One-Step Fast-Synthesized Foamlike Amorphous Co(OH)2 Flexible

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One Step Fast Synthesized Foam-like Amorphous Co(OH)2 Flexible Film on Ti Foil by Plasma Assisted Electrolytic Deposition as a Binder-free Anode of High Capacity Lithium Ion Battery Tao Li, and Xueyuan Nie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05482 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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ACS Applied Materials & Interfaces

One Step Fast Synthesized Foam-like Amorphous Co(OH)2 Flexible Film on Ti Foil by Plasma Assisted Electrolytic Deposition as a Binder-free Anode of High Capacity Lithium Ion Battery Tao Li, Xueyuan Nie* Department of Mechanical, Automotive & Materials Engineering, University of Windsor, Windsor, Ontario N9B 3P4, Canada KEYWORDS: Lithium ion battery; Anode; Plasma assisted electrolytic deposition; Amorphous; Cobalt hydroxide

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ABSTRACT: This research prepared an amorphous Co(OH)2 flexible film on a Ti foil using plasma assisted electrolytic deposition within 3.5 minutes. Amorphous Co(OH)2 structure was determined by XRD and XPS. Its areal capacity testing as binder and adhesive free anode of lithium ion battery shows that the cycling capacity can reach 2000 µAh/cm2 and keep at 930 µAh/cm2 after 50 charge-discharge cycles, which is benefited from the emerging Co(OH)2 active material and amorphous foam-like structure. The research introduced a new method to synthesize amorphous Co(OH)2 as the anode in fast manufactured low-cost lithium ion battery.

1. INTRODUCTION Lithium ion battery (LIB) as an energy storage system for portable devices and automobile industry has attracted huge research interest.1 To achieve high lithium ion storage, researchers started focusing on developing anode materials of LIB by nanotechnologies; for instance, anode nanomaterials such as Co3O4, CoO with different structures (nanowire and nanoparticles) have been prepared or combined with graphene.2-4 Although those nanomaterials exhibited high capacity, the lengthy manufacture time and high cost likely limit their commercial applications, and they have to be mixed with binder and adhesive in LIB manufacture.3 Besides their extra cost, the binder may decompose in long term and weaken ion diffusion and thus depreciate the capacity of LIB.5 Therefore, development of binder-free low-cost flexible electrode is desirable.6 The Co(OH)2 which was previously studied for supercapacitor applications,7 was recently discovered to be a good candidate of anode materials for LIB.8 As for phase structures, amorphous active materials are found beneficial to improve specific capacities over a wide potential window of LIB because of short-range structural ordering and ability to accommodate lattice distortions without phase transitions.9


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The plasma electrolytic deposition (PED) as a surface coating technology has been used to grow oxide ceramic coatings on Mg, Al and Ti for wear resistance and corrosion prevention.10 In this research, the plasma assisted electrolytic deposition as a modified PED was innovatively used for one step preparation of an amorphous Co(OH)2 flexible film on a Ti foil (Figure 1c) as an inexpensive active anode material, where a Ti foil (Figure 1a) was immersed in a Co-containing aqueous solution and applied with high current and voltage (Figure 1b) to generate plasma discharges, leading to the formation of amorphous Co(OH)2 on Ti surface.

Figure 1. Schematic illustration of experimental process (a-c), growth of Co(OH)2 on Ti foil by plasma assisted electrolytic deposition (b1-b3). 2. RESAULT AND DISCUSSIONS The growth process of Co(OH)2 on Ti foil by the plasma assisted electrolytic deposition is schematically shown in Figure 1b1-b3. As the Figure 1b1 shows, when the current (voltage up to 500V) is introduced, numerous plasma discharging sparks with core temperature 5000-6000K formed on the Ti surface would melt the Ti surface spot by spot,11 and dissociate the H2O.10 2H2O+2e-→ 2OH-(aq) +H2(g) (1) Although in the weak acidic electrolyte, this reaction still can be carried out because the anode reaction in plasma assisted electrolytic deposition is different with normal electrolysis of water. In


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plasma reaction, the formation of OH- is due to the high temperature plasma discharging-induced H2O decomposition.12,13 The OH- will combine with Co2+ by high temperature plasma, and the reaction is:14 Co2+(aq)+2OH-(aq)→ Co(OH)2(s) (2) Subsequently, the Co(OH)2 starts growing on the Ti substrate, and some gas vapor formed in the plasma can be trapped in the Co(OH)2 melt (Figure 1b2), and then these small gas mass spits out to cause the formation of pores and channels in the Co(OH)2 coating (Figure 1b3).13

