Lithium battery and preparation method thereof, charging method and power vehicle

Abstract

The present application provides a lithium battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte located between the positive electrode sheet and the negative electrode sheet. Wherein, the negative electrode material layer of the negative electrode sheet contains a lithium-silicon composite negative electrode active material, and the surface of the negative electrode material layer has a protective layer or the surface of the lithium-silicon composite negative electrode active material has a protective layer. Wherein, the protective layer comprises a polymer matrix and a lithium salt; in a state where the lithium battery is fully charged, the lithium-silicon composite negative electrode active material contains lithium metal and lithium-silicon alloy Li 4.4 Si, and the molar proportion of lithium metal in the lithium-silicon composite negative electrode active material is 15%-95%. The lithium battery has a high energy density, a long cycle life and high safety performance. The present application also provides a charging method for supporting the lithium battery and a power vehicle including the lithium battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium batteries, and in particular to a lithium battery and a preparation method, a charging method and a power vehicle thereof. Background Art

[0002] Lithium batteries have been widely used in portable electronic products such as mobile phones and laptops, as well as new energy vehicles. At present, commercial lithium batteries generally use graphite as the negative electrode active material, and in order to ensure the efficient deintercalation of lithium ions in the positive and negative electrodes during the battery cycle, the effective capacity of the graphite negative electrode is generally greater than that of the positive electrode (that is, the N / P ratio of the battery is generally greater than 1) to prevent the precipitation of lithium dendrites at the negative electrode and affect the cycle performance. However, this makes the volume and weight of the negative electrode active material in the battery relatively high, limiting the improvement of the energy density of lithium-ion batteries. It is difficult to exceed 350mAh / g, and it can no longer meet people's growing demand for endurance and standby.

[0003] Lithium metal has a high theoretical specific capacity (3861mAh / g) and the most negative electrochemical potential (-3.04V, relative to the standard hydrogen electrode), and is considered to be the best choice for the next generation of high energy density battery negative electrode materials. At present, some institutions use lithium metal with a volume share far lower than that of traditional negative electrodes, or even lithium-free negative electrodes (Lithium free), such as CN201911075192.1. Although this can obtain high energy density lithium batteries, the cycle performance of the resulting batteries is poor, which hinders the commercialization of high energy density metal lithium batteries. Summary of the invention

[0004] In view of this, the present application provides a lithium battery and a preparation method thereof, a charging method and a power vehicle to solve the problem of poor cycle performance of current metal lithium batteries.

[0005] Specifically, in the first aspect, the present application provides a lithium battery, including a positive electrode sheet, a negative electrode sheet, and a separator and an electrolyte located between the positive electrode sheet and the negative electrode sheet, wherein the negative electrode material layer of the negative electrode sheet contains a lithium silicon composite negative electrode active material, the surface of the negative electrode material layer has a protective layer or the surface of the lithium silicon composite negative electrode active material has a protective layer, and the protective layer includes a polymer matrix and a lithium salt; when the lithium battery is fully charged, the lithium silicon composite negative electrode active material contains lithium element and lithium silicon alloy Li 4.4 Si, and the molar proportion of the lithium element in the lithium-silicon composite negative electrode active material is 15%-95%.

[0006] The lithium battery provided in the first aspect of the present application contains the above-mentioned lithium-silicon composite negative electrode active material, so that the lithium battery has high energy density, long cycle life and high safety performance.

[0007] In a second aspect, the present application also provides a method for preparing a lithium battery, comprising the following steps:

[0008] Applying a mixed slurry containing a silicon-based material, a conductive agent and a binder on a negative electrode current collector, and after drying and rolling, forming a silicon-based material layer on the negative electrode current collector;

[0009] In a glove box, the lithium film and the silicon-based material layer are subjected to hot pressing treatment, so that the lithium element of the lithium film is completely transferred to the silicon-based material layer, and reacts with the silicon-based material in situ to form a negative electrode material layer containing a lithium-silicon composite negative electrode active material, thereby obtaining a negative electrode sheet;

[0010] Wherein, before the silicon-based material layer and the lithium thin film are subjected to hot pressing treatment, a protective layer is formed on the surface of the silicon-based material layer; or after the negative electrode material layer is formed, a protective layer is formed on the surface of the negative electrode material layer; the protective layer comprises a polymer matrix and a lithium salt;

[0011] The negative electrode sheet is assembled into a lithium battery; wherein, when the lithium battery is fully charged, the lithium-silicon composite negative electrode active material contains lithium and lithium-silicon alloy Li 4.4 Si, and the molar proportion of the lithium element in the lithium-silicon composite negative electrode active material is 15%-95%.

[0012] The preparation method described in the second aspect of the present application has a simple process, is easy to control, and is suitable for large-scale industrial preparation.

[0013] In a third aspect, the present application also provides a charging method for the aforementioned lithium battery, comprising the following steps:

[0014] When the lithium battery is required to have a long cycle life, the charging cut-off voltage V of the lithium battery is controlled. s Satisfies the following formula:

[0015] V s =cV b +a×c×K+b×c×(dQ / dV) / (3.6×CA), wherein, when the lithium battery exhibits a long cycle life characteristic, at the V s Under this condition, the negative electrode of the lithium battery does not precipitate lithium, and V s <V h ;

[0016] Among them, V h is the upper limit voltage of the charge that the lithium battery can withstand, CA is the nominal capacity of the lithium battery when discharged at 0.33C, V bis the reference voltage at which no lithium is precipitated from the negative electrode of the lithium battery under the real-time charging capacity, K is the internal resistance growth rate of the real-time DC internal resistance of the lithium battery during charging and its factory DC internal resistance, dQ / dV is the real-time differential value of the charging capacity and the charging voltage of the lithium battery, c is the calibration factor of the real-time cell temperature of the lithium battery during charging, a is the calibration factor of K, and b is the calibration factor of (dQ / dV) / CA.

[0017] The charging method provided in the third aspect of the present application can ensure that the lithium battery has as long a driving range as possible under a long life.

[0018] In a fourth aspect, the present application further provides a power vehicle, whose battery system includes at least one first battery unit, which includes a plurality of lithium batteries as described in the first aspect of the present application and a first charging control device.

[0019] A power vehicle with the first battery unit can adjust the charging cut-off voltage for charging each lithium battery of the first battery unit according to actual cruising range requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic diagram of the structure of a lithium battery provided in an embodiment of the present application.

[0021] Figure 2 This is a discharge curve of the lithium battery provided in the embodiment of the present application.

[0022] Figure 3 A schematic diagram of the structure of a power vehicle provided in an embodiment of the present application.

[0023] Figure 4 Another structural schematic diagram of a power vehicle provided in an embodiment of the present application. DETAILED DESCRIPTION

[0024] Described below are exemplary embodiments of the present application. It should be noted that, for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present application. These improvements and modifications are also considered to be within the scope of protection of the present application.

[0025] The technical solutions in the embodiments of the present application will be described in detail below in conjunction with the drawings in the embodiments of the present application.

[0026] The present application provides a lithium battery, wherein the negative electrode sheet of the lithium battery includes a lithium-silicon composite negative electrode active material. Figure 1The lithium battery 100 includes a negative electrode sheet 10, a positive electrode sheet 20, and a separator 30 and an electrolyte (not shown) located between the positive electrode sheet 20 and the negative electrode sheet 10. Generally, the negative electrode sheet 10 includes a negative electrode current collector 11 and a negative electrode material layer 12 disposed on the negative electrode current collector 11, and the negative electrode material layer 12 contains a lithium silicon composite negative electrode active material, and optional conductive agents and binders, etc. Similarly, the positive electrode sheet 20 includes a positive electrode current collector 21 and a positive electrode material layer 22 disposed on the positive electrode current collector 21, and the positive electrode material layer 22 contains a positive electrode active material, and optional conductive agents and binders, etc.

