Pioneer work with the lithium battery began in 1912 under
G.N. Lewis but it was not until the early 1970s that
the first non-rechargeable lithium batteries became commercially
available. Attempts to develop rechargeable lithium batteries
followed in the 1980s, but failed due to safety problems.
Lithium is the lightest of all metals, has the greatest electrochemical
potential and provides the largest energy density per weight.
Rechargeable batteries using lithium metal anodes (negative
electrodes) are capable of providing both high voltage and
excellent capacity, resulting in an extraordinary high energy
density.
After much research on rechargeable lithium batteries during
the 1980s, it was found that cycling causes changes on the
lithium electrode. These transformations, which are part of
normal wear and tear, reduce the thermal stability, causing
potential thermal runaway conditions. When this occurs, the
cell temperature quickly approaches the melting point of lithium,
resulting in a violent reaction called ‘venting with flame’.
A large quantity of rechargeable lithium batteries sent to
Japan had to be recalled in 1991 after a battery in a mobile
phone released flaming gases and inflicted burns to a person’s
face.
Because of the inherent instability of lithium metal, especially
during charging, research shifted to a non-metallic lithium
battery using lithium ions. Although slightly lower in energy
density than lithium metal, the Li-ion is safe, provided
certain precautions are met when charging and discharging.
In 1991, the Sony Corporation commercialized the first Li-ion
battery. Other manufacturers followed suit. Today, the Li-ion
is the fastest growing and most promising battery chemistry.
The energy density of the Li-ion is typically twice
that of the standard NiCd. Improvements in electrode active
materials have the potential of increasing the energy density
close to three times that of the NiCd. In addition to high
capacity, the load characteristics are reasonably good and
behave similarly to the NiCd in terms of discharge characteristics
(similar shape of discharge profile, but different voltage).
The flat discharge curve offers effective utilization of the
stored power in a desirable voltage spectrum.
The Li-ion is a low maintenance battery, an advantage
that most other chemistries cannot claim. There is no memory
and no scheduled cycling is required to prolong the battery’s
life. In addition, the self-discharge is less than half compared
to NiCd and NiMH, making the Li-ion well suited for
modern fuel gauge applications.
The high cell voltage of Li-ion allows the manufacture
of battery packs consisting of only one cell. Many of today’s
mobile phones run on a single cell, an advantage that simplifies
battery design. Supply voltages of electronic applications
have been heading lower, which in turn requires fewer cells
per battery pack. To maintain the same power, however, higher
currents are needed. This emphasizes the importance of very
low cell resistance to allow unrestricted flow of current.
Chemistry variations — During recent years, several
types of Li-ion batteries have emerged with only one
thing in common — the catchword 'lithium'. Although strikingly
similar on the outside, lithium-based batteries can vary widely.
This book addresses the lithium-based batteries that are predominantly
used in commercial products.
Sony’s original version of the Li-ion used coke, a
product of coal, as the negative electrode. Since 1997, most
Li-ions (including Sony’s) have shifted to graphite.
This electrode provides a flatter discharge voltage curve
than coke and offers a sharp knee bend at the end of discharge
(see Figure 2-5). As a result, the graphite system
delivers the stored energy by only having to discharge to
3.0V/cell, whereas the coke version must be discharged to
2.5V to get similar runtime. In addition, the graphite version
is capable of delivering a higher discharge current and remains
cooler during charge and discharge than the coke version.
For the positive electrode, two distinct chemistries have
emerged. They are cobalt and spinel (also known as manganese).
Whereas cobalt has been in use longer, spinel is inherently
safer and more forgiving if abused. Small prismatic spinel
packs for mobile phones may only include a thermal fuse and
temperature sensor. In addition to cost savings on a simplified
protection circuit, the raw material cost for spinel is lower
than that of cobalt.
Figure 2-5:
Li-ion discharge characteristics.
The graphite Li-ion only
needs to discharge to 3.0V/cell, whereas the coke version
must be discharged to 2.5V/cell to achieve similar performance.
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