Preparation method of lithium squarate nanosheets, lithium squarate nanosheets prepared therefrom and applications thereof

Abstract

The present invention provides a method for preparing lithium squarate nanosheets, the lithium squarate nanosheets prepared thereby, and applications thereof. The method for preparing the lithium squarate nanosheets includes: mixing squaric acid, a lithium source, and a template regulator in an alcohol, and then performing a solvothermal reaction to obtain the lithium squarate nanosheets. The present invention uses squaric acid and a lithium source as raw materials, an alcohol as a medium, and successfully synthesizes ultrathin two-dimensional lithium squarate nanosheets by a solvothermal method with the assistance of a template regulator. The preparation method of the present invention is simple, highly applicable, has inexpensive and easily available raw materials, is energy-saving and environmentally friendly, is conducive to large-scale industrial production, and has significant economic and social benefits. The prepared ultrathin two-dimensional lithium squarate nanosheets have a thin thickness, a large specific surface area, a low de-lithiation potential, and high lithium supplementation performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery materials, and particularly relates to a preparation method of lithium squarate nanosheets, the lithium squarate nanosheets prepared thereby, and applications thereof. Background Art

[0002] In the field of modern battery technology, lithium-ion batteries have become the preferred energy storage solution due to their high energy density, long cycle life, and low self-discharge rate. However, with the continuous improvement of battery performance requirements, traditional lithium-ion batteries have shown deficiencies in some aspects, especially in irreversible lithium loss and high energy density. To address these issues, researchers have been exploring new materials and technologies, and the development and application of lithium replenishing agents have become a prominent research direction.

[0003] Lithium replenishing agents are commonly used materials in traditional lithium-ion batteries to supplement lithium ions, and they play a crucial role in the charge and discharge processes of the batteries. As a known cathode lithium replenishing agent material, lithium squarate has a higher specific capacity than traditional cathode materials and is suitable as a lithium replenishing agent to compensate for irreversible loss of lithium ions and improve energy density. However, the thickness and structure of micron-scale lithium squarate often limit its conductivity and the diffusion rate of lithium ions, resulting in limited charge and discharge rates of the battery and potentially affecting the cycle stability and life of the battery. To overcome these challenges, researchers have explored different preparation methods to improve the performance of lithium squarate.

[0004] CN113443973A discloses a lithium squarate and its preparation method and applications. The lithium squarate prepared by treating an aqueous solution by freeze-drying method has a de-lithiation potential reduced from 4.2 V to 3.8 V by reducing the size of lithium squarate to the nanoscale (50 - 100 nm). However, the high cost and unfavorable mass production of this method limit its application in large-scale production.

[0005] CN118324621A discloses a lithium squarate and its preparation method and applications. By modifying the precursor distribution - mixing heat treatment and then hydrothermal treatment, a saturated solution of lithium squarate is obtained, and lithium squarate is obtained by recrystallization after hydrothermal treatment. Adding it as a lithium replenishing agent improves the capacity of the battery, but there are many operation steps, and agglomeration and accumulation of lithium squarate inevitably occur during the subsequent drying process with water as the solvent.

[0006] CN115417754A discloses a lithium squarate, a preparation method thereof, a positive electrode sheet and a lithium battery. Lithium squarate with an average particle size of 100-500 nm is prepared by a recrystallization method and applied to the lithium iron phosphate system, effectively improving the first charge capacity and Coulomb efficiency of the full battery, reducing the first irreversible lithium loss, and improving the cycle and energy density. Although this method shows potential in improving battery performance, the operation is relatively cumbersome, the equipment requirements are high, and there are challenges in large-scale production.

[0007] In view of this, the present invention is specifically proposed. Summary of the Invention

[0008] The object of the present invention is to provide a preparation method of lithium squarate nanosheets, the lithium squarate nanosheets prepared therefrom and their applications. The preparation method of the lithium squarate nanosheets of the present invention prepares ultrathin lithium squarate nanosheets by a solvothermal synthesis method. The preparation process is simple, has high stability, is easy to operate, and has low cost. The thickness of the prepared lithium squarate nanosheets can be reduced to about 5 nm. The ultrathin structural design enables lithium ions to diffuse faster in the electrode material, thus significantly improving the charge and discharge rate of the battery.

