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Technology Background Paper
Source:TEG NF,KS,CH
Version 2.0
WARNING:
Toshiba considers the information in this paper correct to the best of its
knowledge at the time of printing. Toshiba assumes no liability for errors
omissions or discrepancies, nor for any damages incurred based upon use of
the information contained herein.
Introduction
Lithium-Ion batteries: portable power for the '90s
Today's small, lightweight computer components make it possible to create
a subnote-book system that equals the capabilities of an advanced desktop
system. A 486-based notebook computer, such as Toshiba's powerful Portégé
models, includes a full-colour liquid crystal display, local-bus graphics,
high-capacity hard disk drive, expansion adaptor, built-in pointing device
and a PCMCIA slot that may contain a fax and/or data modem, network
adapter or other options.
But one stumbling block to portable computing remains: most batteries
don't hold a charge long enough to let you work on a presentation as you
relax during a four-hour flight and still have sufficient charge remaining
to make show it when you arrive at your destination. With the advent of a
new, long-lasting battery technology called lithium-ion (Li-ion) , this
situation has now ended. Toshiba's Portégé Series are the first computers
to feature this technology. The more recently announced T200 Series of
Dynapad pen PCs also incorporate lithium-ion batteries.
This paper looks at the demands on mobile batteries, the options available
and describes the advantages of lithium-ion technology in detail. It also
considers the general structure of batteries, summarizes the deficiencies
of the older battery technologies and compares the running time of Li-ion
batteries with those for nickel-cadmium (NiCd) and nickel-hydride (NiMH)
batteries.
Finally, many companies and journalists have confused lithium-ions with
lithium metal and drawn the wrong conclusions about safety. This document
sets out the true situation and explains, in everyday ways, why lithium-
ion batteries are not dangerous. This is because the lithium-ion
batteries which Toshiba uses DO NOT CONTAIN ANY PURE LITHIUM METAL. This
has been internationally recognized. Details of this recognition and other
information about the benefits of lithium-ion technology are contained on
the following pages.
Part I Traditional battery technologies
1.1Battery technology
Everybody knows what a battery does, don't they? Well, yes and no. A
battery provides a store of energy, often for mobile use, but not many
people really understand how it does this. Basically a battery is a
chemical situation which can exist in two states. In it's natural, long-
term state all the chemicals are clearly tied up in a stable condition. As
such they have no excess electrons and therefore no electrical power. When
a charge is applied, additional energy is added and the chemical state of
the battery is moved away from the natural condition. The battery is fully
charged.
As this state gradually deteriorates back to the original position the
extra energy which was needed to change the "natural" state is given off
in the form of electricity (an electron flow). The process can be compared
to rolling a ball up a gentle slope. This requires energy, but the ball
naturally rolls back down again. It is not a natural state for the ball to
sit on a slope. In neither state is the ball dangerous. Nor is a battery
inherently dangerous.
1.2Battery types
All batteries use basically the same process. Their efficiency depends on
a range of characteristics:
-How much energy do you need to change the chemical states?
-How quickly can power be added to "charge" the battery?
-How much of the "unnatural" state can be maintained and for how long?
-At what rate does the state change back?
-How often can the battery change between the different states?
Lead-acid batteries were the first widely used batteries and are very well
known but heavy. The acid can be dangerous if not properly handled.
Various forms of zinc batteries have also been used. Nickel cadmium was,
at its time, a breakthrough technology because it allows more cycles than
previous metal combinations. Nickel hydride was a further improvement with
several improved characteristics. To exploit battery characteristics in
the best possible way companies have developed batteries of ever
increasing sophistication. Lithium-ion models are the latest in this row.
The search for better batteries continues. As technology offers more
chemical options and technological products demand more mobility, battery
technology has become a major industry. An ideal battery would:
-be easy to charge,
-be able to take a high power concentration,
-hold this power indefinitely,
-have no unpleasant environmental effects,
-and be very light.
Meeting all of these requirements is a major challenge. Several
technologies used in mobile computers fall at one or other of the hurdles.
The traditional nickel-based batteries have reasonable technical
characteristics (the first three points) but are heavy and environmentally
suspect. Lithium-ion technology not only has dynamically improved
technical characteristics but is very light and goes a long way to meeting
environmental concerns. Details of the three main contenders are contained
in part III, Battery comparisons.
