Field of the invention

This invention relates to battery monitoring and particularly but not exclusively for monitoring the operation of battery cells.

Background

Lithium-ion battery (LIB) packs as those used in modern electric vehicles (EVs) must be monitored at all times to ensure the vehicle safety and normal operation. Each EV battery pack can have significant numbers (e.g. thousands) of individual LIB cells, which are electrically connected in series or parallel to provide the voltage and current required to power the vehicle.

One of the main safety issues with EV is the flammability of LIB cells, which contain flammable and toxic organic electrolytes. As a result, LIB cells must be switched off at temperatures above about 50 °C. This requires constant monitoring of the cell temperature.

Due to the lack of viable technical solutions, it is typical for the cell temperature to be monitored only at a module or pack level using thermo-couples which are placed at specific points inside the battery pack. Such technical solutions do not provide information about the temperature of individual cells. As a result, the cells may be used below their capacity to ensure that they do not exceed the recommended maximum operating temperature. This reduces the driving range of the EV.

In some safety-critical applications, such as in electric aircraft, thermo-couples can be placed on each individual battery cell. This leads to excessive wiring, which adds significant weight and takes up valuable space inside the pack. As a result, the pack energy density is reduced. In addition, attaching a thermocouple to the surface of the cell is not always reliable, especially in the case of cylindrical cells which have non-flat, curved surfaces.

An important indicator of the battery cell state of health (SOH) is the geometry of the cell. Cells start to deform when they are damaged or beginning to age. LIBs often comprise graphite negative electrodes (anodes), which expand during charging due to intercalation of lithium ions. During successive charging, the anode increases its thickness and gradually the whole cell, typically composed of low-elasticity materials, starts to deform. There can be build-up of pressure inside the cell. Other anodes (silicon, silicon-carbon composite, amorphous carbon, carbon-tin alloys) and mixed metal oxide cathodes also experience volume strain during intercalation and de-intercalation of lithium ions. The level of volume strain depends on the chemistry and crystalline structure of the electrode material, the cell format and cell fabrication processes.

The prior art discloses strain gauges which monitor geometrical changes in the cell geometry.

US 9,660,299 discloses a strain-based testing of battery cells, and strain-based estimation of the battery state of health. The capacity degradation of a battery is estimated as the difference between the strain measurements performed at specified times of the battery life.

US 10,120,035 discloses a strain-based test method to reduce the battery degradation rate by implementing a battery revival cycle in the form of thermal treatment.

US 10,170,733 discloses a method for attaching a strain gauge on the surface of flexible packaging of a battery cell to maximise the transmission of strain from the cell to the strain gauge. The strain gauge is elongated along an axis and measures deformations of the cell in one specific direction. US 2017/0098872A1 discloses an RFID tag which contains strain sensors. The tag is placed on the surface of a LIB cell, and upon interrogation by an RFID reader, strain information is wirelessly transmitted to an external device for analysis.

Metal strain gauges used in the prior art have several drawbacks. They cannot be easily fixed on the surface of the cell, especially on cylindrical, curved cells. Also, they cannot be easily fixed on other critical locations such as the edges of pouch or prismatic cells. Metal strain gauges have low sensitivity (gauge factor <10) which is not sufficient to measure minute changes in the cell geometry. Therefore, the metal gauges require signal amplification, for example by using a Wheatstone bridge, which adds cost and complexity to the battery monitoring system. Metal strain gauges also have issues with robustness. They consist of fine and delicate wires which are easily damaged and prone to corrosion. Finally, metal strain gauges require extensive wiring, which increases cost and adds weight to the battery pack, reducing its energy density.

Another problem in today’s battery packs is that it is difficult to measure the state of charge (SOC) and SOH of individual cells, especially when cells are connected in parallel or series. Cells may have different characteristics and capacities due to manufacturing differences. Individual damaged or degraded cells (weak cells) in such packs can rapidly deteriorate the whole pack.

The most simple and widespread method to determine SOC is through open circuit voltage (OCV) measurement, which relies on the principle of a distinct relationship between cell OCV and SOC. Importantly, an accurate OCV measurement is only possible while the battery is not in use. This method is only useful for battery chemistries in which this relationship exists, for example, batteries based on nickel-manganese-cobalt oxide chemistry (NMC).