[0027] The surface of the negative electrode material layer 12 also has a protective layer 13 (see Figure 1 ), or the surface of the lithium silicon composite negative electrode active material is provided with a protective layer, the protective layer comprising a polymer matrix and a lithium salt. The protective layer can guide the flow of lithium ions, control the uniform deposition of lithium ions on the surface of the negative electrode sheet, effectively inhibit the growth of lithium dendrites on the surface of the negative electrode sheet 10 and prevent them from piercing the diaphragm and causing internal short circuit of the battery, and can reduce the occurrence of side reactions between the negative electrode and the electrolyte, alleviate the volume expansion of the negative electrode during the cycle, and improve the cycle performance and safety performance.

[0028] The protective layer can better suppress the cycle attenuation and internal short circuit problems of the battery caused by lithium plating at the negative electrode. Among them, the protective layer is almost insoluble in the battery electrolyte. Specifically, the polymer matrix may include one or more of polyethylene oxide (PEO), polysiloxane, polyvinylidene fluoride, polymethyl methacrylate, polyacrylonitrile and its derivatives and copolymers, but is not limited to this. The lithium salt has ion conductivity and may include one or more of lithium nitrate (LiNO3), lithium sulfide (Li2S), lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI), lithium fluoride (LiF), lithium phosphate (Li3PO4), etc. In some embodiments, the aforementioned protective layer may also contain an inorganic filler to increase the lithium ion transmission channel, improve mechanical properties, etc. The inorganic filler may be at least one of an oxide (such as silicon dioxide, aluminum oxide, titanium dioxide, etc.), a hydroxide (such as aluminum hydroxide, magnesium hydroxide) and a salt.

[0029] In the present application, the lithium-silicon composite negative electrode active material contains lithium and silicon. When the lithium battery 100 is fully charged, the lithium-silicon composite negative electrode active material contains lithium and lithium-silicon alloy Li 4.4 Si, and the molar proportion of the lithium element in the lithium-silicon composite negative electrode active material is 15%-95%.

[0030] Among them, "fully charged" means that the positive electrode of the battery is charged to 100% SOC (State of Charge). At this time, the capacity of the positive electrode of the battery is fully utilized, the energy density of the battery is very high, and "lithium deposition" occurs at the negative electrode, and this part of "lithium deposition" is active lithium, which can exert capacity at the negative electrode end. Since the negative electrode of the battery in this application, when the battery is fully charged, in addition to allowing the active lithium ions released from the positive electrode to be stored in the negative electrode in the form of an alloy-type material, also accepts direct deposition of lithium in the form of lithium metal in the negative electrode. Therefore, in the negative electrode material layer prepared in this application, the amount of the lithium-silicon alloy material Li x Si (0 < x ≤ 4.4) is less, which can significantly improve the energy density of the battery. In addition, with the setting of the above-mentioned protective layer, the "lithium deposition" and the side reaction with the electrolyte at the negative electrode of the battery can be inhibited, and the risk of the deposited lithium dendrites piercing the diaphragm can be reduced. Therefore, the lithium battery of the embodiment of this application has good cycle performance, safety performance, etc. while having a high energy density.

[0031] In the implementation of this application, when the lithium battery 100 is not fully charged, such as when the SOC value of the positive electrode charging of the battery is lower than the first threshold, the lithium-silicon composite negative electrode active material does not contain lithium metal. At this time, the lithium-silicon alloy in the lithium-silicon composite negative electrode active material can be represented by the chemical formula Li x Si, 0 < x ≤ 4.4. The "first threshold" is the critical value of the positive electrode charging SOC when metallic lithium just precipitates at the negative electrode end during battery charging, that is, the SOC value when the lithium-silicon alloy of the battery negative electrode is fully filled with lithium ions (that is, the lithium-silicon alloy is specifically Li 4.4 Si, and at this time, it can also be said that the battery negative electrode is charged to 100% SOC) and the lithium ions on the positive electrode side are not all released.

[0032] Among them, when the SOC of the positive electrode charging of the battery is lower than the first threshold, the battery does not have lithium deposition, the energy density of the lithium battery is not fully exerted, and only the capacity of the lithium-silicon alloy Li x Si at the negative electrode end is exerted, and the volume expansion borne by the negative electrode end is relatively weak, and the side reaction with the electrolyte is weak. In this way, the lithium battery can perform charge and discharge cycles for a relatively large number of times at a lower energy density (still much higher than the energy density of the battery using graphite as the negative electrode at present), that is, it has a long cycle life. Therefore, the lithium battery provided by this application can take into account the characteristics of "long cycle life" and the above-mentioned "high energy density", and these two characteristics can be freely selected in combination with the battery management system of the lithium battery to meet the full-life cycle requirements of power vehicles.

[0033] In the present application, when the positive electrode of the lithium battery is charged at 100% SOC, the lithium-silicon composite negative electrode active material has an adjustable proportion of lithium metal element, and its molar proportion is in the range of 15%-95%. Accordingly, according to the molar proportion of the lithium metal element, the above-mentioned first threshold value can be adjusted accordingly. Optionally, the first threshold value is also in the range of 15%-95%. See Figure 2 The lithium battery discharge curve shown. Figure 2 In the 2016 study, the lithium battery using lithium-silicon composite negative electrode active materials had a discharge inflection point at a discharge capacity of 58 mAh. The "discharge inflection point" refers to the minimum value of dV / dQ in the battery discharge curve. Before this inflection point, the battery's energy density was high (i.e., the product of battery voltage and battery capacity), and the lithium-silicon alloy Li 4.4 The capacity of Si and lithium; after this inflection point, the SOC of the battery is low, and the negative electrode side only plays the role of lithium-silicon alloy Li x The capacity of Si decreases, and the energy density of the battery becomes lower, but the cycle life of the battery is longer.

[0034] Optionally, when the lithium battery 100 is fully charged, the lithium-silicon alloy Li 4.4 The molar proportion of Si in the lithium-silicon composite negative electrode active material is 5%-85%. In some embodiments, when the lithium battery is fully charged, the lithium-silicon alloy Li 4.4 The sum of the molar proportions of Si and lithium in the lithium-silicon composite negative electrode active material is 100%. In other words, when the lithium battery is fully charged, the lithium-silicon composite negative electrode active material is composed of lithium and lithium-silicon alloy Li 4.4 At this time, the lithium-silicon composite negative electrode active material contains only lithium and silicon elements (that is, it can be formed by in-situ compression of silicon and metallic lithium).

