Cell evaluation method

Abstract

A method and apparatus for evaluation of an electrochemical cell comprising a signal generation unit (202, figure 2) to generate a perturbation (multi-frequency) signal to be passed through the electr

Description

TECHNICAL FIELD Embodiments of the present disclosure relate to methods and apparatus for cell evaluation, and in particular methods and apparatus for evaluating a state of the electrochemical cell. BACKGROUND An electrochemical cell is a device that generates electrical energy using one or more chemical reactions. Examples of electrochemical devices include batteries, fuel cells, and electrolysers. Electrochemical devices are critical to reducing carbon emissions, facilitating the process of electrification, and thus providing energy security. In many applications of electrochemical cells, several cells are connected in series or parallel to form a pack. However, a pack may suffer from safety concerns and / or may suffer from reduced lifespan if there are differences between the connected cells. For example, weaker batteries connected in the pack may deplete or overcharge faster, leading to performance imbalances or safety risks. Such issues may arise throughout the lifetime of a cell, for example due to degradation. Accordingly, such issues may significantly impact the use of retired batteries or batteries being reconfigured into new packs, which may have different usage histories. Alternatively or additionally, such differences may be innate, for example as a result of differences in materials and / or manufacturing of fresh cells. As a result, performance inconsistency is inevitable during the life cycle of electrochemical devices (wherein said lifecycle may include: manufacturing, usage in real life applications, and recycling for a second life). For in-service cells, real-time evaluation may be needed for isolating weaker batteries and preventing further deterioration. Techniques such as X-Ray Computed Tomography (CT) and neutron imaging may provide insight into a cell’s evolution of internal structural changes during thermal runaway, ageing, and abuse usage. However, such techniques may be unaffordable, time-consuming, and / or resource-consuming. For example, in techniques such as using a CT scan to evaluate a cell unaffordable equipment may be needed. Furthermore, some devices may need to be operated in a full range to get a suitable performance curve, which may be time- and resourceconsuming. Other available techniques such as acoustic testing may be non-destructive, but may be sensitive to both sensor installation and environmental noise; such sensitivities may impact the accuracy and reliability of the measurements. Current diagnostic tools may also cause cell degradation during the diagnostic process. In 5 particular current diagnostic tools may require the cell to be charged and discharged for a whole cycle, which may in turn cause battery degradation. Current diagnostic tools may alternatively or additionally require the cell to be in a steady state during the entire diagnostic process. Accordingly, a cell undergoing such a process cannot be 10 used which may be inconvenient or inefficient (where the cell is new or undergoing second life testing). Such processes are also incompatible with testing partially-formed cells, which cannot be kept in a steady state. Accordingly, a reliable, simple, and fast diagnostic tool may be needed. Such a tool may be 15 used to support the building of packs or other systems requiring a high consistency across single cells and to support a safer electrochemical aging process. SUMMARY It is an aim of the present disclosure to provide a method of evaluating the state of a cell and corresponding apparatus which at least partially addresses one or more of the challenges discussed above. It is a further aim of the present disclosure to provide a method and apparatus for evaluating the state of a cell which is more reliable, simpler, and / or faster than known diagnostic methods and apparatus. A first embodiment of the present disclosure provides a method of evaluation of an electrochemical cell. The method comprises generating a perturbation signal, wherein the perturbation signal is a multi-frequency signal and passing the perturbation signal through the electrochemical cell. The method further comprises measuring an electrical signature of the electrochemical cell, wherein the electrical signature is a voltage response and / or current response of the chemical cell that is induced by the passing of the perturbation signal through the electrochemical cell, and evaluating a state of the electrochemical cell directly using the electrical signature. A second embodiment of the present disclosure provides an apparatus for evaluating an electrochemical cell. The apparatus comprises a signal generation unit configured to generate a perturbation signal to be passed through the electrochemical cell, wherein the perturbation signal is a multi-frequency signal. The apparatus further comprises an electrical signature measurement unit configured to measure an electrical signature of the electrochemical cell, wherein the electrical signature is a voltage response and / or current response of the chemical cell that is induced by the passing