Battery separators shutdown

In lithium-based cells, the essential function of battery separator is to prevent electronic contact, while enabling ionic transport between the positive and negative electrodes. It should be usable on highspeed winding machines and possess good shutdown properties. The most commonly used separators for primary lithium batteries are microporous polypropylene membranes. Microporous polyethylene and laminates of polypropylene and polyethylene are widely used in lithium-ion batteries. These materials are chemically and electrochemically stable in secondary lithium batteries. 

Conventional shutdown temperatures are around 130°C. However, a microporous polyolefin battery separator with a shutdown temperature of 95-110°C and a melt integrity of more than 165°C can be made from a basic UHMWPE formulation (38). 

K.V. Nguyen and C.G. Wensley, Shutdown battery separator made with a blend of polymer and oligomer, US Patent 6749961, assigned to Celgard Inc. (Charlotte, NC), June 15, 2004. 

Figure 20.14 shows a typical short-circuit curve for an 18,650 Li-Ion cell with a shntdown separator. The cell does not have other safety devices (e.g., CID, PTC), which nsnally work before separator shnt down. As soon as the cell is short circnited externally through a very small shunt resistor, the cell starts heating because of the large current drained through the cell. The shut down of the separator, which occnrs aronnd 130°C, stops the cell from heating further. The current decrease is caused by an increase of battery internal resistance dne to separator shutdown. The separator shutdown helps to avoid the thermal runaway of the cell. 

Battery separators are characterized by numerous properties, including material nature, membrane stractural and functional properties. Material nature includes chemical stability, crystalline structure, hydrophilicity, thermal shrinkage, melting point, M and Mv,/M of polyolefin materials. Structural properties include thickness, porosity, pore size, pore shape, pore tortuosity, and pore distribution. Functional properties include mechanical strength, electrical resistivity, air permeability, thermal shutdown, electrolyte wettability and retention. Many of the above properties are affected with each other and may be in a trade-off relationship. For example, the mechanical strength is affected in opposite manner by the thickness, porosity and permeability, as required by the battery performance. 

Yu WC, Geiger MW (1996) Shutdown, bilayta- battery separator. US Patent 5,565,281... 

While a battery separator s materials are usually inert and do not influence electrical energy storage or output, its properties can have an important influence on safety. There are three types commonly used [46] (i) high-temperature sohd-polymer electrolytes (SPEs) such as poly(ethylene oxide) (PEO), (ii) microporous shutdown separators, which are composed of poly(ethylene) (PE) or laminates of poly(propylene) (PP) and PE, (iii) gel polymers such as poly(vinyhdene fluoride), PVdF, and (iv) ceramic separators. Table 27.2 shows the types of separators used in secondary Hthium-based batteries. 

Short-circuit tests with lithium-ion batteries have been reported recently [35]. This work shows that the separator provides shutdown when the battery is subjected to an external short circuit with the PTC bypassed. The large increase in impedance of the separator is attributed to the temperature rise in the battery. 

Sony s Introduction of the rechargeable lithium-ion battery in the early 1990s precipitated a need for new separators that provided not only good mechanical and electrical properties but also added safety through a thermal shutdown mechanism. Although a variety of separators (e.g., cellulose, nonwoven fabric, etc.) have been used in different type of batteries, various studies on separators for lithium-ion batteries have been pursued in past few years as separators for lithium-ion batteries require different characteristics than separators used in conventional batteries. 

The shutdown property of separators is measured by measuring the impedance of a separator while the temperature is linearly increased. Figure 7 shows the actual measurement for Celgard 2325 membrane. The heating rate was around 60 °C/min, and the impedance was measured at 1 kHz. The rise in impedance corresponds to a collapse in pore structure due to melting of the separator. A 1000-fold increase in impedance is necessary for the separator to stop thermal runaway in the battery. The drop in impedance corresponds to opening of the separator due to coalescence of the polymer and/or to penetration of the separator by the electrodes this phenomenon is... 

To overcome the poor mechanical properties of polymer and gel polymer type electrolytes, microporous membranes impregnated with gel polymer electrolytes, such as PVdF. PVdF—HFP. and other gelling agents, have been developed as an electrolyte material for lithium batteries.Gel coated and/ or gel-filled separators have some characteristics that may be harder to achieve in the separator-free gel electrolytes. For example, they can offer much better protection against internal shorts when compared to gel electrolytes and can therefore help in reducing the overall thickness of the electrolyte layer. In addition the ability of some separators to shutdown... 

In lithium ion rechargeable batteries, shutdown separators are used as part of the overall battery safety system. These devices prevent, or substantially reduce the likelihood of thermal runaway, which may arise from short circuiting caused by physical damage, internal defect, or overcharging. The shutdown separators, will shutdown by a sufficient pore closure to substantially stop ion or current flow within the cell (37). 

Strong enough for tightly wound cells. Moreover, the shutdown temperature of the separator seans to be very high, and thus not suitable for Li-Ion batteries. 

An abnormal increase in cell temperatnre can occur from internal heating caused by either electrical abuse - overcharge or short circuit - or mechanical abuse - nail penetration or crush. Higher cell temperature also could be a result of external heating. For this reason, battery packs containing Li-Ion cells are designed with safety control circuits that have redundant safety features (PTC, CID, vent, thermal fuse, etc.). Shutdown separators are one of the safety devices inside the cell and act as a last line of defense. The separator shut down is irreversible, which is fine for poly-ethylene-based separators, which melt around 130°C. 

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