WO2017144117A1

WO2017144117A1 - Lithium-ion battery formation process

Info

Publication number: WO2017144117A1
Application number: PCT/EP2016/054111
Authority: WO
Inventors: Takahiro Sakurai
Original assignee: Toyota Motor Europe
Priority date: 2016-02-26
Filing date: 2016-02-26
Publication date: 2017-08-31
Also published as: JP2019506717A; JP6741768B2

Abstract

A method of performing a formation process for a lithium-ion cell (10) comprising an anode (12), a cathode (16), an electrolyte (22) and a separator (20), the formation process including: - performing a charge-discharge cycle, wherein the cell is charged up to a first predetermined voltage level (V₁) and discharged until a second predetermined voltage level (V₂) being lower than the first predetermined voltage level, - determining a resistance R of the cell during discharge, wherein R = (V₁-V₂)/I, I being the discharge current, and - repeating the charge-discharge cycle until the determined resistance of the cell reaches a predetermined lower resistance limit (R_min).

Description

LITHIUM-ION BATTERY FORMATION PROCESS

FIELD OF THE DISCLOSURE

[0001] The present disclosure is related to rechargeable cells, in particular to lithium ion batteries or cells, and more particularly to an improved method for initially charging such batteries (formation process).

BACKGROUND OF THE DISCLOSURE

[0002] Lithium-ion batteries are part of a family of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge and from the positive electrode to the negative electrode when charging.

[0003] There are various types of lithium-ion battery. The anode comprises generally carbon and the cathode comprises a lithium compound. The anode and the cathode are separated by a separator made from a porous polymer, such as a micro-perforated plastic sheet, which allows ions to pass through. The anode, cathode and separator are immersed in an electrolyte.

[0004] Lithium-ion batteries are classified according to the cathode material.

[0005] Once the lithium-ion battery is assembled, before the battery is suitable to be used, the lithium-ion battery may be put through at least one precisely controlled charge/discharge cycle to activate the working material. This step is called the formation process. This formation process provides the initial full charge of the battery.

[0006] During the formation process, a solid electrolyte interface (SEI) is formed on the anode. The SEI formation is important for the lifetime of the lithium-ion battery or cell.

[0007] Methods for initial charging, i.e., for the formation process, of a lithium-ion battery have been proposed.

[0008] Typically, the battery is charged at a constant charge rate. The charge rate is also expressed as a C-rate, which represents a charge or a discharge rate equal to the capacity of a battery in one hour. It has been found that the SEI is best formed at small C-rate, which means that the initial charging is performed over an extended period of time. Indeed, fully charging a battery at a C-rate equal to C/5 would take approximately five hours. The battery is charged at a small C-rate up to the fully charged voltage of the battery in order for the SEI to form on the carbon anode during the first charge and then the battery is held constant at the fully charged voltage until the current drops below a threshold. The battery is then left to rest for two hours and is discharged at a small C-rate to a pre-set voltage, i.e., the discharge cut- off voltage. This formation process may be cycled at least once.

[0009] In order to reduce the manufacturing time of lithium-ion batteries, so-called dynamic forming processes have been proposed. In such processes, the battery is charged at a small C-rate up to the end of SEI layer formation on the anode, corresponding to a threshold voltage value, and then, a large C-rate is used to charge the battery up to the fully charged voltage. For example US 2015/060290 discloses such a formation protocol which still involves at least charging the battery up to the fully charged voltage at least twice, and resting the cell for two hours between each charge/discharge of the cell, the total duration of the dynamic formation process being greater than forty hours. However, in US 2015/060290, the end of SEI layer formation voltage value on the anode being determined by a method using differences of temperature, the determination is an approximate.

[0010] Additives have also been added to the electrolyte to improve the formation of the SEI and therefore enhancing the anode stability.

[0011] A further charging method of a secondary battery is known from JP 2011-222358 (A). In the method a lithium ion secondary battery is initially charged during high temperature ageing. The SEI film formation is determined by an impedance variation and the charging method is changed when the SEI film formation is not completed. However, impedance measurements require further equipment like sensors, etc., and may lead therefore to higher costs.

SUMMARY OF THE DISCLOSURE

[0012] Currently, it remains desirable to reliably and efficiently perform the formation process while having a battery that will exhibit good properties over a large number of charge/discharge cycles.

[0013] Therefore, according to embodiments of the present disclosure, a method of performing a formation process for a rechargeable cell, in particular a lithium-ion cell having an anode, a cathode, an electrolyte and a separator is provided. The method including: performing a charge-discharge cycle, wherein the cell is charged up to a first predetermined voltage level Vi and discharged until a second predetermined voltage level V₂ being lower than the first predetermined voltage level;

- determining a resistance R of the cell, wherein R = (Vi-V₂)/I, I being the discharge current , and

repeating the charge-discharge cycle until the determined resistance of the cell reaches a predetermined lower resistance limit.

