| The depletion of fossil energy sources and the environmental problems caused by their combustion has led to the widespread use of rechargeable batteries(especially lithium-ion batteries)for converting and storing clean energy sources,such as wind and solar power.Although lithium-ion batteries are commercially available,the limited reserves of lithium are hardly sufficient to meet the rapidly growing demand for energy storage.Sodium-ion batteries,abundant and cheap sodium resources,are a viable alternative to lithium-ion batteries.The significant growth in demand for flexible electronics such as foldable smartphones,flexible health monitors and wearable devices has boosted research into flexible sodium-ion batteries.However,the development of flexible sodium ion batteries is seriously hampered by the relative lack of flexible anodes capable of fast and reversible storage of sodium ions.Antimony-based anode materials with abundant resources,high theoretical capacity and suitable discharge voltage have been widely used in the research of sodium ion battery anodes.However,during charging and discharging,antimony-based materials with suboptimal conductivity exhibit large volume expansion;the poor flexibility of simple antimony-based anode materials makes it challenging to constitute flexible antimony-based electrodes,limiting their development in flexible sodium ion batteries.In order to solve the above problems,this thesis starts from the structural modulation and type selection of antimony-based materials,and improves the sodium storage performance of antimony-based materials as anode materials for sodium ion batteries by their compounding with two-dimensional(2D)layered Ti3C2Tx(MXene)which has excellent electrical conductivity and good mechanical properties.The antimony-based materials can inhibit the re-stacking of layered Ti3C2Tx and improve the wettability of the electrolyte.In this study,unique,flexible antimony-based electrode materials of 1D-2D parallel heterostructures with ideal ion transport properties and excellent mechanical strain tolerance were designed and prepared.The morphology,composition,structure and sodium storage properties of the electrode materials were analysed using various characterisation tools;the reasons for the enhanced electrochemical sodium storage of the electrode materials were analysed on an atomic scale by density flooding theory;and ex-situ XPS,XRD,EIS and Raman characterisation tools explored the sodium storage mechanism of the electrode materials.The research of the thesis is as follows:1.Synthesis and sodium storage performance of flexible Sb2O3/Ti3C2Tx filmTo address the volume expansion and unfavourable flexibility of antimony-based anode materials,a 1D-2D parallel heterostructure combined with ex-situ electrostatic self-assembly strategy was designed to construct a flexible Sb2O3/Ti3C2Tx film.Sb2O3/Ti3C2Tx composite was also synthesized by growing Sb2O3 in situ on the layered Ti3C2Tx surface.Firstly,hydrochloric acid and lithium fluoride were used to etch bulk MAX(Ti3AlC2)to obtain a few-layer Ti3C2Tx.Meanwhile,1D Sb2O3 nano wires were synthesized by hydrothermal method to promote the directional transport of sodium ions and electrons.The 1D Sb2O3 surface was modified to carry a positive charge,and then,it was self-assembled and compounded with negatively charged MXene nanosheets to obtain the Sb2O3/Ti3C2Tx film.The morphology,structure and composition of the flexible films were analysed and characterised.The electrochemical properties of the material were tested to investigate the sodium storage performance of the film electrodes.The results show that the surface-modified electrostatic self-assembly strategy can obtain flexible Sb2O3/Ti3C2Tx films and avoid the oxidation and accumulation of Ti3C2Tx.The Sb2O3 nanowires can inhibit the re-stacking of Ti3C2Tx and improve its wettability to the electrolyte;Ti3C2Tx improves the electrical conductivity of Sb2O3 and inhibits the volume expansion of Sb2O3 during the sodiation process.As a result,the flexible Sb2O3/Ti3C2Tx films exhibit excellent sodium storage performance:the discharge specific capacity is 394.7 mA h g-1 after 100 cycles at a current density of 0.1 A g-1 and 202.2 mA h g-1 after 500 cycles at 1.0 A g-1.The flexible Sb2O3/Ti3C2Tx films with a 1D-2D parallel heterogeneous structure solve the problems of volume expansion and lack of flexibility of antimony-based anode materials.2.Synthesis and sodium storage performance of flexible Sb8O11Cl2/Ti3C2Tx filmTo address the formation of excess irreversible solid state electrolyte(SEI)films and low conductivity sodium oxide,and the damage to the electrode and electrolyte by reactive oxygen species during the discharge of Sb2O3-based electrode,Sb8O11Cl2 nano wires were synthesised using an anion-replacement hydrothermal method and flexible Sb8011Cl2/Ti3C2Tx films were prepared using an ex-situ electrostatic self-assembly method.The results of systematic analysis and characterisation of the morphology,structure,components and sodium storage properties of this flexible film show that the replacement of some of the oxygen by chlorine was able to change the surface charge distribution of antimony oxide and improve the products of the conversion reaction process,thus improving the stability of the SEI film and reducing the occurrence of some of the side reactions.As a