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

Abstract

TiNb 2 O 7 (TNO) is a competitive candidate of a fastcharging anode due to its high specific capacity. However, the insulator nature seriously hinders its rate performance. Herein, the La 3+ -doped mesoporous TiNb 2 O 7 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 La 3+ 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 La 3+ 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.

Phosphorus-Doped FeOF Nanoparticle-Based Cathodes for Lithium Storage

“…44 Moreover, the R ct in the two electrolytes (Figure S6) was investigated by EIS spectra and showed that can be expressed as eq 1. 12,20 The peak of R 2 is attributed to the Li x Fe(O 1−n P n )F conversion to Fe 0 and LiF, and the reaction process is outlined in eqs 2 and 3. 16 The whole process of the first discharge can be described as eq 4.…”

“…The CV curves (Figure d) of the P-F6 electrode show two reduction peaks at ∼2.55 ( R 1 ) and 1.39 V ( R 2 ). The peak of R 1 corresponds to the Li + insertion into Fe­(O 1– n P n )F formation Li x Fe­(O 1– n P n )­F, which can be expressed as eq . , The peak of R 2 is attributed to the Li x Fe­(O 1– n P n )F conversion to Fe 0 and LiF, and the reaction process is outlined in eqs and . The whole process of the first discharge can be described as eq .…”

“…are a typical conversion-type electrode material, and they can achieve outstanding theoretical capacities due to multiple electron transfers (multiple Li + ) when the MF x is converted to M 0 and Li y F after discharge. − Among all MF x materials, iron trifluoride (FeF 3 ) has been widely used as a cathode material due to its high working potential (∼2.7 V) and theoretical capacity (712 mA h g –1 when reacted with three Li + ) . When using O to partially substitute F in FeF 3 , the ionicity of Fe–F is reduced and the lattice defects are increased, which can integrate the advantages of fluoride and oxide. , However, the rate performance and cycle stability of FeOF are still unsatisfactory because of the low ionic/electronic conductivities, the aggregation of Fe nanoparticles, side reactions of Fe with electrolytes, and the high electronegativity of Fe–F. , Significant efforts have been made to overcome these obstacles, including doping heteroatoms, morphology design, preparation of nanomaterials and composite structures, and so forth. ,, Ion doping has been widely regarded as an effective strategy for enhancing structural stability and improving electronic structure/properties; thus, heteroatom doping may be a straightforward way to improve electrochemical performance. − Especially, non-metal dopants might induce the reconstruction of charge distribution around the sites of the parent material, which may improve the redox activity of the material. − However, the relevant electrochemical mechanism caused by non-metal doping has not been clearly understood. Phosphorus has higher electrical conductivity and the ability to reduce the adsorption energy of Li + to facilitate the interfacial kinetics of Li + intercalation. − Moreover, introducing P into the material can alter the charge balance and endow the material with certain pseudocapacitive properties, thereby improving the rate performance of the material. , The weak M–P bond enhances the electrochemical activity and reversible redox reaction characteristics of electrode materials, which can improve the rate performance of LIBs .…”

Enhanced Energy Storage Performance of Zr-Doped TiNb2O7 Nanospheres Prepared by the Hydrolytic Method

“…In order to relieve the issues suffered by bulk TiNb 2 O 7 , various strategies have been tried, such as introducing an oxygen vacancy, − regulating the morphology, ,− doping with alien ions, − etc. Oxygen vacancy is an intrinsic defect which can trigger lattice distortion and electron concentration improvement .…”

“…Elements’ doping differs from morphology regulating in modification mechanism; the former can not only introduce extra energy bands to reduce the band gap but also increase the crystal lattice parameter to facilitate ion migration. From previous literature, various dopants have been incorporated into bulk TNO, including Tb 4+ , V 5+ , W 6+ , and La 3+ , and the modified electrode materials displayed bigger Li + diffusion coefficients. Taking into account the structural distortion after doping, the ion radius of heteroatoms should be slightly larger than the Ti 4+ ion (0.53 Å) and Nb 5+ ion (0.69 Å).…”