Components of a Lithium-ion cell – Part 4 | Separator

A Lithium-ion cell is built of various components and sub-components. This article discusses the functionality and importance of selecting the right type of (sub) components.

A Lithium-ion cell has four major components:

• Cathode – Positive electrode
• Anode – Negative electrode
• Electrolyte – Medium for the movement of lithium ions
• Separator – Prevents contact between cathode and anode

The earlier parts in this series talked about the Cathode, Anode and Electrolyte. In this article (part 4), author Rahul Bollini discusses the Separator.

Out of the four major components of a Lithium-ion cell, the separator is likely the least discussed. Thought of as a plastic material, the separator seems very simple from the outside.

However, when studied in depth, the separator is a fascinating component with multiple parameters. Choosing from the various parameters makes the number of possible combinations of separator specifications very high. No single type of separator can cater for all kinds of Lithium-ion cells.

What is a separator?

What are the types of separators?

There are three major types of separators, Dry, Coated and Wet, as described below:

• Dry separator: It is manufactured by melting the polymer and then stretching it in a single direction. It is the oldest, simplest and cheapest technology of separators for Lithium-ion cells, and it is still popular today. A dry separator tends to be thicker and can have multiple layers.

• Coated separator: In a coated separator, the base film (dry separator) has an external coating of ceramic (alumina or boehmite), PVDF-HFP (Polyvinylidene Fluoride-Hexafluoropropylene) and nanofiber (aramid). The coated separator has a higher temperature meltdown (>200°C). Listing the commercially available combinations of coating on separators:

• Ceramic coating on one side of the base film but no coating on other side of base film.
• Ceramic coating on both sides of the base film.
• Ceramic + PVDF coating on both sides of the base film.
• Base film has ceramic coating above it on one side and PVDF coating above the ceramic coating. There is no coating on the side of the base film.
• Ceramic coating on one side of base film, PVDF coating on the other side of base film.
• Base film has ceramic coating above it on one side and PVDF coating above the ceramic coating. It is the same arrangement on another side as well.
• Nanofiber coating on both sides of the base film.
• The base film has a nanofiber coating above it on one side and a PVDF coating above the ceramic coating. It has the same arrangement on the other side as well.

• Wet separator: It is manufactured by mixing the polymer resins with paraffin oil and other additives, heating the mixture, making it into a thin sheet-like film, removing the additives to create micropores and then stretching the film.

The wet separator has a very tensile strength in TD (transverse direction) or CD (cross-machine direction) when compared to the dry separator, which can be as high as 3200Kg/cm^2 in wet separator as compared to around 200Kg/cm^2 in the dry separator. Wet separators are very popular in low thickness, and their thickness starts from 4μm. High-energy density cells use low-thickness separators to improve the volumetric energy density.

The manufacturing setup of wet separators is very expensive, and the process of manufacturing wet separators is expensive and complicated. Hence, wet separators can cost 3x to 5x the cost of dry separators.

Important parameters of separators

• Material of the separator – can be PP (polypropylene) or a mix of PP and PE (polyethylene).
• No. of layers –  can go up to 3 layers. For example, PP/PE/PP tri-layer separator.
• Overall thickness of the separator – can vary from 4μm to 40μm.
• Porosity – can vary from 30% to 60%. Porosity is in the nm scale, and the pores close when the cell experiences high temperature. Generally, it is close to 40%.
• Gurley value – is the time required for a specific amount of air to pass through a specific area at a specific pressure, expressed in seconds. Its value ranges from 120 to 350 seconds/100ml in dry separators, and from 90 to 180 seconds/100ml in wet separators
• Electrolyte wettability – a critical characteristic of the separator because the absorption of the electrolyte is important for the transportation of lithium ions. The wettability process undergoes as soon as the electrolyte filling is done in the cell.
• Ionic conductivity – measured in Milli-Siemens per centimetre.
• TD or CD shrinkage at various temperatures after one hour, e.g. at 90°C, the shrinkage should be less than 1% after one hour.
• MD (machine direction) shrinkage at various temperatures after one hour. e.g. at 90°C, the shrinkage should be less than 5% after one hour.
• Tensile strength in MD, e.g. is about 2000 Kg/cm^2.
• Tensile strength in TD or CD e.g. can be up to 200 Kg/cm^2 in the dry separator.
• Puncture strength – is measured in grams. Its value ranges from 280 to 550 grams in dry separators, and from 280 to 780 grams in wet separators.

Facts about lithium-ion cell separators

• Widest of all the three materials, winded (cylindrical and prismatic cells) or stacked (pouch cell) inside a Lithium-ion cell, meaning its width is more than the width of the cathode and the anode of the cell.
• Separators are made up of polyolefin material, which is either polypropylene or polyethylene or both combined. These materials have very good chemical and mechanical stability and are cost-effective.
• LFP chemistry cells, most of the time, use a single-layer PP dry separator. It is the cheapest type of separator out there.
• NMC and NCA chemistry cells tend to use different types of separators in the range of coated separators and wet separators.
• Tri-layer separator made up of PP/PE/PP is very popular because, in case of high temperatures being reached inside the cell, the middle layer of PE melts and shuts down the cell operation without losing mechanical integrity. It is possible because the melting point of PE (around 130°C) is lesser than PP (around 160°C).
• In a solid-state battery, the solid electrolyte placed between the electrodes eliminates the use of a separator.

Separators are a customized product, and a cell manufacturer generally shares their requirement with a separator manufacturer. Selection of the separator for the Lithium-ion cell is an art because there are no fixed definitions. Also, there can be more than one type of separator that can work well with a type of Lithium-ion cell.

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Also Read –

1. Components of a Lithium-ion cell – Part 1 | Cathode
2. Components of a Lithium-ion cell – Part 2 | Anode
3. Components of a Lithium-ion cell – Part 3 | Electrolyte