Controlling Solid-Electrolyte-Interphase Layer by Coating P-Type

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Controlling Solid-Electrolyte-Interphase Layer by Coating P-type Semiconductor NiOx on Li4Ti5O12 for High-Energy-Density Lithium-Ion Batteries Mi Ru Jo, Gi-Hyeok Lee, and Yong-Mook Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10207 • Publication Date (Web): 01 Dec 2015 Downloaded from http://pubs.acs.org on December 5, 2015

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ACS Applied Materials & Interfaces

Controlling Solid-Electrolyte-Interphase Layer by Coating P-type Semiconductor NiOx on Li4Ti5O12 for High-Energy-Density Lithium-Ion Batteries Mi Ru Jo,† Gi-Hyeok Lee,† Yong-Mook Kang*,† †

Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 100-715,

Republic of Korea

KEYWORDS : Li4Ti5O12, nickel oxide, lithium rechargeable battery, solid electrolyte interphase (SEI), surface coating.

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ABSTRACT Li4Ti5O12 is a promising anode material for rechargeable lithium batteries due to its well-known zero strain and superb kinetic properties. However, Li4Ti5O12 shows low energy density above 1 V vs. Li+/Li. In order to improve the energy density of Li4Ti5O12, its low-voltage intercalation behavior beyond Li7Ti5O12 has been demonstrated. In this approach, the extended voltage window is accompanied by a decomposition of liquid electrolyte below 1 V, which would lead to an excessive formation of solid electrolyte interphase (SEI) films. We demonstrate an effective method to improve electrochemical performance of Li4Ti5O12 in a wide working voltage range by coating Li4Ti5O12 powder with p-type semiconductor NiOx. Ex situ XRD, XPS, and FTIR results show that the NiOx coating suppresses electrochemical reduction reactions of the organic SEI components to Li2CO3, thereby promoting reversibility of the charge/discharge process. The NiOx coating layer offers a stable SEI film for enhanced rate capability and cyclability.


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INTRODUCTION Carbonaceous materials and transition metal oxides have been regarded as the important electrode materials for a portable power source application, and have recently shown their potential in the areas of electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs).1-5 However, a rigorous safety standard for a large-scale energy storage application calls for alternative anode materials to prevent any possible safety hazards. In this regard, spinel Li4Ti5O12 can be a promising material because of its high operating voltage range and zero volume changes during the charge/discharge process.6-8 Unfortunately, Li4Ti5O12 exhibits a low theoretical capacity of 175 mAh g-1 via a two-phase-equilibrium reaction between Li4Ti5O12 and Li7Ti5O12. In order to improve the intrinsic low capacity, there has been an increasing interest in extending the electrochemical utilization of Li4Ti5O12 below 1 V vs. Li+/Li. It has been reported that the reversible capacities of Li4Ti5O12 could be increased when discharged to 0.01 V.9-14 Ge et al. demonstrated the charge/discharge reaction mechanisms at the low voltage range and elucidated the corresponding high capacities beyond its theoretical limit.9 However, the extended voltage window is inevitably accompanied by a decomposition of liquid electrolyte that forms the solid-electrolyte interphase (SEI) films, a lower initial coulombic efficiency (ICE), and a poor cycling performance.15-17 Formation of the SEI layer is a function of various intrinsic and extrinsic variables such as electronic potential, current density, temperature, surface area, and etc.18-22 While introducing electrolyte additives are the most effective way to control chemistry of the SEI layer, protective surface coatings on electrode materials have been also extensively investigated to reduce the formation of the SEI layer.23,24 Since conventional operating voltage of Li4Ti5O12 (> 1 V vs. Li+/Li) is higher than that of the electrolyte decomposition, surface coating on Li4Ti5O12 has


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been mostly conducted in order to improve electronic conductivities of Li4Ti5O12 powder and the electrode.25,26 Indeed, Park et al. have shown that the SEI layer formed above 1 V vs. Li+/Li is reversibly decomposed during the following charge, so net capacity loss by the formation of the SEI layer is not significant.27 However, protective surface coating is necessary when the working voltage is lowered below 1 V vs. Li+/Li because robust inorganic components of the SEI layer are dominant at a low voltage region. In this work, we have introduced nickel oxide (NiOx) coating on Li4Ti5O12 particles in order to study electrochemical properties and surface stability in an extended voltage window (between 0.01 and 3 V vs. Li+/Li). There are several organic and inorganic chemical components in the SEI layer including Li2CO3, Li2C2O4, LiF, LiOH, Li2O, LiOCO2CH3, and LiOCO2C2H5.28 Most of the species form by reduction of the liquid electrolyte that involves complex reduction reactions with Li+ and e-. In this regard, our strategy is to minimize electron concentration on the surface, thereby reducing the formation of the SEI layer. NiOx conducts electricity via hole transports (p-type),29,30 so we have introduced a thin layer of NiOx to demonstrate that p-type coating materials can reduce irreversible Li+ loss at the anode and enhance the electrochemical performance of Li4Ti5O12.