Figure 2. (a) XRD pattern, XPS spectra of (b) Co 2p and (c) O 1s, (d) expanded Co 2p spectrum. From the XRD pattern (Figure 2a), it seems that the Co(OH)2 on Ti foil is amorphous structure, and only one weak peak at 19 two theta degrees might be attributed to the (001) crystal plane of β-Co(OH)2.15 To further determine the structure, it is necessary to analyze it by XPS. The XPS data has been shown in Figure 2b-c. In Figure 2b, the peak on 781.1 eV can be attributed to Co


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2p3/2 of Co(OH)2.14 In Figure 2c, two peaks at 531.5 eV and 532.9 eV observed in O 1s spectrum represent the bound hydroxide groups and the structural water in Co(OH)2.14,16 The peaks at 786 eV and 802.9 eV in expanded Co 2p spectrum (Figure 2d) are the satellite peaks of Co(OH)2 that are the certification of presence of Co2+ oxidation state.14,17 Therefore, according to the analysis of XPS data, it has clearly indicated that our flexible film on Ti substrate is Co(OH)2, and the formation of amorphous phase can be explained as the quenching process that the high temperature plasma spark would melt Co(OH)2 which is then fast cooled by electrolyte without enough time for crystallization. Similar phenomena can be seen in fast solidification for preparation of an amorphous alloy in metal casting.18

Figure 3. The surface SEM images of Co(OH)2 on Ti substrate, (a-b) surface structure, (c) cross sectional structure and (d) surface structure after 50 charging-discharging process. Morphologies of Co(OH)2 are shown by SEM images in Figure 3. Figure 3a indicates that the Co(OH)2 film contains high density of porous structures. Under a higher magnification (Figure


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3b), some volcanic vent liked pores can be obviously seen. This is because the squirting process of gas mass brings out the Co(OH)2 melt which deposit around pores in plasma assisted electrolytic deposition process.13 Cross-sectional image (Figure 3c) shows the thickness of Co(OH)2 film on Ti substrate (bright area) is 50 µm, and the cross-section has pores and multiple channels with foam-like structure. The porosity of Co(OH)2 film is around 36.5% (Figure S1). After 50 chargedischarge cycles, the pores in Figure 3d turn to be smaller and shallower than those in Figure 3b, however, the general porous structure has unobvious change and the integrated film is also kept porous structure without exfoliation.

Figure 4. Electrochemical testing of Co(OH)2 anode in lithium ion battery, (a) cycling performance (0-3V, 400 µA/cm2), (b) galvanostatic cycling profiles (0-3V, 400 µA/cm2), (c) rate capability and (d) cyclic voltammetry (0.1mV/s).


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The electrochemical testing results are shown in Figure 4a-d, In Figure 4a, the cycling performance at current density 400 µA/cm2 indicates that initial discharge capacity arrived to 2000 µAh/cm2, and at the last 5 cycles of the test, the capacity still kept at 930 µAh/cm2. This result is higher than other reported areal discharge capacity of binder free TiO2 nanotree and TiO2/CoO nanotube anode materials on Ti and Ti-Co alloy foil.19,20 The high areal capacity can be explained as the large area of pores connected to numerous internal channels in the Co(OH)2 film (Figure 3b-c), which provides enough lithium ion diffusion paths in the charge-discharge process that warrants the lithium ions to effectively react with Co(OH)2.The degradation of capacity at late stage might be due to the partially blocking to ion diffusion path in pores (Figure 3d) during intercalation and deintercalation of lithium ion. The galvanostatic cycling profiles (Figure 4b) show that the initial discharge capacity is higher than initial charge capacity by 350 µAh/cm2, which is due to formation of solid electrolyte interface (SEI).21 After 10 cycles, the columbic efficiency kept at 98%. In Figure 4c, the rate capability shows that even if the testing current density is 1200 µA/cm2, the discharge capacity still kept at 250 µAh/cm2. The discharge capacity of the last 10 cycles under 400 µA/cm2 came back to 1000 µAh/cm2 that is coincident with the result in Figure 4a. Such a result indicates that our Co(OH)2 film anode has a good rate capability. The cyclic voltammetry (CV) curve is exhibited by Figure 4d where the CV curve started to overlap after two cycles. The enlarged scanning area in subsequent cycles compared with first two cycles can be explained by the activation process.22 After two cycles, the peak at 2.15V in the anodic process is corresponding to reaction of Co to Co(OH)2, and 1.1V in the cathodic process is corresponding to reaction of Co(OH)2 to Co.23 Therefore, the conversion reactions of Co(OH)2 in the lithium ion battery can be written as follows:8 𝑙𝑙𝑙𝑙𝑙𝑙ℎ𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖

