Investigation On Interfacial Chemistry And Charge-discharge Mechanism Of Anode Materials/Electrolyte For Potassium Ion Batteries

With the development of novel potassium-ion battery technologies,traditional electrolytes cannot meet the requirements of high capacity,high rate,and long term cycle stability of potassium-ion batteries.Specific anode materials can only operate with the matched electrolyte systems.In addition,the electrolytes play a role on the interfacial chemical reaction and intercalation mechanism of anode materials for potassium ion batteries.However,the research on the matching between electrolyte and cathode materials in the literature is still not systematic and in-depth.Given that,the electrochemical potassium-ion storage behaviors of two anode materials,namely the mesocarbon microspheres（MCMB）and Ge S₂@Ni S@N-C,have been systematically investigated in varied electrolytes.Specifically,the anode-electrolyte interfaical chemistry and the potassium-ion storage mechanism have been studied.The main contents of this work are as follows:（1）The interfacial chemistry and intercalation mechanism of mesocarbon microspheres（MCMB）were investigated in four different electrolytes.Through the in-situ X-ray diffraction（XRD）experiments,the sequential and stacking K⁺intercalation mechanism of MCMB anode has been revealed in ester-based electrolytes,which realize the complete transformation from graphite to KC₈.The electrochemical data also demonstrates the high specifica capacity,superior rate capability and excellent cycle performances for MCMB anode in ester-based electrolytes.In contrast,MCMB anodes undergo a K⁺-solvent co-intercalation mechanism in ether-based electrolytes,which cause graphite layer exfoliation,successive parasitic reaction and capacity fading.The co-intercalation behavior can be effectively suppressed by conditioning the high-concentration electrolytes.In addition,the results suggest that the accurate control of the concentration and composition of fluorine-containing additives（such as Fluoroethylene carbonate,FEC）is nessesary,otherwise unsatisfactory capacity and cycle performance may be exhibited for the MCMB electrodes.Interface analyses show that solid electrolyte interface（SEI）have been formed on the surface of MCMB anodes in all electrolytes.The components of SEI in ester electrolytes contain the K_xPF_y,K_xPO_yF_z,KF,alkyl carbonate,alkoxide,and the components of SEI in ether electrolytes are mainly the-SO₂-,-SO₂-CF₃/-SO₂-F,SO_x,S,sulfide,KN_xO_y,KF,alkyl carbonate and alkoxide.The composition and content of these SEI species also change continuously from the surface to the bulk.（2）The mesoporous and compositional gradient nitrogen-doped carbon-coated germanium disulfide-nickel sulfide（Ge S₂@Ni S@N-C）microspheres have been successfully prepared by the combined hydrothermal mineralization process,dopamine coating process and Kirkendall effect-induced high-temperature interdiffusion vulcanization process.The outer nitrogen-doped carbon shell is formed by high-temperature pyrolysis of polydopamine（PDA）,and the inner components compose of the heterogeneous interface-rich Ge S₂-Ni S composites.The composition presents a gradient distribution（the outer Ni S phase is enriched,and the inner Ge S₂phase is enriched）.Varied potassium-ion storage behaviors have been revealed for the Ge S₂@Ni S@NC composites in two electrolytes,namely the KFSI-EC/DEC and KPF₆-EC/DEC.The distinct charge-discharge profiles indicate varied potassium-ion intercalation mechanism.The composites exhibit a high initial coulombic efficiency（58.7%）and an excellent rate capability(also 269.0 m Ah g^-1 specific capacity at high current density of 1.0 A g^-1)in KFSI-EC/DE electrolyte,while the pottassium ion storage performances in KPF₆-EC/DEC electrolyte are unsatisfactory(the initial Coulombic efficiency is 44.3%and the specific capacity is only 8.3 m Ah g^-1 at a high current density of 1.0 A g^-1).The interface analyses show that the inorganic components of SEI films in KFSI-EC/DEC electrolyte are the K_xNO_y,KF,S,sulfide,SO_y,-SO₂-and sulphate,while the SEI components in KPF₆-EC/DEC electrolyte contain the K_xPF_y,K_xPO_yF_z,KF,sulfide,S,SO_x,-SO₂-and sulphate.The different SEI components derived from the decomposition of varied pottasium salts affect the structure and stability of the SEI,which further determines the electrochemical potassium-ion storage mechanism and properties.