Patent Description:
Metal ion batteries, for example, lithium ion batteries may be vulnerable to fires as a result of thermal runaway. Thermal runaway may occur when a fault within a battery causes an exothermic reaction, with the reaction rate increasing as temperature increases, to a point where the battery is increasing in temperature at a greater rate than the battery loses heat to its surroundings. The battery temperature will continue to rise, thereby causing the exothermic reaction rate to also increase. As thermal runaway occurs, gases are produced, and usually vented to the surroundings of the battery to prevent pressure increasing within the battery to a point where the battery casing ruptures. The gases may be flammable, for example methane, or ethylene dihydrogen. The decomposition of the battery during thermal runaway may also produce sparks, which can exit the battery via the same route as the gases, which may cause ignition of the gases, resulting in a fire and/or explosion.

The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved battery safety arrangement.

The present invention provides, according to a first aspect, a metal ion battery cell comprising a plurality of electrodes and an electrolyte encased within a housing, the housing comprising a safety valve or vent configured to allow gas build up within the housing to vent outside the housing, and a fabric band surrounding the housing, multiple times wherein the fabric band covers the safety valve or vent, such that gas may pass through the fabric band, but sparks are contained by the fabric band.

The fabric band prevents sparks generated by the battery cell during a thermal runaway, which pass through the safety valve, from igniting gases which may be proximate to the battery cell. Therefore, the chances of the gases igniting are reduced or eliminated.

The metal ion battery may be a lithium ion battery cell. The lithium ion battery cell may be a prismatic lithium ion battery cell. The lithium ion battery cell may be a cylindrical lithium ion battery cell.

The lithium ion battery cell may be a pouch lithium ion battery cell. The metal ion battery may be a sodium ion battery cell, a potassium ion battery cell, an aluminium ion battery cell, or a magnesium ion battery cell.

The fabric band surrounds, or is wrapped around, the housing a multiple number of times, for example, <NUM> to <NUM> windings around the housing. Such an arrangement may allow a thin and flexible fabric to be used as the fabric band, whilst still providing the necessary spark suppression due to the increased thickness of multiple layers covering the safety valve. There may be a plurality of fabric bands wrapped around the housing in order to create a number of windings around the housing. Alternatively or additionally, a single band may be wrapped around the housing a number of times. The skilled person will appreciate that the thickness and the density of the fabric band may be varied in order to provide the required gas permeability and spark arresting characteristics, for example increasing the density of the fabric band may allow a decrease in the thickness of the fabric band, and vice versa. Such variation comprises routine experimentation, and allows the skilled person to easily adapt the fabric band to meet specific use requirements.

The fabric band may be elastic, for example due to the weave of a fibre making up the fabric band. The fabric band may stretch if the housing it surrounds increases in size, which may happen during a thermal runaway. Whilst a limited amount of stretch may be allowed, the fabric band may be arranged such that beyond a stretch limit, the band resists the expansion of the housing it surrounds, potentially supporting the structural integrity of the housing.

The fabric band may comprise a heat resistant material, for example being heat resistant up to <NUM> degrees Celsius. The heat resistance will prevent the increased temperature of a battery cell during a thermal runaway event from damaging, for example igniting, the fabric band, and will ensure that the fabric band is still able to arrest sparks during the thermal runaway event. The fabric band may comprise a felt material needled to or between one or more woven fabrics, for example a woven glass or silica fabric. The felt material may be manufactured by matting, condensing, and/or pressing fibres together. The felt material may be a non-woven material.

The fabric band may be combined with a separate felt filter, the felt filter positioned to cover the safety valve or vent. The fabric band may secure the felt filter in position with respect to the safety valve or vent. The combination of the fabric band and separate felt filter may improve the spark arresting ability of the fabric band, for example by the felt material partially or fully absorbing a spark before the spark reaches the fabric band. The fabric band may be combined with a plurality of separate felt filters, for example with the fabric band being wrapped around the battery housing a plurality of times, with a felt filter being placed covering the safety valve or vent each winding of the fabric.

