Crystallographic Engineering Enables Fast Low-Temperature Ion Transport of TiNb2O7 for Cold-Region Lithium-Ion Batteries

Abstract

TiNb2O7 represents an up-and-coming anode material for fast-charging lithium-ion batteries, but its practicalities are severely impeded by slow transfer rates of ionic and electronic especially at the low-temperature conditions. Herein, we introduce crystallographic engineering to enhance structure stability and promote Li+ diffusion kinetics of TiNb2O7 (TNO). The density functional theory computation reveals that Ti4+ is replaced by Sb5+ and Nb5+ in crystal lattices, which can reduce the Li+ diffusion impediment and improve electronic conductivity. Synchrotron radiation X-ray 3D nano-computed tomography and in situ X-ray diffraction measurement confirm the introduction of Sb/Nb alleviates volume expansion during lithiation and delithiation processes, contributing to enhancing structure stability. Extended X-ray absorption fine structure spectra results verify that crystallographic engineering also increases short Nb-O bond length in TNO-Sb/Nb. Accordingly, the TNO-Sb/Nb anode delivers an outstanding capacity retention rate of 89.8% at 10 C after 700 cycles and excellent rate performance (140.4 mAh g−1 at 20 C). Even at −30 °C, TNO-Sb/Nb anode delivers a capacity of 102.6 mAh g−1 with little capacity degeneration for 500 cycles. This work provides guidance for the design of fast-charging batteries at low-temperature condition.

1. Introduction

Lithium-ion batteries (LIBs) are regarded as the most encouraging energy storage systems by virtue of exceptional cycle life and high energy density [1, 2]. Anode materials represent a critical challenge in advancing LIBs technology [3]. Graphite, as a result of its excellent specific capacity and low cost, has been extensively used as a commercial anode material in LIBs [4]. Nevertheless, graphite typically has a safety risk of dendrite formation, which is associated with low operating voltages [5, 6]. Especially in fast-charging and low-temperature operation, the danger increases significantly as result of high polarization, which leads to graphite anode insertion potentials below 0 V and causes it hard to charge batteries restricted by cutoff voltage [7–9]. Li4Ti5O12, as a promising anode material, exhibits larger working voltage (1.55 V), rapid Li+ transport kinetics, and good low-temperature performance, contributing to cold-region performance and enhanced safety by the suppression of lithium dendrite formation and solid electrolyte interphase (SEI). However, the relatively poor theoretical capacity (175 mAh g−1) restricts its commercial applicability [10]. Thus, exploring anode materials with high safety, superior rate performance, excellent low-temperature performance, and a significantly high theoretical capacity for fast-charging LIBs is critical.

TiNb2O7 (TNO) is first put forward by Goodenough and has been deemed as a promising anode material due to its exceptional structural stability and restrictions on the formation of lithium dendrites [11–13]. Meanwhile, the theoretical specific capacity of TNO is 387.6 mAh g−1 due to the high valence states of Ti4+ and Nb5+, which can undergo five-electron transfer during the electrochemical reaction [13–15]. Though theoretical specific capacity of TNO is comparable to graphite, TNO exhibits unsatisfactory rate capability and cycling lifespan because of its intrinsic low ionic and electronic conductivities. Nanosized or nanoarchitectured design is an effective method to enhance electrochemical reactions by reducing ion diffusion lengths [16–19], whereas TNO possesses the lower compaction density and volume capacity on account of more lattice voids and porous morphology, coupled with the close working potential to the decomposition window of the electrolyte which aggravates the interfacial side reactions, posing significant challenges for practical adoption [20, 21]. Doping engineering represents a straightforward approach to enhance ion and electron transport kinetics in TiNb2O7-based anodes [20, 22–26], exemplified by compounds such as 5Cu-TNO@NC [27], Tb0.01-TNO [28], Ce0.01-TNO [15, 29], Zr0.05-TNO [30], and Fe5-TNO [17]. Nevertheless, there are relatively few studies focusing on the synergistic effects of co-doping and crystallographic engineering to simultaneously improve ionic and electronic conductivities while maintaining structural stability, especially under low-temperature conditions.