The present invention relates to a battery pack comprising flat electrochemical cells, each cell comprising a plurality of overlapping laminated layers, a plurality of electrodes and a casing containing said layers provided with electrode passage openings.

The present disclosure relates to lithium-ion batteries, but it is clear that the invention can be applied to any suitable battery type.

Lithium-based batteries, both primary and secondary, are characterized by a high stored energy density, which allows them to be used in countless applications of high energy and power, such as electric vehicles and aircraft, battery powered tools, and the like. However, the high energy density also makes the batteries susceptible to fire with flame release and chemically aggressive compounds in the event of battery failure or damage. This behaviour is to some extent intrinsic; in fact, if the energy contained in the battery is released suddenly, for example from an accident which physically damages the battery, the same high amount of energy necessarily produces heat to a significant extent.

In fact, it is known that lithium-ion batteries, in the event of mechanical damage which short circuits positive and negative current carriers, release a large amount of energy which leads to a rapid overheating of the electrodes. When this occurs, temperatures above 200 °C are reached, generally in fractions of a second. In these conditions the constituent elements gasify, causing the explosion of the battery and the fire of its metal parts, in what is called "thermal runaway". Since the thermal degeneration of the battery produces oxygen, such a fire is particularly difficult to control. At the same time, hydrofluoric acid, which is highly corrosive and toxic, is generally found among the combustion products.

Moreover, the battery systems requested by the market are increasingly larger. One example is the shipping industry, where there is a strong demand for ships with hybrid propulsion systems, such as to i allow an electric propulsion when the ship moves near an urban environment. In such systems, the storage of between 10 and 15 MWh is requested. However, as the stored energy increases, the number of cells increases and the need to subtract heat and manage the temperature increases, as does the complexity of the connections. This is why the trend is to increase the size of the individual cells, but this makes the thermal regulation of the single cell increasingly problematic.

At the same time, in high power applications, it is often necessary to cool the battery when it is subjected to a rapid discharge, to avoid the overheating thereof due to a high current flow. In the opposite situation, it may be necessary to quickly preheat a battery conserved at temperatures below 0 °C, because at low temperatures the charging process is dangerous.

For the management of these thermal aspects of the batteries in use, state-of-the-art cooling/heating systems are known, comprising heat exchangers placed in contact with the outside of the batteries and provided with ducts for the circulation of a cooling/heating fluid therein. However, such systems have some disadvantages, in particular they significantly increase the weight of the battery pack, contrary to the demand by electric mobility for ever lower weights.

A particular type of cell is the so-called "pouch" cell in which the layers are stacked and the conductive tabs are welded to the electrodes and brought to the outside in a completely sealed manner by a soft and waterproof case which adheres to the components of the cell because under vacuum. The soft case offers a simple, flexible and lightweight solution to battery design and the pouch cell as a whole makes more efficient use of space and reaches a packaging efficiency of 90-95 percent, the highest among batteries. The elimination of the metal casing present in the cylindrical coils reduces the weight. However, due to the lack of a rigid structure, the cell needs to be somewhat supported and by a certain tolerance to be able to expand slightly in the battery compartment. This is because in the ion transfer process corresponding to the charging and discharging process, the thicknesses of the anode and cathode vary, causing a dimensional variation of the battery itself.

Pouch batteries typically have a higher capacity with respect to cylindrical batteries and lower production costs. The cylindrical cell has a high specific energy, good mechanical stability and lends itself to automated production but in the composition of battery packs it necessarily creates gaps between the cells which then decrease the energy density.

The pouch cell, on the other hand, is light and economical, but more complex to be integrated in a battery system which controls its pressure and temperature. In fact, the operation of a battery substantially involves a reversible controlled combustion, in which the fuel is a metal which generates a lot of energy in oxidation, superior for example to carbon oxidation. There is thus necessarily a transport of matter inside the battery during the charging/discharging steps thereof, consisting of the ions which move from the anode to the cathode and vice versa. Although the amount of matter moved is minimal, the battery necessarily changes in volume, depending on the type of technology used. For this reason, pouch batteries need compression systems which keep the inner layers in close contact with each other to compensate for the changes in volume and avoid the risk of swelling, which would increase the internal impedance.

The soft case which completely covers the pouch cell with the exception of the electrodes is typically made of a layered sheet of plastic and aluminium, similar to a food package. The primary purpose of the case is to absolutely avoid the penetration of moisture, which would inevitably lead to damaging the cell, with the consequent risk of fire and explosion. The aluminium layer is necessary with respect to a simple polymer case since the latter still has a minimum water permeability, unlike metal, and can therefore not be used in the currently known batteries. The use of a soft case makes the pouch cell of poorly defined shape and this does not allow it to be inserted into systems requiring dimensional precision.

