High areal capacity and low-temperature operation are crucial for the commercial viability of lithium-ion batteries (LIBs), particularly for high-energy density applications. However, thick electrodes, necessary for achieving high areal capacity, often suffer from sluggish mass and charge transfer kinetics, leading to increased polarization and reduced utilization of the active material. These challenges are exacerbated at low temperatures due to decreased ionic conductivity of the electrolyte, hindered desolvation processes, and reduced solid-state Li⁺ diffusion in the active materials. Conventional graphite anodes, for instance, show significantly decreased fast-charging capabilities at low temperatures and high rates, potentially leading to lithium plating and capacity decay. Niobium-based oxides, particularly TiNb₂O₇, are promising anode materials due to their safe lithiation potential and suitable channels for Li⁺ migration. Despite their advantages, their limited areal capacity and poor low-temperature performance (below -30°C) remain significant obstacles. The large bandgap of TiNb₂O₇ results in substantial charge transfer barriers, hindering efficient Li⁺ accommodation, especially at low temperatures. Furthermore, poor electronic conductivity leads to large voltage polarization, further decreasing performance. Nano-engineering approaches have been explored to enhance kinetics by reducing ion transport distances and increasing surface area. However, nanoparticles often suffer from low tap density, leading to poor volumetric energy density. Therefore, there is a critical need to develop strategies for enhancing the charge transfer kinetics of micron-sized TiNb₂O₇ to enable high-areal-capacity batteries with superior low-temperature performance. This study focuses on delocalized electronic engineering to manipulate the active electronic states of TiNb₂O₇, aiming to significantly improve low-temperature ion diffusion kinetics while maintaining a high tap density.

Several studies have investigated strategies to improve the performance of lithium-ion batteries at low temperatures and high areal capacities. Sun et al. (2017) demonstrated the use of three-dimensional holey-graphene/niobia composites for ultrahigh-rate energy storage, highlighting the importance of conductive networks. Yang et al. (2021) focused on constructing efficient aligned conductive networks to facilitate depolarized high-areal-capacity electrodes. Zhang et al. (2023) coupled multiscale imaging analysis and computational modeling to understand thick cathode degradation mechanisms. Shi et al. (2020) developed low-tortuous, highly conductive, and high-areal-capacity electrodes using aligned carbon fiber frameworks. Wu et al. (2021) provided a comprehensive overview of the design principles for high-performance thick electrodes. Zhou et al. (2019) explored few-layer bismuthene with anisotropic expansion for high-areal-capacity sodium-ion batteries. Recent work has also focused on low-temperature electrolytes (Li, 2023; Holoubek, 2021; Huang, 2024; Hubble, 2022), exploring approaches to improve Li de-solvation and transport. Previous research on niobium-based oxides, like TiNb₂O₇, has also shown promise (Cui, 2023; Han, 2011; Liang, 2022), but these studies did not address the crucial aspects of high areal capacity and low-temperature performance comprehensively.

The researchers synthesized TiNb₂O₇@N (TNO@N) microflowers through a hydrothermal reaction followed by nitriding treatment. The synthesis involved dissolving niobium pentachloride and oxalic acid in water, mixing with a solution of tetrabutyl titanate and oxalic acid in ethanol, and adding ammonium fluoride. The resulting solution was hydrothermally treated, washed, and dried to obtain a precursor, which was then annealed at 750 °C to produce TNO and further treated in an Ar/NH₃ atmosphere to obtain TNO@N. A solid TNO sample was prepared for comparison by mixing Nb₂O₅ and TiO₂ and annealing at 1225 °C. Various characterization techniques were employed, including X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), annular bright-field (ABF) imaging, electron paramagnetic resonance (EPR), X-ray absorption near-edge structure (XANES) spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), and Brunauer-Emmett-Teller (BET) analysis. Density functional theory (DFT) calculations were performed to investigate the electronic structure and Li⁺ diffusion kinetics. Femtosecond transient absorption (TA) spectroscopy was used to study carrier dynamics. Electrochemical measurements including cyclic voltammetry (CV), galvanostatic charge/discharge tests, electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT) were conducted to evaluate the electrochemical performance of the TNO@N electrodes, including those with high mass loading, at different temperatures. Finite element modeling (FEM) simulations were utilized to study Li⁺ concentration distribution in different electrode morphologies. In situ XRD was performed to analyze crystal structure evolution during cycling, and ex situ XPS and TEM were used to investigate the elemental valences and structural changes after various discharge/charge states. Pouch cells were also assembled using TNO@N as the anode and LiNi_0.8Co_0.1Mn_0.1O₂ as the cathode to assess practical applications.

