Low Temperature Pyrolysis Carbonization Of Lignocellulose And Its Lithium Storage Properties

Lithium-ion batteries stand out among energy storage devices due to their high energy density,long cycle life,no memory effect and many other advantages,and occupy a dominant position in the field of energy storage.At present,commercial lithium batteries mainly use graphite as anode material,but its theoretical capacity is low(370 mAh g^-1),which is difficult to meet people's demand for high-capacity lithium ion batteries.Although many inorganic materials,such as silicon-based,tin-based non-metallic and metal oxide anode materials,provide higher capacity,these materials have large volume expansion,resulting in poor cycling performance.In addition,the excessive use of some heavy metal oxides may cause serious environmental problems.A class of organic materials with reversible REDOX structure,such as organic sulfide,free radical compounds,conjugated carbonyl compounds,etc.,can also be used as anode materials for lithium-ion batteries because of their superlithization characteristics and high theoretical specific capacity.However,due to its poor electrical conductivity,it is difficult to give full play to its superlithization potential.Is it possible to develop an organic material with both conductive and superlithized characteristics as the anode material for lithium-ion batteries?Lignocellulose is a renewable material with abundant sources and low cost,which is widely used in the preparation of electrode materials for energy storage batteries.This paper mainly carried out the study of pyrolytic carbonization and its lithium storage performance at low and medium temperatures,including(1)the chemical and crystalline structure transformation and electrical conductivity of microcrystalline cellulose carbonization at a low temperature of 250-500?;(2)the crystalline structure transformation and electrical conductivity of wood and bamboo biomass materials at a medium temperature of 600-1000?.Compared with polyacrylonitrile(PAN),(3)the lithium storage performance of carbonized microcrystalline cellulose and its activated materials at low temperature was studied.The detailed content of the paper is as follows:(1)Microcrystalline cellulose(MCC),an important component of lignocellulosic materials,was used as the precursor material,and its structural transformation and electrical properties were investigated with the change of carbonization temperature and carbonization time by pyrolysis carbonization at a low temperature of250-500?.Results show that the MCC derived carbon with the increase of carbonization temperature and coking time,although no big change of surface topography,but its apparent color has experienced a white to pale yellow-brown-brown-red wine-black color change,after the main pyrolysis interval of MCC beta1,4 glycosidic bond rupture,almost completely this suggests that the cellulose is fully decomposed.The carbonyl group(C=O)vibration peak appeared at 250?for 8 h.With the increase of carbonization temperature and carbonization time,the C/O ratio in the MCC derived carbon also gradually increases,and the MCC changes from the original I-type long-range ordered crystal structure to the disordered stratified structure.According to the electrochemical impedance test,with the increase of carbonization temperature and carbonization time,the conductivity of MCC derived carbon increased gradually.(2)To continue to explore the changes of microstructure and electrical properties of microcrystalline cellulose by carbonization at medium temperature(600?1000?),and to derive lignocellulosic biomass materials,such as wood,bamboo,etc.,to prepare lignocellulosic derived carbon by direct carbonization at a temperature gradient of 600?1000?.To explore the contribution of cellulose carbonization to the carbonization of lignocellulosic biomass materials.The lignocellulose-derived char has abundant pore structure,and the obtained char is amorphous.In the same conditions of carbonized PAN material for comparison,the electrical conductivity of four point probe method is used to test the material,the test results show that under the same carbonization temperature,the electrical conductivity of lignocellulose materials from PAN carbon than higher,and the type of the lignocellulose materials its conductivity also has certain differences,it is associated with the content of the material composition.(3)Using microcrystalline cellulose as raw material and KHCO₃ as activator,cellulose-derived porous carbon materials with excellent morphology were prepared by direct carbonization and chemical activation methods,respectively.In the direct carbonization materials,500-4h materials show excellent performance of lithium storage.The reversible specific capacity can be maintained at 425 mAh g^-1 after 500cycles at the current density of 0.1 A g^-1 and 102 mAh g^-1 even at the current density of 2 A g^-1 when the battery is assembled with 500-4h material as anode material.However,the samples activated by KHCO₃ all have rich pore structures.Although the specific surface area of these materials is not high,they all show an amorphous shape and the internal defect structure increases.Electrochemical performance test results show that the optimal performance after activation is at 400?carbonization for 3 h,MCC:KHCO₃ is 1:4.At the current density of 0.1A g^-1,the reversible specific capacity of cellulose-derived carbon can reach up to 1045 mAh g^-1,which is 181%higher than the theoretical specific capacity of graphite.This result is not only due to the contribution of abundant pore structure.In addition,the oxygen-containing functional groups such as C=O,O-C=O and unsaturated bonds on the surface of cellulose carbon contribute extra capacity in the process of charge and discharge,thus realizing the superlithization.However,the capacity of the activated materials increases gradually and then decreases with the cycle.Among them,the capacity keeps increasing with the cycle mainly because the lithium ion keeps diffusing with the reciprocating process of charge and discharge,which leads to the gradual activation of the active material,so the active sites inside the material are gradually exposed.The subsequent capacity decline is due to the partial pulverization of the active material and the quasi-reversibility of the redox reaction of oxygen-containing functional groups in the material as the cycle proceeds,which leads to the capacity decline of the electrode.