[0009] In order to achieve the above object of the present invention, the following technical solutions are specifically adopted:

[0010] In a first aspect, the present invention provides a preparation method of lithium squarate nanosheets, and the preparation method of the lithium squarate nanosheets includes:

[0011] Mix squaric acid, a lithium source and a template regulator in an alcohol, and then carry out a solvothermal reaction to obtain lithium squarate nanosheets.

[0012] Preferably, the molar ratio of lithium in the squaric acid and the lithium source is 1:(2.0-2.4).

[0013] Preferably, the lithium source includes any one or a combination of at least two of lithium oxide, lithium hydroxide, lithium carbonate, lithium bicarbonate or lithium acetate.

[0014] Preferably, the addition amount of the template regulator accounts for 0.1-5.0% of the total mass of the squaric acid and the lithium source.

[0015] Preferably, the template regulator includes any one or a combination of at least two of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, sodium dodecylsulfonate, oleic acid or polyethylene glycol.

[0016] Preferably, the addition amount of the alcohol is 1-5 times the total mass of the squaric acid and the lithium source.

[0017] Preferably, the alcohol is selected from any one or a combination of at least two of ethanol, ethylene glycol, glycerol, pentaerythritol, trimethylolethane, xylitol or sorbitol.

[0018] Preferably, the mixing is carried out under stirring conditions, the rotation speed of the stirring is 200 - 1500 rpm, the temperature of the stirring is 5 - 80 °C, and the time of the stirring is 0.5 - 2 h.

[0019] Preferably, the temperature of the solvothermal reaction is 100 - 160 °C, and the time of the solvothermal reaction is 4 - 12 h.

[0020] In a second aspect, the present invention provides a lithium squarate nanosheet, which is prepared by the preparation method of the lithium squarate nanosheet as described in the first aspect.

[0021] Preferably, the thickness of the lithium squarate nanosheet is 6 nm or less, preferably 4 - 6 nm, and more preferably 5 nm.

[0022] In a third aspect, the present invention provides an application of the lithium squarate nanosheet as described in the second aspect as a lithium supplement material for the positive electrode of a lithium battery.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The ultra-thin lithium squarate nanosheet prepared by the present invention can be used as a lithium supplement agent for the positive electrode of a lithium battery. The lithium supplement agent is efficient, non-toxic, has a simple preparation process, high stability, is easy to operate, and has a low cost. It has broad application prospects and potential industrialization value, and can be extended to the synthesis of other materials.

[0025] (2) The present invention uses the additional lithium ions provided by the lithium squarate nanosheet during the charging process of the lithium battery to compensate for the irreversible lithium ion consumption caused by side reactions or interface formation in the battery system, so as to improve the actual capacity of the full battery and effectively improve the cycle life and energy density of the lithium ion battery; starting from affecting the electrochemical performance of the battery, the morphology structure of the lithium supplement additive is selectively regulated and designed, so that the obtained positive electrode material can improve the cycle life and energy density while meeting the conventional performance of the lithium ion battery.

[0026] (3) By controlling the preparation method of lithium squarate, the present invention can, on the one hand, ensure the stability of the obtained lithium squarate, and on the other hand, control the morphology of lithium squarate, so that the obtained lithium squarate as a lithium supplement additive has better electrochemical performance. In addition, it can avoid the presence of crystal water in the obtained lithium squarate, avoid the problem that crystal water is difficult to remove, and will not affect the performance of the battery. Description of the Drawings

[0027] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0028] Figure 1 It is the X-ray diffraction (XRD) pattern of the lithium squarate nanosheets provided in Example 1 and the lithium squarate sheets provided in Comparative Example 1.

[0029] Figure 2 It is the scanning electron microscope (SEM) image of the lithium squarate nanosheets provided in Example 1.

[0030] Figure 3 It is the scanning electron microscope (SEM) image of the lithium squarate sheets provided in Comparative Example 1.

[0031] Figure 4 It is the scanning electron microscope (SEM) image of the lithium squarate sheets provided in Comparative Example 2.

[0032] Figure 5 It is the atomic force microscope (AFM) image and the corresponding height profile of the lithium squarate nanosheets provided in Example 1.

[0033] Figure 6 It is the nitrogen (N2) adsorption-desorption isotherm curve and the specific surface area of the lithium squarate nanosheets provided in Example 1 and the lithium squarate sheets in Comparative Example 1.

[0034] Figure 7 It is the X-ray diffraction (XRD) pattern of the freshly prepared lithium squarate nanosheets provided in Example 1 and the lithium squarate nanosheets stored in air for 180 days.

