Real-time XRD Studies of Li-O 2. Electrochemical Reaction in Nonaqueous. Lithium-Oxygen Battery

Transcription

1 Supporting Information Real-time XRD Studies of Li-O 2 Electrochemical Reaction in Nonaqueous Lithium-Oxygen Battery Hyunseob Lim, 1,2 Eda Yilmaz 1, and Hye Ryung Byon 1* 1 Byon Initiative Research Unit, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama , Japan 2 Department of Chemistry and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang , South Korea hrbyon@riken.jp 1

2 Experimental methods Materials: Super P carbon was received from TIMCAL, and nafion and lithium metal was purchased from ion power and Honjo, respectively. Trilayer of polymer membrane (PP/PE/PP, Celgard C480) was received from Celgard, and glassy-fiber separator (GF/D) was purchased from Whatman. Triglyme and DME were received from UBE, and PC and DMC were purchased from Kishida Chemical as the battery grade. Lithium bis(trifluoromethanesulfonyl)imide was purchased from Kanto Chemical. Redox titration for the measurement of Li 2 O 2 purity: The accurate purity of Li 2 O 2 powder (Aldrich. ~90%) was measured by potassium permanganate (KMnO 4 )-based redox titration. Li 2 O 2 dissolved DI water made a LiOH and H 2 O 2 like the following reaction. Li 2 O 2 + H 2 O (cold) LiOH + H 2 O 2 Sufficient amount of sulfuric acid were also added in the Li 2 O 2 solution, which inhibited MnO - 4 ion reduction to MnO 2 during the titration. By slow dropping of KMnO 4 aqueous solution to the Li 2 O 2 solution, the concentration of H 2 O 2 could be determined like the following two half-reactions. H 2 O 2 (aq) O 2 (g) + 2 H + (aq) +2 e (oxidation) MnO 4 (aq) + 8 H + (aq) + 5 e Mn 2+ (aq) + 4 H 2 O (l) (reduction) The endpoint of titration could be determined from the color change of Li 2 O 2 solution to be pink. Linear voltammogram measurement: The cathode electrodes were prepared by 1 wt% of lithium acetate (Wako Chemicals), lithium formate (Aldrich), or lithium carbonate (Nacalai Tesque) in Super P/nafion (molar ratio of 6/4). Li-O 2 battery cells were assembled with a glass-fiber separator and Li metal in addition to 0.5 M LiTFSI in triglyme in an Arfilled glove box. The measurement was performed in V vs. Li/Li + at 0.5 mv s -1 of scan rate using the potentiostat (WonATech). 1 H NMR spectroscopy: Fully discharged electrodes in Li-O 2 XRD cell were disassembled in an Ar-filled glove box, washed with acetonitrile, and dried in vacuum (60 o C for 2 h). The electrodes were immersed in D 2 O for 3 days. 1 H liquid NMR spectroscopy was performed using a Varian NMR system (500 MHz). 2

3 Evaluation of efficiency on Li-O 2 electrochemical reaction using the reference of Li 2 O 2 electrode For quantitative analysis of Li-O 2 efficiency using XRD, the reference of Li 2 O 2 electrodes was prepared by mixing of Super P, nafion, and Li 2 O 2 powder (Aldrich) at the mass ratio of 6/4/1. The mole number of Li 2 O 2 (N Li2O2 ) used in the electrode was estimated from the mass of Li 2 O 2 considering the measured purity (~92.3%) by KMnO 4 -based titration. Note that neither LiOH nor Li 2 CO 3 impurity XRD peak was observed in this Li 2 O 2 powder ground using a mortar and pestle for 10 min. The XRD pattern of the reference Li 2 O 2 electrode is shown in Figure S3. After deconvolution of the XRD pattern, the Li 2 O 2 (101) peak area (A Li2O2 ), showing the highest signal-to-noise ratio among the Li 2 O 2 peaks, is integrated by fitting of a Lorentzian function. The unit peak area of Li 2 O 2 (101) per 1 mole of Li 2 O 2 (da/dn (reference)) can be simply estimated by the following equation (eq. S1). da dn (reference)= A Li2O2 N Li2O2 (eq. S1) Considering a complete two-electron process of Li-O 2, the ideal mole number of Li 2 O 2 per time (dn/dt (ideal)) can be calculated by the following equation (eq. S2): dn dt (ideal)= I 2 F (eq. S2) where I is the supplied current ( A = C h -1 ), and F is the Faraday constant (96,485 C mol -1 ). Therefore, an ideal gradient of Li 2 O 2 (101) peak area per time (da/dt (ideal)) indicated as the ideal rate of Li 2 O 2 (101) formation/decomposition can be estimated from the following equation (eq. S3). da da dn (ideal)= (reference) (ideal) (eq. S3) dt dn dt In Figure S3, the fitted peak area of Li 2 O 2 (101) turns out to be ~27.0. Considering the mole number of Li 2 O 2, 2.20 µmol from 1.11 mg of the Li 2 O 2 reference-electrode mass, the ideal slope of Li 2 O 2 (101), i.e., da/dt (ideal), is ~24.2 h -1. The efficiency of Li-O 2 reaction can be evaluated from Li 2 O 2 (101) peak areas by the comparison of in-situ XRD measurement with the calculated reference value after full discharge. To be simplified, we first normalize Li 2 O 2 peak areas, as shown in Figure 3(a) and 5(b), by the following equations (eq. S4 and S5): 3

