Method for recovering performance of aged nickel-rich NMC positive electrode material for lithium ion secondary battery

Abstract

The invention relates to a method for recovering the performance of an aged nickel-rich NMC positive electrode material for a lithium ion secondary battery. The method comprises the following steps: forming a positive electrode by using an aged nickel-rich NMC positive electrode material and carbon black; and exposing the positive electrode to diethyl phosphamide at an exposure temperature of less than 160 DEG C to obtain a lithium phosphate protective layer on the positive electrode.

Description

Technical Field

[0001] The present disclosure relates to a method for restoring the performance of aged nickel-rich NMC cathode materials for lithium-ion secondary batteries. Background Art

[0002] Lithium-ion secondary batteries use nickel-manganese-cobalt as the positive electrode material. In fact, this material allows the energy density to be increased while allowing the cost of the positive electrode to be reduced. NMC111 is a material with the chemical composition LiNi 0.33 Mn 0.33 Co 0.33 Typical cathode material for O2.

[0003] Nickel-rich NMC is becoming more and more popular in current research due to its higher specific capacity and energy. The so-called nickel-rich NMC refers to a lithium-nickel-manganese-cobalt composition with a molar nickel content of at least 0.5, such as a composition of LiNi 0.5 Mn 0.3 Co 0.2 O2's NMC532.

[0004] However, a higher nickel content increases the reactivity of the material, especially when in contact with atmospheric conditions. This reaction has an impact on the performance of secondary batteries. In fact, aged nickel-rich NMC shows an overpotential peak at the beginning of the charging process, while fresh nickel-rich NMC cathodes do not exhibit an overpotential peak.

[0005] Overpotential peaks can have a significant impact on cell production, especially during the activation process.

[0006] In fact, during the formation process (activation of the battery's working materials), i.e. during the first charge / discharge cycle until the battery is fully charged, a solid electrolyte interface (SEI) is formed on the negative electrode by controlling the current and voltage. For the performance and life of lithium-ion batteries or cells, the formation of a high-quality SEI is desirable.

[0007] The overpotential peak that occurs in the first cycle may have an adverse effect on the quality control of the formation process.

[0008] One solution is to store the cathode material under dry conditions. However, this solution increases the cost of storage equipment and still requires the cathode material to be used as quickly as possible to reduce material surface modification.

[0009] Currently, it is still desired to increase the storage time of cathode materials while reducing the cost of storage equipment. Summary of the invention

[0010] In this regard, the present invention relates to a method for restoring the performance of an aged nickel-rich NMC positive electrode material (nickel-rich NMC cathode material) for lithium-ion secondary batteries. The method comprises: forming a positive electrode with an aged nickel-rich NMC positive electrode material, a binder and carbon black; and exposing the positive electrode to diethylphosphoramidate at an exposure temperature greater than 160° C. (degrees Celsius) to obtain a lithium phosphate protective layer on the positive electrode.

[0011] The so-called nickel-rich NMC refers to a lithium-nickel-manganese-cobalt composition with a molar nickel content of at least 0.5.

[0012] As a non-limiting example, a LiNi 0.5 Mn 0.3 Co 0.2 O2 NMC532, with chemical composition LiNi 0.6 Mn 0.2 Co 0.2 O2 NMC622, or LiNi 0.8 Mn 0.1 Co 0.1 O2's NMC811.

[0013] The so-called aged nickel-rich NMC positive electrode material refers to the nickel-rich NMC positive electrode material that has an overpotential peak during the formation process. Newly made nickel-rich NMC positive electrode materials and / or nickel-rich NMC positive electrode materials stored under dry conditions will not have an overpotential peak.

[0014] As a non-limiting example, the aged nickel-rich NMC positive electrode material may be a nickel-rich NMC positive electrode material that has been stored for at least one month under normal atmosphere (i.e., not under dry conditions). Therefore, it can be understood that the aged nickel-rich NMC material has reacted and there are impurities at least on the surface of the aged nickel-rich NMC positive electrode material.

[0015] By providing the above method, the nickel-rich NMC material can be stored for a longer period of time and the nickel-rich NMC material can be stored in a storage device that does not need to have humidity control (ie, under dry conditions).

[0016] By exposing the positive electrode to diethylphosphoramide, it is possible to avoid shorting the electron path that may occur when the positive electrode material is exposed before the positive electrode is formed. It is understood that diethylphosphoramide (DEPA) reacts with lithium impurities (e.g., lithium hydroxide (LiOH), lithium carbonate (Li2CO3)) present on the surface of the aged nickel-rich NMC positive electrode material. It is understood that no lithium precursor is added in the formation of the protective layer.

[0017] As a non-limiting example, the binder may be polyvinylidene fluoride (PVDF).

[0018] According to some embodiments, the exposing may be performed by a single precursor atomic layer deposition technique, and the exposure temperature is equal to or less than 200°C.

