LETTER pubs.acs.org/NanoLett
Ultrathin Coatings on Nano-LiCoO2 for Li-Ion Vehicular Applications )
Isaac D. Scott,†,^ Yoon Seok Jung,‡,^ Andrew S. Cavanagh, Yanfa Yan,‡ Anne C. Dillon,‡ Steven M. George,§ and Se-Hee Lee*,† †
Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, Colorado 80309-0427, United States National Renewable Energy Laboratory Golden, Colorado 80401, United States § Department of Chemistry and Biochemistry, Department of Chemical and Biological Engineering, and Department of Physics, University of Colorado at Boulder, Boulder, Colorado 80309-0215, United States )
‡
bS Supporting Information ABSTRACT: To deploy Li-ion batteries in next-generation vehicles, it is essential to develop electrodes with durability, high energy density, and high power. Here we report a breakthrough in controlled full-electrode nanoscale coatings that enables nanosized materials to cycle with durable high energy and remarkable rate performance. The nanoparticle electrodes are coated with Al2O3 using atomic layer deposition (ALD). The coated nano-LiCoO2 electrodes with 2 ALD cycles deliver a discharge capacity of 133 mAh/g with currents of 1400 mA/g (7.8C), corresponding to a 250% improvement in reversible capacity compared to bare nanoparticles (br-nLCO), when cycled at this high rate. The simple ALD process is broadly applicable and provides new opportunities for the battery industry to design other novel nanostructured electrodes that are highly durable even while cycling at high rate. KEYWORDS: Atomic layer deposition, LiCoO2, rate performance, Li-ion battery, capacity fade, nanotechnology
L
ithium-ion batteries (LIBs) are generally employed in portable devices and more recently in electric vehicular applications. Consumer demand is driving research efforts for higher storage capacity, faster recharging times, greater cycling stability, and higher power output.1 Unfortunately, the power capability of LIBs is generally hindered by diffusion in bulk and/or micrometer-sized materials.2 Although there is an ongoing search for compositionally new electrode materials, the ability to fabricate nanoversions of known materials can also greatly enhance power performance by simply increasing the surface-to-volume ratio. This allows for an increase in electrode-electrolyte contact area3 and shortens the Liþ-ion insertion distances thus enhancing the power density compared to micrometer-sized particles.4-7 Unfortunately, for high voltage cathode materials, increasing the electrode surface area of course allows for significantly more unfavorable side reactions (e.g., dissolution of species within the active material).8,9 Coating nanoparticles with a stabilizing surface layer is one way to alleviate such problems, as long as the coating is “ultrathin” and therefore does not reduce the overall capacity and also allows for a high rate of Li-ion diffusion. Methods applied to micrometer-sized particles including metal oxides,10-12 phosphates,12-14 and fluorides15 generally employ “sol-gel” wet-chemical methods. Despite the numerous reports dedicated to sol-gel coatings, it is apparent that the lack of control over surface coverage, thickness, and uniformity prohibits sol-gel methods for nanoscale materials. All of these issues may simply be addressed by employing the scalable technique of atomic layer deposition (ALD). Thus ultrathin ALD coatings r 2010 American Chemical Society
