How to Manage Stress in Li-Ion Batteries

Abstract

The present invention relates to a system (1) for applying a variable pressure to an electric battery (2) comprising at least one electrochemical cell, the electric battery being inserted between first and second parallel end plates (4) and (5). According to the invention, the system comprises a first assembly (7) comprising a first working wall (8), at least a first spring (10) interposed between the first working wall (8) and a first end plate (4), and a link (11) extending parallel to the first working wall (8) and moved by an actuator (12), said link (11) being connected to said first working wall (8) by at least one cam (9), a first end (16) of which is rotatably mounted to the link (11) and a second end (17) of which is rotatably mounted about an axis of rotation rigidly connected to a stationary wall (19), said second end (17) abutting against the first working wall (8).

Description

[Technical field]

[0001] The present invention relates to a method for managing the pressure applied to a lithium-ion battery.

[0002] When a Li-ion battery is charged, lithium is mixed with silicon in the chemical formula Li x They form silicon alloys of Si and Si. The formation of these alloys results in an increase in the volume of the Si particles that can reach up to 300% of the initial volume of the silicon particles. When a Li battery is discharged, the Li x The Si alloy is delithiated, which causes a reduction in the size of the silicon particles. As a result, a significant, quasi-reversible change in volume, similar to breathing, occurs on the scale of the silicon electrodes and cells over the course of the battery charge-discharge cycle. This change in volume over the life of the battery (continuous charging-discharging) results in high mechanical stresses in the Li-ion battery. The Si particles can split, detach from the current collectors, and / or become electronically disconnected from the percolation network of the electrodes. These phenomena result in a decrease in battery performance. Furthermore, parasitic reactions occur, causing an irreversible increase in volume and therefore premature aging of the battery.

[0003] Patent application DE102009035482A1 describes a lithium-ion electric battery for motor vehicles comprising a number of battery cells sandwiched between two end plates forming a stack of cells. More specifically, a lithium-ion battery comprises a number of battery cells inserted between two plates thereby forming a stack of cells. A device makes it possible to apply a pressure load to the stack of cells in the stacking direction using an active actuator, by means of a spring. The actuator is controlled by a service life / pressure characteristic diagram or by measurements of temperature and / or pressure within the stack of cells, said measurements being dependent on the charge / discharge cycles.

[0004] The method according to the invention makes it possible to follow the changes in the volume of the Si particles through the various charging and discharging cycles of the electric battery by applying a constant optimal pressure, so that the performance of the Li-ion battery is maintained over time and the aging of said battery is significantly reduced.

[0005] The subject of the present invention is a system for applying a variable pressure to an electric battery comprising at least one electrochemical cell, the electric battery being inserted between first and second end plates parallel to each other.

[0006] According to the invention, the system comprises a first assembly with a first actuating wall extending parallel to two end plates and arranged opposite the first end plate, said first assembly comprising at least one first spring inserted between the first actuating wall and the first end plate, said first actuating wall extending parallel to the first actuating wall and capable of moving towards or away from the first end plate under the effect of a movement of a link moved by an actuator to exert a pressure on said first plate, said link being connected to said first actuating wall by at least one cam, a first end of which is rotatably mounted on the link and a second end of which is rotatably mounted around a rotation pin fixed to a fixed wall parallel to the first actuating wall, said second end abutting the first actuating wall. The principle of the method according to the invention is that a uniform pressure can be exerted on one of the two end plates of the electric battery through at least one spring that can be compressed more or less as a function of the movement of the link. Specifically, movement of the link in one direction moves the at least one cam in a first direction, which moves the first actuating wall closer to the first end plate, causing compression of the at least one spring, which in turn increases the pressure on the first end plate. Moving the link in the opposite direction moves the at least one cam in a second direction, which moves the first actuating wall away from the first end plate, relaxing the at least one spring, which in turn decreases the pressure on the first end plate. Preferably, the link is a straight, elongated rod, and the actuator is configured to be able to translate the rod in both directions along the longitudinal axis of the rod. In the system according to the invention, the first actuating wall is inserted between the link and the first end plate, and the at least one cam is inserted between the link and the first actuating wall. Preferably, the first system comprises a plurality of springs, the number of which is greater than five, and a plurality of cams, the number of which is greater than five. According to a particular embodiment, there are as many cams as there are springs.

