A lithium / silicon / carbon composite anode and a lithium-ion battery comprising the same

Abstract

The present invention relates to a lithium / silicon / carbon composite negative electrode and a lithium-ion battery comprising the same. The lithium / silicon / carbon composite negative electrode is formed by in-situ polymerization of an ultra-thin lithium with lithium hexafluorophosphate generated in-situ on the surface and a substrate infiltrated with an oxygen heterocyclic monomer solution. The raw materials of the substrate include a graphite / silicon monoxide composite material, a conductive additive, and a binder. Compared with conventional silicon / carbon negative electrodes, the lithium / silicon / carbon composite electrode, when combined with a lithium-free positive electrode, effectively improves the uniformity of lithium deposition and the structural stability of the negative electrode, and enhances the cycling performance of the battery. By means of in-situ polymerization, the ultra-thin lithium foil and the substrate are combined, and the generated polymer layer has good adhesion and filling properties, making the contact between lithium and the substrate closer, reducing the porosity, and evenly distributing the interfacial electric field. At the same time, it effectively conducts lithium ions, promotes the reaction between metallic lithium and the substrate, accelerates the integration of the negative electrode, improves the lithiumation efficiency of the substrate, and reduces the generation of dead lithium.

Description

Technical Field

[0001] The present invention belongs to the field of electrochemical power sources, and particularly relates to a lithium / silicon / carbon composite negative electrode and a lithium ion battery comprising the same. Background Art

[0002] In recent years, with the rapid development of fields such as advanced communication terminals and electric vehicles, there is an urgent need for rechargeable batteries with high energy density that have high safety and fast charge and discharge capabilities. Among many electrode materials, metallic lithium is regarded as the ultimate choice for negative electrode materials due to its lowest potential (-3.04 V vs. standard hydrogen electrode) and extremely high theoretical specific capacity (3860 mAh g -1 ). Developing metallic lithium batteries is expected to significantly alleviate the energy density anxiety commonly existing in the field of secondary batteries. However, in conventional liquid electrolytes, the application of metallic lithium faces significant obstacles. First, due to the "host-free" characteristic of the electrode reaction, during the repeated deposition / dissolution process on the surface of metallic lithium, large polarization and electric field effects often lead to uneven deposition, which is extremely likely to induce the growth of lithium dendrites and cause safety problems. Second, highly active metallic lithium can undergo spontaneous chemical / electrochemical reactions with the electrolyte and the active substances shuttling from the positive electrode, resulting in rapid consumption of the positive and negative electrode materials and a sharp decline in capacity. In addition, the metallic lithium negative electrode often exhibits a large volume effect during the cycling process, making it difficult to form a stable solid electrolyte interphase (SEI). Once the electrolyte membrane ruptures, fresh lithium metal will continuously be exposed to the electrolyte, being affected by corrosion and pulverization, resulting in a large amount of dead lithium, which affects the performance of the battery and poses a safety hazard.

[0003] Many reports have been made on the research of the electrochemical stability of metallic lithium negative electrodes. For example, introducing additives into the electrolyte can effectively improve the Coulombic efficiency, but it cannot fundamentally solve problems such as side reactions between metallic lithium and electrolyte components during long-term use. Therefore, designing a suitable "host" for lithium to deposit in the form of an alloy or a lithium-containing compound is expected to significantly improve its electrochemical behavior and fundamentally solve the dendrite problem of metallic lithium.

[0004] Currently, commercially available positive electrode materials are basically lithium-containing positive electrodes, such as lithium cobalt oxide (LiCoO 2 ), lithium iron phosphate (LiFePO 4 ), lithium nickel cobalt manganese oxide (LiNi x Co y Mn 1-x-y O 2)Ternary cathode materials, etc. The advantages of lithium-containing cathode materials are that they have a stable structure and can be used in combination with insertion anodes with a stable structure such as graphite. However, at present, the energy density of commercial lithium-ion batteries based on lithium-containing cathodes and insertion anodes has approached the upper limit of the theoretical value and cannot meet the usage requirements for higher battery life. Cathode materials without lithium, such as elemental sulfur (S), elemental selenium (Se), sulfur-selenium compounds (Se x S y ), iron sulfide (Fe x S y ), titanium disulfide (TiS 2 ), molybdenum disulfide (MoS 2 ), manganese dioxide (MnO 2 ), vanadium pentoxide (V 2 O 5 ), etc. Their advantages are low cost and high energy density, and they are a type of cathode with commercial prospects. However, cathodes without lithium must be used in combination with lithium-containing anodes. The most commonly used lithium-containing anode at present is elemental metallic lithium. However, the application of metallic lithium in conventional systems faces major obstacles: due to the host-free property and non-uniformity of metallic lithium deposition, the surface of metallic lithium cannot effectively accommodate lithium ions, and dendrites are easily generated, resulting in pulverization, and ultimately leading to poor cycle stability of the battery. At present, in order to overcome the problem of matching cathode materials without lithium and metallic lithium, the existing technology generally introduces additives into the electrolyte and constructs a protective layer on the surface of metallic lithium through an in-situ electrochemical process to promote electric field homogenization and induce uniform deposition of lithium. However, this strategy cannot solve the key problem of additive consumption. Once the additives are completely consumed, new protective layers cannot be continuously generated in the subsequent process, and the original protective layer will crack and degrade during the electrochemical process of the anode, exposing fresh metallic lithium, further exacerbating dendrite formation and pulverization. Therefore, the above strategy cannot fundamentally solve the dendrite problem of metallic lithium and cannot be used as a long-term means to construct a lithium-free cathode-matched lithium metal battery.