[0035] Generally, the N / P ratio of a conventional lithium battery (using graphite or silicon as the negative electrode) is generally greater than 1 to prevent the precipitation of lithium dendrites and poor cycle performance when the N / P ratio is less than 1. When the N / P ratio of the battery is greater than 1, the volume of the negative electrode active material in the entire battery accounts for a large proportion, generally more than 37%. For example, when the negative electrode uses graphite, the volume of graphite in the entire battery accounts for 44%-48%. When the negative electrode active material is silicon, its volume in the battery can reach 37%-44%. However, in the lithium battery of the present application, due to the use of the aforementioned lithium-containing single substance and Li 4.4Si lithium silicon composite negative electrode active material, the N / P of the lithium battery is less than 1, so that the volume proportion (less than 37%, for example, less than 20%) and mass proportion of the negative electrode active material in the lithium battery can be small, thereby significantly improving the energy density of the battery and increasing its endurance; and based on the presence of the aforementioned protective layer, when the negative electrode "lithium is precipitated", the side reaction of lithium single substance with electrolyte and its disordered growth to pierce the diaphragm can be suppressed, thereby making the battery cycle capacity better.

[0036] It should be noted that the N / P ratio of the lithium battery 100 in the present application is less than 1, which specifically means that the ratio of the capacity of the aforementioned lithium-silicon composite negative electrode active material to the capacity of the positive electrode active material is less than 1. Among them, the capacity of the lithium-silicon composite negative electrode active material corresponding to the N / P ratio refers to the capacity of the negative electrode just inserted with lithium to form a lithium-silicon alloy Li 4.4 The negative electrode capacity when the positive electrode is in the form of Si and no single lithium is precipitated (the lithium ions in the positive electrode have not been completely released at this time) is the negative electrode capacity corresponding to the above-mentioned first threshold.

[0037] Optionally, in the lithium battery 100, the volume ratio of the lithium silicon composite negative electrode active material to the positive electrode active material is 0.1375-0.825. The ratio of the thickness of the positive electrode sheet 20 to the thickness of the negative electrode sheet 10 is 8:1-4:3. This can better ensure that the N / P ratio of the lithium battery is less than 1, which is conducive to improving the energy density of the battery.

[0038] The electrolyte of the lithium battery 100 generally contains a solvent and a second lithium salt. In the embodiment of the present application, the solvent in the electrolyte of the lithium battery 100 is a non-carbonate solvent. Specifically, the solvent in the electrolyte includes an ether solvent, which may include at least one of a non-halogenated ether solvent and a fluoroether solvent. Among them, the side reaction rate between carbonate solvents and the metallic lithium negative electrode is fast, and sharp lithium dendrites are easily generated at the negative electrode of the battery, piercing the battery separator and causing the battery to spontaneously combust. Ether solvents have good compatibility with metallic lithium, and the side reaction between them and lithium metal is much lower than the side reaction between carbonate solvents and lithium metal, which can effectively inhibit the consumption of active lithium in the cycle process, while improving the uniformity and density of lithium ion deposition, and avoiding the formation of sharp lithium dendrites that pierce the battery separator and cause safety risks.

[0039] Optionally, the non-halogenated ether solvent can be selected from one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, dipropylene glycol dimethyl ether, tripropylene glycol monomethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc., but is not limited to this. Optionally, the fluoroether solvent can be selected from 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, hexafluoroisopropyl ether, tetrafluoroethyl-tetrafluoropropyl ether, 2,2,2-trifluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, difluoromethyl-2,2,3,3-tetrafluoropropyl ether, 2,2,3 ,3,3-pentafluoropropyl methyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1,1,2,3,3,3-pentafluoropropyl difluoromethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether and bis(2,2,2-trifluoroethyl) ether, etc., but not limited thereto.

[0040] Among them, the second lithium salt in the electrolyte can be selected from one or more of lithium bis(fluorosulfonyl)imide (LiN(SO2F)2), lithium bis(trifluoromethylsulfonyl)imide (Li(CF3SO2)2N), lithium bis(perfluoroethylsulfonyl)imide (Li(C2F5SO2)2N), lithium bis(oxalatoborate) (LiB(C2O4)2, LiBOB), lithium trifluoromethylsulfonate (LiCF3SO3), lithium perfluorobutylsulfonate (LiC4F9SO3), tris(trifluoromethylsulfonyl)methyl lithium LiC(CF3SO2)3, etc., but is not limited to these.

[0041] Optionally, the lithium-silicon composite negative electrode active material is formed by in-situ pressing of silicon-based materials and metallic lithium, wherein the silicon-based materials may include but are not limited to silicon, silicon oxide, silicon-based non-lithium alloys (such as silicon-germanium alloy, silicon-magnesium alloy, silicon-copper alloy, silicon-iron alloy, etc.) or other silicon compounds.

[0042] In some embodiments, the negative electrode material layer 12 containing the lithium silicon composite negative electrode active material is formed by in-situ hot pressing of a lithium film (such as a lithium foil or a lithium film attached to a release film) and an initial negative electrode material layer containing a silicon-based material. At this time, the lithium element of the lithium film can be completely transferred to the initial negative electrode material layer, and react in-situ with the silicon-based material to form the lithium silicon composite negative electrode active material. In other embodiments, the lithium silicon composite negative electrode active material is formed by in-situ pressing of a mixture of metallic lithium powder and silicon-based material (which can be a wet slurry or a dry powder). At this time, the above-mentioned negative electrode material layer 12 can be formed by coating a mixed slurry of silicon-based material and lithium powder on the negative electrode current collector, and after drying and pressing, reacting in-situ on the negative electrode current collector to form a negative electrode material layer containing a lithium silicon composite negative electrode active material; or, the lithium silicon composite negative electrode active material formed by in-situ pressing of a mixture of metallic lithium powder and silicon-based material is coated, dried, and pressed into a sheet before being made into a negative electrode sheet.

[0043] When it is necessary to form a negative electrode material layer 12 having a protective layer on the surface, the lithium thin film attached to the release film can be in situ hot pressed with the silicon-based material layer with a protective layer on the surface (the silicon-based material layer is the aforementioned initial negative electrode material layer, which contains silicon-based material and optional binder and conductive agent). In other embodiments of the present application, the negative electrode material layer having a protective layer on the surface can also be formed by forming a protective layer on the surface of the negative electrode material layer containing lithium silicon composite negative electrode active material.

[0044] In the present application, the above-mentioned negative electrode current collector 11 and positive electrode current collector 21 are independently selected from metal single substance foil or alloy foil. Exemplarily, the negative electrode current collector 11 can be specifically copper foil, and the positive electrode current collector 21 can be specifically aluminum foil. The positive electrode active material can be at least one of lithium iron phosphate, lithium manganese phosphate, lithium iron manganese phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium cobaltate, lithium manganate, lithium nickel manganate, lithium nickel cobalt manganate (NCM), lithium nickel cobalt aluminum (NCA), etc. The binder and the conductive agent in the negative electrode sheet 10 and the positive electrode sheet 20 can be made of existing conventional materials. For example, the conductive agent can be one or more of conductive carbon black (such as acetylene black, Ketjen black), carbon nanotubes, carbon fibers, graphite and furnace black. The binder can independently adopt one or more of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyolefins (such as polyethylene, polypropylene, etc.), sodium carboxymethyl cellulose (CMC) and sodium alginate.

[0045] The lithium battery provided in the embodiment of the present application, because it contains the above-mentioned lithium silicon composite negative electrode active material and protective layer, the proportion of lithium silicon composite negative electrode active material in the lithium battery can be relatively low, which is beneficial to improving the energy density of the battery, and the negative electrode of the battery does not deposit lithium under low SOC state, showing a better cycle life; the energy density of the battery is very high under high SOC state, and the side effect of lithium deposition is alleviated due to the provision of the protective layer. Therefore, the lithium battery of the embodiment of the present application can take into account high energy density, long cycle life, high safety, etc.

