Cell Construction

Components

The basic components of a battery are the electrodes with terminals to connect to the external circuit, a separator to keep the electrodes apart and prevent them from shorting, the electrolyte which carries the charged ions between the electrodes and a case to contain the active chemicals and hold the electrodes in place.

• Case

The case may simply be a robust container made from glass, plastic or metal, insulated from the electrodes, which does not form part of the current path through the battery. Lead acid batteries typically have such containers. A metallic case however can be used as one of the electrodes, saving materials, as in the case of Leclanché (Zinc- Carbon) cells. Examples of the range of cases used for rechargeable Lithium cells are shown below.

 

	Plastic case Robust. Easy packaging, Flammable, Inexpensive. Stacked or jelly roll electrodes. Retains heat, Poor thermal dissipation, Sizes up to 1000AH. Used by: Thunder Sky, International Battery, China Hipower, HuanYu
	Cylindrical steel case Robust. High energy density cells but packs wasteful of space, Space allows cooling air flow, Expensive, Sizes up to 200 Ah Used by: Gaia, PHET, LifeBatt, BAK, SAFT, A123
	Prismatic metal case (Steel / Aluminium) Robust, Easy packaging, Good space utilisation, Expensive, Jelly roll or stacked electrodes. High energy density, Good heat dissipation, Sizes up to 200 Ah Used by: BYD, HYB, Lishen, Toshiba, Varta
	Pouch cell - Also known as Lipo cells Vulnerable, Inexpensive, Design freedom on dimensions, Difficult packaging, High energy density but reduced by support packaging needed, Prone to swell and leak, Less danger of explosion (cell bursts), Good heat dissipation, Made in very high volumes, Economical for small volumes, Sizes up to 240 Ah Used by: Kokam, BAHUP, ATL, Yoku, EIG, Enerdel, LG and many others
	Small cylindrical and prismatic cells Metal cans, Low cost products in standard shapes, Made in very high volumes, Complex and expensive packaging and BMS electronics due to low unit capacity, Sizes up to 5 Ah. Used by: ATL, BAK, B&K, BYD, Lishen, Panasonic, Sony, Sanyo, Toshiba, Samsung, Valence and many others

 

• Electrodes

The electrodes material may be a rigid metallic grids as in Lead acid batteries or the active electrode material may impregnated into or coated onto a spiral rolled metallic foil which simply acts as a current collector as in many Nickel and Lithium based cells. See also Battery Manufacturing

• Separator

The separator may be a mechanical spacer, fibreglass cloth or a flexible plastic film made from nylon, polyethylene or polypropylene. It must be porous and very thin to permit the charged ions to pass without impediment and it should take up the minimum of space to allow for the maximum use of the available space for the active chemicals. At the same time it must be resistant to penetration by burrs or dendrite growths on the electrode plates or from contamination of the electrode coating to prevent the possibility of short circuits between the electrodes. These characteristics should be maintained at high operating temperature when softening of the plastic material could clog the pores or reduce its resistance to penetration. The breakdown or penetration of the separator is a potential area of weakness in high power cells and special separator materials have been developed to overcome this problem.

 

The Separator .



• Terminals

There are many ways of connecting to the electrodes ranging from spring contacts, through wires or tags to mechanical studs. The main requirement is that the terminals should be able to handle the maximum current without overheating, either the terminal itself or the electrode connected to it. This needs careful design of the connection to the electrodes to take off the current through the maximum possible area of electrode material so as not to cause any hot spots. See also notes about external connections in the section on Battery Pack Design.

• Electrolyte

For many years all electrolytes were in aqueous or gel form. Recently solid polymer electrolytes have been developed which do not suffer from leakage or spillage. As well as being safer in case of an accident and they also bring new degrees of freedom to cell design allowing mechanical designs to be shaped to fit into odd shaped cavities. Polymer electrolytes are typically used in Lithium batteries.

Internal Construction

Broadly speaking the cell voltage is controlled by the cell chemistry, the capacity is governed by the weight of the active chemicals and the current carrying capacity (or power) is governed by the area of the electrodes. To a lesser extent all three of the above parameters as well as the thermal properties are affected by the internal impedance of the cell and this depends on the conductivity of the electrolyte and the layout and resistance of the components in the current path.

Thus the size shape and materials used in its construction all affect the ultimate electrical performance of a cell.

Electrodes (Energy/Power Trade-Offs)

For a given cell chemistry and within the space available inside a given cell case, the cell performance can be optimised for capacity or power.

• Increasing the surface area of the electrodes increases the cell's current handling capability. Thus the cell can both deliver more power and it can be charged more quickly.
• Increasing the volume of electrolyte in the cell increases the cell's energy storage capacity.

The prime trade off is between the area of the electrodes and the volume of the electrolyte which can be contained within the volume available in the cell case.

High power cells require electrodes with a large surface area as well as enlarged current collectors which take up more of the available space within a given cell, displacing the electrolyte and reducing the cell capacity.