Co(OH)2+2Li++2e- �⎯⎯⎯⎯⎯�Co+2LiOH (3) ACS Paragon Plus Environment

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𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑ℎ𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖

Co+2LiOH-2e- �⎯⎯⎯⎯⎯⎯⎯⎯�Co(OH)2+2Li+ (4)

It is noted that the CV curve shape of amorphous Co(OH)2 is not very similar to the crystalline

Co(OH)2 CV shape.23 The slight changes of position of anodic and cathodic peaks of amorphous Co(OH)2 compared with crystalline Co(OH)2 is because the long term disorder of amorphous Co(OH)2 is more flexible for lithiation and delithiation.24 The broadened anodic and cathodic peaks are attributed to amorphous Co(OH)2 that can sustain a higher stress/strain than crystalline Co(OH)2 in charging-discharging process.25 This is also beneficial to the high areal capacity. 3. CONCLUSIONS In this research work, the Co(OH)2 film was prepared on a Ti foil using a plasma assisted electrolytic deposition technique within only 3.5 min. The XRD and XPS analyses suggest that the active materials film is amorphous Co(OH)2. SEM observations identify the foam-like structure with large number of pores and internal channels. This type of structure offered enormous surface areas and lithium ion diffusion paths and thus high cycling capacity. The results indicate that our fast manufacturing process can produce a porous amorphous Co(OH)2 flexible film on a Ti foil, which can be used as a binder and adhesive free anode material for a low-cost and highperformance lithium ion battery. However, the degradation of cycling capacity needs to be addressed in future research. SUPPORTING INFORMATION Experimental section and porosity analysis are given in supporting information file. Corresponding Author * [email protected] (X. Nie)


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOLEGENMENTS This research was supported by Natural Sciences and Engineering Research Council (NSERC), Canada. Thanks to Dr. Tricia Breen Carmichael and Dr. S. Holger Eichhorn in Department of Chemistry & Biochemistry, University of Windsor for access to battery packaging and XRD testing. REFERENCES (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652-657. (2) Li, Y.; Tan, B.; Wu, Y. Mesoporous Co3O4 Nanowire Arrays for Lithium Ion Batteries with High Capacity and Rate Capability. Nano lett. 2008, 8, 265-270. (3) Sun, X.; Hao, G.; Lu, X.; Xi, L.; Liu, B.; Si, W.; Ma, C. High-defect Hydrophilic Carbon Cuboids Anchored with Co/CoO Nanoparticles as Highly Efficient and Ultra-stable Lithium-ion Battery Anodes. J. Mater. Chem. A 2016, 4, 10166-10173. (4) Wu, Z.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Zhou, G.; Li, F.; Cheng, H. Graphene Anchored with Co3O4 Nanoparticles as Anode of Lithium Ion Batteries with Enhanced Reversible Capacity and Cyclic Performance. ACS nano 2010, 4, 3187-3194.


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(20) Madian, M.; Giebeler, L.; Klose, M.; Jaumann, T.; Uhlemann, M.; Gebert, A.; Oswald, S.; Ismail, N.; Eychmuller, A.; Eckert, J. Self-organized TiO2/CoO Nanotubes as Potential Anode Materials for Lithium Ion Batteries. ACS Sustain. Chem. Eng. 2015, 3, 909-919. (21) Ni, S.; Lv, X.; Li, T.; Yang, X.; Zhang, L. The Investigation of Ni (OH) 2/Ni as anodes for High Performance Li-ion Batteries. J. Mater. Chem. A 2013, 1, 1544-1547. (22) Zhou, M.; Xu, Y.; Wang, C.; Li, Q.; Xiang, J.; Liang, L.; Wu, M.; Zhao, H.; Lei, Y. Amorphous TiO2 Inverse Opal Anode for High-rate Sodium Ion Batteries. Nano Energy 2017, 31, 514-524. (23) Bai, Y.; Liu, W.; Yu, C.; Wang, T.; Feng, J.; Xiong, S. One-Pot Solvothermal Synthesis of ZnO@ α-Co (OH)

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One-Step Fast-Synthesized Foamlike Amorphous Co(OH)2 Flexible

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