GB2615798A

GB2615798A - Thermal insulator for battery cells

Info

Publication number: GB2615798A
Application number: GB2202240.4A
Authority: GB
Inventors: Porter Kevin
Original assignee: Tecman Speciality Materials Ltd
Current assignee: Tecman Speciality Materials Ltd
Priority date: 2022-02-18
Filing date: 2022-02-18
Publication date: 2023-08-23
Also published as: GB202202240D0

Abstract

A laminated thermal insulator is provided for use in a battery pack. The insulator comprises a compressible thermally insulating inner layer sandwiched between two anisotropic layers, the anisotropic layers having a higher thermal conductivity in a direction parallel to the anisotropic layers than in a direction perpendicular to the anisotropic layers. The anisotropic layer may comprise graphite or copper. The thermally insulating inner layer may comprise an aerogel. At least one of the two anisotropic layers may comprise a connector flap that extends beyond an outer edge of at least one of the battery cells for enabling thermal connection to a heat sink. The heat sink may comprise a heat absorbing material, such as a phase change material.

Description

Thermal insulator for battery cells

TECHNICAL FIELD

The present invention relates to a thermal insulator for use in a battery pack. The invention further relates to a battery pack comprising at least two battery cells, separated by and in thermal contact with such a thermal insulator.

BACKGROUND

In recent years, the popularity of electric vehicles has increased considerably. A main advantage of electric vehicles over traditional internal combustion engine vehicles is that they use electric energy to drive the wheels and propel the vehicle and do not need to burn hydrocarbon-containing fuels. Consequently, electric vehicles do not emit carbon dioxide while driving. To enable generating the required electric energy on demand, most electric vehicle comprise a sizeable battery pack with a plurality of chargeable lithium-ion batteries.

In designing electric vehicles, much time and effort goes to providing a battery pack that is relatively small and lightweight, while providing a high energy storage capacity that allows the vehicle to be driven over large distances without requiring the batteries to be recharged. An important challenge in the design of battery packs for electric vehicles is to combine high density energy storage and quick charging with effective thermal management to avoid overheating of individual battery cells and risking thermal runaway of larger parts of the battery pack.

Known solutions for avoiding such thermal runaway include the provision of thermally insulating material between adjacent battery cells. Such thermally insulating material helps to avoid the thermal breakdown of a single cell to propagate to its direct neighbours. The international patent application published as WO 2020/184695 Al discloses a battery pack with battery cells in the form of pouch cells, insulated by a multi-layer composite material with high thermal conductivity in x-y direction for a more even distribution of heat over the pouch surface and low thermal conductivity in the z direction to limit heat transfer to neighbouring pouch cells. The composite material is compressible to accommodate volumetric changes of the pouch cells during use and comprises an inner layer of an aerogel material with a thermal conductivity of 25 mW/m*K or less. The inner layer is sandwiched between two graphene based nano carbon coatings with a thermal conductivity of 50 W/m-K or more. The inner layer of the composite material serves to reduce heat transfer between cells, while the nano carbon coatings are provided to ensure an even distribution of the heat over the full surface of the pouch cell.

The composite material described in WO 2020/184695 Al helps to better distribute excessive heat produced in a defect cell, and therewith allow the battery management system of the vehicle to successfully activate some emergency measures that may prevent the excessive heat production to propagate to other cells. However, this composite material still comes with a few important disadvantages. The graphene based nano carbon coating is expensive and a complex investment heavy coating process is needed to reliably coat the thermally insulating material. Furthermore, the composite material is only able to better distribute the excessive heat a little bit better, but the amount of excessive heat generated and then transferred to neighbouring cells remains the same.

It is an aim of the present invention to address one or more disadvantages associated with

the prior art.

SUMMARY OF THE INVENTION

According to an aspect of the invention there is provided a laminated thermal insulator for use in a battery pack. The insulator comprises a compressible thermally insulating inner layer sandwiched between two anisotropic layers, the anisotropic layers having a higher thermal conductivity in a direction parallel to the anisotropic layers than in a direction perpendicular to the anisotropic layers.