of the perturbation signal through the electrochemical cell device. The apparatus further comprises an evaluation unit configured to evaluate a state of the electrochemical cell directly using the electrical signature. Further embodiments provide methods and apparatus for evaluating the state of a cell as discussed herein. BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the present disclosure, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings 5 in which: Figure 1 is a flow diagram of a method of cell evaluation, in accordance with embodiments; Figure 2 is a schematic diagram of an evaluation apparatus, in accordance with embodiments; 10 Figure 3 is a schematic diagram of an evaluation unit, in accordance with embodiments; Figure 4A and Figure 4B (collectively referred to as Figure 4) are diagrams of an evaluation arrangement, in accordance with embodiments; and 15 Figure 5A, Figure 5B, and Figure 5C (collectively referred to as Figure 5) are diagrams of a further evaluation arrangement and associated measurements, in accordance with embodiments. DETAILED DESCRIPTION Embodiments herein provide methods and apparatus for evaluating the state or performance of an electrochemical cell. Methods and apparatus of the present embodiments may be used with a cell or pack of cells. Embodiments herein may be used for one or more purposes including: evaluating cell state, diagnosing a cell state, evaluating cell performance, and monitoring cell state and / or performance. Embodiments herein may provide a reliable, simple, and fast diagnostic tool for evaluating cell state. For the purpose of explanation, details are set forth in the following description in order to provide a thorough understanding of the embodiments disclosed. It will be apparent, however, to those skilled in the art that the embodiments may be implemented without these specific details or with an equivalent arrangement. Figure 1 depicts a method S100 in accordance with embodiments. As shown in Figure 1, the method may comprise generating a perturbation signal (Step S102). The perturbation signal may be a multi-frequency signal. The method may further comprise passing the perturbation signal through an electrochemical cell (Step S104). The electrochemical cell through which the perturbation signal is passed may be the electrochemical cell to be evaluated. Accordingly, the method may further comprise measuring an electrical signature of the electrochemical cell (Step S106). The electrical signature may be a voltage response and / or current response of the chemical cell, that is induced by the passing of the perturbation signal through the electrochemical cell device. The method may then comprise evaluating a state of the electrochemical cell directly using the electrical signature (Step S108). In specific embodiments, the method may comprise evaluating a state of the electrochemical cell by conducting a comparison. For example, in specific embodiments the method may further comprise comparing the electrical signature of the electrochemical cell with a predetermined electrical signature template. Such a comparison may be appropriate for use cases such as providing a health score for the electrochemical cell (for example, by comparing the electrical signature of the electrochemical cell with that of a new or fresh electrochemical cell of the same or other suitable types). Alternatively or additionally, such a comparison may be appropriate for the use case of tracking battery performance over time (for example, by comparing the electrical signature of the electrochemical cell with that of a new electrochemical cell or electrochemical cell of the same or similar age). Accordingly, for appropriate use cases an appropriate predetermined electrical signature may be obtained and compared with the measured electrochemical cell electrical signature. The predetermined electrical signature may therefore form an expected electrical signature response or the like. Similarly, the predetermined electrical signature may provide a prediction of the potential or likelihood of future electrochemical cell failures. Alternatively or additionally, specific embodiments may further comprise comparing the electrical signature of the electrochemical cell with a further electrical signature of a further electrochemical cell. Such a comparison may be appropriate for use cases such as comparing a plurality of cells. For example, a comparison between a plurality of cells may be needed to evaluate a compatibility of the plurality of cells for second life usage such as forming a pack. In addition, a comparison between a plurality of cells may be used to evaluate a consistency of cell performance within a batch and thus may provide an analysis of a consistency of a cell production method. For example, a batch of cells may be tested for variation in performance quality within the batch or to provide an overall performance quality score in comparison to other batches. Accordingly, present embodiments may be used for the detection of any of the following aspects of one or more cells: inconsistency during manufacture of the cell(s), inconsistency during operation of the cell(s), and degradation inconsistency of the cell(s). In some embodiments, the method may comprise selecting