seem to be ideal for stabilization of nanosized electrode materials for LIBs that are safe and durable while enabling high-energy density and high power. ALD is a method to grow conformal thin films with atomic thickness (angstrom level control) using sequential, self-limiting surface reactions.16-18 Recently, we reported that ALD on micrometer-sized LiCoO219 and natural graphite20 significantly enhanced both the durability and safety of LIBs. For high-volume expansion nanosized MoO3,21 the rate capability was also shown to be improved. Another exceptional benefit of ALD is that it allows for direct deposition on as-formed composite electrodes.20 The electrically insulating ALD film therefore does not disrupt the original electrical pathways constructed between the components in the electrode. In this study, nano-LiCoO2 is employed to demonstrate the effectiveness of ALD coatings because it is a standard battery cathode material and is widely studied. Importantly, this represents the first study where ALD has been shown to profoundly increase the rate performance (250% improvement) for nanosized particles, as opposed to the generally employed micrometer-sized particles.19 Also, we clearly demonstrate that conformal ultrathin metal-oxide coatings applied with direct ALD on nanosized LiCoO2 electrodes increases the voltage window compared to uncoated micrometer and nanosized particles. Received: August 25, 2010 Revised: October 29, 2010 Published: December 17, 2010 414
dx.doi.org/10.1021/nl1030198 | Nano Lett. 2011, 11, 414–418
Nano Letters
LETTER
Nanosized LiCoO2 powders (nLCO) were prepared by a molten salt method, similar to the procedure described by Liang et al.22 Reagent-grade CoO (Sigma, 50 nm), LiOH 3 H2O (Aldrich), and KNO3 (Aldrich) were used as the starting material and mixed in an agate mortar with a molar ratio of 1:1:4. The mixture was heated to 700 C for 5 h in a buffer furnace in an Al2O3 combustion boat and then cooled to ambient temperature. The obtained product was dispersed in deionized water by magnetic stirring for 1 h and then filtered and washed with additional deionized water and butanal to remove the residual fluxes. The final powder was dried at 130 C in a vacuum oven for 24 h. The peaks in the X-ray diffraction (XRD) pattern of the bare nanosized LiCoO2, which from here on are referred to as br-nLCO, reveal the formation of a hexagonal layered structure with a R3m space group. This data corresponds to JCPDS data No. 160427. No additional impurity peaks are detected as displayed in Figure 1a. Field-emission scanning electron microscopy (FESEM) images (Figure 1b) show that the average particle size of br-nLCO is ∼400 nm and has a particle-size distribution of approximately 200-700 nm. As a comparison, Figure 1c shows bare bulk LiCoO2 with an average particle size of ∼5 μm (Aldrich) from here on referred to as br-bLCO. Both br-nLCO and br-bLCO share the same polyhedral morphology.
Figure 1. (a) XRD pattern of the as-synthesized nano-LiCoO2 powder (br-nLCO). The peak positions of JCPDS no. 16-0427 are indicated by vertical marks. FE-SEM images of (b) br-nLCO; (c) bulk LiCoO2.
Figure 2. (a) TEM image of the Al2O3 ALD-coated nanosized LiCoO2 particles prepared by 6 ALD cycles on the bare powders. The EDS signals of areas labeled as p1 and p2 are shown in the right. HR-TEM images of (b) the bare LiCoO2 particles and (c) the Al2O3-coated nanosized LiCoO2 particles by 6 ALD cycles on the bare powders. 415
dx.doi.org/10.1021/nl1030198 |Nano Lett. 2011, 11, 414–418
Nano Letters
LETTER
Figure 3. Cycling performance of the electrodes: uncoated bulk LiCoO2 (br-LCO), uncoated nano-LiCoO2 (br-nLCO), and Al2O3coated nano-LiCoO2 with 2 ALD cycles performed on the electrode (2ALD-nLCO). The electrodes were charged-discharged between 3.3 and 4.5 V (vs Li/Liþ) at a rate of 500 mA/g (2.8C).
Figure 4. Variations in discharge capacities versus charge-discharge cycle number for different LiCoO2 electrodes cycled at different rates between 3.3 and 4.5 V (vs Li/Liþ) at room temperature. Current densities (mA/g) are indicated at the top. In the “post region” starting at the 22nd charge-discharge cycle, the current density was gradually increased by 16, 32, 134, 266, 500, 800, and 1400 mA/g in one cycle.
