Patent Publication Number: US-H1544-H

Title: Thermal battery cells containing cathode materials in low-melting nitrate electrolytes

Description:
The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of electrochemistry, and more particularly relates to thermally activated electrochemical cells having a low melting nitrate electrolyte containing novel cathode materials, resulting in an increase in cell potential. 
     2. Description of the Prior Art 
     Thermally activated electrochemical cells or batteries have been used quite extensively in military applications, such as a power source for arming devices, because of their long shelf life and compactness, and capability of withstanding shock and vibration. Batteries of this type typically include an electrolyte which, under normal storage conditions, is solid and does not conduct electricity. When the battery and/or the electrolyte is heated to a predetermined temperature, as by igniting a built-in pyrotechnic heat source such as an electric match, squib or percussion primer, the electrolyte, upon changing to a molten state, becomes conductive and ionically connects the electrodes to provide the desired electromotive force. 
     Nitrate salts have been proposed for use in thermal batteries because of their low melting points. See U.S. Pat. No. 4,260,667 to Miles and Fletcher. For example, potassium nitrate-lithium nitrate (KNO 3  -LiNO 3 ) mixtures melt at temperatures as low as 124° C. The use of a lower melting electrolyte can shorten a battery&#39;s activation time and reduce the weight of heat sources and insulation. 
     A particular problem area of thermal battery cells is the lack of high performance cathode materials. Adding silver salts to electrolytes as cathode materials to improve cell potentials has previously been unsuccessful because the cathodic reactions involve the reduction of silver ions to the free metal in reversible electrode reactions, such as 
     
         AgNO.sub.3 +e-⃡Ag+NO.sub.3 
    
     Consequently, no net cathodic current can flow in such cells at cathode potentials more positive than the silver ion/silver reversible potential. Additionally, the added silver salts migrate and diffuse to the anode and form silver metal films on the anode surface that interfere with cell operation. Many divalent and trivalent metal ions cannot be used at high temperatures in molten nitrate electrolyte cells since they react rapidly with the nitrate melt to form the metal oxide. 
     U.S. Pat. No. 4,416,958 to Miles and Fletcher discloses a thermally activated electrochemical cell having a low melting point electrolyte. The electrolyte is composed of a layer of a mixture of lithium perchlorate and lithium nitrate adjacent the anode and of a layer of a mixture of lithium perchlorate, lithium nitrate, and silver nitrate adjacent to the cathode of the cell. 
     The article &#34;Cyclic Voltammetric Studies of Nitrato Complexes of Various Metal Ions in Molten LiNO 3  +KNO 3  at 180° C.&#34; by M. H. Miles et al, J. Electoanal Chem., 221 (1987) 115-128, discloses addition of various metal ions such as Co ++ , Cu ++ , Au +++ , Mn ++ , La +++ , and Ce +++   ions to molten nitrates, and the effect of such additions. 
     However, the above article does not disclose or teach the application of the principles or concepts that are disclosed therein to thermal batteries, particularly employing lithium or calcium anodes. 
     One object of the invention accordingly is the provision of an improved thermal electrochemical cell. 
     Another object is to provide a novel thermal electrochemical cell utilizing low melting nitrate electrolytes. 
     A still further object is the provision of improved thermal electrochemical cells incorporating certain cathode materials in the electrolyte. 
     Yet another object is to provide thermal electrochemical cells having nitrate electrolyte and containing certain metal salts as cathodic material, to increase the potential of the cell. 
     SUMMARY OF THE INVENTION 
     According to the present invention, it has now been found that addition of certain metal ions in the form of salts to low melting nitrate electrolytes in an electrochemical cell containing a lithium or calcium anode, when activated by heating, produces cathodic currents at cathode potentials significantly more positive than the reversible potential for the metal ion/metal reaction, as in the case of silver salts, as noted above, thereby producing greater cell potentials than previously achieved. 
