Composite positive electrode, preparation method of composite positive electrode, lithium battery and preparation method of lithium battery

Abstract

The invention provides a composite positive electrode and a preparation method thereof, and a lithium battery and a preparation method thereof, and relates to the technical field of lithium ion battery materials. Wherein the composite positive electrode comprises an inorganic oxide solid electrolyte and a solid electrolyte coating of a lithium supplement agent, and meanwhile, the high-temperature safety, the rate capability and the cycling stability of the lithium battery are improved; compared with a conventional electrode coating, the solid electrolyte coating has the advantages that internal short circuit caused by contact of a positive plate and a negative plate of the lithium battery at high temperature is avoided, negative effects of the coating on impedance and capacity of the battery are avoided, influence of a lithium supplement agent on electronic conductance of the plate is reduced, and defluorination of a binder in a pulping process is reduced; the solid electrolyte coating provided by the invention can provide lower impedance and longer cycle life for the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion battery materials, and in particular to a composite positive electrode and a preparation method thereof, a lithium battery and a preparation method thereof. Background Art

[0002] In recent years, the energy and energy density of energy storage and power lithium batteries in the market have increased year by year for the sake of reducing costs and improving product competitiveness. With the increase in the energy storage of battery cells, thermal safety has become an important hidden danger restricting its application. Among them, the diaphragm, as a polymer material, is the safety shortcoming of the battery cell. Once the diaphragm melts due to thermal runaway, the chemical energy stored in the positive and negative electrode materials in direct contact will be quickly converted into thermal energy, causing the battery temperature to rise sharply, causing comprehensive thermal runaway of the battery cell itself and the module.

[0003] In order to suppress short circuits in lithium batteries above the melting temperature of the separator (PE: 130°C, PP: 170°C), an effective means is to introduce a protective coating between the positive and negative electrodes. The coating includes a high-temperature resistant inorganic oxide solid electrolyte, which has the effect of blocking the positive and negative electrodes. Its thermal stability is better than that of traditional polymer-based separators.

[0004] CN117810516A discloses a double-layer protective layer for a lithium metal negative electrode, wherein the double layer of the protective layer is composed of inorganic oxide particles and an organic polymer binder, wherein the "flexible layer" close to the lithium metal negative electrode side has a high proportion of organic polymer, and the "rigid layer" away from the lithium metal negative electrode side has a high proportion of inorganic oxide particles, and the two together play a role in inhibiting the growth of lithium dendrites. The prepared "diaphragm-free" battery can pass 1mm needle puncture and 150°C hot box test.

[0005] CN108832174A discloses a coating including inorganic ceramic electrolyte powder, binder and lithium salt, which can be applied on the surface of positive electrode and negative electrode respectively. After drying, the composite electrode is thermally composited to prepare an all-solid-state battery, thereby improving the safety and ionic conductivity of the battery.

[0006] However, in order to ensure the safety of the coating on the pole piece, the coating should be mainly composed of high-density inorganic oxide materials to ensure its own thermal stability; the coating should have sufficient thickness to ensure that it can evenly cover conventional pole pieces with a roughness of micron level; the construction of such inorganic ceramic coatings will introduce additional mass and thickness to the battery, resulting in a loss of energy density of the battery cell. Summary of the invention

[0007] In view of the above technical problems, the purpose of the present invention is to provide a composite positive electrode and a preparation method thereof, a lithium battery and a preparation method thereof. The positive electrode material of the composite positive electrode is completely coated with a solid electrolyte coating on the side away from the current collector. On the one hand, the coating can isolate the contact between the positive and negative electrodes and avoid thermal runaway. On the other hand, compared with conventional coatings, the coating of the present invention contains a lithium supplement, which can improve the energy density and cycle life of the battery.

[0008] In a first aspect, the present application provides a composite positive electrode:

[0009] A composite positive electrode, comprising a current collector, a positive electrode layer containing an active material and a solid electrolyte coating, wherein the solid electrolyte coating comprises an inorganic oxide, a lithium supplement and a polymer having an adhesive effect in a mass ratio of (5-15): (1-3): 1, wherein the lithium supplement is lithium oxalate Li 2 C 2 O 4 、Lithium oxide Li 2 O, Lithium iron oxide Li 5 FeO 4 , Lithium-rich nickel oxide LiNiO 2 One or more of the .

