J Sol-Gel Sci Technol (2010) 56:320–326 DOI 10.1007/s10971-010-2308-4

ORIGINAL PAPER

Preparation of LiCoPO4 powders and films via sol–gel M. S. Bhuwaneswari • L. Dimesso • W. Jaegermann

Received: 8 June 2010 / Accepted: 11 August 2010 / Published online: 20 August 2010 Ó Springer Science+Business Media, LLC 2010

Abstract Lithium intercalation materials are of special interest as cathodes in rechargeable batteries. An uncomplicated sol–gel process has been used for the synthesis of Li–Co phosphates powders and, for the first time, of LiCoPO4 films. The powders were prepared from aqueous solutions, containing Li, Co and phosphate precursors to which acid citric and ethylene glycol was added, during the drying process at 75 °C. The X-ray diffraction patterns of the prepared powders confirmed the presence of LiCoPO4 with an olivine-like structure as main phase. The morphological investigations of the powder showed a plateletlike structure with an average grain size of 0.75 lm. The films of LiCoPO4 were deposited onto ITO glass substrates with the combination of the dip-coating process under the same conditions. Finally, the films were annealed in inert atmosphere at 300 °C. The morphological investigations reveal a smooth and homogeneous surface of the prepared Li–Co phosphate films. The preliminary electrical investigation on the prepared LiCoPO4 films showed lithium ions electrochemical activity in the range 3.0–4.5 V. Keywords Sol–gel
Cathode materials
Phosphates
Film
Coating

1 Introduction By searching new and promising cathode materials that can replace the presently used transition metal oxide based materials in lithium batteries, lithium 3d-metal M. S. Bhuwaneswari
L. Dimesso (&)
W. Jaegermann Materials Science Department, Darmstadt University of Technology, Petersenstrasse 32, 64287 Darmstadt, Germany e-mail: [email protected]

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orthophosphates have gained considerable interests [1, 2]. Among the orthophosphates, LiFePO4 and LiCoPO4 offer a relatively high theoretical capacity (170 and 167 mAhg-1 respectively) at reasonably high operating voltages suitable for use in electric and in hybrid electric vehicles [3]. However, there are some disadvantages for this class of cathode materials with an olivine-type structure. For instance, the most common example of these compounds, LiFePO4, suffers from a significant difference between the theoretical and practical capacities, which is due to poor diffusion of lithium ions through LiFePO4/FePO4 interfaces and its poor electrical conductivity (LiFePO4 can be considered as in insulating phase). This can be partly improved by adding some small amounts of carbon [4, 5] or metals [6] that increase the conductivity of the cathode material or by decreasing the particle size that is an efficient way to overcome the diffusion limitations problems for LiFePO4 [7]. Based on the comparison of different examples of LiMPO4 (M=Fe, Mn, Ni and Co), Okada et al. [8] have shown that LiCoPO4 has the highest redox potential. LiCoPO4 is the only example of this class of materials that is suitable for 5 V performances. In fact it can be considered as one of the best examples of 5 V cathode materials because of its high specific capacity and voltage. However some problems should be overcome before commercialization of this cathode material. Generally, there is insufficient technology for the fabrication of a 5 V lithium battery for use in practical applications. For example, the nonaqueous medium needed for the 5 V performances is still problematic, because of the instability of common electrolytes at such a high voltage performance. Another further problem for LiCoPO4 as cathode material is its significant capacity fades at high temperatures. Although many researchers have devoted to the investigation of the

