Introduction to Carbonaceous and Si Anode Materials

2.2-1 Carbonaceous Anode

Nowadays, carbonaceous materials are used as anode materials in commercial lithium ion rechargeable battery. Carbonaceous materials have advantage of high safety, the lightest and low electrochemical potential close to that of metallic lithium [10, 11].

The physicochemical property of carbon depends on its surface structure and chemical composition. The important factors determining application of carbons as anode material are morphology of the carbon: particle shape and size, pore-size distribution and pore-opening, Brunauer-Emmett-Teller (BET) surface area and content of surface species and impurities [10, 11].

The basic building block of carbons is a planar sheet of carbon atoms conjugated by sp² bond, the structure are shown as Fig. 2-3 (called graphene or basal plane). Each planar sheet is held up by van der Waal forces and is stacked in ordered or disordered to form crystallites. As lithium ions intercalate into the graphene layers, the layer structure changes and the structure are resilient enough for lithium ions to insert or deinsert. Then, the graphite served as a host structure for lithium ions intercalation.

Figure 2-3. The faces of graphite crystallite.

The graphene crystallite has two different edge sites: armchair and zigzag sites. While charging, the lithium ions must travel to the outer edges of the graphene sheet before coming to rest (intercalating) between the sheets. At the edge sites, the reactivity is much higher than at the basal plane. Thus, physical and chemical properties of carbon change with the ratio of basal-plane to edge-plane area.

Carbonaceous anode materials used in rechargeable lithium battery can be categorized into graphite, non- graphitized glass-like carbon (hard carbon) and graphitic carbon (soft carbon) [12].

Graphitic carbon is one of the most commonly used anode materials in commercialize. Graphite has the advantages of low cost and high columbic efficiency. The theoretical specific capacity of graphite is 372 mAh/g [13, 14].

During intercalation process, the Lithium ion intercalates into graphite, and Li⁺

Li⁺

Armchair Face Zigzag Face

then it forms LiC6 intercalation compound. That means every six carbon atoms can host only one lithium ion. Thus, graphite shows poor relatively low specific capacity.

Hard carbons can be used with Consumer electronics, such as personal computers. They can be prepared by a thermal decomposition of petroleum pitch. Generally, hard carbons show higher capacities than graphite, which had reversible specific capacities for lithium of up to 600mAh/g [15-17]. They are made up of small single layers. Therefore, the adsorption of lithium occurs on both sides of graphene sheets, which result in more than one absorption per six carbons and greater storage capacity. However, the voltage profile of hard carbon is mainly composed of two regimes, a sloped regime in a voltage range of 1.0–0.1 V. In this region, it only has capacity around 150–250mAh/g. The other one is a plateau region with a capacity around 100–400mAh/g [15-17].

Therefore, hard carbon materials have disadvantages such as low initial columbic efficiency and low tap density.

The soft carbons (graphitic carbon) basically comprise of sp²-hybridized carbon atoms. They are arranged in a planar “honeycomb-like” network, i.e., a

“graphene” layer is formed [18]. Van der Waals force provides a weak cohesion of the graphene layers, leading to the layered graphite structure. The carbon atoms are arranged hexagonally in a planar condensed ring system. The layers are stacked parallel to each other. The atoms within the rings are bonded covalently, whilst the layers are loosely bonded together by van der Waals forces. The high degree of anisotropy in graphite results from the two types of bonding acting in different crystallographic directions.

2.2-2 Si Anode

Silicon anode materials have attracted much attention in recent years due to its high theoretical capacity, low-voltage reaction plateau, low cost and environmental friendliness. It has high theoretical capacity of 4200 mAh/g, which is ten times of commercial graphite (372 mAh/g).

However, achieving this capacity is difficult at ambient temperature [19].

At ambient temperature, Dahn’s and Obrovac’s did detailed examination of the crystal structure. Their results proved that the fully lithiated phase of silicon at room temperature is Li15Si4, not Li22Si5, which shows maximum capacity of 3579 mAh/g-silicon [20-21]. Fig. 2-4 and Table 2-1 show typical binary Li-Si phase diagram and its corresponding crystallography data [22]. The results of Rietveld refinement of Li15Si4 are listed in table 2-2 with a space group of I 4 3d (cubic).