Currently, the most used technical solution is the assembly of a series of stacked pouch cells and their insertion in a containment box provided with springs which keep the cells in compression. As is evident, while in the cylindrical cell the presence of wrapped layers allows to rely on the elasticity of the material to keep the layers in adhesion, in the pouch cell the presence of springs is necessary, with a consequent inevitable increase in the battery pack weight.

The presence of the containment box, typically made of metal for fire safety reasons, as well as electrical connections and compression springs, degenerate the energy density. If the cell itself has a good energy density, for example 270 Wh/Kg, the complete battery pack can also drop to 150 Wh/Kg.

In the automotive industry, the trend is to use pouch cells of rather large size, with large copper and aluminium electrodes which, protruding outside the soft case to allow electrical contact, represent a good heat conductor which can be used to extract heat from inside the cell. Such electrodes are then cooled to cool the entire cell, but the thermal regulation thus carried out is inefficient on the centre of the cell.

Document WO 2018/185001 A1 describes a series of cell support elements stacked together and provided with cooling ducts in contact with each cell. The hydraulic circuit has a common delivery and a common return. The circuit is pressurized and the collectors at the common return are thermolabile so that if an abnormal thermal event of a battery occurs, they break and cause a significant increase in the cooling flow of that battery. However, the cell is of the classic type and is completely separated from the cooling circuit, which envisages the use of water as a cooling liquid. This increases the overall weight of the battery pack and requires always keeping the cells separate from the cooling circuit since an entry of water into the cell would cause an immediate explosion. Document DE 10 2020 005245 A1 discloses a battery module with a plurality of electrochemical cells housed in a vessel filled with a pressurized fluid. The cells are of the bag or pouch type and are spaced apart from each other with as small a distance as possible, but sufficient to allow, in a possible maximum expansion of the individual cells, that the cells do not come into contact with each other. The pressurized fluid is also a thermal regulation means for cooling the cells. The document is silent on the features of the casings of the individual cells and of the liquid, and therefore does not describe a battery pack capable of carrying out a suppression of the thermal runaway of a cell if a degenerative phenomenon of the cell itself is accidentally triggered.

It is therefore the object of the present invention to overcome the drawbacks presented by the currently known batteries described above, by creating a pack of pouch-type lithium-ion secondary batteries which allows, by virtue of its structure, to exercise a thermal regulation and which is at the same time light and economical.

The invention achieves the above objects with a battery pack as described at the beginning, in which the casing of the cell is made of thermoplastic material having a liquefaction temperature between 120 °C and 60 °C lower than the thermal runaway temperature of the electrochemical cell, the cell being immersed in a pressurized non-polar and non-water-based liquid, which liquid performs the dual function of thermal regulation of the cell and suppression of the thermal runaway if a degenerative phenomenon of the cell itself is accidentally triggered and the thermoplastic material is completely impermeable to the non-polar and non-water-based liquid.

According to an exemplary embodiment, the casing comprises two rigid half-shells which can be coupled to each other and each having a flat peripheral edge and a central recess, so that, in a coupled condition, the two central recesses form a central seat for housing said layers and the two mutually contacting peripheral edges form a peripheral flange, the peripheral flange being provided with one or more said passage openings of said electrodes. This allows to have a mechanically defined shape for each cell.

In an embodiment, the flat peripheral edges are such that they allow the secure support of a flat peripheral gasket, by virtue of the fact that the openings are made by moulding and do not alter the flatness of the peripheral edge.

According to an exemplary embodiment, the openings are formed in the peripheral flange by recesses obtained in the thickness of one or both of the peripheral edges of the two half-shells.

According to an improvement, a gasket element is included between the passage opening and the electrode.

In a preferred embodiment, the two half-shells are made of thermoformed polymeric material.

This makes manufacturing the casings simple and economical. It is possible to mould the material accurately and thus have a structure with mechanically defined edges.

In a preferred embodiment, the polymeric material is polyethylene. However, polyethylene terephthalate or other polymers having a melting temperature between 120 °C and 160 °C can be used.

According to an embodiment, a plurality of spacer frames is included of a shape corresponding to the peripheral flange and provided with gasket elements on opposite contact surfaces of two peripheral flanges of two cells side by side and of a thickness such as to form between the two said cells a watertight gap with respect to the outside of the battery pack.

