Lithium Ion Battery Dry Rooms
Introduction
In 2009 the Department of Energy provided over $2 billion in grants and $25 billion in funding for low interest loan guarantees for the proliferation of the manufacture of alternate energy vehicles. The lion’s share of that funding went to automobile manufacturers and suppliers to that industry for the process development and construction of facilities for the manufacture of battery packs for battery operated vehicles.
Manufacturing Process Overview
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. 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 each other. 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 each other. For this reason the anodes and cathodes are usually processed in different rooms. The metal electrode foils are delivered on large reels, typically about 500 mm wide, with copper for the anode and aluminum 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 first stage is to mix the electrode materials with a conductive binder to form slurry which is spread on the surface of the foil as it passes into the machine. 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.
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.
Prismatic cells are often used for high capacity battery applications to optimize 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.
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.
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).
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. 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.
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.