Effects of aliphatic side groups structures of different pitches on their mesophase transformation and the microstructure of needle cokes

The aliphatic side groups components in raw pitches have been considered to make some significant influence on the structure and properties of the prepared needle cokes. In order to investigate the influence of the different aliphatic side groups structures of raw pitch on the mesophase transformation process and the microstructure of needle cokes, three graphitized needle cokes (GNCs) were prepared from petroleum pitch (PP), coal tar pitch (CTP) and ethylene tar pitch (ETP), especially, via mesophase transformation, coking, calcination and graphitization processes. The molecuar structures of the three feedstocks and their derived mesophase pitches were analyzed by infrared spectrometer (IR), nuclear magnetic resonance spectrometer (NMR) and time-of-flight mass spectrometry (TOF-MS), and the microsrstalline structure evolutions during preparation of GNCs as well as the electrochemical properties of the three GNCs as lithium anode were investigated. Molecular structure analysis results shown that both PP and ETP possessed abundant aliphatic side groups. However, the substitued components in PP were mainly short CH₃ aliphatic groups and naphthenic structures while ETP had some longer substitued aliphatic side groups and more olefin components. Besides, CTP demeonstated the highest aromaticity degree with limitted aliphatic side groups content. Further molecular structure analysis results showed that these abundant CH₃ aliphatic groups consisted in PP effectively reduced the CH-π interaction of aromatic molecules and improved their stacking orientation of its masophase pitch, thus promoting its graphitization of the consequent needle coke. However, the olefin components and longer aliphatic chains of ETP reduced its mesophase transform efficiency and the molecular flatness of the mesophase pitch, then being not conducive to the crystal development during graphitization. Although containing lowest CH₃ aliphatic groups, CTP exhibited moderate mesophase transform efficiency and graphite crystal structure of their needle coke due to the strong π-π interaction of aromatic molecules. Consequently, the crystal size (L_c and L_a) of graphitized cokes shown a trend of PP-GNC>CTP-GNC>ETP-GNC while their graphite layer spacing (d₀₀₂) vales presented an opposite trend. As a result of its largest crystal size, PP-GNC has the highest first discharge capacity and reversible capacity as a lithium negative electrode of 358.89 and 356.89 mAh/g at current density of 0.05 A/g, respectively, while ETP-GNC demonstrated better cyclic perfomance at current density of 0.1 A/g and rate performance at current density at current density>1 A/g due to its larger graphite layer spacing and lower crystal orientation. These results indicate that PP-GNC is more suitable using in for high capacity density lithium anode material, while ETP-GNC may has better potential for use for lithium anode material with high power density or long cycle performance.