A Facile Way to Synthesize Carbon-Coated LiMn0.7Fe0.3PO4/Reduced Graphene Oxide Sandwich-Structured Composite for Lithium Ion Batteries

Abstract

Introduction Olivine-type LiMnPO4 which is a cathode material for lithium ion batteries exhibits a higher operating potential (4.1 V vs. Li+/Li) than LiFePO4 thus has gathered more attention in recent years. However, due to the low electron conductivity of LiMnPO4 and the Jahn-Teller effect of Mn3+, it is difficult to achieve satisfied cycle performance. Fe doping of 10-30 mol% is promising as a solution to this problem [1]. Graphene is a single atomic monolayer of graphite and has been found a variety of applications in energy conversion and storage devices, due to the excellent properties such as high carrier mobility (~10,000 cm2 V−1 s−1 at room temperature), structural flexibility, chemical and thermal stability, mechanical strength and ultrahigh theoretical specific surface area (2630 m2g−1) [2]. Previous studies have reported the improved performance of LiMPO4 (M = Fe, Mn and Co by combining with graphene. In particular, it is attractive that the active material is supported on both sides of graphene to form a sandwich structure. One is that active material particles can be highly loaded and interconnected by the highly conductive matrix formed by graphene. The other one is that the sandwich structure can be beneficial for suppressing the graphene restacking and active material particles agglomeration. It is reported that such a graphene-metal oxide nanoparticle composite material can greatly improve the performances of the anode. However, there is still lack of a facial way to obtain ideally sandwich-structured composite material for cathode [3]. We will introduce a facial and high efficient way to synthesize carbon-coated LiMn0.7Fe0.3PO4 (LMFP)/reduced graphene oxide (rGO) sandwich-structured composite for high power lithium ion batteries [4]. Results and discussion The synthesis of sheet-like carbon-coated LMFP/rGO composite material is shown in Fig. 1. First, 0.3 g of LMFP powder (without carbon coating, average particle size 200 nm), 3 mL of oxidized graphite suspension and 0.04 g of sucrose are dispersed in 100 mL of water. Thereafter, by ultrasonic treatment for 2.5 hours, the oxidized graphite gradually peels off to form graphene oxide, and at the same time, the LMFP nanoparticles are supported on the surface of the GO by interaction with the GO functional group. The dissolved sucrose is coated with GO and LMFP particles to form a polymeric coating. After lyophilization, carbon coated LMFP/rGO composite material (LMFP/rGO@C) having a sandwich structure was obtained by heat treatment at 700 degrees under a reducing atmosphere. LMFP/rGO@C, acetylene black and binder were mixed at a mass ratio of 8:1:1 and coated onto Al foil to obtain a cathode electrode. Electrochemical characteristics were evaluated by preparing a 2032 type coin cell using Li foil as the counter electrode. The electrolyte was 1 mol dm-3 LiPF6/EC-DEC (1: 2). SEM image of the synthesized LMFP/rGO@C composite material is shown in Fig. 2. It is clearly observed that all the LMFP particles are uniformly dispersed on the rGO surface at high density (50 to 100 particles mm-2), forming a sandwich structure. The total content of rGO and carbon coating is estimated to be about 7 wt% from TG. Figure 3 shows the discharge rate performance of LMFP/rGO@C and carbon coated only LMFP (LMFP@C). It was confirmed that the discharge capacity of LMFP/rGO@C is greatly improved compared with LMFP@C. This should be due to the formation of a good three-dimensional conductive network by graphene. Acknowledgement We thank Taiheiyo Cement Co., Ltd for the providing of LiMn0.7Fe0.3PO4 powders and NIMS for the TEM measurement. Reference 1) Gong, et.al., Energy Environ. Sci. 4, 3223 (2011). 2) S. Han, et.al., Small. 9 1173–1187 (2013). 3) Z.-S. Wu, et.al. Nano Energy. 1 (2012). 4) D. Ding, et.al., submitted. Figure 1