Lithium Ion Conductive Glass-Ceramics

This invention relates to a lithium ion conductive glass-ceramics suitable for use as all solid electric cell, sensors and electrochemical devices of various types.

Recent development in electronics has brought about high-performance electronic devices of a compact and light-weight design and, as a power source of such electronic devices, development of an electric cell of a high energy density and a long life is strongly desired for.

Lithium has the highest oxidation-reduction potential of Li/Li.sup.+ of all metal elements and has the smallest mass per 1 mol and, therefore, lithium cell can provide a higher energy density than other types of cells. Moreover, if a lithium ion conductive solid electrolyte is used, this electrolyte can be made very thin and, therefore, a cell of a thin film can be formed and increase in energy density per unit volume can thereby be realized.

A lithium ion cell which has been realized to date uses an organic electrolyte solution as its electrolyte and this makes it difficult to achieve a cell of a compact design such as a thin film design. This lithium ion cell has additional disadvantages that it has likelihood of leakage of electrolyte solution and likelihood of spontaneous combustion. If this lithium ion cell is replaced by a cell employing an inorganic solid electrolyte, a wholly solid cell of a high reliability will be realized. For this reason, studies and developments of a solid electrolyte having a high conductivity have been vigorously made for realizing a wholly solid lithium cell. Among known solid electrolyes, Li.sub.3 N single crystal (Applied Physics letter, 30(1977) 621-22) and LiI-Li.sub.2 S-P.sub.2 S.sub.5, LiI-Li.sub.2 S-SiS.sub.4 and LiI-Li.sub.2 S-B.sub.2 S.sub.3 glasses (Mat. Res. Bull., 18(1983) 189) have high conductivity of 10.sup.-3 S/cm or over. These materials, however, have the disadvantages that preparation and handling of these materials are difficult and that decomposition voltage of these materials is so low that, when they are used for a cell, a sufficiently high terminal voltage cannot be obtained.

An oxide lithium solid electrolyte does not have the above described disadvantages and has a decomposition voltage which is higher than 3V and, therefore, it has possibility of usage as a wholly solid lithium cell if it exhibits a high conductivity at room temperature. It is known in the art that conductivity in an oxide glass can be increased by increasing lithium ion concentration. However, there is limitation in increase in the lithium ion concentration even if rapid quenching is employed for glass formation and conductivity of this glass at room temperature is below 10.sup.-6 S/cm at the highest. An oxide ceramic having the highest conductivity at room temperature is Li.sub.1+X Al.sub.X Ti.sub.2-X (PO.sub.4).sub.3. When X is 0.3, the conductivity thereof is 7.times.10.sup.-4 S/cm at room temperature (J. Electrochem. Soc., 137 (1990) 1023). Oxide ceramics are superior in conductivity to glasses but have the disadvantages that they require a troublesome process for manufacturing and that they are difficult to form, particularly to a thin film.

In short, the prior art lithium ion solid electrolytes have the problems that they are either low in conductivity, hard to handle, hard to form to a compact design such as a thin film.

It is, therefore, an object of the invention to provide glass-ceramics which have solved these problems and exhibit a very high lithium ion conductivity in the order of 10.sup.-3 S/cm at room temperature.

As described above, Li.sub.1+X Al.sub.X Ti.sub.2-X (PO.sub.4).sub.3 ceramics exhibit conductivity of 10.sup.-4 S/cm or over at room temperature. These ceramics, however, have pores and a bad grain boundary which can not be eliminated completely and existence of these pores and grain boundary results in a decrease in conductivity. If, therefore, glass-ceramics including the above crystal are provided, there will be no pores and the grain boundary will be improved and, as a result, a solid electrolyte having a higher conductivity is expected to be provided. Besides, glass-ceramics which share a feature of glass can be easily formed into various shapes including a thin film by utilizing this feature of glass. For these reasons, glass-ceramics are considered to have practical advantages over ceramics made by sintering.

As a result of studies and experiments made by the inventor of the present invention on the basis of the above described basic concept, the inventor has succeeded in obtaining glass-ceramics having a very high lithium ion conductivity in the order of 10.sup.-3 S/cm at room temperature by producing glasses including the ingredients of the above described crystal and causing the crystal phase to grow from these glasses by heat treating these glasses.

