Control of AlPO4-nanoparticle coating on LiCoO2 by using water or ethanol
Introduction
Nano-sized inorganic compounds have attracted a great deal of scientific and technical interest as a result of their unique physical and chemical properties that bulk materials may not possess [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Because nanoparticles have a large surface area to volume ratio, the state of the surface molecules plays a key role in determining their properties. In addition, many studies on nanoparticle coatings with oxides or monomer shells aimed at optimizing the properties of the nanoparticles have been reported [1], [2], [3], [4], [5], [6], [7]. While most of these studies have been focused on polymer-supported metal-nanoparticle coatings on metallodielectric spheres, there are few reports of the direct coating of oxide nanoparticles on macro-sized inorganic compounds [16], [17], [18], [19], [20].
The recent increase in mobile electronics has led to the rapid expansion in the demand for Li batteries. In Li-ion cells, the cathode material is the most important part determining the cell capacity and safety, and LiCoO2 is the most widely used material. However, the thermal stability and electrochemical properties of the cathode materials, such as the cycle-life performance and rate capability, largely depend on their powder size, i.e., BET surface area [21], [22], [23], [24], [25], [26], [27], [28], [29]. A cathode material with a larger powder size has larger thermal stability upon charging, but the rate capability and cycle-life performance deteriorate at a higher current rate [29]. The former result is associated with a decreased exothermic reaction of the cathode/electrolyte interface, and the latter is related to the increased Li-diffusion length. Metal-oxide (Al2O3, ZrO2, TiO2, etc.) coatings have been reported to be effective in overcoming these electrochemical shortcomings [30], [31], [32], [33]. Although this method improves the capacity retention and Li diffusivity during cycling, it does not show any noticeable enhancement in thermal stability at the overcharged state (12 V). Recently, a direct nanoparticle coating on the powders in a water-based solution with a uniform nanoscale (∼20 nm) coating on the cathode surface was reported [34], [35], [36], [37], [38].
This paper reports that the AlPO4-nanoparticle crystallinity can be controlled by varying the solvent, and examines its effects on the electrochemical and thermal properties while our previously work that solely dealt with the dependence of AlPO4 coating concentration (1.2–3 wt.%) on electrochemical cycling above 4.6 V [39]. None of previous coating papers did not report any correlation between crystallinity of coating material and electrochemical properties.
Section snippets
Experimental
Aluminum nitrate (Al(NO3)3·9H2O, 1 g) and ammonium phosphate ((NH4)2HPO4, 0.33 g) were dissolved in either distilled water or ethanol, and were mechanically mixed, until a white-colored AlPO4-nanoparticle dispersed solution was observed. These were mixed with LiCoO2 (with an average particle size of ∼10 μm and BET surface area of 0.2 m2/g), which were followed by drying at 130 °C for 6 h and annealing at 700 °C for 5 h, respectively. The estimated AlPO4 to LiCoO2 ratio was 0.3 wt.%.
The cathodes for the
Results and discussion
Fig. 1 shows transmission electron microscopy (TEM) images of the AlPO4 nanoparticles prepared in water and ethanol. The size distribution of the particles prepared in water and ethanol is approximately 3–5 nm and 10–20 nm, respectively, indicating that the particle size is greatly affected by the solvent. The AlPO4 nanoparticles in water instantly began to precipitate from the dissolved Al(NO3)3·9H2O and (NH4)2HPO4 according to the following reaction:Al(NO3)3·9H2O + (NH4)2HPO4 → AlPO4 (↓) + HNO3 + 2NH4NO3
Conclusions
The electrochemical properties and phase transitions of the coated LiCoO2 above 4.6 V were affected by the solvent used for the AlPO4-nanoparticle synthesis. The capacity retention of the coated LiCoO2 powders prepared in ethanol was better than that prepared in water. The AlPO4-coating layer prepared in water or ethanol can suppress Co dissolution effectively, so the electrochemical properties of the coated cathodes are much better than the bare cathode. The difference between the coated
Acknowledgement
This work was supported by grant # R05-2004-000-10029-0 from the Ministry of Science and Technology and University IT Research Center project.
References (46)
- et al.
Chem. Phys. Lett.
(1998) - et al.
J. Power Sources
(1999) - et al.
J. Power Sources
(1999) - et al.
J. Power Sources
(2001) Electrochem. Commun.
(2003)Electrochmica. Acta
(2003)- et al.
J. Power Sources
(2004) - et al.
Electrochim. Acta
(2002) - et al.
Electrochim. Acta
(2004) - et al.
J. Power Sources
(2001)