Holey Reduced Graphene Oxide/Carbon Nanotube/LiMn0.7Fe0.3PO4 Composite Cathode for High-Performance Lithium Batteries

Abstract

Introduction 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) [1]. However, when the graphene sheet has large area, the wrapping of active materials may hinder the transportation of Li+ between active particles and electrolyte, especially at high charge/discharge rates.Herein, holey graphene oxide (h-GO) [2], which is made by a green wet ball-milling of GO in one step without using any catalysts or chemicals, is combined with carbon nanotubes (CNTs) and LiMn0.7Fe0.3PO4 (LMFP) to make a composite cathode for lithium batteries [3]. Results show that the LMFP cathode with h-GO/CNT shows a remarkedly improved electrochemical performances due to the facilitated Li+ transport pathway, compared to that with conventional GO/CNT. This study provides a new approach for fabricating holey graphene and can open up new possibilities for applications on power sources. Results and discussion The synthesis of h-GO and LMFP composite material is shown in Fig. 1. Graphite oxide suspension (~0.6 mg/mL) was obtained by mixing with purified water and after the ultrasonic treatment for 3 hours. Subsequently, the ball-milling of 50 mL GO suspension was performed in a planetary ball-mill machine with 30–120 g of 100 mm size zirconia balls for 20 min at 400 rpm (the corresponding weight ratio of balls to GO aqueous suspension is 0.6−2.4). The balls were fully removed by sieve afterward. Finally, the mixture was freeze-dried for 12 hours. LMFP, h-GO, CNTs and binder were mixed at a mass ratio of 95.6%: 2%: 0.4%: 2% 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 h-GO is shown in Fig. 2. It is found that irregular micropores can be generated on the surface of GO after the ball-milling of GO aqueous suspension. At the conference, we will show that the more balls used, the more and bigger holes could be observed. That is, the hole size and surface density could be controlled by the adjustment of the weight ratio of balls to GO aqueous suspension. We will also show that LMFP electrode with h-GO/CNT can deliver more than half of discharge capacities than that with conventional GO/CNT at a higher discharge rate (>=20 C), also superior to that with h-GO or CNT only. The use of h-GO made by the other method, such as the chemical etching method, the performances will also be discussed. The scalable and robust synthesis method of h-GO and the novel properties of h-GO/CNT reported here greatly advance the practicality of using it as electrodes in lithium batteries as well as other energy storage technologies. Acknowledgment We thank Taiheiyo Cement Co., Ltd for the providing of LiMn0.7Fe0.3PO4 powders and NIMS for the TEM measurement. Reference 1. Han, et.al., Small. 9 (2013). 2. Y. Xu, et al., Nat. Commun. 5 (2014). 3. D. Ding, et.al., J. Power Sources. 449 (2020). Figure 1