Capacity restoration method for lithium metal battery and lithium metal battery

Abstract

Disclosed in the present invention is a capacity restoration method for a lithium metal battery. A lithium metal battery has a conventional charging rate of C1, and a conventional discharging rate of C2, and the lithium metal battery capacity restoration method comprises: primary discharging: performing discharging at the conventional discharging rate C2 until a full-discharge state is reached; charging: performing charging at a charging rate C3 until a preset charging cut-off voltage is reached, and maintaining the preset charging cut-off voltage for constant voltage charging until the charging current is less than 0.025C, wherein 0.05C1≤C3≤0.8C1, and C is the nominal capacity of the lithium metal battery; and secondary discharging: performing discharging at a discharging rate C4 until a preset discharging cut-off voltage is reached, 0.05C1≤C4≤0.8C1. In this way, the capacity can be restored in a timely manner during the use of lithium metal batteries, and the restoration process can be accurately controlled on the basis of the actual condition of the batteries, thereby effectively improving the cycle performance of lithium metal batteries and prolonging the cycle life thereof. Further disclosed in the present invention is a lithium metal battery, the capacity of which may be restored by means of the capacity restoration method.

Description

Lithium metal battery capacity repair method and lithium metal battery TECHNICAL FIELD

[0001] The present application relates to the field of lithium metal batteries, in particular to a lithium metal battery capacity repair method and a lithium metal battery. BACKGROUND

[0002] With the popularity of consumer electronics such as notebook computers, mobile phones, handheld game consoles, tablet computers, mobile power supplies and drones, people have increasingly strict requirements for the electrochemical devices (e.g., batteries) therein. For example, people not only require the batteries to be light, but also require the batteries to have short charging times and long working lives.

[0003] Among many batteries, lithium metal has the lowest chemical potential (-3.04V) and very high reactivity, and is highly concerned in the research and development field due to high energy density. However, the cycle performance of lithium metal batteries is unstable, and the cycle life is short, which is also a major problem limiting the application of lithium metal batteries.

[0004] The cycle performance deterioration and cycle life shortening in lithium metal batteries are mainly caused by the following factors: 1. The lithium metal negative electrode reacts with the electrolyte to form a solid-state electrolyte interface (SEI), the active lithium is lost, and due to the formation of lithium dendrites and the expansion and contraction of the pole piece, the SEI film is also continuously broken and regenerated, and the electrolyte and active lithium are further lost; 2. Lithium metal as a negative active material, in the process of repeated deposition and stripping, part of the lithium metal loses electrochemical activity and becomes dead lithium; 3. With the continuous increase of battery impedance during the cycle process, the battery polarization continuously increases.

[0005] At present, there are also various ways to improve the cycle performance of the battery, such as constructing an artificial SEI film on the surface of the pole piece, designing the composition and structure of the pole piece, etc., which prepares for the above problems in the manufacturing stage. However, in the running process of the lithium metal battery, the behavior of lithium ions, the interaction between the electrolyte and the pole piece, the occurrence of side reactions and other factors that affect the performance of the battery are difficult to control, and if only prevention is carried out in the manufacturing stage, the effect may not be as expected, and it cannot be flexible and dynamic to adjust and optimize the lithium metal battery. Moreover, the deteriorated battery performance that has already occurred cannot be timely handled. In addition, how to repair the battery capacity is also a problem worth considering.

[0006] At the same time, if the improvement of the battery structure and composition can be combined with the capacity repair method, the battery performance can also be better improved in the whole process from manufacturing to application. SUMMARY

[0007] To solve the above technical problems, the embodiment of the present application discloses a lithium metal battery capacity repair method, the conventional charge rate of the lithium metal battery is C1, the conventional discharge rate of the lithium metal battery is C2, and the lithium metal battery capacity repair method comprises the following steps:

[0008] Primary discharge: discharging at a conventional discharge rate C2 until a full discharge state is reached;

[0009] Charging: charging at a charge rate C3 until a preset charge cut-off voltage is reached, maintaining constant voltage charging at the preset charge cut-off voltage until the charging current is <0.025C, wherein 0.05C1≤C3≤0.8C1, and C is the nominal capacity of the lithium metal battery;

[0010] Secondary discharge: discharging at a discharge rate C4 until a preset discharge cut-off voltage is reached, wherein 0.05C1≤C4≤0.8C1.

[0011] The above technical solution can repair the capacity of the lithium metal battery in a timely manner during its use, can process existing battery performance degradation factors, and can accurately control the repair process according to the actual situation of the battery, effectively improving the cycle performance of the lithium metal battery and prolonging its cycle life.

[0012] Optionally, the lithium metal battery capacity is repaired by the lithium metal battery capacity repair method every X cycles, wherein 0≤X≤150.

[0013] Optionally, in the secondary discharge step, before the preset discharge cut-off voltage is reached, it further comprises discharging at a discharge rate C4 for t3 hours, and then maintaining static for a t4 time period, wherein 0.35C / C4≤t3≤0.85C / C4, and 6h≤t4≤30h.

[0014] Optionally, in the primary discharge step, it further comprises maintaining static for a t1 time period after the full discharge state is reached, wherein 2min≤t1≤30min; and / or, in the charging step, it further comprises maintaining static for a t2 time period after the charging current is <0.025C, wherein 2min≤t2≤30min; and / or, in the secondary discharge step, it further comprises maintaining static for a t5 time period after the preset discharge cut-off voltage is reached, wherein 2min≤t5≤30min.