The anisotropic outer layers are laminated to the thermally insulating inner layer, for example using an adhesive layer for bonding between the layers. This makes it significantly easier to manufacture the thermal insulator in large volumes. The process of unwinding a continuous roll of anisotropic material, applying an adhesive coating and then laminating the layers together is a simple low-cost method using established manufacturing processes. Also, when deciding to use a different material for either the inner layer or the anisotropic layers, this can easily be achieved by just replacing a supply roll used in the production process. The laminated thermal insulator can therefore easily be adapted to the preferences and technical requirements of different users.

A further important advantage of the current invention is that it makes it possible to provide a laminate thermal insulator wherein at least one of the anisotropic layers extends beyond at least one edge of the thermally insulating inner layer. The surface area that extends beyond the edge of the inner layer makes it possible to connect the anisotropic layer to an external heat sink. All heat produced in a battery cell contacting the anisotropic layer on the outer face of the laminated thermal insulator can thus be conducted away from the battery cell and its direct neighbours, for example into a heat sink. Instead of just preventing the heat from being transferred to adjacent battery cells, it is effectively conducting it away first thus reducing the amount of thermal energy that has to be insulated against to prevent propagation to its neighbour cell. The thermally insulating properties of the inner layer will only be really used after the heat conducting capacity of the anisotropic layer and the heat sink is exceeded.

In embodiments of the laminated thermal insulator according to the invention, the anisotropic layer comprises graphite. Alternatively, copper or other suitable materials may be used.

Preferably, the thermal conductivity of the anisotropic layers is greater than or equal to 100 W/m*K, 500 W/m*K, or even 1000 W/m*K, in the direction parallel to the anisotropic layers. The anisotropic layers may be adhered to the thermally insulating inner layer by use of an adhesive, e.g. a pressure sensitive adhesive or a heat activated adhesive.

The thermally insulating inner layer may, for example, comprise an aerogel. Many aerogels are very suitable for this purpose because of their excellent thermally insulating properties, their low weight, and their compressibility. The aerogel may be encapsulated in a wrapping material. Preferably, the thermal conductivity of the thermally insulating inner layer is less than or equal to 50 mW/m*K, 25 mW/m*K, or even 10 mW/m*K.

According to a further aspect of the invention, a battery pack is provided comprising at least two battery cells, separated by and in contact with a laminated thermal insulator as described above. Preferably, at least one of the two anisotropic layers comprises a connector flap that extends beyond an outer edge of at least one of the battery cells for enabling thermal connection to a heat sink. The heat sink may, for example, be an engineered heat sink or comprise a heat absorbing material, such as a metallic substance or a phase change material. The battery cells are preferably prismatic cells or pouch cells with a relatively large and substantially flat outer surface that allows for efficient heat exchange with the laminated thermal insulator.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a cross section of an embodiment of a laminated thermal insulator according to the invention.

Figures 2a, 2b, and 2c show cross sections of further embodiments of the laminated thermal insulator according to the invention.

Figure 3 shows a cross section of part of a battery pack wherein a laminated thermal insulator as shown in Figure 2a is used.

DETAILED DESCRIPTION

Figure 1 shows a cross section of an embodiment of a laminated thermal insulator 100 according to the invention. The insulator 100 comprises a compressible thermally insulating inner layer 10 sandwiched between two anisotropic layers 20. It is to be noted that the Figures are schematic drawings and have not been drawn to scale.

