A Novel Family of Structurally Characterized Lithium Cobalt Double

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A Novel Family of Structurally Characterized Lithium Cobalt Double Aryloxides and the Nanoparticles and Thin Films Generated Therefrom Timothy J. Boyle,* Mark A. Rodriguez, David Ingersoll, Thomas J. Headley, Scott D. Bunge, Dawn M. Pedrotty, Sacha M. De’Angeli, Sara C. Vick, and Hongyou Fan Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Boulevard, SE, Albuquerque, New Mexico 87106 Received September 10, 2002. Revised Manuscript Received July 23, 2003

The reaction between LiN(SiMe3)2 and Co(N(SiMe3)2)2 in THF followed by the addition of an aryl alcohol (HOAr) yielded: [Co[(µ-OAr)2Li(THF)x]2 [where x ) 2: OAr ): OC6H4Me-2 (oMP, 1), OC6H4(OCHMe2)-2 (oPP, 2), OC6H3(Me)2-2,6 (DMP, 3); where x ) 1: OAr ) OC6H3(OCHMe2)2-2,6 (DIP, 4)]. Undertaking the same synthesis in pyridine (py) led to the same general structure with py molecules substituted for the THF solvent molecules: [Co[(µOAr)2Li(py)2]2 (OAr ) oMP (5), oPP (6)]. For 1-6, the tetrahedrally (Td) bound Co atom bridges two OAr ligands to each of the Li atoms. The Li atom adopts a Td or trigonal bipyramidal (TBP) geometry, as determined by the number of bound solvent molecules. For the more sterically demanding ligands with py as the solvent, the mononuclear monometallic species [Co[(µ-OAr)2(py)x]2 (OAr ) DMP (7), x ) 2; DIP (8), x ) 3) were observed adopting a Td or TBP geometry, respectively, based on the number of bound solvent molecules. Increasing the steric bulk of the OAr to OC6H3But2-2,6 (DBP) in either THF or py led to the isolation of the previously characterized [Li(DBP)(solv)]2. Calculated XRD powder patterns were generated from the single-crystal structure of 1-8 and used to verify the identity of the bulk powder. Attempts to use these compounds for MOCVD applications were not successful due to low volatility of these compounds. Attempts to generate nanoparticles of the spinel phase of LiCoO2 by injecting a py solution of 5 or 6 into boiling methyl-imidazole (MeIm)/H2O 95:5) led to nanoparticles of Co(OH)2 only, as characterized by XRD and TEM. Thin films of the spinel phase of LiCoO2 were formed by spin-cast deposition methods using 5 or 6 dissolved in a py/toluene mixture onto platinized silicon wafers followed by firing at 700 °C. Cyclic voltammetry revealed two irreversible oxidation processes followed by one reversible process, the latter of which is found to occur at about 4.2 V. XRD analysis of the thin film, both before and after electrochemical cycling, revealed only minimal variations in the crystal structure of the film after cycling. This family of compounds, even though stoichiometrically incorrect, have been shown to be useful for single-source applications in the solid and liquid techniques.

Introduction Lithium ion (or “rocking chair”) batteries are of increasing interest due to the relative environmental friendliness of their components in comparison to existing power sources (i.e., Pb-acid, Ni-Cd).1-9 The spinel phases of LiMn2O4 or the layered oxide phases of LiMO2 * To whom correspondence should be addressed. (1) Doughty, D. H. SAMPE J. 1996, 32, 75. (2) Ohzuku, T. Lithium Batteries. New Materials, Developments and Perspectives; Elsevier Science B. V.: Dordrecht, The Netherlands, 1994. (3) Salomon, M.; Scrosati, B. Gazz. Chim. Ital. 1996, 126, 415. (4) Takehara, Z. J. Power Sources 1997, 68, 82. (5) Jacoby, M. Chem. Eng. News 1998, 76 (Aug 3), 37. (6) Boyle, T. J.; Ingersoll, D.; Alam, T. M.; Tafoya, C. J.; Rodriguez, M. A.; Vanheusden, K.; Doughty, D. H. Chem. Mater. 1998, 10, 2270. (7) Boyle, T. J.; Voigt, J. A.; Section 52-2 (Electrochemical, Radiational, and Thermal Energy Technology): U.S. Patent 5630994, 1997; Vol. Main IPC C01G045-12, p 6. (8) Boyle, T. J.; Ingersoll, D.; Rodriguez, M. A.; Tafoya, C. J.; Doughty, D. H. J. Electrochem. Soc. 1999, 146, 1683. (9) Tullo, A. H. Chem. Eng. News 2002, 25.

(M ) Co and Ni) are often used as the cathode material for these batteries.1-18 In standard configurations, the bulk powders of these ceramic materials are compacted into the desired design. However, due to the miniaturization of devices which require smaller, more powerful power sources, thin film batteries are of increasing interest. (10) Park, Y. J.; Kim, J. G.; Kim, M. K.; Kim, H. G.; Chung, H. T.; Park, Y. J. Power Sources 2000, 87, 69. (11) Kang, K.; Dai, S. H.; Wan, Y. H. J. Inorg. Mater. 2001, 16, 586. (12) Zhang, W. M.; Yang, Y. H.; Sun, S. X.; Liu, Z. P.; Song, X. Y. Chin. J. Inorg. Chem. 2000, 16, 873. (13) Ammundsen, B.; Paulsen, J. Adv. Mater. 2001, 13, 943. (14) Scrosati, B. Electrochim. Acta 2000, 45, 2461. (15) Thackeray, M. M. Prog. Solid State Chem. 1997, 25, 1. (16) Koksbang, R.; Barker, J.; Shi, H.; Saidi, M. Y. Solid State Ionics 1996, 84, 1. (17) Chen, C. H.; Buysman, A. A. J.; Kelder, E. M.; Schoonman, J. Solid State Ionics 1995, 80, 1. (18) Armstrong, R. A.; Bruce, P. G. Nature 1996, 381, 499.

