Supplementary Information Comparative study on Na 2 MnPO 4 F and Li 2 MnPO 4 F for rechargeable battery cathodes Sung-Wook Kim a,b, Dong-Hwa Seo a, Hyung-Sub Kim c, Kyu-Young Park d, and Kisuk Kang a,b* a Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea b Research Institute of Advanced Materials, Seoul National University, Seoul 151-742, Republic of Korea c Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea d Graduate School of EEWS, KAIST, Daejeon 305-701, Republic of Korea * Corresponding author: Prof. K. Kang e-mail: matlgen1@snu.ac.kr, tel: 82-2-880-7088 1
S1. Diffusion modeling using first principles calculations of Na 2 MnPO 4 F and Li 2 MnPO 4 F First principles calculations were performed using Vienna ab-initio simulation package program with the spin-polarized GGA+U calculation using the plane-wave basis set and the projector-augmented-wave potential (Anisimov et al., Phys. Rev. B 1991, 44, 943-954. & Zhou et al., Phys. Rev. B 2004, 70, 235121). The onsite coulomb term, U, and the exchange term, J, of Mn was 5.5 ev and 1 ev, respectively, which was widely used in Mn-based polyanion compounds (Seo et al., Chem. Mater. 2010, 22, 518-523. & Seo et al., Phys. Rev. B 2011, 83, 205127). a 2b c super-cell with 16 formula units of Na x MnPO 4 F and Li x MnPO 4 F (x = 0, 1, 2) was calculated. The plain-wave basis with a kinetic energy cut-off of 500 ev and gamma K-point was used to ensure that total energy is converged to 2 mev per one formula unit. Nudged elastic band (NEB) method was used to verify the activation barrier for Na and Li ion diffusion (Seo et al., Chem. Mater. 2010, 22, 518-523. & Seo et al., Phys. Rev. B 2011, 83, 205127). One Na (or Li) ion diffusion was modeled in the super cell of Na(or Li) 31/16 MnPO 4 F at various diffusion paths. Five images of Na (or Li) ions were used to trace the diffusion paths.neb calculation of Na 2 MnPO 4 F was carried out using a 2b c super-cell with 16 formula units. One of Na ion among 32 Na ions was removed from the super-cell to form vacancy site and the Na ion diffusion from the neighbor Na ion site to the vacancy site was considered. Na 2 MnPO 4 F has 4 different Na sites as shown in Figure S1a. The Na ion diffusion through Na1-Na4 chain, Na2-Na3 chain, and inter-chain was modeled. 7 diffusion paths were calculated in the single chain diffusion (Na1-Na4 and Na2-Na3 chain) as shown in Figure S1b and c, and 18 diffusion paths were calculated in the interchain diffusion as shown in Figure S1d. 5 trace images for Na ion diffusion were 2
created with linear interpolation between the initial and final states of the path and corresponding energy were calculated with NEB method. Energy from initial to final states during Na ion diffusion in NEB calculation was summarized in Table S1a. At each path, maximum energy difference among all Na ion positions was regarded as the activation barrier for the Na ion diffusion. For example, activation barrier in b1 path is 486 mev which is energy difference between image0 and image3 states. Among various diffusion paths under consideration, (i) b1, b2, b5, and b7 in Na2-Na3 chain, (ii) c2, c3, c4, and c6 in Na1-Na4 chain, and (iii) d2, d4, d8, and d10 in inter-chain seem to be possible in Na ion diffusion due to their low activation barriers (< 650 mev). Na ions can move to b-direction along the F ion backbone in Na2-Na3 and Na1-Na4 chains and, in addition, they can move from a chain to another chain through inter-chain diffusion paths. Na ions in Na 2 MnPO 4 F were substituted to Li ions and the structure was relaxed to obtain Li 2 MnPO 4 F for the NEB calculation. NEB calculation on Li 2 MnPO 4 F was also carried out with the same