This invention relates to a container for an energy storage device. More specifically, the present invention relates to a container for electrochemical cells.

Electrochemical cells housed within energy storage devices comprise reactive and hazardous materials that require careful packing to ensure product safety. The packing should contain the hazardous material and also accommodate for changes in temperature and volume of the material as the cells charge and discharge. As well as having a desired robustness, the packaging is also required to have a low weight and volumetric efficiency, so as not to diminish the overall energy density of the energy storage device. The packing and design of the packaging should also avoid adding any unnecessary resistance gains to energy storage device. Progress in energy storage technology has produced electrochemical cells with an increased energy density. This can lead to higher operating temperatures and large volume changes of active materials during a charge/discharge cycle compared to conventional electrochemical cells. Conventional packaging of energy storage may not be able to safely and efficiently accommodate cells with increased energy density.

According to a first aspect of the present invention, there is provided an energy storage device comprising: a container, a mandrel, at least one sheet of separator material, and two or more discrete electrodes, the container comprising a base and an inner surface forming an internal space; the mandrel is positioned in the container, and the mandrel and the inner surface are spaced apart to define a cavity within the container; the sheet of separator material is arranged about the mandrel to provide a plurality of discrete separator layers within the cavity, and wherein at least one electrode is provided to occupy the space between each of the discrete separator layers, and at least a portion of an external surface of a container has a curved profile.

The present invention provides a high energy density electrochemical storage device that can tessellate with other similar devices when combined in an array, whilst also having an external surface that would have reduced contact with adjacent energy storage devices when in an array. Although this would reduce the overall energy density of the array, the void between devices caused by the curved portion of the external surface allows for a cooling fluid to flow between devices and draw away any excess heat from the device during a charge/discharge.

The combination of discrete electrode sheets and a compressible mandrel of the present invention allows for a cell comprising a stack of efficiently connected high energy density electrodes stored within a resilient protective container, in contrast to typical wound cells that rely on a continuous sheet of anode/cathode electrode material wound within the separator material. As such the device may comprise two or more discrete positive electrodes and two or more discrete negative electrodes. During a charge and discharge cycle, the collection energy dense electrode material within each device is expected to produce an amount of heat that would need to be radiated away from the device. In addition, an air or fluid gap between devices allows for isolation in the event of thermal runaway of a device in an array.

The face of the mandrel may be curved. The curved face of the mandrel can provide a uniform stack pressure over the surface of the electrodes within the packaging. In addition, the mandrel can adapt its shape effectively to relieve any build-up of pressure during cell expansion. The mandrel can be shaped such that its surfaces are concentric with the curves of the internal surface of the container. Furthermore, the shape of the mandrel may also be concentric with the overall external shape of the container. The present invention does not rely on a component of the electrochemical cell to be wound around the mandrel, the mandrel main function is to support the separator material against the inner surfaces of the container by contracting or expanding according to the electrode volume during a charge or discharge.

The mandrel may have a single surface and be circular or arced in shape, or alternatively the mandrel may be shaped to have a second surface. For instance, the second surface of the mandrel may be curved such that the cross-sectional shape of the mandrel is elliptical. In an alternative arrangement, the second mandrel surface may be flat and rest against another inner wall or surface of the container. Alternatively, the second mandrel surface may contact the separator material to provide a compression force. The separator material contacting the second mandrel surface may be the same or different separator material that contacts the first mandrel surface.

The inner surface of the container may be curved, such that the container is generally cylindrical in shape. The mandrel may be positioned such that its longitudinal axis aligns with the longitudinal axis of the container. In this embodiment, the cavity is generally pipe shaped, and the separator material sheet(s) are wound around the mandrel to fill the cavity. The device in this embodiment is relatively easy to construct. However, due to the spiral of the separator material sheet(s) and the difference in size of each separator layer, the size of the electrodes increases along the packing axis away from the mandrel.

In an alternative embodiment, the container may be shaped such that is comprises a second inner surface opposite the inner surface. For example, the container might be cube or cuboid in shape with opposite faces being curved. In this alternative embodiment, the packing axis may pass through the container from the inner surface to the second inner surface, and the mandrel may be positioned along the packing axis to provide a second cavity. At the least one sheet of separator material may be arranged in the second cavity to provide a second plurality of separator layers along the packing axis, and the one or more electrodes are provided between the second separator layers. Having a generally cube or cuboid shaped container allows for the electrodes to be of a similar size. Furthermore, in this embodiment, the arrangement of the sheet(s) of separator material about the mandrel can take several forms. For example, a sheet of separator material can be wound about the mandrel, or a sheet of separator material could be folded in the cavity. When there is more than one cavity, at least one sheet of separator material may be provided in each cavity. Each cavity comprises a at least one sheet of separator material. Furthermore, discrete rolls of separator material may be provided in each cavity. Having more than one sheet of separator material allows for different packing arrangements within the cavity/cavities and also means that damage of electrodes or separator material in one part of the device may be isolated. The inner surface and/or the second inner surface of the container may be concave. By curving the surfaces which face the mandrel surfaces, a more uniform pressure can be applied to the cells within the cavities. This provides uniform pressure between the electrodes and the separator material and improves the efficiency of the cell.

