Sodium Ion Conductor, Solid Electrolyte Strengthened With Zirconia

This invention relates to the field of ceramics and particularly to the field of solid electrolytes.

Sodium electrolyte materials such as .beta.-Al.sub.2 O.sub.3, B"-Al.sub.2 O.sub.3, and Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12 are ceramics that are capable of conducting electricity because their crystal structures include a sodium ion (Na.sup.+) conductor. Consequently, they are of great interest for use in high energy density, high power density batteries such as sodium sulfur batteries which require conductive ceramics.

Unfortunately, these conductive ceramics are weak, and strength degradation during their use leads to reduced battery lifetime. Surface cracks in the ceramic electrolyte can cause current focusing during battery charging which accellerates the crack growth and leads to degradation. Structural failure of the ceramic due to external stresses can lead to catastrophic sodium sulphur reactions.

Thus, these new high performance batteries need stronger conductive ceramics in order to improve their reliability, increase their current density, and allow the use of thinner-wall electrolytes.

It is an object of the invention to provide an improved sodium ion conductor, solid electrolyte.

It is an object of the invention to provide a stronger .beta.-Al.sub.2 O.sub.3 ceramic.

It is an object of the invention to provide a stronger .beta."-Al.sub.2 O.sub.3 ceramic.

It is an object of the invention to provide a stronger Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12 ceramic.

According to the invention, the fracture toughness and strength of three solid electrolytes (.beta.-Al.sub.2 O.sub.3, .beta."-Al.sub.2 O.sub.3, and Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12) are increased by incorporating grains of tetragonal ZrO.sub.2 in the structure. During cracking, the stress field around the crack tip causes the tetragonal ZrO.sub.2 to transform to the monoclinic structure which is the stable form at room temperature. This transformation increases the energy required for the crack to propagate and thereby retards its growth, resulting in a stronger and tougher material.

ZrO.sub.2 is added to the solid electrolyte in the amount of from 5 to 40% of the volume. To retain its metastable tetragonal structure at room temperature rather than its equilibrium stable monoclinic structure, the grain size of the ZrO.sub.2 is kept less than about 2 .mu.m.

Additionally, a rare earth oxide such as Y.sub.2 O.sub.3, CeO.sub.2, La.sub.2 O.sub.3 and/or Er.sub.2 O.sub.3 is included in the composition. These oxides are dissolved in the ZrO.sub.2. A sufficient amount is included in the ceramic to insure the formation of the tetragonal structure in the constrained state at the expense of the stable monoclinic structure. However, large amounts of rare earth oxide produce a third form of ZrO.sub.2, namely the cubic structure. The amount of rare earth oxide must be held below the quantity that produces the cubic structure.

These and other objects and features of the invention will be apparent from the following detailed description.

.beta.-Al.sub.2 O.sub.3 and .beta."-Al.sub.2 O.sub.3 are sodium aluminates. .beta.-Al.sub.2 O.sub.3 has a composition range defined by Na.sub.2 O-Al.sub.2 O.sub.3 phase diagrams. The .beta.-Al.sub.2 O.sub.3 phase crystallizes in thin hexagonal plates and has a lower ratio of Na.sub.2 O to Al.sub.2 O.sub.3, for example 1 Na.sub.2 O.9 Al.sub.2 O. The .beta."-Al.sub.2 O.sub.3 has a rhombohedral crystal structure and has a higher ratio of Na.sub.2 O to Al.sub.2 O.sub.3. Additionally, the .beta."-Al.sub.2 O.sub.3 has a third ingredient such as Li.sub.2 O or MgO which tends to promote the .beta." phase. Examples of .beta."-Al.sub.2 O.sub.3 are 1/4 Li.sub.2 O.1 Na.sub.2 O.5 Al.sub.2 O.sub.3 and 1/6 Li.sub.2 O.1 Na.sub.2 O.61/3Al.sub.2 O.sub.3.