diffusion model of Na 2 MnPO 4 F. Table S1b summarizes the calculated results on Li 2 MnPO 4 F. Li 2 MnPO 4 F had lower activation barrier compared to Na 2 MnPO 4 F. This might be due to small ionic size of the Li ions compared to the Na ions. As the activation barriers got lowered, surprising behavior in the b-direction diffusion was identified. The b-direction diffusion of Na ions along the F ion backbone was possible in Na 2 MnPO 4 F, and additional diffusion paths of Li ions along the b- direction across the F ion backbone was possible in Li 2 MnPO 4 F (b3, b4, b6, c1, c5, c7). It means that Li ion diffusion is much easier than Na ion diffusion in A 2 MnPO 4 F framework. Inter-chain diffusion paths in Na 2 MnPO 4 F and Li 2 MnPO 4 F were identical. 3
Figure S1. (a) Occupation sites of Na ions in Na 2 MnPO 4 F and (b-d) calculated Na ion diffusion path through (b) Na2-Na3 chain, (c) Na1-Na4 chain, and (d) inter-chain. Solid arrows indicate the possible diffusion paths with reasonable activation barrier (< 650 mev) and dotted arrows indicate the impossible diffusion paths with too high activation barrier (> 1eV). 4
Table S1a. Calculated energy by NEB in Na 2 MnPO 4 F. Energy difference to the initial state was tabulated for convenience. Path notations are corresponding to Figure S1b-d. 5
Table S1b. Calculated energy by NEB in Li 2 MnPO 4 F. Energy difference to the initial state was tabulated for convenience. Path notations are identical to those used in Figure S1b-d and Table S1 in Li 2 MnPO 4 F structure. 6
S2: Synthesis and characterization of Na 2 MnPO 4 F and Li 2 MnPO 4 F Na 2 MnPO 4 F was synthesized by conventional solid state reaction. Stoichiometric amount of Na 2 CO 3 (Aldrich, 99%), NaF (Aldrich, 99%), Mn(C 2 O 4 ) 2H 2 O (Alfa aesar, Mn 30%), and NH 4 H 2 PO 4 (Aldrich, 98%) were homogeneously mixed with planetary ball-mill (Fritsch, Pulverisette7) at 500 RPM for 12 h with 10 wt.% of pyromellitic acid hydrate (Alfa aesar, 96%) as an organic additive. The mixture was heated to 300 C for 2 h in ambient Ar for the precursor decomposition. The heated mixture was grounded, pelletized, and then heated again to 600 C for 12 h in ambient Ar to obtain Na 2 MnPO 4 F phase. Li 2 MnPO 4 F phase was obtained by exchanging Na ions to Li ions in Na 2 MnPO 4 F. Na 2 MnPO 4 F was introduced into 1M LiBr (Aldrich, 99%) in 1-hexanol (Aldrich, 99%) with Na/Li ratio of 1/10, and the mixed solution was aged for 2 days at 100 C with stirring at 200 RPM in a refluxing system. The ion-exchanged Li 2 MnPO 4 F was washed with 1-hexanol, ethanol, and acetone for several times, and dried. Crystallinity of the synthesized powders was determined by X-ray diffraction (XRD, Bruker, New D8 Advanced) with Cu-Kα radiation operated at 40 ma and 40 V. Rietveld refinement was performed to evaluate lattice parameters using full pattern matching mode in Fullprof program (Rietveld, J. Appl. Crystallogr. 1969, 2, 65-71). Particle morphology was identified by scanning electron microscope (Carl Zeiss, SUPRA 55VP) and transmission electron microscope (JEOL, JEM-3000F). Residual carbon content was identified by carbon/surfer determinator (ELTRA, CS-800) and Na/Li ratio after the ion-exchange was identified by inductively coupled plasma-mass spectroscope (HP4500). Test electrode for characterizing electrochemical property was prepared by casting and drying the homogeneous slurry composed of 70 wt.% of Na 2 MnPO 4 F (or Li 2 MnPO 4 F), 20 wt.% of carbon black (Super P), and 10 wt.% of polyvinylidene 7
