Preparation And Electrochemical Performance Of Transition Metal Oxides/Carbon Nanotubes As Anode Materials For Lithium Ion Batteries

Lithium-ion batteries?LIBs?have been widely utilized in portable electronic devices?mobile phones,portable computers,cameras?.At the same time,LIBs also display a spacious application prospects in power sources of electric vehicle,which are attributable to its high working voltage,high energy density,light weight,no memory effect,durable cycling life and environmental friendliness.Transition metal oxides?TMOs?have been extensively researched owing to its higher theoretical capacity.However,the conductivity of most TMOs is not good,which will speed up the loss of capacity.The conversion reaction mechanism will cause a very large volume expansion in the charge and discharge process,resulting in the crushing of the electrode and damaging the integrity of material.Therefore,improving the conductivity and the cycling stability of TMOs is the key requirements of modification.In addition,carbon nanotubes have good capability of lithium storage because of its unique structure?one-dimensional tubular?,high conductivity,and high specific surface.Thus,in order to obtain LIBs with exceptional electrochemical performance,it is possible to prepare nanocomposites with a certain structure as anode materials for LIBs by combining high specific capacity TMOs with good conductivity carbon nanotubes.This will be a very promising approach.Hence,this paper is devoted to using of comparatively a facial approach to prepare nanocomposites of TMOs and multi-walled carbon nanotubes?MWNTs?as anode materials for LIBs.The main contents and results are as follows:?1?MnO/MWNTs nanocomposites with uniform MnO nanoparticles grown on MWNTs were successfully fabricated by a simple thermal decomposition method.Among them,the functional acid-treated MWNTs and ammonium hydroxide played a vital role in the growth of the particles on the MWNTs.Calcination in an inert atmosphere was essential as well.It can be seen from SEM and TEM that the MnO particles in MnO/MWNTs had a size of about 20-50 nm,while the MnO particles which were directly decomposed were about 20-70 nm,and the agglomeration were obvious between the particles.The presence of C-O bonds in the test of XPS showed the formation of chemical bonds of MnO nanoparticles on the outer surface of MWNTs.The results of EIS revealed that the electronic conductivity and lithium ion transfer of MnO/MWNTs electrode were promoted due to the addition of MWNTs.Electrochemical measurement indicated that MnO/MWNTs nanocomposites acting as anode material for LIBs had delivered predominant electrochemical cycling stability and rate capability compared to MnO nanoparticles and acid-treated MWNTs.Excellent performance appeared in a high reversible capacity of 801.6 mAh g^-1 after cycling 100 cycles at 100mA g^-1,as well as the average reversible capacities of 581.7,533.2,496.5417.3 and 287.9 mAh g^-1 at corresponding current densities of 100,200,400,800 and 1600 mA g^-1.When the current density was switched to 100 mA g^-1,the capacities reached 729.0 mAh g^-1.Therefore,the MnO/MWNTs nanocomposites prepared by the simple synthesis method in this experiment have good application prospects as anode materials for LIBs.?2?Fe₃O₄ nanoparticles were introduced inside MWNTs?Fe₃O₄@MWNTs?successfully by innovative wet chemical injecting method and thermal decomposition method.It was clearly seen from the TEM that carbon atoms at the tips of the capped MWNTs were completely eroded away by the advanced alkali treatment,which facilitated the injection of the solution into the channels of MWNTs from the open ends.Meanwhile,long-time stirring was beneficial for Fe3+ into the cavity of MWNTs with the aid of the capillary force.After calcining in inert atmosphere,Fe₃O₄ nanaoparticles with a diameter of about 1015 nm were obtained.Fe₃O₄@MWNTs exhibited excellent electrochemical performance compared to pure Fe₃O₄ nanoparticles and alkali treated MWNTs.The results showed that at the current density of 100 mA g^-1,the specific capacity of Fe₃O₄@MWNTs reached 757.7 mAh g^-1 after 150 cycles.Meanwhile,Fe₃O₄@MWNTs obtained a reversible capacity of 514.0 mAh g^-1 after 100 times at the current density of 500 mA g^-1,and 190.8 mAh g^-1 after cycling 500 times at 1 A g^-1.It can be concluded that the structure of Fe₃O₄ nanoparticles encapsulated in the nanochannel of MWNTs can give full play to the high conductivity of MWNTs and the restriction effect of the nanoscaled tube on the volume expansion of filled Fe₃O₄ nanaoparticles during the charge and discharge process,thus maintaining the structural stability.