Study On Preparation And Electrochemical Performance Of Graphene-based Carbon/Silicon Composites

Lithium-ion batteries are one of the most promising energy storage devices of electric vehicles (EVs) or hybrid electric vehicles (HEVs) owing to their relatively high energy density, good cycle life and high power performance. Among all anode materials in lithium-ion batteries, silicon has the highest theoretical capacity (4200 mAh·g-1 for Li22Si5 at high temperatures or 3579 mAh·g-1 for Li15Si4 at room temperatures) and a low working potential (< 0.5 V vs Li+/Li). However, silicon has low electrical conductivity. Moreover, the large volume change (> 300%) during lithiation/delithiation leads to high internal stress, electrode pulverization and subsequent loss of electrical contact between the active material and current collector, resulting in poor reversibility. The solid-electrolyte interphase (SEI) of silicon anode will rupture due to the volume change during cycling, leading to continual formation of very thick SEI films. The excessive growth of SEI causes low coulombic efficiency, higher resistance to inoic transport, and it will eventually result in rapidly declining capacity. Recent work has shown that the electrochemical performances of silicon-based anode can be improved by reducing the Si particles size to the nanoscales, preparing nano-structured Si materials, or mixing Si into various metal or carbonaceous materials. Herein, we develop a facile carbon-thermal method to prepare various graphene-based carbon/silicon composites. The composition and structure of various composites are characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Raman spectrum, X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmer-Teller (BET) surface area measurement. The electrochemical performances of various composites anode are characterized by galvanostatic charge-discharge, cyclic votammetry (CV) and impedance measurements. The thesis is consisted of the following parts:(1) N-doped graphene-based carbon materials (G) are prepared from a liquid acrylonitrile homopolymer (LPAN) by carbon-thermal method. LPAN are cured in air at low temperature (120℃) and intermediate temperature (220℃). In this stage, LPAN linear molecular gradually converts to a mesh ladder polymer with high molecular weight by intra-molecular and intermolecular cross linking. XPS scans indicate the existence of C, O and N in graphene-based carbon materials, and most of carbon atoms are assigned to sp2 C species. N atoms are bonded to carbon atoms and in π conjugated system. Pyridinic N and pyrrolic N are dominant in the N-doped graphene-based carbon, which are favorable for facilitatitng the electronic conductivity of carbon layer and the charge transfer at the interface in Li-ion batteries. Graphene-based carbon materials display generally layered structures. With the carbonization temperature increasing, the surface of graphene-based carbon materials becomes more regularly and more graphitization. The electrochemical performance of the graphene-based carbon materials is evaluated using galvanostatic charge-discharge cycles. Different from graphite, there are no obvious voltage plateaus in the charge/discharge curves of graphene-based carbon materials, that similar behavior is observed for graphene. With the carbonization temperature increasing, the initial coloumbic efficiency and cycle stabiliy increase, while the initial charge and discharge capacities decrease.(2) A facial one-step carbon-thermal method is employed to coat Si nanoparticles with nitrogen-doped (N-doped) graphene-based carbon (G-Si) derived from LPAN precursor. When the temperature rises to 1300℃, Si and LPAN react with each other and silicon carbide (SiC) are found in the composites While for the mixture of graphite and Si, SiC start to form in the composites from 900℃. G-Si composites are produced at 800℃ for 5 h. The Si content in the G-Si composites, determined by TG in air, was calculated to be 43 wt%,29 wt% and 20 wt% for Si:LPAN=1:6,1:10,1:14, respectively. The corresponding G-Si composites are referred to as G-43%Si, G-29%Si and G-20%Si. The thickness of the graphenen-based carbon layer of the G-43%Si composite is about 3-4 nm, while the thickness of the layers in the other two composites are 5-8 nm.