Platelike LiMPO4 (M = Fe, Mn, Co, Ni) for Possible Application in

Article pubs.acs.org/JPCC

Platelike LiMPO4 (M = Fe, Mn, Co, Ni) for Possible Application in Rechargeable Li Ion Batteries: Beyond Nanosize V. A. Alyoshin,† E. A. Pleshakov,† H. Ehrenberg,‡ and D. Mikhailova*,§ †

Department of Chemistry, M. V. Lomonosov Moscow State University, Leninskie Gory, 119991, Moscow, Russia Institute for Applied Materials-Energy Storage Systems (IAMESS), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany § Max-Planck Institute for Chemical Physics of Solids, Nothnitzer Strasse 40, 01187 Dresden, Germany ‡

ABSTRACT: The peculiarities of LiMPO4 (M = Fe, Mn, Co, Ni) formation with controlled platelike crystal shape using layered M(OH)2 hydroxides as templates were studied. Thin, platelike crystals of NH4MPO4·H2O with Mn, Fe, and Co were formed in aqueous solutions as intermediate products, whereas for NH4NiPO4·6H2O, particles in the form of spherulites were observed. For replacement of ammonium groups by lithium cations, a solid-state reaction between NH4MPO4·xH2O and Li2CO3 at elevated temperatures was used. The obtained LiMPO4 particles repeat completely the shape of the intermediated NH4MPO4·xH2O crystals. The surface of platelike particles represents a cellular structure and consists of agglomerated, slightly randomized crystallites of several hundred nanometers, which are oriented along the c-axis perpendicular to the plate surface. It was shown in the LiFePO4 example that the cathode materials, consisting of platelike particles and reduced graphene oxide (RGO), have the discharge capacity of 145 mAh/g in Li ion batteries after 10 cycles with 0.1C. This value can successfully compete with the best literature results reported for nanosized LiFePO4/RGO cathode materials. A possible explanation of that is discussed.

1. INTRODUCTION The complex phosphates LiMPO4 (M = Fe, Mn, Co, Ni) with the olivine structure have been known since the end of the 20th century as cathode materials for lithium ion batteries.1 The longitudinal channels along the b-axis between MO6-octahedra are filled by Li ions, which move in this direction during battery charge or discharge,2.3 The main disadvantage of the phosphates is their very poor electronic conductivity, so it is necessary to prepare electronically conductive composite materials. The common ways to improve the electronic conductivity are (i) heterogeneous doping of LiMPO4,4 (ii) coating with an electron conductive phase, for example, carbon,5 Co2P,6 or Fe2P,7 (iii) minimization of particle size,8 (iv) preparation of two-dimensional and three-dimensional LiMPO4/CNTs structures,9,10 or (v) preparation of porous particles.11 Recent developments in the synthesis of composites with lithium metal phosphates LiMPO4 (M = Fe, Co, Ni, Mn) coated on nanostructured carbon architectures (unordered and ordered carbon nanotubes, amorphous carbon, carbon foams) are summarized in ref 12. In the present work we develop another way to overcome the challenge with insulating properties of LiMPO4, which includes the controlled preparation of quasi-two-dimensional (2D) thin plates of phosphates. The composite materials containing these plates combined with graphene sheets will reduce the total electrical resistivity and, therefore, improve electrochemical performance, because of very close contact between thin plates and graphene particles. © 2014 American Chemical Society

In order to control effectively the shape, size, and orientation of the crystallites, special crystallization methods should usually be applied. Some of them, such as solvothermal13,14 and hydrothermal techniques15,16 or crystallization in supercritical carbon dioxide,11 require complex facilities. However, even these synthesis procedures are unstable toward the resulting morphology: a variety of experimental parameters influence the shape and size of obtained olivines. For example, the complexity of the formation of LiMPO4·xH2O thin plates is discussed in refs 14 and 15. Recently, several works succeeded in preparation of ultrathin LiMPO4 nanosheets, oriented along [010], for Li ion batteries using a solvothermal synthesis in high-pressure hightemperature supercritical fluids.17,18 A much more simple and promising approach for synthesis of two-dimensional platelike LiMPO4 includes the crystallization of NH4MPO4·xH2O as intermediate substances13,19−21 with subsequent replacement of the ammonium groups by lithium cations. Although this method has been already used in several works, the main information concerning the cation replacement, such as details of the structure transformation and molar volume change, are not completely understood. Moreover, the competition between the thermally activated substitution of the ammonium groups in NH4MPO4·xH2O on lithium cations and the evaporation of water and ammonia molecules with Received: May 12, 2014 Revised: July 10, 2014 Published: July 14, 2014 17426

