METHOD FOR FORMING A LI-ION BATTERY CELL

Abstract

A Li-ion battery cell includes cathode and anode materials, a separator, and an electrolyte including a mixture of a polyethylene oxide and an oxide of formula LivLasZnOn. A method of forming the cell includes the following successive cycling steps: (a) at least two successive charge and discharge cycles of the cell at a first cycling rate C/x, the charge/discharge steps being limited in time to x/2; (b) at least two successive charge and discharge cycles of the cell at a second charging rate C/y, different from the first cycling rate, where y is lower than x, the charge/discharge steps being limited in time to y/2; and (c) at least two successive charge and discharge cycles of the cell at a third cycling rate C/z different from the first and second charging rates, where z is lower than x and y, the charge/discharge steps being limited in time to z/2.

Claims

1-14. (canceled)

15. A method for forming a lithium-ion battery cell comprising a cathode material, an anode material, and a solid electrolyte, the electrolyte comprising a mixture of a polyethylene oxide and an oxide of formula Li.sub.7La.sub.3Zr.sub.2O.sub.12, the method comprising the following successive cycling steps: (a) at least two successive charge/discharge cycles of the cell at a first cycling rate C/x, the charge/discharge steps being limited in time to x/2; (b) at least two successive charge/discharge cycles of the cell at a second charge rate C/y, different from the first cycling rate, where y is lower than x, the charge/discharge steps being limited in time to y/2; and (c) at least two successive charge/discharge cycles of the cell at a third cycling rate C/z different from the first and second charge rates, where z is lower than x and y, the charge/discharge steps being limited in time to z/2.

16. The method of formation as claimed in claim 15, wherein the first cycling rate is of C/40 applied for 20 h of charge/discharge.

17. The method of formation as claimed in claim 15, wherein the second cycling rate is of C/20 applied for 10 h of charge/discharge.

18. The method of formation as claimed in claim 15, wherein the third cycling rate is of C/10 applied for 5 h of charge/discharge.

19. The method of formation as claimed in claim 15, further comprising the following cycling step: (d) at least two successive charge/discharge cycles of the cell at a fourth cycling rate different from the first, second and third charge rates.

20. The method of formation as claimed in claim 19, wherein the fourth cycling rate is of C/5 applied for 2.5 h of charge/discharge.

21. The method of formation as claimed in claim 19, further comprising the following cycling step: (e) at least two successive charge/discharge cycles of the cell at a fifth cycling rate different from the first, second, third and fourth charge rates.

22. The method of formation as claimed in claim 21, wherein the fifth cycling rate is of C/2 applied for 1 h of charge/discharge.

23. The method of formation as claimed in claim 21, wherein one or more of the cycling steps (a), (b), (c), (d) and (e) comprise at least five successive charge/discharge cycles of the cell.

24. The method of formation as claimed in claim 15, wherein the successive cycling steps are carried out at an increasing cycling rate.

25. The method of formation as claimed in claim 15, wherein the electrolyte comprises lithium bis(trifluoromethanesulfonyl)imide LiTFSI.

26. The method of formation as claimed in claim 15, wherein the cathode comprises a catholyte comprising a mixture of polyethylene oxide and oxide Li.sub.7La.sub.3Zr.sub.2O.sub.12.

27. The method of formation as claimed in claim 26, wherein the catholyte comprises lithium bis(trifluoromethanesulfonyl)imide LiTFSI.

28. The method of formation as claimed in claim 15, further comprising a cathode comprising a mixture of nickel-manganese-cobalt active material, of carbon black and of catholyte comprising a mixture of polyethylene oxide and of Li.sub.7La.sub.3Zr.sub.2O.sub.12.

Description

[0021] Other aims, advantages and features will become clear from the description given hereunder, which is provided purely for purposes of illustration, referring to the appended drawings in which:

[0022] FIG. 1 shows the variation of the voltage as a function of the capacity of the cell that has not undergone any preconditioning during five successive charge/discharge cycles at a cycling rate of C/20.

[0023] FIG. 2 shows the variation of the capacity of the cell that has not undergone any preconditioning as a function of the number of charge/discharge cycles, at different cycling rates.

