Page 408 - Handbook of Battery Materials

380 13 Rechargeable Lithium Anodes

showed that lithium covered with Li 2 CO 3 is superior both in terms of storage and
discharge because of its stability and because a lithium anode has a low impedance
[3, 4].

13.3
Surface of Lithium Coupled with Electrolytes

Lithium metal is chemically very active and reacts thermodynamically with any
organic electrolyte. However, in practice, lithium metal can be dissolved and
deposited electrochemically in some organic electrolytes [5]. It is generally believed
that a protective film is formed on the lithium anode which prevents further
reaction [6, 7]. This film strongly affects the lithium cycling efficiency.
According to the solid electrolyte interphase (SEI) model presented by Peled
[8], the reaction products of the lithium and the electrolyte form a thin protective
film on the lithium anode. This film is a lithium-ion conductor and an electronic
insulator, whose nature prevents any further chemical reaction. Aurbach et al. and
many other research workers have tried to identify the chemical products com-
posing the protection film [9–20] using Fourier transform infrared spectroscopy
(FTIR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The
protective films differ depending on the kind of electrolyte, and mainly con-
sist of Li 2 CO 3 , LiX (X: halogen), ROLi, and ROCOOLi (R: alkyl group). LiPF 6
solute forms LiF, and LiAsF 6 solute forms some As compounds as the pro-
tective films. Chemical composition and physical structure of the surface films
are affected by H 2 O content of electrolyte solutions and remaining water inside
the cell. Ethylene carbonate (EC) and propylene carbonate (PC) are electrolyte
solvents with very similar chemical structures but providing different lithium
cycling efficiencies. Aurbach et al. have reported differences between the lithium
surface films in EC and PC, namely that CH 3 CH(OCO 2 Li) CH 2 OCO 2 Li and
(CH 2 OCO 2 Li) 2 are detected in the lithium surface film with dry PC and EC,
respectively [21].
The reaction of the electrolyte with lithium and the resulting film properties
affect the cycle life of the lithium cell. Shen et al. [22] have examined the
stability (reactivity) of the electrolytes by open-circuit storage tests for the
Li/TiS 2 cell system by microcalorimetry and alternate current (AC) impedance
spectroscopy. They used tetrahydrofuran (THF)-and 2-methyl-tetrahydrofuran
(2Me THF)-based electrolyte, with additives such as 2-methylfuran (2MeF),
EC, PC, and 3-methylsulfolane (3MeS), and LiAsF 6 as the solute. The heat
output of the cells on open circuit for a day (short-term reactivity) or a
year (long-term reactivity) is lower for EC/2MeTHF than for 2MeTHF or
PC/2MeTHF. Also, the cell with EC/2MeTHF has a lower SEI resistivity of
2
2
51 cm 2 than that with 2MeTHF (119 cm ) or PC/2MeTHF (214 cm ).
The cycle life increases with decreases in heat output and resistivity. They
indicate that these measurements are effective in determining electrolyte
stability.