Improving Energy Density Of Sodium-ion Batteries Based On Cathode Pre-sodiation And Anode Interface Tuning Strategies

Sodium-ion batteries（SIBs）have attracted enormous attention owing to their advantages of abundant resource,low cost and good comprehensive performance.Nevertheless,SIBs with hard carbon anode are always faced with the reduced energy density induced by low initial Coulombic efficiency（ICE）.In this thesis,a series of strategies and methods are proposed from aspects of cathode sodium compensation additive and solid electrolyte interface（SEI）film in order to improve energy density of SIBs.Meanwhile,the related mechanisms are also investigated deeply.The main research results are shown as follows:（1）Commercial sodium citrate is proposed as a self-sacrificial cathode additive for the first time.SC is electrochemically decomposed at4.0 V（vs.Na⁺/Na）and delivers an actual sodium compensation capacity of302 m Ah g^-1,corresponding to the utilization rate of 97%.After adding 10wt%SC,the constructed full-cell with commercial hard carbon（CHC）anode and Na₃V₂（PO₄）₂F₃/r GO cathode shows the improved energy density from 149 to 178 Wh kg^-1 without sacrificing other electrochemical properties.As a low-cost,eco-friendly,safe and high-utilization sodium compensation reagent,SC has promising commercial application prospects in SIBs.（2）1,2,4-Triazole sodium（TS）is firstly investigated as a multifunctional cathode sodiation additive.TS can interact with polyvinylidene fluoride（PVDF）binder and N-methyl-2-pyrrolidone（NMP）solvent,and this process not only induces the defluorination of PVDF,but also enables TS to convert into Na₂CO₃.The defluorination and crosslink of PVDF can bond active material and conductive agents tightly to form an excellent conductive framework,reducing the electrochemical polarization.A facile method to prepare free-standing electrode is also inspired by such a desirable electrode structure.TS derived Na₂CO₃particles,which are uniformly distributed on the surface of cathode,leave no damage to the inner electrode structure during the decomposition process.The energy density of CHC//Na₃V₂（PO₄）₃ full-cell is enhanced from 180 to 196 Wh kg^-1.Also,the decomposition of Na₂CO₃ promotes the formation of stable Na F-rich cathode electrolyte interface（CEI）film.（3）Based on the esterification reaction between sodium carboxymethyl cellulose（CMC）and polyethylene oxide（PEO）,a new binary CMC/PEO binder with rich Na⁺transport channels is prepared to improve ICE and rate performance of CHC.Conventional PVDF binder will undergo severer electrochemical defluorination process during the initial discharge process of anode in ether-based electrolyte compared with ester-based electrolyte,resulting in a lower ICE（66.6%vs.74.1%）.In contrast with PVDF,CMC/PEO binder can not only effectively inhibit the decomposition of electrolyte and improve CHC of ICEs in ether-and ester-based electrolyte（85%and 77.2%）,but also induce the formation of SEI film with lower content of inorganic components（Na₂CO₃ and Na F）,which is beneficial to reducing the interface impedance and accelerate Na⁺diffusion.Eventually,the rate performance of CHC is elevated.（4）The electrochemical performance of CMC/PEO binder in ester-based electrolyte is improved by optimizing the composition of electrolyte.Compared with Na Cl O₄/PC,Na Cl O₄/PC/5%FEC,Na PF₆/EC/DEC and Na PF₆/EC/PC electrolytes,a thinner and stabler SEI layer is formed in Na PF₆/EC/PC/DEC electrolyte,facilitating Na⁺diffusion kinetics.Hence,CHC anode with CMC/PEO binder shows higher ICE（79.4%）,more outstanding cycling stability(a capacity retention of 98.6%after 300 cycles at 200 m A g^-1)and rate performance(161 m Ah g^-1 at 300 m A g^-1).The constructed CHC//Na₃Fe₂（PO₄）P₂O₇full-cell exhibits a capacity retention of 93.4%after 100 cycles at 200 m A g^-1,further demonstrating the good compatibility between CMC/PEO binder and Na PF₆/EC/PC/DEC electrolyte.