A lithium ion conductive glass-ceramics achieving the above object of the invention comprise in mol %

and contains Li.sub.1+X (Al, Ga).sub.X Ti.sub.2-X (PO.sub.4).sub.3 (where X is 0 to 0.8) as a main crystal phase.

In one aspect of the invention, said glass-ceramics comprise in mol %

In another aspect of the invention, said glass-ceramics comprise in mol %

The inventor of the present invention has further succeeded in obtaining glass-ceramics having the high lithium ion conductivity in the order of 10.sup.-3 S/cm at room temperature by producing glasses including ingredients of P.sub.2 O.sub.5, SiO.sub.2, TiO.sub.2, M.sub.2 O.sub.3 (where M is Al or Ga) and Li2O and causing a crystal phase of a conductive crystal Li.sub.1-X+Y M.sub.X Ti.sub.2-X Si.sub.Y P.sub.3-Y O.sub.12 to grow from the glasses by hear treating these glasses.

Therefore, in another aspect of the invention, here is provided a lithium ion conductive glass-ceramics comprising in mol %

and containing Li.sub.1+X+Y M.sub.X Ti.sub.2-X Si.sub.Y P.sub.3-Y O.sub.12 (where 0<X.ltoreq.0.4 and 0<Y.ltoreq.0.6) as a main crystal phase.

In another aspect of the invention, said glass-ceramics comprise in mol %

In another aspect of the invention, said glass-ceramics comprise in mol %

According to the invention, there are provided lithium ion conductive glass-ceramics which exhibit a very high conductivity in the order of 10.sup.-3 S/cm at room temperature. In addition to having the high conductivity, the glass-ceramics made according to the invention have such an excellent formability that they can be easily formed into various shapes including a thin film, and they are thermally and chemically stable so that they are suitable for use as electrolytes of wholly solid cells, sensors and other various electrochemical devices.

The compositions of the glass-ceramics made according to the invention are expressed on the basis of compositions of oxides as in their base glasses. The above described content ranges of the respective ingredients have been selected for the reasons stated below.

In the ternary system P.sub.2 O.sub.5 -TiO.sub.2 -Li.sub.2 O, glass forming region exists in a very narrow range and the composition identical with that of Li.sub.1+X Al.sub.X Ti.sub.2-X (PO.sub.4).sub.3 does not form glass when X is 0 (Bulletin of the Chemical Society of Japan, 51(1978) 2559). In the P.sub.2 O.sub.5 -TiO.sub.2 -(Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3)-Li.sub.2 O system including Al.sub.2 O.sub.3 or Ga.sub.2 O.sub.3, a glass forming range has not been reported yet. Neither has been reported any glass-ceramic which has been prepared from such systems for obtaining a high lithium ion conductivity.

The inventor of the present invention has examined the glass forming range of the P.sub.2 O.sub.5 -TiO.sub.2 -(Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3)-Li.sub.2 O system by employing a conventional glass melting method and obtained lithium ion solid electrolytes of a high conductivity which can be glassified within the following composition ranges (expressed in mol %) and can grow, as a result of heat treatment, Li.sub.1+X (Al, Ga).sub.X Ti.sub.2-X (PO.sub.4).sub.3 as a main crystal phase.

In the case of the system including Al.sub.2 O.sub.3,

In the case of a system including Ga.sub.2 O.sub.3,

In the case of a system including both Al.sub.2 O.sub.3 and Ga.sub.2 O.sub.3,

It has been found that a glass forming region exists beyond the above described composition ranges but, after a heat treatment, an electrolyte having a high conductivity could not be obtained from such composition range outside of the above described composition ranges. The above described composition ranges of the glass-ceramics of the invention have been determined on the basis of these experiments.

In this system, a part of Al or Ga ingredient may be replaced by one of such trivalent metal elements as B, In, Sc, Fe and Cr. In this case, however, the amount of Al or Ga replaced by such metal element should not exceed 5%. If the amount of the replaced metal element exceeds 5%, conductivity will drop significantly.

A method for manufacturing the conductive glass-ceramics of the P.sub.2 O.sub.5 -TiO.sub.2 -(Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3)-Li.sub.2 O system will now be described.

