Title of Invention	"A COMPOSITION USEFUL AS A SOLID ELECTROLYTE IN ELECTROCHEMICAL DEVICES"
Abstract	A composition useful as a solid electrolyte in electrochemical devices which comprises [l-y{(l-xi) Li2S04:(xi) Ag2S04}:y{(l-x2) Ag2S04: (x2)AgI}] where, Xi is in the range of 0.6 to 0.9 mole%, X2 is in the range of 0.3 to 0.60 mole%, and y is in the range of 0.1 to 0.15 weight%

Full Text

This invention relates to a composition useful as a solid electrolyte in electrochemical devices. The invention relates to a solid electrolyte mixture of Li2SO4 and Ag2SO4 in the proportion from (1-x)Li2S04:(x)Ag2S04 where, x ranging from 0.6 to 0.9. and is useful for electrochemical devices such as solid state batteries and electrochemical sensors. The solid state batteries offer attractive advantages over their aqueous/non-aqueous counterparts. Many of the problems associated with primary batteries have been ascribed to the presence of liquid electrolytes. These include cell leakage, corrosion, self discharge, drying out of the cell, loss of electrolyte at elevated temperature and several restrictions on the capability for useful discharge at very low temperatures. Solid state batteries are becoming increasingly popular due to their known advantages over mentioned liquid-electrolyte based batteries . For a successful operation of all solid state batteries", a suitable solid electrolyte is of prime importance. Most appropriate electrolytes for solid state batteries are characterized by the following properties: 1) High ionic conductivity and negligible electronic conductivity. ?) Good chemical stability (thermodynamic), 3) Good mechanical strength, 4) Ease of fabrication, and 5) relatively inexpensive. The research efforts for the development of materials which fulfill the above basic requirements have increased over the last two decades. It has been found that sulphate based materials are more suitable than others. For the present, the most important conductors which exhibits high ionic conductivity at room temperature in solid state is the silver ion conductors. Up to the present, a large number of silver ion conductors based on silver iodide (AgT) have been reported (T. Takahashi, J. Appl. Eleotrochem. 3 (1973) 79 and P. Mcgeehin and A. Hooper. J. Materials Science. 12 (1977)1). Silver iodide is generally a mixture of the beta and the gamma-phases, and has a low conductivity below 145°C. At 160°, it transforms to the alpha-phase, and above this temperature, its conductivity increases suddenly to about 10° (ohm cm)~1 which is five to six orders of magnitude greater than that at room temperature (T. Takahashi, Pure & Appl Chem., 50 (1978) 1091). The first success in finding a high ionic conductivity in solid at room temperature was achieved by introducing the sulfide ion into the lattice of silver iodide, Agl (T. Takahashi, Pure & Appl Chem., 50 (1978) 1091). If one makes silver iodide to react with silver sulfide at relatively high temperature in 1:1 molar ratio, a compound Ag3SI is obtained which show extremely high Ag ion conductivity at 25°C which is stable at lower temperatures. The major drawback associated with this electrolyte is electronic conductivity in this compound with deviation from the stoichiometric composition. another drawback with Ag3SI which exhibits low electronic conductivity must be synthesized by heating silver iodide (Agl) and silver sulfide (Ag2S) under an appropriate vapour pressure of sulfur thus hindering vaporization of sulfur at synthesizing temperature. Though its ionic conductivity is very high, it was found that it decomposes by iodine with resulting decrease in conductivity. The silver compounds Agyl4PO4 and Ag19Il15P2O7 belonging to binaries of Agl with Ag3PO4 and Ag2PpOy respectively possess high ionic conductivity. However they are found to be light sensitive which is a chief drawback. The introduction of the potassium and rubidium ions into the lattice of silver iodide has been found to be very effective in creating a high silver ion conductivity at room temperatures.B. Owens, "Advances in Electrochemistry and Electrochemical Eng." Vol8. Wiley Interscience, New York (1971) p. 1.). The most well known example of this group is RbAg4I5. This compound is prepared by fusing a mixture of four moles of Agl and one mole of Rbl in vacuum at 50G°C, before cooling abruptly to room temperature and annealing afterward at 165°C for approximately 10 h. The above stated materials are either chemically unstable (decomposes under external applied field) or the preparation techniques are complex. In comparison, the sulphates are thermodynamically more stable and easy to prepare. Presently sulphate based binary solid electrolytes are being prepared by adding salts such as Agl, AgCl, AgBr etc in an appropriate mole ratio, (1-x) Me2SO4: xAgX) wherein x ranging from 0.1 - 1.0, and fusing at elevated temperature such as 700 to 900°C followed by slow cooling the melt to room temperature or by quenching the melt as described by (A. Khandkar , V.B. Tare, A.Navrotsky and J. B, Wagner, Jr, J, Electrochem. Soc. 131 (1984) 2683, K. Shahi and J.B. Wagner, Jr. J. Phys. Chem. Solids, 43(1982)713). The following is the main drawback with this technique. The solid solutions or binaries prepared by this technique have poor ionic conductivity at room