Preparation And Performance Of Nb₂O₅ And The Derivative Materials For Li-ion Capacitor

Li-ion capacitors have potential applications in energy conversion and storage due to their excellent electrochemical properties.There are two kinds of electrode materials in the Li-ion capacitors,one is capacitive and the other is battery-type.Most of the capacitive materials are active carbons,and the battery-type materials are traditional materials for Li-ion batteries.Due to the different mechanisms for energy storage,the kinetic rates of the cathode and anodes do not match.Therefore,the improvement of the energy density and power density is limited.With the objective to the preparation of battery electrode materials showing high-rate capability,we investigated orthorhombic?T?Nb₂O₅ materials which can demonstrate pseudocapacitive behavior of Li-ion intercalation,and TiNb₂O₇ materials which are derivative materials of Nb₂O₅.The main research achievements in this work are summarized below:?1?T-Nb₂O₅ nanorod films are directly grown on the carbon fiber cloth by a facile hydrothermal method and subsequent annealing treatment.The nanorod films on the carbon cloth maintain a three-dimensional and porous structure.The charge/discharge results show that the capacity retention rate is about 73%at 20 C?compared with that at 1 C?,and about 80%of the capacity can be retained after 2500 cycles at a high rate of 10 C.The cyclic voltametric and impedance results indicate that the T-Nb₂O₅ films have superior pseudocapacitive intercalation which is responsible for the improvement of high rate capability.With the T-Nb₂O₅ films as anodes and the active carbons as cathodes,the Li-ion capacitors can show very high gravimetric energy density and power density(about 95.55 Wh·kg^-1 and 5350.9 W·kg^-1 respectively).Meanwhile,the T-Nb₂O₅ film electrodes maintain good flexibility for the support of carbon cloth,and nearly no change is observed for the charge/discharge behaviors of the model Li-ion capacitors.?2?The ordered arrays of TiNb₂O₇ nanorod clusters are directly grown on the carbon cloth by a facile hydrothermal method and subsequent annealing treatment.After proper annealing,the TiNb2O7 nanorod clusters show excellent high-rate performance,and significantly improved long-cycling stability.The capacity retention rate is about 65.90%at an ultra-high rate of 40 C?compared with that at 1 C?,and87.11%of the initial capacity is retained after 2000 cycles at 10 C.The superior high-rate capability of the TiNb₂O₇ nanorod clusters can be mainly ascribed to the improved electronic conductivity for the compact contact between the nanorods and conductive carbon fibers?to reduced charge-transfer resistance?,and the nano-scaled size of TiNb₂O₇ nanorods?to greatly reduced Li-ion diffusion length?.The improved long-cycling stability may be originated from the improved structural stability of the TiNb2O7 nanorod clusters.By the way,the TiNb2O7 electrodes can also show good flexibility which is inherited from the carbon cloth.