Starting materials are weighed at a predetermined ratio and mixed uniformly and the mixed materials are thereafter put in a platinum crucible and heated and melted in an electric furnace. First, gas components coming from the raw materials are evaporated at 700.degree. C. and then the temperature is raised to 1400.degree. C. to 1450.degree. C. and the materials are melted at this temperature for about one to two hours. Then the melt is cast onto a stainless steel plate to form a sheet glass. The resultant glass is subjected to heat treatment within the temperature range from 800.degree. C. to 1000.degree. C. for 10 to 72 hours and lithium ion conductive glass-ceramics containing Li.sub.1+X (Al, Ga).sub.X Ti.sub.2-X (PO.sub.4).sub.3 as a main crystal phase were thereby produced.

A heat treatment at a higher temperature within the above described temperature range will be desirable if micro cracks are not produced because a heat treatment at a higher temperature will reduce the heat treating time. Generally speaking, a heat treatment performed at a temperature which is higher by about 300.degree. C. than a crystallization temperature of the glass will be most effective because it will provide the highest conductivity.

In the case of the glass-ceramics according to the invention made of the P.sub.2 O.sub.5 -SiO.sub.2 -TiO.sub.2 -(Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3)-Li.sub.2 O system, the above described composition ranges have been selected because, within these composition ranges, dense glass-ceramics containing Li.sub.1+X+Y M.sub.X Ti.sub.2-X Si.sub.Y P.sub.3-Y O.sub.12 (where 0<X.ltoreq.0.4, 0<Y.ltoreq.0.6) as a main crystal phase and exhibiting a high lithium ion conductivity at room temperature were obtained by heat treating glasses of the same composition ranges. It has been found that the same crystal can be precipitated even in a composition range outside of the above described composition ranges but this crystal does not constitute a main crystal phase of a glass-ceramic produced and conductivity of this glass-ceramic is not sufficiently high.

In this system, SiO.sub.2 is a very important ingredient. By adding SiO.sub.2, the glass forming range is broadened and, moreover, melting property and thermal stability of the glass are improved and an excellent conductivity in the order of 10.sup.-3 S/cm can be obtained.

A part of Al or Ga may be replaced by one of such trivalent metal elements such as B, In, Sc, Fe and Cr or one of such divalent metal elements as Mg and Zn. Likewise, a part of Ti may be replaced by Zr and a part of Si may be replaced by Ge. In these cases, however, the amount of Al, Ga, Ti or Si replaced by such metal element should not exceed 5%. If the amount of the replaced metal element exceeds 5%, conductivity will drop significantly.

For improving the melting property of the glass, As.sub.2 O.sub.3, Sb.sub.2 O.sub.3, Ta.sub.2 O.sub.5, CdO or PbO may be added. The amount of such ingredient however should not exceed 3%. If the amount of such ingredient exceeds 3%, conductivity of the glass-ceramic will decrease as the amount of addition of the ingredient increases.

A method for manufacturing the conductive glass-ceramics of the P.sub.2 O.sub.5 -SiO.sub.2 -TiO.sub.2 -(Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3)-Li.sub.2 O system will now be described.

Starting materials are weighed at a predetermined ratio and mixed uniformly and the mixed materials are thereafter put in a platinum crucible and heated and melted in an electric furnace. First, gas components coming from the raw materials are evaporated at 700.degree. C. and then the temperature is raised to 1400.degree. C. to 1500.degree. C. and the materials are melted at this temperature for about one to two hours. Then the melt is cast onto stainless steel plate to form a sheet glass. The glass thus produced is thereafter subjected to heat treatment by heating it under a temperature ranging from 680.degree. C. to 800.degree. C. for about twelve hours and subsequently heating it under a temperature which is higher by 200.degree. C. to 350.degree. C. for about twenty-four hours and glass-ceramics containing Li.sub.1+X+Y M.sub.X Ti.sub.2-X Si.sub.Y P.sub.3-Y O.sub.12 as a main crystal phase and having a high lithium ion conductivity is produced.

This two-step heat treatment method is applicable also to the production of the glass-ceramics of the P.sub.2 O.sub.5 -TiO.sub.2 -(Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3)-Li.sub.2 O system. Conversely, the glass-ceramics of the P.sub.2 O.sub.5 -SiO.sub.2 -TiO.sub.2 -(Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3)-Li.sub.2 O system can be produced by employing the one step heat treatment method described above with respect to the glass-ceramics of the P.sub.2 O.sub.5 -TiO.sub.2 -(Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3)-Li.sub.2 O system.

Examples of the glass-ceramics made according to the invention will now be described. It should be noted that these examples are illustrative only and the scope of the invention in no way is restricted by these examples.

