Abstract:
A thermal battery which comprises a number of stacked cells, each cell consisting essentially of an anode with a lithium compound, a lithium free pyrotechnic heat source pellet which includes a cathode precursor and an all lithium electrolyte layer separating between the anode and the pyrotechnic heat source pellet. Upon ignition of the heat source, an oxide of the cathode precursor is obtained which is lithiated by the lithium supplied by the all lithium electrolyte ion source.

Description:
[0001]     This is a continuation-in-part of U.S. patent application Ser. No. 10/704,612 filed Nov. 12, 2003, which is a continuation-in-part of U.S. patent application Ser. No. 10/138,582 filed May 6, 2002, which claims priority from IL patent application No. 142994 filed May 6, 2001. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to the improvement of thermal batteries, more particularly to the increasing of power and operational longevity of “cathode-less” thermal batteries.  
         [0003]     Thermal batteries are thermally activated, primary reserve, hermetically sealed power sources generally consisting of series or series-parallel arrays of cells. Each cell comprises of an anode, an electrolyte-separator, a cathode and a pyrotechnic heat source.  
         [0004]     Typically, the electrolyte is a mixture of alkali metal halides, for example an eutectic mixture of KCl-LiCl which melt at about 350° C. Common cathode materials among others are iron disulfide and cobalt sulfide. Transition metal oxides such as iron oxide can be used as cathode material as well.  
         [0005]     Lithium-iron or lithium alloys such as lithium-aluminum and lithium-silicon are generally used as anode materials. The pyrotechnic material is a mixture of high surface area iron powder and potassium perchlorate. The main role of this pyrotechnic material is to provide when being ignited the thermal cell with the exact heat needed to melt the electrolyte.  
         [0006]     The exothermic mixture used in the pyrotechnic material burns at a controlled rate, it must not melt, should not produce gas, it must have a caloric output that can be closely controlled. It has to be insensitive enough to be pelleted without igniting, then be able to be lit reliably over a wide temperature range and after ignition it must be electrically conductive, because the burnt pyrotechnic pellets form the inter-cell electrical connectors.  
         [0007]     Near stochiometric ratios of the components of the exothermic mixture (i.e. 63 wt % Fe and 37 wt % KClO 4 ) produce temperatures high enough to melt iron, so to moderate the reaction, to control the rate of burning and to produce a conductive ash, mixtures containing no more than 17% of perchlorate can be used with iron, which upon burning form a thermodynamically stable oxide Fe 0.947 O, which is dispersed within the particles of the excessive iron.  
         [0008]     Recently, U.S. Pat. No. 5,770,329 to Harney, incorporated by reference for all purposes as if fully set forth herein, describes a thermal battery in which the pyrotechnic pellets serve also as cathode precursor wafers which after the generation of heat supply the cathode material of the battery cells, thus eliminating the need for distinct cathode layers.  
         [0009]     Thermal batteries, which are produced according to “329” patent suffer from two major inherent drawbacks which limit their power delivery capability as well as their operation life time:  
         [0010]     Firstly the concentration of the cathode material (i.e. the iron oxide) formed by the burning of the exothermic mixture in the cathode-precursor pyrotechnic pellet is low.  
         [0011]     This is so because, as was said before, it is practically impossible to increase the amount of the oxidizing compound (e.g. KClO 4 ) relative to the amount of the iron powder over a weight ratio of 17:83 respectively. Thus the maximum weight concentration of the FeO formed in the matrix of the pyrotechnic pellet can reach only to about 30%.  
         [0012]     Secondly, the utilization of the formed cathode material by Li +  ions during battery operation (the intercalation process) is low; This is so because the “329” patent does not teach an appropriate way to “flood” the formed FeO particles with surrounding Li +  ions.  
         [0013]     An attempt according to the “329” patent to replace some or all of the KClO 4  in the pyrotechnic pellet with LiClO 4  results with an Li +  ion concentration of at most of about 8%. Other trials to increase the Li +  concentration by stuffing the pyrotechnic pellet with non oxidizing lithium salts (as LiCl) have the following drawbacks:  
         [0014]     They lower too much the specific caloric value of the pellet and by this increase the battery rise time, they decrease the pellets electronic conduction and besides, the embedded lithium salts in the pyrotechnic pellet absorbs humidity extensively.  
