METHOD FOR DETECTING LITHIUM FORMATION ON AN ELECTRODE IN A LITHIUM-ION BATTERY

Abstract

The invention relates to a method for detecting lithium formation in a lithium-ion battery for at least one charging regime 1a, called "basic regime", comprising the steps of: applying the basic regime 1a, having a first current I1a, and a charging regime 2a, called "secondary regime", having a second current I2a; acquiring the voltage and the charge quantity Q of the battery, during the basic regime 1a and the secondary regime 2a; determining a ratio between on the one hand the difference between the voltage in the basic regime 1a and that in the secondary regime 2a, and on the other hand the difference between the first charging current I1a and the second charging current I2a, as a function of the charge quantity Q; and detecting the appearance of lithium as a function of said ratio. Figure 2

Description

Title of the invention: METHOD FOR DETECTING A FORMATION OF LITHIUM ON AN ELECTRODE IN A LITHIUM-ION BATTERY

[0001] The invention relates to a method for detecting a formation of lithium, in particular metallic lithium, on an electrode in a lithium-ion battery. The invention further relates to a lithium-ion battery management system configured to implement the method according to the invention.

[0002] In the general context of decarbonization of transport, there is an increasingly significant demand for electric vehicles that are both quickly rechargeable and have a long range. This demand concerns both light vehicles, such as bicycles, scooters, or other motorized two-wheelers, as well as heavy vehicles, for example cars of all sizes, or trucks. Lithium-ion battery technology (or "Li-ion batteries") is currently the most promising thanks to its performance in terms of mass or volume energy density and power.

[0003] Lithium-ion batteries operate on the principle of the alternating intercalation of lithium ions between a material forming a positive electrode and a material forming a negative electrode. Typically, in normal operation, the positive electrode has a high electrical potential compared to that of metallic lithium; and the negative electrode has an electrical potential close to that of metallic lithium, while remaining higher. The lithium ions flow from the positive electrode to the negative electrode, to be integrated into the latter.

[0004] However, during rapid charging of lithium-ion batteries, the potential of the negative electrode is likely to fall lower than that of metallic lithium: the lithium ions then no longer integrate into the negative electrode and are deposited on its surface. An unwanted deposit of metallic lithium forms on the negative electrode, in particular in the form of dendrites. This phenomenon of lithium formation (or "lithium plating" in English) causes serious damage to the battery if it occurs frequently during charging. Thus, in practice, an accelerated loss of battery capacity is observed, and a higher probability of short circuits between the positive and negative electrodes. It is therefore crucial, in particular for manufacturers of electric vehicles, to be able to detect the formation of lithium on the negative electrode in Li-ion batteries throughout their life.This is particularly in order to adjust the fast charging programs to avoid the formation of lithium.

[0005] A method for real-time detection of the phenomenon of lithium deposition on the negative electrode is known. It is notably described in patent publication US 10,126,367 B2. The method is based on the derivative of the charging potential (or "charging voltage derivative" in English) and consists of monitoring during a charge at a constant charge rate, the variation dU / dQ of the voltage U at the terminals of the battery per unit of charge exchanged as a function of the quantity of charge Q stored in the Li-ion battery. The appearance of a peak on the dU / dQ curve indicates the start of a lithium deposition on the negative electrode. However, this method is effective on Li-ion batteries having an open circuit voltage or OCV (for "Open Circuit Voltage" in English) which is regular. This characteristic is strongly linked to the material of the positive electrode.Thus, this is the case for Li-ion batteries whose positive electrode is made of NMC 622, i.e. comprising 60% nickel, 20% manganese and 20% cobalt. On the other hand, Li-ion batteries whose positive electrode is made of certain materials can present several strong variations in slope in the dU / dQ curve making it difficult to identify the significant peak of lithium formation. This is particularly the case when the positive electrode is made of NMC 811. [Fig.l] represents curves acquired on a Li-ion battery having a positive electrode made of NMC 811 and a negative electrode made of graphite on a full charge for different charge regimes. The curves are acquired for charge regimes having a charge rate of 0.5C, 0.75C, IC, 1.25C and 1.5C. An IC charge rate corresponds to a charge current by which full charge is reached after 1h.A charge rate of 0.5C corresponds to a charge current by which full charge is reached after 1 / 0.5h, or 2h. Lithium formation corresponds to the first P peak of each curve. However, the dU / dQ curve is subject to several other phenomena than lithium formation, in particular to natural variations in the equilibrium potential (open circuit voltage (OCV)) of the positive electrode material; which is reflected in particular by the two peaks appearing in the second part of the curve. This can make it difficult to identify the P peak significant for lithium formation. Furthermore, this identification is only sufficiently precise from a charge rate higher than IC.

