|Comparison of battery performance using the older generation of monolayer separator and the new co-extruded multi-layer separator. Click to enlarge.|
ExxonMobil Chemical introduced two new grades of its co-extruded separator film for lithium-ion batteries at the Advanced Automotive Battery Conference (AABC) in Tampa, Florida (12-16 May). (Earlier post.)
The two new grades—V20CFD and V20EHD—developed by ExxonMobil Chemical and its Japanese affiliate, Tonen Chemical, join the two grades introduced last year (earlier post) and offer an improved balance of properties, including enhanced permeability, improved strength, reduced heat shrinkage and higher rupture temperature to improve battery safety and performance.
The separators are produced using a proprietary wet, bi-orientation manufacturing process. The films in the new family are co-extruded using specially tailored, high-heat resistant polymers for improved separator properties.
|Properties of Co-extruded Separator Grades|
|Air permeability (Gurley)||sec/100cc||180||270||170||290|
|Heat shrinkage MD (@105%deg;C)||%||3.5||3.5||3.0||3.0|
|Heat shrinkage TD (@105°C)||%||1.5||1.5||1.5||1.0|
|Tensile strength MD||kgf/cm2||700||1,100||650||1.050|
|Tensile strength TD||kgf/cm2||800||1,300||750||1,300|
Separators play a critical role in lithium-ion batteries; separators must offer good permeability to support power-related performance, while strength and thermal integrity are necessary from an abuse and safety perspective.
ExxonMobil Chemical has been developing separator films since 1991, with its film being used by Sony in the first commercial lithium-ion battery (LIB). Over the subsequent years, the company built up a portfolio of monolayer separator grades, tailored for differing requirements of the battery manufacturers.
The new family of co-extruded films will also continue to multiply, according to Pat Brant, chief scientist for ExxonMobil Chemical, based on customer need and input.
Those needs can vary with a number of factors, including battery format and chemistry. For example, the company anticipated that the two new, thinner grades in its V series would likely be applied in cells for mild hybrids or power tools, while the thicker original pair would be used for EV or PHEV cells. However, said Dr. Brant, the company is in discussions with a potential customer who wants to apply the thinner films in cells destined for higher-performance battery packs.
One of the value propositions of the co-extruded films is their ability to contain an exothermic event in battery. Even in the event of the eventual use of electrode or electrode-electrolyte pairings that carry less of a risk of a thermal event, Brant says, the co-extruded separators have value to add in terms of their strength, potential lighter-weight, and porosity which can enable improved performance.
In addition to the tuning of the new family of co-extruded separators, the company is also at work on the following next generation of separator films.