Preparation Of Sodium Titanate Nanosheets Embedded With Iron Oxide And The Application In Energy Storage Devices

With the development of new energy technologies,more and more energy storage devices have been developed and used.Supercapacitors are a kind of electrochemical energy storage devices which bridge batteries and traditional physical capacitors.They are widely used due to their high power density,long lifecycle,fast charging speed and environment friendliness.The study of supercapacitors mainly focused on the preparation and development of high-performance electrode materials.In general,the electrode materials of supercapacitors are carbon materials,transition metal oxides and conducting polymers.Each type of materials has their own advantages and shortcomings,such as the specific capacitance of carbon materials is not high and the conductivity of transition metal oxides is low.To tackle these issues,the design or/and modification of electrode materials are proposed.Among them,the design and preparation of composite materials is currently the most potential strategy.In this thesis,the sodium titanate nanosheets embedded with iron oxide were synthesized by hydrothermal method.The composition and structure of the nanosheets were studied.The electrochemical properties of the material in different solutions were investigated.Then asymmetric supercapacitors assembled using the sodium titanate nanosheets embedded with iron oxide as the negative electrode and LiMn₂O₄ or?Cu,Ni?O as the positive electrode are test their performance.The main contents of this thesis are as follows:?1?The sodium titanate nanosheets embedded with iron oxide were grown on the titanium substrate directly by hydrothermal method.It was found that the sodium titanate nanosheets had a layered structure that provided the channel for solution ions to embed and release,and the iron oxide in the material was oxidized and reduced to provide the capacitance during.During the reaction,the deformation of the sodium titanate with the layered structure is small,and the embedded iron oxide is effectively protected,maintaining the stability of the material and exhibiting good cycle performance.The material works in the sodium sulfate solution at a voltage window of-1.0 to 0.2 V,with a pair of symmetrical redox peaks,and the discharge curve is S-type.Without any further modification,after the2000 cycles test,there is still 78%retention.?2?The intercalation and deintercalation of different cations in the material and the effects of anions were investigated.Li₂SO₄,Na₂SO₄,K₂SO₄,LiCl,NaCl,KCl,LiOH,NaOH and KOH solutions were selected.It was found that Li+,Na+and K+could effectively insert into and remove from the material and form a pair of redox peaks,and showed good reversibility.Anions have a certain effect on the electrochemical process,capacitance and impedance of the material.Compared to the neutral solution,the redox peaks of the material in the alkaline solution are more symmetrical and the cycling performance is better.After2000 cycles,the retention are 80%,78%,85%,85%,82%,90%,89%,87%and 98%respectively.?3?An asymmetric supercapacitor consists of the sodium titanate nanosheets embedded with iron oxide as the negative electrode and LiMn₂O₄ as the positive electrode with a 1mol L^-1 Li₂SO₄ solution as electrolyte.The results show that the supercapacitor has a high voltage window of 1.9 V and when the current density is 0.5 mA cm^-2,the area capacitance is 92.6 mF cm^-2.Similarly,an asymmetric supercapacitor based on the sodium titanate nanosheets embedded with iron oxide as the negative electrode and?Cu,Ni?O as the positive electrode with a 3mol L^-1 NaOH solution as electrolyte is fabricated.It works at a voltage region of 0-1.65V.When the current density is 0.5 mA cm^-2,the area capacitance reaches104.9 mF cm^-2.It also has good rate capability and cyclic stability.After 1500 cycles,the capacitance increases by 24%.