Abstract:
Compositions in the NbF 5  (TaF 5 )-P 2  O 5  system which are liquid at room temperature and exhibit relatively high ionic conductivities, useful for example, as electrolytes in batteries or other electrochemical cells, are described.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to ionically conductive materials useful for electrochemical cell electrolytes or the like, and particularly to a new fused salt system which is molten and exhibits good conductivity at low temperatures. 
     Pentavalent metal fluorides are very low-melting, hygroscopic compounds which readily form eutectics with other fluoride salts. Many of these are liquid at or near room temperatures. See, for example, Ray W. Thoma, Advances in Molten Salt Chemistry, Vol 3, Chapter 6 (phase diagrams of binary and ternary fluoride systems). The melting temperatures of SbF 5 , TaF 5 , and NbF 5  are all below 100° C., with SbF 5  exhibiting a boiling point below 150° C. 
     Unfortunately, the reactivity of these fluoride salts with water makes them very difficult to work with. Accordingly, few practical applications involving all-fluoride systems have been pursued. 
     P 2  O 5  has recently been found to be useful in the formation of oxyfluoride glasses exhibiting very low softening temperatures. U.S. Pat. No. 4,314,031 discloses examples of such glasses, based on combinations of SnF 2  and P 2  O 5 , which exhibit glass transition temperatures below 100° C. 
     SUMMARY OF THE INVENTION 
     The present invention is founded on the discovery of a region of composition in the NbF 5  -P 2  O 5  composition system wherein the constituents react to form a stable, ionically conductive liquid phase at room temperature. These liquids are found to be ionically rather than electronically conducting, and while the current carrying species has not been positively identified, it is likely that fluorine or a fluoride complex is involved. Such characteristics make these compositions interesting candidates for electrolytes in batteries or other electrochemical cells. 
     Broadly, compositions in accordance with the invention offering a combination of low melting temperature and low electrical resistivity are those comprising, in weight percent, about 65-95% of one or more pentavalent metal fluorides selected from the group consisting of NbF 5  and TaF 5 , 5-25% P 2  O 5 , and at least about 85% total of NbF 5  +TaF 5  +P 2  O 5 . Optional constituents which may be introduced into the composition without adverse effects on properties include up to about 15% total of halogens selected from the group Cl, Br and I, up to about 2% total of elements selected from the group consisting of B and Li, and up to about 3% S. Hydrogen, which may be present as water in commercially available NbF 5  starting materials, can be present in amounts as high as 2% of the total batch weight. 
     A conductivity cell including a pair of platinum electrodes and a cell electrolyte consisting of a molten NbF 5  -P 2  O 5  composition within the above-described composition range exhibited conductivity characteristics suggesting that the electronic component of conductivity of these electrolytes is quite small. An electrochemical battery cell incorporating one such electrolyte demonstrated an open circuit voltage as high as 4.1 volts, versus a theoretical value of 4.87 volts for the particular cell configuration tested. 
    
    
     DETAILED DESCRIPTION 
     Niobium pentafluoride, the preferred pentavalent fluoride for use in the compositions of the invention, is a hygroscopic salt melting at about 72° C. It is typically supplied in the form of a grey powder, packed under an inert gas, which may be either dry or partially hydrated as received. To prepare compositions including this salt without introducing additional water, it is desirable to use a glove box filled with dry nitrogen to compound the batches and, if possible, to melt the batches under dry nitrogen. 
     Table I below sets forth examples of binary NbF 5  -P 2  O 5  compositions which can be prepared and melted under dry nitrogen to provide fused salt mixtures exhibiting good fluidity and conductivity at room temperature. 
     
                       TABLE I______________________________________                                A.C.Melt   Composition             (Weight %)                       Melt     ResistivityNumber NbF.sub.5  P.sub.2 O.sub.5                       Appearance                                (ohm-cm)______________________________________1      75.4       24.6      amber fluid                                2292      79.7       20.3      amber fluid                                6253      81.6       18.4      amber fluid                                714______________________________________ 
    
     Compositions such as above described are readily melted at temperatures in the range of about 90°-280° C., typically 140°-160° C., and the melts are normally clear and of good quality. Teflon® fluorocarbon-lined containers constitute suitable vessels for melting such compositions. 
     The electrical resistivities reported in Table I are room temperature values which are determined in a conductivity cell between platinum electrodes. Because of cell polarization under d.c. fields, a.c. conductivity measurements are used to determine the electrical resistivities of the fluids; the values reported in Table I are those determined at 10 KHz. 
     Compositions containing TaF 5  in place of NbF 5  can also be melted to provide ionically conductive liquids, although the conductivity of these liquids is not as high and some of the compositions are waxy solids rather than liquids at room temperature. Table II below reports two illustrative compositions in this system, the appearances being those exhibited at room temperature. 
     
