Building Three-Dimensional Graphene Materials for Lithium Metal Secondary Batteries

Author:Deng Wei

Supervisor:liu zhao ping zhou xu feng


Degree Year:2018





The energy density of conventional Li-ion battery is approaching its theoretical limitation,bringing the development bottlenecks for electronic devices and electrical vehicles,thus,how to further enlarge the energy density of Li metal battery required more advanced Li battery,such as Li metal battery.Metallic Li is considered as the ultimate one owing to high theoretical capacity,low density,and negative electrochemical potential.However,issues resulted from characteristic deposition/dissolution features of Li metal have hindered development of anode.Meanwhile,among secondary rechargeable Li metal batteries,lithium-sulfur(Li-S)battery has been spotlighted in recent years by reasons of ultrahigh theoretical energy density and abundance of nontoxic elemental sulfur;nonetheless,the dissolution and shuttle of polysulfides,large volumetric expansion and insulating nature of sulfur hinder the evolution of practical Li-S battery.Routines need to be found to enhance the performance of Li metal anode and sulfur cathode.3D graphene,as one of the most researched materials,is extensively regarded as new candidates to practically applied in energy storage materials.Multiple self-assembly methods endow them with kinds of microstrucutres and excellent physiochemical properties beyond those individual graphene sheets,which can adjust with the requirements of different electrode materials.Unfortunately,owing to assembly process can not be precisely regulated and controlled,usually we can’t get the desirable microstructure for anticipated application.Thus,in this thesis,we focused on how to control pores parameters of 3D graphene materials through controllable assembly process and then enhance the performance of Li metal anode and sulfur cathode.The main contents and results are as follows:(1)Graphene oxide(GO)sheets with different lateral sizes by controlling the delamination conditions of graphite oxide were used as building blocks to form 3D graphene with adjustable pore size,by hydrothermal self-assembly method.The pore sizes of 3D graphene can be effectively controlled by simply altering the sheet sizes of GO,and the smallest average pore size is500 nm.We investigate the electrochemical properties of two distinctive pore sizes as cathode materials in lithium-sulfur battery.Consequently,smaller pores supported high sulfur content(65 wt%)displays higher specific capacity,better rate capability and cycling stability than those of the samples with larger pores.It can retain a high reversible capacity of 606.7 mAh g-1 after 300charge/discharge cycles at 1 C with 99.0%Coulombic efficiency.(2)Bulk porous graphene materials with oriented pore structure and arrangement of graphene sheets are prepared by marrying electrolyte-assisted self-assembly and shear-force-induced alignment of GO sheets,and the super elasticity and anisotropic mechanical,electrical,and thermal properties induced by this unique structure are systematically investigated.Its application in pressure sensing exhibits ultrahigh sensitivity of 313.23 kPa-1 for detecting ultralow pressure variation below 0.5 kPa.Based on the filtration-induced graphene oriented arrangements,we further grown CNTs on both sides of graphene sheets,finally obtained a free-standing carbon host containing a hierarchical porous carbon network integrated with an in situ formed ultrathin graphene shell.When the host was applied for sulfur cathodes,the rationally designed electrode possesses a relatively high areal sulfur loading of3.6 mg cm-2,and shows excellent rate capability at6.0 mA cm-2 and cyclic stability over 200 cycles.And as the graphene nano-shell blocks the diffusion of polysulfides,the anti-self-discharge capability of the cell is remarkably improved.(3)Porous graphene scaffold with cellular chambers for incorporating Li metal is presented.Using such a unique host,ultrathin Li layers of 3μm in thickness are anchored on graphene,which provides much more reaction sites for Li ions compared with that of bulk Li.The high current density can be effectively dissipated by the graphene scaffold to remarkably improve the rate capability of Li anode.The symmetrical Li cell using such a Li anode can run stably for 200 cycles at 5 mA cm-2and even 70 cycles at 10 mA cm-2 in an unmodified carbonate-based electrolyte.Lithium-ion capacitor cells using this anode also show outstanding rate capability and cycling stability,which can work at an ultrahigh current density of 30 A g-1 and keep steady for over 4000 cycles at 3.75 A g-1.(4)We propose a simple method of incorporating Li in commercial carbon fiber cloth(CFC)to realize high-rate and stable metallic Li anodes by confining stripping/plating of Li in micro-channels between carbon fibers and dissipating high current densities through conductive carbon fiber networks.The symmetrical cell using this novel anode can run stably for over 1800 h(900 cycles)under 1 mA cm-2 and even320 h(800 cycles)at 5 mA cm-2.When it is paired with commercial activated carbon,the as-made Li ion capacitor coin cell can deliver high rate capability(up to 30 A g-1)and long-term cycling stability for over 5000 cycles at 10 A g-1,and large pouch cell can operate as ultrafast charge(1 min)battery with high energy density of50 Wh kg-1.To further enlarge its areal capacity of lithium anode,we fabricated graphene nested carbon fiber cloth current collector with surface decoration by lithiophilic seeds for metallic Li,which is able to sustain long-term(over 1500h)reversible stripping/plating of Li metal under ultrahigh areal capacity(12 mAh cm-2).(5)3D graphene scaffold fabricated by CVD method was employed to controllably incorporate with Li metal,and then paired with 3D graphene nested in hollow carbon fiber(obtained from annealing cotton foam)supported sulfur cathode of high areal loading of10 mg cm-2 as Li-S full battery.The incoporated Li metal mass(as lower as 3.0 mg cm-2)varied with the decoration amount of ZnO NPs,owing to surface tension regulation of 3D graphene scaffold inhibit further liquid Li metal climbing into graphene scaffold.The graphene/Li anodes paired with HCG/S cathodes exhibits long-term cycling stability for over 80 cycles at 0.5 mA cm-2,and keep the capacity stably over 6mAh cm-2.As for the mass of two electrodes,the energy density of this cell is over 1000 Wh kg-1 and after 80 cycles the energy density keeps at 700 Wh kg-1.