High-Performance Ga2O3 Anode for Lithium-Ion ... - ACS Publications

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High-Performance Ga2O3 Anode for Lithium Ion Batteries Xun Tang, Xin Huang, Yongmin Huang, Yong Gou, James Pastore, Yao Yang, Yin Xiong, Jiangfeng Qian, Joel D. Brock, Juntao Lu, Li Xiao, Héctor D. Abruña, and Lin Zhuang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16127 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Applied Materials & Interfaces

High-Performance Ga2O3 Anode for Lithium Ion Batteries Xun Tang1, Xin Huang3,4, Yongmin Huang1, Yong Gou1, James Pastore2,4, Yao Yang2, Yin Xiong2, Jiangfeng Qian1, Joel D. Brock3,4, Juntao Lu1, Li Xiao*1,2, Héctor D. Abruña*2, Lin Zhuang*1 1

College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China

2

Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York, 14853, USA

3

School of Applied and Engineering Physics, Cornell University, Ithaca, New York, 14853, USA 4

Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York, 14853, USA

*

E-mail: [email protected], [email protected], [email protected]

Abstract There is a great deal of interest in developing battery systems that can exhibit self-healing behavior, thus enhancing cycleability and stability. Given that gallium (Ga) is a metal that melts near room temperature, we wanted to test if it could be employed as a self-healing anode material for lithium ion batteries (LIBs). However, Ga nanoparticles (NPs), when directly applied, tended to aggregate upon charge/discharge cycling. To address this issue, we employed carbon-coated Ga2O3 NPs as an alternative. By controlling the pH of the precursor solution, highly dispersed and ultra-fine Ga2O3 NPs, embedded in carbon shells, could be synthesized through a hydrothermal carbonization (HTC) method. The particle size of the Ga2O3 NPs was 2.6 nm, with an extremely narrow size distribution, as determined by high-resolution

transmission

electron

microscopy

(HRTEM)

and 1

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Brunner−Emmet−Teller (BET) measurements. A LIB anode based on this material exhibited stable charging and discharging, with a capacity of 721 mAh/g after 200 cycles. The high cyclability is due not only to the protective effects of the carbon shell, but also to the formation of Ga0 during the lithiation process, as indicated by operando X-ray absorption near-edge spectroscopy (XANES). Keywords: gallium oxide; hydrothermal carbonization; self-healing; lithium-ion batteries; anode materials

2


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Introduction Rechargeable Li-ion batteries (LIBs) represent one of the most attractive technologies for advanced electrochemical energy storage.1-3 Despite their widespread use in numerous technologies, the energy density of commercial LIBs cannot fulfill the increasing demand of electronic and especially automotive applications. 4-5 The development of new anode and cathode materials with higher capacity is critical for meeting these needs. 6-8 In terms of the anode, the theoretical capacity of graphite (LiC6) is limited to 372 mAh/g. 9-10. In an attempt to improve anode capacity, great efforts have been devoted to the investigation of alternative materials that can store lithium through conversion reactions (e.g., Fe3O4, Co3O4, MnO2, etc.)

11-19

or

reversible alloying reactions (e.g., Si, Ge, etc.) 20-28. Among potential candidates gallium (Ga) appears to be an especially interesting and attractive one. The melting point of Ga is 29.8°C. It is the only nontoxic metal that can melt near room temperature (while mercury is a liquid metal at room temperature, it is highly toxic). This unique feature makes it possible to avoid structural collapse and particle pulverization during lithiation/delithiation cycles, and thus can potentially enable stable long-term and stable cycling. Zhang and co-workers demonstrated the self-healing behavior of a Ga droplet by in situ TEM observations, in which the Ga droplets could “heal” the cracks formed in the lithiated solid state once the electrode returned to the liquid state after delithiation. 29

However, Ga nanoparticles (NPs) tend to aggregate due to their low surface energy,

30-32

thus electrodes made from monodispersed Ga NPs exhibited poor cyclability. 33 In the present work, our strategy has been to replace the Ga NPs with Ga2O3 NPs

in the as-prepared anode. The Ga2O3 NPs were also coated with carbon, so that Ga NPs, generated during the lithiation process, would not aggregate thus enhancing cycling stability. A hydrothermal carbonization (HTC) method was used to synthesize ultra-fine and highly dispersed Ga2O3 NPs embedded in carbon (Ga2O3@C). From TEM and BET measurements, these Ga2O3 NPs were around 2.6 nm in diameter with a very narrow size distribution. The delithiation capacity was 721 mAh/g after 200 cycles at 3



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a current density of 0.5 A/g (ca 0.7C). The enhanced cyclability, relative to Ga NPs, is attributed to the optimized structure of Ga2O3@C. The generation of Ga0 during the lithiation process was observed by operando XANES.

