Polydopamine-derived N-doped carbon-coated porous TiNb2O7 microspheres as anode materials with superior rate performance for lithium-ion batteries

Insight into the Li-Storage Property of Surface-Modified Ti2Nb10O29 Anode Material for High-Rate Application

Ti-based anode materials are considered to be an alternative to graphite anodes to accomplish high-rate application requirements. Ti2Nb10O29 (TNO15) has attracted much attention due to its high lithium storage capacity through the utilization of multiple redox couples and a suitable operating voltage window of 1.0 to 2.0 V vs Li/Li+. However, poor intrinsic electronic conductivity has limited the futuristic applicability of this material to the battery anode. In this work, we report the modification of TNO15 by introducing oxygen vacancies and using few-layered carbon and copper coatings on the surface to improve its Li+ storage property. With the support of the galvanostatic intermittent titration technique (GITT), we found that the diffusion coefficient of carbon/copper coated TNO15 is 2 orders of magnitude higher than that of the uncoated sample. Here, highly conductive copper metal on the surface of the carbon-coated oxygen-vacancy-incorporated TNO15 increases the overall electronic and ionic conductivity. The prepared TNO15-800-C-Cu-700 half-cell shows a significant rate capability of 92% when there is a 10-fold increase in the current density. In addition, the interconnected TNO15 nanoparticles create a porous microsphere structure, which enables better Li-ion transportation during charge/discharge process, and experiences an enhancement after the carbon and copper coating on the surface of the primary TNO15 nanocrystallites.

Bulk Modification of Porous TiNb2 O7 Microsphere to Achieve Superior Lithium-Storage Properties at Low Temperature

TiNb2 O7 , as a promising alternative of Li4 Ti5 O12 , exhibits giant potential as low-temperature anode due to its higher theoretical capacity and comparable structural stability. However, the sluggish electronic conductivity still remains a challenge. Herein, bulk modification of Cu+ doping in porous TiNb2 O7 microsphere is proposed via a simple one-step solvothermal method with subsequent calcination treatment. The results show that the electronic conductivity is improved effectively due to the reduced band gap after doping, while enhanced lithium-ion diffusion is achieved benefiting from the increased interplanar spacing. Therefore, the optimal sample of Cu0.06 Ti0.94 Nb2 O7 exhibits a high reversible capacity of 244.4 mA h g-1 at 100 mA g-1 after 100 cycles, superior rate capability, and long-term cycling stability at 1000 mA g-1 at room temperature. Particularly, it can also display good performance in a wide temperature range from 25 to -30 °C, including a reversible capacity of 76.6 mA h g-1 at -20 °C after 200 cycles at 200 mA g-1 . Moreover, Cu0.06 Ti0.94 Nb2 O7 //LiFePO4 full cell can deliver a high reversible capacity of 177.5 mA h g-1 at 100 mA g-1 . The excellent electrochemical properties at both ambient and low-temperatures demonstrate the great potential of Cu+ -doped TiNb2 O7 in energy-storage applications.

Hybrid nanotubes constructed by confining Ti0.95Nb0.95O4 quantum dots in porous bamboo-like CNTs: superior anode materials for boosting lithium storage

Current commercial lithium ion battery (LIB) anodes comprising graphite and Li4Ti5O12 inevitably suffer from safety risk and low energy density. Hence, a novel anode material of Ti0.95Nb0.95O4/C hybrid nanotubes was developed via a modified sol-gel method combined with subsequent calcination. The hybrids consist of Ti0.95Nb0.95O4 quantum dots that are homogeneously embedded in the walls of porous bamboo-like CNTs. The high capacity feature of multiple redox couples of Ti-Nb-O based anodes is demonstrated by ex situ XPS in the hybrids. With the advantages of stimulative lithium storage, increased conductivity and robust mechanical properties due to the unique hybrid structure, the hybrids exhibit a high capacity (516.8 mA h g-1 at 0.2 A g-1), superior long-term cycling stability (142.7 mA h g-1 at 5 A g-1 after 3000 cycles) and an ultra-high rate capability (234.6 mA h g-1 at 1 A g-1 and 125 mA h g-1 at 8 A g-1). Meanwhile, the hybrids showed superior electrochemical performance compared with the reported Li4Ti5O12 and Ti-Nb-O based anodes. Furthermore, the GITT measurements revealed the fast Li+ transport for the charge-discharge processes of the hybrids. Such prominent merits of the Ti0.95Nb0.95O4/C hybrid nanotubes make them more likely candidates that can replace graphite and Li4Ti5O12 anodes in LIBs.

Ultrathin Carbon-Coated Porous TiNb2O7 Nanosheets as Anode Materials for Enhanced Lithium Storage

TiNb₂O₇ has been considered as a promising anode material for next-generation high power lithium ion batteries for its relatively high theoretical capacity, excellent safety and long cycle life. However, the unsatisfactory electrochemical kinetics resulting from the intrinsic sluggish electron transport and lithium ion diffusion of TiNb₂O₇ limit its wide application. Morphology controlling and carbon coating are two effective methods for improving the electrochemical performance of electrode materials. Herein, an ultrathin carbon-coated porous TiNb₂O₇ nanosheet (TNO@C) is successfully fabricated by a simple and effective approach. The distinctive sheet-like porous structure can shorten the transport path of ions/electrons and provide more active sites for electrochemical reaction. The introduction of nanolayer carbon can improve electronic conductivity and increase the specific surface area of the porous TiNb₂O₇ nanosheets. Based on the above synergistic effect, TiNb₂O₇@C delivers an initial discharge capacity of 250.6 mAh g^-1 under current density of 5C and can be maintained at 206.9 mAh g^-1 after 1000 cycles with a capacity retention of 82.6%, both of which are superior to that of pure TiNb₂O₇. These results well demonstrate that TiNb₂O₇@C is a promising anode material for lithium ion batteries.

Superimposed Effect of La Doping and Structural Engineering to Achieve Oxygen-Deficient TiNb2O7 for Ultrafast Li-Ion Storage

TiNb₂O₇ (TNO) is a competitive candidate of a fast-charging anode due to its high specific capacity. However, the insulator nature seriously hinders its rate performance. Herein, the La³⁺-doped mesoporous TiNb₂O₇ materials (La-M-TNO) were first synthesized via a facile one-step solvothermal method with the assistance of polyvinyl pyrrolidone (PVP). The synergic effect of La³⁺ doping and the mesoporous structure enables a dual improvement on the electronic conductivity and ionic diffusion coefficient, which delivers an impressive specific capacity of 213 mAh g^-1 at 30 C. The capacity retention (@30C/@1C) increases from 33 to 53 and 74% for TNO, M-TNO, and La-M-TNO (0.03), respectively, demonstrating a step-by-step improvement of rate performance by making porous structures and intrinsic conductivity enhancement. DFT calculations verify that the enhancement in electronic conductivity due to La³⁺ doping and oxygen vacancy, which induce localized energy levels via slight hybridization of O 2p, Ti 3d, and Nb 4d orbits. Meanwhile, the GITT result indicates that PVP-induced self-assembly of TNO accelerates the lithium ion diffusion rate by shortening the Li⁺ diffusion path. This work verifies the effectiveness of the porous structure and highlights the significance of electronic conductivity to rate performance, especially at >30C. It provides a general approach to low-conductivity electrode materials for fast Li-ion storage.