[0010] The solid electrolyte coating on the positive electrode surface includes three components: a solid electrolyte of an inorganic oxide material, a polymer, and a lithium supplement. In the manufacture of the battery, the components of the coating are first evenly dispersed into a slurry, and after the positive electrode slurry is applied to the current collector, dried and rolled, it is then sprayed on the surface of the positive electrode layer, and the solid electrolyte coating is manufactured after drying.

[0011] The positive electrode material of the composite positive electrode is completely coated by a solid electrolyte coating on the side away from the current collector. On the one hand, the coating can isolate the positive and negative electrodes from contact and avoid thermal runaway. On the other hand, compared with conventional coatings, the coating of the present invention contains a lithium supplement, which can improve the energy density and cycle life of the battery. The present application improves the high temperature safety, rate performance and cycle stability of the lithium battery by designing a positive electrode coating including an inorganic oxide solid electrolyte and a lithium supplement.

[0012] Furthermore, the mass ratio of the inorganic oxide, the lithium supplement agent and the polymer is (9-12):(1-2):1.

[0013] Furthermore, the mass ratio of the inorganic oxide, the lithium supplement agent and the polymer is 12:2:1.

[0014] Furthermore, the surface density of the solid electrolyte coating is 5-20 g / m 2 , thickness is 5-25 μm.

[0015] 5. The composite positive electrode according to claim 4, characterized in that the surface density of the solid electrolyte coating is 5-20 g / m 2 , thickness is 5-25 μm, and porosity is 37.89-53.77%.

[0016] Furthermore, the surface density of the solid electrolyte coating is 11.42-17.18 g / m 2 The thickness is 11.66-17.33 μm and the porosity is 44.51-53.77%.

[0017] Furthermore, the current collector includes carbon-coated aluminum foil and PET aluminum foil; the positive electrode slurry includes a positive electrode main material, a conductive agent, a binder and a dispersant.

[0018] The inorganic oxide includes one or more of LATP, LAGP, LLZO, and LLTO; the polymer includes one or more of PVDF, PMMA, PAN, PI, and Nafion.

[0019] In a second aspect, the present application provides a process for preparing a composite positive electrode:

[0020] The solid electrolyte coating of the composite positive electrode is prepared by spraying the solid electrolyte coating slurry on the surface of the positive electrode material, and the specific process is as follows:

[0021] S1. 5-15 parts by mass of an inorganic oxide solid electrolyte, 1 part by mass of a polymer, and 1-3 parts by mass of a lithium supplement are dispersed in N-methylpyrrolidone to prepare a solid electrolyte coating slurry, wherein the mass ratio of N-methylpyrrolidone to the polymer is maintained at 93: (3-10);

[0022] S2. Using spraying technology, the evenly dispersed slurry is sprayed on the surface of the rolled positive electrode layer. After secondary baking and secondary rolling, it is cut and punched to obtain a composite positive electrode.

[0023] In a third aspect, the present application provides a method for preparing a lithium battery:

[0024] A method for preparing a lithium battery includes the method for preparing the composite positive electrode of the present application or uses the composite positive electrode of the present application.

[0025] In a fourth aspect, the present application provides a lithium battery:

[0026] A lithium battery comprises a composite positive electrode obtained by the composite positive electrode preparation method described in the present application or comprises the composite positive electrode described in the present application.

[0027] By constructing a coating with a high-temperature resistant oxide solid electrolyte as the main phase on the surface of the electrode, the positive and negative electrodes can still be blocked by the coating above the melting temperature of the diaphragm, thus avoiding the occurrence of high-temperature internal short circuit; during normal use of the battery, the lithium replenisher in the coating can replenish lithium and make up for the increase in internal resistance and capacity loss of the battery cell caused by the coating; at the same time, by setting the lithium replenisher in the solid electrolyte coating instead of mixing it with the positive electrode material, the negative impact of the lithium replenisher on the electrode impedance due to low electronic conductivity can be avoided; the mixing of the lithium replenisher and the solid electrolyte is beneficial to the ion transition on the surface of the lithium replenisher, improving the lithium replenishment effect, and through the synergistic effect of the solid electrolyte and the lithium replenisher, the final effect of high safety and high specific energy is achieved.