[0046] In a second aspect, the present invention provides a method for preparing the above-mentioned lithium battery, comprising the following steps:

[0047] Applying a mixed slurry containing a silicon-based material, a conductive agent and a binder on a negative electrode current collector, and after drying and rolling, forming a silicon-based material layer on the negative electrode current collector;

[0048] In a glove box, the lithium film and the silicon-based material layer are subjected to hot pressing treatment, so that the lithium element of the lithium film is completely transferred to the silicon-based material layer, and reacts with the silicon-based material in situ to form a negative electrode material layer containing a lithium-silicon composite negative electrode active material, thereby obtaining a negative electrode sheet;

[0049] Wherein, before the silicon-based material layer and the lithium thin film are subjected to hot pressing treatment, a protective layer is formed on the surface of the silicon-based material layer; or after the negative electrode material layer is formed, a protective layer is formed on the surface of the negative electrode material layer; the protective layer comprises a polymer matrix and a lithium salt;

[0050] The negative electrode sheet is assembled into a lithium battery; wherein, when the lithium battery is fully charged, the lithium-silicon composite negative electrode active material contains lithium and lithium-silicon alloy Li 4.4 Si, and the molar proportion of the lithium element in the lithium-silicon composite negative electrode active material is 15%-95%.

[0051] Wherein, the silicon-based material may include but is not limited to silicon alone, silicon oxide, silicon-based non-lithium alloy (such as silicon-germanium alloy, silicon-magnesium alloy, silicon-copper alloy, silicon-iron alloy, etc.) or other silicon compounds (such as fluorine-containing silicon oxide, lithium hexafluorosilicate, silicon carbide, silicon boride), etc. When the silicon-based material used contains other elements besides silicon (such as oxygen and fluorine), the lithium-silicon composite negative electrode active material also contains these elements accordingly. Exemplarily, when the silicon-based material is silicon alone, when the SOC of the battery is lower than the first threshold value, the lithium-silicon composite negative electrode active material contains only the lithium-silicon alloy Li x When the lithium battery is fully charged, the lithium-silicon composite negative electrode active material is only composed of lithium and Li 4.4 For example, when the silicon-based material is silicon oxide, when the SOC of the battery is lower than the first threshold, the lithium-silicon composite negative electrode active material contains a lithium-silicon alloy Lix Si and Li2O, Li2SiO3, etc.; when the lithium battery is fully charged, the lithium silicon composite negative electrode active material contains Li 4.4 Si, lithium element and Li2O, Li2SiO3, etc.

[0052] In an embodiment of the present application, when the silicon-based material layer with a protective layer on the surface and the lithium thin film are in-situ hot pressed, under hot pressing, the protective layer and the silicon-based material layer present a porous structure, and the lithium element of the lithium thin film can enter the silicon-based material layer and react in situ with the silicon-based material to form the lithium-silicon composite negative electrode active material, and finally form a negative electrode material layer with a protective layer, which contains the lithium-silicon composite negative electrode active material.

[0053] The lithium film that is heat-pressed with the silicon-based material layer can be lithium foil directly, or a lithium film attached to a release film, and preferably a lithium film attached to a release film to avoid loss of lithium elements caused by direct contact between the lithium film and the pressing equipment.

[0054] The protective layer may be formed on the negative electrode material layer by liquid coating, vapor deposition or electrodeposition.

[0055] Among them, "assembling the negative electrode sheet into a lithium battery" specifically includes: stacking the positive electrode sheet, the separator and the negative electrode sheet in sequence to form a bare battery cell; placing the bare battery cell in a battery shell, injecting an electrolyte, and sealing the battery shell to obtain a lithium battery.

[0056] The method for preparing a lithium battery provided in the embodiment of the present application has a simple process and is easy to control, and is suitable for large-scale industrial preparation of the above-mentioned lithium battery that can achieve both high energy density and long cycle life.

[0057] In a third aspect, an embodiment of the present application provides a charging method for the above-mentioned lithium battery, comprising the following steps:

[0058] When the lithium battery is required to have a long cycle life, the charging cut-off voltage V of the lithium battery is controlled. s Satisfies the following formula:

[0059] V s =cV b +a×c×K+b×c×(dQ / dV) / (3.6×CA), wherein, when the lithium battery exhibits a long cycle life characteristic, at the V s Under this condition, the negative electrode of the lithium battery just does not precipitate lithium, and V s <V h ;

[0060] Among them, V his the upper limit voltage of the charge that the lithium battery can withstand, CA is the nominal capacity of the lithium battery when discharged at 0.33C, V b is the reference voltage at which no lithium is precipitated from the negative electrode of the lithium battery under the real-time charging capacity, K is the internal resistance growth rate of the real-time DC internal resistance of the lithium battery during charging and its factory DC internal resistance, dQ / dV is the real-time differential value of the charging capacity and the charging voltage of the lithium battery, c is the calibration factor of the real-time cell temperature of the lithium battery during charging, a is the calibration factor of K, and b is the calibration factor of (dQ / dV) / CA.

[0061] It should be noted that the above V s It is also the battery voltage when the negative electrode of the lithium battery is charged to the point where lithium is just precipitated (i.e., the positive electrode of the battery is charged to the first threshold). h It is also the battery voltage when the lithium battery is fully charged (i.e., the positive electrode is charged to 100% SOC), that is, the cut-off voltage corresponding to the maximum capacity of the positive electrode, or the rated voltage. s Capacity C s Less than the lithium battery at a voltage of V h Capacity C h .

[0062] When the lithium battery is required to exhibit a long cycle life characteristic, the charging cut-off voltage is V s Under this condition, the negative electrode of the lithium battery does not precipitate lithium. At this time, the lithium battery is not fully charged, but only charged to a lower SOC. The volume expansion of the negative terminal is relatively weak, and the side reaction with the electrolyte is weak. Therefore, the lithium battery can be charged and discharged for a large number of times, that is, it has a long cycle life. When the lithium battery is required to exert high energy density characteristics, the charging cut-off voltage of the lithium battery is V h , at the V h Under voltage, the negative electrode generally contains lithium-silicon alloy and a certain amount of lithium.

[0063] Therefore, as the charging cut-off voltage V h As the number of times the lithium battery is charged (i.e., fully charged) increases, the active lithium element is continuously consumed. s When charging a lithium battery, charge the negative electrode of the battery to a V that is just high enough for lithium to precipitate. s According to the above formula, increase, so that V s With V h The gap can ensure that the battery can be increased with V without damaging the long cycle life of the battery. sAs the charging cut-off voltage, the energy density of the battery can be increased, so that the power vehicle using the lithium battery can have a longer driving range. For example, when the charging cut-off voltage of the lithium battery is V h , then when the lithium battery is required to exert its long cycle life characteristics, the charging cut-off voltage V s It should be adjusted higher according to the above formula; in addition, if the lithium battery has never been charged to the cut-off voltage V h When charging, if the lithium battery needs to exert its long cycle life characteristics, the above V s Remain unchanged.

[0064] For assembled batteries, their CA, V h The above parameter V b , dQ / dV, K, a, b, c can be obtained through the charging control equipment of the lithium battery, such as the battery management system (BMS). The BMS can monitor the battery status information, such as the battery charging current, real-time charging voltage, temperature, internal resistance, etc. This information can be obtained through the acquisition module of the BMS and can be stored in the controller of the BMS. Among them, dQ / dV represents the amount of electricity charged under unit voltage, which can be calculated based on the current charging capacity point data (charging current and charging voltage) and the previous charging capacity point data in the same charging process obtained by the battery BMS.

