Battery cell and preparation method therefor, and lithium battery comprising battery cell

Abstract

A battery cell, comprising a positive electrode sheet, a separator, and a negative electrode sheet. The positive electrode sheet comprises a positive electrode current collector and positive electrode active material layers located on two surfaces of the positive electrode current collector; the positive electrode active material layers include a first active material layer and a second active material layer which are stacked; the first active material layer is in direct contact with the surface of the positive electrode current collector; the first active material layer comprises a ternary active material, and the specific surface area of the ternary active material is 0.3-0.9 ㎡ / g; the second active material layer comprises a manganese iron phosphate material, and the specific surface area of a lithium manganese iron phosphate material is 20-24 ㎡ / g. The present invention further provides a lithium battery comprising the battery cell.

Description

A battery cell, a preparation method thereof, and a lithium battery using the same

[0001] This application claims priority to the Chinese patent applications filed with the China Patent Office on September 7, 2023, with application number 2023111613075, and the Chinese patent application filed with the China Patent Office on September 7, 2023, with application number 22023111543551. The entire contents of the above applications are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of lithium batteries, and in particular to a battery cell, a preparation method thereof, and a lithium battery using the same. Background Art

[0003] With the development of new energy technologies, the demand for lithium batteries has gradually increased. The main lithium batteries currently in use are ternary lithium batteries and lithium iron phosphate batteries. Among them, ternary lithium batteries have high energy density and excellent rate performance, but they are prone to spontaneous combustion during use and are prone to fire and explosion during needle puncture, heating, and thermal runaway tests, resulting in poor safety. Secondly, ternary lithium batteries have a short life cycle. Studies have shown that after thousands of cycles of charge and discharge, ternary lithium batteries experience a high rate of capacity degradation, even to around 60% of the rated capacity. This, to a certain extent, limits the application of ternary lithium batteries. The safety performance of lithium iron phosphate batteries is superior to that of ternary lithium batteries, but their energy density is relatively low.

[0004] Therefore, blending ternary materials with lithium iron manganese phosphate (LiFeMnPO4) is an effective improvement approach to achieve both high energy density and high safety in battery cells. In the prior art lithium-ion battery pole piece manufacturing process, LiFeMnPO4 is typically blended with the ternary materials before being slurried and coated. Technical issues

[0005] At present, the slurry and coating process for mixing lithium manganese iron phosphate and ternary materials is immature. The internal resistance of the battery cells manufactured by this process is relatively large, and the cycle life and electrochemical performance are poor. In addition, the safety of the batteries prepared by this process also needs to be improved. For example, when the mass ratio of lithium manganese iron phosphate to ternary materials is 5:1, the thermal runaway pass rate is only 40%, which seriously limits the application of the battery cells. Technical Solutions

[0006] In a first aspect, the present application provides a battery cell, which adopts the following technical solution:

[0007] A battery cell comprising a positive electrode sheet, a separator, and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and positive electrode active material layers located on both surfaces of the positive electrode current collector, the positive electrode active material layers comprising a first active material layer and a second active material layer stacked, the first active material layer being in direct contact with the surface of the positive electrode current collector;

[0008] The first active material layer includes a ternary active material, and the specific surface area of ​​the ternary active material is 0.3-0.9 m2 / g; the second active material layer includes a manganese iron phosphate material, and the specific surface area of ​​the manganese iron lithium phosphate material is 20-24 m2 / g.

[0009] In a second aspect, the present application provides a method for preparing a battery cell, which adopts the following technical solution:

[0010] A method for preparing a battery cell comprises the following steps:

[0011] Step 1: slurrying the lithium manganese iron phosphate material and the ternary active material separately to obtain lithium manganese iron phosphate slurry and ternary active slurry respectively;

[0012] Step 2: coating the lithium manganese iron phosphate slurry and the ternary active slurry on both surfaces of the positive electrode current collector along the thickness direction of the positive electrode current collector; wherein the ternary active slurry is coated on the surface of the positive electrode current collector, and the lithium manganese iron phosphate slurry is coated on the surface of the ternary active slurry, and pressing to obtain a positive electrode sheet with a positive electrode active material layer coated on both sides;

[0013] Step 3: Assemble the positive electrode sheet, the separator, and the negative electrode sheet to obtain a finished battery cell.

[0014] In a third aspect, the present application provides a lithium-ion battery, which adopts the following technical solution:

[0015] A lithium-ion battery comprises a battery cell, wherein the battery cell is any of the battery cells described above.

[0016] In a fourth aspect, the present application provides a battery cell adopting the following technical solution: a battery cell comprising a positive electrode sheet, a separator and a negative electrode sheet, characterized in that: the positive electrode sheet comprises a positive electrode collector and a lithium manganese iron phosphate coating and a ternary material coating alternately coated along the width direction of the positive electrode collector.

[0017] In a fifth aspect, the present application provides a method for preparing a battery cell, which adopts the following technical solution:

[0018] A method for preparing a battery cell comprises the following steps:

[0019] Step 1: slurrying the lithium iron manganese phosphate material and the ternary material separately to obtain lithium iron manganese phosphate slurry and ternary slurry respectively;

[0020] Step 2: Coating lithium iron manganese phosphate slurry and ternary slurry on the surface of the positive electrode current collector along the length direction of the positive electrode current collector, the lithium iron manganese phosphate slurry and the ternary slurry are arranged at intervals along the width direction of the positive electrode current collector, and pressing to obtain a positive electrode sheet with a surface coated with lithium iron manganese phosphate coating and ternary material coating;

[0021] Coating a pure graphite slurry and a silicon-doped graphite slurry on the surface of the negative electrode current collector along the length direction of the negative electrode current collector, and spacing the pure graphite slurry and the silicon-doped graphite slurry along the width direction of the positive electrode current collector, and pressing to obtain a negative electrode sheet having a surface coated with the pure graphite coating and the silicon-doped graphite coating;

[0022] Controlling the lithium manganese iron phosphate slurry coating area on the positive electrode current collector to correspond to the pure graphite slurry coating area on the negative electrode current collector, and controlling the ternary slurry coating area on the positive electrode current collector to correspond to the silicon-doped graphite slurry coating area on the negative electrode current collector;

[0023] Step 3: Assemble the positive electrode sheet, the separator, and the negative electrode sheet to obtain a finished battery cell.

[0024] In a sixth aspect, the present application provides a lithium-ion battery, which adopts the following technical solution:

[0025] A lithium-ion battery comprises the above-mentioned battery cell. Beneficial effects

[0026] (1) Currently, in order to improve the safety of ternary active materials, lithium manganese iron phosphate materials are generally mixed with ternary active materials to obtain a mixed slurry, and the mixed slurry is coated on the positive electrode current collector, and pressed to obtain a positive electrode sheet containing the mixed coating. In the actual production process, the present applicant found that as the slurry is completed and the stirring stops, the mixed slurry will undergo sedimentation, flocculation, and other phenomena, resulting in large particles, which will have a significant impact on subsequent coating and other processes. Micropores that are not visible to the naked eye are easily generated at the interface between the slurry and the current collector, which in turn greatly damages the safety performance and cycle performance of the battery cell.