     The greatest increase in cell potential is accomplished employing a cathode material comprising Cu ++ , Fe +++ , Cr +++   or Au +++   ions and a low melting fused salt nitrate electrolyte such as the eutectic mixture of LiNO 3  and KNO 3 . Other ions such as Co ++ , Eu +++ , La +++ , Ni ++ , Mn ++ , Ce +++ , Pr +++ , Nd +++ , Gd +++ , Sm +++   and Tb +++   can also be used, but yield smaller cell potentials. Such ions are added in the form of a suitable salt of such metal ions to the low melting fused nitrate salt electrolyte. The cathode materials can be added directly to the fused salt electrolyte at various concentrations, as noted hereinafter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
     The cathode material comprising one or more of the above ions, or mixtures thereof, is added to the low melting fused nitrate electrolyte in the form of a salt which is soluble in the electrolyte, such as a nitrate or a halide, e.g. chloride, salt, and diffuses in the electrolyte. 
     The following metal salts have been found useful as a source of the desired metal ions in the nitrate melts: CuCl 2 , CuBr 2 , FeCl 3 , Fe(NO 3 ) 3 , CrCl 3 , AuCl 3 , Co(NO 3 ) 2 , EuCl 3 , LaCl 3 , Ni(NO 3 ) 2 , MnCl 2 , CeCl 3 , PrCl 3 , NdCl 3 , GdCl 3 , Sm(NO 3 ) 2 , and TbCl 3 . Any of these cathode materials, such as CuCl 2  or Co(NO 3 ) 2 , can be added directly to the fused nitrate electrolyte in concentrations ranging from about 2×10 -4  to about 2×10 -1  m(molal). The cathodic current increases directly with increase in concentration. 
     The electrochemical cells of the present invention employ low melting nitrate salts. Generally, mixtures of nitrate salts are employed which melt at temperatures not substantially greater than 200° C. Specific examples of such nitrate electrolyte salts are mixtures such as LiNO 3  /KNO 3  (42-58 mole %), LiNO 3  /NaNO 3  (56-44 mole %), LiNO 3  /NaNO 3  /KNO 3  (37.5-18-44.5 mole %) and LiNO 3  /RbNO 3  (30-70 mole %). Such nitrate electrolytes can also include a nitrite component such as NANO 2 , e.g. the mixture LiNO 3  /KNO 2  (40-60 mole %). All of such mixtures have melting points below 200° C. 
     The cells of the present invention preferably are operated at temperatures not greater than about 200° C. Higher temperatures cause decomposition of the nitrate melts in the presence of the added metal ions. Thus, cells of this invention are operated at temperatures between 124° C. and 200° C. The performance of the cathode materials varies in different nitrate electrolyte melts and at different temperatures. 
     Although applicant is not certain as to the particular theory of operation of the above cathodic materials in the low melting nitrate electrolytes, the increase in the cell potential obtained employing the cathode materials of the invention is achievable because there is no reversible reduction of metal ions to the free metal as in the case of the silver salts. Instead, the metal salt cathode materials of the present invention combine with the low melting nitrates to form nitrato complexes of metal ions which are reduced in irreversible cathodic reactions. 
     The metal salts comprising the cathode material according to the invention, can be employed in electrochemical cells having Li or Ca anodes because the metal ion of the cathode material is not reduced to the free metal. This prevents any metal film from forming on the anode surface. Instead, the nitrato complexes of the metal ions are reduced to the metal oxide or oxide species such as FeO + . Many of the metal oxide products thus produced in the present invention are sufficiently soluble in the electrolyte melt, and do not passivate the cell electrodes, and the cathode materials hereof can accordingly be allowed to mix with the electrolyte throughout the entire cell. 