[0007] According to a possible feature of the invention, the link is an elongated rod and the actuator translates said link along its longitudinal axis, the movement of the link under the action of the actuator being along an axis parallel to the first actuating wall and the first end plate, and the movement of the first actuating wall relative to the first end plate being along an axis perpendicular to the axis along which the link moves.

[0008] According to a possible feature of the invention, each cam comprises a rod connecting a first end to a second end, said second end comprising an elongated segment arranged around a pivot pin. The second end, in the form of an elongated segment, is fixed with respect to the rod of the cam. The pivot pin is arranged at one end of the elongated segment, so that the rod of the cam is located on one side of said pivot pin and the elongated segment is located on the other side of said pivot pin. The movement of the link causes the rotation of the rod of the cam, which in turn, due to the presence of the pivot pin, causes the rotation of the elongated segment. Since this elongated segment is in contact with the first working wall, the rotation of the elongated segment of the cam causes the working wall to move in one direction or the other depending on the direction of rotation of the elongated segment.

[0009] According to a possible feature of the invention, the system comprises a computer adapted to control the actuator to move the link in a desired direction and magnitude in order to place the first working wall at a given distance from said first end plate with the aim of compressing each first spring more or less to exert a desired pressure on said first end plate. This computer is programmed to essentially take into account the charging or discharging phase of the electric battery. Depending on the relevant phase, the computer triggers at the exact moment the actuator, via the link and the cam, moves the first working wall to increase or decrease the pressure exerted by said at least one first spring on the first end plate.

[0010] According to a possible feature of the invention, the system comprises a second assembly with a second working wall extending parallel to the two end plates and arranged opposite the second end plate, said second assembly comprising at least one second spring inserted between the second working wall and the second end plate, the direction of the force exerted by said at least one second spring being parallel to the direction of the force exerted by said at least one first spring, said second working wall abutting a fixed wall parallel to said second working wall. This second assembly allows to equalize the overall pressure exerted by the springs on the electric battery, due to the presence of two assemblies each capable of exerting a pressure on either side of the electric battery.

[0011] According to a possible feature of the invention, the system comprises twelve first springs interposed between the first working wall and the first end plate and twelve second springs interposed between the second working wall and the second end plate.

[0012] According to a possible feature of the invention, the first spring is aligned along an axis parallel to the end plates and the second spring is also aligned along an axis parallel to said end plates, such that the first and second actuating walls exert a uniform pressure on the first and second end plates, respectively.

[0013] According to a possible feature of the invention, each first spring and each second spring is formed from a disc spring, or other types of springs can be used, such as, for example, a helical spring or a compressed foam.

[0014] According to a possible feature of the invention, the system comprises a fixed frame surrounding an electric battery, a first actuating wall, a second actuating wall, the at least one first spring and the at least one second spring, the second actuating wall being in contact with a second flat wall of the frame, the at least one pivot pin around which the second end of the at least one cam is mounted is fixed to a first flat wall of the frame parallel to the second flat wall, and a link is positioned behind the first flat wall of the frame whilst being external to the frame.

[0015] Another subject of the invention is the use of the system according to the invention throughout a cycle including a charging phase and then a discharging phase of a Li-ion electric battery.

[0016] According to the invention, the method comprises the steps of: - a start step in which a cell has dimension x1 and the system exerts a force F1; - charging an electric battery, in which the force exerted by the system increases to a value F2, which is greater than F1, because the cells of the battery expand during the charging phase and now have a dimension x2, which is greater than x1; - initiating discharge of the battery, the step including pre-activating a computer controlled system to translationally move the link in a direction that rotates the cam in a first direction that moves the first actuation wall toward the first end plate, whereby the system immediately applies a force F3 to the battery that is greater than force F2; a step at the end of the discharge of the battery in which the system applies a force F4 to the battery that is smaller than force F3, this force F4 applied at the end of the discharge being due to the volume lost by the cell during this discharge phase; - operating a computer controlled system to translate the link in an opposite direction that rotates the cam in a second direction that moves the first actuation wall away from the first end plate, whereby said system immediately applies a force F1 to the battery that is less than force F4 to return the cell to a pre-charged state; and Includes.

