Xiaobin Liu,Zhenguo Gao,Jingcai Cheng,Junbo Gong,3,,Jingkang Wang,3

1 State Key Laboratory of Chemical Engineering,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

2 CAS Key Laboratory of Green Process and Engineering,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

3 The Co-Innovation Center of Chemistry and Chemical Engineering of Tianjin,Tianjin 300072,China

Keywords:Battery chemicals Lithium-ion batteries Crystallization Fluorine-containing chemicals

ABSTRACT With the development of digital products,electric vehicles and energy storage technology,electronic chemicals play an increasingly prominent role in the field of new energy such as lithium-ion batteries.Electronic chemicals have attracted extensive attention in various fields.Characteristics of high-end electronic chemicals are high purity and low impurity content,which requires a very strict separation and purification process.At present,crystallization is a key technology for their separation and purification of electronic chemicals.In this work,the representative fluorine-containing compounds in cathode and anode materials,separator and electrolyte of lithium-ion batteries are introduced.The latest technologies for the preparation and purification of four kinds of fluorine-containing battery chemicals by crystallization technology are reviewed.In addition,the research prospects and suggestions are put forward for the separation of fluorine-containing battery chemicals.

1.Introduction

Electronic chemicals generally refer to the chemical materials used in the electronic industry.Electronic chemicals have a wide variety of characteristics,which can be divided into semiconductor(integrated circuit (IC)/discrete device/sensor),flat panel display(FPD),liquid crystal display (LCD)/organic light-emitting diode(OLED),printed circuit board (PCB) and new energy battery (NEB)according to their terminal applications [1,2].According to the varieties and specifications,it can be divided into semiconductor materials,magnetic materials and intermediates,capacitor chemicals and battery chemicals.In addition,electronic chemicals also have the characteristics of strong specificity,high technical threshold,strong functionality and strict quality requirements.In particular,the control of purity and metal ion content of electronic chemicals is very strict[3].For example,the content of metal ions(including Na,Mg,Ca,Fe,Ni,Al,K,etc.)in wet electronic chemicals specified by the Semiconductor Equipment and Material International (SEMI) should be at ppb (10-9g·ml-1) level [4].This is because the presence of impurity ions in electronic chemicals will significantly affect the synthesis of downstream products or the yield and performance of processed products,which leads to higher preparation difficulty of electronic chemicals.Therefore,chemicals used in the electronic industry need to reach the electronic grade.

As one of the important application fields of electronic chemicals,new energy battery has become a hot spot of scientific research [5].According to the China market share report of electronic chemicals used in various fields in 2018,China’s imports of the new energy battery industry accounts for 60%,as shown in Fig.1[6].Battery chemicals used in new energy cells can be mainly divided into lithium-ion battery chemicals,alkaline manganese battery chemicals,fuel cell chemicals,nickel-hydrogen battery chemicals,etc.Among them,the most mature and valuable technology is the lithium-ion battery,which mainly includes positive and negative electrodes,separator,binder and electrolyte[7].With the continuous upgrading of lithium-ion battery technology and the continuous development of new lithium-ion batteries,the preparation and purification technology of battery chemicals used in the field of lithium-ion batteries has attracted more and more attention.The further development and innovation of the technology of preparation and purification of battery chemicals will contribute to the further development of the lithium-ion battery field.

Fig.1.Market share of electronic chemicals used in NEB,PCB,FPD,IC and others in 2018 [6].

As a new type of chemical material with excellent performance,fluorine-containing chemicals can effectively improve the electrochemical performance of lithium-ion batteries[8].The fluorine element with high electronegativity in the cathode material of the battery is combined with the alkali metal or alkaline earth metal(lithium) with electronegativity in the anode material,which forms a large potential difference between the two poles,so that the fluorine series lithium-ion battery has a high energy density and voltage [9].The literature shows that lithium iron phosphate(LiFePO4) and ternary metal oxide (LiNi1-x-yCoxMnyO2) are the most mature cathode materials for lithium-ion batteries [10,11].The latest research progress shows that the fluorination of cathode materials or coating and doping with fluoride can enhance the anti hydrofluoric acid (HF) corrosion and anti-oxidation effect of electrode materials [12,13],and fluorine-containing lithium salt electrode may develop into a new generation of cathode materials,as shown in Table 1 [14].At present,many researchers have studied the electrode of lithium fluoride,and the performance of the battery has been effectively improved by improving the electrode[28].Separator and binder are key components in batteries.Table 1 lists the commonly used fluorine-containing chemicals for electrode binder and separator.Polyvinylidene fluoride (PVDF) is one of the most widely used separators and binders because of its wide working voltage,high oxidation–reduction stability,high temperature stability,good processability and easy film formation [29,30].