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capacity fadings of LiMn2O4 and more recently to LiMxMn2-xO4 spinels (M=Cr, Fe, Co, Ni and Cu), such investigations are not usually performed on LiMPO4 cathode materials and studies of such novel materials are still in preliminary stages [9]. The oxides have been synthesized mostly by solid state reaction involving prolonged heat treatment at high temperatures (T [ 800 °C) to form desired phase of the material without the presence of any unwanted impurities. The disadvantages of these solid state processes are the absence of control of the microstructure, the overall agglomeration of the particles, contaminations due to possible reaction with the crucible and lithium volatilization during a long heat treatment cycle. The wet-chemistry technique offering improvement in the terms of material preparation has been extensively used as a low-temperature synthesis route (T \ 200 °C) to novel metastable structures of various oxides, organic modified ceramics and ceramic polymers [10, 11]. Oxides with unusual valence states can be prepared which may be beneficial for some applications. The numerous advantages of low-temperatures methods including intrinsic ease of use, low cost, versatility in terms of composition and structure manipulations, and ease of various processing following synthesis, make them especially appealing for preparing solid-state ionic materials. Various forms of the synthesized materials, including powders and thin films coated on different substrates, can be prepared with considerable ease. In very recent years, several low temperature preparation techniques have been introduced to get pure products [12 and references herein]. In these techniques the starting materials are dissolved in a liquid medium (organic aqueous or in mixture of both) which ensures the uniform mixing of the reactants. The metal ions present in the solution are subsequently trapped into a solid phase compound which has been formed by the chemical or electrochemical reaction performed in a liquid medium by addition of chelating agent, at low temperature, generally below 120 °C. Depending upon either the way of preparation of the solid phase or the nature of that phase, the solution technique has been called precipitation, sol–gel, xero-gel, aerosol etc. In the sol–gel method, the aqueous or alcoholic solution of complexing agent (carboxylic or hydrocarboxylic or polyhydroxy acid) and the metal salts, namely nitrates or acetates or hydroxides turn to gel on evaporation of the solvent. The hydroxy- and carboxylic acid groups present in the complexing agent could form a chemical bond with the metal ions and form viscous resins on evaporation of the solvent, which are usually called as precursors. On heating up to the decomposition point of the complex, normally below 400 °C the organic components convert into CO2 and H2O gases. The abrupt formation of

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these gases bloats the material and greatly expands its structure to produce an ash-like morphology. The good stoichiometric control and production of submicronized particle with narrow particle size distribution in relatively shorter processing time at a lower calcinations temperature are the advantages of the method. These submicron-sized particles enable maximization of the particle surface area to volume ratio, the electrolyte-cathode material interfacial area and the particle to particle utilization uniformity during cycling, consequently improving the performances of these cathode materials [13–16]. While the use of inorganic cathodes is not novel, the use of these electrodes in film form with an inorganic electrolyte also as film is a novel approach that has unique advantages. Thin films electrodes are necessary if one wishes to discharge cells at high rates. Discharge of thicker electrodes will require higher current densities leading to excessive polarization and cell degradation. The sol–gel method has been widely recognized as useful technique for producing advanced materials in ‘‘bulk-shape’’ as well as ‘‘coatings’’ in addition with dipcoating or spin-coating [17, 18] on different substrates, such as glass, stainless steel or noble metals. The advantage of the dip-coating process lies in the easy set-up of the system and in the control of the process that allow to deposit homogeneous films with a thickness from few nanometers till hundred nanometers in a very reproducible way. The use of water dispersions reduces firstly the costs of the equipment construction because there is no need to eliminate organic solvents. Moreover the viscosity of the water suspensions is higher than the viscosities of the sol– gel starting solutions therefore thicker films can be expected [19]. Although the functional coating through the sol–gel method is one of the most extensively studied areas, and some cathode as well as anode materials have been successfully developed [20–25], the authors have not found any literature references dealing with the preparation of LiCoPO4 films via sol gel-deposition. In this study the preparation, the structural and morphological characterization of LiCoPO4 as nanocrystalline powders and for the first time as films deposited in combination with dip-coating on ITO glasses are presented.

2 Experimental The motivation for sol–gel processing is primarily the potentially higher purity, homogeneity and the lower processing temperatures compared with traditional glass melting or ceramic powder methods. For the preparation of the LiCoPO4 powders, all precursors were simple salts, and precursor solutions were

