Uncovering the secrets of battery performance: working principle and performance evaluation of battery separators
Uncovering the secrets of battery performance: working principle and performance evaluation of battery separators
The components of a battery are a cathode and an anode, which are separated by a separator. The separator is wetted by an electrolyte, which forms a catalyst that facilitates the movement of ions from the cathode to the anode during charging and vice versa during discharge. Ions are atoms that have lost or gained electrons and become electrically charged. While ions can pass freely between the electrodes, the separator is a non-conductive insulator.
A small amount of current may flow through the separator, which is called self- discharge , and all batteries have self-discharge to varying degrees. During long-term storage, self-discharge will eventually deplete the battery. Figure 1 shows the building blocks of a lithium-ion battery, including the separator and the ion flow between the electrodes.
Figure 1. Ion flow through a lithium-ion separator
Battery separators provide a barrier between the anode (negative electrode) and cathode (positive electrode) while enabling lithium ions to be exchanged from one side to the other.
Early batteries were all flooded, including lead-acid and nickel-cadmium. With the development of sealed nickel-cadmium batteries in 1947 and maintenance-free lead-acid batteries in the 1970s, the electrolyte was absorbed into a porous separator, which was pressed against the electrodes to enable the chemical reaction. The tightly wound or stacked separator/electrode arrangement forms a solid mechanical unit that performs similarly to a flooded battery, but is smaller and can be mounted in any orientation without leaking. Gases generated during charging are absorbed, and if outgassing can be prevented, there is no water loss.Early separators were made of rubber, fiberglass mat, cellulose, and polyethylene plastic. Wood was the original choice, but it deteriorates in the electrolyte. Nickel-based batteries use porous polyolefin film, nylon, or cellophane separators. Absorbent glass mat (AGM) in sealed lead-acid batteries uses fiberglass mat as a separator, soaked in sulfuric acid.Early gel lead-acid batteries, developed in the 1970s, convert the liquid electrolyte into a semi-hard paste by mixing sulfuric acid with a silica gelling agent. Gel and AGM batteries differ slightly in performance; Gel batteries are often used in UPS, while AGM batteries are used in starters and deep cycle applications.Commercially available lithium-ion batteries use polyolefins as separators. This material has excellent mechanical properties, good chemical stability and is low cost. Polyolefins are a class of polymers produced from olefins by polymerizing the olefin ethylene. Ethylene comes from petrochemical sources; polyolefins are made from polyethylene, polypropylene or a laminate of the two materials.Lithium-ion separators must be permeable, with pore sizes ranging from 30 to 100 nm. (Nm stands for nanometer, 10 -9, or one millionth of a millimeter or about 10 atoms thick.) A porosity of 30% to 50% is recommended. This allows for adequate liquid electrolyte and allows the pores to close if the battery overheats.
The components of a battery are a cathode and an anode, which are separated by a separator. The separator is wetted by an electrolyte, which forms a catalyst that facilitates the movement of ions from the cathode to the anode during charging and vice versa during discharge. Ions are atoms that have lost or gained electrons and become electrically charged. While ions can pass freely between the electrodes, the separator is a non-conductive insulator.
A small amount of current may flow through the separator, which is called self- discharge , and all batteries have self-discharge to varying degrees. During long-term storage, self-discharge will eventually deplete the battery. Figure 1 shows the building blocks of a lithium-ion battery, including the separator and the ion flow between the electrodes.
Figure 1. Ion flow through a lithium-ion separator
Battery separators provide a barrier between the anode (negative electrode) and cathode (positive electrode) while enabling lithium ions to be exchanged from one side to the other.
Early batteries were all flooded, including lead-acid and nickel-cadmium. With the development of sealed nickel-cadmium batteries in 1947 and maintenance-free lead-acid batteries in the 1970s, the electrolyte was absorbed into a porous separator, which was pressed against the electrodes to enable the chemical reaction. The tightly wound or stacked separator/electrode arrangement forms a solid mechanical unit that performs similarly to a flooded battery, but is smaller and can be mounted in any orientation without leaking. Gases generated during charging are absorbed, and if outgassing can be prevented, there is no water loss.Early separators were made of rubber, fiberglass mat, cellulose, and polyethylene plastic. Wood was the original choice, but it deteriorates in the electrolyte. Nickel-based batteries use porous polyolefin film, nylon, or cellophane separators. Absorbent glass mat (AGM) in sealed lead-acid batteries uses fiberglass mat as a separator, soaked in sulfuric acid.Early gel lead-acid batteries, developed in the 1970s, convert the liquid electrolyte into a semi-hard paste by mixing sulfuric acid with a silica gelling agent. Gel and AGM batteries differ slightly in performance; Gel batteries are often used in UPS, while AGM batteries are used in starters and deep cycle applications.Commercially available lithium-ion batteries use polyolefins as separators. This material has excellent mechanical properties, good chemical stability and is low cost. Polyolefins are a class of polymers produced from olefins by polymerizing the olefin ethylene. Ethylene comes from petrochemical sources; polyolefins are made from polyethylene, polypropylene or a laminate of the two materials.Lithium-ion separators must be permeable, with pore sizes ranging from 30 to 100 nm. (Nm stands for nanometer, 10 -9, or one millionth of a millimeter or about 10 atoms thick.) A porosity of 30% to 50% is recommended. This allows for adequate liquid electrolyte and allows the pores to close if the battery overheats.
