Negative plate as well as preparation method and application thereof

Abstract

The invention provides a negative electrode plate and a preparation method and application thereof, the surface of the negative electrode plate is covered with a solid electrolyte interface film, the solid electrolyte interface film comprises lithium carbonate, and the mass percentage content of the lithium carbonate in the solid electrolyte interface film is 30%-45%. The internal resistance of the battery can be reduced.

Description

Technical Field

[0001] The present invention relates to a negative electrode sheet, and in particular to a negative electrode sheet and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries (Li-ion batteries), due to their high energy density, long cycle life, and low self-discharge rate, have become the primary power source of choice for portable electronic devices, electric vehicles, and energy storage systems. The properties of the anode material in Li-ion batteries significantly impact the overall performance and lifespan of the battery. Traditional anode materials primarily include graphite, silicon-based materials, and metal oxides. However, these materials still face challenges in practical application, such as high direct current internal resistance (DCIR), limited rate capability, and limited cold-crank efficiency.

[0003] Therefore, a negative electrode sheet that helps improve the battery's direct current internal resistance (DCIR), rate performance, and cold start efficiency is an important research direction. Summary of the Invention

[0004] The present invention provides a negative electrode sheet and a preparation method and application thereof, which are helpful to reduce the direct current internal resistance (DCIR) of the battery, enhance the rate performance, and improve the cold start efficiency.

[0005] The present invention provides a negative electrode sheet, the surface of which is covered with a solid electrolyte interface film, the solid electrolyte interface film includes lithium carbonate, and the mass percentage of the lithium carbonate in the solid electrolyte interface film is 30% to 45%.

[0006] The present invention provides a battery, comprising the above-mentioned negative electrode sheet.

[0007] Optionally, the battery comprises a lithium-ion battery.

[0008] Optionally, the battery further comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode active material layer, and the positive electrode active material layer comprises lithium iron phosphate.

[0009] The present invention provides a method for preparing the above-mentioned battery, comprising: 1) introducing a supercritical fluid into an electrode core injected with an electrolyte, wherein the supercritical fluid is introduced for 20 to 40 minutes at a rate greater than or equal to 0.03 L / h, during which the temperature of the supercritical fluid is adjusted within a range of A to B°C at a rate of 0.5 to 2°C / min, and then allowed to stand for 10 minutes to 1 hour, wherein A is greater than or equal to the critical temperature of the supercritical fluid and is less than 40°C, and B is greater than 45°C, and the electrode core includes a first negative electrode sheet; 2) repeating step 1) at least 4 times, wherein the supercritical fluid in at least one step 1) includes carbon dioxide; 3) further allowing to stand for 3 to 6 hours, and then performing formation to obtain the battery.

[0010] Optionally, the A~B℃ is 32~50℃.

[0011] Optionally, step 2) includes: repeating step 1) 4 to 8 times.

[0012] Optionally, before step 1), the process includes: stacking and winding the positive electrode sheet, the separator, and the negative electrode sheet in sequence to form the electrode core, placing the electrode core into a battery casing, injecting electrolyte into the electrode core, and then standing at 35-45° C. for 1-2 hours.

[0013] Optionally, the supercritical fluid further includes one or more of oxygen and acetylene.

[0014] The present invention provides a method for preparing the negative electrode sheet according to claim 1, comprising: disassembling a battery obtained according to the above-mentioned battery preparation method to obtain the negative electrode sheet.

[0015] The present invention provides a negative electrode sheet, a preparation method, and an application thereof. The solid electrolyte interface film (SEI film) of the negative electrode sheet includes an inorganic component, lithium carbonate (Li2CO3), and the mass percentage of lithium carbonate in the SEI film is 30% to 45%, which is 10% to 20% higher than the lithium carbonate content in the SEI film in the prior art. This helps to reduce the direct current internal resistance (DCIR) of the battery, improve rate performance, improve cold start efficiency, and increase life. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 Schematic diagram of supercritical fluid molecules promoting the movement of electrolyte molecules provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0018] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below. The specific embodiments listed below are only for describing the principles and features of the present invention. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts are within the scope of protection of the present invention.

