Design And Synthesis Of High Perfor Mance TiO₂ Based Anode Material For Lithium-ion Batteries

As one of the most important energy storage devices,lithium ion batteries(LIBs)have been intensively studied in recent years due to their numerous merits,such as high energy density,environmental benignity,and high power density.So far,the tremendous research efforts have been devoted to developing next-generation non-carbon anode materials to replace conventional carbonaceous materials.TiO2 has attracted considerable attention and extensively investigated due to its high safety,nontoxicity,easy to synthesis,low cost,and structurally stable.However,the intrinsically low conductivity,slow reaction power and relatively low theoretic capacity restrain its further practical applications.Based on the above factors,TiO2 in this paper is nano-sized,composited with conductive carbon materials,and hollow mesoporous micro-nanostructures to improve and improve its electrochemical performance.The main research contents of this paper are as follows:(1)We have successfully prepared TiO2 nanocrystalline-assembled mesoporous nanosphere(TiO2 NMNs)through hydrolysis and polymerization reactions of isopropyl titanate and hexadecylamine,and a hydrothermal reaction is performed to crystallize amorphous TiO2 and remove HDA.The nano-crystallization of TiO2 particles is realized.At the same time,it is assembled into sub-micron spheres,which effectively avoids the phenomenon of aggregation of TiO2 nanocrystals during synthesis and electrochemical testing,and it enhances the stability of the structure.With the dissolution of hexadecylamine,many uniform mesopores were obtained,and thus have a large specific surface area of 153 m2 g-1.As anode material for LIBs,TiO2 NMNs present the enhanced Li storage capabilities with high specific capacity and good cycling performance.At the 100th cycle,the discharge capacity is 194.1 mAh g-1 with CE of 99.8%,presenting a capacity retention ratio of 75%with respect to the first discharge.The average discharge capacity over 100 cycles is 219.2 mAh g-1.Which is mainly ascribed to the small size of TiO2 nanocrystalline and the great specific surface area of the mesopore nanosphere.The small crystallite size and mesoporous structure greatly improve the reaction kinetics of TiO2 and remarkably enhance the intercalation ability.These significantly increase the actual capacity of TiO2.(2)By coating hydrous titania and PPy on the carbon nanotubes,the hydrothermal method crystallizes TiO2 and dissolves hexadecylamine.TiO2 nanocrystals coated on the surface of CNTs are obtained,and a large number of pores exist between the crystal grains.Then,it is coated with a layer of PPy and carbonized.Three-layer nanocable structure CNT@TiO2@C was successfully fabricated.It can clearly distinguish the three-layer structure and see that there are a large number of mesopores by TEM.The BET test results showed a specific surface area of 611.6 m2 g-1 and an average pore diameter of 8.95 nm.As anode material for lithium ion batteries,CNT@TiO2@C exhibits excellent lithium storage performance in terms of specific capacity,cycling stability and rate capability.At the 500th cycle,it still delivers discharge capacity of 236 mAh g-1,presenting a high capacity retention of 64.3%and a low capacity loss rate of 0.07%per cycle(versus the 2nd cycle).The excellent cycling performance should be attributed to the supporting effect of the internal CNT and the protecting effect of the external C shell.At 10 C,the discharge capacity is stabilized at 187 mAh g-1.The dual conductivity paths formed by the internal CNT and the external C shell greatly improve electronic conductivity of the TiO2,significantly promoting rate capability of TiO2.70.39%of the total capacity is calculated by the kinetics of the CV curve to arise from the capacitive contribution.The large specific surface area of the mesoporous TiO2 reinforce lithium pseudocapacitive(interfacial)storage of TiO2,leading to the great increase in reversible capacity of TiO2.This high capacitive contribution ratio well explains practical capacity of CNT@TiO2@C much higher than the theoretical capacity.These demonstrate the superiority of the composite structure.(3)Nano tube-in-tube(NTT)CNT@TiO2@CNT was successfully synthesized through chemical bath deposition combined with etching and calcining.It can be clearly seen that the hollow tube-in-tube structure and the outer carbon tube are tightly coated with TiO2 nanotubes by TEM.The unique one-dimensional(1-D)hollow nanotube(NT)structure endows TiO2 with enhanced surface-to-volume ratios and very short Li+transport path.As anode material for lithium ion batteries,CNT@TiO2@CNT exhibits excellent lithium storage performance,ultrahigh rate capability,high specific capacity and very stable cycling performance.The NTTs deliver an initial discharge capacity of 296 mAh g-1 at 10 C;subsequently show stable cycling performance and retain at 180.3 mAh g-1 up to 3000 cycles,exhibiting an excellent capacity retention of 60.8%and a very low capacity loss rate of 0.013%per cycle.The average discharge capacities of 88 and 55 mAh g-1 were delivered at the current rate of 200 C and 300 C.When the current again goes back to 1 C,the discharge capacity can still return to 371 mAh g-1,recovering to 90%of the initial 1 C.The excellent cycle performance and rate performance might be attributed to the sandwich gap between internal and external nanotubes as a unique reservoir can allow electrolyte infiltration and fast mass transfer,meeting the electrochemical reaction demand even at ultrahigh-rate charge-discharge.The NT structure increases surface-to-volume ratios of TiO2.This provides lots of active sites and shortens Li+diffusion path;promotes electrochemical reaction kinetics of TiO2.The external carbon nanotube well withstands expansion-contraction of TiO2 in charge-discharge process,strongly enhancing structure stability of TiO2 NT.The tightly contact between external carbon NT and TiO2 NT greatly promotes the charge transfer and remarkably enhances electron conductivity of TiO2.78.5%of the total capacity is calculated by kinetics to arise from the capacitive contribution.This results demonstrate that for the lithium ion storage of the NTTs,the surface capacitive effect is a predominant process.This explains superior ultrahigh rate capability and high specific capacity of the NTTs.