result,the flexible Sb8O11Cl2/Ti3C2Tx films exhibit superior sodium storage performance:a discharge-specific capacity of 420.5 mA h g-1 after 100 cycles at a current density of 0.1 A g-1 and 222.3 mA h g-1 after 500 cycles at 1.0 A g-1.The anion replacement strategy inhibits unfavourable side reactions at Sb2O3-based electrodes,thereby improving the electrode’s sodium storage performance.3.Synthesis and sodium storage performance of flexible Sb2Se3/Ti3C2Tx filmTo address the unfavourable side reactions caused by reactive oxygen species in Sb8O11Cl2-based electrodes,Sb8011Cl2 was replaced by Sb2Se3 nanowires in this study.The flexible Sb2Se3/Ti3C2Tx film was obtained using a surface-modified electrostatic self-assembly strategy,which will more effectively avoid the occurrence of unfavourable side reactions and improve the initial coulombic efficiency and sodium storage performance:the initial coulombic efficiency of the flexible Sb2Se3/Ti3C2Tx film electrode was as high as 81.5%at a current density of 0.1 A g-1,a discharge specific capacity of 461.0 mA h g-1 after 100 cycles at a current density of 0.1 A g-1 and 233.3 mA h g-1 after 500 cycles at 1.0 A g-1.The reason for the improved sodium storage performance of the electrode material was verified through theoretical calculations and electrochemical mechanism analysis.The Ti3C2Tx exhibited excellent adsorption performance on sodium polyselenide and suppressed the shuttle effect;Ti3C2Tx compounded Sb2Se3 improved the sodium storage capacity and sodium ion diffusion kinetic performance of the electrode.Electrochemical tests and ex-situ Raman spectroscopy revealed the reversible sodiaion/desodiation behaviour of the Sb2Se3/Ti3C2Tx electrode.The replacement of Sb8O11C12 with Sb2Se3 nanowires avoids unfavourable side reactions and improves the electrode’s initial coulombic efficiency and sodium storage performance.4.Synthesis and sodium storage performance of flexible Sb2S3/Ti3C2Tx filmTo solve the problem of insufficient capacity of Sb2Se3/Ti3C2Tx-based batteries,Sb2S3(946 mA h g-1),which has a similar sodium storage mechanism and higher theoretical capacity than Sb2Se3,was used to replace Sb2Se3.Sb2S3 nanowires were synthesized by hydrothermal method,and flexible Sb2S3/Ti3C2Tx films were obtained by surface modification electrostatic self-assembly strategy.The morphology,structure and composition of the material were analysed and characterised.The electrochemical performance of the composite electrode was investigated by testing the electrochemical properties of the material.Theoretical calculations and electrochemical mechanism analysis were carried out to verify the reasons for the enhanced sodium storage performance of the electrode material.Ex-situ XPS,EIS,TEM and SEM characterisation were used to analyse the reversible sodiation/desodiation behaviour of the Sb2S3/Ti3C2Tx film electrode.The results show that Ti3C2Tx first encapsulates Sb2S3 nanowires and then forms a sandwich structure;the Ti3C2Tx composite Sb2S3 improves the sodium ion storage capacity,the diffusion kinetics of sodium ions and electrons,and suppresses the shuttle effect.Based on the synergistic effect of Ti3C2Tx and Sb2S3,the sodium storage performance of the electrode was improved:a discharge specific capacity of 581.0 mA h g-1 after 100 cycles at a current density of 0.1 A g-1 and 464.2 mA h g-1 after 500 cycles at 1.0 A g-1.The flexible Sb2S3/Ti3C2Tx film anode was assembled with a Na3V2(PO4)3/carbon cathode to form a full cell,which exhibited higher energy and power densities than similar sodium ion full cells.5.Synthesis and sodium storage performance of flexible Sb/S-Ti3C2Tx filmIn order to solve the problem of limiting the capacity of flexible sodium ion full batteries due to the lack of conductivity of surface modifier and semiconductor Sb2S3,the reduction of Sb2S3/Ti3C2Tx to flexible Sb/S-Ti3C2Tx films with high conductivity,high specific capacity and suitable discharge potential was investigated by electrostatic self assembly-temperature reduction strategy.The Sb2S3/Ti3C2Tx electrodes were first prepared by an electrostatic self-assembly strategy,and then the flexible Sb/S-Ti3C2Tx films were obtained by low-temperature thermal reduction.The morphology,structure and composition of the Sb/S-Ti3C2Tx were analysed and characterised.The electrochemical performance of the composite electrode was investigated by testing the electrochemical properties of the material.The reasons for the enhanced sodium storage performance of the Sb/S-Ti3C2Tx electrode were verified by theoretical calculations and electrochemical mechanism analysis.The reversible sodiation/desodiation behaviour of the Sb/S-Ti3C2Tx thin film electrode was analysed using ex-situ XRD,EIS,TEM and XPS characterisation.The results show that S-Ti3C2Tx improves the sodium ion storage capacity and sodium ion diffusion kinetic performance.Based on the synergistic effect of S-Ti3C2Tx and Sb,the Sb/S-Ti3C2Tx film electrode exhibited excellent sodium storage performance:a discharge specific capacity of 563.5 mA h g-1 after 100 cycles at a current density of 0.1 A g-1 and 456.0 mA h g-1 after 500 cycles at 1.0 A g-1.The flexible Sb/S-Ti3C2Tx film anode was assembled with the Na3V2(PO4)3/rGO cathode to form a flexible full cell,which exhibited better energy and power densities than the materials in the previous chapters. |
PDF Full Text Request