Nitrogen-doped carbon-coated hierarchical Li₄Ti₅O₁₂-TiO₂ hybrid microspheres as excellent high rate anode of Li-ion battery

Abstract

Nitrogen-doped carbon-coated Li₄Ti₅O₁₂-TiO₂ (LTO-TO) hybrid microspheres were prepared by heat treating the dry mixture of urea and chemically lithiated dandelion-like TiO₂ microspheres in a stainless steel autoclave at 550 °C for 5 h. The hybrid materials were tested as anode of Li-ion batteries. As compared to the pristine sample, the N-doped carbon-coated LTO-TO microspheres exhibited higher specific capacity at both low and high current rates. Discharge capacities of 184 and 123 mAh g⁻¹ were obtained at 0.2 C and 20 C, respectively. Moreover, the LTO-TO/C electrode showed excellent cycle performance, with a discharge capacity of 121.3 mAh g⁻¹ remained after 300 cycles at 5 C, corresponding to an average capacity degradation rate of 0.073% per cycle. These high specific capacity, excellent rate capability and cycle performance demonstrated the high potentiality of the N-doped carbon-coated LTO-TO microspheres as anode material of both energy storage-type and power-type Li-ion batteries.

Introduction

Titanium based compounds [1], including titanium oxides in various polymorphs [2], [3], [4], [5], lithium titanate [6], [7], [8], lithium zinc titanate [9], [10], and sodium titanate [11], [12], have been extensively studied as potential anode materials of Li-ion and Na-ion batteries because of their high structural stability, reasonable high capacity, low cost and environmental friendliness. Among them, spinel-type Li₄Ti₅O₁₂ (LTO) has attracted special interest as anode of Li-ion batteries owing to its relatively high discharge-charge platforms near 1.55 V (vs. Li/Li⁺) and zero strain effect during Li⁺ intercalation/extraction, which leads to an excellent reversibility, long-term stability and high safety [13]. However, the intrinsic low electronic conductivity (10⁻¹³ S cm⁻¹) and medium Li⁺ ion diffusion coefficients (10⁻¹³–10⁻⁹ cm² S⁻¹) of LTO limit its rate capability, especially in bulk-phase [14], [15].

So far, two main strategies have been adopted to enhance the rate performance of LTO. One is coating various carbon material or metals on the surface of LTO to improve its electronic conductivity [15], [16]. The other is to fabricate various nanostructures to reduce the Li⁺ ion diffusion distance, so that to facilitate the Li intercalation/extraction reaction in LTO [17], [18], [19]. To this end, carbon-coated LTO nano/microspheres with both high tap density, hierarchical porous structure and good electronic conductivity have received considerable attentions owing to their ability to preserve both high volumetric energy density and high rate performance [20], [21], [22], [23], [24], [25]. For examples, Xia et al. [24] obtained carbon-coated nanoporous LTO microspheres by a carbon pre-coating process combined with a spray drying method. The resulting sample delivered a reversible capacity of 160 mAh g⁻¹ at 0.2 C (1 C =175 mA g⁻¹), and at 20 C it retained 79% capacity of that at 0.2 C. Sha et al. [25] prepared carbon-coated acanthosphere-like LTO microspheres via a two-step hydrothermal process followed by calcination and obtained a high discharge capacity of 145.6 mAh g⁻¹ at 40C.

Similar to LTO, TiO₂ (TO) also has high Li⁺ insertion potential (1.75 V vs. Li/Li⁺) and low volume change (3–4%) during the charge/discharge process [2]. Meanwhile, the open frameworks, rich Li-ion diffusion channels in TiO₂ crystal structures and its high theoretical specific capacity (335 mAh/g) make Li₄Ti₅O₁₂-TiO₂ (LTO-TO) composite more advantageous than LTO for fast and high amount of lithium storage. For these reasons, hybridizing Li₄Ti₅O₁₂ with TiO₂ has been intensively studied in recent years to improve the performance of LTO anode [26], [27], [28], [29], [30]. However, concerning to the hierarchical LTO-TO nano/microsphere structures, only few works have been reported. Liao et al. [27] proposed a two-step hydrothermal method to prepare carbon-coated hierarchical LTO-TO microspheres consisting of nano-size octahedron-like crystals. The composite anode delivered a discharge capacity of 230 mAh g⁻¹ at 0.2 C and 120 mAh g⁻¹ at 10 C, respectively. Zhang et al. [30] synthesized hierarchical carambola-like LTO-TO composites through controlling the hydrothermal reaction time between LiOH and tetrabutyl titanate (TBT). The obtained products showed discharge capacities of 115.1 and 91.2 mAh g⁻¹ at current rates of 20 and 40 C, respectively, while preserving a stable reversible capacity of 171.7 mAh g⁻¹ after 200 charge-discharge cycles at 1 C. These results indicated that an innovative structural design of LTO-TO composite anode was of vital importance to the improvement of its lithium storage performance.

Recently, coating a thin nitrogen-doped carbon layer on LTO surface has also been proven of an effective way to improve the Li storage performance of LTO. On one hand, the N atoms may offer excess electrons, which increase the electronic conductivity and electron mobility of the carbon layer; On the other hand, the electro-negativity of nitrogen is much stronger than that of carbon, which leads to the improved adsorption energy and decreased energy barrier for Li absorption at the electrode-electrolyte interface [31]. Zhao et al. [32] prepared porous LTO sample by a spray-drying method and coated it with N-doped carbon through an ionic liquid 1-ethyl-3-methylimidazolium dicyanamide. The resulting LTO material with a very thin and uniform N-doped carbon coating layer exhibited superior rate capability and excellent cycling performance in a half cell. Li et al. [20] prepared N-doped carbon-coated LTO microspheres using polydopamine as both the carbon and nitrogen sources and achieved a discharge capacity of 123 mAh g⁻¹ at 30 C. The However, the above mentioned methods might be not suitable for mass production because of the expensive reagents used, such as the ionic liquid and polydopamine. Meanwhile, to our best knowledge, a study concerning to N-doped carbon-coated LTO-TO composite anode with hierarchical structures has not been well studied so far.

In this work, chemically lithiated TiO₂ microspheres assembled by ultrathin nanoflakes were first synthesized by a facile hydrothermal method according to the recent reports [33], [34]. N-doped carbon was then coated onto LTO-TO composite microspheres by treating the dry mixture of urea and lithiated TiO₂ microspheres in a stainless steel autoclave at 550 °C for 5 h following the method described in a recent report [35]. As compared to the pristine sample, the N-doped carbon-coated LTO-TO microspheres exhibited higher specific capacity and excellent rate performance, which delivered a discharge capacity of 200 mAh g⁻¹ at 0.2 C and 123 mAh g⁻¹ at 20 C, respectively, demonstrating its high potential application in both energy storage-type and power-type Li-ion batteries.