                       TABLE II______________________________________                                A.C.Melt   Composition             (Weight %)                       Melt     ResisitivityNumber NbF.sub.5  P.sub.2 O.sub.5                       Appearance                                (ohm-cm)______________________________________4      85.6       14.4      grey melt;                                --                       suspended                       particles5      94.3        5.7      waxy solid                                52 × 10.sup.3______________________________________ 
    
     The water content of the fluoride salt can have important effects on the melting characteristics of these compositions and the properties of the resultant fluid electrolytes. Compositions exhibiting properties such as reported in Table I were compounded using freshly opened, dry NbF 5  purchased from Cerac, Inc., Milwaukee, Wis., 53201, and having a negligible water content. With this salt, nearly all of the starting materials will go into the melt with the exception of a thin, transparent surface skin, although difficulty may be encountered in melting compositions containing more than about 82 weight percent NbF 5 . 
     Table III below reports compositions melted with NbF 5  which had been obtained from the same source, but which had been permitted to age for several months prior to use. Chemical analysis of this starting material indicated that 0.1-0.2% hydrogen (expressed as water) was present. 
     
                       TABLE III______________________________________                                A.C.Melt   Composition             (mole %)  Melt     ResistivityNumber NbF.sub.5  P.sub.2 O.sub.5                       Appearance                                (ohm-cm)______________________________________6      75.4       24.6      amber fluid                                --7      88.7       11.3      amber fluid                                4.8 × 10.sup.38      89.8       10.2      amber fluid                                10.sup.39      93.6       6.4       amber fluid                                10.sup.310     86.3       13.7      amber fluid                                4.7 × 10.sup.3______________________________________ 
    
     Utilizing this starting material, compositions containing up to 95 weight percent NbF 5  were readily melted, although the melts were covered with thick white crusts with some particulate material suspended in the melt. However, volume resistivity values were somewhat higher with these water-containing compositions, typically in the range of about 1-5×10 3  ohm-cm. 
     It is also found that these compositions can be compounded with highly hydrated NbF 5  (2% H by weight, expressed as water), one batch composition being calculated to contain about 63% NbF 5 , 12% H 2  O and 25% P 2  O 5 . Although the resulting melts are very fluid at room temperature and exhibit very low resistivities, (in the range of about 10-100 ohm-cm), they are not truly homogeneous. Melts prepared from highly hydrated NbF 5  typically have the appearance of white milky liquids, suggesting that some phase separation has occurred. 
     The deliberate addition of optional constituents other than hydrogen to the above-described composition is also possible, and can be helpful to enhance or modify selected properties of the melts. Table IV below records examples containing additions of Cl, Br, I, S, Li and B which formed electrically conductive liquids at room temperature. All of these compositions were substantially water-free. The compositions are reported in weight percent on an elemental basis, with the appearance of each melt also being recorded. 
     
                       TABLE IV______________________________________Ele-ments 11      12      13    14    15    16    17______________________________________Nb    39.2    37.1    35.4  37.4  33.1  37.4  37.0F     37.2    35.4    33.7  38.2  46.4  42.4  34.9P     8.7     8.2     7.8   11.7  10.2  7.0   11.1O     11.2    10.6    10.1  12.1  9.9   12.1  15.0Cl    3.9     --      --    --    --    --    --Br    --      8.5     --    --    --    --    --I     --      --      12.8  --    --    --    --S     --      --      --    --    --    --    2.0Li    --      --      --    0.7   0.4   0.4   --B     --      --      --    --    --    0.6   --Melt  yellow  brown   black milky amber milky darkAp-   liquid  liquid  liquid                       liquid                             liquid                                   liquid                                         liq-pear-                                         uidance______________________________________ 
    
     Composition 17 from Table IV above is of particular interest because it reacts spontaneously at room temperature to form a melt, and exhibits a room temperature electrical resistivity lower than that of an unsubstituted analog composition consisting only of NbF 5  and P 2  O 5 . 
     The Drawing consists of a log resistivity plot for a melt corresponding in composition to that of Melt 17 in Table IV (having the molar composition Nb 51  P 48  F 235  S 8  O 120 ) and a melt of very similar composition but containing no sulfur (having a molar composition Nb 50  P 50  F 250  O 125 ). The plot covers log resistivity over the range from about 20°-65° C., as measured at 100 KHz. The significantly lower resistivity of the sulfur-containing analog is evident from these data. 
     Electrochemical cells utilizing molten NbF 5  -P 2  O 5  as a liquid cell electrolyte were fabricated and tested, these cells incorporating a lithium anode and a carbon monofluoride cathode, and having a theoretical open circuit voltage of 4.87 volts. The cells demonstrated rechargeable behavior and exhibited open circuit voltages as high as 4.1 volts.