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prepared using reagent grade chemicals e.g. Co(NO3)3
6H2O and LiH2PO4. The solution was added under stirring in air to an aqueous solution of citric acid. Ethylene glycol was then added to the solution for an ethylene glycol/citric acid molar ratio of 1/1. The water was slowly evaporated a 75 °C under air. When nearly dry, the solution turned to a gel. The gel was dried by maintaining it at 75 °C. A very homogeneous mixture, containing Li, Co and phosphate in the stoichiometric proportions of LiCoPO4 is then produced. The homogeneous mixture was progressively heattreated in an inert atmosphere at 750 °C for ca. 10 h to yield nanocrystalline LiCoPO4 phase coated with some amount of carbon. The films were deposited from the solutions prepared according to the procedure above described. During the drying the films were deposited on the substrate by optimizing the dip-coating conditions. The optimal conditions were by dip-coating for 1 h with a dip rate of 40 mm/min. Finally the so produced films were annealed at 300 °C in nitrogen atmosphere with a heating rate of 0.2 °C/min. The structural characterization of the powders has been performed by X-ray powder diffraction (XRD) using a STOE STADI/P powder diffractometer (Mo Ka1 radiation, ˚ ). A scanning electron microscope (SEM) k = 0.71069 A Philips XL 30 FEG has been used to investigate the morphology of the powders and of the films. Preliminary electrochemical measurements have been carried out as well with a multichannel potentiostatic–galvonostatic system VMP (PerkinElmer Instruments, USA) ina two-electrodes cell. The cell contained the deposited film cathode and a lithium foil as both counter and reference electrodes. The electrolyte consisted of 1 M LiPF6 dissolved in non aqueous solutions of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a weight ratio of 1:1. All the cells were assembled in an argon filled glove-box.

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converted into polyester by heating it in polyhydroxy alcohol where the metal ions are uniformly distributed. The method has been initially invented to prepare oxide powders of titanates and niobates [26]. The main advantage of this method is the homogenously trapping of Li-ions into polyesters eliminating the need for long-range diffusion during the formation of lithium transition metal oxides. Therefore, at relatively low temperature the precursor can form a homogeneous single phase of precise stoichiometry. More recently Audemer et al. [27] have proposed to use an aqueous solution of Fe(NO3)3
9H2O and LiH2PO4 stirred together with an aqueous solution of citric acid to which ethylene glycol was then added. Key to this process is the fact that both the LiFePO4 (as extension LiCoPO4) precursors and the monomers are to be water-soluble. After slow evaporation of the water at 80 °C under air, the solution turns to a gel due to the polymerization between citric acid and ethylene glycol. Subsequent drying of this gel leads to a very homogeneous mixture, containing Li, Fe and phosphate in the stoichiometric proportions of LiFePO4 together with the carbon bearing polymer. A treatment in an inert (N2) or slightly reducing (N2/H2) atmosphere between 600 and 800 °C yields thin conductive carbon layers uniformly coated at the surface. The advantages of this method are the control of the process parameters, the low cost of the starting materials as well as the preparation of films by dip-coating during the evaporation of the water. The XRD pattern of the synthesized LiCoPO4 is shown in Fig. 1. The Bragg peaks of the sample measured after the calcinations show LiCoPO4 as the major phase. However the presence of one or more secondary phases

LiCoPO4

3 Results and discussion We approached the investigation by first characterizing the powders prepared through the chosen sol–gel process as previously described. Due to the lack of literature concerning LiCoPO4 films prepared by sol–gel/dip-coating process the data obtained from the powders could be useful for the preparation of good quality LiCoPO4 films.

Intensity (arb.units)

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111 020

222 410 301 112 221 230

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331 203

401

630 304

3.1 Preparation and characterization of LiCoPO4 powders 10

Among the variety of sol–gel methods, the ‘‘Pechini process’’ has been found to be able to prepare materials of high quality and purity. This process is an extension of sol–gel process in which the chelates of the metals are further

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15

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25

30

35

40

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2θ (degree) Fig. 1 X-Ray diffraction pattern of the prepared LiCoPO4 powder. The indexed reflections refer to a orthorhombic olivine-like structure. (asterisks) indicates secondary phases (possibly Co2P2O7 as in [28])

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(indicated with (*) in Fig. 1) can be observed. The formation of the secondary phases is probably due to a progressive delithiation process occurring at higher temperatures as reported by Bramnik et al. [28]. During the thermal investigation on the Li–Co–P–O system, the authors reported the high thermal stability of the olivinelike phase even at higher temperatures (above 800 °C). On the other hand, due to the delithiation process the authors observed at T = 90 °C the formation of a lithium-poor phase (indicated as LizCoPO4) [29]. At higher temperatures (*250 °C) the formation of the new phase Co2P2O7 was revealed. The formation of this phase resulted from the decomposition of the lithium-poor phases LizCoPO4 and CoPO4 and its presence was observed even after cooling down to room temperature. When the LiMPO4 particles are prepared in micronlevel size, lithium ions cannot insert in the core region of the particles from the surface successfully especially at high current rate. Since the diffusion rate of lithium ions is determined from the chemistry and the crystalline structure, it is important to utilize the available lithium ion-sites in the LiMPO4 particle by forming nanometer size, narrow distribution and highly crystallined phase in order to reduce unwanted diffusion limitation caused by large and irregular shaped particles. A typical SEM micrograph of the Co-containing phosphate is shown in Fig. 2. The picture shows a platelet-like structure of the grains that seem to be better interconnected through the grain boundaries. The average grain size was estimated to be 0.75 lm confirming the data previously reported [28]. An analogous morphology has been observed by Kim et al. [30], investigating LiFePO4/Carbon

