Communication pubs.acs.org/cm
Facile Synthesis of the High-Pressure Polymorph of Nanocrystalline LiFePO4 at Ambient Pressure and Low Temperature Benjamin Voß, Jörg Nordmann, Alexander Kockmann, Jennifer Piezonka, and Markus Haase* Institute of Chemistry, University of Osnabrueck, Barbarastr. 7, 49076 Osnabrueck
Dereje H. Taffa and Lorenz Walder* Institute of Chemistry, University of Osnabrueck, Barbarastr. 7, 49076 Osnabrueck S Supporting Information *
KEYWORDS: LiFePO4, synthetic methods, lithium, high-pressure chemistry, intercalations
O
livine phase LiFePO4 is used as cathode material in Li-ion cells owing to its high storage capacity (170 mAh g−1), low cost, low toxicity, and high redox potential of 3.5 V versus Li+/Li.1 When olivine phase LiFePO4 is electrochemically delithiated, FePO4 in the heterosite phase is obtained, and the respective lithium intercalation/deintercalation reaction has been thoroughly investigated by several groups.2 It is generally accepted that powders consisting of grains in the nanometer size regime show improved electrochemical properties as the mean diffusion length of the lithium ions is shorter in the nanocrystalline material.3,4 FePO4 is known to crystallize in at least five different polymorphs, that is, the orthorhombic heterosite phase (olivine phase) which converts to the trigonal berlinite phase on heating, the FePO 4 high-pressure phase having CrVO4 structure (space group Cmcm, No. 63), and two more recently discovered phases (monoclinic and orthorhombic) obtained by the dehydration of phosphosiderite and strengite (i.e., two modifications of FePO4·2H2O), respectively.5,6 The high-pressure phase and the new monoclinic and orthorhombic phases are reported to show intermediate electrochemical activity with respect to lithium intercalation.6,7 Heterosite is known to be the electrochemically most active phase whereas berlinite is the least active material. When the berlinite phase is exposed to pressures exceeding 12 000 bar at room temperature, however, the mean size of the crystalline domains is reduced from 120 to 16 nm, and this decreased particle size improved the electrode performance of the material.8 At pressures above 25 000 bar, the FePO4 highpressure phase of CrVO4 type starts to form, along with a second phase of amorphous nature.9 Pure CrVO4 type FePO4 is obtained by applying high temperatures and pressures, that is, by treating berlinite or the monoclinic phase of FePO4 at 800 to 900 °C and 20 000 to 60 000 bar.5,7,10,11 Electrochemical Li insertion into the high-pressure phase of FePO4 yields the Cmcm phase of LiFePO4.7 The latter material has also been prepared directly by solid-state synthesis at pressures of 65 000 bar and temperatures of 900 °C. This route yields particle sizes in the range of several micrometers, which show current densities below those achieved with olivine phase LiFePO4.12 © 2012 American Chemical Society
In this communication, we report on the formation of nanocrystalline Cmcm phase LiFePO4 in a simple liquid-phase synthesis performed at 150 °C and ambient pressure. The synthesis procedure is similar to the one used earlier for the preparation of lanthanide phosphate nanoparticles.13−15 A total of 1.988 g (10 mmol) of FeCl2·4H2O and 0.466 g (11 mmol) of LiCl were dissolved in 10 mL of methanol, and the solution was combined with 30 mL diphenylether and 50 mmol of tri-noctylphosphine oxide. The methanol was removed at a rotavap at 40 °C. Subsequently, the water content was further reduced by heating the solution to 100 °C under vacuum for 1 h at a Schlenk-line. Thereafter, the solution was cooled and kept under nitrogen at a temperature of 50 °C. A phosphoric acid containing solution was prepared by combining 1.96 g (20 mmol) of phosphoric acid with 27 mL (80 mmol) of tri-nhexylamine and 10 mL of dihexylether and by evacuating the mixture at a rotavap at 50 °C for 1 h to remove residual water from the phosphoric acid. Finally, both solutions were combined and heated up to 150 °C under nitrogen for 1 h. During this time a precipitate is formed which was separated at room temperature by centrifugation. After washing several times with toluene and methanol, the resulting green-grayish powder was dried at room temperature. For comparison, also olivine phase LiFePO4 was prepared according to the procedure published by Kang and Ceder.16 X-ray powder diffraction measurements of the products were performed on an X’Pert Pro Diffractometer (Panalytical) with Bragg−Brentano geometry using Cu Kα (λ = 1.5406 Å) radiation (40 kV, 40 mA) and a 2Θ step size of 0.0334°. The instrumental resolution function was determined with Y2O3 powder as standard. Lattice parameters and the average crystallite size were evaluated by Rietveld fits using the FullProf software (version Feb. 2007. LLB, Juan Rodriguez Carvajal, Saclay France).17 Figure 1 displays the powder X-ray diffraction data of the powder obtained by the liquid-phase synthesis. Despite the Received: July 14, 2011 Revised: February 7, 2012 Published: February 7, 2012 633
dx.doi.org/10.1021/cm202015g | Chem. Mater. 2012, 24, 633−635
Chemistry of Materials
Communication
In Figure 2, the first results on the electrochemical behavior of the newly prepared nanocrystalline high-pressure phase
Figure 1. XRD data and Rietveld fit of the nanosized high-pressure phase LiFePO4 powder. The observed X-ray powder diffraction pattern (circles), the Rietveld fit (black line), and the residuum (gray line) are given. Short vertical lines indicate the peak positions according to ICSD 97766.
