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Lithium ion conductivity in lithium nitride
Volume 58A, number 4
PHYSICS LETTERS
6 September 1976
LITHIUM ION CONDUCTIVITY IN LITHIUM NITRIDE B.A. BOUKAMP and R.A. HUGGINS Center for Materials Research, Stanford University, Stanford, California USA Received 29 June 1976 The ionic conductivity of polycrystalline lithium nitride has been determined using ac techniques and complex conductivity is quite high, so that this material may be an interesting lithium-conducting
plane analysis. The ionic solid electrolyte.
There is a considerable amount of interest in the use of solid ionic conductors for a number of practical applications, such as high specific energy storage systems, or miniature batteries with an extended lifetime. Because of its low atomic weight and very electropositive nature, lithium is a potentially desirable anode material for use in these high specific energy cells. The number of known useable solid lithium conductors is small, however, and the material with the highest known conductivity at ambient temperature, lithium-exchanged single crystal beta alumina, is not stable at elevated temperatures [1]. Lithium orthosilicate doped with lithium phosphate has recently been found to possess relatively high lithium ion conductivity at intermediate temperatures [2], as does solid L1A1C14 [3]. It is the purpose of the present note to report some measurements on lithium nitride which indicate that it also might be of interest as a lithium ion-conducting solid electrolyte. The crystal structure of Li 3N, as initially proposed by Zintl and Brauer [4] and confirmed recently by Rabenauand Schulz [5], consists of hexagonal Li2N layers connected by Li ions which form N-Li-N bridges, arid thus is very open, with large intersecting empty tunnels in two dimensions; see fig. 1. The ionicity of Li3N is not settled. If it were purely ionic, i.e., all electron shells are filled, it should be either colorless or whit~.The actual color of Li3N ranges between ruby red to violet black [4, 6] The crystal structure also suggests strong directional, i.e., predominantly covalent, bonding. Based on these arguments, Krebs2 hybrid [7] proposed bonding scheme orbitalsa for the lithium with atomsresonant in the Usp 2N layers. Recent NMR measurements -
Fig. 1. Drawing illustrating the crystal structure of Li
3N.
Smaller circles represent the lithium atoms. (Burkert et al. [6]) are in good agreement with this model, rather than with an alternative proposal by Suchet [8]. 7 NMR lines was reMotional narrowing Li indicates that the ported by Bishop et al. of [9]the which lithium ions are very mobile. The steady state line narrows between —70 and —20°C,and their analysis of the data gives an activation energy for the lithium diffusion of 0.55 eV, and an extremely high jump frequency of the order of 10—15 sec. In a recent analysis of the same data, Hendrickson and Bray [10] suggested long range diffusion of lithium inside the tunnels, using a newly developed phenomenological description of the motional line narrowing. They arrived at a slightly higher activation enthalpy, 0.57 eV. Ionic conductivity measurements were performed by Masdupuy [11, 12] ,who reported the activation enthalpy for ionicconductivity conduction to be 0.53 eV, below about 440°C.His measurements are, however, subject to serious doubt, as they were per231
Volume 58A, number 4
PHYSICS LE’FTER~
formed on a loose powder of U3N. In fact, the small magnitude of the ionic conductivity measured by Masdupuy is not consistent with the high jump frequency and the low temperature at which the motional narrowing occurs in the NMR experiments. It seemed, therefore, worthwhile to remeasure the ionic conductivity on pressed and sintered pellets of Li3 N. We report here measurements of the ionic conductivity of samples of two different materials. The first was Li3N obtained from ROC/RIC Company, and was asserted to be only 96% pure. Debye-Scherrer X-ray analysis showed the presence of a few lines in addition to those given for pure Li3 N by Zintl and Brauer [4] This material is probably lithium-rich, as was found previously for other samples [9, 13]. We prepared the second material from pure lithium ribbon (Foote Mineral 99.9% mm) by direct reaction with nitrogen. The reaction is very rapid at temperatures close to the melting point of lithium, but also proceeds at room temperature under very clean conditions. A minimal amount of water vapor is reported to be necessary for initiating the reaction [14]. The reaction mechanism has been studied by Frankenburger [15] ,who found the reaction rate to be limited by the diffusion of nitrogen through cracks formed in the reaction product, due to a 30% decrease in specific volume, After reaction by heating at 170—180°Cin a pure nitrogen atmosphere at 5—6 atm pressure for several hours, following the procedure given by [16 and 171 the yield was crushed to a fine powder. The X-ray pattern of this material was very close to the results of Zintl ~nd Brauer [4J. All handling was performed in a controlled atmosphere glove box under helium, or in a specially designed apparatus to avoid contact with air. Pellets 0.95 cm in diameter and between 0.2 and 0.6 cm thick were pressed a steel die at under a pressure 2. They wereinthen sintered punof 2700 kg/cmat 650°C.These pellets always turned fled nitrogen dark blue-violet black in color, and had densities between 65 and 80% of the theoretical value. lonically
6 September 1976 1000
500 I I I
I
300 200 I
100
50
0
I
C 2 1 0
-
—
1
-‘~
-
~
‘TE
-2 -
-
°
.
