Article pubs.acs.org/JPCC
Enhanced Cycleabity in Lithium Ion Batteries: Resulting from Atomic Layer Depostion of Al2O3 or TiO2 on LiCoO2 Electrodes Ho-Ming Cheng,† Fu-Ming Wang,*,‡,§ Jinn P. Chu,† Raman Santhanam,∥ John Rick,‡ and Shen-Chuan Lo⊥ †
Department of Materials Science and Engineering, ‡Sustainable Energy Center, and §Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan ∥ Maxwell Technologies, San Diego, California, United States ⊥ Materials and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan ABSTRACT: This study reports the preparation of Al2O3 and TiO2 coatings on the as-prepared LiCoO2 electrodes using atomic layer deposition (ALD). A thin Al2O3 ALD coating was shown to eliminate capacity fading effectively during repeated charging and discharging, whereas a TiO2 coating led to significant improvement only at high cycle numbers. An analysis of the differential capacity versus potential curves suggests that this poorer cycling performance could be related to the participation of the TiO2 thin film in the redox reaction. Graphical representation of the energy levels of the various ALD coatings on LiCoO2 during charging and discharging indicated that the redox current is impeded at the Al2O3−LiCoO2 junction, whereas electrons and holes were energetic enough to flow into the TiO2 because of the smaller band gap energy. The barrier between the valence band maxima of TiO2 and LiCoO2 expands as the charge−discharge cycle number increases, eventually making TiO2 redox-inactive. These conclusions are supported by both XPS spectra and the cycle performance in the established literature references. Our results suggest that large band gap materials should be considered to be potentially useful ALD coatings on cathode materials.
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diffusion of Li+ within the electrode. Ultrathin coating also helps diminish charge-transfer resistance and the path length through which the electron and Li+ ion must travel within the electrode. For the application on anode, TiN and Al2O3 ALD coatings, which act as passivation films, have been successfully deposited on lithium titanate spinel (Li4Ti5O12) powder and electrodes.19,20 Natural graphite anodes can also be covered with Al2O3 ALD coating layer, which manifested the propylene carbonate-compatibility and the improved performance at high temperature.21 The success of the Al2O3 ALD coating on silicon, when used as a potential anode material, suggests that the alumina thin film can play the role of an artificial solid electrolyte interface layer, which can provide much better lithium ion conductivity.22,23 Surface protection by Al2O3 ALD coating on MoO3 has been shown to lead to less mechanical degradation upon cycling.24 For the applications on cathode, the surface of the Li(Ni1/3Mn1/3Co1/3)O2 is protected from dissolution and HF attack with as few as four ALD layers of alumina.25 An Al2O3-coated nano-LiCoO2 electrode with a 250% improvement in reversible capacity (compared with a bare electrode at 7.8 C) has been reported, demonstrating that
INTRODUCTION LiCoO2 has been used as the cathode material in commercial lithium ion batteries because of its high capacity, good rate capability, and safety.1,2 However, if more than half of the lithium-ions are extracted (LixCoO2, x < 0.5; >4.2 V vs Li+/Li), Co4+ will dissolve into the electrolyte, resulting in structural changes to the LiCoO2 particles and deterioration of the battery’s performance.3,4 The surface modification of LiCoO2, by metal oxide coating, helps prevent Co dissolution and electrolyte decomposition, thereby reducing the decrease in the lithium ion battery’s energy density and the working voltage. Some of the metal oxides reported as surface coating candidates are: Al2O3,5−9 TiO2,7,8 ZnO,8 SiO2,10 and ZrO2.11−13 Recent studies related to the surface modification of LiCoO2 have focused variously on sol−gel methods,5,14 heat treatment,6 and pulsed laser deposition.15 However, it is difficult to control the surface coverage when using these methods with nanoscale materials.16 Sol−gel coating, currently the major coating strategy, requires large amounts of both the solvent and the precursor, in addition to post-heat-treatment after coating. Atomic layer deposition (ALD) has the advantage of depositing a layer-by-layer homogeneous film coating on a 3D specimen in the gas phase.17,18 The ALD technique not only requires a minimal amount of precursor but also offers conformal and ultrathin coating layers with the minimum addition of nonreactive material. These virtues facilitate the fast © 2012 American Chemical Society
Received: November 3, 2011 Revised: February 11, 2012 Published: March 20, 2012 7629
dx.doi.org/10.1021/jp210551r | J. Phys. Chem. C 2012, 116, 7629−7637
The Journal of Physical Chemistry C
Article
Figure 1. Schematic diagram of the ALD steps for coating Al2O3 thin film on a LiCoO2 electrode.27
precursor, was forced into the reaction chamber. TMA reacted directly with the hydroxyl groups of the LiCoO2 particles and the conductive carbon (graphitic KS-6) of the electrode, forming a Al-CH3 monolayer and methane. The excess aluminum precursor and methane were removed with a vacuum. While purging with water vapor, as the oxygen reactant source, the electrode was exposed to steam to hydrolyze the aluminum precursor layer, that is, Al-CH3, on the LiCoO2 and KS-6 surface, to form −Al−OH and methane. Excess water and methane were again removed under vacuum. By repeating this sequence, the Al2O3 film was formed in a layer-by-layer manner on the LiCoO2 electrode; however, because of the lack of active −OHs on the PVdF available to react with TMA (see our previously published XPS data27), Al2O3 does not grow on the binder. The electrode materials were tested by assembling coin-type cells with potentials compared with a lithium metal reference electrode; the range is from 3.0 to 4.5 V with a constant current rate of 0.2 C. A porous Celgard 2320 film and lithium metal served as the separator and anode, respectively. The electrolyte consisted of 1.0 M LiPF6 (Tomiyama Pure Chemical, Japan) dissolved in an ethylene carbonate/propylene carbonate/ diethyl carbonate (3:2:5 in volume) mixed solvent. TiO2 films were grown using the same procedure but with titanium isopropoxide [Ti(OCH(CH3)2)4] as the Ti precusor, as shown in Figure 2. XPS was performed on the Al2O3 and TiO2 ALDcoated electrodes, both before and after five charge−discharge cycles, to study the chemical composition of the surface coatings.
surface ALD coating is a promising way to advance the cycle performance of lithium ion batteries.16 Recently, the technique has also been used to coat LiCoO2 particles using a rotary ALD reactor.26 Although this method provides thorough coverage, electrical conductive pathways can be disrupted, thereby slowing the associated charge-transfer kinetics. This study uses the ALD technique to deposit Al2O3 and TiO2 films directly on LiCoO2 electrodes and then investigates the electronic band structures of the coatings and active materials, during charging and discharging, to illustrate differences in the electrode’s cycle performance. Finally, we suggest a criterion appropriate for judging the suitability of potential ALD coatings on the cathode materials.
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EXPERIMENTAL DETAILS Al2O3 films were grown directly on the as-prepared LiCoO2 electrodes (L106, LICO, Taiwan) using ALD (Cambridge Nanotech Savannah) at 120 °C. The LiCoO2 cathode electrode comprised: LiCoO2 (91 wt %), poly(vinylidene) fluoride [(PVdF, 4 wt %), Kureha Chemical, Japan] as a binder, and conductive carbon [(KS-6, 5 wt %), Showa, Japan]. The precursors used for the Al 2 O 3 ALD were water and trimethylaluminum [(TMA), Al(CH3)3]. A low reaction temperature was used because the PVdF requires a low processing temperature (
