Lithium Battery Manufacturing
The processes used for manufacturing Lithium batteries are very similar to those used in the production of Nickel Cadmium cells and Nickel Metal Hydride cells with some key differences associated with the higher reactivity of the chemicals used in the Lithium cells.
The anodes and cathodes in Lithium cells are of similar form and are made by similar processes on similar or identical equipment. The active electrode materials are coated on both sides of metallic foils which act as the current collectors conducting the current in and out of the cell. The anode material is a form of Carbon and the cathode is a Lithium metal oxide. Both of these materials are delivered to the factory in the form of black powder and to the untrained eye they are almost indistinguishable from eachother. Since contamination between the anode and cathode materials will ruin the battery, great care must be taken to prevent these materials from coming into contact with eachother. For this reason the anodes and cathodes are usually processed in different rooms.
Particle size must be kept to a minimum in order to achieve the maximum effective surface area of the electrodes needed for high current cells. Particle shape is also important. Smooth spherical shapes with rounded edges are desirable since sharp edges or flaky surfaces are susceptible to higher electrical stress and decomposition of the anode passivating SEI layer, which can lead to very large heat generation and possible thermal runaway when the cells are in use.
The metal electrode foils are delivered on large reels, typically about 500 mm wide, with copper for the anode and aluminium for the cathode, and these reels are mounted directly on the coating machines where the foil is unreeled as it is fed into the machine through precision rollers.
The coating process is shown in the diagram below
The first stage is to mix the electrode materials with a conductive binder to form a slurry which is spread on the surface of the foil as it passes into the machine. A knife edge is located just above the foil and the thickness of the electrode coating is controlled by adjusting the gap between the knife edge and the foil. Since it is not unusual for the gravimetric or volumetric energy storage capacity of the anode material to be different from that of the cathode material, the thickess of the coating layers must be set to allow the energy storage per unit area of the anode and cathode electrodes to be matched.
From the coater, the coated foil is fed directly into a long drying oven to bake the electrode material onto the foil. As the coated foil exits the oven it is re-reeled.
The coated foils are subsequently fed into slitting machines to cut the foil into narrower strips suitable for different sizes of electrodes. Later they are cut to length. Any burrs on the edges of the foil strips could give rise to internal short circuits in the cells so the slitting machine must be very precisely manufactured and maintained.
In the best factories cell assembly is usually carried out on highly automated equipment, however there are still many smaller manufacturers who use manual assembly methods.
The first stage in the assembly process is to build the electrode sub-assembly in which the separator is sandwiched between the anode and the cathode. Two basic electrode structures are used depending on the type of cell casing to be used, a stacked structure for use in prismatic cells and a spiral wound structure for use in cylindrical cells.
See Cell Constuction for a description of Stacked and Wound cells.
The assembly process for prismatic and cylindrical cells is illustrated in the following diagram.
- Prismatic Cells
Prismatic cells are often used for high capacity battery applications to optimise the use of space. These designs use a stacked electrode structure in which the anode and cathode foils are cut into individual electrode plates which are stacked alternately and kept apart by the separator. The separator may be cut to the same size as the electrodes but more likely it is applied in a long strip wound in a zig zag fashion between alternate electrodes in the stack.
While this case design makes optimum use of space when used in a battery pack, it has the disadvantage that it uses multiple electrode plates which need a clamping mechanism to connect all the anodes together and to the main terminal post and a similar mechanism for the cathodes. This all adds to the complexity and labour content of the cell and consequently to the costs.
Some prismatic cells are also made by the simpler method of winding the electrodes on a flat mandrel. (See below)
Stacked electrodes are also used for the production of pouch cells.
- Cylindrical Cells
For cylindrical cells the anode and cathode foils are cut into two long strips which are wound on a cylindrical mandrel, together with the separator which keeps them apart, to form a jelly roll (Swiss roll in the UK). Cylindrical cells thus have only two electrode strips which simplifies the construction considerably.
A single tab connects each electrode to its corresponding terminal, although high power cells may have multiple tabs welded along the edges of the electrode strip to carry the higher currents.
The next stage is to connect the electrode structure to the terminals together with any safety devices and to insert this sub-assembly into the can. The can is then sealed in a laser welding or heating process, depending on the case material, leaving an opening for injecting the electrolyte into the can.
The following stage is to fill the cell with the electrolyte and seal it. This must be carried out in a "dry room" since the electrolyte reacts with water. Moisture will cause the electrolyte to decompose with the emission of toxic gases. Lithium Hexafluoride (LiPF6) for instance, one of the most commonly used electrolyte materials, reacts with water forming toxic hydrofluoric acid (HF).
Afterwards the cell is given an identification with a label or by printing a batch or serial number on the case.
Once the cell assembly is complete the cell must be put through at least one precisely controlled charge / discharge cycle to activate the working materials, transforming them into their useable form. Instead of the normal constant current - constant voltage charging curve, the charging process begins with a low voltage which builds up gradually. This is called the Formation Process. For most Lithium chemistries this involves creating the SEI (solid electrolyte interface) on the anode. This is a passivating layer which is essential for moderating the charging process under normal use.
During formation, data on the cell performance such as capacity and impedance, are gathered and recorded for quality analysis and traceability. The spread of the performance measurements also gives an indication of whether the process is under control. (Beware of manufacturers who use this process for sorting their cells into different performance groups for sale with alternative specifications).
Although not the prime purpose of formation, the process allows a significant percentage of early life cell failures due to manufacturing defects, the so called "infant mortalities", to occur in the manufacturer's plant rather than at the customers' premises.
Tight tolerances and strict process controls are essential throughout the manufacturing process. Contamination, physical damage and burrs on the electrodes are particularly dangerous since they can cause penetration of the separator giving rise to internal short circuits in the cell and there are no protection methods which can prevent or control this.
Cleanliness is essential to prevent contamination and cells are normally manufactured in clean room conditions with controlled access to the assembly facilities often via air showers.
Apart from the production test equipment, a battery manufacturer should be expected to have a materials laboratory equipped to carry out a full analysis of the materials used in the production of the cells as well as to carry out failure analysis. The following list shows some of the major equipment used.
- Scanning electron microscope (SEM) for investigating the physical structure of the materials
- Mass spectrometer for analysing the chemical content of the materials
- Calorimeters for checking the thermal properties of the materials and the cells
- Programmable charge/discharge cycle test equipment to exercise the cells and verify their lifetime
- Environmental chambers and vibration tables for investigating the performance of the cells under their expected operating conditions
- Mechanical stress tesing equipment
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