Many LIBs, for example those based on LiFePO4 chemistry, exhibit voltage plateaus which depend on the particular cell chemistry. This results in relatively static OCV over the normal operating voltage of the cell. Cells with flat or shallow sloping OCV curves provide certain benefits to users, however, the correlation between SoC and OCV does not provide the necessary accuracy to determine the cell SOC.

Another common method known as “ampere-counting” involves the precise measurement and logging of battery current throughout its lifetime to predict SOC. In this case, the initial SOC must be known, and small errors in current measurement accrued over the battery lifetime will lead to significant errors in SOC. Side reactions in the cell can lead to additional deviations from the actual SOC.

Therefore, there is a need in the industry for a battery cell monitoring method and devices which can provide accurate estimation of the cell SOC and/or SOH while the cell is working; regardless of the cell chemistry, and fully decoupled from measurements of the operating parameters such as OCV.

Description of the invention

A first aspect of the invention provides a battery cell fitted with a sensor, the sensor comprising a percolative sensing element.

The sensor of the invention, the percolative sensing element may be a percolative resistance sensor; that is, external stimuli induce a change in the percolation properties of the conductive network of the sensor, and therefore change in the electrical resistance of the sensor.

In particular, the percolative sensor may be used to monitor the operation of battery cells, for example rechargeable lithium-ion battery (LIB) cells and lithium/sulphur cells. The sensors are usable to monitor battery packs and modules comprising two or more battery cells. The percolative sensor may comprise a conductive coating which changes its resistance upon the application of strain, stress, temperature, or other physical stimuli. The conductive coating can be made from graphene, carbon, silver, and I or other conductive materials, and mixtures thereof.

Changes in the sensor resistance can be measured and correlated with the battery cell state- of-charge (SOC), state-of-health (SOH), ageing, and degradation processes taking place in the cell during its lifetime. In addition, certain changes of the sensor resistance are indicative of anomalous behaviour and imminent safety-critical events such as fires and explosions.

SOH herein means the general condition of a battery and its ability to deliver the performance characteristics of a fresh battery.

The percolative sensor can monitor and predict an individual battery cell. As a result, the battery management system (BMS) enables greater control of cell balancing and reconfiguration, which increases the performance of the battery system, enabling greater lifetimes and reducing the chance of full system failure.

Percolating networks comprise electrically conductive particles that overlap with each other to create an electrically conductive network. The network is able to maintain conductivity even when deformed due to continual contact among the electrically conductive particles. The percolation threshold is the lowest concentration of conductive material at which an electrical path is formed through the sensor.

The external stimuli can be temperature, mechanical impact such as strain, stretch or load, or others. These stimuli cause a change in the sensor geometry, which leads to change in the particle density and distribution within the percolation network. As a result, the electrical resistance of the network changes.

Percolation sensors are advantageous because they are more sensitive than conventional metallic foil strain gauges, which measure strain based on purely geometric changes of a conductive wire and require a Wheatstone bridge to carry out the measurements. The excellent strain sensitivity of percolative sensors arises from several factors. Firstly, it has been shown in the research literature that conductivity in a percolated system shows a power law dependence on particle density through the percolation threshold. Therefore, highly nonlinear conductivity changes are expected to result from minor geometrical changes to a sample, due to changing of the number density of the conductive particles (M. Hempel et al, A novel class of strain gauges based on layered percolative films of 2D materials, NanoLetters, 2012 Nov 14;12(11):5714-8. doi: 10.1021/nl302959a). In contrast, metal strain gauges show only linear dependence of resistance on geometrical changes.

Choi et al (Transducers 2019, Eurosensors XXXIII, 23-27 June 2019) disclose a carbon nanotube (CNT) based strain sensor for monitoring strain in LIB pouch cells. The strain sensor is formed by spraying a CNT network onto a polydimethylsiloxane (PM DS) substrate. At temperatures of 25 to 50°C the rate of change of resistance to temperature (DR/DT) was of the order of 0.1.

Other effects also contribute to high sensitivity in percolative systems; including opening up of inter-particle junctions leading to a reduced number of critical spanning conductive clusters.

Advantageously, percolative sensors are able to provide a meaningful reading without use of amplifiers, or very sensitive measurement devices, such as a Wheatstone bridge. Accordingly, not only is our sensor simpler than prior art sensors but it also allows simpler measurement and hence less complex and expensive wiring and installation.

