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Home , Difluorophosphate, Difluorophosphoric acid, Monofluorophosphate

(19) TZZ _T

(11) EP 2 826 747 A1

(12) EUROPEAN PATENT APPLICATION published in accordance with Art. 153(4) EPC

(43) Date of publication: (51) Int Cl.: 21.01.2015 Bulletin 2015/04 C01B 25/455 (2006.01)

(21) Application number: 12871541.4 (86) International application number: PCT/JP2012/057408 (22) Date of filing: 14.03.2012 (87) International publication number: WO 2013/136533 (19.09.2013 Gazette 2013/38)

(84) Designated Contracting States: • SHOGAMI, Kazuhiko AL AT BE BG CH CY CZ DE DK EE ES FI FR GB Izumiotsu-shi GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO Osaka 595-0075 (JP) PL PT RO RS SE SI SK SM TR • SATOH, Tomoya Designated Extension States: Izumiotsu-shi BA ME Osaka 595-0075 (JP)

(71) Applicant: Stella Chemifa Corporation (74) Representative: Winter, Brandl, Fürniss, Hübner, Osaka-shi, Osaka 541-0047 (JP) Röss, Kaiser, Polte - Partnerschaft mbB Patent- und Rechtsanwaltskanzlei (72) Inventors: Alois-Steinecker-Strasse 22 • NISHIDA,Tetsuo 85354 Freising (DE) Izumiotsu-shi Osaka 595-0075 (JP)

(54) METHOD FOR PRODUCING

(57) A process for preparing difluorophosphate com- from the by solid-liquid separa- prising reacting difluorophosphoric acid with at least one tion, the precipitate being precipitated by crystallization salt, as a raw material, selected from a halide salt, a car- operation in the difluorophosphoric acid, and removing bonate, a , a hydroxide and an oxide of an the difluorophosphoric acid contained in the precipitate alkali metal, an alkaline earth metal or an onium in the by distillation to obtain difluorophosphate. difluoraphosphoric acid, then separating a precipitate EP 2 826 747 A1