[0024] FIG. 3 shows the variation of the voltage as a function of the capacity of the cell during the cycling steps of preconditioning of the cell according to a first embodiment.

[0025] FIG. 4 shows the variation of the voltage as a function of the capacity of the cell that has undergone preconditioning according to the first embodiment, at different cycling rates.

[0026] FIG. 5 shows the variation of the capacity of the cell that has undergone preconditioning according to the first embodiment as a function of the number of charge/discharge cycles, at different cycling rates.

[0027] FIG. 6 shows the variation of the voltage as a function of the capacity of the cell during the cycling steps of preconditioning according to a second embodiment.

[0028] FIG. 7 shows the variation of the voltage as a function of the capacity of the cell that has undergone preconditioning according to the second embodiment, at different cycling rates.

[0029] FIG. 8 shows the variation of the capacity of the cell that has undergone preconditioning according to the second embodiment as a function of the number of charge/discharge cycles, at different cycling rates.

[0030] All-solid Li-ion batteries generally comprise a cathode, an anode and a solid electrolyte.

[0031] The method of formation according to the invention relates to a Li-ion battery cell comprising a solid electrolyte comprising a mixture of polyethylene oxide (PEO) and oxide of formula Li.sub.7La.sub.3Zr.sub.2O.sub.12 (LLZO).

[0032] Preferably, the anode comprises lithium metal.

[0033] According to a preferred embodiment, the electrolyte comprises a lithium salt, for example lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0034] The cathode is prepared by mixing an active material, a catholyte and a filler of active material.

[0035] Preferably, the active material comprises a material of formula LiNiMnCoO.sub.2, corresponding to a mixture of nickel, manganese and cobalt such as, for example, NMC622.

[0036] The filler of active material may comprise carbon black, for example carbon black C65.

[0037] The catholyte preferably comprises a mixture of polyethylene oxide and oxide of formula Li.sub.7La.sub.3Zr.sub.2O.sub.12.

[0038] Preferably, the catholyte further comprises lithium bis(trifluoromethanesulfonyl)imide, thus advantageously forming a PEO-LiTFSI:LLZO mixture.

[0039] The method for forming a Li-ion battery cell according to the invention comprises a step of preconditioning of the cell comprising at least three successive steps (a), (b) and (c).

[0040] Step (a) comprises at least two successive charge/discharge cycles of the cell at a first cycling rate C/x, each charge/discharge step being limited in time to x/2, step (b) comprises at least two successive charge/discharge cycles of the cell at a second charge rate C/y different from the first cycling rate, where y is lower than x, each charge/discharge step being limited in time to y/2, and step (c) comprises at least two successive charge/discharge cycles of the cell at a third cycling rate C/z different from the first and second charge rates, where z is lower than x and y, each charge/discharge step being limited in time to z/2.

[0041] It was found, surprisingly, that the preconditioning of the cells comprising at least the three steps (a), (b) and (c) as described above allows formation of an SEI on the surface of the cells, the quality of which helps to stabilize the polymer with which the cell is provided.

[0042] In particular, it was observed that the stability of the polymer at high battery operating potentials, between 3.7V and 5V, was improved. The battery performance and in particular the energy density are thus improved.

[0043] Advantageously, the successive cycling steps are carried out at an increasing cycling rate.

[0044] Advantageously, the cycling rate during cycling step (a) is C/40 and the cycling rate during cycling step (b) is C/20.

[0045] The cycling rate or “charge or discharge rate” is designated C/n, where C is the capacity of the battery in A.h, i.e. the amount of electrical energy that it is capable of returning after receiving a full charge and where n refers to a time in h.

[0046] Advantageously, the cycling rate during cycling step (c) is C/10.

[0047] Preferably, the method for forming the Li-ion battery cell further comprises a successive step (d) comprising at least two successive charge/discharge cycles of the cell at a fourth cycling rate different from the first, second and third charge rates.

[0048] Advantageously, the cycling rate during cycling step (d) is C/5.

[0049] According to a preferred embodiment, the method for forming the Li-ion battery cell further comprises a successive step (e) comprising at least two successive charge/discharge cycles of the cell at a fifth cycling rate different from the first, second, third and fourth charge rates.