[0019] In recent years, researchers have begun to focus on strategies to improve battery performance by introducing lithium compounds into anode materials. Among them, lithium carbonate (Li2CO3) has become a popular additive due to its good chemical stability and conductivity.

[0020] Based on this, an embodiment of the present invention provides a negative electrode sheet, the surface of which is covered with a solid electrolyte interface film, the solid electrolyte interface film includes lithium carbonate, and the mass percentage of lithium carbonate in the solid electrolyte interface film is 30%~45%.

[0021] According to research and analysis, the solid electrolyte interface membrane (SEI membrane) of the above-mentioned negative electrode sheet includes the inorganic component lithium carbonate (Li2CO3), and the mass percentage of lithium carbonate in the SEI membrane is 30%~45%, which is 10%~20% higher than the content of lithium carbonate in the SEI membrane in the prior art. Lithium carbonate (Li2CO3) has high ionic conductivity and can form relatively smooth lithium ion transmission channels in the SEI membrane at low temperatures, reducing the transmission resistance of lithium ions at the interface between the SEI membrane and the electrode, and at the same time optimizing the ion migration number in the SEI membrane, thereby increasing the proportion of lithium ions in the transmission process; in addition, the inorganic component lithium carbonate can enhance the mechanical strength and flexibility of the SEI membrane, thereby helping to reduce the direct current internal resistance (DCIR) of the battery, improve rate performance, improve cold start efficiency, and at the same time increase life.

[0022] Illustratively, the mass percentage of the above-mentioned lithium carbonate in the above-mentioned solid electrolyte interface membrane can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45% or a range consisting of any two of them.

[0023] An embodiment of the present invention further provides a battery comprising the aforementioned negative electrode sheet. Based on the negative electrode sheet, the battery has a low direct current internal resistance (DCIR).

[0024] It is understandable that the battery further comprises a positive electrode sheet, a separator and an electrolyte. The embodiments of the present invention do not specifically limit the positive electrode sheet, separator and electrolyte, and conventional positive electrode sheets, separators and electrolytes in the art are applicable to the present invention.

[0025] In some embodiments, the positive electrode sheet includes a positive electrode active material layer, wherein the positive electrode active material layer includes lithium iron phosphate. The battery has a low direct current internal resistance (DCIR).

[0026] Specifically, the battery may include a lithium-ion battery.

[0027] During the battery manufacturing process, it is generally necessary to place the battery in an environment of 38~50℃ for 24~72 hours after liquid injection to allow the electrolyte to better penetrate into the pores of the electrode, especially the micropores. The above-mentioned standing time after liquid injection accounts for about 20% of the entire production cycle. In addition, in order to meet the current requirements of electric vehicle range, the battery electrode surface density and electrode compaction are getting higher and higher. In this case, in order to achieve the same infiltration effect, the general shelving process will take longer, resulting in a significant extension of the production cycle.

[0028] Therefore, an embodiment of the present invention also provides a method for preparing the above-mentioned battery, comprising: 1) introducing a supercritical fluid into the electrode core into which the electrolyte is injected, wherein the supercritical fluid is introduced for 20 to 40 minutes and the introduction rate is greater than or equal to 0.03 L / h, and the temperature of the supercritical fluid is adjusted within the range of A to B°C at a rate of 0.5 to 2°C / min, and then allowed to stand for 10 minutes to 1 hour, wherein A is greater than or equal to the critical temperature of the supercritical fluid and A is less than 40, and B is greater than 45, and the electrode core includes a first negative electrode sheet; 2) repeating step 1) at least 4 times, wherein the supercritical fluid in at least one step 1) includes carbon dioxide; 3) allowing to stand for another 3 to 6 hours, and then undergoing formation to obtain the above-mentioned battery.