Al2O3 ALD films were grown directly on the LiCoO2 composite electrodes. Each ALD cycle deposits a uniform Al2O3 layer of approximately 1.1-2.2 Å in thickness.23,24 A transmission electron microscopy (TEM) images of bare LiCoO2 particles and Al2O3 ALD-coated LiCoO2 particles coated by 6 ALD cycles are shown in Figure 2. In the typical coated powders by wet-chemical methods, nonuniform agglomeration of a few to tens of nanometers thick porous coating species is observed.25,26 In contrast, the surface of Al2O3 ALD-coated LiCoO2 in Figure 2a shows smooth curvature. Also small circular cross sections, labeled p1 and p2 in Figure 2a, were examined for the presence of alumina with energy dispersive X-ray spectroscopy (EDS). The fact that a clear Al peak is observed only in p1 but not in p2 demonstrates controlled deposition of a very thin layer of Al2O3 upon the surface of the LiCoO2 nanoparticle. Comparison of the HRTEM images of the bare LiCoO2 (Figure 2b) and the Al2O3 ALD coated LiCoO2 (Figure 2c) also strongly supports conformal ultrathin layer of the coating. The lattice fringes in the surface region for bare LiCoO2 (Figure 2b) are identical to those in the bulk region. In contrast, conformal ∼1-2 nm thick film which has clearly different lattice fringes from those in the bulk region is clearly observed for Al2O3 ALD coated LiCoO2 (Figure 2c). The LiCoO2 composite electrode was prepared by spreading LiCoO2 powder, acetylene black, and polyvinylidene fluoride (PVDF, binder) (70:15:15 weight ratio) on a piece of Al foil (Supporting Information). Figure 3 compares the chargedischarge cycling performance of br-bLCO, br-nLCO, and Al2O3-coated nLCO by 2 ALD cycles when cycled between 3.3 and 4.5 V (vs Li/Liþ) at a current rate of 16 mA/g (0.1C) for the first three charge-discharge cycles and 500 mA/g (2.8C) for the subsequent cycles. Increasing the voltage above the standard 4.2 V allows LiCoO2 to achieve a greater initial specific capacity, but also generally results in undesirable surface reactions including Co dissolution. For the uncoated LiCoO2, decreasing the particle size from ∼5 μm (br-bLCO) to ∼400 nm (br-nLCO) alone leads to significant improvement in capacity retention. The bulk powders (br-bLCO) lose almost all of their capacity after 50 charge-discharge cycles. In contrast, br-nLCO retains 76% capacity after 50 charge-discharge cycles. (Capacity comparisons are
normalized to the fourth charge-discharge cycle.) This size effect on cycling performance is of course magnified by employing higher rates (500 mA/g = 2.8C, 1C = 180 mA/g). Sony commercial LIBs, for example, cycled at 3C show a quick decrease in capacity in the first few cycles.27 For bulk powders, even a slight increase in impedance from surface passivation can seriously degrade the performance of a material at high rates because of the long Liþ-ion diffusion lengths. For the nanopowders, however, increased surface area resulting in a short Liþ-ion diffusion length can buffer this abrupt capacity fade22 as long as increased surface reactions do not prove to be detrimental. Here, the stable capacity retention of br-nLCO is dramatically enhanced with ultrathin Al2O3 coatings resulting from 2 ALD cycles (2 ALD-nLCO) on the electrode as seen in Figure 3. The Al2O3-coated nLCO retains 100% capacity, compared to the fourth discharge capacity, after 200 charge-discharge cycles. In contrast, br-nLCO loses almost all capacity after the same number of cycles. The improvement in cycling performance following ALD on LiCoO2 is also consistent with previous results found coating micrometer-sized particles, but cycling was performed at a significantly lower rate, 140 mA/g.19 Finally, the rate-performance of the electrodes was tested using different current densities. Figure 4 shows variations in discharge capacities versus charge-discharge cycle number for different LiCoO2 electrodes cycled at different rates between 3.3 and 4.5 V (vs Li/Liþ) at room temperature. The discharge voltage profiles of the br-bLCO, br-nLCO, and Al2O3-coated nLCO by 2 and 6 ALD (6 ALD-nLCO) cycles at three different current rates are also depicted in Figure 5. The decrease of particle size from ∼5 μm to ∼400 nm clearly reduces the overpotential and thereby increases the capacity. The improved kinetics by reducing the size of particles is explained by a decrease of the characteristic time constant (t) as given by t = L2/D where L and D is the diffusion length and the diffusion coefficient, respectively. Increased interfacial area between LiCoO2 and electrolyte can also contribute to accelerate charge transfer reaction. When compared to the bare electrode (Figure 5b), the Al2O3-coated electrode with 2 ALD cycles (Figure 5c) 416
dx.doi.org/10.1021/nl1030198 |Nano Lett. 2011, 11, 414–418
Nano Letters
LETTER
Figure 6. Rate capability of the LiCoO2 electrodes as a function of particle size and Al2O3 ALD-coating thickness: uncoated bulk LiCoO2 (square, br-bLCO), uncoated nano-LiCoO2 (circle, br-nLCO), and Al2O3-coated nano-LiCoO2 with 2 ALD (triangle), and 6 ALD (diamond) cycles on the electrodes.