In order to better understand the present invention, and to show more clearly how the invention may be put into effect, the invention will now be described, by way of example, with reference to the following drawings:

Figure 1 is an exploded view of a schematic of an energy storage device of the present invention;

Figures 2a-d are schematics of alternative layouts of separator material and mandrel within the energy storage device;

Figures 3a-c are schematics of alternative embodiments of energy storage devices of the present invention; and

Figures 4a and b are schematics of arrays of alternative container shapes.

Figure 1 shows an energy storage device 1 comprising a container 2, a compressible mandrel 3, separator material 4, and discrete electrodes 5. The container 2 has a casing 6, a base 7 and a cap 8 that together form the shell of the energy storage device 1. The casing 6 is formed of robust material to avoid external objects from piercing or rupturing the device 1. The casing 6 could be deep drawn/rolled/shaped and formed with the base 7 and cap 8 so as to form an internal space 9 for holding the electrochemical cell components, namely the compressible mandrel 3, separator material 4, and electrodes 5. The casing 6 has an inner surface 10 facing into the space 9. The base 7 and cap 8 are provided over the open ends of the casing 6 to enclose the electrochemical cell components 3,4,5 within the container 2. Although the base 7 and cap 8 are shown as separate parts from the casing 6, it is conceivable that the casing 6 may include, or be attached to, a preformed base 7 and cap 8.

The mandrel 3 has a first mandrel surface 11 and second mandrel surface 12 connected by an arm 13. The mandrel 3 is formed from a single piece of pliable material, such as a plastic or metal. The cross section of the mandrel 3 generally has the shape of an S and its outline is elliptical. The mandrel 3 has a longitudinal axis L which is normal to the S shape formed by the curved surfaces 11, 12 and the arm 13. The mandrel 3 extends along its longitudinal axis L such that it is similar in length to the container 2. The general cross-sectional shape of the mandrel 3 is the same along the entire length of its longitudinal axis L.

The mandrel 3 is formed so that it can be placed in the internal space 9 of the container 2. When the mandrel 3 is positioned within the internal space 9, a cavity 9a remains between the mandrel surfaces 11, 12 and the inner surface 10 of the container 2. Due to the shape of the mandrel 3, columns of hollow dead space exist between the mandrel surfaces 11, 12 and the arm 13 which extend along the longitudinal axis L. The hollow columns allow space for the mandrel 3 to collapses, as well as providing access for welding to at least part of the base 7 when the electrochemical cell components 3,4,5 are placed within the container 2. The mandrel 3 is compressible in the direction of a packing axis P, which will be described in more detail in relation to the separator material 4. Generally speaking, the mandrel 3 can compress and/or deform such that the general elliptical shaped outline of its cross section decreases in size. The volume of the space 9 taken up by the mandrel 3 decreases as the mandrel 3 compresses. Furthermore, the mandrel surfaces 11, 12 can deform under extreme pressure such that the curvature or arc can change according to compression forces applied to the surface.

The separator material 4 as presented in Figure 1 is a continuous sheet of electronically insulating porous material. The separator material 4 is rolled and positioned within the cavity 9a between the container 2 and the mandrel 3. The separator material 4 is wound around the mandrel 3 about a winding axis W which overlies the longitudinal axis L of the mandrel 3 when the electrochemical cell container 1 is in its complete form. As the sheet of separator material 4 is wound about winding axis W, layers of separator material are formed as the sheet rolls over itself. In the complete energy storage device 1, the separator material 4 is arranged in the container 2 to provide a plurality of separator layers placed along the packing axis P. this creates spaces 14 between layers of the separator material 4 which are occupied by the electrodes. Electrodes 5 are positioned along the packing axis P within the spaces 14 of the wound separator material 4. For simplicity, only two electrodes 5 (one anode and one cathode along with the separator material 4 forming a cell) are shown in Figure 1. However, an electrochemical cell container 1 of the present invention may contain many electrodes 5, forming multiple electrochemical cells.

The electrodes 5 each comprise a tab 15a, 15b which can be secured to the internal surfaces of the base 7 and cap 8. By providing a tabs 15a, 15b on each electrode 5, the current path length for each electrode 5 is reduced and the internal resistance of the cell decreases.