Printed by Jouve, 75001 PARIS (FR) 1 EP 2 826 747 A1 2

Description Literature 2). [0005] In Non-patent Literature 1, 1-ethyl-3-methylim- TECHNICAL FIELD idazolium chloride is reacted with potassium difluoro- phosphate in acetone, potassium chloride which is a by- [0001] The present invention relates to a process for 5 product is removed by filtration, the resulting acetone so- preparing a difluorophosphate. lution is passed through an alumina column, and acetone is distilled off to obtain 1-ethyl-3-methylimidazolium dif- BACKGROUND ART luorophosphate. Performance of a battery or a capacitor is remarkably influenced by impurities in an electrolytic [0002] In recent years, a salt having a melting point at 10 solution, and thus it is desirable to reduce impurities as near room temperature or a salt having a melting point low as possible when an ionic liquid is used as an elec- less than room temperature (ionic liquid) is found. The trolytic solution. Ionic liquids are hardly volatile and are ionic liquid is comprised of a cation and an anion, and liquid state in a wide temperature range. It is difficult to exists as a liquid state even at room temperature because reduce impurities in the ionic liquid by purification such a bonding strength is very weak. When the structures of 15 as distillation or recrystallization. Therefore, it is neces- a cation and an anion are designed as a bonding strength sary to use a starting material having high purity in order become poor, it is possible to vary a melting point of the to prepare an ionic liquid having high purity. Potassium salt and obtain an ionic liquid. Furthermore, it is said that difluorophosphate used in Non-patent Literature 1 is de- the properties of an ionic liquid can be controlled inten- sirable to contain impurities as low as possible. tionally by varying a combination of a cation and an anion, 20 [0006] Processes for preparing difluorophosphoric ac- or introducing a substituent into each ion. id salt are disclosed, for example, in Patent Literatures [0003] Ionic liquids are hard to volatilize and have the 1 to 5 and Non-patent Literatures 3 to 7. characteristic that they exist stably to high temperatures [0007] In Patent Literature 1, a process is disclosed for more than several hundred degrees centigrade. Ionic liq- preparing potassium difluorophosphate by mixing and uids differ from so-called "liquid" such as water or organic 25 melting potassium and potassium solvent in characteristics and are called "third liquid". Use metaphosphate. However, this process is not deemed of the ionic liquids as a lubricant or application to a reac- an excellent process in view of product purity and pro- tion solvent or extraction separation medium are inves- ductivity because of contamination of impurities from a tigated employing hard volatilization and excellent ther- melting pot and high-temperature environment of 700 °C. mal stability of the liquids. In addition, the ionic liquid is 30 [0008] Patent Literatures 2 to 5 disclose processes for salt and has ionic conductivity because it is comprised preparing lithium difluorophosphate by reacting lithium only of ions. Therefore, it is possible to use ionic liquid hexafluorophosphate or phosphorus pentafluoride with itself as an electrolytic solution. Investigation is actively any of lithium metaphosphate, silicon dioxide or lithium conducted for using an ionic liquid as an electrolytic so- carbonate in an organic electrolytic solution. However, lution of a battery or a capacitor, or as a plating bath. 35 these reactions require 40 to 170 hours for obtaining di- Conventionally an aqueous electrolytic solution or an or- salt and are not suitable for indus- ganic electrolytic solution has been used for an electro- trial production. lytic solution of a battery and a capacitor, the aqueous [0009] Non-patent Literature 3 or 4 discloses a process electrolytic solution will be restricted in water decompo- for preparing difluorophosphoric acid salt by reacting di- sition voltage, and the organic electrolytic solution en- 40 phosphorus pentaoxide with ammonium or acid counters to a problem in heat resistance and safety. Ionic . However, these processes produce a liquid are preferable in view of safety because they are lot of monofluophosphate, phosphate and water as by- incombustible and nonvolatile, and are also high in elec- products, require severe purification process and are not trochemical stability, they are suitable as an electrolytic effective methods. Non-patent Literature 5 discloses a solution of an electric double layer capacitor or a battery 45 process for preparing difluorophosphoric acid salt by re- to use under particularly high temperature environment. acting P2O3F4 (difluorophosphoric acid anhydride) with [0004] Ionic liquids composed of various cations and oxide or hydroxide such as Li2O or LiOH. However, dif- anions are investigated in order to apply ion liquids as luorophosphoric acid anhydride used herein is very ex- an electrolytic solution of a battery and a capacitor. Re- pensive and high-purity one is hardly available, and thus cently, thecharacteristic of an ionic liquid whichis 1-ethyl- 50 this process is unfavorable for industrial production. 3-methylimidazolium difluorophosphate having difluoro- [0010] Non-patent Literature 6 discloses a process for phosphate as an anion was reported (Non-patent Liter- preparing difluorophosphoric acid salt by reacting alkali ature 1). It is disclosed that 1-ethyl-3-methylimidazolium metal chloride with excess of difluorophosphoric acid and difluorophosphate is equal in electrical conductivity and removing hydrogen chloride (by-product) and unreacted voltage resistance to 1-ethyl-3-methylimidazolium55 difluorophosphoric acid by drying with heat at a reduced tetrafluoroborate which is known as a representative ion- pressure. However, it is difficult to obtain difluorophos- ic liquid, and is reported that it is suitable as an electrolytic phoric acid salt having high purity even if starting difluor- solution of an electric double layer capacitor (Non-patent ophosphoric acid having high purity is used, since a lot