[0015] Optionally, the electrolyte composition of the lithium metal battery comprises 1wt%-95wt% of fluorine ether compounds and 1wt%-45wt% of ether compounds in terms of mass percentage, and the electrolyte further comprises lithium salt with a molar concentration of 0.1M-5M, wherein the molecular structure of the fluorine ether compound is shown in formula (I), and the molecular structure of the ether compound is shown in formula (II).

[0016] wherein R1 and R2 are each independently selected from C1-C10 haloalkyl or C2-C10 haloalkenyl, and R3 and R4 are each independently selected from C1-C10 alkyl or C1-C10 alkoxy.

[0017] Optionally, the electrolyte further comprises 0.01wt%-10wt% of an additive, by mass percentage, the additive being one or more of fluoroethylene carbonate, vinyl ethylene carbonate, lithium bisoxalato borate, lithium bisfluorosulfonylimide, lithium tetrafluoroborate, methylene methane disulfonate, 4-trifluoromethylvinyl carbonate, 1,3,2-dioxazolothiophene-2,2-dioxide, fluoroethylene carbonate, ethylene sulfite, vinylene carbonate, succinic anhydride, propylene sulfite, propenyl-1,3-sultone, bis(trimethylsilyl) sulfite, lithium nitrate, N-methyl, butylpyrrolidinium bis-trifluoromethylsulfonylimide salt, N-methyl, propylpiperidinium bis-trifluoromethylsulfonylimide salt, 1,3-dioxane, 1,4-dioxane, 1,2-bis(cyanoethoxy)ethane, hexanedinitrile, 1,3,5-pentanetricarbonitrile, mesoconic dinitrile, and 1,2,3-tris(2-cyanoethoxy)propane.

[0018] Optionally, the fluoroether compound is one or more of 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and perfluorodiethyl ether; and / or, the ether compound is one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, and diethylene glycol dimethyl ether; and / or, the lithium salt is one or more of lithium perchlorate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis-trifluoromethylsulfonylimide, lithium bisfluorosulfonylimide, lithium bisoxalato borate, lithium bisfluorosulfonylimide, lithium tetrafluoroborate, lithium trifluoromethylsulfonate, and lithium difluorophosphate.

[0019] Optionally, the fluoroether compound has a molecular structure as shown in formula (I-a), the ether compound has a molecular structure as shown in formula (II-a) or formula (II-b), and the lithium salt is lithium bis-trifluoromethylsulfonylimide,

[0020] Optionally, the electrolyte composition of the lithium metal battery comprises 55wt%-87wt% of the fluoroether compound and 13wt%-45wt% of the ether compound, by mass percentage, and the electrolyte further comprises lithium bis-trifluoromethylsulfonylimide at a molar concentration of 1.5M-3.5M.

[0021] According to another specific embodiment of the present application, the embodiments of the present application disclose a lithium metal battery which can be capacity repaired in the capacity repair mode of the lithium metal battery as described above.

[0022] By adopting the technical scheme, capacity repair can be performed on the lithium metal battery in a timely manner in the use process of the lithium metal battery, so that the lithium metal battery runs more stably, has a longer service life, and is more reliable in application. BRIEF DESCRIPTION OF DRAWINGS

[0023] FIG. 1 shows a scanning electron microscope diagram of lithium metal deposition under electrolyte of a lithium metal battery in an embodiment of the present application;

[0024] FIG. 2 shows a scanning electron microscope diagram of lithium metal deposition under electrolyte of a conventional lithium metal battery. DETAILED DESCRIPTION

[0025] The present application is described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0026] In order to make the objects, technical solutions and advantages of the present application clearer, the embodiments of the present application will be further described in detail below.

[0027] In the embodiments of the present application, the "normal charge rate" refers to the charge rate used under normal use conditions, which can be different for different batteries and applications. For example, the normal charge rate of a common lithium metal battery is usually ≥ 0.2C.

[0028] In the embodiments of the present application, the "normal discharge rate" refers to the discharge rate used under normal use conditions, which can be different for different batteries and applications. For example, the normal discharge rate of a common lithium metal battery is usually ≥ 0.2C.

[0029] In the embodiments of the present application, the "nominal capacity" refers to the amount of charge that a battery can release under normal conditions (such as specific temperature, current, voltage, and discharge rate). For lithium metal batteries, the nominal capacity is usually expressed in ampere-hours (Ah) or milliampere-hours (mAh). For example, a battery with a nominal capacity of 2000 mAh means that under ideal conditions, the battery can continuously output a current of 2000 mA for one hour.

[0030] In embodiments of the present application, "full discharge state" refers to a state in which a battery has released most or all of its available charge during discharge, reaching a minimum level of charge at which the battery can be discharged.

[0031] In embodiments of the present application, "full charge cut-off voltage" refers to a voltage value at which a charger or charge management system automatically stops charging a battery during charging, when the battery voltage reaches a specific high voltage level, in order to prevent overcharging and protect the safety and life of the battery.

[0032] In embodiments of the present application, "discharge cut-off voltage" refers to a voltage threshold at which a battery management system or related circuit automatically stops the battery from supplying power externally, when the battery voltage drops to a specific low voltage level during discharge, in order to prevent damage caused by over-discharge.

[0033] In embodiments of the present application, "haloalkyl" refers to a group formed by replacing one or more hydrogen atoms in an alkyl group with a halogen atom, such as chloromethane, bromoethane, etc.

