Characteristics
Lithium is the lightest of metals and it floats on water. It also has
the greatest electrochemical potential which makes it one of the most
reactive of metals. These properties give Lithium the potential to
achieve very high energy and power densities in high power battery
applications such as automotive and standby power.
Many variations of the basic Lithium chemistry
have been developed to optimise the cells for specific applications or
perhaps in some cases to get around the patents on the original
technology. Lithium metal reacts violently with water and can ignite
into flame. Early commercial cells with metallic lithium cathodes were
considered unsafe in certain circumstances, however modern cells don't
use free Lithium but instead the Lithium is combined with other elements
into more benign compounds which do not react with water.
The typical Lithium-ion cells use Carbon for its anode and Lithium
Cobalt dioxide or a Lithium Manganese compound as the cathode. The
electrolyte is usually based on a Lithium salt in an organic solvent.
Lithium batteries have now taken their place as the rechargeable
battery of choice for portable consumer electronics equipment. Though
they were expensive when introduced, volume production has brought the
prices down.
Advantages
In many ways Lithium is almost the perfect cell chemistry and many
variants exist. Practical Lithium based rechargeable batteries were
first demonstrated in the 1970's, and they are now used in very high
volumes in low power applications such as mobile phones, laptops,
cameras and other consumer electronic products. They have many
attractive performance advantages which make them also ideal for higher
power applications such as automotive and standby power.
- High cell voltage of 3.6 Volts means fewer cells and associated
connections and electronics are needed for high voltage batteries. (One
Lithium cell can replace three NiCad or NiMH cells which have a cell
voltage of only 1.2 Volts)
- No liquid electrolyte means they are immune from leaking.
- Very high energy density (About 4 times better than Lead acid). For
example a 3.5 ton electric powered LDV light van uses 750Kg of Lead acid
batteries. The same capacity could be provided by less than 200 Kg of
Lithium batteries, allowing the van an increased payload of half a ton.
Alternatively. The van's range of only 50 miles could be quadrupled by
using the same weight of Lithium batteries.
- Very high power density. As above.
- Very small batteries also available. Solid state chemistry can be printed on to ceramic or flexible substrates to form thin film batteries with unique properties.
- Low weight
- Can be optimised for capacity or rate.
- Individual cells up to 1000Ah capacity available.
- Can be discharged at the 40C rate or more. The high discharge rate
means that for automotive use the required cold cranking power or boost
power for hybrid vehicles can be provided by a lower capacity battery.
- Fast charge possible.
- Can be deep cycled. The cell maintains a constant voltage for over
80% of its discharge curve. It thus delivers full power down to 80% DOD
versus 50% for Lead acid. This means that in practice, for a given
capacity, more of the stored energy is usable or that the battery will
accept more starting attempts or boost power requests before becoming
effectively discharged.
- Very low self discharge rate. Can retain charge for up to ten years.
- Very high coulombic efficiency (Capacity discharged over Capacity
charged) up to 95% or more. Thus very little power is lost during the
charge/discharge cycles.
- No memory effect. No reconditioning needed.
- Tolerates microcycles
- Long cycle life. 1000 to 3000 deep cycles. (But see Lithium titanate
below). Cycle life can be extended significantly by using protective
circuits to limit the permissible DOD of the battery. This mitigates
against the high initial costs of the battery.
- Does not need reconditioning as do nickel based batteries.
- Variants of the basic cell chemistry allow the performance to be tuned for specific applications.
- Available in a wide range of cell constructions with capacities from
less than 500 mAh to 1000 Ah from a large number (over 100) of
suppliers world-wide.
-
Versatile and highly scalable. Can be adapted to practically any
voltage, power and energy requirement, with power to energy ratios
ranging from very high power (i.e. 10kW / kWh) to very high energy.
- Very fast response to charge and discharge calls.
Shortcomings
Internal impedance higher than equivalent NiCads
For high power applications which require large high cost batteries
the price premium of Lithium batteries over the older Lead Acid
batteries becomes a significant factor, impeding widespread acceptance
of the technology. This in turn has discouraged investment in high
volume production facilities keeping prices high and has for some time
discouraged take up of the new technology. This is gradually changing
and Lithium is also becoming cost competitive for high power
applications.
Stability of the chemicals has been a concern in the past. Because
Lithium is more chemically reactive special safety precautions are
needed to prevent physical or electrical abuse and to maintain the cell
within its design operating limits. Lithium polymer cells with their
solid electrolyte overcome some of these problems.