2 3 EP 2 826 747 A1 4 of monofluorophosphoric acidsalt or fluoridesalt remains Non-patent Literature as impurities in the desired difluorophosphoric acid salt. [0011] Non-patent Literature 7 discloses a process for [0014] preparing potassium difluorophosphate by melting and reacting potassium dihydrogenphosphate and ammoni- 5 Non-patent Literature 1: K. Matsumoto and R. Hag- um fluoride. The reaction temperature is about 170 °C iwara, Inorganic Chemstry, 2009, 48, 7350-7358 and is mild compared with the reaction condition of patent Non-patent Literature 2: No. 77, The Electrochemi- Literature 1,hence easy topractice industrially.However, cal Society of Japan, Preliminary report, 1I18 it is not effective in view of treatment of a large quantity Non-patent Literature 3: Ber. Dtsch. Chem., Ges. of by-produced ammonia gas and remaining of a large 10 B26 (1929) 786 quantity of . Thus, there is problem in Non-patent Literature 4: Zh. Neorgan. Khim., 7 the purity of the final product. (1962) 1313 [0012] Further, difluorophosphoric acid salt having Non-patent Literature 5: Journal of Fluorine Chem- high purity is usable not only as a starting material for an istry, 38 (1988) 297-302 ionic liquid but as an additive for an electrolytic solution 15 Non-patent Literature 6: Inorganic Nuclear Chemis- of a lithium secondary battery. In recent years, in the try Letters, vol. 5 (1969) 581-585 application field of the lithium secondary battery, a further Non-patent Literature 7: The Japan Society for An- technological advance is seen in the improvement of out- alytical Chemistry, 43th Annual Meeting, Lecture put density and the energy density, and the restriction of summary, 536 (1994) capacity loss with use expansion of the electronic equip- 20 ment such as a mobile phone, personal computer, digital SUMMARY OF THE INVENTION camera toone in-vehicleuse. Particularly,since the prod- ucts in-vehicle use might be exposed to the environment PROBLEM TO BE SOLVED BY THE INVENTION that is more severe than those of consumer products use, high reliability is required in a life cycle and storage char- 25 [0015] An object of the present invention is to provide acteristics of the products. A non-aqueous electrolytic a process for preparing a difluorophosphoric acid salt solution in which a lithium salt is dissolved in an organic having high purity which is usable as a starting material solvent is used as an electrolytic solution for a lithium for preparing an ionic liquid and as an additive for an secondary battery. Because the non-aqueous electrolyt- electrolytic solution of a lithium secondary battery. ic solution decomposes and causes extraordinary reac- 30 tion to influence on the performance of the lithium sec- MEANS FOR SOLVING THE PROBLEM ondary battery, it is attempted to improve life cycle and storage characteristics of the battery by adding various [0016] The present invention provides the following. additives to the non-aqueous electrolytic solution. Patent Literature 6 discloses it is possible to form a film on a 35 1. A process for preparing difluorophosphate com- positive electrode and a negative electrode by using a prising reacting difluorophosphoric acid with at least non-aqueous electrolytic solution containing as an addi- one salt, as a raw material, selected from a halide tive at least one of lithium and lith- salt, a carbonate, a phosphate, a hydroxide and an ium difluorophosphate and to suppress decomposition oxide of an alkali metal, an alkaline earth metal or of the electrolytic solution caused by the contact of the 40 an onium in the difluoraphosphoric acid, then sepa- non-aqueous electrolytic solution and a positive elec- rating a precipitate from the difluorophosphoric acid trode active material and negative electrode active ma- by solid-liquid separation, the precipitate being pre- terial. As a result, it is possible to suppress the self dis- cipitated by crystallization operation in the difluoro- charge and enhance storage characteristics. , and removing the difluorophos- 45 phoric acid contained in the precipitate by distillation Prior Art to obtain difluorophosphate. 2. A process for preparing difluorophosphate where- Patent Literature in a starting salt is at least one selected from a halide, a carbonate, a phosphate, a hydroxide and an oxide [0013] 50 of an alkali metal. 3. A process for preparing difluorophosphate where- Patent Literature 1: DE-813848 inthe alkali metal is at least one selected from lithium, Patent Literature 2: JP 2005-53727 A sodium and potassium. Patent Literature 3: JP 2005-219994 A 4. A process for preparing difluorophosphate where- Patent Literature 4: JP 2005-306619 A 55 in a starting salt is or a starting salt and difluorophos- Patent Literature 5: JP 2006-143572 A phoric acid are added to the difluorophosphoric acid Patent Literature 6: JP Patent No. 3439085 solution obtained by solid-liquid separation after crystallization operation, and then the operation in