[0034] In embodiments of the present application, "haloalkenyl" refers to a group formed by replacing one or more hydrogen atoms in an alkenyl group with a halogen atom, such as chloroethylene, bromopropylene, etc.

[0035] In embodiments of the present application, "alkyl" refers to a saturated carbon-hydrogen chain, such as methyl (CH3-), n-propyl (CH3CH2CH2-), isopropyl ((CH3)2CH-), etc.

[0036] In embodiments of the present application, "alkenyl" refers to a straight-chain or branched group containing a carbon-carbon double bond, such as ethenyl (H2C=CH-).

[0037] In embodiments of the present application, "alkoxy" refers to a group formed by replacing one hydrogen atom in an alkyl group with an oxygen atom, such as methoxy (CH3O-).

[0038] The first aspect of the present application discloses a lithium metal battery capacity repair method, wherein the conventional charge rate of the lithium metal battery is C1 and the conventional discharge rate is C2, specifically, C1≥0.2C and C2≥0.2C. Specifically, the lithium metal battery capacity repair method is performed when the lithium metal battery is in an idle state, i.e., the lithium metal battery is not in a power supply state. For example, the battery can be provided with or connected to a control system. When the control system determines that the battery is not in a working state at this time, the capacity repair can be automatically performed to avoid conflicts with the working state of the battery and to avoid interference with its normal operation.

[0039] The lithium metal battery capacity repair method includes the following steps:

[0040] Primary discharge: discharge at a regular discharge rate C2 until full discharge state;

[0041] Charging: charge at a charge rate C3 until a preset charge cut-off voltage is reached, maintain the preset charge cut-off voltage for constant voltage charging until the charging current < 0.025C, wherein 0.05C1≤ C3≤ 0.8C1, C is the nominal capacity of the lithium metal battery;

[0042] Secondary discharge: discharge at a discharge rate C4 until a preset discharge cut-off voltage is reached, wherein 0.05C1≤ C4≤ 0.8C1.

[0043] Specifically, the above charging step and secondary discharge step are repeated 1-5 times to complete the repair. More specifically, the above charging step and secondary discharge step are repeated 2 times.

[0044] First, in the primary discharge step, the lithium metal battery is discharged at a regular discharge rate C2 to a full discharge state, which is a constant capacity process, and the capacity that can be used before capacity repair can be obtained. The state of the battery can be evaluated first, and the subsequent repair process can be adjusted accordingly. After capacity repair, it can also be compared with the first circle of normal discharge after capacity repair. The capacity value that can be improved by capacity repair can be clearly obtained, and the subsequent running state of the battery can be further evaluated, which is convenient for starting the capacity repair program at the appropriate time.

[0045] Secondly, in the charging and secondary discharging steps, charging and discharging are carried out at a rate C3 and C4 less than the conventional charging rate C1, which is the main process for capacity repair. In the normal charging and discharging process, the charging and discharging rate is fast, and the speed of lithium ions shuttling between the positive and negative electrodes is also fast, which leads to the deposition of lithium ions without time to find a suitable place after reaching the electrode sheet. For example, lithium ions reach the negative electrode surface and directly deposit without being reduced into lithium metal, leading to the accumulation of lithium ions. Moreover, after repeated cycles of lithium metal batteries, the surface charge of the lithium metal negative electrode is different, and during the recharging process, lithium ions preferentially deposit on the exposed, fresh lithium metal surface, so there will be uneven lithium deposition, and local areas will appear as strip or needle-shaped lithium morphology, known as lithium dendrites. Then, during the discharging process, lithium tends to start peeling off at the bottom of the strip or needle-shaped lithium block, and when the lithium at the bottom is completely peeled off, the lithium at the tip loses electrical contact and becomes dead lithium, losing active lithium, leading to gradual attenuation of the cycle, thus producing dead lithium. As a result, this will lead to an increase in battery internal resistance, a decrease in charging and discharging efficiency, a decrease in energy density, and even safety hazards. However, by charging and discharging at a smaller rate C3 and C4, the deposition rate of lithium ions is slower, and the deposition behavior of lithium ions is more controllable, which can re-establish the electrochemical connection of the dead lithium that has lost electrical contact and restore some of the dead lithium to active lithium, thereby restoring some of the capacity. In addition, during the operation of the battery, lithium may be surrounded by an insulating SEI film, thereby losing contact with the electrode material and losing active lithium. However, the SEI film is not airtight, and the SEI film is also in dynamic change, so charging and discharging at a rate C3 and C4 less than the conventional charging rate C1 can re-establish the electrochemical connection through controllable lithium ion behavior and repair the capacity.

