Preparation and characterization of LiNi0.8Co0.2O2/PANI microcomposite electrode materials under assisted ultrasonic irradiation
Graphical abstract
PANI/LiNi0.8Co0.2O2 microcomposites prepared under ultrasound irradiation are formed by oxide particles in contact with the conducting polymer procuring connectivity that enhances electrical and electrochemical properties of the resulting materials.
Introduction
The Ni-rich oxides of the Li–Ni–Co system present characteristics such as high cell voltage, high and variable oxidation states, high electronic conductivity at room temperature, making them interesting for applications as electrode in rechargeable lithium batteries [1], [2], [3], [4]. However, their commercialization is currently limited due to structural impediments associated with the cationic disorder in the octahedral layer occupation [5], [6], [7]. Among different synthetic strategies proposed as alternatives to overcome such limitations, we have recently proposed a chemical method for preparation of a Ni-rich mixed oxide phase with structural cationic ordering that enhances its behavior as lithium battery positive electrode [8]. On the other hand, it is known that Ni–Co mixed oxides present a close packed crystalline structure in which octahedral sites are occupied by lithium ions. As the structural features of these materials negatively influence lithium diffusion, their use as battery cathodes imposes one to work at low current (typically 30–50 μA). In addition, these oxides have high relative density and very poor mechanical properties making them less attractive for practical purposes.
To overcome the above problems we aim to contribute preparing hybrid materials of the LiNi0.8Co0.2O2 nickel-rich phase combined with a classic conducting polymer such as polyaniline (PANI). Doped PANIs are conducting materials of interest for use in secondary batteries [9], [10] that have been also satisfactorily combined with inorganic redox oxides, giving systems suitable for electrochemical Li intercalation. The first PANI-based nanocomposite was reported by Kanatzidis and co-workers [11]. These authors described the intercalative polymerization of aniline in V2O5 xerogel giving PANI-nanocomposites of good electrical conductivity. Later on Nazar and co-workers demonstrated the feasibility of these materials for reversible electrochemical lithium insertion and therefore their applicability as electrodes for rechargeable lithium batteries [12], [13]. The lithium chemical diffusion coefficient is higher for the nanocomposite than for the oxide alone by one order of magnitude, this effect being particularly remarkable for high cycling rates [13]. On the basis of this approach, the development of conducting nanocomposites received significant attention, deserving new studies using other polymers combined at the molecular (nanometer) level with different inorganic solids [14], [15], [16], [17]. Many of these systems involve inorganic hosts exhibiting a layered structure susceptible to be exfoliated by the guest polymer intercalation. Non-intercalable and three-dimensional structured metal oxides could be also combined with conducting polymers such as PANI or polypyrrole (PPy) giving rise to the so-called microcomposites, i.e. materials combined at the micrometer scale, or even nanocomposites taking into account the micrometer or nanometer scale size of the involved oxide particles [18], [19], [20], [21], [22]. The resulting solids can exhibit better electrical, electrochemical, mechanical, morphological and thermal properties than the former components (inorganic solid and guest polymeric species) and some of them have been also tested in rechargeable lithium batteries applications [23], [24], [25]. Concerning Ni and Co dioxides/PANI systems, Ramachandran and co-workers [26] have reported PANI intercalation by previous treatment of the host solids with ammonium peroxodisulfate. As discussed below, the same method applied to LixNi0.8Co0.2O2 phases does not produce intercalation compounds.
As sonochemistry appears as a powerful tool for preparation of a great variety of materials [27] this strategy has been applied here to prepare PANI/LiNi0.8Co0.2O2 composites by polymerization of PANI in the presence of LiNi0.8Co0.2O2 as a microparticulated solid assisted by ultrasound irradiation. This type of approach has been recently applied to the preparation of comparable materials, for instance, PANI/TiO2 composites providing compounds of enhanced conductivity, e.g. at room temperature [28]. The final objective of the present work is to prepare electroactive PANI/LiNi0.8Co0.2O2 microcomposites showing enhanced electrical conductivity and preserving their reversible electrochemical Li-insertion behavior, which make them improved electrode materials for Li-rechargeable batteries.
Section snippets
Synthesis
LiNi0.8Co0.2O2 is synthesized by a procedure previously described following a Li–Ni–Co mixed citrate route for the preparation of the Li0.7Ni0.8Co0.2O2 phase [8] that has been modified in the present case to reach the desired stoichiometry. In this way, the thermal decomposition of the citrate precursor has been carried out here in sealed Au crucibles adding 0.3 mole/formula of LiOH·H2O to procure additional Li. The LiNi0.8Co0.2O2/PANI composite (1:1 oxide/polymer intended ratio) was prepared
Results and discussion
In a first set of experiments we have applied the procedure reported by Ramachandran et al. [26] trying to prepare PANI/LiNi0.8Co0.2O2 composites with the aim to obtain nanocomposites based on the in situ intercalative polymerization of PANI within the mixed oxide layers. It should be remarked that although this last procedure was apparently successfully applied to obtain intercalation compounds of related solids, such as LiNiO2 and LiCoO2 [26], in our case the resulting compounds consisted of
Concluding remarks
Concerning the preparation of the LiNi0.8Co0.2O2/PANI composites, the ultrasound treatment appears to be more effective than conventional methods, such as magnetic stirring, to produce more homogeneous materials containing a large amount of the metal oxide particles of small size. The composites prepared under these conditions can be regarded as microcomposites instead of nanocomposites in view of the size of most part of the oxide particles and the lack of intercalation/exfoliation of the
Acknowledgments
Financial support from the CSIC and the Havana University (CITMA) through a Spanish–Cuban cooperation (reference 2001CU0007), the CICYT (Spain, MAT2003-06003-C02-01 project) and the UNESP-MES cooperation are gratefully acknowledged.
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