Nanocomposite Gel Electrolytes Based on Fumed Silica for Lithium-Ion Batteries

Abstract

Nanocomposite gel electrolytes based on silica particles dispersed in lithium bis͑perfluoroethylsulfonyl͒imide ͑LiBETI͒ salt + mixed-carbonate solvent are examined as an electrolyte system for a lithium-ion battery. Gel behavior is observed with both hydrophilic ͑A200͒ and hydrophobic ͑R805͒ fumed silicas. The silica nanoparticles affect a small decrease in conductivity but increase mechanical strength significantly ͑elastic modulus ϳ10 5 Pa͒. Chronoamperometry and linear sweep voltammetry results show that an Al current collector is stable in 1 M LiBETI carbonates up to ϳ5 V. Cycling Li͑Ni͒/electrolyte/Li cells shows that silica nanoparticles improve the coulombic efficiency and interfacial stability in the order: 10% R805 LiBETI gel Ͼ LiBETI liquid Ͼ LiPF 6 liquid. Cycling Li/LiFePO 4 cells shows that both liquid and 10% R805 gel electrolytes provide good capacity and cycle performance, but the average charge/discharge voltages for the latter are more stable. Both Li/LiMn 2 O 4 and Li/graphite cells have less capacity fade using LiBETI than LiPF 6 electrolyte. The gel electrolyte provides better cycle performance than its liquid counterpart because of its increased interfacial stability due to improved rheology and ability to scavenge residual moisture. Silica-based LiBETI carbonate nanocomposite gel electrolytes appear to be a promising candidate for lithium-ion batteries. Lithium-ion batteries are widely used in portable electronics and are under scrutiny for application in electric vehicles ͑EVs͒ and hybrid electric vehicles ͑HEVs͒ since they have relatively high-energy and -power density compared with other batteries. 1 Most commercial lithium-ion cells today use liquid electrolytes. Cur-rently, mixed-organic carbonate solutions with LiPF 6 salt are the dominant electrolyte for lithium-ion batteries since they offer high-ionic conductivity, stable solid electrolyte interface ͑SEI͒ with graphite, and good voltage stability. 2 However, these electrolytes are not a good candidate for use in large-size batteries due to the ther-mal instability of LiPF 6 , 3 which is reported to undergo thermal de-composition at 60°C. 4 Since LiPF 6 is thermally unstable, an inten-sive effort has led to more stable substituted lithium salts. Among them, lithium bis͑oxalato͒borate ͑LiBOB͒, lithium bis͑per-fluoroethylsulfonyl͒imide ͑LiBETI͒, LiFAP ͓LiPF 3 ͑CF 2 CF 3 ͒ 3 ͔, and Li 2 B 12 F 9 H 13 are the leading candidates. 4-7 However, concerns re-main for these salts: e.g., LiBOB has limited solubility in organic solvents, which negatively impacts conductivity and potentially hin-ders cell-rate capability. 8 LiBETI, a lithium salt developed by 3M, has been under extensive investigations both for lithium metal and lithium-ion batteries. LiBETI is one family member of lithium imide salts, which have excellent ionic dissociation, even in low dielectric solvents, and can plasticize a polymer. 9,10 In comparison with other lithium salts, LiBETI is reported to form a thinner and smoother interface film with lithium, and thus have higher lithium cycling efficiency. 11,12 Thermogravimetric analysis results reveal that Li-BETI is more thermally stable than LiPF 6 and LiBOB. 13 Gnanaraj et al. 14 have used accelerating rate calorimetry and found that LiBETI electrolytes are more stable than LiPF 6 and LiFAP. Gel electrolytes are attractive for use in lithium-ion batteries since they are reported to provide improved safety and alleviate leakage problems compared to the liquid counterpart. 15 Third-generation lithium-ion cells using gel electrolytes have been already commercialized by Sony. 16 Our group has successfully developed a type of nanocomposite gel electrolyte using fumed silica nanoparticles. 17-20 The gelling mechanism is the formation of a three-dimensional continuous network by interaction of silica sur-face groups rather than the solvent swelling of conventional poly mer chains, e.g., poly͑vinylidenefluoride-co-hexafluoropropylene ͑PVDF-HFP͒. 21 Gels affected by use of nanoparticulate solid fillers differ significantly from those using polymers, especially at low and high temperature. At low temperature polymer chains tend to a glassy state while at high temperature polymer chains melt with consequential decrease in elastic modulus. In comparison, solid fill-ers can prevent crystallization at low temperature and yet still sup-port a three-dimensional network at high temperature. 22 Silica-based composite gel electrolytes exhibit desirable mechanical elastic modulus GЈ in excess of 10 5 Pa and are processable with shear-thinning behavior. 18,20 Fumed silica nanoparticles ͑ϳ12 nm primary particle͒ are an amorphous, nonporous type silica with a native hy-droxyl group that may be functionalized with a variety of moieties.