The thermally insulating inner layer 10 may, for example, comprise an aerogel. Other suitable insulating materials may be used for this inner layer 10 too. Important selection criteria for the employed insulation material may include, its thermal conductivity, weight, cost, compressibility, flexibility, strength, and durability. The thermal conductivity of the thermally insulating inner layer preferred to be less than or equal to 50 mW/m*K, 25 mW/m*K, or even 10 mW/m-K. Many aerogels are very suitable for use in this inner layer 10 because of their excellent thermally insulating properties, their low weight, and their compressibility. Compressibility is an important feature of the inner layer 10 as it helps to ensure proper contact between the battery cells and the anisotropic layers when the battery cells shrink or expand as a result of changes in temperature. The compressible material is preferably resilient, such that it is able to retain its original shape when the battery cells cool down and the space between two neighbouring cells expands. Many aerogels have these desirable properties and are thus particularly suitable for use in the inner layer 10. Furthermore, aerogels are very lightweight, which helps to reduce the overall weight of the battery pack and the vehicle it is used in. Because aerogels are typically fragile and difficult to handle without being damaged, the aerogel may be encapsulated in a protective wrapping material. Such wrapping material may or may not have thermally insulating properties. The wrapping material is preferably easy to apply, lightweight, flexible, and inexpensive. Advantageously, a wrapping material is used that adheres well to the anisotropic layers 20.

The anisotropic layers 20 are made of materials that have a higher thermal conductivity in the plane of the material (x and y directions) than in a direction perpendicular thereto (z direction). Preferably, the thermal conductivity of the anisotropic layers is greater than or equal to 100 W/m*K, 500 W/m*K, or even 1000 W/m*K, in the direction parallel to the anisotropic layers. For example, graphite sheets show the desired anisotropic behaviour and are suitable for use in laminated thermal insulators 100 as shown in Figures 1 to 3. Alternatively, copper or other suitable materials may be used. The use of an anisotropic material brings the advantage that the heat conducting outer layers 20 of the laminated thermal insulator 100 immediately disperse any heat generated at a single location in the x and y directions without having to rely on the thermally insulating properties of the insulating inner layer 10. Any heat transfer in the z direction is already blocked before the heat reaches the inner layer 10. Similarly, any residual heat that may still be transferred through the inner layer 10 to the other side of the thermal insulator 100 will be distributed by the second anisotropic layer 20 in the x and y direction only, and not in the z-direction. Consequently, it will take even longer before excessive heat at one side of the thermal insulator 100 can reach an object at the other side of the thermal insulator 100.

Furthermore, the laminated thermal insulator 100 is easy to manufacture in large volumes using known laminating processes. The anisotropic layers 20 may be adhered to the thermally insulating inner layer 10 by use of an adhesive, e.g. a pressure sensitive adhesive or a heat activated adhesive. A further advantage of using a laminating process instead of a coating process is that, when deciding to use a different material for any of the layers, this can easily be achieved by just replacing a supply roll used in the production process. The laminated thermal insulator 100 can thus easily be adapted to the preferences and technical requirements of different users.

Figures 2a, 2b, and 2c show cross sections of further embodiments of the laminated thermal insulator according to the invention. Because the anisotropic layer 20 is a laminated layer and not a coated layer, it is possible to use an anisotropic layer 20 with a larger surface are than the inner layer 10. When applied to the inner layer 10, one or more flaps or flanges 21 of the anisotropic material may thus extend beyond at least one edge of the thermally insulating inner layer 10. In the embodiment of Figure 2a, both anisotropic layers 20 of the thermal insulator 100 extend beyond the same edge of the inner layer 10. In the embodiment of Figure 2b, the two anisotropic layers 20 extend beyond two opposing edges of the inner layer 10. In Figure 2c, both anisotropic layers 20 extend at the same two edges of the inner layer 10. Many other variations will be apparent to the skilled person.

The surface area that extends beyond the edge of the inner layer 10 makes it possible to connect the anisotropic layer 20 to an external heat sink (see Figure 3). When not connected to a heat sink, the flap or flange 21 may still release heat to its immediate surroundings. All heat produced in objects insulated with insulators such as disclosed in Figures 2a, 2b, and 2c is not just evenly distributed over a larger surface area but can effectively be led away from the heat-producing object. Preferably, the size, location, and shape of the extending flap or flange 21 is optimised for enabling an easy and effective connection to the nearby heat sink.