         [0015]     Thus, the batteries according to the invention of the “329” patent are applicable only at small current densities and have a short life-time.  
         [0016]     The present invention removes these drawbacks and provides other related advantages.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention provides a “cathode-less” thermal battery, which is based on a conventional iron-KClO 4  pyrotechnic components and on an all-lithium electrolyte separator.  
         [0018]     In accordance to the present invention there is provided a thermal battery of the type heated to an operating temperature having a plurality of stacked cells, each cell consisting essentially of: (a) an anode which includes a lithium compound; (b) a substantially lithium free pyrotechnic heat source, the pyrotechnic heat source includes a cathode precursor, and (c) a separator separating between said anode and said pyrotechnic heat source, said separator includes an LiF-LiCl-LiBr mixture having the lowest melting point for a mixture of such constituents.  
         [0019]     In accordance with the present invention there is provided a method for improving power and operational life-time of a “cathode less” thermal battery, the method comprises the steps of:  
         [0020]     (a) providing a “cathode less” thermal battery, said thermal battery having a pyrotechnic heat source wherein a metal in said pyrotechnic heat source performs as a cathode precursor; (b) igniting said pyrotechnic heat source obtaining an oxide of said metal, and (c) using an all-lithium electrolyte as a sole lithium ion source for a lithization of said oxide of said metal.  
         [0021]     It is an object of the present invention to provide a thermal battery capable of sustaining current densities up to about 1000 mA/cm 2 .  
         [0022]     It is another object of the present invention to provide a thermal battery with an extended life-time.  
         [0023]     It is still another object of the present invention to provide a more reliable thermal battery.  
         [0024]     It is a further object of the present invention to provide a thermal battery having reduced dimensions at a reduced production cost.  
         [0025]     Other objects and goals of the present invention will become apparent upon reading the following description in conjunction with the following figures.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     The invention is herein described, by way of examples, with reference to the accompanying drawings, wherein:  
         [0027]      FIGS. 1A and 1B  show a traverse section of a thermal cell of a “cathode-less” thermal battery and of a conventional thermal battery respectively;  
         [0028]      FIG. 2  shows a discharge curve of a “cathode-less” battery which is described in example 1;  
         [0029]      FIG. 3  shows a discharge curve of a “cathode-less” battery which is described in example 2;  
         [0030]      FIG. 4  shows a discharge curve of “cathode-less” battery which is described in example 4;  
         [0031]      FIG. 5  shows a discharge curve of a “cathode-less” battery which is described in example 5;  
         [0032]      FIG. 6  shows a discharge curve of a “cathode-less” battery which is described in example 6, and  
         [0033]      FIG. 7  shows a discharge curve of a “cathode-less” battery which is described in example 7. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0034]     The present embodiments herein are not intended to be exhaustive and to limit in any way the scope of the invention, rather they are used as examples for the clarification of the invention and for enabling of other skilled in the art to utilize its teaching.  
         [0035]      FIG. 1A  depicts a typical cell  10  of what is being referred hereinafter as a “cathode-less” thermal battery, in which a cathode  11  of a cell of a conventional thermal battery, shown in  FIG. 1B  to which reference is now made, becomes redundant.  
         [0036]     As explained above, a prerequisite for satisfactorily operation of a “cathode-less” thermal battery  10 , in which an oxidized pyrotechnic precursor (not shown) in pyrotechnic pellet  12  is used as the cathode material, is a high rate of lithization of the formed oxidized pyrotechnic precursor particles by surrounding Li +  ions (not shown). This demands high concentration of Li +  ions in pyrotechnic pellet  12 .  
         [0037]     It was also explained why it is unlikely that a major contribution to the needed Li +  ion concentration can originate form lithium salts, which were dispersed in advance in pyrotechnic pellet  12 .  
         [0038]     In the present invention we make use of the fact that it is only a narrow layer  12 ′ of pyrotechnic pellet  12  beneath the pyrotechnic pellet surface  13  which contacts electrolyte layer  14 , which takes place in the Li +  intercalation process. The rest  12 ″ of the thickness of pyrotechnic pellet  12  is substantially intact with respect to any electrochemical change and merely conduct electrons.  