temperature, because this technique neither help in increasing solid solubility leading to the formation of defects (vacancies or lattice distortions) nor reduces the grain size leading to an increased highly conducting interfacial region which are the key factors for the ion migration in polycrysta11ine solids. The main object of the present invention is to provide a composition useful as a solid electrolyte in electrochemical devices which obviates the drawbacks of known electrolytes. Another object of the present invention is to provide a composition having enhanced ionic conductivity. Accordingly the present invention provides a composition useful as a solid electrolyte in electrochemical devices 'which comprises [1-y { ( 1-x 1 )L i2SO4 : ( x., )Ag2S04} :y{ ( 1-x2)Ag2SO4: (x?)Ag1} ] where X1 is in the range of 0.6 to 0.9 mole% , X2 is in the range of 0.3 to 0.60 mole%, and y is in the range of O.1 to 0.15 wt % . Lithium and silver sulphates as well as silver iodide used may of G.R. grade i.e. 99.9% pure. The composition of the present invention is not a mere admixture .but has a synergistic effect of providing properties which are not merely the aggregation of the properties of the individual ingredients. Solid electrolyte is prepared by using the composition of the present invention as follows: Drying of Li2SO4, Ag2S04 and Agl at 100-150°C for a period of 36 to 48 hrs, mixing composition in [1-y((1-x1)Li2S04:(x1)Ag2S043:y{(1-x2)Ag2SO4:(x2)AgI)] where X1, is in the range of 0.6 to 0.9 mole% . X2 is in the range of 0.3 to 0.60 mole%, and y is in the range of 0.1 to 0.15 wt % in organic solvent (nonreactive with salts) and heating at 1OO -20G°C for 6-8 hrs for complete drying of resultant mixture, heating the dried mixture in crucible at a temperature 10-20°C above the melting point of the mixture for a period of one to two hours to obtain the homogeneous melt, quenching the molten mass by rapid quenching technique, followed by crushing the resultant mass. The entire process was carried out in dark The complete drying of the Li2S04, Ag2SO4 and Agl is affected by repetitive cycles of heating and weighing till the constant. weight is achieved. The composition was heated to obtain a homogeneous melt. which is effected preferably at 2O°C above melting point. he following detailsare given by way of illustration and should not be construed to limit the scope of the present invention. The initial ingredients Li2SO4, Ag2SO4, and AgT of AR grade are dried at 20O°C for 48 hrs. The complete drying of the materials is confirmed by repetitive cycles of heating and weighing. The dried materials of appropriate mole ratio- are mixed thoroughly by wet mixing process in an agate mortar under acetone (99.9% pure) followed by a second drying process at 200°C for 6-8 hours. The mixture is then heated in a crucible using a temperature controlled electric furnace. The temperature of the crucible is maintained at 2Q°C above the melting point for an hour so as to obtain complete homogeneous melt. The clear transparent molten mass is quenched using twin rollers. The samples obtained in the form of thin flakes are crushed and pelletized at 5-1O tons/sq cm pressure using stainless steel die and punch. These pellets were subjected to the measurement of ac conductivity as a function of temperature in the frequency range from 5Hz to 13 MHz with the help of computer controlled HP 4192A If impedance analyzer. Purely ionic nature of these samples is confirmed by the transport number measurements using do polarization technique (proposed by Wagner, Int. Committee of Electrochemical Thermodynamics and Kinetics, Prof. 7th meet , Butterworths) and Tubandt's method. A detailed impedance analysis as depicted in table-1 reveals that (70L i2SO4: 30Ag2S04)+12 . 5wt% (48Ag2SO4: 52AgI) composition offers a maximum conductivity. Table-1 (Table Removed) 2Li2S04:8Ag2SO4 composition Note: the conductivity of 2Li:,S04:8AgpSO/1 composition at 4O is very small (i.e. around 10 Inference Our results suggest that the high ionic conductivity can be achieved relatively at lower temperature by the addition of silver sulphate-silver iodide eutectic (48-52mole%). Particularly, the high ionic conductivity can be synthesized by addition of 7-15 mol% of 48Ag2SO4:52AgI (eutectic). The main advantages of the present Invention are: 1. The electrolytes are electrochemically stable against the iodine cathode and silver anode 2. The electrolytes are thermodynamical stable 3. Provides higher ionic conductivity 4. Electronic contribution to the total conductivity is negli gible. We Claim: 1. A composition useful as a solid electrolyte in electrochemical devices which comprises [l-y{(l-xi) Li2S04:(xi) Ag2S04}:y{(l-x2) Ag2S04: (x2)AgI}] where, X1 is in the range of 0.6 to 0.9 mole%, X2 is in the range of 0.3 to 0.60 mole%, and y is in the range of 0.1 to 0.15 weight% 2. A composition useful as a solid electrolyte in electrochemical devices substantially as herein described.

Documents:

643-del-1996-abstract.pdf

643-del-1996-claims.pdf

643-del-1996-complete specification (granted).pdf

643-del-1996-correspondence-others.pdf

643-DEL-1996-Correspondence-PO.pdf

643-del-1996-description (complete).pdf

643-del-1996-form-1.pdf

643-DEL-1996-Form-2.pdf

643-del-1996-form-3.pdf

643-DEL-1996-Form-4.pdf

643-del-1996-form-6.pdf