A lithium-ion battery with a long cycle life and a method for extending the cycle life of a lithium-ion battery

Abstract

The present invention relates to a lithium-ion battery with a long cycle life, which includes a positive electrode, a negative electrode, a separator, an electrolyte, a battery case, electrode tabs, and a battery management system. The negative electrode is made of graphite, a lithium storage agent, a conductive agent, a binder, and a current collector. The lithium storage agent is a high-capacity alloy-based negative electrode material. The discharge cut-off voltage is controlled by the battery management system. When the battery capacity decays to a set value, the discharge cut-off voltage of the lithium-ion battery is automatically reduced. By controlling the discharge cut-off voltage of the battery in stages, the present invention controls the staged release of active lithium in the lithium storage agent and timely supplements the lithium consumed during the battery cycle, significantly extending the cycle life of the lithium-ion battery. It is convenient, simple, effective, and highly operable for improving the cycle life of the lithium-ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly relates to a lithium-ion battery with a long cycle life and a method for extending the cycle life of a lithium-ion battery. Background Art

[0002] Lithium-ion batteries have the advantages of high energy density, long cycle life, low self-discharge, fast charge and discharge, good safety, etc. Since their commercialization in 1991, they have developed rapidly and are widely used in various fields such as digital products, electronic devices, electric vehicles, energy storage, etc. Lithium-ion batteries mainly achieve reversible charge and discharge through the reversible shuttling of lithium ions between the positive and negative electrodes. During charging, lithium ions are removed from the positive electrode active material and migrate through the electrolyte to be embedded in the negative electrode, and vice versa during discharging.

[0003] The cycle life is an important indicator for evaluating the performance of lithium-ion batteries. Since the commercialization of lithium-ion batteries, their cycle life has been greatly improved through the progress of battery materials and battery technologies. For practical applications, the longer the cycle life of lithium-ion batteries, the better. Especially for some special fields, lithium-ion batteries are required to have a longer cycle life to reduce the use cost. For example, the application of lithium-ion batteries in large-scale energy storage is of great significance for building a smart grid, improving energy utilization efficiency, and realizing a low-carbon economy. However, lithium-ion batteries for energy storage generally require more than 3500 charge-discharge cycles and a service life of more than 10 years, which poses higher requirements for lithium-ion battery technology.

[0004] In actual use, lithium-ion batteries mainly perform charge-discharge cycles by controlling the charging cut-off voltage / current and the discharging cut-off voltage. As the number of cycles increases, the active lithium in the battery is continuously consumed, resulting in continuous attenuation of the battery capacity. It is generally considered that the life of a lithium-ion battery ends when its capacity decays to 80% of its initial capacity. To improve the cycle life of lithium-ion batteries, Patent CN201610796029.4 provides a method of adding a supplementary active lithium material (LiCoO2) to the lithium iron phosphate cathode to extend the cycle life of lithium-ion batteries. However, this method requires the voltage to be charged to 4.45 - 4.8V during the first charge, which is much higher than the charging cut-off voltage (3.7 - 4.0V) of the lithium iron phosphate cathode, and may cause damage to the structure of the lithium iron phosphate cathode. In addition, the structure of the supplementary active lithium material collapses during the first charge and cannot replenish lithium in subsequent cycles. The above two reasons may limit the extent to which it can extend the cycle life of lithium-ion batteries. CN202210046115.9 provides a graphite anode material, which consists of two particle sizes of graphite particles. The second particle size has a larger specific surface area and more pores, and can store more electrolyte, thereby improving the long-cycle performance of the graphite anode material. CN202111349454.6 provides a lithium iron phosphate battery with a long cycle life, which optimizes the electrolyte, anode material, and cell design, and uses different electrolytes for the first and second injections to reduce the internal resistance and achieve a longer cycle life.

[0005] However, the limitation of the above-mentioned existing technologies is that they basically aim to improve the battery cycle life by improving the materials. However, this method is not easy to implement in commercial batteries. Considering factors such as cost, quality control, and safety, it is more convenient for industrialization if the cycle life of the battery can be improved on the basis of existing commercial batteries without changing the materials as much as possible.

