Lithium compounds

This invention relates to lithium compounds composed of a lithium halide and lithium imide. The compounds are high in lithium ionic conductivity and are used as solid electrolytes. Particularly excellent ones of the lithium compounds are those represented by the general formula: EQU yLi.sub.2 NH.(1-y)LiX where X represents Cl, Br or I and y denotes the molar fraction of lithium imide ranging from about 0.35 to about 0.98 when X is chlorine, from about 0.25 to about 0.35 when X is bromine or from about 0.33 to about 0.75 when X is iodine.

BACKGROUND OF THE INVENTION 
This invention relates to lithium compounds, and particularly to lithium 
compounds which have high lithium ionic conductivity and are useful as 
lithium ionic conductive solid electrolytes. 
A variety of lithium compound have been expected as highly promising 
material for use as lithium ionic conductor on an industrial basis. 
Namely, when a solid lithium ionic conductor is used in various types of 
electrochemical devices, there will be obtained following several 
characteristic features that could not be attained in the past. For 
example, there will be no fear of leak or salting of electrolyte, the 
device can be used for a longer period of time and can have a much smaller 
and thinner configuration. Therefore, solid lithium ionic conductors are 
expected to have a number of applications such as very thin type cells and 
electrochromic displays. 
When such a solid electrolyte is used for a cell or the like, it must meet 
several requirements. However, hitherto there have not been found any 
lithium compound that can meet all these requirements, and there is a keen 
demand for the availability of a lithium compound which has a sufficient 
lithium ionic conductivity at room temperature. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a novel lithium compound which 
has sufficient lithium ionic conductivity. 
Another object of this invention is to provide a solid electrolyte which 
has sufficient lithium ionic conductivity. 
These and other objects of the invention are attained by a lithium compound 
composed of a lithium halide and lithium imide.

DETAILED DESCRIPTION OF PREFFERED EMBODIMENTS 
The compound according to this invention is composed of a lithium halide 
(herein LiCl, LiBr and LiI are generally referred to as lithium halide)and 
lithium imide, and is produced by mixing specified amounts of the two 
component substances with each other and heating the resulting mixture. 
Lithium imide, Li.sub.2 NH, can be prepared by the method reported by B. A. 
Boukamp et al. (Phys. Lett. 72A, 464 (1979)). 
In this method, lithium nitride (Li.sub.3 N) is heated, for example, at 
500.degree. C. for about 30 min in a gaseous 50/50 mixture of nitrogen and 
hydrogen, to obtain Li.sub.2 NH by the following reactions (1) and (2). 
EQU Li.sub.3 N+H.sub.2 .fwdarw.Li.sub.2 NH+LiH (1) 
EQU 4LiH+N.sub.2 .fwdarw.2Li.sub.2 NH+H.sub.2 (2) 
To bring the reaction (2) to complete, it is useful to conduct further 
heating in nitrogen at about 600.degree. C. for about 3 hours. 
A product obtained in this way was filled in glass capillaries and the 
product was identified by X-ray powder diffraction method (the 
Debye-Scherrer method) to confirm the formation of Li.sub.2 NH. 
Each of predetermined amounts of the thus obtained Li.sub.2 NH was 
intimately mixed with a predetermined amount of LiCl, LiBr or LiI to form 
different mixtures. Each of the mixtures was pressure-molded into a 
disk-shaped pellet by using a die of 15 mm in diameter and a molding force 
of 2 tons. All the operations such as mixing of raw materials and molding 
of each mixture were carried out in a nitrogen atmosphere. In addition, 
since LiBr contained water, it was previously dehydrated by a prethermal 
treatment carried out in nitrogen gas at 400.degree. C. for 15 hours. 
Then, the resulting pellets were heat-treated in a nitrogen atmosphere at 
400.degree.-500.degree. C. for 30 hours to produce six kinds of compounds. 
Each of the compounds thus obtained was analyzed by the X-ray powder 
diffraction method to determine the d-value (interplanar spacing) and the 
relative intensity I/Io. The results are shown in Table 1, in which X 
represents the species of halogen in the lithium halide used as raw 
material, and y denotes the molar fraction of lithium imide in the general 
formula yLi.sub.2 NH.multidot.(l-y)LiX which represents the compound of 
this invention. 
