Preparation And Li Storage Performance Of Transition Metal Oxide/Carbon Composites

Lithium ion batteries have been widely used in consumer electronics and power supply because of their advantages with high energy density,long cycle life,and pollution-free.However,when commercialized graphite acts as an anode for Li-ion batteries（LIBs）,it can not meet the demand of high energy density due to the low theoretical capacity(372 m Ah g^-1).Transition metal oxides have attracted wide attention owning of their high theoretical capacity,abundant resources and environmental protection.But their slow kinetics and poor stability hinder their practical applications.This paper takes MnO、CoO and Co₃O₄ as the research object to improve the lithium storage performance through the combination strategy of nanostructure design and conductive carbon composite.The main work is as follows:（1）Using glucose as carbon source,urea as nitrogen source and Mn CO₃ as manganese source,porous N-doped carbon nanosheets wrapped MnO cubes in 3D carbon networks were successfully fabricated by one-step pyrolysis strategy.This unique novel nanostructure not only provides pores and conductive frameworks to promote fast ion transport and enhance the charge transfer,but also effectively supplies an elastic buffer space to accommodate volume expansion of active materials during charge/discharge.In addition,kinetic analysis and other tests show that pseudocapacitance controlled lithium storage is an important storage method,and further oxidation of Mn²⁺to higher valence states during cycling also contributes a large specific capacity.Therefore,the composite anode exhibits high specific capacity(917 m Ah g^-1 after 190 cycles at 0.2 A g^-1),superior rate performance(328 m Ah g^-1 at5.0 A g^-1)and long cycle life(capacity retention of 105%after 550 cycles at a current density of 1 A g^-1).Besides,using the composite as anode and commercial Li Fe PO₄ as cathode to assemble lithium full battery,the full-cell also shows excellent rate and cycle performance.The simple and low-cost strategy has great potential in synthesizing other unique composite nanostructures.（2）Using MnSO₄.H₂O as manganes source,Dopamine hydrochloride as nitrogen-doped carbon source and carbon nanotubes as substrate,flexible self-supporting composite films with nitrogen-doped carbon coated MnO quantum dots uniformly anchored on carbon nanotubes were prepared by electrostatic interaction,vacuum filtration and annealing.The synergistic effect of MnO quantum dots,nitrogen-doped carbon layers and carbon nanotubes is beneficial to accommodate volume expansion of MnO during charge-discharge and ensure almost the whole surface of MnO quantum dots to attend electrochemical reaction.Meanwhile,it can also enhance electron conductivity and shorten lithium ion diffusion distance.In addition,kinetic analysis and characterization tests show that capacitive contribution for Li storage in the freestanding film is calculated to be high and further oxidation of Mn²⁺to higher valence state upon cycling generates lots of specific capacity,thus endowing it with excellent rate,ultrahigh capacity and outstanding cycling stability.Therefore,the composite anode delivers ultrahigh capacity of 1741 mAh g^-1 after 120 cycles at 0.1A g^-1,high rate capability of 858 m Ah g^-1 at 3.2 A g^-1 after 120 cycles at 0.1 A g^-1,and long cycling stability with a capacity of 136.5%at 1.0 A g^-1 after 1000 cycles.This unique nanostructure design is considered to have great potential in the universal synthesis of carbon coated metal oxide quantum dots and carbon nanotubes.（3）The composite film consisting of CoO nanocrystals and carbon nanotubes was successfully prepared via a four-step process including electrostatic adsorption,co-precipitation assisted self-assembly,vacuum filtration,and annealing.Microstructure analysis showed that CoO nanocrystals with an average diameter of about 20 nm were dispersed and tightly anchored on the surface of interconnected carbon nanotubes without agglomeration,leading to almost the complete surface of CoO nanocrystal being available for electrochemical reactions.On the one hand,this unique composite provides a conductive carbon network to enhance electronic conductivity and accommodate volume expansion.On the other hand,the composite has a large specific surface area and abundant active sites,which enhances pseudocapacitance-controlled lithium storage.When used as anode material for LIB half cells,the composite film anode shows super high capacity and excellent cycle stability at 0℃,25℃and 55℃.Moreover,when the composite film and activated carbon were used as the anode and the cathode,the full-cell exhibits high energy density of 91 Wh kg^-1 and high power density of 13.9 k W kg^-1 as well as high capacity retention.Our results show that the development of CoO-based composites has great potential in Lithium ion hybrid capacitors.（4）Freestanding film anode of the N-doped carbon coated hollow Co₃O₄nanocrystals with average diameter of 12 nm and anchored tightly on highly conductive carbon nanotube substrates was synthetized by two-step annealing and Kirkendall effect.Such nanocomposite possesses large specific surface area to supply ample active sites,highly conductive carbon networks to enhance electric conductivity,short ion diffusion distance to improve ion transport,and large interior space to accommodate volume expansion during repeatedly electrochemical reaction.Therefore,when it acts as an anode for lithium ion batteries,the freestanding anode exhibits high specific capacity(1468 m Ah g^-1 after 130 cycles at 0.2 A g^-1),superior rate capability(271 m Ah g^-1 at 12.5 A g^-1),and long cycle life(905 m Ah g^-1 after 350cycles at current density of 1.0 A g^-1).Moreover,the assembled Li full cell using the freestanding composite as anode and Li Fe PO₄ as the cathode display superior electrochemical performances.Our results show that the prepared freestanding composite anode has great potential in lithium-ion batteries.