Between electric cars, cell phones and
laptops it seems as if batteries are everywhere. This is not going to change
any time soon. Global electricity use is skyrocketing and smart phones, tablets
and e-readers are all becoming more common. In addition, batteries are finding
applications in energy storage as the renewable energy sector continues to
grow. Engineers and scientist have developed many novel technologies to supply
our storage needs, but none seems to have established itself as the ultimate
technology. Flywheel, compressed air and thermal storage are all strong
contenders for grid-scale storage while lithium-ion, nickel-cadmium and
nickel-metal-hydride batteries compete for portable electricity storage. What
is all comes down to is that we still have not found an optimal way to store
our electricity. This article will discuss the technology and potential of
lithium batteries.
Until the 1990s nickel-cadmium (NiCad)
batteries were practically the only choice in rechargeable batteries. The major
problem with these devices was that they had a high temperature coefficient.
This meant that the cells' performance would plummet when they heated up. In
addition, cadmium, one of the cell's main elements, is costly and
environmentally unfriendly (it is also used in thin film panels).
Nickel-metal-hydride (NiMH) and lithium-ion emerged as competitors to NiCad in
the 90s. Since then a mind numbing number of technologies have appeared on the
market. Amongst these lithium-ion batteries stand out as a promising candidate
for a wide range of uses.
Lithium-ion
cells have been used in hundreds of applications including electric cars,
pacemakers, laptops and military microgrids. They are extremely low maintenance
and energy dense. Unfortunately commercial lithium ion cells have some serious
drawbacks. They are very expensive, fragile and have short lifespans in
deep-cycle applications. The future of many budding technologies, including
electric vehicles, depends on improvements in cell performance.
Technology
A battery is an electrochemical device. This
means that it converts chemical energy into electrical energy. Rechargeable
batteries can convert in the opposite direction because they use reversible
reactions. Every cell is composed of a positive electrode called a cathode and
a negative electrode called an anode. The electrodes are placed in an
electrolyte and connected via an external circuit that allows electron flow.
Early lithium batteries were high temperature cells with molten
lithium cathodes and molten sulfur anodes. Operating at around 400 degrees
celcius, these thermal rechargeable batteries were first sold commercially in
the 1980s. However, electrode containment proved a serious problem due to
lithium's instability. In the end temperature issues, corrosion and improving
ambient temperature batteries slowed the adoption of molten lithium-sulfur
cells. Though this is still theoretically a very powerful battery, scientists
found that trading some energy density for stability was necessary. This lead
to lithium-ion technology.
A lithium-ion battery generally has a graphitic carbon anode,
which hosts Li+ ions, and a metal oxide cathode. The electrolyte consists of a
lithium salt (LiPF6, LiBF4, LiClO4) dissolved in an organic solvent such as
ether. Since lithium would react very violently with water vapor the cell is
always sealed. Also, to prevent a short circuit, the electrodes are separated
by a porous materials that prevents physical contact. When the cell is
charging, lithium ions intercalate between carbon molecules in the anode.
Meanwhile at the cathode lithium ions and electrons are released. During
discharge the opposite happens: Li ions leave the anode and travel to the
cathode. Since the cell involves the flow of ions and electrons, the system
must be both a good electrical and ionic conductor. Sony developed the first
Li+ battery in 1990 which had a lithium cobalt oxide cathode and a carbon
anode.
Overall lithium ion cells have important
benefits that have made them the leading choice in many applications. Lithium
is the metal with both the lowest molar mass and the greatest electrochemical
potential. This means that Li-ion batteries can have very high energy density.
A typical lithium cell potential is 3.6V (lithium cobalt oxide-carbon). Also,
they have a much lower self discharge rate at 5% than that of NiCad batteries
which usually self discharge at 20%. In addition, these cells don't contain
dangerous heavy metals such as cadmium and lead. Finally, Li+ batteries do not
have any memory effects and do not need to refilled. This makes them low
maintenance compared to other batteries.
Unfortunately lithium ion technology has
several restricting issues. First and foremost it is expensive. The average
cost of a Li-ion cell is 40% higher than that of a NiCad cell. Also, these
devices require a protection circuit to maintain discharge rates between 1C and
2C. This is the source of most static charge loss. In addition, though lithium ion batteries are powerful and stable, they have a
lower theoretical charge density than other kinds of batteries. Therefore
improvements of other technologies may make them obsolete. Finally, they have a
much shorter cycle life and a longer charging time than NiCad batteries and are
also very sensitive to high temperatures.