A change in the percolation network can also be induced by temperature changes, especially when the percolation network (e.g. a network of graphene flakes) is embedded into a polymer matrix. Polymers exhibit phase transitions such as glass transitions and melting which can lead to softening of the polymer matrix and increased segmental motion of the polymer chains. This can help the graphene flakes re-orient through sliding and increase the contact area between individual particles, leading to more conductive percolation network. It is found that simple and low-cost percolative sensors as those disclosed herein can exhibit surprisingly high sensitivity and ability to detect minute changes in temperature and geometry, as those observed in battery cell such as lithium-ion battery (LIB) cells.

A further aspect of the invention provides a battery pack comprising plural battery cells and comprising a percolative sensor comprising a sensing element.

The sensing element may located on or within a component of the battery pack, for example on a cell or on a wall of the pack proximate a cell, thereby to monitor or time changes within the battery pack.

A yet further aspect of the invention provides a method of monitoring the state of health of a battery cell, the method comprising locating a percolative sensor comprising a sensing element on or proximate to the battery cell and monitoring the resistance of the sensing element, for example monitoring the resistance over time.

A further aspect of the invention provides a strain sensor, the strain sensor comprising an elongate body formed into a loop having a substantially rectangular (e.g. square) form, a first end of the elongate body proving a first contact and a second end of the elongate body providing a second contact, the elongate body comprising a network of electrically conductive particles.

Brief Description of the Drawings

The invention will now be described, by way of example only, and with reference to the following figures:

Figure 1 is a schematic view of a first embodiment of sensing element;

Figure 2 is a schematic view of a second embodiment of sensing element;

Figures 3A to 3D are respectively a three cell battery pack (3A), charge/discharge cycle (3B), a graph of the temperature of the cells measured using thermocouples (3C) and a graph of the resistance of the cells measured using a sensor of the invention (3D);

Figures 4A and 4B are respectively a graph of the temperature of cells measured using thermocouples and a graph of the resistance of the cells measured using the sensor of the invention;

Figures 5A to 5D are respectively a view of a cell with sensors and thermocouples attached (5A); a graph of the temperature of the cells measured using thermocouples (5B); a graph of the resistance of the cells measured using the sensor of the invention (5C), and a chargedischarge cycle (5D).

Figures 6A to 6D are a series of graphs showing measurements taken by the sensing element of the invention at different discharge rates.

Figure 7 is a graph showing measurements taken by the sensing element of the invention at a different charge rate.

Detailed description of the sensor

The sensor, e.g. the percolative sensor in this invention is defined as the combination of one or more of the following components:

(a) a sensing element

(b) sensor substrate on which the sensing element is deposited

(c) means of attaching the sensor element to an object to be monitored

(d) means of attaching the sensor substrate to an object to be monitored

(e) sensor encapsulation (f) means of electrically connecting the sensing element to the readout system.

The sensing element is the main component of the sensor, and it is present in all embodiments. The other components are optional and may not be present in all embodiments.

In this invention, the object to be monitored is a battery cell, such as LIB cell. The battery cell can be of any format, such as cylindrical, pouch, or prismatic cell, but the sensor of the invention has particular utility in monitoring the temperature, strain and/or state-of-health (SOH) of cylindrical and prismatic cells.

The object to be monitored might also be a component of a battery pack (e.g. a wall, a cell holder, or a cooling plate) located next to a battery cell.

The sensing element contains the conductive percolation network. The sensing element is made from an electrically conductive composition, for example, an electrically conductive ink or an electrically conductive paste, or electrically conductive elastomer or electrically conductive mouldable composition. The sensing element may be formed by printing an electrically conductive composition onto a substrate or onto an object, for example, screen printing, stencil printing, or ink jet printing. The sensing element may be coated or deposited onto an object using other techniques known in the art. In embodiment, the sensing element can be injection-moulded over a substrate.

In embodiments, the sensing element may comprise electrically conductive particles. In embodiments, the electrically conductive particles may comprise or consist of metallic particles, for example, silver particles, gold particles, copper particles, nickel particles, platinum particles, and/or palladium particles, or combinations thereof. Additionally, or alternatively, the electrically conductive particles may comprise metal-clad particles or metal alloys particles or combinations thereof. The electrically conductive particles may comprise particles of an inorganic conductor, such as indium tin oxide (ITO) or fluorine doped tin oxide (FTO).