[0017] It will of course be appreciated that the step of charging the electric battery may be triggered for any initial charge state of the battery, not necessarily zero, and similarly, the step of discharging the battery may be stopped regardless of the discharge state of the battery, not necessarily zero.

[0018] Reference is now made to a detailed description of preferred embodiments of a system for applying variable pressure, in accordance with the present invention, with reference to the following drawings, in which: [Brief description of the drawings]

[0019] [Figure 1] FIG. 2 is a schematic diagram of a system for applying variable pressure to an electric battery according to the present invention, the cell having a dimension x1 and the system applying a force F1. [Figure 2a] 2 is a diagram showing an example of forces applied to the cells of an electric battery over time during charging and discharging phases of the battery. [Figure 2b] 2 is a diagram showing an example of the change in width of a cell of a battery over time during the charging and discharging phases of the battery; [Diagram 3] 1 is a schematic diagram of a system according to the present invention when an electric battery has finished charging, the cell has an increase in size to a dimension x2 greater than x1, and the applied force is a force F2 greater than F1. [Figure 4] FIG. 2 is a schematic diagram of a system according to the present invention in which a force F3, greater than F2, is applied to a cell, the cell being of dimension x2, as the cell begins to discharge. [Diagram 5]FIG. 2 is a schematic external view of a system according to the invention when an electric battery has finished discharging and the cells have returned to dimension x1, and said system is applying a force F4. [Figure 6] FIG. 1 is a schematic diagram of a system according to the present invention in which an actuator reduces pressure on a cell as the cell begins to charge, said cell having dimension x1 and the system exerting a force F1. [Figure 7a] 1 is a diagram showing an example of forces applied to a cell of an electric battery over time during charging and discharging phases of the battery, the cell expanding to dimension x3 instead of x1 due to aging. [Figure 7b] FIG. 2 is a diagram showing an example of the change in dimensions of a battery cell over time during the charging and discharging phases of the battery, where the cell expands to dimension x3 instead of x1 due to aging. [Figure 8] FIG. 1 is a schematic diagram of a system according to the present invention when an electric battery is about to start charging, the cells are of dimension x3, and a force F1 is applied. [Figure 9] FIG. 2 is a schematic diagram of a system according to the invention when an electric battery has finished charging and the cell has, due to an increase in its size, a dimension x4 which is greater than x3, and the force applied then is a force F2 which is greater than F1. [Figure 10] FIG. 1 is a schematic diagram of a system according to the present invention in which a force F3, greater than F2, is applied to a cell when the cell is about to begin discharging, the cell being of dimension x4. [Figure 11] FIG. 2 is a schematic external view of a system according to the present invention when an electric battery has finished discharging and the cells have returned to dimension x3, and said system is applying a force F4. [Figure 12] FIG. 1 is a schematic diagram of a system according to the present invention in which an actuator reduces pressure on a cell as the cell begins to charge, the cell being of dimension x3 and the system applying a force F1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The electric battery 2 is composed of a series of perfectly aligned and contacting Li-ion cells 3. The electric battery 2 comprises a first end plate 4 and a second end plate 5 that enclose the cells 3 of the electric battery 2. Schematically, the two end plates 4, 5 have substantially the same dimensions as the cells 3, and the cells 3 are inserted between and parallel to the two end plates 4, 5.

[0021] The system 1 for applying variable pressure according to the invention is capable of applying pressure to a first end plate 4 and a second end plate 5, - Frame 6, a first assembly 7 comprising a first actuating wall 8, a number of cams 9, a number of first springs 10, a link 11, an actuator 12 capable of moving said link 11, and a computer 13 adapted to control said actuator 12 as a function of the phase undergone by the battery 2, i.e. charging or discharging, from the beginning of its life to the end of its life, and a second assembly 40 comprising a second actuation wall 14 and a plurality of second springs 15; Equipped with:

[0022] The link 11 corresponds to an elongated straight rod, and an actuator 12 is capable of moving said link 11 translationally in one direction or the other along its longitudinal axis according to commands received from a computer 13.

