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2.2.1.2 LITHIUM-ION BATTERIES
STRUCTURE AND CHARACTERISTICS
The lithium-Ion battery has the highest potential for use in future
hybrid and battery electric vehicles due, among other characte-
ristics, to its relatively good energy density (50 to 200 Wh/kg) and
power density (up to 5,000 W/kg). Moreover, the achievable high
cyclic stability (approx. 3,000 cycles to 80 percent DoD), as well as
a high cycle efficiency (approx. 96 percent at 80 percent DoD), are
arguments for the use of this battery in the vehicle. However, lithi-
um batteries are relatively expensive. The battery cells consist of 2
electrodes (negative anode and positive cathode) that are divided
by a separator, and usually contain an electrolyte. Rechargeab-
le lithium batteries can be classified depending on the selected
electrode material, as well as the separator and electrolytes, as
lithium-ion batteries (lithium-ion with liquid electrolyte, as well as
lithium-ion polymer with gel electrolyte), and lithium-metal batte-
ries (lithium-metal with liquid electrolyte, and lithium-polymer with
polymer electrolyte). The different properties and characteristics
of the cells depend on material selection.
CONSUMER AREA
In the area of consumer products, lithium-ion batteries have been
used for some time. In this sector, a lithium-cobalt-oxide (LiCoO
2
)
cathode, a lithium-graphite anode, an organic electrolyte, and a
polyethylene separator are usually used. Japan, Korea and Chi-
na are driving this market in terms of technology and production.
However, there are characteristics relating to safety, service life,
and aging of the cells that are problematic for use of these consu-
mer product cells in an automobile; these characteristics are not
significant in the consumer sector due to short innovation cycles
or low loads. Since new processes and procedures, as well as
new challenges, are necessary for the new concepts and materi-
als, for some time, the extent to which the previous manufacturers
of consumer lithium batteries will be able to profit from their know-
how in handling the component materials and production of small
cells, has been an open question.
AUTOMOBILE APPLICATIONS
For electromobility, not only new cell chemicals are demanded,
also new cell types are required. In this area the trend is toward
larger cells. In addition to the round cell which has often been
used up to this point in time, it is now mostly prismatic cells or
pouch cells (so-called coffee bags) that are being developed for
the vehicle sector.
The conflicting objectives between power density and energy
density that are evident in Fig. 4, result in a differentiation of high
energy and high power batteries depending on the selected cell
chemistry and cell structure: While the former enables a long ran-
ge of the vehicle (desirable for a battery electric vehicle), the latter
permits strong power consumption and output, (required for recu-
peration or boost functions in the hybrid, for example).
In this regard, the different usable battery capacity of the different
concepts must also be considered. For hybrids this is approxi-
mately 10 percent with an extremely high cycle count (i.e. only ap-
proximately 10 per cent of the nominal available capacity is used),
for battery electric vehicles, the figure is up to 80 percent with a
lower number of charge cycles.
DIFFERENT CELL CHEMICALS
In addition to the LiCoO
2
battery there are a number of approaches
that have achieved significant improvements in the characteristics
of the battery, particularly in terms of energy density, through the
use of new materials for the cathode and electrode. The lithium
iron phosphate cell is cited as an example; it is also characterized
by the fact that „thermal runaway“ (uncontrolled temperature in-
crease from an exothermal reaction due to released oxygen, that
can cause the cell to explode) does not occur at increased cell
temperature.
Fig. 7 provides an overview of four different Li-Ion battery types
and their respective characteristics. High material costs of the
aluminum and copper foil used for the anode and cathode, as well
as electrolyte and separator costs and the complex production
procedures and high machining costs, result in high prices for
lithium-ion vehicle cells.
Chapter 2