[0006] Alloy-based anode materials (such as silicon, germanium, tin, etc.) have extremely high theoretical specific capacities. For example, the theoretical specific capacity of silicon is 4200 mAh / g, the theoretical specific capacity of germanium is 1600 mAh / g, and the theoretical specific capacity of tin is 994 mAh / g, all of which are much higher than that of traditional graphite anodes (372 mAh / g). Current research mainly focuses on using high-capacity alloy-based anodes to replace or partially replace traditional graphite anodes to improve the energy density of lithium-ion batteries, and has received extensive attention. However, research on using high-capacity alloy-based anodes as lithium storage agents to extend the cycle life of lithium-ion batteries has not been reported so far. Summary of the Invention

[0007] Aiming at the problem that the long cycle life of lithium-ion batteries cannot be effectively improved in the prior art, the present invention provides a lithium-ion battery with a long cycle life and a method for extending the cycle life of lithium-ion batteries. By introducing a high-capacity alloy-based negative electrode material as a lithium storage agent into the negative electrode, the discharge cut-off voltage is gradually reduced in stages during the battery cycle, so that the active lithium stored in the lithium storage agent is gradually released in stages, supplementing the active lithium irreversibly consumed during the long cycle of the battery, thereby achieving the purpose of extending the cycle life of the lithium-ion battery.

[0008] In order to achieve the above-mentioned invention purpose, it is specifically realized through the following technical solutions:

[0009] The first object of the present invention is to provide a lithium-ion battery with a long cycle life, including a positive electrode, a negative electrode, a separator, an electrolyte, a battery case, a tab, and a battery management system. The negative electrode is made of graphite, a lithium storage agent, a conductive agent, a binder, and a current collector. The lithium storage agent is a high-capacity alloy-based negative electrode material; the discharge cut-off voltage is controlled by the battery management system, and when the battery capacity decays to a set value, the discharge cut-off voltage of the lithium-ion battery is automatically reduced.

[0010] Furthermore, the battery is controlled by the battery management system. When the battery capacity decays to 70-90% of the initial rated capacity, the discharge cut-off voltage of the lithium-ion battery is automatically reduced, and the reduction range is 0.01-0.2V; preferably, when the battery capacity decays to 80-85% of the initial rated capacity, the battery management system automatically reduces the discharge cut-off voltage of the lithium-ion battery, and the reduction range is 0.05-0.1V. The initial rated capacity refers to the capacity corresponding to the first discharge from the fully charged state to the set first-stage discharge cut-off voltage after the lithium-ion battery undergoes normal formation steps.

[0011] Furthermore, the first-stage discharge cut-off voltage is the voltage corresponding to when the discharge capacity of a lithium-ion battery composed of a graphite negative electrode without a lithium storage agent and the corresponding positive electrode material is 60-90% of the full discharge capacity.

[0012] Furthermore, the positive electrode material of the lithium-ion battery is any one or a combination of two or more of lithium iron phosphate, lithium vanadium phosphate, lithium manganese phosphate, lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel manganate, nickel cobalt manganese ternary positive electrode material, nickel cobalt aluminum ternary positive electrode material, and lithium-rich manganese-based positive electrode material; the negative electrode active material of the negative electrode is a graphite material.

[0013] In a preferred embodiment of the present invention, the positive electrode material of the lithium-ion battery is lithium iron phosphate, and in the negative electrode active material of the negative electrode, graphite accounts for 80-99wt%.

[0014] In the present invention, a high-capacity alloy-based anode material is introduced into the negative electrode sheet of a lithium-ion battery as a lithium storage agent. The lithium storage agent mainly serves to store active lithium, and the addition amount of the lithium storage agent is 0.5-20 wt% of the total weight of the electrode material (including graphite, lithium storage agent, conductive agent, and binder), preferably 0.95-8.55 wt%.

[0015] The lithium storage agent is a high-capacity alloy-based substance, such as a silicon-based substance (silicon, silicon monoxide, silicon-carbon composite, carbon-coated silicon, carbon-coated silicon monoxide, doped silicon, doped silicon monoxide, silicon alloy), a germanium-based substance (germanium, germanium alloy, carbon-coated germanium, germanium-carbon compound), and a tin-based substance (tin, tin alloy, carbon-coated tin, tin-carbon compound), or a combination of two or more of them. Preferably, considering capacity and technological maturity, a silicon-based substance is preferably used as the lithium storage agent; more preferably, carbon-coated silicon monoxide.