the perturbation signal for generation based on the type of electrochemical cell. That is, for some types of electrochemical cell a suitable frequency range or other aspect of the perturbation signal may already be known. Such ranges may be known, for example, from previously performed evaluations and / or implementations of present embodiments. In specific embodiments, the suitable frequency range may comprise particular frequency subranges ranges of interest (for example subranges where electrochemical cells of said type exhibit a phase shift and / or amplitude change of interest). Accordingly, where such a suitable perturbation signal or aspect thereof is known, such a signal may be selected for use in present embodiments. In further specific embodiments, the type of electrochemical cell may be one or more of: a lithium (Li) cell, a sodium (Na) cell, a flow cell, and a metal-air cell. The electrochemical cell may be either a traditional-type cell or a solid state cell. For example, a traditional lithium cell may comprise a liquid electrolyte and graphite anode or cathode. Alternatively, a zero-excess lithium metal traditional lithium cell may comprise a copper foil, on which Li is deposited from the cathode; the anode of a traditional cell could alternatively be comprised of hard carbon or Li metal. Such a lithium cell may have significantly different performance and degradation properties to a traditional lithium cell comprising a liquid electrolyte and graphite anode. Furthermore, a solid state lithium cell may comprise a solid state electrolyte in place of a liquid electrolyte. For example, the solid state lithium cell may comprise a solid electrolyte such as a Li ion conducting solid material; the solid material may be a ceramic or polymer. The above differentiations between traditional and solid state lithium cells may also be applicable to sodium cells, lithium-sulphur cells, metal-air cells including zinc-air cells and lithium-air cells. Lithium cells may also be referred to as lithium ion cells, and similarly sodium cells may be referred to as sodium ion cells. In specific embodiments, the electrochemical cell may be a high-rate anode cell such as a niobium oxide anode cell (or a doped equivalent anode cell) and / or a lithium titanate (LTO) anode cell. In addition, present embodiments may be applied to fuel cells and electrolysers. Suitable fuel cells and electrolysers may employ either solid polymer or ceramic electrolytes, for example high temperature solid oxide type fuel cells and electrolysers. As detailed above, the perturbation signal may be a multi-frequency signal. Accordingly, specific embodiments may comprise generating or synthesizing the multi-frequency signal. In such embodiments, the frequency points may need to be carefully chosen, for example to provide a suitable electrical signature for cell evaluation. In order to obtain a suitable frequency range, for example to determine a frequency range comprising one or more suitable frequency points, specific embodiments may comprise a calibration process. The calibration process may comprise performing Electrochemical Impedance Spectroscopy (EIS) on a calibration electrochemical cell to obtain a frequency range. The calibration electrochemical cell may be of the same type as the electrochemical cell. The EIS may be performed over a frequency range and the result of the EIS may then be analysed, for example to determine one or more frequency subranges of interest. The calibration process may further comprise selecting one or more subranges of the frequency range to obtain perturbation frequency ranges, and using the perturbation frequency ranges in the generation of the perturbation signal. Alternatively or additionally, the calibration process may comprise applying a chirp signal to a calibration electrochemical cell to obtain a frequency range, wherein the calibration electrochemical cell is of the same type as the electrochemical cell. The chirp signal may be a signal with a frequency sweep over time. The calibration process may then further comprise measuring and / or analysing an output response from the calibration electrochemical cell, for example to determine one or more frequency subranges of interest, and selecting one or more subranges of the frequency range to obtain perturbation frequency ranges. Further alternatively or additionally, the calibration process may comprise applying a plurality of single-frequency sinusoidal perturbations to a calibration electrochemical cell. The calibration electrochemical cell may be of the same type as the electrochemical cell. The sinusoidal perturbations may be applied to the calibration electrochemical cell at a few discrete frequencies. The calibration process may then further comprise measuring the electrical response of the calibration electrochemical cell and analysing the measurements to identify one or more frequencies where the calibration electrochemical cell exhibits a phase shift and / or amplitude change of interest. The calibration process may then further comprise selecting one or more subranges of the frequency range to obtain perturbation frequency ranges based on the analysis. In specific embodiments, the multi-frequency perturbation signal may be generated by generating