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the above item 1 is repeated. (2) difluorophosphoric acid, (3) in the difluorophosphoric acid, [0017] The present process is characterized in that the (4) separating a precipitate from the difluorophos- starting salt and difluorophosphoric acid are reacted in phoric acid by solid liquid separation, the precipitate the difluorophosphoric acid, and the crystal is precipitat- 5 being precipitated by crystallization operation in the ed from the difluorophosphoric acid solution having the difluorophosphoric acid, formed difluorophosphate dissolved therein by crystalli- (5) and removing the difluorophosphoric acid con- zation operation. tained in the precipitate by distillation to obtain dif- [0018] As mentioned above, in the conventional proc- luorophosphate. esses of difluorophosphate, it was impossible to obtain 10 difluorophosphate having high purity due to an undesired [0024] The precipitate precipitated by the above crys- formation of a fluoride, monofluorophosphate or phos- tallizationoperation contains difluorophosphoric acid and phate. Usually, when insufficient in purity, purity can be impurities in addition to the desired difluorophosphate. enhanced by recrystallization. For recrystallization, a sol- [0025] Examples of the above salts, as a raw material, vent is required which suitably dissolves a product and 15 are halide salt, carbonate, phosphate, hydroxide and ox- does not react with the product. The present inventors ide of an alkali metal, an alkaline earth metal or an onium. have carefully investigated organic and inorganic sol- At least one of these can be used. vents usually used, but could not find a crystallization [0026] Examples of the above alkali metals are Li, Na, solvent which is suitable for crystallization of difluoro- K, Rb and Cs. Among them, preferable are Li, Na, K in phosphate. In non-patent literature 4, alkali metal chlo- 20 view of cost and availability. ride is reacted with difluorophosphoric acid to form dif- [0027] Examples of the above alkaline earth metals luorophosphate, which is washed with ether and then are Be, Mg, Ca, Sr, Ba and Al. Among them, preferable recrystallized in dehydrated alcohol for purification. are Mg, Ca, Ba and Al in view of cost and safety. When we have tried recrystallization of difluorophos- [0028] Examples of the above oniums are ammonium, phate in alcohol, we confirmed formation of impurity ion 25 phosphonium and sulfonium. + by ion chromatography. The structure of the impurity ion [0029] Examples of ammoniums are NH 4 , secondary is not clear, it is assumed to be formed by the reaction ammonium, tertiary ammonium and quaternary ammo- of difluorophosphoric acid ion and alcohol. nium. Examples of quaternary ammoniums usable in the [0019] After repeated trial and error, we found difluor- present invention are tetraalkyl ammonium, imidazolium, ophosphoric acid is suitable for crystallization solvent of 30 pyrazolium, pyridinium, triazolium, pyridazinium, thiazo- difluorophosphate, and made possible to prepare difluor- lium, oxazolium, pyrimidinium and pyrazinium, but is not ophosphate having high purity easily by using difluoro- limited to these examples. phosphoric acid as both of reaction solvent and crystal- [0030] Example of phosphoniums usable in the lization solvent. present invention is tetraalkyl phosphonium. 35 [0031] Example of sulfoniums usable in the present EFFECT OF THE INVENTION invention is trialkyl sulfonium. [0032] Examples of halide salts as a raw material are [0020] According to the present process, it is possible fluoride, chloride, bromide and iodide. Among them, pref- to industrially prepare difluorophosphate having high pu- erable are fluoride and chloride in view of molecular rity easily. 40 weight. [0021] Particularly, difluorophosphate is extremely [0033] Examples of as a raw material are useful as a raw material for ionic liquid or an additive for orthophosphate, monohydrogenphosphate, dihydrogen- electrolytic solution of lithium secondary cell. Thus, dif- phosphate, metaphosphate, monohydrogenmetaphos- luorophosphate produced by the present invention is phate, dihydrogenmetaphosphate, phosphinate, meta- highly valuable to use. 45 phosphinate and monofluorophosphate. Among them, preferable are orthophosphate and dihydrogenphos- EMBODIMENT OF CARRYING OUT THE INVENTION phate in view of cost and availability. [0034] In the present invention, halide, carbonate, [0022] A detailed description will be given of the em- phosphate, hydroxide and oxide are usable singly or in bodiment of the invention, but is not limited to the follow- 50 at least two of them. ing contents and is modified suitably within the scope of [0035] Regarding the ratio of the starting salt and dif- the invention. luorophosphoric acid, the starting salt and difluorophos- [0023] The present process is characterized in that phoric acid are mixed and reacted in mole amounts which corresponds to saturated solubility of difluorophosphate (1) at least one salt, as a raw material, selected from 55 in difluorophosphoric acid. It is preferable to use 0.01 to a halide salt, a carbonate, a phosphate, a hydroxide one mole, more preferably 0.03 to 0.5 mole, and partic- and an oxide of an alkali metal, an alkaline earth ularly preferably 0.05 to 0.3 mole of the starting salt per metal or an onium, is reacted with mole of difluorophosphoric acid.

