Flow controllers in lithium-ion battery anode production

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Rechargeable lithium-ion batteries are never far away from us: in gadgets like smartphones and tablets, and in vehicles like electric cars and electrical bicycles. The anode and cathode, and therefore the materials they are made of, determine largely the capacity of these batteries.

Carbon in the form of graphite has always been the workhorse anode material for lithium-ion batteries. However, silicon with space available for more lithium ions than carbon is knocking on the door. This comes at the expense that silicon anodes swell and shrink more severely when charging or discharging than graphite, making the material mechanically more vulnerable than graphite. To combine the good properties of both materials, silicon/carbon composites as anode material are of high interest and are under development.

Bronkhorst devices are essential in the manufacturing of silicon/carbon composite anodes at leading Chinese anode manufacturer Foshan. Read why.

Application requirements

The chemical vapor deposition (CVD) process used for silicon/carbon composite anode manufacturing requires optimum control of the precursors supply. Especially an accurate flow control of the silane precursor is required, typically with 1% of full-scale accuracy. Moreover, this accuracy needs to be retained over time, so repeatability is important as well. Special attention is required for the highly reactive nature of silane.

Important topics

Better control of CVD process, therefore a more reliable process

High accuracy and repeatability of especially the silicon-precursor silane flow rate

Flexibility due to ease of scaling-up

Process solution

In the CVD process for silicon/carbon composite anode manufacturing, gaseous compounds as silane (SiH4), nitrogen (N2) and acetylene (C2H2) are supplied to the heated reactor where they react to solid silicon and carbon compounds that deposit as a composite material in a porous carbon substrate. Bronkhorst thermal EL-FLOW mass flow controllers are used for this precursor supply, which are calibrated to suit the precursor type and flow range.

Essentially, accuracy is a prerequisite to keep the CVD process under control. The thermal principle of the EL-FLOW series allows for an accurate supply of the precursors, especially for silane as main precursor. There is a direct relationship between thermal properties of the precursor and its mass. This has the advantage that a mass flow rate is controlled, which is not affected by fluctuating process conditions like variations in temperature or pressure.

Bronkhorst has gained extensive experience with the use of silane as precursor, for example in the photovoltaics (solar cell) market. As it is highly reactive and easily flammable, the EL-FLOW mass flow controller allows for a safe handling of this hazardous compound. For example, in the event that the highly reactive silane source is polluted with small amounts of water and air, tiny silica (SiO2) particles can be formed in the silane stream. Means such as filters are available to prevent the interior of the mass flow controllers from being clogged. Moreover, air (O2) leaking from the outside into the silane stream is prevented by a very good leak integrity of the Bronkhorst instruments.

As accuracy is key in this application, the used thermal principle of the Bronkhorst EL-FLOW device for precursor mass supply is better than differential pressure flow control where the accuracy is affected by zero-point drift, especially when powder particles are being generated inside the sensor.

If you need more robust mass flow controllers for this application, MASS-STREAM devices are available, working with a similar thermal flow control principle, and also with an accuracy of 1% of full scale.

"Bronkhorst is our preferred supplier for gas mass flow control. Their high precision, short lead times, stable and reliable performance (backed by a 3-year warranty), and effective technical services make them an excellent choice for us. We anticipate even closer collaboration with Bronkhorst as industrialization continues to develop."

- Foshan SAPFIT Technology Co. Ltd -