broadband random noise and passing the broadband random noise through a passband filter to isolate specific frequency bands. In further specific embodiments, the multifrequency signal may be a stacked signal; using a stacked signal may provide faster measurements. Alternatively, the multi-frequency perturbation signal may be a chirp signal or other multifrequency signal that may sweep over time and / or frequency. The range of the frequency points may be selected to cover the Ohmic, charge transfer and mass transfer resistance frequency points of the cell electrical signature. In further specific embodiments comprising multiple electrochemical cells, multi-frequency signal generation may be realised by varying the signal frequency over time to cover multiple frequency bands, adding up the sine wave of the electrical signature of each electrochemical cell together and / or using advanced algorithms like the Walsh function to lower the amplitude of the overall signal and avoid affecting the normal operation of the electrochemical cells. In specific embodiments, multi-frequency signal generation may be realised by sweeping the signal frequency between a maximum frequency and a minimum frequency within a predetermined time period. The method S100 of Figure 1 may be performed by any suitable apparatus, for example an evaluation apparatus 200. An example of a suitable evaluation apparatus 200 is depicted in Figure 2. As depicted in Figure 2, the evaluation apparatus 200 performing the methods of embodiments may comprise a signal generation unit 202, an electrical signature measurement unit 204, and an evaluation unit 206. In specific embodiments, the signal generation unit 202 may be one of a potentiostat or galvanostat. Alternatively or additionally, the electrical signature measurement unit 204 may be one or more of: an ammeter, a voltmeter, and / or a potentiometer. Further alternatively or additionally, the evaluation unit 206 may be a computer. Accordingly, in specific embodiments an evaluation apparatus 200 may comprise a signal generation unit 202 configured to generate a perturbation signal to be passed through the electrochemical cell, wherein the perturbation signal is a multi-frequency signal. The evaluation apparatus 200 may further comprise an electrical signature measurement unit 204 configured to measure an electrical signature of the electrochemical cell, wherein the electrical signature is a voltage response and / or current response of the chemical cell that is induced by the passing of the perturbation signal through the electrochemical cell device. The evaluation apparatus 200 may additionally further comprise an evaluation unit 206 configured to evaluate a state of the electrochemical cell directly using the electrical signature. An example of an evaluation unit 206 suitable for use in the evaluation apparatus 200 is depicted in Figure 3. As shown in Figure 3, the evaluation unit 306 may comprise a processor or processing circuitry 306A, interfaces 306B, and a memory or non-transitory machine-readable medium 306C storing a computer program or instructions 306D. The steps of the method of present embodiments, for example the relevant steps as depicted in Figure 1, may be performed in accordance with the computer program 306D stored on the memory 306C, and may be executed by the processor 306A in conjunction with one or more interfaces 306B. In specific embodiments, the method may further comprise passing the perturbation signal through the electrochemical cell for a duration based on the minimum frequency forming the perturbation signal. For example, in specific embodiments the perturbation signal may be pulsed through the electrochemical cell. In such a case where the perturbation signal is a pulsed signal, the duration of each pulse may be limited by the lowest frequency component or minimum frequency forming the multi-frequency perturbation signal. For example, in a case where a frequency component of a multi-frequency perturbation signal was 1 Hz, an associated measurement for said component may need to be for at least 1s. Any measurements associated with higher frequency signals in the multi-frequency perturbation signal may be complete within that 1s period. However, were a frequency component of 0.25Hz to be included in the perturbation signal, a measurement period of 4s would be needed. Accordingly, a single pulse of the perturbation signal may have a time duration corresponding to the lowest frequency component or minimum frequency forming the multi-frequency perturbation signal. The sampling time, or duration of signal acquisition, associated with a perturbation signal may therefore be at least one full period of the lowest frequency forming the multi-frequency perturbation signal. This may ensure that the measured electrical signature includes all necessary data and electrical responses for evaluating the state of the electrical cell and / or improving the accuracy of the evaluation. In specific embodiments, the method may further comprise passing the perturbation signal through the electrochemical cell for a maximum duration of 1 second. Alternatively or additionally, for a pulsed perturbation signal the sampling rate associated with the perturbation signal may be at least double the