[0065] V b V is the reference voltage at which the negative electrode of the lithium battery does not precipitate lithium at the real-time charging capacity. b It can be obtained by calibration based on the capacity of the lithium-silicon composite negative electrode active material without precipitation of lithium. Furthermore, the battery with the highest design capacity of the positive electrode and an excess of 10% of the silicon-based negative electrode (i.e., the aforementioned lithium battery with N / P=1.1) can be charged to obtain a curve of its charging capacity and the real-time battery voltage during the charging process. According to the curve, the real-time battery voltage (i.e., the voltage reference value) at any charging capacity can be obtained, which is V b . V at each charge capacity b It can be pre-existing in the BMS.

[0066] Parameters a, b, and c are empirical values ​​and dimensionless. Optionally, the value range of a is 0.02-1.2, the value range of b is -0.008 to -0.15, and the value range of c is 0.8-1.5. Parameters a, b, and c can be used to obtain the corresponding calibration factors corresponding to the battery under the current state information based on the established correspondence between the battery state information and the corresponding calibration factors. It should be noted that in the above formulas, a, b, c, and K are all obtained for the lithium battery at the same time point / time period during the same charging process.

[0067] For example, lithium batteries generally have a rated operating temperature range, such as between 10°C and 40°C. If the temperature of the lithium battery is higher, such as above a threshold temperature (such as 42°C), such as 45°C, then the value of c should be 0.92, so that the adjusted charging cut-off voltage V s It is better, which can not only limit the occurrence of lithium precipitation, but also give full play to the capacity before lithium precipitation. Exemplarily, the BMS can pre-store the correspondence between the real-time cell temperature of the battery during charging and the temperature calibration factor c, based on which the temperature calibration factor of the lithium battery cell at the current charging temperature can be known. The following Table 1A shows the correspondence between the cell temperature and the temperature calibration factor.

[0068] Table 1A

[0069] Cell temperature (℃) Cell temperature calibration factor c -10 1.3 0 1.28 10 1.15 20 1.13 25 1.1 30 1 40 0.95 45 0.92

[0070] The above internal resistance growth rate K is the growth ratio of the real-time DC internal resistance of the lithium battery collected to its factory DC internal resistance (also called "DC internal resistance in the initial state"). For example, the factory DC internal resistance of the lithium battery is recorded as R b , the real-time internal resistance collected at a certain charging time point is recorded as R c , then the internal resistance growth rate K is (R c -R b ) / R b Similarly, the correspondence between the internal resistance growth rate K and the internal resistance calibration factor b can be pre-stored in the mobile terminal. For example, the following Table 1B shows the correspondence between the internal resistance growth rate and the internal resistance calibration factor.

[0071] Table 1B

[0072]

[0073]

[0074] Similarly, the correspondence between the differential of charge per unit capacity ((dQ / dV) / CA) and its calibration factor b can be pre-stored in the BMS. For example, the following Table 1C shows the correspondence between (dQ / dV) / CA and its calibration factor b.

[0075] Table 1C

[0076]

[0077] The charging method of the lithium battery comprises: when the charging voltage of the lithium battery reaches V s When the lithium battery is required to exert high energy density characteristics, the lithium battery is continuously charged to the V h If it is not necessary for the lithium battery to exert its high energy density characteristics (i.e., maintain its long cycle life characteristics), stop charging the lithium battery.

[0078] Whether the lithium battery needs to exert its high energy density characteristics can be remotely turned on by the user during the charging process, or the mode selection setting can be made before charging. This will be explained in detail below when introducing the power vehicle.

[0079] In a fourth aspect, an embodiment of the present application further provides a power vehicle 300, wherein the battery system of the power vehicle includes at least one first battery unit, and the first battery unit includes a plurality of lithium batteries as described in the first aspect of the present application. The battery system of the power vehicle can communicate with the vehicle drive unit 301. Exemplarily, the power vehicle 300 can be a pure electric vehicle or a hybrid electric vehicle. The vehicle drive unit 301 can be an electric motor.

[0080] In some embodiments, see Figure 3 , the battery system of the power vehicle 300 only includes the first battery unit 1. The first battery unit 1 can be a "module-free" battery pack or a "module-equipped" battery pack. When the first battery unit 1 is a "module-equipped" battery pack, multiple lithium batteries 100 can be connected in series, in parallel, or in combination to form a modular battery pack. Figure 3 As shown, the first battery unit 1 includes a plurality of lithium batteries 100 and a first charging control device 110. The first charging control device 110 is used to monitor the status information of each lithium battery, such as voltage, current, internal resistance, temperature, etc., and control the charging status of each lithium battery 100. The first charging control device 110 can be specifically a BMS (Battery Management System) of the first battery unit 1, or it can be a separate module and electrically connected to the BMS of the second battery unit (in this case, the two can be connected via a CAN bus).

[0081] When the battery system of the power vehicle 300 only includes the first battery unit 1, the lithium battery can be controlled to be fully charged or charged at a lower SOC according to the actual cruising range requirement of the power vehicle, which can meet the long cruising range requirement when necessary and ensure a long cycle life under short cruising range requirements.

[0082] Specifically, the first charging control device 110 is used to control the charging cut-off voltage of the lithium battery to be V when the power vehicle is about to run in the first mode. s The first charging control device is also used to control the charging cut-off voltage of the lithium battery to be V when the power vehicle is running in the second mode. h , and V s <V h , at the V s Under this condition, the negative electrode of the lithium battery does not precipitate lithium, and the lithium battery has a long cycle life characteristic, V h is the upper limit voltage of charging that the lithium battery can withstand; wherein, in the first mode, the cruising range provided by the first battery unit for the power vehicle is less than the cruising range provided by the first battery unit for the power vehicle in the second mode. Here, the first mode can be called the short cruising range mode, and the second mode can be called the long cruising range mode.

[0083] The first mode is a mode in which the power vehicle is used more frequently, such as daily short-distance commuting; the second mode is less frequently used and is usually enabled when driving long distances during holidays. When the power vehicle needs to operate in the short-range mode, the first battery unit 1 is controlled during the charging process of the power vehicle, and the lithium battery is not charged to the upper limit voltage but to a lower SOC, which can ensure that lithium is not precipitated from the negative electrode of the battery, thereby facilitating the lithium battery of the first battery unit to exert its long cycle life characteristics. When the power vehicle needs to operate in the long-range mode, the lithium battery is fully charged to ensure that the first battery unit exerts its high energy density characteristics. In this way, although the cycle life of the lithium battery in the "long endurance mode" is not as long as its cycle life in the "short endurance mode", due to the lower frequency of use of the "long endurance mode" and the provision of the above-mentioned protective layer, the side reaction of the lithium battery 100 with the electrolyte under a high energy density state is suppressed, and the risk of lithium plating piercing the diaphragm is reduced, so that the first battery unit of the power vehicle as a whole can have a larger number of low SOC cycles and a larger number of full charge cycles, and release a long endurance mileage for the vehicle when the vehicle needs it.

[0084] For example, the cruising range of the power vehicle in the first mode may be 400-800 km, and the cruising range of the power vehicle in the second mode may be 800-1200 km. In addition, the first mode may also be referred to as the "daily mode", which is the most commonly used mode for power vehicles. The second mode may also be referred to as the "holiday mode", which is the mode occasionally used by power vehicles.