The electrolyte is a medium in which conductive ions shuttle between positive and negative electrodes during charging and discharging.The addition of fluorine in the electrolyte can make the lithium-ion battery have good overall performance and solid electrolyte interface(SEI)[31-33].It can also improve the low temperature and high temperature characteristics of lithium-ion batteries,avoid electrode self-discharge,and has the advantages of relatively non-toxic,no harm to the environment and low cost[34,35].Fluorine can make the usually combustible electrolyte salt become nonflammable,which helps to improve the safety of high voltage lithium-ion batteries [36,37].In addition,the addition of fluorine-containing high-purity solvents and functional additives can effectively improve the flame retardancy and stability of lithium-ion batteries,making lithium-ion batteries safer [38,39].The fluorine-containing chemicals in the electrolyte components reported in the literature are listed in Table 2.

Table 1 Electrode,separator and binder of fluorinated material

Table 2 Fluorine containing chemicals in electrolyte components

The electrochemical properties of fluorinated compounds as electrolytes for lithium-ion batteries reported in the literature show that LiPF6is still the most popular commercial electrolyte[87].LiBF4has become one of the potential substitutes due to its safety and stability at high temperatures[88].In addition,the solvent and functional additive,taking fluoroethylene carbonate(FEC)as an example,are the key factors to improve the safety performance of lithium batteries and have attracted the attention of researchers [89-91].At present,lithium-ion batteries have been widely used in various fields,and all countries have formulated the industrial policy goal of the next generation of lithium-ion batteries.The further development of the preparation and purification technology of fluorine-containing chemicals in lithium-ion batteries is the only way to achieve this goal.

In this paper,the preparation and purification techniques of PVDF,LiPF6,LiBF4and FEC,which are representative fluorinecontaining materials in electrode and separator materials,electrolyte salts and additives of lithium-ion batteries,are reviewed and discussed.It also provides prospects and possible strategies for the further development of the purification technology of fluorine-containing chemicals in lithium-ion batteries,so that lithium-ion batteries with different formulations have a long life,high energy density,high power and sufficient safety under competitive manufacturing costs.The summary of fluorinecontaining chemicals in lithium-ion batteries will encourage further research in the promising field of rechargeable advanced lithium-ion battery materials.

2.Preparation of Fluorine-Containing Lithium-Ion Battery Chemicals

Four kinds of fluorine-containing chemicals,PVDF,LiPF6,LiBF4and FEC,used in lithium-ion batteries are introduced,and the basic preparation methods of these fluorine-containing lithium-ion battery chemicals are reviewed.

2.1.Binder and separator:PVDF

PVDF was first commercialized by pennwalt company in 1961,and its preparation technology was basically mature in 1970s[92].At present,the commonly used industrial synthesis methods are emulsion polymerization and suspension polymerization.The reaction equation is as follows:

The structure of PVDF molecular chain is composed of -CH2--CF2-repeat unit.Due to the asymmetry of VDF monomer structure,there are several monomer connection modes in the polymerization process,such as head–tail connection(-CH2-CF2--CH2-CF2-),tail–tail connection (-CF2-CH2-CH2-CF2-),headhead connection (-CH2-CF2-CF2-CH2-) and some branched structures.The head and tail structures of PVDF are disadvantageous to the subsequent crystallization and purification because they destroy the regular structure of PVDF.