[0027] First, by controlling the specific surface area of ​​the lithium iron manganese phosphate material and the ternary active material, the interface fusion degree between the lithium iron manganese phosphate slurry and the ternary active slurry, and the interface fusion degree between the ternary active slurry and the surface of the positive electrode collector are high, and the number of micropores that are not visible to the naked eye at the interface of the positive electrode collector-first active material layer and the interface of the second active material layer-first active material layer is reduced, thereby improving the stability and safety of the positive electrode sheet during the cycle; at the same time, the wetting effect of the electrolyte on the positive electrode active material layer is improved, so that the lithium desorption rate of the second active material layer and the first active material layer in the positive electrode active material layer is coordinated, avoiding the situation where the lithium ion extraction and lithium insertion speeds in the active material layer are inconsistent during the cycle, and avoiding the accumulation of lithium ions in the lithium iron phosphate coating or the first active material layer in the positive electrode active material layer, thereby improving the cycle performance and safety performance of the battery. Second, because the conductivity of the ternary active slurry is better than that of the lithium iron manganese phosphate slurry, after coating it on the side close to the positive electrode current collector, it can significantly improve the rate performance of the battery. The second active material layer is set in the outermost layer. Under the action of external force, it can form a certain buffering effect on the first active material layer close to the surface of the positive electrode current collector. In addition, the lithium iron manganese phosphate material has excellent thermal stability. When the second active material layer is located above the first active material layer, it can evenly alleviate the heat spread of the ternary active material, thereby further improving the safety of the battery cell. Third, because the lithium iron manganese phosphate material and the ternary active material are slurried independently, and the lithium iron manganese phosphate slurry and the ternary active slurry are simultaneously coated on the surface of the positive electrode current collector along the thickness direction of the positive electrode current collector, the contact internal resistance between the large particles of the ternary active slurry and the small particles of the lithium iron manganese phosphate slurry in the mixed slurry can be reduced, reducing the temperature rise and improving the cycle life of the battery cell.

[0028] (2) Currently, lithium manganese iron phosphate material and ternary material are mixed to obtain a mixed slurry, and the mixed slurry is coated on the positive electrode current collector, and pressed to obtain a positive electrode sheet containing a mixed coating; however, the particle size of the lithium manganese iron phosphate material and the ternary material in the mixed slurry is quite different, and the distribution in the mixed system is uneven, which has a negative impact on the uniformity of the mixed coating; and when the mixed slurry is coated on the current collector, due to the different sedimentation rates of the lithium manganese iron phosphate material and the ternary material and the effect of the intermolecular interaction force, the uniformity and stability of the coating components on the surface of the positive electrode current collector are further reduced, which has a negative impact on the stability and internal resistance of the battery cell.

[0029] First, by slurrying the lithium iron manganese phosphate material and the ternary material separately, the negative impact of the mixed slurry on the stability and electrical performance of the positive electrode sheet can be avoided; secondly, lithium iron manganese phosphate has excellent thermal stability. After being coated on the surface of the current collector with the ternary material at intervals, the lithium iron manganese phosphate area can serve as a buffer zone to prevent the heat spread of the ternary material, thereby improving the safety of the battery cell; finally, for the existing process of mixing the slurry first and then coating, due to factors such as the different expansion rates of the lithium iron manganese phosphate and the ternary material over time, there are problems such as loosening of the coating in the later stage of the cycle. In this application, the lithium iron manganese phosphate and the ternary material coating intervals are set independently, which can avoid problems such as loosening of the coating caused by the expansion rate of the lithium iron manganese phosphate and the ternary material, thereby improving the stability of the electrode during the cycle and improving the cycle performance of the battery cell; in summary, the battery cell prepared by the present application can simultaneously have excellent safety performance, rate performance and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG1 is a schematic cross-sectional view of a positive electrode current collector in Example 1 in which positive electrode active material layers are coated on both surfaces.

[0031] FIG2 is a schematic diagram of the structure of the positive electrode sheet coated along the coating direction in the battery cell of Example 11.

[0032] FIG3 is a schematic diagram of the structure of the negative electrode sheet coated along the coating direction in the battery cell of Example 11.

[0033] Description of reference numerals:

[0034] 1. Positive electrode current collector; 2. Positive electrode active material layer; 21. First active material layer; 22. Second active material layer; 11. Lithium manganese iron phosphate; 12. Ternary material; 3. Negative electrode current collector; 31. Graphite; 32. Silicon-doped graphite. Modes for Carrying Out the Invention

[0035] Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as modified by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that can vary depending upon the desired properties sought to be obtained.

[0036] As used herein, "and / or" means one or all of the mentioned elements.

[0037] As used herein, “including” and “comprising” encompasses the case where only the stated elements are present and also the case where there are other elements other than the stated elements.

[0038] All percentages in the present invention are by weight unless otherwise stated.

[0039] Unless otherwise indicated, as used in this specification, "a," "an," "an," and "the" are intended to include "at least one" or "one or more." For example, "a component" refers to one or more components, and thus more than one component is contemplated and may be employed or used in the practice of the described embodiment.

[0040] In some embodiments, the chemical formula of the lithium manganese iron phosphate material is LiFe 1-x-z Mn x D z PO4, 0.2≤x≤0.8, z>0; wherein, the D comprises at least one of Mg, Ti, V, Ni, Co, Al, Nb, Y, Mo, Sr, La, Zr, and B; the Fe 1-x-z Mn x D z Denoted as M, the molar ratio of M / P is 0.90-1.15, the molar ratio of Li / M is 1.01-1.10, and the molar ratio of Li / P is 0.95-1.10.

[0041] In some embodiments, the surface density of the first active material layer is 40-100 g / ㎡.

[0042] In some embodiments, the surface density of the second active material layer is 100-360 g / ㎡.

[0043] By controlling the surface density of the second active material layer and the first active material layer, the inconsistent charge transfer efficiency of the two active material coatings can be avoided during the cycle, thereby reducing the impact on the battery's capacity, output power and other performance. More importantly, the equivalent resistance of the positive and negative electrodes of the battery is reduced, avoiding heat generation and loss inside the battery, and even possible short circuits and safety accidents of the battery, thereby improving the stability and safety performance of the battery.

[0044] In some embodiments, the compaction density of the first active material layer is 3.0-3.6 g / m³.

[0045] In some embodiments, the compaction density of the second active material layer is 2.0-2.6 g / m³.

[0046] By controlling the compaction density of the second active material layer and the first active material layer, the number of micropores at the interface between the first active material layer and the positive electrode current collector is reduced, and a dense and uniform transition interface is provided between the second active material layer and the first active material layer. Furthermore, the first active material layer has excellent surface contact performance with the positive electrode current collector, thereby reducing the internal resistance of the battery and improving the cycle stability and safety performance of the battery. Furthermore, due to the combined effect of the compaction density and the surface density, the diffusion distance of lithium ions can be reduced, the concentration difference between the positive and negative electrode coating materials is reduced, and thus the diffusion resistance is reduced, the wettability of the electrolyte to the positive electrode active material layer is improved, and the diffusion rate of lithium ions in the battery cell is increased, which has a positive effect on the cycle life of the battery cell.

[0047] In some embodiments, the surface of the negative electrode sheet is coated with a negative electrode active material layer, and the specific surface area of ​​the negative electrode active material constituting the negative electrode active material layer is 1-2 m2 / g.