[0029] According to research and analysis, by controlling the temperature range A~B℃, A is greater than or equal to the critical temperature of the supercritical fluid and A is less than 40, and B is greater than 45. Within this temperature range, the density and viscosity of the supercritical fluid will decrease with the increase of temperature. Therefore, adjusting the temperature of the supercritical fluid within the above temperature range A~B℃ can cause the supercritical fluid's own physical properties (state) to change, triggering irregular and violent movement of the supercritical fluid molecules, making the supercritical fluid jump in the electrolyte like popping candy, promoting the electrolyte to circulate, shuttle, and move into the electrode micropores. The specific process can be referred to Figure 1 , thereby making the pores between the pole pieces filled with electrolyte faster, and because the movement of critical fluid molecules is irregular, it can promote the electrolyte to tend to evenly infiltrate the micropores of the pole piece, thereby improving the wetting effect of the battery pole piece, shortening the wetting time, and helping to improve the cycle life, safety performance and offline capacity. In addition, the residual carbon dioxide can participate in the formation of inorganic component lithium carbonate Li2CO3 in the SEI film during the formation stage, so that the mass percentage of Li2CO3 in the SEI film can reach 30%~45%, which is an increase of 10%~20% compared with the existing technology, which helps to reduce the direct current internal resistance (DCIR) of the battery, improve the rate performance, improve the cold start efficiency, and increase the life of the battery.

[0030] Therefore, the above-mentioned battery preparation method can prepare a battery with low direct current internal resistance (DCIR), high rate performance, good cold start efficiency and long life.

[0031] The supercritical pressure of the supercritical fluid is P and the supercritical temperature is T. When the supercritical pressure P remains constant, within the range where the temperature of the supercritical fluid is higher than the supercritical temperature T, the density and viscosity of the supercritical fluid will decrease with the increase of temperature. Therefore, it can be understood that in the above preparation method, the supercritical pressure of the supercritical fluid needs to be controlled to be constant at an appropriate value so that the supercritical fluid satisfies the density and viscosity that decrease with the increase of temperature within the temperature range of A~B℃. Then, within the temperature range of A~B℃, adjusting the temperature of the supercritical fluid can change the physical properties (state) of the supercritical fluid itself, triggering irregular and violent movement of the supercritical fluid molecules, thereby promoting the infiltration of the electrolyte into the electrode, shortening the infiltration time, and improving the infiltration effect. For example, the supercritical pressure of carbon dioxide (CO2) is 7.38 MPa, and the supercritical temperature is 31.1°C. When supercritical carbon dioxide is under constant pressure, its temperature A~B°C is 31.1°C~50°C, such as 31.1°C, 32°C, 33°C, 35°C, 40°C, 45°C, 50°C or a range consisting of any two of them. Preferably, when A~B°C is 32~50°C, the density and viscosity of supercritical carbon dioxide will decrease with increasing temperature. Therefore, by controlling the supercritical carbon dioxide to maintain a constant pressure and then adjusting its temperature within the range of 31.1°C~50°C, the physical properties of supercritical carbon dioxide itself can be changed, triggering irregular and violent movement of carbon dioxide molecules, thereby promoting the infiltration of electrolyte into the electrode, shortening the infiltration time, and improving the infiltration effect.

[0032] In the above preparation method, before step 1), the following steps may be further included: stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence and winding them to form a core, placing the core into a battery casing, injecting the electrolyte into the core, and then standing at 35-45°C for 1-2 hours.

[0033] The embodiments of the present invention do not particularly limit the positive electrode sheet, separator and electrolyte. Conventional positive electrode sheets, separators and electrolytes in the art are applicable to the present invention.

[0034] In step 1), by controlling the supercritical fluid introduction rate to be greater than or equal to 0.03L / h, it can be ensured that the irregular movement of the supercritical fluid molecules is sufficient to promote the infiltration of the electrolyte into the electrode, shorten the infiltration time, and improve the infiltration effect. At the same time, controlling the supercritical fluid introduction time to 20~40min and then letting it stand for 10min~1h will help promote the electrode to better accept the infiltration of the electrolyte, shorten the infiltration time, and improve the infiltration effect.

[0035] It is understandable that the first negative electrode sheet is an original negative electrode sheet, for example, a negative electrode sheet that has just been prepared and has not yet been formed.

[0036] In addition, the supercritical fluid in at least one step 1) includes carbon dioxide. For example, step 1) is repeated 4 times, wherein the supercritical fluid in step 1) may include carbon dioxide once, twice, three times or four times. Furthermore, if step 1) is repeated 4 times, the supercritical fluid in only two steps 1) includes carbon dioxide, while the supercritical fluid in the other two steps 1) may use other types of supercritical fluids, and similar effects can still be achieved.