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composites prepared via polyol method. The authors claimed that particles with nanoplate-like shapes, a uniform size distribution and an average grain size of 0.3 lm can improve the electrochemical performance of the cathode material by favoring the lithium diffusion through the particles. 3.2 Preparation and characterization of LiCoPO4 films As described above, the films were deposited on the substrate by dip-coating for 1 h with a dip rate of 40 mm/min from the solutions prepared as described in the previous paragraph. After the deposition, the films were annealed at 300 °C in nitrogen atmosphere with a heating rate of 0.2 °C/min. Such annealing conditions have been used in order to obtain LiCoPO4 films of good homogeneity from a chemical and morphological point of views. Typical SEM micrographs of LiCoPO4 films are shown in Fig. 3. The morphological investigation revealed the good crystallization of the LiCoPO4 particles. Moreover, a dense and smooth surface of the film (Fig. 3b) can be observed. Such an effect could result from strains relaxations of the film during the amorphous/crystallized transition as often encountered when passing through such post crystallization step [32, 33] or to a difference in the thermal dilatation between the film and the substrate. From the Atomic Force (AF) data, a roughness of 83 nm has been calculated for the LiCoPO4 films. The morphology plays an important role on the electrical properties of the films. West et al. [33] observed on LiCoPO4 films deposited on Pt by magnetron sputtering the presence of a granular structure of the phase resulting from the aggregation of the LiCoPO4 grains with heating to both minimize the surface energy of the grains and as-deposited residual film stress. This transition was observed for samples annealed in inert atmosphere as well as for samples annealed in air. The authors obtained a dramatic improvement of the electrical properties after annealing at 700 °C for 1 h. 3.3 LiCoPO4 films

Fig. 2 SEM micrograph of the synthesized LiCoPO4 powder

In the Li–Fe–P–O system, well crystallized films with LiFePO4 as single phase have been obtained by annealing at 500 °C under Ar [31]. While this annealing temperature has been reported as the optimal one, the authors experienced that the heating rate also plays an important part in avoiding traces of secondary phases. In fact, XRD and electrochemical measurements revealed the presence of Li3Fe2(PO4)3 when the amorphous films were heated faster than 2 °C/min. A similar influence of the thermal treatment has been observed on the Co-containing phosphate system. Because of the delithiation process by increasing the temperature,

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decomposition of LizCoPO4 is accompanied by the formation of an additional amount of crystalline LiCoPO4. This is in agreement with the increasing intensity of LiCoPO4 reflections as observed in the diffractions patterns in the temperature range 100–200 °C. 3.4 Electrochemical measurements

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1 Cycle nd

2 Cycle 4

rd

3 Cycle

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-3

the formation of a Li-poor phase has been observed by heating at T = 90 °C. In the range 90–160 °C this phase disappears completely and, simultaneously, the amount of LiCoPO4 increases. The chosen annealing temperature should minimize the delithiation process and consequently the formation of secondary phases. In fact, as reported in [28] and confirmed by neutron diffraction and magnetic measurements [29], the transformation of the olivine-like LizCoPO4 and CoPO4 to the monoclinic Co2P2O7 means a significant structure rearrangement, including the coupling of isolated PO4 tetrahedra to P2O7 units via additional corner sharing. In situ synchrotron diffraction reveals that the crystallization of Co2P2O7 proceeds not simultaneously with the decomposition of LizCoPO4 and CoPO4, but at slightly higher temperatures. Two possible reaction paths can be considered: (a) the lithium-poor olivine-like phases undergo amorphisation at temperatures below 200 °C and transform to crystalline Co2P2O7 upon further heating; or (b) the lithium poor-phases decompose below 200 °C, forming initially amorphous Co2P2O7 which crystallizes at temperatures above 200 °C. In both cases, the

I (x10 mA)