[0048] By controlling the specific surface area of ​​the negative electrode active material, it can be coordinated with the positive electrode sheet to keep the lithium de-lithiation and lithium insertion rates of the lithium battery consistent during the cycle process, so that lithium ions can participate more in the cycle process, avoiding the formation of lithium dendrites on the negative electrode surface, and improving the safety of the ternary lithium battery.

[0049] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a pure graphite coating and a silicon-doped graphite coating alternately disposed along a width direction of the negative electrode current collector.

[0050] In some embodiments, the vertical projection of the lithium manganese iron phosphate coating on the diaphragm overlaps with the vertical projection of the pure graphite coating on the diaphragm, and the vertical projection of the ternary material coating on the diaphragm overlaps with the vertical projection of the silicon-doped graphite coating on the diaphragm.

[0051] Since the ternary material has a large gram capacity, when coating the surface of the negative electrode sheet, silicon-doped graphite and pure graphite are alternately coated, and the pure graphite area corresponds to the lithium manganese iron phosphate area, and the silicon-doped graphite area corresponds to the ternary material area, so that the N / P ratios of the two materials are consistent, thereby further improving the safety performance and cycle life of the battery cell.

[0052] In some embodiments, the thickness of the lithium manganese iron phosphate coating and the thickness of the ternary material coating both meet the range of 35-135 μm.

[0053] To a certain extent, the thickness of the coating on the positive electrode sheet can be controlled, the diffusion distance of lithium ions can be reduced, and the concentration difference between the positive and negative electrode coating materials can be reduced, thereby reducing the diffusion resistance and increasing the diffusion rate of lithium ions in the battery cell, which has a positive effect on the cycle life of the battery cell.

[0054] In some embodiments, the thickness of the lithium iron manganese phosphate coating is the same as the thickness of the ternary material coating.

[0055] The capacity of the lithium battery can be maintained within a reasonable range, lithium plating can be avoided during the cycle of the lithium battery, and the cycle performance and safety performance of the lithium battery can be improved.

[0056] In some embodiments, the ratio of the width of the lithium iron manganese phosphate coating to the width of the ternary material coating is (1-8):1.

[0057] By controlling the width of the lithium manganese iron phosphate coating and the ternary material coating, the lithium manganese iron phosphate can evenly alleviate the heat release of the ternary material, so that the surface of the positive electrode will not experience local excessive temperature rise, significantly improving the safety and cycle performance of the lithium battery.

[0058] In some embodiments, the mass ratio of the lithium manganese iron phosphate slurry used in the lithium manganese iron phosphate coating to the mass ratio of the ternary slurry used in the ternary material coating is (1-9):1.

[0059] By adjusting the amount of lithium manganese iron phosphate slurry and ternary slurry, lithium manganese iron phosphate can improve the safety performance of ternary battery cells without reducing the cycle performance and electrochemical performance of ternary battery cells, so that the battery cells can obtain higher energy density and have excellent cycle performance and safety performance.

[0060] In some embodiments, the lithium iron manganese phosphate slurry includes a lithium iron manganese phosphate material, and the chemical formula of the lithium iron manganese phosphate material is LiFe 1-x-z Mn x D z PO4, M is Fe 1-x-z Mn x D z , 0.2<x<0.8, z>0; wherein the molar ratio of Li / P is 0.95-1.10, the molar ratio of M / P is 0.90-1.15, and the molar ratio of Li / M is 1.01-1.10; wherein D includes at least one of Mg, Ti, V, Ni, Co, Al, Nb, Y, Mo, Sr, La, Zr, B, etc.

[0061] Example

[0062] Example 1

[0063] 1. Preparation of positive electrode

[0064] Step 1: The chemical formula of lithium manganese iron phosphate material is LiFe 0.15 Mn 0.8 Ti 0.05 PO4, M = Fe 0.15 Mn 0.8Ti 0.05 , wherein the molar ratio of M / P is 0.92, the molar ratio of Li / M is 1.03, the molar ratio of Li / P is 0.95, and the specific surface area of ​​the lithium iron manganese phosphate material is 20 m2 / g; the above-mentioned lithium iron manganese phosphate material and the lithium nickel cobalt manganese oxide material with a specific surface area of ​​0.3 m2 / g are slurried separately to obtain lithium iron manganese phosphate slurry and ternary active slurry respectively;

[0065] Step 2: Coat lithium iron manganese phosphate slurry and ternary active slurry on both surfaces of the carbon-coated aluminum foil along the thickness direction of the carbon-coated aluminum foil, with the mass ratio of lithium iron manganese phosphate slurry to ternary active slurry being 1:1. The ternary active slurry is coated on both surfaces of the carbon-coated aluminum foil, and the lithium iron manganese phosphate slurry is coated on the surface of the ternary active slurry. After pressing, positive electrode active material layers are formed on both surfaces of the carbon-coated aluminum foil. In the positive electrode active material layers, the first active material layer has an area density of 100 g / m2 and a compaction density of 3.6 g / m³; the second active material layer has an area density of 100 g / m2 and a compaction density of 2 g / m³. The coating structure of the positive electrode sheet of Example 1 is shown in Figure 1.

[0066] 2. Preparation of negative electrode sheet

[0067] The negative electrode slurry was prepared as follows: artificial graphite with a specific surface area of ​​1 m2 / g was added to a vacuum mixer with a conductive agent, acetylene black, a thickener, and a binder, SBR, at a mass ratio of 96.4:1:1.2:1.4, and then deionized water was added to the mixture. The mixed slurry was stirred in a vacuum mixer until it was uniform, thereby obtaining the negative electrode slurry of this embodiment.

[0068] The above-mentioned negative electrode slurry is evenly coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature and then transferred to an oven for further drying. After drying in the oven, a negative electrode sheet semi-finished product is obtained, and then the negative electrode sheet semi-finished product is cold pressed and cut to obtain the negative electrode sheet to be assembled.

[0069] 3. Preparation of battery cells and lithium-ion batteries

[0070] A commercially available polyethylene film is used as a separator for a lithium-ion battery, and the positive electrode sheet, separator, and negative electrode sheet are assembled to obtain a finished battery cell. After packaging, liquid injection, standing, formation, and capacity separation, a finished battery is obtained.

[0071] Example 2

[0072] The difference between this embodiment and embodiment 1 lies in the preparation of the positive electrode sheet and the preparation of the negative electrode sheet. The preparation of the positive electrode sheet in this embodiment adopts the following steps:

[0073] Step 1: The chemical formula of lithium manganese iron phosphate material is LiFe 0.75 Mn 0.2Mg 0.05 PO4, M = Fe 0.75 Mn 0.2 Mg 0.05 The molar ratio of M / P is 1.05, the molar ratio of Li / M is 1.05, the molar ratio of Li / P is 1.10, and the specific surface area of ​​the lithium iron manganese phosphate material is 24 m2 / g. The lithium iron manganese phosphate material and the lithium nickel cobalt manganese oxide material with a specific surface area of ​​0.9 m2 / g are slurried separately to obtain lithium iron manganese phosphate slurry and ternary active slurry respectively.

