Page 509 - Handbook of Battery Materials

482 16 The Anode/Electrolyte Interface

− −
e − e
C 3 O 3 H 4 (C 3 O 3 H 4 ) + Li 2 CO 3 + CH 2 = CH 2
2Li
+
2Li
−
2[(C 3 O 3 H 4 ) ] − OCO 2 CH 2 CH 2 CH 2 CH 2 OCO 2
LiOCO CH CH CH CH OCO Li
2
2
2
2
2
2
−
e − CH 2 =CH 2
CH 2 = CH 2 H C−CH 2 [CH − CH ]
2
2
2 n
−
− + 2e 3e 0
AsF 6 + 3Li 3LiF + AsF 3 + 6LiF + As
3Li
1 + 2e −
O + 2Li Li 2 O
2 2
− 1
e
H 2 O + Li + LiOH + H 2
2
− 3e − 0
BF 4 + 4Li B + 4LiF
− −
PF 6 + H 2 O HF + PF + OH
Li + HF LiF + 1 H
2 2
Li CO + 2HF 2LiF + H CO 3
3
2
2
LiOH + HF LiF + H 2 O
Li 2 O + 2HF 2LiF + H 2 O
Figure 16.1 Electrolyte decomposition reactions.
The charge needed to complete the formation of the SEI (about 10 −3 mAh cm −2
[8, 12]) increases with the real surface area of the electrode and decreases with
increase in the current density and with decrease in the electrode potential (below
the SEI potential). In practice, it may take from less than a second to some hours to
build an SEI 2–5 nm thick. When lithium is cut while immersed in the electrolyte,
the SEI forms almost instantaneously (in less than 1 ms [13, 14]). On continuous
plating of lithium through the SEI during battery charge, some electrolyte is
consumed in each charge cycle in a break-and-repair process of the SEI [1, 2],
and this results in a faradaic efficiency lower than 1. When a battery is made
with commercial lithium foil, the foil is covered with a native surface film. The
composition of this surface film depends on the environment to which the lithium
is exposed. It consists of Li 2 O, LiOH, Li 2 CO 3 ,Li 3 N, and other impurities. When
this type of lithium is immersed in the electrolyte, the native surface film may react
with the solvent, salts, and impurities to form an SEI, whose composition may
differ from that of electrodeposited lithium in the same electrolyte. The formation
of SEI on carbonaceous anodes is discussed in Section 16.3.