Page 18 - TeamStrunz_e_mobil_structure_study_en

New-generation battery technologies are frequently based on
metallic anodes, for example, lithium-sulfur or lithium air. Signi-
ficantly higher energy densities are achieved with these anodes.
Currently, recharging is still a challenge in this regard: During the
recharging process, non-uniform deposition of the lithium occurs.
This causes so-called dendrites to form; the dendrites penetrate
the separator after multiple charges and thus can cause an in-
ternal short circuit and failure of the cell. Currently lithium-sulfur,
in particular, and lithium-oxygen are being discussed from the
medium-term and long-term perspectives, respectively. However,
alongside performance of the cells, safety and required service
life are the highest priorities for the automotive manufacturers.
These requirements must be fundamentally fulfilled before series
production of the cells will become probable.
The Lithium-sulfur cell, in the charged state, consists of a metal-
lic lithium anode that is connected to a sulfur bearing cathode.
This offers a high proportion of active material, as well as a large
surface area and good conductance. The result is a theoretical
energy density of 1675 Wh/kg, as well as a power density of 3900
W/kg. Cell voltage is between approximately 2.2 and 2.5 Volts. As
yet unresolved is a parasitic shuttle mechanism that occurs in ad-
dition to the desired chemical reaction. This results in a high self-
discharge of the cell and poor efficiency.
Actual values currently achieved are 350 Wh/kg and 300 full cycles
(Sion Power Corporation). Developments are particularly aimed at
reducing the cost structure of the electrodes, increasing the actu-
al energy density (approximately 600 Wh/kg is considered possible
in this regard), increasing cycle stability (> 2000 cycles is the goal)
and, in addition, solving the problem of the so-called „shuttle me-
chanism“ [Tübke (2011); Hagen (2011)].
Lithium-oxygen batteries currently hold out the promise of the
greatest energy densities and thus the longest ranges for a given
weight. The anode consists of a pure lithium metal, while the ca-
thode consists of a porous Mn
-/C mixture, and is replaced by air
supplied from the outside. The energy density that can theoreti-
cally be achieved is 5200 Wh/kg with consideration of the oxygen
(11,140 Wh/kg without consideration of the oxygen). The cell volta-
ge is approximately 2.9 Volts.
To date values of approximately 700 Wh/kg at 300 full cycles have
been attained (PolyPlus Battery in Berkeley, CA) [Tübke (2011)]. It
is expected that commercially implementable systems will require
at least 20 more years of development time [Möller (2011)].
Further developments are aimed at enabling higher voltages in the
cells. This would mean that a lower number of cells would suffice.
Here the target values are 4.5 to 5 Volts. High-voltage materials are
already considered as advanced in terms of technical application
(for example, LiNiMnCoO
, LiCoPO
and LiNiPO
), however, they
still need to be integrated into the area of future technologies for
automotive manufacturing [Hartnig (2011)].
The voltage range cannot be arbitrarily increased because cur-
rent materials, in particular, become unstable at high voltages and,
moreover, no electrolytes exist that are stable at high voltages
[Möller (2011)].
The separator/electrolyte consists of a polymer that prevents den-
dritic growth of the lithium during recharging and should also en-
sure the safety of the battery.
One advantage of the LMP battery is its high efficiency that can be as
much as 99.7 percent. The lithium metal polymer battery (LMP) has
been known for some time, however up to this point in time it has not
been able to find an application in automotive manufacturing, due to
unresolved problems. For example, one reason has been that current
can only flow in the cells above a temperature of 60 degrees Celsius.
The technology also became known when the company DMB ad-
vertised that they had solved the problems, with a 600 km drive from
Munich to Berlin: Due to a special layer-like structure (using 4-layer
technology) and a new type of material combination. The general sa-
fety and functionality was confirmed by the Federal Institute for Ma-
terials Research and Testing (Bundesanstalt für Materialforschung)
and DEKRA8. However, scientists and research experts doubt the
presentation of the record range [see Winter (2011) for example].
Chapter 2