A METHOD FOR SYNTHESISING AN ESSENTIALLY V2O5-FREE VANADIUM OXIDE HAVING A MEAN VANADIUM OXIDATION STATE OF AT LEAST +4 BUT LOWER THAN +5 FROM NH4VO3, SAID VANADIUM OXIDE PREFERABLY CONSISTING ESSENTIALLY OF V6O13, VO2 OR ANY MIXTURE THEREOF

Abstract: A new method for synthesising an essentially V2O5-free vanadium oxide having a mean vanadium oxidation state of at least +4 but lower than +5 from NH4VO3 is disclosed. By this method NH4VO3 is heated to a reaction temperature sufficient for thermal decomposition of NH4VO3, and at said reaction temperature, the pressure is kept on at least 0.5 MPa. The method enables production of single-phase V6O13, single-phase VO2 as well as any mixture thereof in a remarkably simple manner, as it primarily involves careful control of temperature and pressure conditions. The synthesis method of the invention can thus be easily scaled up to any industrial requirement.

Description: A method for svnthesising an essentially V^-J oxide having a mean vanadium oxidation state of at least +4 hut lower than +5 from NK-VC . said vanadium oxide pref¬ erably consisting essentially of vO^,- V0 or any mixture thereof. The present invention is concerned with a method for syn- thesising an essentially V 2 0 5 -free vanadium oxide having a mean vanadium oxidation state of at least +4 but lower than +5 from NH 4 V0 3 , said vanadium oxide preferably con- sisting essentially of V 6 0 13 , V0 2 or any mixture thereof. Vanadium oxides having a mean vanadium oxidation state be¬ tween +5 and +4 have attracted much attention as potential active cathode materials in secondary lithium batteries. These materials are generally prepared from NH 4 V0 3 (ammoni- um metavanadate) . Single phase V 2 0 5 is readily synthesised by heating NH 4 V0 3 in air (see e.g. K.C. Khulbe and R.S. Mann, Can. Jour. Chem. Vol. 53 (1975), p. 2917). The lower oxides, especi¬ ally V 6 0 13 (mean oxidation state +4- j ) and V0 2 (mean oxida- tion state +4), are more difficult to obtain as single- phase materials (see e.g. U. von Sac en and J.R. Dahn, J. Power Sources, Vol. 26 (1989), p. 461). In J. Thermal Anal., Vol. 16 (1979) 659 Trau discusses the problem of obtaining phase-pure V 6 0 13 under thermal decom- position of NH 4 V0 3 without using added reduction reactants. The use of S0 2 as a reducing agent for V 2 0 5 to form V 6 0 13 is technically very complicated; it is difficult to stop the reduction and obtain a homogeneous single-phase V 6 0 13 pow¬ der. Trau instead suggests thermal decomposition in a stream of pure nitrogen gas with slow heating to 500-550°C, and a post-treatment with boiling NaOH,.,, to remove V 2 0 5 as proposed by Yankelvich et al. in Ukr. Khim. Zh. , Vol. 42 (1976) 659. Brown et al. reports in J. Thermal Anal, Vol. 6 (1974) 529 the formation of lower oxides using an NH 3 atmosphere when decomposing NH 4 V0 3 . US-A-4 486 400 describes a process for preparing stoi- chiometric V 6 0 13 by thermal decomposing ammonium metava- nadate in a nitrogen atmosphere to produce nonstoichiomet- ric V 6 0 13 , followed by heating the obtained nonstoichiomet- ric V 5 0 13 in an CO/C0 2 -atmosphere having a composition giv¬ ing an oxygen partial pressure equal to the oxygen partial pressure over stoichiometric V 6 0 13 to form stoichiometric V 6 0 13 . US-A-4 035 476 describes the preparation of agglomerated vanadium oxides of the formula V 2 0 x , wherein x is between 3.8 and 4.6, by thermal decomposition of ammonium polyva- nadate ( (NH 4 ) 2 .0.3V 2 0 5 .nH 2 0) at a temperature of 600 to 900°C and permitting the solid decomposition products and reduc- ing agents to react. JP 62-197317 describes the production of V 6 0 13 or V 2 0 4 by introducing NH 4 V0 3 , optionally mixed with <15% V 2 0 5 , into a reaction vessel, heating to a temperature of 380 to 750°C at a rate of 0.5 to 30 K/min, keeping this temperature for 30 min to 3 hours and keeping the pressure at about 3 atm or below by means of a pressure-control valve. Impurities in the cathode material generally have a nega- tive influence on the overall battery performance, espe¬ cially the battery capacity declines faster in the pres¬ ence of impurities. In particular it is has been shown that even small amounts of other V0 X phases, especially V 2 0 5 , has a negative influence on the lithium intercalating properties of V 6 