Figure 3 shows a cross section of part of a battery pack 200 wherein a laminated thermal insulator 100 as shown in Figure 2a is used. The battery pack 200 comprises a plurality of battery cells 210, three of which are shown in this schematic drawing. The battery cells 210 are preferably prismatic cells or pouch cells with a relatively large and substantially flat outer surface that allows for efficient heat exchange with the laminated thermal insulator 100.

Neighbouring battery cells 210 are separated by and in thermal contact with a laminated thermal insulator 100 as described above. In this exemplary embodiment, both anisotropic layers 20 of each thermal insulator 100 comprise a connector flap 21 that extends beyond an outer edge of the battery cell 210 it is connected to. This extending connector flap 21 makes it possible to thermally connect the anisotropic layers 20 to a heat sink 230.

The heat sink 230 may, for example, be an engineered heat sink or comprise a heat absorbing material, such as a metallic substance or a phase change material. Preferably, two anisotropic layers 20 of the same thermal insulator 100 are connected to separate heat sinks 230 to avoid creating a thermal connection between the two anisotropic layers 20 at both sides of the same thermal insulator 100. By using these heat sinks 230, excessive heat generated in one of the battery cells 210 can be effectively led away from the cells 210 and outside the battery pack 200. Due to the anisotropic properties of the anisotropic layers 20 of the thermal insulators 100, the thermally insulating properties of the inner layer 10 of that thermal insulator 100 are only used after the heat rejecting capacity of the anisotropic layer 20 and the connected heat sink 230 is exceeded. As a result, thermal runaway of the neighbouring cells 210 is prevented and a much safer battery pack 200 is obtained.

It is noted that, although the invention has been explained above with reference to its use in a battery pack 200 for an electric vehicle, the thermal insulator 100 according to the invention can be used with similar technical advantages in other battery packs or in different technology wherein thermal insulation is needed for avoiding locally produced heat to spread to other parts of the same device.

Claims

CLAIMS1. A laminated thermal insulator for use in a battery pack, the insulator comprising a compressible thermally insulating inner layer sandwiched between two anisotropic layers, the anisotropic layers having a higher thermal conductivity in a direction parallel to the anisotropic layers than in a direction perpendicular to the anisotropic layers.
2. A laminated thermal insulator as claimed in claim 1, wherein at least one of the anisotropic layers extends beyond at least one edge of the thermally insulating inner layer.
3. A laminated thermal insulator as claimed in any preceding claim, wherein the anisotropic layer comprises graphite or copper.
4. A laminated thermal insulator as claimed in any preceding claim, wherein the anisotropic layers are adhered to the thermally insulating inner layer by use of an adhesive.
5. A laminated thermal insulator as claimed in claim 4, wherein the adhesive is a pressure sensitive adhesive or a heat activated adhesive.
6. A laminated thermal insulator as claimed in any preceding claim, wherein the thermally insulating inner layer comprises an aerogel.
7. A laminated thermal insulator as claimed in claim 6, wherein the thermally insulating inner layer comprises an aerogel, encapsulated in a wrapping material.
8. A laminated thermal insulator as claimed in any preceding claim, wherein a thermal conductivity of the thermally insulating inner layer is less than or equal to 25 mW/m K.
9. A laminated thermal insulator as claimed in any preceding claim, wherein a thermal conductivity of the anisotropic layers is greater than or equal to 100 W/m*K in the direction parallel to the anisotropic layers.
10. A battery pack comprising at least two battery cells, separated by and in thermal contact with a laminated thermal insulator as claimed in any preceding claim.
11. A battery pack as claimed in claim 10, wherein at least one of the two anisotropic layers comprises a connector flap that extends beyond an outer edge of at least one of the battery cells for enabling thermal connection to a heat sink.
12. A battery pack as claimed in claim 11, further comprising the heat sink and wherein the heat sink is connected to the connector flap.
13. A battery pack as claimed in claim 12, wherein the heat sink comprises a heat absorbing material, such as a phase change material.
14. A battery pack as claimed in any of claims 10 to 13, wherein the battery cells are prismatic cells or pouch cells.