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Several routes to thin films of ceramic materials exist, including metal-organic chemical vapor deposition (MOCVD), laser ablation, sputtering, aerosol-assisted CVD, and solution routes (or the so-called “sol-gel”) methods. Metal alkoxides have been found to be excellent solution route precursors to ceramic materials due to their high solubility, low decomposition temperatures, cross-linking ability, ease of modification, and commercial availability.19-24 Using a variety of metal alkoxides, we have developed several novel solution routes to LiCoO2 materials,25 as well as demonstrated the utility that metal alkoxides may have in MOCVD methods for production of thin films.26 For both methodologies it would be beneficial to have a “single-source” precursor to LiCoO2 to simplify processing of smaller batteries. Once developed, these single-source precursors may also be exploited for other applications. It has been wellestablished that nanoscale crystals of the appropriate size exhibit fundamental modifications of the allowed electronic energy level structure when compared to the bulk material. Changes in the electronic density of states can significantly impact the electronic and optical behavior of these materials, often resulting in phenomena that are unattainable in the corresponding bulk material. The fundamental issues operative in electrochemistry are also anticipated to be influenced by quantum level effects in such structures. While a great deal of work is available demonstrating the use of metal alkoxides as useful precursors to homometallic and heterometallic27-36 ceramic nanoparticles, little work has been proffered on the use of single-source heterometallic alkoxides as precursors to complex nanoceramic materials.37-39 We have recently reported on the structural properties of a number of solvated lithium aryloxides, [Li(OAr)(19) Bradley, D. C. Chem. Rev. 1989, 89, 1317. (20) Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P. Metal Alkoxides; Academic Press: New York, 1978. (21) Bradley, D. C.; Mehrotra, R. C.; Rothwell, I. P.; Singh, A. Alkoxo and Aryloxo Derivatives of Metals; Academic Press: San Diego, 2001. (22) Chandler, C. D.; Roger, C.; Hampden-Smith, M. J. Chem. Rev. 1993, 93, 1205. (23) Hubert-Pfalzgraf, L. G. New. J. Chem. 1987, 11, 663. (24) Turevskaya, E. P.; Yanovskaya, M. I.; Turova, N. Y. Inorg. Mater. 2000, 36, 260. (25) Boyle, T. J. Unpublished results. (26) Gallegos, J. J. I.; Ward, T. L.; Boyle, T. J.; Francisco, L. P.; Rodriguez, M. A. Adv. Mater. CVD 2000, 6, 21. (27) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. (28) Rockenberger, J.; Scher, E. C.; Alivisatos, P. J. Am. Chem. Soc. 1999, 121, 11595. (29) O’Brian, S.; Brus, L.; Murray, C. B. J. Am. Chem. Soc. 2001, 123, 12085. (30) Liu, C.; Zou, B.; Rondinone, A. J.; Zhang, Z. J. J. Am. Chem. Soc. 2001, 123, 4344. (31) Dutremez, S.; Gerbier, P.; Guerin, C.; Henner, B.; Merle, P. Adv. Mater. 1998, 10, 465. (32) Schehl, M.; Diaz, L. A.; Torrecillas, R. Acta Mater. 2002, 50, 1125. (33) Rodriguez-paez, J. E.; Caballero, A. C.; Villegas, m.; Moure, C.; Duran, P.; Fernandez, J. F. J. Eur. Ceram. Soc. 2001, 21, 925. (34) Kim, e. J.; Oh, S. H.; Hahn, S. H. Chem. Eng. Commun. 2001, 187, 171. (35) Kim, E. J.; Hahn, S. H. Mater. Lett. 2001, 49, 244. (36) Hu, M. Z. C.; Harris, M. T.; Byers, C. H. J. Colloid Interface Sci. 1998, 198, 87. (37) Shen, H.; Mathur, S. J. Phys. IV 2002, 12, 1. (38) Meyer, F.; Hempelmann, R.; Mathur, S.; Veith, M. J. Mater. Chem. 1999, 9, 1755. (39) Terry, K. W.; Lugmair, C. G.; Gantzel, P., K,; Tilley, T. D. Chem. Mater. 1996, 8, 4.

Boyle et al.

(solv)x]n,40 and shown the wide variety of structure types that are available ranging from squares to hexagons to cubes to hexagonal prisms to fused hexagonal-cube geometries. There are only a few reports of crystallographically characterized cobalt alkoxides, including [Co(OR)2]2‚solv, where OR ) OC(C6H11)3,41 OCPh3,41 OSiPh3,41 OCPh3,41 and [{Co[(µ3-OC6H2(O-2)(But2-2,5)][THF]2}2{Co[(µ3-OC6H2(O-2)(But2-2,5)][THF]}2].42 Furthermore, there are only two reports of a Li2Co species, both of which employ siloxide ligands: Co[Li(µ-OSi(Ph)2)2OSi(Ph)2)(py)2]243 and Co[Li(µ-OSi(Ph)2)2OSi(Ph)2)(TMEDA)]2.44 While there are numerous compounds that possess both Li and Co atoms (59 using the CCSD V 5.22, October 2001), none of these compounds utilize only alkoxides as the supporting ligands. Alkoxy ligated species are of interest due to their clean conversion to the oxide and uniformity of processing. We therefore undertook an investigation of the synthesis of LiCoO2 double-aryloxide precursors. The reaction pathway studied involved the amide alcohol exchange shown in eq 1, which was similar to that used for the formation of the [Li(OAr)(solv)x]n40 species. The reaction products were crystallographically identified as Co[(µ-OAr)2Li(THF)x]2 [where x ) 2: OAr ): OC6H4Me-2 (oMP, 1), OC6H4(OCHMe2)-2 (oPP, 2), OC6H3(Me)2-2,6 (DMP, 3); where x ) 1: OAr ) OC6H3(OCHMe2)2-2,6 (DIP, 4)], Co[(µ-OAr)2Li(py)2]2 [OAr ) oMP (5), oPP (6)], and Co(OAr)(py)x [OAr ) DMP, x ) 2 (7); DIP, x ) 3 (8)]. This report discusses the syntheses and characterizations of 1-8, in detail. solv

