Environmental Engineering Reference
In-Depth Information
Table 4.16 Lithium-polymer comparison to lead-acid battery
(Delphi Automotive)
Attribute
Lead-acid
Delco Freedom SLI
Lithium-polymer
Delphi Automotive PLI
Power (W/kg) at 50% SOC
107
930
W/L at 50% SOC
233
1,860
Energy (Wh/kg) at 2 h rate
27
80
Wh/L at 2 h rate
59
160
The costs of lithium-ion battery systems are today at least four times higher
than that of SLI batteries. The LiPo battery discussed above has cost metrics of
$400-550/kWh without its supporting subsystems and $500-700/kWh with sup-
porting battery management unit in a 42 V PowerNet. From Table 4.16 we see
that LiPo has a power to energy ratio (
P
/
E
= 12 and higher) well into the range of
hybrid applicability. LiPo is capable of high pulse power because the cell struc-
ture used is composed of a number of bicells in parallel instead of plates. These
bicells rely on thin film technology to create a tight contact with the electrolyte
with minimum free electrolyte. The electrodes are immersed in a polymer matrix
akin to a sponge that retains the liquid electrolyte. Variations in the electrode
thickness then have direct bearing on cell power and energy characteristics.
Thin electrodes are high power while thicker electrodes, more volume of micro-
pores, have higher energy. The bicell laminations can be made to any length or
width. Prismatic cell construction is readily obtained, so that very thin, flat geo-
metries can be fabricated that makes installation easier. The cell electrode
described forms the basic structure of the electronic double layer capacitor: the
ultra-capacitor.
For comparison,
P
/
E
~ 3 is typical of BEV application. Higher voltage energy
storage modules have gravimetric and volumetric cost metrics double the respec-
tive values at 42 V. This must be factored into any voltage level selection.
According to the US Advanced Battery Consortium [33], LiPo technology is
seen as the most promising long term battery on the basis of performance and life
testing. But it has implementation issues related to its thin film construction and
requires further progress in new electrode and electrolyte materials, improved
laminate manufacturing process and safe means of transporting from manufacturer
to vehicle integrator. In the past decade, two battery chemistries have emerged that
have the potential to deliver the
P
/
E
targets needed for hybrid propulsion: NiMH
and lithium ion. NiMH offers high power capability because it has good ionic
conductivity in the potassium hydroxide electrolyte. Lithium ion, however, suffers
from poor ionic transport unless very thin foil electrodes are used. Lithium ion does
possess better energy density than NiMH.
The concern over shipping safety has been addressed by the Department of
Energy, Advanced Battery Readiness Working Group. This group recommended