mild reaction conditions, the pattern is not consistent with the olivine phase of LiFePO4 or any polymorph of FePO4 but well matches the data of the high-pressure phase of LiFePO4. In fact, a Rietveld fit based on the Cmcm space group of the highpressure phase reproduces the experimental data very well. Values for the lattice constants, the mean particle size, and other parameters resulting from the Rietveld fit are summarized in Supporting Information Table S1 and are consistent with the data published for the same material prepared at 65 000 bar. The chemical composition of the powder was determined by ICP-OES measurements indicating a Li/Fe ratio very close to one (see Supporting Information for details). The corresponding X-ray powder diffraction data and the Rietveld refinement of the olivine phase synthesized for comparison are given in the Supporting Information (Figure S2 and Table S2). In contrast to the conventional method, several grams of the material can be easily prepared in a single batch at low costs. Moreover, the liquid-phase synthesis yields particles with a mean domain size of only 20 nm. TEM images show, however, that the particles are strongly agglomerated as already indicated by the precipitation of the particles during synthesis. To verify that our material in fact displays lithium intercalation, preliminary electrochemical experiments were performed in Swagelok type cells using lithium metal as the negative and reference electrode. The positive electrode was prepared, following a modified literature procedure,18 by thoroughly mixing in a mortar a small amount of N-methyl pyrrolidone with finely powdered carbon, polyvinylidene fluoride, and active material (LiFePO4) in a mass ratio of 70:15:15. The resulting suspension was cast on roughened aluminum discs and dried in an oven at 90 °C for several hours. A commercial solution of 1 M LiPF6 in EC (ethylene carbonate) and DMC (dimethyl carbonate) in a volume ratio of 2:1 (Selectilyte LP30) was used as electrolyte. The cell was assembled in a moisture-free atmosphere inside a Glove box. Cyclic voltammetry experiments were carried out using a PGSTAT 20 (Ecochemie, The Netherlands) run under GPES 4.9. The potential was scanned between 2 and 4.5 V at rates of 0.05 to 0.2 mV/s.
Figure 2. Cyclic voltammograms of nanosized LiFePO4 batteries in 1 M LiPF6 + EC/DMC (1:1) at scan rate of 0.1 mV/s. (A) Olivine phase pellet (3.1 mg active material), (B) β-phase pellet (1.82 mg active material, C = 32 mAh/g).
LiFePO4 are presented and compared with that of the olivine phase. The cyclic voltammogram of the olivine phase LiFePO4 given in Figure 2a displays one pair of characteristic cathodic/ anodic peaks at approximately 3.2 and 3.6 V at a 0.1 mV/s scan rate.19 The total peak potential separation, ΔEp, amounts to 0.4 V, with 0.2 V related to the cell resistance (Supporting Information Figure S3) and 0.2 V related to the Li(de)insertion. Using the same experimental conditions, the CV given in Figure 2b was obtained for our high-pressure phase LiFePO4. The cathodic and anodic peaks appear at 2.9 and 3.4 V, respectively. The peaks are broader and structured, pointing to surface phenomena and/or a nonhomogeneous sample leading to different insertion dynamics. However, the midpoint potential, 3.15 V, is still readable and obviously shifted by 0.25 V as compared to olivine, in agreement with calculations.3,7 The charge capacity of the high pressure phase is in the range of 30 mAh g−1, that is, approximately 25% of an optimized olivine electrode, and corresponds also to a Δx of 0.2.7 To the best of our knowledge, this is the first reversible Li-(de)intercalation Li-intercalation cyclic voltammogram on a Cmcm LiFePO4 sample displaying reasonable capacity and kinetics. The process is reversible over many cycles without any loss of charge capacity (see Supporting Information). Arroyo y de Dompablo et al. have reported very slow kinetics and essentially no charge capacity (Δx < 0.08) in the interesting 2−4 V potential region for micrometer sized Cmcm phase FePO4 (originally present in the delithiated state).5,7 In our experiments we start the electrochemical experiment with nanometer sized Cmcm phase LiFePO4 in the lithiated state. Higher reversibility and increased charge capacity may therefore be related to smaller Li-diffusion paths within nanocrystalline vs microcrystalline Cmcm FePO4, respectively. Since our particles are not well separated (see above), however, it is likely that a large fraction of the particle surface is not 634
dx.doi.org/10.1021/cm202015g | Chem. Mater. 2012, 24, 633−635
Chemistry of Materials
Communication
(18) Delmas, C.; Maccario, M.; Croguennec, L.; Le Cras, F.; Weill, F. Nat. Mater. 2008, 7, 665−671. (19) (a) Jin, B.; Gu, H. B.; Zhang, W.; Park, K. H.; Sun, G. P. J. Solid State Electrochem. 2008, 12, 1549−1554. (b) Garcia-Moreno, O.; Alvarez-Vega, M.; Garcia-Alvarado, F.; Garcia-Jaca, J.; GallardoAmores, J. M.; Sanjuan, M. L.; Amador, U. Chem. Mater. 2001, 13, 2455.
electrochemically accessible and the electroactivity is therefore limited to a Δx of about 0.3. Further investigations on the scope and limits of the new nanocrystalline high-pressure phase active material with emphasis on the Li (de)insertion kinetics are planned.
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ASSOCIATED CONTENT
S Supporting Information *
XRD-Pattern of HP-LiFePO4, olivine LiFePO4, results of Rietveld fit of XRD data, resistance of the cell, and repetitive cycling (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: (+) 49 541 969 3323. E-mail:
[email protected] (M.H.),
[email protected] (L.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. C. Salvatore (CELGARD LLC, Sélestat, France) for the donation of a Celgard membrane (type 2340). The ICP-OES measurements were performed by Deutsches Institut für Lebensmitteltechnik e.V.
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REFERENCES
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