[0006] A method for detecting the formation of lithium on the negative electrode during charging of a Li-ion battery is therefore sought which is sufficiently precise, in particular for Li-ion batteries such that their OCV is irregular.

[0007] To this end, the invention proposes a method for detecting lithium formation on an electrode in a lithium-ion battery, comprising the detection of lithium formation for at least one charging regime, called “basic regime”, comprising the steps consisting of: i. apply the basic regime, having a first load current, and a regime charging, called “secondary regime”, having a second charging current, the absolute value of the difference between the charging rate in the basic regime and the charging rate in the secondary regime being less than a threshold; ii. acquire the battery terminal voltage and the battery charge quantity, during the basic regime and the secondary regime; iii. determining a ratio between, on the one hand, the difference between the voltage at the terminals of the battery in the basic regime and that in the secondary regime, and on the other hand the difference between the first charging current and the second charging current, as a function of the quantity of charge; and iv. for the basic regime, detect the appearance of lithium formation based on the value of said ratio.

[0008] Said ratio has the advantage of being relatively constant at the start of a charge, in particular when the charge begins from a small quantity of charges stored in the battery, then of collapsing when there is formation of lithium on the negative electrode. This strong variation of the ratio makes it possible to better identify the appearance of a lithium deposit. The variation of said ratio has the advantage of being more regular than that of the dU / dQ curve used in the prior art. Furthermore, the method according to the invention is effective, whatever the OCV of the material of the positive electrode.

[0009] According to one embodiment, the appearance of a lithium deposit corresponds to a last maximum, in particular a last local maximum, of said ratio, preceding a fall of at least 10% of said ratio.

[0010] According to one embodiment, the absolute value of the difference between the load rate of the basic regime and the load rate of the secondary regime is less than 0.5 and preferably less than 0.25.

[0011] According to one embodiment, the application of the basic regime and the secondary regime is carried out alternately during a charge of the lithium-ion battery.

[0012] According to a variant, the secondary regime is applied over negligible durations compared to the durations over which the basic regime is applied.

[0013] According to a variant, the method comprises the emission of a lithium formation detection signal when the value of the ratio becomes less than or equal to a threshold.

[0014] According to a variant, said threshold is a function of the quantity of charge Q of the battery and is equal to:

[0015] rs(Q) = aR0(1-0.05-^-)

[0016] where Qtot is the maximum charge quantity of the battery in mAh, Ro is the initial value of the ratio when charging the battery from a state of charge less than 0.1Qtot, a is a constant between 0.75 and 0.95.

[0017] According to one variant, the method comprises: i. when the battery is charged from a state of charge less than or equal to 0.1Qtot: - the determination of the parameter Ro by calculating the average of said ratio for load quantities less than or equal to 0.2Qtot; - storing the Ro parameter in a memory; and - the use of the Ro parameter stored in memory to determine it reduction of the threshold rs(Q) for charge quantities greater than 0.2Q early? ii. or, when charging the battery from a state of charge greater than 0.1Qtot, using the last Ro parameter stored in memory for the detection of lithium formation during charging.

[0018] According to one variant, the method comprises: i. the comparison between the last two values ​​of the Ro parameter each determined during a charge of the battery from a state of charge less than or equal to 0.1Qtot, and ii. when the difference between the last two values ​​of the Ro parameter is greater than a threshold, the emission of a battery fault signal.

[0019] According to a variant, the charging of the lithium-ion battery is a rapid charge having decreasing charging current levels, the method comprising the detection of the formation of lithium for a plurality of basic regimes, each basic regime corresponding to one of said current levels.

[0020] According to one embodiment: i. the application of the basic regime is carried out on a first complete charge of the battery, the voltage at the battery terminals being acquired as a function of the quantity of charge of the battery, ii. the application of the secondary regime is carried out on a second full charge of the battery, the voltage at the battery terminals being acquired as a function of the quantity of charge of the battery.

[0021] According to one variant, the method comprises: i. the detection of a lithium deposit for a plurality of basic regimes; ii. the storage in a memory of a table comprising, for each of the plurality of basic regime, the respective charge quantity from which a lithium deposit appears; and iii. upon subsequent charging of the battery, using said table to detect the occurrence of lithium formation on an electrode as a function of the charging current applied and the amount of battery charge.

[0022] Lithium-ion battery management system configured to implement a method for detecting a formation of lithium on an electrode according to the invention at during a lithium-ion battery charge.

