Study Of Controlled Preparation And Electrochemical Properties Of Nb₂O₅ Nanotubes As Anode Materials For Fast-charging Li-Ion Batteries

The fast-charging demand for lithium-ion batteries is extremely growing for modern electric transportation and wearable electronics.However,the low operating voltage plateau(0.1 V vs.Li/Li⁺),poor rate performance,and severe safety concerns caused by unstable solid electrolyte interface(SEI)film of commercial graphite anodes have deeply limited their development when achieving the fast-charging goal.Therefore,exploring long-life and fast-charging anode materials is essential to satisfy the needs of lithium-ion batteries.Among the diverse anodes,orthorhombic niobium pentoxide(T-Nb₂O₅)as an intercalation-type material has been widely studied because of its excellent chemical stability,safety voltage window,and abundant redox pairs(Nb⁵⁺/Nb⁴⁺,Nb⁴⁺/Nb³⁺).However,the slow Li-ions diffusion kinetics and poor electronic conductivity(3×10^-6 S cm^-1)of T-Nb₂O₅ hinder its further development in the future fast-charging lithium-ion batteries.In this work,we design and construct the T-Nb₂O₅ nanotubes and their composites to address those problems.The relationship between the morphology and lithium storage kinetics is explored in detail and the difference in their electrochemical properties at room temperature and high temperature is clarified.This work opens a new avenue towards further development of intercalation-pseudocapacitive nanostructured materials for high-rate and ultra-stable energy-storage devices.The main contents and results are as follows:(1)To improve the diffusion rate of lithium ions in T-Nb₂O₅ materials,the tunable orthorhombic niobium pentoxide nanotubes(T-Nb₂O₅ NTs)were fabricated by atomic layer deposition(ALD)using the electrospun polyacrylonitrile(PAN)nanofibers as the sacrificial templates.By changing the number of ALD cycles,the wall thickness of T-Nb₂O₅ NTs can be precisely adjusted.The relationship between the wall thickness and electrochemical properties was investigated in detail.Electrochemical kinetic analysis shows that the lithium storage in T-Nb₂O₅ NTs is dominated by surface and intercalation pseudocapacitance.The results revealed that the morphology of T-Nb₂O₅ crystals has a significant influence on the Li-ions insertion/extraction kinetics and the electrochemical capability in lithium-ion batteries,and providing new ideas for the design of future high-rate anode materials.(2)Based on the optimized T-Nb₂O₅ NTs,a continuous and uniform Nitrogen-doped carbon film was successfully coated on the surface of T-Nb₂O₅ NTs using the ionic liquid1-ethyl-3-methylimidazolium dicyanamide salt(EMIm-dca)with high mobility and good thermal stability as the carbon source.Nitrogen-doped carbon coated T-Nb₂O₅ NTs(T-Nb₂O₅ NTs@NC)electrode materials were synthesized by heat treatment in N₂atmosphere.The constructed Nitrogen-doped carbon films further enhanced the electron and ion transport efficiency of T-Nb₂O₅ NTs,and the electrodes exhibited more excellent electrochemical performance.This work revealed the difference in the electrochemical performance of T-Nb₂O₅ NTs@NC at room temperature and high temperature.We investigated the reason for the decay of the electrochemical performance of this material at high temperature.The rapid capacity decay of T-Nb₂O₅ NTs@NC at high temperature was effectively tackled by adjusting the potential window.Besides,the T-Nb₂O₅ NTs@NC as the anode,assembled with the cathode of lithium iron phosphate(LFP)in a full-cell device also shows great potential in fast-charging lithium-ion batteries.