[0074] Step 2: Lithium iron manganese phosphate slurry and ternary active slurry are coated on both surfaces of the carbon-coated aluminum foil along the thickness direction of the carbon-coated aluminum foil, and the mass ratio of lithium iron manganese phosphate slurry to the ternary active slurry is 9:1; wherein, the ternary active slurry is coated on both surfaces of the carbon-coated aluminum foil, and the lithium iron manganese phosphate slurry is coated on the surface of the ternary active slurry. After pressing, positive electrode active material layers are formed on both surfaces of the carbon-coated aluminum foil; and in the positive electrode active material layer, the surface density of the first active material layer is 40g / ㎡ and the compaction density is 3.3g / m³; the surface density of the second active material layer is 360g / ㎡ and the compaction density is 2.6g / m³.

[0075] The negative electrode active material used in the preparation process of the negative electrode sheet is artificial graphite with a specific surface area of ​​2㎡ / g.

[0076] The rest of the parts are consistent with those in Example 1.

[0077] Example 3

[0078] The difference between this embodiment and embodiment 1 lies in the preparation of the positive electrode sheet and the preparation of the negative electrode sheet. The preparation of the positive electrode sheet in this embodiment adopts the following steps:

[0079] Step 1: The chemical formula of lithium manganese iron phosphate material is LiFe 0.35 Mn 0.6 V 0.05 PO4, M = Fe 0.35 Mn 0.6 V 0.05 The molar ratio of M / P is 0.95, the molar ratio of Li / M is 1.01, the molar ratio of Li / P is 0.96, and the specific surface area of ​​the lithium iron manganese phosphate material is 21 m2 / g. The lithium iron manganese phosphate material and the lithium nickel cobalt manganese oxide material with a specific surface area of ​​0.6 m2 / g are slurried separately to obtain lithium iron manganese phosphate slurry and ternary active slurry respectively.

[0080] Step 2: Lithium iron manganese phosphate slurry and ternary active slurry are coated on both surfaces of the carbon-coated aluminum foil along the thickness direction of the carbon-coated aluminum foil, and the mass ratio of lithium iron manganese phosphate slurry to the ternary active slurry is 4:1; wherein, the ternary active slurry is coated on both surfaces of the carbon-coated aluminum foil, and the lithium iron manganese phosphate slurry is coated on the surface of the ternary active slurry. After pressing, positive electrode active material layers are formed on both surfaces of the carbon-coated aluminum foil; and in the positive electrode active material layer, the surface density of the first active material layer is 70g / ㎡ and the compaction density is 3.0g / m³; the surface density of the second active material layer is 160g / ㎡ and the compaction density is 2.3g / m³.

[0081] The negative electrode active material used in the preparation process of the negative electrode sheet is artificial graphite with a specific surface area of ​​1.5㎡ / g.

[0082] The rest of the parts are consistent with those in Example 1.

[0083] Example 4

[0084] The difference between this embodiment and embodiment 1 lies in the preparation of the positive electrode sheet and the preparation of the negative electrode sheet. The preparation of the positive electrode sheet in this embodiment adopts the following steps:

[0085] Step 1: The chemical formula of lithium manganese iron phosphate material is LiFe 0.15 Mn 0.8 Mg 0.02 Ti 0.03 PO4, M = Fe 0.15 Mn 0.8 Mg 0.02 Ti 0.03 The molar ratio of M / P is 0.94, the molar ratio of Li / Mn is 1.10, the molar ratio of Li / P is 1.03, and the specific surface area of ​​the lithium iron manganese phosphate material is 22 m2 / g. The lithium iron manganese phosphate material and the lithium nickel cobalt manganese oxide material with a specific surface area of ​​0.7 m2 / g are slurried separately to obtain lithium iron manganese phosphate slurry and ternary active slurry respectively.

[0086] Step 2: Lithium manganese iron phosphate slurry and ternary active slurry are coated on both surfaces of the carbon-coated aluminum foil along the thickness direction of the carbon-coated aluminum foil, and the mass ratio of lithium manganese iron phosphate slurry to the ternary active slurry is 1:1; wherein, the ternary active slurry is coated on both surfaces of the carbon-coated aluminum foil, and the lithium manganese iron phosphate slurry is coated on the surface of the ternary active slurry, and active material layers are formed on the two surfaces of the carbon-coated aluminum foil after pressing; and in the positive electrode active material layer, the surface density of the first active material layer is 100 g / ㎡ and the compaction density is 3.0 g / m³; the surface density of the second active material layer is 100 g / ㎡ and the compaction density is 2.0 g / m³.

[0087] The negative electrode active material used in the preparation process of the negative electrode sheet is artificial graphite with a specific surface area of ​​2㎡ / g.

[0088] The rest of the parts are consistent with those in Example 1.

[0089] Example 5

[0090] The difference between this embodiment and embodiment 3 is that, in the process of preparing the positive electrode sheet, in the positive electrode active material layer, the surface density of the first active material layer is 115 g / ㎡ and the compaction density is 3.6 g / m³, and the surface density of the second active material layer is 92 g / ㎡ and the compaction density is 2.0 g / m³; the rest are consistent with embodiment 3.

[0091] Example 6

[0092] The difference between this embodiment and embodiment 3 is that, in the process of preparing the positive electrode sheet, and in the positive electrode active material layer, the surface density of the first active material layer is 100 g / ㎡ and the compaction density is 3.8 g / m³, and the surface density of the second active material layer is 100 g / ㎡ and the compaction density is 1.8 g / m³; the rest are consistent with embodiment 3.

[0093] Example 7

[0094] The difference between this embodiment and embodiment 3 is that, in the process of preparing the positive electrode sheet, in the positive electrode active material layer, the surface density of the first active material layer is 372 g / ㎡ and the compaction density is 3.0 g / m³, and the surface density of the second active material layer is 31 g / ㎡ and the compaction density is 2.0 g / m³; the rest are consistent with embodiment 3.

[0095] Example 8

[0096] The difference between this embodiment and embodiment 3 is that, in the process of preparing the positive electrode sheet, in the positive electrode active material layer, the surface density of the first active material layer is 100 g / ㎡ and the compaction density is 3.8 g / m³, and the surface density of the second active material layer is 360 g / ㎡ and the compaction density is 1.8 g / m³; the rest are consistent with embodiment 3.

[0097] Example 9

[0098] The difference between this embodiment and embodiment 3 is that, in the process of preparing the positive electrode sheet, the mass ratio of lithium manganese iron phosphate slurry to the ternary active slurry is 12:1; the rest is consistent with embodiment 3.

[0099] Example 10

[0100] The difference between this embodiment and embodiment 3 is that, in the process of preparing the positive electrode sheet, the mass ratio of lithium manganese iron phosphate slurry to the ternary active slurry is 0.5:1; the rest is consistent with embodiment 3.

[0101] Comparative Example 1

[0102] The difference between this comparative example and Example 3 is that both sides of the carbon-coated aluminum foil are coated with the ternary active slurry; the rest are consistent with Example 3.

[0103] Comparative Example 2

[0104] The difference between this comparative example and Example 3 is that both sides of the carbon-coated aluminum foil are coated with lithium manganese iron phosphate slurry; the rest are consistent with Example 3.

[0105] Comparative Example 3

[0106] The difference between this comparative example and Example 3 is that the second active material layer in the positive electrode active material layer is in direct contact with the surface of the carbon-coated aluminum foil, and the first active material layer is in direct contact with the surface of the second active material layer; the rest are consistent with Example 3.

[0107] Comparative Example 4

[0108] The difference between this comparative example and Example 3 is that the specific surface area of ​​the lithium manganese iron phosphate material constituting the second active material layer is 2 m2 / g, and the specific surface area of ​​the ternary active material constituting the first active material layer is 16 m2 / g; the rest are consistent with Example 3.