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[0036] In the process for preparing difluorophosphate of the present invention, it is possible to reuse the dif- of the present invention, the reaction of the starting salt luorophosphoric acid solution having dissolved difluoro- and difluorophosphoric acid is conducted at preferably phosphate therein and obtained by solid-liquid separa- -50 to 110 °C, more preferably at 0 to 80 °C, particularly tion after crystallization operation. Namely, the difluoro- preferably at 0 to 40 °C. The reaction time is 0.5 to 40 5 phosphoric acid solution obtained by solid liquid separa- hours, preferably 1 to 20 hours. tion becomes low in concentration of difluorophosphoric [0037] In the process for preparing difluorophosphate acid. Therefore, it is possible to prepare difluorophos- of the present invention, the temperature of crystalline phate by adding a starting salt or a starting salt and dif- out the difluorophosphate from difluorophosphoric acid luorophosphoric acid to the solution to perform the reac- solution wherein difluorophosphate is dissolved in dif- 10 tion of difluorophosphoric acid and the starting salt, and luorophosphoric acid by crystallization operation is pref- conducting the similar crystallization and filtration oper- erably -100 to 100 °C, more preferably -80 to 80 °C, par- ations. ticularly preferably -50 to 50 °C. [0038] In the process for preparing difluorophosphate EXAMPLES of the present invention, the crystals precipitated from 15 crystallization operation contain difluorophosphoric acid [0043] The present invention will be described with ref- used as crystallization solvent and impurities by-pro- erence to the following Examples. The materials or duced. It is necessary to remove these impurities by dry- amounts recited in the examples are a simple example ing operation. The drying temperature is preferably 0 to and do not limit the scope of the invention unless recited 100 °C, more preferably 0 to 80 °C, particularly preferably 20 limitative. 0 to 60 °C. [0039] It is preferable to conduct the drying in an inert Reference Example 1 Purification of difluorophosphoric gas such as nitrogen and argon or in stream of these acid by distillation gases. Although performed at ordinary pressure or under reduced pressure, the drying is preferably conducted un- 25 [0044] In order to enhance the purity, difluorophos- der reduced pressure to promote removal of volatiles. phoric acid was distilled for use as crystallization solvent. [0040] In the process for preparing difluorophosphate To a round-bottom flask made of PTFE was placed 400 of the present invention, an organic solvent can be added g of difluorophosphoric acid (reagent: Fluorochem Inc.) to difluorophosphoric acid solution in order to vary solu- and distilled at 40 °C under reduced pressure, and 313 bility of the difluorophosphate or enhance filtability in the 30 g of fraction was obtained in a round-bottom flask made filtration operation. The solvent is not limited insofar as of PTFE and cooled at -20 °C. The fraction was subjected the solvent does not react with the starting salt, difluor- to anion analysis by ion chromatography (Dionex KK, ophosphoric acid and difluorophosphate, and does not DX-500, column AS-23) and purity of difluorophosphoric affect operability of the present process. Examples of the acid was measured by area normalization method of di- solvent are hydrocarbons, ethers, nitriles and carbon- 35 fluorophosphoric acid ion. The difluorophosphoric acid ates. was 99 % purity in relative area. [0041] As for the purity of difluorophosphoric acid used in the process for preparing difluorophosphate of the Example 1 present invention, higher one is preferable. The difluor- ophosphoric acid can be prepared by the conventionally 40 [0045] To a 500 ml vessel made of PFA was placed known methods, for example, by the method disclosed 300 g of difluorophosphoric acid obtained by distillation on page 536 of J.C.BAILAR et. al., COMPREHENSIVE operation in Reference Example 1 and thereto was add- INORGANIC CHEMISTRY vol.2. Namely, phosphoric ed 25 g of lithium chloride (reagent: Wako Pure Chemical anhydride is reacted with three times moles of anhydrous Industries, Ltd.). The reaction solution is filtered to re- to obtain a mixture of monofluorophos- 45 move insolubles and the resulting filtrate was cooled from phoric acid and difluorophosphoric acid. The mixture was 25 °C to -30 °C to precipitate the crystals. The slurry was distilled at, for example, 51 °C /100 mmHg to enhance subjected to solid-liquid separation to obtain crystals and purity of difluorophosphoric acid. As recited in non-patent the crystals was dried at 40 °C under reduced pressure Literature 4, it is possible to further enhance purity of using a round-bottom flask made of PTFE. The obtained difluorophosphoric acid by repeated distillation opera- 50 crystals of lithium difluorophosphate were subjected to tions. The higher the purity of difluorophosphoric acid anion analysis by ion chromatography (Dionex KK, DX- used in the process for preparing difluorophosphate is, 500, column AS-23) and purity of lithium difluorophos- the higher the purity of difluorophosphate obtained. The phate was measured by area normalization method of content of difluorophosphoric acid is preferably at least difluorophosphoric acid ion. The lithium difluorophos- 95 %, more preferably at least 98 %, and especially pref- 55 phate crystals was 97 % purity in relative area. erably at least 99 % by measured with ion chromatogra- phy. [0042] In the process for preparing difluorophosphate

