Single crystalline lithium titanate nanostructure with enhanced rate performance for lithium ion battery

Abstract

Here we report the synthesis and electrochemical performance of 40 nm near-uniform lithium titanate (Li₄Ti₅O₁₂) single crystals. The cubic spinel Li₄Ti₅O₁₂ nanocrystals are transformed without size changes at 400 °C from cubic NaCl type (Li_0.4H_0.6)₂TiO₃ nanocrystals that are prepared from hydrothermal synthesized α-Li₂TiO₃. The prepared Li₄Ti₅O₁₂ nanocrystals show reduced charge-transfer impedance and exhibit favorable performance under high current charging/discharging due to the reduced Li⁺ ion diffusion path.

Introduction

Secondary lithium ion batteries (LIBs) have been of great potential for applications in the hybrid electric vehicles (HEVs) and electric vehicles (EVs), due to the unique features such as high energy density and efficiency, long cycling life and environmental friendliness [1], [2], [3]. The key challenges for LIBs have been the insufficient power density and safety issues which are aroused from the intrinsic low conductivities of materials and the side reactions with the electrolytes under extreme operation conditions, respectively [4], [5], [6], [7], [8]. To solve these problems, nanostructured forms of materials with stable structure and inert reactivity to electrolytes have attracted tremendous attention, because these nanomaterials have shown excellent rate capability as well as safety due to the reduced diffusion time for Li⁺ ions and surface current density in terms of τ ∝ 1/R² and i ∝ R, respectively [9], [10], [11]. Nanostructured spinel Li₄Ti₅O₁₂ is one of the examples which have been regarded as potential candidate for anode materials in power LIBs [12], [13], [14], [15], [16], [17].

The outstanding features of Li₄Ti₅O₁₂ nanostructure can be summarized as follows: (1) the zero-strain structure in which the insertion/desertion of Li⁺ ions occur on tetrahedral 8a and octahedral 16c sites with tiny lattice change [18], [19], [20], [21], [22]; (2) the potential based on Ti³⁺/Ti⁴⁺ redox couple is 1.5 V verse Li metal, so high that no reduction of organic electrolytes into surface electrolyte interface (SEI) could occur, and Li dendrites are unlikely to form even when over discharged [23], [24]; (3) the Li⁺ insertion/desertion reactions are highly reversible compared to graphite and alloys, ensure this material with high columbic efficiency and long cycling life [15]; (4) nanostructures of Li₄Ti₅O₁₂ show favorable rate capability as the Li⁺ diffusion path becomes short and the surface contact area is large [2], [5], [8]. These features make Li₄Ti₅O₁₂ nanomaterials the ideal candidate for anode in high power LIBs, even though the 175 mAh g⁻¹ specific capacity is not in favor of this spinel material. Recently, efforts have been made to the synthesis and electrochemical investigation of nanostructured Li₄Ti₅O₁₂ materials, including 1D nanorods [25], [26], 2D nanosheets [27], hollow spheres [28], [29] and assembled microspheres [30].

While the perspective of nanostructured Li₄Ti₅O₁₂ is fascinating, the synthesis of Li₄Ti₅O₁₂ nanomaterials with uniform morphology and single crystalline nanostructure has been a challenge. The reason for the difficulty may be this: sintering of the material accompanying the essential high-temperature calcinations (500–1000 °C) in conventional approaches would destruct the morphologies of the nanostructures and result into wide size-distributed materials [31], [32], [33]. The most effective solution to the sintering is to prepare Li₄Ti₅O₁₂ nanostructure at the convenient temperature low enough to prevent the sintering. As a result, convenient low-temperature approaches for uniform single crystalline Li₄Ti₅O₁₂ nanostructure would be of importance for the application of this spinel material in high power LIBs.

In this effort, we propose a low-temperature strategy for near-uniform Li₄Ti₅O₁₂ single crystals with 40 nm in size. By taking advantage of metastable cubic (Li_0.4H_0.6)₂TiO₃ nanocrystals delithiated from α-Li₂TiO₃ as the intermediate, well dispersed crystalline Li₄Ti₅O₁₂ nanocrystals are obtained at 400 °C for 2 h. The as-synthesized sample nanocrystals are phase pure cubic spinel Li₄Ti₅O₁₂. It has been clear that the sizes, surface areas and morphologies of the samples before and after the calcination treatment are in well accordance by various characterizations, indicating the calcination at 400 °C is sintering-resistant. Impedance spectroscopy (EIS) and galvanostatic tests indicate that the 40 nm single crystalline Li₄Ti₅O₁₂ nanomaterials exhibit reduced charge-transfer impedance and enhanced rate performance comparing with the reference sample synthesized at 700 °C.