Single-crystalline LiFePO 4 Nanosheets for High-rate Li-ion Batteries

Transcription

1 /8 SUPPORTING INFORMATION Single-crystalline LiFePO 4 Nanosheets for High-rate Li-ion Batteries Yu Zhao, Lele Peng, Borui Liu, Guihua Yu* Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 7872, USA. *Author to whom correspondence should be addressed: ghyu@austin.utexas.edu These authors contributed equally.

2 2/8 SUPPORTING INFORMATION Methods. Preparation of bulk LiFePO 4 powders. The preparation of bulk LiFePO 4 powders was carried out under solvothermal reaction with diethylene glycol as solvent. Firstly, FeCl 2 4H 2 O (3 mmol, Fisher Scientific) and LiCl (3 mmol, Acros) were dissolved in diethylene glycol (20 ml, Acros) with magnetic stirring to form light green transparent solution. Afterwards, LiH 2 PO 4 (3 mmol, Fisher Scientific) was dissolved in deionized water (0.5 ml) and slowly added into diethylene glycol with magnetic stirring until greyish green homogenous suspension formed. The preparation of the precursor solution was carried out under Ar atmosphere with the presence of traces of L-ascorbic acid (Acros, 5% with respect to the weight of FeCl 2 4H 2 O) to avoid the oxidation of Fe 2+. Then, the precursor solution was transferred into a glass vial which was directly put into the Teflon pot. Finally, the Teflon pot was put into the stainless steel autoclave and kept at 200 C for sufficient time. Typically, solvothermal treatment for 24 hours yields light greenish white, phase pure bulk LiFePO 4 powders. Preparation of LiFePO 4 nanosheets and carbon coating. Single-crystalline LiFePO 4 nanosheets could be readily obtained by the exfoliation of bulk LiFePO 4 powders. In a typical procedure, bulk LiFePO 4 powders ( g) and 2-propanol (0 ml, Aldrich) were mixed and exposed to ultrasonication. The white suspension was collected by centrifuging, and dried in vacuum. The carbon coating was achieved using polyethylene oxide (Mw. 600,000, Aldrich) as carbon source. In a typical procedure, polyethylene oxide (0% in weight ratio), the as-prepared LiFePO 4 nanosheets and proper amount of deionized water were mechanically mixed in Thinky centrifugal mixer (ARE-30) for 30 min, dried in vacuum, and annealed in Ar/H 2 (90/0, v/v) atmosphere at 650 C for 2 hours. Cell assembly. The electrode was prepared by mixing the as-prepared carbon-coated LiFePO 4 nanosheets, Super P carbon (Timcal Graphite & Carbon), and polyvinylidene fluoride binder (Fisher Scientific) with a weight ratio of 80: 5: 5 in N-methyl-2-pyrrolidone (anhydrous, Fisher Scientific). The mixture was thoroughly mixed in Thinky centrifugal mixer to form the homogenous slurry, which was then casted on aluminium foil and dried in vacuum at 5 C for 2 hrs. The electrochemical performance was evaluated with a standard CR2032 coin cell using metallic lithium as the anode, Celgard 2500 as the separator, and LiPF 6 in : ethylene carbonate and diethyl carbonate (BASF Corp.) as the electrolyte. Characterizations. X-ray powder diffraction patterns were performed on a Philips X-ray diffractometer (APD 3520) equipped with Cu Kα radiation. SEM and TEM observations were carried out on Hitachi S scanning electron microspcope (S-5500) and JEOL transmission electron microscope (200F), respectively. Raman and FTIR measurements were performed on Renishaw Raman system equipped with a 633 nm argon ion laser and Thermal Scientific FT-IR spectrometer (Nicolet TM is TM 5), respectively. BET surface area was tested with a NOVA surface area analyzer (Quantachrome Instruments NOVA4000). Electrochemical characterization was performed on LANHE battery cycler (CT200A) and Bio-logic potentiostat (VMP3) equipped with impedance modules. To measure the tap density, a certain amount of the carbon-coated LiFePO 4 nanosheets were placed in a small measuring cylinder and tapped for 0 min by hand. The measured volume of the tapped powder and its mass were used to calculate the tap density of the product.

3 3/8 SUPPORTING INFORMATION S. Effect of reaction time on the phase structure of the LiFePO 4 powders Intensity / a.u. 8 hrs 6 hrs 24 hrs 48 hrs Theta / degree Figure S. XRD patterns of the LiFePO 4 powders obtained at different reaction time: 8 hrs (black), 6 hrs (red), 24 hrs (blue), and 48 hrs (purple). Black dash lines represent the major characteristic diffraction differences of α and β LiFePO 4. All diffraction peaks can be assigned to β LiFePO 4 for reaction time of 8 hrs, and gradually transformed into α LiFePO 4 for reaction time in excess of 6 hrs. S2. TEM image of the carbon-coated LiFePO 4 nanosheets Figure S2. TEM image of carbon-coated LiFePO 4 nanosheets, showing the carbon layer thickness ~ 5 nm.

4 4/8 SUPPORTING INFORMATION S3. Morphology information of commercial LiFePO 4 /C powder a b 2 um 500 nm Figure S3. (a) Low- and (b) high-magnification SEM image of commercial LiFePO 4 /C powders revealing that the particles are in irregular shapes and the particle size varies in the range of tens to hundreds of nanometers. S4. Impedance spectra of LiFePO 4 nanosheets and commercial LiFePO 4 /C powders c 0 Z Im / ohm c r el r 0 W r 2 Z W 00 0 Nanosheets Commercial powder Z Re / ohm Figure S4. Nyquist plots of LiFePO 4 nanosheets/li metal (black) and commercial LiFePO 4 /Li metal (red) half-cells measured in the frequency region of Hz. Inset: simplified-contact-randles equivalent circuit for the simulation of the Nyquist plots.