dx.doi.org/10.1021/jp504587f | J. Phys. Chem. C 2014, 118, 17426−17435

The Journal of Physical Chemistry C

Article

Figure 1. Scheme of LiMPO4 formation using M(OH)2 templates, via NH4MPO4·H2O (M = Mn, Fe, Co).

pyrophosphate M2P2O7 formation has never been discussed. According to DTA studies of the thermal decomposition of NH4MPO4·H2O (M = Fe,22 Co23), a metal hydrogen phosphate MHPO4 is formed during the first step, after ammonia and water escaped, and transformed into M2P2O7 under further heating. Therefore, formation of admixtures or a significant deformation of the crystals could be expected during thermally activated replacement of NH4+ by Li+ ions. The present work is focused on understanding of crucial parameters leading to the formation of LiMPO4 thin plates in the three-step process M(OH)2 → NH4MPO4·xH2O → LiMPO4. The knowledge obtained in the present study allows one to control reliably the platelike shape of LiMPO4 during the synthesis, which is very important for the application of these materials as cathodes in rechargeable lithium ion batteries. Further, the composite materials containing macrosize LiFePO4 thin plates and a reduced graphene oxide were tested in Li ion batteries. We have shown that these composites provide good electrochemical performance comparable with the results for nanosize LiFePO4 cathodes with graphene; see for example refs 24−29.

MSO4 + 2NH3 + 2H 2O → M(OH)2 ↓ + (NH4)2 SO4 ,

M = Fe (1)

M(NO3)2 + 2NH3 + 2H 2O → M(OH)2 ↓ +2NH4NO3 , M = Mn, Co, Ni

(2)

The ammonia solution was added dropwise with vigorous stirring until full hydroxide precipitation and appearance of the characteristic color of ammonium-containing complex compounds of Co and Ni. In the case of the iron hydroxide, a gel-like white precipitate in a blue solution was formed immediately. The precipitation of the manganese hydroxide was accompanied by formation of a light-cream-colored solution. The intermediate NH4MPO4·xH2O compounds were formed via reaction of metal hydroxides with NH4H2PO4 under normal conditions in an aqueous solution: M(OH)2 + NH4H 2PO4 → NH4MPO4 ·H 2O + H 2O, M = Fe, Mn, Co

(3)

In the case of NH4NiPO4·xH2O, the compound with x = 6 was formed:

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. A three-step process includes precipitation of the M(OH)2 templates as the first step, crystallization of NH4MPO4·xH2O intermediate products as the second step, followed by replacement of NH4 groups by lithium ions for the LiMPO4 formation as the third step. Although the experiments were mostly carried out on LiFePO4, in order to trace the regularities of crystallization, some other 3d metal phosphates with M = Mn, Co, Ni were involved in this study. The starting reagents for synthesis were Mohr’s salt [(NH4)2Fe(SO4)2·6H2O], metal nitrates [Mn(NO3)2·6H2O, Co(NO3)2·6H2O, and Ni(NO3)2·6H2O], ammonium dihydrogen phosphate (NH4H2PO4), lithium carbonate (Li2CO3) with a degree of purity more than 99.9% (metals basis), and a concentrated ammonia solution. The 0.5 M aqueous solutions of metal salts and the 1 M solution of NH4H2PO4 were used in the experiments. The preparation of ammonium phosphates of iron and manganese, filtration, and drying were carried out in a glovebox filled with nitrogen. The dried samples are stable in air. The syntheses of cobalt and nickel compounds were carried out in air. Metal hydroxide templates were precipitated by adding a concentrated ammonia solution to the 0.5 M solution of a corresponding divalent metal:

