12598
J. Phys. Chem. C 2010, 114, 12598–12603
Influence of Particle Size and Crystal Orientation on the Electrochemical Behavior of Carbon-Coated LiFePO4 Stefania Ferrari,† Rodrigo Lassarote Lavall,† Doretta Capsoni,† Eliana Quartarone,† Aldo Magistris,† Piercarlo Mustarelli,*,† and Patrizia Canton‡ Dipartimento di Chimica Fisica “M. Rolla”, UniVersita` degli Studi di PaVia, and IENI-CNR, PaVia, Italy, and Dipartimento di Chimica Fisica, UniVersita` Ca’ Foscari, Venezia, Italy ReceiVed: March 22, 2010; ReVised Manuscript ReceiVed: May 20, 2010
We investigate the influence of particle size and crystal orientation on the electrochemical behavior of carboncoated LiFePO4 prepared by a hydrothermal synthesis in the presence of a polymeric surfactant and a source of carbon. We evaluated the charge/discharge profiles of two samples, one constituted by particles in the micrometer range with a platelet-like shape (large ac facet and (020) crystal orientation) and another made of sub-micrometer-rounded particles with a random crystal orientation. At low current rates the crystal orientation seems to be the prevailing factor, whereas at high current rates smaller particles can allow a shorter electronic conduction path, so reducing the resistance experienced by Li ions during diffusion. 1. Introduction The use of lithium-ion batteries can be a suitable choice for the 21st century “low-emission” society.1 Among the cathode materials, LiFePO4 is indeed the most promising one for largescale applications, e.g., in hybrid vehicles.2-5 Recent reports gave experimental evidence of one-dimensional lithium diffusion,6 which calls for the need of high crystal orientation, and highlighted the possibility to change the well-established twophase room-temperature insertion process into a single-phase one by lowering the particles size,7-9 which suggests that nanocrystalline structures play a major role in improving cathode performance. Preferential lithium diffusion has been also demonstrated by means of first principles calculations.10-13 The LiFePO4 olivine structure has a theoretical capacity of 170 mAhg-1, presents suitable thermal stability, no toxicicity being environmentally benign, and low cost.3,4 The main problem is related to its low intrinsic electronic conductivity that can be increased by different approaches, such as the addition of a carbon source to provide a homogeneous carbon coating around the particles, the doping with some elements, and the modulation of particles size and texture.4,14-17 However, a better understanding of the mechanism involved in the process of charge/discharge during device cycling is mandatory for the preparation of an optimized material. This has been a very active research field in recent years, but the subject is still under investigation despite the fact that several models have been proposed.2,3,18-22 Among them, the domino cascade model is relevant, since it changed the way we look at the mechanisms involved in the charging/discharging of lithium single transition metal phosphates.23 Besides the peculiarities of each model, however, there is a general agreement that Li ions move in the tunnels along the b direction and are extracted or inserted at the interface (phase boundary) where LiFePO4 and FePO4 crystalline structures coexist. The phase boundary motions and nucleation phenomena govern the dynamic of the process.11 * To whom correspondence should be addressed. Phone: +39 0382 987205. Fax: +39 0382 987575. E-mail:
[email protected]. † Universita` degli Studi di Pavia. ‡ Universita` Ca’ Foscari.