2.2.Electrolyte salt:LiPF6

LiPF6is a kind of white crystal in solid state (Fig.2) or powder with strong deliquescence properties.It is easily soluble in water and soluble in organic solvents such as low concentration methanol,ethanol,acetone and carbonate.LiPF6is the most widely used electrolyte for lithium batteries,which has the characteristics of relative safety and high ionic conductivity [93,94].

The main synthetic methods of LiPF6include the gas–solid method,hydrogen fluoride solvent method,organic solvent method and complex method.The reaction equations,reaction conditions,advantages and disadvantages of each method are given in Table 3.At present,the high impurity content of LiPF6is still the key to limit its application and promotion.The purity of the product obtained by the hydrogen fluoride solvent method is high,so it has become the most commonly used synthesis method of LiPF6in industry.

2.3.Electrolyte salt:LiBF4

LiBF4is a kind of lithium salt electrolyte,which has the advantages of insensitive to water in the environment,good stability,low toxicity,high safety,and can form a surface oxide film on the surface of the electrode to prevent corrosion [105,106].LiBF4crystal belongs to space group p3121 of the cubic system,in which BF4-is similar to XY4molecular type and has four vibration modes,as shown in Fig.3 [107,108].

Fig.2.Schematic diagram of LiPF6 crystal structure [95].

There are four main methods to prepare LiBF4:gas–solid reaction method,aqueous solution method,organic solvent method and hydrogen fluoride solution reaction method,as shown in Table 4.At present,LiBF4is mainly prepared by the organic solvent method and hydrogen fluoride solution reaction method.Because the purity of LiBF4synthesized by these methods is not high,the subsequent purification process is still needed to realize the production of electronic grade LiBF4.

2.4.Functional additive:FEC

FEC is colorless and transparent liquid under normal pressure,easy to absorb water,and solid in winter.FEC can be used as an additive in the electrolyte of Li-ion battery,which can inhibit the decomposition of electrolyte,reduce the impedance of the battery,improve its low temperature resistance,and significantly improve the specific capacity and cycle stability of the battery;It can be used as electrolyte solvent to improve the charge–discharge cycle characteristics and current efficiency of secondary batteries,capacitors and other chemical devices [115,116].There are three main preparation methods of FEC:direct fluorination,electrochemical fluorination and halogen exchange.The reaction equations,reaction conditions,advantages and disadvantages of each method are given in Table 5.At present,the most widely used preparation process in the industry is the halogen exchange method.

Table 3 Synthesis method,reaction equation,reaction conditions,advantages and disadvantages of LiPF6

Table 4 Synthesis method,reaction equation,reaction conditions,advantages and disadvantages of LiBF4

Table 5 Synthesis method,reaction equation,reaction conditions,advantages and disadvantages of FEC

3.Preparation and Purification of Fluorine Containing Chemicals for Lithium-Ion Batteries

The chemicals used in lithium-ion batteries have strict requirements on the purity,electrochemical properties and metal impurity content.The fluorine-containing chemicals obtained by the above basic preparation methods cannot meet these conditions.Therefore,many researchers have improved the preparation methods of these chemicals by crystallization and other preparation and purification processes.So that the purity of fluorine-containing chemicals can reach the electronic grade requirement.In this section,the preparation,purification process and electrochemicalproperties of electronic grade PVDF,LiPF6,LiBF4and FEC were reviewed.size less than 100 μm was obtained(Fig.4).It was used as the binder for a lithium battery.The crystallinity of the modified particles was 48.07% and 53.82%,respectively.

Fig.3.Four vibration modes of BF4- [107,108].

Fig.4.Structure of surface modified PVDF with core–shell structure [133].

3.1.Binder and separator:PVDF

PVDF is a semi-crystalline polymer,and the crystallinity is generally about 40% [126,127].When PVDF is used as membrane in battery,the higher its crystallinity is,the more difficult ion migration is,which will easily lead to the increase of internal resistance of the battery.At the same time,the higher the crystallinity,the internal stress of the electrode will make the active material of the electrode partially or completely peel off from the collector,resulting in the deterioration of capacity [128,129].For PVDF as binder,its characteristics are high thermal stability,electrochemistry and adhesion[130].The adhesion increased with the increase of crystallinity[131].Therefore,in addition to the requirements of high purity,we should also pay attention to its crystallinity in the preparation of PVDF.