TABLE 1 
__________________________________________________________________________ 
Compound .circle.1 
Compound .circle.2 
Compound .circle.3 
Compound .circle.4 
Compound .circle.5 
Compound .circle.6 
X = Cl, y = 0.45 
X = Cl, y = 0.75 
X = Br, y = 0.25 
X = Br, y = 0.55 
X = I, y = 0.33 
X = I, y = 0.75 
d-value.sup.(1) 
I/Io.sup.(2) 
d-value.sup.(1) 
I/Io.sup.(2) 
d-value.sup.(1) 
I/Io.sup.(2) 
d-value.sup.(1) 
I/Io.sup.(2) 
d-value.sup.(1) 
I/Io.sup.(2) 
d-value.sup.(1) 
I/Io.sup.(2) 
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3.04 100 2.98 100 5.12 8 4.91 10 5.42 8 6.00 47 
2.97 30 2.58 18 4.43 5 3.13 100 4.72 7 3.67 96 
2.66 6 1.82 35 3.13 100 2.71 42 3.33 100 3.13 100 
2.63 20 1.556 
14 2.67 9 1.91 17 2.84 8 2.59 17 
2.57 33 1.489 
2 2.56 3 1.81 7 2.72 6 2.38 25 
1.86 23 1.290 
2 2.21 12 1.629 
16 2.36 14 2.11 28 
1.82 12 1.184 
3 2.03 5 1.561 
4 2.16 6 1.99 23 
1.63 2 1.98 3 1.410 
2 2.11 5 1.83 16 
1.586 
11 1.806 
25 1.354 
3 1.93 26 1.751 
11 
1.551 
7 1.703 
2 1.238 
3 1.817 
5 1.637 
15 
1.521 
3 1.564 
6 1.208 
3 1.669 
8 1.579 
6 
1.487 
4 1.495 
2 1.596 
3 1.450 
6 
1.314 
2 1.399 
7 1.574 
3 1.383 
10 
1.286 
3 1.277 
2 1.493 
10 1.347 
9 
1.207 
4 1.182 
5 1.363 
4 1.220 
5 
1.178 
5 1.151 
2 1.323 
3 1.193 
3 
1.149 
3 1.042 
2 1.262 
9 1.136 
5 
0.989 
1 1.231 
2 
1.183 
2 
1.113 
3 
1.058 
2 
__________________________________________________________________________ 
Note .sup.(1) : dvalues are in Angstrom unit. 
Note .sup.(2) : Relative intensity 
The d-values and relative intensities for Li.sub.2 NH, LiCl, LiBr and LiI 
used as raw materials are reported respectively on ASTM Cards 6-0417, 
4-0664, 6-0319 and 1-0592. However, the results shown in Table 1 are 
entirely different from the values listed on the ASTM Cards. This 
difference confirms that novel compounds were formed by the process 
described above. 
When the d-values for the compounds 2 , 3 , 5 and 6 in Table 1 were 
calculated assuming the compounds to be of cubic lattice types having the 
lattice constant a of 5.16, 8.86, 9.45 and 10.36 .ANG. respectively, the 
resulting d-values agreed very closely with the measured values shown in 
Table 1 and, moreover, the Miller indices obtained by indexing showed a 
common regularity in all cases of the compounds. 
Namely, each of the diffraction lines for the compounds 2 , 3 , 5 and 
6 shown in Table 1 had the Miller indices h, k, l that were either all 
even or all odd (in no case, an even number and an odd number were 
coexistent in a single set of the Miller indices). 
Accordingly, it was recognized that the lattice types of the compounds 2 , 
3 , 5 and 6 were all face-centered cubic (fcc) and the values of the 
lattice constant a were 5.16 .ANG., 8.86 .ANG., 9.45 .ANG. and 10.36 .ANG. 
respectively. 
On the other hand, the compounds 1 and 4 in Table 1 had so complicated 
crystal structures that their lattice types could not be identified. 
In the next place, the above procedure was repeated while changing the 
valves of X and y in the general formula set forth above, to obtain the 
compositional ranges in which the compounds can exist as stable ones. The 
results are shown in FIGS. 1, 2 and 3. In these Figures, the symbols C, B, 
I and N represent LiCl, LiBr, LiI and Li.sub.2 NH, respectively. 
FIG. 1 shows the results for the case of X=Cl, in which the symbols .DELTA. 
and indicate that the compound yLi.sub.2 NH.(l-y)LiCl was able to exist 
stably as single phase of unidentified structure and as a single phase 
face-centered cubic (fcc), respectively. The symbol indicates that one 
crystal type phase predominated over the other. 
As is clear from FIG. 1, when X is Cl, the compound can exist as stable 
one, if y is in the range of about 0.35 to about 0.98. 
When y became 0.30, a considerable amount of LiCl was found to be 
coexistent and, on the other hand, when y exceeded 0.98, Li.sub.2 NH was 
found to be coexistent. Therefore, the range of y in which the compound 
can exist as a single one is about 0.30 to about 0.98, when X is Cl. 
FIG. 2 shows the results for the case of X=Br, in which the symbols 
.quadrature. and indicate that the compound yLi.sub.2 NH.(l-y)LiBr could 
exist stably with the crystal structure of face-centered cubic (fcc) 
lattice and of unidentified type, respectively. The symbol + indicates 
As is clear from FIG. 2, when X is Br, the range of y in which the compound 
can exist stably with face centered cubic (fcc) lattice or with an 
unidentified crystal structure was from about 0.25 to about 0.55. When y 
was decreased to 0.15, an LiBr phase came to appear and, on the other 
hand, when y was 0.6 or larger, a phase of Li.sub.2 NH became coexistent 
with the matrix. 