The felt filter may comprise a mineral wool, for example an alkaline earth silicate wool, alumino silicate wool, polycrystalline wool, or Kaowool <RTM>. The felt filter may comprise organic binders. The felt may be manufactured by matting, condensing, and/or pressing fibres together. The felt may be a non-woven material. The felt filter may be flexible and/or resiliently deformable. The felt filter may be relatively rigid compared to fabric materials. The felt filter may be heat resistant, for example up to <NUM> degrees Celsius. The heat resistance will prevent the increased temperature of a battery cell during a thermal runaway event from damaging, for example igniting, the felt filter, and will ensure that the felt filter is still able to arrest sparks during the thermal runaway event.

The felt filter may have a thickness of between <NUM> to <NUM>. The felt filter may have a density chosen to allow gas to flow through at a rate which prevents backpressure at the safety valve or vent. The felt filter may have a density chosen to prevent sparks from passing through the felt filter. The felt filter may be easy to cut and shape, to allow easy application to the metal ion battery cell.

The safety valve may be a one way valve arranged to open at <NUM>. 9MPa, or <NUM>. 9MPa ±<NUM>. 2MPa pressure within the lithium ion battery cell, or any other opening pressure as conventionally used for safety valves for metal ion battery cells. The safety vent may provide an aperture though which excess gases may pass at atmospheric pressure.

According to a second aspect, the invention comprises a plurality of metal ion battery cells, each metal ion battery cell comprising a plurality of electrodes and an electrolyte encased within a housing, the housing comprising a safety valve or vent configured to allow gas build up within the housing to vent outside the housing of the first aspect of the invention, the plurality of metal ion battery cells arranged to form a cell stack, and a fabric band surrounding the cell stack to cover a plurality of safety valves or vents such that gas may pass through the fabric band, but sparks are contained by the fabric band, wherein the the fabric band surrounds the cell stack a multiple number of times. For example, the invention may comprise four lithium ion battery cells linked in series and forming a cell stack. The plurality of lithium ion battery cells may be arranged such that the safety valves or vents are linearly aligned, such that a fabric band extends in a linear fashion so as to cover a plurality of safety valves. The plurality of metal ion battery cells may comprise a power module. The plurality of metal ion cells may comprise a battery pack. The battery pack may comprise one or more power modules.

According to a third aspect, the invention comprises a power module, the power module comprising a plurality of metal ion battery cells, the plurality of metal ion battery cells encased within a housing, the housing comprising a safety valve or vent configured to allow gas build up within the housing to vent outside the outing, and a fabric band surrounding the housing, wherein the fabric band covers the safety valve or vent such that gas may pass through the fabric band, but sparks are contained by the fabric band, wherein the fabric band surrounds the power module a multiple number of times.

According to a fourth aspect, the invention comprises a battery pack, the battery pack comprising one or more power modules, each power module comprising a plurality of metal ion battery cells, the one or more power modules encased within a housing, the housing comprising a safety valve or vent configured to allow gas build up within the housing to vent outside the housing, and a fabric band surrounding the housing, wherein the fabric band covers the safety valve or vent such that gas may pass through the fabric band, but sparks are contained by the fabric band wherein the fabric band surrounds the cell stack a multiple number of times.

According to a fifth aspect of the invention there is also provided a method of manufacturing a metal ion battery cell according to the first aspect of the invention, the method comprising the steps of: applying a fabric band to a metal ion battery cell, such that a safety valve or vent on the metal ion battery cell is covered by the fabric filter and the fabric band surrounds the housing a multiple number of times.

As another example, any features described with reference to the first aspect of the invention may be incorporated into the second, third, fourth, or fifth aspects of the invention, and vice versa.

<FIG> shows a cross-sectional view of a metal ion battery cell, in this case a lithium ion prismatic cell <NUM>. The cell comprises a plurality of electrodes and an electrolyte within a housing <NUM>. Terminal connections <NUM> and <NUM> extend from the housing <NUM>. A safety valve <NUM> is located on a top cover of the housing <NUM>, and is arranged to vent gas from the housing <NUM> when the pressure reaches a threshold value to prevent undue pressure build up within the housing. A fault may increase the temperature within the cell <NUM>, leading to a flammable gas being generated by the electrodes and electrolyte, which is vented through the safety valve <NUM> into an area proximate to the cell <NUM>. This is shown by the cloud <NUM> shown in <FIG>. As the temperature of the cell <NUM> rises, the exothermic reaction also increases, leading to further temperature rises. At some point, the temperature will rise to a level at which sparks <NUM> are generated within the cell <NUM> and pass through the safety valve. The sparks <NUM> may ignite the gas <NUM>, leading to a fire <NUM> as shown in <FIG>.