[0019] Typically, the reaction between the lithium precursor and diethylphosphoramide occurs at about 300° C. With the atomic layer deposition technique, the reaction temperature can be reduced to below 200° C. Furthermore, in contrast to conventional dual-precursor atomic layer deposition, there is no need for annealing at 600° C., which is disadvantageous for coatings made directly on the cathode because the annealing process may decompose the binder.

[0020] In some embodiments, the atomic layer deposition technique may be an air space atomic deposition technique.

[0021] It should be understood that the atmospheric atomic deposition technique is a single precursor atmospheric atomic deposition technique.

[0022] In some embodiments, the carrier gas for diethylphosphoramide can be argon or nitrogen that has been preheated at the exposure temperature.

[0023] As a non-limiting example, argon is 99.999% pure argon. As a non-limiting example, nitrogen is 99.99% pure nitrogen.

[0024] In some embodiments, the carrier gas may be argon or nitrogen.

[0025] As a non-limiting example, argon is 99.999% pure argon. As a non-limiting example, nitrogen is 99.99% pure nitrogen.

[0026] The carrier gas may be purged between two supply locations in the spatial atomic layer deposition apparatus for exposure to diethylphosphoramide.

[0027] In some embodiments, the time for which the cathode is exposed to diethylphosphoramide may be in the range of 0.5 seconds to 6 seconds.

[0028] In some embodiments, the cathode can be dried at a temperature in the range of 65°C to 75°C, such as at 70°C, for at least 24 hours prior to exposure to diethylphosphoramide.

[0029] In some embodiments, the formation of the positive electrode may include: performing slurry preparation using an aged nickel-rich NMC positive electrode material, a binder, and carbon black; and applying the slurry to form a layer, drying the layer, and calendaring the layer to obtain the positive electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described hereinafter with reference to the accompanying drawings, in which like numerals represent like elements, and in which:

[0031] Figure 1 The function relationship between voltage and capacity of NMC532 aged for three years and three months in the first charge is shown;

[0032] Figure 2 A flowchart showing a method according to an embodiment of the present disclosure;

[0033] Figure 3 A schematic diagram showing a spatial atomic deposition head;

[0034] Figure 4 The function relationship between the voltage and the charge in the first charge of the NMC532 aged for three years with a protective layer and the NMC532 aged for three years without a protective layer is shown;

[0035] Figure 5 shows the current-voltage curve of NMC532 aged for three years with a protective layer measured at 0°C;

[0036] Figure 6 The measurement results of the electrochemical impedance spectroscopy of NMC532 aged for three years with a protective layer are shown. DETAILED DESCRIPTION

[0037] Figure 1 The function relationship between the voltage V (expressed in V (volts)) and the charge Q (expressed in mA.h (milliampere-hours)) of the NMC532 electrode aged for three months (dashed line) and three years (flat line) in the first charge of the electrode is shown. It can be seen that the curve of the NMC532 electrode aged for three years has an overpotential peak at about 0.3 mA.h. It can also be seen that the NMC532 electrode aged for three months also has an initial overpotential, but the voltage value is lower than that of the NMC532 electrode aged for three years.

[0038] Figure 2 A flow chart showing a method 100 for restoring the performance of an aged nickel-rich NMC positive electrode material for a lithium-ion secondary battery. The method includes step 110, which uses an aged nickel-rich NMC positive electrode material, a binder and carbon black to form a positive electrode 10 (see Figure 3 ). The step 110 of forming a positive electrode may include: a step 102 of preparing a slurry using an aged nickel-rich NMC positive electrode material, a binder and carbon black, and a step of applying the slurry to form a layer 104, drying the layer 106 and calendering the layer 108 to obtain a positive electrode.

[0039] The step 110 of forming the positive electrode is well known. Calendering 108 helps increase the density of the positive electrode.

[0040] The method includes: step 114 of exposing the positive electrode 10 to diethylphosphoramide at an exposure temperature greater than 160° C. to obtain a lithium phosphate protective layer on the positive electrode.

[0041] As an example, Figure 3 As shown, exposure of an aged nickel-rich NMC positive electrode can be performed in an atmospheric space atomic layer deposition apparatus 20. More specifically, Figure 3 The deposition head 22 of the atmospheric space atomic layer deposition apparatus 20 is schematically shown, wherein the cathode 10 rotates under the deposition head 22. Diethylphosphoramide 24 is simultaneously supplied at different positions of the cathode 10 at atmospheric pressure using a carrier gas 26. In addition, a carrier gas 28 is supplied between different positions where diethylphosphoramide 24 is supplied, thereby purging the system. The exhaust of the gas is marked as 29.

[0042] As a non-limiting example, the aged nickel-rich NMC positive electrode material may be an NMC532 positive electrode material aged for three years. The binder may be polyvinylidene fluoride. The positive electrode 10 obtained based on the NMC532 positive electrode material, polyvinylidene fluoride and carbon black is hereinafter named as NMC532 positive electrode.