2Li[N(SiMe3)2] + Co[N(SiMe3)2]2 + 6HOAr 98 Co[(µ-OAr)2Li(solv)x]2 (1) solv ) THF or py Even though the stoichiometries of these species were not correct for LiCoO2, this was not expected to be a major hindrance since excess lithium is often added to materials systems to compensate for lost Li during processing. Several experiments were undertaken to explore the utility of these double-aryloxide precursors as single-source precursors to nanoparticles and thin films (sol-gel and MOCVD) of LiCoO2. The synthesis, characterization, and application of these precursors to various material forms is presented. Experimental Section All compounds described below were handled under an atmosphere of argon with rigorous exclusion of air and water using standard Schlenk line and glovebox techniques. FT-IR data were obtained on a Bruker Vector 22 spectrometer using KBr pellets under an atmosphere of flowing nitrogen. Elemental analysis was performed on a Perkin-Elmer 2400 CHNS/O elemental analyzer. All solvents were freshly distilled from the appropriate drying agent immediately prior to use and stored over rigor(40) Boyle, T. J.; Pedrotty, D. M.; Alam, T. M.; Vick, S. C.; Rodriguez, M. A. Inorg. Chem. 2000, 39, 5133. (41) Sigel, G. A.; Bartlett, R. A.; Decker, D.; Olmstead, M. M.; Power, P. P. Inorg. Chem. 1987, 26, 1773. (42) Olmstead, M. M.; Power, P. P.; Sigel, G. A. Inorg. Chem. 1988, 27, 580. (43) Abrahams, I.; Lazell, M.; Motevalli, M.; Simon, C. K.; Sullivan, A. C. Khim. Geterotsikl. Soedin. SSR 1999, 1089. (44) Hursthouse, M. B.; Mazid, M. A.; Motevalli, M.; Sanganee, M.; Sullivan, A. C. J. Organomet. Chem. 1990, 381, C43.

Lithium Cobalt Double Aryloxides

Chem. Mater., Vol. 15, No. 20, 2003 3905 Table 1. Data Collection Parameters for 1-4

compound

1

2

3

4

chemical formula formula weight temp (K) space group

C44H60CoLi2O8 789.73 168(2) P(1 h) triclinic 11.176(3) 11.198(3) 17.746(5) 83.618(5) 83.750(5) 75.875(5) 2132.7(10) 2 1.230 0.452 6.10 15.71

C48H68CoLi2O8 845.83 168(2) P2/n monoclinic 17.338(5) 13.320(3) 20.600(5)

C52H76CoLi2O8 901.94 168(2) P2(1)/n monoclinic 16.0665(17) 14.3438(15) 22.459(2)

C56H84CoLi2O6 926.04 168(2) Pbcn orthorhombic 17.1013(13) 16.7999(13) 19.3768(15)

101.699(5)

91.746(2)

4659(2) 4 1.206 0.418 5.78 10.44

5173.3(9) 4 1.158 0.380 3.71 10.30

a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (Mg/m3) µ, (Mo KR) (mm-1) R1a (%) wR2b (%) a

5567.0(7) 4 1.105 0.353 5.14 11.95

R1 ) ∑||Fo| - |Fc||/∑|Fo| × 100. b wR2 ) [∑w(Fo2 - Fc2)2/∑(w|Fo|2)2]1/2 × 100.

ously dried 4-Å sieves: including pyridine (py) from CaH2 and tetrahydrofuran (THF) from sodium benzophenone. The following chemicals were used as-received (Aldrich), stored, and handled under an argon atmosphere: H-oMP, H-oPP, H-oBP, H-DMP, H-DIP, H-DBP, LiN(SiMe3)2, 1-methyl imidazole (MeIm), and CoCl2. Co(N(SiMe3)2)2 was synthesized through the reaction of CoCl2 and 2 equiv of LiN(SiMe3).41 General Synthesis. One equivalent of Co(N(SiMe3)2)2 was added to a stirring solution of 2 equiv of Li(N(SiMe3)2) dissolved in the desired solvent. The reaction mixture turns from a pale yellow to a dark purple color. After complete dissolution (∼5 min), 6 equiv of the appropriate HOAr was added with varying reports of color changes (vide infra). The reaction was stirred for 12 h, the volume of the solution was reduced by slow evaporation of the volatile portion of the reaction, or the reaction mixture was placed in a freezer at -35 °C to yield crystalline material. Co[(µ-oMP)2Li(THF)2]2 (1). Used Co(N(SiMe3)2)2 (0.50 g, 1.3 mmol), Li(N(SiMe3)2 (0.44 g, 2.6 mmol), and H-oMP (0.86 g, 7.9 mmol) in THF (∼6 mL). Crystalline yield 56% (0.61 g). Solution color did not change upon addition of H-oMP. FT-IR (KBr, cm-1): 3367 (w), 2980 (s), 2876 (s), 1592 (s), 1568 (sh, w), 1483 (s), 1437 (sh, w), 1288 (s), 1188 (m), 1150 (w), 1111 (m), 1000 (s), 979 (w), 921 (w), 896 (sh, m), 861 (s), 775 (s), 719 (m), 598 (m), 556 (m), 481 (m), 446 (m). Elemental analysis for C44H64O8Co. Calcd: 67.76, C; 8.27, H. Found: 66.21, C; 7.55, H. Co[(µ-oPP)2Li(THF)2]2 (2). Used Co(N(SiMe3)2)2 (0.50 g, 1.3 mmol), Li(N(SiMe3)2 (0.44 g, 2.6 mmol), and H-oPP (1.1 g, 7.9 mmol) in THF (∼6 mL). Crystalline yield 49% (0.50 g). Solution color remains unchanged upon addition of H-oPP. FT-IR (KBr, cm-1): 2961 (s), 2870 (sh, m), 1592 (s), 1483 (s), 1443 (s), 1380 (w), 1346 (m), 1290 (s), 1261 (s), 1149 (w), 1086 (m), 1047 (s), 893 (s), 845 (s), 758 (s), 745 (s), 725 (sh, w), 598 (s), 545 (m), 461 (w). Elemental analysis for C52H76O8Li2Co. Calcd: 69.24, C; 8.49, H. Found: 68.95, C; 8.28, H. Co[(µ-DMP)2Li(THF)2]2 (3). Used Co(N(SiMe3)2)2 (0.50 g, 1.3 mmol), Li(N(SiMe3)2 (0.44 g, 2.6 mmol), and H-DMP (0.97 g, 7.9 mmol) in THF (∼6 mL). Crystalline yield 55% (0.65 g). Solution changes to blue color upon addition of H-DMP. FTIR (KBr, cm-1): 2958 (br, s), 159 (m), 1466 (s), 1269 (s), 1231 (m), 1093 (s), 1044 (m), 916 (w), 847 (s), 753 (s), 689 (m), 578 (w), 526 (m), 477 (m). Elemental analysis for C48H68O8Li2Co. Calcd: 68.16, C; 8.10, H. Found: 67.51, C; 7.99, H. Co[(µ-DIP)2Li(THF)]2 (4). Used Co(N(SiMe3)2)2 (0.50 g, 1.3 mmol), Li(N(SiMe3)2 (0.44 g, 2.6 mmol), and H-DIP (1.4 g, 7.9 mmol) in THF (∼6 mL). Crystalline yield 26.6% (0.30 g). Solution turns darker purple upon addition of H-DIP. FT-IR (KBr, cm-1): 3045 (w), 2960 (s), 2847 (m), 1588 (w), 1431 (s), 1383 (w), 1360 (w), 133.89 (m), 1266 (m), 1204 (m), 1204 (w), 1149 (w), 1108 (m), 1042 (s), 932 (w), 885 (w), 845 (m), 753 (s), 684 (m), 579 (w), 431 (w). Elemental analysis for C64H100O8Li2Co. Calcd: 72.63, C; 9.14, H. Found: 72.56, C; 9.07, H.

