Evaluation of Molecular Structural Effects on Needle Coke Mesophase Stacking

Molecular simulations were performed to investigate the molecular structural effects on needle coke mesophase stacking.The simulation results showed that the stacking states of anthracene trimer and tetramer accumulations were orderly,while the stacking states of anthracene dimer,pentamer,and hexamer accumulations were disorderly.Anthracene trimer and tetramer in the model compounds were two of the most ideal needle coke mesophase constituents.It was also found that the introduction of methyl side chains in anthracene trimer derivatives was not conducive to the formation of a mesophase crystal.To sum up,the molecules which had similar structures to anthracene trimer or tetramer with no alkyl chains are ideal constituents of needle coke mesophase.

Boosting of reversible capacity delivered at a low voltage below 0.5 V in mildly expanded graphitized needle coke anode for a high-energy lithium ion battery

The rate performance and cycle stability of graphitized needle coke(GNC)as anode are still limited by the sluggish kinetics and volume expansion during the Li ions intercalation and de-intercalation process.Especially,the output of energy density for lithium ion batteries(LIBs)is directly affected by the delithiation capacity below 0.5 V.Here,the mildly expanded graphitized needle coke(MEGNC)with the enlarged interlayer spacing from 0.346 to 0.352 nm is obtained by the two-step mild oxidation intercalation modification.The voltage plateau of MEGNC anode below 0.5 V is obviously broadened as compared to the initial GNC anode,contributing to the enhancement of Li storage below the low voltage plateau.Moreover,the coin full cell and pouch full cell configured with MEGNC anode exhibit much enhanced Li storage ability,energy density and better cycling stability than those full cells configured with GNC and commercial graphite anodes,demonstrating the practical application value of MEGNC.The superior anode behaviors of MEGNC including the increased effective capacity at low voltage and superior cyclic stability are mainly benefited from the enlarged interlayer spacing,which not only accelerates the Li ions diffusion rate,but also effectively alleviates the volume expansion and fragmentation during the Li ions intercalation process.In addition,the above result is further confirmed by the density functional theory simulation.This work provides an effective modification strategy for the NC-based graphite to enhance the delithiation capacity at a low voltage plateau,dedicated to improving the energy density and durability of LIBs.

Intercalation mechanisms of lithium into graphitized needle cokes

A needle coke was graphitized at different heat treatment temperature (2 000℃ to 3 000℃). The electrochemical intercalation mechanism of Li into the graphitized coke has been studied in Li|1 mol·L 1 LiClO 4+ethylene carbonate/diethylene carbonate|graphite cells, using an in situ X Ray diffraction (XRD) technique.The study of Li C intercalation processes of the graphitized coke reveals that there are three major types of intercalation behavior.The first is uniformly intercalated at all Li C compounds in graphitized coke heated at 2 250℃;the second is obviously staging phenomenon during intercalation for the graphitized coke heated at 2 750℃; the third is cointercalation of solvated Li ion at high potential (>0.3V) and then lithium electrochemical intercalation at lower potential for that heated at 3 000℃, resulting in the decrease of capacity and efficiency of graphite negative electrode for lithium ion secondary battery.