Silicon could act as anode materials in lithium-ion batteries to enhance capacity of battery (~3500 mAh/g with the formation of Li15Si4). However, Silicon anode suffers from volume expansion during cycling. Upon insertion and extraction of lithium, the volume expands to 400% of the initial size. This is almost highest volume expansions among the common alloy anodes. The huge expansion causes the crack of silicon, irreversible capacity of the first cycle, decrease of conductivity and capacity significantly during subsequent cycles. These reasons hinder the application of silicon anode on lithium ion rechargeable battery.

For example, Ryu et al. showed that commercial bulk silicon powder consists of 10 um particles as anode materials, and the first discharge capacity (lithiation) was about 3260 mAh/g [23]. However, the first cycle coulombic efficiency was only 35%. The specific capacity of 1170 mAh/g was obtained during the first charge (de-lithiation). The drastic capacity loss was observed in the subsequent cycles. After 10 cycles, the capacity dropped to lower than 200 mAh/g, which is insufficient for an anodes material.

To solve crack of silicon during cycling, much effort has been devoted. In order to increase the performance and cyclability of silicon materials, some research focus on nano-sized silicon. Synthesized nano-scaled morphology silicon and/or form silicon/carbon composite are one of the effective methods.

For example, Peng, KQ et al. proposed silicon nanowire arrays prepared by metal-induced chemical etching. The nanowire has not only good conductivity but also nanometer-scale rough surfaces. These two features facilitate charge transport and insertion/extraction of Li ions. The electroless-etched SiNWs anode showed higher charge capacity and longer cycling stability than the conventional Si wafer [24].

For example, Guo, H et al. proposed Silicon thin films deposited on rough Cu foil by a radio frequency magnetron sputtering. The thin thicknesses are in range 1000-5300 angstrom. They found that the stability and reversible discharge capacity are depend on the film thickness, and thinner ones have larger accommodation capacity. A Si film with 3120 angstrom provides a

reversible specific capacity over 3500 mA h/g with excellent cycleability under 0.5 C charge/discharge rates [25].

Figure 2-4. Li-Si phase diagram [26].

Table 2-1. Li-Si crystallography data [26].

Phase Composition (wt.% Si) Pearson symbol Space group Li

Table 2-2. Results of the Rietveld refinement of Li15Si4.

Parameters Dahn et al. [26] Obrovac and Christensen [27]

Lattice constant 10.777 Å 10.685 Å

2.2-3. Silicon/Carbon Composite Anode

During charge, Organic solvents easily decompose on the negative electrodes. When appropriate organic solvents are used as the electrolyte, the solvent decomposes on initial charging and forms a solid layer called the solid electrolyte interphase (SEI) [28]. The SEI layer is electrically insulating and it usually provides ionic conductivity. It prevents further decomposition of the electrolyte during charge. However, as silicon anode suffers dramatic volume changes on cycling, the SEI layer will be broken on expansion.

Carbon coating or composite with silicon can help to reduce the SEI film formation and irreversible capacity. As mentioned, Carbon has less surface area than nano-silicon, thus side reactions in electrode and electrolyte take place less.

Besides, carbon possesses good electronic conductivity and it buffers huge volume expansion of silicon during lithiation and de-lithiation processes.

Therefore, in recent years, more research have focus on fabricate Si/C composites.

For example, Dan Thien, N., et al. proposed a carbon fiber-interwoven amorphous nano-SiOx/graphene structure [29]. They prepared a simple and facile room temperature synthesis of amorphous SiOx nanoparticles using silica, followed by their homogeneous dispersion with graphene nanosheets and carbon fibers in room temperature aqueous solution. Transmission and scanning electron microscopic(TEM and SEM) imaging reveal that amorphous SiOx primary nanoparticles are 20-30 nm in diameter and carbon fibers are

interwoven throughout the secondary particles of 200-300 nm, connecting SiOx nanoparticles and graphene nanosheets. This carbon fiber-interwoven nano-SiO0.37/graphene electrode exhibit good cycling performance and rate-capability under 5C rate. After over 50 cycles, the discharge capacities is still 1579-1263 mAh/g at the C/5 rate. It has capacity retention of 80% and Coulombic efficiencies of 99% and nearly sustained microstructure.