EXPERIMENTAL SECTION Materials: Li4Ti5O12 powder, Ni(NO3)2.6H2O (≥97%), and NH4OH (30%) were purchased from Samchun Pure Chemical Company and Sigma-Aldrich Corporation, and they were used without further purification.


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Synthesis of the NiOx-coated Li4Ti5O12: Ni(NO3)2.6H2O (0.05 g (10 wt%; corresponded to 2 wt% after decomposition of precursor) and 0.10 g (20 wt%; corresponded to 4 wt% after decomposition of precursor)) was dissolved in 50 ml of distilled water (D. I. water). 0.5 g of Li4Ti5O12 was added to the solution and stirred for 2 h. After the stirring, 2 ml of ammonium hydroxide (NH4OH) was added to the solution at room temperature for 2 h. The final solution was centrifuged at 3,000 rpm for 10 min. The supernatant was discarded, and the precipitates were washed with D. I. water and dried at 60 oC overnight. The dried powder was heated from room temperature to 500 oC at the rate of 5 oC min-1 in the ambient atmosphere, and then maintained at that temperature for 3 h. The tube furnace was then allowed to cool to room temperature. Materials Characterization: The phase purity of the NiOx-coated Li4Ti5O12 samples was analyzed by powder X-ray diffraction (XRD, Rigaku, Ultima IV) with Cu K radiation. Powder morphologies of the NiOx-coated Li4Ti5O12 samples were characterized with field-emission scanning electron microscope (FE-SEM; JEOL, JSM-6700F, operated at 10 and 30 kV) and transmission electron microscope (TEM, JEOL, JEM-3010 electron microscope with an acceleration voltage of 300 kV). X-ray photoelectron spectroscopy (XPS) data was collected with PHI5000 VersaProbeTM equipment (ULVAC-PHI) with a monochromatic Al Kα (1486.6 eV) light source. Fourier transform infrared (FT-IR) spectroscopy was measured in ATR mode (Varian FTS 1000 FT-IR spectrometer) directly on the electrode surfaces after dimethyl carbonate (DMC) rinsing. For ex-situ XRD, XPS, and FT-IR tests, the electrodes in different states of charge and discharge were separated and further rinsed with DMC prior to the measurements. TGA (thermogravimetric analysis) has been done to observe the mass change of NiOx coated Li4Ti5O12 composites without calcination and Ni precursor during calcination. Each


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sample was scanned at a heating rate of 10 oC min-1 within an appropriated temperature range under ambient atmosphere. (PerkinElmer STA 6000) Electrochemical Measurements: Electrochemical measurements were carried out with CR2032-type coin cells using Li metal as the reference and counter electrodes. The working electrode was made by casting the anode slurries containing active material (90 wt%), acetylene black (3 wt%) as a conductive agent, and a PVDF binder (7 wt%) dissolved in N-methyl2pyrrolidone (NMP). This slurry was pasted onto a copper foil current collector and dried at 120 °C for 5 h under vacuum (10-3 Torr). Electrode discs with 12 mm diameter were punched, and the average loading density of the active materials in the NiOx-coated Li4Ti5O12 electrode is 4.4 mg cm-2. A porous polypropylene (PP) membrane was used as a separator, and coin half-cells were assembled in an Ar-filled glove box. A 1 M solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, v/v) was employed as an electrolyte. Galvanostatic chargedischarge cycling tests were performed between 0.01 and 3 V vs. Li+/Li at the rate of 0.1 C (0.1 C-rate corresponds to 17.5 mA g-1). All the voltage values in this paper refer to lithium half-cells. In situ XRD patterns were obtained at a beam line of 9A with a wavelength of 0.7653 Å at the Pohang Light Source facility using a Mar345 image plate detector. For easy comparison, 2θ values of all XRD patterns were recalculated and converted to the corresponding angles of λ=1.54 Å, which is the wavelength of a conventional X-ray tube source with a CuKα radiation.

RESULTS AND DISCUSSION Figures 1a and 1b show TEM images of the pristine Li4Ti5O12 and the NiOx-coated Li4Ti5O12 samples, respectively. Figure 1b shows the NiOx layer on the surface of Li4Ti5O12. NiOx forms a


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thin and uniform coating layer on Li4Ti5O12, and the thickness ranges from 5 to 10 nm. Thermogravimetric analysis (TGA) profiles are shown in Figure S2. Ni precursor was completely decomposed and removed around 350 oC and thusafter the residual Ni content, estimated by TGA, was approximately 24 wt%. Also, The NiOx coated Li4Ti5O12 composites without calcination underwent 6.4% weight loss at the same temperature. When two samples were compared, NiOx coating layer seems to correspond to roughly 2~3 wt% after decomposition of Ni precursor. Ni 2p XPS spectrum of the NiOx-coated Li4Ti5O12 is shown in the Figure 1c. First, it confirms the presence of the Ni element on the surface. Second, it shows that the oxidation state of Ni is 2+ or 3+, which is why NiOx (2

Controlling Solid-Electrolyte-Interphase Layer by Coating P-Type

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