[0028] Beneficial effects: 1. The present invention improves the high temperature safety, rate performance and cycle stability of lithium batteries by designing a solid electrolyte coating including an inorganic oxide solid electrolyte and a lithium supplement;

[0029] Compared with conventional electrode coatings, this solid electrolyte coating avoids the internal short circuit caused by the contact between the positive and negative electrodes of the lithium battery at high temperature, while avoiding the negative effects of the coating on the battery impedance and capacity, reducing the impact of lithium supplements on the electronic conductivity of the electrodes and reducing the defluorination of the binder in the pulping process; the solid electrolyte coating of the present application can give the battery lower impedance and higher cycle life.

[0030] 2. By optimizing the dosage of inorganic oxides, lithium supplement agents and polymers in the solid electrolyte coating of the composite positive electrode, the FSDC, ICE and Retention performance of the battery using the composite positive electrode are further improved, the DCR is reduced, and the comprehensive performance of the lithium battery is improved.

[0031] 3. By optimizing the surface density, thickness and porosity of the solid electrolyte coating of the composite positive electrode, the FSDC, ICE and Retention performance of the battery using the composite positive electrode are further improved, the DCR is reduced, and the overall performance of the lithium battery is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the structure of the composite positive electrode of the present invention / Example 1. Description of the accompanying drawings

[0033] 101. current collector; 102. positive electrode layer; 103. solid electrolyte coating. DETAILED DESCRIPTION

[0034] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] Embodiment 1: A soft-pack lithium battery, comprising the following preparation steps:

[0036] S1. Preparation of negative electrode sheet:

[0037] The negative electrode active material (graphite + silicon oxide, mass ratio 93.5:6.5), conductive agent (carbon black), binder (styrene-butadiene rubber + sodium carboxymethyl cellulose, mass ratio 1:1) are dispersed in water at a mass ratio of 97:1:2, and a uniformly dispersed negative electrode slurry is prepared according to the pulping process. The negative electrode slurry is coated on copper foil, baked, rolled, cut, and punched to obtain a negative electrode sheet.

[0038] S2. Preparation of composite positive electrode:

[0039] The composite positive electrode comprises a current collector 101 , a positive electrode layer 102 containing active materials and a solid electrolyte coating 103 in sequence.

[0040] The specific preparation process is as follows:

[0041] The positive electrode active material (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ), conductive agent (carbon black), binder (polyvinylidene fluoride), dispersed in an appropriate amount of N-methylpyrrolidone at a mass ratio of 96:2:2, and a uniformly dispersed positive electrode slurry was prepared according to the slurry making process. The positive electrode slurry was coated on the composite current collector foil, baked, rolled, and set aside.

[0042] Prepare PVDF N-methylpyrrolidone glue, the mass ratio of PVDF to N-methylpyrrolidone is 7:93, then add inorganic oxide LATP and lithium-rich ferrite (lithium supplement) to the polymer PVDF glue to prepare a uniformly dispersed solid electrolyte coating slurry, wherein the mass ratio of LATP (inorganic oxide), lithium-rich ferrite (lithium supplement) and PVDF (polymer) is 5:1:1;

[0043] The solid electrolyte coating slurry is sprayed onto the surface of the positive electrode sheet after rolling, and then baked and rolled for a second time, and then cut and punched to obtain a composite positive electrode.

[0044] S3 dry cell assembly:

[0045] The negative electrode sheet and composite positive electrode prepared above were stacked with a boehmite|PE|boehmite (3+9+3 μm) separator or without any separator to prepare a soft-pack battery with a theoretical capacity of 6.3 Ah.

