Research On The Preparation,Microstructure Controlling Of Porous Nano LiMn₂O₄–based Cathode Materials And Their Electrochemical Properties In Aqueous Electrolyte

LiMn₂O₄-based material is an ideal electrode material for the aqueous energy storage systems(such as aqueous lithium-ion batteries,hybrid capacitor and etc.)due to its cheap,inherently safe and compatible with the aqueous electrolyte.Although current research results show that nano LiMn₂O₄-based materials exhibit high specific capacity,its low lithium ion conductivity,severe Jahn-Taller effect,disproportion reaction of Mn³⁺and etc.inevitably lead to the unsatisfactory rate capability and cycling stability,especailly at large current density.According to the energy storage mechanism,the coordination of surface absorption-desorption process and bulk Faradaic reaction should be fully exerted in order to improve the electrochemical performance of LiMn₂O₄-based material in aqueous electrolyte.And in order to better coordinate the effect of surface absorption-desorption process and bulk Faradaic reaction,the relationship between structure and electrochemical performance should be clarified.However,due to different preparation strategies,reactants and etc.,currently reported LiMn₂O₄-based materials show great differences on their structures and electrochemical performance.Moreover,in current research,the relationship between structure(such as morphology,crystallinity,specific surface area,component and etc.)and their electrochemical energy storage as well as electrochemical performance are still lack of systematically study.In view of this,this work aims to improve the rate capability and cycling stability,especailly at large current density.The relationship between microstructue,morphology,component and electrochemical energy storage as well as electrochemical properties are systematically investigated.The main contents are as follow:1)The effect of crystallization and microstructure on electrochemical performance of LiMn₂O₄.3D porous LiMn₂O₄(denoted as LMO)is prepared using our previously reported 3D network mesoporous Mn O₂ in air by a pyrolysis at 250 ^oC.The relationship between structure and electrochemical performance indicate that well-defined nanocrystals and rich pores are beneficial for the improvement of electrochemical performance,especailly at large current densities.Therefore,due to the proper particle size with perfect crystallization(?50 nm),LMO formed at 265 ^oC exhibits the best comprehensive electrochemical performance:a high capacities of102 m Ah g^-1 at a current density of 5 C and maintain 57.3%of its initial capacity at70 C with a greatly improved cycling stability of 70.7%at large current density 13.5C after 5000 cycles in 1M Li₂SO₄.2)The relationship between crystallite size and electrochemical performance of LiMn₂O₄.Using 3D network mesoporous Mn O₂ as reactant and Na Cl as confined template,homogeneous cubic nanocrystals assembled 3D porous LMO with different crystallite size are prepared by pyrolysis in air.The relationship between structure and electrochemical performance indicate that LMO with 50 nm shows the best comprehensive electrochemical performance at large current densities(including in aqueous electrolyte and organic electrolyte).As a cathode material,it can exhibit a high capacity up to 120.9 m Ah g^-1 at current density of 0.5 A g^-1 and maintain64.2%of its initial capacity at 15 A g^-1 with an enhanced cycling stability of 80.1%at large current density of 3 A g^-1after 5000 cycles in 1M Li₂SO₄.Meanwhile,it can exhibit a specific capacity of 112.3 m Ah g^-1 at a current density of 0.1 A g^-1 and maintain 72.4%of its initial capacity at 3 A g^-1 with a well cycling stability of84.6%at current density of 1 A g^-1after 1000 cycles in 1M Li PF₆.3)The relationship between component,valance state and electrochemical energy storage as well as electrochemical performance of Li_1+xMn_2-xO₄ cathode materials.Spinel lithium-rich maganese oxides Li_1+xMn_2-xO₄(denoted as LLMO,x=0,0.06,0.12,0.24,0.33)with the same morphologies,crystallite size and microstructures are successfully prepared by solid-state method.The relationship between component,valance state and electrochemical performance indicate Mn⁴⁺is helpful to improve the lithium ion transportation efficiency and structure stability,while Mn³⁺is benefical for the Faradic reaction and higher specific capacity.Therefore,the ratio of Mn³⁺and Mn⁴⁺in LLMO should be reasonably optimized from the perspective of the improvement of comprehensive electrochemical performance.Considering the relationship of specific capacity and Li content(the specific capacity of LLMO(y)and its Li-rich content(x)coincide with a linear relationship y=-260.55x+104.74)as well as its rate capability and cycling stability,LLMO with Li/Mn ratio around0.5-0.6(corresponding to the Mn³⁺/Mn ratio in the range of 35.4%-50%)shows better comprehensive electrochemical performance.4)The influence of Mt(Co?Ni)doping on Mn valence controlling and electrochemical performance of LMO.3D porous Li(Mt_xMn_1-x)₂O₄(Mt=Ni or Co,x=0,0.06,0.14,0.25)with similar morphology are prepared by solid state pyrolysis in air.The results indicate that the rate capability and cycling stability of Mt(Co?Ni)doped Li(Mt_xMn_1-x)₂O₄are obviously improved.Furthermore,the specific capacities of Li(Ni_xMn_1-x)₂O₄ and Li(Co_xMn_1-x)₂O₄(y)and dopants Mt(Co?Ni)content(x)coincide with a linear relationship y=-420.65x+121.61(0?x?0.25)and y=-213.66x+122.13(0?x?0.25),respectively.Li(Mt_xMn_1-x)₂O₄with dopant Mt(Co?Ni)content less than 3%is demonstrated more favourable for the comprehensive electrochemical performance,especailly for Li(Co_xMn_1-x)₂O₄.