Your battery has several internal components, one of them being the battery separator. Most batteries have a separator with several functions, as you'll soon find out later in this article. The battery separator also affects how the battery performs, such as battery cycle. This article will also take a look at: why a battery requires a separator, and how battery separators are made. Components of a battery separator. Different battery separators for different battery types (Li-ion, lead-acid, etc.). The differences between a battery separator and an isolator. Let’s dive right in!
1.What is a battery separator?
First things first - before diving in too deep on battery separators, it would be best if you’d first understand the basics. The battery separator is an ion-permeable electronic insulating film between the cathode and anode, which plays a crucial role in the electrochemical and safety performance of the battery.Therefore, it is also known as the "third electrode" of the battery.
Positioning the separator between the two electrodes is essential because it helps prevent the battery from electrical short-circuiting during electrolysis and limiting excessive current. A good battery separator is well balanced between porosity (ability to transport) and mechanical robustness. In addition to that, pore size and permeability are aspects that must not be ignored. Permeability of the battery separator should ensure that the battery's performance is not limited. A battery separator should also be able to withstand high temperatures.
2.Types of battery separators
There are different types of battery separators. Check them out:
① Microporous separator
The separator is a functional membrane material with a microporous structure, and the thickness is generally 8-40 μm. In the battery system, it plays the role of separating the positive and negative electrodes, blocking the passage of electrons in the circuit during charging and discharging, and allowing the free passage of lithium ions in the electrolyte. The microhole can be selectively closed when the battery is charged or discharged or the temperature rises to limit excessive current and prevent short circuit.
② Nonwovens (nylon, cotton, glass, etc.)
Nonwovens separators are widely used in lead-acid, alkaline batteries, nickel-metal hydride and supercapacitors and other fields. They have the advantages of high temperature resistance and high porosity. Non-woven separators are expected to be suitable for large batteries due to their excellent heat resistance. Especially energy storage and power batteries that require large and long cycle life, they are promising for high-energy lithium battery systems due to their excellent heat resistance.
③ Ion exchange membranes
Ion exchange membrane is one of the key materials of flow cells, which can separate the cathode and anode, and achieve the construction of a complete circuit in the battery structure by selectively permeating ions.
④ Supported liquid membranes
The supported liquid membrane is to firmly adsorb the liquid membrane in the micropores of the porous support, and on both sides of the membrane are the feed liquid phase and the stripping phase that are immiscible with the membrane.
⑤ Polymer films
Advanced polymer films are important components of flexible electronic devices, such as wearable devices, electronic skins, and flexible batteries.
⑥ Solid ion conductors
Solid ion conductors can selectively pass lithium ions through ion channels. Because they are solid and capable of efficiently transporting lithium ions, they are expected to be the raw materials for next-generation batteries, such as high-voltage batteries or lithium metal batteries.
⑦ Separators used for non-aqueous
Now, you may be thinking to yourself, "How are battery separators made?" That’s what we will cover in the next section of this article.
3.How are battery separators made?
Battery separators are made from either organic, inorganic, or naturally occurring materials. When making battery separators, leading battery manufacturers must consider whether the material is both electrochemically and chemically stable for use inside a battery. The separator has to be stable with the electrolyte and the electrodes.
On top of that, separators also need to be robust enough to withstand high tension during the battery manufacturing process. Pore size also matters - an ideal battery separator’s pores should be smaller than the ion size of electrode materials, including electrode active materials, conductive additives, etc. Now, battery separators are produced using two primary ways: dry process and wet process.
4.Dry vs. wet battery separator production - what’s the difference?
The micropore preparation technology is the core of the lithium battery separator preparation process. According to the separator pore formation mechanism, the separator production process can be currently divided into two types: dry production and wet production. The primary difference between the two types of battery separator production is that dry production requires the manufacturer to first exturd, then anneal, and finally stretch the material.
On the other hand, wet battery separator production has more work. It requires the manufacturer to mix, heat, exturd, stretch and remove additives from the separator being produced. The difference between the two is that the dry production is to obtain crystallization of polyethylene, while the wet production is to heat and melt liquid hydrocarbons to form a mixture. Therefore, dry production of a battery separator process for polymers is ideal for polymers that possess high crystallinity. On the other hand, wet production of a battery separator process is suitable for polymers with both crystallinity and amorphous.
5.Why does a battery require a separator?