1.3The "cell memory" problem
The first battery-powered computers used rechargeable nickel-cadmium
(NiCd) batteries. Users of computers powered with NiCd batteries, however,
found that their batteries seem to "remember" the last discharge level and
couldn't be recharged beyond that level.
This is because recharging a NiCd battery before it is fully discharged
causes electrode crystallization in the charged battery cells.
Crystallization prevents some of the cells from accumulating a full
recharge. Since the cells in a battery discharge sequentially, the amount
of charge remaining varies between the cells. The total amount of power
stored after the recharge is, therefore, less than a total charge.
Recharging the battery before it is fully discharged, results in an even
shorter battery life. Repeatedly recharging a partially charged battery
results in rapidly decreasing operating time. Users of rechargeable NiCd
batteries must therefore be diligent about fully discharging the battery
before recharging it. This requirement can conflict with real-world needs.
Ignoring this requirement reduces the useful life of the battery.
Until Toshiba introduced Li-ion batteries in the T3400 Series, NiMH
batteries were the longest running batteries. A NiMH battery powers a
fully loaded 486-based notebook for approximately three hours. But NiMH
batteries also exhibit this "memory" problem, although not as severely as
with NiCd batteries. NiMH batteries are also still quite heavy.
1.4Environmental concerns
When a battery no longer holds a charge, it must be replaced. Early
batteries using lead and mercury cause problems because they contain
metals which are environmentally dangerous. Users of NiCd batteries must
also comply with strict regulations governing the safe disposal of these
batteries because the presence of heavy metals such as cadmium in land
fills and their possible migration into the ground water supply present
major environmental concerns. Although NiMH batteries are currently free
of regulations there are also concerns about the disposal of the nickel
metal part.
With growing environmental concerns the race has been on to find a
chemical combination which would create a battery function but not include
any dangerous metals. Lithium-ion batteries come close to that ideal.
Battery research is still continuing, however, and there are reports of
other types of batteries which may have even more acceptable environmental
standards. At the moment none of these is in the production stage and
whatever is said about the small amounts of lithium in the compounds or
the other metals used, none is in the same class of pollutant as cadmium
or nickel.
In addition, many of the components used are natural and as such
environmentally friendly. The carbon based electrode and the organic based
solvent used for the electrolytic fluid are both natural substances which
should decompose harmlessly. The small amounts of copper and aluminium are
also non-toxic. The casing is made of materials which do not produce
polluting gases when incinerated.
1.5The weight of portable power
Another important aspect of battery technology is the weight. Basically
users have been offered a rather steep trade-off between weight and
battery life: the longer the desired battery life, the more a notebook PC
had to weigh. It is not unusual for a battery to make up 15 to 20 % of
the weight of a mobile PC. In some cases it has even been more. Even so,
a battery life of maximum three hours is still the most common. The weight
problem arises because both types of commonly used battery so far required
large amounts of nickel. In contrast lithium-ion batteries need very
little lithium, which is anyway a lighter element.
The benefits of this can be seen in the two products Toshiba has so far
launched with lithium-ion batteries. Both the Portégé subnotebook Series
and the T200 pen PC Series are PCs which have light weight as a primary
specification. Their whole concept is based around this. They are designed
to be carried constantly. With the new batteries, the weight for both
products was kept down to 1.8 kg (2.0 kg for colour). Despite this the
Portégé Series offers battery life up to 4.5 hours for the colour TFT
models and up to an impressive 6 hours for the monochrome version. The
T200 reaches 3.5 and 3 hours even including the extra power needs of the
interactive screen. The battery progress in the T200 Series is more
impressive when compared to the T100X, the predecessor model. With 386
SXLV processor and no colour, two nickel cadmium batteries were delivered
to reach an acceptable battery lifetime. Now one battery manages a better
time with a 486 processor and colour version.
Part II The lithium-ion solution
2.1Lithium versus lithium ion
Lithium-ion batteries solve many of the problems that battery
manufacturers have faced. Early attempts to produce batteries based on
lithium faced great problems because pure lithium metal can be very
dangerous, especially when it gets wet. As a raw element lithium is
highly reactive but, like other elements in its chemical group, in
combination with other materials, lithium becomes a very stable medium.