[0023] Other characteristics and advantages of the present invention will appear more clearly on reading the description which follows in relation to the following appended figures:

[0024] [Fig-1]: [Fig.l], already described, illustrates an example of a method according to the prior art;

[0025] [Fig.2]: [Fig.2] represents a current delivered to a Li-ion battery in a first example of a method according to the invention;

[0026] [Fig.3]: [Fig.3] represents the voltage at the terminals of the battery in the first example of a method according to the invention;

[0027] [Fig.4]: [Fig.4] represents the ratio R in the first example of the method according to the invention;

[0028] [Fig.5]: [Fig.5] represents a current delivered to a Li-ion battery in a example of a method according to the invention;

[0029] [Fig.6]: [Fig.6] represents the voltage at the terminals of the battery in the example of process illustrated in [Fig.5];

[0030] [Fig.7]: [Fig.7] represents the ratio R in several examples of the process according to the invention;

[0031] [Fig.8]: [Fig.8] represents the negative potential of the battery in several examples of the method according to the invention;

[0032] [Fig.9]: [Fig.9] represents a current delivered to a Li-ion battery in a example of a method according to the invention;

[0033] [Fig. 10]: [Fig. 10] represents the voltage at the terminals of the battery as a function of the quantity of charges stored, for several examples of the method according to the invention;

[0034] [Fig. 11]: [Fig. 11] represents the ratio R for several examples of the method according to the invention;

[0035] [Fig. 12]: [Fig. 12] represents the negative potential of the battery for several examples of the method according to the invention.

[0036] A first example of a method according to the invention will be described in relation to Figures 2 to 4. The method makes it possible to detect the formation of lithium, in particular metallic lithium, on a negative electrode of a lithium ion battery.

[0037] For this purpose, the method comprises detecting the formation of lithium on a charging regime, called basic regime 1a, having a first charging current Iu. As for example illustrated in [Fig.2], the basic regime 1a is applied to the Li-ion battery with the first charging current Ila. Another charging regime, called "secondary regime" 2a, having a charging current I2a is further applied to the Li-ion battery. In [Fig.2], the y-axis represents the values ​​of the current i(t) in mA and the values ​​of the charge quantity Q(t) in mAh.

[0038] The secondary regime 2a is chosen so that the load rate C1 of the basic regime 1a and the load rate C2 of the secondary regime 2a are close. In this respect, the

[0039]

[0040]

[0041]

[0042]

[0043] secondary regime 2a may have a higher or lower charging current I2a than the charging current Iu in the basic regime la. In other words, the basic regime la and the secondary regime 2a are chosen so that the absolute value of the difference C2-C1 between the charging rate Cl in the basic regime la and the charging rate C2 in the secondary regime 2a is less than a threshold. As for example illustrated in [Fig.3], the voltage Uia at the terminals of the battery is acquired during the basic regime la, as is the voltage U2a at the terminals of the battery during the secondary regime 2a. As for example illustrated in [Fig.2], the charge quantity Q of the battery is also acquired, in particular in parallel with the acquisition of the voltages Uia, U2a at the terminals of the battery during the basic regime la and the secondary regime 2a. From the acquired values, for a state of charge Q of the battery, a ratio R(Q) is determined between, on the one hand, the difference AU between the voltage U[a at the terminals of the battery during the basic regime la and the voltage U2a at the terminals of the battery during the secondary regime 2a, and on the other hand the difference AI between the first charging current I[a and the second charging current I2a: n _ AU _ 20^ ia AI ~ [Fig.4] illustrates the evolution of the ratio R as a function of the quantity of charges Q of the battery. The shape of the graph makes it possible to detect the appearance of a lithium deposit on the negative electrode. In particular, when the quantity of charges Q increases, the curve remains relatively stable, in particular to within 0.1x10 3Q, before falling relatively sharply, which is significant of a formation of lithium on the negative electrode of the battery. Thus, the quantity of charges for which there is lithium formation is determined as a function of the value of the ratio R. In particular, the amount of charge for which a lithium deposit appears corresponds to a last maximum, in particular a last local maximum, of the ratio R preceding a fall of at least 10% of said ratio. By considering a low fall value, the formation of lithium can be detected as early as possible, but there is then the risk of false detections due to intrinsic variations in the ratio curve. By taking a higher fall value, a false detection can be avoided, but possibly to the detriment of the accuracy of the detection of lithium formation. The charge rate Cl of the basic regime 1a and that C2 of the secondary regime 2a are preferably close so that the detection of lithium formation by the ratio R is significant. For example, the absolute value of the difference between the charge rate Cl of the basic regime 1a and the charge rate C2 of the secondary regime 2a is less than or equal to 0.5 and preferably less than 0.25. However, the absolute value of the difference between the charge rate Cl of the basic regime 1a and the charge rate C2 of the secondary regime 2a is preferably large enough to give a significant result, taking into account the precision with which the voltages Uia U2a are acquired.