maximum frequency forming the multifrequency perturbation signal. For example, in a perturbation signal where a frequency component of a multi-frequency perturbation signal was 1000Hz, the sampling rate may be at least 2000Hz. Such a sampling rate may reduce the likelihood of aliasing and may improve the accuracy of the reconstruction of the signal (for example, following the Nyquist Sampling Theorem). As discussed above, present embodiments may be applied to multiple use cases. Accordingly, in some embodiments the electrochemical cell may be one or more of: a newly formed cell, a partly-formed cell, and a used cell. For example, the method of present embodiments may be applied to a newly formed cell in order to compare newly formed cells within a batch (such as to determine batch consistency), and / or to determine the quality of or provide a quality score for the newly formed cell. Alternatively or additionally, the method of present embodiments may be applied to a partly-formed cell, for example to determine whether faults have occurred in the formation process. More specifically, the method of present embodiments may be used to evaluate external-film formation of one or more cells. Further alternatively or additionally, the method of present embodiments may be applied to used cells, for example to compare used cells (such as to determine whether the cells would be compatible with each other in a pack), to determine the quality of the used cell, provide a health score for the used cell, and / or track cell performance over time. Similarly, present embodiments may be applied in various industries and thus to different forms of electrochemical cell. In specific embodiments, the electrochemical cell may be one or more of: an electrolytic cell, a battery or other electrochemical power storage device, an electrolyser, a supercapacitor, a solar cell, and a fuel cell. Electrolytic cells may power devices such as carbon capture devices, peroxide synthesis devices and / or ammonia synthesis devices. Accordingly, present embodiments may be applied to any electrochemical cell for which EIS may be used to understand internal processes including: electrode kinetics, charge transfer, diffusion mechanisms, and other dynamic behaviours. In accordance with the above use cases, the method of specific embodiments may be performed while the electrochemical cell is in a steady state, a charging state, and / or a discharging state. Accordingly, the state of the electrochemical cell during which the method of present embodiments is performed may be selected based on the intended use case. Present embodiments allow for evaluation of a cell performance even when the cell is not in a steady state (that is, even when the cell is in full operation). That is, present embodiments may be implemented during the operation of the electrochemical cell. Present embodiments may therefore be used for “in situ” or “on site” use cases such as cell state tracking. As discussed above, present embodiments may be more versatile than known methods as they may be implemented on electrochemical cells during operation. Present embodiments directly analyse and process the electrical signature of the electrochemical cell. The electrical signature is the voltage response and / or current response of the electrochemical cell and thus the voltage / current response signal of the electrochemical cell is directly analysed or processed in the time domain and precise impedance calculation may not be required. Such direct processing may also remove the need for repeat measurements. Furthermore, the use of multi-frequency signals may allow for excitation to be performed in one second or less which may reduce the likelihood of the electrochemical cell undergoing changes during the evaluation method that may impact measurement results. By way of example, where a typical charge or discharge time for a cell is taken as one hour and a device in accordance with present embodiments has a one second time resolution, the state of the charge of the cell will only change by 1 / 3600th during each measurement period. Accordingly, the electrochemical cell may effectively be modelled as in a steady state operation during the measurement period. Such models may even be applied to high rates of charge / discharge (for example 10C or 6-minute discharge) due to the short measurement period of present embodiments. Present embodiments may be applied to one or more cells. In such embodiments comprising a plurality of cells, it will be understood that an electrical signature such as a voltage or current response may be collected for each individual electrochemical cell and / or several cells can be collected as a group and saved in a specific sample range. In particular an electrical signature may be collected for each individual electrochemical cell to provide improved resolution. An electrochemical cell may be collected for several cells connected as a group to reduce the number of sampling sensors needed and thus provide reduced cost. Accordingly, present embodiments may be applied to a plurality of connected cells, for example connected either in series or in parallel. For such embodiments, the method may further comprise measuring a plurality of electrical signatures for the