Table 2. Data Collection Parameters for 5 and 6 compound

5

6


C48H48CoLi2N4O4 817.71 168(2) P2(1)/c monoclinic 18.331(4) 10.268(2) 23.168(5) 92.996(5) 4354.6(16) 4 1.247 0.441 6.32 11.30

C224H256Co4Li8N16O16 3719.69 168(2) P2(1) monoclinic 14.4894(18) 37.823(5) 19.364(2) 91.384(3) 10609(2) 2 1.164 0.370 5.16 5.83

a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd(Mg/m3) µ, (Mo KR) (mm-1) R1a (%) wR2b (%)

a R1 ) ∑||F | - |F | |/∑|F | × 100. b wR2 ) [∑w(F 2 - F 2)2/ o c o o c ∑(w|Fo|2)2]1/2 × 100.

Co[(µ-oMP)2Li(py)2]2 (5). Used Co(N(SiMe3)2)2 (11.3 g, 29.8 mmol), Li(N(SiMe3)2 (10.0 g, 59.8 mmol), and H-oMP (38.8 g, 358 mmol) in py (∼60 mL). Crystalline yield 40.6% (19.8 g). Solution turns from dark blue to purple upon addition of H-oMP. Elemental analysis for C48H48CoLi2N4O4. Calcd: 70.50, C; 5.91, H; 6.85, N. Found: 65.27, C; 5.79, H; 5.55, N. Co[(µ-oPP)2Li(py)2]2 (6). Used Co(N(SiMe3)2)2 (11.3 g, 29.8 mmol), Li(N(SiMe3)2 (10.0 g, 59.8 mmol), and H-oPP (48.8 g, 358 mmol) in py (∼60 mL). Crystalline yield 41.7 (23.2 g). Solution turns from dark blue to purple upon addition of H-oPP. Elemental analysis for C48H48CoLi2N4O4. Calcd: 72.33, C; 6.93, H; 6.03, N. Found: 64.22, C; 5.60, H; 5.21, N. Co(DMP)2(py)2 (7). Used Co(N(SiMe3)2)2 (1.00 g, 2.63 mmol), Li(N(SiMe3)2 (0.880 g, 5.26 mmol), and H-DMP (1.93 g, 15.8 mmol) in py (∼6 mL). Solution turns from dark blue to reddishpurple upon addition of H-DMP. Co(DIP)2(py)3 (8). Used Co(N(SiMe3)2)2 (1.00 g, 2.63 mmol), Li(N(SiMe3)2 (0.880 g, 5.26 mmol), and H-DIP (2.82 g, 15.8 mmol) in py (∼6 mL). Solution turns from dark blue to reddishpurple upon addition of H-DIP. X-ray Crystal Structures.45 Tables 1-3 list the data collection parameters for 1-4, 5-6, and 7-8, respectively. Metrical data for all compounds can be found in the Supporting Information. All crystals were mounted onto a thin glass fiber from a pool of Fluorolube and placed immediately under a liquid N2 stream on a Bruker AXS diffractometer. Structural solutions were performed using the following software: SMART Version 5.054, SAINT+ 6.02 (7/13/99), SHELXTL 5.1 (10/29/ 98), XSHELL 4.1 (11/08/2000), and SADABS within the SAINT+ package.45 Each structure was solved using direct methods, yielding the Co, O, N, and some of the C atoms with subsequent Fourier synthesis yielding the remaining C atom

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Table 3. Data Collection Parameters for 7 and 8 compound

7

8


C26H28CoN2O2 459.43 183(2) C2/c monoclinic 14.293(3) 12.398(3) 14.148(3)

C39H49CoN3O2 650.74 168(2) P1 h triclinic 10.702(2) 10.969(2) 16.108(3) 72.975(3) 84.058(3) 81.564(4) 1784.8(7) 2 1.211 0.517 4.65 11.64

a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (Mg/m3) µ, (Mo KR) (mm-1) R1a (%) wR2b (%)

109.211(4) 2367.4(9) 4 1.289 0.749 3.24 7.41

R1 ) ∑||Fo| - |Fc||/∑|Fo| × 100. b wR2 ) [∑w(Fo2 - Fc2)2/ ∑(w|Fo|2)2]1/2 × 100. a