[0085] In the process of charging the lithium battery, if the first charging control device 110 does not receive the instruction of the power vehicle 300 to enable the second operation mode, the default charging cut-off voltage for charging the lithium battery 100 is V s During or before charging the lithium battery 100, if the first charging control device 110 receives an instruction from the power vehicle 300 to enable the second operation mode, the charging cut-off voltage for charging the lithium battery is controlled to be V h .

[0086] The "instruction for the power vehicle to enable the second operating mode" can be set before charging, or remotely enabled during the charging process. Specifically, the instruction can be issued by the user of the power vehicle by pressing the mode button on the vehicle operating panel, or by the user remotely operating an intelligent terminal that can communicate with the vehicle (for example, when the charging voltage of the lithium battery is close to V s When the car is in long-range mode, the car's intelligent network system will push information such as "whether to turn on long-range mode" to the user).

[0087] In some embodiments of the present application, if the charging cut-off voltage of the lithium battery during the i-th charging is V h , then the charging cut-off voltage for the lithium battery during the i+1th charging is adjusted to V s When the V s The following formula should be satisfied:

[0088] V s =cV b +a×c×K+b×c×(dQ / dV) / (3.6×CA),

[0089] Wherein, CA is the nominal capacity of the lithium battery when discharged at 0.33C, V b is the reference voltage of the lithium battery at which no lithium is precipitated from the negative electrode under the real-time charging capacity, K is the internal resistance growth rate of the real-time DC internal resistance of the lithium battery during the i+1th charging process and its factory DC internal resistance, dQ / dV is the real-time differential value of the charging capacity and the charging voltage of the lithium battery during the i+1th charging process, c is the calibration factor of the real-time cell temperature of the lithium battery during the i+1th charging process, a is the calibration factor of K, and b is the calibration factor of (dQ / dV) / CA.

[0090] As described above in this application, in this case, adjusting V s Mainly to reduce V s With V h The gap is reduced to ensure that the lithium battery has as long a mileage as possible under long life.

[0091] In other embodiments, see Figure 4The battery system of the power vehicle 300 further includes at least one second battery unit 2, wherein the second battery unit 2 includes a plurality of second single cells 200, and the negative electrode active material of the second single cell 200 includes graphite and / or silicon-based materials. The silicon-based material includes one or more of elemental silicon, silicon oxide, silicon-based alloy, and silicon-carbon composite material. The second single cell 200 is a conventional lithium battery, which does not use metallic lithium (lithium elemental and / or lithium-silicon alloy) as the negative electrode active material, and its energy density is lower than the energy density of the lithium battery 100 provided in the first aspect of the present application.

[0092] Similar to the first battery unit 1, the second battery unit 2 may be a "module-free" battery pack or a "module-equipped" battery pack, which may include a plurality of second single cells 200 and a second charging control device 210 for monitoring the charging of the second single cells 200. Similarly, the second charging control device 210 may be a BMS of the second battery unit 2 or an independent module electrically connected to the BMS of the second battery unit. Of course, the second charging control device 210 may also be integrated in the same controller as the first charging control device 110.

[0093] In some embodiments of the present application, the power vehicle 300 operates in a first mode, and the power vehicle 300 is powered only by the second battery unit 2; the power vehicle 300 operates in a second mode, and the first battery unit 1 and the second battery unit 2 jointly power the power vehicle 300, or the power vehicle 300 is powered only by the first battery unit 1; wherein the cruising range of the power vehicle in the first mode is less than the cruising range in the second mode.

[0094] Optionally, the first charging control device 110 is used to control the charging cut-off voltage of the lithium battery 100 of the first battery unit 1 to be the above V before or during charging the lithium battery 100 of the first battery unit 1 when it is known that the power vehicle will operate in the second mode. h That is, the lithium battery 100 can be fully charged when it is charged. Of course, the first charging control device 110 is used to control the charging cut-off voltage of the lithium battery 100 of the first battery unit 1 to be the above V before or during the charging of the lithium battery 100 of the first battery unit 1, and when the first battery unit 1 and the second battery unit 2 jointly power the power vehicle 300 in the second mode. s However, in this case, the cruising range of the vehicle in the second mode is less than the range of the second battery unit 2 and the battery charged to V h The cruising range that can be provided by the first battery unit 1.

[0095] When the power vehicle 300 does not need a particularly long cruising range (such as using the vehicle for commuting), only the second battery unit 2 with low energy density can be used to power the vehicle, so that the vehicle can withstand more charging and discharging times and has a longer service life; when the vehicle occasionally needs a long cruising range (such as using the vehicle for long-distance travel during holidays), the first battery unit 1 and the second battery unit 2 can be used to jointly power the vehicle or only the first battery unit 1 can be used to power the vehicle, so that the vehicle can obtain more sufficient power and can selectively achieve the purpose of long cruising range without sacrificing the overall cycle life. Therefore, the power vehicle can selectively use different battery units of the battery system according to different cruising ranges, so that the high-energy-density battery units with low usage frequency in the power vehicle can show a longer cycle life, so that the overall battery system has both long service life and long cruising range.

[0096] The embodiments of the present application are further described below with reference to a plurality of embodiments.

[0097] Example 1

[0098] (1) Production of positive electrode

[0099] 960 g of the positive electrode active material ternary NCM 622, 30 g of the binder PVDF, 5 g of the acetylene black conductive agent, and 5 g of the carbon fiber conductive agent were added to 2000 g of the solvent NMP (nitrogen methyl pyrrolidone), and then stirred in a vacuum mixer to form a stable and uniform positive electrode slurry;

[0100] The positive electrode slurry was evenly and intermittently coated on both sides of the aluminum foil (aluminum foil size: width 160mm, thickness 16μm) by slit coating equipment; then dried at a temperature of 393K, and after being pressed by a roller press, a positive electrode material layer with a thickness of 135μm was formed on the aluminum foil to obtain a positive electrode sheet. The positive electrode sheet was then cut into a rectangular electrode sheet with a size of 48mm*56mm, and the electrode ears were spot welded in the width direction.

[0101] (2) Preparation of electrolyte

[0102] In a glove box filled with argon (H2O content ≤ 5ppm, O2 content ≤ 5ppm), take ethylene glycol dimethyl ether (DME), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), and difluoroethyl acetic acid (DFEA), and mix them in a volume ratio of DME:TTE:DFEA=30:50:20, and then add 60wt% of lithium bis(fluorosulfonyl)imide LiN(SO2F)2 to the mixed solution to obtain an electrolyte.

[0103] (3) Preparation of negative electrode sheet

[0104] a. First, add 1000g of silicon oxide powder to 2000g of water, then add 50g of polyacrylic acid (PAA) binder and 20g of acetylene black conductive agent, and stir vigorously to form a uniform and stable negative electrode slurry. Use a slit coating device to evenly and intermittently coat the negative electrode slurry on both sides of a copper foil (copper foil size: width 160mm, thickness 8μm); then dry at a temperature of 393K, and after roller pressing, a silicon oxide negative electrode material layer with a thickness of 60μm is formed on the copper foil to obtain a negative electrode sheet SA1.

[0105] b. First, add 200g of PEO with a molecular weight of 600,000 into 2000g of acetonitrile, then add 16g of nano-alumina powder and 10g of anhydrous lithium nitrate powder, and stir vigorously to obtain a uniform and stable protective layer slurry. The protective layer slurry is evenly and intermittently coated on the negative electrode sheet SA1 to obtain the negative electrode sheet SA2.