plurality of cells, wherein each electrical signature is associated with one or more of the plurality of cells. In specific embodiments, each electrical signature may be associated with a corresponding cell of the plurality of cells. In further specific embodiments, the method may further comprise passing the perturbation signal through each of the plurality of cells concurrently and / or measuring the plurality of electrical signatures for the plurality of cells concurrently. Accordingly, it may be ensured that all cells receive the same perturbation signal. Further, the likelihood of differences in the perturbation signal and / or differences cell state across the plurality of cells (e.g. temperature differences, being charged / discharged to different levels) introducing erroneous excitation behaviours may be reduced. Such embodiments may comprise an electrical signature measurement unit corresponding to each individual cell, which may provide improved resolution. Various embodiments comprising a plurality of cells is depicted in Figure 4. That is, it will be understood that the excitation and response signals for each cell may be compared simultaneously. Alternatively or additionally, the excitation and response signals may be sampled simultaneously, for example to avoid inaccuracies in directly comparing the response signals. Such inaccuracies may arise, for example, due to inconsistencies in the excitation signals caused by the connection method. In specific embodiments, only cells with the same excitation signal may provide an accurate comparison when their response signals are directly compared. For example, in series-connected cells the voltage response signals may be directly compared under current perturbation. In contrast, in parallel-connected cells the current response signals may be directly compared under voltage perturbation. Figure 4A depicts an embodiment in which the method is applied to a plurality of cells that are not connected. As shown in Figure 4A, an electrical signature is obtained for each of the individual cells. That is, an individual electrical signature is obtained for each cell of the plurality of cells. An evaluation is obtained for each cell, based on the associated individual electrical signature for said cell. Figure 4B depicts an embodiment in which the method is applied to a plurality of cells connected in a pack. As shown in Figure 4B, a multi-channel electrical signature may be obtained. This multi-channel electrical signature may comprise measuring a plurality of electrical signatures for the plurality of cells, wherein each electrical signature is associated with one or more of the plurality of cells. An evaluation may then be obtained for the pack of cells, based on the multi-channel electrical signature. This evaluation may, for example, be a health score or quality assessment for the pack of cells (for example by comparing the multichannel electrical signature with a predetermined signature template). Alternatively, a difference between one or more of the electrical signatures may be calculated (for example to evaluate a consistency between the cells in the pack). In specific embodiments, a machine learning algorithm such as a cluster algorithm or classification algorithm may be trained and / or used to group batches of electrochemical cells based on their associated electrical signatures. Such grouping may be used, for example, to determine whether fresh or used cells are compatible for use in packs and / or to aid in pack manufacture. In particular, a battery pack that is formed from electrochemical cells that are consistent and / or more compatible with one another may provide a more reliable performance than a battery pack formed from randomly selected electrochemical cells. In specific embodiments as detailed above, present methods and apparatus may be applied to a plurality of cells forming one or more packs. The pack may, for example, be a battery pack. In more specific examples, the battery pack may be for use in an electric vehicle. Further, the method may be performed while the battery pack is installed in the electric vehicle. As present embodiments may be used during battery charging / discharging and do not impact normal operation, specific embodiments may comprise online deployments such as live battery health and / or performance monitoring, for example in an automotive battery management system. Figure 5 presents a schematic diagram of a multi-channel and multi-frequency electrical excitation response setup in accordance with embodiments, and example results obtained by said system. In this specific embodiment, a battery pack comprising eight cells (C1 through to C8) has their electrical signature measured, wherein the battery pack is from an automotive pack. The specific embodiment of Figure 5A comprises a GAMRY Reference 3000 acting as a potentiostat and / or galvanostat (that is, acting as a signal generation unit 202 as shown in Figure 2). However, it will be appreciated that other suitable signal generation units such as a commercial potentiostat may be used. This specific embodiment further comprises a computer acting as an evaluation unit 206 as shown in Figure 2, and a voltage input module acting as an electrical signature measurement unit 204 