Nanocomposite Based Electrolytes for Lithium-Ion Batteries
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Attempts at dispersing
smectites into PEO based systems has also met with limited success even with low-molecular weight PEO.5 Most likely the conductivities were low due to a layered assembly of the smectite particles rather than a disperse system, resulting in poor lithium ion mobility. The focus of this work has been to disperse the lithium form of hectorite into highdielectric organic solvents (carbonates) to create a physically gelled nanocomposite electrolyte. It is believed that a high-dielectric solvent should adequately solvate the lithium ions, preventing collapse of the dispersed hectorite platelets. EXPERIMENT Synthetic sodium hectorite was donated by Hoechst (SKS-21, 88 meq/100 g, 250 nm avg. size). Ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) were obtained from Aldrich and dried using 4 A molecular sieves. Lithium hexafluorophosphate (LiPF 6) was obtained from Aldrich and vacuum dried at approximately 120'C for 24 hours. Water content was measured using Karl-Fischer titration. Li Hectorite/carbonate nanocomposites were produced using the general procedure outlined in Figure 1. Water concentration was varied in the PC composites to study the effect on conductivity and transference number. Conductivity measurements were performed with a twoelectrode (platinum) cell. Li+ transference numbers were measured in Li metal/electrolyte/Li
137 Mat. Res. Soc. Symp. Proc. Vol. 575 02000 Materials Research Society
metal button cells using the steady-state current method of Bruce and Vincent. 6 Elastic and viscous moduli and dynamic yield stress were measured using methods previously described. 7 RESULTS Conductivity LiHect based composite electrolytes exhibit room-temperature conductivities in excess of 10 4 S/cm (Figure II). In comparison, these conductivities are approximately 30 times less than conductivities of Li÷ molar equivalent LiPF 6 electrolytes. Among the three solvents examined (PC, 1:1 EC:PC (v:v), and 2:1:1 EC:PC:DMC (v:v:v)), there does not appear to be a strong solvent dependence for the LiHect conductivity within the typical reproducibility (+/- 10%) of the composite conductivity measurements.
The optimum Lillect concentration for maximum conductivity is approximately 0.4 M
Composite Formation Procedure Aqueous cation exchange (NaHect to Lilect) and dry
Disperse in mixture of EC (or PC) +_H I +H 2
Dry to form concentrated composite in
( P C) EC(orPC)
"*Carbonate solvents studied: * PC Dilute to * 1:1 EC:PC (v:v) desired (v:v:v) • 2::1 C:P:DMC(v~~v)composition * 2: 1:1 EC:PC:DMC
and
"*Solvent abbreviations: * EC - ethylene carbonate • PC - propylene carbonate * DMC - dimethyl carbonate
concentration
Figure I. General procedure for producing Li Hectorite/carbonate nanocomposites. 10.2
LiPF6
_••=5•-
4
E
PC
10-3
--
1:1 EC:PC 2:1:1 EC:PC:DMC
-
"U 0 C.
Li Hectorite
-
10-4
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Li concentration (M) 0.0
0.2
0.4 0.6 0.8 1.0 1.2 g Li Hectorite / ml solvent
Figure II. Comparison of LiHect composite conductivity with
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