CN104557995B - Preparation method of lithium oxalyldifluoroborate - Google Patents

Abstract

A preparation method of lithium difluorooxalate borate, which involves stirring and mixing reaction of lithium bisoxalate borate and lithium fluoride in a non-aqueous solvent, and the molar ratio of lithium bisoxalate borate to lithium fluoride is in the range of 1:1.90 to 1:2.10 , The reaction time is 10‑24 hours, and the reaction generates difluorobisoxalate lithium borate and lithium oxalate. The non-aqueous solvent is benzene, toluene or xylene; the reaction temperature is from 50-85°C, and the reaction time is 12-16 hours. The invention has a unique process circuit, is environmentally friendly, has a wide range of process control, can easily separate the two products of the reaction, and can easily prepare electrolyte salts whose quality meets the requirements of lithium batteries. The lithium difluorobisoxalate borate solution produced by the invention has less content of chlorine compounds and free acids and controllable moisture.

Description

A kind of preparation method of lithium difluorooxalate borate

1. Technical field

The invention relates to a method for preparing an electrolyte lithium difluorooxalate borate LiODFB for a lithium ion battery and a lithium ion battery using the electrolyte.

2. Background technology

The electrolyte for lithium-ion batteries mainly uses lithium hexafluorophosphate, which has been commercially used for more than two decades. For example, CN200680042560.7 discloses a method for preparing an electrolyte solution for lithium-ion batteries. Lithium hexafluorophosphate is prepared by reacting phosphorus trichloride and lithium chloride in a non-aqueous organic solvent, and reacting the reaction product formed in the solvent with hydrogen fluoride.

However, lithium hexafluorophosphate LiPF ₆ has poor thermal stability and is prone to deliquescence, and thermal decomposition and deliquescence products will damage battery performance and pollute the environment. In addition, it must be combined with ethylene carbonate (EC) to form a stable solid electrolyte interfacial film (SEI film) on the surface of the carbon anode, and the melting point of EC is 37 °C, which severely limits the low-temperature performance of the battery. Boron-containing lithium salts have attracted extensive attention from researchers because of their good thermal stability and high electrical conductivity in common solvent systems. Therefore, the use of lithium borate as a dielectric for lithium-ion batteries has become a hot topic of current research. Dozens of boron-containing lithium salts have been studied, but the most promising ones are LiBF4, LiB(C2O4)2 (LiBOB for short) and LiBC2O4F2 (LiODFB for short).

Among the lithium borate salts, lithium tetrafluoroborate and lithium bisoxalate borate (LiBOB) were first proposed. Compared with the commercial lithium hexafluorophosphate LiPF ₆ , it has certain advantages, and has received great attention in the application of lithium-ion batteries: lithium bisoxalate borate has good thermal stability, and the thermal decomposition temperature can reach as high as 300 ° C-enhanced the battery safety; does not contain F elements, does not produce HF to corrode electrode materials and current collectors, improves the cycle life of the battery, and reduces the cost of the battery; it can form a relatively stable SEI film on the surface of the carbon negative electrode, which can be used in pure It is used in PC solvent, which broadens the temperature range of the battery; the synthetic raw materials are cheap and easy to obtain, the preparation process is simple, and it is environmentally friendly. The electrolyte prepared by lithium tetrafluoroborate LiBF4 has a lower charge transfer resistance (the main part of the internal resistance of the battery at low temperature), and the assembled battery has better low-temperature performance than LiPF6, which is the most widely used electrolyte in industry except LiPF6. Lithium salt. But the high temperature performance is slightly worse.

Lithiumoxalyldifluoroborate (also known as LiODFB, LiDFOB, LiFOB) was first proposed by American scholar ShengShui Zhang under the above conditions, CAS No: 409071-16-5, chemical formula LiBC ₂ O ₄ F ₂ , molecular weight 143.77 g/mol, decomposition temperature 240°C, see An unique lithium salt for their improved electrolyte of Li-ion battery, ShengShui Zhang Electrochemistry Communications8(2006) 1423-1428. And mentioned the reaction of boron trifluoride and lithium oxalate and purified by recrystallization. It is also foreseen that because LiODFB contains half of LiBF4 and half of LiBOB in its molecular structure, its properties are also well combined with the advantages of LiBF4 and LiBOB two lithium salts. You can also refer to the research progress of boron-containing lithium salts for lithium-ion battery electrolytes such as Cui Xiaoling from Tianjin University in China; Xie Hui et al., New lithium salts that can be used in lithium-ion batteries: LiODFB, etc.

