Tuning Bulk Redox and Altering Interfacial Reactivity in Highly Fluorinated Cation-Disordered Rocksalt Cathodes

• Published in: ACS applied materials & interfaces Vol. 15; no. 15; pp. 18747 - 18762
• Main Authors: Crafton, Matthew J., Huang, Tzu-Yang, Yue, Yuan, Giovine, Raynald, Wu, Vincent C., Dun, Chaochao, Urban, Jeffrey J., Clément, Raphaële J., Tong, Wei, McCloskey, Bryan D.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 19.04.2023; American Chemical Society (ACS)
• Subjects: cathodes; cathode−electrolyte interface; cation-disordered rocksalt; differential electrochemical mass spectrometry; electric potential difference; electrochemistry; electrolyte degradation; electrolytes; ENERGY STORAGE; Energy, Environmental, and Catalysis Applications; fluoride-scavenging; fluorine; high voltage; hysteresis; manganese; mass spectrometry; nuclear magnetic resonance spectroscopy; oxygen; oxygen redox; titration; titration mass spectrometry; X-ray photoelectron spectroscopy; differential electrochemical mass spectrometry; cathode−electrolyte interface; oxygen redox; high voltage; titration mass spectrometry; fluoride-scavenging; cation-disordered rocksalt; electrolyte degradation
• ISSN: 1944-8244; 1944-8252; 1944-8252
• DOI: 10.1021/acsami.2c16974

Lithium-excess, cation-disordered rocksalt (DRX) materials have been subject to intense scrutiny and development in recent years as potential cathode materials for Li-ion batteries. Despite their compositional flexibility and high initial capacity, they suffer from poorly understood parasitic degradation reactions at the cathode–electrolyte interface. These interfacial degradation reactions deteriorate both the DRX material and electrolyte, ultimately leading to capacity fade and voltage hysteresis during cycling. In this work, differential electrochemical mass spectrometry (DEMS) and titration mass spectrometry are combined to quantify the extent of bulk redox and surface degradation reactions for a set of Mn2+/4+-based DRX oxyfluorides during initial cycling with a high-voltage charging cutoff (4.8 V vs Li/Li+). Increasing the fluorine content from 7.5 to 33.75% is shown to diminish oxygen redox and suppresses high-voltage O2 evolution from the DRX surface. Additionally, electrolyte degradation processes resulting in the formation of both gaseous species and electrolyte-soluble protic species are observed. Subsequently, DEMS is paired with a fluoride-scavenging additive to demonstrate that increasing fluorine content leads to increased dissolution of fluorine from the DRX material into the electrolyte. Finally, a suite of ex situ spectroscopy techniques (X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectroscopy, and solid-state nuclear magnetic resonance spectroscopy) are employed to study the change in DRX composition during charging, revealing the dissolution of manganese and fluorine from the DRX material at high voltages. This work provides insight into the degradation processes occurring at the DRX–electrolyte interface and points toward potential routes of interfacial stabilization.