As starting materials, NH.sub.4 H.sub.2 PO.sub.4, TiO.sub.2, Al(OH).sub.3 and Li.sub.2 CO.sub.3 were used. These starting materials were weighed to constitute a composition of 39P.sub.2 O.sub.5 -8.5Al.sub.2 O.sub.3 -39TiO.sub.2 -13.5Li.sub.2 O in mol %. The materials were mixed uniformly and then put in a platinum crucible and heated and melted in an electric furnace. First, CO.sub.2, NH.sub.3 and H.sub.2 O coming from the raw materials were evaporated at 700.degree. C. Then the temperature was raised to 1450.degree. C. and the materials were melted by heating them at this temperature for 1.5 hour. Thereafter, the melt was cast onto a stainless steel plate to form a uniform sheet glass. The glass was annealed at 550.degree. C. for two hours for removing thermal stress of the glass.

The glass thus produced was cut into specimens each having the size of 20.times.20 mm. The specimens of glass were polished on both surfaces and subjected to heat treatment under various heat conditions. The crystal phase which precipitated in the specimens was determined by the powder X-ray diffraction method. As a result, it was found that the precipitated crystal phase under all heat conditions was Li.sub.1+X Al.sub.X Ti.sub.2-X (PO.sub.4).sub.3. Electrical conductivity of the glass-ceramic was measured within a range from 10.sup.-2 -3.times.10.sup.+7 Hz by the complex impedance. Resistance of the specimens (sum of grain resistance and grain boundary resistance) was determined from the Cole-Cole Plot and the conductivity was calculated by the equation .sigma.=(t/A)(1/R) (where .sigma. is conductivity, t is thickness of the specimen, A is electrode area and R is resistance of the specimen). As a result, the specimen which was heat treated at 1000.degree. C. for 12 hours exhibited the highest conductivity of 1.3.times.10.sup.-3 S/Cm at room temperature (Table 1, Example No. 1).

As the starting materials, NH.sub.4 H.sub.2 PO.sub.4, TiO.sub.2, Al(OH).sub.3, Ga.sub.2 O.sub.3 and Li.sub.2 CO.sub.3 were used to produce a glass-ceramic by employing the same manner as in Example 1. The crystal phase which grew in specimens of this glass-ceramic was determined to be Li.sub.1+X (Al, Ga).sub.X Ti.sub.2-X (PO.sub.4).sub.3. The specimen which was heat treated at 950.degree. C. for 12 hours exhibited the highest conductivity of 1.0.times.10.sup.-3 S/cm (Table 1, Example No. 2).

As starting materials, NH.sub.4 H.sub.2 PO.sub.4, SiO.sub.2, TiO.sub.2, Al(OH).sub.3 and Li.sub.2 CO.sub.3 were used. These starting materials were weighed to constitute a composition of 32P.sub.2 O.sub.5 -8SiO.sub.2 -41TiO.sub.2 -5Al.sub.2 O.sub.3 -14Li.sub.2 O in mol %. The materials were mixed uniformly and then put in a platinum crucible and heated and melted in an electrical furnace. First, CO.sub.2, NH.sub.3 and H.sub.2 O coming from the raw materials were evaporated at 700.degree. C. Then the temperature was raised to 1450.degree. C. and the materials were melted by heating them at this temperature for 1.5 hour. Thereafter, the melt was cast onto a stainless steel plate to form a uniform sheet glass. The glass was annealed at 550.degree. C. for two hours for removing thermal stress of the glass.

The glass thus produced was cut into specimens each having the size of 20.times.20 mm. The specimens of glass were polished on both surfaces and subjected to heat treatment at a temperature of 800.degree. C. for 12 hours and then at 1000.degree. C. for 24 hours to produce a dense glass-ceramic. The crystal phase precipitated in the specimens was determined by the powder X-ray diffraction to be Li.sub.1+X+Y Al.sub.X Ti.sub.2-X Si.sub.Y P.sub.3-Y O.sub.12. The glass-ceramic exhibited a very high conductivity of 1.5.times.10.sup.-3 S/cm at room temperature (Table 2, Example No. 3).

FIG. 1 shows an X-ray diffraction pattern of the glass-ceramic of Example 3.

Specimens of glass-ceramics were prepared in a manner similar to Example 3. The compositions and conductivities of these specimens as well as the composition and conductivity of Example 1 are shown in the following Tables 2 and 3.