[0043] As a non-limiting example, the gap between the deposition head 22 and the anode 10 is about 25 μm (micrometers) and the carrier gas 26 and the carrier gas 28 are both argon gas of 99.999% purity.

[0044] As a non-limiting example, a NMC532 cathode may be exposed for 666 ms (milliseconds) in one cycle and obtain a lithium phosphate (Li3PO4) protective layer through ten cycles.

[0045] As a non-limiting example, the exposure temperature may be 195°C.

[0046] The following describes the performance of a three-year aged NMC532 cathode with a lithium phosphate protective layer and a three-year aged NMC532 cathode without a lithium phosphate protective layer, the NMC532 cathode being exposed to ten 666 ms cycles of diethylphosphoramide.

[0047] The three-year aged NMC532 cathode without lithium phosphate protective layer obtained by exposure to diethylphosphoramide showed 0.85±0.02mA.h / cm at 1C. 2 The average capacity is 39±4Ω / cm 2 The charge transfer resistance Rct and 49±4Ω / cm 2 The total resistance of

[0048] The three-year aged NMC532 cathode with lithium phosphate protective layer obtained by exposure to diethylphosphoramide exhibited 0.88 ± 0.02 mA.h / cm at 1C. 2 The average capacity is 36.3±0.4Ω / cm 2 The charge transfer resistance Rct and 45±1Ω / cm 2 The total resistance of

[0049] Due to the lithium phosphate protective layer, the average capacity is increased by 3%, the charge transfer resistance is reduced by 7%, and the total resistance is reduced by 8%. Figure 4 As shown in Figure 2, the overpotential peak is significantly reduced. Figure 4 In the figure, the curve of the NMC532 positive electrode aged for three years without a lithium phosphate protective layer is a flat straight line, and the curve of the NMC532 positive electrode aged for three years with a lithium phosphate protective layer is a bold line.

[0050] Regarding the charge transfer resistance Rct and the total resistance, measurements were performed using a cell assembly having the following specifications.

[0051] -Button cell type (CR2032) battery pack;

[0052] - Negative electrode: lithium metal, thickness 0.45 mm (millimeter), diameter 15 mm;

[0053] -Cathode: NMC532 aged for three years with lithium phosphate protective layer and NMC532 aged for three years without lithium phosphate protective layer, 1.0 mA.h / cm 2 ; Diameter 13mm (1.33cm 2 );

[0054] -Separator: Celgard-2500, diameter 17 mm;

[0055] -Electrolyte: LP40, 20 μL.

[0056] The cycling scheme was as follows: wait 6 hours, formation at C / 3, capacity check at 1C, set to SOC (state of charge) 55%, both devices at 25°C, and set to 0°C for 5 hours, then return to 25°C after measurement.

[0057] like Figure 5 As shown, the total resistance of the cell is measured by IV measurement (current-voltage). Different currents are applied to the cell and the voltage drop is measured. The current is plotted as a function of the voltage drop, and the slope is the total resistance of the cell.

[0058] The charge transfer resistance R was measured using EIS (electrochemical impedance spectroscopy) at 20kHz-0.1Hz (hertz) and an amplitude of 10mV. ct . R ct=(R2-R1) / 1.33, unit is Ω / cm 2 .

[0059] The present disclosure describes specific exemplary embodiments, but these embodiments may be modified without departing from the overall scope of the invention as defined in the claims. In particular, individual features of the various illustrated / mentioned embodiments may be combined in other embodiments. Therefore, the description and drawings should be regarded as illustrative rather than restrictive.

Claims

1. A method for restoring the performance of an aged nickel-rich NMC positive electrode material for a lithium-ion secondary battery, characterized in that: The method comprises: forming a positive electrode using an aged nickel-rich NMC positive electrode material, a binder, and carbon black; and The positive electrode is exposed to diethylphosphoramide at an exposure temperature greater than 160° C. to obtain a lithium phosphate protective layer on the positive electrode.

2. The method according to claim 1, characterized in that: The exposure is performed by a single precursor atomic layer deposition technique, and the exposure temperature is equal to or less than 200°C.

3. The method according to claim 2, characterized in that Atomic layer deposition technology is an atmospheric space atomic deposition technology.

4. The method according to claim 2 or 3, characterized in that: The carrier gas for diethylphosphoramide is argon or nitrogen preheated at the exposure temperature.

5. The method according to claim 3, characterized in that: The carrier gas is argon or nitrogen.

6. The method according to any one of claims 1 to 5, characterized in that The time for which the positive electrode is exposed to diethylphosphoramide is in the range of 0.5 seconds to 6 seconds.

7. The method according to any one of claims 1 to 5, characterized in that Prior to exposure to diethylphosphoramide, the cathode was dried at a temperature in the range of 65°C to 75°C for at least 24 h.

8. The method according to any one of claims 1 to 6, characterized in that The formation of the positive electrode includes: slurry preparation using aged nickel-rich NMC cathode material, binder and carbon black; and The slurry is applied to form a layer, and the layer is dried and rolled to obtain a positive electrode.

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