The Size and Shape Effect of LiMnPO 4 Nanoparticles on the Lithium Ion Diffusion

Advanced lithium ion batteries require higher safety, lower cost, longer durability and lower toxicity to apply larger applications [1]. LiMnPO 4 can be an alternative cathode material due to its stable structure, low material cost, lower toxicity, high theoretical capacity (170 mAh/g), high operating voltage (4.1 V vs. Li) and good capacity retention. However, it suffers from poor electronic and ionic conductivity [2, 3]. Its poor ionic conductivity can be overcome by employing nano-particles in order to shorten Li-ion path lengths [4, 5]. Enhancement in electron transport is achieved by carbon coated nanocomposite cathode material. Most high-performing LiMnPO 4 materials were so far achieved by adding a large amount of carbon (15 – 30 wt%) in order to increase the electronic conductivity [6-9]. Recently, we reported < 30 nm sized nano-LiMnPO 4 reached 97 % of theoretical capacity with 10 wt% of carbon additive in total in the electrodes [10]. It shows that controlling the particle size of LiMnPO 4 and the construction of nanocomposite of nano-LiMnPO 4 and carbon are critical to maximize the electrochemical properties. Therefore, we investigated further to address the following questions; i) which direction is favorable for lithium ions in different shapes of nano-LiMnPO 4 ? ii) What is the desired composite structure to improve the electrochemical properties ? Since olivine LiMnPO 4 materials have a preferred direction of lithium ion diffusion in the lattice, it can be different depending on the shapes and sizes of nano-LiMnPO 4 shown in Fig. 1. Chemically exfoliated graphene from graphite flake was applied to nano-LiMnPO 4 , forming a thin coating on the surface of the active material shown in Fig. 2. We determined the lithium ion diffusion coefficients in terms of shapes and sizes of LiMnPO 4 nanomaterials. We also studied the influence of nanocomposite structures, which affected significantly the electrochemical behavior of LiMnPO 4 cathode. Several processes of making nanocomposites have been investigated to understand the kinetics of diffusivity in LiMnPO 4 cathode. References: [1] G.-A. Nazri, G. Pistoia, Lithium batteries: science and Technology, Kluwer Academic Publishers, USA, 2004. [2] M. Yonemura, A. Yamada, Y. Takei, N. Sonoyama, R. Kanno, J. Electrochem. Soc. 151 (2004) A1352-A1356. [3] P.P. Prosini, M. Lisi, D. Zane, M. Pasquali, Solid State Ionics 148 (2002) 45-51. [4] N.-H. Kwon, T. Drezen, I. Exnar, I. Teerlinck, M. Isono, M. Graetzel, E