[0035] Figure 8 It is the first charge curve of the lithium squarate nanosheets provided in Example 1 and the lithium squarate sheets in Comparative Example 1. Specific Embodiments

[0036] Unless otherwise defined herein, scientific and technical terms used in conjunction with the present invention shall have the meanings commonly understood by those of ordinary skill in the art. The meanings and scopes of the terms should be clear. However, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or extrinsic definition. In this application, unless otherwise specified, the use of "or" means "and / or". In addition, the use of the term "including" and other forms is non-restrictive.

[0037] It should be noted that specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar generalizations without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0038] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0039] In a first aspect, the present invention provides a method for preparing lithium squarate nanosheets, and the method for preparing lithium squarate nanosheets includes:

[0040] After mixing squaric acid, a lithium source, and a template regulator in an alcohol, a solvothermal reaction is carried out to obtain lithium squarate nanosheets.

[0041] In the present invention, ultrathin lithium squarate nanosheets are successfully synthesized by a solvothermal method with squaric acid and a lithium source as raw materials, alcohol as a medium, and with the assistance of a template regulator. The solvothermal synthesis method can prepare ultrathin lithium squarate nanosheets with a certain thickness and high specific surface area at a relatively low cost by controlling the reaction conditions. If the ultrathin lithium squarate nanosheets prepared by the solvothermal synthesis method can successfully overcome existing challenges and achieve large-scale production, they have the potential to become one of the key innovations to promote the progress of lithium-ion battery technology.

[0042] As an optional implementation manner, the molar ratio of lithium in the squaric acid to the lithium source is 1:(2.0 - 2.4), for example, it can be 1:2.0, 1:2.1, 1:2.2, 1:2.3, 1:2.4, etc.

[0043] It should be noted that the present invention controls the molar ratio of squaric acid to the lithium source at 1:(2.0 - 2.4) to obtain ultrathin lithium squarate nanosheets. If it is too low, the size of the obtained material is non-uniform, and if it is too high, the lithium salt is excessive and there are impurities in the product.

[0044] As an optional implementation manner, the lithium source includes any one or a combination of at least two of lithium oxide, lithium hydroxide, lithium carbonate, lithium bicarbonate, or lithium acetate.

[0045] As an optional implementation manner, the addition amount of the template regulator accounts for 0.1 - 5.0% of the total mass of the squaric acid and the lithium source, for example, it can be 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, etc.

[0046] It should be noted that the addition amount of the template regulator in the present invention is controlled within 0.1-5.0%, and ultrathin lithium squarate nanosheets are obtained based on this concentration. If it is too low, the size of the obtained material is uneven, and if it is too high, the product material agglomerates.

[0047] As an optional implementation manner, the template regulator includes any one or a combination of at least two of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, sodium dodecylsulfonate, oleic acid or polyethylene glycol.

[0048] As an optional implementation manner, the addition amount of the alcohol is 1-5 times the total mass of the squaric acid and the lithium source, and for example, it can be 1 time, 1.5 times, 2 times, 2.5 times, 3 times, 3.5 times, 4 times, 4.5 times, 5 times, etc.

[0049] As an optional implementation manner, the alcohol is selected from any one or a combination of at least two of ethanol, ethylene glycol, glycerol, pentaerythritol, trimethylolethane, xylitol or sorbitol.

[0050] It should be noted that since the product lithium squarate is soluble in water but insoluble in organic alcohols, alcohol is selected as the medium in the synthesis process. First, the polarity of alcohol is relatively weak, which helps to reduce the interaction between the solvent and the layered substance, thus facilitating the exfoliation of the layered structure to form ultrathin two-dimensional nanosheets. Second, the surface energy of alcohol may be more matched with the surface energy of some layered substances, which helps to maintain the stability of the nanosheets during the exfoliation process. Finally, in the alcohol solvent, the template regulator can form ordered aggregates such as micelles, reverse micelles, microemulsions, etc., and these aggregates can serve as microreactors or templates to realize the regulation of the morphology of nanomaterials. In this case, the template regulator can reduce the energy loss during the exfoliation process and stabilize the exfoliated flakes, thus making the exfoliation process more effective. In summary, the present invention regulates the crystal phase, morphology and particle size distribution of the material through the specific chemical action of the alcohol solvent, so as to optimize the electrochemical activity and stability of the material.

[0051] As an optional implementation manner, the mixing is carried out under stirring conditions, and the rotation speed of the stirring is 200-1500 rpm, and for example, it can be 200 rpm, 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, 1400 rpm, 1500 rpm, etc.