blocking electrodes were applied by sputtering 0.3 micron molybdenum layers on both sides of the pellets. The conductivity was measured as a function of frequency at various temperatures, generally between 20 Hz and 50 kHz, using either a 1608 General Radio 232
-
LiEN (PURE) 96%) •:. MASDUPUY
—
5 -
U
-6
-
-‘
—J— I
0
I
3 10 /T I
I
1
-
U
-
I
-1 (K ) I
I
I
I
2
3 Fig. 2. Temperature dependence of the ionic conductivity of Li3N and lithium beta alumina [1]. Data are shown for two
different samples prepared from the 96% pure material. impedance bridge or a specially designed arrangement [18] using a P.A.R. 129 two phase lock-in amplifier. In some cases the frequency range was extended down to 0.01 Hz by using a PDP-8E digital computer for analysis. Although measurements were carried out in a nitrogen atmosphere at one atmosphere, the ionic conductivity did not seem to depend on the nitrogen pressure. Conductivity values were determined by use of the complex plane method of data analysis [19], whichfrom provides a clear polarization separation ofeffects. bulk ionic conductivity interfacial The resulting conductivity values are plotted as log aT versus 1/Tin fig. 2. It is evident that our data indicate a much higher conductivity for Li 3N than was reported by Masdupuy, whose results are also shown for comparison. We found the 96% material to have a higher conductivity than the purer Li3N, mdicating the influence of some unknown dopant. Even though they are certainly not optimized, these materials have remarkably high values of ionic conductivity, even
Volume 58A, number 4
Conductivity
Material
Activation
enthalpy (eV)
PHYSICS LETTERS Table 1 data for lithium nitride Conductivity at 25°C (ohm cmY’
Conductivity at 100°C 1 (ohm cm~
6 September 1976
This work was supported by the Advance Research Projects Agency through the Office of Naval Research under Contract N00014-67-A-Ol 12-0075. References
Masdupuy
(powder)*, 370450°C 0.66 430—550°C 1.32 Li 3N (pure) 0.61 3.7 X 108 3.4 X 10-6 5 Li3N (96%) 0.63 3.0 X i0’ 3.8 x ioValues obtained by least squares fitting and re-analysis of his data. — —
—
*
exceeding that of single crystal lithium beta alumina [I] at temperatures above 240°C,as shown in fig. 2. The activation enthalpy values obtained by a least squares fit of both our data and those by Masdupuy, are shown in table 1. Samples measured using solid lithium electrodes showed, using the complex plane analysis method, essentially no interfacial polarization. This information, as well as the results of very low frequency measurements, clearly indicate that the electronic conductivity Is at least several orders of magnitude lower than the ionic conductivity. Further work is being undertaken in our laboratory to produce better samples, so that the intrinsic properties can be measured with more precision, and to investigate the influence of doping. The relatively high values of lithium ionic conductivity found in this material, even on samples that are only moderately dense, lead to the possibility that it might be of practical use as a solid electrolyte in various lithium-based systems. It also may be useful as an electrochemical transducer for nitrogen in the absence of oxygen.
