Basic Research On Cathode Materials For Aluminum-Based Batteries Based On AlCl₃/[EMIm]Cl Ionic Liquids

Currently,in the new secondary battery,the metal aluminum has attracted a lot of attention and research because of its high safety and low cost.The cathode materials for aluminum-based batteries are one of the key materials that determine their energy density and cost.Therefore,the electrochemical properties and energy storage mechanism of AlCl3/[EMIm]Cl electrolyte aluminum-based battery cathode materials(graphite,oxides,sulfides and selenides)are studied in detail.The results of the study are summarized as the following:(1)The use of n-butyllithium improves the microstructure and interlayer structure of graphite.The results show that pre-lithiation will expand the interlaminar structure of graphite,which will facilitate the insertion and extraction of polyaluminum anions,increase the utilization of electrode materials,and improve the electrochemical performance of graphite aluminum-based batteries.At the current density of 200 mA g"1,the specific specific capacity of lithiated graphite is 68.3 mAh g-1,which is significantly higher than that of graphite.After 500 cycles,the capacity retention rate is 95.1%at the current density of 500 mA g-1,and its coulombic efficiency is basically stabilized at 100%.It is found by cyclic voltammetry that the two pairs of redox peaks of lithiated graphite are attributed to the reversible intercalation/deintercalation of Al2Cl7-and AICl4-,respectively.(2)The electrochemical performance of the 3D AIV3O9 aluminum battery transition metal oxide cathode material shows that the specific discharge capacity is 327 mAh g-1 at the current density of 100 mA g-1.After 100 cycles,the initial discharge capacity is reduced to 195.5 mAh g-1,and the corresponding capacity retention is 59.8%.The pseudocapacitance behavior of the electrode material is analyzed by cyclic voltammetry.The results show that the capacity contributed by the pseudocapacitance increases from 21.6%at 0.2 mV s'1 to 90.7%(5 mV s-1)with increasing scan rate.For its energy storage mechanism,XPS and ex-situ XRD studies show that Al3+ reversibly intercalates/deintercalates into AIV3O9 electrode materials,and its phase transition at the electrode material interface during charge and discharge is partially reversible.(3)The microsphere MoS2 aluminum battery transition metal sulfide is composed of layered nanosheets with good dispersibility,and the thickness of the nanosheet is only several tens of nanometers.According to its electrochemical study,the first specific discharge capacity is 253.6 mAh g-1 at the current density of 20 mA g-1,and the reversible specific capacity is 66.7 mAh g-1 after 100 cycles.In addition,it is worth noting that by analyzing the non-in-situ XPS etching technique of the microsphere MoS2 electrode under different charge and discharge states,it is found that Al3+ is first inserted in the A1 position(constructed by the S-Mo-S ionic bond)of the smaller vacancy during discharge,and then inserted in the A2 position of the larger vacancies(constructed by Van der Waals forces between the MoS2-MoS2 layers).During the charging process,Al3+ first escapes from the A2 position to the A1 position,and then escapes from the A1 position to the electrolyte.However,Al3+ is only inserted and extracted at the A2 position inside the electrode material,and the phase change of the electrode material occurs only at the electrode material interface.(4)For selenide aluminum-based cathode materials,one-dimensional nanofibers selenium coated with carbon are prepared by using a simple wet chemical method,and one-dimensional hollow Se@C nanofibers were obtained after subsequent calcination.Using Se@CT and metallic aluminum to form Al-Se novel secondary batteries in the electrolytes of room temperature ionic liquids,it is believed that the oxidation intermediate of Se may not only be Se22+in the Al-Se battery,but actually there may be other oxidation intermediate products,such as,Se4+,Se2+,and Se82+,etc.In addition,the carbon layer in the Se@CT can play a structural stability role and provide the possibility to accommodate more electrochemical reactions of Se,and it can also slow the volume expansion of Se during the electrochemical process.Therefore,Se@CT shows excellent electrochemical performance in Al-Se batteries.At the current density of 200 mA g-1,its first discharge capacity is as high as 447.2 mAh g-1,the operating voltage is about 1.6 V,its energy density is 708.8 Wh kg-1,the capacity can still be 162.9 mAh g-1 after 200 cycles at the current density of 500 mA g-1,and the capacity retention rate is as high as 83.5%.For its energy storage mechanism,through electrochemical performance and XPS analysis,it is found that during the first cycle,the Se changed from a circular structure to a chain structure,making its discharge voltage relatively low.However,during the second cycle,it changes between the chain structures and its discharge voltage is relatively high.In order to highlight its electrochemical properties,it is found that by comparing the electrochemical properties of the relevant aluminum battery cathode electrode materials in the literature,its electrochemical properties are much better than those of the Se@CMK-3 in Al-Se batteries and the carbon materials,oxides,and sulphide electrode materials in aluminum batteries.(5)Through the analysis of the energy storage mechanism of the graphite-based aluminum battery cathode material for AlCl3/[EMIm]Cl electrolyte,it is found that the charge-discharge mechanism is different from that of the lithium-ion battery,and it is somewhat far-fetched called "aluminum-ion battery" from the reaction mechanism.It is basically similar to the dual ion battery currently under study,and therefore should be referred to as a dual ion battery based on an AlCl3/[EMIm]Cl aluminum salt electrolyte.By designing a simple and novel graphite-graphite dual ion battery(GGDIB),the charge-discharge mechanism,pseudocapacitance and self-discharge phenomenon of GGDIB are studied.The results show that the capacity contributed by the pseudocapacitance increases from 22.66%at the scan rate of 0.2 mV s'1 to 46.56%(at the scan rate of 1 mV s-1)as the scan rate increases,and the self-discharge rate of this GGDIB is 16.82%h-1.In addition,this GGDIB also exhibits excellent electrochemical performance,it provides a reversible discharge capacity of 76.5 mAh g-1 at the current density of 200 mA g-1,the specific discharge capacity is still 62.3 mAh g-1 after 1000 cycles at the current density of 500 mA g-1,and the capacity retention rate is as high as 98.42%.