Wang et al.[132] reported a preparation method of PVDF for lithium battery binder.PVDF was synthesized by polymerization and crystallization of water,emulsifier (perfluorooctanoic acid),vinylidene fluoride monomer,initiator(persulfate(potassium persulfate,ammonium persulfate or sodium persulfate)or di tert butyl peroxide).The test results of the three samples were shown in Table 6.The results showed that the crystallinity of PVDF prepared by this method was lower than that of PVDF on the market,and it had a higher electrochemical stability voltage,which was about 0.2 V higher than that of PVDF on the market.

Table 6 Parameter comparison between PVDF synthesized by Chinese invention patent CN104530276B rand commercial PVDF

Yang et al.[133]reported a core–shell structure lithium battery binder with PVDF surface modification and its preparation method.PVDF particles were prepared by suspension polymerization,and then PVDF particles were copolymerized by vinyl ester monomer.The surface modified PVDF with core–shell structure with particle

3.2.Electrolyte salt:LiPF6

3.2.1.Preparation of LiPF6by improved solvent method

Zhao improved the preparation of LiPF6by HF solvent method[134].The process includes six parts:anhydrous hydrogen fluoride(AHF) refining,LiF synthesis,PCl5synthesis,LiPF6synthesis,LiPF6crystallization/washing/drying and post-treatment.The electronic grade LiPF6crystal with high quality and consistent crystal structure was obtained by gradient temperature control and ultrasonic assisted crystallization.The total reaction equation was shown in Eq.(2).The synthesized LiPF6crystal was characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD).The results showed that the product had high crystallinity and does not contain other impurities.LiPF6was dissolved in the mixed solution of ethylene carbonate (EC):diethyl carbonate (DEC):dimethyl carbonate (DMC)=1:1:1 to form 1 mol·L-1solution,which was used as the electrolyte of high capacity lithium ion container to investigate its electrochemical performance.In asymmetric Li-ion capacitors,EDLC was formed whenions are adsorbed and desorbed by AC cathode.

Liu et al.[135]used calciumfluoride(CaF2)instead of HF to react with PCl5at 350°C to produce PF5,and then added the PF5into the LiF suspension in the prepared nitrile to produce LiPF6.The reactions were expressed by Eqs.(3) and (4).The results showed that the purity of LiPF6was 99.9%,the content of metal impurities was ppm(10-6g·ml-1)level,such as Na,K and Ca were lower than the required value.

Liu et al.[136]prepared PF5from CaF2and phosphorus pentoxide (P2O5) at 280 °C.LiPF6was synthesized by the reaction of LiF and PF5in acetonitrile (C2H3N) at room temperature.The synthesized LiPF6was characterized by infrared spectroscopy (IR) and XRD.The results of atomic absorption spectrometry and ion chromatography showed that the purity of LiPF6was 99.98%.The results showed that compared with commercial LiPF6,the selfmade LiPF6had higher purity,lower impurity content and better thermal stability.

3.2.2.Preparation of LiPF6by improved complexing method

Tian improved the preparation process of lithium hexafluorophosphate by complexing method(Eqs.(5)–(7))[137].The crystallized product was confirmed by IR and XRD.The purity of LiPF6determined by atomic absorption spectrometry was more than 99.9%.The LiPF6synthesized by this method was dissolved in EC and DMC,and the electrolyte was obtained based on the optimal ratio.LiCoO2was used as cathode material to study the charge–discharge and cycle life of Li-ion battery.The results showed that the initial charge and discharge capacities were 144.3 mA·h·g-1and 144.6 mA·h·g-1,respectively.After 20 cycles,the capacity of the battery was about 88%.