[0052] As an alternative embodiment, the mixing is carried out under stirring conditions, and the temperature of the stirring is 5 to 80 °C, for example, it can be 5 °C, 10 °C, 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, etc.

[0053] As an alternative embodiment, the mixing is carried out under stirring conditions, and the time of the stirring is 0.5 to 2 h, for example, it can be 0.5 h, 0.6 h, 0.8 h, 1 h, 1.2 h, 1.4 h, 1.6 h, 1.8 h, 2 h, etc.

[0054] As an alternative embodiment, the temperature of the solvothermal reaction is 100 to 160 °C, for example, it can be 100 °C, 105 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, 135 °C, 140 °C, 145 °C, 150 °C, 155 °C, 160 °C, etc.

[0055] As an alternative embodiment, the time of the solvothermal reaction is 4 to 12 h, for example, it can be 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, etc.

[0056] It should be noted that by precisely controlling the conditions of the hydrothermal reaction, the morphology and properties of the nanomaterials can be effectively regulated. By controlling an appropriate extended hydrothermal time in the present invention, it helps to inhibit stacking, and the growth of the material in the two-dimensional plane direction is more obvious, which is beneficial to the preparation of ultrathin nanosheets.

[0057] It can be understood that in the above embodiments of the present application, squaric acid and a lithium source are used as raw materials, with an alcohol as a medium assisted by a template regulator, and under hydrothermal conditions, the product lithium squarate grows in a two-dimensional direction, and finally an ultrathin two-dimensional nanosheet structure is formed.

[0058] It should be noted that the preparation method of the present invention is not limited to the preparation of lithium squarate nanosheets, and is also applicable to the preparation of other lithium and sodium salt nanosheets. For example, when squaric acid, oxalic acid, etc. act with lithium and sodium salts, the corresponding sodium squarate, lithium oxalate, and sodium oxalate nanosheets can also be obtained.

[0059] In a second aspect, the present invention provides a lithium squarate nanosheet (the chemical formula of the lithium squarate is Li2C4O4), and the lithium squarate nanosheet is prepared by the preparation method of the lithium squarate nanosheet as described in the first aspect.

[0060] In the present invention, ultrathin lithium squarate nanosheets with a certain thickness and high specific surface area are prepared. The ultrathin lithium supplement agents have unique advantages. They are usually made of extremely thin materials with a thickness ranging from only a few nanometers to dozens of nanometers. This ultrathin structural design enables lithium ions to diffuse more quickly in the electrode material, thus significantly improving the charge and discharge rate of the battery. In addition, the ultrathin lithium supplement agents also help to increase the energy density of the battery because they can provide more lithium ions to participate in the charge and discharge reactions without increasing the volume of the battery.

[0061] More importantly, due to the ultrathin characteristics of the lithium squarate nanosheets, the volume change during the charge and discharge process is small, which helps to reduce the fatigue and degradation of the battery materials and extend the service life of the battery. At the same time, the preparation process of the lithium squarate nanosheets described in the present invention is more refined, and the composition and structure of the materials can be controlled more precisely, thereby optimizing the performance of the battery. This method can not only improve the conductivity of the material and the diffusion rate of lithium ions, but also maintain the structural stability of the material, thus improving the charge and discharge rate and cycle stability of the battery.

[0062] As an optional implementation manner, the morphology of the ultrathin lithium squarate nanosheets is a sheet structure.

[0063] As an optional implementation manner, the thickness of the lithium squarate nanosheets is 4 - 6 nm, for example, it can be 4 nm, 4.2 nm, 4.4 nm, 4.6 nm, 4.8 nm, 5 nm, 5.2 nm, 5.4 nm, 5.6 nm, 5.8 nm, 6 nm, etc., and preferably 5 nm.

[0064] As an optional implementation manner, the specific surface area of the lithium squarate nanosheets is 80 - 85 m 2 / g, for example, it can be 80 m 2 / g, 80.5 m 2 / g, 81 m 2 / g, 81.5 m 2 / g, 82 m 2 / g, 82.5 m 2 / g, 83 m 2 / g, 83.5 m 2 / g, 84 m 2 / g, 84.5 m 2 / g, 85 m 2 / g, etc.