[0044] The first example 3 of the method is according to a first embodiment in which the basic regime 1a and the secondary regime 2a are applied alternately during a charge of the battery. In other words, during the same charge, the basic regime 1a and the secondary regime 2a are applied successively. Thus, by monitoring the ratio R between the basic regime 1a and the secondary regime 2a, it is possible to detect the appearance of a lithium deposit during the charge itself. In particular, the method according to this embodiment allows real-time detection of the appearance of a lithium deposit during the charge. In particular, in the context of the present application, the term "charge" means the uninterrupted application of an electric current to the terminals of the Li-ion battery in order to increase the quantity of charges stored in the battery.

[0045] In particular, the secondary regime 2a is applied over negligible durations compared to the durations over which the basic regime 1a is applied. In particular, the basic regime 1a is a nominal charging regime. The secondary regime 2a is therefore sufficiently short so as not to disturb the charging of the battery with the basic regime. In particular, the ratio between a duration of application of the secondary regime and a duration of application of the basic regime is less than or equal to 102. The application of the secondary regime 2a takes in particular the form of pulses during the basic regime 1a. For example, an occurrence of the secondary regime 2a has a duration of between 1s and 10s. Thus, in [Fig.2], the duration of a pulse corresponding to the secondary regime 2a is for example equal to 5s.

[0046] Preferably, the secondary regime 2a is applied periodically. Thus, the ratio R can be determined regularly during charging, and then allow regular detection of a lithium deposition on the negative electrode. For example, the secondary regime 2a is applied with a period Ts of between 50s and 1000s. In particular, a period Ts shorter than 50s could lead to disturbances of the basic regime 1a, while periods greater than 1000s would not allow sampling precise enough to detect the moment of lithium deposition.

[0047] Preferably, the difference between the charge rate C1 of the basic regime 1a and the charge rate C2 of the secondary regime 2a is less than or equal to 0.5 and preferably less than 0.25. Thus, in [Fig.2], the difference AI between the charge current I2a in the secondary regime 2a and the charge current 11a in the basic regime 1a is for example equal to 10mA.

[0048] As for example illustrated in [Fig.3], each application of the secondary regime 2a induces a voltage U2a greater than the voltage Uia of the basic regime la. By calculating the voltage difference AU between the value of the voltage U[a before the application of the secondary regime 2a and the value of the voltage U2a during the application of the secondary regime 2a, and by relating it to the difference between the charging current I2a in the secondary regime 2a and the charging current Iu in the basic regime la, we obtain the ratio illustrated in [Fig.4]. To avoid too long an application of the secondary regime 2a and thus a disturbance of the battery charge, only one voltage value U2a is preferably measured per application of the secondary regime 2a. For example, the measurement is carried out one second after the application of the secondary regime 2a. However, one could consider taking the average of several measurements of the voltage U2a.

[0049] In [Fig.7], the ratio R is shown in the first process example 3 with a base regime 1a having a charge rate of IC and a secondary regime 2a having a charge rate of 1.25C. For comparison, the ratio R is also shown in a second process example 4 according to the first embodiment with the base regime having a charge rate of 0.75C and the secondary regime 2a having a charge rate of IC; and the ratio R in a fast charge example 5 described later. The second process example 4 according to the first embodiment is identical to the first example 3, except for the charge rates in the base regime and the secondary regime.

[0050] By comparing the curves with each other, it can be seen in particular that the higher the charge rate, the more the drop in the R ratio indicating lithium formation is marked and appears earlier. In particular, the drop in the R ratio remains sufficiently marked for a basic regime having a charge rate greater than or equal to a threshold, for example 0.75C, in particular for an example of a battery of the NMC 811-graphite type. However, the minimum charge rate for which the drop in the R ratio remains significant depends on the type of Li-ion battery and its state of aging.

[0051] In particular, the battery is capable of receiving a maximum charge quantity Qtot. In particular, for quantities of stored charges Q less than 0.2Qtot, the ratio R is substantially constant for a given charge quantity regardless of the charging regime considered. This is particularly the case under normal operating conditions, i.e. outside of any lithium formation on the negative electrode. For example in [Fig.7], it is thus observed that for a quantity of stored charges less than 0.2Qtot, the ratios R are relatively close. It is further observed that the ratios R decrease slightly on average from an initial value Ro, for example 4.2x10 3Q, regardless of the charging regime considered, before reaching the lithium formation zone.

[0052] Such a decrease is for example represented in [Fig.4] and 7 by a straight line r, for example with the equation:

[0053] r ( Q) = R{ 1-0.05^}

[0054] Where Qtot is the maximum charge quantity of the battery in mAh, Ro is the initial value of the ratio when charging the battery from a state of charge less than 0.1Qtot.

