Development of high capacity rechargeable battery anode would significantly improve the advanced energy technologies and enhance the energy efficiency of many economic markets, particularly electric grid storages and renewables. Recently, major research has been directed in finding new anode materials capable of hosting more lithium and hence, delivering higher energy densities. Graphite, consisting of commensurately-stacked infinite layers of graphene, is the commonly used anode material in lithium-ion batteries (LIBs) due to its high energy density and high power density, though with limited capacity (372 mAh/g). The limited capacity is caused by low diffusion of lithium into interplanar spaces of commensurately-stacked graphene layers. Here we present a feasible fabrication technique of novel graphene structures using commercial Nickel and Copper powders by CVD method. High-quality 3D porous structures were created, which consist of up to 93% of incommensurately-stacking multilayer graphene (IMLG) (Figs. 1a,b). This IMLG graphene can be considered as a new type of “graphite-like” structure with new physical and electronic properties, where the interplanar interaction forces are significantly weaker as compared to graphite. IMLG pristine structures were applied as an anode material in LIBs coin cells exhibits four times higher reversible capacity up to 1540 mAh/g (Sample 1 in Fig. 1) than the theoretical capacity of graphite. This high capacity remains stable throughout long-term cycling under high current density. Structural and binding analyses of IMLG electrodes revealed that lithium atoms reversibly intercalate/de-intercalate within incommensurately-stacked layers, followed by a further flexible rearrangement of layers for a long-term stable cycling. The retained capacity remains over 93% throughout hundred cycles by reaching up to 100% Coulombic efficiency for LiC2 stoichiometry (1116 mAh/g capacity) (Samples 2,3 in Fig. 1). C-rate testing of high capacity cells demonstrated stability under higher current densities which make these cells feasible. Graphene foam of 19-93% incommensurate fraction was tested throughout hundred cycles and revealed an increase of reversible capacity from 410 mAh/g up to 1540 mAh/g by a decrease of stacking commensurateness of layers (Fig. 2). Specific capacity shows the nonlinear dependence of incommensurate fraction by reaching the maximum capacity of 1674 mAh/g that corresponds to Li3C4. This nonlinear dependency allows us to consider also other factors affecting the reversible capacity such as the number of graphene layers that participate in charge transfer. Based on our observations on lithiated graphene electrodes, the X-ray probing and HRTEM analysis led us to propose a new model of Lithium insertion into multilayer structures where capacity varies N number of graphene layers according to Li(N+1)C2N formula in which all carbon hexagons are occupied by lithium atoms. The specific capacity increases as the number of graphene layers decrease achieving maximal in a bi-layer configuration corresponding to Li3C4 stoichiometry. In fact, incommensurate graphene layers provide fully intercalation of lithium without any damage in graphene structure and are capable of stable long-term charge-discharge cycling. An effective capacity increase over six times compare to commercial graphite promises the feasibility of the rapid development of lightweight, cost-efficient, high-capacity secondary batteries.
Journal: TechConnect Briefs
Volume: 1, Advanced Materials: TechConnect Briefs 2017
Published: May 14, 2017
Pages: 95 - 98
Industry sector: Advanced Materials & Manufacturing
Topics: Carbon Nano Structures & Devices, Graphene & 2D-Materials