         [0039]     Thus it is needed to supply Li +  ions to layer  12 ′ only, rather then to entire pellet  12 .  
         [0040]     Because the limited volume of layer  12 ′ and the intimate contact of interface  13  with electrolyte wafer  14 , it is possible to build up the concentration of Li +  ions which is needed in region  12 ′, by Li +  ion transfer from electrolytic layer  14 , provided that the Li +  ion flux toward layer  12 ′ will be sufficiently high.  
         [0041]     To accomplish such high flux, a separator, which is composed of a mixture of LiF, LiCl and LiBr in an appropriate weight ratio which corresponds to a composition having the lowest melting point of any mixtures of these constituents was used.  
         [0042]     The preparation, properties and use of such a mixture of lithium salts in conventional thermal batteries is prior art which is described e.g. in the proceedings of the 33 rd  international power sources symposium, 13-16 June 1988, published and distributed by the Electrochemical society Inc, pages 369-392, incorporated by reference for all purposes as if fully set forth herein.  
         [0043]     There, such mixture of lithium salts is alternatively referred to as an “all-lithium electrolyte” or as an “all-lithium eutectic” (page 373 therein) and the applicability of the all-lithium electrolyte to (conventional) thermal batteries with extreme current density is demonstrated.  
         [0044]     It was also reported that lithiating of the cathode layer by the all-lithium electrolyte is so extensive that it proceeds at an appreciable rate even at room temperature, see e.g. U.S. Pat. No. 5,900,331 to Krieger.  
         [0045]     We disclosed in the present invention that the beneficial effect of the all-lithium electrolyte in extending “cathode-less” batteries life-time as compared to an electrolyte having the conventional LiCl-KCl eutectic composition (45% wt: 55% wt respectively) is far more pronounced then is anticipated in accordance to the enrichment (by about 55%) in the Li +  ion content of the all-lithium electrolyte, as compared to the LiCl-KCl eutectic.  
         [0046]     The way in which such non-anticipated improvement in batteries performance occurs will now be explained:  
         [0047]     One problem associated with the all-lithium electrolyte is its relative high melting point (445° C., as compared to the melting point of 350° C. for the LiCl-KCl eutectic). This causes the discharge voltage of a conventional battery employing the all-lithium electrolyte to drop as soon as the temperature of the battery cools below 460° C.  
         [0048]     In Japanese Patent No. 02 021568 A to Tsukamoto Hisashi, a method is disclosed to extend the life-time of an all-Li electrolyte conventional thermal battery (in which the electrolyte in the cathode is the first to freeze) which comprises of adding KCl to the cathode material.  
         [0049]     The KCl added reacts with the lithium ions which enters the cathode layer during the battery discharge to produce a LiCl-KCl eutectic which has a relatively lower melting point then the all-Li electrolyte, thus the performance at low temperatures is improved and the battery gains a longer life-time.  
         [0050]     The point to emphasise here is that in accordance with the present invention, the oxidizing agent in the pyrotechnic pellet is KClO 4 , which upon oxidation converts into KCl, thus a situation similar to that described in Japanese Patent No. 02021568 A is reproduced in-situ.  
         [0051]     Consequently, from one hand an all-lithium electrolyte is the favourable electrolyte for a “cathode-less” battery, which needs an extensive lithization. From the other hand the initially lithium free pyrotechnic heat source which contains KClO 4  is a favourable candidate to be effected by the all-lithium electrolyte.  
         [0052]     This cooperative effect which results from a combination of a pyrotechnic pellet having KClO 4  as the oxidizing agent together with an all-lithium electrolyte, to provide the highest performing “cathode-less” electrochemical cell, is neither rendered obvious nor can it be anticipated by a skilled person in the art of thermal batteries.  
         [0053]     Generally, the cells of the improved batteries according to the present invention include an anode  9 , a substantially lithium free pyrotechnic heat source  12 , which includes a cathode precursor, and an all-lithium electrolyte separator  14  which is disposed between anode  9  and pyrotechnic heat source  12 .  
         [0054]     Pyrotechnic cathode-precursor according to the present invention comprises a high surface area iron powder.  