[0055] When lithium begins to form on the negative electrode, there is a relatively sharp drop in the ratio R. The last maximum before the drop in the ratio R is notably around 8 mAh for the first method example 3. However, to avoid false detections, it is possible to choose to trigger the emission of a lithium formation detection signal when the value of the ratio R becomes less than or equal to a threshold rs. The threshold rs is for example defined by the expression:

[0056] fj .(0 =<zRo (ioo5rq

[0057] Where a is a constant between 0.75 and 0.95.

[0058] Thus, the threshold rs is in particular a translation of the straight line r corresponding to the shape of the ratio R under normal operating conditions, with a reducing factor a. For example, the reducing factor a is equal to 0.90. In particular, crossing the threshold rs will cause the emission of a signal detecting a lithium deposit on the negative electrode. In the event of heating of the battery during charging, the shape of the ratio R may vary, the threshold rs can then be adapted. In particular, heating of the battery results in a greater average decreasing slope of the ratio R before the lithium formation zone. However, a sudden drop in the ratio R will still be significant of lithium formation. The threshold rs is preferably corrected to take heating into account.

[0059] The detection of the appearance of a lithium deposit can be done by a means other than crossing a threshold. For example, it can be done by determining the local slope of the ratio R, in particular after filtering the noise. Or, by the detection by an artificial intelligence of an S-shaped profile of the ratio R, said profile being significant of the appearance of the lithium deposit.

[0060] The initial value Ro of the ratio R can be determined during charging. This is particularly the case when charging starts from a battery charge quantity that is less than 0.1Qtot. The initial value Ro can then correspond to the average of the ratio R for charge quantities less than or equal to 0.2Qtot. The initial value Ro thus determined is then stored in a memory, particularly a memory of a battery management system, to be used in determining the threshold rs for charge quantities greater than 0.2Qtot.

[0061] Alternatively, the initial value Ro used in the load may be a value stored in memory, in particular in a memory of a management system battery. This is particularly the case when the battery is charged from a battery charge quantity that is greater than 0.1Qtot. In particular, the initial value Ro then corresponds to the last value stored in memory during a previous charge from a charge quantity less than 0.1Qtot.

[0062] The determination of the initial value Ro can be carried out at each charge of the battery starting from an initial quantity of stored charges less than or equal to 0.1Qtot. The determination of the initial value Ro can also be carried out every X charges starting from an initial quantity of stored charges less than or equal to 0.1Qtot, X being a natural integer.

[0063] The change in the initial value Ro during battery charging can be significant of battery aging. For example, the last two values ​​of the parameter Ro, each determined during a battery charge, can be compared. A large difference between these last two values ​​of the parameter Ro can be significant of a sudden and significant deterioration of the battery. Thus, it can be predicted that when the difference between the last two values ​​of the parameter Ro is greater than a threshold, a battery fault signal is emitted, in particular by a battery management system.

[0064] The first method example 3 has been described with a basic regime 1a, having a constant first charging current 1¼, and a secondary regime 2a having a constant charging current I2a, as for example illustrated in [Fig.2]. However, the basic regime 1a could have a variable charging current Iu, as well as the secondary regime 2a, with a difference C2-C1 between the charging rate C1 of the basic regime 1a and the charging rate C2 of the secondary regime 2a remaining below a threshold as described previously, and in particular remaining constant.

[0065] Figures 5 and 6 show a third example of fast charging 5, in which the charging current varies during charging. The example of fast charging 5 is notably implemented on a Li-ion battery of the NMC 811-graphite type. In [Fig.5], the y-axis represents the values ​​of the current i(t) in mA and the values ​​of the charge quantity Q(t) in mAh. In particular, as for example illustrated in [Fig.5], the charging starts with a relatively high current, then the current is reduced in stages during charging. Such a charging profile makes it possible to adapt the charging current to avoid lithium formation on the negative electrode. The charging current profile can be determined at the beginning of the battery life. However, over time such a current profile may become ineffective due to the aging of the battery.The method according to the invention can be applied for each current level and thus make it possible to verify in real time that lithium formation is not taking place, and thus make it possible to verify that the charging current profile of the fast charge 5 is always effective in avoiding lithium formation in the Li-ion battery.

[0066] In particular, the third method example is applied with a first basic regime 1a, having a constant charging current Iu corresponding to the first current level. Then, the third method example is applied with a second basic regime 1b, having a constant charging current Iib corresponding to the second current level, and so on for the other current level(s).