[0005] Currently, there are patents that prepare composite metal lithium anodes by designing three-dimensional current collectors. For example, Patent CN104716330A prepares a three-dimensional porous current collector for loading metallic lithium. However, the deposition product of lithium in such anodes is still metallic lithium, and it cannot completely solve problems such as dendrite growth and volume expansion caused by the host-free effect. Patent CN105374991A prepares a composite metal lithium anode with a composite carbon skeleton as the substrate. However, the carbon skeleton used in the above methods has no electrochemically redox activity itself, and its ability to combine with lithium is weak, and it cannot form a stable lithium-containing compound. Lithium still precipitates in the form of a dendrite-prone elemental state essentially, so it cannot fundamentally change the host-free characteristics of lithium deposition and cannot solve key problems such as lithium dendrites and pulverization. Patent CN104466095A uses a hot pressing process to prepare a composite lithium foil. During the electrode forming process, lithium is extremely easy to react with water and oxygen in the air at high temperature, generating electrochemically inert by-products such as lithium oxide, lithium hydroxide, and lithium carbonate, resulting in the consumption of active lithium, increasing the cost while reducing the performance of the battery.

[0006] In the prior art, there are anodes with a lithium supplementation function through ultra-thin lithium (i.e., pre-lithiated anodes), such as the anodes described in CN114361398, CN114079041A, CN114171712A, CN114171798A, and CN113451544A. Generally, the anode substrate and ultra-thin lithium are compounded together by physical pressure methods such as roll pressing through a binder. The purpose is to supplement lithium to the anode. However, the above methods cannot overcome the problems of uneven contact and poor bonding ability between lithium and the anode substrate: due to the unevenness of the surface of the original anode substrate, there are a large number of voids between the metallic lithium introduced by the roll pressing method and the substrate, and ions / electrons cannot effectively transport between the substrate and metallic lithium through the voids, resulting in uneven local electric fields, making it impossible for effective bonding between metallic lithium and the anode substrate through redox reactions, and a large amount of "dead lithium" and un-lithiated anode substrates appear. After assembling the above composite anode with a lithium-free cathode to form a battery, serious lithium consumption occurs during the cycling process, reducing the capacity and cycle life of the battery; at the same time, the "dead lithium" inside the composite anode and the un-lithiated anode substrate constitute an electrochemically inert layer with electron / ion insulation, greatly increasing the internal resistance of the battery and leading to rapid capacity decay. Summary of the Invention

[0007] To solve the defects of lithium having no host, uneven deposition, and easy dendrite formation in lithium batteries assembled with cathodes without lithium in the prior art, the present invention proposes a lithium / silicon / carbon composite anode. By modifying an initiator on the surface of ultrathin lithium, wetting the substrate with a monomer solution, and compounding the substrate and the ultrathin lithium by in-situ polymerization, the defects of the conventional rolling method are overcome, the deposition / dissolution behavior of lithium ions is improved, and the Coulomb efficiency and stability during the battery cycling process are enhanced.

[0008] The present invention achieves the purpose through the following technical solutions:

[0009] A lithium / silicon / carbon composite anode is formed by in-situ polymerization of ultrathin lithium with lithium hexafluorophosphate generated in-situ on its surface and a substrate infiltrated with an oxygen heterocyclic monomer solution. The raw materials of the substrate include a graphite / silicon monoxide composite material, a conductive additive, and a binder.

