Multielectron, Cation and Anion Redox in Lithium-Rich Iron Sulfide Cathodes

Area of Science:

• Materials Science
• Electrochemistry
• Solid-state Chemistry

Background:

• Conventional lithium-ion (Li-ion) cathodes rely on metal cation redox for charge storage.
• Higher charge capacities are sought by enabling storage of multiple lithium ions per transition metal.
• Resource-sensitive elements like cobalt and nickel are being phased out in battery materials.

Purpose of the Study:

• To investigate novel lithium-rich layered iron sulfides for high-capacity Li-ion battery cathodes.
• To elucidate the charge storage mechanisms, including cation and anion redox contributions.
• To assess the cycling stability and potential advantages over existing cathode materials.

Main Methods:

• Synthesis and characterization of Li₂FeS₂ and LiNaFeS₂.
• Electrochemical testing including charge-discharge cycling.
• Ex situ and operando structural and spectroscopic analyses (e.g., S K-edge spectroscopy).

Main Results:

• Li₂FeS₂ and LiNaFeS₂ reversibly store ≥1.5 electrons per formula unit.
• Charge storage involves reversible Fe²⁺ oxidation and S²⁻ anion redox (2 S²⁻ → (S₂) ²⁻).
• Distinct structural responses to redox processes were observed between Li₂FeS₂ and LiNaFeS₂, impacting cycling stability.

Conclusions:

• Lithium-rich iron sulfides offer a promising avenue for high-capacity, stable Li-ion battery cathodes.
• Anion redox plays a crucial role in achieving high charge storage in these materials.
• The developed materials avoid critical elements and operate within a stable electrochemical window, facilitating mechanistic studies.