[0067] The third method example can be implemented with a secondary regime 2a having a charging current I2a greater than the charging current Iu of the basic regime 1a, or with a secondary regime 2a' having a charging current I2a- less than the charging current Iu of the basic regime 1a. In particular, charging of the battery can be implemented with the third method example using only positive pulses, i.e. with a secondary regime 2a having a charging current greater than that of the basic regime 1a; or only negative pulses, i.e. with a secondary regime 2a' having a charging current I2a- less than that of the basic regime 1a; or both.

[0068] [Fig. 6] represents the voltage U at the terminals of the battery. The voltage increases progressively during charging, with jumps corresponding to the changes in current level. The voltage exhibits other variations corresponding to the application of the secondary regimes. In particular, positive current pulses induce a voltage U2a in the secondary regime 2a; negative current pulses induce a voltage U2a' in the secondary regime 2a'. Thus, in particular, each application of the secondary regime 2a induces a voltage U2a higher than the voltage Uia of the basic regime 1a or a voltage U2a- lower than the voltage Uia of the basic regime 1a, depending on the polarity of the pulse.

[0069] [Fig.7] represents the evolution of the ratio R as a function of the battery charge for the fast charge example 5. For this graph, only the positive pulses are taken into account, the results being similar for the negative pulses. It can be seen that curve 5 only reaches the threshold rs at the end of charging, indicating however that the last current level should be adjusted to avoid the formation of lithium on the negative electrode.

[0070] [Fig.8] represents the potential Un of the negative electrode during charging for the fast charging example 5 and the first example 3 of the method according to the first embodiment with a charging rate of IC for the basic regime, and the second example 4 of the method according to the first embodiment with a charging rate of 0.75C for the basic regime. The passage of the potential Un of the negative electrode below the zero value is indicative of the appearance of a lithium deposit on the negative electrode. It can be seen that the passage below the zero value corresponds to the last maximum of the ratio R, preceding a sharp drop in the latter. In normal use of a battery, the potential Un of the negative electrode is gener- not normally accessible. In particular, achieving such a potential requires an invasive device in the battery, such as a reference electrode, which impacts the battery's footprint. In addition, its reliability over its lifetime may no longer be guaranteed.

[0071] Another example of a method according to the invention will now be described in relation to Figures 9 to 12. These figures show graphs acquired on a Li-ion battery of the NMC 811-graphite type. However, similar results can be obtained on a Li-ion battery of another type. The method makes it possible to detect the formation of lithium, in particular metallic lithium, on a negative electrode of a lithium-ion battery.

[0072] For this purpose, the method comprises detecting the formation of lithium on a charging regime, called "basic regime" 1a. As for example illustrated in [Fig.9], the basic regime 1a is applied to the Li-ion battery with the first constant charging current 1a. Another charging regime, called "secondary regime" 1a, having a constant charging current 1uA is further applied to the Li-ion battery.

[0073] The secondary regime 11a is chosen so that the charge rate C1 of the base regime 1a and the charge rate C2 of the secondary regime 11a are close. In this respect, the secondary regime 11a may have a higher or lower charge current I2a than the charge current I[a in the base regime 1a. In other words, the base regime 1a and the secondary regime 11a are chosen so that the absolute value of the difference C2-C1 between the charge rate C1 in the base regime 1a and the charge rate C2 in the secondary regime 11a is less than a threshold. For example, in [Fig.9], the base regime 1a has a charge rate of IC and the secondary regime 11a has a charge rate of 1.25C.

[0074] The voltage Ula at the terminals of the Li-ion battery is acquired during the basic regime la, as is the voltage Ulla at the terminals of the battery during the secondary regime lia. The charge quantity Q of the battery is also acquired, in particular in parallel with the acquisition of the voltages Ula, Ulla at the terminals of the battery during the basic regime la and the secondary regime lia. As for example illustrated in [Fig. 10], the voltage Ula at the terminals of the battery during the basic regime la and the voltage Ulla at the terminals of the battery during the secondary regime lia are obtained in particular, as a function of the charge Q of the battery.From the acquired values, we determine, for a state of charge Q of the battery, a ratio R(Q) between on the one hand the difference AU between the voltage Uia at the terminals of the battery during the basic regime la and the voltage Uua at the terminals of the battery during the secondary regime lia, and on the other hand the difference AI between the first charging current F, and the second charging current Ina: .

[0075] AU _ UIlaUIa AI hla-Iln

[0076] [Fig. 11] illustrates the evolution of the ratio R as a function of the charge Q of the battery. Each curve is identified by the charge rate of the basic regime used to determine the ratio R. The shape of the graph makes it possible to detect the formation of a lithium deposit on the negative electrode for the basic regime. In particular, towards an increasing charge quantity, the curve corresponding to the basic regime IC undergoes variations before reaching a local maximum P, then falling relatively sharply, which is significant of a formation of lithium on the negative electrode of the battery. Thus, the charge quantity from which there is lithium formation can be determined as a function of the value of the ratio R.