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Example 2 Example 5

[0046] To 215 g of filtrate obtained by solid-liquid sep- [0049] To a 500 ml vessel made of PFA was placed aration in crystallization operation of Example 1 was add- 300 g of difluorophosphoric acid obtained by distillation ed 2.4 g of lithium chloride (reagent: Wako Pure Chemical 5 operation in Reference Example 1 and thereto was add- Industries, Ltd.). The reaction solution is filtered to re- ed 61 g of sodium bromide (reagent: Wako Pure Chem- move insolubles and the resulting filtrate was cooled from ical Industries, Ltd.). The reaction solution is filtered to 25 °C to -30 °C to precipitate the crystals. The slurry was remove insolubles and the resulting filtrate was cooled subjected to solid-liquid separation to obtain crystals and from 25 °C to -30 °C to precipitate the crystals. The slurry the crystals was dried at 40 °C under reduced pressure 10 was subjected to solid-liquid separation to obtain crystals using a round-bottom flask made of PTFE. The obtained and the crystals was dried at 40 °C under reduced pres- crystals of lithium difluorophosphate were subjected to sure using a round-bottom flask made of PTFE. The ob- anion analysis by ion chromatography (Dionex KK, DX- tained crystals of sodium difluorophosphate were sub- 500, column AS-23) and purity of lithium difluorophos- jected to anion analysis by ion chromatography (Dionex phate was measured by area normalization method of 15 KK, DX-500, column AS-23) and purity of sodium difluor- difluorophosphoric acid ion. The lithium difluorophos- ophosphate was measured by area normalization meth- phate crystals was 97 % purity in relative area. od of difluorophosphoric acid ion. The sodium difluoro- phosphate crystals was 94 % purity in relative area. Example 3 20 Example 6 [0047] To a 500 ml vessel made of PFA was placed 300 g of difluorophosphoric acid obtained by distillation [0050] To a 500 ml vessel made of PFA was placed operation in Reference Example 1 and thereto was add- 300 g of difluorophosphoric acid obtained by distillation ed 22 g of lithium carbonate (reagent: Wako Pure Chem- operation in Reference Example 1 and thereto was add- ical Industries, Ltd.). The reaction solution is filtered to 25 ed 33 g of calcium chloride (reagent: Wako Pure Chem- remove insolubles and the resulting filtrate was cooled ical Industries, Ltd.). The reaction solution is filtered to from 25 °C to -30 °C to precipitate the crystals. The slurry remove insolubles and the resulting filtrate was cooled was subjected to solid-liquid separation to obtain crystals from 25 °C to -30 °C to precipitate the crystals. The slurry and the crystals was dried at 40 °C under reduced pres- was subjected to solid-liquid separation to obtain crystals sure using a round-bottom flask made of PTFE. The ob- 30 and the crystals was dried at 40 °C under reduced pres- tained crystals of lithium difluorophosphate were subject- sure using a round-bottom flask made of PTFE. The ob- ed to anion analysis by ion chromatography (Dionex KK, tained crystals of calcium difluorophosphate were sub- DX-500, column AS-23) and purity of lithium difluoro- jected to anion analysis by ion chromatography (Dionex phosphate was measured by area normalization method KK, DX-500, column AS-23) and purity of calcium difluor- of difluorophosphoric acid ion. The lithium difluorophos- 35 ophosphate was measured by area normalization meth- phate crystals was 95 % purity in relative area. od of difluorophosphoric acid ion. The calcium difluoro- phosphate crystals was 95 % purity in relative area. Example 4 Comparative Example 1 [0048] To a 500 ml vessel made of PFA was placed 40 300 g of difluorophosphoric acid obtained by distillation [0051] The experiment was conducted in the same operation in Reference Example 1 and thereto was add- manner as in Example 1 except that the filtrate was not ed 14 g of lithium hydroxide (reagent: Wako Pure Chem- subjected to crystallization but was directly concentrated ical Industries, Ltd.). The reaction solution is filtered to and dried at 40 °C under reduced pressure using a round- remove insolubles and the resulting filtrate was cooled 45 bottom flask made of PTFE. The obtained crystals of lith- from 25 °C to -30 °C to precipitate the crystals. The slurry ium difluorophosphate were subjected to anion analysis was subjected to solid-liquid separation to obtain crystals by ion chromatography (Dionex KK, DX-500, column AS- and the crystals was dried at 40 °C under reduced pres- 23) and purity of lithium difluorophosphate was measured sure using a round-bottom flask made of PTFE. The ob- by area normalization method of difluorophosphoric acid tained crystals of lithium difluorophosphate were subject- 50 ion. The lithium difluorophosphate crystals was 85 % pu- ed to anion analysis by ion chromatography (Dionex KK, rity in relative area. DX-500, column AS-23) and purity of lithium difluoro- phosphate was measured by area normalization method INDUSTRIAL APPLICABILITY of difluorophosphoric acid ion. The lithium difluorophos- phate crystals was 93 % purity in relative area. 55 [0052] Difluorophosphate produced by the present in- vention is extremely useful as a raw material for ionic liquid or an additive for electrolytic solution of lithium sec- ondary cell and is highly valuable to use.

