Synthesis And Lithium-Storage Performance Of Lignin-Based Porous Carbon/Silica Composites

The porous carbon has been widely used in lithium ion batteries?LIBs?,supercapacitors and other energy storage fields owing to its abundant pores,high specific surface area,high conductivity and good stability.Lignin,a kind of renewable natural high polymers,is rich in aromatic rings and with a high carbon content of 50 to 60%,so it is the ideal precursor for the preparation of the porous carbon.However,carbonizing lignin directly will result in serious collapse and aggregation of its structure.As the anode material of LIBs,nano-silica?SiO₂?has the advantages of high specific capacity,high rate capability,stable structure and abundant reserves.But there is serious aggregation and volume expansion of nano-SiO₂ in the process of lithiation/delithiation,resulting in a rapid capacity fading.It was found that the combination of nano-SiO₂ and lignin-based carbon could not only inhibit the volume expansion and aggregation of nano-SiO₂,but also prepare lignin-based porous carbon?LPC?by using nano-SiO₂ as templates,which was expected to show high specific capacity,high rate capability and stable cycling performance to meet the demand of lithium ion power batteries.Therefore,the research about the preparation,lithium-storage performance and related mechanisms of lignin-based porous carbon/silica composites?LPC/SiO₂?had great theoretical significance and applicable value.In this paper,Using the industrial alkali lignin?AL?derived from pulping liquor as the raw material,and SiO₂ particles with a size of 30 nm as the active materials and templates,series of LPCs and LPC/SiO₂ were prepared by mixed solvent thermal reaction,carbonization and etching to remove the templates.Due to their unique composition and structural characteristics,the prepared LPC and LPC/SiO₂ both showed excellent lithium-storage performance.The main conclusions are as follows:?1?In the preparation of LPC/SiO₂ composites,the dispersion of nano-SiO₂ was the key to their performance.For this reason,the AL was modified to prepare the electropositive quaternized alkali lignin?QAL?in this paper,and then a series of quaternized alkali lignin/silica composites?QAL/SiO₂?with stable and uniform structure was prepared by ethanol-water mixed solvent thermal reaction with electronegative nano-SiO₂.SiO₂ particles with a size of about 30 nm were uniformly distributed in the three-dimensional lignin network.There was electrostatic interaction,hydrogen bonding interaction,Si-?interaction and a small amount of Si-O-C bond between nano-SiO₂ and QAL molecules.?2?Using the QAL/SiO₂ with different SiO₂ contents?10⁶0 wt%?as precursors,we successfully prepared series of LPCs with different pore structures and morphologies after carbonization and removal of all the SiO₂ templates.The BET surface area of LPCs ranged from 292 to 1107 m²·g^-11 and the pore volume ranged from 0.25 to 2.53 cm³·g^-1.The pores in all LPCs were uniform with diameter from 20 to 30 nm.With the increase of SiO₂ contents,the BET surface area and pore volume of LPCs increased firstly and then decreased.When the SiO₂ content of the precursor was 50 wt%,LPC-50 had the largest BET surface area(1107m²·g^-1)and the most abundant mesoporous structure(2.53 cm³·g^-1).When used as anode materials of LIBs,LPC-50 possessed the largest specific capacity and reached 521 mAh·g^-1after 100 cycles at a current density of 200 mA·g^-11 which was 1.4 times that of graphite anode materials.At a large current density of 1 A·g^-1,LPC-50 had a specific capacity of 289mAh·g^-1.EIS result showed that adding the SiO₂ templates enlarged the charge transfer resistance(R_ct)of LPCs and reduced their diffusion coefficients of lithium ions(D⁺_Li).?3?Using the QAL/SiO₂ composite with a fixed SiO₂ content of 50 wt%as the precursor and nano-SiO₂ as active materials and templates,we prepared LPC/SiO₂-39.5,LPC/SiO₂-20.9,LPC/SiO₂-9.9 with SiO₂ contents of 39.5 wt%?20.9 wt%?9.9 wt%respectively by controlled the etching time.TEM and SEM showed that LPC/SiO₂ had a uniform and abundant mesoporous with nano-SiO₂?about 30 nm?dispersed internally and uniformly.With the decrease of SiO₂ content,the BET surface area of LPC/SiO₂ increased from 559 to 887 m²·g^-1and the pore volume increased from 0.82 to 1.75 cm³·g^-1.Among all the LPC/SiO₂ composites,LPC/SiO₂-9.9 had the best cycling performance,which reached a specific capacity of 731mAh·g^-11 after 100 cycles at 100 mA·g^-1.At an utral large current density of 5 A·g^-1,the specific capacity of LPC/SiO₂-9.9 was 233 mAh·g^-11 after 1000 cycles.EIS result showed that compared with SiO₂,LPC/SiO₂ possessed the smaller R_ctt ranging from 90.4 to 317.5?and the larger D_Li⁺ranging from 2.53×10^-1616 to 5.02×10^-1515 cm²·s^-1.According to XPS result,the main lithium-storage mechanism of SiO₂ in LPC/SiO₂ was that SiO₂ firstly transformed with Li⁺irreversibly into Li₂O,Li₄SiO₄ and Si,and then the alloying reaction between Si and Li⁺occurred reversibly.It provided a new development direction for the preparation of anode materials of LIBs with high specific capacity,high rate capability,stable cycling performance,low cost and environmental protection.