[1] M.S. Whittingham and RA. Huggins, in Solid state chemistry, ed. by R.S. Roth and SJ. Schneider, Nat. Bur. Stand. Spec. Pub. 364 (1972), p. 139. [2] Mat. Y-W. Res. Hu, l.D. Bull.Raistrick and R.A. Huggins, submitted to [3] W. Weppner and R.A. Huggins, submitted to I. Electrochem. Soc. [4] E. Zintl and G. Brauer, Z. Elektrochem. 41(1935)102. [5] A. Rabenau and H. Schulz, J. Less Common Metals, in press. [6] P.K. Burkert, H.P. Fritz and G. Steflanak, Z. Naturforsch. B25 (1970) 1220. [71H. Kxebs, Fundamentals of inorganic crystai chemistry, (McGraw-Hill, London 1968), p. 252. [8] J.P. Suchet, Acta Cryst. 14(1961) 651. [9] S.G. Bishop, PJ. Ring and PJ. Bray, .1. Chem. Phys. 45 (1966) 1525. [101J.R. Henrickson and PJ. Bray, J. Magn. Res. 9 (1973) 341.
[11] F. Gallais and E. Masdupuy, Compt. Rendus 227 (1948) 635. [121E. Masdupuy, Ann. Chimie (Paris) 13 Series 2(1957) 1527. [13] P.3. Haigh, R.A. Forman and R.C. Frisch, J. Chem. Phys. 45 (1966) 813. [141M. Fromont, Rev. Chim. Miner. 4 (1967) 447. [15] W. Frankenburger, Z. Elektrochem. 32(1926)481. [161S.A. Kutolin and A.!. Vulikh, Prom. Khim. Reaktivov Osobo Chist. Veshchestv 13 (1968) 26. [171S.A. Kutolin and A.I. Vulikh, 2h. Prikl. Khim. 41(1968) 2529. [181 B.A. Boukamp, Internal Report, Stanford University (1975). [191I.D. Raistrick and R.A. Huggins, in Proc. Symp. and Workshop on Advanced Battery Research and Design, Argonne National Laboratory, March, 1976. P. B-277.
233
PHYSICS LETTERS
6 September 1976
LITHIUM ION CONDUCTIVITY IN LITHIUM NITRIDE B.A. BOUKAMP and R.A. HUGGINS Center for Materials Research, Stanford University, Stanford, California USA Received 29 June 1976 The ionic conductivity of polycrystalline lithium nitride has been determined using ac techniques and complex conductivity is quite high, so that this material may be an interesting lithium-conducting
plane analysis. The ionic solid electrolyte.
There is a considerable amount of interest in the use of solid ionic conductors for a number of practical applications, such as high specific energy storage systems, or miniature batteries with an extended lifetime. Because of its low atomic weight and very electropositive nature, lithium is a potentially desirable anode material for use in these high specific energy cells. The number of known useable solid lithium conductors is small, however, and the material with the highest known conductivity at ambient temperature, lithium-exchanged single crystal beta alumina, is not stable at elevated temperatures [1]. Lithium orthosilicate doped with lithium phosphate has recently been found to possess relatively high lithium ion conductivity at intermediate temperatures [2], as does solid L1A1C14 [3]. It is the purpose of the present note to report some measurements on lithium nitride which indicate that it also might be of interest as a lithium ion-conducting solid electrolyte. The crystal structure of Li 3N, as initially proposed by Zintl and Brauer [4] and confirmed recently by Rabenauand Schulz [5], consists of hexagonal Li2N layers connected by Li ions which form N-Li-N bridges, arid thus is very open, with large intersecting empty tunnels in two dimensions; see fig. 1. The ionicity of Li3N is not settled. If it were purely ionic, i.e., all electron shells are filled, it should be either colorless or whit~.The actual color of Li3N ranges between ruby red to violet black [4, 6] The crystal structure also suggests strong directional, i.e., predominantly covalent, bonding. Based on these arguments, Krebs2 hybrid [7] proposed bonding scheme orbitalsa for the lithium with atomsresonant in the Usp 2N layers. Recent NMR measurements -
Fig. 1. Drawing illustrating the crystal structure of Li
3N.