[0065] It should be noted that the preparation method of ultrathin lithium squarate nanosheets provided by the present invention through the above-mentioned embodiments uses squaric acid and lithium salts as precursors, alcohol as the medium, and under the assistance of a template regulator, lithium squarate is synthesized under hydrothermal conditions, enabling the prepared lithium squarate nanosheets to exist stably, having an ultrathin nanosheet morphology, with a microscopic thickness of 5 nm, further increasing the specific surface area of the material, providing more contact areas and active sites, having good stability, and also providing a new idea for the synthesis of similar lithium and sodium supplement agents.

[0066] In a third aspect, the present invention provides an application of the lithium squarate nanosheets as described in the second aspect in use as a lithium supplement material for the positive electrode of a lithium battery.

[0067] It should be noted that the ultrathin lithium squarate nanosheets provided by the present invention through the above-mentioned embodiments can be obtained under different concentrations and hydrothermal conditions, and the reaction time is not long, and stable lithium squarate can be obtained, enabling batch production. The synthesis method is simple and easy to operate. The lithium squarate nanosheets of the present invention are efficient, non-toxic, have a simple preparation process, high stability, are easy to operate, have low costs, and have broad application prospects and potential industrialization value. The ultrathin lithium squarate nanosheets provided by the present invention reduce the de-lithiation potential of the material, improve its specific capacity, have an obvious lithium supplement effect, can be applied to the lithium supplement of the positive electrode of lithium-ion batteries, and have good market prospects.

[0068] The present invention will be further described below through examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or directly purchased from the market.

[0069] Example 1

[0070] This example provides a kind of lithium squarate nanosheets, which are prepared by the following steps: Add 0.03 g of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (P123) and 7.4 g of lithium carbonate into a polytetrafluoroethylene liner, stir and add 50 mL of ethanol, then add 11.4 g of squaric acid, stir at a speed of 300 rpm at room temperature for 2 h, then put it into a steel sleeve, heat it in an oven at 120 °C for 4 h. After the reaction is completed, let it cool naturally to room temperature, and after centrifugal washing and drying in an oven, the lithium squarate nanosheets are obtained.

[0071] Example 2

[0072] This example provides a lithium squarate nanosheet, which is prepared by the following steps: Weigh 0.02 g of CTAB and 13.6 g of lithium bicarbonate, add them to a polytetrafluoroethylene inner liner, stir and add 50 mL of ethylene glycol, then add 11.4 g of squaric acid. After stirring at 60 °C and a rotation speed of 600 rpm for 1 h, put it into a steel sleeve and heat it in an oven at 140 °C for 8 h. After the reaction is completed, let it cool naturally to room temperature. After centrifugal washing and drying in the oven, the lithium squarate nanosheet is obtained.

[0073] Example 3

[0074] This example provides a lithium squarate nanosheet, which is prepared by the following steps: Weigh 0.04 g of oleic acid and 4.8 g of lithium hydroxide, add them to a 100 mL polytetrafluoroethylene inner liner, stir and add 50 mL of ethylene glycol, then add 11.4 g of squaric acid. After stirring at 60 °C and a rotation speed of 800 rpm for 1 h, put it into a steel sleeve and heat it in an oven at 120 °C for 10 h. After the reaction is completed, let it cool naturally to room temperature. After centrifugal washing and drying in the oven, the lithium squarate nanosheet is obtained.

[0075] Example 4

[0076] This example provides a lithium squarate nanosheet. The difference from Example 1 is only that the molar ratio of squaric acid to lithium carbonate is 1:1.5, and the other steps are exactly the same as those in Example 1.

[0077] Example 5

[0078] This example provides a lithium squarate nanosheet. The difference from Example 1 is only that the molar ratio of squaric acid to lithium carbonate is 1:2.5, and the other steps are exactly the same as those in Example 1.

[0079] Example 6

[0080] This example provides a lithium squarate nanosheet. The difference from Example 1 is only that lithium carbonate is replaced with an equal mass of lithium oxide, and the other steps are exactly the same as those in Example 1.

[0081] Example 7

[0082] This example provides a lithium squarate nanosheet. The difference from Example 1 is only that the content of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (P123) is adjusted to 0.01 g, and the other steps are exactly the same as those in Example 1.

[0083] Example 8

[0084] This example provides a lithium squarate nanosheet, which is only different from Example 1 in that the content of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (P123) is adjusted to 1 g, and other steps are exactly the same as those in Example 1.

[0085] Example 9

[0086] This example provides a lithium squarate nanosheet, which is only different from Example 1 in that the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (P123) is replaced with an equal mass of polyvinylpyrrolidone, and other steps are exactly the same as those in Example 1.

