Elucidating Gas Reduction Effects of Organosilicon Additives in Lithium-Ion Batteries

Abstract

Lithium-ion batteries (LIBs) with nonaqueous liquid electrolytes are prone to gas generation at elevated voltages and temperatures, degrading battery performance and posing serious safety risks. Organosilicon (OS) additives are an emerging candidate solution for gassing problems in LIBs, but a detailed understanding of their functional mechanisms remains elusive. In this work, we present a combined computational and experimental study to elucidate the gas-reducing effects of OS additives. Cell volume measurements and gas chromatography-mass spectrometry reveal that OS additives can substantially reduce gas evolution in LIBs, particularly CO₂ regardless of source. Through density functional theory calculations, we identify multiple plausible pathways for CO₂ evolution, including (1) nucleophile-induced ring-opening of ethylene carbonate (EC) and the subsequent electro-oxidation and (2) direct electro-oxidation of lithium carbonate (Li₂CO₃). Correspondingly, we find that OS additives function via two primary mechanisms: (1) scavenging of nucleophiles such as superoxide (O₂^•-), peroxide (O₂^2-), and carbonate ion (CO₃^2-); (2) oligomerization with ethylene carbonate oxide ion and ethylene dicarbonate ion. Moreover, we discover that OS additives possess strong lithium coordination affinity, which helps further reduce the nucleophilic reaction energies and hence increases their nucleophile-scavenging efficiency. Finally, we provide a mechanistic interpretation for the enhanced gas-reduction effects observed with fluorinated OS compounds, corroborated by surface analysis results from X-ray photoelectron spectroscopy. Our study offers the first molecular-level insights into how OS additives contribute to reduced gas formation in LIBs, paving the way for improved safety and performance of LIBs.

Figure 1

Molecular structures of the organosilicon (OS) nitrile additives considered in this work. Degrees of fluorination in the silyl group are labeled nonfluorinated (NoF), monofluorinated (1F), difluorinated (2F), and trifluorinated (3F), respectively.

Figure 2

(a) Gas volume increase after 4 weeks of storage at 60 °C relative to after formation in 4.3 V SC-NMC811/Gr pouch cells with control 1 and 3% NoF-OS/1F-OS/2F-OS/3F-OS electrolytes. (b) CO₂ volume in a separate experiment (also 4 weeks storage at 60 °C in 4.3 V SC-NMC811/Gr pouch cells) with control 2 and 3% 1F-OS/1% 2F-OS/0.75% 3F-OS electrolytes, with ¹³C-labeled EC and GC-MS employed to quantify CO₂ originating from EC (¹³CO₂, m/z 45) versus other sources (¹²CO₂, m/z 44).

Figure 3

(a, b) Statistics of OS in Li⁺ first solvation shells, (c, d) average Li⁺ coordination numbers of various species, and (e, f) statistics of Li⁺ with different anion coordination states in 1 M LiPF₆ 1:1:1 (%vol) EC:EMC:DEC solutions. (a, c, e) Control and 1/2/5% NoF-OS electrolytes; (b, d, f) control and with 5% NoF-OS/1F-OS/2F-OS/3F-OS electrolytes.

Figure 4

(a, b) Free energy profiles of S_N2-type ring-opening reactions of (a) noncoordinated EC and (b) Li⁺-coordinated EC via nucleophilic substitution (superoxide, peroxide, carbonate ion) at an ethylene carbon. (1) and (2) in (b) refer to the monodentate and bidentate conformers of the transition state/intermediate state structures. Red and blue highlights indicate bond breaking and formation, respectively. (c) Electrochemical oxidation pathways of ethylene carbonate oxide, ethylene dicarbonate, and carbonate ion. Oxidation potentials are referenced to the Li⁺/Li redox couple.

Figure 5

(a) Optimized geometries of NoF-OS, Li⁺-NoF-OS, NoF-OS-EC, and NoF-OS-PF₆^– in their native and oxidized states. (b) Oxidation potentials of the aforementioned species, referenced to the Li⁺/Li redox couple. The horizontal line at 4.4 V vs Li⁺/Li is a typical upper voltage limit for NMC811 cathodes under normal working conditions.

Figure 6

Relative free energies of the intermediate states (I) and products (P) of various chemical reactions between noncoordinated (gray-boxed)/Li⁺-coordinated (pink-boxed) OS additives and various nucleophiles. (a) Bimolecular nucleophilic substitution (S_N2) via backside attack of an F atom attached to Si. (b) Nucleophilic addition (A_N) via backside attack of a methyl group attached to Si. (c) A_N via backside attack of the −(CH₂)₃–C≡N moiety attached to Si. (d) A_N via attack of the cyano-C. (e) Same as (a), with the nucleophile being the [O₂^•–]/[CO₃^•–] part of ethylene carbonate oxide ion and the [CO₃^•–] part of ethylene dicarbonate ion. Nonreacting additive moieties are shown as squiggly lines in the molecular structures.

Figure 7

XPS surface analysis of NMC811 cathodes after formation in SC-NMC811/Gr pouch cells for control 1 and 3% NoF-OS/1F-OS/2F-OS/3F-OS electrolytes: (a) N 1s and Si 2s spectral regions; (b) percent composition of nitrogen and silicon out of all surface elements.