Investigation On High-Rate Performance And Mechanism Of Rechargeable Graphene-Aluminum Ion Batteries

With the increasing demand for fast charging of energy storage devices,shortening the charging time and improving the high-rate performance of batteries have become an important trend in renewable energy storage fields.Rechargeable aluminum-ion（Al-ion）batteries are considered as a promising energy storage technology to replace lithium-based batteries owing to their low cost,easy fabrication,high theoretical capacity,high safety,fast-charging capability and long cycle life.This dissertation aims to design and prepare rechargeable Al-ion batteries with high-rate capability from the four aspects of cathode,anode,interface and electrolyte;explore the mechanism of high-rate reactions;and expects to open an avenue for future liquid/solid-state Al-ion batteries with high-rate capability.Its main contents and conclusions are as follows:（1）Based on the influence of graphite structure on the electrochemical performance of Al-ion batteries,three-dimensional（3D）graphene cathode was prepared by chemical vapor deposition（CVD）method and dried with supercritical carbon dioxide（CO₂）.The performance of the 3D graphene cathode was then optimized via selecting templates,regulating gas flow rate for CVD growth,controlling the mass and density of cathode.Finally,the Al-ion battery based on this 3D graphene cathode achieved a breakthrough in the high-rate performance.It can deliver a specific capacity of 200 m Ah g^-1,an energy density of 385.5 Wh kg^-1 and a power density of 748.7 k W kg^-1.In addition,it was found that“fast discharge”is a key factor restricting the high-rate performance of batteries（specific capacity decays by 84%）;a“fast charge-slow discharge”strategy was proposed,which can further improve the“fast charge”performance of the battery and achieve a high charging current up to 1000 A g^-1.（2）In order to solve the problems of high saturation voltage and low specific capacity caused by serious polarization of Al anode during fast charging,a surface activation method for Al anode with liquid metal was proposed.The results of SEM and OM showed that the deposition mode of Al atoms changes from the granular nucleation mode guided by the surface defects on pure Al anode to the explosive dendritic growth mode on the activated Al anode,which can greatly improve the charging and discharging efficiency of activated anodes.As a result,the batteries with this activated anode exhibit higher Coulombic efficiency in initial cycles,lower saturation voltages,a 500%higher specific capacity at high rates（with the same cut-off voltage）,and long-term stability over 45,000 cycles.（3）Surface-enhanced Raman spectroscopy was used to track the events at the anode surface during charging and discharging.It was found that a triple complex of aluminum(Al₃Cl₁₀^-)is involved in the high-rate reactions of Al-ion batteries,which’s occurrence and frequency depend on the current density and the nature of the anode surface.Thus,a new equation including this triple complex for the reaction on anodes at high rates was proposed,as well as a new theory that the arrangement of anions in the electric double layer has polymer-like properties and can be regulated according to the demand of electrochemical reactions.（4）A novel synthesis method of free-standing gel polymer electrolyte（GPE）is proposed,which can increase the ionic conductivity to 1.69×10^-2 S cm^-1 and regulate the interface of Al anode with high-level fractal morphology during charging.Full batteries with a structure of Al/GPE/3D graphene are proved to possess the feature of flexibility,be stable under current densities from 20 to 200 A g^-1(with specific capacities of 122 m Ah g^-1),and can provide a high energy density of 469 k W kg^-1,an ultrafast charging rate of 0.24 s(at 1000 A g^-1)and the stability over 20,000 cycles.Moreover,high-flux operations are found particularly useful for activating the interface between GPE and anodes in solid-state Al-ion batteries,which can reduce voltage surge（～0.3 V）,increase specific capacities（～10%）,improve Coulombic efficiency,elevate discharge voltage plateaus and increase energy/power density in output（～17%）.