WO2017144120A1

WO2017144120A1 - Lithium-ion battery high temperature aging process

Info

Publication number: WO2017144120A1
Application number: PCT/EP2016/054119
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: JP6741770B2; JP2019506722A

Abstract

The invention relates to a method of performing a high-temperature aging process of a lithium-ion cell (10) comprising an anode (12), a cathode (16), an electrolyte (22) and a separator (20), wherein a solid electrolyte interface (24) is formed on the anode. The method comprises: providing the solid electrolyte interface (24) with Lithium Fluoride (LIF) at a first predetermined concentration; and heating the anode until the concentration of the Lithium Fluoride (LIF) of the solid electrolyte interface (24) reaches a second predetermined concentration being smaller than the first predetermined concentration. The invention further relates to a lithium-ion cell (10).

Description

LITHIUM-ION BATTERY HIGH TEMPERATURE AGING 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 of performing a high-temperature aging process of such batteries, especially after 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 cutoff voltage. This formation process may be cycled at least once.

[0009] Additives have also been added to the electrolyte to improve the formation of the SEI and therefore enhancing the anode stability.

[0010] It is further known to perform a high temperature aging process of the battery after the formation process, in order to complete the battery.

[0011] JP 2014-071975 describes for example a manufacturing method of a secondary battery, where the resistance of the positive electrode of the battery is decreased below 5.5 ΓΠΩ by high temperature aging. However, in the method it is not possible to estimate the capacity stability, i.e. the capacity retention. Moreover the method makes it difficult to identify the origin of resistance, as it is difficult or even impossible to separate between the resistance of the positive electrode and the negative electrode by using impedance measurement depending on the type of batteries. SUMMARY OF THE DISCLOSURE

[0012] Currently, it remains desirable to reliably controlling the reduction of the resistance of the positive electrode and at the same time the increment of the capacity stability.

[0013] Therefore, according to embodiments of the present disclosure, a method of performing a high-temperature aging process of a rechargeable cell, in particular a lithium-ion cell having an anode, a cathode, an electrolyte and a separator is provided. The method includes:

- providing the solid electrolyte interface with Lithium Fluoride (LiF) at a first predetermined concentration;

- heating the anode until the concentration of the Lithium Fluoride (LiF) of the solid electrolyte interface reaches a second predetermined concentration being smaller than the first predetermined concentration.

[0014] By providing such a method, the Lithium Fluoride on the anode (negative electrode) can improve the capacity stability. At the same time, since the concentration of the Lithium Fluoride (LiF) of the anode may be controlled by controlling the heating temperature and heating time, the resistance of the anode can be controlled, as well. Hence, both the capacity stability and the resistance of the anode can be reliably controlled such that these two characteristics have at the end of the high-temperature aging process a desirable level or are in a desirable range.

[0015] Desirably heating is stopped, when the Lithium Fluoride (LiF) of the solid electrolyte interface reaches the second predetermined concentration.

[0016] It is also possible that the complete cell is heated in the method instead of only the anode.

[0017] The determination of the heating temperature and heating time may be performed prior to applying a high-temperature aging process to a cell of a certain cell type. For example, test cells of the same cell type may be heated by a fixed temperature and different heating times, respectively. The resulting characteristics of the test cells regarding the resistances of their anodes and their capacity stabilities, i.e. capacity retentions, may be measured subsequently. Those of the test cells which have values of said characteristics in a desirable range may be used to determine the target concentration of the Lithium Fluoride (LiF), i.e. the second predetermined concentration.

[0018] The SEI of the anode may be provided with Lithium Fluoride (LiF) at the first predetermined concentration during performing a formation process for the lithium-ion cell.

[0019] In other words, the Lithium Fluoride (LiF) may be added to the anode during the formation process of the cell, i.e. before the anode is heated. In particular also the SEI itself may be formed during the formation process.

[0020] The method may be performed for completion of the lithium-ion cell after the formation process of the lithium-ion cell. The formation process may in particular include the first charging of the cell.

[0021] The second predetermined concentration may be between 2 at% (atomic %) and 7 at%.

[0022] In this range a completed cell can have a desirable capacity stability and a desirable resistance of its anode.

