Page 543 - Handbook of Battery Materials

516 16 The Anode/Electrolyte Interface

ordered and less conductive film, as found previously [1, 2] for nonaqueous solu-
tions. In another test, an Li/CPE/Li cell was stored for over 3000 h with almost no
change in R SEI . At temperatures above or near the eutectic temperature of the PE,
−2
C SEI values were in the range 0.1–2 µFcm . However, below this temperature
or for CSEs, which are stiffer than CPEs, the SEI capacitance could be as low as
−2
0.001 µFcm . The conductivity of the SEI (σ ESI ) was found to be three to four
orders of magnitude lower than that of the CPE and did not change much with the
salt concentration [5]. The replacement of Al 2 O 3 by MgO, or LiI by LiBr, had little
effect on σ SEI . However, both changes result in a severe decrease in C SEI , probably
due to the stiffness of these CPEs. The apparent energy for ionic conduction in the
−1
◦
◦
SEI measured at 130 C > T > 60 C was found to be 7–11 kcal mol . The addition
of copolymers such as poly(butyl acrylate) (PBA) and poly(methyl acrylate) (PMA)
and the variation of the EO : Li ratio from 6 : 1 to 10 : 1 were found to have some
effect on the SEI properties. In general, C SEI increases and R SEI decreases with
increasing organic content of the CPE. The stiffer CPE, which contained PBA, had
the highest SEI resistance (due to low θ values), and the lowest conductivity.
Figure 16.17 presents the effect of EC and PEGMDE on the Li/CPE interphase.
The addition of EC was followed by a decrease by 1 order of magnitude in R SEI
◦
and L SEI .Both R SEI and L SEI were stable for over 500 h of storage at 120 C. Similar
behavior was observed in CPEs containing PEGDME with a MW of 500 and 2000.
High SEI stability was achieved in these electrolytes for 1200 h of storage. The
positive effect of plasticizers may result from better wetting of the lithium metal by
the PE, that is, an increase in the contact area. In addition, the formation of a thin and
stable SEI composed mainly of lithium carbonate is expected in CPE containing EC.
The interfacial properties of gel electrolytes containing EC immobilized in a
polyacrylonitrile (PAN) matrix with a lithium (bis)trifluoromethane sulfonimide
(LiTFSI) salt have been studied [137]. SEI stability appeared to be strongly dependent
2
on the LiTFSI concentration. A minimum value of R SEI of about 1000 cm was
obtained after 200 h of storage of an electrolyte containing 14% salt. This value was

80 2000
without EC(2) 1500
60
R SEI (ohmcm 2 ) 40 R SEI L SEI 1000 L SEI [Å]

with EC(1)
20
R SEI L SEI 500
0 0
0 200 400 600 800
time [hr]
Figure 16.17 Effect of EC on the apparent thick-
ness and stability of SEI [6]. CPE composition: (1)
LiI/P(EO) 9 P(MMA) 0.5 (EC) 2 + 6% (v/v)Al 2 O 3 .(2)
LiI/P(EO) 9 P(MMA) 0.5 + 6%(v/v)Al 2 O 3 .