fluoride binder dissolved in adequate amount of N-methyl pyrrolidone (Aldrich, 99.5%) onto Al foil. Cell performance of Na 2 MnPO 4 F was measured in Na-cell using Swagelok-type cell composed of the test electrode, Na metal counter electrode, polymer membrane separator (Celgard 2400), and 1M NaClO 4 (Aldrich, 98%) in propylene carbonate (Aldrich, 99.7%) as an electrolyte. Cell performance of Li 2 MnPO 4 F was measured in Li-cell using CR2016 coin-cell. Li metal and 1M LiPF 6 in 1:1 (v/v) mixture of ethylene carbonate and dimethyl carbonate (Techno Semichem) were used as the counter electrode and the electrolyte, respectively. The test cells were operated by battery cycler (WonA Tech, WBCS 3000) at room temperature with a current rate of 10 ma g -1. Direct synthesis of Li 2 MnPO 4 F (using Li 2 CO 3 (Aldrich, 99%) and LiF (Aldrich, 99%) and LiNaMnPO 4 F (using Li 2 CO 3 and NaF) was also tried using the same solid-state reaction for Na 2 MnPO 4 F. Figure S3 shows the XRD patterns of both Li 2 MnPO 4 F and LiNaMnPO 4 F. Figure S2. XRD patterns of solid-state synthesized Li 2 MnPO 4 F (black) and LiNaMnPO 4 F (red). Pink reverse triangle, blue diamond, and green open-circle correspond to the peak position of LiF, LiMnPO 4, and Na 2 MnPO 4 F, respectively. 8
S3: Structure prediction of Li 2 MnPO 4 F To predict crystal structure of Li 2 MnPO 4 F, three crystal structures reported on A 2 MPO 4 F framework (Pbcn, Pnma, and P21/n) were considered. Crystallographic information of Pbcn, Pnma, and P2 1 /n structures was obtained from Na 2 FePO 4 F (Ellis et al., Chem. Mater. 2010, 22, 1059-1070), Li 2 NiPO 4 F (Dutreilh et al., J. Solid State Chem. 1999, 142, 1-5), and Na 2 MnPO 4 F (Yakubovich et al., Acta Crystallogr. C 1997, 53, 395-397) respectively. All of Na ions and transition metal ions were substituted to Li ions and Mn ions, respectively, to obtain crystallographic information for Li 2 MnPO 4 F for the first principles calculations. Structure relaxation was carried out using the GGA+U with the same condition described in S1. Calculated energy of Li 2 MnPO 4 F obtained from Pbcn, Pnma, and P2 1 /n structure was approximately -59.350, -59.540, and -59.556 ev per one formula unit, respectively. It means that P2 1 /n structure is the most stable among the three structures, and this well agrees with experimental result. Calculated crystal structures are illustrated in Figure S3 and calculated XRD pattern are shown in Figure 3b in the manuscript. Figure S3. Illustrations of Li 2 MnPO 4 F structures calculated from (a) Pbcn, (b) Pnma, and (c) P2 1 /n space group. 9
S4: Lattice parameter evaluation of Na 2 MnPO 4 F and Li 2 MnPO 4 F Lattice parameters of synthesized Na 2 MnPO 4 F and Li 2 MnPO 4 F were evaluated by using full pattern matching technique of Rietveld refinement with Fullprof program as shown in Figure S4. Figure S4. Rietveld refinement of (a) Na 2 MnPO 4 F and (b) Li 2 MnPO 4 F in full pattern matching mode. 10
S5: Ion-exchange of Na 2 MnPO 4 F at different conditions Ion-exchange of Na 2 MnPO 4 F was performed in the LiBr solution as described in the S2. Figure S5 shows the XRD patterns of Na 2 MnPO 4 F before and after the ion-exchange reaction. When aged at 80 C for 5 days, only small amount of Na ions was exchanged to Li ions and no significant change in XRD patterns was observed. Na/Li ratio was approximately 3/1, indicating that Li 0.5 Na 1.5 MnPO 4 F was obtained. However, when aged at 170 C for 2 days, original structure of Na 2 MnPO 4 F completely disappeared. The newly appeared patterns were not comparable to those of calculated XRD patterns of Li 2 MnPO 4 F shown in Figure 3b in the manuscript, and, even more, LiF phase was identified. Other XRD peaks were not clearly identified, but XRD peaks of various LiMn x O y, Mn x O y, and Mn x P y compounds were partially overlapped. This implies that the ion-exchange reaction of Na 2 MnPO 4 F was not occurred but chemical reaction occurred. It is speculated that Li ions in the solution attacked Na 2 MnPO 4 F to decomposed Na 2 MnPO 4 F forming LiF and other compounds at this harsh condition. Figure S5. XRD patterns of Na 2 MnPO 4 F before (black) and after the ion-exchange at 80 C for 5 days (red) and at 170 C for 2 days (blue). 11