[0010] In the present invention, a reaction is carried out on the surface of the ultrathin lithium so that its surface is uniformly modified with lithium hexafluorophosphate capable of initiating the polymerization of the oxygen heterocyclic monomer. By in-situ polymerization, the ultrathin metallic lithium foil entirely covers the surface of the substrate material, keeping the anode surface flat and the metallic lithium from falling off. Compared with traditional methods such as direct rolling, in the present invention, lithium hexafluorophosphate with an initiating function is introduced on the surface of the ultrathin lithium, and a contact-type in-situ polymerization reaction occurs with the oxygen heterocyclic active monomer on the surface of the anode substrate, inducing the formation of a uniform polymer interface layer at the contact interface. The highly wettable precursor can effectively fill the voids between the metallic lithium and the anode substrate, improve the interfacial contact, thereby constructing a continuous carrier cross-interface migration path, enabling an effective interfacial electrochemical reaction between the metallic lithium and the anode substrate, accelerating the integration process of the anode, avoiding the formation of dead lithium by quickly lithiating the anode substrate, and further forming a composite anode with good interfacial contact and stable structure.

[0011] Furthermore, the ultrathin lithium with lithium hexafluorophosphate generated in-situ accounts for 50 - 75 wt% of the lithium / silicon / carbon composite anode, and on the surface of the ultrathin lithium with lithium hexafluorophosphate generated in-situ, P accounts for 1.4 - 2.0%, and F accounts for 9.5 - 11.2%; preferably, P accounts for 1.5 - 1.7%, and F accounts for 9.8 - 10.4%.

[0012] Furthermore, the graphite content in the graphite / silicon monoxide composite material accounts for 40 - 80%; the graphite / silicon monoxide composite material accounts for 70 - 99 wt% of the substrate, the conductive additive accounts for 0.5 - 20 wt% of the substrate, and the binder accounts for 0.5 - 20 wt% of the substrate.

[0013] Preferably, the graphite content in the graphite / silicon monoxide composite is 50-70%; the graphite / silicon monoxide composite accounts for 80-95 wt% of the substrate, the conductive additive accounts for 5-10 wt% of the substrate, and the binder accounts for 5-10 wt% of the substrate.

[0014] Preferably, the conductive additive is selected from one or more of Super P, Ketjen black, graphene, and conductive carbon nanotubes; the binder is selected from one or more of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber / sodium carboxymethyl cellulose (CMC / SBR), sodium alginate (SA), and gelatin, and the solvent is selected from one or more of N-methylpyrrolidone and ethanol.

[0015] Further, the thickness of the ultrathin lithium is 10-100 μm, preferably 10-20 μm.

[0016] Further, the in-situ polymerization is the in-situ generation of lithium hexafluorophosphate (LiPF 6 ) on the surface of the ultrathin lithium. The silicon-carbon substrate is infiltrated with the monomer solution. The ultrathin lithium with lithium hexafluorophosphate modified on its surface and the silicon-carbon substrate infiltrated with the monomer solution are pressed tightly under a certain pressure to initiate in-situ polymerization, obtaining a lithium / silicon / carbon composite negative electrode.

[0017] Further, the oxacyclic monomer is selected from at least one of 1,3-dioxolane, 1,4-dioxane, and tetrahydrofuran; the solvent of the oxacyclic monomer solution is selected from at least one of ethylene glycol dimethyl ether, ethylene glycol monomethyl ether, triethylene glycol dimethyl ether, methyl carbonate, ethylene carbonate, propylene carbonate, caprolactone, caprolactam, tetrahydrofuran, trioxymethylene, ethylene oxide, propylene oxide, dioxolane, and dioxane, with a volume concentration of 30-50%.

[0018] The second object of the present invention is to provide a method for preparing the above lithium / silicon / carbon composite negative electrode, comprising the following steps:

[0019] (S1) In a closed container, under an inert atmosphere, a fluoride solution is dip-coated on the surface of the ultrathin lithium, and the surface of the ultrathin lithium is fluorinated through an in-situ chemical reaction to form a uniform LiF layer. Subsequently, the surface-fluorinated metallic lithium is placed in a dry inert atmosphere, and gaseous PF 5 is introduced. Through the chemical reaction between LiF and PF 5 (LiF + PF 5 → LiPF 6 ), lithium hexafluorophosphate is uniformly formed on the surface of the ultrathin lithium;

[0020] (S2) The graphite / silicon monoxide composite, the conductive additive, the binder, and the solvent are ground and mixed, coated on the surface of the current collector, and dried to obtain a substrate material;

[0021] (S3) The substrate material is immersed in a solution of oxacyclic monomers, and after sufficient immersion, it is taken out.

[0022] (S4) The ultrathin lithium surface-modified with lithium hexafluorophosphate obtained in step (S1) and the substrate infiltrated with the monomer solution obtained in step (S3) are pressed tightly, and the lithium / silicon / carbon composite negative electrode with in-situ interfacial polymerization can be obtained by placing it at room temperature.