The effective surface area of an electrode can be increased without increasing its physical size by making its surface porous and using materials with very fine particle size. This can increase the effective surface area of the electrodes by 1000 to 100,000 times enabling higher current rates to be achieved.

High capacity cells require large volumes of electrolyte which must be accommodated between the electrodes. This has a double effect in reducing the cell power handling capability. First, the electrodes must be smaller and further apart to make space for the extra electrolyte and hence they can carry less current. Secondly, because of the increased volume of the electrolyte, it takes longer for the chemical actions associated with charging and discharging to propagate completely through the electrolyte to complete the chemical conversion process. (More details are given in the section on Charging Times).

As an example - Lithium Ion cells optimised for capacity may typically handle peak currents of 2C or 3C for short periods, whereas Lithium Ion cells optimised for power could possibly deliver pulsed currents of 30C to 40C.

Four of the most common constructions are shown below. Over the years there have been many thousands of variants of these basic types used for many different cell chemistries.

High power cells usually incorporate special safety devices. See "Designed in" safety measures.

Bobbin Electrodes

Zinc - Carbon (Leclanché) Cell

The bobbin construction has been used for over a century since the introduction of the Zinc- Carbon (Leclanché) Cell and a more recently with the newer Alkaline cells which use a more complex version of this type. It is a cylindrical construction utilizing an internal cylindrical electrode usually in the form of a rod which is immersed in the electrolyte which is in turn contained in an external electrode in the form of a cylindrical cup arranged as a sleeve inside the cell container. A separator sheath prevents contact between the electrodes.

Its advantage is that it is simple, it stores a large amount of electrolyte which gives it a high capacity and hence long life but the electrode surfaces are very small which results in a high internal resistance and limits the currents it can deliver. However, this small area also results in a low rate of self discharge and these batteries therefore have a long shelf life. It is ideal for many applications requiring primary cells and is manufactured in very large quantities.

Flat Plate Electrodes

Cells Used in Lead Acid Battery

Source Eurobat

Flat plate cells typically used in Lead Acid batteries also have over a hundred years of history and development. The electrodes are made in the form of flat plates suspended in the electrolyte which is held in a suitable container which does not usually take part in the chemical reaction. A separator between the plates prevents them from touching eachother and short circuiting. This is another simple construction which is also used by many different cell chemistries. Its main advantage is that it can be scaled up to very large sizes, larger plates providing for higher currents and larger containers allowing high storage capacities.

Spiral Wound Electrodes (Also called Jelly-roll or Swiss-roll construction)

LG Spiral Wound Cylindrical Cell

In the quest for higher current carrying capacity, it is necessary to increase the active surface area of the electrodes, however the cell case size sets limits on the size of electrodes which can be accommodated. One way of increasing the electrode surface area is to make the electrodes and the separator from long strips of foil and roll them into a spiral or cylindrical jelly-roll shape. This provides very low internal resistance cells. The downside is that since the electrodes take up more space within the can there is less room for the electrolyte and so the potential energy storage capacity of the cell is reduced. This construction is used extensively for secondary cells. The example above shows a Lithium-Ion cell but this technology is also used for NiCads, NiMH and even some Lead acid secondary cells designed for high rate applications.

Spiral wound construction not limited to cylindrical shapes. The electrodes can be wound onto a flat mandrel to provide a flattened shape which can fit inside a prismatic case. The cases may be made from aluminium or steel.

This construction is ideally suited for production automation.

Button Cells and Coin Cells

Most button and coin cells follow the same basic layout in their construction as shown in the diagram above. They are used where small size is required. As with bobbin cells they have a relatively high capacity but deliver low power.

Multiple Electrode Cells

• Monopolar configuration

In this configuration the battery is constructed form individual cells with external connections joining the cells to form series and parallel chains.

• Stacked Electrodes

This is an example of monopolar configuration. Reminiscent of Volta's pile, the positive and negative plates are sandwiched together in layers with separators between them, inserted into the case, and sealed. This construction provides an energy efficient form factor and is typically used in high power Lithium Polymer cells with the solid polymer electrolyte separating the cells.

• Bipolar configuration

In bipolar batteries the cells are stacked in a sandwich construction so that the negative plate of one cell becomes the positive plate of the next cell. Electrodes, often called duplex electrodes, are shared by two series-coupled electrochemical cells in such a way that one side of the electrode acts as an anode in one cell and the other side acts as a cathode in the next cell. The anode and cathode sections of the common electrodes are separated by an electron-conducting membrane which does not allow any flow of ions between the cells and serves as both a partition and series connection.

 

This is an efficient design which reduces the number of plates and eliminates external connections, reduces the weight of the battery, increases the energy density and reduces costs. The current pathway is also shorter and resistance losses will be less enabling the battery to handle higher power. Bipolar construction is usually adopted for Fuel Cells and Flow Batteries in which the electrolyte flows over, or is pumped through the cells, but the technique has recently been trialled on much simpler, smaller scale automotive and portable batteries with several other cell chemistries using single electrolytes.

Bipolar Cell Construction In a Flow Battery                                         Source: Regenesys

 

Bipolar cells suffer from higher self discharge due the shunt current which occurs through the electrolyte manifold because the electrodes of the same polarity are at different potentials.