For example, Holzapfel, M et al. proposed two types of nano-size silicon prepared via thermal vapour deposition either with or without a graphite substrate are presented [30]. They showed superior reversible charge capacity and cycle life when it served as negative electrode. Then the lithiation reaction was applied on the materials. Raman spectroscopy, dilatometry and differential electrochemical mass spectrometry (DEMS) was used to investigate the materials. The Si/graphite compound material shows relatively high kinetics upon discharge, and the result showed relative volume change and low gas evolution of the nano silicon based electrode.

Many efforts have made on the research of Silicon/Carbon Composite anode materials. Table 2-3 lists some review recent paper proposed in 2013.

Table 2-3. Papers of Si/C composites anode material proposed in 2013 [31-47].

Paper title Description

Preparation and

Silicon powders and different amounts of multiwalled carbon nanotube were mechanically alloyed in polyacrynitrile solution via high speed planetary ball milling. Produced composite was characterised via X-ray diffraction pattern, scanning electron microscopy, energy dispersive spectroscopy and thermogravimetric analysis.

Si/graphene composite prepared by magnesium thermal reduction of SiO2 as anode material for lithium-ion batteries [32].

Nanosized Si/graphene composite was prepared by magnesium thermal reduction of the in-situ generated SiO2 particles on graphene sheets, in which about 5 nm-silicon nanopartides were homogeneously loaded on graphene sheets. This unique structure can not only accommodate the large volume changes, but also maintain electronic conductivity during Li-ion insertion/extraction. The composite exhibits an excellent cycling stability with a capacity of 1374 mAh/g over 120 cycles.

Porous silicon/carbon composite was prepared via a mechanochemical reaction between Li13Si4 and SiCl4 under ball milling. Specific surface area of the composite can be adjusted by controlling the particle size distribution of Li13Si4.

The results indicated that the composite material exhibited a considerably high reversible capacity of 1 900 mAh/g and an excellent cycling stability with only 7.6%

capacity decay after 50 cycles at a current density of 300 mA/g.

Silicon nanoparticles supported on graphitic carbon paper as a hybrid anode for Li-ion

rechargeable batteries [34].

A hybrid anode consisting of silicon nanoparticles supported on a woven graphitic carbon paper is reported.

The silicon in the hybrid electrode exhibits a specific capacity of similar to 1300 mAh/g which decreases as the loading increases to 0.6 mg, but still showing good cyclability. The structural and morphological changes of graphite and silicon within the hybrid electrode during charge and discharge are also presented.

Silicon Oxycarbides in Hard-Carbon

Microstructures and Their Electrochemical Lithium Storage [35].

Silicon oxycarbide (Si-O-C) glasses have been obtained by pyrolysis of powdery blends, composed of polysiloxane resin and phenol aralkyl resin, to 1000 degrees C. Electrochemical measurements showed that delithiation capacities linearly increased to >800 mAh/g with increasing the silicon content in these Si-O-C glasses. Interestingly, the carbon-rich Si-O-C glasses had delithiation capacities of up to 580 mAh/g with good cyclability. Li-7 NMR analyzes indicated that the carbon-rich Si-O-C glasses have more hard carbon-like features and that at least two electrochemically active sites store lithium species.

Cross-Linked

Cross-linked poly(acrylic acid) (PAA) with polycarbodiimide (PCD) is utilized as a binder for the Si/graphite composite electrode. Cross-linkage of PAA can be modulated by the addition of the PCD as a cross-linker into the slurry. Initial reversible capacity of Si/graphite composite electrodes is increased with suppressed electrolyte decomposition by the use of cross-linked PAA. The Si-composite electrode with optimal amount of 1 wt% PCD cross-linker delivers more than 1,000 mAh/g of reversible capacity with improved capacity retention.

Caramel Popcorn Shaped Silicon Particle with Carbon Coating as a High Performance Anode Material for Li-ion

rechargeable batteries [37].

Caramel popcorn shaped porous silicon particles with carbon coating are fabricated by a set of simple chemical methods as active 'anode material.

