Research On LaMnO₃ Perovskite And LaMnO₃/Graphene Nanocomposite For Hybrid Battery Supercapacitor Device

Energy storage devices with excellent electrochemical performance（high energy and power density without sacrificing the cycling life）is of great importance.As one of these devices,hybrid battery-supercapacitor device（HBSD）that typically consists of a high-energy density battery-type electrode and a high-power density capacitor electrode,has attracted enormous attention due to its potential applications in future electric vehicles,smart electric grids,and electronic devices.With proper electrodes design,HBSD provides unique advantages such as high performance,safety,cheapness,and environment-friendly.At present,LaMnO3 perovskite oxide has proven to be an effective energy storage material due to its ability to accommodate large numbers of oxygen and cations vacancies,and therefore significantly increases the charge storage sites.Unfortunately,LaMnO3 often suffer from poor rate capability,rapid capacity fading and low cyclic stability due to their low ionic/electronic conductivity.As a result,the development of LaMnO3 perovskite with high energy/power densities and stable cyclability remains a significant challenge.On the other hand,reduced graphene oxide（rGO）has been widely investigated as capacitor-type electrode materials for HBSD due to its high electrical conductivity and large surface area.In this thesis,a hybrid battery-supercapacitor device consists of reduced graphene oxide（rGO）as capacitor-type electrode and LaMnO3（LMO）composite as battery-type electrode has been designed and fabricated.Before constructing the full hybrid device,the performance of the capacitor electrode and battery electrode were optimized as follow:For the capacitor electrode（rGO）,two different reducing agents,hydrazine（H-rGO）and ammonia（U-rGO）were used to reduce graphene oxide.Electrochemical tests revealed that H-rGO has exceptional high rate capacity(the specific capacitance is about 250 F g^-1 at 5 A g^-1 and 90%retention after 2000 cycle at 10 A g^-1).The excellent electrochemical performances are attributed to the high surface area of rGO and good electronic conductivity,which suggests their promising application for HBSD as a capacitor electrode.For the battery electrode（LMO）,four different strategies were proposed to improve the electrochemical performance of LMO,and can be summarized as below:First of all,the effective parameters on the synthesis of LMO including the molar ratio of citricacid/metal nitrate,calcination temperature and calcination time were investigated,and the effects of these parameters on the specific capacity of LMO were evaluated,and the optimum parameters was obtained by the Taguchi method.Based on the analysis results,the citric acid/metal nitrate molar ratio of 0.5 and calcination temperature of 750^?C for 4 hours are found to be effective to enhance the specific capacity of LMO.The confirmation test is conducted to validate the test result,which revealed that the capacitance of LMO was efficiently improved(250 C g^-11 at 5 A g^-1).After optimizing the synthesis parameters,LMO was incorporated into three-dimensional nitrogen-doped reduced graphene oxide（N-rGO）to improve the electrochemical performance of the LMO electrode.The process is driven by the electrostatic interaction between positively charged N-rGO and negatively charged LMO.Microscopic investigations confirmed that LMO particles were wrapped by N-rGO sheets.Electrochemical characterizations revealed that the specific capacity of the LMO/rGO nanocomposite(480.5 C g^-1 at 5 A g^-1)has been dramatically improved compared with pristine graphene and LMO.The nonstoichiometric effects were also investigated,a series of LaMn1±xO3 perovskite（x=0,0.05,0.1）have been synthesized and comprehensively characterized by a number of spectroscopic and microscopic tools.It was found that the nonstoichiometric LaMn1.1O3（LM1.1O）showed much higher specific capacity(300 C g^-1 at 5 A g^-1)than stoichiometric LaMnO3perovskite(250 C g^-1 at 5 A g^-1).Detailed chemical analysis demonstrated that the presence of point defects such as oxygen and cation vacancies,and a high Mn⁴⁺/Mn³⁺ratio contributed to the excellent electrochemical performance.To further improve the electrochemical performance of the LM1.1O as a novel HBSDelectrode material,more efficient approach to prepare nonstoichiometric LaMn1.1O3（LM1.1O）perovskite encapsulated by graphene sheets was proposed.In this approach,LM1.1O nanoparticles were first functionalized by aminopropyltrimethoxysilane（APS）to induce a positive charge on the LM1.1O nanoparticles surface.Then the positively charged LM1.1O is combined with graphene oxide.LM1.1O/rGO was obtained after hydrothermal reduction.For comparison,LM1.1O/N-rGO was also prepared（by direct mixing of LM1.1O with N-rGO）.Due to the unique structure,high chemical stability and large surface area,as-prepared LM1.1O/rGO nanocomposite possesses superior electrochemical performances with high specific capacity(648 Cg^-1 at 5 A g^–1)and excellent cycling stability(85%at 10 A g^–1).LM_1.1O/rGO was then used as a battery electrode,and graphene-based three-dimensional porous structure（rGO）was used as a capacity electrode material to fabricate HBSD.Thereby,the LM1.1O/rGO//rGO HBSD was successfully fabricated with an ultrahigh energy density of 53.47 W h kg^-1(power density of 374 W kg^-1),which also remains of 23.3 W h kg^-1 even at a high power density of 2035 W kg^-1.These results revealed that as-prepared HBSD could be used as an effective electrochemical energy storage device for future applications such as a hybrid electric vehicle and portable electronic devices.