The Research On Low-Cost Electrode For Sodium-Ion Battery

The increasing demands for energy and environment deterioration have become a contradiction in social development.Under this premise,the"peaking carbon-dioxide emissions"goal will promote the leapfrogging development of renewable energy,such as wind and solar,which puts forward higher requirements for the grid reliability and brings new opportunities for the large-scale application of rechargeable energy storage systems.Although lithium-ion batteries（LIBs）have some advantages of high energy density and long cycle life,the insufficient reserves and uneven distribution of lithium resources limit the large-scale application in grid-scale energy storage.Based on the advantages of sodium resources for reserves and distribution,the low-cost sodium-ion batteries（SIBs）have received widespread attention in renewable energy consumption,distributed energy storage power stations and other energy storage fields.Developing low-cost and high-performance electrode for SIBs play a key role in practical application.Meanwhile,in-depth studies of the sodium storage behavior and failure mechanism of key electrode materials will help to further improve the comprehensive performance and accelerate the application of SIBs.In this thesis,we researched a series of low-cost electrode materials for SIBs,including exploring the high-salt-concentration preparation and regulation of vacancy defects of Na₂MnFe（CN）₆,revealing the failure mechanism of Na₂MnFe（CN）₆ and the storage mechanism of hard carbon/Mg_0.5Ti₂（PO₄）₃ anode materials.The concrete work is as follows:（1）Research on regulating vacancy defects of Na₂MnFe（CN）₆:Due to the three-dimensional open framework for rigid ion conduction,easy synthesis,and high structural stability,Na₂MnFe（CN）₆（Mn PBA）possesses high theoretical energy density and has become a potential low-cost cathode material for SIBs.However,the high-concentration Fe（CN）₆vacancy defects limit the electrochemical performance.Firstly,we systematically compared the effects of different experimental conditions,including chelator conditions,PH value,aging time,and aging temperature,on the important characteristics under the high-salt-concentration co-precipitation reaction,such as crystallinity,vacancy defect concentration,particle morphology and electrochemical properties.The results illustrate that the aging time and temperature play crucial roles on the physicochemical properties of Na₂MnFe（CN）₆,especially for morphology and concentration of vacancy defects.The micro-spherical Na₂MnFe（CN）₆ prepared at ageing temperature of 80°C for ageing time of 20 h（PBAT-80）achieved theoretical reversible capacity of 170 m Ah g^-1.Then,using thermal and spectroscopic analyses,the high crystallinity and low vacancy defect concentration were further demonstrated to play a positive role in improving the thermal stability and reducing the formation of surficial CEI films.In addition,the products by high-salt-concentration preparation are beneficial to increase the yield of the product per volume(100 g L^-1),which will guide for the subsequent scale-up production.（2）Research on the failure mechanism of Na₂MnFe（CN）₆:Although micro-spherical Na₂MnFe（CN）₆ exhibited theoretical reversible capacity,the capacity retention of the half cells showed a significant decrease during long cycles.In the next part,we systematically explored the failure mechanism of Mn PBA electrodes for half cells.Firstly,based on the galvanostatic cycle,capacity-limited cycle,and GITT curves,we found that the increase of polarization voltage during charging process is the main reason for the decrease of reversible capacity.Secondly,to determine the effects of the impedance for counter/working electrode on the decaying of polarization voltage,reversible capacity and rate capability,the changes of counter/working electrode impedance were independently investigated using three-electrode impedance spectra.The results verified that the accumulation of CEI film tend to dynamically increase during long cycles,which is main failure mechanism.For comparison,the impedance of counter electrode is more stable and gradually decreases during cycling.In addition,the change of impedance in charged state and the evolution of ex-situ XPS elementary measurement further demonstrate the continuous accumulation of CEI film.Finally,there is no significant crack of particle morphology,dissolution of transition metal ions,and irreversible phase transition,which is not the main cause for cell failure.This work provides guidance for subsequent performance modification work.（3）Study on the synthesis of high-performance hard carbon material and the sodium storage mechanism:Based on the advantages of high reversible capacity,low working potential,and high cycling stability,hard carbon is the main candidate as low-cost anode materials.However,the storage mechanism of the disordered carbon layer structure is still controversial,which needs to be further refined.In the third part,we select natural kapok fiber with a hollow tube structure as biomass precursor and use a two-step carbonization method to obtain high-performance hard carbon materials,which have excellent reversible capacity(292.5 m Ah g^-1),initial Coulombic efficiency,capacity retention,and rate capability.The variation of Na-ions diffusion coefficient was simulated using GITT and CV methods.And the fitting results showed that the electrochemical process can be divided into two storage behaviors:diffusion-controlled process of voltage platform and adsorption-controlled process of voltage slope.The relationship between the hard carbon structure and the sodium storage behavior was further investigated by combining the characterization of the ex-situ microstructure,energy spectrum analysis,and electron spin resonance.Sodium stores in the hard carbon as ionic state in the slope region pseudocapacitive adsorption at defect sites and quasi-metallic sodium state in the low-voltage plateau by diffusion in the micropores.Moreover,based on the results of thermal analyses,the physic state of metallic sodium in the hard carbon could be quasi-liquid metal at room temperature.These results guide to design high-performance hard carbon anode materials for SIBs.（4）Study on the storage mechanism of alkali metal ions in NASICON-structure Mg_0.5Ti₂（PO₄）₃:Due to the high structural stability,high ionic conductivity,and easy availability,the NASICON-type Ti-base phosphates materials are promising for applications in high-rate area.In the fourth part,firstly,a series of complex NASICON electrode materials A_xTi₂（PO₄）₃(A=Li,Na,K,Mg_0.5)were synthesized by solid phase reaction.Among them,Mg_0.5Ti₂（PO₄）₃（MTP）has the best capacity retention in both LIBs and SIBs systems.Then,the structural evolution and atomic occupancy of the MTP electrode in LIBs/SIBs during intercalation-extraction process were systematically investigated using in-situ/ex-situ XRD and ex-situ neutron diffraction.The discussions of structural evolution and the refinement results of atomic occupancy demonstrate the multi-process reactions in LIBs and the 2-phase changes in SIBs.Moreover,the results of element analysis,ND and DFT calculation verify that Mg-ions are not replaced by ions from electrolyte and maintain in M1 positions during charge/discharge.These results demonstrate that,besides transition metal ions and polyanions,ion type and distribution in M1 site can also significantly influence the electrochemical behavior of NASICON materials.The clarifications of structural evolution and storage mechanism will be of great importance for the further designs on the NASICON-structure electrodes.