[0023] Further, the fluoride in step (S1) is selected from at least one of metal fluorides (such as iron fluoride (FeF 3 ), magnesium fluoride (MgF 2 ), calcium fluoride (CaF 2 ), zinc fluoride (ZnF 2 ), aluminum fluoride (AlF 3 ), chromium fluoride (CrF 3 ), manganese fluoride (MnF 2 ) and at least one of fluorine-containing polymers (such as at least one of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE)). The solvent of the fluoride solution is selected from at least one of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DEF), N-methylpyrrolidone (NMP), and tetrahydrofuran (THF); the concentration of the fluoride solution is 0.1 - 0.2 M. The time for dip-coating the ultrathin lithium surface with the fluoride solution is 15 - 30 min.

[0024] Further, the current collector in step (S2) is selected from one or more of flat copper foil, carbon-coated copper foil, and three-dimensional copper foil current collectors.

[0025] Further, the infiltration time in step (S3) is 1 - 2 h; the oxacyclic monomers are selected from at least one of 1,3-dioxolane, 1,4-dioxane, and tetrahydrofuran; the solvents of the oxacyclic monomer solution are selected from at least one of ethylene glycol dimethyl ether, ethylene glycol monomethyl ether, triethylene glycol dimethyl ether, methyl carbonate, ethylene carbonate, propylene carbonate, caprolactone, caprolactam, tetrahydrofuran, paraformaldehyde, ethylene oxide, propylene oxide, 1,3-dioxolane, and 1,4-dioxane, and the volume concentration is 30 - 50% of the solution.

[0026] Further, in step (S4), the pressing method is selected from the roll pressing method using a hydraulic balance electric pair-roll machine (MSK-2300, roll shaft gap 50-200 μm) or the cold pressing method using a flat hot and cold press (MSK-131, pressure range 0.05-0.8 Mpa). The standing time at room temperature is 2-5 h. Since the selected cationic ring-opening polymerization reaction system has the characteristic of room temperature initiation, lithium hexafluorophosphate on the lithium surface can initiate the ring-opening polymerization of oxacyclic monomers when contacting the solution in the substrate at room temperature, and a lithium / silicon / carbon composite negative electrode with in-situ interfacial polymerization is obtained.

[0027] The third object of the present invention is to provide a lithium-ion battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode is a lithium-free positive electrode, and the negative electrode is the above-mentioned lithium / silicon / carbon composite negative electrode.

[0028] Further, the lithium-free positive electrode is selected from sulfur (S), selenium (Se), sulfur-selenium compounds (Se x S y ), iron sulfide (Fe x S y ), titanium disulfide (TiS 2 ), molybdenum disulfide (MoS 2 ), manganese dioxide (MnO 2 ), vanadium pentoxide (V 2 O 5 ); the separator is a polyethylene separator, a polypropylene separator, a polyethylene / polypropylene composite separator, hydroxymethyl cellulose, or cellulose acetate; in the electrolyte, the electrolyte is selected from one or more of lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalato)borate, and lithium bis(fluorosulfonyl)imide salt, and the solvent is selected from one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, dimethyl carbonate, diethyl carbonate, ethylene carbonate, and propylene carbonate.

[0029] The lithium / silicon / carbon composite electrode provided by the present invention has the following advantages compared with the prior art:

[0030] (1) Compared with the conventional silicon / carbon negative electrode, the lithium / silicon / carbon composite electrode can be compatible with both lithium-free positive electrodes (such as elemental sulfur, elemental selenium, etc.) and lithium-containing positive electrodes (such as lithium iron phosphate, lithium cobaltate, etc.); especially with lithium-free positive electrode materials, it can effectively improve the deposition uniformity and structural stability of the matched lithium-containing negative electrode, and thus improve the cycle performance of the battery.

[0031] (2) Compared with conventional lithium metal anodes (such as lithium metal foils or lithium / copper composite tapes with copper foils as current collectors), the lithium / silicon / carbon composite anode can deposit lithium in the form of stable lithium-containing compounds on the anode side, effectively avoiding dendrites generated during the deposition / dissolution process of elemental lithium, and greatly improving the deposition efficiency of lithium metal on the anode side.

[0032] (3) In the present invention, the ultra-thin lithium foil and the substrate are compounded by in-situ polymerization. Compared with the traditional rolling method, the advantages of this method are as follows: the generated polymer layer has good adhesion and filling properties, which can make the contact between lithium and the substrate closer, reduce the porosity, make the interfacial electric field distribution more uniform, effectively conduct lithium ions, promote the reaction between lithium metal and the substrate, accelerate the integration of the anode, improve the lithiation efficiency of the substrate, and reduce the generation of dead lithium.

