Elsevier

Electrochimica Acta

Volume 213, 20 September 2016, Pages 432-438
Electrochimica Acta

Effect of heat pre-treatment conditions on the electrochemical properties of mangrove wood-derived hard carbon as an effective anode material for lithium-ion batteries

https://doi.org/10.1016/j.electacta.2016.07.138Get rights and content

Abstract

Anodic hard carbon for lithium-ion batteries with high discharge capacity was successfully developed using mangrove green tree as a cheap raw material. To obtain commercial-level first-cycle Coulombic efficiency, the effects of heat pre-treatment in the low temperature range of 450–600 °C at various pressures, to produce optimized char precursor before carbonization, were closely investigated, focusing on synthetic yield, structural and physical properties, first-cycle Coulombic efficiency, and discharge capacity of the resulting hard carbon. The hard carbon with heat pre-treatment involving optimum fabrication conditions of 500 °C in inner gas at 0.7 MPa for 39 days exhibited higher synthetic yield of 30.0 wt% and better first-cycle Coulombic efficiency of 81% and discharge capacity of 424 mAh/g than that prepared through direct one-step carbonization, due to low H/C and Odiff/C ratios and a pore structure promoting lithium-ion insertion/desertion.

Introduction

Hard carbon is considered as one of the most promising candidate anodic materials for lithium-ion batteries (LIBs) due to its high reversibility, good rate performance, natural abundance, and low cost of its precursors [1], [2], [3]. Moreover, various raw materials could be used as precursors for hard carbon, such as epoxy resin, phenol formaldehyde resin, organic materials, isotropic pitch, and biomass [4], [5], [6], [7]. However, the substantial irreversibility, the low 1st cycle Coulombic efficiency, of hard carbon still restricts its commercial application, despite its substantial merits.

The electrochemical performance of hard carbon strongly depends on its structure, porosity, and heteroatom contents, which can be controlled by selecting ideal precursors and controlling preparation conditions [8]. Matsubara et al. suggested that the improvement of electrochemical properties depends on carbonization conditions, and stated that this could markedly influence lithium storage in electrodes due to the varying structures of carbonaceous materials [9]. Guo et al. found that some properties of hard carbon, such as porosity and graphite-like micro-crystallites, are associated with excellent electrochemical performance [10]. Therefore, substantial efforts have focused on the raw materials suitable for use as carbon precursors and the optimal preparation process for enhancing their electrochemical performance, especially the first-cycle Coulombic efficiency.

To date, hard carbon obtained from biomass sources such as sugar [11], [12], [13], cotton wool [14], pinecone hull [15], peanut shell [16], and rice husk [17] has been reported. In recent years, Indonesian mangrove charcoal has also attracted attention as a precursor of hard carbon due to its abundance and low cost. The total area of mangroves in the year 2000 was 137,760 km2, including 118 countries and territories in the tropics and subtropics. The largest expanse of mangroves is in Asia (42%), particularly Indonesia (23%), followed by Africa (20%), North and Central America (15%), Oceania (12%), and South America (11%) [18]. In our previous work, we found out that mangrove charcoal-derived hard carbon carbonized under vacuum shows higher available discharge capacity and lower irreversibility (58 mAh/g) than those carbonized under an argon atmosphere due to the formation of a pyrolytic carbon film on the surface, and lower specific surface area and amounts of heteroatoms and hydroxyl groups [19], [20]. These findings suggest that mangrove-charcoal-derived hard carbon is a potential candidate anode material for LIBs. However, previous studies only focused on the anodic performance of mangrove-charcoal-derived hard carbon whose starting material was mangrove charcoal prepared by distillation in the ground.

For commercial applications, it is important to develop a process for preparing mangrove charcoal with stable properties on a massive scale from raw mangrove wood. However, the direct production of the hard carbon for Li-ion battery using mangrove green tree has always suffered low preparation yield and low 1st cycle Coulombic efficiency. Therefore, it has been strongly required to establish the stable production process of mangrove derived hard carbon from mangrove green tree. In this study, we aimed to optimize the conditions for preparing charcoal from raw mangrove wood to improve the first-cycle Coulombic efficiency of mangrove-derived hard carbon. In particular, the effect of heat pre-treatment of the green mangrove tree at the temperature range of 450–600 °C, which can affect the physical and structural properties of formed charcoal, was closely examined to improve the electrochemical performance of resulting hard carbon.

Section snippets

Sample preparation

Indonesian mangrove wood (Rhizophora apiculata) was used as received without any pre-pyrolysis. The mangrove wood was pyrolysed by two different procedures as follows:

To prepare non-pre-pyrolysed mangrove charcoal (NP-MC), the pyrolysis of raw mangroves was carried out in a stainless steel tube bomb at 800 °C (heating rate: 5 °C/min) for 1 h. After grinding to a particle size of less than 45 μm, the NP-MC samples were heat-treated at 1000 °C (heating rate: 15 °C/min) for 1 h in a vacuum (30 Pa). To

Effects of pre-pyrolysis of hard carbon on the electrochemical performance of LIBs

To confirm the effects of heat pre-treatment conditions before the preparation of hard carbon at 1000 °C on the performance of electrode materials, raw mangrove wood was heated by two different procedures. The mangrove charcoal not subjected to heat pre-treatment (NP-MC) was heated from room temperature to 800 °C at 5 °C/min for 30 min. In contrast, the heat-pretreated mangrove charcoal (P-MC) was heated from room temperature to 500 °C at a pressure of 0.7 MPa for 1 day. Fig. 1 shows the effect of

Conclusions

The conditions for heat pre-treatment of mangrove-derived hard carbon were investigated to maximize its first-cycle Coulombic efficiency and discharge capacity. The optimum heat pre-treatment conditions involved heat-treating raw mangrove wood at 500 °C and 0.7 MPa by inert gas for 39 days. Hard carbon prepared under these conditions exhibited a first-cycle Coulombic efficiency of 80.0% and discharge capacity of 424 mAh g−1, likely resulting from low H/C and Odiff/C ratios and a pore structure that

Acknowledgement

This work was supported by a grant for the Human Resources Development Program (No. 20114010203130) of the South Korean Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry, and Energy, South Korea.

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