(3) It is clear that the particle size of Si is at the nanometer scale and the Si nanoparticles are homogeneously encapsulated by uniform N-doped graphene-based carbon in G-Si composites. There are many pores in the G-Si composites which can provide void space to accommodate the volume change of Si during discharge-charge cycles. The electrochemical performance of the G-Si composite as anode for lithium-ion batteries is evaluated using galvaostatic charge-discharge cycles at 200 mA·g-1 between 0.01 and 2 V. G-Si composite anode displays an initial coulombic efficiency of 82%, which is about three times greater than its pristine counterpart. The initial discharge capacity is 1319 mAh·g-1 and the capacity is 819 mAh·g-1 after 50 cycles for G-29% Si compostie, while the capacity decreases to 478 mAh·g-1 after 30 cycles for the G-43% Si composite. After 50 electrochemical cycles, the Si nanoparticles are uniformly distributed on graphene-based carbon networks in the G-29% Si and G-20% Si composites, while some of the Si nanoparticles are not coated by graphene-based carbon in G-43% Si composite. CV curves of G-Si composites are carried out at a scan rate of 0.1 mV·s-1 in the range of 0.01-1V. A broad peak appeared at potential of 0.5-0.9 V in first negatively going sweep process corresponds to the formation SEI. Two distinctive reduction peaks at 0.4 and 0.5 V are assigned to the reversible reaction of Li with Si. The excellent cycle performance and the extremely high initial coulombic efficiency of the G-Si composties are mainly attributed to the following reasons: firstly, the graphene-based carbon provide the electrode with more flexibility as well as electrical contact, and protect the electrode from cracking; secondly, the graphene-based carbon can prevent agglomeration of the Si nanoparticles and accommodate the volume change of Si particles effectively due to the sheet structure and good mechanical properties; thirdly, the G-Si composites provide void space, which is formed during carbonization, to accommodate the volume change of Si during discharge-charge cycling.(4) A sacrificial template method is used to conformally coat Si nanoparticles with a SiO2 sacrificial layer and then with a LPAN layer, which is subsequently carbonized to form nitrogen-doped graphene-based carbon coating. After removing the SiO2 sacrificial layer by hydrofluoric acid (HF) treatment, the G-void-Si composite is obtained. Preparation conditions of G-void-Si composites are investigated, including amount of TEOS, type of carbon precursor and amount of LPAN. The average thickness of the SiO2 coating layers are tuned by altering the mass ratio of TEOS to Si in the solution. When the mass ratio of TEOS:Si is higher than 10:1, the volume ratio of the void space to Si nanoparticles in the obtained composite is 3:1 or more. The SiO2 layer functions as a sacrificial template for generating void space to buffer the volume expansion of the silicon. The average void space between the Si cores and the hollow shells increases with TEOS amount, while the capacity of G-void-Si decreases with the TEOS amount. C-void-Si composite is obtained by using dopamine as a precursor, and G-void-Si composite is produced by using LPAN as a precursor. Si nanoparticles are well encapsulated in hollow carbon in C-void-Si composite, while Si nanoparticles are well coated by hollow graphene-based carbon in G-void-Si composite. The composites as anode for lithium-ion batteries are evaluated using galvaostatic charge-discharge cycles at 200 mA·g-1 between 0.01 and 1 V. The composites show initial charge and discharge capacities of 1497.3 and 1535.3 mAh·g-1 for G-void-Si and 689.3 and 700.9 mAh·g-1 for C-void-Si, giving a capacity retention of 66.7% and 45% after 100 cycles respectively. The more amount of LPAN is added, the better cycling performance G-void-Si composite displays, however the lower capacity G-void-Si composite exhibits.