[0046] Specifically, charging is performed at a charging rate C3 until a preset charging cutoff voltage is reached, and then constant voltage charging is performed at the preset charging cutoff voltage until the charging current < 0.025C, because although the preset charging cutoff voltage is reached on the surface, in fact, due to the existence of battery impedance, the preset charging cutoff voltage may not actually be reached, the repair of this process does not achieve the desired effect, so constant voltage charging can be continued at the preset charging cutoff voltage until the charging current < 0.025C, ensuring that the capacity repair in this step is fully performed. Wherein, "charging current < 0.025C" means that the charging current is less than 0.025 times the nominal capacity of the battery. For example, if the nominal capacity of a battery is 2000mAh, the charging current < 0.025C means that the charging current is less than 50mA. More specifically, the preset charging cutoff voltage is the full charge cutoff voltage. Wherein, 0.05C1≤C3≤0.8C1, which can adapt to the needs of different batteries, ensuring the efficiency and safety of the repair process. And if C3 is too small, the charging and discharging speed of the lithium metal battery is too slow, the reaction time on the negative electrode surface is too long, and the interface side reaction for too long time will cause too much by-product, resulting in too much loss of active lithium involved in the reaction, ultimately affecting the cycle performance; if C3 is too large, the charging and discharging speed of the lithium metal battery is too fast, the fast charging cannot restore the dead lithium that has lost electrochemical activity, and the fast discharging will form more dead lithium, thereby affecting the cycle performance. Further, the charging current < 0.25C can further ensure the recovery effect in the charging step.

[0047] Specifically, in the secondary discharging step, the preset discharging cutoff voltage is the battery specified discharging cutoff voltage. In addition, C4 satisfies 0.05C1≤C4≤0.8C1, if C4 is too small, the charging and discharging speed of the lithium metal battery is too slow, the reaction time on the negative electrode surface is too long, and the interface side reaction for too long time will cause too much by-product, resulting in too much loss of active lithium involved in the reaction, ultimately affecting the cycle performance; if C4 is too large, the charging and discharging speed of the lithium metal battery is too fast, the fast charging cannot restore the dead lithium that has lost electrochemical activity, and the fast discharging will form more dead lithium, thereby affecting the cycle performance.

[0048] Further, based on the capacity determination process of the primary discharging step, C3, C4 and the charging current value can be actually selected in the subsequent charging step and the secondary step, which can realize more targeted capacity repair for different batteries, and improve the reliability of the repair. In the capacity repair method of the present application, the primary discharging step, the charging step and the secondary discharging step cooperate with each other, and can adaptively repair the batteries in different working conditions and different states, which is more effective and safe.

[0049] In some embodiments of the present application, the lithium metal battery is subjected to capacity restoration in the above-mentioned manner every X cycles, wherein 0≤X≤150. That is, the battery is subjected to one complete charging and discharging cycle by charging at a charging rate C1 to full charge and then discharging at a discharging rate C2 to full discharge. During the use of the battery, capacity restoration is required every time X complete charging and discharging cycles are completed. If X>150, the battery already has a large amount of dead lithium, and the surface of the dead lithium is coated with an electron-insulating SEI during the cycle, so the restoration effect may be limited. Preferably, 30≤X≤80. In this range, the restoration can be performed in time before the performance of the battery begins to decrease significantly, and the effect of capacity restoration can be maximized, thereby ensuring the long-term stable operation of the battery and taking into account the economic benefits and effect realization.

[0050] In some embodiments of the present application, in the second discharging step, before reaching the preset discharging cut-off voltage, a step of discharging at C4 for t3 hours is further included, followed by a step of keeping still for t4 hours, wherein 0.35C / C4≤t3≤0.85C / C4 and 6h≤t4≤30h. By controlling the discharging time in the range of 0.35C / C4 to 0.85C / C4, the effective recovery of the capacity of the battery can be further ensured, which helps to activate the active substances inside the battery and improve the electrochemical performance thereof. After the t3 hours of discharging is completed, the battery enters a period of keeping still for t4 hours. The length of this period is set to be between 6 hours and 30 hours. The introduction of the period of keeping still is to provide a buffering and recovery stage for the battery after the discharging process. During this period, the chemical substances inside the battery can be redistributed and stabilized, which helps to improve the overall stability and safety of the battery and improve the cycle performance of the battery.

[0051] In some embodiments of the present application, in the first discharging step, a step of keeping still for t1 hours after reaching the full discharge state is further included, wherein 2min≤t1≤30min; and / or, in the charging step, a step of keeping still for t2 hours after the charging current is less than 0.025C is further included, wherein 2min≤t2≤30min; and / or, in the second discharging step, a step of keeping still for t5 hours after reaching the preset discharging cut-off voltage is further included, wherein 2min≤t5≤30min.

[0052] Preferably, in the first discharging step, a step of keeping still for t1 hours after reaching the full discharge state is further included, wherein 2min≤t1≤30min; in the charging step, a step of keeping still for t2 hours after the charging current is less than 0.025C is further included, wherein 2min≤t2≤30min; and in the second discharging step, a step of keeping still for t5 hours after reaching the preset discharging cut-off voltage is further included, wherein 2min≤t5≤30min.

[0053] By the above-mentioned manner, the battery can be further given a buffer period and a recovery period, so as to ensure the stability and uniformity of the active ingredients.

[0054] In a specific embodiment of the present application, an electrolyte is also provided for cooperating with the above-mentioned capacity repairing method of lithium battery. In the electrolyte, the electrolyte components include 1wt%-95wt% fluorine ether compound and 1wt%-45wt% ether compound in terms of mass percentage, and further include 0.1M-5M molar concentration of lithium salt. The molecular structure of the fluorine ether compound is shown in formula (I), and the molecular structure of the ether compound is shown in formula (II),

[0055] wherein R1 and R2 are each independently selected from C1-C10 haloalkyl or C2-C10 haloalkenyl, and R3 and R4 are each independently selected from C1-C10 alkyl or C1-C10 alkoxy.