[0037] Supercritical carbon dioxide helps to form a SEI film with a high lithium carbonate content on the surface of the first negative electrode sheet, thereby obtaining the above-mentioned negative electrode sheet. In addition, the supercritical state of carbon dioxide is easy to achieve, and the supercritical pressure and supercritical temperature of supercritical carbon dioxide are relatively suitable. Under appropriate supercritical pressure, A~B℃ can be adjusted to a temperature range suitable for battery electrolyte infiltration, thereby promoting better electrolyte infiltration and will not adversely affect other battery performance.

[0038] In step 2), repeating step 1) at least four times can help the supercritical fluid fully promote the infiltration of the electrolyte and help form the aforementioned negative electrode sheet. The surface of the negative electrode sheet is covered with a solid electrolyte interface film, which includes lithium carbonate. The mass percentage of lithium carbonate in the solid electrolyte interface film can reach 30% to 45%, which helps to reduce the direct current internal resistance (DCIR) of the battery, improve the rate performance, and improve the cold start efficiency.

[0039] Furthermore, in the above step 2), step 1) can be repeated 4 to 8 times.

[0040] It is understandable that the various parameters of step 1) repeated multiple times, including at least the type of supercritical fluid, introduction time, introduction rate, temperature range, temperature adjustment rate, standing time, etc., can be the same or different. For example, the various parameters of the first step 1) and the various parameters of the second step 1) can be the same or different, and the various parameters between other sub-steps 1) can also be the same or different.

[0041] In step 3), the battery is allowed to stand for another 3 to 6 hours to promote the electrode to fully accept the infiltration of the electrolyte under the action of the supercritical fluid, which helps to promote the electrode to better accept the infiltration of the electrolyte, shorten the infiltration time, and improve the infiltration effect, thereby helping to reduce the direct current internal resistance (DCIR) of the battery, improve the rate performance, and improve the cold start efficiency.

[0042] Next, the battery of the embodiment of the present invention can be obtained through chemical formation.

[0043] The above-mentioned formation process can be carried out according to conventional processes in the art, for example, through a formation process of 0.05C constant current charging for 3 hours, 0.1C constant current charging for 3.75 hours, 0.3C constant current charging to 3.7V, and constant voltage charging for 30 minutes to obtain the above-mentioned battery. It is understood that the various parameters in the above-mentioned formation process are all average values ​​within the conventional error.

[0044] The above preparation method does not use conventional processes such as heating and pressurizing pre-charging, but instead uses a supercritical gas-assisted formation process to improve the wettability of the electrolyte in the electrode sheet, while increasing the Li2CO3 content in the SEI film, thereby preparing the above-mentioned battery. Based on the aforementioned supercritical fluid and negative electrode sheet, the battery has corresponding advantages, which will not be repeated here.

[0045] The supercritical fluid may also include one or more of oxygen and acetylene. In specific implementations, carbon dioxide can be replaced with oxygen or acetylene, and its critical pressure and critical temperature can be adjusted according to the aforementioned requirements of A to B°C. This allows the supercritical oxygen or supercritical acetylene to satisfy the requirement that its density and viscosity decrease with increasing temperature within the temperature range of A to B°C. Furthermore, adjusting the temperature of the supercritical fluid within the temperature range of A to B°C can change the physical properties (state) of the supercritical fluid, inducing irregular and violent motion of the supercritical fluid molecules, thereby promoting electrolyte infiltration into the electrode, shortening the infiltration time, and improving the infiltration effect. Furthermore, the type of supercritical fluid can be changed during the repetition of step 1) at least four times. For example, supercritical carbon dioxide can be used in the first step 1), supercritical oxygen in the second step 1), supercritical acetylene in the third step 1), and supercritical carbon dioxide in the fourth step 1). It is understood that the order of changing the supercritical fluid type can be adjusted according to actual circumstances and is not limited to the above example.

[0046] An embodiment of the present invention further provides a method for preparing the aforementioned negative electrode sheet, comprising disassembling a battery obtained according to the aforementioned method to obtain the negative electrode sheet. The surface of the negative electrode sheet is coated with a solid electrolyte interface film (SEI) comprising lithium carbonate, with the mass percentage of lithium carbonate in the SIE film being 30% to 45%. The advantages of this negative electrode sheet are not further elaborated here.

