Nano Res.

Electronic Supplementary Material

Engineering the surface of LiCoO2 electrodes using atomic layer deposition for stable high-voltage lithium ion batteries

Jin Xie1, Jie Zhao1, Yayuan Liu1, Haotian Wang2, Chong Liu1, Tong Wu1, Po-Chun Hsu1, Dingchang Lin1,

Yang Jin1, and Yi Cui1,3 ()

1 Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA 2 Department of Applied Physics, Stanford University, Stanford, CA 94305, USA 3 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA Supporting information to DOI 10.1007/s12274-017-1588-1

Table of contents:

1. Additional experimental details 2. Figure S1: EDX line scan of LiAlO2 coated LiCoO2 3. Figure S2: Thickness measurement and calculation 4. Figure S3: SEM characterizations and particle-size distribution analysis of LiCoO2 5. Figure S4: Long-term cycling performance of coated and pristine LiCoO2 electrodes 6. Figure S5: Electrochemical characterizations of Co3O4 electrodes 7. Figures S6 andS7: Additional Raman characterizations 8. Figure S8: Electrochemical characterizations of PI and PVDF bound electrodes

Additional experimental details

Materials synthesis and preparation:

Co3O4 nanoparticles (10–30 nm powder, Alfa Aesar), Super C65 (Imerys) and polyimide (DuPont) were dried under vacuum for 24 h to remove trapped moisture prior to use. Co3O4 working electrodes were made using a typical slurry method by mixing Co3O4, super C65 and polyimide with a mass ratio of 90:5:5 in N-methyl- pyrrolidone (NMP) solvent.

Electrochemical measurements:

Battery cycling performance of Co3O4 electrodes was evaluated by the galvanostatic cycling of coin cells (CR 2032) with Li foil as the counter electrode. The electrolyte consists of 1.0 M LiPF6 in ethylene carbonate (EC) and

Address correspondence to yicui@stanford.edu

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diethyl carbonate (DEC) with 1:1 volume ratio. Battery cycling data was collected using a LAND 8-channel battery tester with the potential window of 2.75–4.60 V vs. Li+/Li at room temperature. The specific capacity was calculated based on the weight of Co3O4. EIS was conducted using a Biologic VSP potentiostat over the frequency range of 0.01 Hz to 1 MHz.

Materials characterizations:

The thickness of ALD Al2O3 films on silicon wafers was characterized using a Woollam M2000 Spectroscopic Ellipsometer in clean room.

EDX line scan of LiAlO2 coated LiCoO2

Figure S1 EDX line scan of a LiCoO2 particle with 200 cycles ALD LiAlO2 coating.

In addition to the EDX mapping result (main text, Fig. 1), EDX line scan was also performed to show the core-shell structure after 200 cycles ALD LiAlO2 coating on LiCoO2.

Thickness measurement and calculation

Figure S2 (a) The thickness of ALD Al2O3 coating on Si wafers measured by the ellipsometer; (b) calculated areal loading densities of Al and Li in ALD Al2O3 films and ALD LiAlO2 films with different ALD cycle numbers.

The thickness of ALD Al2O3 films on Si wafer was measured by the ellipsometer (Fig. S2(a)). With a close to linear relationship between film thickness and ALD cycle numbers, the growth rate of ALD Al2O3 was determined to be ~0.106 nm/cycle. The areal loading densities of Al, assuming a bulk density of 3.95 g·cm–3 for Al2O3, were summarized and compared to those of LiAlO2 coatings in Fig. S2(b). The areal loading of Li and Al in LiAlO2 films were measured directly using ICP-MS. For all LiAlO2 ALD deposition, alternate Li2O sub-cycle and Al2O3 sub-cycle were introduced with a 1:1 ratio. For a given ALD LiAlO2 deposition, the total number of

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ALD cycles was counted by adding all Li2O sub-cycles and Al2O3 sub-cycles. The growth rate of LiAlO2 in our ALD deposition condition was stable within the first 200 cycles (Fig. 1 of main text). While previous literature reported unstable growth rates caused by the hygroscopic and reactive LiOH component [S1], such effect was not severe in this study, at least for the initial 200 cycles. The relatively stable deposition rate is possibly benefited from the prolonged Ar purging time after each precursor injecting and the relatively thin film thickness at limited cycle numbers (

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In addition to the cycle performance shown in the main text, additional long-term cycle performance was also presented in Fig. S4 for 100 cycles with both 2.75–4.50 V and 2.75–4.60 V electrochemical windows. In the long-term cycling tests, LiAlO2 coated LiCoO2 cathodes have shown improved stability compared to pristine LiCoO2 cathodes.

Electrochemical characterizations of Co3O4 electrodes

Figure S5 Electrochemical characterizations of pristine Co3O4 nanoparticle electrodes. (a) Cycle performance of Co3O4 nanoparticle electrode tested with a 2.75–4.60 V vs. Li+/Li electrochemical window at a cycling rate of 50 mA·g–1; (b) voltage vs. capacity plots of Co3O4 nanoparticle electrode at 1st, 10th and 50th cycle; (c) EIS characterization of Co3O4 nanoparticle electrode after 1st charge cycle.