[0033] (4) The preparation process of the present invention is simple, the raw materials are easy to obtain, and it is convenient for large-scale production. Description of the Drawings

[0034] Figure 1 SEM and EDS Mapping pictures of the lithium hexafluorophosphate layer in-situ generated on the surface of lithium metal;

[0035] Figure 2 XPS spectrum and elemental content analysis of the lithium hexafluorophosphate layer in-situ generated on the surface of lithium metal;

[0036] Figure 3 Scanning electron microscope photograph of the surface of the substrate material;

[0037] Figure 4 Scanning electron microscope photograph of the cross-section of the substrate material;

[0038] Figure 5 Scanning electron microscope photograph of the surface of the lithium / silicon / carbon composite electrode with ultra-thin lithium loaded on the surface in Example 1;

[0039] Figure 6 Scanning electron microscope photograph of the cross-section of the lithium / silicon / carbon composite electrode with ultra-thin lithium loaded on the surface in Example 1;

[0040] Figure 7 Charge-discharge curve of the lithium metal secondary battery using the lithium / silicon / carbon composite electrode matching the sulfur cathode in Example 1;

[0041] Figure 8 Charge-discharge curve of the lithium metal secondary battery using the lithium metal anode with a flat copper foil substrate matching the sulfur cathode in Comparative Example 1. Detailed Embodiments

[0042] The present invention will be further described below in conjunction with specific embodiments.

[0043] In the following examples, the experimental methods are conventional methods unless otherwise specified; the reagents and materials can all be obtained commercially.

[0044] Example 1

[0045] (I) Preparation of Lithium / Silicon / Carbon Composite Electrode and Its Application in Metal Lithium Batteries

[0046] (S1) Under a dry atmosphere, a dimethyl sulfoxide (DMSO) solution containing iron fluoride (FeF 3 ) (FeF 3 concentration 0.1 mol / L) was uniformly dip-coated on the surface of ultrathin metallic lithium (20 μm), and left standing for 15 min to fluorinate the surface of metallic lithium to obtain a lithium fluoride (LiF) layer. Subsequently, the surface solution was washed off with a small amount of 1,2-dimethoxyethane (DME), and the surface-fluorinated metallic lithium was air-dried at room temperature and placed in a sealed container filled with an inert gas. PF 5 was introduced into the container and left standing for 1 h to allow the LiF on the surface of metallic lithium to react fully with PF 5 to obtain ultrathin metallic lithium surface-modified with lithium hexafluorophosphate (LiPF 6 ).

[0047] As can be seen from Figure 1 , the surface of metallic lithium is uniformly covered with a nano-particle layer. EDS Mapping analysis shows that the main elements of the particles are P and F, preliminarily proving that the generated substance is LiPF 6 . Further, Figure 2 (a) is the surface XPS spectrum of the modified ultrathin metallic lithium. As can be seen from the figure, the characteristic peaks of P 2p and F 1s elements exist on the lithium surface, and the peak positions are consistent with those of LiPF 6 , which can prove that ultrathin metallic lithium surface-modified with LiPF 6 is successfully obtained through S1. From the XPS element content analysis of Figure 2 (b), it can be seen that the element ratios of P and F on the surface of metallic lithium are approximately 1.7% and 10.4%, indicating that the generated LiPF 6 layer is thin and the proportion of non-active elements is low, thus not affecting the intrinsic electrochemical performance of metallic lithium.

[0048] (S2) Under a dry atmosphere, the active materials graphite / silicon monoxide (graphite: 70%, silicon monoxide: 30%), conductive additive Super P, and binder sodium carboxymethyl cellulose (CMC) / styrene-butadiene rubber (SBR) (m CMC :m SBR = 1:1) were mixed at a mass ratio of 8:1:1, water was added as a dispersant, and after grinding, it was uniformly coated on the surface of a copper foil current collector. It was placed in an 80 °C oven and vacuum-dried for 24 h to obtain the base material of the composite negative electrode.Figure 3 SEM image of the surface of the obtained base material. Figure 4 SEM cross-sectional image of the obtained base material. It can be seen that the surface of the untreated original base is uneven, with local protrusions and depressions. If the base is directly mechanically roll-pressed and mixed with metallic lithium, the uniformity of the contact interface will be significantly affected.

[0049] (S3) Immerse the surface of the negative electrode base obtained in step (S2) in a 1,3-dioxolane monomer solution in ethylene glycol dimethyl ether with a volume concentration of 50%, and soak for 2 h to achieve sufficient immersion. Then take it out for standby.

[0050] (S4) In a dry atmosphere, cover the surface of the ultra-thin lithium layer containing LiPF 6 obtained in step (S1) entirely on the surface of the base soaked with the monomer solution obtained in step (S3). Use a hydraulic balance electric pair-roller machine (MSK-2300, roller shaft gap 140 μm) to roll-press the metallic lithium until it is closely attached to the base material, keeping the surface of the negative electrode flat and the metallic lithium not falling off. Let it stand at room temperature for 5 h to complete the interfacial polymerization reaction, obtaining a lithium-containing composite-phase negative electrode, which is the lithium / silicon / carbon composite negative electrode of the present invention. Figure 5 SEM image of the surface of the obtained negative electrode. Figure 6 SEM cross-sectional image of the obtained negative electrode. It can be seen from Figure 5 that the surface of the metallic lithium composite on the negative electrode base is flat, indicating that the in-situ constructed polymer layer can effectively improve the uniformity of the composite of the base and metallic lithium, avoiding problems such as wrinkling and cracking of metallic lithium caused by protrusions and pores on the base surface. It can be seen from Figure 6 that the metallic lithium is in close contact with the base, and there is a uniform and dense polymer layer on the interface, which plays a role in effectively adhering the metallic lithium to the base.