Smaller circles represent the lithium atoms. (Burkert et al. [6]) are in good agreement with this model, rather than with an alternative proposal by Suchet [8]. 7 NMR lines was reMotional narrowing Li indicates that the ported by Bishop et al. of [9]the which lithium ions are very mobile. The steady state line narrows between —70 and —20°C,and their analysis of the data gives an activation energy for the lithium diffusion of 0.55 eV, and an extremely high jump frequency of the order of 10—15 sec. In a recent analysis of the same data, Hendrickson and Bray [10] suggested long range diffusion of lithium inside the tunnels, using a newly developed phenomenological description of the motional line narrowing. They arrived at a slightly higher activation enthalpy, 0.57 eV. Ionic conductivity measurements were performed by Masdupuy [11, 12] ,who reported the activation enthalpy for ionicconductivity conduction to be 0.53 eV, below about 440°C.His measurements are, however, subject to serious doubt, as they were per231
Volume 58A, number 4
PHYSICS LE’FTER~
formed on a loose powder of U3N. In fact, the small magnitude of the ionic conductivity measured by Masdupuy is not consistent with the high jump frequency and the low temperature at which the motional narrowing occurs in the NMR experiments. It seemed, therefore, worthwhile to remeasure the ionic conductivity on pressed and sintered pellets of Li3 N. We report here measurements of the ionic conductivity of samples of two different materials. The first was Li3N obtained from ROC/RIC Company, and was asserted to be only 96% pure. Debye-Scherrer X-ray analysis showed the presence of a few lines in addition to those given for pure Li3 N by Zintl and Brauer [4] This material is probably lithium-rich, as was found previously for other samples [9, 13]. We prepared the second material from pure lithium ribbon (Foote Mineral 99.9% mm) by direct reaction with nitrogen. The reaction is very rapid at temperatures close to the melting point of lithium, but also proceeds at room temperature under very clean conditions. A minimal amount of water vapor is reported to be necessary for initiating the reaction [14]. The reaction mechanism has been studied by Frankenburger [15] ,who found the reaction rate to be limited by the diffusion of nitrogen through cracks formed in the reaction product, due to a 30% decrease in specific volume, After reaction by heating at 170—180°Cin a pure nitrogen atmosphere at 5—6 atm pressure for several hours, following the procedure given by [16 and 171 the yield was crushed to a fine powder. The X-ray pattern of this material was very close to the results of Zintl ~nd Brauer [4J. All handling was performed in a controlled atmosphere glove box under helium, or in a specially designed apparatus to avoid contact with air. Pellets 0.95 cm in diameter and between 0.2 and 0.6 cm thick were pressed a steel die at under a pressure 2. They wereinthen sintered punof 2700 kg/cmat 650°C.These pellets always turned fled nitrogen dark blue-violet black in color, and had densities between 65 and 80% of the theoretical value. lonically
6 September 1976 1000
500 I I I
I
300 200 I
100
50
0
I
C 2 1 0
-
—
1
-‘~
-
~
‘TE
-2 -
-
°
.