Bipolar batteries are also called "Layer Cells" and "Stacked Wafer Cells"

History

Electrode Interconnections

The internal resistance of the cell and the distribution of the currents and potentials within the cell are affected by the cell geometry which must be optimised to provide robust connections, minimum length current paths, and even distribution of currents to avoid local hot spots and uneven potentials which could give rise to unbalanced, damaging chemical reactions at different parts of the electrodes.

Sealed Cells and Recombinant Cells

Most batteries these days seal the electrolyte into the cell case. This is to prevent leakage of electrolyte and to prevent any gaseous products of charging and discharging from escaping into the atmosphere. The cell may also include a catalyst to promote recombination of these gaseous products. Such cells are called recombinant cells.

Safety Vents

When a cell is sealed, high internal pressures may build up due to the release of gases and due to expansion caused by high temperatures. As a safety precaution sealed cells usually incorporate a safety vent to allow excess pressure to be reduced in a controlled way.

Cell Casing

As with the internal cell construction there are many variants of case designs. Some standard, low power packages are shown below.

Cylindrical Cells

The picture above shows the most common cylindrical cell sizes from a range of manufacturers. The shapes may be standardized but the voltages and chemistries are not. The four largest sizes can each house Leclanché, Alkaline, NiCad, NiMH and Lithium chemistries with voltages ranging from 1.2 to 3.7 Volts. The first two chemistries are used for primary cells and are based on a bobbin construction. The remaining chemistries are used for secondary cells with spiral wound electrodes. Care is needed to match the chargers to the appropriate cells.

Cylindrical cells provide strong mechanical stability, good energy density and low costs.

It is estimated that 90% of portable, battery-operated devices require AA, C, or D battery sizes.

Multi-cell Batteries

The two batteries shown above are actually made up from groups of cylindrical cells. The type 23A contains eight 1.5 Volt cells and the PP3 contains six 1.5 Volt cells.

Common Household-Battery Sizes

Button Cells and Coin Cells

Button and coin cells are available in a range of sizes, voltages and cell chemistries. (Zinc -Air cells are also available but not shown)

Pouch Cells

Valence pouch cells

Prismatic Cells

Thin Film Batteries

Thin film printing technology is now being used to apply solid state Lithium polymer chemistry to a variety of substrates to create unique batteries for specialist applications. Thin film batteries can be deposited directly onto chips or chip packages in any shape or size, and flexible batteries can be made by printing on to plastics, thin metal foil or even paper. Because of their small size, the energy storage and current carrying capacity of thin film batteries is low but they have unique properties which distinguish them from conventional batteries including:

• All solid state construction
• The battery can be integrated into the circuit for which it provides the power
• Bendable batteries are possible
• Can be made in any shape or size
• Long cycle life and operating life
• Operate over wide temperature range
• High energy and power densities
• Cost and capacity are proportional to the area
• No safety problems

Thin film batteries have a wide range of uses as power sources for consumer products and for micro-sized applications. The battery on the right from Oak Ridge Micro-Energy is designed for use in implantable medical devices.
	Other applications include non-volatile memory backup and sensors.   Flexible film batteries like that shown on the left from Infinite Power Solutions are suitable for powering smart cards and radio frequency identification (RFID) tags.

Under development for ten years, thin film batteries are just recently becoming available in commercial quantities.

High Power Batteries

Timescales and Costs

In order to achieve economical market shares and at the same time guarantee high quality standards, cells are generally manufactured on high volume automated production lines. The timescale to develop and prove out an all new battery technology can be typically ten years and the costs of setting up a production line to manufacture the new batteries can be anywhere from $100 million to $300 million or more. Sales volumes need to be in the hundreds of millions to break even.

Nomenclature

Cell identification.

There is considerable confusion about naming standards for cells with different systems used in Europe, the USA and Japan as well as manufacturers ' own standards.

One convention is two letters followed by a series if numbers.

The first letter represents the cell chemistry. The second letter represents the shape of the cell.
The numbers represent the dimensions of the cell in millimetres. For cylindrical cells the first two digits are the diameter and the remaining digits the length. For prismatic cells the first two digits represent the thickness, the second pair the height and the last pair the width.

Because of the plethora of "standards" the only safe course in identifying a cell is to consult the manufacturers' data sheets.

Examples

• Common Primary Cells

See Battery Case Sizes for dimensions of common primary cells.

• Cylindrical Cells
  LC18650 is a common Li-ion cell in a Cylindrical can   Size (diameter18mm height 65.0mm)

See Cylindrical Cell Sizes for a listing of typical cylindrical cell sizes and capacities

• Prismatic Cells
  LP083448 is a Li-ion cell in a Prismatic can   Dimensions( thickness 8mm height 48mm width 34 mm)

See Prismatic Cell Sizes for a listing of typical prismatic cell sizes and capacities.

See Power Cell Sizes for examples of high power prismatic cells. (High power cylindrical cells are also available)

See also Battery Pack Design