Magnetite, Fe 3 O 4, is a promising anode material for lithium ion batteries due to its high theoretical capacity (924 mA h g −1 ), high density, low cost and low toxicity. However, its application as high capacity anodes is still hampered by poor cycling performance. To stabilize the cycling performance of Fe 3 O 4 nanoparticles, composites comprising Fe 3 O 4 nanoparticles and graphene sheets (GS) were fabricated. The Fe 3 O 4 /GS composite disks of μm dimensions were prepared by electrostatic self-assembly between negatively charged graphene oxide (GO) sheets and positively charged Fe 3 O 4 -APTMS [Fe 3 O 4 grafted with (3-aminopropyl)trimethoxysilane (APTMS)] in an acidic solution (pH = 2) followed by in situ chemical reduction. Thus prepared Fe 3 O 4 /GS composite showed an excellent rate capability as well as much enhanced cycling stability compared with Fe 3 O 4 electrode. The superior electrochemical responses of Fe 3 O 4 /GS composite disks assure the advantages of: (1) electrostatic self-assembly between high storage-capacity materials with GO; and (2) incorporation of GS in the Fe 3 O 4 /GS composite for high capacity lithium-ion battery application.

It is challenging to design silicon anode exhibiting stable cycling behavior, high volumetric and specific capacity, and low volume expansion for Li-based batteries.Herein, we designed Si/C-IWGN composites (Si/C composites internally-wired with graphene networks). For this purpose, we used simple aqueous sol-gel systems consisting of varying amounts of silicon nanoparticles, resorcinol-formaldehyde, and graphene oxide. We found that that a small amount of graphene (1-10 wt%) in Si/C-IWGNs efficiently stabilized its cycling behavior. The enhanced cycling stability of Si/C-IWGNs could be ascribed to the following facts: 1) ideally dispersed graphene networks were formed in the composites, 2) these graphene networks also created enough void spaces for silicon to expand and contract with the electrode thickness increase comparable to that of graphite. Furthermore, properly designed Si/C-IWGNs exhibited high volumetric capacity of ~141% greater than that of commercial graphite.Finally, a hybrid sample, Si-Gr, consisting of a high capacity Si/C-IWGN and graphite was prepared to demonstrate a hybrid strategy for a reliable and cost-effective anode with a capacity level required in high-energy Li-ion cells. The Si-Gr hybrid exhibited not only high capacity (800-900 mAh g -1 at 100 mA g -1 ) but also high electrode volumetric capacity of 161% greater than that of graphite. 6 large fraction of void spaces between nanoparticles and graphene networks in these composites. Since, Si/C composites are internally-wired with graphene networks, they are denoted as Si/C-IWGNs hereafter. The Si/C-IWGN samples were simply prepared by carbonizing the composite gels formed in aqueous mixtures consisting of SiNPs, resorcinol(R)-formaldehyde (F) as the carbon precursor and a small amount of graphene oxide (GO) in one-pot reactions as shown in Scheme 1. Various Si/C-IWGN samples were prepared with different contents of SiNPs (40 or 50 wt%) and graphene (1, 5 and 10 wt%). Two types of gelation catalysts (C), Na 2 CO 3 or NH 4 OH, with different concentration (R/C ratio = 100-500 in molar) were used for forming composite gels.Various electrochemical responses of Si/C-IWGNs, such as cycling stability, volumetric as well as specific capacity, Coulombic efficiency, and electrode thickness increase, are thoroughly compared with those of the following reference samples: 1) control composites, Si/C composites, which were prepared without GO addition in the gel formation process in Scheme 1, and 2) commercial graphite. Finally, we have demonstrated a hybrid strategy to develop a reliable and low-cost anode materials consisting of mixtures of high capacity Si/C-IWGNs and commercial graphite. Experimental

A series of Si/graphene sheet/carbon (Si/GS/C) composites was prepared by electrostatic self-assembly between amine-grafted silicon nanoparticles (SiNPs) and graphene oxide (GO). The Si/GS derived from carbonization of Si/GO assemblies showed limited cycling stability owing to loose cohesion between SiNPs and graphene, and increased impedances during cycling. To counteract the cycling instability of Si/GS, an additional carbon-gel coating was applied to the Si/GO assemblies in situ in solution followed by carbonization to yield dense three-dimensional particulate Si/GS/C composite with many internal voids. The obtained Si/GS/C composites showed much better electrochemical performances than the Si/GS owing to enhanced cohesion between the SiNPs and the carbon structures, which reduced the impedance buildup and protected the SiNPs from direct exposure to the electrolyte. A strategy for practical use of a high-capacity Si/GS/C composite was also demonstrated using a hybrid composite prepared by mixing it with commercial graphite. The hybrid composite electrode showed specific and volumetric capacities that were 200% and 12% larger, respectively, than those of graphite, excellent cycling stability, and CEs (>99.7%) exceeding those of graphite. Hence, electrostatic self-assembly of SiNPs and GO followed by in situ carbon coating can produce reliable, high-performance anodes for high-energy LIBs. Energy storage via rechargeable batteries will play an increasingly important role in the future not only to power advanced mobile electronic devices, power tools, sensors for internet-of-things devices, medical implants, military and aerospace devices, drones, e-bikes, many electrified vehicles, and so on, but also to store energy from intermittent renewable resources (solar and wind power) and for back-up energy supplies for smart electric grids.1-3 Among many rechargeable batteries, lithium-ion batteries (LIBs) offer the highest energy density to date and reasonably long cycle life and power capability. [4][5][6] Nonetheless, the demands for high-energy LIBs are increasing, especially the demand for electric vehicles to carry sufficient energy comparable to that of internal combustion vehicles. 1,2,[7][8][9] To further enhance the energy content of an LIB, electrode materials capable of delivering high capacity at a high working voltage (a higher-potential cathode coupled with a lower-potential anode vs. Li/Li + ) have to be incorporated, and the cell design has to be optimized to maximize the packing density. Noticeable progress has been made on the cathode side recently by optimizing the composition and structure of Ni-rich LiNi x Co y Mn z O 2 10,11 and Li-rich layered oxide 12,13 cathodes, which has resulted in LIBs with a much higher energy content. On the other hand, all LIB anodes are still made of graphite, whose theoretical capacity (372 mAh g −1 ) has been achieved since its first introduction in 1991. Therefore, a breakthrough in terms of the energy density of LIBs would require adoption of new anodes having a m...

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