| More and more researchers have focused on rechargeable aluminum-ion battery which is regarded as a potential candidate for alleviating partial resources shortage in lithium-ion battery.Aluminum-ion battery has many intrinsic advantages,such as abundant natural resources,large gravimetric/volumetric energy density and high safety,nontoxicity,etc.Up to date,one of the most essential challenges in aluminum-ion battery is the development of suitable positive electrode materials.Among the potential candidates for positive electrodes,elemental selenium holds more favorable energy storage capabilities,such as ideal volumetric capacity and high theoretical specific capacity of 1357 mA h g-1.Therefore,in view of the key problems existing in aluminum-ion batteries,the electrochemical properties of the selenium and transition metal vanadium selenide positive electrode materials were studied in this work.On this basis,from the perspective of controlling the dissolution of intermediates in electrolytes and blocking the migration of intermediates,some measures such as solid electrolyte,modified separator and carbon material coating were adopted to improve the performance of aluminum-selenium batteries.The results and progress are listed as follows:(1)The electrochemical performance of selenium nanowire cathode in aluminum-ion battery shows that the first-cycle discharge specific capacity of the ionic liquid electrolyte is 267 mA h g-1 under the current density of 200 mA g-1.After 100 cycles,the discharge specific capacity of the ionic liquid aluminum-selenium battery attenuates to 32 mA h g-1,and the corresponding capacity retention is 11.9%,The results show that the aluminum-selenium battery has higher charging and discharging platforms,but serious capacity attenuation.The energy storage mechanism of aluminum selenium battery is clarified by studying the change of valence state of selenium positive material.In the charging process,there are two stages.First,Se is oxidized to Se22+during charge to 1.77 V.By charging to 2.0 V,Se2+ is oxidized to Se4+.In the discharging process,Se4+is reduced to Se2+,and finally Se22+is reduced to Se.Combined with the thermodynamic calculation,the total reaction of aluminum selenium battery is:3Se+7A12Cl7-(?)4Al+3SeCl3AlC14+7AlCl4-.Meanwhile,we confirmed that the attenuation of capacity is probably caused by the dissolution of positive products(Se2C12 and SeCl4)in the charging and discharging process.(2)Due to the serious attenuation of aluminum-selenium battery in ionic liquid electrolyte,aluminum-selenium battery was constructed by gel-polymer solid electrolyte.It was found that aluminum-selenium battery with gel-polymer solid electrolyte in has stable,lower self-discharge and obvious charging and discharging platforms.Under the current density of 200 mA g-1,the first-cycle discharge specific capacity of solid electrolyte aluminum-selenium battery is 386 mA h g-1.After 100 cycles,the discharge specific capacity of the solid electrolyte aluminum-selenium battery attenuates to 79 mA h g-1,and the corresponding capacity retention is 20.5%.Therefore,the performance of aluminum-selenium batteries has been improved by using solid electrolyte.(3)A novel prototype of separators modified with CMK-3 porous carbon coating was developed,aiming to substantially promoting the rechargeable capacity and cycling stability of the aluminum-selenium batteries.As anticipation,the as-assembled aluminum-selenium batteries with CMK-3 modified separators presented an enhanced specific capacity up to 1009 mA h g-1 at the current density of 1000 mA g-1,with a retained specific capacity of 270 mA h g-1 over 500 cycles,and the corresponding capacity retention is 26.8%.CMK-3 modified separators can effectively inhibit the shuttle effect caused by soluble selenium chloroaluminate compound and alleviate the capacity attenuation of aluminum selenium battery.CMK-3 modified separators can block products dissolved into the negative electrode in the charging and discharging process,and adsorption intermediate as a second electrode reaction at the same time to achieve more effective active material recycling and higher utilization rate of selenium.Therefore,CMK-3 modified separators can effectively improve the performance of aluminum-selenium battery.The results of the mechanism suggest that aluminum-selenium batteries with the presence of CMK-3 modified separators have well suppressed the unexpected shuttle effects induced by soluble selenium chloroaluminate compounds.(4)Se@CMK-3-400 and Se@CMK-3-600 composites were successfully prepared with mesoporous carbon CMK-3 as the carrier at 400 and 600? by thermal fusion perfusion method.The mass percentage of selenium in the composites was 71.20%and 64.53%,respectively.Se@CMK-3-600 composite shows good cyclic stability in aluminum-ion batteries.The initial discharge specific capacity of Se@CMK-3-600 positive electrode was increased to 540 mA h g-1.After 100 cycles,the discharge specific capacity remained stable at 115 mA h g-1.Furthermore,the CMK-3 modified separator is applied to Se@CMK-3-600 battery to achieve double protection.The initial discharge specific capacity of the Se@CMK-3-600 positive electrode was increased to 1295 mA h g-1 at a current density of 1000 mA g-1,and the specific capacity is remained stable at 651 mA h g-1 after 400 cycles,and the corresponding capacity retention is 50.3%.It is found that the double protection can effectively improve the electrochemical performance of aluminum-selenium batteries.(5)The brick-like VSe2 as positive electrode for aluminum-ion battery can deliver two discharge voltage plateaus of-1.2 V and-0.6 V.The initial specific discharge capacity is 650 mA h g-1 at a current density of 100 mA g-1.The reversible capacity is about 50 mA h g-1 with coulombic efficiency of about 94%after 100 cycles.The electrochemical mechanism of the battery was measured by analyzing the positive electrode before and after cycled.During the charging process,the Se2-in the cathode material were oxidized to Se,and V4+was oxidized to V5+.After discharging,Se was probably assigned to Sen2-and V5+was reduced into V4+.Therefore,the capacity of the battery is provided by the change of valence states of Se and V during repeated charge/discharge cycles.The battery capacity will decline because irreversible side reactions will be occurred during the charging and discharging process. |
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