Synchronous Manipulation of Ion and Electron Transfer in Wadsley–Roth Phase Ti‐Nb Oxides for Fast‐Charging Lithium‐Ion Batteries

Abstract Implementing fast‐charging lithium‐ion batteries (LIBs) is severely hindered by the issues of Li plating and poor rate capability for conventional graphite anode. Wadsley–Roth phase TiNb2O7 is regarded as a promising anode candidate to satisfy the requirements of fast‐charging LIBs. However, the unsatisfactory electrochemical kinetics resulting from sluggish ion and electron transfer still limit its wide applications. Herein, an effective strategy is proposed to synchronously improve the ion and electron transfer of TiNb2O7 by incorporation of oxygen vacancy and N‐doped graphene matrix (TNO− x @N‐G), which is designed by combination of solution‐combustion and electrostatic self‐assembly approach. Theoretical calculations demonstrate that Li+ intercalation gives rise to the semi‐metallic characteristics of lithiated phases (Li y TNO− x ), leading to the self‐accelerated electron transport. Moreover, in situ X‐ray diffraction and Raman measurements reveal the highly reversible structural evolution of the TNO− x @N‐G during cycling. Consequently, the TNO− x @N‐G delivers a higher reversible capacity of 199.0 mAh g−1 and a higher capacity retention of 86.5% than those of pristine TNO (155.8 mAh g−1, 59.4%) at 10 C after 2000 cycles. Importantly, various electrochemical devices including lithium‐ion full battery and hybrid lithium‐ion capacitor by using the TNO− x @N‐G anode exhibit excellent rate capability and cycling stability, verifying its potential in practical applications.

were conducted on the electrochemically lithiated TNO -x @N-G and graphite electrodes to study the thermal stability, which were carried out on Netzsch STA 449 F3 from 30 to 200 °C at a scanning rate of 5 °C min -1 .

Electrochemical measurement
The working electrode slurry was prepared by mixing active materials (70 wt%), acetylene black (20 wt%) and bi-component binders of same weight of carboxyl methyl cellulose (CMC) and styrene-butadiene resin (SBR) (10 wt%) using deionized water as the solvent. The slurry was spread on Cu foil with an applicator, and dried for 10 h at 80 °C in a vacuum oven. The loading mass of active materials was 1.3 ~ 1.5 mg cm -2 . CR2016 type coin cells were used to measure the electrochemical properties of the samples assembled in an Ar-filled glove box. Li metal was used as the counter electrode and celgard 2400 was used as the separator. The electrolyte was l M LiPF 6 dissolved in the mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with the volumetric ratio of 1: 1. The cells were tested using a charge/discharge unit (Neware BTS battery charger, Shenzhen, China) in the potential range from 1.0 to 3.0 V at various current rates. The theoretical capacity of TiNb 2 O 7 is calculated to be 232.2 mAh g -1 based on one electron transfer per transition metal atom, and 1 C is simplified to be 200 mA g -1 . Cyclic voltammetry (CV) tests were conducted on the CHI 1030C electrochemical workstation at the scan rate of 0.1 mV s -1 . Electrochemical impedance spectra (EIS) were measured on the Autolab 302N in the frequency range from 100 mHz to 100 kHz. S6 The LiFePO 4 cathode was prepared by mixing active materials (80 wt%), acetylene black (10 wt%) and poly(1,1-difluoroethylene) (PVDF, 10 wt%) using N-methylpyrrolidone (NMP) as the solvent. The slurry was spread on Al foil with an applicator, and dried overnight at 80 °C in a vacuum oven. The active carbon cathode was also fabricated by similar method except that carbon cloth was used as the current collector. For the assembly of CR-2032 type lithium-ion full cells, the TNO and TNO -x @N-G electrodes were used as anode, and the LiFePO 4 electrode was used as cathode, respectively. In lithium-ion full cells, the capacity ratio of the negative electrode to positive electrode (N/P ratio) was controlled to be 0.80 ~ 0.90. Hybrid lithium-ion capacitors were also assembled into CR-2032 type coin cells with prlithiated TNO -x @N-G as the anode (pre-cycled in half cells for 10 cycles, and disassembled inside the glove box at a lithiation state at 1.0 V (vs. Li + /Li), and active carbon electrode as the cathode. In hybrid lithium-ion capacitors, the N/P ratio was carefully controlled to be about 1.