[0050] Advantageously, the cycling rate during cycling step (e) is C/2.

[0051] Preferably, one or more of the cycling steps (a), (b), (c), (d) and (e) comprise at least five successive charge/discharge cycles of the cell.

[0052] During the various preconditioning steps, the duration of each charge/discharge cycle carried out at a cycling rate of C/n is preferably equal to n/2.

[0053] The present invention is illustrated in a nonlimiting manner by the following examples of the method for forming a Li-ion cell.

EXAMPLE 1

1. Preparation of the Cell

[0054] The Solid Electrolyte

[0055] The electrolyte used is prepared by mixing polyethylene oxide (PEO), the oxide of formula Li.sub.7La.sub.3Zr.sub.2O.sub.12 (LLZO) and a lithium salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0056] The percentage by volume of PEO-LiTFSI is 90 vol % and the percentage by volume of LLZO oxide is 10 vol %.

[0057] The Anode

[0058] The anode is formed from lithium metal.

[0059] The Cathode

[0060] The cathode is prepared by mixing an active material, a catholyte and an electron conductor.

[0061] NMC622 of formula LiNi.sub.0.6Mn.sub.0.2Co.sub.0.2O.sub.2 is selected as the active material and corresponds to a nickel-manganese-cobalt mixture comprising 60 mol % of nickel, 20 mol % of manganese and 20 mol % of cobalt.

[0062] Carbon black C65 is selected as the electron conductor.

[0063] Furthermore, the cathode comprises a catholyte comprising polyethylene oxide, the oxide of formula Li.sub.7La.sub.3Zr.sub.2O.sub.12 and lithium bis(trifluoromethanesulfonyl)imide to form a PEO-LiTFSI:LLZO mixture.

[0064] The proportions of NMC622, carbon black C65 and catholyte in the cathode are shown in Table 1. The amount of NMC622 present at the cathode is adjusted in order to obtain a surface capacity of 1.6+0.6 mAh.cm.sup.−2, assuming a theoretical capacity of 166 mAh/g for NMC622.

TABLE-US-00001 TABLE 1 NMC622 Carbon black Catholyte Percentage by weight 74  6 20 (wt %) Percentage by volume 50 10 40 (vol %)

[0065] Cell Assembly

[0066] A 16 mm dia. electrolyte disk and a 13 mm dia. cathode disk are assembled before being added to a 14 mm dia. anode disk to form a cell.

2. Galvanostatic Cycling, Evaluation and Result

[0067] A series of five charge/discharge cycles is carried out on the cell obtained according to the method of formation described in paragraph 1, at a rate of C/20. The cycling tests are carried out at 70° C. and in a voltage window between 2.7V and 4.2V.

[0068] In FIG. 1, curves A, B, C, D and E correspond respectively to the variation of the voltage as a function of the capacity of the cell during the first, second, third, fourth and fifth charge/discharge cycles.

[0069] It is noted that there is instability of the voltage during the charge cycles beyond 3.7V. During discharge, the capacity reaches on average 43 mA.h.g.sup.−1, or only 26% of the expected experimental capacity for a cell comprising NMC622 as active material, which is 166 mA.h.g.sup.−1.

[0070] Five additional charge/discharge cycles are carried out on the cell at a cycling rate of C/20, during which the capacity slowly increases until it reaches 85 mA.h.g.sup.−1.

[0071] As can be seen in FIG. 2, which shows the variation of the capacity of the cell as a function of the number of cycles at rates of C/20, C/10, C/5, C/2 and 1 C, the values of discharge capacity at C/20 are similar to a cycling rate of C/10 but gradually decrease as the capacity level C increases, i.e. about 20 mA.h.g.sup.−1 at C/2 and 1 C.

[0072] The voltage instability observed during the first charge/discharge cycles can be explained by the degradation of the composite electrolyte and, in particular, of the polyethylene oxide.

EXAMPLE 2

1. Preparation of the Cell

[0073] The method of preparation of the cell described in detail in example 1 is reproduced for the method of preparation of the cell in example 2. Furthermore, an additional step of preconditioning of the cell, described further hereunder, is carried out.

