NMR Characterization And Ion Loss Studies Of Cathode Materials For Sodium Ion Batteries

The increasing demand for energy and onsumption of fossil fuels are severe problems worldwide,and thus,efforts to develop high-energy-density storage systems to satisfy future energy requirements are underway.Lithium-ion batteries with high energy densities and extended cycle lives dominated the market in terms of portable electronic devices in recent decades,and they also recently attracted attention as practical battery systems for use in electric/hybrid vehicles.However,the large-scale use of lithium-ion batteries should soon be hampered by limited lithium resources.As a result,researchers are considering alternatives to lithium-ion batteries,and one of the most promising alternatives is the sodium-ion battery.As an element in the same group as lithium and in close proximity to it in the periodic table,sodium exhibits similar physicochemical properties to those of lithium.However,current sodium-ion batteries are still lacking in terms of theoretical specific capacity and cycling stability compared to those of lithium-ion batteries.Therefore,in this thesis,two typical cathode materials for use in sodium-ion batteries（phosphate and oxide systems）,which are currently the most popular and commercially viable,were investigated using nuclear magnetic resonance（NMR）spectroscopy as a critical characterization tool.The first part of this dissertation re-examined the solid-state NMR signals of sodium superionic conductor-type Na3V2（PO4）3.We first varied the parameters of the NMR experiments and observed that the ²³Na NMR spectrum of the synthesized Na3V2（PO4）3 not only displayed two signals at 80 and approximately–11 ppm but also a signal at 16 ppm.The signal at approximately–11 ppm was because of impurities in the synthetic process,based on a comparison of the trends of the signals observed using different parameters and 2D exchange spectroscopy studies.Moreover,based on T2relaxation studies,the signals at 80 and 16 ppm were assigned to Na at the M2（18e）and M1（6b）sites of Na3V2（PO4）3,respectively.Finally,the local environmental evolution of Na and P in Na3V2（PO4）3 during charging and discharging was investigated in detail.The second part of this dissertation built on the first and explored the electrochemical properties of Na3V2（PO4）3.Based on constant-current long cycle,rate,and electrochemical impedance spectroscopy studies at 30℃ in a potential window of2.5–4.0 V,impurities introduced during the synthesis of Na3V2（PO4）3 affect the interfacial charge transfer impedance.This increases the potential polarization during charging/discharging,resulting in the incomplete conversion of V³⁺to V⁴⁺under the current charge/discharge conditions and a decrease in specific capacity and cycle stability as charging/discharging progresses.To obtain a higher specific capacity for Na₃V₂（PO₄）₃,the potential window was extended to 1.0–4.0 V and the V³⁺/V²⁺redox reaction was introduced.However,this drastically increased the removal of the V ions from the lattice and their dissolution in the electrolyte during charging/discharging,significantly reducing the cycle stability.Although cycling at a low temperature of–20℃ could inhibit this phenomenon to a certain extent,the cycling stability at low temperatures was still poor.The third part of this dissertation is a targeted modification of the findings of the second study.Firstly,by replacing 1/4 of the V with Cr to form Na3V1.5Cr0.5（PO4）3,the V-ion dissolution was reduced compared to that of Na3V2（PO4）3 after the introduction of the V³⁺/V²⁺redox reaction at 30℃.The cycling performance was significantly improved,but it remained unsatisfactory.Cycling at a low temperature of–20℃ could further reduce the dissolution of V ions,thus yielding an enhanced cycle stability.Then,Na3V2（PO4）2F3,which was formed by replacing 1/3（PO4）with F,exhibited significantly reduced dissolution of V ions compared to that of Na3V2（PO4）3 after the introduction of the V³⁺/V²⁺redox reaction at 30℃ and a good cycle stability.However,the electrochemical performance of Na3V2（PO4）2F3 at low temperatures（-20℃）was unsatisfactory,and that of the Na3V2（PO4）3 half-cell assembled with a V-ion-doped glass fiber separator was more promising at 30 and-20℃.This suggests that reducing V-ion dissolution during charging/discharging may improve the electrochemical performance of Na3V2（PO4）3.The fourth part of this dissertation applied the concept of the dissolution of metal ions influencing the cycle stability to an Na0.83Li0.12Ni0.22Mn0.66O2 system with a typical P2 phase.Solid-state NMR techniques revealed that the proportions of Li⁺ions distributed in the AM（Alkali Metal）and TM（Transition Metal）layers varied with the charge/discharge cycle.For potential windows of 2.0–4.2 and 2.0–4.6 V,the proportion of Li⁺in the TM layer continuously decreases as charge/discharge cycling progresses.A significant amount of Li⁺is removed from the lattice and dissolved in the electrolyte,ultimately leading to a reduced cycle stability.However,the first cycle to activate the oxygen reaction（in a potential window of 2.0–4.6 V）followed by subsequent charge/discharge cycles in a potential window of 2.0–4.2 V should reduce Li⁺dissolution,resulting in an improved cycle stability.