[0023] The first predetermined concentration may be greater than 7 at%, preferably greater than 10 atomic %. The SEI may be provided with this concentration during the formation process. [0024] The anode may be heated with a predetermined temperature, e.g. greater than 30°C, in particular 60°C. At this temperature level desirable results with regard to capacity stability and resistance of the anode have been achieved, while at the same time the heating time could be limited to an acceptable range.

[0025] The anode may be heated for a predetermined heating time. This heating time may be chosen as a function of the Lithium Fluoride (LiF) concentration of the SEI. Hence, based on a determined concentration of the Lithium Fluoride (LiF) the heating time can be calculated. Such a determination may be done for a specific cell type in a pre-experiment, e.g. by heating several test cells at different heating periods.

[0026] The predetermined heating time may be in particular between 30 min and 200 hours, desirably between 5 and 24 hours.

[0027] The concentration of the Lithium Fluoride (LiF) may be measured by XPS.

[0028] X-ray photoelectron spectroscopy (XPS) is a qualitative and quantitative analysis technique that allows measuring the elemental composition on the surface of a sample. XPS may detect light elements such as lithium and may measure the elemental composition at the parts per thousand range. XPS has also the advantage that the surface chemistry of the sample may be analysed without requesting additional treatments of surface preparation.

[0029] The cell, in particular the test cells, may be disassembled for the XPS measurement. This may be done after the heating procedures have been carried out at the test cells, in order to analyse their resulting Lithium Fluoride (LiF) concentrations.

[0030] The disclosure further relates to a rechargeable cell, in particular to a lithium-ion cell comprising:

an anode (12) with a solid electrolyte interface (24),

a cathode (16),

an electrolyte (22), and

a separator (20), wherein

the solid electrolyte interface comprises Lithium Fluoride (LiF) with a concentration between 2 at% and 7 at%. [0031] In this concentration range of Lithium Fluoride (LiF) the cell can have a desirable capacity stability and resistance of its anode.

[0032] The anode may be formed by the high-temperature aging process as described above.

[0033] The anode may comprise graphite.

[0034] The cathode may comprise ϋΝο_1/30⁾ι/3 ηι/3θ2.

[0035] The separator may be made of a film comprising polyethylene.

[0036] The electrolyte may comprise a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, in particular present in equal volume ratio.

[0037] The electrolyte may comprise LiPF₆, in particular at 1 mol/L.

[0038] It is intended that combinations of the above-described elements and those within the specification may be made, except where otherwise contradictory.

[0039] 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.

[0040] 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

[0041] Fig. 1 shows a lithium ion cell;

[0042] Fig. 2 shows a flow chart illustrating an exemplary method according to embodiments of the present disclosure; and

[0043] Fig. 3 shows a XPS spectrum indicating the Lithium Fluoride (LiF) concentration of the SEI.

DESCRIPTION OF THE EMBODIMENTS

[0044] 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.

[0045] 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.

[0046] 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. LiNil/3Col/3Mn 1/302. The cathode current collector 18 may be made of aluminum. The separator 20 may be made of a film comprising polyethylene.

[0047] 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).

[0048] 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. This means the SEI 24 is desirably formed before the high temperature aging process according to the disclosure is carried out.

[0049] Additionally, the SEI 24 comprises Lithium Fluoride (LiF). After the formation process the LiF may have a concentration of > 7 at%, e.g. 10 at% or more. The LiF may have been added to the SEI 24 during the formation process. The LiF may originate from the electrolyte 22. As it will be described in the following, the concentration of the LiF of the SEI 24 is reduced during the high temperature aging process according to the disclosure.

[0050] Fig. 2 shows a flow chart illustrating an exemplary method according to embodiments of the present disclosure. This method is preferably carried out with test cells of the same cell type, in order evaluate for this cell type a suitable heating time and heating temperature in the high temperature aging process according to the disclosure. This evaluation is based on the measured LiF concentration. Once these parameters of a suitable heating time and heating temperature are known for the test cells, subsequent high temperature aging processes of regular cells of the same cell type may be controlled based on these parameters.