[0056] In the electrolyte, the fluoroether compound with the above structure has good stability to the lithium metal negative electrode and is difficult to react with the active lithium metal, and a tight positive electrode protective film can be formed at the positive electrode. The fluoroether compound can play the role of a diluent in the electrolyte due to its low viscosity, thereby reducing the viscosity of the electrolyte. The ether compound with the above structure has good stability to the lithium metal negative electrode and is difficult to react with the active lithium metal, and the ether compound has very good solubility to the lithium salt and can fully dissolve the lithium salt. The ether compound with the above structure is a very good lithium metal electrolyte solvent, and the ether compound forms a five-membered ring structure when solvating lithium, which is more stable. The lithium salt provides lithium ion concentration in the electrolyte, and under the transport of the electrolyte, lithium ions are transferred from the positive electrode to the negative electrode, thereby enabling normal charging and discharging of the battery. The lithium salt can also be reduced at the negative electrode to form a negative electrode protective film, thereby improving the cycle performance of the lithium metal battery. The inventors have found that by designing the above components and further designing the content of each component, the electrolyte can fully cooperate with the battery capacity repair method to improve the effect of battery capacity repair. In particular, the inventors have found that, as shown in FIG. 1, in the electrolyte of the present application, the deposited lithium metal core size is large, and the lithium core diameter is about 5-10 um. Such morphology of lithium core is not easy to form lithium dendrites, and the battery performance is stable. In the traditional electrolyte, as shown in FIG. 2, the lithium core size is small, and lithium dendrites are easily generated. The inventors further consider that the lithium core particles in the electrolyte of the present application are large, and the contact between the particles will gradually deteriorate after repeated charging and discharging. Therefore, in cooperation with the above capacity repair method, the battery capacity can be greatly improved. Through the electrolyte, the battery performance can be optimized during battery manufacturing, and through the capacity repair method, the battery capacity can be repaired in time during battery application, and the electrolyte and the capacity repair method also have a synergistic effect. The effect of the capacity repair method is better in the electrolyte system, thereby ensuring the performance of the battery throughout the whole process from battery manufacturing to use.

[0057] Preferably, in the electrolyte, the electrolyte components include 55wt%-95wt% of the fluoroether compound and 5wt%-45wt% of the ether compound, and the molar concentration of the lithium salt is 0.8M-3.5M. The component electrolyte has a higher degree of cooperation with the capacity repair method, and the repair effect is better.

[0058] In some embodiments of the present application, the electrolyte further comprises 0.01wt%-10wt% of an additive by mass percentage, the additive including but not limited to one or more of fluoroethylene carbonate, ethylene sulfite, vinylene carbonate, succinic anhydride, propylene sulfite, propylene-1,3-sultone, bis(trimethylsilyl) sulfite, lithium nitrate, N-methyl, butylpyrrolidinium bis(trifluoromethylsulfonyl) imide salt, N-methyl, propylpiperidinium bis(trifluoromethylsulfonyl) imide salt, 1,3-dioxane, 1,4-dioxane, 1,2-bis(cyanoethoxy)ethane, hexanedinitrile, 1,3,5-pentanetricarbonitrile, mesoxalonitrile, and 1,2,3-tris(2-cyanoethoxy)propane. These additives play an important role in lithium battery electrolytes, such as improving the high-temperature performance, cycle life, and safety performance of the battery. By precisely controlling the type and content of the additive, the overall performance of the battery can be optimized to meet the needs of different application scenarios. Preferably, the additive is fluoroethylene carbonate (FEC).

[0059] In some embodiments of the present application, the fluoroether compound includes but is not limited to one or more of 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and perfluorodiethyl ether.

[0060] In some embodiments of the present application, the ether compound includes but is not limited to one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, and diethylene glycol dimethyl ether.

[0061] In some embodiments of the present application, the lithium salt includes but is not limited to one or more of lithium perchlorate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(trifluoromethylsulfonyl) imide, lithium bis(fluorosulfonyl) imide, lithium bis(oxalato)borate, lithium bis(fluorooxalato)borate, lithium tetrafluoroborate, lithium trifluoromethylsulfonate, and lithium difluorophosphate.

[0062] These fluoroether compounds, ether compounds, and lithium salts are better suited for use in electrolytes and are more suitable for use in combination with battery capacity repair methods.

[0063] In some embodiments of the present application, the molecular structure of the fluoroether compound is shown in formula (I-a), the molecular structure of the ether compound is shown in formula (II-a) or formula (II-b), and the lithium salt is lithium bis(trifluoromethylsulfonyl) imide,

[0064] In some embodiments of the present application, the molecular structure of the lithium salt is shown in formula (III-a):

[0065] Further, the electrolyte further comprises an additive, and the molecular structure of the additive is shown as formula (IV-a):

[0066] The electrolyte with the component structure of the above formula not only has stronger cooperation between components, better battery performance, but also can fully cooperate with the capacity repair method of the battery, and the repair effect is obvious.

[0067] Specifically, in the electrolyte, the fluoroether compound with the molecular structure of formula (I-a) accounts for 55wt%-87wt%, the ether compound with the molecular structure of formula (II-a) or formula (II-b) accounts for 13wt%-45wt%, and the molar concentration of lithium salt lithium bis-trifluoromethylsulfonylimide is 1.5-3.5M. More specifically, the electrolyte further comprises 0-2wt% of the additive with the molecular structure shown as formula (IV-a) in terms of mass percentage. Through the combination of the above components, the cycle performance of the battery can be effectively improved, and the capacity repair effect is obvious.