[0109] Comparative Example 5

[0110] The difference between this comparative example and Example 3 is that both sides of the carbon-coated aluminum foil are coated with the same coating, and the slurry forming the coating is obtained by mixing lithium manganese iron phosphate material powder and ternary active material powder in a mass ratio of 4:1 and then slurrying; the rest are consistent with Example 3.

[0111] Detection method

[0112] 1. Acupuncture test

[0113] The lithium batteries prepared in Examples 1-10 and Comparative Examples 1-5 were subjected to a needle penetration performance test. The test method is as follows: The lithium-ion battery was placed in a 25°C constant temperature chamber and allowed to stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. The lithium-ion battery that reached a constant temperature was charged at a constant current and constant voltage of 1C to the upper limit voltage (4.2V), with a cut-off current of 0.05C. The fully charged lithium-ion battery was transferred to a nail penetration tester, and the test environment temperature was maintained at 25°C ± 2°C. A steel nail with a diameter of 5mm was uniformly penetrated through the center of the lithium-ion battery at a speed of 25mm / s. The nail was held for 1 hour. The lithium-ion battery was considered to have passed if it did not catch fire or explode. Ten lithium-ion batteries were tested each time, and the number of lithium-ion batteries that passed the needle penetration test was used as an indicator to evaluate the safety performance of the lithium-ion battery. The needle penetration test results are recorded in Table 1.

[0114] 2. Energy density test

[0115] The energy density test of the lithium batteries prepared in Examples 1-10 and Comparative Examples 1-5 was conducted as follows: (1) at room temperature of 25 ± 2°C, charge to 4.2V at 0.33C constant current and constant voltage, with a cut-off current of 0.05C, and let it stand for 30 minutes; (2) discharge to 2.5V at 0.33C constant current, and let it stand for 30 minutes; (3) repeat step (1) to step (2) three times, and take the average value ΔE of the three discharge energies E; (4) use an electronic scale to measure the mass M (in kg) of the test object; (5) calculate the discharge energy density PED (in Wh.kg) of the test object. -1 The calculation formula is as follows: PED=△E / M; record the energy density in Table 1.

[0116] 3. Cycle performance test

[0117] The lithium batteries prepared in Examples 1-10 and Comparative Examples 1-5 were subjected to a cycling performance test. The test method was as follows: The lithium-ion battery was placed in a 25°C constant temperature box and allowed to stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. (1) 1C constant current and constant voltage charge was applied to 4.2V, 0.05C cutoff current, and the battery was left for 30 minutes; (2) 1C constant current discharge was applied to 2.5V, and the battery was left for 30 minutes; (3) Steps (1) to (2) were repeated until the capacity retention rate was 80% of the initial capacity. The number of cycles was recorded in Table 1.

[0118] Table 1

[0119] No. Needle penetration rate / % Energy density / Wh.kg-1 Cycle life / cycle Example 190 236 2550 Example 2 100 211 2500 Example 3 100 222 2530 Example 4 90 235 2535 Example 5 90 238 2310 Example 6 100 236 2300 Example 7 100 234 2320 Example 8 100 235 2305 Example 9 100 20 22 498 Example 10 30 24 3 2156 Comparative Example 1 10 250 2040 Comparative Example 2 100 195 2015 Comparative Example 3 50 220 2025 Comparative Example 4 100 221 2216 Comparative Example 5 60 221 2100

[0120] In combination with Examples 1-4, Comparative Examples 1-5, Figure 1 and Table 1, it can be seen that by arranging the lithium iron manganese phosphate slurry and the ternary active slurry along the thickness direction of the positive electrode current collector and controlling the specific surface area of ​​the lithium iron manganese phosphate material and the ternary active material, the safety performance, energy density and cycle performance of the battery cell can be improved; this is because the above-mentioned coating method can avoid the problem of increased internal resistance of the battery cell caused by the mixed slurry, and by controlling the specific surface area of ​​the lithium iron manganese phosphate material and the ternary active material, the interface fusion degree between the second active material layer and the first active material layer and the surface area of ​​the first active material layer and the positive electrode current collector are improved. The high degree of interface fusion between the surfaces avoids the formation of harmful micropores between the interfaces, thereby improving the stability and energy density of the positive electrode during the cycle; in addition, by stacking the second active material layer on the first active material layer, the second active material layer can effectively slow down the heat release rate of the ternary active material during the charge and discharge process, thereby improving the thermal stability of the battery cell; and the second active material layer is arranged at the outermost layer of the positive electrode active material layer. Under the action of external force, it can form a certain buffering effect on the first active material layer close to the surface of the positive electrode current collector, thereby further improving the safety of the battery cell while increasing the life cycle of the battery.

[0121] In combination with Examples 3, 5-8 and Table 1, it can be seen that the adjustment of the compaction density and the surface density in Examples 5-8 can have a relatively positive impact on the safety performance and energy density of the positive electrode sheet. This is because by regulating the compaction density and the surface density, the probability of micropores appearing at the interface between the positive electrode active material layer and the surface of the positive electrode current collector can be reduced, so that the battery cell has both excellent safety performance and energy density.

[0122] Furthermore, by adjusting the compaction density and surface density, it is possible to avoid situations where the thickness of the positive electrode active material layer is too large or too small. This is because, although increasing the electrode thickness can increase the specific gravity of the active material and thus improve the energy density of the battery, it will have a negative impact on the electrochemical performance of the battery. Thicker electrodes will increase the electrolyte consumption near the current collector side, thereby increasing the electrode capacity loss. On the one hand, the thicker the electrode, the greater the difference in electrochemical rate within the electrode, further deepening the non-uniformity of the electrochemical reaction within the positive electrode active material layer, thereby reducing the battery capacity. On the other hand, it may cause inconsistent volume changes, accumulate stress within the electrode, increase impedance, further weaken the discharge capacity of the battery, and may cause more side reactions, leading to battery capacity degradation.

[0123] Combining Examples 3, 9-10 and Table 1, it can be seen that when the ratio of lithium manganese iron phosphate material to ternary active material is too low, the battery safety performance decreases and the energy density increases; when the ratio is too high, the safety performance of the lithium battery is improved but the energy density decreases; therefore, according to different application scenarios and design principles, different blending ratios can be adopted to enable the lithium battery to achieve excellent cycle performance while taking into account high energy density and high safety.

[0124] Example 11

[0125] 1. Preparation of positive electrode

[0126] Step 1: slurrying the lithium iron manganese phosphate material and the lithium nickel cobalt manganese oxide material separately to obtain lithium iron manganese phosphate slurry and ternary slurry respectively;

[0127] Step 2: Coating lithium iron manganese phosphate slurry and ternary slurry on the surface of the positive electrode current collector along the length direction of the positive electrode current collector, the mass ratio of lithium iron manganese phosphate slurry to ternary slurry is 5:1, the lithium iron manganese phosphate slurry and the ternary slurry are spaced apart along the width direction of the positive electrode current collector, and the ratio of the width of the lithium iron manganese phosphate coating to the width of the ternary material coating is 4:1, and pressing to obtain a positive electrode sheet coated with the lithium iron manganese phosphate coating and the ternary material coating, wherein the thickness of the lithium iron manganese phosphate coating is 85 μm and the thickness of the ternary material coating is 90 μm;

[0128] Lithium manganese iron phosphate slurry includes lithium manganese iron phosphate material, the chemical formula of lithium manganese iron phosphate material is LiFe 1-x-z Mn x D z PO4, M is Fe 1-x-z Mn x D z , x=0.6, z=0.05, where the molar ratio of Li / P is 1.05, the molar ratio of M / P is 1.05, the molar ratio of Li / M is 1.05, and D is Mg. The slurry coating on the positive electrode sheet is shown in Figure 2.