[0016] Further preferably, the carbon-coated silicon monoxide particles have a particle size of 1-20 μm, a carbon content of 1-8 wt%; the thickness of the carbon coating layer is 5-30 nm.

[0017] According to the present invention, the lithium storage agent can be introduced in various ways, such as: 1) pre-mixing the lithium storage agent with the graphite material or mechanically mixing it during the preparation of the slurry for introduction; 2) the lithium storage agent can also be introduced by forming a composite negative electrode with graphite through high-temperature sintering or deposition.

[0018] In the present invention, the lithium stored in the lithium storage agent can be obtained in various ways, such as: 1) obtained from the positive electrode during charging; 2) it can also be obtained through prelithiation or lithium supplementation techniques, including prelithiation or lithium supplementation of the negative electrode (such as adding metallic lithium powder, ultra-thin metallic lithium, using a prelithiated alloy negative electrode, electrochemical prelithiation, etc.); 3) adding a lithium supplementation reagent (Li5FeO4, Li5Fe5O8, Li2NiO2, Li2C2O4, Li3N, Li2O, etc.) to the separator or the positive electrode.

[0019] In the present invention, the positive electrode of the lithium-ion battery includes a positive electrode active material, a conductive agent, a binder, a positive electrode additive, and a current collector. The active material, additive, binder, and conductive agent are dispersed, mixed, stirred, the viscosity is adjusted, coated, dried, roll-pressed, and slit, etc., to obtain the positive electrode sheet of the lithium-ion battery.

[0020] In the present invention, the negative electrode of the lithium-ion battery includes a negative electrode active material, a lithium storage agent, a conductive agent, a binder, and a current collector. The active material, lithium storage agent, binder, and conductive agent are dispersed, mixed, stirred, the viscosity is adjusted, coated, dried, roll-pressed, and slit, etc., to obtain the negative electrode sheet of the lithium-ion battery.

[0021] The second object of the present invention is to provide a method for extending the cycle life of a lithium-ion battery. During the cycle of the above-mentioned long-cycle-life lithium-ion battery, when the battery capacity decays to 70-90% of the initial rated capacity, preferably 80-85%, the discharge cut-off voltage is reduced through the battery management system to restore the battery capacity. When the battery capacity decays to the set value of the initial rated capacity again, the discharge cut-off voltage is reduced again through the battery management system, and the above operations are repeated until the discharge cut-off voltage is reduced to 1.5-2.5V, or the active lithium in the lithium storage agent is completely released.

[0022] Furthermore, the method for extending the cycle life of a lithium-ion battery includes the following steps:

[0023] (S1) Control the initial discharge cut-off voltage to be the voltage corresponding to when the discharge capacity of a lithium-ion battery composed of a graphite negative electrode without a lithium storage agent and a corresponding positive electrode material is 60-90% of the full discharge capacity;

[0024] (S2) During the cycle charge and discharge process, control the battery through the battery management system. When the battery capacity decays to 70-90% of the initial rated capacity, reduce the battery discharge cut-off voltage to restore the battery capacity;

[0025] (S3) Repeat step (S2) until the discharge cut-off voltage is reduced to 1.5-2.5V, or the active lithium in the lithium storage agent is completely released. At this time, further reducing the discharge cut-off voltage will contribute little to the battery cycle life.

[0026] Furthermore, in step (S1), when the positive electrode material is lithium iron phosphate, its initial discharge cut-off voltage is 2.9-3.1V, preferably 2.95-3.0V.

[0027] Furthermore, in step (S2), the reduction amplitude of the discharge cut-off voltage is 0.01-0.2V, preferably 0.05-0.1V. The reduction amplitude of the discharge cut-off voltage is such that during the entire cycle of the lithium-ion battery, the discharge cut-off voltage is reduced 3-20 times, preferably 5-15 times. If the reduction amplitude of the discharge cut-off voltage is too large, the lithium storage agent will participate in the battery cycle too deeply prematurely, which is not conducive to significantly extending the battery cycle life. In addition, if the reduction amplitude of the discharge cut-off voltage is too large, the capacity that the battery can deliver after reducing the discharge cut-off voltage may exceed the initial rated capacity.