blocking electrodes were applied by sputtering 0.3 micron molybdenum layers on both sides of the pellets. The conductivity was measured as a function of frequency at various temperatures, generally between 20 Hz and 50 kHz, using either a 1608 General Radio 232
-
LiEN (PURE) 96%) •:. MASDUPUY
—
5 -
U
-6
-
-‘
—J— I
0
I
3 10 /T I
I
1
-
U
-
I
-1 (K ) I
I
I
I
2
3 Fig. 2. Temperature dependence of the ionic conductivity of Li3N and lithium beta alumina [1]. Data are shown for two
different samples prepared from the 96% pure material. impedance bridge or a specially designed arrangement [18] using a P.A.R. 129 two phase lock-in amplifier. In some cases the frequency range was extended down to 0.01 Hz by using a PDP-8E digital computer for analysis. Although measurements were carried out in a nitrogen atmosphere at one atmosphere, the ionic conductivity did not seem to depend on the nitrogen pressure. Conductivity values were determined by use of the complex plane method of data analysis [19], whichfrom provides a clear polarization separation ofeffects. bulk ionic conductivity interfacial The resulting conductivity values are plotted as log aT versus 1/Tin fig. 2. It is evident that our data indicate a much higher conductivity for Li 3N than was reported by Masdupuy, whose results are also shown for comparison. We found the 96% material to have a higher conductivity than the purer Li3N, mdicating the influence of some unknown dopant. Even though they are certainly not optimized, these materials have remarkably high values of ionic conductivity, even
Volume 58A, number 4
Conductivity
Material
Activation
enthalpy (eV)
PHYSICS LETTERS Table 1 data for lithium nitride Conductivity at 25°C (ohm cmY’
Conductivity at 100°C 1 (ohm cm~
6 September 1976
This work was supported by the Advance Research Projects Agency through the Office of Naval Research under Contract N00014-67-A-Ol 12-0075. References
Masdupuy
(powder)*, 370450°C 0.66 430—550°C 1.32 Li 3N (pure) 0.61 3.7 X 108 3.4 X 10-6 5 Li3N (96%) 0.63 3.0 X i0’ 3.8 x ioValues obtained by least squares fitting and re-analysis of his data. — —
—
*
exceeding that of single crystal lithium beta alumina [I] at temperatures above 240°C,as shown in fig. 2. The activation enthalpy values obtained by a least squares fit of both our data and those by Masdupuy, are shown in table 1. Samples measured using solid lithium electrodes showed, using the complex plane analysis method, essentially no interfacial polarization. This information, as well as the results of very low frequency measurements, clearly indicate that the electronic conductivity Is at least several orders of magnitude lower than the ionic conductivity. Further work is being undertaken in our laboratory to produce better samples, so that the intrinsic properties can be measured with more precision, and to investigate the influence of doping. The relatively high values of lithium ionic conductivity found in this material, even on samples that are only moderately dense, lead to the possibility that it might be of practical use as a solid electrolyte in various lithium-based systems. It also may be useful as an electrochemical transducer for nitrogen in the absence of oxygen.
[1] M.S. Whittingham and RA. Huggins, in Solid state chemistry, ed. by R.S. Roth and SJ. Schneider, Nat. Bur. Stand. Spec. Pub. 364 (1972), p. 139. [2] Mat. Y-W. Res. Hu, l.D. Bull.Raistrick and R.A. Huggins, submitted to [3] W. Weppner and R.A. Huggins, submitted to I. Electrochem. Soc. [4] E. Zintl and G. Brauer, Z. Elektrochem. 41(1935)102. [5] A. Rabenau and H. Schulz, J. Less Common Metals, in press. [6] P.K. Burkert, H.P. Fritz and G. Steflanak, Z. Naturforsch. B25 (1970) 1220. [71H. Kxebs, Fundamentals of inorganic crystai chemistry, (McGraw-Hill, London 1968), p. 252. [8] J.P. Suchet, Acta Cryst. 14(1961) 651. [9] S.G. Bishop, PJ. Ring and PJ. Bray, .1. Chem. Phys. 45 (1966) 1525. [101J.R. Henrickson and PJ. Bray, J. Magn. Res. 9 (1973) 341.
[11] F. Gallais and E. Masdupuy, Compt. Rendus 227 (1948) 635. [121E. Masdupuy, Ann. Chimie (Paris) 13 Series 2(1957) 1527. [13] P.3. Haigh, R.A. Forman and R.C. Frisch, J. Chem. Phys. 45 (1966) 813. [141M. Fromont, Rev. Chim. Miner. 4 (1967) 447. [15] W. Frankenburger, Z. Elektrochem. 32(1926)481. [161S.A. Kutolin and A.!. Vulikh, Prom. Khim. Reaktivov Osobo Chist. Veshchestv 13 (1968) 26. [171S.A. Kutolin and A.I. Vulikh, 2h. Prikl. Khim. 41(1968) 2529. [181 B.A. Boukamp, Internal Report, Stanford University (1975). [191I.D. Raistrick and R.A. Huggins, in Proc. Symp. and Workshop on Advanced Battery Research and Design, Argonne National Laboratory, March, 1976. P. B-277.
233