[0051] In step SI, the formation process of the cell is carried out. This includes initial charging of the cell and hence desirably formation of the SEI 24.

[0052] Then, in step S2 a high temperature aging process of the cell 10 is started. For example the cell 10 or at least its anode 12 may be heated, e.g. at [0053] In step S3 the LiF concentration of the SEI 24 is determined. This may be done by a XPS measurement. For this purpose the cell 10 is desirably disassembled. For the purpose of disassembly several samples (i.e. test cells) may be obtained from the same production lot, i.e. being from the same cell type. In the cell production process (in particular including the high temperature ageing process), cells are produced from a certain production lot having multiple cells (e.g. 20 cells). In case of the high temp ageing, there is temperature and time variation among multiple lots. Therefore, even if the temperature and time are determined in advance, the LiF measurement is needed for each lot to make it more accurate. Therefore one or desirably several samples (i.e. test cells) may be obtained from the same production lot.

[0054] It is further determined in step S3, whether the determined LiF concentration is equal to or smaller than a predetermined concentration (a second predetermined concentration according to the disclosure). If this is not the case heating is continued by returning to step S2.

[0055] However, in case the determined LiF concentration is equal to or smaller than the predetermined concentration, heating is stopped. The high temperature aging process is completed.

[0056] The LiF concentration is reduced by heating the anode. The characteristics of the test cells regarding their resistances of their anodes and their capacity stabilities are functions of the LiF concentration. Therefore the LiF concentration can be used as a threshold to determine, whether the high temperature aging process is completed. Those of the test cells which have values of said characteristics in a desirable range may be used to determine the target concentration of the Lithium Fluoride (LiF), i.e. the second predetermined concentration.

[0057] It has been found that said characteristics are in a desirable range, when the LiF concentration at the end of the high temperature aging process is in the range of 2 to 7 at%. This has been determined in a test, as it is schematically described in the following.

[0058] A given number of test cells 10 (i.e. samples) of the same type having the same components were heated in the high temperature aging process with the same temperature, in particular more than 30 °C, e.g. 60°C, for different heating times, e.g. between 0 and 100 hours. All test cells have undergone before the same formation process, e.g. they have been charged with a charge rate of 1 C up to 4 V each.

[0059] Table 1 summarizes the resulting characteristics of the cells after the high temperature aging process.

[0060]

Tablel: high temperature aging condition [0061] As it can be seen in this test, samples 1 to 4 have a capacity retention in an acceptable range, i.e. at least 98% (acceptable values are highlighted). This first candidate group of samples have maximally been heated for 24 hours (cf. sample 4). Their minimum LiF concentration is 2.5 at%.

[0062] However, only samples 3 to 5 have a reaction resistance in an acceptable range, i.e. maximally 3.5 mQ (acceptable values are highlighted). This second candidate group of samples have minimally been heated for 5 hours (cf. sample 3). Their maximum LiF concentration is 6.3 at%. Accordingly, and in particular due to further corresponding tests, a suitable LiF concentration at the end of the high temperature aging process has been found to be in the range of 2 to 7 at%.

[0063] The LiF concentration has been determined by XPS measurement. The disassembled anodes of the samples were dipped in a solution of ethyl methyl carbonate for 10 minutes and dried. They were set inside a glove box and brought to measurement in a closed chamber. The anodes were then ready for XPS analysis.

[0064] The X-ray intensity used during the XPS analysis was 1500 eV (electronvolt) and the X-ray diameter was 200 μπι (micrometre). The detected angle of the photoelectron was 45 degrees.

[0065] Fig. 3 shows a XPS spectrum indicating the Lithium Fluoride (LiF) concentration of the SEI. [0066] The LiF concentration relevant for the present disclosure can be measured by the lowest peak between 680 eV and 700 eV, in particular between 680 eV and 695 eV, in the XPS spectrum of fig. 3.

[0067] Accordingly, this peak is representative of LiF concentration and may therefore be used to determine the characteristics of the test cells regarding the resistances of their anodes and their capacity stabilities.

