Pre-baked anode based on petroleum coke used as lithium-ion battery anode material

Non-calcined petroleum coke can serve as an anode material for lithium-ion batteries (LIBs). Nevertheless, this method results in materials with insufficient conductivities and low Coulombic efficiencies during the initial cycle. To address these challenges, the usage of pre-baked carbon anodes as a material for anodes in LIBs is proposed in this study. The surface features of the pre-baked anode (i.e. wrinkle-like filaments) aid in reducing the volume expansion of the electrode during the lithium-ion insertion–removal process. Furthermore, the treatment increases the particle contact area, improving the conductivity of the pre-baked anode. At a current density of 3 A g−1, the pre-baked anode demonstrated an initial discharge capacity and a stable discharge capacity of 548.7 and 134.5 mAh g−1, respectively, after 100 cycles. The capacity of the anode (after 1000 cycles) consistently varies within a narrow range at a current density of 3 A g−1, indicating the stability of the electrode capacity over extended use. Therefore, this study provides valuable insights into exploring potential applications of pre-baked anode materials.


Introduction
With the rapid social and economic development, the use of traditional energy sources, such as fossil fuels, has resulted in escalating environmental problems [1,2].Pollution poses a momentous threat to humankind as well as the environment and interferes with production and living activities [3,4].Currently, the comprehensive and efficient use of petroleum by-products is receiving increasing attention.Since 1990, lithium-ion batteries (LIBs) have been recognised as a highly promising and reliable energy storage devices, playing a leading role in various fields such as portable electronics, smart grids and hybrid electric vehicles [5][6][7].For almost 30 years, there has been rapid progress in the materials and production technologies used for the preparation of LIBs [8,9].
The electrochemical performance of LIBs is closely linked to the structure and performance of their anode materials [10][11][12].Currently, carbon materials are the preferred anode materials for LIBs owing to their high cycle efficiency, long cycle life and solid safety performance.Because of the limited lithium resources, alternative battery technologies, such as sodium-ion batteries, potassium-ion batteries (PIBs) and lithium-selenium batteries (LSeBs), are being examined as substitutes for LIBs.
Various carbon materials have been used in several secondary batteries.For example, biomass carbon materials have been employed as negative electrode materials in PIBs.In addition, shredded stalkless puffball biomass sponge has been used as a carbon source in a hydrothermal reaction, in which doping was controlled to create an electrode material with a high specific capacity, stable long cycle performance and excellent rate performance.This novel carbon material has the potential to become a cost-effective PIBs electrode material that can be used for large-scale production [13].Similarly, polyacrylonitrile are carbon materials used as highperformance flexible energy storage materials for PIBs.A new approach involving enhancing the electrode flexibility by adjusting the porosity of carbon nanofibers has been proposed for developing flexible materials [14].For example, a three-dimensional porous carbon has been used as a Se host in LSeBs.N and Co increase the electrical conductivity of the porous carbon and exhibit an adsorption effect on polyselenide.The aforementioned Co-doped porous carbon may be used to construct other metal-doped carbon materials engineered for multiple applications [15].These examples show that numerous carbon materials are employed in several batteries, demonstrating the importance of carbon materials in the domain of energy storage.
Petroleum coke is a by-product of petroleum refining.It comprises more than 80% carbon, 1.5%-8% hydrogen and some amounts of oxygen, nitrogen, sulphur and metallic elements [16].Petroleum coke can be easily graphitised because it has a micro-crystalline lattice constant similar to that of natural graphite.Compared to natural graphite, the resistivity of petroleum coke after graphitising considerably decreases and its true density increases.These characteristics makes petroleum coke a good candidate for producing graphite electrodes with low resistivities [17,18].Pre-baked anodes made from premium petroleum coke with low sulphur, ash and volatile contents are suitable as anode materials in the electrolytic aluminium sector and as anode materials for LIBs.This increases the value of petroleum coke [19,20] and ensures its safety to a certain extent by securing the supply chain of raw materials amid the considerable increase in the demand for LIB anode materials.
Herein, we propose a pre-baked material as an anode material for LIBs.Currently, there is insufficient research in this area.Thus, the objective of this work is to facilitate the use of pre-baked anode materials in LIBs.

