant (electro)chemical reactivity will ultimately lead to premature cell failure during extended cycling.

Understanding interfacial stability is an especially challenging issue common to all solid-state battery systems due to the inability of many experimental techniques to adequately interrogate the chemical properties of buried interfaces.

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Interfacial Incompatibility and Internal Stresses in All‐Solid‐State Lithium Ion Batteries

“…In 2011, K.H. Due to significant cation interdiffusion and structural changes, the reaction products were revealed to be Li 2 CO 3 , La 2 Zr 2 O 7 , and LaCoO 3 , [234] as described in Figure 13a. These reaction products restrained Li insertion and extraction across the interface, leading to a poor electrochemical performance.…”

“…High-temperature processing withdrew Li + ions from the LiNiO 2 and drove them to move toward A-site vacancies within the LLTO, destroying the stoichiometry of LiNiO 2 . [234] Copyright 2018, American Chemical Society. Apart from the chemical reaction, the interfacial impedance at the oxide SSE/cathode material inter-faces may also arise from electrochemical reaction during battery operation.…”

Reducing the Interfacial Resistance in All‐Solid‐State Lithium Batteries Based on Oxide Ceramic Electrolytes

“…The annealing of the ceramic electrolyte‐coated LiCoO 2 can increase the crystallographic texture in the LiCoO 2 film, thus enhancing the transfer of Li + inside the cathode layer. The control of the annealing temperature is needed to suppressing the cross‐diffusion of elements across the cathode interface at high temperature (>500 °C) . The disadvantage of PLD technology is its low efficiency in coating a cathode layer with thicknesses of tens of micrometers because this process takes a long time.…”