Lithium Battery Knowledge

Batteries

Cylindrial cells

The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder has the ability to withstand internal pressures without deforming. Figure 1 shows a cross section of a cell.

Figure 1: Cross section of a lithium-ion cylindrical cell


The cylindrical cell design has good cycling ability, offers a long calendar life, is economical but is heavy and has low packaging density due to space cavities.

Typical applications for the cylindrical cell are power tools, medical instruments and laptops. Nickel-cadmium offers the largest variety of cell choices, and some popular formats have spilled over to nickel-metal-hydride. To allow variations within a given size, manufacturers use fractural cell length, such as half and three-quarter formats.


The established standards for nickel-based batteries did not catch on with lithium-ion and the chemistry has established its own formats. One of the most popular cell packages is the 18650, as illustrated in Figure 2. Eighteen denotes the diameter and 65 is the length of the cell in millimeters. The Li-manganese version 18650 has a capacity of 1,200–1,500mAh; the Li-cobalt version is 2,400–3,000mAh. The larger 26650 cells have a diameter of 26mm with a length of 65mm and deliver about 3,200mAh in the manganese version. This cell format is used in power tools and some hybrid vehicles.


Figure 2: Popular 18650 lithium-ion cell

The metallic cylinder measure 18mm in diameter and 65mm the length. The larger 26650 cell measures 26mm in diameter.

Lead acid batteries come in flooded and dry formats; portable versions are packaged in a prismatic design resembling a rectangular box made of plastic. Some lead acid systems also use the cylindrical design by adapting the winding technique, and the Hawker Cyclone is in this format. It offers improved cell stability, higher discharge currents and better temperature stability than the conventional prismatic design.

Cylindrical cells include a venting mechanism that releases excess gases when pressure builds up. The more simplistic design utilizes a membrane seal that ruptures under high pressure. Leakage and subsequent dry-out may occur when the membrane breaks. The re-sealable vents with a spring-loaded valve are the preferred design. Cylindrical cells make inefficient use of space, but the air cavities that result with side-by-side placement can be used for air-cooling.


Pouch cell

In 1995, the pouch cell surprised the battery world with a radical new design. Rather than using a metallic cylinder and glass-to-metal electrical feed-through for insulation, conductive foil tabs welded to the electrode and sealed to the pouch carry the positive and negative terminals to the outside. Figure 5 illustrates such a pouch cell.

Figure 3: The pouch cell


The pouch cell offers a simple, flexible and lightweight solution to battery design. Exposure to high humidity and hot temperature can shorten service life.

The pouch cell makes the most efficient use of space and achieves a 90 to 95 percent packaging efficiency, the highest among battery packs. Eliminating the metal enclosure reduces weight but the cell needs some alternative support in the battery compartment. The pouch pack finds applications in consumer, military, as well as automotive applications. No standardized pouch cells exist; each manufacturer builds the cells for a specific application.

Pouch packs are commonly Li-polymer. Its specific energy is often lower and the cell is less durable than Li-ion in the cylindrical package. Swelling or bulging as a result of gas generation during charge and discharge is a concern. Battery manufacturers insist that these batteries do not generate excess gases that can lead to swelling. Nevertheless, excess swelling can occur and most is due to faulty manufacturing, and not misuse. Some dealers have failures due to swelling of as much as three percent on certain batches. The pressure from swelling can crack a battery cover, and in some cases break the display and electronic circuit board. Manufacturers say that an inflated cell is safe. While this may be true, do not puncture a swollen cell in close proximity to heat or fire; the escaping gases can ignite. Figure 6 shows a swelled pouch cell.


Figure 4: Swelling pouch cell

Swelling can occur as part of gas generation. Battery manufacturers are at odds why this happens. A 5mm (0.2”) battery in a hard shell can grow to 8mm (0.3”), more in a foil package.

To prevent swelling, the manufacturer adds excess film to create a “gas bag” outside the cell. During the first charge, gases escape into the gasbag, which is then cut off and the pack resealed as part of the finishing process. Expect some swelling on subsequent charges; 8 to 10 percent over 500 cycles is normal. Provision must be made in the battery compartment to allow for expansion. It is best not to stack pouch cells but to lay them flat side by side. Prevent sharp edges that could stress the pouch cell as they expand.


Summary of Packaging Advantages and Disadvantages

A cell in a cylindrical metallic case has good cycling ability, offers a long calendar life, is economical to manufacture, but is heavy and has low packaging density.

The prismatic pouch pack is light and cost-effective to manufacture. Exposure to high humidity and hot temperature can shorten the service life. A swelling factor of 8 - 10 percent over 500 cycles is normal.


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.



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