[0014] By providing such a method, repeating the charge-discharge cycle may improve the capacity retention, i.e. the lifetime of the cell can be increased.

[0015] The resistance is desirably determined during discharging, e.g. based on one or several measured values of the current during discharging. The current may namely be based on the constant current discharge. The current is usually most stable during discharging. However, the resistance may also be calculated at the end of or after ending discharging.

[0016] The resistance may also be determined based on an averaged value of the current during discharging. This way of determination regularly provides the most exact resistance calculation. For example, the average may be based on the values between Is after starting discharging and 10s (end). The start point of discharging (0s) may be avoided because discharge current is usually unstable at this time.

[0017] The determination of the predetermined lower resistance limit may be performed prior to applying a formation process to a cell. For example, test cells may be charged by applying different numbers of charge-discharge cycles to the test cells, respectively. The resulting resistance of the test cells may be measured subsequently. The SEI, which is formed by the application of the charge-discharge cycles, decreases the resistance. The resistance saturates with an increasing number of charge-discharge cycles indicating the completion of the SEI formation process, e.g. after the third cycle. Hence, the lower resistance limit may be determined to correspond to this saturated value of the resistance or to a value at which the resistance starts to saturate. The latter value may be determined for example by adding a tolerance range, e.g. 1, 2 or 3%, to the actual saturated value. [0018] Thus, the lower resistance limit may be used to initially charge cells having the same configuration and the same components as the cells used to determine the lower resistance limit. Due to a suitable choice of the lower resistance limit, the determination of the required number of charge-discharge cycles until completion of formation is accurate, as the resistance can be reliably determined, without the requiring expensive measurement equipment.

[0019] Accordingly, the charge-discharge cycles can be limited to a minimum number which is necessary to complete the formation process. This limitation allows reducing the formation process duration.

[0020] When the determined resistance of the cell reaches the predetermined lower resistance limit, the cell may be charged up to a predetermined final voltage level, which is higher than the first predetermined voltage level.

[0021] The cell may reach at the predetermined final voltage level a fully charged capacity.

[0022] Accordingly, once, the SEI formation is completed, the cell may be charged up to a fully charged capacity.

[0023] In the charge-discharge cycle the cell may be charged at a first predetermined charge rate and discharged at a second charge rate, the second charge rate being greater than the first charge rate.

[0024] Charging the cell up to the first predetermined voltage level, in particular with the first predetermined charge rate, allows for the formation of the SEI on the anode. This first predetermined rate allows for the formation of a SEI with good electrochemical properties while not extending too much the duration of the full formation process.

[0025] The second predetermined charge rate allows reducing the formation process duration, as the second predetermined rate is greater than the first predetermined rate.

[0026] Each of the charge/discharge rates may be any value, preferably below 5 C.

[0027] The first charge rate may be smaller than 2 C, preferably smaller than or equal to 1 C.

[0028] The second charge rate may be equal to or greater than 2 C, in particular equal to or greater than 3 C. [0029] In addition, also the step of charging the cell up to the predetermined final voltage level may be done with a third predetermined charge rate being greater than the first predetermined charge rate. The third predetermined charge rate may be equal to or greater than 2 C, in particular equal to or greater than 3 C.

[0030] The first predetermined voltage level may be below the voltage of a fully charged state of the cell, e.g. 4 V. In particular the first predetermined voltage level may be between 2 V and 3.5 V, more particularly 3.2 V.

[0031] The second predetermined voltage level may be greater than 0 V and smaller than 3 V, in particular 2 V.

[0032] Such a value is preferable because SEI can be formed in the beginning of formation. The SEI piles up on the surface of the anode. Electro chemistry of charge-discharge occurs on the anode surface. Moreover additives make SEI on the surface and improve the electro chemistry like catalyst. Additives make SEI below 3V mostly. From these facts, SEI to improve performance can be formed in the beginning of formation. SEI cannot grow forever. If it reaches a saturated thickness, the growth stops.

[0033] The predetermined final voltage level may be 4 V.

[0034] The second predetermined voltage level may be determined by, when the cell has the first predetermined voltage level, discharging the cell in the charge-discharge cycle for a predetermined time interval, in particular between 5s and 30s, more particularly 10s.

[0035] In this way the second predetermined voltage level may be determined based on the discharge time. Such an embodiment only requires a simple control method. This is possible because the second predetermined voltage level may have any level lower than that one of the first predetermined voltage level.

[0036] The predetermined lower resistance limit may be determined as being a value, at which the determined resistance becomes constant when repeating the charge-discharge cycle.