This is particularly so in the form lithium ion.
A daily example makes the difference between the two forms of lithium
clear. Sodium is an element from the same chemical group as lithium. On
its own, it is dangerous and, like lithium, especially so when in contact
with water. Although it is not quite as reactive as lithium, the
comparison does not fall on this point. If you were to eat sodium it would
burn your throat out, yet each of us eats sodium ions every day. Indeed we
need them to survive. Most of us normally ask for the salt rather than
sodium chloride, but it amounts to the same thing.
The lithium element in the batteries is a very small amount and is trapped
in combination with other metals in oxides and in the lithium fluoride
(which uses fluorine, the material used by dentists and in water supplies
to hinder tooth decay). This is the reason why the batteries have been
declared safe by several institutions (see below). There is no danger of
reactions with water or of the lithium oxides and ions becoming too hot
(see safety measures below). Reports that characterize lithium batteries
as dangerous are either referring to first generation lithium batteries
(with lithium metal) or are confused about the differences in the
technologies.
2.2Stability recognized
Like sodium ions (salt), lithium ions are chemically very stable. There
is no danger in using this combination and this has been internationally
recognized. The first rulings were on transportation of such batteries in
the United States. There are strict regulations there, particularly
concerning flights. The Department of Transport of the U.S. Government has
a Dangerous Materials Transport Section which specifically deals with such
issues. After analysis of the components of lithium-ion batteries they
announced that they "are not considered lithium batteries" and therefore
judged them "to be exempt from dangerous materials regulations for
transport within the United States".
Following on this, the International Air Transport Association (IATA) was
also asked to rule on the danger of lithium-ion batteries. They first
dealt with the issue at the 56th Dangerous Goods Board meeting of October
1990. After noting the US ruling and then making their own inquiries with
Sony, the original developers, IATA concluded at the following (57th)
meeting that "for transportation, the batteries will be considered 'Dry
Batteries' and therefore will not be subject to the IATA Dangerous goods
regulations." According to battery manufacturers, any restrictions
against the transportation of solid lithium batteries should not therefore
apply to the newer lithium-ion batteries.
2.3Lithium-ion structure
A lithium-oxide compound forms the positive electrode (anode) and a
carbon-based structure constitutes the negative electrode (cathode). An
organic liquid electrolyte containing lithium ions separates the
electrodes. This is lithium fluoride dissolved in propylene carbonate.
Movement of the lithium ions between the opposing electrodes stores and
discharges energy.
The light weight of the electrodes and liquid electrolyte combined with
the highly efficient ion-flow mechanism offer a considerable improvement
over the heavier nickel-based components and water/acid reactions of
earlier batteries. This results in more energy per kilogram than either
NiCd or NiMH batteries can provide.
2.4Impressive characteristics
All of the chemicals used in lithium-ion batteries are very light. This
allows them to satisfy one of the major demands on new battery
technologies. Lithium-ion batteries also demonstrate other benefits. As a
result, they typically weigh only half as much as their nickel
counterparts for the same power. With an average operating voltage of 3.6
V per cell, they also offer three times as much power per cell as nickel-
based cells. For power hungry uses this is excellent news. For mobile PCs,
this means that fewer cells need to be used to achieve the power level
needed.
Another major benefit of lithium-ion batteries is that there is no memory
problem. All nickel-based batteries show some sort of memory development
after repeated usage. Lithium-ion batteries show no such tendency. This is
highly useful for PC users who may not always be able to completely
exhaust their batteries before having to reload for the next mobile
working period. Because the batteries remain full active they also do not
have to be replaced so often. Finally, the self-discharge rate, another
important measure of the efficiency of a battery is also better for
Lithium ion. Nickel-based batteries lose around two and a half times as
much as lithium-ion models within a set time period.
2.5Environmentally friendly.
Li-ion batteries contain no heavy metals. Lead, cadmium and mercury are
absent. The small lithium-oxide compound electrode, carbon-based electrode
and electrolyte liquid can be disposed of without taking any special
precautions. They are not dangerous for humans. Also, because lithium-ion
batteries generally have a longer battery-life, disposal is less frequent
than required with other technologies.