0 13 and V0 2 . Thus, although several methods for the manufacturing of lower vanadium oxides are known there still exists a need for an improved synthesis method for the preparation of lower vanadium oxides of high purity, i.e. essentially free of V 2 0 5 . Accordingly, it is an object of the invention to provide a method for synthesising an essentially V 2 0 5 -free vanadium oxide having a mean vanadium oxidation state of at least +4 but lower than +5 from NH 4 V0 3 , said vanadium oxide pref¬ erably consisting essentially of V 6 0 13 , V0 2 or any mixture thereof. It has been shown that this object is accomplished by a method described above, in which NH 4 V0 3 is heated to a re¬ action temperature sufficient for thermal decomposition of NH 4 V0 3 , and in which, at said reaction temperature, the pressure is kept on at least 0.5 MPa. It is a further advantage of the method according to the invention that it does not involve any time-consuming and thus costly separation processes. Nor does it involve the use of added reducing agents in order to obtain the de¬ sired purity, but only carefully controlled temperature and pressure conditions. The synthesis method of the in¬ vention can thus be easily scaled up to any industrial re- quirement. The formation of vanadium oxides from NH 4 V0 3 involves two processes: the decomposition of ammonium metavanadate, followed by the reduction of the formed vanadium species. These processes can proceed simultaneously and the reac¬ tion routes are often quite complicated. The formation of V 6 0 13 from NH 4 V0 3 presumably proceeds through a reduction of vanadium from oxidation state +5 resulting in formation of N 2 and the lower oxide (V 6 0 13 ) having a mean oxidation state of +4y, according to the following total reaction scheme: 18NH 4 V0 3 → 3V 6 0 13 + 14NH 3 + 15H 2 0 + 2N 2 (1) It is very likely that the same type of reaction route is followed when V0 2 is formed, thus giving rise to the fol¬ lowing total reaction scheme: 18NH 4 V0 3 → 18V0 2 + 12NH 3 + 18H 2 0 + 3N 2 (2) It has surprisingly been shown that the structure of the vanadium oxide produced by carrying out the process ac¬ cording to the invention can be controlled in a remarkably simple manner by varying the pressure under which the product is synthesised: When the pressure is kept within the range of 0.5 to 2.5 MPa, preferably 1.0 to 2.0 MPa, e.g. about 1.5 MPa, the product obtained is single phase V 6 0 13 . At pressures below 0.5 MPa, V 2 0 5 is produced together with V 6 0 13 . At pressures above 2.5 MPa V0 2 is formed together with V 6 0 13 . When the desired product is single phase V0 2 the pressure should be at least 3.5 MPa, preferably in the range of 3.5 MPa to 7.0 MPa. In the present context the expression "single phase" is used to designate vanadium oxides which according to X-ray analysis contain virtually no other crystal phases (less than the detectability limit for XRD of approximately 2%) than the predominant one. The reaction temperature should be selected so that effi¬ cient decomposition of NH 4 V0 3 occurs. Accordingly, NH 4 V0 3 is preferably heated to a temperature of at least 250 °C, more preferably to a temperature in the range of 300 to 800 °C, even more preferably 425 to 550 °C. The applied heating rate is advantageously lower than 2 K/min. Preferably the heating rate is in the range of 0.1 to 2 K/min, more preferably in the range of 0.5 to 1 K/min. The period at which the NH 4 V0 3 is kept at the reaction tem¬ perature may vary according to the desired end product. For synthesis of single phase V 6 0 13 a reaction period of 10 s to 24 h is preferably employed, whereas for synthesis of single phase V0 2 a reaction period of 2 h to 5 d is pref¬ erably employed. The heating is preferably performed under such conditions that the solid decomposition product and the produced de¬ composition gas, including NH 3 , from the NH 4 V0 3 starting material are permitted to react. In a preferred embodiment such conditions are ensured by heating the NH 4 V0 3 in a closed reaction chamber, preferably equipped with means for controlling the pressure in the chamber, such as an adjustable relief valve. In this embodiment the reaction chamber is preferably filled with an amount of NH 4 V0 3 powder corresponding to 1/2 to 9/10 of the reaction chamber volume, so that the