673886040@qq.com
Battery Separators
The building blocks of a battery are the cathode and anode, which are isolated by a separator. The separator is moistened with electrolyte and forms a catalyst that promotes the movement of ions from cathode to anode on charge and in reverse on discharge. Ions are atoms that have lost or gained electrons and have become electrically charged. Although ions pass freely between the electrodes, the separator is an isolator with no electrical conductivity. The small amount of leakage that may pass is self-discharge that is present on all batteries and will eventually deplete the charge on a prolonged storage. Figure 1 illustrates the building block of a lithium-ion cell with the separator and ion flow between the electrodes.Figure 1. Ion flow through the separator of Li-ion. Battery separators provide a barrier between the anode (negative) and the cathode (positive) while enabling the exchange of lithium ions from one side to the other. Courtesy of CELGARD, LLC
Early batteries were flooded, including lead acid and nickel-cadmium. With the development of the sealed nickel-cadmium in 1947 and the maintenance-free lead acid in the 1970s, the electrolyte was absorbed into a porous separator that was pressed against the electrodes. The tightly wound or stacked separator/electrode arrangement forms a solid mechanical unit that offers similar performance to the flooded type but is smaller, maintenance-free and can be installed in any orientation without leakage.
Early separators were made of rubber, glass fiber mat, cellulose and polyethylene plastic. Wood was the original choice but it deteriorated in the electrolyte. Nickel-based batteries use separators of porous polyolefin films, nylon or cellophane. The absorbed glass mat (AGM) in the sealed lead acid version uses a glass fibre mat as separator that is soaked in a sulfuric acid. The preceding gelled lead acid developed in the 1970s converts the liquid electrolyte into a semi-stiff paste by mixing the sulfuric acid with a silica-gelling agent. There are slight differences in performance between gel and AGM. Gel batteries are commonly used in UPS and AGM in starter and deep-cycle applications.
Commercially available Li-ion cells use polyolefin as a separator. This material has excellent mechanical properties, good chemical stability and is low-cost. A polyolefin is a class of polymer that is produced from olefin by polymerizing olefin ethylene. Ethylene comes from a petrochemical source; polyolefin is made from polyethylene, polypropylene or laminates of both materials.
The Li-ion separator must be permeable and the pore size ranges from 30 to 100nm. (Nm stands for nano-meter that is one billionth of a meter or about 10 atoms thick.) The recommended porosity is 30 to 50 percent, an absorbency holds sufficient liquid electrolyte but is not too porous to prevent the pores from closing should the cell overheat.
The shut-down mechanism by closing the pores on excessive heat is achieved with polyethylene (PE) by a melting process that occurs when the core reaches 130°C (266°F). This stops the transport of ion and halts current flow, effectively shutting the cell down. Without this provision, the heat in the failing cell could rise to the thermal runaway threshold and vent with flame. The internal safety fuse also helps pass the stringent UN Transportation Testing for Lithium Batteries that includes altitude simulation, as well as thermal, vibration, shock, external short circuit, impact, overcharge and forced discharge tests.
The separator should be as thin as possible as not to add dead volume and still provide sufficient tensile strength to prevent stretching during the winding process and offering good stability throughout life. The pores must be uniformly spread on the sheet to ensure even current distribution throughout the entire separator area. Furthermore, the separator must be compatible with the electrolyte and allow easy wetting. Dry areas can create hot spots through elevated resistance, leading to possible cell failure.
Most batteries for mobile phones and tablets have a single polyethylene separator. Since ca. 2000, larger industrial batteries deploy a trilayered separator that provides enhanced fuse protection on thermal extremes and on multi-cell configurations. Figure 2 illustrates the PP/PE/PP trilayer separator consisting of polyethylene in the middle that is sandwiched by outer polypropylene (PP) layers. While the inner PE layer shuts down a 130°C by closing the pores, the outer PP layers stay solid and do not melt until reaching 155°C (311°F).
Figure 2: Side view of PP/PR/PP trilayer. Combining separator material with different melting properties adds to safety. PE melts before PP to close the pores and stop current flow.
Courtesy of Dalhousie, Handbook of Batteries
In ca. 2008, further improvements were made by adding the ceramic-coated separator. Ceramic particles do not melt and this addition provides a further safety level. Ceramic coating is also used on the high-voltage lithium cobalt oxide (LCO) that charges up to 4.40V/cell instead of the traditional 4.20V/cell. The ceramic coating works in tandem with the PE and PP layers and is placed next to the positive side to prevent electrical contact.
To boost the energy density further, separators are getting thinner. A thickness of 25.4μm (1.0 mil) is common but some separators go down to 20μm, 16μm and now even 12μm by not significantly compromising the properties of the cell. (One micron, also known as µm, is one millionth of a meter.)
Ultrathin separators raise safety concerns and the reader is reminded of the massive Sony call-back. In 2006, a one-in-200,000 cell-breakdown triggered a recall of almost six million lithium-ion packs. Sony, the maker of the lithium-ion cells in question, pointed out that on rare occasions microscopic metal particles may come into contact with other parts of the battery cell, leading to an electrical short circuit within the cell. The Sony cells in question had a separator thickness of between 20–25µm.
Lithium polymer
Marketing promotes lithium polymer as a new battery breed, but much of this is hype. Polymer goes back to the 1970s when the traditional separator was replaced with a plastic-like dry film. The true polymer battery does not conduct at room temperature and needs to be heated to 60°C (140°F) and higher to promote ion exchange and induce of current flow.
To make Li-polymer battery conductive at room temperature, gelled electrolyte is added and Li- polymer becomes a hybrid. This causes a blur in the market place, but since the characteristics of the two battery systems are identical, the users need not worry.
Li-polymer is not a unique battery chemistry but represents a different architecture that can be built on many systems, including lithium iron phosphate (LiFePO4). Li-polymer packs for the consumer market are commonly lithium cobalt oxide (LCO). Thin pack profile is an advantage but fabrication costs tend to be higher than with regular Li-ion because polymers are more difficult to handle than a simple sheet of plastic. In addition, polymer separators increase the electrolyte resistance by a factor of four to five. This limits the polymer battery to low current applications.
Research with solid electrolyte (SE) continues and attempts are made by using metallic lithium as anode material. Solid lithium has a higher energy density than in modified lithium-ion form, but lithium anodes have been tried before and battery manufacturers were forced to discontinue production because of safety issues. Lithium tends to form metal filaments, or dendrites, that cause short circuits. Scientists are trying to overcome this invasion by using specially designed separators and other remedies.
The key objectives for the so-called “solid state lithium ion battery” are achieving sufficient conductivity at room temperature and below and delivering a high enough cycle count, a weak point with most new battery designs. Prototypes of the solid state lithium ion only reach 100 cycles. Targeted applications are load leveling for renewable energy source and fulfilling the emotional need of personal transportation in cars that are non-polluting, charge in minutes and do not prompt range anxiety. Commercialization can take 10 years or longer.
Tcbest:
Shenzhen Tcbest Technology Co,.Ltd is one of the largest battery manufacturer in China, is committed to research, develop, produce and sale environmental alkaline batteries, carbon zinc batteries, lithium button cells, ni-mh, ni-cd rechargeable batteries, polymer lithium battery and charger, etc. is the most complete varieties of battery manufacturers in China.