[0051] (S5) Under high-purity argon, using sulfur as the positive electrode active material, Super P as the conductive additive, and PVDF as the binder, drop an electrolyte with ethylene glycol dimethyl ether as the solvent and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (molar concentration 1 M) as the solute on the positive electrode side. Then sequentially add a Celgard separator and the lithium / silicon / carbon composite electrode obtained in step (S4), and stack them in sequence in the battery case. Completely seal the above battery case and conduct battery performance tests.

[0052] (II) Performance test of metal lithium battery

[0053] Test the electrochemical performance of the battery in a battery test system. The test temperature is 25 °C, and the battery capacity and charge-discharge current are calculated based on the mass of sulfur. Figure 7Figure 0 shows the charge-discharge curves of the battery in Example 1 at a rate of 0.1C. Record its first-cycle discharge capacity, first-cycle Coulombic efficiency, discharge capacity after 20 cycles, and capacity retention rate. The test results of the obtained battery are listed in Table 1.

[0054] Example 2

[0055] Other conditions are the same as those in Example 1, except that the active material used in step (S2) is graphite / silicon monoxide (graphite content: 50%, silicon monoxide: 50%).

[0056] Example 3

[0057] Other conditions are the same as those in Example 1, except that the pressing method in step (S4) is changed to the cold pressing method of a flat hot and cold press (MSK-131, pressure range 0.6 Mpa).

[0058] Example 4

[0059] Other conditions are the same as those in Example 1, except that in (S1), in a dry atmosphere, a solution of dimethyl sulfoxide (DMSO) containing FeF 3 (FeF 3 concentration 0.1 mol / L) is uniformly dip-coated on the surface of ultra-thin metallic lithium (20 μm), allowed to stand for 30 min, and then the surface solution is washed off with a small amount of 1,2-dimethoxyethane (DME), that is, the dip-coating time of the ultra-thin metallic lithium with the FeF 3 solution is changed from 15 min to 30 min.

[0060] Example 5

[0061] Other conditions are the same as those in Example 1, except that in (S1), the dip-coating time of the ultra-thin metallic lithium with the FeF 3 solution is changed from 15 min to 10 min.

[0062] Comparative Example 1

[0063] Step 1) In a dry atmosphere, the active material graphite / silicon monoxide (graphite: 70%, silicon monoxide: 30%), conductive additive Super P, and binder sodium carboxymethyl cellulose (CMC) / styrene-butadiene rubber (SBR) (m CMC : m SBR = 1:1) are mixed in a mass ratio of 8:1:1, water is added as a dispersant, and after grinding, it is uniformly coated on the surface of a copper foil current collector. It is placed in an 80°C oven and vacuum-dried for 24 h to obtain the base material of the composite negative electrode. Figure 1 Figure 44 is the surface SEM image of the obtained base material. Figure 2 Figure 46 is the cross-sectional SEM image of the obtained base material.

[0064] Step 2) In a dry atmosphere, a 20-μm-thick ultra-thin lithium is integrally covered on the surface of the above substrate. The lithium metal is roll-pressed by a hydraulic balance electric pair-roller machine until it is closely attached to the substrate material, ensuring that the surface of the negative electrode is flat and the lithium metal does not fall off. A lithium-containing composite-phase negative electrode is obtained.

[0065] Step 3) Under high-purity argon, using sulfur as the positive electrode active material, Super P as the conductive additive, and PVDF as the binder, an electrolyte with ethylene carbonate, dimethyl carbonate, and diethyl carbonate (v / v / v = 1:1:1) as the solvent and lithium hexafluorophosphate (molar concentration of 1 M) as the solute is dropped on the positive electrode side. Subsequently, a Celgard separator and the lithium / silicon / carbon composite electrode obtained in Step 2 are sequentially stacked in the battery case. The above battery case is completely sealed, and battery performance tests are carried out.

[0066] Comparative Example 2

[0067] Preparation of a battery using pristine lithium metal as the negative electrode:

[0068] Under high-purity argon conditions, using sulfur as the positive electrode active material, Super P as the conductive additive, and PVDF as the binder, an electrolyte with ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (v / v / v = 1:1:1) as the solvent and lithium hexafluorophosphate (molar concentration of 1 M) as the solute is dropped on the positive electrode side. Subsequently, a Celgard separator and the pristine lithium metal negative electrode are sequentially stacked in the battery case. The above battery case is completely sealed, and battery performance tests are carried out.

