Design And Synthesis Of High-performance Flexible Free-standing Electrodes For Energy Storage

In recent years,flexible/wearable electronics such as roll-up displays,medical cards,smart textiles,soft robotics,bio-sensors,and the Internet products are increasingly entering our daily life.Energy storage device is one of the core components of flexible/wearable electronic device,which plays a crucial role in the performance of flexible/wearable electronic devices.The lithium batteriey assembled with conventional electrode,which contains the addition of conductive agent and binder,is rigid.It severely restricts the futher development of flexible/wearable electronic device.Although lithium metal can work as the anode in lithium metal batteries independently,it is easily broken under the deformation state because of poor mechanical property.Moreover,the uneven lithium plating/stripping on the surface of lithium metal generates the dendrites,which exists serious safety hazard for lithium metal batteries.Therefore,the fundamental way for flexible lithium batteries is to develop flexible free-standing electrode without conductive agent and binder.However,there are still remaining challenges for flexible free-standing electrode,such as low content of active materials,low area capacity,weak adhesion between substrate and active materials,etc.In this dissertation,two strategies are employed to solve the problems of low content of active materials in the flexible free-standing electrode and the resulting low area capacity:i)constructing multilayer active materials on flexible substrates;ii)adopting lithium metal as active material,which has 3860 m Ah g^-1 high specific capacity,combining with flexible substrate to construct the composite lithium metal anode.The major contents and research results are listed below:(1)Multilayer flexible free-standing electrode with in situ formed interlayer.In this work,a multilayer structure of Ni O@Ni O/NF electrode is proposed to increase the content of active materials and reduce the weight of substrate by forming an interlayer in situ.The electrode is composed of three parts:an inner Ni foam conductive framework,interlayer Ni O layer,and outer few-layer Ni O nanoflowers.The interlayer derived from partial oxidation of Ni foam substrate not only reinforces the attachment of the active layer on Ni foam but also contributes capacity to the whole electrode,leading to excellent stability and areal capacity.When used as the anode of lithium-ion battery,ultrahigh reversible capacity of 1.98 m Ah cm^-2 is achieved at 1.2 m A cm^-2.The electrode still maintains good integrity and flexibility after 900 cycles at 2.5 m A cm^-2,showing good structure stability.(2)Lightweight flexible free-standing electrode with core-multishell structure.In the previous system,the construction of Ni O interlayer improves the content of active materials and the stability of electrodes,however,the bulk morphology is unfavorable for specific capacity of electrode.In this work,the flexible electrode with an architecture of substrate/reduced graphene oxide(r GO)/bimetallic sulfides nanosheets/r GO/bimetallic sulfides nanosheets/r GO from the inside to the outside is successfully prepared.Owing to the double-layer nanosheets,the content of active materials reaches to 20 wt.%in the electrode.Three alternating r GO layers effectively improve the stability and conductivity of the electrode.When matched with flexible Li Fe PO4/SWCNT cathode,the flexible pouch cell shows gravimetric energy density of 127 Wh kg^-1 under deformation state.Furthermore,the lithium storage mechanism of the electrode and the synergistic effect in CMS are systematically studied through cyclic voltammograms,X-ray photoelectron spectroscopy measurements and DFT theoretical calculation,which provides valuable guidance for the exploration of bimetallic sulfides.(3)High-performance flexible lithium metal anode based on porous carbon cloth substrate.The excessive increase of active materials in electrode will seriously damage the stability of electrode structure.The fundamental way to solve this problem is to adopt active materials with higher theoretical specific capacity,such as lithium metal.In this work,a porous carbon fabric substrate PCF@r GO@Ni with uniform Ni nanoparticles and r GO for lithium metal anode is successfully prepared.The substrate can homogeneously combine 10 m Ah cm^-2 lithium metal without dendrites formation.DFT theoretical calculation proves that PCF@r GO@Ni combined with Li possesses excellent energy stability.COMSOL multi-physical field simulation shows that the porous structure of PCF@r GO@Ni can uniform distribution of lithium-ion flow,lithium-ion concentration and electric field during Li plating.Furthermore,the symmetric cells based on PCF@r GO@Ni/Li exhibit overlength cyclic stability for 8300 h.(4)Flexible pouch full cell based on PCC@V₂O₅ cathode.The performance of flexible energy storage device not only depends on anode materials,but also cathode materials.In this work,a flexible cathode PCC@V2O5 is successfully synthesized via growing the V2O5 nanowires on porous carbon cloth.The pore-forming treatment not only reduces the weight of carbon cloth,but also improves the surface activity of substrate,resulting in sufficient growth sites for V2O5 nanowires.When paired with PCF@r GO@Ni/Li anode,the obtained lithium metal flexible pouch cell with dimensions of 3.0×3.0 cm² exhibits 8 m Ah capacity under severe deformation,showing the great potential for practical application in lithium metal batteries.