positions. The hydrogen atoms were fixed in positions of ideal geometry and refined within the XSHELL software.45 The final refinement of each compound included anisotropic thermal parameters on all non-hydrogen atoms. See appropriate table or Supporting Information for additional details. Nanoparticles. Approximate 0.20 M solutions of 5 or 6 in a py mixture (33/67 v/v) was added directly to a stirring, boiling solution of MeIm/water (95/5 v/v). The reaction was heated for 0.5 h after addition and then allowed to cool to room temperature. The reaction mixture was centrifuged and the insoluble powder washed with hexanes. The insoluble material was re-slurried in hexanes. An aliquot of which was placed onto a TEM grid that was then dried in an oven (140 °C) overnight. The resultant particles were studied by transmission electron microscopy (TEM) on a JEOL 2010 with 200-kv accelerating voltage, equipped with a Gatan slow scan CCD camera. Film Production. Standard spin-coat deposition routes46 were used to generate thin films with a brief description following. The appropriate precursors were dissolved in their parent solvent at a 0.20 M concentration. Multilayered films of the desired composition were spin-coat-deposited, in air, onto Pt(1700 Å)/Ti(300 Å)-coated, thermally oxidized SiO2/Si substrates using a photoresist spinner (3000 rpm for 30 s). After each deposition, the films were heated on a hot plate (300 °C for ∼5 min) and allowed to cool to room temperature before the deposition of the next layer. After the final layer received the 300 °C treatment (for this study three layers were used), the film was crystallized in a tube furnace at 650 °C for 30 min in air. X-ray diffraction (XRD) was used to confirm the phase purity and orientation of the final films. Electrochemical Measurements. Electrochemical measurements were made using the multilayered thin films (vide supra) with a portion of the wafer being masked-off to prevent deposition of the thin oxide film over the entire Pt surface. Electrical contact to the oxide thin film was then made by directly attaching to the exposed Pt surface, which underlies the entire sample. When electrochemical measurements were made, only that portion of the sample having the thin oxide film was immersed into the electrolyte. All electrochemical measurements were made in an open three electrode in an Ar-filled glovebox. Lithium metal was used as both the counter and reference electrodes, and all voltages reported here are referenced to it. The electrolyte used was LP40 Selectpur Battery Electrolyte obtained from EM Industries and is 1 M LiPF6 in diethyl carbonate/ethylene carbonate. All electrochemical measurements were made using a Parr Model 273A (45) The listed versions of SAINT, SMART, XSHELL, and SADABS Software from Bruker Analytical X-ray Systems Inc., 6300 Enterprise Lane, Madison, WI 53719, were used in analysis. (46) Boyle, T. J.; Al-Shareef, H. N. J. Mater. Sci. 1997, 32, 2263.

Figure 1. Thermal ellipsoid plot of 1. Thermal ellipsoids are drawn at the 30% level.

potentiostat under Corware software control. After completion of electrochemical cycling, the sample was removed from the glovebox and immediately rinsed with isopropyl alcohol and then allowed to air-dry. The electrochemical results described here are qualitative in nature due to the fact that the amount of material being characterized is not known.

Results and Discussion Due to the dearth of characterized Co(OR)241 and the desire for a single-source LiCoO2 precursor, we undertook the synthesis of a series of “LiCo(OAr)3” double aryloxides. We chose to study the OAr derivatives because of previous success with the [Li(OAr)(solv)x]n family40 and the wide range of variability in steric bulk available for the phenoxide substituents. Several synthetic pathways to mixed alkoxides exist; however, we chose to investigate the amide alcohol exchange mechanism due to the clean exchange and flexibility to alter the pendant aryloxide ligand that was noted for the [Li(OAr)(solv)x]n systems.40 Synthesis. Initial attempts to generate a singlesource precursor to LiCoO2 materials focused on exploiting the hyper-oligomerization often observed for metal alkoxides due to the large ratio of size to charge. Therefore, “Co(OAr)2” and the [Li(OAr)(THF)x]n40 were synthesized and mixed, but this failed to yield a homogeneous solution. Alternatively, it was ventured that an in situ method would generate soluble complexes; thus, an amidealcohol exchange in a 1:1 Li:Co stoichiometric ratio was investigated. The Li and Co amide precursors were mixed followed by addition of the HOAr. A number of HOAr precursors were investigated including H-oMP, H-oPP, H-OC6H4(CMe3)-2 (H-oBP), H-DMP, H-DIP, and OC6H3(CMe3)2-2,6 (H-DBP). From these mixtures, Co[Li(OAr)(THF)x]2 was isolated for OAr ) oMP (1), oPP (2), and DMP (3) where x ) 2 and DIP (4) where x ) 1. Figures 1 and 2 display the thermal ellipsoid plots of 1 and 4, respectively, showing the two types of coordination around the Li atoms. For the oBP and DBP derivatives, only the “Li(OAr)(THF)” derivative was isolated as characterized by single-crystal X-ray diffraction, which implies that the “Co(OAr)2(THF)x” derivatives were also synthesized but not isolated.. Attempts to generate the 1:1 derivative have not been successful.




Increasing the Li:Co stoichiometry to 2:1 led to an increased yield for some of the samples that we were interested in. When the steric bulk was further increased through the introduction of tert-butyl groups in the ortho position, only the [Li(OAr)(THF)x]2 (x ) 2, oBP; x ) 1, DBP)40 species were isolated. The Co(OAr)2(THF)x compounds are purple in color and mixed phases could be separated visually since the “Li(OAr)(THF)x”40 species were colorless; however, the “Co(OR)2(THF)x” crystals were not of X-ray quality. The pyridine adducts were then generated to investigate structural variations as was noted for the [Li(OAr)(solv)x]n families.40 These compounds were synthesized by following the reaction in eq 2 or by redissolution of the appropriate THF adduct in py. The Co[(µ-OAr)2Li(py)2]2 structure was also isolated for the oMP (5, see Figure 3) and oPP (6) ligands. Interestingly, for the sterically demanding ligands (DMP and DIP) coupled with a strong Lewis base (py), only cobalt aryloxides were isolated as Co(DMP)2(py)2 (7) with Co(DIP)2(py)3 (8), shown in Figures 4 and 5, respectively. Solid State Structures. Tables 1-3 list the data collection parameters for 1-4, 5 and 6, and 7 and 8, respectively. Additional information concerning the

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crystal structures can be found in the Supporting Information. The structures of 1-6 all possess the same central core and will be discussed collectively. Representative structures of each type of arrangement as noted for 1, 4, and 5 observed are pictured in Figures 1-3, respectively. Each molecule consists of a tetrahedral (Td) Co metal center that bridges to two Li metal centers by two µ-OAr ligands. The coordination of the Li atom is dependent upon the number of bound solvent molecules, which for 1, 2, 3, 5, and 6, the Li atoms are Td but for 4 it is only 3-coordinated, forming a trigonal planar geometry. The reduction in solvation for 4 is attributed to the sterically demanding CHMe2 groups in both ortho positions of the phenoxide ring. The central core observed for 1-6 is identical to what was reported for the bidentate siloxanes43,44 and for the alkyl derivative Co{Li(µ-C(SiMe3)2(TMEDA)}2.;47 however, in these compounds the Li metal centers were Td bound. Both of the sterically hindered Co(OAr)2 isolated in this study proved to be monomeric, using py solvent molecules to fill their coordination spheres. For 7, the (47) Hay-Motherwell, R. S.; Wilkinson, G.; Hussain, B.; Hursthouse, M. B. Polyhedron 1990, 9, 931.

