Abstract:
A non-aqueous electrolyte for an electrolytic capacitor contains as solute a salt obtained from an amine and a trialkyl phosphate.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an electrolytic capacitor and more particularly to a non-aqueous electrolyte for an electrolytic capacitor, especially an aluminum electrolytic capacitor. 
     As is well-known, electrolytic capacitors comprise an anode of a so-called valve metal which has an oxide film on it in contact with an electrolyte. Two of the most commonly used metals are aluminum and tantalum. Aluminum is widely used because of its lower density, lower cost, availability in high-purity form, and relative ease of reformation. 
     Many electrolytes for aluminum capacitors have unsatisfactory low- and/or high- temperature characteristics. Thus, aqueous electrolytes are restricted generally to operation above the freezing-point and below the boiling-point of the solvent, i.e., water. While ethylene glycol has a lower freezing-point and a higher boiling-point than water, its resistivity at low temperatures is unsatisfactory. N,N-dimethylformamide, another preferred solvent, has a low freezing-point but a boiling-point below that of ethylene glycol. 
     Beside having a low-freezing and a high-boiling point, an electrolyte solvent should have a high dielectric constant and be chemically neutral to the electrodes and the dielectric oxide. Such solvents include 4-butyrolactone, 3-methoxypropionitrile, propylene carbonate, methyl carbitol, and butyl cellosolve. 
     Unfortunately, many solutes which have been used in the past with non-aqueous solvents, especially dimethylformamide, are not necessarily useful with other non-aqueous solvents, e.g., 4-butyrolactone. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an electrolyte with improved high- and low- temperature operating characteristics. 
     It is a further object of this invention to provide solutes suitable for use in non-aqueous solvents, especially 4-butyrolactone. 
     These objects have been attained through the use as solutes of salts obtained from a trialkylphosphate and an amine. These materials may be prepared by refluxing of a trialkylphosphate and an amine for 12-24 hrs. In the course of this reaction, the trialkylphosphate is converted to a dialkylphosphate anion, and the amine is alkylated to an alkyl-substituted ammonium ion. For example, the reaction of piperidine and triethylphosphate yields N-ethylpiperidinium diethylphosphate which is the product that would result from the neutralization of diethylphosphoric acid with N-ethylpiperidine. The other product that might have been expected, an amide, is the product of ammonolysis of triethylphosphate. However, both 100 MHz proton NMR and phosphorus NMR spectra indicate that the reaction proceeds quite cleanly to give the first product with less than 1% of the amide being formed. The reaction takes place with primary, secondary, and tertiary amines. When tertiary amines are used, the cation formed is a quaternary ammonium ion, and this has been confirmed via picrate derivatives. In all cases, the products were viscous, colored syrups. Conductivities were between 4400 and 6300 Ω cm for the undiluted products. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Referring now to the appended drawing, a capacitance section 10 is shown in a partly unrolled condition. Anode 11 is of aluminum or tantalum and has an insulating oxide layer on its surface. Cathode 12 may be also made of aluminum or tantalum and is preferably etched. The anode is also preferably etched. Films 13 and 14 are spacers and may be of paper, polymer film or a combination of these. Tabs 15 and 16 are connected to electrodes 11 and 12 respectively to function as terminals for capacitance section 10. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the examples that follow, the solutes are listed in the tables according to starting materials. Thus, N-butylpiperidinium dibutylphosphate is listed as piperidine-tributylphosphate, and N-ethyltributylammonium diethylphosphate is listed as tributylamine-triethylphosphate. The amines used are piperidine, piperazine, morpholine, N-methylmorpholine, ethanolamine, triethylamine, and tributylamine. The phosphates used are trimethylphosphate, triethylphosphate, and tributylphosphate. The salts obtained from the specific products listed in the tables are N-butylpiperidinium dibutylphosphate, N-methylpiperidinium dimethylphosphate, N-ethylpiperidinium diethylphosphate, N-ethyl-2-hydroxyethylammonium diethylphosphate, N-methyl-2-hydroxyethylammonium dimethylphosphate, tetraethylammonium diethylphosphate, N-methyltriethylammonium dimethylphosphate, N-ethyltributylammonium diethylphosphate, N-ethyl,N-methylmorpholinium diethylphosphate, N-ethylmorpholinium diethylphosphate, and N,N&#39;-diethylpiperazinium bis-diethylphosphate. These salts were evaluated in the following solvents: N,N-dimethylformamide (DMF), 4-butyrolactone (BLO), 3-methoxypropionitrile (MPN), acetonitrile (ACN), ethyleneglycol (glycol), propylene carbonate (Pr carbonate), diethyleneglycolmethylether (Me carbitol), ethyleneglycol monobutylether (Bu cellosolve), and mixtures of these. 
     EXAMPLE 1 
     Resistivity data at 25° C. in ohm-cm (Ω cm) are presented for various salts in a variety of solvents. The amount of solute in whole-number percent in each solvent is given also in Table I. In most cases, about 1 gm water may be present. 
     