[0023] Each cam 9 has a first end 16 articulated to the link 11 and a second end 17 with an elongated segment connected to said first end 16 and said second end 17 by a connecting rod 18. The second end 17 is fixedly connected to said connecting rod 18 such that the elongated segment is inclined relative to the connecting rod 18. The elongated segment of the second end 17 of each cam 9 passes around an axis of rotation on a connecting pin fixed to a first wall 19 of the frame 6, which first wall is parallel to the first working wall 8 and to the first end plate 4 of the electric battery 2.

[0024] The first actuating wall 8 is flat and is arranged parallel to and facing the first end plate 4 of the battery 2. A series of first springs 10, for example possibly twelve in number, are inserted between the first end plate 4 of the electric battery 2 and the first actuating wall 8. These first springs 10 are aligned along the first actuating wall 8 and the first end plate 4 of the battery 2. A link 11 extends parallel to the first actuating wall 8 and the first end plate 4 of the electric battery 2, and the first actuating wall 8 is arranged between the link 11 and the first end plate 4.

[0025] The second working wall 14 is flat and is arranged parallel to and facing the second end plate 5 of the electric battery 2. A series of second springs 15, for example possibly twelve in number, are inserted between the second end plate 5 of the battery 2 and the second working wall 14. These second springs 15 are aligned along the second working wall 14 and the second end plate 5 of the battery 2. Under the effect of the second springs 15, the second working wall 14 is pressed against a second wall 20 of the frame 6, which second wall is parallel to a first wall 19 of the frame 6, to which the rotation pin of the cam 9 is fixed. This second wall 20 of the frame 6 is parallel to the second working wall 14 and the second end plate 5 of the battery 2. In other words, the second working wall 14 is inserted between the second wall 20 of the frame 6 and the second end plate 5 of the battery 2.

[0026] The first spring 10 and the second spring 15 may be formed, for example, from disc springs. The first spring 10 exerts a force in a direction perpendicular to the plane of the first working wall 8 and the first end plate 4, and the second spring 15 exerts a force in a direction perpendicular to the plane of the second working wall 14 and the second end plate 5.

[0027] The cams 9 are mounted in the system 1 according to the invention such that their second ends 17 , which comprise an elongated segment, abut against the first actuating wall 8 .

[0028] The principle of operation of such a system is that a computer 13 to which an actuator 12 is connected sends to that actuator 12 an instruction in the form of a signal to move the link 11 in one direction or the other. The link 11 is then translated along its longitudinal axis in the specified direction and by the desired magnitude, thereby pivoting the cams 9. The rotation of the connecting rod 18 of each cam 9 causes the pivoting of the second end 17 of said cam, which is fixed to said rod 18 by the presence of a rotation pin fixed to a first wall 19 of the frame 6, which pivots the first actuating wall 8 towards or away from the first end plate 4 of the electric battery 2. The first spring 10 is then either compressed or released, exerting a pressure of greater or lesser magnitude on said first end plate 4. The second assembly 40 then reacts in the same way as the first component 7, in other words when the first spring 10 is compressed under the effect of the pivoting of the cam 9, the second spring 15 is also compressed and when the first spring relaxes, said second spring 15 is also relaxed. The second assembly 40 plays the role of a relay that counterbalances the pressure exerted by the first assembly 7 on the battery 2.