[0046] S4 battery cells are made of:

[0047] The moisture content in the glove box is controlled to be no more than 10 ppm, ethylene carbonate, propylene carbonate, ethyl methyl carbonate and dimethyl carbonate are stirred and mixed in a mass ratio of 30 / 5 / 45 / 20, and purified and dehydrated with a molecular sieve to obtain a solvent component;

[0048] In a glove box, a solvent, fluoroethylene carbonate, and lithium hexafluorophosphate were mixed in a mass ratio of 82.5 / 5 / 12.5, and stirred until clear to obtain an electrolyte;

[0049] In a protective atmosphere with a moisture and oxygen content of no more than 10 ppm, the electrolyte is injected into the dry battery cell, and the soft-pack lithium battery is obtained after heat sealing.

[0050] Embodiment 2, a soft-pack lithium battery:

[0051] Compared with Example 1, the difference in Example 2 is that the mass ratio of LATP, lithium-rich lithium ferrite, and PVDF is 9:1:1.

[0052] Embodiment 3, a soft-pack lithium battery:

[0053] Compared with Example 1, the difference in Example 3 is that the mass ratio of LATP, lithium-rich lithium ferrite, and PVDF is 12:1:1.

[0054] Embodiment 4, a soft-pack lithium battery:

[0055] Compared with Example 1, the difference of Example 4 is that the mass ratio of LATP, lithium-rich lithium ferrite, and PVDF is 15:1:1.

[0056] Embodiment 5, a soft-pack lithium battery:

[0057] Compared with Example 1, the difference of Example 5 is that the mass ratio of LATP, lithium-rich lithium ferrite, and PVDF is 12:2:1.

[0058] Embodiment 6, a soft-pack lithium battery:

[0059] Compared with Example 1, the difference in Example 6 is that the mass ratio of LATP, lithium-rich lithium ferrite, and PVDF is 12:3:1.

[0060] Comparative Example 1, a soft-pack lithium battery:

[0061] Compared with Example 5, the difference in Comparative Example 1 is that the positive electrode sheet structure does not include a solid electrolyte coating.

[0062] Comparative Example 2, a soft-pack lithium battery:

[0063] Compared with Example 5, the difference in Comparative Example 2 is that the mass ratio of LATP, lithium-rich lithium ferrite, and PVDF is 12:0:1.

[0064] Comparative Example 3, a soft-pack lithium battery:

[0065] Compared with Comparative Example 2, the difference of Comparative Example 3 is that lithium-rich lithium iron oxide is used in the positive electrode slurry, and lithium-rich lithium iron oxide is not used in the solid electrolyte coating.

[0066] Specific composite positive electrode sheet preparation process: positive electrode active material (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ), lithium iron-rich lithium ferrate, conductive agent (carbon black), binder (polyvinylidene fluoride), dispersed in an appropriate amount of N-methylpyrrolidone at a mass ratio of 93:3:2:2, and prepared a uniformly dispersed positive electrode slurry according to the slurry making process. The positive electrode slurry is coated on the composite current collector foil, baked, and rolled;

[0067] Prepare PVDF N-methylpyrrolidone glue, the mass ratio of PVDF to N-methylpyrrolidone is 7:93, then add LATP to the PVDF glue to prepare a uniformly dispersed solid electrolyte coating slurry, wherein the mass ratio of LATP to PVDF is 12:1;

[0068] The solid electrolyte coating slurry is sprayed onto the surface of the positive electrode sheet after rolling, and then baked and rolled for a second time, and then cut and punched to obtain a composite positive electrode.

[0069] Comparative Example 4, a soft-pack lithium battery:

[0070] Compared with Example 5, the difference of Comparative Example 4 is that LiTFSI is used to replace the lithium-rich lithium ferrite in equal amounts;

[0071] The specific preparation process of the solid electrolyte coating slurry is as follows: prepare PVDF N-methylpyrrolidone glue solution, the mass ratio of PVDF and N-methylpyrrolidone is 7:93, and then add LATP and LiTFSI to the PVDF glue solution to prepare a uniformly dispersed solid electrolyte coating slurry, wherein the mass ratio of LATP, LiTFSI and PVDF is 12:2:1.

[0072] The electrochemical performance and safety performance of the composite positive electrode of the present invention are illustrated below by using data from examples and comparative examples.