pro¬ duced decomposition gas efficiently displaces the air in the reaction chamber. The invention will be further described with reference to examples and the drawing in which: Fig. 1 shows an apparatus for performing the process ac¬ cording to the invention; Fig. 2 shows a real (upper curve) and a simulated (lower curve) X-ray diffractogram of a single-phase V 6 0 13 sample produced according to an embodiment of the invention; and Fig. 3 shows a real (upper curve) and a simulated (lower curve) X-ray diffractogram of a single-phase V0 2 sample produced according to another embodiment of the invention. The reaction chamber used for the synthesis of the vana¬ dium oxide according to the invention is shown in Fig. 1. The chamber, tubes and all welds are made of stainless acid resistant steel. The chamber could only be opened and closed at the stainless steel high-vacuum coupling (CF 16, VACUTECH), which was sealed with replaceable copper gas¬ kets, six steel screws (Unbraco M4) and 8 mm silver steel nuts. The experimental set-up shown in Fig. 1 was used for the synthesis of V 6 0 13 (Example 1). The desired overpressure was regulated with a relief valve (NUPRO R3A) fitted with viton standard seals and pre-set with exchangeable springs (NUPRO, blue Kl-A). Prior to an experimental run all parts were cleaned at 60°C in a basic detergent (Labkemi RBS 25) for 60 minutes in an ultra-sound bath and thereafter rinsed in water and ethanol. A slightly different experimental set-up was used for the V0 2 synthesis(Example 2). The pressure was controlled with a blanking flange (Balzer DN 16 CF) mounted directly on the reaction chamber. The final products produced according to the following ex¬ amples 1 and 2 were characterised in terms their phase- purity and degree of crystallinity by X-ray diffraction (XRD) using CuKα x radiation and a STOE & CIE GmbH STADI powder diffractometer equipped with a curved position sen¬ sitive detector. The mean oxidation state for the vanadium of the final products was determined by titration. The V 6 0 13 and V0 2 products were dissolved in 0.5M H 2 S0 4 and 3M HC1, respec¬ tively. The dissolved samples were first titrated up to V(+5) with a standard Ce(S0 4 ) 2 ' 4H 2 0 (Merck, 0.10M) solution, followed by titration down to V(+4) with a solution of FeS0 4 ' 7H 2 0 (Merck, p.a.), controlled against standard KMn0 4 (Merck, 0.02M). Three subsequent titrations were perfor¬ med. Grain-size distribution and morphology of the final prod- ucts were analysed by scanning electron microscopy (SEM) on samples pressed onto a carbon film using a Zeiss DSM 960A scanning electron microscope. EXAMPLE 1 Synthesis of single-phase V 6 0 13 . Using the apparatus shown in Fig. 1., single-phase V 6 0 13 was synthesised in the following manner: NH 4 V0 3 powder (Gesellschaft fur Electrometallurgie, MBH, 99.9%) was introduced in the chamber (having a volume of 940 cm 3 ) in an amount corresponding to 2/3 of the chamber volume. Then the reaction chamber was sealed, connected to the relief valve arrangement, placed in a temperature con¬ trolled furnace and heated to a temperature of 500 °C at a heating rate of 0.5 K/min. The relief valve was adjusted so as to control the pressure in the reaction chamber to 1.5 MPa. The temperature was kept at 500°C for 1 minut. The chamber was then allowed to cool to room temperature in an airstream and dismantled. The obtained dark (bluish-black) powder had sintered into agglomerates and had to be scraped out of the chamber. The phase purity of the resulting product was checked by X-ray diffraction (XRD). In fig. 2 is shown the X-ray dif¬ fractogram for the obtained V 6 0 13 product in the upper curve, while the lower curve is a corresponding simulated X-ray diffractogram. XRD showed that the V 6 0 13 obtained was phase-pure and highly crystalline. A mean vanadium oxidation state of 4.30 (±0.01) was deter¬ mined by titration. SEM studies of the morphology showed needle-like crystals propagating in the direction of the short b-axis in the monoclinic V 6 0 13 unit cell. However, very different lengths were observed; from 5 to 50 μm for adjacent crystals. The crystals compacted together on sintering to form spherical powder grains with a maximum dimension of up to 300 μm. EXAMPLE 2 Synthesis of single-phase V0 2 . The reaction chamber shown in Fig. 1 was