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LITHIUM-ION BATTERY (LI-ION)
The Li-ion battery is the latest advance in battery technology and
is already established and widespread in the field of portable
devices. Many experts see it as a highly promising candidate in
powering electric or HEVs within the next few years. Of all the
rechargeable battery types currently available, Li-ion batteries
exhibit the highest specific energy. As in Ni-MH batteries, the
positive electrode of Li-ion batteries is also formed as an
insertion electrode. Another special feature is the use of nonaqueous
„aprotic“ electrolytes, since both metallic Li as well as
Li-C intercalation compounds produce a violent reaction with
water. The cathode matrix can comprise a number of materials,
such as for example LiNi XMn YCo ZO 2, LiMn 2O 4, or Li XFePO 4.
A porous polymer separates the cathode from the negative
electrode, which is commonly made of graphite or in the
newest configurations of Sn or Ti compounds. In both
electrodes, lithium ions are absorbed and released reversibly.
During charging and discharging processes, the formation of
protective layers at the phase boundaries between electrodes
and electrolyte plays a key role. These protective layers are
formed by the spontaneous reaction of the highly reactive
lithium compounds with the electrolyte. Ideally, these protective
layers – Solid Electrolyte Interphase (SEI) – possess high,
(exclusively) ionic conductivity and force the lithium ions to
completely lose the solvate envelope before being intercalated
into the solid. In this way, they separate the electrodes from the
electrolyte and prevent the intercalation of solvated ions, which,
due to their large volume, would destroy the structure of the
electrode. The considerably larger volume requirement due to
the solvate envelope means that such processes lead to an
„opening up“ of the layered structure of the graphite electrode
and thus to rapid and irreversible damage of the electrode.
The thermodynamic instability of the organic electrolyte
components with regard to lithium or highly reactive lithium
compounds also represents a problem for the gas-tight cell
construction of Li-ion batteries. A gas-tight design is necessary
to operate such cells with non-aqueous (aprotic) electrolytes, in
ENERGY STORAGE
order to prevent the intake of humidity from the air and the
immediate violent reaction of water with the charged lithium
anode, resulting in the formation of hydrogen gas and lithium
hydroxide. On the other hand, partial reaction of Li with
propylene carbonate or ethylene carbonate – two components
frequently used in electrolytes – can occasionally cause the
evolution of gaseous propylene or ethylene and thus an
undesired increase in pressure which, in certain circumstances,
can lead to cell damage.
In fact, besides lifetime issues, safety concerns are still
hampering the introduction of Li-ion batteries to the automotive
market on a large scale. In contrast to Ni-MH batteries,
prototype vehicles equipped with Li-ion technology often use
batteries produced on small scale. Up-scaling, however, leads to
significant quality problems which so far have not been
completely overcome. In addition Li-ion batteries have a very
high energy content which can cause hazardous conditions
when released spontaneously.
To overcome this safety issue, the worldwide research
community puts strong efforts in introducing more stable
compounds, developing new cell and system design, and
performing advanced abuse tests to assess the real safety level
of a cell or an entire energy storage system. One key result of
these activities is the steady improvement of the performances
and safety of these electrochemical systems.
R&D process in developing electrochemical energy storage systems for
automotive application
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