[0073] 1. Physical properties of solid electrolytes

[0074] The physical properties of the solid electrolyte coating on the composite cathode surface are characterized by the following methods:

[0075] Surface density: For each embodiment and comparative example, 5 pole pieces before / after spraying were weighed and the average value M0 / Mn (g) was taken, where the average value of the comparative example 1 group was M0 (g), and S was the pole piece area (m2).

[0076] Single lateral density ρ1 (g / m2) = (Mn-M0) / (S*2).

[0077] Porosity of solid electrolyte coating: The thickness of 5 positive electrode sheets of each embodiment and comparative example before and after spraying, baking and rolling the solid electrolyte coating was measured using a screw micrometer. The thickness difference was the coating thickness L1 (μm).

[0078] For an embodiment or comparative example in which the mass ratio of the solid electrolyte, the polymer, and the lithium supplement in the slurry formula is M solid electrolyte: M polymer: M lithium supplement, and the volume densities are ρ solid electrolyte, ρ polymer, ρ lithium supplement (g / cm3) respectively:

[0079] Measured density ρ2 (g / cm3) = ρ1 / L1;

[0080] Compacted density ρ3 (g / cm3) = (M solid electrolyte + M polymer + M lithium supplement) / (M solid electrolyte / ρ solid electrolyte + M polymer / ρ polymer + M lithium supplement / ρ lithium supplement);

[0081] Porosity P = (1-ρ2 / ρ3) * 100%;

[0082] The relevant test results are shown in Table 1.

[0083] Table 1. Summary of physical properties of solid electrolyte coatings in various examples and comparative examples

[0084] Experimental Group Solid electrolyte coating corresponds to the positive electrode <![CDATA[Areal density (g / m 2 )]]> Thickness(μm) <![CDATA[Measured density (g / cm 3 )]]> Porosity(%) A01 Example 1 6.76 6.66 1.01 40.17 A02 Example 2 11.42 12.33 0.92 53.77 A03 Example 3 11.96 11.66 1.02 52.28 A04 Example 4 17.56 15.5 1.13 49.79 A05 Example 5 17.18 17.33 0.99 44.51 A06 Example 6 19.54 20.16 0.96 37.89 A07 Comparative Example 1 / / / / A08 Comparative Example 2 17.36 12.83 1.35 51.6 A09 Comparative Example 3 / / / / A10 Comparative Example 4 13.36 12.65 1.05 40.9

[0085] Porosity mainly affects the proportion of electrolyte that the coating can absorb in the final battery. The higher the porosity, the lower the internal resistance of the battery. Within a reasonable range, the higher the better. Surface density mainly involves the additional mass introduced by the coating, which affects the final energy density of the battery. The lower the better while maintaining the uniformity of the coating. The greater the thickness, the greater the additional internal resistance of the battery introduced by the coating. The lower the better while maintaining the uniformity of the coating.

[0086] From A01 to A04, it can be seen that when the ratio of lithium supplement and polymer is kept unchanged, the surface density and thickness of the coating show an upward trend with the increase of solid electrolyte content; the pores first increase significantly and then tend to stabilize at around 50%. This is caused by the reduction of the polymer ratio and the inability to fill the pores between the rigid particles.

[0087] By observing A08, A05, and A06, it can be seen that when the ratio of solid electrolyte to polymer remains unchanged, increasing the ratio of lithium supplement will lead to a decrease in porosity and a significant increase in spray thickness. This is the result of the high alkalinity lithium supplement causing the viscosity of the coating slurry to increase, and the mixing of particles of different particle sizes leading to an increase in particle stacking density. However, the appropriate introduction of low-density lithium-rich ferrite can also reduce the overall density of the coating. Comparing A05 and A10, it can be seen that when the ratio of solid electrolyte to ceramic remains unchanged, replacing lithium manganese iron phosphate with lithium salt will lead to a decrease in coating thickness and an increase in density. This is because the lithium salt has a plasticizing effect on the polymer, significantly reducing the crystallinity and mechanical strength of the polymer, thereby changing the overall physical properties of the coating.