[0037] In other words, the predetermined lower resistance limit may correspond to a saturation value of the determined resistance, when the number of charge-discharge cycles is increased. [0038] Indeed, when the determined resistance does not decrease any more with increasing number of charge-discharge cycles, it may be concluded that the SEI formation on the anode is completed.

[0039] Before performing the charge-discharge cycle, an additive may be added to the electrolyte for improving a solid electrolyte interface build-up on the anode, in particular the additive being selected from an oxalate salt, the additive being more particularly a Lithium bis(oxalato)borate.

[0040] The additive may present a decomposition potential at a smaller voltage than the electrolyte.

[0041] These additives may improve the formation of the SEI on the anode and provide a SEI having better in-use characteristics than SEI formed from the electrolyte only.

[0042] It is intended that combinations of the above-described elements and those within the specification may be made, except where otherwise contradictory.

[0043] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure, as claimed.

[0044] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the description, and serve to explain the principles thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Fig. 1 shows a lithium ion cell;

[0046] Fig. 2 shows a flow chart illustrating an exemplary method according to embodiments of the present disclosure;

[0047] Fig. 3 shows an exemplary and schematic voltage - time diagram of the charge-discharge cycles applied to a cell.

DESCRIPTION OF THE EMBODIMENTS

[0048] Reference will now be made in detail to exemplary embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. [0049] Fig. 1 shows a schematic representation of an exemplary lithium ion cell 10. The lithium ion cell 10 includes an anode 12 fixed on an anode current collector 14 and a cathode 16 fixed on a cathode current collector 18. The anode 12 and the cathode 16 are separated by a separator 20, the anode 12, the cathode 16 and the separator 20 being immersed in an electrolyte 22.

[0050] Typically, the anode 12 is made of a carbonaceous material and/or graphite. The anode current collector 14 may be made of copper. The cathode 16 may be made of an intercalated lithium compound, e.g. ϋΝί_1/3α)ι/3Μηι/3θ2. The cathode current collector 18 may be made of aluminum. The separator 20 may be made of a film comprising polyethylene.

[0051] The electrolyte 22 may be a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate present in equal volume ratio. The electrolyte may also comprise LiPF₆ at 1 mol/L (mole/litre).

[0052] On the anode 12, a solid electrolyte interface (SEI) 24 is formed. The SEI 24 is formed during the formation process of the cell, i.e., during the initial charging of the cell.

[0053] Additive may be added to the electrolyte 22 to improve the formation of the SEI.

[0054] According to some embodiments, the additive provided in the electrolyte may be selected from an oxalate salt.

[0055] Examples of oxalate salts may include lithium salts of bis(oxalato)borate:

[0057] In particular, the additive may be Lithium bis(oxalato)borate, more in particular added at 5 wt% (weight percent) to the electrolyte 22.

[0058] Lithium ions present in the electrolyte 22 move from the anode 12 to the cathode 16 during discharge of the cell 10 and from the cathode 16 to the anode 12 when charging the cell 10.

[0059] Fig. 2 shows a flow chart illustrating an exemplary method according to embodiments of the present disclosure.

[0060] In step SI, the cell 10 is charged at a first predetermined rate Ci up to a first predetermined voltage level Vi, e.g. to 3.2 V. [0061] For example, the first predetermined rate Ci may be equal to 1 C. This first predetermined rate Ci allows for the formation of a SEI 24 with good electrochemical properties while not extending too much the duration of the full formation process.

[0062] Then, in step S2 the cell 10 is discharged at a second predetermined rate C₂ until a second predetermined voltage level V₂ is reached, e.g. 2 V.

[0063] During or after the cell 10 is discharged in step S2 the resistance R of the cell is determined. For this purpose the formula R = (Vi-V₂)/I is used. I is the discharge current and may be measured during the discharging in step S2.

[0064] In step S3 it is determined, whether the resistance R is greater than a predetermined lower resistance limit Rmi_n. If this is the case, the method moves back to step SI, i.e. the cell is charged up again to the first predetermined voltage level Vi. Accordingly, the charge-discharge cycle is repeated.

[0065] However, in case the resistance R is not greater but reaches the predetermined lower resistance limit R_min (i.e. is equal to or smaller than R_min), the cell may be charged up to a predetermined final voltage level Vfin in step S4. V_fin may be higher than the first predetermined voltage level, e.g. 4 V. This voltage level may correspond to a fully charged state of the cell. Hence, when this voltage level is reached, the cell is fully charged for the first time.

[0066] With each charging procedure in step SI the formation of the SEI is further advanced. The formation of the SEI decreases the resistance until the SEI formation is completed. Then a saturation level of the resistance is reached. Therefore the resistance can be used as a threshold to determine, at what time the formation of the SEI is completed. Based on the saturation level the predetermined lower resistance limit R_min (being this threshold) may be determined.

