Uncovering The Electrochemical Interfacial Properties Of Graphite/Electrolyte Interface In Potassium-ion Battery

The physical and chemical properties of the graphite/electrolyte interphase（solid electrolyte interphase,SEI）are the key factors to the electrochemical performance of graphite anode for potassium-ion batteries,analogous to lithium-ion batteries.It is vital to construct an electrochemically stable interphase by modifying the structure and composition of SEI.Clarifying the structure,composition,formation,evolution processes of the SEI on graphite surface and elucidating key influentional factors are the theoretical basis and prerequisites for the design of high-performance potassium ion batteries.However,there is still a lack of systematic research on the above issues and in-depth understanding of the key influencing factors,especially the internal mechanism on the effects of the binders and electrolytes on physical and chemical properties of the SEI.To address the above-mentioned bottleneck issues,this article focuses on the regulation of physical and chemical properties of SEI using various spectroscopic characterization techniques（in-situ Fourier transform infrared spectroscopy（FTIR）and Raman mapping）,X-ray photoelectron spectral deep profiling（XPS）),combined with in-situ electrochemical impedance analysis（EIS）and high-precision electrochemical test,to accurately capture and clearly analyze the influence of binders and electrolytes on the composition,structure,formation,and evolution process of SEI on graphite.The effects of binders and electrolytes on the physicochemical characteristics of SEI were revealed,and a clear relationship among the binders/electrolytes,physicochemical characteristics of SEI and electrochemical potassium storage properties were established.The specific research content and important conclusions are as follows:（1）In terms of binders,sodium alginate（SA）and sodium carboxymethyl cellulose（CMC）constructs an more electrochemically more stable SEI on the surface of the graphite,which significantly improves the electrochemical potassium storage characteristics,compared to polyvinylidene fluoride（PVDF）.Among them,the SA/graphite composite electrode still has a reversible capacity of 238 m Ah/g after 100cycles at 0.2 C,and the capacity retention reaches 85.4%,which is higher than that of PVDF（81.7%）.Moreover,the initial coulombic efficiency of the graphite anode has also been significantly improved from 37.69%（PVDF）to 62.42%（SA）and 73.87%（CMC）.The results show that SA and CMC can promote the formation and enrichment of K₂CO₃and polyethylene oxide polymers（PEO）,and the mechanism by which SA and CMC induce EC to form K₂CO₃ while reduce the formation of alkyl potassium carbonate（PEDC）was revealed.The advantages of SA and CMC in improving the potassium storage performance of graphite and the characteristics of SEI are due to:K₂CO₃ and PEO polymers from CMC-and SA-SEI with higher strength and wider distribution leading to the formation of a dense structure and stable SEI layer.The large number of-COONa functional groups in SA and CMC binders also helps to consume HF generated by electrolyte decomposition,thus avoiding structural damage of SEI and further reducing electrolyte consumption.（2）In terms of electrolytes,the cycle stability and coulombic efficiency of graphite anode in 3 M KFSI and 1 M KFSI in EC/DEC electrolyte are significantly higher than that of 0.8 M KPF₆ in EC/DEC electrolyte.The coulombic efficiency has also increased from 66.27%（0.8 M KPF₆）to 71.12%（1 M KFSI）and 74.22%（3 M KFSI）.The reversible capacity of 3 M KFSI reaches 268.5 m Ah/g after 100 cycles at 0.2 C,much higher than that of 0.8 M KPF₆（248.9 m Ah/g）.The capacity at 0.1 C is 261.1 m Ah/g,and can still reach 199 m Ah/g even at a high current density of 1 C,which is much higher than other types of electrolytes（0.8 M KPF₆:179 m Ah/g,1 M KFSI:53.6m Ah/g）.The results reveal that the FSI^-anionic groups preferentially decompose to form an SEI layer rich in KF components,and the decomposition potential of EC and DEC is obviously lagging in KFSI-based electrolytes.In addition,the SEI layer derived from KFSI-based electrolyte is rich in KF and K₂CO₃,and has a suitable content of PEO polymers,thus constitutes a dense and stable SEI.Attributeing to the SEI with good mechanical stability,chemical/electrochemical stability,and excellent ionic conductivity derived from the KFSI-based electrolyte,the graphite anode exhibits better potassium storage performance.