Ni(OH)2 + NH4H 2PO4 + 4H 2O → NH4NiPO4 · 6H 2O,

M = Ni (4)

For obtaining of LiMPO4, a stoichiometric mixture of NH4MPO4·xH2O and Li2CO3 was heated at 600−650 °C in an Ar flow for 12 h: NH4MPO4 ·x H 2O + 0.5Li 2CO3 → LiMPO4 + 0.5CO2 + x H 2O + NH3

(5)

2.2. X-ray Powder Diffraction (XRD). Phase analysis and determination of cell parameters at room temperature were carried out using a Huber G670 X-ray diffractometer (Gemonochromator, Image Plate detector, Cu Kα1 radiation, λ = 1.540 56 Å). All diffraction patterns have been analyzed by using the software package WinPLOTR. 2.3. Scanning Electron Microscopy (SEM). The morphology of the particles was studied by an electron microscope LEO SUPRA 50VP (Carl Zeiss) at an accelerating voltage of 5−15 kV and JEOL JSM-6490LV (W cathode, 30 kV, S-UTW-Si-(Li) detector). 2.4. Transmission Electron Microscopy (TEM) and Electron Diffraction. The particle morphology studies and 17427



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Figure 2. SEM pictures and XRD patterns of NH4MPO4·xH2O (M = Mn, Fe, Co, Ni), together with experimental, theoretical, and differential curves. All obtained materials represent single-phase samples. Structural models for Rietveld refinement were used from ref 32 for M = Mn, Fe, and Co and from ref 33 for M = Ni.

2.6. Thermogravimetric Analysis (TG−MS). The mass loss, thermal effects, and mass spectra of gaseous products during sample decomposition were studied in the temperature range from 20° to 800 °C, using a NETZSCH STA 409 PC/PG thermoanalyzer. Experiments were carried out in alumina crucibles in an Ar flow with the heating rate of 10 °C/min.

electron diffraction were performed using an electron microscope JEOL JEM-2000 FX II. 2.5. Chemical Analysis. The cation composition of the samples was controlled by the inductively coupled plasma method with mass spectroscopic analyzer (ICP-MS, PerkinElmer ELAN DRC II) using HNO3 (65%, p.a.) for dissolving the samples. 17428



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Table 1. Unit Cell Dimensions for NH4MPO4·xH2O (M = Mn, Fe, Co, Ni) compd

a, Å

b, Å

c, Å

V, Å3

V/Z, Å3

NH4MnPO4·H2O NH4FePO4·H2O NH4CoPO4·H2O NH4NiPO4·6H2O

8.8061(2) 8.8224(8) 8.7773(2) 11.1584(14)

5.7257(9) 5.6602(11) 5.6185(6) 6.9160(4)

4.9107(9) 4.8267(6) 4.7953(8) 6.1034(17)

247.60 241.03 236.48 471.01

123.80 120.51 118.24 235.50

Figure 3. Morphology of NH4FePO4·H2O crystallites dependent on ammonia concentration: (a) 0.02 M, (b) 0.2 M, and (c) 2.0 M solution.

2.7. Electrochemical Characterization. The electrochemical tests were performed in a three-electrode cell with a lithium anode and lithium reference electrode connected to a Potentiostat Autolab PGstat 302. As a positive electrode, suspensions in N-methylpyrrolidone of a mixture of platelike LiFePO4, reduced graphene oxide, and PVdF (85:10:5 weight ratio) were prepared in an ultrasonic bath, put on the steel mesh with 8 mm diameter, and dried in vacuum at 100 °C. A mixture of EMI-TFSI and Li-TFSI (both from Sigma-Aldrich, 99+%) was used as electrolyte. The cells were successively charged and discharged in a GCPL mode (galvanostatic cycling with potential limitation) at a constant current that corresponds to the intercalation or extraction of one Li per formula unit during 10 h (0.1C rate).

