Device for measuring thickness changes of battery cells

Abstract

A device to measure the thickness change of a battery cell 19 under external mechanical pressure. Pressure on the cell is continuously monitored by a load cell 14. Thickness change is measured central

Description

20 02 25 Background of the Invention An important factor in testing battery cells is obtaining knowledge about their mechanical properties. Especially generation 3b materials for lithium-ion cells (silicon) show increased volume change upon cycling. This quantity is crucial information for integration of the technology in battery modules. Furthermore, external pressure has shown to be an important influencing factor for battery cell performance, in particular also for the volume change. Investigation of novel cell chemistries and electrode materials always starts in a lab. After testing the principal properties on coin cells, single-, bi- or multi-layer pouch cells are the cell format of choice for testing cell performance on full cell level. The present invention proposes a device which is able to measure the thickness change of a lab-scale battery cell under well-defined external mechanical pressure (i.e. the pressure can be freely adjusted by the user). The pressure is applied centrally and is evenly distributed over the whole cell without imposing any inhomogeneities. For measuring the thickness change directly at the centre of the cell, a contactless displacement sensor is used. The cell is contacted to an electrical cycler and an arbitrary current profile can be applied. The reaction of the cell regarding its thickness and the corresponding change in the cell pressure can be measured with this device. The specific construction of the set-up allows a stable, reproducible, long-term measurement under well-defined pressure without damaging the cell. In addition, the modular design of the set-up allows to remove and insert the cell in pressurised state. The cell is located between two holder plates which can be connected by spiral springs. This enables an easy transport of the cell. State of the Art Understanding the thickness change of a battery cell is critical for characterisation of new cell chemistries and for their upscaling and integration into modules / packs / vehicles. The thickness change needs to be known for different external parameters for considering this change when designing a battery module / pack. This is especially true for battery materials such as silicon-based anodes which show increased thickness change upon lithiation compared to graphite. The method of choice for measuring thickness change of battery cells is dilatometry. This method is already well-established in the field and several measurement set-ups have been published already. A thorough overview on previous experimental set-ups is given in two review papers by Michael et al. in 2021 (DOI: 10.1002 / batt.202100027) which deals with dilatometry only; and Popp et al. (DOI; 10.1016 / j.est.2020.101859) which reviews mechanical testing methods in general, and dilatometry specifically. There are several types of sensors on the market for different applications starting with simple dial gauges with resolution in the pm-range to high-precision electrochemical dilatometers with nano-meter resolution. These types of sensors have been used in several specifically developed research applications used for various cell types and formats ranging from half-cells to automotive cells. It has also been shown in literature that external mechanical pressure on the cell has significant impact on the cell performance and lifetime. Here it is especially important that the applied pressure is homogeneous and continuous, i.e. pressure hotspots and pressure release over lifetime must be avoided. Therefore, in many published measurement set-ups it is possible to pressurise the cell in some way. Load cells are used to measure the force / pressure change when the cell is cycled as a response to its thickness change. Here, different constraints situations can be investigated, once where the cell is allowed to expand / contract. Here, the force change will be small. The other situation is that the cell expansion / contraction is prohibited, and the dimensional change can be measured by a change in measured force. There are also several patent applications in this field: CN209570286U (Device for measuring battery expansion force distribution) provides a device for measuring the force change of a constrained cell upon volume change. The change in expansion force can be measured locally resolved, whereas the thickness change is not measured. CN111122036A (Battery cell cyclic expansion detection method and detection device) proposes a device which is able to measure the expansion force change of a battery cell during cycling. LD CN213748386U (Novel expansion tester) proposes a device where the thickness change of a pressurised cell can be measured. It provides a framework with a support platform where a cell is located. The cell can be pressurised and when it changes its dimensions, the platform moves along two supporting rods, and the change is measured with a displacement sensor. CN111351460A (High-precision battery cell expansion displacement testing device) provides device for a pressurized cell combined with a measurement of the cell's thickness change. Here, the framework consists of two sets of plates. The first is used for the pressure application, which is split into six separated springs with separated pressure plates in the drawing. For each of the pressure plates there is a displacement sensor which is led through a hole in the pressure plate and is directly pressing on the specimen on different points of the cell. WO20211236Q9A1 (BENCH FOR MECHANICALLY CHARACTERIZING THIN OBJECTS WITH INCREASED RELIABILITY) proposes a device to measure the expansion of a thin object (which can be