Study On The Performance And Mechanism Of Potassium Storage In Soft Carbon Materials

Potassium ion batteries（PIBs）operate at voltages close to those of lithium ion batteries and are highly promising high-energy-density electrochemical energy storage systems.Among the anode materials,carbon-based materials are considered to be the closest ones to practical potassium storage materials.How to construct potassium storage carbon materials with good performance is still a key issue.In-depth analysis of the structural evolution of carbon materials during the process of（de）uuuu potassiation reaction and elucidation of the potassium storage reaction mechanism are of great theoretical and practical significance for the design of potassium storage carbon anode materials from the fundamental point of view.Quantitative resolution of the amorphous phase structural characteristics of soft carbon materials is a key challenge in studying their potassium storage mechanisms.In this paper,a series of soft carbon materials were prepared and their structures and properties were analyzed.Based on the evolution of local structure and long-range structure,the potassium storage mechanism and properties of different structural carbon materials（e.g.graphite and soft carbon）were explored combining with advanced characterization techniques such as solid-state NMR spectroscopy,Raman spectroscopy and XRD.The details are as follows:i.Optimized design of high-performance soft carbon materials.Soft carbon with a turbostratic structure was prepared by a simple high-temperature calcination method using pitch powder as the precursor.The carbonization temperature,constant temperature holding time and heating rate of the calcined samples were optimized,and the potassium storage performance of soft carbon was improved.The electrode binder and electrolyte were improved to solve the problem of significant capacity decay during cycles.What’s more,high-performance PIBs were developed,which exhibited a high reversible capacity of 323 m Ah g^-1at the current density of 13.95 m A g^-1and providing a cycling capacity of199 m Ah g^-1after 100 cycles at 139.5 m A g^-1with a capacity retention rate of 99%.Even under the condition of 279 m A g^-1,the reversible capacity of 185 m Ah g^-1can be maintained after 200 cycles,and the coulomb efficiency was as high as 99.8%.Applying the material,the material shows superior energy density potential compared to graphite by simulating full cells.ii.Reaction mechanism of high-performance soft carbon potassium storage.In-depth analysis of the microscopic reaction mechanism of soft carbon materials will provide an important theoretical basis for the rational design of the materials.In view of the amorphous structural characteristics of soft carbon materials,we applied solid ¹³C NMR spectroscopy with Raman and XRD techniques to analyze the structural evolution of the carbon framework in the process of potassium embedding in soft carbon materials.It is revealed that the insertion of potassium ions into the material firstly induces a transformation of the disordered region to short-range ordered stacking,involving both the pristine local unorganized and organized carbon layers.Subsequently,potassium ions intercalate into the rearranged carbon structure,finally producing the nano-sized KC₈.Moreover,a remarkable capacity of 322 m Ah g^-1with a low mid potassiation voltage of<0.3 V is present for the prepared soft carbon,which is on account of the underlying potassium storage sites,including the disordered stacking carbon as a main component of the soft carbon.These results suggest that regulating the disordered stacking region in the turbostratic structure of soft carbon is a critical issue for further improving the performance.