FIG. 3 shows the results for the case of X=I, in which the symbols .circle. 
and indicate that the compound yLi.sub.2 NH.(l-y)LiI could exist stably 
with face centered cubic (fcc) structure having a lattice constant of 9.45 
.ANG. and of 10.36 .ANG., respectively. The symbol indicates that one 
lattice type predominated over the other. 
It was recognized that the compound yLi.sub.2 NH.(l-y)LiI could exist 
stably as one having face centered cubic (fcc) structure when y was in the 
range of about 0.33 to about 0.75. When y was 0.25 and when it was 0.85, a 
small but appreciable amount of LiI and Li.sub.2 NH, respectively, was 
found to be coexistent with the matrix. 
As described above, the lithium compound of this invention is produced by 
mixing specified amounts of a lithium halide and lithium imide with each 
other and heating the resulting mixture. 
This reaction proceeds at a temperature of at least about 300.degree. C., 
and the reaction rate becomes very high at temperatures of 350.degree. C. 
or higher. 
However, at a temperature of about 750.degree.-800.degree. C., although the 
lithium halide remains stable, Li.sub.2 NH will decompose. Therefore, it 
should be avoided to heat the mixture to a temperature of 
700.degree.-800.degree. C. or higher. 
In addition, the reaction is carried out in an inert gas such as nitrogen, 
argon and helium or in a hydrogen atmosphere. When the reaction is 
conducted in a vacuum or at a reduced pressure, raw materials or reaction 
products will be more likely to decompose. Therefore, it is preferable to 
carry out the reaction at an atmospheric pressure or at a pressure 
approximate to it. 
The compound of the above-mentioned general formula in which X is C1 has a 
melting point slighlty differing in accordance with the value of y but 
substantially within the range of about 490.degree. to 600.degree. C. 
Similarly, the melting points of the compounds corresponding to X=Br and 
X=I, respectively, are 450.degree.-460.degree. C. and 
410.degree.-550.degree. C. 
When the compounds are heated to these respective temperatures, they are 
melted, and yet they remain stable without decomposing. However, when the 
temperature is raised to about 750.degree.-800.degree. C. or higher, the 
compounds will decompose, which is considered to arise from the 
decomposition and the resulting escape or evaporation of NH present in the 
ionic crystals constituting the compounds. 
EXAMPLE 1 
Each of the above mentioned compounds 1 to 6 was molded into a 
disk-shaped pellet of 2 to 3 mm in thickness by using a die of 10 mm in 
diameter and a molding face of 2 tons. An electrode was formed by 
uniformly dispersing Pb-added PbI.sub.2 on one side of the disk-shaped 
compound, and the whole was pressure-molded at a molding pressure of 2 
tons to obtain a disk-shaped double-layer pellet consisting of a layer of 
the compound and a layer of Pb-added PbI.sub.2. 
Furthermore, a piece of metallic lithium was pressure-bonded to the other 
side of the pellet to form anode, whereby a cell having a structure of 
PbI.sub.2 (Pb).vertline.the above compound.vertline.Li was obtained. 
When the e.m.f. of each of the thus obtained cells was measured at room 
temperature, an e.m.f. of 1.90 V was obtained, which was coincident with 
the theoretical value. Thus, it was confirmed that the compounds 1 to 6 
are lithium ionic conductors useful as solid electrolyte. 
EXAMPLE 2 
Each of the compounds 1 to 6 was molded into a disk-shaped pellet of 2 
to 3 mm in thickness by using a die of 10 mm in diameter and a molding 
force of 1 ton. 
The disk-shaped pellets thus obtained were then provided with silver 
electrodes respectively on both sides thereof. The resulting assemblies 
were served to AC-impedance measurements in an electric furnace provided 
with temperature control means, to determine the temperature dependence of 
lithium ionic conductivity of each of the compounds 1 to 6 . 
The measurements were carried out in the conditions of a frequency 1 KHz, a 
loaded voltage of 100 mV rms and an atmosphere of nitrogen. FIG. 4 shows 
the temperature dependence of ionic conductivity of the compounds 1 to 
6 which had been sintered at 300.degree. to 500.degree. C. for 10 min. 
In FIG. 4, the lines 1 to 6 indicate the Arrhenius plots for the ionic 
conductivity of the compounds 1 to 6 , respectively. 
As is clear from FIG. 4, among the compounds according to this invention, 
the compounds 5 and 6 have high ionic conductivities of 
1.0.times.10.sup.-1 Sm.sup.-1 and 6.0.times.10.sup.-3 Sm.sup.-1, 
respectively, and they are superior in temperature dependence of ionic 
conductivity. 
Accordingly, among the compounds of this invention, those containing iodine 
as halogen X in the general formula are particularly suitable for various 
applications such as room-temperature types of solid state battery, 
electrochromic display, voltamometer, capacitor, alkaline ion selective 
film and coulometer.