4 A normalized (t n )= A measured (t n ) 100 (eq. S4) A ideal (t n, fully discharged ) A ideal (t n, fully discharged )= da dt (ideal) t n, fully discharged (eq. S5) where A measured (t n ) is the measured value of Li 2 O 2 (101) peak area at discharge/recharge time on n-times of cycle and A ideal (t n,fully-discharged ) is the reference value of ideal Li 2 O 2 (101) peak area from eq. S3 at fully discharged time on n-times of cycle (i.e. ~ for 31.5 h of the first discharge and ~ for 11.5 h of the second discharge). The efficiency (%) of Li 2 O 2 formation is the normalized peak-area value at the fully discharged time in Figure 3(a) and 5(b), which can be also expressed as the following equation (eq. S6): Efficiency(%)= A normalized (t n, fully discharged )= A (t ) measured n, fully discharged 100 (eq. S6) A ideal (t n, fully discharged ) where A measured (t n,fully-discharged ) is the measured value of Li 2 O 2 (101) peak area at fully discharged time on n-times of cycle. The number of electrons on Li-O 2 electrochemical reaction The numbers of electrons used for formation/decomposition of a Li 2 O 2 (n e - /(+/ ) Li 2 O 2 ) can be estimated from the following equation (eq. S7): n A normalized = 2 A ideal (eq. S7) where A normalized / is every five point of time-dependent Li 2 O 2 (101) peak area and A ideal / n is time-dependent ideal Li 2 O 2 (101) peak area in Figure 3(a) and 5(b). The error bars The error bars (σ +,NOE,σ,NOE ) plotted in Figure 3(b) and Figure S7(b) are calculated based on standard errors of harmonic mean (σ H, A/ ) as considering the formation/decomposition rate of Li 2 O 2 ( A normalized /) from the following equations (eqs. S8, S9 and S10). σ H, A/ = E( A i ) 2 σ A/ n (eq. S8) σ +,NOE = 2 ( A ideal + A normalized A ideal + ( A normalized +σ H, A/ )) (eq. S9) σ,noe = 2 ( A ideal + A normalized A ideal + ( A normalized σ H, A/ )) (eq. S10) 4

5 Figure S1. Digital images of (a) components of Li-O 2 XRD cell and (b) assembled Li-O 2 XRD cell loaded on the parallel beam XRD instrument. 5

6 Figure S2. The behaviors of XRD peak areas in Li 2 O 2 (101), Li 2 O 2 (100), and the comparison of (101) with (100) (from left to right) during the first discharge and recharge. Linear increasing and non-linear decreasing patterns are similar as both Li 2 O 2 (101) and (100). The A and A max indicate XRD peak area and maximum peak-area. 6

7 Figure S3. XRD patterns of the reference Li 2 O 2 electrode (bottom) and of the 8 th scanned insitu cell (top, ~4 h later than the starting of the first discharge). The Li 2 O 2 (101) peak area from reference Li 2 O 2 electrode (green) was fitted by a Lorentzian function. The mass of Super P/nafion in the reference Li 2 O 2 electrode was almost identical to that of Super P/nafion electrodes. The assembly of reference Li 2 O 2 electrode on Li-O 2 XRD cell and the measurement condition of XRD were also exactly same as in-situ experiments. 7

8 Figure S4. 1 H liquid NMR spectroscopy of discharge byproducts in D 2 O extracted from the fully discharged electrode performed in 0.5 M LiTFSI in triglyme. 1 H chemical shift values (δ) of acetate (CH 3 CO 2 D) and formate (HCO 2 D) are 1.92, 8.46 ppm, respectively. 8

9 Figure S5. Linear voltammograms (from 2.5 to 5.0 V vs. Li/Li + ) of (a) Super P/nafion, (b) Li 2 O 2 (9 wt%) + Super P/nafion, (c) CH 3 CO 2 Li (1 wt%) + Super P/nafion, (d) HCO 2 Li (1 wt%) + Super P/nafion, and (e) Li 2 CO 3 (1 wt%) + Super P/nafion with 0.5 M LiTFSI of triglyme at 0.5 mv s -1 of scan rate. The cells were filled with Ar gas. The oxidation peaks are shown over 3.5 V vs. Li/Li + to all samples. The oxidation of triglyme occurs in V vs. Li/Li + but apparently severe over 4.75 V vs. Li/Li (a) (b) Current (ma) Current (ma) Current (ma) Potential (V vs. Li/Li + ) (c) (e) Potential (V vs. Li/Li + ) Current (ma) Potential (V vs. Li/Li + ) 0.4 (d) Potential (V vs. Li/Li + ) Current (ma) Potential (V vs. Li/Li + ) 9

10 Figure S6. Proposed electrochemical reaction scheme for the first discharge/recharge of Li- O 2 battery with Super P/nation cathode in ether-based electrolyte. 10

11 Figure S7. Efficiency of Li-O 2 electrochemical reaction using 0.5 M LiTFSI of DME on the first discharge/recharge evaluated from Li 2 O 2 peak areas of in-situ XRD patterns. (a) Timedependent Li 2 O 2 (101) peak area. The red dashed line indicates the ideal rate of Li 2 O 2 (101) formation/decomposition. (b) The values of number of electron used to formation/decompose a Li 2 O 2 (n e - /(+/ )Li 2 O 2 ) calculated from every five point of time-dependent Li 2 O 2 (101) peak areas in (a). The red dashed line guides for the ideal two-electron process of Li-O 2. The filled circle and empty square dot indicate Li 2 O 2 (101) formation and decomposition, respectively. 11

12 Figure S8. XRD graph of fully discharged cathode in 0.5 M LiTFSI of PC/DMC. 300 Intensity (cps) θ (degree) 12