[0088] Comparing A05 and A10, the role of the lithium replenisher is to make up for the positive electrode active lithium lost in the SEI film at the electrolyte | electrode interface during the first charge and discharge process, and the role of the lithium salt is to improve the ionic conductivity of the coating itself; the addition of lithium replenisher can better improve the cycle stability of the battery and reduce the internal resistance; lithium salt can reduce the internal resistance of the battery, but lithium salt is hygroscopic. Including lithium salt in the coating slurry will inevitably introduce decomposition impurities or moisture, resulting in poor battery cycle effect.

[0089] 2. Data of lithium battery at 25℃

[0090] The lithium battery including the positive electrode sheet of the embodiment and comparative example of this patent is manufactured and data is collected by the following steps:

[0091] At 25°C, the battery was charged to 4.25V at 0.1C constant current and 0.05C at 4.25V constant voltage, and the first charge capacity at 0.1C was QFCC (Ah).

[0092] Discharge at 0.1C constant current to 2.8V, and obtain 0.1C constant capacity QFDC (Ah);

[0093] Then it was charged to 4.25V at a constant current of 0.3C, charged to 0.05V at a constant voltage of 4.25V, and then discharged to 2.8V at a constant current of 0.3C to obtain the 0.3C constant capacity Q2nd grading (Ah).

[0094] Then the battery was charged at a constant current of 0.3C for 1h to 30% SOC. After standing for half an hour, the battery steady-state voltage U0 (V) was recorded. Then, the battery was discharged at a rate of 2.5C for 10s to obtain the battery voltage U1 (V0).

[0095] Subsequently, the battery was charged to 4.25 V at a constant current of 0.3 C, charged to 0.05 V at a constant voltage of 4.25 V, and then discharged to 2.8 V at a constant current of 0.3 C. After 100 cycles, the capacity Q100 cycles (Ah) was obtained.

[0096] M positive electrode active material mass = (M positive electrode sheet - M solid electrolyte coating - M current collector) * A active material mass ratio in positive electrode slurry;

[0097] First discharge capacity FSDC (mAh / g) = 1000*QFDC / M positive electrode active material mass;

[0098] Coulomb efficiency ICE (%) = QFDC / QFCC*100%;

[0099] Battery offline DCR (mΩ) = (U1-U0) / 2.5 / 6.3*1000;

[0100] Battery 100 cycle capacity retention rate Retention (%) = (Q100 cycles / Q2nd grading)*100%;

[0101] The relevant test results are shown in Table 2.

[0102] Table 2. Summary of relevant data of battery manufacturing corresponding to positive electrodes of various embodiments and comparative examples

[0103] Experimental Group positive electrode Diaphragm FSDC(mAh / g) ICE(%) DCR(mΩ) Retention(%) A01 Example 1 have 185.6 82 9.3 80.6 A02 Example 2 have 192.8 82.1 9.2 87 A03 Example 3 have 192.9 82.3 8.7 90.1 A04 Example 4 have 191.7 82.4 11.3 89.2 A05 Example 5 have 193.3 81.7 8.2 95.9 A06 Example 6 have 184.9 80.6 9.4 69.5 A07 Example 5 none 194.5 81.6 7.6 98.6 A08 Comparative Example 1 have 192 82.7 11.2 91.3 A09 Comparative Example 2 have 188 83.4 13.9 87 A10 Comparative Example 3 have 187.5 82.4 10.1 92.3 A11 Comparative Example 4 have 189.4 82.53 9.7 84.5

[0104] By observing A08 and A09, we can see that in the absence of a lithium supplement, LATP can also play a small role in replenishing lithium, resulting in a slight improvement in the initial efficiency of the battery, but the solid electrolyte coating significantly increases the internal resistance of the battery, reducing the battery's capacity and cycle stability.

[0105] By observing A08 and A01 to A04, it can be seen that when the mass ratio of lithium supplement agent to polymer is 1:1, the battery capacity in grams is basically unchanged, and the first effect is slightly reduced due to the increase in the first charge capacity in grams. However, except for Example 4, the impedance of the other composite positive electrode groups is reduced compared with Comparative Example 1. Among them, Example 3 has the lowest battery internal resistance and the longest cycle life, which is the result of the best comprehensive performance of coating component ratio, coating thickness and porosity.