[0069] Comparative Example 3

[0070] (S1) In a dry atmosphere, the active materials graphite / silicon monoxide (graphite: 70%, silicon monoxide: 30%), the conductive additive Super P, and the binder sodium carboxymethyl cellulose (CMC) / styrene-butadiene rubber (SBR) (m CMC :m SBR = 1:1) are mixed in a mass ratio of 8:1:1. Water is added as a dispersant, and after grinding, it is uniformly coated on the surface of a copper foil current collector. It is placed in an 80°C oven and vacuum-dried for 24 h to obtain the substrate material of the composite negative electrode.

[0071] (S2) The lithium metal particles are heated and melted at 180°C, and the melted liquid lithium metal is coated on the surface of the substrate obtained in Step (S1) by a casting method. The liquid lithium metal is naturally cooled to room temperature to obtain a lithium-containing composite-phase negative electrode.

[0072] Comparative Example 4

[0073] (S1) In a dry atmosphere, the active material graphite / silicon monoxide (graphite: 70%, silicon monoxide: 30%) is mixed with the conductive additive Super P and the binder sodium carboxymethyl cellulose (CMC) / styrene-butadiene rubber (SBR) (m CMC : m SBR = 1:1) in a mass ratio of 8:1:1. Water is added as a dispersant, and after grinding, it is evenly coated on the surface of the copper foil current collector. It is placed in an 80°C oven and vacuum dried for 24 h to obtain the substrate material of the composite negative electrode.

[0074] (S2) Using the substrate material obtained in step (S1) as the cathode and a lithium metal sheet as the anode, 5 mAh / cm 2 of metallic lithium is deposited on the substrate by electrochemical deposition.

[0075] Application Example Performance Test of Lithium Metal Batteries

[0076] The electrochemical performance of the battery is tested in a battery test system. The test temperature is 25°C, and the battery capacity and charge-discharge current are calculated based on the mass of sulfur. Record its first-cycle discharge capacity, first-cycle Coulomb efficiency, discharge capacity after 20 cycles, and capacity retention rate. The test results of the obtained batteries are listed in Table 1.

[0077] Table 1

[0078]

[0079] By comparing Example 1 with Comparative Example 1, it can be seen that the present invention uses an electrochemically active material as the substrate, and compared with the traditional inactive substrate material (copper), it can significantly improve the deposition of lithium, thereby enhancing the Coulomb efficiency and cycle stability of the battery. By comparing Example 2 with the comparative example, directly using metallic lithium as the negative electrode, dendritic growth is serious and the cycle stability is poor. From Comparative Example 3 and Comparative Example 4, it can be seen that when using the melt-rolling coating method or the electrochemical deposition method, the morphology and content of lithium on the substrate surface are uncontrollable and it does not have the scalability of industrial scale. Although its cycle stability is improved to a certain extent, it is still not satisfactory.

[0080] In summary, the present invention creatively provides a lithium / silicon / carbon composite electrode with an electrochemically active material as the substrate and an ultrathin lithium coating on the surface. During the use of the battery, the electrochemically active material can combine with lithium ions and store the metallic lithium deposited on the negative electrode side in the form of alloying, which is conducive to the dispersed deposition of lithium ions, avoiding the dendritic and pulverization phenomena caused by the uneven deposition of lithium, and enhancing the Coulomb efficiency and cycle stability of the lithium metal battery. On this basis, the raw materials used in the present invention are easily available, the preparation process is simple, the cost is controllable, it is suitable for large-scale production and application, and has good commercial prospects.

Claims

1. A lithium / silicon / carbon composite anode, characterized in that, it is a composite formed by in-situ polymerization of ultrathin lithium with lithium hexafluorophosphate generated in-situ on the surface and a substrate infiltrated with an oxygen heterocyclic monomer solution. The raw materials of the substrate include a graphite / silicon monoxide composite material, a conductive additive, and a binder; the lithium / silicon / carbon composite anode is prepared by a preparation method including the following steps: (S1) In a sealed container, under an inert atmosphere, a fluoride solution is dip-coated on the surface of ultrathin lithium, and the surface of the ultrathin lithium is fluorinated through an in-situ chemical reaction to form a uniform lithium fluoride layer. Subsequently, the surface-fluorinated metallic lithium is placed under a dry inert atmosphere, and gaseous PF 5 is introduced. Through the chemical reaction between LiF and PF 5 , lithium hexafluorophosphate is uniformly formed on the surface of the ultrathin lithium; (S2) Grind and mix the graphite / silicon monoxide composite material, the conductive additive, the binder, and a solvent, coat it on the surface of a current collector, and obtain a substrate material after drying; (S3) Immerse the substrate material in a solution of an oxygen heterocyclic monomer, and take it out after sufficient immersion; (S4) Press the ultrathin lithium with lithium hexafluorophosphate modified on the surface obtained in step (S1) and the substrate infiltrated with the monomer solution obtained in step (S3), and place it at room temperature to obtain a lithium / silicon / carbon composite anode with in-situ interfacial polymerization.