[0074] Preconditioning

[0075] The cell is prepared according to a first embodiment of preconditioning. Two charge/discharge cycles are carried out at a first cycling rate of C/40 according to a first step (a).

[0076] Each charge/discharge cycle of step (a) is carried out for 20 hours.

[0077] In a second step (b), two charge/discharge cycles are carried out at a second cycling rate of C/20, where each cycle is carried out for 10 hours.

[0078] Finally, in a third and final step (c), two charge/discharge cycles are carried out at a third cycling rate of C/10, where each cycle is carried out for 5 hours.

[0079] The variation of the voltage as a function of the capacity of the cell during the cycling steps of preconditioning according to the first embodiment is shown in FIG. 3. It can be seen that the results obtained during this preconditioning show better stability in comparison with the results in example 1. The voltage reached is 3.8V.

2. Galvanostatic Cycling, Evaluation and Result

[0080] After preconditioning of the cell, charge and discharge tests at the cycling rates C/20, C/10, C/5, C/2 and 1 C are carried out. The variation of the voltage as a function of the capacity of the cell and the variation of the capacity of the cell as a function of the number of charge/discharge cycles at the different rates are shown in FIG. 4 and FIG. 5, respectively.

[0081] The voltage profiles as a function of the capacity at cycling rates C/20 and C/10 show greater stability during charging relative to the cell in example 1, for which no preconditioning was carried out, reaching 3.9V and 3.8V respectively.

[0082] Voltage instability persists at cycling rates C/S and C/2, but is nevertheless lower than in example 1 without preconditioning.

[0083] An average capacity of 120 mA.h.g.sup.−1 is obtained on discharge at the cycling rate C/20, or 73% of the expected experimental capacity. This therefore represents a significant increase in capacity relative to a cell that has not been preconditioned.

[0084] The preconditioning as described in example 2 has thus made it possible to improve the performance of the cell.

EXAMPLE 3

1. Preparation of the Cell

[0085] The method of preparation of the cell described in detail in example 1 is reproduced for the method of preparation of the cell in example 3. Furthermore, a step of preconditioning of the cell, presented in more detail hereunder, is carried out.

[0086] Preconditioning

[0087] The cell is prepared according to a second embodiment of preconditioning. Five charge/discharge cycles are carried out at a first cycling rate of C/40 according to a first step (a). Each charge/discharge cycle of step (a) is carried out for 20 hours.

[0088] In a second step (b), five charge/discharge cycles are carried out at a second cycling rate of C/20, where each cycle is carried out for 10 hours.

[0089] In a third step (c), five charge/discharge cycles are carried out at a third cycling rate of C/10, where each cycle is carried out for 5 hours.

[0090] In a fourth step (d), five charge/discharge cycles are carried out at a fourth cycling rate of C/5, where each cycle is carried out for 2 hours.

[0091] In a fifth step (e), five charge/discharge cycles are carried out at a fifth cycling rate of C/2, where each cycle is carried out for 1 hour.

[0092] The variation of the voltage as a function of the capacity of the cell during the cycling steps of preconditioning according to the second embodiment is shown in FIG. 6. It can be seen that the results obtained during this preconditioning show very good stability in comparison with the results in example 1, but also an improvement relative to the stability of the cell in example 2. The voltage reached is 4.2V.

2. Galvanostatic Cycling, Evaluation and Result

[0093] After preconditioning of the cell, charge and discharge tests, at cycling rates C/20, C/10, C/5, C/2 and 1 C, are carried out. The variation of the voltage as a function of the capacity of the cell and the variation of the capacity of the cell as a function of the number of charge/discharge cycles at the different rates are shown in FIG. 7 and FIG. 8 respectively.

[0094] The potential of the cell may reach 4.2V in charging regardless of the charge rate, C/20 to C/2, polarization increasing for charge rate 1 C.

[0095] In discharge, an average capacity of 150 mA.h.g.sup.−1 is obtained at cycling rate C/20, or 91% of the expected experimental capacity and 81% at C/10. The performance of the cell is thus clearly improved relative to the performance of the cell that has not undergone any preconditioning.