These issues have sparked interest in
other chemistries, such as lithium-air, lithium-polymer and lithium-iron. Since
I do not have time to go through all these devices, we'll briefly look at
lithium-air batteries. In these systems, Li is oxidized at the anode, releasing
electrons that travel through an external circuit. Li+ ions then flow to the
cathode where they reduce oxygen, forming the intermediary compound lithium
peroxide. In theory, this allows for a truly reversible reaction to take place,
improving the performance of lithium-air batteries in deep-cycle applications.
However, much like Li+ cells, these batteries suffer from short lives. This is
due to the formation of oxygen radicals that decompose the cell's organic
electrolyte. Fortunately two lithium-air batteries developed independently in
2012 by Jung et al., a team of researchers from Rome and Seoul, and Peter
Bruce, who led a group at St. Andrews, seem to have solved this problem. Both
the groups' batteries underwent approximately 100 charging and discharging
cycles without losing much of their capacity. Bruce's device lost only 5%
capacity during tests. The batteries also have higher energy density than their
lithium ion counterparts. This is a sign that the future of energy storage may
reside with powerful, resilient lithium-air chemistry. However we will first
have to overcome durability, cost and weight problems.
Implementation
Though novel lithium battery chemistries
are being developed and marketed, Li+ batteries remain near the top of the food
chain for now. As we mentioned previously, this technology is often considered
the first choice for electric vehicles and electronic devices due to its energy
density. Tesla's Roadster contains no less than 6831 lithium ion batteries.
Arranged into packs of 69, the cells are capable of taking the vehicle from 0
to 60 mph in just 3.9 seconds. Just in case you were wondering, 69 goes into
6831 exactly 99 times. Also, if you are reading this article on your laptop, it
is likely that it is powered by a lithium cell.
The major drawback to current Li
batteries is their susceptibility to aging effects, especially when heated. You
may have noticed that laptop and cell phone life deteriorates dramatically
after a few years. This is largely due to aging. This issue has made the
technology ill suited for backup and grid-scale power. Despite this, Li-ion
batteries have competed for energy storage projects with alternative
technologies such as thermal, flywheels and compressed air storage. Most of
these installations have been in California. Silent Power's Li+ cells are being
used to dampen power fluctuations in Sacramento and Greensmith has installed
1.5 megawatts of grid-balancing lithium-ion batteries throughout the state. In
addition, AES Energy Storage has installed, or is in the process of installing,
76MW of Li+ battery capacity worldwide with 500MW in development. The main
benefit of this technology is the fact that we understand it well and have the
immediate resources for it to work. In large scale projects lithium-ion
batteries have been most successful in sites where there are severe space
restrictions or minimal maintenance capabilities.
In the near future it seems as if
lithium ion technology is set to continue to dominate many applications. Li+
batteries are a proven concept, unlike some other technologies that have
remained cloistered in the lab. The possible emergence of electric vehicles and
the booming demand for electronics will undoubtedly have positive effects on
the industry. Unfortunately, all good things come to an end. Analysts forecast
that the technology will lose some of its competitive edge once infant
technologies such as aluminium-ion, zinc-bromine and lead-carbon come on the
market. For example on the topic of lithium ion batteries in storage
applications, Lux Research said the following:
"Li-ion batteries developed for
transportation applications are energy dense storage devices. Stationary
storage projects rarely value this metric, resulting in wasted value for
grid-tied Li- ion battery systems. Rapidly evolving technologies with
equivalent or superior performance metrics and substantially lower costs and
higher resource availability will take over the majority of the grid storage
market in the coming years."
Though they are unlikely to be used in
many grid scale storage projects, Li-ion batteries will certainly play a large
role in our future. Their high cost will probably drop as the concept continues
to mature and the devices become more widespread. A study by Mckinsey research
found that 1/3 price reductions could be achieved through economies of scale
alone. In any case lithium ion batteries are going to have to fight to keep the
advantage they currently have.
Lithium is just a small part of the
global picture. There are currently many competing concepts in the world of
energy storage, each with their own pros, cons and background.
ติดต่อผู้ดูแลเว็บไซต์ได้ที่ webdungdong@gmail.com|บริษัท ดั้งโด่งดอทคอม จำกัด|ติดต่อลงโฆษณา| ดั้งโด่งดอทคอม@2020
Copyright © 2001-2013 Comsenz Inc. All Rights Reserved. Powered by Discuz! X3.2 R20140618, Rev.27, Thzaa City 1 Style