FFD IM - Freudenberg Forschungsdienste SE & Co. KG

More documents

Recommendations

Info

Batteries at FFD – under scrutiny InnOvAtIve LIthIuM IOn bAttery sepArAtOrs 4 Innovative lithium ion battery separators Introduction Battery separators are porous surface-type components which prevent electrical contact between the positive and negative electrodes of a battery cell yet, at the same time permit an unobstructed transport of ions between the electrodes. Ideally, separators are not involved in electrochemical reactions inside a battery, although they influence battery properties such as energy density, power density, lifetime and operational reliability to a very considerable extent. In the case of lithium ion batteries for classic applications in the 4C market (computers, cameras, cellular phones, cordless tools), separators are primarily microporous polyolefin-based (PP and / or PE) membranes. The ten largest lithium ion battery manufacturers alone need more than 450 million m 2 of separators per year (2011), with growth rates of more than 10%. The advantages of microporous membranes are their thickness (25 µm or less) and their small pore sizes (< 1 µm). The disadvantages of these membranes are their low melting point (approx. 120°C for PE and 160°C for PP), their high shrinkage rate and, in particular, their low puncture resistance with regard to electrode particles, for example. In 2011, on behalf of the French Ministry of the Environment, the French institute INERIS established that conductive particles which force their way through a separator have been one of
the main causes for internal short-circuits of lithium ion batteries. This has resulted in very cost-intensive recall campaigns by battery manufacturers. Since conventional polyolefin-based separators have the weaknesses described above, FFD, in cooperation with Freudenberg Vliesstoffe KG, has since 2007 been developing separators, particularly for large-volume lithium ion batteries. Due to their high energy content, these impose totally new requirements on the safety of batteries and thus also of separators. Freudenberg’s technology With Freudenberg’s approach (see Figure 1), an ultra-thin temperature-stable PET wet nonwoven is coated with anorganic particles and an organic bonding agent. The anorganic particles ensure excellent temperature stability and the organic bonding agents create a level of flexibility which, despite the anorganic particles, does not make the materials brittle but highly flexible. Figure 1: Coating of a nonwoven Due to temperature stability, the separators developed by Freudenberg feature a very low shrinkage rate. Figure 2 depicts a comparison between a Freudenberg separator and commercial PO membranes after a storage period of one hour at 160°C. What can be clearly seen is the shrinkage rate of the wet membrane (1, PE) and the unidirectionally stretched dry membranes (2, PP and 3, PE / PP multilayer). In both cases, the stress levels of the membranes which are frozen at room temperature are increasingly released, resulting in shrinkage and thus in a high safety risk. In contrast to conventional PO separators, Freudenberg’s products do not show any detectable shrinkage, a fact which predestines them for safety-related applications. Figure 2: Comparison of the shrinkage rates of different lithium ion separators puncture resistance of separators – resistance desired In order to be able to determine and assess the relevant safety properties of separators, the Physical Testing Department has, over the last few years, developed and improved a series of specific test methods, including the mixed penetration test (see Figure 3), with which the puncture resistance of a separator can be determined. Innovative lithium ion battery separators 5
Page 1 and 2: Issue 2_2012 FFD IM DIALOG More inf
Page 3: Editorial Dear Business Partners, T
Page 7 and 8: cyclical lifetime and a high curren
Page 9 and 10: As non-contact machine guards rFID
Page 11 and 12: In our case, active transponders ar
Page 13 and 14: examples of suitable machinery Old
Page 15 and 16: test method as a standard, we are c
Page 17 and 18: News from the Senior Scientists Fig
Page 19 and 20: Figure 5: Axi-symmetrical FE model
Page 21 and 22: Location: FST Academy Building 75 F
Page 23: Impressum Freudenberg Forschungsdie

FFD IM - Freudenberg Forschungsdienste SE & Co. KG

Create successful ePaper yourself

Delete template?

Save as template?