a battery cell) with a pressure applied to it, which can also be measured. CN113418432A (Thickness measuring device) proposes a device which can measure the thickness change of a battery cell under pressure using a set-up of four steel plates connected by an optical axis. The thickness change is measured using thickness gauges. CN207688816U (Laminate polymer battery's thickness detection device) proposes a device where the thickness change can be measured for a battery cell with pressure applied to it. Description of the Invention 20 02 25 The invention presents a device to measure the thickness change of battery cells of various designs (e.g. pouch cell, cylindrical cell, prismatic cell) under well-defined external mechanical pressure for arbitrary current profiles. Well-defined in that regards means that a specific pressure can be set by the user before the measurement. First, the device itself is described that shows the following properties and functions: Figure 1 shows the whole measurement device. The device consists of the cell under test 19 with its electrical contacts 18 which is located in a cell holder 17 consisting of two plates. The holder itself consists of several parts and is described in more detail below. The device itself consists of a stable framework to prevent the influence of external influences. The framework consists of a base plate 7, two connection plates 6, and a top plate 5. The base plate 7 and the top plate 5 are connected to the connection plates 6 by four connection blocks 8. On top of the top plate 5 there is a clamp plate 2 which serves as holder for the displacement sensor 1. Force / pressure on the cell is applied by the central part of the set-up by tightening the preload sleeve 3 and a preload elastic element 11 and is monitored with a load cell 14. As connection between the load cell 14 and the cell holder 17, a pressure block 15 is used. As connection between the load cell 14 and the preload elastic element 11, a receptacle 13 is used. For optimum force transmission, a thrust bearing 9 and a shaft spring thrust bearing 10, are used. To maintain the applied force, a shaft nut 4 is tightened. The force application is ring-shaped and through the inner part of the ring, a measuring shaft 12 is connected to the holder 17 and led to the displacement sensor 1. When the cell under test 19 changes its thickness, the upper holder plate, and thus the measuring shaft 12 moves up or down, and the displacement sensor 1 detects a change in the position of the measuring shaft 12. This means, when the cell under test 19 gets thicker, the distance between the measuring shaft 12 and the displacement sensor 1 gets smaller and vice versa. This distance can be recalculated into a thickness change during post-processing of the measurement data. During the measurements the cell contacts 18 are connected to an electrical cycler and an arbitrary current profile can be applied. The electrical data can be synchronised with the thickness change and the force data to obtain a connection between the cell's electrical and mechanical behaviour. An additional feature of the proposed measurement set-up is that it allows two different kinds of measurement with only slight adaptation of the set-up. As embodiment of the elastic element 11, either a spiral spring or a stiff shell can be used. When the elastic element 11 is a spiral spring, the cell can change its thickness, which is measured with the capacitive displacement sensor 1 (referred to as constant pressure measurement, as the applied pressure to the cell does not significantly change). When the elastic element 11 is a stiff shell, it prevents the thickness change of the cell and allows to measure the increase of mechanical pressure with the load cell (referred to as constant thickness measurement, as the cell thickness does not significantly change). The replacement of the elastic element 11 can be done fast without further changes of other components of the measurement setup, specifically the pressure application unit. It makes the set-up versatile and useable for a large variety of measurements. Figure 2 shows an image of the cell holder. The cell under test 19 is located between the upper cell holder plate 20 and the lower cell holder plate 21. For electrical insulation, there are two thin insulating layers 27, 28 between the cell under test 19 and the holder plates 20, 21. Within the set-up, the force 20 02 25 is applied by the central spring 12, as described above. The special construction of the set-up itself and the cell holder 17 allows a transport of the cell under test 19 in pressurised state within the cell holder 17. To do this, the upper holder plate 20 and the lower holder plate 21 can be connected by 4 fitted screws 24 which are fixed by hex nuts with flange 25. Force / pressure is applied by four compressions springs 23. To prevent excess pressure, optional fitted washers 26 can be introduced between the two plates. Pressure application is done either by applying a defined torque to all four fitted screws 24, if the cell under test 19 is outside the set-up, or by hand-fasting the 4 fitted screws 24 equally when the cell under test 19 is already under pressure within the set-up. The innovative aspect of this set-up is the ring-shaped and concentric design with the simultaneous application of pressure to the cell and the measurement of the thickness change of the cell happening alongside a single axis and at the same point on the cell while being able to monitor both quantities and combine it with electrical data. The specific construction assures an even pressure distribution without a sensor, or any other component touching the cell under test 19 and therefore avoids pressure hot spots over the course of the cell's cycle life. The stable framework leads to a minimum bending of the set-up during pressure application and minimises also external