In-situ electrochemical XRD and Raman spectroscopy measurement
The TNO -x @N-G material was coated on the stainless steel grid with the same slurry preparation procedure mentioned above for in-situ electrochemical XRD and Raman spectral measurements. The configuration of the in-situ electrochemical XRD cell and Raman spectral cell were described in the previous reports. [2] The XRD pattern measurement and the galvanostatic charging and discharging were simultaneously performed in the in-situ electrochemical XRD cell. Every XRD pattern was measured simultaneously with galvanostatic charging and discharging on S7 Rigaku Ultima IV with the X-ray radiation wavelength of 0.154 nm (Cu K), the X-ray radiation power of 1200 W, and the 2 angle scanning rate at 2 min -1 with the step size of 0.02. The in-situ electrochemical XRD test were conducted by a charge/discharge unit (Neware BTS battery charger, Shenzhen, China) in the potential range from 3.0 V to 1.0 at a current density of 20 mA g -1 . Every Raman spectral was measured with an acquisition time of 600 s and an interval time of 300 s on Renishaw inVia Raman microscope with the laser wavelength of 532 nm, the laser power of 0.1 mW and the grating of 1800 T.

Ex-situ XRD measurement
The CR2016 type coin cells were assembled as the procedures above. The coin cells were stopped at the lithiation state y=1.6 and 2.0 in the discharging and the charging and the electrodes were taken from the coin cell for the ex-situ XRD measurement with the sealing window. The ex-situ XRD measurement condition is described as follows: the X-ray radiation wavelength is 0.154 nm (Cu K), the X-ray radiation power is 1200 W and the 2 angle scanning rate at 2 min -1 with the step size of 0.02 on Rigaku Ultima IV.

Theoretical calculations
All first-principles calculations were performed within the Vienna Ab Initio Simulation Package (VASP) based on density functional theory (DFT). [3] The     Table S1. The comparison of the TNO-x@N-G and other previously reported TiNb 2 O 7 -based materials in terms of cycling stability and lifespan.

References
Capacity retention Carbon Coated Porous Titanium Niobium Oxides as Anode Materials of Lithium-Ion Batteries for Extreme Fast Charge Applications [5] 80.6% after 500 cycles at 6 C Coarse-grained reduced Mo Ti1−Nb2O7+ anodes for high-rate lithium-ion batteries [6] 73% after 500 cycles at 10 C Interstitial and substitutional V5+-doped TiNb2O7 microspheres: A novel doping way to achieve high-performance electrode [7] 71.7% after 2000 cycles at 10 C Ionic Liquid-Directed Nanoporous TiNb2O7 Anodes with Superior Performance for Fast-Rechargeable Lithium-Ion Batteries [8] 74% after 1000 cycles at 5 C Multiscale Designed Niobium Titanium Oxide Anode for Fast Charging Lithium Ion Batteries [9] 78.3% after 100 cycles at 0.3 C Nanoscale assembling of graphene oxide with electrophoretic deposition leads to superior percolation network in Li-ion electrodes: TiNb2O7/rGO composite anodes [10] 81.1% after 150 cycles at 0.5 C Superior performance of ordered macroporous TiNb2O7 anodes for lithium ion batteries: Understanding from the structural and pseudocapacitive insights on achieving high rate capability [11] 82% after 1000 cycles at 10 C TiNb2O7 hollow nanofiber anode with superior electrochemical performance in rechargeable lithium ion batteries [12] 80.6% after 900 cycles at 10 C TiNb2O7 nano-particle decorated carbon cloth as flexible self-support anode material in lithium-ion batteries [13] 81% after 500 cycles at 0.5 C Controlled fabrication and performances of single-core/dual-shell hierarchical structure m-TNO@TiC@NC anode composite for lithium-ion batteries [14] 64.8% after 200 cycles at 1 C Our work 86.5% after 2000 cycles at 10 C S15 Figure S9. Specific capacity of the TNO -x electrode (a) at 0.5 C and (b) at 10 C. S16 Figure S10. Schematics of electrode configurations of the TNO and TNO -x @N-G electrodes.