A more recently discovered sodium ion conductor, solid electrolyte has the general formula Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12. The preparation and properties for this electrolyte for x=2 (Na.sub.3 Zr.sub.2 Si.sub.2 PO.sub.12, called nasicon) is described in an article in the American Ceramic Society Bulletin, Vol. 59, No. 5, May 1980 entitled "Phase Transformation in an Na.sub.3 Zr.sub.2 Si.sub.2 PO.sub.12 Ceramic" by G. Desplanches, et al.

These three ceramics are sodium ion conductors and can be used as the electrolyte in sodium-sulphur batteries. However, they do not have sufficient mechanical strength to permit optium battery design and high operating reliability.

It has been discovered that these solid electrolytes can be strengthened by incorporating metastable grains of ZrO.sub.2 in their microstructure. During cracking, the stress field around the crack tip causes the constrained tetragonal ZrO.sub.2 to transform to the monoclinic structure which is the stable crystal structure of ZrO.sub.2 at room temperature. This is called a stress-induced, martensitic transformation. This transformation increases the energy required for the crack to propagate and thereby increases the material's resistance to fracture.

Zirconium dioxide exists in three crystal forms, the cubic form which is stable at very high temperatures, the tetragonal form which is also stable at high temperatures, and the monoclinic form which is stable at low temperatures. According to the present invention, the ZrO.sub.2 must be in the metastable tetragonal form rather than in the monoclinic form normally obtained during cooling after sintering or annealing the ceramic. To accomplish this, the grain size of the ZrO.sub.2 is kept below about 2 .mu.m and a rare earth oxide is added to the composition.

By keeping the grain size small, retention of the metastable tetragonal structure is promoted by the strain energy arising from the elastic constraint imposed by the surrounding material on the transformation volume and shape change. The transformation of ZrO.sub.2 from the high temperature tetragonal structure to the low temperature monoclinic structure involves a large shear strain and a substantial volume increase. If the individual grains of ZrO.sub.2 are small, then they tend to remain in the stable high temperature tetragonal form during cooling of the ceramic because the surrounding matrix constrains their expansion and shape change which is required to form new monoclinic grains. Theoretical calculations by the present inventor (F. F. Lange, "Stress-Induced Phase Transformation: Theory of Phase Retention and Fracture Toughness," Office of Naval Research Report #6, Contract N00014-77-C-0441, October, 1979) explain in greater detail the relationship between grain size, the restraining matrix, and the retention of the tetragonal phase of ZrO.sub.2.

When rare earth oxides are added to the ceramic, they are held in solid solution in the ZrO.sub.2 grains. This increases the critical grain size required to retain the tetragonal ZrO.sub.2 in the matrix. Consequently, tetragonal ZrO.sub.2 containing a rare earth additive can be retained in the solid electrolyte matrix with grain sizes up to about 2 .mu.m. Without the rare earth additive, such large grains would normally transform to the monoclinic structure when the ceramic is cooled from its fabrication temperature.

Y.sub.2 O.sub.3 is one of a family of 16 rare earth oxides. Actual tests have shown that Y.sub.2 O.sub.3 does promote the retention of tetragonal ZrO.sub.2. A comparison of the phase diagrams ("Phase Diagrams for Ceramist", complied by the National Bureau of Standards and published by The American Ceramic Society) of ZrO.sub.2 --Y.sub.2 O.sub.3 with ZrO.sub.2 --CeO.sub.2, with ZrO.sub.2 --Er.sub.2 O.sub.3, and with ZrO.sub.2 --La.sub.2 O.sub.3 shows similar behavior in that these four metal oxides each tend to lower the temperature of the transformation of ZrO.sub.2 from the tetragonal to the monoclinic structure. It is therefore concluded that the addition of CeO.sub.2, Er.sub.2 O.sub.3, or La.sub.2 O.sub.3 to the ZrO.sub.2 would also favor the retention of tetragonal ZrO.sub.2 in a manner analogous to the Y.sub.2 O.sub.3 addition. As in the case of Y.sub.2 O.sub.3, the amount added must be sufficient to cause the retention of tetragonal ZrO.sub.2 , but not so much as to cause the formation of substantial amounts of cubic ZrO.sub.2. According to actual tests with Y.sub.2 O.sub.3 and to the phase diagrams, Y.sub.2 O.sub.3 is beneficial in the range of about 2 to 5.5 m/o, CeO.sub.2 is beneficial in the range of about 3 to 20 m/o, Er.sub.2 O.sub.3 is beneficial in the range of about 1 to 4 m/o, and La.sub.2 O.sub.3 is beneficial in the range of about 2 to 5 m/o. Mole percent is defined as: ##EQU1##