In embodiments the electrically conductive particles may comprise or consist of non-metallic particles, for example, carbon or carbon-based particles such as graphene particles, graphite particles, multi-layered graphene particles, graphene-oxide particles, carbon nanotubes, carbon fibres, carbon black, or combinations thereof.

In embodiments, the sensing element may comprise electrically conductive organic polymer, for example poly-pyrrole, or poly(3,4-ethylenedioxythiophene) (PEDOT).

The electrically conductive particles may be particles of any shape, such as spherical and/or regular or irregular flakes. The electrically conductive particles may be nanomaterials. Spherical electrically conductive particles may have a particle size (/.e. a nominal diameter) of less than 25 microns, say less than 10 micron, less than 1 micron, less than 500nm, or less than 100nm. In some embodiments, electrically conductive flake particles have a nominal lateral diameter of less than 50 microns, say less than 25 micron, or less than 10 micron, or less than 1 micron. In embodiments, the electrically conductive particles may comprise or consist of nanoparticles, e.g. nanospheres, nanofibers, and/or nanoflakes.

In embodiments, the electrically conductive particles comprise graphene particles, preferably few-layer and multi-layer graphene particles. In embodiments, the graphene particles may comprise exfoliated particles, for example, exfoliated graphite. In embodiments, the graphene particles may have a particle size of less than 50 microns, say less than 25 micron, or less than 10 micron, or less than 1 micron.

In this application we intend the term particle size to mean the largest transverse dimension of the particle. The term ‘nominal diameter’ is intended to mean the smallest diameter of a sphere which would entirely encapsulate the particle. Thus is the particle has a sphericity of 1 its diameter will be the nominal diameter, whereas as the sphericity deviates from unity the nominal diameter will be measured as the maximum transverse dimension. The sensing element may further comprise a non-electrically conductive matrix, for example a polymeric matrix. The polymeric matrix may comprise an acrylic polymer, a polyester polymer or polyether polymer. The polymeric matrix may comprise a urethane polymer. The polymeric matrix may comprise a combination of different polymers, for example, a combination of an acrylic polymer and a urethane polymer.

The sensing element may comprise electrically conductive particles retained or suspended in a polymeric matrix. In embodiments, the sensing element may comprise metallic particles suspended in a polymeric matrix. In embodiments, the sensing element may comprise non- metallic particles suspended in a polymeric matrix. In embodiments, the sensing element may comprise graphene particles suspended in a polymeric matrix, for example, an acrylic matrix or a polyurethane matrix, or combinations thereof.

The polymer matrix may comprise from 50 to 99.5 w/w% of the sensing element.

In an embodiment, for example when the electrically conductive particles are CNTs, the polymer matrix may comprise up to 99.5 w/w%, the CNTs providing the balance. In another embodiment, for example when the electrically conductive particles are graphene, the graphene particles may comprise up to 50 w/w% of the sensing element, the balance being provided by the polymer matrix. In a preferred embodiment graphene particles may be present in an amount of 10 to 45 w/w% of the sensing element. We have found that the sensitivity of the sensor can be tuned by changing the amount of electrically conductive particles.

Indeed, we believe, although we do not wish or intend to be bound to any particular theory, that the presence of the polymer matrix affords the sensing element with high sensitivity sufficient to allow it to effectively monitor the strain in rigid-bodied cells (e.g. cylindrical and prismatic cells).

The sensing element may be fabricated by printing, for example screen printing, gravure printing, 3-d printing (or other additive manufacture). The sensing element may be fabricated in a roll-to-roll process.

The size and shape of the sensing element are determined by the geometry and dimensions of the object which will be monitored, and in such a way that any changes in the sensor resistance are maximised in order to increase its sensitivity; while ensuring that the resistance of the sensor is within a range suitable for the readout system. Preferably, the shape and the size of the sensor element are defined in such a way that its resistance is in the range of 1 Ohms to 500,000 Ohms, preferably in the range 10 Ohms to 150,000 Ohms, most preferably in the range 1 ,000 Ohms to 10,000 Ohms.