2. The lithium / silicon / carbon composite anode according to claim 1, characterized in that, the ultrathin lithium with lithium hexafluorophosphate generated in-situ on the surface accounts for 50-75 wt% of the lithium / silicon / carbon composite anode.

3. The lithium / silicon / carbon composite anode according to claim 1, characterized in that, the graphite content in the graphite / silicon monoxide composite material accounts for 40-80%; the graphite / silicon monoxide composite material accounts for 70-99 wt% of the substrate, the conductive additive accounts for 0.5-20 wt% of the substrate, and the binder accounts for 0.5-20 wt% of the substrate.

4. The lithium / silicon / carbon composite anode according to claim 3, characterized in that, the graphite content in the graphite / silicon monoxide composite material accounts for 50-70%; the graphite / silicon monoxide composite material accounts for 80-95 wt% of the substrate, the conductive additive accounts for 5-10 wt% of the substrate, and the binder accounts for 5-10 wt% of the substrate; the sum of all components is 100%.

5. The lithium / silicon / carbon composite anode according to claim 1, characterized in that, the conductive additive is selected from one or more of SuperP, Ketjenblack, graphene, and conductive carbon nanotubes; the binder is selected from one or more of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene-butadiene rubber / sodium carboxymethyl cellulose, sodium alginate, and gelatin.

6. The lithium / silicon / carbon composite anode according to claim 1, characterized in that, the thickness of the ultrathin lithium is 10-100 μm; and on the surface of the ultrathin lithium with lithium hexafluorophosphate generated in-situ, P accounts for 1.4-2.0%, and F accounts for 9.5-11.2%.

7. The lithium / silicon / carbon composite anode according to claim 6, characterized in that, on the surface of the ultrathin lithium with lithium hexafluorophosphate generated in-situ, P accounts for 1.5-1.7%, and F accounts for 9.8-10.4%.

8. The lithium / silicon / carbon composite anode according to claim 6, characterized in that, the thickness of the ultrathin lithium is 10-20 μm.

9. The lithium / silicon / carbon composite anode according to claim 1, characterized in that, The oxacyclic monomer is selected from at least one of 1,3-dioxolane, 1,4-dioxane, and tetrahydrofuran; the solvent of the oxacyclic monomer solution is selected from at least one of ethylene glycol dimethyl ether, ethylene glycol monomethyl ether, triethylene glycol dimethyl ether, methyl carbonate, ethylene carbonate, propylene carbonate, caprolactone, caprolactam, tetrahydrofuran, trioxane, ethylene oxide, propylene oxide, 1,3-dioxolane, and 1,4-dioxane; the volume concentration of the oxacyclic monomer is 30-50%.

10. The lithium / silicon / carbon composite negative electrode according to claim 1, characterized in that, the pressing method is selected from the roll pressing method using a hydraulic balance electric pair roll machine or the cold pressing method using a flat hot and cold press.

11. The lithium / silicon / carbon composite negative electrode according to claim 1, characterized in that, the fluoride in step (S1) is selected from metal fluorides and fluorine-containing polymers; the solvent of the fluoride solution is selected from at least one of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and tetrahydrofuran; the concentration of the fluoride solution is 0.1-0.2 M; the time for dip-coating the ultra-thin lithium surface with the fluoride solution is 15-30 min.

12. The lithium / silicon / carbon composite negative electrode according to claim 11, characterized in that, the metal fluoride is selected from at least one of iron fluoride, magnesium fluoride, calcium fluoride, zinc fluoride, aluminum fluoride, chromium fluoride, and manganese fluoride; the fluorine-containing polymer is selected from at least one of vinylidene fluoride and polytetrafluoroethylene.

13. The lithium / silicon / carbon composite negative electrode according to claim 1, characterized in that, in step (S4), the room temperature placement time is 2-5 h.

14. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, the negative electrode is the lithium / silicon / carbon composite negative electrode according to any one of claims 1-13.

15. The lithium-ion battery according to claim 14, characterized in that, the positive electrode is a lithium-free positive electrode.

16. The lithium-ion battery according to claim 15, characterized in that, the lithium-free positive electrode is selected from sulfur, selenium, sulfur-selenium compounds, iron sulfide, titanium disulfide, molybdenum disulfide, manganese dioxide, or vanadium pentoxide.

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