[0087] Example 10

[0088] This example provides a lithium squarate nanosheet, which is only different from Example 1 in that the content of the ethanol is reduced to 20 mL, and other steps are exactly the same as those in Example 1.

[0089] Example 11

[0090] This example provides a lithium squarate nanosheet, which is only different from Example 1 in that the content of the ethanol is increased to 80 mL, and other steps are exactly the same as those in Example 1.

[0091] Example 12

[0092] This example provides a lithium squarate nanosheet, which is only different from Example 1 in that the ethanol is replaced with an equal volume of glycerol, and other steps are exactly the same as those in Example 1.

[0093] Example 13

[0094] This example provides a lithium squarate nanosheet, which is only different from Example 1 in that it is heated at 100 °C for 12 h in an oven, and other steps are exactly the same as those in Example 1.

[0095] Example 14

[0096] This example provides a lithium squarate nanosheet, which is only different from Example 1 in that it is heated at 160 °C for 4 h in an oven, and other steps are exactly the same as those in Example 1.

[0097] Comparative Example 1

[0098] This comparative example provides a lithium squarate sheet, which is prepared by the following steps: 11.4 g of squaric acid and 7.4 g of lithium carbonate are respectively dissolved in 25 mL of deionized water. The squaric acid aqueous solution is slowly added to the lithium carbonate aqueous solution in several portions under a 60 °C water bath. After complete addition, stirring is continued for 2 h to obtain a lithium squarate solution, and the lithium squarate sample is obtained by evaporation, concentration, crystallization and drying.

[0099] Comparative Example 2

[0100] This comparative example provides a lithium squarate sheet, which is only different from that of Example 1 in that the ethanol is replaced with water of the same volume, and the other steps are exactly the same as those of Example 1.

[0101] Comparative Example 3

[0102] This comparative example provides a lithium squarate sheet, which is only different from that of Example 1 in that the triblock copolymer of polyethylene oxide - polypropylene oxide - polyethylene oxide (P123) is no longer added, and the other steps are exactly the same as those of Example 1.

[0103] Test Example 1

[0104] Test samples: Lithium squarate nanosheets provided in Examples 1 to 14 and lithium squarate sheets provided in Comparative Examples 1 to 3.

[0105] Test items: X-ray diffraction, scanning electron microscopy and atomic force microscopy.

[0106] The test results are shown in Table 1 below and Figures 1 to 7 as follows:

[0107] Table 1

[0108]

[0109]

[0110] As shown in Table 1, the thickness of the lithium squarate nanosheets prepared by the preparation methods provided in Examples 1 to 3 of the present invention is 5.0 - 6.0 nm, and the specific surface area of the lithium squarate nanosheets is 81.5 - 85.0 m 2 / g. This shows that the preparation method of the ultrathin lithium squarate nanosheets provided by the present invention through the above embodiments uses squaric acid and lithium salt as precursors, alcohol as the medium, and under the assistance of a template regulator, synthesizes lithium squarate under hydrothermal conditions, enabling the prepared lithium squarate nanosheets to exist stably, having an ultrathin nanosheet morphology, a microscopic thickness of 5 nm, further increasing the specific surface area of the material, providing more contact areas and active sites, and having good stability.

[0111] Figure 1 is the X-ray diffraction (XRD) pattern of the lithium squarate nanosheets provided in Example 1 and the lithium squarate sheets provided in Comparative Example 1. From Figure 1 the comparison, it can be found that the prepared samples are all pure phases and are consistent with the lithium squarate standard card, but the small size of the nanosheets and the preferred orientation of the sample crystal phase result in more burrs in the diffraction peaks.

[0112] Figure 2 is the scanning electron microscopy (SEM) image of the lithium squarate nanosheets provided in Example 1; Figure 3It is the scanning electron microscope (SEM) image of the squarylium lithium tablet provided by Comparative Example 1; Figure 4 It is the scanning electron microscope (SEM) image of the squarylium lithium tablet provided by Comparative Example 2. First, from Figure 2 and Figure 3 's comparison, it can be seen that under the same magnification condition, Comparative Example 1 shows a layered body phase structure, while the squarylium lithium nanosheets prepared in Example 1 are in a separated flake structure. Secondly, from Figure 2 and Figure 4 's comparison, it can be seen that since the product squarylium lithium is itself soluble in water, it cannot grow directionally under the action of the template regulator, thus unable to form nanosheets, and the final form is the recrystallized thick sheet after drying.

