Method of preparation of a compound on the base of a derivative of vanadiumoxide for cathodes of thermal cells

0001The present invention relates to a process for the preparation of a compound based on a vanadium oxide derivative for a thermal cell cathode according to the preamble of claim 1. Such a process is known, for example from document DE-A- 2,142,922.

0002Vanadium oxide, in particular vanadium pentoxide, cannot be used directly as cathodic active material in a thermal cell, on the one hand because of its lack of electronic conductivity, and, on the other hand, because of its reactivated in molten chloride media.

0003Mixing with an electronic conductor of graphite or carbon black type, as proposed in the document above, may seem an obvious solution for those skilled in the art to improve the conductivity of the cathodes. However, when mixing V₂O₅ and carbon black, and trying to use this composition in a thermal cell cathode, we obtain unstable and not very conductive mixtures. In a stack, the result is always reflected in successive stages of discharge potential. In addition, mixtures containing an amount of graphite or carbon black sufficient to improve the conductivity are always very difficult to tablet.

0004Other methods have been proposed for preparing vanadium oxide derivatives which can serve as cathode materials for thermal cells.

0005Patent US-A-4 315 905 proposes in particular to manufacture an electronic conductive product, to be used as a cathode material or as a cathode coating, by heating vanadium pentoxide in a reducing or neutral gas atmosphere (hydrogenated nitrogen or helium). A mixture is thus obtained containing on the one hand V₆O₁₃ and on the other hand an unidentified V₂O₅ derivative, but no example of use of this compound is given.

0006Document EP-A-0 145 261 (application filed on November 6, 1984) describes the use of a cathode material chosen from one or more of the following compounds:<maths id="math0001"><math display="inline"><mrow><msub><mrow><mtext>V</mtext></mrow><mrow><mtext>₆</mtext></mrow></msub><msub><mrow><mtext>O</mtext></mrow><mrow><mtext>13 + x</mtext></mrow></msub><msub><mrow><mtext>(O ≦ x ≦ 0.5), VO</mtext></mrow><mrow><mtext>2 ± z</mtext></mrow></msub><msub><mrow><mtext>(O ≦ z ≦ 0.05), Li</mtext></mrow><mrow><mtext>y</mtext></mrow></msub><mtext>V₂O₅ (0.88 ≦ y ≦ 1), V₃O₇ and LiV₃O₈.</mtext></mrow></math><img file="EP0343586B1_D0001.tif" /></maths> However, the compounds described above are not satisfactory for use as a cathode thermal cell material. In particular, they do not have a stable discharge voltage, as shown by the discharge examples given in the patent. Regardless of the compound used, the discharge curves have a large slope and multiple stages. In addition, the preparation process requires several successive operations before the final cathode mixture containing the electronic conductor and a suitable quantity of electrolyte can be obtained; its preparation time, which is around 8 hours to several days, is prohibitive.

0007The object of the present invention is to implement a process for the easy preparation of a compound based on a vanadium oxide derivative, capable of being used as cathode material in a thermal cell and having good stability of the discharge voltage.

0008The subject of the present invention is a process for the preparation of a compound based on a vanadium oxide derivative capable of being used as cathode material in a thermal cell, process according to which a pulverulent mixture containing pentoxide is produced. vanadium, a carbonaceous material and electrolyte from said cell, and characterized<ul id="ul0001" list-style="dash"><li>in the fact that in said mixture, said carbon material is in a proportion by weight of V₂O₅ of between 3% and 20%, and said electrolyte is a mixture of alkali halides found in a proportion by weight of V₂O₅ of between 15 % and 50%,</li><li>and by the fact that a heat treatment of said pulverulent mixture is carried out at a temperature higher than the melting temperature of said mixture of halides, said temperature being between 365 ° and 650 ° C, for a period of between 15 minutes and 2 hours.</li></ul>

0009Preferably,<ul id="ul0002" list-style="dash"><li>Said carbonaceous material is smoke black, such as carbon black, acetylene or benzene.</li><li>Said mixture of alkali halides is a mixture of lithium and potassium halides, chosen from the LiCl-KCl eutectic, LiCl-KCl mixtures containing between 50% and 80% LiCl in molar fraction, ternary LiCl- mixtures LiBr-LiF, LiCl-LiBr-KBr.</li><li>The proportion by weight of said carbon material is between 5% and 10%.</li><li>The proportion by weight of said mixture of alkali halides is between 30% and 45%.</li><li>Said treatment temperature is between 410 ° C and 550 ° C and the duration of said heat treatment is between 1/2 hour and 1 hour.</li></ul>

0010The compound thus obtained is ground after cooling until a particle size less than 100 microns is obtained. It is then put in the form of a pellet to be used as a cathode.

0011The present invention also relates to a thermal cell comprising the aforementioned cathode and in addition a lithium or lithium alloy anode chosen from lithium-aluminum, lithium-silicon and lithium-boron compounds, and an electrolyte composed of a gelling agent. and said mixture of alkali halides.

0012Said gelling agent can be chosen from SiO₂, MgO, Al₂O₃.