Zheng et al.[138] used pyridine,hexafluorophosphoric acid(HPF6) and lithium hydroxide as raw materials to prepare electronic grade LiPF6by complexing the dissociation method.The synthesis route was as follows:firstly,pyridine reacts with HPF6to form pyridine hexafluorophosphoric acid,then lithium pyridine hexafluorophosphate was formed by the exchange of pyridine hexafluorophosphoric acid with lithium hydroxide,and lithium pyridine hexafluorophosphate was decomposed under room temperature and negative pressure to obtain LiPF6.The structure of the product was analyzed by nuclear magnetic resonance(NMR) and XRD.The content of lithium ion in the product was determined by atomic absorption spectrophotometer (AAS).The purity of the product was obtained by further conversion.The content of Fe,K,Na and other impurities in the product was determined by flame AAS.Compared with the Chinese national standard HG/T4066-2008 (Table 7),the product with high purity can be obtained by the reported process.

Table 7 Comparison of parameters between LiPF6 product synthesized by Zheng et al.and Chinese national standard

3.2.3.Purification of LiPF6by crystallization and its application

The purification process of electronic grade LiPF6has been widely concerned.In order to reduce the acid and metal impurities inside or attached on the surface of LiPF6crystal,Huang et al.[139]disclosed the purification method of LiPF6.In this method,LiPF6was dissolved in carbonate and(or)ether solvent under the protection of dehydrated protective gas.0.1–10 volume of alkane or ether solvent was added to the transparent liquid obtained by filtering out insoluble matter to make LiPF6nucleate and crystallize.Repeat the above steps,and finally,it can get high purity LiPF6by vacuum drying.The inorganic and organic impurities in LiPF6can be removed by repeated crystallization,but the residual HF impurities in LiPF6cannot be removed.

Li et al.[140]disclosed a purification method of LiPF6.The crude LiPF6was purified with ether solvent,carbonate solvent or C2H3N as the organic solvent,and the insoluble substances in LiPF6were removed.Then the free acid was removed by adding silazane to react with residual HF acid in the crude LiPF6.The metal impurities were removed by cooling crystallization and vacuum crystallization,and the crystallization rate and crystal quality of LiPF6were improved.This purification method can effectively remove the acid and metal impurities in the interior or on the surface of LiPF6crystal,reduce the content of insoluble matter,and improve the purity of LiPF6to more than 99.9%.

In order to solve the problem of low purity of LiPF6in the existing technology,Mo et al.[141]provided a LiPF6and its crystallization and preparation method for lithium-ion battery electrolyte and lithium-ion battery.The LiPF6mother liquor was obtained by hydrogen fluoride solvent or organic solvent method.The LiPF6crystal with the purity of 99.9% was prepared by adjusting the operating conditions such as the speed of mother liquid dropping into the mould,stirring speed,cooling time,cooling rate and ultrasonic frequency.SEM characterization showed that the crystal of the product was regular hexagonal cube.The particle size of the crystal was 10–230 μm.The prepared LiPF6was prepared with EC and DMC as 1 mol·L-1LiPF6/(EC+DMC) electrolyte,and the lithium-ion battery of iron phosphate was assembled.The electrochemical test results showed that with the increase of discharge ratio,the capacity retention rate of the battery is more than 90%,which met the requirements of high rate impulse discharge.The lithium-ion battery had better cycling performance at room temperature.After 500 cycles,the capacity retention rate of the battery was over 88.69%.

In to the above method of improving the original process,electrochemical extraction technology can also be used to prepare chemicals [142-147].It is a substance required for preparation by extracting/releasing ions from different solvents using different electrodes.According to the research,when lithium manganese oxide (LiMn2O4) is used as the electrode material,the selectivity of extracting Li+is high to ensure the requirements of high purity[142-144].This is expected to solve the problem of high cost and high energy consumption for synthesizing LiPF6using traditional methods.

Zhao et al.[148] assembled a double electrolytic cell with LiMn2O4(cathode) and polyaniline (PANI,anode).LiPF6was prepared by ion extraction and ion release.Firstly,LiMn2O4and PANI+-electrodes(ion extraction)were obtained by discharging in an anode chamber filled with potassium hexafluorophosphate (KPF6)aqueous solution and a cathode chamber with lithium chloride(LiCl) aqueous solution as electrolyte.Then put the obtained electrodes into the organic electrolyte under the condition of full charge (ion release).After the two electrodes were washed in N3-dimethylpropane-1,3-diamine (EDC) for 3 times,the synthesized LiPF6electrolyte was concentrated at 60°C.LiFePO4/graphite battery was assembled by dissolving it in EC+DEC to measure its electrochemical performance.The results show that LiPF6prepared by this method is similar to commercial high-purity LiPF6for the reversible capacity and cycle stability of the two electrodes.