[0106] By observing A03, A05, and A06, it can be seen that when the mass ratio of solid electrolyte to polymer is 12:1, increasing the ratio of lithium supplement to polymer to 2:1 can improve the capacity and cycle stability of the battery. However, when it is increased to 3:1, due to the excessive thickness of the coating, the porosity is too low, and the high alkalinity of the lithium supplement causes partial defluorination and gelation of the polymer, which affects the uniformity of the coating and ultimately reduces the cycle stability of the battery cell.

[0107] Observing A05 and A07, in the presence of solid electrolyte coating, removing the separator component in the battery can further reduce the internal resistance and improve the cycle stability of the battery.

[0108] By observing A01, A08, A09, and A10, it can be seen that the conventional strategy of directly adding lithium supplements to the positive electrode slurry can also improve the battery cycle stability and reduce the battery internal resistance, but the effect is not as good as adding lithium supplements to the solid electrolyte coating, which shows that the mixing method with the solid electrolyte improves the interfacial ion transport performance of the lithium supplement and avoids the low electronic conductivity of the lithium supplement affecting the charge transfer inside the electrode.

[0109] By observing A05, A09, and A11, we can see that replacing the lithium supplement component in the coating with lithium salt LiTFSI can also reduce the internal resistance of the battery during the formation stage. This is because the presence of lithium salt improves the ionic conductivity of the coating and reduces the thickness of the coating. However, due to the strong hygroscopicity of LiTFSI, it is inevitable that moisture will rise during the preparation of the composite pole piece and the dry cell, which ultimately affects the cycle stability of the battery.

[0110] 3. Hot box safety test

[0111] The following steps are used to perform the hot box safety test in the embodiments and comparative examples of this patent:

[0112] The formed battery is charged to 4.25V at 0.3C constant current and 0.05C constant voltage, and then hot box test is performed. The test steps are as follows:

[0113] 1. Store the battery at 60°C for 150 minutes;

[0114] 2. Raise the temperature to 130℃ at 5℃ / min and keep warm for 30min;

[0115] 3. Raise the temperature to 150℃ at 2℃ / min and keep warm for 30min;

[0116] 4. Raise the temperature to 170°C at 2°C / min and keep warm for 30 minutes;

[0117] 5. Raise the temperature to 190℃ at 2℃ / min and keep warm for 30min;

[0118] 6. Raise the temperature to 200°C at 2°C / min and keep warm for 30 minutes;

[0119] 7. Stop heating and store for 60 minutes or until the battery temperature drops below 50°C.

[0120] The characteristic temperature of the hot box safety test is defined as follows:

[0121] T1 (℃): starting temperature of battery cell self-heating;

[0122] T2 (℃): The starting temperature of thermal runaway of the battery cell, corresponding to the temperature at which the self-heating rate of the battery cell reaches 1℃ / s;

[0123] T3 (℃): The highest temperature of the battery cell after thermal runaway.

[0124] The relevant test results are shown in Table 3.

[0125] Table 3. Hot box safety test results

[0126] Experimental Group positive electrode Diaphragm Is there thermal runaway? T1 (℃) T2 (℃) T3 (℃) A01 Example 5 have no 68.9 186.7 / A02 Example 5 none no 59.5 185.4 / A03 Comparative Example 1 have yes 70.1 168.7 433.3 A04 Comparative Example 3 have no 66.1 186.4 /

[0127] Observing A01, A02, A03, and A04, it can be seen that regardless of whether there is a solid electrolyte coating, the starting temperature T1 of the battery cell self-heating is in the range of 60-75. This is caused by the thermal decomposition of lithium salts in the electrolyte, the generation of strong acidic substances and the reaction of the lithium storage negative electrode. Since the solid electrolyte coating is located on the positive electrode side, it cannot inhibit the side reaction of the negative electrode and the electrolyte, and the starting temperature of self-heating of each group is relatively consistent.