         [0055]     The iron powder is mixed with a powder of potassium perchlorate to form a physical mixture. It should be noted that other transition metals such as Co, Ni, Mn, Cu, V, Cr and combinations thereof could be used as pyrotechnic cathode precursor with or besides iron as well, in such cases the weight percentage of the potassium perchlorate can be some times higher whenever the heat of formation of the oxides is smaller.  
         [0056]     When using iron, the pyrotechnic cathode-precursor pellets were prepared by blending vacuum dried −200 mesh iron powder with potassium perchlorate having gain size of 4 to 7 μm in a Turbula mixer for about one hour.  
         [0057]     The composition of the mixture was 70%-90% Fe powder, and 30% to 10% KClO 4  respectively, because no lithium additive was used this composition is a substantially lithium free mixture.  
         [0058]     The homogeneous blend is then pressed at 0.5 to 3 ton/cm 2  to obtain 14 or 30 mm diameter pyrotechnic pellets.  
         [0059]     Electrolyte-separator pellets are formed according to well-known procedures. The pellets comprise about 40 wt % to 60 wt % MgO binder with a LiF-LiCl-LiBr mixture with the lowest melting point composition having weight proportions of about 9.6%-22%-68.4% respectively. This mixture of lithium salts is referred hereinafter to as “LiF-LiCl-LiBr eutectic electrolyte”. The separator wafers were prepared by pressing a fused LiF-LiCl-LiBr eutectic electrolyte-MgO mixture at 0.5 to 3 ton/cm 2  into 14 or 30 mm diameter pellets.  
         [0060]     Anodes were prepared by blending lithium-aluminum alloy powder, eutectic LiCl-KCl electrolyte, and iron powder at about 64:16:20 weight ratio, following by pressing the mix at 1.5-2.5 ton/cm 2  into 14 or 30 mm diameter anode pellets.  
         [0061]     All anode and separator pellets were prepared in a dry room environment having a relative humidity of less than 1%.  
         [0062]     The following examples, which are illustrative only and do not limit the invention, demonstrate the performance of “cathode-less” batteries according to the present invention and compares it with the performance of “cathode-less” batteries which their precursor heat source does not include a lithium compound and which has a LiCl-KCl electrolyte in their separator as the lithization agent.  
       EXAMPLE 1  
       [0063]     Two “cathode-less” thermal batteries were assembled. The batteries consist of two sections; a 24V section and a 7V section, made of 30 mm diameter cells. Each cell comprises 0.28 g anode, 0.67 g KCl-LiCl electrolyte-separator (having 55:45 weight percent ratio respectively) and 0.97 g cathode-precursor pyrotechnic (CPP) pellet. The CPP pellet composition was Fe-KClO 4  at a weight fraction of 83% and 17% respectively. This composition provided about 314 cal/pellet at a burning rate of about 100 mm/sec without any significant gas formation.  
         [0064]     The batteries were conditioned at −40° C. and −60° C. and then discharged at a constant load of 0.3 A with several pulses of 5 A up to 13 A. The discharge data are summarised in Table 1 and the discharge curves of the −40° C. battery are shown in  FIG. 2 .  
         [0065]     In Table 1 and the forthcoming Tables; Rise-time and Life-time are defined as the two periods which passed since the activation of the battery until the section&#39;s voltage crosses a predetermined fraction of its nominal value.  
                                                   TABLE 1                           discharge data of “cathode-less” thermal batteries of example 1.                +24 V section   +7 V section            discharge   Rise-time,       Rise-time,           temperature   msec   Life time, sec   msec   Life time, sec               −40° C.   78   53   61   170       −60° C.   81   52   64   150                  
 
       EXAMPLE 2 (the present invention)  
       [0066]     The same batteries as in example 1 were built with the only exception of having the LiF-LiCl-LiBr eutectic electrolyte-separator instead of the KCl-LiCl electrolyte-separator. The discharge data are summarised in Table 2 and the discharge curve of the −40° C. battery is shown in  FIG. 3 .  