[0106] c. In a glove box (H2O content ≤ 5ppm, O2 content ≤ 5ppm), the SA2 negative electrode sheet is laminated with a lithium film (lithium film thickness is 15μm) coated on a PET release film, so that the protective layer is in contact with the lithium film. Under the action of a hot press, all the lithium elements on the lithium film are transferred to the negative electrode sheet SA2 to obtain a negative electrode sheet SA3, wherein the negative electrode material layer of the negative electrode sheet SA3 is 78μm, and the lithium silicon negative electrode active material includes lithium silicon alloy and Li2O. The negative electrode sheet SA3 is cut into a rectangular electrode sheet with a size of 49mm*57mm, and the pole ears are spot welded in the width direction to obtain a negative electrode for assembling a battery.

[0107] (4) Assembling the battery

[0108] The positive electrode sheet obtained in step (1) and the negative electrode sheet obtained in step (3) are alternately stacked together with a separator to obtain a bare cell, wherein the positive and negative electrode sheets are separated by a separator. The bare cell is placed in an aluminum-plastic film outer packaging body, and the electrolyte prepared in step (2) is injected, and then vacuum-sealed, and then placed at 60° C. for 48 hours, pressurized at 60° C., secondary packaging, exhaust, and capacity separation are performed to obtain the lithium battery of Example 1.

[0109] The lithium battery of Example 1 is fully charged, and the lithium-silicon composite negative electrode active material contains lithium and lithium-silicon alloy Li 4.4 Si, and in the lithium-silicon composite negative electrode active material, the molar proportion of lithium is 23%, and the lithium-silicon alloy Li 4.4 The molar proportion of Si is 71%.

[0110] The lithium battery of Example 1 of the present application was subjected to charge and discharge cycle testing by the following method.

[0111] The lithium battery of Example 1 was subjected to charge and discharge cycle test at 25±1°C on the LAND CT 2001C secondary battery performance test device. The steps of the first conventional low SOC cycle are as follows: leave it for 10 minutes; first charge at a constant current of 0.2C to a charging cut-off voltage of 3.95V, and then charge at a constant voltage of 0.05C; leave it for 10 minutes; then discharge at a constant current to a voltage of 3.0V, which is one conventional low SOC cycle. Repeat this step and perform conventional low SOC cycles 30 times. The steps of the high energy cycle are as follows: leave it for 10 minutes; first charge at a constant current of 0.2C to a voltage of 4.25V, and then charge at a constant voltage of 0.05C; leave it for 10 minutes; then discharge at a constant current to 3.0V, which is one high energy cycle. A high energy cycle is performed every 30 conventional cycles. After each high energy cycle, the formula V is used. s =cV b +ac×I×R+bc×(dQ / dV) verifies the charging cut-off voltage of the next normal low SOC cycle.

[0112] When the battery discharge capacity during the cycle is less than 80% of the initial discharge capacity, the cycle is terminated, and the cycle number n is the cycle life of the lithium battery. The capacity retention rate at the cycle number n is recorded, and the ratio of the battery energy density at the cycle number n to the initial energy density is taken as the energy retention rate of the lithium battery.

[0113] Example 2

[0114] A lithium battery, which differs from Example 1 in that: in step (3), 800 g of silicon powder is used to replace silicon oxide powder, and the thickness of the lithium film coated on the PET release film is 12 μm.

[0115] In the fully charged state of the lithium battery of Example 2, the molar proportion of lithium in the lithium-silicon composite negative electrode active material is 24%, and the molar proportion of the lithium-silicon alloy Li 4.4 The molar proportion of Si is 76%.

[0116] The lithium battery of Example 2 is charged according to the charge and discharge schedule provided in Example 1.

[0117] Example 3

[0118] A lithium battery, which differs from Example 1 in that: in step (3), 800 g of silicon powder is used to replace silicon oxide powder, and the thickness of the lithium film coated on the PET release film is 10 μm.

[0119] In the fully charged state of the lithium battery of Example 3, the molar proportion of lithium in the lithium-silicon composite negative electrode active material is 18%, and the molar proportion of lithium silicon alloy Li 4.4 The molar proportion of Si is 82%.

[0120] The lithium battery of Example 3 is charged according to the charge and discharge schedule provided in Example 1.

[0121] Example 4

[0122] A lithium battery, whose structure is the same as that of Example 3, except that the "0.2C constant current charging until the charging cut-off voltage is 3.95V" of the first conventional low SOC cycle in Example 3 is changed to a charging cut-off voltage of 4.0V.

[0123] In order to highlight the beneficial effects of the embodiments of the present application, the following comparative examples are provided:

[0124] Comparative Example 1

[0125] A lithium battery, which differs from Example 1 in that: in step (3), the prepared negative electrode sheet SA1 is directly used as the negative electrode sheet DS1 of the lithium battery of comparative example 1, and the negative electrode of Example 1 does not contain any metal; in step (1), the electrolyte solvent is an ester solvent, specifically a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 4:6.

[0126] The charge and discharge cycle test method of the lithium battery prepared in Comparative Example 1 is as follows: 5 lithium batteries are taken respectively, and the battery is subjected to a charge and discharge cycle test at 0.2C on a LANDCT 2001C secondary battery performance test device at 25±1°C. The steps are as follows: leave for 10 minutes, first charge at 0.2C constant current to a charge cut-off voltage of 4.2V, then charge at 4.2V constant voltage to 0.05C cut-off; leave for 10 minutes, then discharge at constant current to 3.0V, which is 1 charge and discharge cycle.

[0127] Repeat the above charge and discharge steps. When the discharge capacity of the battery during the cycle is less than 80% of the initial discharge capacity, terminate the cycle. The number of cycles n is the cycle life of the lithium battery. Record the capacity retention rate at the number of cycles n, and the ratio of the battery energy density at the number of cycles n to the initial energy density is taken as the energy retention rate of the lithium battery.

[0128] Comparative Example 2

[0129] A preparation of a lithium battery, which differs from Example 1 in that: in step (3), no protective layer is formed on the negative electrode sheet SA1, but the negative electrode sheet SA1 and the lithium thin film covered on the PET release film are hot pressed, and the obtained negative electrode sheet DS2 is assembled to obtain the lithium battery of Comparative Example 2.

[0130] In order to strongly support the beneficial effects brought by the technical solution of the embodiment of the present application, the battery was tested for energy density (testing battery volume and discharge energy) and cycle life. The test results are shown in Table 2.

[0131] Table 2 Energy density and cycle life test data of each group of samples

[0132]

[0133] It can be seen from Table 2 that the lithium battery provided in the embodiment of the present application has the characteristics of high energy density and long cycle life.