     The metal salts of which cathode materials are comprised can be used in conjunction with other cathodic materials such as silver salts, e.g. AgNO 3 , that undergo reduction to silver metal. Under these circumstances, a small concentration of silver salt not to exceed the concentration of the metal salt cathode material hereof should be employed, e.g. a proportion of about 0.01 to about 1 mole of silver salt per mole of metal salt cathode material, and it is essential that the silver ion be kept near the cathode or cathode collector, so it does not plate out on the anode. Many metal ion additives of the present invention, Cu ++ , Fe +++ , Cr +++ , and Au +++ , give cathodic reactions positive to the Ag +  /Ag cathode reaction. 
     In the electrochemical cell of the present invention the low melting nitrate electrolyte containing the metal salt cathode materials hereof is disposed between a lithium or calcium anode, and a cathode, which can be in the nature of a metal current collector such as a nickel screen, with electrical connections to the anode and the cathode. However, if desired, the metal salt cathode material can be used as a solid cathode laver in the electrolyte in solid form, spaced from the anode, and adjacent to the cathode current collector, instead of being added directly to the electrolyte melt. 
     According to a preferred embodiment of the invention, providing a thermal battery cell with a lithium anode, a nickel screen cathode current collector spaced from the anode, and a LiNO 3  /KNO 3  electrolyte containing CuCl 2  cathode material in a concentration of 0.02 m with respect to the electrolyte, the electrolyte can be provided as a disc such as of fiberglass filter paper with the electrolyte containing the cathodic materials absorbed thereon. Such electrolyte can be prepared by dipping the disc into the molten electrolyte having the CuCl 2  cathode material diffused therein, removing and allowing the electrolyte to solidify. The treated fiberglass discs are then placed with their flat surfaces adjacent to each other and sandwiched between the anode and cathode to form the cell as described above. 
     The following are examples of practice of the invention: 
     EXAMPLE 1 
     CuCl 2  as Cathode Material 
     Cyclic voltammetric studies at 50 mV/s (millivolts per second) of 0.020 m CuCl 2  in LiNO 3  /KNO 3  (42-58 mole %), electrolyte m.p.=124° C.) at 180° C. show dramatic increases in the electrochemical reactivity of the LiNO 3  /KNO 3  melt. Reduction waves for the melt begin near +0.3 V (vs. Ag +  /Ag). From the results of the cyclic voltammetric studies, a thermally activated electrochemical cell constructed having a Li anode, LiNO 3  /KNO 3  electrolyte (mole fraction KNO 3  =0.58, m.p.=124° C.) and a CuCl 2  cathode will have an open circuit potential (Eoc)=3.7 V at 180° C. Current densities of 4 mA/cm 2  can be achieved for 0.02 m CuCl 2  and densities of 20 mA/cm 2  can be achieved for 0.1 m CuCl 2 . A cell potential of 3.4 V at 20 mA/cm 2  can be achieved for 0.1 m CuCl 2 . 
     EXAMPLE 2 
     FeCl 3  as Cathode Material 
     Cyclic voltammetric studies at 50 mV/s of 0,020 m FeCl 3  in LiNO 3  /KNO 3  (42-58 mole %) electrolyte, m.p.=124° C.) at 180° C. show dramatic increases in the electrochemical reactivity of the LiNO 3  /KNO 3  melt. Reduction waves for the melt begin near +0.6 V (vs. Ag +  /Ag). From the results of the cyclic voltammetric studies, a thermally activated electrochemical. cell constructed having a Li anode, LiNo 3  /KNO 3  electrolyte (mole fraction KNO 3  =0.58, m.p=124° C. and a FeCl 3  cathode material would have an Eoc=4.0 V at 180° C. Current densities of 2 mA/cm 2  can be achieved for 0.02 m FeCl 3 . A cell potential of 3.8 V at 10 mA/cm can be achieved for 0.1 m FeC13. 
