(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 DIFLUOROPHOSPHATE (57) A process for preparing difluorophosphate com- from the difluorophosphoric acid 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 phosphate, 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 hexafluorophosphate 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- fluorophosphoric acid 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 fluoride or acid counters to a problem in heat resistance and safety. Ionic sodium fluoride. 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 ammonium fluoride. 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.