Preparation of pre-baked anode
First, petroleum coke was crushed and grounded to reduce its size to 50-70 mm.The petroleum coke particles that met the standard were placed in a rotary kiln and calcined at a high temperature (1250 °C-1350 °C) under air atmosphere.The asphalt coke and calcined petroleum coke particles underwent multi-stage crushing and screening.Second, they were mixed according to a specific ratio and sent for the preheating treatment in the preheating spiral.A specific quantity of molten coal tar was then added, and the raw material was moulded and shaped to obtain a raw-material block.Finally, the raw-material blocks were placed into the roasting chamber and covered with coke powder to prevent their oxidation during the high-temperature roasting process.The heating system operated under a negative pressure, and the raw-material blocks were gradually heated to 1000 °C-1050 °C.After reaching this temperature, it was maintained for 30 h.The blocks were then naturally cooled to obtain the pre-baked anode products.

Material characterization
A scanning electron microscope (Phenom Pro, Funa Scientific Instrument (Shanghai) Co., LTD) was used to characterize the morphology of the samples.The structure of the as-prepared samples was characterized by x-ray diffraction (XRD, Rigaku Rint-2000 Advance Powder x-ray Diffractometer) and Raman spectroscopy was measured using a Raman spectrometer (Nicolet 5700, Thermo Fisher Technology (China) Co., LTD).The nitrogen adsorption-desorption measurements undertaken by Micromeritics, ASAP 2460 were used to ascertain the specific surface area and pore size distribution of the materials.

Preparation of LIBs anode materials
The conductive carbon black (Super P), the binder (sodium hydroxymethyl cellulose (CMC) : styrene butadiene rubber (SBR) = 1:1) and the pre-baked anode powder sample as the active substance were dissolved in water in the ratio of 8:1:1.After full stirring, the appropriate electrode material slurry could be obtained.The fully agitated slurry was applied evenly onto the surface of the copper foil using a 100 mm scraper.The coated copper foil was placed into a clean tray, and then the tray was put into a vacuum drying oven for 30 min at 90 °C.The copper foil, already completely dried, underwent sectioning into 12 mm diameter rounds through the employment of a button battery slicing machine, with the active substance mass load standing at approximately 1.2 mg cm −2 .Several electrode sample sheets that had been cut and shaped were placed in a vacuum drying oven for 12 h at 90 °C.Subsequently, the dried electrode sample sheets were promptly transferred to the glove box after being weighed.

Electrochemical measurements
The button cells discussed in this article are packaged in glove boxes containing high-purity Argon gas.The concentration of oxygen and water within the glove boxes is less than 0.01 ppm.The prepared electrode is used as the working electrode, with lithium foil as the opposing electrode and a glass fiber diaphragm.The electrolyte consists of a 1 M LiPF 6 solution mixed with ethylene carbonate and dimethyl carbonate in a 1:1 volume ratio.After packaging, the button battery still needs to be stored in an Ar atmosphere in the glove compartment for a certain duration to allow the electrolyte to penetrate each component thoroughly.The electrochemical performances were tested using the LAND CT2001A multi-channel battery test system (0.01-3.0 V) at a temperature of 25 °C.Cyclic voltammetry (CV) tests ranging from 0.01 to 3.0 V were conducted using CHI 760E at 25 °C.