[0077] In particular, the quantity of charge for which a lithium deposit appears corresponds to a last maximum, in particular a last local maximum, of the ratio R preceding a drop of at least 10% of said ratio. By considering a low drop value, the formation of lithium can be detected as early as possible, but there is then the risk of false detections due to intrinsic variations in the ratio curve. By taking a higher drop value, a false detection can be avoided, but possibly to the detriment of the accuracy of the detection of the formation of lithium.

[0078] The charge rate Cl of the basic regime 1a and that C2 of the secondary regime 1a are preferably close so that the detection of the formation of lithium by the ratio R is significant. For example, the absolute value of the difference between the charge rate Cl of the basic regime 1a and the charge rate C2 of the secondary regime 1a is less than 0.5 and preferably less than 0.25. However, the absolute value of the difference between the charge rate Cl of the basic regime 1a and the charge rate C2 of the secondary regime 1a is preferably large enough to give a significant result, taking into account the precision with which the voltages Ula, UUa are acquired.

[0079] The exemplary method is according to a second embodiment in which the basic regime 1a is applied to a first complete charge of the battery; and the secondary regime 1a is applied to a second complete charge of the battery. In other words, in the second embodiment, the basic regime 1a and the secondary regime 1a are applied during different charges. On the first charge, the voltage Ula at the terminals of the battery is acquired as a function of the charge quantity Q of the battery. On the second charge, the voltage Ulla at the terminals of the battery is acquired as a function of the charge quantity Q of the battery. The method according to the second embodiment is particularly advantageous when it is possible to carry out a calibration of the Li-ion battery, either at the end of the battery production line, or on a vehicle test bench when the battery is mounted therein.Thus, with the method according to the second embodiment, it is possible to determine for . the basic rate, the battery charge from which the lithium deposit appears. This information can then be stored in a memory, in particular in a memory of a battery management system, for use during the life of the battery. For example, the information can be used to stop or change the charging rate, when the battery is charged with the basic rate and the battery charge reaches the previously identified charge, from which a lithium deposit is formed.

[0080] Preferably, the exemplary method is implemented for several basic regimes. For example, [Fig. 10] represents the voltage at the terminals of the Li-ion battery as a function of the quantity of charge Q stored in the battery, for several charging regimes. In particular, [Fig. 10] illustrates the charging regimes having charge rates of 0.1C, 0.5C, 0.75C, IC, 1.25C and 1.5C. For each charging regime applied, the ratio R is determined by taking as the basic regime the charging regime itself, and as the secondary regime, the lower or higher charging regime having the closest charge rate. For example, the ratio R for each charging regime illustrated in [Fig. 10], is represented in [Fig. 11], with the exception of the regime having a charge rate of 0.1C.

[0081] In particular, the exemplary method for detecting lithium formation is effective for a charging regime having a charging rate greater than or equal to a threshold. For example, in [Fig. 11], the ratio R for the 0.5C charging regime does not exhibit a sufficiently marked drop in the ratio for it to be significant, unlike the 0.75C, IC, 1.25C, and 1.5C charging regimes. In particular, the efficiency threshold for an exemplary Li-ion battery of the NNLIC 811-graphite type is 0.75C, whereas with the prior art method illustrated in [Fig.l], detection of lithium formation is only possible for a charging regime having a charging rate greater than or equal to IC. The exemplary method according to the invention therefore allows detection of lithium formation for a wider range of charging regimes than in the prior art.However, this efficiency threshold depends on the architecture of the battery, in particular the composition of the electrodes, and its state of aging.

[0082] For comparison, [Fig. 12] represents the potential Un of the negative electrode during charging for the charging regimes illustrated in [Fig. 10]. The passage of the potential Un of the negative electrode below the zero value is indicative of the start of lithium formation on the negative electrode. It can be seen that the passage below the zero value corresponds to the last maximum of the ratio R, preceding a sharp drop in it.

[0083] Thus, for each of the applied load regimes, we obtain from the shape of the ratio R, the respective quantity of load from which there is formation of a lithium deposit. This information can be stored in table form in a memory, particularly a memory of a battery management system.

[0084] In particular, during a subsequent charge of the battery, the information stored in the table makes it possible to determine the occurrence of lithium formation as a function of the charge current, i.e. the charge rate, and the quantity of charge stored in the battery. Thus, before reaching the quantity of charge from which lithium formation occurs, a change can be made to a more advantageous charge regime, i.e. a charge regime for which the quantity of charge for which lithium formation occurs is higher.