[0067] The predetermined lower resistance limit R_min may be determined as follows.

[0068] A given number of test cells 10 (i.e. samples) having the same components were charged with a different number of applied charge-discharge cycles (i.e. repeat number of charge-discharge cycles), respectively. Table 1 summarizes the resulting resistance of the cells. [0069]

Tablel: formation condition [0070] As it can be seen in this example, after the second repetition, i.e. when the charge-discharge cycle has been applied three times, the resistance starts to saturate, in this example at 293 ιηΩ. Accordingly, for this type of cell, a suitable value for the predetermined lower resistance limit R_mi_n may be 293 ΓΠΩ.

[0071] Fig. 3 shows an exemplary and schematic voltage - time diagram of the charge-discharge cycles applied to a cell.

[0072] At t=0 the battery is charged, starting from 0 V. This charging procedure corresponds to step SI in fig. 2. When the voltage reaches the first predetermined voltage level Vi, e.g. 3.2 V, the cell is discharged again until the second predetermined voltage level V₂ is reached, e.g. to 2 V. This discharging procedure corresponds to step S2 in fig. 2. At the time of discharging or when the second predetermined voltage level V₂ is reached, the resistance may be determined.

[0073] Such a charge-discharge cycle may now be repeated several times, until the determined resistance is equal to or lower than the predetermined lower resistance limit.

[0074] Thus, it is demonstrated that accurate determination of the required number of charge-discharge cycles based on the predetermined lower resistance limit may produce cells with good capacity retention and relatively short duration of formation process.

[0075] Throughout the description, including the claims, the term "comprising a" should be understood as being synonymous with "comprising at least one" unless otherwise stated. In addition, any range set forth in the description, including the claims should be understood as including its end value(s) unless otherwise stated. Specific values for described elements should be understood to be within accepted manufacturing or industry tolerances known to one of skill in the art, and any use of the terms "substantially" and/or "approximately" and/or "generally" should be understood to mean falling within such accepted tolerances.

[0076] Although the present disclosure herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present disclosure.

[0077] It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims.

[0078] The method is described in terms of a single cell. However, it may be easily adapted for batteries having multiple cells. Moreover it may also refer to other cell types than lithium-ion cells.

Claims

1. A method of performing a formation process for a rechargeable cell (10) comprising an anode (12), a cathode (16), an electrolyte (22) and a separator (20), the formation process comprising:

performing a charge-discharge cycle, wherein the cell is charged up to a first predetermined voltage level (Vi) and discharged until a second predetermined voltage level (V₂) lower than the first predetermined voltage level,

- determining a resistance R of the cell during discharge, wherein R = (Vi-V₂)/I, I being the discharge current, and

repeating the charge-discharge cycle until the determined resistance of the cell reaches a predetermined lower resistance limit (R_min).

2. The method according to claim 1, further comprising the step of: when the determined resistance of the cell reaches the predetermined lower resistance limit, charging the cell up to a predetermined final voltage level (Vfj_n), which is higher than the first predetermined voltage level.

3. The method according to claim 1 or 2, wherein the cell reaches at the predetermined final voltage level (Wm) a fully charged capacity.

4. The method according to any of claims 1 to 3, wherein in the charge- discharge cycle the cell is charged at a first predetermined charge rate (Ci) and discharged at a second charge rate (C₂), the second charge rate (C₂) being greater than the first charge rate (Ci).

5. The method according to any of claims 1 to 4, wherein the first charge rate (Ci) is smaller than 2 C, preferably smaller than or equal to 1 C.

6. The method according to any of claims 1 to 5, wherein the second charge rate (C₂) is equal to or greater than 2 C, in particular equal to or greater than 3 C.

7. The method according to any of claims 1 to 6, wherein the first predetermined voltage level (V is below 4 V, in particular between 2 V and 3.5 V, more particularly 3.2 V, and/or

the second predetermined voltage level (V₂) is greater than 0 V and smaller than 3 V, in particular 2 V, and/or

the predetermined final voltage level (V_fm) is 4 V.

8. The method according to any of claims 1 to 7, wherein the second predetermined voltage level (V₂) is determined by, when the cell has the first predetermined voltage level (Vi), discharging the cell in the charge-discharge cycle for a predetermined time interval, in particular between 5s and 30s, more particularly 10s.

9. The method according to any of claims 1 to 8, wherein the predetermined lower resistance limit (R_min) is determined as a value, at which the determined resistance becomes constant when repeating the charge- discharge cycle.

10. The method according to any of claims 1 to 9, wherein before performing the charge-discharge cycle, an additive is added to the electrolyte (22) for improving a solid electrolyte interface (24) build-up on the anode (12), in particular the additive being selected from an oxalate salt, the additive being more particularly a Lithium bis(oxalato)borate.