[0047] The present invention is further described below by way of specific examples and comparative examples. Unless otherwise specified, the reagents, materials, and instruments used below are all conventional reagents, materials, and instruments, all of which are commercially available, and the reagents and materials involved can also be synthesized by conventional synthesis methods.

[0048] The sources of some raw materials in the following examples and comparative examples are as follows:

[0049] The preparation process of the positive electrode sheet and the negative electrode sheet used in the following examples and comparative examples is as follows:

[0050] Positive electrode sheet: The positive electrode active material lithium iron phosphate (LFP), the binder polyvinylidene fluoride (PVDF), the conductive agent carbon black (CB) and carbon nanotubes (CNTs) are dissolved in NMP solvent at a mass ratio of 95.5:2.5:0.5:1.5 to obtain a positive electrode slurry. The positive electrode slurry is then coated on aluminum foil and dried to obtain a positive electrode sheet.

[0051] Negative electrode sheet: The negative electrode active material graphite, the binder sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR), and the conductive agent carbon black (CB) are dissolved in deionized water solvent at a mass ratio of 95:1.5:2:1.5 to obtain a negative electrode slurry, which is then coated on copper foil and dried to obtain a negative electrode sheet;

[0052] Diaphragm: Polyethylene (PE) diaphragm, thickness 14um.

[0053] Example 1

[0054] This embodiment provides a method for preparing a battery, comprising:

[0055] 1) The positive electrode sheet, separator, and negative electrode sheet are stacked and wound in sequence to form a core, which is then encapsulated in an aluminum-plastic film shell. The electrolyte is then injected into the core, which is then allowed to stand at 45°C for 1 hour. Supercritical fluid carbon dioxide is then introduced into the core with the electrolyte injected. The supercritical fluid carbon dioxide is introduced for 30 minutes at a rate of 0.04 L / h. During this period, the temperature of the supercritical fluid carbon dioxide is adjusted within the range of 32-50°C at a rate of 1°C / min, and the core is allowed to stand for 30 minutes.

[0056] 2) Repeat step 1) 4 times;

[0057] 3) Let it stand for another 3 hours, then undergo formation to obtain a battery.

[0058] Example 2

[0059] This embodiment is basically the same as the first embodiment, except that:

[0060] In step 2), repeat step 1) 8 times;

[0061] In step 3), the mixture is allowed to stand for another 6 hours and then subjected to formation to obtain a battery.

[0062] Example 3

[0063] This embodiment is basically the same as the first embodiment, except that:

[0064] In step 3), the mixture is allowed to stand for another 6 hours and then subjected to formation to obtain a battery.

[0065] Example 4

[0066] This embodiment is basically the same as the first embodiment, except that:

[0067] In step 2), repeat step 1) 10 times;

[0068] In step 3), the mixture is allowed to stand for another 6 hours and then subjected to formation to obtain a battery.

[0069] Example 5

[0070] This embodiment is basically the same as the first embodiment, except that:

[0071] In step 1), the supercritical fluid carbon dioxide is introduced for 30 minutes at a rate of 0.03 L / h, during which the temperature of the supercritical fluid carbon dioxide is adjusted within a range of 32°C to 50°C at a rate of 2°C / min, and then allowed to stand for 1 hour;

[0072] In step 2), repeat step 1) 4 times;

[0073] 3) Let it stand for another 3 hours, then undergo formation to obtain a battery.

[0074] Example 6

[0075] This embodiment is basically the same as the first embodiment, except that:

[0076] In step 1), the supercritical fluid carbon dioxide is introduced for 20 minutes at a rate of 0.04 L / h, during which the temperature of the supercritical fluid carbon dioxide is adjusted within a range of 32° C. to 50° C. at a rate of 1° C. / min, and then allowed to stand for 10 minutes.

[0077] In step 2), repeat step 1) 4 times;

[0078] 3) Let it stand for another 3 hours, then undergo formation to obtain a battery.