3. RESULTS AND DISCUSSION 3.1. Shape-Controlled NH4MPO4·xH2O (M = Fe, Mn, Co, Ni). 3.1.1. Structural considerations. A three-step process was proposed for preparation of platelike particles of LiMPO4, including precipitation of M(OH)2 templates as the first step, crystallization of NH4MPO4·xH2O intermediate phases as the second step, followed by replacement of NH4 groups by lithium ions as the third step. The scheme of structural transformations from M(OH)2 into LiMPO4 is presented in Figure 1. In order to assist the crystallization of NH4MPO4·H2O in the form of platelike crystals, the preliminary precipitated brucitelike hydroxides M(OH)2 were used as templates. These hydroxides have a layered structure with a strong chemical bonding within metal−oxygen layers and a weak hydrogen bond between the layers. These peculiarities of the crystal structure lead to the crystallization of M(OH)2 in the form of thin plates. For example, the precipitation of hexagonal thin nanoplates β-Co(OH)2 is reported in detail in previous works.30,31 After transformation of brucite-like hydroxides into NH4MPO4· H2O, the layers of edge-sharing MO6 octahedra remain nearly unchanged, whereas the phosphate and ammonium groups are inserted between these layers. Part of the OH groups transforms into water molecules. The interaction between layers becomes stronger due to the appearance of an ionic interaction with participation of ammonium ions. Such kind of conversion allows one to keep the size and shape of the initial plate with some increase of its thickness. The NH4MPO4·H2O structure32 can be represented as a sequence of metal−oxide octahedral layers, interconnected with

Figure 4. Results of TG−MS studies of the NH4FePO4·H2O + 0.5Li2CO3 mixture in Ar flow at rate 10 °C/min.

phosphate tetrahedra and water molecules. Ammonium groups are placed between the layers. The same mode of metal−oxide octahedral and phosphate tetrahedral layers is characteristic for the olivine LiMPO4 structure as well. Under transformation to LiMPO4, metal−oxide layers move together; the oxygen atoms from the water molecules in the MO6 octahedra are replaced by the oxygen atoms of the PO4 tetrahedra. The channels between MO6 octahedra and PO4 tetrahedra are filled by lithium ions. These peculiarities of the crystal structure can be suitable for keeping the main structural features under transformation and, hence, allow the form of crystals to be maintained. This solid-state exchange reaction requires elevated temperatures, and the thermal instability of NH4MPO4·H2O should be taken into account. Note that in the case of NH4NiPO4·6H2O, nickel cations in NiO6 octahedra are surrounded by oxygen atoms from water molecules, and therefore, much more significant structural changes should accompany water evaporation from the structure. The replacement of the ammonium group by lithium with the formation of the olivine-like structure can take place without structure collapse, despite the changes in the molar volume. 3.1.2. Formation of NH4MPO4·H2O (M = Mn, Fe, Co) with Use of M(OH)2 Templates. The M(OH)2 templates were precipitated according to reactions 1 and 2. Afterward, a 1 M 17429



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Figure 5. SEM and XRD pictures of the reaction products during transformation of platelike NH4FePO4·H2O into LiFePO4 at (a) 300 °C, (b) 400 °C, and (c) 700 °C. The structural models of Li2CO3 and LiFePO4 for Rietveld refinement were taken from refs 36 and 37, respectively. At 400 °C, only amorphous phases were observed.

Mohr’s salt and 0.5 M (NH4)3PO4 in ammonia solution described elsewhere:34

solution of ammonium dihydrogen phosphate with continuous stirring was added to the precipitated metal hydroxide (see reactions 2 and 3). After 12 h, thin lamellar crystals of NH4MPO4· xH2O (M = Mn, Co, Ni), having the same crystal structure, were isolated from the solution (see Figure 2a−c). The hexahydrate NH4NiPO4·6H2O, having another crystal structure, was crystallized in a globular form (Figure 2d). XRD patterns of obtained crystals confirm the single-phase materials (Figure 2a−c); the unit cell parameters are in agreement with the literature data (Table 1). The structural model of NH4CoPO4·H2O from ref 32 was applied for the Rietveld refinement of isostructural NH4MPO4·H2O (M = Mn, Fe, Co) compounds, and the structural model of NH4NiPO4·6H2O was taken from ref 33. 3.1.3. Nontemplate Crystallization of NH4FePO4·H2O. Another method to prepare NH4MPO4·xH2O used in the present work for M = Fe is based on the reaction between stoichiometric amounts of 0.5 M iron(II) sulfate in the form of