0013Unexpectedly, it seems that during the melting of vanadium pentoxide and the mixture of alkali halides, the presence of smoke black plays a very particular role on the nature of the mixture obtained. This could be explained by the existence of superficial carbon-oxygen bonds formed during the high temperature treatment and then facilitating electronic transfers.

0014The mixture of V₂O₅ with carbon and the mixture of alkali halides allows during the heat treatment period a controlled and reproducible decomposition. Carbon may also act as a catalyst during this decomposition.

0015The method according to the invention makes it possible to obtain a complex compound, the properties of which are well suited for use as a thermal cell cathode:<ul id="ul0003" list-style="dash"><li>This complex compound is stable in the temperature range useful for use in thermal cells.</li><li>It exhibits, in constant current discharge, a single stable discharge stage with respect to a lithium electrode; this level can be + 2.5 Volts.</li><li>It is well suited for making thin pellets.</li></ul>

0016Other characteristics and advantages of the present invention will appear during the following description of embodiments given by way of illustration but in no way limitative.

0017In the attached drawing:<ul id="ul0004" list-style="dash"><li>FIG. 1 shows discharge curves of a cathode of the prior art and of a cathode obtained by the method according to the invention,</li><li>FIG. 2 shows a discharge curve for a variant cathode obtained by the method according to the invention,</li><li>FIG. 3 shows discharge curves of a battery of thermal cells according to the invention, and of a battery of cells of the prior art.</li></ul>

0018In a first example of implementation, 100 grams of V₂O₅ powder are intimately mixed for one hour with 50 grams of LiCl-KCl eutectic and 10 grams of acetylene black. This mixture is then placed in an oven and brought to a neutral atmosphere, such as argon, at 550 ° C for 30 minutes. Composition A obtained is cooled in the absence of moisture and ground to a particle size of less than 100 micrometers.

0019A 0.4 mm thick cathode A is then produced by tableting. A monocouple is produced comprising a cathode A, an electrolyte based on the eutectic LiCl-KCl added with a gelling agent such as SiO₂, and an anolyte based on a lithium aluminum alloy LiAl.

0020Illustrated in FIG. 1 in a graph giving on the ordinate the voltage V (in volts) and on the abscissa the time t in seconds, the discharge A of the preceding monocouple with a current density of 100 mA / cm² and at 450 ° C. . There is a voltage plateau at 2.15 volts.

0021The discharge B is shown, under completely similar conditions, of a cathode of the prior art produced by the method described in document EP-A-0 145 261, according to which a mixture comprising VO₂ is started. and γ -LiV₂O₅. Several levels of discharge are observed and the advantage of the composition according to the invention thus appears very clearly.

0022Other compositions were produced starting from the same initial products and in the same proportions as in the first example recited, and by modifying the heat treatment. Table I gives for each treatment the initial voltage of the monocouple Vo (in volts), the voltage of the plateau V (in volts), the capacitance C of the monocouple (in mAh) with a stop voltage of 2.0 volts, and the yield Rt in%.

