Improved Performance of Hydrothermally Synthesized by Ball Milling as a Positive Electrode for Li Ion Battery

LiMnPO4 is anticipated to be a promising cathode material for next generation lithium battery. A reduction of particle size is recognized as a good strategy to improve its performance and it can be achieved by ball milling. However, the ball milling including carbon addition forms small LiMnPO4 particles with large carbon content, which leads to low volumetric energy density of electrode. In this study, carbon-coated LiMnPO4 prepared by hydrothermal route was applied to the ball milling without carbon addition. The reduction of particle size of carbon-coated LiMnPO4 was achieved by the ball milling without destroying the surface carbon layer. The ball-milled LiMnPO4 particle revealed better cathodic performance than non-milled sample. This was attributed to shortening Li ion diffusion path, improvement of structural flexibility, and large surface area of electrode due to reduction of particle size. The ball milling is attested to be a promising method to improve cathodic performance of carbon-coated LiMnPO4. 1. Introduction LiMPO4 compounds (M = Fe, Mn, Co, and Ni) with olivine structure have been attracted as alternative cathode materials for lithium ion batteries owing to their low cost, low toxicity, chemical and thermal stabilities compared with currently used LiCoO2 [1]. Among these compounds, LiMnPO4 has been recognized the most attractive compound due to its high operation voltage determined by Mn3+/4+ redox couple at 4.1？V versus Li/Li+ and is compatible with the system presently used in lithium ion battery [2]. However, the most serious problem of this cathode material is low intrinsic electronic and Li ion conductivities [3, 4]. Various experimental reports have indicated that carbon-coating can provide high electronic conductivity [5–7]. In fact, a large charge-discharge capacity of carbon-coated LiFePO4 synthesized by a hydrothermal synthesis method has been reported [8]. As for improvement of Li ion conductivity, much effort has been paid for particle size reduction to shorten Li ion conduction path [9]. Drezen et al. reported reversible capacities for 140 and 270？nm diameter LiMnPO4 particles prepared by sol-gel method were 81 and 7？mA？h？g？1, respectively [10]. Some groups reported a good performance of ball-milled small LiMnPO4 [11] and LiCoPO4 particles [12]. However, their processes included conductive carbon addition (≤20？wt.%) before the ball milling to obtain a carbon composite. This large amount of carbon causes surely improvement of the electronic conductivity, however, such heavy carbon coating decreases volumetric capacity.