Environmental Engineering Reference
In-Depth Information
Thermal management of ESS packs remains a matter of OEM preference, but
air cooling prevails due to cost considerations. Some examples can be cited in
support of this assertion:
The Ford Escape hybrid discussed here uses Sanyo NiMH cells and a separate
dedicated air cooling loop.
●
The 2010 Fusion hybrid with similar electric drive and ESS does not require a
separate cooling loop.
●
Recent introductions of battery electrics such as the Ford Focus EV, BMW
Mini PHEV conversion and Nissan EV may also use air cooling of lithium ion
packs.
●
Mild hybrids such as the Mercedes S Class and GM's next generation BAS
(belt alternator starter) are, or will be, designed around 120 V, 18-20 kW lithium
ion packs.
10.1.3 Lithium ion
Lithiated transition metal oxides are used as the cathode (positive terminal) in a
lithium ion cell. The metal is typically bound within a host lattice during discharge
and released during charge with no real change or damage to the electrode host.
These lithium ions form the basis of the lithium ion cell chemistry as follows:
LiMn
2
O
4
,
Li
1
x
Mn
2
O
4
þ
x
Li
þ
þ
xe
ð
10
:
18
Þ
C
þ
x
Li
þ
þ
xe
,
Li
x
C
ð
10
:
19
Þ
LiMn
2
O
4
þ
C
,
Li
x
C
þ
Li
1
x
Mn
2
O
4
ð
10
:
20
Þ
Cathode (negative terminal) chemistry is defined by (10.18) and anode
chemistry (positive terminal) by (10.19). Notice in these two expressions that some
fraction of lithium metal is released into solution with an equivalent electron
release to the external circuit at the cathode. Only the lithium ions are able to cross
the separator and fill into pores in the anode host lattice. The anode (10.19) illus-
trates how lithium ions entering the host lattice reunite with electrons from the
external circuit to form a carbon compound. Equation (10.20) illustrates the overall
reaction, known as 'rocking chair' chemistry. The reversible parameter,
x
, in these
equations is on the order of 0.85. On recharging, the carbon compound releases
lithium ions back into solution that traverse the separator and combine with elec-
trons at the cathode, reconstituting the lithium manganese oxide.
Figure 10.13 shows that during charge a lithium ion oxidizes at the positive
lithium-metal oxide (LiMO
2
) electrode and enters solution (the electrolyte con-
sisting of LiPF
6
plus additives) simultaneous with its orbital electron being
removed from the cathode and delivered under external electromotive force (emf)
to the anode. At the negative electrode, the free electron enters the graphitic
structure simultaneous with a lithium ion being removed from solution and reduced
in the anode to metallic lithium between the graphitic carbon layers such that three