FeSO4 + (NH4)3 PO4 + H 2O NH3(aq)

⎯⎯⎯⎯⎯⎯⎯⎯→ NH4FePO4 · H 2O ↓ + (NH4)2 SO4

(6)

We studied the ammonia-concentration-dependent precipitation of NH4FePO4·H2O. The morphology of NH4FePO4· H2O crystallites (Figure 3a−c) changes from thick plates at low ammonia concentration (0.02 M) to spherulites at 2.0 M. At the medium concentration of 0.2 M, thin plates with common edges were precipitated, which are an intermediate form between plates and spherulites. Almost the same morphology of crystals (so-called flowerlike crystals) was observed for precipitation of NH4MnPO4·H2O in the presence of citric acid.35 So, the template technique has undeniable advantages in forming thin plates. 17430



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Figure 6. SEM pictures and XRD results of platelike LiMPO4 (M = Mn, Co) after synthesis at 700 °C. Structural models of LiMnPO438 and LiCoPO439 were applied for the Rietveld refinement.

NH4FePO4 ·H 2O → FeHPO4 + H 2O + NH3

3.2. Replacement of Ammonia Group by Lithium Ions in NH4MPO4·xH2O (M = Fe, Mn, Co, Ni). The main features of the chemical reaction between NH4MPO4·xH2O and Li2CO3 were studied using a template-prepared NH4FePO4·H2O as an example. This reaction is complicated by the low thermal stability of NH4MPO4·xH2O. According to the TG−MS study of the NH4FePO4·H2O + 0.5Li2CO3 mixture with a mass-spectrometric analysis of eliminated gases (Figure 4), the main ions in mass spectra with m/e = 15, 16, 17, 18, and 44 were detected, which corresponded to H2O, NH3, and CO2 molecules of origin escaping (Table 2).

The measured mass loss of 16.8% is somewhat higher than the calculated mass loss of 15.7%. The difference may be caused by a partial interaction with lithium carbonate in the first stage. A small elimination of carbon dioxide was detected at 270 °C before the main reaction at 480 °C, likely as a result of the pyrohydrolysis reaction of lithium carbonate with escaping water. In the second step at 480 °C, FeHPO4 reacts with Li2CO3, giving the olivine FeLiPO4: 2FeHPO4 + Li 2CO3 → 2LiFePO4 + H 2O + CO2 ↑

Table 2. Vapor Composition over NH4FePO4·H2O + Li2CO3 Mixture under Heating m/e

MS ions

molecules of origin

15 16 17 18 44

NH+ NH2+, O+ NH3+, HO+ H2O+ CO2+

NH3 NH3, H2O NH3, H2O H2O CO2

(7)

(8)

The total measured mass loss of 29.1% is in a good agreement with the calculated value of 29.5%. The lithium content x(Li) in final LixFePO4 is 0.95 ± 0.05 according to the ICP results. In order to track the morphology during transformation of platelike NH4FePO4·H2O into LiFePO4 during reaction with Li2CO3, the initial stoichiometric mixtures of NH4FePO4·H2O and Li2CO3 (molar ratio 1:0.5) were heated with 10°/min in a pure argon flow from room temperature up to different temperatures with a maximum of 700 °C, similar to the TG−MS study, and then immediately quenched. The results of XRD and SEM analysis of the samples after heat treatment are presented in Figure 5a−c. The phase composition at temperatures below 400 °C corresponded to the mixture of NH4FePO4·H2O and Li2CO3. The appearance of a white powderlike coating at 300 °C (Figure 5) may be caused by a partial lithium carbonate pyrohydrolysis through water, escaped from the cracks. The decomposition process is accompanied by formation of an amorphous intermediate state: only amorphous products were registered by XRD at 400 °C. The crystalline phase LiFePO4 appeared at temperatures

