Design Of High-performance Carbon Anode For Sodium/potassium Ion Batteries And Study Of Its Energy Storage Mechanis

Under the background of the rapid development of large-scale energy storage power stations and the shortage of lithium-ion battery resources and the high price,low-cost and resource-rich sodium ion batteries（SIBs）and potassium ion batteries（PIBs）have been studied and developed rapidly.In particular,carbon-based materials have become the most promising anode for SIBs and PIBs due to their dual advantages in price and performance.However,the current problems of SIBs and PIBs,such as low initial coulombic efficiency（ICE）,poor rate performance,unclear energy storage mechanism,and differences in electrolyte kinetics,limit practical applications.With the purpose of revealing and solving the scientific problems and application challenges faced by SIBs and PIBs,this paper follows the"bottom-up"research idea of material structure-electrode-electrolyte interface-battery systems and proposes several efficient design strategies.First,through structural control,the sodium storage mechanisms of typical soft carbons,soft carbon precursor-derived refractory disordered carbons,and typical hard carbons were sequentially unlocked,and a general model for the sodium storage mechanism of non-graphitic carbons was established.Based on structural control,further focus is placed on the design of the overall electrode,especially the influence of the optimization of the binder on the electrochemical performance.Subsequently,the truth of the kinetic difference between ester-based electrolytes and ether-based electrolytes is revisited.Finally,the influence of the battery system including structural and test protocol on battery performance evaluation is revealed.The main research results of this paper are as follows:（1）A series of PC-X soft carbon materials are developed by one-step pyrolysis of petroleum coke with low price and ultra-high carbon yield（89%）.The structural relationship between microstructure and sodium storage performance is explored by adjusting the pyrolysis temperature,and the sodium storage mechanism of soft carbon in ether-based electrolytes is revealed.Among them,PC-700 shows the most excellent sodium storage performance,exhibiting a reversible specific capacity of 276.8 m Ah g^-1 and an ICE of 88.1%at 20 m Ag^-1,as well as excellent rate performance and stability(153.2 m Ah g^-1 at 2 A g^-1).As the pyrolysis temperature increases,the sodium storage performance of the soft carbon anode gradually decreases,which is mainly due to the reduction of the microcrystalline interlayer at high temperature,and the decrease of defect and pore content leads to the reduction of sodium storage sites and diffusion channels.Using structure-activity analysis,CV curves at different scan rates and GITT synergistic analysis,the sodium storage mechanism of soft carbon anodes in ether-based electrolytes was confirmed for the first time with"surface adsorption-intercalation-filling"three-segment model.（2）To address the low sodium storage performance of petroleum coke-derived soft carbon anodes,an air-assisted ball milling strategy was successfully developed.This strategy utilizes mechanical force and oxygen species in the air to synergistically suppress the ordering of petroleum coke,converting the soft carbon precursor into a non-graphitizable disordered carbon PCQ-1100.Compared with PC-1100 soft carbon,this strategy significantly improves the sodium storage performance,increasing the reversible capacity from 201.3 to 307.4 m Ah g^-1 in the ether-based electrolyte.PCQ-1100 also exhibits excellent rate performance and excellent cycling stability(81.5%retention after 2000 cycles at 1 A g^-1).Furthermore,the Na（Ni Fe Mn）_1/3O₂//PCQ-1100full cell displays a high capacity of 287.1 m Ah g^-1.More importantly,CV and GITT synergistic analysis further confirmed that soft carbon and non-graphitizable disordered carbon have a general sodium storage mechanism:adsorption-intercalation-pore filling.This work provides new insights and strategies for converting low-cost soft carbon precursors into non-graphitizable disordered carbons for enhanced sodium storage performance.（3）Using petroleum coke precursor as raw material,the integrated electrode PC-X was regulated from microstructural engineering to binder optimization to achieve high ICE and efficient potassium storage.With the strong assistance of sodium carboxymethylcellulose（CMC）aqueous binder,the PC-900 anode exhibits an ultra-high ICE of 80.5%,which is one of the highest values reported for PIBs carbon-based anodes.Meanwhile,the PC-900 anode shows high capacity(304.3 m Ah g^-1),excellent rate capability(138.2 m Ah g^-1 at 10 C),and excellent stability.The ultra-high ICE and excellent sodium storage performance are mainly attributed to the favorable microstructure（low specific surface area,functional group content,and large interlayer spacing）.At the same time,binder optimization also plays a crucial role in reducing irreversible capacity and interfacial impedance,further improving ICE and rate capability.Finally,the potassium storage mechanism of the PC-X electrode is synergistically analyzed by different scan rates CV,electrochemical in-situ XRD and structure-activity relationship.This work not only provides an excellent anode material for potassium storage but also provides a new direction for the development of carbon anodes with ultra-high ICE and large-scale applications of PIBs.（4）A series of practical CPHCs hard carbon anodes were prepared by choosing low-cost,wide-ranging,and renewable biomass polymer cellulose powder as the precursor.Among them,the CPHC-1500 electrode exhibits an ultra-high reversible capacity of 405 m Ah g^-1 and an ICE of 93.8%in the ether-based electrolyte.In addition,CPHC-1300 also exhibits a reversible capacity of 377 m Ah g^-1 with 95%ultra-high ICE and excellent rate performance(204.5 m Ah g^-1 at 2 A g^-1).Using different scan rates CV curves,GITT,electrochemical in-situ XRD and structure-activity analysis synergistically show that the sodium storage behavior of CPHCs in ether-based electrolytes also conforms to the three-stage mechanism of"adsorption-intercalation-pore filling".So far,this study has revealed a common sodium storage mechanism for non-graphitic carbons with different microstructures,ranging from soft to hard carbons.（5）By evaluating the rate capability of SIBs,ether-based electrolytes generally outperform ester-based electrolytes,which has almost become a consensus in previous studies based on half-cell tests.However,we find that contrary to the consensus,ester-based electrolytes exhibit better sodium storage capacity than ether electrolytes in full cells.In-depth analysis of the three-electrode,symmetric cell,and in-situ XRD tests show that the traditional half-cell test results are unreliable due to the interference of Na electrodes.In particular,the Na electrode shows a huge difference in stability between the ester and ether-based electrolytes,and ester-based electrolytes are more severely disturbed than ether-based electrolytes,resulting in the illusion that ester-based electrolytes are far inferior to ether-based electrolytes.More seriously,the three-electrode test,which is considered to be more accurate,also suffers from Na electrode interference.Therefore,a"corrected half-cell test"protocol is proposed to shield the Na electrode interference,revealing the excellent rate capability of hard carbons in very close proximity to ester and ether electrolytes.This work breaks the inherent perception that the kinetic properties of ester-based electrolytes in SIBs are inferior to ethers,reveals the pitfalls of half-cell testing,and proposes a new testing protocol with reliable results that will help advance the commercialization of SIBs.