Operando Monitoring of F^– Formation in Lithium Ion Batteries

Abstract

Online electrochemical mass spectrometry (OEMS) was applied to study the influence of tris­(trimethylsilyl)­phosphate (TMSPa) as an additive in 1 M LiPF6 (fluoroethylene carbonate/diethylene carbonate (DEC)) electrolyte on the gas evolution in Li-rich/NCM full cells during cycling. The results indicate that TMSPa neither influences the solid electrolyte interphase (SEI) formation on the anode nor the surface reconstruction on the cathode but acts as a chemical scavenger for HF and LiF. TMSPa thus lowers the electrolyte acidity and suppresses further LiPF6 decomposition, resulting in lower impedance and higher lithium ion battery (LIB) performance. Furthermore, the selective reactivity of TMSPa toward fluorides leads to the formation of Me3SiF enabling the additive to act as a chemical probe and to study HF/LiF formation operando by OEMS. By this methodology, we were able to identify contributions from SEI formation, proton and reactive oxygen formation >4.2 V, cross-talk between the anode and cathode, and the polyvinylidene fluoride binder to the fluoride formation in LIBs.

Anion Reactivity in Cation‐Disordered Rocksalt Cathode Materials: The Influence of Fluorine Substitution

“…LiPF 6 also commonly degrades to form fluorides such as LiF and HF, so the additional fluoride formed from LMNOF could potentially be insignificant in comparison to the electrolyte‐originating fluoride. [ 26,28,30,35 ] To study the effect of cathode fluorination on fluoride ion formation in a cell containing LiPF 6 , we also conducted fluoride‐scavenging DEMS in cells containing LiPF 6 electrolyte and polyethylene bound cathodes. Now using Me 3 SiF evolution as a proxy for fluoride formation from either fluorine dissolution from LMNOF or degradation of LiPF 6 , we are able to study fluoride formation in another setting.…”

“…Fluoride species like HF have been shown to increase the degradation of LiPF 6 . [ 26,30 ] This reaction leads to the formation of more fluoride species, thereby accelerating itself in an autocatalytic loop. Through this mechanism, an initially small amount of fluoride dissolved from the cathode could therefore greatly increase the extent of electrolyte degradation in the cell.…”

“…Fluoride‐scavenging is a technique whereby an additive that reacts to consume fluoride is added to the cell. [ 26 ] By using an additive that produces a gas when it reacts with fluoride, we can pair fluoride‐scavenging with DEMS. In this work, we use the fluoride‐scavenging additive originally described by Bolli et al, tris(trimethylsilyl) phosphate (TMSPa).…”

“…In this work, we use the fluoride‐scavenging additive originally described by Bolli et al, tris(trimethylsilyl) phosphate (TMSPa). [ 26 ] TMSPa reacts with fluoride to form gaseous trimethylsilyl fluoride (Me 3 SiF), which can then be detected by DEMS. [ 27 ] Using Me 3 SiF evolution as a proxy for fluorine dissolution from the cathode into the electrolyte as fluoride, we can thereby monitor and quantify the formation of fluoride from the cathode.…”

In Situ Construction of Uniform and Robust Cathode–Electrolyte Interphase for Li‐Rich Layered Oxides

“…[ 34–38 ] On the other hand, the (SiO)‐containing molecules can preferentially decompose to facilitate the in situ “pouring” of various fragments to form a crosslinked flexible CEI layer. [ 39,40 ] Subsequently, the “anchoring + pouring” synergy in CEI construction significantly enhance the electrochemical stability of LLOs, by suppressing the irreversible side reactions and TM dissolutions. The effectiveness and mechanism of this CEI strategy is well demonstrated on a representative LLO (Li 1.13 Mn 0.517 Ni 0.256 Co 0.097 O 2 ) as displayed experimentally and theoretically next.…”