The sensing element may have a thickness of from 1 micron to 1000 microns and/or a width of from 1 to 100 mm.

In a preferred embodiment, the shape of the sensing element is rectangular as shown in Figure 1. In this embodiment, the sensing element 1 is a printed graphene film with contacts 2A, 2B at either end.

In another preferred embodiment, the shape of the sensing element is an elongated member which forms a nearly complete loop as shown in Figure 2. In this embodiment, the sensing element 3 is a printed graphene track of width 5mm with contacts 4A, 4B at either terminus.

The percolative sensor in this invention may comprise only one sensing element. It can also comprise an array of two or more sensing elements.

The sensor substrate is the material on which the sensor is deposited. The sensor substrate must be electrically insulating. Preferably, the sensor substrate is made from polymer material which is flexible or stretchable. In another embodiment, the sensor substrate is a polymer- coated metal foil. The mechanical properties of the sensor substrate are important because it must be able to transfer strain, temperature, and other external stimuli from the object to the sensing element. Such external stimuli must not cause deformations, fatigue, or other irreversible changes in the mechanical properties of the substrate.

Examples of suitable sensor substrates include plastic films such as polyester films (polyethylene terephthalate, PET; polyethylene naphthalate, (PEN), polyimide films (e.g. Kapton (RTM) films supplied by DuPont), polycarbonate films, halogenated polymer films (ethylene tetrafluoroethylene, ETFE), thermoplastic urethanes, etc. Other examples include metal foils such as aluminium, copper, steel, etc which are coated with an insulating polymer.

Preferably the substrate has a relatively high Young’s modulus, say above 2.7 GPa. Materials with lower Young’s moduli tend to be easily deformed and do not beneficially allow for the transference of strain to the sensing element.

In some embodiments, the sensor substrate may not be present. Instead, the sensor element is deposited directly on the object to be monitored, for example on the wall of a battery case, which thereby provides a substrate.

The sensor substrate is attached to the object to be monitored using an adhesive. Examples of suitable adhesives include acrylate, acrylic, polyurethane, and silicone adhesives. The adhesive must be able to transfer strain, temperature, and other external stimuli from the object to the sensor substrate. As a consequence, we prefer to use adhesives which form more rigid bonds than those that form more flexible bonds. In this regard acrylic may be preferred. Such external stimuli must not cause deformations, fatigue, or other irreversible changes in the mechanical properties of the adhesive. For optimal performance there should be no air bubbles or other irregularities within the adhesive.

In some embodiments, the sensor substrate is placed directly on the object to be monitored without the use of adhesive. This method of attachment is suitable for high-tack sensor substrates which can make uniform and continuous contact with the surface of the object to be monitored.

In some embodiments, the sensor substrate is provided simply as a means of supporting the sensing element, for example when the sensing elements is placed on the walls or other component/s of a battery pack which are in contact with the battery cells. The sensing element may be provided as an integral component of the battery pack component used to monitor one or more of the battery cells.

The sensor encapsulation consists of a protective layer which is placed on top of the sensing element. The role of the sensor encapsulation is to protect the sensing element from mechanical damage, and from the ambient atmosphere. Graphene films in particular can be sensitive to moisture and other gases, which leads to changes in their electrical resistance. This interferes with the resistance changes due to strain or other mechanical and thermal stimuli.

The encapsulation material must be electrically insulating. Suitable encapsulation materials include non-porous polymer films, resins, and polymer coated metal foils.

In one embodiment, the sensor encapsulation can be moulded over the sensor element. In this case, the sensor encapsulation can be in any 3D shape.

In another embodiment, a layer of adhesive can be inserted between the sensing element and the sensor encapsulation. This makes the sensor assembly more robust.

Each sensing element is electrically connected to the readout system by means of two wires. Optionally, the wires can be printed, coated, or otherwise deposited on the sensor substrate or the object to be monitored, ensuring that there is electrical connection between the wire and the sensing element.

Optionally, electrode pads may be printed or otherwise deposited at the end points of the sensing element to facilitate the attachment of wires. The electrical resistance of the wires must be stable and display minimum changes upon strain or other mechanical or thermal stimuli. Any changes of the electrical resistance of the wires must be at least an order of magnitude smaller than the changes of electrical resistance of the sensing element.