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Co metal center adopts a Td arrangement by coordinating only two py solvent molecules (Figure 4). In contrast, the Co metal centers in 8 coordinate three py molecules, which yields a trigonal bipyramidal geometry (Figure 5). The previously characterized Co(OR)2 were all found to be dinuclear in the absence of a coordinating solvent but were converted to monomeric Td bound species if THF were added,41 except for the bidentate catecholato derivative, which formed a TBP-coordinated Co tetramer in THF.42 This implies that the monodentate OAr derivatives should also be dinuclear if generated in a nonpolar environment. The metrical data of 7 and 8 are within agreement to the literature [Co(OR)]2‚solv and the Co(OR)2(THF)2 species previously reported.41,42 The Li-solv distances (solv ) THF (1-4), average 1.96 Å; py (5 and 6), average 2.08 Å) and the Li-(µOAr) distances (1-4 average 1.90 Å; 5 and 6, average 1.93) were found to be consistent with the metrical values noted for the [Li(OAr)(solv)x]n family of compounds.40 The Co-(µ-OAr) were found to be on average 1.96 Å for 1-4 and 1.95 Å for 5 and 6 and were slightly shorter for 7 and 8 (average 1.91 Å). The Li- - -Co distances of 1-6 were found to be very similar in comparison, independent of the bound solvent ranging from 2.740(9) to 2.866(8) Å, which are in agreement with literature values.43,44,47 For 1-3 and 5 and 6, each of the Co and Li atoms adopt distorted Td arrangements, similar to what was noted for the other species.43,44,47 Characterization. A number of analytical characterization techniques were undertaken to compare the bulk powder of 1-8 with the single-crystal structures, including FT-IR spectroscopy, elemental analyses, multinuclear NMR spectroscopy, electron pair resonance spectroscopy (EPR), and X-ray diffraction studies. The FT-IR spectra of 1-8 resemble the parent alcohol with the notable exception of the broad OH stretches around 3500 cm-1 and the N(SiMe3)2 stretches (1239, 1046, 986 cm-1). The absence of these two peaks is indicative that complete exchange had occurred. Due to the overlap of the OAr and solvent stretches and the complexity of the remainder of the spectra, it is not possible to definitively assign the Co-O-Li stretches or infer structural information for the bulk powder. While several of the elemental analyses for these compounds were found to be acceptable, reproducibility of this analysis is difficult due to the lability of the bound solvent and the volatility of the Li ions.40 Any variations in the analyses reported have been attributed to these phenomena. This limits the utility of elemental analysis to characterize the bulk powder, which was also a problem observed for the [Li(OAr)(solv)]n family of compounds.40 All of the Li-Co double-alkoxide compounds are soluble in toluene, THF, or py. Attempts to collect multinuclear NMR data on redissolved crystalline material of 1-4 in THF-d8 were undertaken. The samples were made as concentrated as possible. For each sample, the 1H and 13C NMR spectra were very broad with a low signal-to-noise ratio. The paramagnetic Co metal center broadened the spectra significantly and made any interpretation of the resulting data difficult. 7Li NMR data were also dramatically affected by the presence of the Co2+ metal center; however, the presence of Li was unequivocally established for 1-4. Solid state13C MAS

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Figure 6. XRD spectrum of bulk powders of 1: (a) observed versus (b) calculated.

spectra also proved to be very broad due to the presence of the paramagnetic Co metal center. Therefore, EPR analyses were attempted but the signal of the Co species was too strong and meaningful data could not be collected on these samples. In a final attempt to characterize the bulk powder, the XRD patterns of 1-6 were collected. Due to the air sensitivity of the compounds, each sample was loaded into a plastic envelope and heat-sealed under an argon atmosphere. The XRD pattern was collected and compared to the calculated pattern generated from the single-crystal X-ray structure. In general, the form of the diffraction curve for the observed pattern was similar to that of the calculated pattern. For an example, see Figure 6 for compound 1. As can be observed, there were minor differences (i.e., intensity variations, changes in peak full-width, and peak position) between the calculated and observed peaks, Figure 6. The peak shifts are an artifact that the powder data were collected at room temperature, while the calculated pattern was based on structural data from -105 °C. This difference may change the unit cell dimensions and shift peak positions along the 2θ axis. Additionally, peak shifts may result from sample displacement of the powder sample in the X-ray beam. Because the samples were loaded into plastic envelopes, the specimen had an inherent curvature when mounted in the diffractometer, which translates into an inherent sample displacement error for some fraction of powder analyzed. This may also explain the small shift observed in some of the 2θ peaks. Most peaks in the pattern were broad, but a few were notably sharp, in particular, the peaks near 8°, 10°, and 16° 2θ, which indexed well to the (010), (002), and (020) hkl’s, respectfully. It is likely that these strong “spikes” in intensity result from large, preferentially oriented, grains present in the specimen. The sample powder was not ground prior to sealing in the plastic envelope because of concern that mechanical grinding would alter the crystallite structure of 1, possibly through loss of bound solvent. Hence, the powder had many large grains; these strongly preferred grains were then responsible for the sharp peaks present in the diffraction patterns. Although the XRD powder pattern shows consistency to that of the calculated powder pattern, the measurement does not reveal amorphous content that may be present within the sample. Hence, the XRD powder data are only useful for commenting on the purity of crystalline phase(s) present. For 7 and 8, the XRD powder diffraction pattern was obtained in a similar manner as discussed for 1-6. The spectra of these homonuclear compounds have only a