     EXAMPLE 3 
     AuCl 3  as Cathode Material 
     From cyclic voltammetric studies similar to Examples 1 and 2, employing the same nitrate electrolyte and the same Li anode as seen in Examples 1 and 2, but using AuCl 3  as cathode material, the cathodic wave begins at +0.8 V (vs. Ag +  /Ag). As set forth in the previous examples, a cell constructed having 0.02 m AuCl 3  will have an Eoc=4.2 V at 180° C. A cell potential of 3.8 V can be achieved at a current density of 10 mA/cm 2  for 0.1 m AuCl 3 . 
     EXAMPLE 4 
     Ni(NO 3 ) 2  as Cathode Material 
     From studies similar to Examples 1 and 2, a new cathodic wave begins at -0.3 V (vs. Ag +  /Ag) for 0.02 m Ni(NO 3 ) 2  added to the LiNO 3  /KNO 3  melt at 180° C. Therefore, a thermally activated cell constructed with a lithium anode and 0.02 m Ni(NO 3 ) 2  in LiNO 3  /KNO 3  will have an Eoc=3.1 V at 180° C. A cell potential of 2.4 V can be achieved at 25 mA/cm 2  for 0.1 m Ni(NO 3 ) 2 . 
     EXAMPLE 5 
     Co(NO 3 ) 2  as Cathode Material 
     From cyclic voltammetric studies similar to Examples 1 and 2, the cathodic wave begins at -0.4 V (vs. Ag +  /Ag) with 0.02 m Co(NO 3 ) 2  added to the LiNO 3  /KNO 3  melt at 180° C. Therefore, a cell constructed with a lithium anode and 0.02 m Co(NO 3 ) 2  will yield an Eoc=3.0 V at 180° C. A cell potential of 2.5 V can be achieved at a current density of 20 mA/cm 2  for 0.1 m Co(NO3) 2 . 
     EXAMPLE 6 
     EuCl 3  as Cathode Material 
     From cyclic voltammetric studies similar to Examples 1 and 2, a new cathodic wave begins at -0.5 V (vs. Ag +  /Ag) with 0.02 m EuCl 3  added to the LiNO 3  /KNO 3  melt at 180° C. Therefore, a cell constructed with a lithium anode and 0 02 m EuCl 3  will have an Eoc=2.9 V at 180° C. A cell potential of 2.4 V can be achieved at a current density of 10 mA/cm 2  for 0.1 m EuCl 3 . 
     EXAMPLE 7 
     MnCl 2  as Cathode Material 
     From cyclic voltammetric studies similar to Examples 1 and 2, the new cathodic wave begins at -0.6 V (vs. Ag +  /Ag) when 0.02 m MnCl 2  is added to the LiNO 3  /KNO 3  melt at 180° C. Therefore, a cell constructed with a lithium anode and 0.02 m MnCl 2  in LiNO 3  /KNO 3  will have an Eoc=2.8 V. A cell potential of 2.5 V can be achieved at a current density of 25 mA/cm 2  for 0.1 m MnCl 2  as the active cathode material. 
     The present invention has the novel feature of producing cathodic currents at potentials significantly more positive than the potential of certain electrode reactions, thereby producing greater cell potentials than could be previously achieved. 
     This is believed due to the formation of nitrato-complexes with the metal ions that allows the nitrate ions to be reduced more readily. Negative electrons can be transferred more readily to nitrate ions that are associated with positive metal ions. The complexed nitrate ions can then be reduced at potentials more positive than potentials where reduction of metal ions occur. 
     The effects of increased cell potential have been observed using the cathode materials of the invention at various concentrations in low melting nitrate electrolytes by adding the cathode materials directly to the electrolyte melt. The use of the cathode materials of the invention does not interfere with the lithium or calcium anodes employed. As previously noted, the use of the cathodic materials of the invention can be practiced in conjunction with other cathodic materials such as silver salts, which undergo reduction to the free metal. 
     Since various changes and modifications can be made in the invention without departing from the spirit of the invention, the invention is not to be taken as limited except by the scope of the appended claims.