Results and discussion
The production of pre-baked anodes was a simple procedure (figure 1) [21,22].Petroleum coke and asphalt coke were used as aggregates, and coal pitch was used as a binder to form a mixture that was then heated to obtain prebaked anode products with high mechanical strength and oxidation resistance as well as low resistivity.The morphology of the resultant petroleum coke and pre-baked anodes was analysed using scanning electron microscopy (SEM).The SEM results showed that the used petroleum coke exhibits amorphous morphology and mostly consists of solid particles with varying sizes and irregular shapes (figures 2(a) and (b)).This is predominantly affected by the product quality during the coking process and depended on the properties of the crude oil [23].Figures 2(c) and (d) present the SEM image of the pre-baked anode, showing its amorphous morphology.The boundary between the independent particles becomes indistinct, and there are no gaps in the contact area between particles.The pre-baked anode exhibits a multi-layered structure, of which the surface has numerous thin, filamentous folds, creating an overall semi-enclosed core-shell structure [24].The surface of the pre-baked anode exhibits folds without any scattered particles.This unique structure allows the surface to contain numerous voids.This structure allows for an uninterrupted transfer of charge and exchange of lithium ions, ensuring excellent electrochemical performance [25].
Figure 3(a) presents the Raman spectra of the petroleum coke and pre-baked anodes, showing typical carbon material spectra.The characteristic peaks (G bands) of petroleum coke and pre-baked anodes were observed at approximately 1594 and 1577 cm −1 , respectively, which correspond to the stretching of the sp 2 atom in a carbon ring or a long chain.The peaks observed at 1332 and 1330 cm −1 represent the D bands of the petroleum coke and pre-baked anodes, respectively.These bands are primarily affected by boundaries, defects and disordered carbon.The intensity ratio of the D and G bands (I D /I G ratio) is a vital criterion that helps in determining the crystallinity of carbon materials.The I D /I G ratios of the petroleum coke and pre-baked anode are 0.87 and 0.67, respectively.Compared to the pre-baked anode, the intensities of the D and G bands are higher.This suggests that the structure of petroleum coke contains more boundaries and defects compared to that of the pre-baked anode, causing periodic destruction of the material lattice.The rapid change in the electric field leads to a strong electron scattering and high resistivity of the materials [26,27].
X-ray diffraction analysis was conducted to examine the crystal structures of the petroleum coke and prebaked anode.Figure 3(b) illustrates that both materials comprises amorphous carbon.The pre-baked anode exhibits two prominent diffraction peaks at 26°and 43°, representing the (002) and (100) crystal faces of disordered carbon, respectively.The strength of the diffraction peak of the (002) crystal plane of the pre-baked anode is higher and its half width is lower than those of the petroleum coke.This suggests that an improvement in the crystallinity, purity and material order of the pre-baked anode compared to the petroleum coke.
We analysed the specific surface area and pore size of the petroleum coke and pre-baked anode using nitrogen adsorption-desorption isotherms.The Brunauer-Emmett-Teller (BET) model (figure S1) shows that the specific surface area of petroleum coke is 0.1649 m 2 g −1 .The Barrett-Joyner-Halenda desorption average pore diameter was calculated based on the obtained results to be 38.789nm.In figure 4(a), the isotherm shows a classic type-IV nitrogen adsorption branch with an adsorption hysteresis loop manifesting in the middle section, which is indicative of the capillary condensation occurring in porous adsorbents.The adsorption-desorption isotherm hysteresis loop closure are divided into type H3, indicating the presence of numerous slit apertures in the material structure.The BET model shows that the pre-baked anode has a large surface area of 3.4348 m 2 g −1 .
As shown in figure 4(b), the pre-baked anode exhibits a wide pore sizes distribution ranging from 1 to 10 nm with a desorption average pore diameter of 6.9193 nm.A higher specific surface area results in an increased number of electrochemical active sites for lithium-ion insertion-extraction, increasing the capacity of the material.The numerous pores in the pre-baked anode create additional paths for lithium-ion diffusion and promote electron transport.The electrochemical performances of the petroleum coke and pre-baked anode as LIB anodes were tested.Figure 5(a) shows the cycling performance of petroleum coke at 3 A g −1 .The initial discharge capacity of the petroleum coke electrode is 79.8 mAh g −1 .The capacity of LIBs generally increases over the first 1000 chargedischarge cycles owing to the activation of the electrode materials [28].During the cycling process, the electrode capacity of the petroleum coke shows a stepwise increase.Similarly, the capacity of the pre-baked anode also increases.However, its capacity fluctuates within a certain range.This phenomenon is consistent with the following results of lithium-ion diffusion coefficient after several cycles [29].This study primarily investigates the performance of the pre-baked anode as an electrode material of lithium-ion battery.To further test the stability of the pre-baked anode during the charge-discharge process, long cycles were tested at 3 A g −1 .As shown in figure 5(c), even after 1000 cycles, the capacity of the pre-baked anode is still 132.5 mAh g −1 , indicating a high capacity retention, good cycling performance and structural stability.Moreover, the capacity of the LIB with the pre-baked anode increases owing to the activation of the electrode material [30,31].Over 1000 chargedischarge cycles, the capacity of the electrode material fluctuates within a small range, showing a good overall stability.In this regard, the performance of the pre-baked anode is slightly better than that of the petroleum coke  electrode.The Coulombic efficiency of the pre-baked anode is very stable (99.8%) over 1000 charge-discharge cycles, indicating the good cycle stability and high Coulombic efficiency of the pre-baked anode at high current densities.