Experimental section Preparation of Ga2O3@C. Ga2O3@C was synthesized by a hydrothermal carbonization (HTC) method followed by a calcination process under an argon atmosphere. In a typical synthesis, 0.4655 g Ga(NO3)3·xH2O (x ≈ 6) (99.9%, Aladdin Chemistry) and 0.7237 g glucose (AR, Sinopharm Chemical) were dissolved in 4 mL of deionized water, and 0.2965 g KOH (GR, Sinopharm Chemical) were dissolved in 3 mL of deionized water. The two solutions were subsequently mixed in a 10 mL Teflon-lined stainless steel autoclave, which was placed in an air-circulated oven at 180oC for 4 h, and then allowed to cool down to room temperature. The product was centrifuged and washed with deionized water and ethanol. The obtained product was dried at 80oC, followed by a heat treatment in a tube furnace under an argon atmosphere. The temperature was first raised from room temperature to 400oC at 5oC/min and maintained at 400oC for 4 hrs.

The temperature was then increased at

5oC/min to 600oC, and maintained at that level for 4 hrs.

After cooling to room

temperature, the final product, Ga2O3@C, was obtained. Four different Ga2O3@C samples were synthesized by controlling the pH of the precursor solution, which were 1.80 (Sample-1), 2.76 (Sample-2), 8.58 (Sample-3), and 10.61 (Sample-4). These values correspond to Ga3+/OH– molar ratios of 1:1, 1:2, 1:3, and 1:4, respectively. Ga2O3 etching for BET tests. 100 mg Ga2O3@C were stirred in 200 mL of HCl solution (0.5 M) for 72 h, centrifuged and washed three times with ultrapure water. The sample was further dried by vacuum freeze-drying before testing. Structural characterization. Products were thoroughly characterized using X-ray diffraction (XRD, RigakuSmartLab, 9kW, Cu Kα, λ = 1.5406 Å, 20° to 80°, 10°/min), field emission scanning electron microscope (FESEM, ZEISS Merlin Compact, EHT = 5 kV) equipped with energy dispersive X-ray spectroscopy (EDX, INCAPentalFETx3, Oxford Instruments), confocal Raman microscopy (Renishaw, 4


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inVia+plus, λ = 532 nm), BET testing (Quantachrome QuadraSorb SI gas sorption system, N2, 77K), high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F30) equipped with electron energy loss spectroscopy (EELS), thermogravimetric analysis (TGA, TA Instruments, Q500, Al2O3 crucibles, 30°C to 800°C, 5°C/min, air flow 40 mL/min), X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi spectrometer with monochromic Al Kα radiation), and operando Ga K-edge X-ray absorption near edge structure spectra (XANES, carried out at the F-3 beamline of the Cornell high energy synchrotron source, CHESS) using in situ X-ray cells. The cells were made from CR2032 coin cell casings by drilling 3 mm holes and attaching Kapton windows with epoxy. The data were processed with the Athena software package. 34 Electrochemical measurements. For the anode, a mixture of active material, conductive carbon black (Super P, TIMCAL) and polyvinylidene fluoride (PVDF) binder in a weight ratio of 80:10:10 was dispersed in N-methylpyrrolidone (NMP), and the resultant slurry was then uniformly doctor bladed on a Cu foil current collector. The active material loading ranged from 1.5 to 2.5 mg/cm2. A Celgard 2400 microporous polypropylene membrane was used as the separator, and Li foil was used as the counter electrode. The non-aqueous electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) mixed solvent (1:1 by weight). The cell assembly was carried out in an argon filled glovebox (where the residual concentrations of water and oxygen were below 0.1 ppm). Galvanostatic cycling experiments were performed on a LAND CT2001A battery test system over the voltage range of 0.005 V to 3.00 V versus Li/Li+ at 25oC. Cyclic voltammograms were recorded using a CHI-1000C potentiostat between 0.005 V to 3 V at a scan rate of 0.2 mV/s.

Results and discussion Morphology control by adjusting the pH of the precursor solution. In typical HTC reactions, the pH of the precursor solution has been reported to play a key role in the morphology and particle size of the resulting products. 35-38 In the present work, 5



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we did not employ buffer solutions to control the pH of the Ga3+ solution, so as to avoid the interference of additional anions. By adding controlled amounts of KOH solution, the molar ratio of OH– / Ga3+ was controlled to be 1:1, 2:1, 3:1, and 4:1. The pHs of the corresponding precursor solutions were determined to be 1.80, 2.76, 8.58, and 10.61, respectively. The Ga2O3@C samples thus synthesized are denoted as Sample-1, Sample-2, Sample-3, and Sample-4, respectively. As shown in Figure S1, the precursor solutions of Sample-1, Sample-2, and Sample-4 were colloid solutions with clear Tyndall phenomena, but the precursor solution of Sample-3 turned out to be an emulsion. Through the HTC reaction, carbonaceous materials are generated that coat the target substrates. 39-41 In this work, there are two components in the samples, Ga2O3 and carbon. Our expectation was that the Ga2O3 NPs would be mostly embedded in carbon, preventing significant morphology changes during charging/discharging. Such a Ga2O3@C structure was only obtainable by careful control of the pH of the precursor solution, as established by the following in-depth characterization. SEM was first employed to provide an overview of the morphology of the as-prepared samples. As shown in Figures 1A-1D, while Samples 1-3 exhibited an aggregated morphology, Sample-4 was highly dispersed and uniform. EDX analyses also revealed that the Ga2O3 and C components are actually separated in Samples 1-3. Specifically, the C (carbon) line profile does not match the Ga or O profiles (insets to Figures 1A, 1B, and 1C). On the contrary, the EDX profiles for C, O, and Ga are essentially the same for Sample-4 (inset to Figure 1D), indicating that the carbon/Ga2O3 distribution is quite uniform. This result is also consistent with the EELS elemental mapping analysis for Sample-4 (Figure S2). The TGA behavior of Sample-4 was also different from that of the other samples. As shown in Figure 1E, the carbon combustion temperature of Sample-4 (440oC) was much lower than that of the other 3 samples (500oC, 480oC, 470oC, respectively) and pure carbon synthesized by this method (560oC), suggesting that the carbon component in Sample-4 is likely smaller in size (or the carbon coating on the Ga2O3 6


