Optimization of the preparation conditions for pitch based anode to enhance the electrochemical properties of LIBs

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Abstract

The anode of a lithium ion battery was prepared by the thermal treatment of PFO (pyrolysis fuel oil) utilizing the following three steps: PFO  pitch  coke  anode. The PFO-based pitch with a high softening point exhibited high discharge capacity and Coulombic efficiency because of turbostratic disorder within the disordered structures. A two-step intermediate heating process during the (pitch  coke) reaction induced the remaining relatively light components to participate in condensation/cross-linking reactions during anode formation (coke  anode). The intermediate heating provided a high discharge capacity and a first-cycle Coulombic efficiency based on a well-ordered and suitable micropore structure for lithium storage sites.

Introduction

During the past several years, lithium ion batteries (LIBs) have been successfully developed for portable electronic devices, and current LIBs are considered promising for the development of batteries for hybrid electric vehicles (HEVs) and energy storage systems (ESSs) applications. In future years, it is anticipated that the global demand for LIBs will greatly explode with the increase in popularity of HEVs and ESSs. However, it is critically important to improve the capacity, long-term stability, high-energy density and safety of LIBs while maintaining low cost for high-power applications. Consequently, this surge in demand for LIBs has increased concern regarding their high costs, such as the unit production cost of anode, cathode and electrolyte components in LIBs [1], [2].

With respect to the anodes, graphite has been the most widely used anode material for LIBs because of its low cost. However, the electrochemical performance properties of graphite materials, such as capacity (<372 mAh g−1) and power, are not sufficient for electrodes in future LIBs applications. To overcome these insufficiencies, several research efforts have been devoted to improve the electrochemical performance of carbonaceous materials by modifying their structural features [3], [4], [5], [6]. The electrochemical properties of carbonaceous materials strongly depend on the morphology, crystallinity and orientation of crystallites, which are different from the raw material, and also preparation conditions [7]. Therefore, the selection of raw materials and an efficient preparation process are very important factors for effective applications of future LIBs.

An inexpensive petrochemical by-product, pyrolysis fuel oil (PFO), which is generated from the petroleum refining process, has attracted considerable interest as a carbon precursor because of its low cost, abundance and high-carbon content with aromatic structure [8], [9], [10], [11]. Thus, great progress may be achieved in producing high electrochemical performance characteristics, low-cost and a high carbonization yield of pitch-based anodes in high-power batteries applications [12], [13], [14], [15], [16]. Furthermore, from a commercial viewpoint, it is crucial to utilize a simple green and low-priced synthesis method, which is based on inexpensive and abundant raw materials.

The petroleum pitch-based disordered carbon anode can be separated into graphitizable carbon (soft carbon) and non-graphitizable carbon (hard carbon) [17], [18]. The electrochemical properties of these disordered carbons are strongly dependent on pitch properties and preparation conditions. The graphitizable carbon that is prepared under optimal conditions has good rate properties and a high Li-storage capacity because of optimal surface area and a crystalline structure. However, a LIB system, which uses pitch-based soft carbon anode, usually suffers from a poor discharge capacity caused by a large hysteresis in the charge/discharge profiles and a low carbonization yield of the pitch [19], [20], [21], [22]. To enhance the capacity of the pitch-based anode, previous experimental studies have reported coating the surfaces of the anodic substrate with metal oxides, such as SnO, ZrO2 and Al2O3 [20], [21]. This coating increases the ion diffusivity of the overall anode. Kim et al. reported that boron-doped carbon prepared from a petroleum residue increased the initial efficiency and discharge capacity because boric acid promoted the thermal condensation and formation of a graphitic carbon structure [22]. Despite great advantages, these preparation processes are complicated, expensive and difficult to industrialize due to additional processes. To reduce preparation costs for enhancing the PFO-based anode, we have developed a cost-effective and simple approach.

We have selected cheap materials, such as pyrolysis fuel oil (PFO), which is a by-product of the naphtha cracking process. For commercial applications, it is important to develop a process for preparing green cokes with stable properties on a massive scale from PFO-based pitch. Pitch-based anode materials require additional thermal processing, such as the coking process that is produced by coke and carbonization. As the reaction proceeds, the pitch undergoes polymerization, domain growth and the formation of larger domains. The purpose of this study is to determine the optimal preparation conditions for the development of cost-effective anode materials using PFO-based pitch. The carbonaceous anode, which is primarily determined by a precursor and the preparation process (such as pyrolysis conditions), strongly influences the electrochemical properties of lithium-ion batteries.

Section snippets

Sample preparation

PFO (Yeochun NCC CO. LTD., produced by NCC (Naphtha Cracking Center, South Korea)) was used as the feedstock for the synthesis of the PFO-based pitch. The PFO-based pitch was heat-treated at 320–420 °C in a 5 L batch-type autoclave reactor under atmospheric conditions with a N2 flow for 1 to 3 h. Additionally the PFO-based pitch was prepared with softening points (S.Ps) at 80, 121, 148, 172 and 210 °C. The prepared samples were further denoted by S.P temperature, such as S.P 80, S.P 121, S.P 148,

Effects of pitch properties on the electrochemical properties of LIBs

Table 1 shows the PFO-based pitch yield, softening point (S.P) and elemental analysis results that were obtained at distillation temperatures ranging from 340 to 420 °C. The conditions for pitch synthesis were considered based on reaction temperature and time for controlling softening point. Generally, over 450 °C, the cokes are obtained instead of pitch because of heavy molecular crosslinking [23]. It is evident that higher distillation temperatures increased the softening point and decreased

Conclusions

The conditions for the preparation of PFO-based pitch were investigated to maximize the discharge capacity and carbonization yields. The carbon yield of the PFO-based anode may be increased with an intermediate heat treatment before the start of volatile loss. A high carbon yield is achieved with an intermediate heat treatment at 500 °C. The PFO-based anode electrochemical properties exhibit typical graphitizable carbon (soft carbon) characteristics. The optimal intermediate heat treatment

Acknowledgments

This work was supported by Korea Evaluation institute of Industrial Technology (KEIT) through the Carbon Cluster Construction project [10083621, Development of preparation technology in petroleum-based artificial graphite anode] funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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    Authors contributed equally to this work.

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