Carbon coating of Li₄Ti₅O₁₂ using amphiphilic carbonaceous material for improvement of lithium-ion battery performance

Abstract

Carbon coating of fine particles of Li₄Ti₅O₁₂ synthesized under hydrothermal condition is carried out by amphiphilic carbonaceous material (ACM) in aqueous solution, followed by carbonization at 800 °C for 2 h. The particles prepared are comprised of highly-crystalline spinel-type Li₄Ti₅O₁₂ with the size in the range of 100–400 nm without any agglomeration, of which surface is uniformly covered by a thin carbon layer. Their electrochemical performance as an anode in lithium-ion batteries is evaluated. The initial discharge capacity of carbon-coated Li₄Ti₅O₁₂ at 20 C rate is 137 mA h g⁻¹ and remains as high as 125 mA h g⁻¹ after 100 cycles (91% retention), exhibiting good rate and cyclic performance. Carbon coating by using ACM as carbon precursor gives the Li₄Ti₅O₁₂ particles an enhanced performance as an anode in lithium-ion batteries, owing to the improvement in electrical conductivity, polarization and ability of dispersion. This non-organic coating process may present a new economic, facile, and green pathway for the preparation of carbon-coated Li₄Ti₅O₁₂ as a high power anode material in lithium-ion batteries.

Introduction

Energy storage is an important problem to realize low carbon society and there have been carried out many challenges [1]. Li-ion batteries (LIBs) have been widely used for portable electric devices, such as mobile phones and notebooks. However, the performance of today’s commercial LIBs still cannot meet the requirements of some industrial applications, such as in electric vehicles and sustainable energy storage, in terms of high power density, long cycle life and high safety [2], [3], [4].

The spinel-type lithium titanate (Li₄Ti₅O₁₂, LTO) has attracted great interest as anode material of rechargeable lithium-ion batteries because of its unique characteristics: (1) zero strain during charging and discharging, (2) excellent cycle reversibility, (3) fast Li⁺ insertion and de-insertion ability and (4) high lithiation voltage plateau at 1.55 V vs. Li/Li⁺, which sufficiently avoid the formation of metallic lithium, therefore improving the safety of Li-ion batteries [5], [6], [7], [8]. All of these merits make Li₄Ti₅O₁₂ more competitive as a safe anode material for long life LIBs. Unfortunately, the high-rate performance of bulk LTO is hindered by its inherent poor electronic conductivity (ca. 10⁻¹³ S cm⁻¹) and moderate Li⁺ diffusion coefficient (10⁻⁸ cm² s⁻¹) [9], which makes the polarization of the electrode serious at the charged/discharged with high current densities. Various research works have focused on developing strategies to overcome this problem, such as reducing particle size [10], [11], doping with other metals or metal oxides [12], [13], [14], mixing with a conductive second phase [15] and coating with conductive materials [16], [17], [18], [19].

Carbon coating of LTO particles is supposed to be the most effective and low cost way, because it improves the surface electronic conductivity and the electrical contact with electrolyte solution, and, in turn, leads to a significantly improved electrochemical performance [20]. However, its effect depends on the amount and the nature of carbon coated. Until now, carbon coating was mainly carried out by either chemical vapor deposition (CVD) [16] or heat treatment of a mixture with an organic precursor at a high temperature under inert atmosphere [17], [18], [19]. Thickness and uniformity of carbon layer is also important factor to get better performance: a non-continuous carbon layer cannot improve the electronic conductivity effectively, while the excessively thick coated layer may restrict the efficient charge transfer/transport [19]. Therefore forming a thin and uniform carbon layer on the LTO surface is critical, in addition to choosing an appropriate carbon precursor to control nature of carbon layer coated.

“Amphiphilic carbonaceous material” (ACM), a kind of water-dispersible carbonaceous material, especially in alkaline aqueous solution, was firstly synthesized from coke by Fujii et al. [21], [22], [23], [24], showing that hydrophilic groups, such as –COOH, –OH and –SO₃H, existed in ACM make it dispersible in alkaline aqueous solutions in nano-scale. These functional groups may provide a better connection with other material to form a homogenous contact through interfacial interaction on the process to carbon coating. Compared with conventional solid carbon precursors, the ACM aqueous solution is more favorable for forming a thin uniform layer on the particle surface. In our previous work, artificial graphite powder was coated by carbon via an ACM aqueous solution, enhancing its electrochemical performance as electrode in lithium batteries [25].

In this work, Li₄Ti₅O₁₂ synthesized via a hydrothermal method was coated with ACM in its aqueous solution, by expecting a marked improvement in anode performance in LIBs, similar to the case of graphite [25]. The high-rate performances of the carbon-coated LTO in LIBs were investigated by galvanostatic charge/discharge tests, cyclic voltammetry and electrochemical impedance spectroscopy.