Fumed oxide-based nanocomposite polymer electrolytes for rechargeable lithium batteries

Abstract

Rechargeable lithium batteries are promising power sources for portable electronic devices, implantable medical devices, and electric vehicles due to their high-energy density, low self-discharge rate, and environmentally benign materials of construction. However, the high reactivity of lithium metal limits the choice of electrolytes and impedes the commercialization of rechargeable lithium batteries. One way to tackle this problem is to develop electrolytes that are kinetically stable with lithium. Composite polymer electrolytes (CPEs) based on fumed oxides presented in this work are promising candidates for rechargeable lithium batteries.

The effects of fumed oxides (SiO2, Al2O3, TiO2) and binary mixtures of oxides (SiO2/Al2O3) on ionic conductivity of CPEs based on poly(ethylene oxide) (PEO) oligomers (Mw =250, 200, 1000, and 2000) + lithium bis(trifluromethylsulfonyl)imide [LiN(CF3SO2)2] (LiTFSI) (Li:O=1:20) are studied using electrochemical impedance spectroscopy (EIS), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy. Fillers show similar effect on conductivity in all systems: no distinguishable effect is found with filler type, and addition of filler decreases conductivity at temperatures above the melting point (Tm) but increases conductivity at temperatures below. The insulating nature of fillers and stiffening of the polymer solvent (as evidenced by FTIR and DSC data) in the presence of fillers cause a decrease in conductivity at temperatures above Tm, which remains constant upon addition of fillers. The increase in conductivity at temperatures below Tm can be attributed to faster ion transport along the filler surface. Addition of fumed oxides increases electrolyte viscosity (and elasticity) and the extent of enhancement varies with filler type: fumed silica shows the strongest and titania the least. Elastic modulus, yield stress, and normalized viscosity of gel-type composite electrolytes decrease with increasing oligomer Mw when electrolytes are amorphous. The reduction in structure strength may be ascribed to the enhanced interactions between surface hydroxyl groups on fumed oxides and polyether oxygens. Thus, the number of accessible ?OH groups is reduced for interactions among fumed oxide particles, which dictates the strength of solid-like structure.

The interfacial stability between electrolyte and lithium is enhanced in the presence of fumed silica. The enhancement in interfacial stability is seen as a decrease in interfacial resistance and cell polarization, and an increase in lithium cycleability and cell capacity. The improved interfacial stability between CPE and lithium is attributed to less lithium corrosion (fillers scavenge water impurities that corrode lithium) and dendrite formation (electrolyte elasiticty inhibits dendrite formation). The extent of the enhancing effect of fumed silica depends on its surface chemistry, with the largest effect seen with hydrophilic fumed silica, which has the largest scavenging capacity and highest elasticity.

The effect on cycle capacity is reported of cathode material (metal oxide, carbon, and current collector) in lithium/metal oxide cells cycled with fumed silica-based composite electrolytes. Cells with composite electrolytes show higher capacity and less cell polarization than those with filler-free electrolyte. Among the three active materials studied (LiCoO2, V6O13, and LixMnO2), V6O13 cathodes deliver the highest capacity and LixMnO2 cathodes render the best capacity retention. Discharge capacity of Li/LiCoO2 cells is affected greatly by cathode carbon type and discharge capacity increases with decreasing carbon particle size. Current collector materials also play a significant role in cell cycling performance: Li/V6O13 cells deliver increased capacity using Ni foil and carbon fiber current collectors in comparison to an Al foil.

In summary, fumed oxide-based nanocomposite electrolytes are promising candidates for lithium battery applications with high room-temperature conductivity, good mechanical strength, stable interface between lithium metal and electrolytes, and reasonable capacity and capacity retention with optimized cathode compositions.