Fig. 3 a and b SEM images of the prepared LiCoPO4 film

Unlike from the LiFePO4, the electrochemical activity of the LiCoPO4 system is still a controversial debate and has not been fully understood. Moreover, the lack of literature on LiCoPO4 films makes it more difficult to compare the obtained results. Extensive investigations on the electrochemical properties of LiCoPO4 powders prepared by sol–gel process have been recently reported [28, 29, 34 and references herein]. The authors observed the presence of two plateaus in the charge curve (corresponding to 0.7 B x B 1.0 and 0 B x B 0.7 respectively in the stoichiometric compound) independently of the electrolyte used during the measurements which support the idea that two-step delithiation is an intrinsic property of the system. This model was supported by using in situ and ex situ XRD to explain the Li extraction/insertion mechanism in LiCoPO4. However, the XRD results showed that the crystalline phase was only a lithium-poor phase; no ‘‘CoPO4’’ was identified even though LiCoPO4 was fully charged [34]. Analogous considerations can be valid for LiCoPO4 films. The preliminary electrochemical measurements revealed lithium ions charge/discharge activity in the prepared LiCoPO4 films. Cyclic voltammograms (CVs) for LiCoPO4 films using Li metal as counter and reference electrode cycled in the range 3.0–4.5 V are shown in Fig. 4.

2

anodic 1

cathodic

0 2,8

3,2

3,6

4,0

4,4

Voltage (V) Fig. 4 Cyclic voltammograms recorded for LiCoPO4 films in the potential range 2.5–4.0 V versus Li?/Li

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The CV curves show that the shape of the first cycle is quite different, while CV-profiles are almost reduplicate in the second and third cycles in the voltage range between 2.8 and 3.6 V. This is possibly due to a structural rearrangement in LiCoPO4 taking place during the first charge process of the LiCoPO4 thin film. The asymmetry between the cathodic and anodic CV plots could imply that the Li-ion diffusion in LiCoPO4 thin films obeys a different mechanism between charge and discharge processes as proved out recently by Xie et al. [35] by measuring the Li-ion chemical diffusion coefficients on thin films prepared by rf-magnetron sputtering. Other factors can affect the electrochemical properties of the LiCoPO4 films as e.g. electrolytes [33]. The difference in the electrochemical performances of a cathode material at different electrolyte components can be attributed to the influence of electrolyte components to form different solid films at the cathode surface. As the Li intercalation process is partly controlled by the Li migration through the films formed on the electrode surface, different solid films properties such as ionic conductivity, compositions, and structures lead to different charge-transfer processes. The electrolytes play a so important role in the electrochemical measurements that the thermodynamic instability of the electrolyte at the operation voltage close to 5 V versus metallic lithium could be the reason for drastic capacity losses. If a side reaction upon charging of the cell occurs, it contributes to the charge passed through the cell and then biases the calculation of the amount of extracted lithium. On the other hand, the degradation of the electrolyte on the cathode side may cause a limitation in mass transport on the cathode/electrolyte interface. The studies on the electrochemical behaviour of lithium metal phosphates as powders and especially as films are at early ages. Further investigations are necessary in order to better understand their chemical, physical and electrochemical properties to meet the requirements for industrial applications.

4 Conclusions A simple Pechini sol–gel method has been used for the synthesis of LiCoPO4 powders as well as, to the best knowledge of the authors, films for the first time. The powders were prepared from aqueous solutions, containing Li, Co and phosphate precursors to which acid citric and ethylene glycol was added, during the drying process at 75 °C. The X-ray diffraction patterns of the prepared powders confirmed the presence of LiCoPO4 with an olivine-like structure as main phase. The morphological investigations of the powders showed a platelet-like structure with an average grain size of 0.75 lm. The films

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of LiCoPO4 were deposited onto ITO glass substrates with the combination of the dip-coating process under the same conditions. The films, annealed in inert atmosphere at 300 °C, showed a smooth and homogeneous surface by performing HREM measurements. The preliminary electrical investigation on the prepared LiCoPO4 films showed lithium ions electrochemical activity in the range 3.0–4.5 V. Although these measurements are not conclusive, and according with the few literature available, the authors can affirm that a structural rearrangement in LiCoPO4 takes place during the first charge process of the LiCoPO4 films; that the asymmetry between the cathodic and anodic CV plots could imply that the Li-ion diffusion in LiCoPO4 thin films obeys a different mechanism between charge and discharge processes.

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