0023The optimal conditions are between 410 ° C and 550 ° C with a duration of between 30 min and 1 hour. <tables id="tabl0001" num="0001"><table frame="all"><title>TABLE I</title><tgroup cols="6" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="26.25mm" /><colspec colnum="2" colname="col2" colwidth="26.25mm" /><colspec colnum="3" colname="col3" colwidth="26.25mm" /><colspec colnum="4" colname="col4" colwidth="26.25mm" /><colspec colnum="5" colname="col5" colwidth="26.25mm" /><colspec colnum="6" colname="col6" colwidth="26.25mm" /><thead valign="top"><row><entry namest="col1" nameend="col2" align="center">Treatment</entry><entry namest="col3" nameend="col3" align="center">Vo (V)</entry><entry namest="col4" nameend="col4" align="center">V bearing (V)</entry><entry namest="col5" nameend="col6" align="center">Stop at 2.0 V / LiAl</entry></row><row><entry namest="col1" nameend="col1" align="center">Temperature</entry><entry namest="col2" nameend="col2" align="center">Duration</entry><entry namest="col3" nameend="col3" /><entry namest="col4" nameend="col4" /><entry namest="col5" nameend="col5" align="center">C (mAh)</entry><entry namest="col6" nameend="col6" align="center">Rt (%) *</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">365 ° C</entry><entry namest="col2" nameend="col2" align="char" char=",">1 h</entry><entry namest="col3" nameend="col3" align="center">2,85</entry><entry namest="col4" nameend="col4" align="left">2,50</entry><entry namest="col5" nameend="col5" align="char" char=",">9,2</entry><entry namest="col6" nameend="col6" align="right">44</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" /><entry namest="col3" nameend="col3" /><entry namest="col4" nameend="col4" align="left">2,25</entry><entry namest="col5" nameend="col5" /><entry namest="col6" nameend="col6" /></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" /><entry namest="col3" nameend="col3" /><entry namest="col4" nameend="col4" align="left">2,15</entry><entry namest="col5" nameend="col5" /><entry namest="col6" nameend="col6" /></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">365 ° C</entry><entry namest="col2" nameend="col2" align="char" char=",">2 h</entry><entry namest="col3" nameend="col3" align="center">material</entry><entry namest="col4" nameend="col4" align="left">no</entry><entry namest="col5" nameend="col5" align="center">pastillable</entry><entry namest="col6" nameend="col6" /></row><row><entry namest="col1" nameend="col1" align="right">550 ° C</entry><entry namest="col2" nameend="col2" align="char" char=",">15 min</entry><entry namest="col3" nameend="col3" align="center">2,32</entry><entry namest="col4" nameend="col4" align="left">2,11</entry><entry namest="col5" nameend="col5" align="char" char=",">14,0</entry><entry namest="col6" nameend="col6" align="right">67</entry></row><row><entry namest="col1" nameend="col1" align="right">550 ° C</entry><entry namest="col2" nameend="col2" align="char" char=",">30 min</entry><entry namest="col3" nameend="col3" align="center">2,30</entry><entry namest="col4" nameend="col4" align="left">2,11</entry><entry namest="col5" nameend="col5" align="char" char=",">13,8</entry><entry namest="col6" nameend="col6" align="right">72</entry></row><row><entry namest="col1" nameend="col1" align="right">550 ° C</entry><entry namest="col2" nameend="col2" align="char" char=",">1 h</entry><entry namest="col3" nameend="col3" align="center">2,22</entry><entry namest="col4" nameend="col4" align="left">2,15</entry><entry namest="col5" nameend="col5" align="char" char=",">19,6</entry><entry namest="col6" nameend="col6" align="right">74</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">550 ° C</entry><entry namest="col2" nameend="col2" align="char" char=",">2 h</entry><entry namest="col3" nameend="col3" align="center">2,25</entry><entry namest="col4" nameend="col4" align="left">2,12</entry><entry namest="col5" nameend="col5" align="char" char=",">12,6</entry><entry namest="col6" nameend="col6" align="right">60</entry></row><row><entry namest="col1" nameend="col1" align="right">650 ° C</entry><entry namest="col2" nameend="col2" align="char" char=",">30 min</entry><entry namest="col3" nameend="col3" align="center">2,32</entry><entry namest="col4" nameend="col4" align="left">2,09</entry><entry namest="col5" nameend="col5" align="char" char=",">7,0</entry><entry namest="col6" nameend="col6" align="right">35</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">650 ° C</entry><entry namest="col2" nameend="col2" align="char" char=",">1 h</entry><entry namest="col3" nameend="col3" align="center">2,38</entry><entry namest="col4" nameend="col4" align="left">2,09</entry><entry namest="col5" nameend="col5" align="char" char=",">9,2</entry><entry namest="col6" nameend="col6" align="right">44</entry></row></tbody></tgroup><tgroup cols="6" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="26.25mm" /><colspec colnum="2" colname="col2" colwidth="26.25mm" /><colspec colnum="3" colname="col3" colwidth="26.25mm" /><colspec colnum="4" colname="col4" colwidth="26.25mm" /><colspec colnum="5" colname="col5" colwidth="26.25mm" /><colspec colnum="6" colname="col6" colwidth="26.25mm" /><tbody valign="top"><row><entry namest="col1" nameend="col6" align="justify">* Rt calculated on 1 F</entry></row></tbody></tgroup></table></tables>

0024In Example 2, 500 grams of V₂O₅ are mixed as above with 250 grams of electrolyte and 25 grams of acetylene black. This mixture was brought to 410 ° C. under a neutral atmosphere for 1 hour. The composition obtained was ground and placed in the form of a pellet to form a cathode C.

0025A stack composed of this cathode, a LiCl-KCl electrolyte tablet with Al₂O₃ gelling agent, and an anode made of a thin sheet of lithium-boron alloy is discharged at 450 ° C at constant current with density 100 mA / cm². The corresponding curve C is given in FIG. 2. A voltage plateau at 2.5 volts is observed.

0026A battery of batteries is then produced obtained by the same process as that described above. The cathode material is that of Example 1. The electrolyte tablets contain 85% of LiCl-KCl eutectic and 15% of SiO₂. The anolyte pellets contain 90% LiAl alloy and 10% LiCl-KCl eutectic.

0027An example of the structure of such a battery is described in "Proceedings of the 32nd International Power Sources Symposium, Electrochemical Society Inc., 1986, pages 686 et seq." By J. Douglass BRISCOE, Daniel GLEN, SAFT America Inc.

0028A battery according to the invention was discharged at room temperature, at a current density of 220 mA / cm², with periodic pulses of current of 2 seconds at 600 mA / cm² (see curve F of FIG. 3). With such discharge regimes, the voltage after 45 seconds of discharge is 27 V (25 V during pulses at high speed). Under the same discharge conditions, a battery using the conventional cathode material FeS₂ and identical on all the other points will have a discharge voltage of 22.2 V under a discharge current density of 200 mA / cm², and 20.2 V at 600 mA / cm² (see curve E in Figure 3).

0029Of course, the invention is not limited to the exemplary embodiments given above.