Biographite Synthesized By Iron-Catalytic Graphitization Using Hydrothermal Carbonization Pretreatment for Lithium Ion Battery Anode
Authors: Nakayasu Yuta , Perianayagam Preethi , Imaizumi Futa , Itoh Takashi , Watanabe Masaru 
Published: December 2024

Graphite is commonly employed as the negative electrode material for lithium-ion batteries (LIBs), facilitating the intercalation and deintercalation of Li+ ions during the charge and discharge processes. However, both natural graphite (NG) and artificial graphite (AG) exhibit certain limitations. NG necessitates drilling to obtain fixed carbon and is geographically constrained, while AG requires high-temperature processing (>2500°C) of petroleum pitch-derived sources, resulting in substantial energy consumption [1]. In recent years, there has been a burgeoning interest in harnessing biomass resources for the synthesis of carbon materials as LIB anodes due to their abundance, cost-effectiveness, and recyclability. In this investigation, we successfully obtained highly crystalline carbon by subjecting Fe-impregnated hydrothermal precursors derived from woody biomass sources to heat treatment at 850°C [2]. Furthermore, we quantitatively assessed the lignin content in Fe-impregnated Japanese cedar sawdust under diverse hydrothermal treatment conditions and characterized the structural properties of the carbonized precursors synthesized at 850°C under varying hydrothermal treatment conditions.
To prepare the samples, Japanese cedar sawdust and an iron nitrate solution were placed in an autoclave, subjected to overnight impregnation, and subsequently subjected to hydrothermal treatment at temperatures ranging from 190°C to 280°C for durations of 4 to 24 hours, with a stirring rate of 300 rpm. To analyze the lignin content, the hydrothermally carbonized precursors were dried, washed with HCl and ultrapure water, dried again, and subjected to the Klason method [3]. The structural characteristics of the dried samples were analyzed using X-ray diffraction (XRD), while the acid-soluble lignin in the filtrate was quantified using UV-vis spectroscopy. Each hydrothermally carbonized precursor was subsequently calcined at temperatures between 850°C and 1000°C for a duration of 1 hour under a nitrogen atmosphere. The calcined samples were then subjected to sequential washing with 2 mol/L HCl, ultrapure water, and subsequent drying. The dried samples, denoted as GC_x_y (with x and y representing the hydrothermal reaction temperature and time), were structurally characterized using XRD.
From the XRD analysis, a broad peak around 22°, attributed to lignin [4], was observed in all hydrothermally carbonized precursors due to the removal of hemicellulose and cellulose during the Klason method. Additionally, the UV spectra of each filtrate sample revealed the presence of a peak derived from lignin at approximately 300 nm. Notably, the sample treated hydrothermally at 190°C for 4 hours exhibited the highest peak intensity. The yield of acid-soluble lignin was calculated from the UV spectra, with the hydrothermally treated precursor at 190°C for 4 hours demonstrating a higher yield of acid-soluble lignin, indicating a relatively less advanced lignin degradation reaction during hydrothermal treatment. Based on the XRD results obtained for each calcined sample, well-defined peaks in the (002) plane, indicative of a highly developed crystal structure resembling graphite, were observed in the samples subjected to hydrothermal treatment at temperatures between 220°C and 280°C for a duration of 4 hours, as well as at 190°C for a duration of 24 hours. This phenomenon can be attributed to the removal of hydroxyl groups from cellulose and lignin in sawdust during hydrothermal treatment, leading to enhanced carbonization [4]. However, no peak on the (002) plane was observed in the sample treated hydrothermally at 190°C for 4 hours, indicating lower crystallinity. This can be attributed to the higher lignin content in the hydrothermally carbonized precursor [5,6]. The interlayer distance of the crystalline carbon (d002) and the crystal size (Lc) were obtained from the XRD analysis. The calcinated sample GC_190_24 exhibited a d002 value closest to that of NG, while GC_250_4 displayed the highest Lc value. In comparison to previous studies, the d002 and Lc values of the samples in this investigation were relatively similar to those of natural graphite, suggesting that the decomposition of lignin through hydrothermal pretreatment is crucial for the synthesis of highly crystalline carbon. The presentation will also discuss the anode characteristics of LIBs.
References:
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