                       Table I______________________________________Salt formed fromreaction of  %        Solvent       Ω cm______________________________________Piperidine-  (40%)    MPN           664 tributylphosphate        (33%)    MPN           1030        (25%)    BLO           1073        (20%)    glycol        1008        (20%)    Pr carbonate  1030        (33%)    Me carbitol   2360Piperidine-  (25%)    BLO           305 trimethylphosphatePiperidine-  (25%)    DMF           337 triethylphosphate        (24%)    MPN           408        (25%)    Bu cellosolve 1115        (24%)    Me carbitol   837        (25%)    glycol        485        (25%)    Pr carbonate  506Ethanolamine-        (26%)    DMF           909 triethylphosphate        (24%)    glycol        734        (33%)    Bu cellosolve 1459        (25%)    Pr Carbonate  2231        (35%)    Me carbitol   1780        (24%)    BLO           1973        (25%)    MPN           1287Ethanolamine-        (33%)    glycol        405 trimethylphosphate        (10%)    DMF           855Triethylamine-        (50%)    glycol        493 triethylphosphate        (12%)    glycol-MPN(40:60                               270                  by wt.)        (21%)    glycol-MPN(26:74                               184                  by wt.)Triethylamine-        (46%)    glycol        311 trimethylphosphate        (65%)    glycol        202        (25%)    DMF           146        (33%)    DMF           75tributylamine-        (20%)    BLO           743 triethylphosphate        (10%)    DMF           570N-methylmorpholine- triethylphosphate        (20%)    BLO           333morpholine-  (25%)    DMF           417 triethylphosphate        (33%)    glycol        686        (25%)    Me carbitol   1502        (25%)    BLO           686        (35%)    Bu cellosolve 2016        (25%)    Pr carbonate  807        (33%)    MPN           601______________________________________ 
    
     EXAMPLE 2 
     Maximum formation voltages (V max ) at various temperatures are given in Table II for aluminum foil along with resistivity in ohm-cm (Ω cm) at 25° C. for the particular formulation. 
     
                       Table II______________________________________Formulation   Ω cm                 Vmax.sub.25                          Vmax.sub.105                                 Vmax.sub.125______________________________________10g   Et.sub.3 PO.sub.4 - tributylamine90g   DMF2g    H.sub.3 BO.sub.31g    H.sub.2 O   396     293    --     49510g   Et.sub.3 PO.sub.4 - piperidine90g   BLO3g    H.sub.2 O   477     372    188    --10g   Et.sub.3 PO.sub.4 - piperazine90g   BLO3g    H.sub.2 O   1580    295    --     26710g   Et.sub.3 PO.sub.4 - morpholine90g   DMF2g    H.sub.3 BO.sub.31g    H.sub.2 O   438     465    425    --10g   Et.sub.3 PO.sub.4 - morpholine90g   BLO         981     500    492    22717g   Et.sub.3 PO.sub.4 - morpholine78g   BLO         610     480    383    43017g   Et.sub.3 PO.sub.4 - morpholine68g   BLO1g    H.sub.2 O   406     450    445    --20g   Et.sub.3 PO.sub.4 - morpholine80g   BLO1g    H.sub.2 O   506     450    455    --40g   Me.sub.3 PO.sub.4 - piperidine160g  BLO3%    H.sub.2 O   324     465    177    --20g   Et.sub.3 PO.sub.4 - tributylamine80g   BLO1%    H.sub.2 O   702     228    174    --35g   Me.sub.3 PO.sub.4 - ethanolamine35g   glycol3%    H.sub.2 O   918     155     94    --______________________________________ 
    
     EXAMPLE 3 
     This example shows the usefulness of this type of electrolyte system for tantalum foil. Methyltriethylammonium dimethylphosphate was prepared by reaction of 70.8 g. of trimethylphosphate and 50.5 g. of triethylamine in 50 ml. of acetonitrile and then removing the acetonitrile. The resistivity was 1020 Ω-cm. A 36% solution of this material is N,N-dimethylformamide had a resistivity of 85 ohm-cm, and the maximum formation voltage for tantalum foil at 25° C. was 145 V. 
     EXAMPLE 4 
     A set of 6 aluminum electrolytic capacitors was constructed using the following electrolyte formulation: 20 g N-ethylpiperidinium diethylphosphate (Et 3  PO 4  -piperidine), 120 g 4-butyrolactone, and 3 g H 2  O. The capacitors were of 250 V rating and made with etched and formed aluminum anode foil. Life test data at 200 VDC and 125° C., capacitance in μF, dissipation factor, and leakage current in μA at 0 hr. and 2000 hr. are presented in Table III. 
     
                       Table III______________________________________    0 hrs.       2000 hrs.Capacitor  μF   DF      μA                           μF DF    μA______________________________________1          12.44   3.91    3.8  12.20 4.19  1.92          11.96   3.88    3.4  11.65 4.79  2.63          11.88   4.07    2.9  11.61 3.99  1.64          12.25   3.81    6.4  11.97 4.34  1.95          12.01   3.50    3.8  11.73 3.64  2.06          11.61   3.98    4.9  11.31 3.84  1.6Average    12.02   3.86    4.2  11.74 4.13  1.9______________________________________ 
    
     It is seen that capacitance and dissipation factor remained virtually unchanged, or changed only slightly, and leakage current improved. Temperature stability (average of the six units) is presented below. 
     
                       Table IV______________________________________Temperature      μF   DF     Impedance Ω                              Impedance ratio______________________________________125° C.      13.14   2.56   101      0.9625° C.      12.59   4.22   105      1.0-55° C.      10.41   61.3   149      1.42______________________________________ 
    
     These results show a 17% capacitance decrease and a 42% impedance increase at -55° C. which is excellent for aluminum capacitors. These tables show that these capacitors are remarkably stable under operating conditions and over a wide temperature range.