Performance Investigation Of Aqueous Zinc-based Energy Storage Devices Based On Ti₃C₂T_x And Nanocellulose Composite Films

With the increasing demand for wearable electronic products（such as smart watches and wearable health monitoring systems）,it is imperative to build flexible energy storage devices with environmental friendliness,low cost,multi-function,and high electrochemical performances.Nanocellulose,derived from biomass with abundant resources,possesses excellent mechanical properties and unique structure,and has shown great potential in constructing functional energy storage systems.Similarly,Ti₃C₂T_x MXene is also a promising electrode material for supercapacitors and batteries.It has the advantages of excellent conductivity,rich terminal group,unique layered structure,large specific surface area,and hydrophilicity,whereas the restacking issue limits its electrochemical performances.In this dissertation,three composite films are prepared by combining nanocellulose and Ti₃C₂T_x,thereby preventing the dense packing of Ti₃C₂T_x.And the supercapacitor and zinc-based energy storage devices are assembled,which were analyzed by a series of electrochemical measurements.In addition,the effect and mechanism of Ti₃C₂T_x/nanocellulose composite films in improving the capacitive performances and inhibiting zinc dendrites and accompanied side reactions are investigated in this dissertation.The main research contents include:（1）Delaminated Ti₃C₂T_x flakes are modified by alkalization and post-annealing treatments to considerably decrease–F and–OH functional groups.In addition,inspired by the architecture of nacres,the modified Ti₃C₂T_x is combined with soybean stalk-derived nanofibrillated cellulose,which can improve mechanical properties,prevent dense packing of Ti₃C₂T_x,and facilitate the transport of electrolyte ions.Therefore,the optimized composite film exhibits large tensile strength（53.9 MPa）,high electrical conductivity(24930 S m^–1),and superior electrochemical performance for supercapacitors and zinc-ion capacitors.In particular,the composite film under zinc-ion capacitors delivers high capacitances of 265.2 F g^–1 at 0.5mA cm^–2,excellent cyclability（94.31%capacitance retention over 10,000 cycles）,and great affordability to bending deformations.（2）Ti₃C₂T_x/nanocellulose hybrid film is prepared by a facile solution casting method and employed as the zinc-free anode for aqueous hybrid Zn-Li batteries.Benefiting from the ultra-low diameter and rich hydroxyl groups of nanocellulose,the hybrid film exhibits better mechanical properties,superior electrolyte wettability,and more importantly,significantly improved zinc plating/stripping reversibility compared to the pure Ti₃C₂T_x film.The hybrid film also dramatically overwhelms the stainless steel as the electrode for reversible zinc deposition.Further analysis shows that the hybrid film can lower the zinc deposition overpotential and promote the desolvation process of hydrated Zn²⁺ions.In addition,it is found that hexagonal Zn thin flakes are horizontally deposited onto the hybrid film owing to the low lattice mismatch between the Ti₃C₂T_x surface and the（002）facet of Zn.Consequently,zinc dendritic growth and accompanied harmful side reactions can be considerably inhibited by the hybrid film,and the assembled Zn-Li hybrid batteries exhibit excellent electrochemical performances.（3）The lithium manganate and lithium salts used in the previous work are relatively expensive.Sodium resources are much more abundant and have much lower costs.And the electrochemical characteristics of sodium and lithium are very close,enabling sodium to be a very suitable alternative to lithium.In this work,a zinc-sodium hybrid battery system was constructed,in which sodium vanadium phosphate was used as the cathode material.Ti₃C₂T_xand nanocellulose are mixed into aqueous solution and then coated onto stainless steel foil to obtain the Zn-free anode.In addition,the electrolyte is composed of high concentrations of sodium perchlorate and zinc perchlorate.This concentrated electrolyte can improve the coordination environment of zinc ions and inhibit the side effects caused by a lot of water molecules.According to a series of electrochemical tests,the assembled Zn-Na hybrid batteries achieve high specific capacity,excellent rate performance,and good cycling stability.