[LAB??] SMART COMPOSITE CASING FOR AIRCRAFT BATTERIES

Thermal runaway is a potential problem for lithium-ion batteries. It can cause damage to the battery and, in extreme cases, lead to fires. Adding a metal enclosure to contain thermal runaway is one method used to make batteries safer, but this also adds weight to the battery system.

To provide a better option, a lightweight composite casing equipped with sensors and capable of resisting a cell thermal runaway event was developed under the framework of the OASIS H2020 project. OASIS (Open Access Single entry point for scale-up of Innovative Smart lightweight composite materials and components) consists of 20 partner organisations and functions as an innovation test bed to help companies develop multifunctional lightweight products based on aluminium and polymer composites. As part of this mission, OASIS partner Thales is leading a showcase to develop the composite battery casing.

The target objectives for the composite casing are: 10-20% higher gravimetric energy density of the battery system (ratio between the stored energy quantity and the battery’s weight) compared to a reference aluminium casing; resistance to thermal runaway of one cell in the battery system and reduced fire propagation risk; and reduced thermal runaway risk through early detection.

?Reducing risk of thermal runaway

Lithium batteries for aircraft applications have to prove capable of containing the thermal runaway effect. When a battery cell runs into thermal runaway, the pressure in the casing strongly increases due to the limited gas evacuation through the venting hole. Flames, fumes and cell electrolyte vapours are produced rapidly, and the air temperature inside the casing increases.

To withstand these demanding conditions, a thermoplastic composite casing with embedded sensors was developed, offering both lightweight and monitoring functions. The casing is composed of two parts: a cylindrical body produced by injection moulding of a short fibre reinforced technical thermoplastic polymer, and a hybrid cover produced from a thermoplastic composite prepreg and an overmoulded polymer. Overmoulding makes it possible to integrate additional functions such as sensors, localised reinforcements and assembly features. The temperature sensor is fully integrated in the composite part, thus simplifying installation, reducing wiring requirements, and allowing more reliable temperature measurements.

A PPS (polyphenylene sulphide) matrix was chosen for both the composite prepreg and the overmoulding matrix. To optimise the casing’s mechanical performance while limiting its weight, a PPS/carbon fibre prepreg was selected. To meet the required dielectric properties, a PPS/glass fibre ply was added on both sides of the carbon prepreg. The overmoulding matrix is a PPS resin reinforced with short glass fibre.

The composite casing cover was manufactured by IPC Centre Technique Industriel de la Plasturgie et des Composites using the stamping/overmoulding process. A specific mould was designed and manufactured for the cover, in two versions to allow both prepreg stamping and the subsequent overmoulding step for integration of the thin temperature sensors.

Tests were performed to validate the materials, the sensors and the final product. The casings were assembled for destructive tests with real cells according, as far as possible, to the DO-311 standard. During this test, a cell thermal runaway was intentionally started, causing a pressure and temperature peak with gas, fume and flames generated inside the casing. The temperature sensor embedded in the cover clearly detected the thermal runaway. The mechanical integrity analysis showed that the casing withstood the pressure and temperature peak and the fire caused by the cell thermal runaway.

Find out more about the technologies showcased in this project in the January-February 2023 issue of JEC Composites Magazine: https://digital-magazine.jeccomposites.com/

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