[0129] 2. Preparation of negative electrode sheet

[0130] Pure graphite slurry is coated on the surface of the negative electrode current collector along the length direction of the negative electrode current collector, and pressed to obtain a negative electrode sheet with a pure graphite coating on the surface.

[0131] 3. Preparation of battery cells

[0132] Assemble the positive electrode sheet, the separator, and the negative electrode sheet to obtain a finished battery cell.

[0133] 4. Assemble lithium batteries

[0134] The above-mentioned battery cells are assembled to obtain a lithium battery.

[0135] The schematic diagram of the coating structure on the surface of the positive electrode sheet in this embodiment is shown in FIG2 ; the schematic diagram of the coating structure on the surface of the negative electrode sheet in this embodiment is shown in FIG3 .

[0136] Example 12

[0137] The difference between this embodiment and embodiment 11 is that the preparation of the positive electrode sheet is different. The positive electrode sheet is prepared by the following steps:

[0138] Step 1: slurrying the lithium iron manganese phosphate material and the lithium nickel cobalt manganese oxide material separately to obtain lithium iron manganese phosphate slurry and ternary slurry respectively;

[0139] Step 2: Coating lithium iron manganese phosphate slurry and ternary slurry on the surface of the positive electrode current collector along the length direction of the positive electrode current collector, the mass ratio of lithium iron manganese phosphate slurry to ternary slurry is 1:1, the lithium iron manganese phosphate slurry and the ternary slurry are spaced apart along the width direction of the positive electrode current collector, and the ratio of the width of the lithium iron manganese phosphate coating to the width of the ternary material coating is 1:1, and pressing to obtain a positive electrode sheet coated with the lithium iron manganese phosphate coating and the ternary material coating, wherein the thickness of the lithium iron manganese phosphate coating is 40 μm and the thickness of the ternary material coating is 35 μm;

[0140] Lithium manganese iron phosphate slurry includes lithium manganese iron phosphate material, the chemical formula of lithium manganese iron phosphate material is LiFe 1-x-z Mn x D z PO4, M is Fe 1-x-z Mn x D z , x=0.3, z=0.1, wherein the molar ratio of Li / P is 0.95, the molar ratio of M / P is 1.15, and the molar ratio of Li / M is 1.10; D is a mixed element of Mg and Ti. The rest of the contents remain the same as in Example 11.

[0141] Example 13

[0142] The difference between this embodiment and embodiment 11 is that the preparation of the positive electrode sheet is different. The positive electrode sheet is prepared by the following steps:

[0143] Step 1: slurrying the lithium iron manganese phosphate material and the lithium nickel cobalt aluminum oxide material separately to obtain lithium iron manganese phosphate slurry and ternary slurry respectively;

[0144] Step 2: Coating lithium iron manganese phosphate slurry and ternary slurry on the surface of the positive electrode current collector along the length direction of the positive electrode current collector, the mass ratio of lithium iron manganese phosphate slurry to ternary slurry is 9:1, the lithium iron manganese phosphate slurry and the ternary slurry are spaced apart along the width direction of the positive electrode current collector, and the ratio of the width of the lithium iron manganese phosphate coating to the width of the ternary material coating is 8:1, and pressing to obtain a positive electrode sheet coated with the lithium iron manganese phosphate coating and the ternary material coating, wherein the thickness of the lithium iron manganese phosphate coating is 135 μm and the thickness of the ternary material coating is 130 μm;

[0145] Lithium manganese iron phosphate slurry includes lithium manganese iron phosphate material, the chemical formula of lithium manganese iron phosphate material is LiFe 1-x-z Mn x D z PO4, M is Fe 1-x-z Mn x D z , x=0.7, z=0.1, wherein the molar ratio of Li / P is 1.10, the molar ratio of M / P is 0.90, and the molar ratio of Li / M is 1.01; D is Zr.

[0146] The rest of the parts are the same as those in Example 11.

[0147] Example 14

[0148] The difference between this embodiment and embodiment 11 is that the preparation of the positive electrode sheet is different. The positive electrode sheet is prepared by the following steps:

[0149] Step 1: slurrying the lithium iron manganese phosphate material and the lithium nickel cobalt aluminum oxide material separately to obtain lithium iron manganese phosphate slurry and ternary slurry respectively;

[0150] Step 2: Coating lithium iron manganese phosphate slurry and ternary slurry on the surface of the positive electrode current collector along the length direction of the positive electrode current collector, the mass ratio of lithium iron manganese phosphate slurry to ternary slurry is 7:1, the lithium iron manganese phosphate slurry and the ternary slurry are spaced apart along the width direction of the positive electrode current collector, and the ratio of the width of the lithium iron manganese phosphate coating to the width of the ternary material coating is 4:1, and pressing to obtain a positive electrode sheet coated with the lithium iron manganese phosphate coating and the ternary material coating, wherein the thickness of the lithium iron manganese phosphate coating is 110 μm and the thickness of the ternary material coating is 100 μm;

[0151] Lithium manganese iron phosphate slurry includes lithium manganese iron phosphate material, the chemical formula of lithium manganese iron phosphate material is LiFe 1-x-z Mn x D z PO4, M is Fe 1-x-z Mn x D z, x=0.7, z=0.1, wherein the molar ratio of Li / P is 1.05, the molar ratio of M / P is 0.95, and the molar ratio of Li / M is 1.03; D is Zr.

[0152] The rest of the parts are the same as those in Example 11.

[0153] Example 15

[0154] The difference between this embodiment and embodiment 11 is that the preparation of the negative electrode sheet is different. The specific steps for preparing the negative electrode sheet are as follows:

[0155] 2. Preparation of negative electrode sheet

[0156] A pure graphite slurry and a silicon-doped graphite slurry were applied to the surface of the negative electrode current collector along its length. The pure graphite slurry and the silicon-doped graphite slurry were spaced apart along the width of the positive electrode current collector. Pressing was performed to obtain a negative electrode sheet coated with both pure and silicon-doped graphite coatings. A positive electrode sheet was obtained in which the vertical projection of the lithium manganese iron phosphate coating on the separator overlapped with the vertical projection of the pure graphite coating on the separator, and the vertical projection of the ternary material coating on the separator overlapped with the vertical projection of the silicon-doped graphite coating on the separator. All other aspects remained consistent with Example 11. The slurry coating on the negative electrode sheet is shown in Figure 31.

[0157] Example 16

[0158] The difference between this embodiment and embodiment 11 is that, during the preparation of the positive electrode sheet, the thickness of the lithium manganese iron phosphate coating and the thickness of the ternary material coating are both 90 μm; the rest are consistent with embodiment 11.

[0159] Example 17

[0160] The difference between this embodiment and Example 11 is that, during the preparation of the positive electrode sheet, the ratio of the width of the lithium manganese iron phosphate coating to the width of the ternary material coating is 0.5:1; the rest remains consistent with Example 11.