slightly modified for the V0 2 synthesis, the flange being closed by a plate. NH 4 V0 3 powder (Gesellschaft fur Electrometallurgie, 99.9%) was introduced in the chamber in an amount corresponding to 9/10 of the chamber volume. Then the chamber was heated in the same manner and at the same rate as in Example 1 to a temperature of 500 °C and annealed at that temperature for 3 days. The obtained V0 2 powder (greyish-black) con¬ sisted of very fine grains and could easily be removed from the reaction chamber. The phase purity of the resulting product was checked by XRD. In fig. 3 is shown the X-ray diffractogram for the obtained V0 2 product in the upper curve, while the lower curve is a corresponding simulated X-ray diffractogram. XRD showed that the V0 2 formed was phase-pure and crystal¬ line. A mean vanadium oxidation state of 4.00 (±0.01) was deter- mined by titration. SEM revealed bulky crystals (5 to 20 μm) in a quite homo¬ geneous and non-sintered powder. In the following Table I the results from the analysis of the products synthesised in Examples I and II are presen¬ ted. Table I Example I II Compound v 6 o 13 V0 2 Crystal Structure C 2/m P 2 τ_ a = 11.911(3)A a = 5.750(1)A b = 3.674(1)A b = 4.5289(8)A c = 10.130(2)A c = 5.381(1)A β = 100.90(1)° β = 122.61(1)° XRD Phase-pure V 6 0 13 Phase-pure V0 2 Mean oxidation state for vanadium +4.30 (±0.01) +4.00 (±0.01) SEM Needles, sintered Bulky powder, non- to form spherical sintered agglomerates

Claims: Claims. 1. A method for synthesising an essentially V 2 0 5 -free vana¬ dium oxide having a mean vanadium oxidation state of at least +4 but lower than +5 from NH 4 V0 3 , said vanadium oxide preferably consisting essentially of V 6 0 13 , V0 2 or any mix¬ ture thereof, c h a r a c t e r i z e d in that NH 4 V0 3 is heated to a reaction temperature sufficient for thermal decomposition of NH 4 V0 3 , at that, at said reaction rempera- ture, the pressure is kept on at least 0.5 MPa. 2. A method according to claim 1 for synthesising single phase V 6 0 13 from NH 4 V0 3 , c h a r a c t e r i z e d in that the pressure is kept within the range of 0.5 to 2.5 MPa, preferably 1.0 to 2.0 MPa. 3. A method according to claim 1 for synthesising a vana¬ dium oxide consisting of a mixture of V 6 0 13 and V0 2 from NH 4 V0 3 , c h a r a c t e r i z e d in that the pressure is kept within the range of 2.5 to 3.5 MPa. 4. A method according to claim 1 for synthesising single phase V0 2 from NH 4 V0 3 , c h a r a c t e r i z e d in that the pressure is greater than 3.5 MPa, preferably in the range of 3.5 to 7.0 MPa. 5. A method according to each of the preceding claims, c h a r a c t e r i z e d in that the NH 4 V0 3 is heated to a temperature of at least 250 °C, preferably to a tempera- ture in the range of 300 to 800 °C, more preferably 425 to 550 °C. 6. A method according to each of the preceding claims, c h a r a c t e r i z e d in that the NH 4 V0 3 is heated from ambient temperature to reaction temperature at a rate lower than 2 K/min, preferably at a rate in the range of 0.1 to 2 K/min, more preferably at a rate in the range of 0.5 to 1 K/min. 7. A method according to each of the preceding claims, c h a r a c t e r i z e d in that the NH 4 V0 3 is kept at the reaction temperature for a period of from 10 s to 24 h for synthesis of single phase V 6 0 13 , and for a period of from of 2 h to 5 d for synthesis of single-phase V0 2 . 8. A method according to each of the preceding claims, c h a r a c t e r i z e d in that heating is performed under such conditions that the solid decomposition product and the produced decomposition gas from the NH 4 V0 3 starting material are permitted to react. 9. A method according to claim 8, c h a r a c t e r i - z e d in that the NH 4 V0 3 is heated in a closed reaction chamber, preferably equipped with means for controlling the pressure in the chamber, preferably an adjustable re- lief valve. 10. A method according to claim 9, c h a r a c t e r i - z e d in that the reaction chamber is filled with an amount of NH 4 V0 3 powder corresponding to 1/2 to 9/10 of the reaction chamber volume.

Inventors: LAMPE-ONNERUD CHRISTINA, LAMPE-ONNERUD, CHRISTINA
Applicants: DANIONICS AS, LAMPE ONNERUD CHRISTINA, DANIONICS A/S, LAMPE-ONNERUD, CHRISTINA
Application year: 1995
Main classification: G01G31
IPC codes: C01G31, H01M4