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NPs is thinner) and thus oxidizes at a lower temperature. 42-43 Based on these SEM and EDX results, it appears that the key for obtaining a highly dispersed and uniform Ga2O3@C sample is to control the pH of the precursor solution to be ca. 10.6. In the following, the structure of these samples, in particular Sample-4, is further described through the use of XRD and HRTEM. Structural analysis of Ga2O3@C. The XRD patterns of the four Ga2O3@C samples (Figure 2A) indicate a face-centered cubic (fcc) structure of γ–Ga2O3 with space group Fd3m (JCPDS#20-0426). Sample-4 was further analyzed by SAED (selected area electron diffraction) (Figure 2B), with diffraction rings corresponding to the {311}, {400} and {440} facets of the fcc structure, consistent to the XRD results. The particle size of Ga2O3, calculated from the width of the (440) peak in the XRD patterns, using the Scherrer formula, was very similar for all samples (2.3±0.1 nm, Table S1), indicating that the pH of the precursor solutions, while critical to the sample morphology, had no observable effect on the particle size of the Ga2O3 NPs, separated or coated by carbon. The morphology and distribution of Ga2O3 NPs were characterized by TEM. Figure 2G-2J display TEM images and the corresponding particle size distribution histograms of Ga2O3 in the four samples. In all samples, Ga2O3 assumed a monodisperse state and approximately spherical shape. According to the statistics from fifty particles, the average particle size of Sample 1-4 was calculated to be 3.05 nm, 2.89 nm, 2.93 nm and 2.75 nm, respectively. Figures 2C-2F display atomically resolved HRTEM images of each of the Ga2O3 samples. The clearly identified {400} facets are typical of the fcc structure of Ga2O3. The HRTEM image for Sample-4 (Figure 2F) is somewhat blurred, relative to the other samples. This is likely because the Ga2O3 NPs in Sample-4 are fully coated with carbon, or embedded in a carbon substrate. In the following, we investigate this point further. In all samples, the Ga2O3 NPs are assumed to be either coated by carbon or just attached to the surface of the carbon substrate. We used HCl solution to wash out the Ga2O3 NPs, and then measured the change in surface area of the samples via BET 7



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measurements. Raman spectroscopy (Figure S5) was used to ensure that the Ga2O3 NPs had been completely leached out. As seen in Figure 3, there were no obvious pores on any sample before leaching the Ga2O3 NPs (dash lines), and the BET surface area of all samples was close to 250 m2/g (Table 1). However, after leaching out the Ga2O3, the BET curves (solid lines) exhibited clear evidence of pores around 2.6 nm in all samples, consistent with the particle size of the Ga2O3 NPs observed by XRD and TEM. These results clearly indicate that the Ga2O3 NPs in the four samples were coated by carbon, such that cavities were generated after leaching the Ga2O3 NPs in a manner analogous to an inverse opal. A negative shift of the binding energy of Ga2O3@C Ga2p3/2 peak, compared with that of bare γ-Ga2O3, was observed by XPS (Figure S4), which further demonstrated the carbon coated structure. The BET results also point to significant increases in the surface area after leaching the Ga2O3 NPs. It is worth noting that while the BET surface area of Sample-1, Sample-2 and Sample-3 increased by less than 200% (from 230.1 m2/g, 264.2 m2/g and 254.0 m2/g to 418.0 m2/g, 592.7 m2/g and 671.7 m2/g, respectively. See Table 1), the BET surface area of Sample-4 increase by more than 400% (234.2 m2/g to 1237.0 m2/g). Such a large difference can only be ascribed to structural differences of Sample-4 relative to the other samples. Since according to TG characterizations (Figure S3), the Ga2O3 loading for each sample was very similar, the much greater increase in the BET surface area of Sample-4 after leaching Ga2O3 strongly indicates that more Ga2O3 NPs are coated by carbon or embedded in the carbon substrate, while for the other samples, the carbon coating is less effective, and more Ga2O3 NPs are bare or are just attached to the carbon substrate. Based on the above analyses, Sample-4 appears to have the optimal structure for lithiation since the active material (Ga2O3) has a small particle size (

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