Stricter regulations on shipping methods than for other cell chemistries.
Degrades at high temperatures.
Capacity loss or thermal runaway when overcharged.
Degradation when discharged below 2 Volts.
Venting and possible thermal runaway when crushed.
Need for protective circuitry.
Measurement of the state of charge of the cell is more complex than
for most common cell chemistries. The state of charge is normally
extrapolated from a simple measurement of the cell voltage, but the flat
discharge characteristic of lithium cells, so desirable for
applications, renders it unsuitable as a measure of the state of charge
and other more costly techniques such as coulomb counting have to be
employed.
Although Lithium cell technology has been used in low power
applications for some time now, there is still not a lot of field data
available about long term performance in high power applications.
Reliability predictions based on accelerated life testing however shows
that the cycle life matches or exceeds that of the most common
technologies currently in use.
These drawbacks are far out weighed by the advantages of Lithium
cells and are now being used in an ever widening range of applications.
Charging Lithium Batteries
Should be charged regularly.
The cell voltage is typically 4.2 Volts
Battery lasts longer with partial charges rather than full charges.
Charging to 4.1 Volts will increase the cycle life but reduces the effective cell capacity by about 10%.
Can not tolerate overcharging and hence should not be trickle charged.
Charging method: Constant Current - Constant Voltage .
Fast chargers typically operate during the constant current charging
phase only when the charging current is at a maximum. They switch off at
the point when the constant voltage, reducing current phase starts. At
this point the battery will only be charged to about 70% of its
capacity.
Applications
Rechargeable Lithium cells are used a wide range of consumer products
including cameras, camcorders, electric razors, toothbrushes,
calculators, medical equipment, communications equipment, instruments,
portable radios and TVs, pagers and PDA's.
They are fast replacing Nickel Metal Hydride cells as the preferred
power in mobile phones. Laptop computers almost exclusively use Lithium
batteries.
Now high power versions of up to 1000Ah capacity and more are
becoming available for use in traction applications in electric and
hybrid vehicles as well as for standby power.
Increasingly used in grid scale energy storage applications.
Costs
The price of Lithium cells continues to fall as the technology gains more acceptance.
The target price for high power cells is around $300/kWh but cell makers are still quite some way from achieving that.
Although Lithium secondary batteries may cost two or three times more
than the cost of equivalent Lead acid batteries and even more when the
necessary battery management electronics are thaken into account, this
is more than compensated for by their longer cycle life which may be
five to ten times the life of Lead acid batteries. Valid cost
comparisons should therefore take into account the lifetime costs as
well as the initial capital costs.
Lithium Cell Chemistry Variants
Lithium's unique properties have been used as a basis of numerous
battery chemistries both for primary and secondary cells. Using nano -
electrode materials provides a bigger active surface area and hence a
higher current carrying capacity. This technology allows current rates
of 10 C or more making the cells suitable for HEV applications.
Lithium-ion
Lithium-ion batteries were designed to overcome the safety problems
associated with the highly reactive properties of Lithium metal.
The essential feature of the Lithium ion battery is that at no stage in
the charge-discharge cycle should there be any Lithium metal present.
Rather, Lithium ions are intercalated into the positive electrode in the
discharged state and into the negative electrode in the charged state
and move from one to the other across the electrolyte.
Lithium-ion batteries thus operate based on what is sometimes called
the "rocking chair" or "swing" effect. This involves the transfer of
Lithium ions back and forth between the two electrodes. The anode of a
Lithium-ion battery is composed of Lithium, dissolved as ions, into a
carbon or in some cases metallic Lithium. The cathode material is made
up from Lithium liberating compounds, typically the three electro-active
oxide materials, Lithium Cobalt-oxide LiCoO2 , Lithium Manganese-oxide LiMn2 O4 , and Lithium Nickel-oxide LiNiO2
Lithium salt constitutes the electrolyte.
The origin of the cell voltage is then the difference in free energy
between Li + ions in the crystal structures of the two electrode
materials.
Lithium-ion cells have no memory effect and have long cycle life and
excellent discharge performance. For safety reasons, charge control
circuitry is required for virtually all Lithium-ion applications.
Lithium-ion technology uses a liquid or gel type electrolyte. This
cell chemistry and construction permits very thin separators between the
electrodes which can consequently be made with very high surface areas.
This in turn enables the cells to handle very high current rates making
them ideal for use in high power applications. Some early cells used
flammable active ingredients which required substantial secondary
packaging to safely contain these potentially hazardous chemicals. This
additional packaging not only increased the weight and cost, but it also
limited the size flexibility. Modern cell chemistries and additives
have essentially eliminated these problems.