[0068] The second aspect of the present application discloses a lithium metal battery, which can be repaired in capacity according to the capacity repair method of the lithium metal battery of the present application, has more stable performance and longer service life.

[0069] In a specific embodiment of the present application, the lithium metal battery further has the electrolyte of the present application.

[0070] The following will be described in conjunction with more specific embodiments.

[0071] Examples 1-13 and Comparative Examples 1-2 are for capacity repair of lithium metal batteries, and the specific preparation method of the lithium battery used in Examples 1-13 and Comparative Examples 1-2 is as follows:

[0072] (1) Preparation of negative electrode sheet

[0073] The metal lithium is compounded on the negative electrode current collector copper foil with a thickness of about 12μm by physical rolling, the copper current collector is single-sided coated with lithium by adjusting the pressure of the roller, and the thickness is controlled to be about 50μm. Then, after cutting and slitting, it is placed in a dry argon atmosphere glove box for storage for standby use.

[0074] (2) Preparation of positive electrode

[0075] The positive active material lithium iron phosphate (LFP), the conductive agent (conductive carbon of Super P), and the binder polyvinylidene fluoride are mixed in a weight ratio of about 97:1.4:1.6, N-methyl pyrrolidone (NMP) is added, and the system is stirred to uniformity under the action of a vacuum stirrer to obtain a positive electrode slurry, wherein the solid content of the positive electrode slurry is about 72 wt%; the positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil; after drying, cold pressing, cutting, and slitting, the positive electrode is obtained by drying at about 85°C under vacuum for about 4h.

[0076] (3) Preparation of the separator

[0077] A polyethylene (PE) with a thickness of about 15μm is used as the separator film.

[0078] (4) Preparation of the electrolyte

[0079] In a dry argon atmosphere, the components shown in Table 1 are mixed in the amounts shown, and then lithium salt is added to the obtained mixed solution to dissolve uniformly, thereby obtaining the electrolyte. In Table 1, the molecular structure of the fluoroether compound is shown in formula (I-a), the molecular structure of the ether compound is shown in formula (II-a) or formula (II-b), and the lithium salt is lithium bis(trifluoromethylsulfonyl)imide,

[0080] The structure of the lithium salt molecule is specifically shown in formula (III-a):

[0081] The molecular structure of the additive is shown in formula (IV-a):

[0082] (5) Preparation and testing of the lithium metal battery

[0083] The positive electrode, the separator, and the metal lithium negative electrode are stacked in order, and then are stacked; after the tab is welded, they are placed in an outer packaging aluminum plastic film, and then are subjected to processes such as vacuum packaging, standing, formation (0.02C constant current charging to 3.3V, and then 0.1C constant current charging to 3.7V), shaping, and capacity testing, thereby obtaining a soft-packaged stacked lithium metal battery.

[0084] The obtained lithium metal battery is subjected to cycle testing on a charge-discharge device, and is charged at a charge rate of 0.5C to 3.7V, and then is discharged at a discharge rate of 1C to 2.5V, and the cycle is repeated.

[0085] The capacity repair method of Examples 1-13 and Comparative Examples 1-2 is selected from the following capacity repair methods, and the capacity repair method actually used in Examples 1-13 and Comparative Examples 1-2 is shown in Table 1.

[0086] Capacity repair method one: every 50 cycles, the lithium metal battery is subjected to capacity repair, discharged at a regular discharge rate C2 until full discharge state. Subsequently, charged at a charge rate C3 (C3 = 0.2C1) to full charge cut-off voltage, at full charge cut-off voltage, constant voltage charging to current <0.025C, followed by discharging at a discharge rate C4 (C4 = 0.2C1) to discharge cut-off voltage, the process is repeated twice to end.

[0087] Capacity repair method two: every 10 cycles, the lithium metal battery is subjected to capacity repair, discharged at a regular discharge rate C2 until full discharge state. Subsequently, charged at a charge rate C3 (C3 = 0.2C1) to full charge cut-off voltage, at full charge cut-off voltage, constant voltage charging to current <0.025C, followed by discharging at a discharge rate C4 (C4 = 0.2C1) to discharge cut-off voltage, the process is repeated twice to end.

[0088] Capacity repair method three: every 200 cycles, the lithium metal battery is subjected to capacity repair, discharged at a regular discharge rate C2 until full discharge state. Subsequently, charged at a charge rate C3 (C3 = 0.2C1) to full charge cut-off voltage, at full charge cut-off voltage, constant voltage charging to current <0.025C, followed by discharging at a discharge rate C4 (C4 = 0.2C1) to discharge cut-off voltage, the process is repeated twice to end.

[0089] Capacity repair method four: every 50 cycles, the lithium metal battery is subjected to capacity repair, discharged at a regular discharge rate C2 until full discharge state. Subsequently, charged at a charge rate C3 (C3 = 2C1) to full charge cut-off voltage, at full charge cut-off voltage, constant voltage charging to current <0.025C, followed by discharging at a discharge rate C4 (C4 = 2C1) to discharge cut-off voltage, the process is repeated twice to end.

[0090] Capacity repair method five: every 50 cycles, the lithium metal battery is subjected to capacity repair, discharged at a regular discharge rate C2 until full discharge state. Subsequently, charged at a charge rate C3 (C3 = 0.05C1) to full charge cut-off voltage, at full charge cut-off voltage, constant voltage charging to current <0.025C, followed by discharging at a discharge rate C4 (C4 = 0.05C1) to discharge cut-off voltage, the process is repeated twice to end.