In use the sensor may be secured to an article to be sensed. For example the sensing element may be secured to a wall of a battery cell such as a prismatic or cylindrical cell. In one embodiment a sensor substrate of the sensor may be secured to a wall of a battery cell. Preferably at least a portion of the sensor at or towards the ends of the sensor are secured to the article. Preferably a major proportion of the sensor is secured to the article and most preferably substantially all (or all) of the sensor is secured to the article.

We have found that the sensor is most effective in measuring strain when the sensor, e.g. the sensor substrate, is securely held in position with respect to the article being monitored.

The sensor may be located on an article to be monitored. For a cylindrical cell the sensor substrate may be located around a major portion of the cell, preferably around greater than 60, 70, 80% of the cell. For a prismatic cell the sensor substrate may also be located around a major portion of the cell. The sensing element may be located around the minor portion of the article for example the sensing element may be located around less than 40, 30, 20 % of the article. Without wishing to be bound by any theory, we believe that by locating the sensor around a major portion of the cell the sensor may be deformed in two dimensions (e.g. in a radial directions and in the axial direction) and thereby the sensor is more able to sensitively monitor the cell.

In a cylindrical cell the sensor may be located towards the negative terminal of the cell.

In order that the invention may be more fully understood, reference is made to the accompanying Examples, as follows:

Example 1.

In this example, one loop-shaped sensor as shown in Figure 2 was mounted on each of three cylindrical LIB cells INR2170-50E. The sensing element was produced by printing a graphene ink (ca. 44 w/w%), while silver ink was used to produce electrodes for connection to the readout system. The sensor substrate was a PET film of thickness 125 microns, and it was wrapped around the metal case of the cell without the use of adhesive. A protective layer of polyimide sheet with acrylic adhesive was placed on top of each sensor so that the acrylic adhesive was facing the sensing element. Finally, each cell was wrapped in an insulation plastic PVC sleeve for safe handling. The three cells were connected in parallel to form a 3- cell pack as shown in Figure 3A. For comparison, one K-type thermocouple was mounted on each cell in the centre of the loop. The thermocouple was attached on the outside of the insulation sleeve using an adhesive tape.

Figure 3B show the charge-discharge curves when the pack was charged at a rate of 0.5C (2.5A) and discharged at a rate of 1C (5A).

Figure 3C shows the temperature readings from the K-type thermocouples during the pack charge and discharge. The temperature rises toward the end of the charging cycle, and there is another significant temperature rise towards the end of the discharge. Cells A and B appear to heat more than cell C.

Figure 3D shows the graphene sensor resistance readings during the same charge-discharge cycle. It can be seen that the graphene sensor replicates exactly the readings from the thermocouples, with the sensors on cells A and B showing more significant changes compared to cell C. The changes of sensor resistance can be presented by the ratio (R _t - Ro)/Ro where Ro is the resistance at the time of sensor installation, and R _t is the resistance at time t. A calibration coefficient can be used to convert the values of (R _t - Ro)/Ro to temperature.

Based on these experiments we can see that the rate of change of resistance with respect to temperature is of the order of 10 [AR/AT ~ (200-20/40-24) =11] which demonstrates the sensitivity of the sensor of the invention.

In this example, the graphene sensor provides a more robust means for monitoring the temperature of individual cells, because it is placed closer to the cell and cannot be easily displaced. In contrast, the thermo-couples can easily detach from the cell surface, slip, or change their location. Additionally, the thermocouples take additional volume which reduces the volumetric energy density of the pack.

Example 2

This example used the same graphene sensor and pack construction as described in Example 1 , however the cells were continuously charged at a constant current, aiming to reach above the safe cut-off voltage of 4.2V.

During overcharge, the thermocouples detected a temperature rise for all three cells, before the safety mechanism was activated and cells switched off. The level of temperature increase, up to 47.5 °C, can be dangerous for the safe operation of the cells. At the same time, the graphene sensors showed a significant resistance, which can be used as an indicator of the cell overcharge.

This example shows that the percolative sensors disclosed in this invention can be used to monitor the safe operation of LIB cells.

Example 3.