In summary, we have demonstrated the feasibility to create Li-ion battery electrodes with durability, high energy density, and high power by employing nanostructured electrode engineering. An ultrathin Al2O3 ALD coating grown directly on a nanosized LiCoO2 composite electrode can act as a protective layer and enable stable high rate capability. We believe our results will enable nanosized electrode materials coated by ALD to be employed in commercial LIBs.
Figure 5. Discharge voltage profiles of the LiCoO2 cycled at different current densities: (a) uncoated bulk LiCoO2 (br-bLCO), (b) uncoated nano-LiCoO2 (br-nLCO), and Al2O3-coated nano-LiCoO2 by (c) 2 ALD and (d) 6 ALD cycles on the electrodes.
further reduces the overpotential and thus increases the capacity. However, 6 ALD cycles results in an even larger overpotential than that of the bare electrode. Considering that the difference in thickness of Al2O3 films between 2 ALD and 6 ALD cycles is between 2.2 and 11 Å, the decrease in rate-performance of 6 ALD-nLCO compared to 2 ALD-nLCO indicates that thick ALD Al2O3 coatings result in poor Liþ conductivity and further demonstrates the need for control of ultrathin coatings at the nanoscale. Importantly, the fact that 2 ALD-nLCO outperforms br-nLCO, suggests that the ultrathin Al2O3 ALD can transport Liþ-ions faster than the SEI (solid electrolyte interphase) formed on the br-nLCO surface. For comparison, the thickness of the SEI layer on LiCoO2 nanoparticles was reported to be ∼2-5 nm.28 From these results, it can be concluded that Al2O3 ALD can suppress the undesirable side reactions and thereby act as a stable “artificial” SEI layer that can quickly transport Liþ-ions. Figure 6 summarizes the discharge capacities at different current rates depending on the particle size and Al2O3 ALD coating. Consistent again with the results in Figures 4 and 5, the rate performance is improved by reducing the particle size and more importantly by introducing an ultrathin ALD coating (2 ALD cycles). The improved rate behavior for the coated electrodes is believed to be caused by the suppression of cobalt dissolution from the LiCoO2 with the formation of an aluminumoxide barrier residing between the LiCoO2 cathode and the liquid electrolyte and by decreasing the particles size from the bulk material. The Al2O3-coated nano-LiCoO2 electrode with 2 ALD cycles exhibits a discharge capacity of 133 mAh/g at 1400 mA/g (7.8C), which corresponds a 250% improvement in reversible capacity compared to the bare nanoparticles (br-nLCO).
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental information. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions ^
These authors contributed equally to this work.