According to the different models, the phase boundary can lie in the bc plane or in the plane ([110], c). Chen and co-workers proposed that lithium is extracted at disordered transition zone on the ac crystal surface as the phase boundary moves in the direction of the a axis, and since Li ions move parallel to b, the ac plane is the only one active for lithium extraction/insertion.22 The rate of boundary diffusion depends on particle size and shape, and because of the anisotropic crystal structure of LiFePO4,24 the crystal orientation of the particles has significant effects on the charge-discharge process and on the cell capacity.25 At the same time, Gibot et al. recently showed that the reduction of the particle size to the nanometer range allows voltage charge/discharge curves characteristic of a single-phase behavior, instead of the usual two-phase one, which has some intrinsic advantages with respect to various storage applications.9 The idea that the overall charge/discharge behavior on cells prepared with LiFePO4 must take into account both the crystal orientation and the particle size has been stressed in the literature.14 Our key feature, here, is to try to separate the influence of: (i) particle size and (ii) crystal orientation in determining the electrochemical behavior of carbon-coated LiFePO4. Since the preparation of particles having the same dimension and different facet orientations is not easy whereas, at the same time, a strong downshift of the particles size may lead to a very complex situation because of the injection of many variables related to the size confinement,9 we decided to follow a sort of compromise by comparing well oriented and nonoriented crystals with dimensions not more different than roughly 1 order of magnitude. Within the limits of our approach, we are reasonably confident to separate, at least at a first approximation, the effects of our parameters of interest on the electrochemical cell performances. The samples were prepared by hydrothermal synthesis in the presence of a polymeric surfactant (polyvinylpyrrolidone, PVP) and a carbon source (glucose). Discharge rates spanning over more than 2 orders of magnitude were investigated. 2. Experimental Methods Three samples of olivine were prepared by hydrothermal synthesis. An uncoated LiFePO4 sample (sample A) was
10.1021/jp1025834 2010 American Chemical Society Published on Web 07/06/2010
Electrochemical Behavior of Carbon-Coated LiFePO4 prepared by the reactive system FeSO4 · 7H2O (Aldrich g99.0%), NH4H2PO4 (Aldrich g99.9%), CH3COOLi · 2H2O (Fluka g99.0%) in the molar ratio 1:1:3 (Fe:P:Li). The lithium excess forms Li2SO4 thus balancing the SO42- ions provided by the iron sulfate. The reagents were dissolved in a polyvinylpyrrolidone (PVP, Aldrich) aqueous solution (25 wt % PVP). The resulting solution was put in a Teflon reactor inside a stainless steel Parr autoclave. The hydrothermal synthesis was performed in N2 atmosphere for 5 h at 170 °C under self-generated pressure. After the reaction, the light green precipitate was filtered and dried in an oven at 80 °C for 24 h. A final thermal treatment was accomplished in a tubular oven at 600 °C in a N2 flow for 5 h. The second sample (sample B) was prepared by the same procedure used for sample A with the addition of glucose (Merck g99.0%) in the molar ratio 1:1:3:1 (Fe:P:Li:C) in the reaction mixture to obtain carbon-coated LiFePO4/C. Before the thermal treatment, a portion of this sample was ball-milled for 30 min at 250 rpm using a planetary mill (jar sealed under argon atmosphere); then it was heated at 600 °C in nitrogen flow for 5 h, as the previous one, thus obtaining the sample indicated as LiFePO4/C milled (sample C). After the thermal treatment at 600 °C, the sample B possesses 8.78 wt % of carbon residue determined by elemental analysis (UNI CEN/TS 15407 2006). The X-ray powder diffraction (XRD) patterns were collected on a Bruker D5005 diffractometer, using Cu KR (KR1 ) 1.5406 Å, KR2 ) 1.5443 Å) radiation, operating at 40 KV and 30 mA in the angular range 10° < 2θ < 100° with an acquisition step of 0.015° and 1 s/step of counting time. A nickel filter and a position sensitive detector were used. The structural and profile refinements were performed on the basis of the Rietveld method with the TOPAS 3.0 software. The Rietveld refinement was performed using a fundamental parameters line profile fitting.26-28 The pattern refinements were carried out starting from the lattice parameter and the atomic position obtained in the study of Streltsov et al.29 Scanning electron microscopy (SEM) micrographs at different magnifications were collected with a Cambridge Stereoscan 200 and with a Zeiss EVO-MA10-HR microscopes on gold-sputtered samples. Transmission electron microscopy (TEM) analysis has been performed using a JEOL JEM3010 at a 300 kV acceleration voltage. The materials structure has been monitored using selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). For TEM analysis, the powder specimen was sonicated in isopropanol and transferred as a suspension to a copper grid covered with a holey carbon film. To prepare the cathode layer, a slurry was made by mixing the LiFePO4 active material with carbon black (Alfa) and poly(vinylidene fluoride) (PVdF, Solvay) in N-methyl-2-pyrrolidone (NMP, Aldrich) with a weight ratio of 70:20:20. The suspensions were spread on an aluminum current collector by using a doctor blade. After the evaporation of the solvent in a oven at 60 °C for 24 h, the foils were transferred to an Arfilled drybox (MBraun,