The rate performance of the petroleum coke electrode at 0.1-5 A g −1 was evaluated.The specific capacity of the petroleum coke electrode considerably decreases with the increase in the current density.As shown in figure 5(b), the rate performance of the petroleum coke electrode is not good.Similarly, the specific capacity of the pre-baked anode decreases with the increase in the current density (figure 5(d)).However, at low current densities, the capacity of the pre-baked anode exhibits a lower decrease during two cycles (i.e. the capacity fluctuation decreases and the stability is improved).When the current density returns to 0.1 Ag −1 , the pre-baked anode can still exhibit a good capacity and good capacity retention (figure 5(d)), demonstrating a good rate tolerance of the pre-baked anode.At low current densities, the rate capacity of the pre-baked anode is lower than that of the petroleum coke (figures 5(b) and (d)), which is due to the difference in the electrode porosity.The porosity of the electrode reduces the capacity of the electrode at low rates but can improve the capacity of the electrode at high rates.The porosity of the pre-baked anode is higher than that of the petroleum coke electrode; thus, the rate capacity of the pre-baked anode is lower than that of the petroleum coke electrode at low current densities [32].
Figure 6(a) shows the charge-discharge profiles of the petroleum coke electrode over the initial three cycles at 3 A g −1 .The initial discharge cycle capacity, charge capacity and initial Coulombic efficiency (ICE) of the petroleum coke electrode are up to 1470 mAh g −1 , 680 mAh g −1 and 46.3%, respectively.The loss in the initial capacity is primarily due to the formation of a solid electrolyte interface (SEI) film and the occurrence of a side reaction during the first cycle.During the initial 1-3 cycles, the discharge stability of the petroleum coke electrode was poor in the initial stage; thus, the petroleum coke electrode exhibited poor reversibility and stability in the initial cycle stage.However, the charge-discharge curves of the 10th and 100th charge-discharge cycle, almost overlap, and the capacity of the petroleum coke electrode considerably decreases, showing a discharge cycle capacity of 95 mAh g −1 .Figure 6(b) shows the charge-discharge curve of the pre-baked anode with a voltage range of 0.01-3 V and a current density of 3 A g −1 .The discharge and charge specific capacities of the pre-baked anode are 548.7 and 375.4 mAh g −1 , respectively, with its ICE being 68.4%, of which 31.3% is the irreversible capacity loss due to the decomposition of the SEI layer, electrolyte and formation of Li 2 O [33].The ICE of the pre-baked anode is considerably higher than that of the petroleum coke.This can be attributed to the fact that the amorphous carbon can form a stable SEI layer during the charge and discharge cycle, which can effectively reduce the transport resistance of lithium ions and prevent the formation of a new SEI layer during the cycle to improve the Coulombic efficiency of the pre-baked anode [34].
To investigate the lithium-storage mechanism, cyclic voltammetry (CV) curves of the petroleum coke electrode and pre-baked anode were obtained at 0.01-3 V and a scan rate of 0.1 mV S −1 in the first four cycles (figures 7(a) and (b)).During the first cycle of the petroleum coke electrode, two distinct cathodic peaks were observed.The peaks at the vicinity of 1.5 V are clear, whereas those near 0.9 V are tiny.During the initial negative (lithiation) scan, an irreversible peak is obtained at approximately 1.5 V.This may be attributed to the decomposition of the electrolytes and the formation of a solid electrolyte interphase as well as some electrode conversion reactions [35,36].The CV curves in the second, third and fourth cycles show an inadequate overlap, indicating that their cyclic stability and reversibility are less than those of the pre-baked anodes.As shown in As illustrated in figure 7(b), the curves show multiple anodic and cathodic peaks associated with lithium-ion insertion-extraction.A strong well-defined cathodic peak is observed at approximately 0.75 V, which can be attributed to the electrolyte decomposition and formation of a solid electrolyte interphase as well as some conversion reactions of the electrode [37].The CV curves of the pre-baked anode overlap after the first cycle, demonstrating the stability of the charge-discharge process.To further explain the excellent rate capability of the pre-baked anode, CV analysis of the pre-baked anode was performed at different scanning rates and its pseudocapacitive contribution was analysed (figure 7(d)).Similar to petroleum coke, the CV curves show similar shapes within 0.1-3 mV S −1 and the peak current increases with the scan rate.Interestingly, the linear relationship between the peak current and scan rate of the pre-baked anode was better than that of the petroleum coke electrode, confirming the superior properties of the pre-baked anode [38].
The performance of the pre-baked anode is generally better than that of the petroleum coke electrode, mainly because the surface of the pre-baked anode material has numerous filamentous folds, which increase the ion transport channel, weaken the effect of volume expansion of the electrode to a certain extent, show better electrochemical performance and simultaneously ensure that the charge transfer and lithium-ion exchange are not hindered.This ensures good electrochemical performance, especially at high magnification.The unique semi-wrapped structure can increase the activated area, which results in the regulation of the lithium-ion flux during the cycle, a reduction in the local current density applied to the negative electrode and a reduction in the potential during nucleation, ultimately achieving a stable cycle that reduces dendrite formation.

