Hydrothermal Synthesis Of Lithium Manganese Oxides As Electrode Materials For Hybrid Supercapacitors

Supercapacitors are new energy storage devices between batteries and traditional capacitors,which have the characteristics of high energy density and power density,short charging time,long cycle life and good performance under low temperature etc.Much research in this area has focused on electrode materials.Lithium manganese oxide is considered as promising electrode material due to its environmental friendship,abundant resources,low cost,and good rate capability.In this paper,Manganese oxides(MnO2)precursors with various morphologies and crystal structures have been successful synthesized via a hydrothermal route by using KMnO4 and various reductants(CO(NH2)2,NaNO2,(NH4)2SO4,MnSO4).Then,these MnO2 precursors were used to hydrothermally synthesize spinel LiMn2O4 samples in order to investigate the effect of various MnO2 precursors on the capacitive properties of LiMn2O4 samples.The main results are as follows:(1)Manganese oxides(MnO2)with various morphologies and crystal structures were synthesized at different hydrothermal temperatures(120 °C,150 °C,180 °C)in 8h by using urea as the reductants.?-MnO2 nanoflowers and ?-MnO2 nanowires were formed at hydrothermal temperature of 120 °C and 180 °C,respectively.MnO2 and activated carbon(AC)were used as the positive and negative electrode in hybrid supercapacitors AC//MnO2.The AC//?-MnO2 and AC//?-MnO2 delivered the initial specific capacitance of 31.2 F·g-1 and 25.4 F·g-1,respectively,in 1 M Li2SO4 neutral electrolyte.After 1000 cycles,the capacitance retention was 97.4% and 94.6%.It was clear that ?-Mn O2 nanoflowers exhibited higher specific capacitance and better cycling performance.When the reaction time was prolonged from 8 h to 36 h,the electrochemical performance of MnO2 became worse and worse.Therefore,?-MnO2 nanoflowers which were synthesized at the condition of 120 °C and 8 h exhibited the best capacitive properties.The work has not been reported to our best knowledge.(2)The ?-Mn O2 nanoflowers were used to hydrothermally prepare spinel Li Mn2O4 samples with various crystal structures at different hydrothermal temperatures(140 ?~ 200 ?)and reaction time(6 h ~ 36 h).The results showed that the optimum temperature was 180 °C and the optimal reaction time was 24 h.The LiMn2O4 electrode obtained at the optimal condition of 180 °C and 24 h was used as the positive electrode in AC//LiMn2O4 hybrid supercapacitors,which delivered the initial specific capacitance of 45.4 F·g-1 in 1 M Li2SO4 neutral electrolyte.After 1000 cycles,the capacitance retention was 97.6%.This method has better electrochemical performance than that of LiMn2O4 obtained by the calcination route.(3)The other reductants(NaNO2,(NH4)2SO4,MnSO4)were used to hydrothermally prepare MnO2 precursors(MO-1,MO-2,and MO-3)with various morphologies and crystal structures in order to investigate the effect of the other MnO2 precursors on capacitive properties of LiMn2O4 samples.The pure LMO-1,impure LMO-2,and impure LMO-3 were synthesized from nanoflower-like MO-1,microflower-like MO-2 and microscale urchin-like MO-3,respectively.The electrochemical tests showed that the initial specific capacitance of AC//LMO-1,AC//LMO-2,and AC//LMO-3 were 41.98 F·g-1,37.12 F·g-1 and 35.07 F·g-1,respectively.After 1000 cycles,the capacitance retention was 98.9%,93.9% and 84.5%.It was clear that LMO-1 electrode showed the best capacitive property.Therefore,MO-1(nanoflower-like ?-MnO2)obtained by using NaNO2 reductant was also suitable for hydrothermal synthesis of spinel LiMn2O4.The work has not been reported to our best knowledge.In conclusion,two nanoflower-like ?-MnO2 precursors were obtained at the hydrothermal condition of 120 °C and 8 h by adding CO(NH2)2 or NaNO2 reductant into KMnO4.Pure spinel LiMn2O4 samples with high crystallinity were successfully from the nanoflower-like ?-MnO2 precursors.The pure LiMn2O4 samples showed excellent electrochemical performance.