[0028] The present invention controls the battery discharge cut-off voltage in stages, controls the staged release of active lithium in the lithium storage agent, and timely supplements the lithium consumed during the battery cycle, significantly extending the cycle life of the lithium-ion battery; the preparation method of the long-cycle-life lithium-ion battery provided by the present invention is compatible with the existing lithium-ion battery preparation process, and can be achieved only by introducing a lithium storage agent during the preparation of the negative electrode and controlling the discharge cut-off voltage in stages during normal cycling. It is convenient, simple, effective, and highly operable for improving the cycle life of lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is the battery cycle program of Example 1;

[0030] Figure 2 is the battery cycle result of Example 8. DETAILED DESCRIPTION OF THE INVENTION

[0031] In the embodiment of the present invention, the lithium iron phosphate cathode is taken as an example, and its actual capacity is 145 mAh / g; natural graphite coated with carbon is used as the anode material, and its actual capacity is 350 mAh / g. The embodiments of the present invention are only further elaboration and explanation of the technology of the present invention, and do not limit the present invention. The present invention is not limited to the following embodiments, and any change within the scope of the claims belongs to the scope of the present invention.

[0032] The carbon-coated silicon monoxide is purchased from Yijin Company, and the grade is HC1500.

[0033] Example 1

[0034] (1) Preparation of lithium iron phosphate cathode electrode: 94 wt% of lithium iron phosphate cathode material, 4 wt% of conductive carbon (Super P), and 2 wt% of PVDF binder are pre-mixed, then N-methylpyrrolidone (NMP) solvent is added for mechanical stirring to make a slurry, the viscosity of the slurry is adjusted, and then it is coated on both sides of an aluminum foil with a thickness of 0.013 mm, and the double-sided surface loading is 30.7 mg / cm 2 , and then the electrode is roll-pressed, and the compaction density is 2.4 g / cm 3 , and after slitting, a lithium iron phosphate cathode electrode is obtained.

[0035] (2) Preparation of composite pole pieces containing lithium storage agent: The lithium storage agent is carbon-coated silicon oxide, and its actual specific capacity is 1480mAh / g. The mass ratio of graphite and silicon oxide is 91:9 for premixing, and then 95wt% of the graphite / silicon oxide mixed negative electrode is mixed with 1.5wt% of the conductive agent (Super P), and 1.1wt% of sodium carboxymethyl cellulose (CMC) glue is added for stirring and mixing, and then 2.4wt% of styrene-butadiene rubber latex (SBR) is added for mixing, and the viscosity is adjusted to obtain a slurry, wherein the mass ratio of the graphite / silicon oxide mixed negative electrode, conductive agent, sodium carboxymethyl cellulose and styrene rubber latex is 95:1.5:1.1:2.4. The prepared slurry is then coated on both sides of a copper foil with a thickness of 0.008mm, and the double-sided surface loading is 11mg / cm 2 The pole piece is rolled and compacted to a density of 1.55 g / cm 3 After cutting, a graphite / silicon dioxide composite negative electrode sheet is obtained.

[0036] (3) Soft-pack battery assembly: The positive electrode sheet, the negative electrode sheet and the polypropylene separator are stacked by a stacking machine and sealed in an aluminum-plastic film bag. After drying, adding electrolyte (1 mol / L lithium hexafluorophosphate / ethylene carbonate / diethyl carbonate / fluoroethylene carbonate), and sealing, a soft-pack battery is obtained.

[0037] (4) Battery formation: The battery was left at room temperature and 45°C for 12 h respectively, and then formed at a rate of 0.01C. After formation, the battery was vented and sealed again.