effects such as vibrations. Placing the set-up in a climate chamber guarantees constant temperature and thus avoid another potential influencing factor on the measurement. In addition, the cell holder 17 can be easily removed from the set-up including the cell under test 19 which enables a transport of the cell under test 19 in pressurised state either from or to the set-up. This has the advantage that the cell can be transported between different labs without relieving the applied pressure. A pressure relief between measurement can change the electrochemical properties of the cell and is thus not recommended. It also allows to interrupt the electrical measurements to test for instance the calendaric ageing of the cell under a certain pressure while enabling the measurement of a different cell within the set-up. Further, easily applicable design modifications such as the selection of the elastic element 11 (spiral spring, stiff shell) in the pressure application unit with a stiff metal pipe (see above) and the adaptation of the cell holder 17 for other cell designs and formats (pouch, prismatic, cylindrical). The proposed set-up differs from existing patents in the following ways. It allows a simultaneous measurement of thickness change and pressure on the cell which distinguishes it from the devices proposed in CN209570286U and CN111122036A, where only the force change is measured. Both measured quantities, i.e. force on the cell and thickness change of the cell are measured at the same representative point in the centre of the cell. The ring-shaped realisation of the central part of the set-up allows pressure and the thickness change are measured on a representative point in the centre of the cell without influencing the thickness change. Pressure distribution is even due to the flat nature of the holder plate. This is a significant difference to CN213748386U on the one hand, where the thickness change of the cell is measured on the side and also to CN111351460A, where the pressure application and also the thickness change measurement are split into several points and thus the situation of central even pressure application is not given. In the latter patent, the pressure application plate has a hole where the displacement sensor is led through and is thus directly pressing on the cell under test which heavily influences the pressure on the cell and is the cause of significant inhomogeneities which influence the cell performance. With the proposed invention these 20 02 25 shortcomings are efficiently avoided, since the pressure application is done centrally and evenly; the cell under test 19 is not touched directly by any sensor and thus there is no interference with the measured quantities. The central measurement of both thickness change and force is also the most significant difference to WO2021123609A1, where the thickness change of the cell is deduced from the distance between two plates and is measured next to the cell at two points and not directly on the cell. The measurement points the thickness change are spatially separated from the point of force application in contrast to the proposed invention. The same is true for CN113418432A and CN207688816U, where the thickness change is not measured at the same point, where the pressure is applied. The proposed invention is designed such that is allows an insertion / removal of the cell under test 19 into / from the set-up in pressurised state, i.e. it enables a safe transport of the cell under test 19 without pressure relief assuring a continuous pressure on the cell which is apparently not the case in WO2021123609A1, CN113418432A, or CN207688816U, where removal or insertion of the cell is not addressed. One main distinctive feature of the proposed invention compared to the patents listed above is the aforementioned ring-shaped, concentric design where the pressure application / measurement and thickness change measurement are done at a single point in the centre of the cell under test 19, i.e., the pressure is applied atone single point and is maintained using an elastic element 11. If the pressure is applied using two or four points, there is always a risk of tilting / canting of the pressure plate which confounds the pressure application and the measurement of the thickness change. Measuring the thickness change at several points next to the cell has the disadvantage that the focus is not placed on the behaviour of the bulk of the cell including effects at the edge of the cell and also potential effects coming from the housing. Inhomogeneities of the cell thickness change can also interfere with the measured quantities. Another advantage is the large versatility where single components such as the elastic element 11 and the cell holder 17 can be easily exchanged to increase the measurement capabilities of the set-up, as has been already described above. The typical steps for measuring the thickness change of a cell under test can be described as follows (see also Fig.3): Step 1: Place the cell under test 19 in the holder. Step 2: Introduce the cell holder 17 into the measurement set-up. Step 3: Apply the desired force by tightening the preload sleeve 3 and thus the elastic element 11 and monitor the applied force with the load cell 14. Fix the applied force by tightening the shaft nut 4. Step 4: Apply the displacement sensor 1 and bring it in the correct position. Step 5: Connect the electrical contacts 18 of the cell under test 19 to an electrical cycler. Step 6: Apply an arbitrary current profile and measure electrical quantities (voltage, current, capacity, ...), thickness change of the cell under test 19 and external mechanical pressure on the cell under test 19 Step 7: Do a post-processing of the measurement data to synchronise the different data sources and obtain a connection between the electrical and mechanical (thickness change, pressure) data. 20 02 25