In this example, two rectangular graphene sensors (1) were attached at different locations of the outside metal case of a cylindrical cell ICR18650-26J (5) as shown in Figure 5A. Each sensing element had an area of 1cm x 2cm. Both sensing elements were printed on the same substrate, a polyimide sheet of thickness 60 microns. The polyimide sheet with the two sensors was wrapped around the metal case of the cell and kept in place using acrylic adhesive. In a next step, a protective layer of a polyimide sheet with acrylic adhesive was placed on top of the sensors so that the acrylic adhesive was facing the sensing elements. Finally, each cell was wrapped in an insulation plastic sleeve for safe handling.

Three K-type thermocouples (100) were also attached to the cell as shown in Figure 5A. Two of the thermocouples were placed at the same height as the sensing element, and one thermocouple was placed close to the top (positive) terminal of the cell.

The cell was cycled as follows: charge at 0.5C, discharge at 1 C, charge at 0.5C, discharge at 2C. 2C was the maximum discharge rate recommended by the manufacturer (as shown in Figure 5D).

As shown in Figure 5B, the thermocouples measured significant rise of the cell temperature, especially during discharge at 2C. At the same time, both graphene sensors showed a significant change of resistance during 2C discharge (Figure 5C), which cannot be explained with the temperature changes alone. Furthermore, the changes in resistance were fully or partially irreversible, and the resistance values did not recover to the original values. These changes in resistance can be attributed to geometrical changes in the cell, where the metal case expands, and is permanently deformed, during fast discharge. Such processes can lead to ageing and deterioration of the battery SoH. This example shows that the graphene sensor disclosed in this invention can be used to monitor the cell SoH and ageing. The changes detected by the sensor are not detectable by standard thermo-couples.

In each of the above examples NMC (Li - nickel manganese cobalt) LIBs were used. To investigate whether other cell chemistries are equally suited to monitoring using the sensor of the invention we have conducted further testing, as follows.

Example 4.

In this example, a single rectangular graphene sensor was attached to the outside of the metal case of a cylindrical lithium iron phosphate cell LFP05 in a similar manner to that shown in Figure 5A. The sensing element had an area of 1cm x 2cm and was printed on a polyimide sheet of thickness 60 microns. The sensing element was formed from a graphene ink having about 35 w/w% graphene. The polyimide sheet with the sensor was wrapped around the metal case of the cell and held in place using acrylic adhesive. Subsequently, a protective polyimide sheet was applied atop the first sheet using acrylic adhesive such that the acrylic adhesive was facing the sensing element. Finally, each cell was wrapped in an insulating plastic sleeve for safe handling.

A single K-type thermocouple was attached to the outside of the insulating sleeve in a similar manner to that shown in Figure 5A.

The cells were cycled as follows

In each case, the upper trace (601 , 611 , 621 , 631) provides the output of the sensor of the invention.

The middle trace (line 602, 612, 622, 632) represents the output of the thermocouple and the lower trace (line 603, 613, 623, 633) represents the ambient temperature.

The evidence shows that at very high discharge rates (e.g. Figure 6D) the sensor of the invention is able to determine a permanent conformation change in the cell because the resistance value does not recover to its initial value.

Indeed, as the discharge rate increases from a ‘normal’ discharge rate (Figure 6A) to a high discharge rate (Figure 6D) the response of the sensor alters

This example shows that the graphene sensor disclosed in the invention can be used to monitor the cell SoH and ageing of lithium iron phosphate cells. Moreover, the changes detected by the sensor are not detectable by standard thermo-couples. Example 5.

This example uses the same graphene sensor and cell construction as described in Example 4, however the cell was both charged at 2C and discharged at 0.5C. The results are shown in Figure 7. As before the upper trace (line 701) is the response of the sensor of the invention, the middle trace (line 702) is the response of the thermocouple and the lower trace (line 703) is the ambient temperature.

A rise in temperature is observed by the thermocouple while at the same time, the sensor showed of the invention showed a significant change in resistance. Again the sensor of the invention was able to detect a difference in the cell after the charge discharge cycle which was not demonstrated by the thermocouple.

The above experiments demonstrate that the sensor of the invention is able to monitor battery cells of different chemistry and detect changes in cells as a result of different charge/discharge conditions. Accordingly, the sensors of the invention are able to effectively monitor SOH of battery cells in use. Integration of the output from the sensors in a BMS will allow for effective, sensitive, real time battery monitoring.