[0128] Without coating protection, the A03 cell experienced thermal runaway at 168.7°C, which was caused by the melting of the ceramic diaphragm, resulting in a short circuit in the cell. Although the boehmite on both sides of the diaphragm can remain thermally stable below 200°C, the shrinkage of the PE-based film destroys the integrity of the diaphragm's coverage of the electrode sheet, and the battery eventually heats up rapidly and catches fire, with the highest temperature reaching 433.3°C; and observing the A01, A02, and A04 cells, it can be seen that when there is a solid electrolyte coating on the surface of the positive electrode sheet, even if the characteristic temperature of the diaphragm ruptures, the coating can still ensure the electronic insulation of the positive and negative electrode sheets, thereby improving the safety of the battery; on the other hand, at around 180°C, the high-nickel positive electrode begins to decompose and release oxygen, and some of the gas reaches the negative electrode side from the pores of the coating, and reacts mildly with the negative electrode, causing the cell self-heating rate to exceed 1°C / s, but it fails to cause a comprehensive thermal runaway. The above experimental conclusions confirm that the solid electrolyte coating improves battery safety.

[0129] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A composite positive electrode, characterized in that: The invention comprises a current collector, a positive electrode layer containing an active material and a solid electrolyte coating in sequence. The solid electrolyte coating comprises an inorganic oxide, a lithium supplement and a polymer with an adhesive effect in a mass ratio of (5-15):(1-3):

1. The lithium supplement is one or more of lithium oxalate Li2C2O4, lithium oxide Li2O, lithium iron-rich lithium ferrate Li5FeO4 and lithium nickel-rich lithium nickelate LiNiO2.

2. The composite positive electrode according to claim 1, characterized in that The mass ratio of the inorganic oxide, the lithium supplement agent and the polymer is (9-12): (1-2):

1.

3. The composite positive electrode according to claim 2, characterized in that: The mass ratio of the inorganic oxide, the lithium supplement agent and the polymer is 12:2:

1.

4. The composite positive electrode according to claim 1, characterized in that: The surface density of the solid electrolyte coating is 5-20 g / m 2 , thickness is 5-25 μm.

5. The composite positive electrode according to claim 4, characterized in that: The surface density of the solid electrolyte coating is 5-20 g / m 2 , thickness is 5-25 μm, and porosity is 37.89-53.77%.

6. The composite positive electrode according to claim 5, characterized in that: The surface density of the solid electrolyte coating is 11.42-17.18 g / m 2 The thickness is 11.66-17.33 μm and the porosity is 44.51-53.77%.

7. The composite positive electrode according to claim 1, characterized in that: The current collector includes carbon-coated aluminum foil and PET aluminum foil; the positive electrode slurry includes a positive electrode main material, a conductive agent, a binder and a dispersant. The inorganic oxide includes one or more of LATP, LAGP, LLZO and LLTO; the polymer includes one or more of PVDF, PMMA, PAN, PI and Nafion.

8. The process for preparing the composite positive electrode according to any one of claims 1 to 7, characterized in that: The solid electrolyte coating of the composite positive electrode is prepared by spraying the solid electrolyte coating slurry on the surface of the positive electrode material, and the specific process is as follows: S1. 5-15 parts by mass of an inorganic oxide solid electrolyte, 1 part by mass of a polymer, and 1-3 parts by mass of a lithium supplement are dispersed in N-methylpyrrolidone to prepare a solid electrolyte coating slurry, wherein the mass ratio of N-methylpyrrolidone to the polymer is maintained at 93: (3-10); S2. Using spraying technology, the evenly dispersed slurry is sprayed on the surface of the rolled positive electrode layer. After secondary baking and secondary rolling, it is cut and punched to obtain a composite positive electrode.

9. A method for preparing a lithium battery, characterized in that: The invention comprises the preparation method as claimed in claim 8 or adopts the composite positive electrode as claimed in any one of claims 1 to 7.

10. A lithium battery, characterized in that: It comprises a composite positive electrode obtained by the preparation method of claim 8 or a composite positive electrode according to any one of claims 1 to 7.

Images