Design And Investigation Of Graphene-based Energy Storage Devices With High Volumetric Energy Density

Nanostructuring of electrode materials has enabled electrochemical energy storage?EES?devices lighter and more energetic,however,it shows limited ability to make an EES device as small as we wish due to the unsatisfactory volumetric performance resulting from the low density of nanomaterials.Among electrode materials,graphene is considered as a promising candidate due to its flexibility to design varies of nanostructures.Densifying graphene network is an efficient way to enhance the volumetric performance.In this respect,the pore structure and density need to be balanced in a densified graphene framework,where the former dictates the ease of ion transport and the later determines the volumetric performance.In this work,we show some typical examples to tune the pore structure of a dense graphene monolith and discuss their applications in EES devices for compact energy storage.Firstly,we presents a new class of porous graphene particles with a three-dimensional microscale network and an ultrahigh specific surface area(2590 m²g^-1),which is obtained by the KOH activation of a compact graphene monolith.As supercapacitor electrodes,such porous graphene particles show high compressibility and little capacitance loss when subjected to a compressive force up to 40 MPa,yielding an excellent volumetric capacitance of 170 F cm^-3 in ionic liquid.Secondly,we develop a novel approach to fabricate a monolithic ultra-thick and dense carbon electrode for symmetric supercapacitors.We found that ZnCl₂ is an ideal sacrificial pore former to tune the specific surface area of the monolithic graphene from 370 to over 1000 m² g^-1 while the monoliths maintain a high density from 1.6 to0.6 g cm^-3.Having a good balance of porosity and density,the directly sliced graphene pellet electrode with a thickness up to 400?m delivers a capacitance of 150F cm^-3 in ionic liquid,corresponding to a volumetric energy density of⁶5 Wh L^-1 for at the device level,the highest value reported to date for supercapacitors.Thirdly,we fabricate high-density graphene bulk materials with“ink-bottle-like”mesopores by H₃PO₄ activation.These pores can effectively confine the polysulfides due to their unique structure with wide body and narrow neck,which shows only a0.05%capacity fade per cycle for 500 cycles?75%capacity retention?for accommodating polysulfides.With a density of 1.16 g cm^-3,a hybrid cathode containing 5.6 mg cm^-2 sulfur delivers a high volumetric capacity of 653 mAh cm^-3.As a result,a cell-level volumetric energy density as high as 517 Wh L^-1 was achievedwith a cathode thickness of 100?m.This is a practical advance to bridge the volumetric performance gap between Li-ion batteries and Li-S batteries.These works show some examples of designing graphene networks for compact energy storage,and may give some ideas to construct high performance EES devices,especially with high volumetric energy densities.