[0079] Example 7

[0080] This embodiment is basically the same as the first embodiment, except that:

[0081] In step 1), the supercritical fluid carbon dioxide is introduced for 40 minutes at a rate of 0.04 L / h, during which the temperature of the supercritical fluid carbon dioxide is adjusted within a range of 32° C. to 50° C. at a rate of 1° C. / min, and then allowed to stand for 1 hour.

[0082] In step 2), repeat step 1) 4 times;

[0083] 3) Let it stand for another 3 hours, then undergo formation to obtain a battery.

[0084] Example 8

[0085] This embodiment is basically the same as the first embodiment, except that:

[0086] In step 1), the supercritical fluid, acetylene, was introduced for 30 minutes at a rate of 0.04 L / h. During this period, the temperature of the supercritical fluid carbon dioxide was adjusted within the range of 36-50°C at a rate of 1°C / min, and then allowed to stand for 30 minutes.

[0087] In step 2), step 1) is repeated 4 times, wherein the supercritical fluid used in step 1) once comprises acetylene, and the supercritical fluid used in step 1) three times comprises carbon dioxide;

[0088] 3) Let it stand for another 3 hours, then undergo formation to obtain a battery.

[0089] Comparative Example 1

[0090] This comparative example provides a method for preparing a battery, comprising:

[0091] The positive electrode sheet, separator and negative electrode sheet are stacked and wound in sequence to form a pole core, which is then placed in an aluminum-plastic film shell for packaging. The electrolyte is then injected into the pole core. The battery restraint force is then set to 0.8 MPa, placed in a 45°C environment, left to stand for 24 hours, and then subjected to formation to obtain a battery.

[0092] Comparative Example 2

[0093] This comparative example is basically the same as comparative example 1, except that:

[0094] The standing time was adjusted to 36 h, and other conditions remained unchanged.

[0095] Comparative Example 3

[0096] This comparative example is basically the same as comparative example 1, except that:

[0097] The standing time was adjusted to 48 h, and other conditions remained unchanged.

[0098] Test example

[0099] The following parameters of each embodiment and comparative example were tested:

[0100] 1) 60°C Cycle Performance: Perform charge and discharge cycles at 1C at 60°C and 82.5% to 100% SOC. Record the discharge capacity at each cycle and calculate the retention rate of the discharge capacity after 1000 cycles relative to the initial discharge capacity at the first cycle. This retention rate is calculated as: discharge capacity after 1000 cycles / initial discharge capacity at the first cycle.

[0101] 2) Cold Cranking Performance: The battery was stored at 65°C and 100% SOC until it degraded to 80% SOH. It was then discharged at 5C for 2s at -30°C and 50% SOC, and the cut-off voltage, also known as the -30°C cold cranking voltage, was recorded.

[0102] 3) Discharge capacity at 1C at 25℃ (25℃1C discharge capacity): At 25℃, charge at 1C constant current to 3.7V, then charge at constant voltage to 0.05C, let stand for 30 minutes, then discharge at 1C constant current to 2.0V, let stand for 30 minutes, and cycle test three times. The third discharge capacity is the calibration capacity, that is, the 25℃1C discharge capacity;

[0103] 4) Capacity retention rate at 10C discharge at 25°C (25°C 10C capacity retention rate): At 25°C, charge at 1C constant current to 3.7V, then charge at constant voltage to 0.05C, let stand for 30 minutes, then discharge at 10C constant current to 2.0V, let stand for 30 minutes, and cycle the test three times. The third discharge capacity is the calibration capacity; 10C discharge capacity / 1C discharge capacity is the 25°C 10C capacity retention rate;

[0104] 5) DC internal resistance (DCIR) after 8C discharge for 10s at 25°C: At 25°C, charge the battery to 3.7V at 1C constant current, then charge it to 0.05C at constant voltage, let it rest for 30 minutes, and record the voltage after the rest as U1. Then discharge the battery at 8C constant current for 10s, and record the voltage after the constant current discharge for 10s as U2. Then (U1-U2) / 2I is the DCIR after 10s discharge, where I is the current 8C.

