Structure And Interface Regulating Of Hard Carbon Anode Materials For Na-ion Batteries

The development of sodium-ion batteries（SIBs）with low cost and high energy density is the trend to promote the development of large-scale energy storage system.As the most promising commercial anode material,hard carbon materials has attracted great attention due to its low working voltage,high reversible capacity,good cycle stability,and abundant raw material.However,hard carbon also have a series of problems such as unclear sodium storage mechanism,low plateau capacity,poor rate performance,low initial coulombic efficiency（ICE）,and high costs.This thesis firstly focuses on tuning hard carbon through the closed pore structure optimization and establishing the carbonation mechanism of biomass to improve the energy density and rate performance for SIBs.In addition,designing strategies of hard carbon including electrolyte additive and electrolyte-electrode interface regulating have been employed to avoid the negative impacts of low initial coulombic efficiency of hard carbon and improve the rate performance.The main research works and results are shown as follows:（1）Low-voltage plateau capacity is the main contributor to high capacity of hard carbon anode for sodium-ion batteries,in which the closed pore structure plays a key role.However,it is still blank about the generation mechanism of closed pore in biomass-derived hard carbon.Herein,employing waste wood-derived hard carbon as a template,the formation mechanism of closed pores and their effect on sodium storage performance are systematically established.The results elucidate that the high crystallinity cellulose in nature wood decomposes to long-range carbon layers,which act as the wall of closed pore.The amorphous component can hinder carbon layer’s graphitization and induce the crispation of long-range carbon layers.The elaborately synthesized sample demonstrates a high reversible capacity of 430 m Ah g^–1 at 20 m A g^–1(plateau capacity of295 m Ah g^–1),as well as excellent rate and cycling performances(85.4%after 400 cycles at 500 m A g^–1).Deep insights into the closed pore formation of hard carbon will greatly forward the development of sodium-ion batteries,further the sustainable applications of biomass for energy storage.（2）Here,a facile strategy of ball milling is successfully adopted to tune the closed pores structure of derived-biomass hard carbon anode to improve the SIBs energy density.Bamboo is selected as the low cost,fast growth and high consistency precursor.By tuning the structural changes of bamboo components and the types of oxygen-containing functional groups by ball milling,the closed pores structure effective conversion to more can be realized at further carbonization processes.Compared with the carbonized original bamboo,the ball milling bamboo carbon capacity was increased from 246 to 358 m Ah g^–1,the capacity of the plateau region was increased by 83 m Ah g^–1,and the samples had better rate performance and cycle performance.The experiment results reveal that amorphous cellulose and the introduction of carbonyl bonds after ball milling are the keys to achieve the highly disordered structure,not only ensuring the cross-linkage during low-temperature splitting decomposition process but also suppressing the carbon structure from melting and rearranging in the high-temperature carbonization process.Most importantly,this simple and effective ball milling strategy can also be extended to other high cellulose biomass precursors to facilitate the low-cost and high-performance hard carbon anode for SIBs and beyond.（3）Hard carbon is the most promising anode for sodium-ion batteries（SIBs）.However,the poor rate capability and low reversible capacity are still big challenges for its wide commerical application in SIBs.In this work,we report a novel method enhancing commercial hard carbon anode through a facile chemical treatment.Commercial hard carbon with an expanded carbon interlayer is realized via controllably introducing oxygen functional groups.When employed as anode for SIB,the modified CHC demonstrates a high reversible capacity of 341 m Ah g^–1 at 20 m A g^–1,which is much higher than pristine HC(270 m Ah g^–1.Excellent rate capability with a capacity of 50 m Ah g^–1 is maintained at 5000 m A g^–1.More importantly,this facile oxidation strategy is also suitable for commercial soft carbon,which also displays significantly enhanced electrochemical performance.The kinetic measurements and theoretical calculation results reveal that the enhanced electrochemical properties should be attributed to the introduced oxygen function groups,which not only facilitate the diffusion of Na ions via enlarging the carbon interlayer distance,but also enhance the absorption capacity to Na ions.（4）Sodium-ion batteries capable of operating at rate and temperature extremes are highly desirable,but elusive due to the dynamics and thermodynamics limitations.Herein,a strategy of electrode–electrolyte interfacial chemistry modulation is proposed.The commercial hard carbon demonstrates superior rate performance with 212 m Ah g^–1 at an ultra-high current density of 5 A g^–1 in the electrolyte with weak ion solvation/desolvation,which is much higher than those in common electrolytes（nearly no capacity in carbonate-based electrolytes）.Even at–20℃,a high capacity of 175 m Ah g^–1（74%of its room-temperature capacity）can be maintained at 2 A g^–1.Such an electrode retains 90%of its initial capacity after 1000 cycles.As proven,weak ion solvation/desolvation of tetrahydrofuran greatly facilitates fast-ion diffusion at the SEI/electrolyte interface and homogeneous SEI with well-distributed Na F and organic components ensures fast Na⁺diffusion through the SEI layer and a stable interface.（5）Commercial DTD was investigated as the electrolyte functional additive to passivate the irreversible Na⁺sites（oxygen functional groups）in SIBs due to the low ICE of commercial hard carbon.The products of DTD preferential decomposition are chemically bonded to the oxygen functional group of hard carbon,which minimizes the irreversible electrolyte decomposition and promotes SEI formation.DFT calculations further indicate that the S clusters have stronger adsorption energy for oxygen functional groups than Na⁺which improve the ICE of the carbon materials.As a result,the electrolyte with 0.5 wt%DTD delivers a higher reversible capacity of 347.9 m Ah g^-1 with an increased ICE of 81.96%and a more stable cycle performance.By adding 0.5 wt%DTD to 1M Na PF₆/DME electrolyte,the energy density of the full battery assembled with NVP cathode and HC anode was significantly improved by 29.6%.Most importantly,this work exhibits a simple,generic,low-cost,and efficient method to improve the electrochemical performance of hard carbon,which may give a promising perspective to boost the practical application in SIBs.