The structural formula of LiODFB is as follows:

CN101648963 discloses a synthesis process for obtaining lithium difluorooxalate borate and lithium tetrafluoroborate, comprising the following steps: (1) compounding fluorine-containing compounds, boron-containing compounds, lithium-containing compounds and oxalate-containing compounds at 0 ～100℃, reaction pressure 0.1～1Mpa, and reaction in the reaction medium, wherein the molar ratio of lithium, fluorine, boron and oxalate ion is 2～3:5～6:2:1; The reaction solution of lithium oxalate borate and lithium tetrafluoroborate; (2) preliminary separation of lithium difluorooxalate borate and lithium tetrafluoroborate in the reaction solution, and then use an organic compound capable of extracting lithium difluorooxalate borate or lithium tetrafluoroborate The solvent is further extracted and separated; (3) Recrystallization and vacuum drying are carried out respectively to obtain battery-grade lithium difluorooxalate borate and lithium tetrafluoroborate. However, the above reaction cannot obtain the desired product smoothly.

3. Contents of the invention

The purpose of the present invention is to propose a method for preparing the electrolyte LiODFB for lithium ion batteries and a lithium ion battery using the electrolyte, especially to obtain a preparation method that is beneficial to industrialization and commercialization, and has conditions and excellent performance in line with the use of lithium power batteries. Quality, reaction conditions and quality of materials are easy to control, especially the use of environmentally friendly raw materials, no pollution during the preparation process.

The technical solution of the present invention is: a preparation method of lithium difluorobisoxalate borate, which is characterized in that lithium bisoxalate borate and lithium fluoride are stirred and mixed in a non-aqueous solvent, and the moles of lithium bisoxalate borate and lithium fluoride are The ratio is in the range of 1:1.90 to 1:2.10, the reaction time is 10-24 hours, and the reaction produces difluorobisoxalate lithium borate and lithium oxalate.

Further, in the production method of lithium difluorobisoxalate borate, the non-aqueous solvent is benzene, toluene or xylene, the reaction temperature is from 50-85°C; the reaction time is 10-24 hours, especially 12-16 hours.

Further, especially, the reflux heating reaction is carried out in a non-aqueous solvent such as benzene, toluene or xylene.

After the reaction, separation and purification by solvents such as EC, DMC, DEC, EMC, such as separating lithium bisoxalate borate and lithium oxalate from the mixture of difluorobisoxalate lithium borate and lithium oxalate, especially by dimethyl carbonate DMC, makes dioxalate The product mixture of lithium borate and lithium fluoride was dissolved in dimethyl carbonate DMC; the solution obtained by filtration was concentrated and vacuum-dried to obtain a white solid of lithium oxalate difluoroborate. The product was confirmed to be lithium oxalate difluoroborate by thermogravimetric analysis and NMR analysis.

The lithium difluorobisoxalate borate solution produced by this method has less content of chlorine compounds and free acids, and the water content is controllable (within 30ppm), so this electrolyte can be used to prepare lithium-ion batteries—especially large-capacity, high-rate discharge , high-temperature and low-temperature conditions use lithium power batteries with good performance, and use this as an effective additive for improving the performance of non-aqueous electrolyte lithium-ion batteries.

The beneficial effect of the present invention is that: the lithium salt LiODFB prepared by the present invention for lithium ion batteries has a unique chemical structure, which combines the advantages of lithium dioxalate borate (LiBOB) and lithium tetrafluoroborate (LiBF4). Compared with LiBOB, the solubility of LiODFB in carbonate and the viscosity of solvent have been significantly improved, so that the lithium-ion battery has better high and low temperature performance and rate discharge performance. Compared with LiBF4, LiODFB can promote the formation of a stable solid electrolyte interface (solid electrolyte interface, SEI), which improves the high-temperature performance of lithium-ion batteries. The novel lithium salt also has the following advantages: good chemical stability with lithium metal, good passivation of aluminum foil at high potential and improvement of safety performance and overcharge resistance of lithium ion batteries. These properties make Li ODFB a lithium salt that is very likely to replace LiPF6 for commercial use. Especially for power lithium batteries. The process circuit of the present invention is unique and environment-friendly, the process control range is wide, the two products of the reaction are easy to separate, and the electrolyte salt whose quality meets the requirements of lithium batteries is easy to prepare.

4. Description of drawings

Figure 1 is the curve of the number of charge and discharge cycles at 1C and the battery capacity (the capacity of the primary battery is 20Ah) of the battery prepared by adding 3% Li ODFB at 60°C; the upper root is ODFB, and the lower curve is compared with lithium hexafluorophosphate cycle.

Figure 2 is a curve of 1C-1C deep charge and discharge and battery capacity at a temperature of 45°C; the top line is the LIODFB electro-hydraulic, and the bottom three are the cycles of the comparative lithium hexafluorophosphate electro-hydraulic.