[0161] Example 18

[0162] The difference between this embodiment and embodiment 11 is that, during the preparation of the positive electrode sheet, the ratio of the width of the lithium manganese iron phosphate coating to the width of the ternary material coating is 10:1; the rest remains consistent with embodiment 11.

[0163] Example 19

[0164] The difference between this embodiment and embodiment 11 is that, in the process of preparing the positive electrode sheet, the mass ratio of the lithium manganese iron phosphate material used to the mass ratio of the ternary material is 0.2:1; the rest are consistent with embodiment 11.

[0165] Example 20

[0166] The difference between this embodiment and Example 1 is that, in the process of preparing the positive electrode sheet, the mass ratio of the lithium manganese iron phosphate material used to the ternary material is 12:1; the rest is consistent with Example 11.

[0167] Example 21

[0168] The difference between this embodiment and Example 11 is that, during the process of preparing the positive electrode sheet and the negative electrode sheet, the vertical projection of the lithium manganese iron phosphate coating on the diaphragm overlaps with the vertical projection of the silicon-doped graphite coating on the diaphragm, and the vertical projection of the ternary material coating on the diaphragm overlaps with the vertical projection of the pure graphite coating on the diaphragm; the rest are consistent with Example 11.

[0169] Comparative Example 6

[0170] The difference between this comparative example and Example 11 is that, during the preparation of the positive electrode sheet, the lithium manganese iron phosphate slurry and the ternary slurry are evenly mixed and coated on the surface of the positive electrode collector to form a mixed coating with a thickness of 90 μm; the rest are consistent with Example 11.

[0171] Comparative Example 7

[0172] The difference between this comparative example and Example 11 is that, during the preparation of the positive electrode sheet, only lithium manganese iron phosphate slurry is coated on the surface of the positive electrode collector to form a lithium manganese iron phosphate coating with a thickness of 90 μm; the rest of the parts are consistent with Example 11.

[0173] Comparative Example 8

[0174] The difference between this comparative example and Example 11 is that, during the preparation of the positive electrode sheet, the surface of the positive electrode collector is only coated with the ternary slurry, and a ternary material coating with a thickness of 90 μm is formed; the surface of the negative electrode collector is only coated with silicon-doped graphite slurry; the rest of the parts are consistent with Example 11.

[0175] Detection method

[0176] 1. Thermal runaway test

[0177] The lithium batteries prepared in Examples 10-21 and Comparative Examples 6-8 were tested for thermal runaway performance. The test method was as follows: the lithium-ion batteries were charged to 100% SOC at a constant current and constant voltage of 1C, then continued charging at a constant current of 1C for 12 minutes. The heating device was then activated to heat the batteries. When the batteries reached thermal runaway or the large surface temperature reached 300°C, the heating device was turned off and the batteries were observed for 1 hour. The batteries were considered to have passed if they did not catch fire or explode. The number of lithium-ion batteries that passed the thermal runaway test was used as an indicator for evaluating the safety performance of the lithium-ion batteries, and the test data is recorded in Table 2.

[0178] 2. Rate performance test

[0179] The lithium batteries prepared in Examples 10-21 and Comparative Examples 6-8 were tested for rate performance using the following method: At 25°C, they were charged at a constant current of 0.33C to 4.2V, then switched to constant voltage charging until the current reached 0.05C, and then discharged at a constant current of 0.33C to 2.8V. The discharge capacity was recorded as Q1. They were then charged at a constant current of 0.33C to 4.2V, then switched to constant voltage charging until the current reached 0.05C, and then discharged at a constant current of 3C to 2.8V. The discharge capacity was recorded as Q2. The rate performance was characterized by the ratio Q2 / Q1, and the test data are recorded in Table 2.

[0180] 3. Cycle performance test

[0181] The lithium batteries prepared in Examples 10-21 and Comparative Examples 6-8 were subjected to a cycling performance test. The test method was as follows: the lithium-ion battery was placed in a 25°C thermostat and allowed to stand for 30 minutes to allow the lithium-ion battery to reach a constant temperature. The lithium-ion battery that had reached a constant temperature was discharged at a constant current of 1C to a voltage of 2.8V, allowed to stand for 30 minutes, and then charged at a constant current of 1C to a voltage of 4.2V. Then, the battery was charged at a constant voltage of 4.2V to a current of 0.05C, allowed to stand for 30 minutes, and discharged at 1C to a voltage of 2.8V. This cycle was repeated until the capacity retention rate reached 80% of the initial capacity. The test data is recorded in Table 2.

[0182] Table 2

[0183] No. Thermal runaway pass rate / % Rate performance / % Cycle life / times Example 118097.22205 Example 127097.62210 Example 1310096.62206 Example 149096.72198 Example 158597.12635 Example 168597.32650 Example 177597.12215 Example 187597.22195 Example 1960981985 Example 209096.41980 Example 218097.11800 Comparative Example 65094.91505 Comparative Example 710093.22185 Comparative Example 81098.32220

[0184] In combination with Examples 11-14, Comparative Examples 6-8, Figure 2 and Table 2, it can be seen that the positive electrode sheet obtained by arranging the lithium iron manganese phosphate slurry and the ternary slurry at intervals perpendicular to the coating tape direction can improve the safety performance, rate performance and cycle performance of the battery cell; this is because the above-mentioned coating method can avoid the problem of increased internal resistance of the battery cell caused by the mixed slurry, and enhances the stability of the electrode sheet during the cycle, thereby improving the cycle performance and rate performance of the battery cell; in addition, the lithium iron manganese phosphate material is used to slow down the heat diffusion of the ternary material in the thermal runaway test, thereby improving the safety performance of the battery cell.

[0185] In combination with Examples 11, 15, 21, Figures 2-3 and Table 2, it can be seen that when the pure graphite and silicon-doped graphite coated on the negative electrode surface correspond to the lithium iron manganese phosphate and ternary materials coated on the positive electrode surface, the safety performance and cycle performance of the battery cell are slightly improved. This is because the gram capacity of the ternary material is large, and the above coating method can keep the N / P ratio of the two materials consistent, which in turn has a positive impact on the comprehensive performance of the battery cell, which is manifested macroscopically as an improvement in the safety performance and cycle performance of the lithium battery.

[0186] In combination with Examples 11 and 16 and Table 2, it can be seen that by adjusting the thickness of the lithium iron manganese phosphate coating and the ternary material coating to be the same, the cycle performance and safety performance of the lithium battery can be further improved. This is because when the thickness of the lithium iron manganese phosphate coating and the ternary material coating in the positive electrode sheet is the same, the migration path of lithium ions in the lithium iron manganese phosphate coating and the ternary material coating can be kept the same, making the lithium insertion state in the two coatings on the surface of the negative electrode sheet more uniform, which is manifested macroscopically as an improvement in the cycle performance and safety performance of the lithium battery.

[0187] In combination with Example 11, Examples 17-18 and Table 2, it can be seen that when the ratio of the width of the lithium iron manganese phosphate coating to the width of the ternary material coating is within an appropriate range, the lithium iron manganese phosphate can fully alleviate the heat released in the ternary material coating area, so that the heat release on the electrode remains uniform, and local overheating does not occur in the battery cell, thereby improving the safety performance of the lithium battery.