[0038] (5) Battery cycle: Use the Xinwei battery tester (model CT-4008-5V12A) according to Figure 1 The setting program shown is used to perform a cycle test on the battery: the battery is cycled at a rate of 0.5C, the charge cut-off voltage is 3.65V, the discharge cut-off voltage is 3.0V, and the initial capacity of the battery is 1.48Ah; when the battery capacity decays to 80% of the initial capacity, that is, 1.184Ah, the charge cut-off voltage of 3.65V remains unchanged, the discharge cut-off voltage is reduced to 2.9V, and a charge and discharge cycle is performed, and the battery capacity is restored to 1.347Ah; the charge and discharge cycle of the lithium-ion battery is continued, and when the battery capacity decays to 80% of the initial capacity again, the charge cut-off voltage remains unchanged, the discharge cut-off voltage is reduced to 2.8V, and a charge and discharge cycle is performed, and the battery capacity is restored to 1.321Ah; the above operation is repeated, the charge cut-off voltage is kept unchanged at 3.65V, and the discharge cut-off voltage is reduced to 2.7V, 2.6V, and 2.5V respectively. When the discharge cut-off voltage drops to 2.5V, the lithium-ion battery maintains a charge cut-off voltage of 3.65V and a discharge cut-off voltage of 2.5V for cycling until the end.

[0039] Figure 1It is the battery cycling program of Example 1.

[0040] Example 2

[0041] The rest is the same as Example 1, except that in step (2), the mass ratio of graphite to silicon suboxide is 95:5, and the double-sided surface loading of the obtained electrode sheet is 12.2 mg / cm 2 .

[0042] Example 3

[0043] The rest is the same as Example 1, except that in step (2), the mass ratio of graphite to silicon suboxide is 99:1, and the double-sided surface loading of the obtained electrode sheet is 13.7 mg / cm 2 .

[0044] Example 4

[0045] The rest is the same as Example 1, except that in step (2), pre-lithiated silicon suboxide is used as the lithium storage agent, with a capacity of 1350 mAh / g and a first Coulombic efficiency of 91%, and the double-sided surface loading of the obtained electrode sheet is 11.2 mg / cm 2 .

[0046] Example 5

[0047] The rest is the same as Example 1, except that in step (3), a polypropylene separator coated with a Li5FeO4 lithium supplement agent is used for soft-pack battery assembly, and the thickness of the lithium supplement agent coating is 5 μm.

[0048] Example 6

[0049] The rest is the same as Example 1, except that after obtaining the coated graphite / silicon suboxide electrode sheet in step (2), stable lithium metal powder is dispersed in toluene solvent and evenly sprayed on the surface of the above electrode sheet (0.2 mg / cm 2 ), and then after drying, rolling and slitting, it is used for soft-pack battery assembly.

[0050] Example 7

[0051] The rest is the same as Example 1, except that 1% of Li5FeO4 lithium supplement agent is added in the preparation of the lithium iron phosphate positive electrode sheet in step (1).

[0052] Example 8

[0053] The rest is the same as in Example 1, except that the cycling program used during the battery cycling in step (5) is as follows: each time the battery capacity decays to 80% of the initial capacity, the discharge cut-off voltage is reduced by 0.05 V. That is, during cycling, the discharge cut-off voltages are 3.0 V, 2.95 V, 2.90 V, 2.85 V, 2.80 V, 2.75 V, 2.70 V, 2.65 V, 2.60 V, 2.55 V, and 2.50 V in sequence. Figure 2 It is the battery cycling result of Example 8. During the cycling process, the discharge cut-off voltage of the battery is sequentially reduced to 3.0 V, 2.95 V, 2.90 V, 2.85 V, 2.80 V, 2.75 V, 2.70 V, 2.65 V, 2.60 V, 2.55 V, and 2.50 V.

[0054] Comparative Example 1

[0055] The rest is the same as in Example 1, except that the cycling program used during the battery cycling in step (5) is as follows: the battery is cycled at a rate of 0.5 C, the charge cut-off voltage is 3.65 V, and the discharge cut-off voltage is 2.5 V; when the battery capacity decays to 80% of the initial capacity, the cycling ends.

[0056] Comparative Example 2

[0057] The rest is the same as in Example 1, except that no lithium storage agent is added in step (2), and the areal loading on both sides of the prepared electrode is 14.1 mg / cm 2 , and in addition, the cycling program used during the battery cycling in step (5) is the same as that in Example 1, that is, the charge cut-off voltage is 3.65 V, the initial discharge cut-off voltage is 3.0 V, and each time the battery capacity decays to 80% of the initial capacity, the discharge cut-off voltage is sequentially reduced to 2.9 V, 2.8 V, 2.7 V, 2.6 V, and 2.5 V through the battery test device.