Mixtures of Y.sub.2 O.sub.3, CeO.sub.2, Er.sub.2 O.sub.3, and La.sub.2 O.sub.3 can be added in various portions within the above ranges to promote the tetragonal ZrO.sub.2.

The composition of the matrix can range from 5 to 40 v/o ZrO.sub.2 (including the rare earth oxide dissolved therein) with the balance being one or more of the above mentioned solid electrolytes. The upper limit of ZrO.sub.2 is the maximum amount of ZrO.sub.2 which can be included in the ceramic without eliminating continuous sodium ion conducting phases in the ceramic. Above this limit, there will be a large increase in resistivity. This limit is estimated to be about 40 v/o ZrO.sub.2.

Volume percentages of ZrO.sub.2 on the high side of the range will produce stronger ceramics, but the resistivity of the ceramic will be lower. Thus, there is a trade-off between strength and resistivity that requires the selection of a composition which gives the optimum combination of properties for a particular battery design.

The following are examples of methods which can be used to fabricate the ceramic composition according to the invention.

NaHCO.sub.3, LiOH, Al.sub.2 O.sub.3, Y(NO.sub.3).sub.3, and ZrO.sub.2 were mixed together in quantities sufficient to produce a first phase of .beta."--Al.sub.2 O.sub.3 occupying 85 v/o of the ceramic and a second phase of ZrO.sub.2 occupying 15 v/o of the ceramic. The ingredients were proportional to yield .beta."--Al.sub.2 O.sub.3 having a composition of 1/6 Li.sub.2 O.1 Na.sub.2 O.61/3 Al.sub.2 O.sub.3 and ZrO.sub.2 containing 3 m/o Y.sub.2 O.sub.3. The mixture was milled and calcined at 1200.degree. C.

The calcined powders were handled under inert gas because it is necessary to control the moisture in the .beta."--Al.sub.2 O.sub.3 powder in order to obtain predictable sintering characteristics. The powders were isostatically pressed at .gtoreq.30,000 psi and then were sintered at 1600.degree. C. to >95% of theoretical density.

X-ray analysis showed that this material contained a mixture of .beta.--Al.sub.2 O.sub.3 and .beta."--Al.sub.2 O.sub.3 (about 50/50) plus tetragonal ZrO.sub.2. The average flexural strength of this material was 50,300.+-.6,500 psi (350 MPa). Its average value of K.sub.c (stress intensity factor) was 5.63 MPam.sup.1/2. This compares with reported values for .beta."--Al.sub.2 O.sub.3 (without ZrO.sub.2) of less than 32,000 psi (220 MPa) for flexural strength and 2.63 for K.sub.c.

The resistivity of the sample prepared per Example I was as shown in Table I below. For comparison, the resistivity of a commercially available sample of .beta."--Al.sub.2 O.sub.3 (without ZrO.sub.2) is shown. For battery applications, a resistivity less than 10 ohm cm at 300.degree. C. is generally considered satisfactory.

Examples II-VI utilize a sol-gel technique to produce amphorous powders which directly crystallize to form .beta."--Al.sub.2 O.sub.3 upon heating to temperatures above 1200.degree. C. Gel derived powders have high surface area and must be handled carefully. They should be handled under inert gas and their moisture content carefully controlled in order to obtain proper sintering characteristics. When done properly, the sol-gel technique should produce a better dispersed ZrO.sub.2 (t) second phase with a finer grain size.