3.3.Electrolyte salt:LiBF4

3.3.1.Improved solvent method for preparation of LiBF4

Liu et al.[149] developed a new solvent method to prepare LiBF4.Using anhydrous C2H3N,LiF and sodium fluoborate (NaBF4)as raw materials,BF3gas with high purity was prepared by heating decomposition of NaBF4.LiBF4-C2H3N solution was obtained by reaction of BF3and self-made high purity LiF in CH3CN solvent.The solution was cooled and crystallized at-20°C to obtain crude LiBF4.After being dissolved and purified in C2H3N,LiBF4was dried in vacuum at 80°C for 24 h to obtain white powder product.Infrared detection and XRD analysis showed that the material is LiBF4(Fig.5).At the same time,the crystal structure of the samples was determined by XRD data.The impurity in the product detected by ion chromatography showed that there were no F-,and other impurity ions.This method effectively avoids the problems of high toxicity,high energy consumption and high impurity content when HF is used as solvent.

Mochida et al.[150] disclosed a process for preparing LiBF4by the reaction of LiF with BF3in non-aqueous organic solvents (tetrahydrofuran,dimethoxyethane,ethyl acetate and propylene carbonate).LiBF4had good solubility in nonaqueous organic solvents and can form complex with BF3.The LiF and BF3impurities were filtered out after the reaction.The saturated solution of BF3in nonaqueous organic solvent was prepared to reduce the solubility of LiBF4and precipitate LiBF4crystals.Anhydrous LiBF4crystal can also be obtained by evaporation crystallization.Because the solubility of LiBF4in ether was only 13 g·L-1at 250 °C,ether was not used in this process.

Zhao et al.[151] studied the reaction of anhydrous lithium sulphate (Li2SO3) and boron fluoride ether (BF3·O(C2H5)2) to prepare LiBF4.Firstly,anhydrous Li2SO3solid was prepared and dissolved in anhydrous ether.Then BF3·O(C2H5)2solution was added to the above solution drop by drop.After reaction at 25 °C for 6 h,LiBF4was obtained by filtration,distillation,concentration,cooling crystallization and drying (155 °C).The nonaqueous solvent method invented by Korean was to react BF3ether complex in organic solvent with LiF to form solid LiBF4[152].Then solid LiBF4was dissolved in AHF and cooled to -10 to -30 °C.F2gas was introduced for the stirring and recrystallization process.The process can remove metal impurities and lithium carbonate with high purity and high yield.

3.3.2.Preparation of LiBF4by improved solution method

Lan et al.[153] provided a new solution synthesis process to prepare electronic grade LiBF4using HF,boric acid (H3BO3) and lithium carbonate (Li2CO3) as raw materials.Firstly,H3BO3was dripped into HF at a constant rate to prepare tetrafluoroboric acid aqueous solution,which reacted with Li2CO3and then evaporated to obtain LiBF4.The lithium ion and other metal impurity ions in LiBF4products were quantitatively analyzed by atomic emission spectrometry,and the purity was analyzed by atomic absorption spectrometry.The purity and yield of LiBF4were 99.59% and 90.08%,respectively.The results showed that the metal impurity content meets the quality requirements of electronic LiBF4(Table 8).