5 5/8 SUPPORTING INFORMATION S5. Electric conductivity & lithium ion diffusion coefficient at open circuit state The electric conductivity of the cell could be determined by the intersection at high frequency with Z RE axis in the Nyquist plots shown in Figure S4. The Li-ion diffusion coefficient at open circuit state could be determined from the slope of linear response of Z Re and ω /2 at low frequency region by Eq. () and Eq. (2): D = R 2 T 2 /2A 2 n 4 F 4 C 2 σ 2 () Z Re = K + σω /2 (2) where D is the diffusion coefficient (cm 2 s ), R is the gas constant (8.3 J mol K ), T is the absolute temperature (298 K), A is the surface area of the cathode (.3 cm 2, Figure S5), n is the number of electrons transferred in the half-reaction for the redox couple (for LiFePO 4, n=), F is the Faraday constant (96485 C mol ), C is the is the molar concentration of Li ions in LiFePO 4 ( mol cm 3 ), K is a constant, ω is frequency, and σ is the Warburg factor which corresponds to the slope of the curve shown in Figure 3a. Figure S5. Nitrogen adsorption isotherms of LiFePO 4 nanosheets. The surface area was measured to be m 2 g. S6. Lithium ion diffusion coefficient during charge/discharge Randles-Sevcik equation, Eq. (3), was used to calculate the lithium ion diffusion coefficient during charge/discharge: I pc = n 3/2 AD /2 Cυ /2 (3) where I pc is the peak-current (A), n is the number of electrons in the charge-transfer step (for LiFePO 4, n = ), A is the total surface area of the electroactive material calculated from the mass loading of LiFePO 4 nanosheets and their corresponding specific surface area (cm 2 ), D is the Li ion diffusion coefficient in LiFePO 4 at 298 K (cm 2 s ), C is the molar concentration of Li ions in LiFePO 4 ( mol cm 3 ), and ν is the scan rate (V s ).

6 6/8 SUPPORTING INFORMATION S7. Rate capability of commercial LiFePO 4 /C powders Specific capacity / mahg C 5C charge discharge Coulombic efficiency 0C Figure S6. Rate capability of commercial LiFePO 4 /C powders at the C-rate ranging from 30 C (C=70 ma g ). 20C Cycle number 30C C Coulombic efficiency / % S8. Representative charge/discharge profiles at various current rate a Potential / V vs Li + /Li C, 20C, 0C, 5C, C, 0.5C Specific capacity / mah g Figure S7. Typical charge/discharge profiles of (a) LiFePO 4 nanosheets, and (b) commercial LiFePO 4 /C powders at current rate ranging from C. b Potential / V vs Li + /Li C, 20C, 0C, 5C, C, 0.5C Specific capacity / mah g

7 7/8 SUPPORTING INFORMATION S9. Cyclic performance of commercial LiFePO 4 /C electrode Specific capacity / mah g charge discharge Coulombic efficiency Cycle number Figure S8. Cyclic performance and corresponding Columbic efficiency of commercial LiFePO 4 /C electrode measured at C-rate of 5. The capacity retention is 84% after 500 cycles based on the capacity of the first and last discharge capacities Coulombic efficiency / %

8 8/8 SUPPORTING INFORMATION S0. Determine the carbon content in the carbon-coated LiFePO 4 nanosheets The LiFePO 4 nanosheets with or without carbon coating are very stable during the TGA experiment (Figure S) when the temperature is below 250 C. The slight weight loss in this temperature range is probably due to the loss of adsorbed water. LiFePO 4 is gradually oxidized with two steps of weight gain between 300 C and 550 C. The obtained product is very stable thereafter. Although the mechanisms occurring at the two steps are not yet fully known, the completely oxidized products of LiFePO 4 are composed of two phases: Li 3 Fe 2 (PO 4 ) 3 and Fe 2 O 3. Therefore, the following oxidative processes for LiFePO 4 during TGA experiment could be proposed according to the following Eq. (4): LiFePO 4 + O2 Li3Fe2 ( PO4 ) 3 + Fe2O3 (4) To determine the carbon content in the LiFePO 4 nanosheets, C-coated LiFePO 4 is oxidized as in the case of LiFePO 4, with carbon being mixed with Li 3 Fe 2 (PO 4 ) 3 and Fe 2 O 3 formed during the oxidation process as represented by Eq. (5): LiFePO 4 / C + O2 Li3Fe2 ( PO4 ) 3 + Fe2O3 + C (5) When heating the LiFePO 4 /C sample to above 550 C, the carbon is oxidized to CO 2. As a consequence, a weight loss associated with the oxidation of carbon takes place in the TG curve according to Eq. (6): LiFePO 4 / C + O2 + no2 Li3Fe2 ( PO4 ) 3 + Fe2O3 + nco2 (5) Above 550 C, total oxidation of both LiFePO 4 and carbon is complete and the overall weight gain in this case is about 3.2% for C-coated LiFePO 4 nanosheets and 5.% for the as-exfoliated LiFePO 4 nanosheets. The carbon content was calculated to be ~.9 wt% based on the weight gain according to Eq. (7): %carbon = weight gain of as-exfoliated LiFePO 4 weight gain of C-coated LiFePO 4 (7) 06 Weight gain / % % 3.2% C coated LiFePO 4 nanosheets 98 LiFePO 4 nanosheets Temperature / C Figure S9. TGA curves of as-exfoliated LiFePO 4 nanosheets (red) and LiFePO 4 nanosheets after carboncoating (blue).