[0029] With reference to FIGS. 1 to 6, a method of using the system 1 according to the invention through a complete cycle of a discharging phase followed by a charging phase of the battery 2, when the cells of said battery 2 have not aged, comprises the following steps: a start step in which a cell has dimension x1 and the system 1 applies a force F1, with reference to Figures 1 and 2a; - charging the battery 2, with reference to figures 2a, 2b and 3, in which the cells of the battery 2 expand during the charging phase and now have a dimension x2 greater than x1, so that the force applied by the system 1 increases to a value F2 greater than F1, this force F2 being the resulting force applied by the system 1 at the end of charging; - with reference to Figures 2a and 4, a step of initiating the discharge of the electric battery 2, comprising a step of pre-activating the system 1 controlled by the computer 13 so as to move the link 11 translationally upwards and thus rotate the cam 9 in a direction that brings the first actuation wall 8 closer to the first end plate 4, during which the system 1 immediately exerts on the battery 2 a resultant force F3 that is greater than the force F2; - a step at the end of discharge, with reference to figures 2a, 2b and 5, in which the cell returns to dimension x1 and the system applies a resultant force F4 to the battery 2 which is smaller than force F3, this force F4 applied at the end of discharge being due to the volume lost by the cell during this discharge phase; - actuating the system 1 controlled by the computer 13, with reference to Figures 2a and 6, to move the link 11 translationally downwards and rotate the cam 9 in a direction to move the first actuation wall 8 away from the first end plate 4, whereby the system 1 immediately applies a force F1 to the battery 2 which is less than the force F4, the system 1 then being in a position ready for charging; Includes.

[0030] 7 to 12, as the cells of the battery 2 age, the volume of the cells tends to increase over time, and this decrease is irreversible. The use of the system 1 according to the invention over a complete cycle of a charging phase followed by a discharging phase of the aged battery comprises the following steps: - a start step, with reference to Figs. 7a, 7b and 8, in which the cell has a dimension x3 which is greater than the initial dimension x1 when not aged and in which the system 1 applies a resultant force F1; - charging battery 2, with reference to figures 7a, 7b and 9, in which the force applied by system 1 increases to a value F2 greater than F1, since the aged cells of battery 2 have expanded during the charging phase and now have a dimension x4 greater than x3, this force F2 being the force applied by system 1 at the end of charging; - with reference to Figures 7a and 10, a step of initiating the discharge of the electric battery 2, comprising a step of pre-activating the system 1 controlled by the computer 13 so as to move the link 11 translationally upwards and thus rotate the cam 9 in a direction that brings the first activation wall 8 closer to the first end plate 4, during which the system 1 immediately applies to the battery 2 a force F3 that is greater than the force F2; - a step at the end of discharge, with reference to figures 7a, 7b and 11, in which the cell returns to dimension x3 at the end of discharge and the system applies a force F4 to the battery 2 which is smaller than force F3, this force F4 applied at the end of discharge being due to the volume lost by the cell during this discharge phase; - actuating the system 1 controlled by the computer 13, with reference to Figures 7a and 12, to rotate the cam 9 in a direction to translate the link 11 downwards and move the first actuation wall 8 away from the first end plate 4, whereby the system 1 immediately applies a force F1 to the battery 2 which is smaller than the force F4; Follow.

[0031] The various steps of the method for managing pressure when the battery cells are aged are identical to the various steps of the method for managing pressure when the battery cells are not aged, only the initial dimension x3 of the battery cells is greater than the initial dimension x1 of the unaged cells due to the initial increase in volume of those aged cells.

[0032] It should be noted that the charging step of battery 2 is not only performed when the state of charge of battery 2 is zero, but may be performed from any state of charge. Similarly, the discharging step is not necessarily performed until the state of charge of the battery has a value of zero. The discharging step may be interrupted at any time, even if the state of charge of the battery is not zero.

Claims

1. A system (1) for applying a variable pressure to an electric battery (2) comprising at least one electrochemical cell, the electric battery being inserted between a first end plate (4) and a second end plate (5) parallel to each other, the system (1) comprising a first assembly (7) extending parallel to the two end plates (4, 5) and comprising a first actuating wall (8) arranged opposite the first end plate (4), the first assembly (7) comprising at least one first spring (10) inserted between the first actuating wall (8) and the first end plate (4), the first actuating wall (8) extending parallel to the first actuating wall (8) and an actuator (11). a pressure applying system (1) that can move toward or away from the first end plate (4) and apply pressure to the first end plate (4) under the effect of movement of a link (11) moved by a motor (12), the link (11) being connected to the first working wall (8) by at least one cam (9), a first end (16) of the cam (9) being rotatably mounted on the link (11) and a second end (17) of the cam (9) being rotatably mounted about a rotation pin fixed to a fixed wall (19) parallel to the first working wall (8), the second end (17) abutting the first working wall (8).