Conclusion
In summary, the pre-baked anode was obtained through an extensive processing of a petroleum coke material.After a series of treatments, the pre-baked LIB anode material was successfully prepared.At a current density of 0.1 A g −1 , the capacity of the pre-baked anode increases instead of decreasing after multiple cycles, clearly showing that the cycle stability of the anode material is better than that of the petroleum coke electrode.After 100 charge-discharge cycles at a current density of 3 A g −1 , the pre-baked anode still exhibits a higher discharge capacity than the petroleum coke electrode (134.5 mAh g −1 ), which is attributed to the special thin silky folds on the surface of the pre-baked anode.This structure reduces the diffusion path of lithium ions and renders it adaptable to the volume change during cycling.Thus, the pre-baked anode has a good application potential in LIBs in addition to its application as an electrolytic cell anode in the electrolytic aluminium industry.

Figure 1 .
Figure 1.Schematic illustration of the preparation process for pre-baked carbon anode.

Figure 3 .
Figure 3. (a) Raman spectra of petroleum coke and pre-baked anode; (b) XRD patterns of the petroleum coke and pre-baked anode.

Figure 5 .
Figure 5. (a) Cyclic performance of petroleum coke electrode at 3 A g −1 ; (b) Rate performance curves at different current densities.(c) Cyclic performance of pre-baked anode electrode at 3 A g −1 ; (d) Rate performance curves at different current densities.

Figure 7 .
Figure 7. Cyclic voltammogram curves at 0.1 mV S −1 in the voltage range of 0.01-3.00V for (a) petroleum coke and (b) pre-baked anode.CV curves under various scan rates from 0.1 to 3.0 mV S −1 for (c) petroleum coke and (d) pre-baked anode.
2.1.Preparation of the materialsPetroleum coke was acquired from the Qilu Branch of Sinopec and the Jinan Branch of Sinopec, while coal pitch was obtained from Xingtai Xuyang Coal Chemical Co., LTD and Henan Baoshun New Energy Co., Ltd.Conductive carbon black Super P was purchased from Tianjin Youmeng Chemical Technology Co., Ltd.Carboxymethylcellulose sodium (C 8 H 16 NaO 8 , AR) and polymerized styrene butadiene rubber (C 12 H 14 , AR) were obtained from Nanjing Mojisi Energy Technology Co., Ltd.All agents were used as received.