Table 8 Metal impurity contents of LiBF4 products synthesized by Lan et al.[153] and electronic LiBF4 products

3.3.3.Preparation of LiBF4by improved hydrogen fluoride solution reaction method

Sang et al.[155]prepared LiBF4by HF solution reaction method.The production of LiBF4was speculated by thermogravimetric analysis,and then the impurities and purity of the product were checked by ion chromatography.The effects of LiF concentration and evaporation crystallization time on the yield of LiBF4were explored.The analysis results showed that the impurity content Angaiah et al.[154]disclosed a process for the preparation of LiBF4,which reacted with HF by saturated LiBO2/LiN2O4or LiBO2aqueous solution.After the reaction,the high purity LiBF4product was obtained by the crystallization method.of LiBF4was ppm level,and the purity of LiBF4was more than 99.9%,which met the quality requirements of electrolyte.

Fig.5.XRD pattern of LiBF4 (a) and standard ion chromatograph of F-,(b) [149].

Yu et al.[156]developed a new process for preparing electronic grade LiBF4by anhydrous HF solvent method in view of the high water content and insoluble matter content of electronic LiBF4.After LiF/AHF solution preparation,absorption reaction,evaporation crystallization,filtration and drying,electronic LiBF4product was obtained.The product characterization results showed that the water,Cr,Fe,Mg,Na,Ni and metal impurity ions were ppm level.They all met the requirements of electronic products.

3.4.Functional additive:FEC

3.4.1.Preparation of electronic grade FEC by improved halogen method

Niu[157]proposed a stable,efficient and high yield method for the preparation of electronic grade FEC by halogen substitution.The purity of industrial FEC was improved by distillation.Under the protection of argon,high-purity fluoride salt with the molar ratio of 1:(1.1–1.3),high-purity FEC and inorganic deacidification agent were added into the reactor.The halogen replacement reaction was carried out at 35–130 °C for 8–15 h.Finally,the product was subjected to vacuum distillation,dehydration and recrystallization to obtain electronic grade product.The preparation of electronic grade FEC without solvent,initiator and catalyst was realized.

Weng et al.[158]disclosed a preparation method of high purity FEC.In this method,metal fluoride and vinyl chloride carbonate were used as raw materials to prepare FEC.FEC with the purity of 99.96% was obtained by recrystallization at -10 °C to 0 °C.The mass ratio of recrystallization solvent to FEC was 1:1–1:7.The recrystallization solvent was one of ether,tert butyl methyl ether,hexafluoropropyl ether and hexafluoropropyl ether.The yield of FEC was 92%.

Wang et al.[159] provided an improved method for preparing FEC for application in lithium ion batteries.Alkali metal fluorohydrides (anhydrous potassium hydrofluoride (KHF2) and/or sodium hydrofluoride (NaHF2)),quaternary phosphonium salts (tetraphenyl phosphine bromide (TPPB) and/or tetra (diethylamine)phosphonium bromide (TPB)) and organic solvents (DEC,C2H3N,etc.)are added to the reaction vessel for mixing.When the temperature was rises to 45–100 °C,chloroethylene carbonate is added into the reaction vessel for reaction.FEC products were obtained by filtration,desolvation,reduced pressure distillation (100–125°C) and melt crystallization (crystallization temperature 12–19°C,sweating temperature 20–23 °C).The purity of this product is more than 99.99% and the yield is more than 90%.This process has the advantages of mild reaction conditions,good safety and short reaction time,which is conducive to industrial application.

3.4.2.Preparation of electronic grade FEC by improved direct fluorination method

Hou et al.[160] disclosed a preparation method for high purity FEC,which used chloroethylene carbonate and AHF liquid as raw materials to carry out liquid–liquid reaction.High purity FEC was prepared by deacidification,crystallization at low temperature and dehydration at high temperature.The purity was more than 99.5% and and the yield was more than 95% for the product.The corrosion-resistant equipment used in this method has a simple structure and low cost,which was suitable for industrial application.

Lin et al.[161] disclosed a preparation method of FEC.The FEC crude product was prepared by reacting vinyl carbonate with chloro-gas with AHF.FEC with the purity of 99% was obtained by decolorization and distillation.Then,the FEC crystal with the purity of 99.95% was obtained by adding crystallization solvent (e.g.toluene,ethylbenzene,n-hexane,dimethyl carbonate and cyclohexane) to the FEC obtained in the distillation kettle.Then the FEC crystal was obtained by cooling and crystallization.The method was simple to operate,easy to control,less pollution to the environment shortens the reaction time and improves the conversion rate of FEC.