2. 2. The pressure application system according to claim 1, wherein the link (11) is an elongated rod and the actuator (12) moves the link (11) translationally along the longitudinal axis of the link (11).

3. 3. A pressure application system according to claim 2, characterized in that each cam (9) comprises a rod (18) connecting said first end (16) to said second end (17), said second end (17) comprising an elongated segment arranged around said pivot pin.

4. 4. A pressure applying system according to any one of claims 1 to 3, characterized in that it comprises a computer (13) adapted to control the actuators (12) to move the links (11) in a desired direction and magnitude in order to position the first actuating walls (8) at a given distance from the first end plate (4) in order to compress each first spring (10) more or less in order to apply a desired pressure to the first end plate (4).

5. 4. A pressure applying system according to claim 1, further comprising a second assembly (40) extending parallel to the two end plates (4, 5) and comprising a second actuating wall (14) arranged opposite the second end plate (5), the second assembly (40) comprising at least one second spring (15) inserted between the second actuating wall (14) and the second end plate (5), the direction of the force exerted by the at least one second spring (15) being parallel to the direction of the force exerted by the at least one first spring (10), and the second actuating wall (14) abutting a fixed wall (20) parallel to the second actuating wall (14).

6. 6. A pressure applying system according to claim 5, characterized in that it comprises twelve first springs (10) inserted between the first working wall (8) and the first end plate (4) and twelve second springs (15) inserted between the second working wall (14) and the second end plate (5).

7. 6. A pressure application system according to claim 5, characterized in that the first spring (10) is aligned along an axis parallel to the end plates (4, 5) and the second spring (15) is also aligned along an axis parallel to the end plates (4, 5).

8. 6. A pressure application system according to claim 5, characterized in that each first spring (10) and each second spring (15) is formed from a disc spring.

9. 6. The pressure applying system according to claim 5, comprising a fixed frame (6) surrounding the electric battery (2), the first actuating wall (8), the second actuating wall (14), the at least one first spring (10), and the at least one second spring (15), wherein the second actuating wall (14) is in contact with a second flat wall (20) of the frame (6), the at least one rotating pin around which the second end (17) of the at least one cam (9) is mounted is fixed to a first flat wall (19) of the frame (6) parallel to the second flat wall (20), and the link (11) is located outside the frame (6) but behind the first flat wall (19) of the frame (6).

10. 4. A method of using a system according to any one of claims 1 to 3 throughout a cycle including a charging phase and a discharging phase of a Li-ion electric battery, comprising: an initial step in which said electrochemical cell has a dimension x1 and said system (1) applies a force F1; charging said electric battery (2), in which the electrochemical cells of said electric battery (2) expand during the charging phase to a dimension x2 greater than x1, so that the force exerted by said system (1) increases to a value F2 greater than F1; - initiating the discharge of the electric battery (2), comprising pre-activating the system (1) controlled by a computer (13) to translate the link (11) in a direction that rotates the cam (9) in a first direction that moves the first actuation wall (8) closer to the first end plate (4), whereby the system (1) immediately applies a force F3 to the electric battery (2) that is greater than the force F2; a step at the end of the discharge of the electric battery (2) in which the system (1) applies a force F4 to the electric battery (2) that is smaller than the force F3, this force F4 applied at the end of the discharge being due to the volume lost by the electrochemical cell during this discharge phase; - operating the system (1) controlled by the computer (13) to translate the link (11) in an opposite direction that rotates the cam (9) in a second direction that moves the first actuation wall (8) away from the first end plate (4), whereby the system (1) immediately applies a force F1, which is smaller than the force F4, to the electric battery (2) to return the electrochemical cell to a pre-loaded state before charging; A method comprising:

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