S6: Average voltage evaluation in Na 2 MnPO 4 F and Li 2 MnPO 4 F Average voltage is strongly related with the energy of ground state relaxed by the first principles calculations. When electrochemical reaction between AMX and MX + A takes place, average voltage, V, can be evaluated by following equation: V = ( E(AMX) E(MX) E(A))/n, (eq. S6) where E is the calculated energy in ev unit, n is the number of electrons participated in the reaction. While calculating the energy at every state of charge in large super cell of Na 2 MnPO 4 F is the most promising, it is not suitable due to limitation of computation resources. For simple calculation, only Na 2 MnPO 4 F, NaMnPO 4 F, and MnPO 4 F phases were handled. The energy of Na 2 MnPO 4 F was obtained by relaxing the reported crystal structure (Yakubovich et al., Acta Crystallogr. C 1997, 53, 395-397) The relaxed energy was approximately -58.005 ev per one formula unit. Since Na 2 MnPO 4 F has the 4 Na ions occupation sites, 6 NaMnPO 4 F structures are possible: Na1-Na2, Na1-Na3, Na1- Na4, Na2-Na3, Na2-Na4, and Na3-Na4 occupied structures. The six structures were fabricated by removing adequate Na ions from the relaxed Na 2 MnPO 4 F structure and then they were relaxed. Among them, Na3-Na4 occupied NaMnPO 4 F had the lowest energy (-53.049 ev per one formula unit). MnPO 4 F structure was fabricated by removing all Na ions from the relaxed NaMnPO 4 F structure. The relaxed energy of MnPO 4 F was -47.055 ev per one formula unit. The energies of Li 2 MnPO 4 F and LiMnPO 4 F were obtained with the same method (-59.556 and -53.802 ev per one formula unit in Li 2 MnPO 4 F and LiMnPO 4 F, respectively). The energies of Li and Na were approximately -1.3 and -1.9 ev per one formula unit calculated in body-centeredcubic structures. By using the equation S6, average voltage could be simply obtained and the results are summarized in Table 2 in the manuscript. 12
S7: High voltage operation of Na 2 MnPO 4 F and Li 2 MnPO 4 F To confirm the more than one Na and Li ions transfer in Na 2 MnPO 4 F and Li 2 MnPO 4 F, respectively, high voltage operation was performed (< 4.8 V vs. Na + /Na for Na 2 MnPO 4 F and < 5.3 V vs. Li + /Li for Li 2 MnPO 4 F). Significant electrolyte decomposition took place near 4.7 V vs. Na + /Na in Na 2 MnPO 4 F (Figure S7a). Charge capacity of approximately 200 mah g -1 was achieved prior to the electrolyte decomposition, indicating that more than one Na ions were extracted (~1.6 Na). Li 2 MnPO 4 F also exhibited larger specific capacity shown in Figure S7b compared to the moderate voltage operation shown in Figure 4(b) in the manuscript. The initial charge capacity was approximately 220 mah g -1, which is corresponded to 1.6 Li ion extraction. The conclusive evidence of the electrolyte decomposition was not observed in the charge-discharge profiles, but differential capacity curve on the first charge shown in inset of Figure S7b clearly shows that the electrolyte decomposition occurred. Figure S7. Charge-discharge profiles of (a) Na 2 MnPO 4 F in Na-cell (1-4.8 V) and (b) Li 2 MnPO 4 F in Li-cell (1-5.3 V) at a current rate of 10 ma g -1 (inset: differential capacity curve on the first charge). Red rectangles indicate that electrolyte was decomposed at high voltage. 13