[0058] Table 1

[0059]

[0060] Table 1 shows the comparison of the cycling performance of the batteries prepared in different examples and comparative examples. By comparing Example 1 with Comparative Example 1 and Comparative Example 2, it can be seen that by cycling in the manner of adding a lithium storage agent to the negative electrode and controlling the discharge cut-off voltage in stages provided by the present invention, the cycling life of the battery is significantly increased. In addition, by comparing Example 1 with Examples 4-7, it can be seen that by combining the present invention with the lithium supplementation and prelithiation technologies, the capacity of the battery can be increased while significantly extending the cycling life of the lithium-ion battery. By comparing Example 1 with Example 8, it can be seen that reducing the reduction amplitude of the discharge cut-off voltage is beneficial to further extending the battery cycling life.

Claims

1. A method for extending the cycle life of a lithium-ion battery, characterized in that, It includes the following steps: (S1) Control the initial discharge cut-off voltage to be the voltage corresponding to 60 - 90% of the full discharge capacity of a lithium-ion battery composed of a graphite negative electrode without a lithium storage agent and the corresponding positive electrode material; (S2) During the charge-discharge cycle, control the battery through the battery management system. When the battery capacity decays to 80 - 85% of the initial rated capacity, reduce the battery discharge cut-off voltage to restore the battery capacity; The amplitude of the reduction in the discharge cut-off voltage is 0.01 - 0.2 V; The amplitude of the reduction in the discharge cut-off voltage is such that during the entire cycle of the lithium-ion battery, the discharge cut-off voltage is reduced 5 - 15 times; (S3) Repeat step (S2) until the discharge cut-off voltage is reduced to 1.5 - 2.5 V, or the active lithium in the lithium storage agent is completely released; The long-cycle-life lithium-ion battery includes a positive electrode, a negative electrode, a separator, an electrolyte, a battery case, electrode tabs, and a battery management system. The negative electrode is made of graphite, a lithium storage agent, a conductive agent, a binder, and a current collector. The lithium storage agent is selected from silicon monoxide, silicon-carbon composite, carbon-coated silicon, carbon-coated silicon monoxide, doped silicon, doped silicon monoxide, silicon alloy, germanium, germanium alloy, carbon-coated germanium, germanium-carbon compound, tin, tin alloy, carbon-coated tin, and tin-carbon compound; the discharge cut-off voltage is controlled by the battery management system, and when the battery capacity decays to a set value, the discharge cut-off voltage of the lithium-ion battery is automatically reduced.

2. The method according to claim 1, wherein In step (S2), the amplitude of the reduction in the discharge cut-off voltage is 0.05 - 0.1 V.

3. The method according to claim 1, wherein The positive electrode material of the lithium-ion battery is any one or a combination of two or more of lithium iron phosphate, lithium vanadium phosphate, lithium manganese phosphate, lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel manganate, nickel cobalt manganese ternary positive electrode material, nickel cobalt aluminum ternary positive electrode material, and lithium-rich manganese-based positive electrode material; the negative active material of the negative electrode is a graphite material.

4. The method according to claim 3, characterized in that, The positive electrode material of the lithium-ion battery is lithium iron phosphate, and in the negative active material of the negative electrode, graphite accounts for 80 - 99 wt%.

5. The method according to claim 1, wherein The addition amount of the lithium storage agent is 0.5 - 20 wt% of the total weight of the electrode material.

6. The method according to claim 1, characterized in that, The addition amount of the lithium storage agent is 0.95 - 8.55 wt% of the total weight of the electrode material.

7. The method according to claim 1, characterized in that, The lithium storage agent is carbon-coated silicon monoxide.

8. The method according to claim 7, wherein The particle size of the carbon-coated silicon monoxide is 1 - 20 μm, the carbon content is 1 - 8 wt%; the thickness of the carbon coating layer is 5 - 30 nm.

9. The method according to claim 1, wherein In step (S1), when the positive electrode material is lithium iron phosphate, its initial discharge cut-off voltage is 2.9 - 3.1 V.

10. The method according to claim 9, characterized in that, When the positive electrode material is lithium iron phosphate, its initial discharge cut-off voltage is 2.95 - 3.0 V.

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