As you've read earlier, the battery separator is a very critical part of the battery, and it mainly plays two roles. First, it can act as an insulating layer to prevent short circuits caused by positive and negative contacts. Second, as a semi-permeable layer, it can improve the concentration difference near the cathode and anode of the battery, which is conducive to the diffusion of ions, thereby improving the storage efficiency of the battery. It is important to note that there is a battery type that does not require a battery separator - the solid-state battery. This battery has a solid electrolyte - it separates the anode from the cathode. That's why this battery type does not require a separator.
6.Which battery performance does the battery separator affect?
Battery separators indeed affect the performance of your battery. For example, the Li-ion battery's separator directly affects how the battery performs. The separator is one of the most critical inner layer components in the structure of lithium batteries. The quality of its performance directly affects the capacity, rate, life and safety of the battery. It actively contributes to the thermal stability of your lithium-ion battery.
Table 1 Correlation comparison between some separator performance and battery performance
7.What is the purpose of battery separators in lead-acid batteries?
Lead-acid batteries use glass fiber mat that has been soaked in sulfuric acid. Its purpose is to separate the battery from a short circuit during electrolysis. Apart from that, lead-acid battery separators allow the transport of sulfate ions from one side to another.
8.What does a separator do to a lithium-ion battery?
In lithium batteries, the separator mainly plays the role of isolating the cathode and anode to prevent short circuits and providing microchannels to support lithium ion migration, which has a key impact on battery safety, rate performance and cycle performance. High-performance lithium batteries require separators with uniform thickness and excellent mechanical properties (including tensile strength and puncture resistance), air permeability, and physical and chemical properties (including wettability, chemical stability, thermal stability, and safety).
Consider this example: your Li-ion battery may be under heavy use, becoming too hot. Since Li-ion batteries are sensitive to heat, thermal runaway may happen. To prevent thermal runway, the separator will then shut down. Li-ion battery separators also enhance the battery's performance. In other words, your Li-ion battery separator also protects the battery from damage.
9.Which material is used as a separator in Li-ion batteries?
Lithium-ion batteries use polymers as separators. There are different types of polymers that a Li-ion battery uses. Check them out: At present, the commercial lithium-ion battery separator products are mostly microporous films made of polyolefin materials. The main raw materials are high molecular weight polyethylene and polypropylene. The products include polyethylene PE single-layer film, polypropylene PP single-layer film and PP/PE/PP multi-layer microporous film composited with PP and PE.
① Polyethylene (PE) battery separator
PE separator has the unique characteristics of balanced MD/TD tensile strength and highly connected pore structure, which can promote the uniform growth of Li and alleviate the uneven distribution of Li+ flux, thereby slowing down the growth of local Li dendrites, and is often used in ternary lithium battery.
② Polypropylene (PP) battery separator
Single-layer PP separator provides better rate capability and remains stable over a wider temperature range, which is commonly found in LiFePO4 batteries.
③ Multilayer composite separator
Multilayer composite separator, namely PP/PE two-layer composite separator or PP/PE/PP three-layer composite separator, combines the advantages of PP film with good mechanical properties, high melting temperature and PE film with softness, good toughness and low closed-cell temperature, increasing the safety performance of the battery. These three polymer films are widely used in Li-ion batteries because of their robustness, porosity, permeability, and pore size.
10.What is the difference between battery isolator and separator?
As its name suggests, a battery isolator prevents over-discharge from your battery by isolating it. For example, if you have a completely discharged automobile battery, the isolator will prevent it from being discharged further. It blocks other types of loads from discharging the battery, that function helps the battery charge.
Here's the difference between a battery isolator and a separator: a battery isolator uses a capacitor (or a group of capacitors) to help charge your battery by preventing parasitic loads from draining your discharged battery. On the other hand, battery separators are more complex. Besides preventing short-circuiting in batteries, battery separators also check if the battery has enough voltage to run and helps the battery charge. People often confuse battery separators with battery isolators because battery separators are called "smart battery isolators."
A battery separator serves a significant function in your battery. It helps prevent short-circuiting in the battery as it is placed between the anode and cathode of your battery. Battery separators are robust, porous, and possess excellent porosity and permeability. They also have good pore size to facilitate ion movement from one side to another. Battery separators are produced primarily in two ways: wet and dry production.
There are different types of battery separators, and each type of battery uses its type of separators. The only battery that does not need a separator is the solid-state lithium-ion battery. Battery isolators and separators are different in that a battery isolator is one-directional and prevents battery over-discharge. On the other hand, the separator is multi-directional and serves several purposes, including checking the battery's voltage, preventing short-circuiting, and preventing battery over-discharge.