                                                   TABLE 2                           discharge data of “cathode-less” thermal batteries of example 2.                +24 V section   +7 V section            discharge   Rise time,   Life time,   Rise time,           temperature   msec   sec   msec   Life time, sec               −40° C.   81   102   54   185       −60° C.   79   102   53   175                  
 
       EXAMPLE  3  (the present invention)  
       [0067]     One “cathode-less” thermal battery was assembled. The battery consists of two strings of 20, 30 mm diameter cells, connected in parallel to give an 28V section. Each cell comprises 0.28 g anode, 0.67 g LiF-LiCl-LiBr eutectic electrolyte-separator and 0.85 g cathode-precursor pyrotechnic (CPP) pellet. The CPP pellet composition was Fe-KClO 4  at a weight fraction of 83% and 17% respectively. This composition provided about 270 cal/pellet at a burning rate of about 100 mm/sec without any significant gas formation.  
         [0068]     The battery was discharged at room temperature at a constant load of 15 A with pulses of 20 A every second (not shown). The battery rise time was 65 msec while the life-time to 28 V was larger than 5 sec. A same “cathode-less” reference battery with LiCl-KCl electrolyte in its separator, collapses at these operation conditions from the first moment of its activation and no life-time time can be defined.  
       EXAMPLE 4  
       [0069]     One “cathode-less” thermal battery was assembled. The battery consists of 10, 30 mm diameter cells. Each cell mainly comprises 0.28 g anode, 0.75 g LiCl-KCl (45:55 weight ratio respectively) electrolyte-separator and 1.54 g CPP pellet. The CPP pellet composition was Fe-KClO 4  at a weight fraction of 86% and 14% respectively. This composition provided about 400 cal/pellet at a burning rate of about 90 mm/sec without any significant gas formation.  
         [0070]     The battery was discharge at room temperature at a constant load of 300 mA with pulses of 0.5 A every 40 sec. The battery life-time to 14V was 25 sec as is shown in  FIG. 4  to which reference is now made.  
       EXAMPLE 5 (the present invention)  
       [0071]     The same battery as in example 4 was built with the only exception of having the LiF-LiCl-LiBr eutectic electrolyte-separator instead of the LiCl-KC electrolyte-separator. The battery life-time to 14V extends to 200 sec. as is shown in  FIG. 5  to which reference is now made.  
       EXAMPLE 6  
       [0072]     Two “cathode-less” thermal batteries were assembled. The batteries consist of three sections (+12V, −12V and +2.8V) made of cells of 14 mm in diameter. Each cell comprises 0.06 g anode, 0.12 g KCl-LiCl electrolyte-separator (35:65 weight ratio respectively) and 0.2 g CPP pellet. The pellet composition was Fe-KClO 4  at weight fraction of 83% and 17% respectively. This composition provided about 65 cal/pellet at a burning rate of about 100 mm/sec without any significant gas formation.  
         [0073]     The batteries were cold conditioned at −40° C. and +25° C. and then discharge through a constant resistance of 150 Ω (+12V and −12V sections) and 1.87 Ω (+2.8V section). The discharge data are summarised in Table 3 and the discharge curve of the −40° C. battery is shown in  FIG. 6  to which reference is now made.  
                                                                                                                       TABLE 3                           discharge data of “cathode-less” thermal batteries of example 6.                +12 V section       +2.8 V section                Rise   Life   −12 V section       Life            discharge   time,   time,   Rise time,   Life time,   Rise time,   time,       temperature   msec   sec   msec   sec   msec   sec                    −40° C.   96   19   68   21   50   15       +25° C.   112   28   82   28   60   15                  
 
       EXAMPLE  7  (the present invention)  
       [0074]     The same batteries as in example 6 were built with the only exception of having the LiF-LiCI-LiBr eutectic electrolyte-separator instead of the LiCl-KCl electrolyte-separator. The discharge data are summarized in Table 4 and the discharge curves of the −40° C. battery are shown in  FIG. 7  to which reference is now made.  
                                                                                             TABLE 4                           discharge data of “cathode-less” thermal batteries of example 7.                +12 V section   −12 V section   +2.8 V section                    Life       Life       Life       discharge   Rise time,   time,   Rise time,   time,   Rise time,   time,       temperature   msec   sec   msec   sec   msec   sec                    −40° C.   96   &gt;40   82   &gt;40   52   19       +25° C.   98   21   68   32   48   19                  
 
         [0075]     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made without departing from the spirit and scope of the invention.