Surface Fluorination of Reactive Battery Anode Materials for Enhanced Stability

Abstract

Significant increases in the energy density of batteries must be achieved by exploring new materials and cell configurations. Lithium metal and lithiated silicon are two promising high-capacity anode materials. Unfortunately, both of these anodes require a reliable passivating layer to survive the serious environmental corrosion during handling and cycling. Here we developed a surface fluorination process to form a homogeneous and dense LiF coating on reactive anode materials, with in situ generated fluorine gas, by using a fluoropolymer, CYTOP, as the precursor. The process is effectively a "reaction in the beaker", avoiding direct handling of highly toxic fluorine gas. For lithium metal, this LiF coating serves as a chemically stable and mechanically strong interphase, which minimizes the corrosion reaction with carbonate electrolytes and suppresses dendrite formation, enabling dendrite-free and stable cycling over 300 cycles with current densities up to 5 mA/cm. Lithiated silicon can serve as either a pre-lithiation additive for existing lithium-ion batteries or a replacement for lithium metal in Li-O and Li-S batteries. However, lithiated silicon reacts vigorously with the standard slurry solvent N-methyl-2-pyrrolidinone (NMP), indicating it is not compatible with the real battery fabrication process. With the protection of crystalline and dense LiF coating, LiSi can be processed in anhydrous NMP with a high capacity of 2504 mAh/g. With low solubility of LiF in water, this protection layer also allows LiSi to be stable in humid air (∼40% relative humidity). Therefore, this facile surface fluorination process brings huge benefit to both the existing lithium-ion batteries and next-generation lithium metal batteries.

A compact inorganic layer for robust anode protection in lithium‐sulfur batteries

“…F 1s spectra follow the same trend (Figure B), exhibiting a F‐S peak (688.6 eV) at the surface and strengthening LiF (684.6 eV) peak in the inner part as a result of FS reduction by Li metal. The high mechanical modulus and surface energy of LiF were proven to suppress Li dendrites in previous publications . N 1s spectra exhibits four peaks at the surface due to its strong oxidizing nature (Figure C), namely, (407.0 eV), (401.6 eV), and their reduction products (403.3 eV) and Li 3 N (399.0 eV).…”

Halide‐Based Materials and Chemistry for Rechargeable Batteries

“…The surface modification of anode materials focused on forming stable SEI layers, which were mostly developed by designing and optimizing electrolyte compositions (see Section 2.3). For example, fluorination treatments were developed for lithium‐metal anodes to form a high‐quality LiF protective layer that could limit the side reactions between the electrolyte and anode material and suppress the formation of lithium dendrites …”

“…A transplantable LiF‐rich layer has been employed to protect the Li anode, successfully suppressing the side reactions between the Li metal and the carbonate‐based electrolytes . A similar impressive achievement was obtained by forming a chemically stable and mechanically strong LiF interphase, which was prepared by fluorinating the surface of the Li metal with in situ generated fluorine gas from a fluoropolymer . Recently, a Li/Al 4 Li 9 ‐LiF nanocomposite anode, which was fabricated through an “overlithiation” of a mesoporous AlF 3 framework, was developed.…”

Halogenid‐basierte Materialien und Chemie für wiederaufladbare Batterien

“…Die Oberflächenmodifikation für Anodenmaterialien fokussiert sich auf die Bildung stabiler SEI‐Schichten, die weitestgehend durch Konstruktion und Optimierung der Elektrolytzusammensetzungen (siehe Abschnitt 2.3) entwickelt wurden. Zum Beispiel wurden Fluorierungsbehandlungen für Lithium‐Metall‐Anoden entwickelt, um eine hochwertige LiF‐Schutzschicht zu bilden, welche die Nebenreaktionen zwischen Elektrolyten und Anodenmaterial begrenzen und die Lithiumdendrit‐Bildung unterbinden konnte …”

“…Eine transplantierbare LiF‐reiche Schicht wurde zum Schutz der Li‐Anode eingesetzt und hat erfolgreich die Nebenreaktionen zwischen dem Li‐Metall und den Carbonat‐basierten Elektrolyten unterbunden . Eine ähnlich beachtliche Errungenschaft konnte durch die Bildung einer chemisch stabilen und mechanisch starken LiF‐Interphase erreicht werden, präpariert durch Fluorierung der Li‐Metall‐Oberfläche mit in situ mit einem Fluorpolymer erzeugtem Fluorgas . Kürzlich wurde eine Li/Al 4 Li 9 ‐LiF Nanokomposit‐Anode, die durch eine “Überlithiierung” eines mesoporösen AlF 3 ‐Gerüsts hergestellt wurde, entwickelt.…”