The ions with m/e = 18 and 44 correspond to water and carbon dioxide molecules, respectively. Ammonia can be detected uniquely by m/e = 15 ion only; the ionic current of this ion is shown in Figure 4. The ionic currents were not calibrated to partial vapor pressures. The mass-spectral results presented in Figure 4 have qualitative character to show the main molecules in the vapor phase. Two stages for the reaction of NH4FePO4·H2O with lithium carbonate were revealed. At the first step the decomposition of NH4FePO4·H2O to FeHPO4 with elimination of ammonia and crystallized water was observed at 270 °C: 17431



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Table 3. Unit Cell Parameters of LiMPO4 (M = Mn, Fe, Co, Ni) 1 2 3 4

sample

a, Å

b, Å

c, Å

V, Å3

V/Z, Å3

Δ, %a

LiMnPO4 LiFePO4 LiCoPO4 LiNiPO4

10.4382(9) 10.3301(12) 10.2021(7) 10.0317(4)

6.1043(16) 6.0063(9) 5.9257(2) 5.8538(1)

4.7426(12) 4.6913(7) 4.7003(1) 4.6768(1)

302.19 291.07 284.15 274.64

75.55 72.77 71.04 68.66

38.98 39.62 39.92 70.84

The relative decrease of molar volume Δ (%) = 100[(VNH4MPO4·xH2O/Z) − (VLiMPO4/Z)]/(VNH4MPO4·xH2O/Z) with x = 1 for M = Mn, Fe, and Co and x = 6 for M = Ni a

Figure 7. TEM images and electron diffraction patterns of a LiFePO4 plate. The areas exposed in the diffraction study are marked.

Figure 8. LiFePO4 plates and RGO sheets for electrochemical studies. 17432



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above 500 °C, as concluded from SEM and XRD measurements (data not shown). The SEM and XRD pictures after heat treatment at 700 °C revealed that although the shape of the particles did not change significantly, the microstructure of the surface became quite different: a smooth NH4FePO4·H2O surface without any visible defects is replaced by a cellular-type microstructure of small crystallites with some porous defects. The same microstructure is clearly seen also for other olivines LiMPO4 with M = Mn and Co (Figure 6a, b), or M = Ni (SEM pictures are not shown). These crystals also keep the initial platelike form after transformation from NH4MPO4·H2O into LiMPO4. This change in the morphology seems to be due to cell shrinkage (see Table 3; the volume per formula unit decreased by about 40% for M = Mn, Fe, Co and about 70% for M = Ni). 3.3. TEM Studies of Platelike LiFePO4. The disorder of crystallites LiFePO4 and their orientation in the cellular structure, which appeared after replacement of ammonium cations by lithium cations, was studied by transmission electron microscopy. TEM images of the platelike LiFePO4 together with electron diffraction patterns are presented in Figure 7. The surface of the plate is nonhomogeneous. Formation of a composite consisting of platelike particles and small secondary nanoparticles could not be excluded, although sample preparation for TEM experiments actually leads to isolation of single particles. Furthermore, we have tested three different surface areas (a−c) of a LiFePO4 plate and obtained practically the same electron diffraction patterns. This fact means that the crystallites orientation is the same in different parts of the plate. According to results of electron diffraction, the crystallites are oriented along the [001] direction perpendicular to the plane of the lamellar particles, and hence, lithium channels are directed along the plate (Figure 7). 3.4. Galvanostatic Cycling of Platelike LiFePO4. The composite LiFePO4/reduced graphene oxide (RGO) material was used for an electrochemical test experiment. The LiFePO4 plates of 7−8 μm were significantly larger than RGO sheets (about 0.5 μm) (see Figure 8a,b). Results of galvanostatic cycling are presented in Figure 9.

