Elsevier

Chemical Engineering Journal

Volume 254, 15 October 2014, Pages 597-604
Chemical Engineering Journal

Effect of pyrolysis temperature on carbon obtained from green tea biomass for superior lithium ion battery anodes

https://doi.org/10.1016/j.cej.2014.06.021Get rights and content

Highlights

  • Pyrolytic carbons obtained from green tea leaves are used as LIB anode materials.

  • Three different pyrolysis temperatures from 700 to 900 °C were adopted.

  • The carbon pyrolyzed at lower temperature exhibits superior rate capability.

  • It is attributed to numerous active sites and pores for lithium ion insertion.

Abstract

Carbonaceous materials pyrolyzed from green tea leaves are fabricated and characterized for their potential application as high-performance anodes in lithium ion batteries (LIBs). Three different pyrolysis temperatures (700, 800, and 900 °C) are employed, and the most efficient pyrolysis temperature is determined through a variety of physical and electrochemical measurements. The carbon pyrolyzed at a relatively low temperature of 700 °C contains numerous functional groups, defects that are different from those in graphitic carbon, and large pores. Consequently, the sample exhibited a relatively large capacity of 471 mAh g1 at the 50th cycle, even though high initial irreversibility was observed. Furthermore, when compared to the extremely low capacity of graphite (12.7 mAh g1), the carbon specimen pyrolyzed at 700 °C displays an excellent high-rate capability of 131 mAh g1 at 10 C. Such a result is attributed to the relatively isotropic structures and large-size pores in the sample, which facilitates the rapid diffusion of lithium ions.

Introduction

Since lithium ion batteries (LIBs) were commercialized by SONY in 1991, they have become important energy storage systems for mobile electronic devices. Graphite, with a theoretical capacity of 372 mAh g1, is widely used as a lithium storage material due to its stable structure, excellent electrical conductivity, good cycle ability, and relatively low cost [1], [2]. However, the capacity and rate capability of graphite limit its application to electric vehicles and large energy storage systems that require high power and high energy density. Recently, numerous studies have been conducted on next-generation high-capacity anode materials such as Si, Sn, their oxides, and transition metal oxides (CuO, MnO2, Fe2O3, Co3O4) [3], [4], [5], [6], [7], [8]. Unfortunately, these materials exhibit significant volumetric changes during charge/discharge processes, which lead to a large irreversible capacity during use when compared to carbonaceous materials. Such a difficult problem must be resolved so that the commercialization of high-capacity non-carbonaceous materials can be realized. For this reason, carbonaceous materials are still being studied as candidates for high-capacity anodes [9], [10], [11], [12].

In a recent study, pyrolytic carbons obtained from biomass precursors such as cotton fibers, rice, coffee, walnuts, sugar, and wood were employed as anode materials in LIBs [13], [14], [15], [16], [17], [18]. Pyrolytic carbons are generally regarded as eco-friendly and low-cost materials that can be synthesized by relatively simple methods. In addition, optimization of the pyrolysis temperature, preprocessing procedure, and precursor chemistry allow for the fabrication of carbonaceous materials with capacities higher than that of graphite, though their characteristics vary with the nature of biomass precursors [16], [17], [18]. Therefore, it is important to control the physical and chemical characteristics of pyrolytic carbons if they are to be used as anodes in LIBs. The electrochemical performance of the pyrolytic carbons was significantly dependent upon the pyrolysis condition and surface morphology [14], [17]. Wang et al. [15] reported that the reversible capacity of pyrolytic carbons was dependent upon the atomic ratio of hydrogen to carbon. According to their results, the binding energy of C–H is nearly the same as that of C–Li, hydrogen can easily be substituted for lithium, and a high H:C ratio gives rise to an increase in capacity. Control over pore size is also important when fabricating pyrolytic carbons [19]. Carbonaceous materials composed of nano-sized pores generally exhibit high initial capacities because they provide a large number of lithium adsorption sites. However, the initial efficiency decreases due to the low lithium adsorption/desorption reversibility of the pores. Another significant factor affecting the capacity of pyrolytic carbon is the degree of graphitization. A low degree of graphitic carbon leads to a high irreversible capacity with low efficiency during charge/discharge cycles [18].

While research into biomass-derived pyrolytic carbons has yielded some progress, the effect of the structure and pore size of pyrolytic carbons on the reaction mechanism with lithium ions is not yet completely understood. In this study, pyrolytic carbon obtained from green tea as a biomass precursor is prepared at different pyrolysis temperatures. The effects of pore size, functional groups, and crystallinity on both the reaction mechanism and electrochemical properties are subsequently investigated through a variety of characterization techniques.

Section snippets

Experiment

Green tea leaves collected from Bosenog in Korea were used as a biomass precursor in this work. First, 5 g of green tea leaves were subject to two retting treatments in 300 mL of deionized water at 100 °C for 5 min to remove impurities in the leaves. The wetted leaves were then dried overnight in a convection oven at 80 °C. The resulting product was pyrolyzed in a furnace at three different temperatures, 700, 800, and 900 °C, for 2 h under a nitrogen atmosphere, as shown in Fig. 1. The specimens

Results and discussion

The effects of the pyrolysis temperature on the functional groups and degree of crystallization for the pyrolytic carbons were analyzed by FT-IR and Raman spectroscopy; the results are displayed in Fig. 2. Regardless of the pyrolysis temperature, the FT-IR spectra of all samples in Fig. 2(a) showed peaks at 3300, 2925, 2850, and 1573 cm1 corresponding to –OH, C–H, and C–O stretching from surface carboxyl groups of the pyrolytic carbons [20], [21]. As the pyrolysis temperature increases, the

Conclusions

Three different pyrolysis temperatures from 700 to 900 °C were adopted to optimize pyrolytic carbon obtained from green tea leaves for a high-performance anode material in lithium ion batteries. The following conclusions can be drawn from this study:

  • (1)

    At a low pyrolysis temperature, the formation of an sp2 graphitic structure in the pyrolytic carbon is reduced due to increases in the H:C atomic ratio and the amount of defects, and a decrease in the stacking of graphene sheets.

  • (2)

    The particle size and

Acknowledgements

This work was supported by the National Research Foundation of Korea Grant, funded by the Korean Government (MEST) (NRF-2009-C1AAA001-2009-0093307). Support was also provided by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0024077).

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