Thereby, the gap which is created between two adjacent cells can be filled by a thermal regulation fluid which can substantially act on the entire half-shell using it as a heat exchange surface.

This is also particularly advantageous in combination with the aforesaid feature of including the flat peripheral edges, because it allows to include the gasket elements of the completely flat spacer frames.

In a further exemplary embodiment, the cells are stacked together, said frame being interposed to two adjacent cells, and two terminal covers included at the opposite ends of the stack, provided with gasket elements adapted to seal on the peripheral flanges of the two end cells, and retention means further being included in the assembled condition of the cells, the frames and the covers.

Thereby, a single body of very compact dimensions is formed without the need for an external box, in which the cells are positioned at a predetermined distance from each other by frames which act as both structural spacers and as watertight elements towards the outside of each gap which is formed between two subsequent cells.

In a further exemplary embodiment, each peripheral flange has at least one area in which it is in contact with two adjacent gaps, one or more through holes being included in such an area such that the two adjacent gaps are put in hydraulic communication.

In an embodiment, all the gaps are placed in series to form a single hydraulic circuit, circulation means of a thermal regulation fluid being included in said hydraulic circuit.

Preferably the hydraulic circuit comprises an inlet at a first gap included near one first said terminal cover and an outlet at a last gap included near one said second terminal cover.

Alternatively, it is possible to connect a small number of gaps to each other and thus form more independent hydraulic circuits.

In an exemplary embodiment, said thermal regulation fluid is nonaqueous.

This is particularly advantageous in combination with the use of half-shells in polymeric material which over time and in the presence of a water-based fluid could not ensure impermeability to water and moisture, putting the cell at risk of rapid degradation and potential damage such as explosion or fire. By virtue of the use of a non-aqueous thermal regulation fluid, to which the polymeric material is completely impermeable, the entry of water or moisture into the cell is completely prevented.

In a preferred exemplary embodiment, the thermal regulation fluid is a vegetable oil.

This allows to have a thermal regulation fluid without water and with high extinguishing power. In a further exemplary embodiment, pressure setting means of the thermal regulation fluid are included.

This allows each cell to be held down, maintaining intimate contact between the constituent layers and preventing swelling. At the same time, the pressurized fluid allows the small changes in volume necessary for the operating life of the cell.

Furthermore, if the cell undergoes abnormal behaviour and develops a potentially very dangerous hot spot, the half-shell of low melting temperature polymeric material melts and is pierced, letting in the pressurized thermal regulation fluid which can then extinguish the fire upon starting. Therefore, the present invention has the great advantage that, by providing a casing of thermolabile material in direct contact with the thermoregulation fluid by virtue of the use of a non-aqueous thermoregulation fluid, it allows to pierce the casing and flood the entire cell in the event of an abnormal and dangerous thermal event.

In a preferred embodiment, the thermal regulation fluid has a pressure between 1 and 3 bar, preferably 2 bar.

In a further exemplary embodiment, two adjacent cells have respective peripheral flange areas provided with holes in positions opposite each other such that the hydraulic circuit has a coil shape.

This imposes an obligatory path to the thermal regulation fluid, which must flow along the opposite faces of each cell and thereby lap all the areas of the battery pack exercising its thermal function.

Therefore, the invention has the advantage of obtaining a much more compact battery pack with respect to the devices known in the state of the art, with a minimum amount of thermal regulation fluid, and a very high efficiency.

These and other features and advantages of the present invention will become clearer from the following description of some non-limiting exemplary embodiments illustrated in the attached drawings in which: fig. 1 shows an exploded view of a cell with its casing; fig. 2 shows two cells, each in assembled condition, and a spacer frame; fig. 3 shows a sectional view of a plurality of cells side by side and supported by spacer frames in assembled condition; fig. 4 shows the complete battery pack in assembled condition; fig. 5 shows a sectional view of the hydraulic circuit formed by the complete battery pack in assembled condition.

The present invention relates to a battery pack comprising flat lithium-ion cells 1 , such as the known pouch-type cells. As with pouch cells, each cell 1 comprises a plurality of overlapping laminated layers 10, a plurality of electrodes 11 and a casing containing the layers.

