Phase-junction engineering triggered built-in electric field for fast-charging batteries operated at −30°C

Highlights

• A mosaic TiNb₂O₇/TiNbN₂ is proposed for fast-charging batteries at −30°C

• The interfacial electric field enables promoted charge-transfer kinetics

• Excellent capacity and cyclability up to 3,000 cycles of TiNb₂O₇/TiNbN₂ is achieved

Summary

The high operational capability of fast-charging lithium-ion batteries (LIBs) at low temperatures (<−30°C) is essential for frequency regulation and peak-load shifting of power grids in cold regions. However, the low-temperature performance is seriously plagued by the sluggish Li⁺ diffusion and high-voltage polarization. Herein, the heterostructure-induced built-in electric field is constructed by selected nitriding of TiNb₂O₇, which demonstrates high accessibility for accelerated low-temperature dynamics. The introduced charged phase-junction interface enables promoted Li⁺ diffusion rates, enhanced Li⁺ adsorption, and reduced lattice strains. The mosaic TiNb₂O₇/TiNbN₂ heterostructure displays an outstanding specific capacity of 241.5 mA h g⁻¹ at −30°C, corresponding to 89% retention compared with that at 25°C. It even delivers a high-capacity retention of 96% at a 5C rate after approximately 1,000 cycles at −30°C. Our work provides new insight into structure-function relationships for developing fast-charging batteries for cold-region accumulation of electric energy.

Introduction

Smart grids can innovate electric systems through intelligent collection and redistribution of electric energy from unstable solar and wind energy conversion. As a key link, lithium-ion batteries (LIBs) have emerged as a high possibility for electric reservoir roles in future grid systems.^1,2 However, it is a huge challenge reducing the battery workability due to the low temperature in cold regions such as Canada, Northern Europe, and Northern China.³ Beyond that, batteries with extremely fast-charging ability and long life spans are highly expected on account of the requirements for multifunctions of frequency regulation and peak-load shifting in the grid energy storage systems.^4,5 Graphite as a commercial anode usually suffers from safety risks of dendrite formation related to the low operating voltage (≈0.1 V vs. Li⁺/Li). Especially, the danger is elevated at fast charging and low-temperature conditions. A promising anode candidate of Li₄Ti₅O₁₂ shows superior rate capability and safety along with low-temperature performance compared with graphite, but its low theoretical capacity (175 mA h g⁻¹) still fails to meet commercial applicability.⁶

Titanium niobium oxides (TiNb₂O₇), first proposed by Goodenough, have been pursued recently due to their high theoretical capacities (387 mA h g⁻¹ with the voltage window of 0–3 V) and rapid Li-ion de-/intercalations because of a stable Wadsley-Roth structure.^7,8 Actually, considering the safety and cycling stability, a practical capacity of approximately 300 mA h g⁻¹ is generally presented in a safe voltage window of 1–3 V, which is nearly twice that of Li₄Ti₅O₁₂.^9,10,11 Although that high energy density and durability of TiNb₂O₇ displays a high possibility for emerging applications in smart grids, the low-temperature performance is expected to be improved.¹² At subzero temperatures (below −30°C), the factors leading to rapid performance decrease mainly include (1) increased viscosity and reduced conductivity of the electrolytes, (2) suppressed de-solvation process of Li⁺ at the electrode-electrolyte interface, and (3) decreased ion diffusion in the bulk phase of active material.^13,14,15 Nanosized or nanoarchitectured (i.e., nanowires, nanospheres, nanorods, and so on) designs have been proven valid.^16,17 Nevertheless, reduced tap density and overall volumetric capacity, along with the increased interface irreversible reaction between electrode and electrolyte, make it hard to be adopted.^18,19 Therefore, exploiting a simple and versatile method to improve the ion/electron transfer of micron-sized TiNb₂O₇ at low temperature is urgently required for the grid requirements.

Constructing heterojunctions by coupling nanocrystals with different band gaps is a promising route toward improving the low-temperature performance of the TiNb₂O₇ anode. The heterostructures can boost a high electrochemical activity arising from the strong interfacial synergistic effect.^20,21,22,23 Beyond that, a built-in electric field (E-field) can be formed within the phase-junction interface, which is expected to provide a driving force for interfacial charge transfer and accelerate rapid low-temperature dynamics for Li⁺ diffusion. More significantly, the electrochemical behavior of the Ti_xNb_yO_z-based anodes below −20°C has been rarely investigated, and the influence of the atomic interfacial E-fields on low-temperature performance is still lacking.

Herein, we proposed a mosaic TiNb₂O₇/TiNbN₂ (TNO/TNN) heterojunction with an atomic interfacial E-field to achieve improved ion-diffusion kinetics for superior Li storage (Figure 1). Density functional theory (DFT) calculations further revealed that the phase-junction interface can significantly reduce the diffusion barrier, improve electrical conductivity, and enhance the Li⁺-de-solvation ability, endowing high Li⁺ diffusion coefficients and fast charge capability of TNO/TNN at low-temperature conditions. Even at −30°C, the TNO/TNN delivered a large reversible capacity (241.5 mA h g⁻¹), superior rate performance (108.6 mA h g⁻¹ at 20C), and inspiring durability (96% after about 1,000 cycles). In addition, TNO/TNN displayed a negligible capacity decay for 3,000 cycles at 25°C. This ingenious structural design provides a new direction for the construction of functional electrodes for fast-charging batteries for cold-region grid energy storage.