Lithium Secondary Cell

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.

See more details below.

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) of almost 100%. 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.

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.

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.
 
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.
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.
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. Lithium Sulphur cells are tolerant of over-voltages. The cell voltage is 2.1 Volts

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  Cells), they offer a very high energy density. Rechargeable versions are now under development. There are potential safety concerns with the metallic Lithium anodes.