[0113] Figure 5 It is the atomic force microscope (AFM) image and the corresponding height profile of the squarylium lithium nanosheets provided by Example 1. From Figure 5 it can be seen that the average thickness of the squarylium lithium nanosheets provided by Example 1 is 5 nm.

[0114] Figure 6 It is the nitrogen (N2) adsorption-desorption isotherm curve and the specific surface area of the squarylium lithium nanosheets provided by Example 1 and the squarylium lithium tablets of Comparative Example 1. It can be seen from the figure that the specific surface area of the squarylium lithium nanosheets prepared in Example 1 is 82.9 m 2 / g, while the specific surface area of the squarylium lithium product prepared in the comparative example is only 10.7 m 2 / g. Obviously, the specific surface area of the squarylium lithium nanosheets prepared in Example 1 is 7.7 times that of the comparative example, which fully shows that the preparation method described in the present invention further increases the specific surface area of the material and provides more contact areas and active sites.

[0115] Figure 7 It is the X-ray diffraction (XRD) pattern of the freshly prepared squarylium lithium nanosheets provided by Example 1 and the squarylium lithium nanosheets stored in air for 180 days. From Figure 7 it can be found that there is no difference in XRD between the squarylium lithium nanosheets exposed to air for a long time and the freshly prepared ones, further showing the excellent air stability of the squarylium lithium nanosheets prepared by the method described in the present invention.

[0116] Test Example 2

[0117] Test samples: Squarylium lithium nanosheets provided by Examples 1 to 14, squarylium lithium tablets provided by Comparative Examples 1 to 3.

[0118] Test items: Squarylium lithium coin cell test.

[0119] Test method:

[0120] Mix the lithium squarate material to be tested (lithium squarate nanosheets, lithium squarate comparative examples), binder (PVDF), and conductive agent in a mass ratio of 80:10:10. Add the dispersant N-methylpyrrolidone (NMP) to make a slurry, uniformly coat it on aluminum foil, dry it, press it into a sheet, then punch the sheet, and dry it in an oven at 80 °C for 12 h for later use. Use a lithium metal sheet as the counter electrode, use a 1 mol / L LiPF6 electrolyte, and the solvent is a ternary mixed solvent, that is, a mixture of EC:DMC:EMC with a volume ratio of 1:1:1. Use a Celgard 2400 microporous polypropylene membrane as the separator and assemble it into a CR2032 coin cell in an argon glove box. The charge-discharge test of the coin cell is carried out on a BlueTEC battery test system. The charge-discharge voltage is 2.0 - 4.4 V, and the charge-discharge rate is 0.1 C. The first charge capacity is tested.

[0121] The test results are shown in Table 2 below and Figure 8 as follows:

[0122] Table 2

[0123]

[0124]

[0125] As shown in Table 2, using the lithium squarate nanosheets provided in Examples 1 - 3 of the present invention as the lithium supplement material for the positive electrode of the lithium battery, the initial charge specific capacity of the prepared half-cell is 436.5 - 446 mAh / g.

[0126] Obviously, for the half-cells corresponding to the lithium squarate nanosheets synthesized by the solvothermal method in Examples 1, 2, and 3 of the present invention, the initial charge specific capacity is much higher than that of Comparative Examples 1 - 2. In addition, under the condition that Example 1 and Comparative Example 2 have the same reaction conditions except for the different solvents (alcohol and water), they show significantly different half-cell performances. This may be because the product lithium squarate itself is easily soluble in water, so it cannot grow directionally under the action of the surfactant, thus unable to form nanosheets, and the final form is a recrystallized thick sheet after drying.

[0127] Test Example 3

[0128] Test samples: Lithium squarate nanosheets provided in Examples 1 - 14, lithium squarate sheets provided in Comparative Examples 1 - 3.

[0129] Test items: Lithium-ion battery test.

[0130] Test method:

[0131] Mix the above samples with lithium iron phosphate cathode material, acetylene black conductive agent, and PVDF in a ratio of 5:75:10:10. After mixing, add them to NMP to prepare a black viscous slurry. Apply the slurry onto an aluminum foil, and after drying treatment, a cathode sheet containing squarylium lithium is obtained. Assemble a button battery with metallic lithium, a separator, and the prepared cathode sheet. The button battery model used is CR2032, the separator model is 2500, and the electrolyte is 1 mol / L lithium hexafluorophosphate (the solvent is a mixed solution composed of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1). After assembly, take the battery out of the glove box, let it stand at 30 °C for 12 h, and then conduct tests on a Land test system. The charge-discharge voltage is 2.0 - 4.4 V, the charge-discharge rate is 0.1 C, and the first reversible capacity is tested.