2.6Lithium-ion battery safety measures
Should the charger and the control circuit built into a battery pack
break down it is possible for an overcharge condition to arise in a
battery. Lithium-ion batteries have three safety measures built in for
this case. Unlike nickel-cadmium and lead-acid batteries, lithium-ion
units only produce carbon-dioxide gas which is inert. It does not react
with either the electrodes or the electrolytic solution. If, due to gas
production in an individual cell, internal pressure rises, a cut-off valve
is forced into operation breaking the electrical circuit between the
positive and negative electrodes in the cell. this first level of
protection prevents an excess accumulation of energy. If for any reason
pressure continues to build, a second safety valve cuts in and the gas is
released outside of the cell and battery.
If the problem is not related to the individual cells but arises out of
other events, the whole battery has an additional safety feature based
upon a positive temperature coefficient thermister (PTC thermister).
Should a short circuit in the battery be caused via external contacts, the
PTC thermister registers the overheating and/or overcurrent. It then cuts
the contacts inside the battery to stop the short-circuiting.
Part III Battery comparisons
Lithium-ion batteries represent a major improvement over traditional
nickel-based version in many ways. In two ways specifically they are
tremendous improvements over all other technologies so far. They have a
much higher power to weight ratio than all other types of standard battery
so far. Simply put, this means that designers are faced with an enviable
alternative: the notebook PC keeps the same weight and battery life is
doubled to six hours or more, or the notebook looses a few hundred grammes
and battery life stays the same. Toshiba chose a lighter PC. That is why
the batteries are used in subnotebooks. The new battery technology plays a
major part in reaching the magical 2 kg weight. This allows Toshiba to but
more quality into the other components.
The following table indicates the overall balance of the different types
of battery based on the main technical characteristics as well as the
perceived acceptability (weight and environmental status).
Characteristic NiCd NiMH Li-ion
Weight ratio (Wh/kg) + + +++
Volume ratio (Wh/l) + ++ ++
Life time (cycles) + + ++
Memory effect -- - +
Discharge rate + + +
Self-discharge rate -- - ++
Fast recharges ++ ++ +
Weight -- - +
Environmental friendliness -- + +
The individual charts on the following pages demonstrate the advantages of
Li-ion batteries in the four specific areas: run-time, power density,
battery life and charge retention. Each chart compares lithium-ion, NiMH
and NiCd batteries, using NiCd performance as the baseline.
3.1 Increased running-time
The following table compares the estimated running time in hours for a
monochrome notebook using a fully charged battery.
<table simplified>
Battery life in hours: NiCd = 2h, NiMH = 4h, Li-Ion = 7h
3.2 Lighter batteries result of higher power density
The new batteries store more power in a lighter weight cell than nickel-
based batteries because the lithium-ion reaction is highly efficient. The
following chart compares power per kilogram of cell weight. NiCd equals
100%. <table simplified>
Battery power density (%): NiCd = 100%, NiMH = 138%, Li-Ion = 240%
Battery power density (%): NiCd = 100%, NiMH = 138%, Li-Ion = 240%
3.3 More recharges lengthen battery life
For those who use a portable computer constantly, the expense of buying
new batteries is a factor in overall satisfaction. The more recharges a
battery will accept relates directly to how long the battery will last.
The longer the battery lasts, the more money the user saves by not having
to buy a new battery. The following chart compares the number of
recharges the three types batteries will accept. <table simplified>
Battery recharge cycles: NiCd = 1000, NiMH = 1000, Li-Ion = 1200
3.4 Charge retained on shelf
Discharge during storage is a problem for those who use a portable
computer infrequently or store extra battery packs. The need to recharge a
battery that hasn't been used increases cost. The following chart shows
how Li-ion batteries offer a ninety percent improvement in charge
retention during storage. <table simplified>
Battery power retention: NiCd = 80, NiMH = 75, Li-Ion = 90
Sources for this paper:
Sony Corporation, Japan
Toshiba Corporation, Japan
A&T Battery Corp., Japan,
International Air Transport Association (IATA), Canada
Duracell Corporation
Varta, Germany
Nigel Fusedale, TEG