few lines, which makes interpretation of these patterns much more difficult since there is potential for the presence of “Li(OR)(solv)x”, ”Co(OAr)2(solv)x”, and/or “Li2Co(OAr)4(solv)x” products. The similarity of these patterns would make differentiation of these species impossible and further interpretation of these patterns was not undertaken. Application. With the single-source precursors in hand, the fundamental utility of these compounds was investigated in the gaseous (MOCVD), solid (nanoparticle precipitation), and liquid (sol-gel) states. The experimental setup used for each are described in the Experimental Section. The results are presented for each method below. MOCVD (Gas). The melting point of 5 was determined to be 116-118 °C. The purple precursor powder was loaded into the MOCVD precursor pot, attached to the reaction chamber, and evacuated down to 50 mTorr. When the MOCVD apparatus stabilized at the set temperatures (105 °C precursor pot; 380 °C substrate), the precursor pot was opened to the vacuum. No sublimation or deposition of 5 was observed. After 5 min, the precursor pot temperature was raised to 115 °C and still no sublimation or deposition was observed. This procedure was repeated until the precursor pot temperature reached 185 °C. No melting, sublimation, or deposition of the precursor was observed. The powder remained purple throughout the procedure. Under the simple condition investigated for this study, the volatility of these compounds was determined to be too low to allow for use as an MOCVD reagent. This is most likely due to preferential loss of the solvent molecules and further oligomerization of the precursor. Nanoparticles (Solid). Several variations of solution and oxidant were used to generate nanoparticles. The solvent ultimately chosen due to its high boiling point and binding ability to metal centers was 1-methyl imidazole (MeIm). After investigation of several oxidants, water proved to be the most useful for generating crystalline material. The precursors (for these studies 5 and 6 were used due to their availability and higher boiling coordinated solvent) were dissolved in a pyridine mixture and then rapidly injected into a boiling solution of (95:5) MeIm/H2O mixture. The purple color of the starting solutions for 5 or 6 in pyridine turned the reaction mixture of the MeIm/H2O light purple immediately upon introduction. After ∼5 min, the reaction mixtures both turned an opalescent purple color. Upon cooling, the solutions were centrifuged and the insoluble material was washed with hexanes. The mother liquor from the reaction with 5 was dark green and that of 6 was dark purple. The isolated powders from both reactions were dark purple and were re-suspended in hexane via sonification. The XRD spectra of the nanoparticles generated from 5 and 6 are shown in Figure 7 along with the calculated pattern for the Co(OH)2 phase (PDF # 74-1057). Both powder samples indexed well to this phase and have been identified as the Co(OH)2 compound. It is interesting to note that the peaks vary in full-width at halfmaximum (fwhm) depending on the hkl observed. In both powders samples the (001) peak is substantially broader than the (100), implying a large aspect ratio for the crystallites. The morphology of the particles (as

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Figure 7. XRD spectrum of Co(OH)2 of nanoparticles from MeIm/H2O using (a) 5 and (b) 6 and (c) a calculated pattern for the Co(OH)2 phase (PDF # 74-1057).

determined by TEM) appears to be platelike with the c-axis likely perpendicular to the plate direction because the plates have a hexagonal shape. The powder XRD data are consistent with this observation, implying that the particles are thin wafers. A quick estimation of the crystallite size based on the Scherrer formula shows that the c-axis direction of the grain is in the ∼10-20nm range while the a-axis has a much larger dimension of ∼60-70 nm for the Co(OH)2 phase in these samples. The crystallinity of the particles produced from the MeIm/H2O mixture was confirmed by the rotational photomicrographs (Figure 8). To obtain TEM images, a drop of the Li-Co dispersion was deposited on a thin carbon-coated copper TEM grid and subsequently heated to 130 °C for 12 h to facilitate removal of residual solvent. The nanocrystals were observed using a Philips CM 30 TEM operating at 300 keV. The TEM images are shown in Figure 8. Particles generated from reaction with complexes 5 and 6 (Figure 8a,b) reveal the presence of nanocrystalline material. The observed morphology is a mixture of disk and rod shaped particles with sizes in rough agreement with those calcualted by the XRD pattern (vide infra). The nanocrystals have a large tendency to aggregate, which is not unusual for nanocrystalline metal oxides. Dark-field TEM studies suggest that each particle is single crystalline in nature. Selected area electron diffraction (SAED) images (Figure 8a,b left inset) also demonstrates the crystalline nature of the nanoparticles. Analysis of the d spacing in these images confirms that the observed rings are consistent with the XRD powder patterns for the Co(OH)2. The chemical composition of the nanocrystals was obtained via X-ray energy dispersive spectroscopy (XEDS) (Figure 8 right inset). An electron beam was focused on several sections of the sample and only the characteristic peaks of Co, O, and Cu (from the copper TEM sample grid) and C (from the grid coating) were detected. Due to the limited scattering of Li, it is very difficult to identify using TEM or XRD. Therefore, an 7Li NMR investigation of the mother liquor was undertaken and the presence of Li was unequivocally confirmed. The shift does not correspond to any of those associated with [Li(OAr)(solv)x]n40 but the solvent is MeIm and several solution variations are possible for an unbalanced eq 1.

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Figure 8. TEM micrographs of nanoparticles from (a) 5 and (b) 6 using MeIm/H2O. Left inset is the SAED powder diffraction pattern; right inset is the XEDS analysis of individual particles.

Figure 9. XRD spectrum of thin film (four layers) of (a) 5 and (b) 6.

While the identity of the Li species is not fully established, these data indicate that a significant amount of Li ion remains in solution for both 5 and 6. Films (Solution). Two OAr products were selected to investigate the utility of these compounds as singlesource solution precursors to LiCoO2 thin films. Due to the lack of the disubstituted py adducts, only the oMP (1, 5) and OPP (2, 6) adducts were used to generate films. Both the THF and py adducts were dissolved in the parent solvent and diluted with toluene to generate the precursor solution. When standard spin-coating methods were followed, films of each were generated and processed to fully crystallize the final ceramic material. The films were slightly cloudy with a few defects that could be visually observed. Each film was analyzed by GIXRD (Figure 9 shows the spectra for 5 and 6) analyses and found to be the spinel phase of LiCoO2 (excess Li is easily pyrolyzed if not stabilized in the LiCoO2 network). The basic promise of a singlesource precursor has been realized and can be fine-tuned by varying the alkoxide ligands.

