The Size and Shape Effect of LiMnPO4 Nanoparticles on the Lithium Ion Diffusion

Abstract

Advanced lithium ion batteries require higher safety, lower cost, longer durability and lower toxicity to apply larger applications [1].LiMnPO4 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 LiMnPO4 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-LiMnPO4 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 LiMnPO4 and the construction of nanocomposite of nano-LiMnPO4 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-LiMnPO4? ii) What is the desired composite structure to improve the electrochemical properties ?Since olivine LiMnPO4 materials have a preferred direction of lithium ion diffusion in the lattice, it can be different depending on the shapes and sizes of nano-LiMnPO4 shown in Fig. 1. Chemically exfoliated graphene from graphite flake was applied to nano-LiMnPO4, 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 LiMnPO4 nanomaterials. We also studied the influence of nanocomposite structures, which affected significantly the electrochemical behavior of LiMnPO4 cathode. Several processes of making nanocomposites have been investigated to understand the kinetics of diffusivity in LiMnPO4cathode.