Lithium-ion Polymer
Lithium-ion polymer batteries use liquid Lithium-ion electrochemistry
in a matrix of ion conductive polymers that eliminate free electrolyte
within the cell. The electrolyte thus plasticises the polymer, producing
a solid electrolyte that is safe and leak resistant. Lithium polymer
cells are often called Solid State cells.
Because there's no liquid, the solid polymer cell does not require
the heavy protective cases of conventional batteries. The cells can be
formed into flat sheets or prismatic (rectangular) packages or they can
be made in odd shapes to fit whatever space is available. As a result,
manufacturing is simplified and batteries can be packaged in a foil.
This provides added cost and weight benefits and design flexibility.
Additionally, the absence of free liquid makes Lithium-ion polymer
batteries more stable and less vulnerable to problems caused by
overcharge, damage or abuse.
Solid electrolyte cells have long storage lives, but low discharge rates.
There are some limitations on the cell construction imposed by the
thicker solid electrolyte separator which limits the effective surface
area of the electrodes and hence the current carrying capacity of the
cell, but at the same time the added volume of electrolyte provides
increased energy storage. This makes them ideal for use in high capacity
low power applications.
Despite the above comments there are some manufacturers who make
cells designated as Lithium polymer which actually contain a liquid or a
gel. Such cells are more prone to swelling than genuine solid polymer
cells.
Other Lithium Cathode Chemistry Variants
Numerous variants of the basic Lithium-ion cell chemistry have been
developed. Lithium Cobalt and Lithium Manganese were the first to be
produced in commercial quantities but Lithium Iron Phoshate is taking
over for high power applications because of its improved safety
performance. The rest are either at various stages of development or
they are awaiting investment decisions to launch volume production.
Doping with transition metals changes the nature of the active
materials and enables the internal impedance of the cell to be reduced.
The operating performance of the cell can also be be "tuned" by
changing the identity of the transition metal. This allows the voltage
as well as the specific capacity of these active materials to be
regulated. Cell voltages in the range 2.1 to 5 Volts are possible.
While the basic technology is well known, there is a lack of
operating experience and hence system design data with some of the newer
developments which also hampers their adoption. At the same time
patents for these different chemistries tend to be held by rival
companies undertaking competitive developments with no signs of industry
standardisation or adoption of a common product. (The original patent
on Lithium Cobalt technology has now expired which is perhaps one
explanation for its popularity).
Lithium Cobalt LiCoO2
Lithium Cobalt is a mature, proven, industry-standard battery
technology that provides long cycle life and very high energy density.
The polymer design makes the cells inherently safer than "canned"
construction cells that can leak acidic electrolyte fluid under abusive
conditions. The cell voltage is typically 3.7 Volts. Cells using this
chemistry are available from a wide range of manufacturers.
The use of Cobalt is unfortunately associated with environmental and toxic hazards.
Lithium Manganese LiMn2O4
Lithium Manganese provides a higher cell voltage than Cobalt based
chemistries at 3.8 to 4 Volts but the energy density is about 20% less.
It also provides additional benefits to Lithium-ion chemistry, including
lower cost and higher temperature performance. This chemistry is more
stable than Lithium Cobalt technology and thus inherently safer but the
trade off is lower potential energy densities. Lithium Manganese cells
are also widely available but they are not yet as common as Lithium
Cobalt cells.
Manganese, unlike Cobalt, is a safe and more environmentally benign cathode material.
Manganese is also much cheaper than Cobalt, and is more abundant.
Lithium Nickel LiNiO2
Lithium Nickel based cells provide up to 30% higher energy density
than Cobalt but the cell voltage is lower at 3.6 Volts. They also have
the highest exothermic reaction which could give rise to cooling
problems in high power applications. Cells using this chemistry are
therefore not generally available.
Lithium (NCM) Nickel Cobalt Manganese - Li(NiCoMn)O2
Tri-element cells which combine slighlty improved safety (better than
Cobalt oxide) with lower cost without compromising the energy density
but with slightly lower voltage. Different manufacturers may use
different proportions of the three constituent elements, in this case
Ni, Co and Mn.
Lithium (NCA) Nickel Cobalt Aluminium - Li(NiCoAl)O2
As above, another tri-element chemistry which combines slighlty
improved safety (better than Cobalt oxide) with lower cost without
compromising the energy density but with slightly lower voltage.
