Environmental Engineering Reference
In-Depth Information
Charge
A
e
Graphene
structure
LiMO
2
structure
e
x
x
e
x
Li
+
ion
x
x
PF
-
6
ion
x
e
Electron
x
x
x
Solvent
x
x
x
Cathode
LiMO
2
Li
1-
x
MO
2
+
x
Li
+
+
x
e
-
Discharge
x
e
Charge
x
x
Porous
separator
Al
current
collector
Anode
C
n
+
x
Li
+
+
x
e
-
C
n
Li
x
Lithium-ion battery
Figure 10.13 Lithium ion electrodynamics at cell level (from Reference 7)
carbon atoms above and three carbon atoms below trap this lithium atom, hence
LiC
6
. During discharge, the lithium de-intercalates from the anode graphite and re-
enters the electrolyte as a free ion. At the cathode, another free lithium ion enters
the electrode structure and reconstitutes it in a self-assembly manner. This is why
lithium ion cells require excess LiMO
2
and binders - the mass transfer must not be
excessive to collapse its structure. Roughly 50% of the cathode mass is therefore
involved in reactions, and one useful rule of thumb is that energy storage is
approximately 1 kWh per 170 g of metallic lithium.
Lithium systems have a nearly reciprocal charge/discharge characteristic, or
'rocking chair' behaviour. A lithium system exhibits very high energy density, very
good pulse power, highest cell potential and excellent cycle life. However, like the
NiMH cell, it requires more capable charge/discharge management, generally under
microprocessor control. A Li ion cell has a potential of 4.1 V open circuit, a gravi-
metric energy density of 125 Wh/kg and in excess of 300 Wh/L. Discharge potential
is generally from 4.1 to 3.0 V or 73%. Cycle life at 100% depth of discharge (DOD)
can exceed 1,000 cycles with a charge retention of 94%. Operating temperature for
Li ion systems is only
20 to
þ
40
C on charge and
20 to
þ
45
C on discharge. It is
of interest that Li ion has a useable SOC that is four times that of an SLI Pb-acid
battery. This is because where a Pb-acid battery may only be operated from 90% to
40% SOC or less, the Li ion battery can easily operate from 100% to 10% or less SOC
before recharge is necessary. This makes the Li ion very suited to hybrid propulsion.
Li ion has the discharge behaviour illustrated in Figure 10.14.
In a lithium ion cell the anode (negative terminal during discharge) is generally
made of carbon (graphite), whereas the cathode (positive terminal during
discharge) consists of a lithium-metal oxide composition. The electrolyte is an