The TNO@N microflowers exhibited a unique morphology consisting of compactly assembled nanosheets, resulting in high tap density while retaining nano-effects for favorable low-temperature performance. XRD, Raman, and XPS analysis confirmed the successful nitrogen doping and the presence of oxygen vacancies in the TNO@N structure. XANES showed an intermediate oxidation state of Nb in TNO@N compared to pure Nb and TNO, indicating electron redistribution. The presence of oxygen vacancies was also directly visualized via ABF imaging. DFT calculations showed that N doping preferentially occupied tetra-coordination sites, and the introduction of oxygen vacancies, especially near triple-coordinated Ti sites, was energetically favorable. Electron localization function (ELF) analysis demonstrated enhanced electron delocalization in TNO@N compared to TNO, indicating improved electronic conductivity. The calculated density of states (DOS) showed the emergence of impurity bands within the bandgap of TNO@N, further confirming improved conductivity. Experimental measurements confirmed that TNO@N exhibited electrical conductivity five orders of magnitude higher than TNO. Transient absorption spectroscopy revealed that the carrier decay lifetime was significantly longer in TNO@N due to oxygen-defect-assisted recombination, promoting charge separation and transfer. The increased concentration of polarons in TNO@N, evidenced by TA spectroscopy, suggests a reduced polaron hopping activation energy, facilitating electron transport. DFT calculations corroborated the enhanced Li⁺ adsorption and faster Li⁺ diffusion in TNO@N. Electrochemical tests demonstrated superior cycling performance of TNO@N at various current densities and temperatures. Even at high mass loadings (10 mg cm⁻²) and low temperatures (−30 °C), TNO@N exhibited high capacity retention and rate capability. At −40 °C, an areal capacity of 1.32 mAh cm⁻² was achieved, showcasing excellent low-temperature performance. Finite element simulations confirmed that the unique microflower structure promoted uniform Li⁺ distribution and faster diffusion kinetics compared to solid spheres. The TNO@N electrode exhibited a significant capacitive contribution to the total capacity, further explaining its superior rate performance. GITT measurements demonstrated significantly higher Li⁺ diffusion coefficients in TNO@N compared to TNO at both room and low temperatures. The superior wettability of TNO@N further facilitated rapid Li⁺ transport at the electrode-electrolyte interface. In situ XRD analysis revealed reversible structural transitions during cycling, with smaller lattice volume variations observed in TNO@N due to the enhanced interlayer spacing. Ex situ XPS analysis provided further support for this understanding.

The findings demonstrate that delocalized electronic engineering of TiNb₂O₇ via nitrogen doping and oxygen vacancy creation is highly effective in enhancing the low-temperature performance of high-areal-capacity lithium-ion batteries. The synergistic effects of these modifications lead to improved electron conductivity, faster Li⁺ diffusion, and enhanced Li⁺ adsorption. The results directly address the challenges associated with thick electrodes and low-temperature operation by providing a pathway to mitigate polarization and enhance kinetics. The combination of experimental and theoretical approaches (DFT and TA spectroscopy) provides strong evidence for the underlying mechanisms, confirming the link between electronic structure modulation and improved electrochemical properties. The excellent performance of the TNO@N electrode, even at ultra-low temperatures and high mass loadings, significantly advances the potential for practical applications of LIBs in cold environments.

This research successfully demonstrated the synthesis and characterization of a novel TiNb₂O₇@N electrode material with significantly enhanced low-temperature performance for high-areal-capacity lithium-ion batteries. The delocalized electronic engineering strategy, combining nitrogen doping and oxygen vacancy creation, proved highly effective in improving electronic conductivity, Li⁺ diffusion kinetics, and Li⁺ adsorption. The material showed excellent performance at low temperatures and high mass loadings, surpassing many previously reported Nb-based electrodes. Future research directions could include exploring other dopants or defect engineering strategies to further optimize the properties of TiNb₂O₇, investigating alternative electrolyte formulations for even better low-temperature performance, and scaling up the synthesis process for commercial applications.

While this study demonstrates significant improvements in low-temperature performance and areal capacity, some limitations should be noted. The study primarily focused on half-cell tests; further investigation with full-cell configurations is needed to fully assess the practical implications of the findings. The long-term stability of the TNO@N electrode under extreme conditions (e.g., extended cycling at very low temperatures and high current densities) could be further investigated. The cost-effectiveness and scalability of the synthesis method for large-scale production should also be evaluated before commercialization. The study used a homemade electrolyte; exploring commercially available electrolytes could be beneficial for practical implementation.