3.4.3.Purification of electronic grade FEC by crystallization and its application

According to the high purity requirements of electronic grade FEC,Shu et al.[162] developed a method of fractional crystallization to prepare electronic grade FEC.Different purity FEC produced in the distillation process was collected by stages.For the middle distillate product (purity ≥99.5%),first crystallization is carried out;second stage crystallization is carried out for the product from the pre-distillation(the fraction collected after distillation);for the product (purity ＜99.5%) produced from post distillation,threestage crystallization was carried out.Then distillation was carried out to remove materials.Finally,the FEC with chromatographic purity of 99.99% can be obtained by decolorization and dehydration.

4.Conclusions and Prospect

At present,some fluorochemicals such as LiPF6and PVDF have been produced on a large scale by many enterprises.In the process of industrialization,the existing equipment needs to be designed or improved according to the reaction.Because some of the processes involved are still using strong corrosive solvents such as HF,it can increase the content of acids and metal elements in the final fluorine-containing battery chemicals.When studying the preparation and purification by crystallization,it is necessary to pay attention to the uniformity of heat transfer in the amplification process,the stirring mode (uneven stirring will lead to the accumulation of solids at the bottom),the addition of crystal seeds and other factors.This will affect the purity,dispersion and uniformity of the product.To solve the above problems,the following methods can be adopted:(1)Select suitable crystal seeds.It is necessary to explore the influence of seed particle size on product performance;(2) Fully heated with draft tube;(3) Optimize the stirring device in the reactor,such as high shear mixer.

For industrial production using electrochemical methods,electrode materials are an important limiting factor.It is necessary to develop electrode materials with low production cost and optimize battery design to realize large-scale production of fluorinecontaining battery chemicals

In conclusion,significant progress has been made in the preparation and purification of PVDF,LiPF6,LiBF4and FEC.This will lay a foundation for future electronic technology innovation and the improvement of the electrochemical performance of lithium-ion batteries.In the face of the increasing demand for high-quality and environment-friendly battery chemicals,there are challenges and opportunities in the development in the future.

(1) Development of the green crystallization process.According to the preparation and purification process of four fluorinecontaining battery chemicals,toxic and corrosive HF solvent would be used in the commercialized process.The next development direction can focus on the source of green,according to the source of elements to find suitable raw materials to prepare electronic grade chemicals.

(2) In order to meet the requirement of ppb level in battery chemicals,it is necessary to obtain high purity raw material.In addition to the current crystallization purification methods such as evaporation crystallization and cooling crystallization,melting crystallization [163-165] and sublimation crystallization [166-169] can also be considered to improve the purification process.In addition,we should also pay attention to the regulation of its internal crystal structure and external morphology.By exploring the effect of impurities on cluster aggregation,crystal nucleation and growth,as well as the inclusion behavior of impurities in the process of particle coalescence and scaling,the molecular crystal particle multi-scale mechanism of impurities affecting the crystallization process is revealed,and the online crystallization feedback control system based on impurity ion purification and the theoretical model of selective adsorption and inclusion of impurities are constructed.

(3) At present,there are few kinds of fluorine-containing chemicals that can be used in lithium-ion batteries.The development and preparation of new fluorine-containing chemicals for electrode materials,separator and electrolyte composition of lithium batteries (lithium-ion batteries,lithiumsulfur batteries,lithium-air batteries and so on) are the directions of further development of new energy batteries in the future.In addition,sodium ion battery has the advantages of similar electrochemical behavior as lithium ion battery and low cost,but its large ion radius leads to slow ion diffusion [170,171].Fluorine containing battery chemicals or modified fluorine containing battery chemicals are helpful to improve the above phenomenon [172,173].It is also very important to develop fluorine-containing chemicals for sodium ion batteries.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (2193000361).

Nomenclature

M relative molecular mass

Mw/Mnmolecular weight distribution coefficient,%

Rininternal resistance in battery,mΩ

UMax-stablemaximum stable electrochemical voltage,V

η rotary viscosity,MPa·s