Lithium Iron Phosphate LiFePO4
Phosphate based technology possesses superior thermal and chemical
stability which provides better safety characteristics than those of
Lithium-ion technology made with other cathode materials. Lithium
phosphate cells are incombustible in the event of mishandling during
charge or discharge, they are more stable under overcharge or short
circuit conditions and they can withstand high temperatures without
decomposing. When abuse does occur, the phosphate based cathode material
will not burn and is not prone to thermal runaway. Phosphate chemistry
also offers a longer cycle life.
Recent developments have produced a range of new environmentally
friendly cathode active materials based on Lithiated transition metal
phosphates for Lithium-ion applications.
Phosphates significantly reduce the drawbacks of the Cobalt
chemistry, particularly the cost, safety and environmental
characteristics. Once more the trade off is a reduction of 14% in energy
density, but higher energy variants are being explored.
Due to the superior safety characteristics of phosphates over current
Lithium-ion Cobalt cells, batteries may be designed using larger cells
and potentially with a reduced reliance upon additional safety devices.
The use of Lithium Iron Phosphate chemistry is the subject of patent
disputes and some manufacturers are investigating other chemistry
variants mainly to circumvent the patent on the LiFePO4 chemistry.
Lithium Metal Polymer
Developed specifically for automotive applications employing 3M
polymer technology and independently in Europe with technology from the
Fraunhofer Institute, they have been trialled successfully in PNGV
project demonstrators in the USA. They use metallic Lithium anodes
rather than the more common Lithium Carbon based anodes and metal oxide
(Cobalt) cathodes.
Some versions need to work at temperatures between 80 and 120ºC for
optimum results although it is possible to operate at reduced power at
ambient temperature.
The Fraunhofer technology uses an organic electrolyte and the cell
voltage is 4 Volts. It is claimed that their the cell chemistry is more
tolerant to abuse.
These products are not yet in volume production.
Lithium Sulphur Li2S8
Lithium Sulphur is a high energy density chemistry, significantly
higher than Lithium-ion metal oxide chemistries. This chemistry is under
joint development by several companies but it is not yet commercially
available. A major issue is finding suitable electrolytes which are not
subject to the numerous unwanted side reactions which plague the current
designs.
Lithium Sulphur cells are tolerant of over-voltages but current versions have limited cycle life. The cell voltage is 2.1 Volts
See also Dissolution of the Electrodes on the New Cell Designs and Chemistries page.
Alternative Anode Chemistry (LTO)
The anodes of most Lithium based secondary cells are based on some
form of carbon (graphite or coke). Recently Lithium Titanate Spinel (Li4Ti5O12) has been introduced for use as an anode material providing high power thermally stable cells with improved cycle life.
This has the following advantages
- Does not depend on SEI Layer for stability
- No restriction on ion flow hence significantly higher charge and
discharge rates possible as well as better low temperature performance.
- Lower internal impedance of the cell
- Higher temperatures can be tolerated.
- No SEI build up over time means very long cycle life possible (10,000 deep cycles)
- Public domain technology (No patent disputes)
Disadvantages are
- Lower anode reactivity means cell voltage reduced to 2.25 Volts when
used with Spinel cathode. (Other cathode chemistries possible)
- 25% to 30% Lower energy density hence bulkier cells
Lithium Air Cells
Originally conceived as primary cells (see Lithium Pimary Cells),
Lithium air cells offer a very high energy density. Rechargeable
versions are now under development which promise energy densities of 10
times more than the current generation of Lithium cells, approaching
that of Gasoline/Petrol.
The anode is Lithium and the cathode is not air but in fact gaseous
Oxygen from the air. Because the cell does not have a solid cathode in
the conventional sense it eliminates the weight and volume of the
cathode as well as its mechanical supporting structure.
This would enable very small batteries to be made with the same range
as current technology, or alternatively, electric drive ranges of
several hundred miles could be obtained from batteries the same physical
size as those available today.
The Lithium is oxidised by the Oxygen during discharging and charging
drives the Oxygen off again, a relatively simple chemistry. There are
however problems in preventing the other constituents of the air from
poisoning the Lithium electrode. There are also potential safety
concerns with the metallic Lithium anodes. The cells demonstrate very
high hysteresis with the charging voltage considerably higher than the
discharge voltage This corresponds to a low Coulombic efficiency,
currently only about 60% to 70%.
The cell voltage is 2.5 Volts.
.See also Energy Density on the New Cell Designs and Chemistries page.
See note on the Toxicity of Lithium
|
 |
|