[0105] 6) Disassemble the battery and check whether there is brown band or lithium precipitation on the negative electrode interface, as well as the content of free electrolyte (i.e., the electrolyte that can be poured out after disassembling the battery) to check the electrolyte infiltration effect. Under the same positive and negative electrode and diaphragm system design, the less free electrolyte content, the better the electrolyte infiltration effect, and the more free electrolyte content, the worse the electrolyte infiltration effect;

[0106] 7) Mass percentage of the inorganic component Li2CO3 in the SEI film: The battery was disassembled, the negative electrode was removed, and after cleaning and drying with DMC, the elemental composition of the SEI film on the surface of the negative electrode was characterized by X-ray photoelectron spectrometry. The Li2CO3 content was calibrated with the carbon spectrum content. The X-ray photoelectron spectrometer used was the PHIGENESIS500 equipment from ULVAC-PHI, with a spatial resolution of ≤5μm, and the measurements were made in High-Resolution scanning mode.

[0107] Test results

[0108] Table 1

[0109]

[0110] Table 2

[0111]

[0112] Data Analysis:

[0113] Compared with the conventional infiltration method (Comparative Examples 1-3), the infiltration time of the electrode core can be reduced to 7 hours by using supercritical fluid (gas), there is no dry area on the surface of the electrode core, the electrode interface is a uniform golden yellow, and the 60°C cycle performance and rate performance are significantly improved; comparing Example 1 and Comparative Example 1, the Li2CO3 content in the SEI film after supercritical gas infiltration is increased by about 20% compared with conventional infiltration; the cycle life is significantly improved, for example, by comparing Example 1 and Comparative Example 1, the capacity recovery rate after 1000 cycles increases by 4.7%.

[0114] The above-mentioned battery can increase the cold start voltage by 14% at -30°C in the end-of-life state (EOL state), reduce DCIR by 23%, increase the 10C discharge capacity retention rate by 5% at 25°C, and increase the 1C discharge capacity by 10% at 25°C.

[0115] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A negative electrode sheet, characterized in that: The surface of the negative electrode sheet is covered with a solid electrolyte interface film, and the solid electrolyte interface film includes lithium carbonate. The mass percentage of the lithium carbonate in the solid electrolyte interface film is 30% to 45%.

2. A battery, characterized in that: The battery comprises the negative electrode sheet according to claim 1.

3. The battery according to claim 2, characterized in that The battery comprises a lithium-ion battery.

4. The battery according to claim 2 or 3, characterized in that The battery further includes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material layer, and the positive electrode active material layer includes lithium iron phosphate.

5. A method for preparing a battery according to any one of claims 2 to 4, characterized in that: include: 1) introducing a supercritical fluid into the electrode core into which the electrolyte is injected, wherein the supercritical fluid is introduced for 20 to 40 minutes at a rate greater than or equal to 0.03 L / h, during which the temperature of the supercritical fluid is adjusted within a range of A to B°C at a rate of 0.5 to 2°C / min, and then allowed to stand for 10 minutes to 1 hour, wherein A is greater than or equal to the critical temperature of the supercritical fluid and A is less than 40°C, and B is greater than 45°C, and the electrode core includes a first negative electrode sheet; 2) repeating step 1) at least 4 times, wherein the supercritical fluid in at least one of the steps 1) comprises carbon dioxide; 3) The battery is allowed to stand for another 3 to 6 hours and then formed to obtain the battery.

6. The method for preparing a battery according to claim 5, wherein: The A~B℃ is 32~50℃.

7. The method for preparing a battery according to claim 5 or 6, characterized in that: Step 2) includes: repeating step 1) 4 to 8 times.

8. The method for preparing a battery according to any one of claims 5 to 7, characterized in that: Step 1) includes: stacking and winding the positive electrode sheet, the separator and the negative electrode sheet in sequence to form the electrode core, placing the electrode core into the battery casing, injecting the electrolyte into the electrode core, and then standing at 35-45°C for 1-2 hours.

9. The method for preparing a battery according to any one of claims 5 to 8, characterized in that: The supercritical fluid further comprises one or more of oxygen and acetylene.

10. A method for preparing the negative electrode sheet according to claim 1, characterized in that: include: The battery obtained by the method for preparing the battery according to any one of claims 5 to 9 is disassembled to obtain the negative electrode sheet.

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