Morphology control of lithium iron phosphate nanoparticles by soluble starch-assisted hydrothermal synthesis

Highlights

• The spheroidal LiFePO₄ particles have been synthesized successfully.

• Soluble starch can provide a template to restrict the growth of LiFePO₄ crystals.

• The particle size of LiFePO₄ synthesized in 0.075 mol L⁻¹ starch solution is ∼100 nm.

• Cycling stability and rate capacity have been greatly improved.

Abstract

Lithium iron phosphate (LiFePO₄) is a potentially high efficiency cathode material for lithium ion batteries, but the low electronic conductivity and one-dimensional diffusion channel for lithium ions require small particle size and shape control during the synthesis. In this paper, well-crystallized and morphology-controlled LiFePO₄ cathode material for lithium-ion batteries is successfully synthesized via a soluble starch-assisted hydrothermal method at 180 °C for 5 h, followed by calcining with phenolic resin at 750 °C for 6 h. In this study, we investigate the effect of five different concentrations of starch solution on controlling morphology of LiFePO₄. Interestingly, the nano-sized LiFePO₄ particles obtained in 0.075 mol L⁻¹ starch solution exhibit a spheroidal microstructure, while the platelet shape LiFePO₄ particles are synthesized in lower or higher concentration of starch solution. The mechanism and process of forming such spheroidal microstructure is discussed. These unique structural and morphological properties of LiFePO₄ lead to high specific capacity and stable cycling performance. Analysis of the electrochemical impedance spectroscopy reveals that nano-sized carbon/polyacene coated LiFePO₄ cathode materials play an critical role in achieving excellent electrochemical performance.

Introduction

The olivine-type lithium iron phosphate (LiFePO₄) has received much attention ever since it was introduced as an alternative cathode material for new generation lithium ion batteries [1], [2], [3], [4], [5], [6]. It processes several advantages over conventional cathodes, such as lower cost, improved safety performance, lower toxicity, and an extremely flat charge–discharge profile at reasonably high potential of ∼3.4 V versus Li/Li⁺. Nevertheless, the pristine compound is a very poor conductor (σ∼10⁻⁹ S cm⁻¹), which limited its electrochemical properties and addressed as major problems to be solved before it could be deployed on a commercial scale [7], [8], [9], [10].

To address the issue of poor conductivity, approaches to enhance the ionic and the electronic conductivity of LiFePO₄ cathode have recently focused on using nanostructured carbon/LiFePO₄ hybrid materials [11], [12], [13], [14], [15], [16], [17], [18], [19]. The concept of the synthetic process for the preparation of nano-sized LiFePO₄via hydrothermal method was first introduced in 2001 by Yang et al. [20] in order to decrease the particle size and shorten the Li⁺ ion diffusion length, which can further improve Li-ion migration rate and the electrode kinetics. So far, those promising hydrothermal strategies adding various organic surfactants in precursor solution to prepare morphology-controlled and size-controlled carbon/LiFePO₄ composites have shown excellent results. For instance, LiFePO₄ nanowires have been synthesized via a surfactant (nitrilotriacetic acid (NTA)) assisted hydrothermal method [21]. Similarly, various morphologies of LiFePO₄crystallies have also been observed as rod-like LiFePO₄ nano-crystallites, [18] platelets, [22] rectangular prisms and spindle, [23], [24] diamond, [25] block, [26] sphere, [27] core/shell, [28] hollow, [29] and bundle particles [30]. The particle geometries of the products are highly dependent on the addition of organic molecules such as glucose, [31] citric acid (CA), [32] ethylene glycol (EG), [22] ascorbic acid [32], [33] and cetyltrimethyl ammonium bromide (CTAB), [34] etc. acting as shape controllers. The introduced organics can also be used as carbon coating materials providing a barrier to restrict the growth of the LiFePO₄ particles during hydrothermal treatment and post-hydrothermal carbon coating to prevent the oxidation of Fe²⁺ to Fe³⁺. However, in spite of these accomplishments, the serious impact on environment and high cost of most surfactants [35], [36], [37] lead to find alternative materials that can not only exhibit the same functions as surfactants but also have no environment pollution. Recently, starch has gained considerable interest because it provides a viable alternative to replace those organic molecules, eliminating the environment pollution. In addition, starch has a wide range of application in many different fields. For example, Zheng et al. [38] and Cui et al. [39] have reported that using starch solution as raw material can prepare carbon microspheres via hydrothermal carbonization process. Inspired from the above research, we have found that the presence of soluble starch can effectively control the crystal growth and direct the plate-shaped LiFePO₄ particles towards a spheroidal microstructure. However, to the best of our knowledge, the formation mechanism of the different morphologies under soluble starch-assisted hydrothermal conditions is not fully understood.

Herein, the soluble starch in the hydrothermal reaction system has the profound effect on size and morphology of LiFePO₄ particles. Remarkably, the main process in this method was carried out in aqueous solution without involving any other environmentally toxic surfactants. This paper will mainly focus on assessing the effect of soluble starch on morphology and electrochemical performances of LiFePO₄ cathode material for rechargeable lithium batteries.