As the cells charge/discharge, the electrodes 5 may expand and contract. As the electrodes 5 expand and occupy more volume within the internal space 9, the mandrel 3 compresses. Similarly, as the electrodes contract, the mandrel 3 expands to re-occupy the volume whilst also providing a constant compressive force along the packing axis P between the separator material 4 and the electrodes 5. The curved mandrel surfaces 11, 12 ensure that a uniform pressure over the surface of the electrodes 5 is maintained.

Various alternative electrochemical cell container 1 arrangements that are within the scope of the present invention are shown schematically in Figures 2a-d. The electrochemical cell containers 1 are shown in cross section along the longitudinal axis L of the mandrel 3, and without electrodes 5 for simplicity. Each electrochemical cell container 1 is shown in an over simplified manner as a squared container 2. However, it is appreciated that the separator material 4 would curve to occupy the internal space 9 of the container 2.

In Figure 2a, two sheets of separator material 4 are wound about the mandrel 3. The mandrel 3 is positioned along the winding axis W of the separator materials 4. The sheets of the separator material 4 are concentric about the longitudinal axis L of the mandrel 3. A multitude of layers 14 is provided between the layers of the wound separator material 4 for housing electrodes 5. The electrodes 5 are arranged along the packing axis P. In Figure 2b, a mandrel 3 is provided with a single curved surface 11. The mandrel arm 13 rests against an internal surface 10 of the container 2. One sheet of separator material 4 is provided in the internal space 9 and is wound around a winding axis W. The winding axis W does not overlie the longitudinal axis L of the mandrel 3. Discrete layers 14 are provided in the roll of separator material 4 for housing electrodes. The electrodes 5 are arranged along the packing axis P.

Figure 2c and d illustrate further alternatives of the present invention, where rolls or folds of separator material sheets 4 are positioned in the cavities 9a about the mandrel 3, the the separator 4 is not wound about the mandrel 3. The device in Figure 2c comprises two rolled sheets of separator material 4 in each cavity 9a. In Figure 2d, the sheets of separator material

4 are folded in the cavity 9a. Electrodes 5 would be placed within the spiral layers or the folds of the separator material 4. In these cases, the mandrel 3 is merely functioning to absorb expansion of the electrode 5 within the device 1, and not providing a bobbin for material 4,5 to be wound around.

The container 2 in Figure 1 is shown as cylindrical but could also form the shape of any prismatic cell. Cross-sectional schematics are shown in Figures 3a-c of devices 1 of the present invention. The layers of separator material 4 are shown as concentric rings instead of a continuous rolled sheet in the cavity 9a merely as a way of simplify the drawing. Electrodes

5 are shown schematically as broken lines and can be positioned anywhere within the layers 14 between the rolled sheets of separator material 4. Figure 3a shows a simplified cross sectional view of the complete device 1 of Figure 1. The inner surface 10 is one continuous surface, and the mandrel surfaces 11, 12 face different regions of the same inner surface 10.

Figure 3b illustrates a device 1 that has a generally cuboid shaped container 2, wherein the inner surfaces 10 facing the mandrel faces 11, 12 are concave. The separator material 4 is folded or wound so that fills the cavities 9b between the mandrel surfaces 1 1,12 and the inner surface 10 of the container 2. The separator material 4 is arranged to provide layers 14 along the packing axis P, the layers being filled with electrodes 5. The curvature of the concave inner surface 10 similar to the curvature of the mandrel surfaces 11, 12 such that a uniform pressure is applied across the surface of the electrodes 5 within the layers 14 of separator material 4.

Figure 3c illustrates a device 1 that has a generally cuboid shaped container 2, wherein the device 1 has only one cavity 9c which is filled with electrochemical cells. The inner surface 10 facing the mandrel face 11 is concave. The separator material 4 is folded or wound so that fills the cavity 9c between the mandrel surfaces 11, 12 and the inner surface 10 of the container 2. The separator material 4 is arranged to provide layers 14 along the packing axis P, the layers being filled with electrodes 5. The curvature of the concave inner surface 10 similar to the curvature of the mandrel surfaces 11,12 such that a uniform pressure is applied across the surface of the electrodes 5 within the layers 14 of separator material 4.

In the examples shown in Figures 3a-c, the curvature of the external casing matches the concave shape of the inner surfaces 10, the external casing may be flatted to provide an external cuboid shape. However, it may be beneficial to keep the curvature of the casing 6.

Figures 4a and b show an array of energy storage devices 1 according to Figures 3b and c respectively. The curvature of the casing 6 allows for gaps 16 between the containers 2 when arranged in an array. The curved casing 6 ensures that physical contact between adjacent containers 2 is reduced. A fluid such as air can be provided in the gaps 16 between the containers 2. The reduced contact between the containers 2 ensures that low heat transfer occurs between adjacent devices 1. In addition, fluid is free to flow over the array of containers and act as a coolant to remove any excess heat given off by the cells within the devices 1.