<FIG> shows a schematic cross-sectional view of a lithium ion prismatic cell <NUM>. The cell comprises a plurality of electrodes and an electrolyte <NUM> within a housing <NUM>. Terminal connections <NUM> and <NUM> extend from the housing. A safety valve <NUM> is located on the top cover of the housing <NUM>. , and is arranged to vent gas from the housing <NUM> when the pressure reaches a threshold value. The cell <NUM> further comprises a fabric band <NUM> which surrounds the housing <NUM> and runs across the top cover of the housing <NUM>, covering the safety valve <NUM>. The dashed lines show how the band surrounds the housing <NUM>, along with running along the bottom cover of the housing <NUM>. The fabric band is a low silica felt needled between two silica fabrics, and is porous enough to all gas to pass through into the surroundings of the cell <NUM>, and also dense enough to absorb any sparks that are generated by the cell and travel out through the safety valve <NUM>. If a fault occurs which results in a flammable gas being generated, the flammable gas will pass through the safety valve <NUM> once the safety pressure within the cell has been reached, and disperse into the environment surrounding the battery cell <NUM>. Any sparks that are generated and pass through the safety valve <NUM> will be absorbed by the felt band <NUM>, reducing or eliminating the risk of the sparks igniting the flammable gas surrounding the cell <NUM>.

<FIG> shows lithium ion cell stack <NUM> made up of four lithium ion cells as described with reference to <FIG>. The cell stack <NUM> may also be referred to as a battery module. A fabric band <NUM> surrounds the cells <NUM> and extends across the top of the cell stack, such that each of the safety valves <NUM> of the four lithium ion cells <NUM> is covered. Applying a single fabric band <NUM> which covers each of the safety valves <NUM> is potentially time saving compared to applying individual foam strips to each of the cells <NUM>. As can be seen in the figure, the fabric band is wrapped around the cell stack a number of times, with each wrap increasing the spark arresting ability of the fabric band. However, the skilled person will appreciate that the fabric band is carefully chosen not to create any significant back pressure at the safety valve, and gas should pass through the layers of fabric band relatively easily.

<FIG> shows a schematic cross-sectional view of a fabric band <NUM>. The fabric band <NUM> comprises a first silica fabric layer <NUM>, and low-silica felt material, <NUM> and a second silica fabric layer <NUM>. The low-silica felt material <NUM> is needled to the first silica fabric layer <NUM> and the second silica fabric layer <NUM>.

<FIG> shows the method steps for manufacturing a cell <NUM> according to the first embodiment of the invention. In step <NUM> a fabric band <NUM> is wrapped around a lithium ion battery cell <NUM>. In step <NUM> the fabric band <NUM> is located such that the safety valve <NUM> on top of the cell <NUM> is covered.

The lithium ion battery cell may be a cylindrical cell. The lithium ion battery cell may be a pouch cell. The invention may be applied to various other metal ion battery cells, in particular metal ion battery cells that potentially generate flammable gases.

In an alternative arrangement, the fabric band may be applied to cover a safety vent. The fabric band may be combined with a felt filter, for example the fabric band may secure a felt filter in a position covering the safety valve or vent in order to further provide spark arresting characteristics.

The fabric band may be applied to cover a safety valve or vent in a housing of a power module, the power module comprising a plurality of metal ion battery cells. The fabric band may be applied to cover a safety valve or vent in a housing of a battery pack, the battery pack comprising a plurality of metal ion battery cells. The battery pack may comprise one or more power modules, each power module comprising a plurality of metal ion battery cells.

Claim 1:
A metal ion battery cell (<NUM>) comprising a plurality of electrodes and an electrolyte (<NUM>) encased within a housing (<NUM>), the housing comprising a safety valve or vent (<NUM>) configured to allow gas build up within the housing to vent outside the housing, and a fabric band (<NUM>) surrounding the housing, wherein the fabric band covers the safety valve or vent (<NUM>), such that gas may pass through the fabric band (<NUM>), but sparks are contained by the fabric band (<NUM>), characterised in that the fabric band (<NUM>) surrounds the housing (<NUM>) a multiple number of times.