[0091] Capacity repair mode six: capacity repair was performed on the lithium metal battery every 50 cycles, and the lithium metal battery was discharged at a regular discharge rate C2 until the full discharge state. Subsequently, the lithium metal battery was charged at a charge rate C3 (C3 = 0.25C1) to the full charge cutoff voltage, and at the full charge cutoff voltage, the lithium metal battery was charged at a constant voltage until the current < 0.025C, and then the lithium metal battery was discharged at a discharge rate C4 (C4 = 0.25C1) for 4 hours to the discharge cutoff voltage, and then the lithium metal battery was rested for 6 hours before the next charging, and the process was repeated twice to end.

[0092] Table 1.

[0093] The lithium metal batteries after capacity repair of Examples 1-13 and Comparative Examples 1-2 were subjected to performance tests in the following manner:

[0094] (1) Lithium metal battery room temperature cycle performance test

[0095] The lithium metal battery was placed in a 25°C constant temperature oven and rested for 30 minutes to allow the lithium metal battery to reach a constant temperature. The lithium metal battery reaching a constant temperature was charged in different charging modes, and then discharged at a constant current of 0.5C to a voltage of 2.5V, which was one charge-discharge cycle. The capacity of the first discharge was taken as 100%, and the charge-discharge cycle was repeated until the discharge capacity decreased to 80%, at which point the test was stopped, and the cycle number was recorded as an index for evaluating the cycle performance of the lithium metal battery.

[0096] (2) Lithium metal battery high temperature cycle performance test

[0097] The lithium metal battery was placed in a 45°C constant temperature oven and rested for 30 minutes to allow the lithium metal battery to reach a constant temperature. The lithium metal battery reaching a constant temperature was charged in different charging modes, and then discharged at a constant current of 0.5C to a voltage of 2.5V, which was one charge-discharge cycle. The capacity of the first discharge was taken as 100%, and the charge-discharge cycle was repeated until the discharge capacity decreased to 80%, at which point the test was stopped, and the cycle number was recorded as an index for evaluating the cycle performance of the lithium metal battery.

[0098] The test results are shown in Table 2.

[0099] Table 2.

[0100] From the test results of Examples 1-5, it can be seen that the use of the electrolyte scheme and capacity repair scheme provided by the present application in the lithium metal battery can significantly improve the room temperature cycle performance and high temperature cycle performance of the lithium metal battery. The capacity repair scheme of the present application can achieve better results if used in combination with the electrolyte with good lithium stability of Example 1-4.

[0101] From Comparative Example 1 and Examples 2-4, it can be seen that the addition of the additive improves the cycle performance of the lithium metal battery to some extent, and the film formation of the additive on the negative electrode is beneficial to the deposition of lithium.

[0102] From Comparative Example 6 and Example 2, it can be seen that the capacity repair frequency of Example 2 is once every 50 cycles, and a large amount of dead lithium has not yet been generated, the cycle number is not large, and the degree of the surface of the dead lithium being wrapped by the SEI film is relatively shallow, so the capacity repair effect is better.

[0103] From Comparative Example 1 and Example 2, it can be seen that when the charge and discharge rate used during capacity recovery is too large, i.e., greater than 0.8C1, the purpose of capacity repair cannot be achieved. When charging at a large rate, the deposition speed of lithium is fast, and the purpose of recovering the connected dead lithium cannot be achieved, the active lithium cannot be recovered, and thus the cycle performance cannot be improved. When discharging at a large rate, the stripping speed of lithium is fast, and it is easy to form dead lithium, causing the loss of active lithium. Moreover, the battery cannot be completely depolarized, and the lithium ions accumulated on the surface of the negative electrode cannot be dispersed. Therefore, the capacity repair effect is not good.

[0104] From Comparative Example 7 and Example 2, it can be seen that the charge and discharge rate of Example 2 is moderate, and the repair effect is good, because under a proper charge and discharge rate, the reaction time on the surface of the negative electrode is moderate, the side reaction does not increase, and the loss of active lithium is small.

[0105] From Comparative Example 2 and Example 2, it can be seen that when the battery is not subjected to capacity repair, the cycle performance differs greatly, a large amount of active lithium cannot be recovered, and the energy loss caused by polarization cannot be recovered, so the cycle performance is not good.

[0106] From Comparative Example 8 and Examples 1-4, it can be seen that the electrolyte of Examples 1-4 contains a fluoroether compound, the viscosity of the electrolyte is moderate, the electrode has good wetting effect, the room temperature cycle performance and high temperature cycle performance of the lithium metal battery are improved, and the compatibility with the capacity repair method is good.

[0107] From Comparative Example 9 and Examples 1-4, it can be seen that the lithium salt concentration in the electrolyte of Examples 1-4 is appropriate, the viscosity of the electrolyte is suitable, the electrode has good wetting, the cycle performance and rate performance of the lithium metal battery are improved, and the compatibility with the capacity repair method is good.

[0108] Example 10 uses a common electrolyte on the market, and compared with Examples 1-4 which also use the capacity repair method 1, the capacity repair effect of Examples 1-4 is better, and the improvement of the battery performance is more obvious.

[0109] Example 11 uses an ether compound with the molecular structure of formula (II-b), and compared with Example 1, the electrode of Example 1 has relatively good wetting effect, and the compatibility with the capacity repair method is also relatively high.

