Elsevier

Electrochimica Acta

Volume 146, 10 November 2014, Pages 630-637
Electrochimica Acta

Increasing reversible capacity of soft carbon anode by phosphoric acid treatment

https://doi.org/10.1016/j.electacta.2014.09.088Get rights and content

Abstract

Soft carbon with a high reversible capacity has been proposed as an anode material for high-power lithium ion batteries. In this work, we investigate the effect of phosphoric acid (H3PO4) addition during preparation on the microstructure and electrochemical performance of soft carbon. H3PO4 facilitates the formation of nanopores by terminating C-C bonding and forming C-Ox-P (0<x≤4) chemical bonds. The newly integrated nanopores are mainly responsible for increasing the reversible capacity of soft carbon and maintaining excellent cycle performance and rate capability. In addition, the proposed synthetic route is simple and cost-effective, which will be helpful for successfully employing soft carbon on a commercial scale.

Introduction

Carbonaceous materials have attracted much attention as anode materials for lithium ion batteries (LIBs) since they were first commercialized in 1991. Carbonaceous materials can generally be classified into three types according to their microstructures: graphite, soft carbon, and hard carbon [1], [2]. Among these types, graphite has been most widely used in current LIBs because of its high theoretical capacity (372 mAh g−1), high initial coulombic efficiency (∼95%), and excellent reversibility. Despite these attractive features, the poor cycle performance and rate capability of graphite still need to be resolved. As an alternative, soft carbon has been proposed for use in high-power LIBs. Even though soft carbon offers a smaller reversible capacity (∼300 mAh g−1) than graphite, it shows excellent cycle performance and capacity retention even at high current densities. Such promising power characteristics mainly originate from its distinctive microstructure, in which short-range-ordered graphite is randomly arranged in a disordered carbon matrix. This structure is completely different from that of graphite, which has a well-developed layered structure and amorphous hard carbon structures [3], [4], [5].

Soft carbon is generally synthesized by pyrolysis of coke or pitch precursors at a temperature range between 700 and 1800 °C. It is widely recognized that the microstructure of soft carbon is determined by the synthesis temperature and directly affects its electrochemical performance. In particular, the reversible capacity of soft carbon decreases with increasing synthetic temperature, and soft carbon synthesized at temperatures below 800 °C exhibits a higher reversible capacity than graphite [6], [7], [8]. However, its low initial coulombic efficiency associated with its poor reversibility hinders its practical use for commercial applications. To overcome those limitations, many studies have focused on tailoring the microstructure of soft carbon in order to control its electrochemical performance.

To improve the reversible capacity of soft carbon, various structural modifications have been proposed. Incorporation of heteroatoms such as B [9], [10], N [11], [12], Si [13], [14], and P [15], [16] is effective in increasing the reversible capacity of soft carbon without causing significant performance fading. The incorporated heteroatoms produce more reversible Li+ storage sites by forming chemical bonds to C atoms in the structure, which directly contributes to the improved reversibility for Li+ insertion and extraction. However, most current approaches for the heteroatom incorporation are based on expensive and/or toxic process such as chemical vapor deposition (CVD) or chemical etching [9], [11], [13]. Thus, there is a strong need for the development of efficient and cost-effective synthetic routes for mass production.

Herein, we introduce a simple method for preparing structurally modified soft carbon by adding phosphoric acid (H3PO4). The correlation between the microstructure and electrochemical properties of the soft carbon is thoroughly investigated based on various structural and electrochemical characterizations. The presented evidence indicates that abundant nanopores can be effectively formed by thermal decomposition of H3PO4 in the soft carbon, which is essential for improving the reversible Li+ storage capacity. Furthermore, the proposed approach is based on a simple and cost-effective process and will be helpful for the employment of soft carbon on a commercial scale.

Section snippets

Experimental

Pristine soft carbon was prepared by heating petroleum coke precursor at 900 °C for 2 h under Ar flow with a heating rate of 10 °C min−1. From the elemental analysis, we confirmed that as-received petroleum coke precursor has a C/H ratio of 3.96 (atomic). To synthesize H3PO4-treated soft carbon, the petroleum coke and 2 wt% (vs. coke mass) of H3PO4 (85% in water, Aldrich) were thoroughly mixed using a homogenizer (SMT Co., Ltd. HF93), and then heated in a furnace under the same conditions.

Results and Discussion

It is well known that soft carbon structures have two different sites for Li+ storage. The first is the stacked graphene layers, where Li+ can be reversibly inserted and extracted, and the second is the nanopores integrated in the disordered carbon matrix, which are responsible for the adsorption and desorption of Li+ clusters [6], [8]. By successfully utilizing both active sites, the soft carbon can exhibit a higher Li+ storage capacity than graphite, that is, a theoretical capacity above 372 

Conclusion

To increase the reversible capacity of soft carbon, a small amount of H3PO4 was added during the pyrolysis of coke precursor. As a result of H3PO4 addition, the crystallinity of soft carbon was slightly decreased due to the formation of C-Ox-P (0<x≤4) bonding. This is responsible for the formation of additional nanopores, which allows a further improvement in the reversible Li+ storage characteristic of the soft carbon. Consequently, the reversible capacity of the soft carbon was enhanced from

Acknowledgements

This work was supported by the IT R&D program of Ministry of Trade, Industry & Energy/KEIT. [10039155, Soft carbon anode material development for high capacity rechargeable battery.

References (24)

  • R. Marom et al.

    A review of advanced and practical lithium battery materials

    J. Mater. Chem.

    (2011)
  • L. Ji et al.

    Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries

    Energy & Environ. Sci.

    (2011)
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