as shown in Figure 2. Figure 5B demonstrates the individual impedance spectrums for each of cells C1 to C8, wherein the EIS measurements span a frequency range from 1 Hz to 10000 Hz. The battery voltage under a multi-frequency signal perturbation includes both the DC component and the multi-frequency voltage response. In order to evaluate the similarity between EIS or voltage response curves, the Euclidean distance between two curves may be calculated, where a larger distance indicates greater dissimilarity. Figure 5C illustrates the corresponding multifrequency voltage responses (in one second) measured individually, with the DC component of the cell voltage subtracted. It may be seen from Figure 5B and Figure 5C that the individual multi-frequency voltage responses all follow the corresponding EIS measurements. However, the measurement time for each impedance spectrum in Figure 5B may take time on the order of several minutes, as the frequency needs to be scanned sequentially from 10,000 Hz to 1 Hz, whereas each multifrequency voltage response measurement in Figure 5C may take only 1 second. The total measurement time for all 8 cells is thus reduced to 8 seconds in a case where the cells are not connected, which is still significantly shorter than the time required for a single impedance spectrum. If the cells are connected and thus perturbed simultaneously, for example in series, the total measurement time for all 8 cells or any number of cells in series (here demonstrated with 8 cells) may be further reduced to a maximum of one second as the measurements may be taken simultaneously. That is, the electrical signature measurements may be taken simultaneously in an embodiment where the number of electrical signature measurement units in the apparatus (e.g. sensors) is greater than or equal to the number of cells / required electrical signature measurements. It will be understood that the specific embodiment comprises 8 cells, however a greater number of cells may be measured (for example, 10, 20, or 50 cells) where hardware requirements are met. Accordingly, present embodiment may provide methods and apparatus for evaluation, diagnosis, and / or monitoring of electrochemical cells which are faster than known methods and systems. In particular, the perturbation signal of present embodiments only needs a time scale on the order of several seconds to get a suitable multi-channel voltage response with a wide range of frequencies. Furthermore, present embodiment may provide methods and apparatus for evaluation, diagnosis, and / or monitoring of electrochemical cells which have a reduced cost compared to known methods and systems. In particular, analysing a voltage or current response directly as in present embodiments may remove the need for additional processing and hardware, and any associated cost. That is, present embodiments may provide a simpler hardware structure than known structures. The perturbation signal may also be generated at low cost using existing appropriate signal generation units. In addition, present embodiment may provide methods and apparatus for evaluation, diagnosis, and / or monitoring of electrochemical cells which have improved accuracy over known methods, as fewer processing steps are needed. For example, in specific embodiments the same perturbation signal may pass through all electrochemical cells connected in series concurrently, which may avoid errors arising from multiple executions. Further, present embodiments may not be sensitive to sensor location and / or environmental noise and thus may be more reliable and robust to noise than known apparatus. Present embodiments may be used as a grading tool for brand-new electrochemical devices, in real application or second-life recycling. Present embodiments may also be used during the normal operation of electrochemical devices and / or cells without interruption or destruction, and thus may provide improved versatility. References in the present disclosure to “one embodiment”, “an embodiment” and so on, indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. It should be understood that, although the terms “first”, “second” and so on may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of the disclosure. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed terms. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “has”, “having”, “includes” and / or “including”, when used herein, specify the presence of stated features, elements, and / or components, but do not preclude the presence or addition of one or more other features, elements, components and / or combinations thereof. The terms “connect”, “connects”, “connecting” and / or “connected” used herein cover the direct and / or 5 indirect connection between two elements. The present disclosure includes any novel feature or combination of features disclosed herein either explicitly or any generalization thereof. Various modifications and adaptations to the foregoing exemplary embodiments of this disclosure may become apparent to those skilled in 10 the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings. However, any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of this disclosure. For the avoidance of doubt, the scope of the disclosure is defined by the claims.