[0085] The graphs illustrated in the figures are acquired at constant temperature, such as 25°C. However, the temperature could be variable while still allowing application of the method according to the invention. The example methods have been described with loading rate values ​​which are illustrative. Other loading rate values ​​can be used while remaining within the spirit of the invention.

Claims

Claims

1. A method for detecting lithium formation on an electrode in a lithium-ion battery, comprising detecting lithium formation for at least one charging regime 1a, 1a, called the "basic regime", comprising the steps of: i. applying the basic regime 1a, 1a, having a first charging current I[a, Ila, and a charging regime 2a, 1a, called the "secondary regime", having a second charging current I2a, Iua, the absolute value of the difference between the charging rate C1 in the basic regime 1a, 1a and the charging rate C2 in the secondary regime 2a, 1a being less than a threshold; ii. acquiring the voltage U across the battery terminals and the charge quantity Q of the battery, during the basic regime 1a, 1a and the secondary regime 2a, 1a; iii.determining a ratio R between, on the one hand, the difference AU between the voltage U[a, Ula at the terminals of the battery in the basic regime la, la and that U2a, UUa in the secondary regime 2a, lia, and on the other hand the difference AI between the first charging current Iu, Ila and the second charging current I2a, Iua, as a function of the quantity of charge Q; and iv. for the basic regime la, la, detecting the appearance of lithium formation as a function of the value of said ratio R.

2. Method according to claim 1, in which the appearance of a lithium deposit corresponds to a last maximum P, in particular a last local maximum, of said ratio R, preceding a fall of at least 10% of said ratio R.

3. Method according to claim 1 or 2, in which the absolute value of the difference between the loading rate C1 of the basic regime 1a, 1a and the loading rate C2 of the secondary regime 2a, 1a is less than 0.5 and preferably less than 0.

25.

4. Method according to one of the preceding claims, in which the application of the basic regime 1a and the secondary regime 2a is carried out alternately during a charge of the lithium-ion battery.

5. Method according to the preceding claim, in which the secondary regime 2a is applied over negligible durations compared to the durations over which the basic regime is applied.

6. Method according to one of claims 4 to 5, comprising the emission of a lithium formation detection signal when the value of the ratio R becomes less than or equal to a threshold rs.

7. Method according to the preceding claim, in which said threshold rs is a function of the charge quantity Q of the battery and is equal to: r$(Q) = 1-0.05-^- j Where Qtot is the maximum charge quantity of the battery in mAh, Ro is the initial value of the ratio when charging the battery from a state of charge less than 0.1Qtot, a is a constant between 0.75 and 0.

95.

8. Method according to the preceding claim, comprising: i. when the battery is charged from a state of charge less than or equal to 0.1Qtot: - determining the parameter Ro by calculating the average of said ratio R for charge quantities Q less than or equal to 0.2Qtot; - storing the parameter Ro in a memory; and - using the parameter Ro stored in memory for determining the threshold rs for charge quantities greater than 0.2Qtot, ii. or, when the battery is charged from a state of charge greater than 0.1Qtot, using the last parameter Ro stored in memory for detecting the formation of lithium during charging.

9. Method according to the preceding claim, comprising: i. the comparison between the last two values ​​of the parameter Ro each determined during a charge of the battery from a state of charge less than or equal to 0.1Qtot, and ii. when the difference between said last two values ​​of the parameter Ro is greater than a threshold, the emission of a battery fault signal.

10. Method according to one of claims 4 to 9, in which the charging of the lithium-ion battery is a rapid charge having stages of decreasing charging currents, the method comprising detecting lithium formation for a plurality of base regimes 1a, 1b, each base regime 1a, 1b corresponding to one of said current levels.

11. Method according to one of claims 1 to 3, in which: i. the application of the basic regime Ia is carried out on a first complete charge of the battery, the voltage U at the terminals of the battery being acquired as a function of the quantity of charge Q of the battery, ii. the application of the secondary regime 11a is carried out on a second complete charge of the battery, the voltage U at the terminals of the battery being acquired as a function of the quantity of charge Q of the battery.

12. Method according to the preceding claim, comprising: i. the detection of a lithium deposit for a plurality of basic regimes; ii. storing in a memory a table comprising, for each of the plurality of basic regimes, the respective charge quantity from which a lithium deposit appears; and iii. upon subsequent charging of the battery, using said table to detect the occurrence of lithium formation on an electrode as a function of the charging current applied and the amount of battery charge.

13. Lithium-ion battery management system configured to implement a method for detecting a formation of lithium on an electrode according to one of claims 1 to 12 during a charge of the lithium-ion battery.

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