[0132] The test results are shown in Table 3 below and Figure 8 as follows:

[0133] Table 3

[0134]

[0135]

[0136] As shown in Table 2, using the squarylium lithium nanosheets provided in Examples 1 - 3 of the present invention as the lithium supplement material for the lithium battery cathode, the first-cycle charge specific capacity of the prepared battery is 200 - 220 mAh / g, the discharge specific capacity is 150 - 160 mAh / g, the lithium supplement rate is more than 29%, and the capacity retention rate is more than 95%. The first-cycle charge specific capacity, first-cycle discharge specific capacity, lithium supplement rate, and cycle capacity retention rate of the full batteries corresponding to Examples 1, 2, and 3 are all higher than those of Comparative Examples 1 and 2. Therefore, the synthesized ultra-thin two-dimensional squarylium lithium nanosheets have high lithium supplement activity and are a lithium supplement material for the cathode with broad application prospects.

[0137] Figure 8 The first charge curves of the squarylium lithium nanosheets synthesized in Example 1 and squarylium lithium of Comparative Example 1 are shown. The de-lithiation potential of squarylium lithium in Comparative Example 1 is 4.2 V, and the charge specific capacity is 162.5 mAh / g. After nanometerization treatment, the ultra-thin squarylium lithium nanosheets significantly improve the electrochemical performance of the material, reduce the de-lithiation potential to 3.9 V, and the charge specific capacity can reach 446.7 mAh / g, increasing by nearly 1.8 times.

[0138] Based on the above experimental examples, the specific charge capacity of the ultrathin lithium squarate provided by the present invention is close to the theoretical value, the de-lithiation potential is low, and it can be compatible with the cathode system; when added to the lithium iron phosphate system, it significantly enhances the charge-discharge efficiency of the first cycle and maintains a high cycle capacity retention rate, with excellent lithium compensation effect, ensuring the cycle stability of the battery. In addition, the preparation process of the present invention is simple, with high stability, easy to operate, and low cost, having broad application prospects and potential industrial value.

[0139] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing lithium squarate nanosheets, characterized in that: The preparation method of the lithium squarate nanosheets comprises: The squarate, lithium source and template regulator are mixed in alcohol and then subjected to a solvothermal reaction to obtain squarate lithium nanosheets; The molar ratio of the squaric acid to the lithium in the lithium source is 1:(2.0-2.4); the added amount of the template regulator accounts for 0.1-5.0% of the total mass of the squaric acid and the lithium source; The lithium source includes any one of lithium oxide, lithium hydroxide, lithium carbonate, lithium bicarbonate or lithium acetate, or a combination of at least two thereof; The template regulator includes any one or a combination of at least two of polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer, hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, sodium dodecylsulfonate, oleic acid or polyethylene glycol; The alcohol is selected from any one or a combination of at least two of ethanol, ethylene glycol, glycerol, pentaerythritol, trimethylolethane, xylitol or sorbitol.

2. The method for preparing lithium squarate nanosheets according to claim 1, characterized in that: The added amount of the alcohol is 1 to 5 times the total mass of the squaric acid and the lithium source.

3. The method for preparing lithium squarate nanosheets according to claim 1, characterized in that: The mixing is performed under stirring conditions, the stirring speed is 200 to 1500 rpm, the stirring temperature is 5 to 80° C., and the stirring time is 0.5 to 2 h.

4. The method for preparing lithium squarate nanosheets according to claim 1, characterized in that: The temperature of the solvent thermal reaction is 100 to 160° C., and the time of the solvent thermal reaction is 4 to 12 hours.

5. A lithium squarate nanosheet, characterized in that: The squarate lithium nanosheets are prepared by the preparation method of squarate lithium nanosheets according to any one of claims 1 to 4.

6. The lithium squarate nanosheet according to claim 5, characterized in that: The thickness of the lithium squarate nanosheets is 4-6 nm.

7. The lithium squarate nanosheet according to claim 5, characterized in that: The specific surface area of ​​the lithium squarate nanosheets is 80 to 85 m 2 / g.

8. Use of the lithium squarate nanosheet according to any one of claims 4 to 7 as a positive electrode lithium supplement material for lithium batteries.

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