[0068] For determining the LiF concentration the total share (concentration) of elements is calculated by between whole energy. From this calculation the phosphorus share (concentration) can be obtained. However phosphorus consists of two peaks. Therefore, separation by Gaussian distribution fitting is needed. And then, the share of the two peaks can be obtained (the relevant lowest peak and the other peak). The following equation may be used for determining the the lowest peak concentration:

(Phosphorus concentration) * (lowest peak share) /100 = lowest peak concentration

[0069] The fitting curve in fig. 3 may be used, because XPS peaks follow the normal distribution (Gaussian distribution). In case of multiple peaks overlapping, Gaussian fitting is necessary to calculate area separately. For instance, the peak of LiF is the right side (lowest peak) between 680 and 700 eV. The peak of P-F is the left peak (highest peak).

[0070] According to an example the resistance of the test cells (i.e. the samples of table 1) was measured by Electro Impedance Spectroscopy. For this purpose the test cells were cooled to 0°C. The cell voltage to measure was 3.5 V. the voltage amplitude was 5 mV. The measuring frequency was 0.01 Hz to 100 kHz. The reaction resistance was obtained from semicircle fitting on a Cole- Cole plot.

[0071] The capacity retention (i.e. capacity stability) was determined by applying a charge-discharge cycle test to the test cells. Said test included charge and discharge between 3 V and 4 V at current rate of 2C, wherein desirably 300 cycles were carried out at room temperature. The capacity retention may be calculated by the equation (Capacity retention) = ((Discharge capacity after cycle test) / (Discharge capacity after formation))* 100 (%).

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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 high-temperature aging process of a rechargable cell (10) comprising an anode (12), a cathode (16), an electrolyte (22) and a separator (20), wherein a solid electrolyte interface (24) is formed on the anode, the method comprising:

- providing the solid electrolyte interface (24) with Lithium Fluoride (LiF) at a first predetermined concentration;

- heating the anode until the concentration of the Lithium Fluoride (LiF) of the solid electrolyte interface (24) reaches a second predetermined concentration smaller than the first predetermined concentration.

2. The method according to claim 1, wherein the solid electrolyte interface (24) is provided with Lithium Fluoride (LiF) at the first predetermined concentration during performing a formation process for the lithium-ion cell (10).

3. The method according to 2, wherein the method is performed for completion of the lithium-ion cell (10) after the formation process for the lithium-ion cell (10), the formation process in particular including a first charging of the cell.

4. The method according to any one of claims 1 to 3, wherein the second predetermined concentration is between 2 atomic % and 7 at%.

5. The method according to any one of claims 1 to 4, wherein the first predetermined concentration is greater than 7 atomic %, desirably greater than 10 atomic %.

6. The method according to any one of claims 1 to 5, wherein the anode is heated with a predetermined temperature which is greater than 30°C, in particular 60°C.

7. The method according to any one of claims 1 to 6, wherein the anode is heated for a predetermined heating time chosen as a function of the Lithium Fluoride (LiF) concentration of the solid electrolyte interface (24), and which is in particular between 30 min and 200 hours, desirably between 5 and 24 hours.

8. The method according to any one of claims, wherein the concentration of the Lithium Fluoride (LiF) is measured by XPS.

9. The method according to any one of claims, wherein the cell (10) is disassembled for the XPS measurement.

10. A rechargable cell (10) comprising:

- an anode (12) with a solid electrolyte interface (24),

- a cathode (16),

- an electrolyte (22), and

- a separator (20), wherein

the solid electrolyte interface (24) comprises Lithium Fluoride (LiF) with a concentration between 2 at% and 7 at%.

11. The cell (10) of claim 10, wherein

the anode is formed according to any one of the preceding method claims 1 to 9.

12. The cell (10) of claim 10 or 11, wherein

the anode (12) comprises graphite and/or the cathode (16) comprises LiNoi ₃Coi 3Mni/₃02.

13. The cell (10) any one of claims 10 to 12, wherein

the separator (20) is made of a film comprising polyethylene.

14. The cell (10) any one of claims 10 to 13, wherein

the electrolyte (22) comprises a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, in particular present in equal volume ratio, and/or the electrolyte (22) comprises LiPF₆, in particular at 1 mol/L.