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Claims

1. A process for preparing difluorophosphate compris- ing reacting difluorophosphoric acid with at least one salt, as a raw material, selected from a halide salt, 5 a carbonate, a phosphate, a hydroxide and an oxide of an alkali metal, an alkaline earth metal or an onium in the difluoraphosphoric acid, then separating a pre- cipitate from the difluorophosphoric acid by solid-liq- uid separation, the precipitate being precipitated by 10 crystallization operation in the difluorophosphoric acid, and removing the difluorophosphoric acid con- tained in the precipitate by distillation to obtain dif- luorophosphate. 15 2. A process for preparing difluorophosphate as de- fined in claim 1 wherein a starting salt is at least one selected from a halide, a carbonate, a phosphate, a hydroxide and an oxide of an alkali metal. 20 3. A process for preparing difluorophosphate as de- fined in claim 1 or 2 wherein the alkali metal is at least one selected from lithium, sodium and potas- sium. 25 4. A process for preparing difluorophosphate wherein a starting salt is or a starting salt and difluorophos- phoric acid are added to the difluorophosphoric acid solution as recited in claim 1 obtained by solid-liquid separation after crystallization operation, and then 30 the operation recited in claim 1 is repeated.

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REFERENCES CITED IN THE DESCRIPTION

This list of references cited by the applicant is for the reader’s convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description

• DE 813848 [0013] • JP 2005306619 A [0013] • JP 2005053727 A [0013] • JP 2006143572 A [0013] • JP 2005219994 A [0013] • JP 3439085 B [0013]

Non-patent literature cited in the description

•K.MATSUMOTO; R.HAGIWARA. Inorganic Chem- • Inorganic Nuclear Chemistry Letters, 1969, vol. 5, stry, 2009, vol. 48, 7350-7358 [0014] 581-585 [0014] • The Electrochemical Society of Japan, Preliminary • The Japan Society for Analytical Chemistry, 43th An- report, 1I18 [0014] nual Meeting, Lecture summary, 1994, 536 [0014] • Ber. Dtsch. Chem., Ges., 1929, vol. B26, 786 [0014] • J.C.BAILAR. COMPREHENSIVE INORGANIC • Zh. Neorgan. Khim., 1962, vol. 7, 1313 [0014] CHEMISTRY, vol. 2, 536 [0041] • Journal of Fluorine Chemistry, 1988, vol. 38, 297-302 [0014]

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