[Supporting information]

Transcription

1 [Supporting information] Proof of ionic transport in interparticles of LiMPO 4 electrodes Kyu T. Lee, Wang H. Kan, Linda F. Nazar *. University of Waterloo, Department of Chemistry, Waterloo, Ontario, Canada N2L 3G1. lfnazar@uwaterloo.ca 1. Experimental. A. Synthesis. 1) Nano-LiMPO 4 and Nano-MPO 4 : LiFePO 4 crystallites, on average nm in dimension, were prepared by a polyol process. Specifically, M Fe-(CH 3 COO) 2, NH 4 H 2 PO 4, and Li-CH 3 COO in tetraethylene glycol were mixed in a stoichiometric molar ratio (1:1:1). The mixture solution was heated at 320 o C for 20 hours under a N 2 atmosphere. The products were centrifuged, washed several times with acetone and dried under vacuum at 160 o C for 1 day. Nano-LiFePO 4 was chemically delithiated to form Nano-FePO 4 using 1M NOBF 4 in acetonitrile. LiMnPO 4 crystallites, on average nm in dimension, were also prepared by the polyol process. Li-CH 3 COO, Mn-(CH 3 COO) 2 4H 2 O and H 3 PO 4 were used for the synthesis of LiMnPO 4, utilizing the same conditions as for LiFePO 4 described above. Nano-LiMnPO 4 was completely chemically delithiated into Nano-MnPO 4 using 1M NO 2 BF 4 in acetonitrile in a slight stoichiometric excess. 1

2 2) Bulk-LiMPO 4 (M = Fe, Mn) LiFePO 4 crystallites (ca. 500 nm in dimension) were synthesized by a conventional solid-state method. Li 2 CO 3, FeC 2 O 4 2H 2 O and NH 4 H 2 PO 4 were ball-milled in a substoichiometric molar ratio (1.3:1:1.1). The powders were heated at 350 o C for 5h under an N 2 atmosphere, and then at 600 o C for 10h under an N 2 atmosphere, resulting in composites composed of two phases: LiFePO Li 3 PO 4. The powders were treated with acetic acid to dissolve the phase-separated Li 3 PO 4 to create a pure LiFePO 4 phase. LiMnPO 4 powders with a size of ca. 500 nm were also synthesized by a conventional solid-state method. Li 2 CO 3, MnC 2 O 4 2H 2 O and NH 4 H 2 PO 4 were used for the synthesis of LiMnPO 4 utilizing the same conditions as for Bulk-LiFePO 4. B. Characterization The morphology of LiMPO 4 nanorods were examined by transmission electron microscopy (TEM) using a Philips CM12 microscope. For Bulk-LiMPO 4, scanning electron microscopy (SEM) was performed on a LEO 1530 field emission SEM instrument. X-ray diffraction patterns were collected on a D8-ADVANCE powder X-ray diffractometer operating at 40 kv and 30 ma and employing Cu-K α radiation (λ= nm). C. Electrochemistry Electrodes were prepared, comprised of 60 wt% LiMPO 4, 30 wt% Super-S carbon and 10 wt% Poly(tetrafluoroethylene) (PTFE) binder. The cathode materials were coated onto a carbon-modified aluminum current collector. The electrolyte was composed of a 1.0 M LiPF 6 solution in a 1:1 ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC). Lithium metal was used as the counter electrode. A current of C/10 (corresponding to a discharge (or charge) of 1 Li in 10 hrs) was used to 2

3 carry out the galvanostatic cycling between 2.5 and 4.2 V vs. Li/Li +.. For the ex-situ XRD analysis of the electrodes, the electrodes were washed with DMC, and then dried under vacuum. D. Calculation of electrode composition The dramatic increase of the FWHM of the LiFePO 4 phase on electrochemical equilibration provides proof of the process shown in Fig. 3 and as described in the text. Otherwise, the FWHM of the LiFePO 4 phase in the electrode would be similar before and after equilibrium, since only 3 mol% Bulk- LiFePO 4 delithiates into Bulk-FePO 4 in the initial process, resulting in 0.97 Bulk-LiFePO 4 and 0.03 Bulk-FePO 4 [8.8 at.% solid-solution region of Nano-Li α FePO 4 (Fig. 1a) 1/3 wt. ratio of FePO 4 /LiFePO 4 in the electrode]. 3

4 Figure S1. XRD patterns of Bulk-LiMPO 4 synthesized by a conventional solid-state method with a substoichiometric molar ratio. (a) LiFePO 4 and (b) LiMnPO 4. (asterisk: Li 3 PO 4 ) 4

5 Figure S2. SEM image of LiFePO 4 synthesized by a conventional solid-state method with a stoichiometric molar ratio. 5

6 Figure S3. XRD patterns of Nano-MPO 4 (M: Fe, Mn) prepared by chemical delithiation. 6

7 Figure S4. XRD patterns of the mixed-crystallite electrodes of [Bulk-LiMPO 4 + Nano-MPO 4 ], before and after the electrochemical equilibrium process. (a) M = Fe and (b) M = Mn. 7