Separator acts as a fuse in lithium-ion batteries
In the event of overheating, shutdown is achieved by closing the pores of the lithium-ion separator through a melting process. When the core temperature reaches 130°C (266°F), the polyethylene (PE) separator melts. This stops ion transport, effectively shutting down the battery. Without this measure, the heat in a faulty cell could rise to the thermal runaway threshold and burn. This internal safety fuse also helps pass the rigorous UN transportation tests for lithium batteries, which include altitude simulation as well as thermal, vibration, shock, external short circuit, impact, overcharge and forced discharge tests.
In the event of overheating, shutdown is achieved by closing the pores of the lithium-ion separator through a melting process. When the core temperature reaches 130°C (266°F), the polyethylene (PE) separator melts. This stops ion transport, effectively shutting down the battery. Without this measure, the heat in a faulty cell could rise to the thermal runaway threshold and burn. This internal safety fuse also helps pass the rigorous UN transportation tests for lithium batteries, which include altitude simulation as well as thermal, vibration, shock, external short circuit, impact, overcharge and forced discharge tests.
Most cell phone and tablet batteries have a single polyethylene separator. Since around 2000, large industrial batteries have used a three-layer separator to provide enhanced fuse protection at extreme temperatures and in multi-cell configurations. Figure 2 shows a PP/PE/PP three-layer separator with polyethylene in the middle and polypropylene (PP) layers on the outside. While the inner PE layer closes its pores at 130°C, the outer PP layer remains solid and does not melt until it reaches 155°C (311°F).
Figure 2: Side view of PP/PE/PP three-layer material.
Combining separator materials with different melting characteristics increases safety. PE melts before PP, closing the pores and preventing the flow of current.Around 2008, further improvements were achieved with the addition of a ceramic coated separator. The ceramic particles do not melt and this addition provides a higher level of safety. Lithium cobalt oxide (LCO) cells also use ceramic coatings and have a charging voltage of up to 4.40V/cell instead of the traditional 4.20V/cell. The ceramic coating works in conjunction with the PE and PP layers and is placed next to the positive electrode to prevent electrical contact.The separator should be as thin as possible to avoid adding dead volume, while still providing sufficient tensile strength to prevent stretching during winding and provide good stability over the service life. The pores must be evenly distributed across the sheet to ensure uniform distribution across the separator area. In addition, the separator must be compatible with the electrolyte and easily wetted. Dry areas may cause hot spots due to increased resistance, which can lead to battery failure.Separators are getting thinner. Thickness is typically 25.4μm (1.0 mil), but some go down to 20μm, 16μm, and even 12μm without significantly compromising battery performance. (A micrometer, also called a µm, is one millionth of a meter.) The separator with the electrolyte in a modern lithium-ion battery accounts for only 3% of the battery's content.Ultra-thin separators pose safety risks. Recall the massive Sony recall, where a one-in-200,000 battery failure rate triggered a recall of nearly six million lithium-ion battery packs. In rare cases, tiny metal particles can come into contact with other parts of the battery cell, causing an electrical short. The separators in the Sony batteries in question were between 20µm and 25µm thick. (A micrometer (µm) is one thousandth of a millimeter.) Some separators were as thin as 10 µm. Micro-shorts on separators examined in the lab were about a millimeter in diameter. A well-designed separator will melt at the point of the short and provide a localized shutdown.
Most cell phone and tablet batteries have a single polyethylene separator. Since around 2000, large industrial batteries have used a three-layer separator to provide enhanced fuse protection at extreme temperatures and in multi-cell configurations. Figure 2 shows a PP/PE/PP three-layer separator with polyethylene in the middle and polypropylene (PP) layers on the outside. While the inner PE layer closes its pores at 130°C, the outer PP layer remains solid and does not melt until it reaches 155°C (311°F).
Figure 2: Side view of PP/PE/PP three-layer material.
Combining separator materials with different melting characteristics increases safety. PE melts before PP, closing the pores and preventing the flow of current.Around 2008, further improvements were achieved with the addition of a ceramic coated separator. The ceramic particles do not melt and this addition provides a higher level of safety. Lithium cobalt oxide (LCO) cells also use ceramic coatings and have a charging voltage of up to 4.40V/cell instead of the traditional 4.20V/cell. The ceramic coating works in conjunction with the PE and PP layers and is placed next to the positive electrode to prevent electrical contact.The separator should be as thin as possible to avoid adding dead volume, while still providing sufficient tensile strength to prevent stretching during winding and provide good stability over the service life. The pores must be evenly distributed across the sheet to ensure uniform distribution across the separator area. In addition, the separator must be compatible with the electrolyte and easily wetted. Dry areas may cause hot spots due to increased resistance, which can lead to battery failure.Separators are getting thinner. Thickness is typically 25.4μm (1.0 mil), but some go down to 20μm, 16μm, and even 12μm without significantly compromising battery performance. (A micrometer, also called a µm, is one millionth of a meter.) The separator with the electrolyte in a modern lithium-ion battery accounts for only 3% of the battery's content.Ultra-thin separators pose safety risks. Recall the massive Sony recall, where a one-in-200,000 battery failure rate triggered a recall of nearly six million lithium-ion battery packs. In rare cases, tiny metal particles can come into contact with other parts of the battery cell, causing an electrical short. The separators in the Sony batteries in question were between 20µm and 25µm thick. (A micrometer (µm) is one thousandth of a millimeter.) Some separators were as thin as 10 µm. Micro-shorts on separators examined in the lab were about a millimeter in diameter. A well-designed separator will melt at the point of the short and provide a localized shutdown.
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