(5) G-void-Si composites are prepared following different approaches. And the electrochemical performances of the as-prepared composties as active anode materials for lithium-ion batteries are investigated. The final silicon core-hollow grpahene-based carbon composites are according denoted G-void-Si(C2) and G-void-Si(C10) with the alcohol and decanol as solution during LPAN coating respectively. And the G-void-Si composites are referred to as G-void-Si-mill and G-void-Si-stir respectively using milling or stirring during LPAN coating. The silicon content of composites is 75.2%,81.7%,82.3% and 78.9% for G-void-Si(C10)-mill, G-void-Si(C10)-stir, G-void-Si(C2)-mill and G-void-Si(C2)-stir, respectively. The structure of G-void-Si(C10) is more intact than G-void-Si(C2)’s, and the structure of G-void-Si-mill is more tightly than G-void-Si-stir’s. The delithiation capacity of G-void-Si(C10)-stir composite decreases from 2022.7 mAh·g-1 to 179.6 mAh·g-1 only after 50 cycles, while the retention capacity of G-void-Si(C10)-mill composite is 530.3 mAh·g-1 after 200 cycles. LPAN do not dissolve in decanol, but dissolves in alcohol. Therefore, in alcohol, LPAN is easy to coat on SiO2-Si particles. LPAN are aggregated and do not coat conformally on SiO2-Si particles in the decanol during stirring. However, in the decanol, milling process improves the LPAN coating on SiO2-Si particles. The G-void-Si composite anode was further tested at various step-wise current densities. Specific capacities of 1568.3 and 70.2 mAh·g-1 can be achieved at high current densities of 4 A·g-1 for G-void-Si(C2)-mill and G-void-Si(C2)-stir composite respectively. When the current density is restored to 200 mA·g-1, the capacities recover to 2188.5 and 383.8 mAh·g-1 for G-void-Si(C2)-mill and G-void-Si(C2)-stir composite respectively. The electrochemical performance can be influenced by the charge cut-off voltage.(6) The preparation condition of G-void-Si composite with excellent electrochemical performance is as follows. Si, TEOS and LPAN with mass ratio of 1:10:2 are milled in alcohol for 10 h. The mixture were cured in air at 220 ℃ for 3 h and carbonized in an argon atmospehere at 1000℃ for 5 h. The initial charge capacity and coulombic efficiency of G-void-Si composite are 2245.5 mAh·g-1 and 73.8% respectively. The charge capacity of G-void-Si composite remains at 1161.3 mAh·g-1 after 200 cycles. The cells were tested by charge-discharge cycling from 200 mA·g-1 to 10 A·g-1 and then in reverse to 200 mA·g-1. Specific capacities of 2181.9,1873.8,1568.3 and 527.7 mAh·g-1 can be achieved at current densities of 1,2,4 and 10 A·g-1. The capacity recovers to 2188.5 mAh·g-1 when the current density is restored to 200 mA·g-1. The excellent electrochemical performance of G-void-Si can be attributed to following reasons. Firstly, G-void-Si composite has a well-defined void space around Si particles, which allows for silicon to expand upon lithiation without pulverization. Secondly, the N-doped graphene-based carbon shell is both electronically and ionically conducting. Thirdly, the graphene-based carbon shell is a self-supporting framework, which allows for the growth of a stable SEI on the electrode.(7) A porous silicon/graphene-based carbon nanosheet (G-porous Si) composite is synthesized by aluminothermic reduction of fumed silica and carbonization of LPAN simultaneously. The fumed silica is amorphous. The aluminothermic reduction occurs at 900 ℃ between Al and SiO2. With reaction temperature increasing, corundum appears and is difficult to remove. With mass ratio of 3:5:10 for Al, SiO2 and LPAN, the aluminothermic reduction is conducted at 900℃ for 5 h to produce G-porous Si composite. The G-porous Si composite has core-shell structure, and the particle of composite is about 50 nm in average. G-porous Si composite as anode for lithium-ion batteries is evaluated using galvaostatic charge-discharge cycles at 200 mA·g-1 between 0.01 and 1 V. The initial charge and discharge capacities are 1162.7 and 1251.7 mAh·g-1 respectively. G-porous Si composite exhibits a reversible capacity of 1036.8 mAh·g-1 after 15 cycles and the initial coulombic efficiency reaches to 92.9%. After 50 cycles, the capacity of composite is about 500 mAh·g-1. The high initial coulombic efficiency and good cycling performance of G-porous Si composite is caused by three aspects. Firstly, the porous silicon can provide a large space to accommodate volume expansion, and to maintain electrode structure integrity during charge and discharge processes. Secondly, the porous silicon has large surface area which is accessible to the electrolyte and a short diffusion length for lithium-ion to transport from electrolyte to silicon. Thirdly, the N-doped graphene-based carbon shell is both electronically and ionically conducting.