Claims

1. A method of evaluation of an electrochemical cell comprising:generating a perturbation signal, wherein the perturbation signal is a multifrequency signal;passing the perturbation signal through the electrochemical cell;measuring an electrical signature of the electrochemical cell, wherein the electrical signature is a voltage response and / or current response of the electrochemical cell that is induced by the passing of the perturbation signal through the electrochemical cell; andevaluating a state of the electrochemical cell directly using the electrical signature.

2. A method as claimed in Claim 1, wherein the method further comprises:comparing the electrical signature of the electrochemical cell with a predetermined electrical signature template.

3. A method as claimed in any preceding claim, wherein the method further comprises:comparing the electrical signature of the electrochemical cell with a further electrical signature of a further electrochemical cell.

4. A method as claimed in any preceding claim, wherein the method further includes:selecting the perturbation signal for generation based on the type of electrochemical cell.

5. A method as claimed in Claim 4, wherein the type of electrochemical cell is one of: a lithium cell, a sodium cell, a flow cell, and a metal-air cell.

6. A method as claimed in any of Claims 4 and 5 .wherein the method further includes a calibration process, the calibration process comprising:performing Electrochemical Impedance Spectroscopy, EIS, on a calibration electrochemical cell to obtain a frequency range, wherein the calibration electrochemical cell is of the same type as the electrochemical cell; andselecting a plurality of subranges of the frequency range to obtain perturbation frequency ranges; andusing the perturbation frequency ranges in the generation of the perturbationsignal.

7. A method as claimed in any preceding claim, wherein the method further comprises: passing the perturbation signal through the electrochemical cell for a duration based on the minimum frequency forming the perturbation signal.

8. A method as claimed in Claim 7, wherein the method further comprises: passing the perturbation signal through the electrochemical cell for a maximumduration of 1 second.

9. A method as claimed in any preceding claim, wherein the electrochemical cell is one or more of: a newly formed cell, a partly-formed cell, and a used cell.

10. A method as claimed in any preceding claim, wherein the electrochemical cell is one or more of: an electrolytic cell, a battery, an electrolyser, a supercapacitor, a solar cell, and a fuel cell.

11. A method as claimed in any preceding claim, wherein the electrochemical cell is one of: a plurality of cells connected in series, or a plurality of cells connected in parallel, and wherein the method further comprises:measuring a plurality of electrical signatures for the plurality of cells, wherein each electrical signature is associated with one or more of the plurality of cells.

12. A method as claimed in Claim 11, wherein each electrical signature is associated with a corresponding cell of the plurality of cells.

13. A method as claimed in any of Claims 11 and 12, wherein the method further comprises: passing the perturbation signal through each of the plurality of cells concurrently.

14. A method as claimed in any of Claims 11 to 13, wherein the method further comprises: measuring the plurality of electrical signatures for the plurality of cells concurrently.

15. A method as claimed in any of Claims 11 to 14, wherein the plurality of cells form a battery pack.

16. A method as claimed in Claim 15, wherein the battery pack is for use in an electric vehicle, optionally wherein the method is performed while the battery pack is installed in the electric vehicle.

17. A method as claimed in any of Claims 1 to 16, wherein the method is performed while the electrochemical cell is in a steady state, a charging state, and / or a discharging state.

18. An apparatus for evaluating an electrochemical cell, wherein the apparatus comprises:a signal generation unit configured to generate a perturbation signal to be passed through the electrochemical cell, wherein the perturbation signal is a multi-frequency signal;an electrical response measurement unit configured to measure an electrical response of the electrochemical cell, wherein the electrical response is a voltage response and / or current response of the chemical cell that is induced by the passing of the perturbation signal through the electrochemical cell device; andan evaluation unit configured to evaluate a state of the electrochemical cell directly using the electrical response.

19. An apparatus as claimed in Claim 18, wherein the apparatus is further configured to perform the method of any of Claims 2 to 17.

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