Separate aqueous solutions were prepared of soluble NaHCO.sub.3 and LiOH. These solutions were blended into an Al(OH).sub.3 isopropanol slurry in proper sequence to form a sol-gel upon setting. The proportions of the ingredients were selected so that after dehydration and calcination at 900.degree. C., .beta."--Al.sub.2 O.sub.3 was produced having a composition of 1/6 Li.sub.2 O.1 Na.sub.2 O.61/3 Al.sub.2 O.sub.3.

The .beta."--Al.sub.2 O.sub.3 produced by the sol-gel technique was mixed by milling with ZrO.sub.2 plus Y(NO.sub.3).sub.3 to produce 3 m/o Y.sub.2 O.sub.3 upon decomposition of the nitrate to the oxide. The mixed powders were isostatically pressed at .gtoreq.30,000 psi and then sintered at 1600.degree. C. to produce a .beta."--Al.sub.2 O.sub.3 +15 v/o ZrO.sub.2 (t) ceramic having over 95% theoretical density. The ZrO.sub.2 in the resulting ceramic was substantially 100 v/o tetragonal.

A sample was prepared as described in Example II above except that sufficient Y(NO.sub.3).sub.3 to produce 1 m/o Y.sub.2 O.sub.3 was included. The resulting ZrO.sub.2 was substantially 100% monoclinic, thus indicating that 1 m/o Y.sub.2 O.sub.3 is not sufficient to provide the desired tetragonal ZrO.sub.2 structure.

A sample was prepared as described in Example II above except that sufficient Y(NO.sub.3).sub.3 to produce 2 m/o Y.sub.2 O.sub.3 was included. About 60 v/o of the ZrO.sub.2 in the resulting ceramic was tetragonal with the remaining 40 v/o being monoclinic, indicating that the amount of Y.sub.2 O.sub.3 in the sample was on the low side of the desired range.

A sample was prepared as described in Example II above except that sufficient Y(NO.sub.3).sub.3 to produce 4 m/o Y.sub.2 O.sub.3 was included. The ZrO.sub.2 in the resulting ceramic was substantially 100 v/o tetragonal. There appeared to be a trace amount of cubic ZrO.sub.2 in the sample indicating that the amount of Y.sub.2 O.sub.3 in the ceramic was approaching the high side of the allowable range.

Separate aqueous solutions were prepared of soluble NaHCO.sub.3, LiOH, and Zr(NO.sub.3).sub.4 plus Y(NO.sub.3).sub.3. These solutions were blended into an Al(OH).sub.3 isopropanol slurry in proper sequence to form a sol-gel upon setting. The sol-gel was directly crystallized to .beta."--Al.sub.2 O.sub.3 plus tetragonal ZrO.sub.2 by heating above 1200.degree. C.

The composite powders were handled in inert gas after calcining, ground to minimize aggregation, isostatically pressed at .gtoreq.30,000 psi into discs and sintered at 1480.degree. C. to produce a .beta."--Al.sub.2 O.sub.3 +15 v/o ZrO.sub.2 (t) ceramic having a density over 95% of theoretical.

The above examples are illustrative of methods which can be used to produce the ceramic which is defined by the attached claims. The proportion of ingredients required to produce various compositions of .beta.--Al.sub.2 O.sub.3, .beta."--Al.sub.2 O.sub.3, and Na.sub.1+x Zr.sub.2 Si.sub.x P.sub.3-x O.sub.12 can be readily determined using known chemical relationships. Similarly, the volume percent of ZrO.sub.2 in the final ceramic can be varied from 5 to 40 v/o by simply using the proper proportion of ZrO.sub.2, Zr(NO.sub.3).sub.4, or other suitable ZrO containing ingredients. Accordingly, it should be clearly understood that the form of the invention described above is illustrative only and is not intended to limit the scope of the invention.