5. Specific implementation

General reaction formula of the present invention is as follows:

LiBC ₄ O ₈ +2LiF＝LiBC ₂ O ₄ F ₂ +Li ₂ C ₂ O ₄

Bisoxalate lithium borate + 2 lithium fluoride = difluorobisoxalate lithium borate + lithium oxalate; bisoxalate lithium borate and lithium oxalate are mixed and reacted with stirring in a non-aqueous solvent, and the molar ratio of bisoxalate lithium borate to lithium fluoride is 1: The range of 1.90～1:2.10 reacts to generate difluorobisoxalate lithium borate and lithium oxalate. The reaction is heated under reflux in benzene, toluene or xylene. After the reaction, the DMC solvent was separated and purified; the solution obtained by filtration was concentrated and vacuum-dried to obtain a white solid of lithium oxalic acid difluoroborate. The product was confirmed to be lithium oxalate difluoroborate by thermogravimetric analysis and NMR analysis. Lithium oxalate can be reused.

Embodiment 1,

Add 193.79g of lithium dioxalate borate and 51.88g of lithium fluoride into 500ml of toluene, stir and heat to reflux for 16h, filter to obtain a solid mixture of lithium oxalate difluoroborate and lithium oxalate, dissolve the product mixture in 500ml of diethyl carbonate and stir After 3h, the solution obtained by filtration was concentrated and vacuum-dried to obtain a white solid of lithium oxalate difluoroborate. The product 52g was confirmed to be lithium oxalate difluoroborate by thermogravimetric analysis and NMR analysis (the same data as ShengShui Zhang). In thermogravimetric analysis, the product begins to decompose in large quantities above 267°C.

Embodiment 2,

Add 203g of lithium dioxalate borate and 51.88g of lithium fluoride into 500ml of xylene, stir and heat to reflux for 12h, filter to obtain a solid mixture of lithium oxalate difluoroborate and lithium oxalate, dissolve the product mixture in 500ml of dimethyl carbonate and stir for 3h , The solution obtained by filtration was concentrated and vacuum-dried to obtain a white solid of lithium oxalate difluoroborate. 49 g of the product was confirmed to be lithium oxalate difluoroborate by thermogravimetric analysis and NMR analysis.

If there is no reflux heating, stirring and heating reaction, under similar reaction conditions (reaction time 12-16h, no essential difference in temperature control at 50°C, 60°C, 70°C, 85°C), the yield is slightly worse.

The main raw material is lithium dioxalate borate (lithium bisoxalate borate LiBOB) using the raw material of the following process, CAS No: 244761-29; C4BO8. Li molecular weight 193.79. It can be synthesized by solid-phase method. The preparation process uses oxalic acid, lithium hydroxide, and boric acid as raw materials. The ratio of the substances is 2.1:1:1. After being mixed by ball milling, it is fired at a high temperature. The firing temperature is 120°C and the dehydration temperature is . 240°C, the resulting product was purified by ethyl acetate to obtain the product. See Zhang Yao et al., Solid Phase Synthesis of Bisoxalate Lithium Borate, Inorganic Salt Industry 2011-4; the cost of raw materials is not high (its purity is over 99%), and the preparation process has no emissions and is environmentally friendly.

The solubility of LiODFB in common carbonate solvents is greater than that of LiBOB, but less than that of LiBF4. LiODFB electrolyte can maintain high conductivity in a wide temperature range: at high temperature, the conductivity of LiODFB electrolyte is close to LiBOB system, but higher than LiBF4. As shown in Figure 2, the number of 1C charge-discharge cycles and battery capacity under the condition of 45°C are obtained. The LiODFB electrolyte is significantly better than the cycle of the lithium hexafluorophosphate electrolyte.

Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Those skilled in the art of the present invention can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be defined by the claims.

Claims (2)

1. a kind of preparation method of difluorine oxalic acid boracic acid lithium is it is characterised in that di-oxalate lithium borate and fluorination in nonaqueous solvent Lithium stirs hybrid reaction, di-oxalate lithium borate and lithium fluoride and mol ratio be 1: 1.90～1: 2.10 scope, Reaction generates difluorine oxalic acid boracic acid lithium and lithium oxalate；Nonaqueous solvent is benzene, toluene or dimethylbenzene；Response time 12-16 hour；And Benzene, toluene or dimethylbenzene are carried out be heated at reflux reaction.

2. the difluorine oxalic acid boracic acid lithium according to claim 1 preparation method it is characterised in that reaction after through carbonic acid two The separating-purifying of methyl ester DMC solvent, the product mixing with di-oxalate lithium borate and lithium fluoride dissolves in dimethyl carbonate DMC；Filter Difluorine oxalic acid boracic acid lithium is obtained after obtaining the concentrated vacuum drying of solution.