Preparation And Electrochemical Performance Study Of KMnF₃-based Potassium Ion Battery Cathode Material

In the field of cathode materials for potassium ion batteries?PIBs?,Mn-based materials are favored by researchers in terms of environmental friendliness and abundant resources.Among them,Mn-based perovskite fluoride material has the advantages of being cathode for PIBs due to its inherent three-dimensional diffusion channel and high theoretical capacity.However,these materials have the problems of dissolution of Mn ions in the electrolyte and poor fluoride conductivity,which affects the electrochemical performance when used as electrode materials.In this dissertation,Mn-based perovskite fluoride KMnF₃ is selected as the matrix material,and various composite materials are synthesized as cathode for PIBs by co-precipitation and homogeneous precipitation methods.The selected synthesis method is green and the process is controllable.The formation mechanism of different morphologies is discussed.The electrode materials are designed from the perspective of improving the stability of the SEI film,inhibiting the dissolution of Mn ions,and improving the electrical conductivity of the material,so as to achieve the improvement of the electrochemical performance for PIBs.Electrochemical Impedance Spectroscopy?EIS?is used to explain the interface problems in the process of charging and discharging.The attribution of each time constant in EIS is explained,and then the analysis of the capacity decay mechanism of PIBs is realized,and the method for further improving the electrochemical performance is given.The reasonable interpretation of EIS will promote the study of such materials as PIBs electrode materials,which is of great significance for the further development of efficient energy storage systems.The specific research content and results are as follows:?1?First-principles are based on density functional theory?DFT?and predict the possibility of KMnF₃ as cathode for PIBs.Based on calculations,KMnF₃ nanomaterials are synthesized by co-precipitation method,the synthesis conditions are optimized,and the formation mechanism of the nanomaterials is speculated.When used as cathode for PIBs,its reversible capacity in the first cycle is low,and shows the rapid capacity fading.According to the fitting data of EIS equivalent circuit diagram,the attribution of different time constants is inferred: the high frequency region semicircle belongs to the migration process of K ions through the SEI film,and the low frequency region oblique line or arc belongs to the charge transfer process.EIS clarifies the reason for the poor electrochemical performance,that is,the instability of the solid electrolyte interface?SEI?film intensifies the dissolution of Mn ions in the electrolyte and increases the irreversible capacity.Charge transfer resistance is an important parameter that affects the electrochemical performance.?2?In order to suppress the dissolution of Mn ions in the electrolyte and improve the stability of SEI film,based on the first-principles calculation,Co-doped KMnF₃ is synthesized,combined with the aforementioned optimized co-precipitation method.Through the control of the synthesis conditions,the gradient distribution of Co ions is realized,thereby suppressing the dissolution of Mn ions in the electrolyte,and improving the charge-discharge cycle performance.The charge and discharge capacity of the 60 th cycle reached 118.2 and 103.4 m Ah·g-1?current density: 40 m A·g-1?.Using EIS to prove the attribution of different time constants,the fitting trend analysis of the equivalent circuit diagram confirms that the mid-frequency region semicircle belongs to the Schottky contact.The EIS results also confirmed that the special gradient structure can promote the stability of the SEI film,but the long-cycle stability still needs to be improved.?3?EDTA-assisted homogeneous precipitation method is used to synthesize Ni-doped and Co-doped KMnF₃ separately,and the battery rate performance and long cycle stability are improved through morphology control.In the synthesis process,EDTA acts as a buffer and chelating agent.On the one hand,the difference between the stability constants of EDTA and Ni,Co,Mn can be used to achieve a slow and controlled release of Mn.On the other hand,EDTA can be adsorbed on the surface of the particles to create a steric effect,preventing the particles from agglomerating.Ni doping can form K?Mn0.95Ni0.05?F3 with a surface buffer layer structure,which helps to suppress the dissolution of Mn inside the particles during charge and discharge,and significantly improves its charge and discharge performance.The prepared K?Mn0.95Ni0.05?F3/CNTs composite electrode is particularly outstanding for the improvement of rate charge-discharge performance.After 35 m A·g-1 to 280 m A·g-1 rate charge and discharge,when the current density is reduced to 35 m A·g-1 again,the reversible capacity can still be restored to 107.6 m Ah·g-1.Co doping can form K?Mn0.95Co0.05?F3 with a hollow yolk-shell structure,which helps to buffer the volume change during repeated charge and discharge processes,and makes the formed SEI film have good stability.At a current density of 35 m A·g-1,the charge-discharge capacity remained at 88.2 m Ah·g-1 and 83.7 m Ah·g-1 after 200 charge-discharge cycles,respectively.?4?On the basis of first-principles calculations,a significant improvement in the conductivity and electrochemical performance of KMnF₃ is achieved through O doping.A new etching route is introduced during the synthesis of KMn O0.125F2.875@C composite material to promote the uniform dispersion of KMn O0.125F2.875 nanoparticles in the carbon coating.The carbon coating layer can not only improve the conductivity of the material,but also prevent Mn ions from directly contacting the electrolyte,which may inhibit the dissolution of Mn ions in the electrolyte.The structure design is expected to obtain electrode materials with long cycle stability.After 100 cycles,the charge and discharge capacity can still be maintained at 120.2 and 108.0 m Ah·g-1,respectively,at a current density of 35 m A·g-1.EIS assigns the mid-frequency region semicircle to electronic conductivity,which realizes the correlation between EIS and the basic parameters of the battery.?5??-MnO₂/KMnF₃ composite with mixed valence of Mn ions is constructed to achieve comprehensive improvement of electrochemical performance.The composite electrode material has improved conductivity and stable,fast liquid absorption performance,showing high capacity,good cycle performance and rate performance during the charge and discharge process.After 200 cycles,the charge and discharge capacity at a current density of 100 m A·g-1 can be as high as 90 m Ah·g-1or more.In order to further improve the stability of the SEI film,the effect of succinonitrile film-forming additives on the interface properties is investigated.The-CN group of succinonitrile strongly interacts with the Mn ions on the electrode surface,which helps to form the dense and stable SEI film,thereby effectively reducing the direct contact between the electrolyte and the cathode,and suppressing the oxidative decomposition of electrolyte.The EIS results show that due to the addition of succinonitrile,the ion hopping energy barrier tends to be stable at different electrode potentials,and the migration of K+ through the SEI film is less affected by temperature.These research results will provide new ideas and directions for the improvement of electrochemical performance.