Claims

20 02 251. Device to determine the thickness change of a battery cell under test 19 under well-defined, evenly distributed external mechanical pressure while being electrically cycled, comprising a central pressure application unit consisting of a preload sleeve 3, a shaft nut 4, a thrust bearing 9, a shaft spring thrust bearing 10, a preload elastic element 11, a receptacle 13, a load cell 14 concentrically aligned with a measuring shaft 12, a pressure block 15 that applies the pressure evenly by an elastic element 11 and an upper holder plate 20a pressure monitoring unit using a load cell 14 that monitors the applied pressure onto the cella central thickness change measurement unit that measures the thickness change of the cell under test 19 via movement of a measuring shaft 12 which is detected by a displacement sensor 1 directly above of the measuring shaft 12 held by a clamp plate 2, wherein the preload sleeve 3, the shaft nut, the thrust bearing 9, the shaft spring thrust bearing 10, the preload elastic element 11, the receptacle 13, the load cell 14, and the pressure block 15 are arranged substantially along a single axis such that a substantially homogeneous pressure distribution is applied to the battery cell under test 19 and a measurement of both relevant quantities force on and thickness change of the cell under test 19 are measured at the same central point.

2. Device to determine the thickness change of a battery cell under test 19 according to claim 1, characterized bya specific modular design with a cell holder 17 which allows to easily insert or remove the cell under test 19 from the set-up inside the mentioned cell holder 17 in a defined pressurised state allowing a safe transport without pressure relief.

3. Device to determine the thickness change of a battery cell under test 19 according to claim 1 and 2, characterized bya specific modular design for selecting the central elastic element 11 to be a spiral spring or a stiff shell,in case of a spiral spring is used, the thickness change of the cell under test 19 is the main measurement quantity monitored with the capacitive displacement sensor 1, andin case of a stiff shell is used, the pressure change is the main measurement quantity monitored with the load cell 14.

4. Device to determine the thickness change of a battery cell under test 19 according to claim 1, 2 and 3, characterized bya specific modular design for the cell holder 17 to adapt it to different cell geometries (pouch, prismatic, cylindrical).

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