’ ACKNOWLEDGMENT The studies conducted by the authors from the University of Colorado-Boulder are supported by the DARPA Center on Nanoscale Science and Technology for Integrated Micro/ Nano-Electrochemical Transducers (iMINT) funded by DARPAN/MEMS S&T Fundamentals Program (N66001-10-14007)(Dr. Tayo Akinwande, Program Manager). This work was performed in part at the University of Colorado's Nanomaterials Characterization Facility. NREL is grateful for support from the U.S. Department of Energy under subcontract number DE-AC36-08GO28308 through DOE Office of Energy Efficiency and Renewable Energy Office of the Vehicle Technologies Program. 417
dx.doi.org/10.1021/nl1030198 |Nano Lett. 2011, 11, 414–418
Nano Letters
LETTER
’ REFERENCES (1) Johnson, B. A.; White, R. E. J. Power Sources 1998, 70, 48. (2) Lee, Y.; Kim, M. G.; Cho, J. Nano Lett. 2008, 8, 957. (3) Chan, C. K.; Peng, H.; Twesten, R. D.; Jarausch, K.; Zhang, X. F.; Cui, Y. Nano Lett. 2007, 7, 490. (4) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; Van Schalkwijk, T. Nat. Mater. 2005, 4, 366–377. (5) Taberna, P. L.; Mitra, S.; Poizot, P.; Simon, P.; Tarascon, J. -M. Nat. Mater. 2006, 5, 567. (6) Guo, Y. G.; Hu, Y. S.; Sigle, W.; Maier J. Adv. Mater. 2006, 19, 2087. (7) Lee, S. H.; Kim, Y. H.; Deshpande, R.; Parilla, P. A.; Whitney, E.; Gillaspie, D. T.; Kones, K. M.; Mahan, A. H.; Zhang, S.; Dillon, A. C. Adv. Mater. 2008, 20, 3627. (8) Fey, G. T.-K.; Lu, C.-Z.; Huang, J.-D.; Kumar, T. P.; Chang, Y.-C. J. Power Sources 2005, 146, 65. (9) Xiao, L.; Yang, Y.; Zhao, Y.; Ai, X.; Yang, H.; Cao, Y. J. Solid State Electrochem. 2008, 12, 149. (10) Cho, J.; Kim, Y. J.; Kim, T. J.; Park, B. W. Angew. Chem., Int. Ed. 2002, 40, 3367. (11) Kim, S. S.; Kadoma, Y.; Ikuta, H.; Uchimoto, Y.; Wakihara, M. Electrochem. Solid-State Lett. 2001, 4, A109. (12) Li, C.; Zhang, H.; Fu, L.; Liu, H.; Wu, Y.; Ram, E.; Holze, R.; Wu, H. Electrochem. Acta 2006, 51, 3872. (13) Cho, J.; Kim, Y. W.; Kim, B.; Lee, J. G.; Park, B. W. Angew. Chem., Int. Ed. 2003, 42, 1618. (14) Lee, S. E.; Kim, E.; Cho, J. Electrochem. Solid-State Lett. 2007, 10, A1. (15) Sun, Y. K.; Lee, Y. S.; Yoshio, M.; Amine, K. Electrochem. SolidState Lett. 2002, 5, A99. (16) Ritala, M.; Leskela, M. In Atomic Layer Deposition; Academic Press: San Diego, 2001. (17) George, S. M.; Ott, A. W.; Klaus, J. W. J. Phys. Chem. 1996, 100, 13121. (18) George, S. M. Chem. Rev. 2010, 110, 111. (19) Jung, Y. S.; Cavanagh, A. S.; Dillon, A. C.; Groner, M. D.; George, S. M.; Lee, S. H. J. Electrochem. Soc. 2010, 157, A75. (20) Jung, Y. S.; Cavanagh, A. S.; Riley, L. A.; Kang, S. -H.; Dillon, A. C.; Groner, M. D.; George, S. M.; Lee, S. H. Adv. Mater. 2010, 22, 2172. (21) Riley, L. A.; Cavanagh, A. S.; George, S. M.; Jung, Y. S.; Yan, Y.; Lee, S. H.; Dillon, A. C. ChemPhysChem. 2010, 11, 2124. (22) Liang, H.; Qiu, X.; Chen, H.; He, Z.; Zhu, W.; Chen, L. Electrochem. Commun. 2004, 6, 789. (23) Ott, A. W.; Klaus, J. W.; Johnson, J. M.; George, S. M. Thin Solid Films 1997, 292, 135. (24) Groner, M. D.; Fabrequette, F. H.; Elam, J. W.; George, S. M. Chem. Mater. 2004, 16, 639. (25) Chen, Z.; Dahn, J. R. Electrochem. Solid-State Lett. 2002, 5, A213. (26) Chen, Z.; Qin, Y.; Amine, K.; Sun, Y.-K. J. Mater. Chem. 2010, 20, 7606. (27) Ning, G; Haran, B.; Popov, B. J. Power Sources 2003, 117, 160. (28) Liu, N.; Li, H.; Wang, Z. X.; Huang, X. J.; Chen, L. Q. Electrochem. Solid-State Lett. 2006, 9, A328.
418
dx.doi.org/10.1021/nl1030198 |Nano Lett. 2011, 11, 414–418