[0110] Example 13 has a certain time of standing when capacity repair is carried out on the lithium battery, lithium deposition is more uniform, the capacity repair effect is also better, and the battery performance is obviously improved.

[0111] Although the application has been illustrated and described with reference to certain preferred embodiments thereof, it should be understood by the skilled in the art that the foregoing is a further detailed description of the application in connection with the specific embodiments, and is not intended to limit the specific embodiments of the application. Those skilled in the art can make various changes in form and details without departing from the spirit and scope of the application, including making a number of simple deductions or substitutions.

Claims

1. A lithium metal battery capacity restoration method, characterized in that, The conventional charge rate of the lithium metal battery is C1, the conventional discharge rate of the lithium metal battery is C2, the lithium metal battery capacity repair method comprises the following steps: Primary discharge: discharge at a conventional discharge rate C2 until a full discharge state; Charging: charging at a charge rate C3 until a preset charging cutoff voltage is reached, maintaining the preset charging cutoff voltage for constant voltage charging until the charging current < 0.025C, wherein 0.05C1≤C3≤0.8C1, and C is the nominal capacity of the lithium metal battery; Secondary discharge: discharging at a discharge rate C4 until a preset discharge cutoff voltage is reached, wherein 0.05C1≤C4≤0.8C1.

2. A lithium metal battery capacity restoration approach as claimed in claim 1, wherein, Wherein, The lithium metal battery is repaired in capacity every X cycles, wherein 0≤X≤150.

3. The lithium metal battery capacity restoration approach of claim 1, wherein, In the secondary discharge step, before the preset discharge cutoff voltage is reached, it further comprises discharging at a discharge rate C4 for t3 hours, and then maintaining static for a t4 period, wherein 0.35C / C4≤t3≤0.85C / C4, 6h≤t4≤30h.

4. The lithium metal battery capacity repair method of claim 1, wherein, In the primary discharge step, it further comprises maintaining static for a t1 period after the full discharge state is reached, 2min≤t1≤30min; And / or, in the charging step, it further comprises maintaining static for a t2 period after the charging current < 0.025C, 2min≤t2≤30min; And / or, in the secondary discharge step, it further comprises maintaining static for a t5 period after the preset discharge cutoff voltage is reached, 2min≤t5≤30min.

5. The lithium metal battery capacity restoration approach of claim 1, wherein, The electrolyte composition of the lithium metal battery comprises 1wt%-95wt% of fluorine ether compounds and 1wt%-45wt% of ether compounds by mass percentage, and the electrolyte further comprises lithium salt with a molar concentration of 0.1M-5M, The molecular structure of the fluorine ether compound is shown in formula (I), and the molecular structure of the ether compound is shown in formula (II), Wherein, R1 and R2 are each independently selected from C1-C10 halogenated alkyl or C2-C10 halogenated alkenyl, and R3 and R4 are each independently selected from C1-C10 alkyl or C1-C10 alkoxy.

6. A lithium metal battery capacity restoration approach as claimed in claim 5, wherein, The electrolyte further comprises 0.01wt%-10wt% of an additive by mass percentage, and the additive is one or more of fluoroethylene carbonate, ethylene sulfite, lithium difluoro oxalate borate, lithium tetrafluoroborate, methanedi sulfonate, 4-trifluoromethyl vinyl carbonate, 1,3,2-dioxazolothiophene-2,2-dioxide, fluoroethylene carbonate, ethylene sulfite, vinylene carbonate, succinic anhydride, propylene sulfite, propylene-1,3-sulfonic acid lactone, bis(trimethylsilyl) sulfate, lithium nitrate, N-methyl, butyl pyrrolidine bis-trifluoromethyl sulfonimide salt, N-methyl, propyl piperidine bis-trifluoromethyl sulfonimide salt, 1,3-dioxane, 1,4-dioxane, 1,2-bis(cyanoethoxy)ethane, hexanedinitrile, 1,3,5-pentanetricarbonitrile, trans-butenedinitrile, and 1,2,3-tris(2-cyanoethoxy)propane.

7. A lithium metal battery capacity restoration method as claimed in claim 5, wherein, The fluoroether compound is one or more of 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and perfluorodiethyl ether; And / or, the ether compound is one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, and diethylene glycol dimethyl ether; And / or, the lithium salt is one or more of lithium perchlorate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis-trifluoromethylsulfonylimide, lithium bisfluorosulfonylimide, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium tetrafluoroborate, lithium trifluoromethylsulfonate, and lithium difluorophosphate.

8. A lithium metal battery capacity restoration approach as claimed in claim 5, wherein, The molecular structure of the fluoroether compound is shown in formula (I-a), the molecular structure of the ether compound is shown in formula (II-a) or formula (II-b), and the lithium salt is lithium bis-trifluoromethylsulfonylimide, 9. A lithium metal battery capacity restoration approach as claimed in claim 8, wherein, The electrolyte composition of the lithium metal battery includes 55wt%-87wt% of the fluoroether compound and 13wt%-45wt% of the ether compound by mass percentage, and the electrolyte further includes lithium bis-trifluoromethylsulfonylimide with a molar concentration of 1.5M-3.5M.

10. A lithium metal battery, characterized in that, The lithium metal battery can be capacity-restored according to the lithium metal battery capacity-restoring method as claimed in any one of claims 1-9.

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