The battery cycling performance of Co3O4 nanoparticle electrodes was tested in Fig. S5(a). In spite of the small particle sizes of Co3O4, it was unable to store Li ions within the electrochemical window of 2.75–4.60 V vs. Li+/Li. In the voltage vs. capacity plots (Fig. S5(b)), Co3O4 electrodes exhibited typical capacitance behavior without any lithiation/delithiation plateau. After 1st charge cycle, electrochemical impedance spectrum of Co3O4 electrodes was tested and shown large impedance.

Additional Raman characterizations

Figure S6 Raman characterizations of pristine LiCoO2 electrode (black trace), pristine LiCoO2 electrode after 120 s laser illumination at 5% of its maximum power (blue trace), pristine LiCoO2 electrode after 120 s laser illumination at 10% of its maximum power (pink), and pristine LiCoO2 electrode after 120 s laser illumination at 25% of its maximum power (red). Raman spectrum of each sample was recorded after laser illumination using the same 532 nm wavelength laser at 5% of its maximum power.

Intense laser illumination could possibly convert LiCoO2 to other phases including Co3O4 [S2, S3]. To study whether Co3O4 peaks observed in Fig. 5 of main text were due to battery cycling or laser effect, a laser induced conversion was studied in Fig. S6. For 120 s laser illumination at 5% or 10% of its maximum power, the peaks of LiCoO2 remained unaffected. However, LiCoO2 to Co3O4 conversion was found when the laser intensity was increased to 25% of its maximum power for 120 s illumination on LiCoO2. To avoid such laser induced conversion during our Raman mapping (Fig. 5 of the main text and Fig. S7), we have limited the laser intensity to 1% of its maximum power for a total of 20 minutes’ data acquisition time to avoid laser induced phase transformation.

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Figure S7 Raman characterizations. (a) LiAlO2 coated LiCoO2 electrode after 50 cycles of battery cycling; (b) pristine LiCoO2 electrode after 50 cycles of battery cycling; (c) pristine LiCoO2 electrode prior to battery cycling; (d) pristine Co3O4 electrode prior to battery cycling. Raman spectrum of each sample was collected using a 532 nm laser at 1% of its maximum power for a total of 20 minutes’ data acquisition time to avoid laser induced phase transformation.

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Raman spectra of LiAlO2 coated LiCoO2 electrode after 50 cycles and pristine LiCoO2 electrode after 50 cycles were presented in Figs. S7(a) and S7(b), respectively. In addition to LiCoO2 maps and Co3O4 maps shown in Fig. 5 of the main text, carbon maps were included. Standard samples including pristine LiCoO2 electrode before cycling and pristine Co3O4 electrode before cycling were tested in Figs. S7(c) and S7(d), respectively. In Fig. S7(c), LiCoO2 showed characteristic Co-O stretching (A1g) at 593 cm–1 and O–Co–O bending (Eg) at 484 cm–1 [S4]. In Fig. S7(d), Raman mapping of standard Co3O4 electrodes showed characteristic peaks at 473, 513, 606, and 677 cm–1, which are corresponding to Eg, F2g, F2g and A1g mode of Co3O4 [S5]. No Co3O4 impurities peaks were detected for pristine LiCoO2 electrode before cycling. The contrast between Figs. S7(b) and S7(c) highlighted that the Co3O4 phase indeed arose due to repeated battery cycling with a wide electrochemical window, rather than laser induced phase transformation.

Electrochemical characterizations of PI and PVDF bound electrodes

Figure S8 (a) Optical images of PI bound LiCoO2 electrodes and PVDF bound LiCoO2 electrodes after ALD coating; (b) cycle performance of PI bound LiCoO2 electrodes after 20 cycles ALD LiAlO2 coating and PVDF bound LiCoO2 electrodes after 20 cycles ALD LiAlO2 coating at a current density of 50 mA·g–1.

Polyimide binder was chosen in this study because of its high thermal stability. We did control experiments with both PVDF bound electrodes and PI bound electrodes (Fig. S8). Both PVDF bound and PI bound samples shown good uniformity and adhesion after ALD coating (Fig. S8(a)). PI bound sample did show slightly better cycling stability compared to PVDF bound sample (Fig. S8(b)).

References

[S1] Comstock, D. J.; Elam, J. W. Mechanistic study of lithium aluminum oxide atomic layer deposition. J. Phys. Chem. C 2013, 117, 1677–1683.

[S2] Song, S. W.; Han, K. S.; Fujita, H.; Yoshimura, M. In situ visible Raman spectroscopic study of phase change in LiCoO2 film by laser irradiation. Chem. Phys. Lett. 2001, 344, 299–304.

[S3] Lemos, V. Comment on 'in situ visible Raman spectroscopic study of phase change in LiCoO2 film by laser irradiation'. Chem. Phys. Lett. 2004, 400, 268–270.

[S4] Baddour-Hadjean, R.; Pereira-Ramos, J. P. Raman microspectrometry applied to the study of electrode materials for lithium batteries. Chem. Rev. 2010, 110, 1278–1319.

[S5] Hadjiev, V. G.; Iliev, M. N.; Vergilov, I. V. The Raman spectra of Co3O4. J. Phys. C: Solid State Phys. 1988, 21, L199–L201.