Three-dimensional Macroporous Graphene-based Composites: Controllable Synthesis And Electrochemical Energy Storage

Graphene, with large specific surface area, excellent electrical conductivity and high chemical stability, has been considered as an ideal carbon material for a variety of applications including electronics, energy storage/conversion, biology, environment, etc. However, the strong aggregation and restacking of graphene sheets poses a barrier to obtain naturally single graphene layer which seriously hamper the practical applications of graphene. The three-dimensional（3D） macroscopic frameworks built up by graphene sheets, such as graphene papers, aerogels or foams, can not only mitigate the effect of aggregation and restacking between graphene sheets, but also provide highly interconnected channels for the transportation of charge carriers, which are appealing for the energy storage and conversion devices. Herein, the inorganic nanoparticles with different compositions were firstly anchored on the graphene oxide sheets（GO） by in-situ growth, resulting into hybrid nanosheets（M/GO）. The 3D macroporous inorganic nanoparticle/graphene hybrids were then fabricated via the hydrothermal assembly or ice-templating using the M/GO sheets as the building blocks. Such 3D graphene-based composites were utlized for both LIBs anode and cathode materials. The relationship between the structure and electrochemical performance of these macroporous hybrids was investigated. The contents are listed as follows:（1） 3D hierarchical tin oxide/graphene frameworks（SnO₂/GFs） were built up from the in-situ synthesis of two-dimensional（2D） SnO₂/graphene nanosheets followed by a hydrothermal assembly. SnO₂/GFs exhibited a high capacity of 800 mA h g–1 for up to 120 cycles of charge/discharge at 0.1 A g–1. The outstanding electrochemical behavior was attributed to the 3D frameworks with mesopores, macropores and large surface area, which not only prevented the π-π stacking of graphene and the agglomeration of SnO₂ nanoparticles, but also facilitated fast ion and electron transport in 3D pathways.（2） One strategy to enhance the electrochemical performance of electrode materials is to improve the loading amount of active compoment on hybrids. However, the large quantity loading of inorganic NPs will greatly weaken the interactions between the graphene sheets, and consequently reduce the stability and controllability of the resultant 3D macroporous hybrids. 3D macroporous graphene/SnO₂ frameworks（MGTF） with high SnO₂ loading and controllable macroporous structure were fabricated assisted by various polymer(polyethylenimine（PEI）, poly（vinyl alcohol）（PVA）, Pluronic F127（F127）, and graphene oxide（GO） as cross-linkers. With PEI as the cross-linker, the resulting N-doped MGTFPEI featured a high SnO₂ content（69%） with 5-10 μm macropores. Remarkably, N-doped MGTFPEI manifested an ultra-high capacity of 1227 mA h g–1 even after 120 cycles at a current density of 0.1 A g–1.（3） We developed an unprecedented fabrication strategy towards highly oriented macroporous graphene monoliths hybridized with carbon-coated SnO₂（C/Sn O₂/GM）, employing an ice-templating co-assembly of M/GO nanosheets（metal oxide/phosphates nanoparticles decorated on graphene oxide） and PVA, and followed by thermal treatment. The carbon coating layer could be controlled by adjusting the ratio of PVA/GO. When the ratio of PVA/GO was 1:1, the obtained C/SnO₂/GM with highly oriented macropores and carbon coated structure showed an ultrahigh capacity of 1665 mA h g^-1 at a current density of 0.2 A g^-1 for 200 cycles. Even at an ultrafast charge rate of 10 A g^-1, a decent capacity of 300 mA h g^-1 was achieved. Such remarkable electrochemical performance benefited from the unique highly oriented macroporous structure and the integrated carbon coated layer, which could reduce the electrode polarization at high current density, and protect the SnO₂ nanoparticles from agglomeration and reinforce the stability of the whole composite electrode.（4） The fabrication protocol described above further rendered the construction of high performance of other inorganic nanoparticles/graphene hybrids with unique highly oriented macroporous architectures and carbon coating structure, such as C/FePO₄/GM and C/Fe₃O₄/GM hybrids. Serving as the cathode in LIBs, C/FePO₄/GM delivered a highly reversible capacity of 175 mA h g^-1 at a current density of 0.05 A g^-1 for 150 cycles and an excellent capacity of 57 mA h g^-1 at 2 A g^-1; This superior performance was attributed to highly oriented 3D macroporous graphene frameworks with good conducting pathways. C/Fe₃O₄/GM also displayed an excellent electrochemical activity when used as the anode electrode materials for LIBs. It delivered a capacitance of about 1120 mA h g^-1 after 270 cycles at a current density of 0.5 A g^-1; and the capacity was maintained at 462 mA h g^-1 at a high current density of 8 A g^-1, which further valified that the highly oriented macroporous and cabon coated structure could greatly improve the electrochemical performance.