So, thin micron-sized LiFePO4 plates with Li channels along the plate can provide the same electrochemical performance as the nanoparticles. The platelike form of LiFePO4 particles allows close surface contact with graphene sheets, yielding better electric contact along the plate and higher electrical conductivity between plates. The low thickness and high surface area reduce the electric resistance of the material, and the porous microstructure of LiFePO4 plates reduces the lithium diffusion path. Lithium ions can diffuse into platelike LiFePO4, not only from the lateral side but also from any part of the plate through the pores (see Figure 10).

Figure 10. Scheme of the composite containing platelike LiFePO4 and RGO.

4. CONCLUSIONS In the present work, the fundamental principles of platelike LiMPO4 formation are discussed. The layered M(OH)2 compounds were transformed into NH4MPO4·xH2O by intercalation of phosphate and ammonium groups and further into LiMPO4 by replacing ammonium groups by lithium, with preservation of initial octahedral metal−oxygen layers. The transformation of M(OH)2 into platelike NH4MPO4·xH2O particles with a smooth surface takes place in aqueous solutions at room temperature. The same procedure could be applied for the preparation of NH4Fe1−yMnyPO4·H2O (0 ≤ y ≤ 1) solid solutions with platelike crystallites. Transformation of NH4MPO4·xH2O into LiMPO4 via replacement of NH4+ by Li+ from Li2CO3 requires elevated temperatures. The substitution of ammonium groups on lithium ions does not change the shape of the crystals, but leads to a recrystallization with appearance of a cellular structure and pores on the crystal surface. This reaction includes two steps: thermal decomposition of the ammonium salt with the metal hydrophosphate formation and a subsequent reaction with lithium carbonate. Cracks and scratches appeared on the crystal surface during the decomposition of ammonium salt; the subsequent increasing temperature led to a recrystallization. The formed LiFePO4 crystallites are oriented along the [001] direction perpendicular to the plane of the lamellar particles. Lithium channels are extended along the plate. The discharge capacity values of composites, containing microsized, thin, platelike LiFePO4 and RGO obtained at 0.1C, are very close to the best literature values reported for nanosized LiFePO4/graphene cathodes cycled at the same discharge rate. The advantage of 2D-like olivines with Li ion transport channels on nanometer scale was traditionally attributed to the potentially enhanced mass transport properties. For example, the high specific capacity, excellent rate capability, and stable cyclability of LiFePO4 nanosheets oriented along the b-axis were explained by the nanometer-scale Li ion diffusion pathways.17,18 In contrast, our platelike LiFePO4 particles show the high specific discharge capacity oriented along the c-axis. Their enhanced electrochemical properties originated primarily from enhanced

Figure 9. Charge−discharge curves of the LiFePO4/RGO composite with platelike LiFePO4 cycled in a galvanostatic mode with 0.1C.

The discharge capacity of 145 mAh/g reached after 10 cycles for the platelike microsized LiFePO4/RGO with 0.1C is very close to the best literature values reported for nanosized LiFePO4/graphene cathode materials.29 17433



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electric conductivity from RGO in the composite. The LiFePO4 plates provide a close contact with graphene sheets, which leads to better electric contact along the plate and, hence, better electrical conductivity of the composite. Note that use of the composite consisting of platelike LiFePO4 and conventional carbon black revealed much lower discharge capacity of ca. 80−60 mAh/g (data not shown), confirming the hypotheses of enhanced electrical conductivity of the composite with reduced graphene oxide. Additionally, the porous microstructure of LiFePO4 improves the lithium ion diffusion inside the plates.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 351 4646 4424. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the RFBR grant 12-08-01258a, and partially supported by the M. V. Lomonosov MSU Program of Development and the Helmholtz Initiative for Mobile and Stationary Energy Storage. The authors are thankful to T. Shatalova (M. V. Lomonosov MSU) for the support in carrying out of TG-MS analyses, D. Itkis and D. Semenenko (M. V. Lomonosov MSU) for help in electrochemical measurements, A. E. Sarapulova (MPI CPfS) and O. Drozhin (M. V. Lomonosov MSU) for help in scanning electronic microscopy measurements, and N. Khasanova (M. V. Lomonosov MSU) for a fruitful discussion.

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Platelike LiMPO4 (M = Fe, Mn, Co, Ni) for Possible Application in

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