The relationship between the redox activity and electrochemical stability of solid electrolytes for solid-state batteries (1908.10144v1)

Abstract: All-solid-state Li-ion batteries promise safer electrochemical energy storage with larger volumetric and gravimetric energy densities. A major concern is the limited electrochemical stability of solid electrolytes and related detrimental electrochemical reactions, especially because of our restricted understanding. Here we demonstrate for the argyrodite, garnet and NASICON type solid electrolytes, that the favourable decomposition pathway is indirect rather than direct, via (de)lithiated states of the solid electrolyte, into the thermodynamically stable decomposition products. The consequence is that the electrochemical stability window of the solid electrolyte is significantly larger than predicted for direct decomposition, rationalizing the observed stability window. The observed argyrodite metastable (de)lithiated solid electrolyte phases contribute to the (ir)reversible cycling capacity of all-solid-state batteries, in addition to the contribution of the decomposition products, comprehensively explaining solid electrolyte redox activity. The fundamental nature of the proposed mechanism suggests this is a key aspect for solid electrolytes in general, guiding interface and material design for all-solid-state batteries.

• The paper demonstrates that an indirect decomposition pathway through (de)lithiated states enhances the practical stability window of solid electrolytes.
• The study uses DFT, XRD, NMR, and molecular dynamics to verify that redox changes occur before stable decomposition products form.
• The findings suggest improved ASSB designs by leveraging redox activity to mitigate ionic conductivity barriers and optimize battery performance.

Analyzing Electrochemical Stability and Redox Activity of Solid Electrolytes in Solid-State Batteries

This paper explores the interplay between the redox activity and electrochemical stability of solid electrolytes in all-solid-state batteries (ASSBs). The paper investigates three types of solid electrolytes: argyrodite, garnet, and NASICON. These electrolytes are critical components of ASSBs, touted as safer alternatives to traditional lithium-ion batteries, owing to their high volumetric and gravimetric energy densities.

Core Findings

Indirect Decomposition Pathway

A significant finding of this research is the proposal of an indirect decomposition pathway for solid electrolytes. Through Density Functional Theory (DFT) simulations and experimental validation, the authors demonstrate that solid electrolytes undergo an indirect decomposition process through intermediate (de)lithiated states, rather than direct decomposition into stable products. This indirect pathway is kinetically favorable and results in a larger practical electrochemical stability window than what is predicted by direct decomposition analysis.

Redox Activity and Electrochemical Stability

The paper highlights that the redox activity of the solid electrolytes is closely tied to their electrochemical stability. The paper specifically examines the argyrodite Li6PS5Cl solid electrolyte, where lithiation upon reduction of phosphorus and delithiation upon oxidation of sulfur occurs before the formation of stable decomposition products. This redox activity effectively extends the electrochemical stability window, which is shown to be 1.25 V through experimental galvanostatic cycling.

Experimental Verification

To verify the model, various techniques were employed, including XRD, solid-state NMR, and molecular dynamics simulations. These methods provided insights into the structural and chemical changes during the (de)lithiation process and confirmed the presence of (de)lithiated phases that contribute to reversible cycling capacity.

Broader Implications and Applications

Practical Implications

The insights from this paper could significantly impact the design and optimization of ASSBs. By understanding the interplay between redox activity and stability, researchers can design solid electrolytes and interfaces that minimize the formation of decomposition products, which pose a barrier to ionic conductivity and increase internal resistance—detrimental to battery performance.

Theoretical Implications

The proposed decomposition mechanism is not limited to the studied electrolytes but suggests a general approach for understanding solid electrolyte behavior. It presents a paradigm where solid electrolytes' electrochemical stability is primarily dictated by their redox activity, tied to intrinsic kinetic barriers, rather than the stability of decomposition products.

Future Directions

The research opens avenues for further inquiries into kinetic barriers at the interfaces of solid electrolytes and electrodes. It also prompts exploration into novel electrolyte materials that could leverage the described (de)lithiation processes for enhanced stability and performance. Moreover, the understanding of electrochemical stability would benefit from further studies into how variations in solid electrolyte compositions and configurations affect their redox pathways and resulting stability windows.

In conclusion, this paper advances the understanding of solid-state battery technology by shedding light on the fundamental aspects of solid electrolyte behavior. It underscores the necessity of considering redox activity in designing materials for efficient and stable battery interfaces and paves the way for innovations in energy storage systems.