Figure 10. Cyclic voltammagram of 6 for first and second cycles in the voltage range of 3.2-4.1 V at a scan rate of 2 mV s-1.

Films generated from the pyridine adducts (5 and 6) produced the most useful films for electrochemical testing. These films may be of higher quality in comparison to the THF adducts due to improved wetting that has been previously noted for other systems.46,48-51 Upon immersion of a sample produced from 6 into the electrolyte, an open circuit voltage of approximately 3.2 V was observed, which is typical for LiCoO2 in the fully (48) (a) Boyle, T. J. U.S. Patent 5858451, 1999. (b) Boyle, T. J. U.S. Patent 5858323, 1999. (c) Boyle, T. J. U.S. Patent 5683614, 1997. (49) Boyle, T. J.; Al-Shareef, H. N.; Buchheit, C. D.; Cygan, R. T.; Dimos, D.; Rodriguez, M. A.; Scott, B.; Ziller, J. W. Integr. Ferroelectr. 1997, 18, 213. (50) Boyle, T. J.; Buchheit, C. D.; Rodriguez, M. A.; Al-Shareef, H. N.; Hernandez, B. A.; Scott, B.; Ziller, J. W. J. Mater. Res. 1996, 11, 2274. (51) Boyle, T. J.; Clem, P. G.; Rodriguez, M. A.; Tuttle, B. A.; Heagy, M. D. J. Sol-Gel Sci. Technol. 1999, 16, 47.


Figure 11. Cyclic voltammagrams of 6 after five cycles with the voltage range expanded to 4.4 V recorded at 1 mV s-1.

discharged state. With use of this as an initial voltage, a series of cyclic voltammagrams (CV) were recorded at scan rates of 1 and 2 mV s-1. These slow scan rates were used in an effort to ensure equilibrium. Figure 10 shows the first two CVs obtained on a single sample in the voltage range of 3.2-4.1 V at a scan rate of 2 mV s-1. On the first cycle two oxidation waves are present, having peak potentials of ∼4.00 and 4.05 V. There is a single reduction wave, although this could in fact be multiple waves, having a peak potential of about 3.88 V. Upon continued cycling over this same potential range, the sample becomes completely inactive. This is seen by examination of the second CV recorded with this sample (Figure 10). Continued cycling over this range shows behavior identical to the second CV recorded. To further evaluate the electrochemical behavior, the potential window was opened to 4.4 V and additional CVs recorded. The fifth CV over this expanded voltage range, which was recorded at 1 mV s-1, is shown in

Chem. Mater., Vol. 15, No. 20, 2003 3911

Figure 11. There is a single reversible redox process observable, having an oxidation peak potential of ∼4.23 V and a reduction at ∼3.85 V (note: the term “reversible” for this work is not the Nernstian sense but instead is in terms of a battery). The oxide can be repeatedly cycled (charged and discharged) over this voltage range, and although the fifth cycle is shown in Figure 11, this particular electrode was subjected to nine complete cycles. In general, it appears that the film of 6 is reversible in the battery sense. To explore the structural changes that occurred during cycling, GIXRD analyses of the samples before and after cycling were undertaken. A sample that had undergone nine complete cycles between 3.2 and 4.4 V at 1 mV s-1 was removed from the electrochemical cell after the potential on the electrode was poised at 3.2 V for sufficient time to ensure that its state of charge was similar to the material prior to the outset of cycling. After removal from a glovebox, the sample was immediately rinsed with isopropyl alcohol to remove excess electrolyte solution. An XRD pattern of this sample was collected and compared to the XRD of the sample prior to cycling; see Figure 12. A slight increase in the lattice spacing after having been cycled is observable but there does not appear to be any significant structural transformation that can provide a ready explanation of the electrochemical behavior. Further work is underway to provide a more complete picture of the structural changes that occur during electrochemical cycling of these films. Summary and Conclusion A novel family of Li-Co double aryloxides were identified as Co[(µ-OAr)2Li(solv)x]2. (solv ) THF or py and OAr ) a series of ortho substituted phenols). It is of note that the complex structures of the [Li(OAr)(solv)x]n precursors are not preferentially retained in the presence of the Co2+ cation and, independent of the

Figure 12. XRD spectrum of film from 6: (a) prior to and (b) post-electrochemical cycling.

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stoichiometry used, the 2:1 Li:Co structures were isolated. This may be a reflection of the large cation size of Co2+ and its low charge necessitating oligomerization even in the presence of strong Lewis basic solvents. Interestingly, the py did not disrupt the general structure, as was noted for the [Li(OR)(solv)x]n families until sufficiently large ligands were used, which resulted in the first Co(OAr)2(py)x compounds. While several standard analytical means were undertaken to characterize 1-8, the presence of the paramagnetic Co2+ metal centers, the low molecular weight Li cations, and the volatile solvent molecules metal centers thwarted these attempts. Therefore, it was necessary to use the idealized XRD pattern generated from the crystal structure and compare it to the experimental patterns obtained on sealed samples of 1-8. On the basis of these data, we were able to verify the similarity of the bulk powder and the single-crystal X-ray structure between 1 and 6. We have investigated the general utility of these precursors as single-source materials in gas, solid, and liquid materials production methodologies. Initial attempts to utilize these compounds as MOCVD reagents were not successful. However, 5 and 6 were found to generate Co(OH)2 nanoparticles through a precipitation

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route that utilizes H2O and MeIm as a solvent. Solution routes revealed these species may be easily used to produce high-quality LiCoO2 thin films that demonstrate electronic cycling. Further work investigating the utility of these compounds is underway. Acknowledgment. For support of this research, the authors would like to thank the Office of Basic Energy Sciences of the Department of Energy and the U.S. Department of Energy under Contract DE-AC0494AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Department of Energy. The authors would also like to thank the following persons for their technical assistance and helpful discussions concerning the various areas listed: Dr. T. M. Alam (SNL-NMR), Dr. T. L. Dunbar (SNL-EPR), J. Segal (UNM-Films and NMR), L. Matzen (Rice-nanoparticles), and Prof. A. Rheingold (UDel-X-ray). Supporting Information Available: X-ray crystallographic files for structures 1-8 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. CM020902U

A Novel Family of Structurally Characterized Lithium Cobalt Double

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