The casing is divided into two rigid, thermoformed half-shells 12 which can be assembled together by shape coupling. Each rigid half-shell 12 has a flat peripheral edge 120 which completely surrounds it and a central tray-like recess 121 , complementary to the set of layers forming the cell. In the coupled condition of the two half-shells 12 the two peripheral edges 120 are brought into mutual contact and lie on a single plane, while the central recesses 121 extend symmetrically in opposite directions with respect to said plane. Thereby the two central recesses 121 form a central housing seat of the constituent layers 10 of the cell and the two peripheral edges 120 in mutual contact form a peripheral flange 122.

The two thermoformed half-shells 12 are made of polymeric material having a low melting temperature, in particular melting temperature between 120°C and 160°C, such as polyethylene or polyethylene terephthalate. The two half-shells 12 are preferably heat- sealed together along the peripheral edges 120, but can also be glued or fixed to each other by any fastening means known in the state of the art capable of keeping the casing sealed with respect to the outside.

The casing formed by the assembly of the two half-shells 12 is provided with passage openings 124 of the electrodes 11 . Such openings 124 are formed in the peripheral flange 122 by shaping one or both of the peripheral edges 120. To ensure the watertight seal on the electrodes 11 , each passage opening 124 is internally provided with a gasket element 125 adapted to interpose between the walls of the passage opening 124 and the electrode 11 .

One side of the peripheral edge 120 of each half-shell 12 has a plurality of holes 126 such that in the assembled condition of the casing the peripheral flange 122 has a perforated area.

As shown in figure 2, the battery pack comprises a plurality of spacer frames 2 of rectangular shape and dimensions corresponding to the peripheral flange 122. The spacer frame 2 is adapted to be interposed between two cells 1 side by side, contacting the respective peripheral flanges 122. The spacer frame 2 is provided with gasket elements 20 on opposite contact surfaces of two peripheral flanges 122 of two cells 1 side by side.

The thickness of the spacer frame 2, i.e., the distance between the two opposite contact surfaces, is such that, in the assembled condition of the battery pack, a watertight gap 30 is formed between the two cells 1 with respect to the outside of the battery pack, as shown in section in figure 3.

To assemble the battery pack, the cells 1 are stacked together as shown in figure 4, i.e., placed on planes parallel to each other and aligned along a longitudinal axis. A spacer frame 2 is placed between two adjacent cells 1 throughout the stack, so as to form an alternating series of cells 1 and frames 2. At the opposite ends of the stack there are two end cells 1 and a first and a second terminal cover 40 and 41 are included with gasket elements adapted to seal on the peripheral flanges 122 of such end cells 1. Both terminal covers are shaped so as to identify a first gap included between the first terminal cover 40 and the corresponding end cell 1 and a last gap included between the second terminal cover 42 and the corresponding further end cell 1 .

The covers 40 and 41 are preferably provided with reinforcing ribs 401.

Retention means are further included in the assembled condition of the cells 1 , the spacer frames 2 and the terminal covers 40 and 41 . In the example in the figure, such means consist of metal bars or bolts 42 io fixed on special eyelets 43 included on both of the terminal covers 40 and 41.

In the stack thus formed, the holes 126 present on each peripheral flange 122 put all the gaps 30 in hydraulic communication with each other to form a single hydraulic circuit 3.

The hydraulic circuit 3 comprises an inlet 31 at the first gap and an outlet 32 at the last gap, the inlet and the outlet being formed in the example of the figures by connection vents to an external circulation circuit of a thermal regulation fluid.

The regulation fluid is preferably a non-aqueous liquid, in particular consisting of vegetable oil, i.e. , other non-aqueous and self-extinguishing liquids.

The external circuit is also provided with means for pressurizing the thermal regulation fluid, so that such a fluid flows pressurized in the hydraulic circuit 3 of the battery pack, in particular with a pressure between 1 and 3 atm, preferably 2 atm.

As shown in figure 5, the hydraulic circuit 3 has a coil configuration. To obtain this configuration, the cells 1 are arranged in the stack such that two adjacent cells 1 have the respective peripheral flange areas 122 provided with holes 126 in positions opposite each other with respect to a longitudinal plane of the battery pack.

In a preferred embodiment, the cell 1 including its casing has a thickness of 11 mm while the gap 30 measures 1 mm between the two recesses 121 of two adjacent cells 1 . This size of the gaps 30 has been shown to be sufficient for effective thermal regulation and at the same time allows for a minimum weight of thermal regulation fluid and reduced volumes.

The battery pack thus formed is very compact and can ensure a mass efficiency of around 90%, with only 10% by weight of structure in addition to the single cell. This means that if the single cell has an energy density of 270 Wh/Kg, the entire battery pack has a fully respectable capacity of 250 Wh/Kg. ii