[0188] In combination with Example 11, Examples 19-20 and Table 2, it can be seen that when the ratio of the amount of lithium iron manganese phosphate to the amount of the ternary material is within an appropriate range, the safety performance, rate performance and cycle performance of the battery cell are more balanced; although lithium iron manganese phosphate has excellent safety performance and can alleviate the problem of poor safety performance of the ternary material, when the ratio of the two is too large, the safety of the lithium battery is only slightly improved, and when the ratio of the two is too small, the safety performance of the lithium battery is greatly reduced. At the same time, when the ratio of the two is too large or too small, the cycle performance of the lithium battery decreases significantly; this is because, when the ratio of the two is within an appropriate range, the heat released during the charge and discharge process is more uniform, and the lithium iron manganese phosphate material and the ternary material can achieve excellent synergy; it shows that only when the ratio of the two materials is within an appropriate range can the battery cell achieve a balance in safety performance, rate performance and cycle performance.

Claims

1. A battery cell, comprising a positive electrode sheet, a separator and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer located on both surfaces of the positive electrode current collector, wherein the positive electrode active material layer comprises a first active material layer and a second active material layer stacked, wherein the first active material layer is in direct contact with the surface of the positive electrode current collector; in, The first active material layer includes a ternary active material, and the specific surface area of ​​the ternary active material is 0.3-0.9 m2 / g; the second active material layer includes a manganese iron phosphate material, and the specific surface area of ​​the manganese iron lithium phosphate material is 20-24 m2 / g.

2. A battery cell according to claim 1, wherein: The chemical formula of the lithium manganese iron phosphate material is LiFe 1-x-z Mn x D z PO4, 0.2≤x≤0.8, z>0; wherein, D comprises at least one of Mg, Ti, V, Ni, Co, Al, Nb, Y, Mo, Sr, La, Zr, and B; and Fe 1-x-z Mn x D z Denoted as M, the molar ratio of M / P is 0.90-1.15, the molar ratio of Li / M is 1.01-1.10, and the molar ratio of Li / P is 0.95-1.

10.

3. A battery cell according to claim 1, wherein: The surface density of the first active material layer is 40-100 g / ㎡.

4. A battery cell according to claim 1, wherein: The surface density of the second active material layer is 100-360 g / ㎡.

5. A battery cell according to claim 1, wherein: The compaction density of the first active material layer is 3.0-3.6 g / m³.

6. A battery cell according to claim 1, wherein: The compaction density of the second active material layer is 2.0-2.6 g / m³.

7. A battery cell according to claim 1, wherein: The surface of the negative electrode sheet is coated with a negative electrode active material layer, and the specific surface area of ​​the negative electrode active material constituting the negative electrode active material layer is 1-2 m2 / g.

8. A method for preparing a battery cell according to any one of claims 1 to 7, comprising the following steps: Step 1: slurrying the lithium iron manganese phosphate material and the ternary active material separately to obtain lithium iron manganese phosphate slurry and ternary active slurry respectively; Step 2: coating the ternary active slurry and the lithium manganese iron phosphate slurry on both surfaces of the positive electrode current collector along the thickness direction of the positive electrode current collector; wherein, The ternary active slurry is coated on the surface of the positive electrode current collector, and the lithium manganese iron phosphate slurry is coated on the surface of the ternary active slurry, and pressed to obtain a positive electrode sheet with positive electrode active material layers coated on both sides; Step three: Assemble the positive electrode sheet, the separator, and the negative electrode sheet to obtain a finished battery cell.

9. A battery cell according to claim 8, wherein: In the step 2, the ratio of the mass of the lithium iron manganese phosphate slurry to the mass of the ternary active slurry is 1-9:

1.

10. A lithium-ion battery, comprising a battery cell, wherein the battery cell is the battery cell according to any one of claims 1 to 8.

11. A battery cell comprising a positive electrode sheet, a separator and a negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode collector and a lithium manganese iron phosphate coating and a ternary material coating alternately coated along the width direction of the positive electrode collector.

12. A battery cell according to claim 11, wherein: The negative electrode sheet includes a negative electrode current collector and a pure graphite coating and a silicon-doped graphite coating alternately arranged along a width direction of the negative electrode current collector.

13. A battery cell according to claim 12, wherein: The vertical projection of the lithium manganese iron phosphate coating on the diaphragm overlaps with the vertical projection of the pure graphite coating on the diaphragm, and the vertical projection of the ternary material coating on the diaphragm overlaps with the vertical projection of the silicon-doped graphite coating on the diaphragm.

14. A battery cell according to claim 11, wherein: The thickness of the lithium iron manganese phosphate coating and the thickness of the ternary material coating both meet the range of 35-135 μm.

15. A battery cell according to any one of claims 11 or 14, wherein: The thickness of the lithium iron manganese phosphate coating is the same as the thickness of the ternary material coating.

16. A battery cell according to claim 11, wherein: The ratio of the width of the lithium iron manganese phosphate coating to the width of the ternary material coating is (1-8):

1.

17. A battery cell according to claim 11, wherein: The mass ratio of the lithium manganese iron phosphate slurry used in the lithium manganese iron phosphate coating to the mass ratio of the ternary slurry used in the ternary material coating is (1-9):

1.

18. A battery cell according to claim 17, wherein: The lithium iron manganese phosphate slurry includes a lithium iron manganese phosphate material, and the chemical formula of the lithium iron manganese phosphate material is LiFe 1-x-z Mn x D z PO4, M is Fe 1-x-z Mn x D z , 0.2<x<0.8, z>0; wherein the molar ratio of Li / P is 0.95-1.10, the molar ratio of M / P is 0.90-1.15, and the molar ratio of Li / M is 1.01-1.10; wherein D includes at least one of Mg, Ti, V, Ni, Co, Al, Nb, Y, Mo, Sr, La, Zr, B, etc.

19. A method for preparing a battery cell according to any one of claims 11 to 18, comprising the following steps: Step 1: slurrying the lithium iron manganese phosphate material and the ternary material separately to obtain lithium iron manganese phosphate slurry and ternary slurry respectively; Step 2: coating lithium iron manganese phosphate slurry and ternary slurry on the surface of the positive electrode current collector along the length direction of the positive electrode current collector, the lithium iron manganese phosphate slurry and the ternary slurry are arranged at intervals along the width direction of the positive electrode current collector, and pressing to obtain a positive electrode sheet with a surface coated with lithium iron manganese phosphate coating and ternary material coating; Pure graphite slurry and silicon-doped graphite slurry are coated on the surface of the negative electrode current collector along the length direction of the negative electrode current collector, the pure graphite slurry and the silicon-doped graphite slurry are arranged at intervals along the width direction of the positive electrode current collector, and pressed to obtain a negative electrode sheet with a surface coated with a pure graphite coating and a silicon-doped graphite coating; Controlling the lithium manganese iron phosphate slurry coating area on the positive electrode current collector to correspond to the pure graphite slurry coating area on the negative electrode current collector, and controlling the ternary slurry coating area on the positive electrode current collector to correspond to the silicon-doped graphite slurry coating area on the negative electrode current collector; Step three: Assemble the positive electrode sheet, the separator, and the negative electrode sheet to obtain a finished battery cell.

20. A lithium ion battery, comprising a battery cell, wherein the battery cell is the battery cell according to any one of claims 11 to 18.

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