[0134] The above-described embodiments only express several exemplary implementations of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the present application. It should be noted that, for a person of ordinary skill in the art, several modifications and improvements may be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A method for charging a lithium battery, characterized in that: The following steps are involved: The lithium battery comprises a positive electrode sheet, a negative electrode sheet, and a separator and an electrolyte located between the positive electrode sheet and the negative electrode sheet. The negative electrode material layer of the negative electrode sheet contains a lithium-silicon composite negative electrode active material. The surface of the negative electrode material layer has a protective layer or the surface of the lithium-silicon composite negative electrode active material has a protective layer, wherein the protective layer comprises a polymer matrix and a lithium salt; and when the lithium battery is fully charged, the lithium-silicon composite negative electrode active material contains lithium and a lithium-silicon alloy Li 4.4 Si, and the molar proportion of the lithium element in the lithium-silicon composite negative electrode active material is 15%-95%; When the lithium battery is required to have a long cycle life, the charging cut-off voltage V of the lithium battery is controlled. s Satisfies the following formula: V s =cV b +a×c×K+b×c×(dQ / dV) / (3.6×CA), wherein, when the lithium battery exhibits a long cycle life characteristic, at the V s Under this condition, the negative electrode of the lithium battery does not precipitate lithium, and V s <V h ; Among them, V h is the upper limit voltage of the charge that the lithium battery can withstand, CA is the nominal capacity of the lithium battery when discharged at 0.33C, V b is the reference voltage at which no lithium is precipitated from the negative electrode of the lithium battery under the real-time charging capacity, K is the internal resistance growth rate of the real-time DC internal resistance of the lithium battery during charging and its factory DC internal resistance, dQ / dV is the real-time differential value of the charging capacity and the charging voltage of the lithium battery, c is the calibration factor of the real-time cell temperature of the lithium battery during charging, a is the calibration factor of K, and b is the calibration factor of (dQ / dV) / CA.

2. The method for charging a lithium battery as claimed in claim 1, characterized in that: When the charging voltage of the lithium battery reaches the V s When the lithium battery is required to exert high energy density characteristics, the lithium battery is continuously charged to the V h If the lithium battery does not need to exert its high energy density characteristics, stop charging the lithium battery; wherein, when the lithium battery exerts its high energy density characteristics, the lithium silicon composite negative electrode active material contains lithium, and the charging cut-off voltage of the lithium battery is greater than the V s .

3. The method for charging a lithium battery as claimed in claim 1, characterized in that: When the lithium battery is fully charged, the lithium silicon alloy Li 4.4 The molar proportion of Si in the lithium-silicon composite negative electrode active material is 5%-85%.

4. The method for charging a lithium battery as claimed in claim 1, wherein: The polymer matrix includes one or more of polyethylene oxide, polysiloxane, polyvinylidene fluoride, polymethyl methacrylate, polyacrylonitrile and derivatives and copolymers thereof; the lithium salt includes one or more of lithium nitrate, lithium sulfide, lithium chloride, lithium bromide, lithium iodide, lithium fluoride and lithium phosphate.

5. The method for charging a lithium battery as claimed in claim 1, characterized in that: The N / P ratio of the lithium battery is less than 1.

6. The method for charging a lithium battery as claimed in claim 1, characterized in that: The solvent in the electrolyte includes an ether solvent, wherein the ether solvent includes at least one of a non-halogenated ether solvent and a fluorinated ether solvent.

7. The method for charging a lithium battery as claimed in claim 1, characterized in that: When the SOC of the positive electrode of the lithium battery is charged below a first threshold, the lithium-silicon composite negative electrode active material does not contain lithium alone; wherein the first threshold is 15%-95%.

8. The method for charging a lithium battery as claimed in claim 1, wherein: The lithium battery is prepared by the following preparation method: Applying a mixed slurry containing a silicon-based material, a conductive agent and a binder on a negative electrode current collector, and after drying and rolling, forming a silicon-based material layer on the negative electrode current collector; In a glove box, the lithium film and the silicon-based material layer are subjected to hot pressing treatment, so that the lithium element of the lithium film is completely transferred to the silicon-based material layer, and reacts with the silicon-based material in situ to form a negative electrode material layer containing a lithium-silicon composite negative electrode active material, thereby obtaining a negative electrode sheet; Wherein, before the silicon-based material layer and the lithium thin film are subjected to hot pressing treatment, a protective layer is formed on the surface of the silicon-based material layer; or after the negative electrode material layer is formed, a protective layer is formed on the surface of the negative electrode material layer; the protective layer comprises a polymer matrix and a lithium salt; The negative electrode sheet is assembled into the lithium battery.

9. The method for charging a lithium battery as claimed in claim 8, characterized in that: The silicon-based material includes at least one of silicon element, silicon oxide, silicon-based non-lithium alloy and other silicon-containing compounds.

10. A power vehicle, characterized in that: The battery system of the power vehicle includes at least one first battery unit, the first battery unit includes a plurality of lithium batteries and a first charging control device; wherein the lithium battery includes a positive electrode sheet, a negative electrode sheet, and a separator and an electrolyte located between the positive electrode sheet and the negative electrode sheet, the negative electrode material layer of the negative electrode sheet contains a lithium-silicon composite negative electrode active material, the surface of the negative electrode material layer has a protective layer or the surface of the lithium-silicon composite negative electrode active material has a protective layer, the protective layer includes a polymer matrix and a lithium salt; and when the lithium battery is fully charged, the lithium-silicon composite negative electrode active material contains lithium and a lithium-silicon alloy Li 4.4 Si, and the molar proportion of the lithium element in the lithium-silicon composite negative electrode active material is 15%-95%; The first charging control device is used to control the charging cut-off voltage of the lithium battery to be V when charging the lithium battery of the first battery unit before or during charging and knowing that the power vehicle will run in the first mode. s ; The V s Satisfies the following formula: V s =cV b +a×c×K+b×c×(dQ / dV) / (3.6×CA), Wherein, CA is the nominal capacity of the lithium battery when discharged at 0.33C, V b is the reference voltage of the lithium battery at which no lithium is precipitated from the negative electrode under the real-time charging capacity, K is the internal resistance growth rate of the real-time DC internal resistance of the lithium battery during charging and its factory DC internal resistance, dQ / dV is the real-time differential value of the charging capacity and the charging voltage of the lithium battery, c is the calibration factor of the real-time cell temperature of the lithium battery during charging, a is the calibration factor of K, and b is the calibration factor of (dQ / dV) / CA; In the V s Under this condition, the negative electrode of the lithium battery just does not precipitate lithium, and V s <V h , V h It is the upper limit voltage of charging that the lithium battery can withstand.

11. The power vehicle according to claim 10, characterized in that: The first charging control device is also used to control the charging cut-off voltage of the lithium battery to be V when the lithium battery of the first battery unit is charged or during the charging process and it is known that the power vehicle will run in the second mode. h ; Among them, the cruising range of the power vehicle in the first mode is shorter than the cruising range in the second mode.

12. The power vehicle according to claim 10, characterized in that: The battery system of the power vehicle further includes at least one second battery unit, wherein the second battery unit includes a plurality of second single cells, and the negative electrode active material of the second single cells includes graphite and / or silicon-based materials.

13. The powered vehicle according to claim 12, characterized in that: The power vehicle operates in the first mode, and the power vehicle is powered only by the second battery unit; the power vehicle operates in the second mode, and the power vehicle is powered by both the first battery unit and the second battery unit, or only by the first battery unit; Among them, the cruising range of the power vehicle in the first mode is shorter than the cruising range in the second mode.

14. The powered vehicle according to claim 13, characterized in that: Before or during charging of the first battery unit, when the first charging control device learns that the power vehicle will operate in the second mode and only the first battery unit is used to power the power vehicle, the charging cutoff voltage of each lithium battery of the first battery unit is controlled to be the V h .

15. The powered vehicle according to claim 13, characterized in that: Before or during charging of the first battery unit, the first charging control device learns that the power vehicle will operate in the second mode and the first battery unit and the second battery unit jointly power the power vehicle, and controls the charging cutoff voltage of each lithium battery of the first battery unit to be the V s or the V h .

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