Si particles are 'etched to form a porous structure, which provide space for the volume expansion and liquid electrolyte diffusion. A layer of amorphous carbon is formed inside the pores, which gives an excellent isolation between the Si particle and electrolyte, so that the formation of the SEI layer is stabilized. This novel structure enhances the mechanical properties of the Si particles, and the crack phenomenon caused by the volume change is significantly restrained. Therefore, an excellent cycle life is achieved.

Mesoporous, Si/C composite anode for Li battery obtained by 'magnesium-thermal' reduction process [38].

Mesoporous silicon was synthesized by a 'one spot' magnesium-driven reduction reaction from SBA-15 mesoporous silica template. Then, carbon coated in order to confer it electronic conduction. Mesoporous Si/C composite material showed an initial discharge capacity of about 1500 mAh/g at 0.1 C (=420 mA/g), which tend at a plateau at around 600 mAh/g after 100 cycles with an excellent coulombic efficiency.

Improving

microstructure of

silicon/carbon nanofiber composites as a Li battery anode [39].

Silicon/carbon nanofiber (Si/CNF) nanocomposite material as a anode for rechargeable lithium ion batteries was investigated. Amorphous silicon layers were uniformly coated via chemical vapor deposition on both the exterior and interior surfaces of the CNF.

The resulting Si/CNF composites exhibited capacities near 800 mAh/g for 100 cycles. Based upon the experimental analysis and theoretical calculation, they have proposed several interfacial engineering approaches to improve the performance of the electrodes by optimizing the microstructure of this nanocomposite.

Silicon-conductive nanopaper for Li-ion rechargeable batteries [40].

In this study, flexible and conductive nanopaper aerogels with incorporated carbon nanotubes (CNT) was fabricated. Such conductive nanopaper is made from aqueous dispersions with dispersed CNT and cellulose nanofibers, followed by deposition of a thin-layer of silicon through a plasma-enhanced CVD (PECVD) method. The open channels allow for an excellent ion accessibility to the surface of silicon.

A stable capacity of 1200 mA h/g for 100 cycles in half-cells is achieved. Such flexible anodes based on earth abundant materials and aqueous dispersions could potentially open new opportunities for low-cost energy devices.

In Situ Synthesis and Cell Performance of a Si/C

Core-Shell/Ball-Milled Graphite Composite for Lithium Ion Batteries [41].

A high-capacity silicon-carbon core shell (Si/C) supported by ball-milled graphite (BMG) was synthesized in situ using a hydrosilylation reaction.

The Si/C/BMG sample effectively absorbed high volumetric expansion/contraction generated. After 50 charge/discharge cycles, the Si/C/BMG electrodes still had a very high capacity of 1615 mAh/g.

Lithium insertion into

Carbon-rich silicon oxycarbide ceramics (SiOC) was prepared via thermal conversion of polyorganosiloxane.

The ceramics demonstrate high lithiation capacity and reliable rate capability when used as anode material in Li-ion rechargeable batteries.

A high-performance lithium-ion battery anode based on the core-shell heterostructure of silicon-coated vertically aligned carbon

nanofibers [43].

A high-performance hybrid lithium-ion anode material using coaxially coated silicon shells on vertically aligned carbon nanofiber (VACNF) cores was reported. The unique "cup-stacking" graphitic microstructure makes VACNFs a good lithium-ion intercalation medium and, more importantly, a robust bush-like conductive core to effectively connect high-capacity silicon shells for lithium-ion storage. A high specific capacity of 3000-3650 mAh/g, comparable to the maximum value of amorphous silicon, has been achieved. About 89% of the capacity is retained after 100 charge-discharge cycles at the C/1 rate.

Nano silicon carbide: a new lithium-insertion anode material on the horizon [44].

Cubic (3C polytype) nano SiC was prepared by a chemical vapour deposition (CVD) method. It delivers a reversible lithium insertion capacity of about 1200 mAh/g over 200 cycles.

A simple method for improving the electrochemical performance of silicon nanoparticle-core/carbon-shell (Si-core/C-shell) nanofibers was reported. Additional conductive paths between the Si nanoparticles were formed by incorporating a small percentage of multi-walled carbon nanotubes (MWNTs)into the Si nanoparticle core. The electrical conductivity of a single Si-core/C-shell nanofiber showed a more than five times increase according to MWNT addition. A galvanostatic

charge-discharge test demonstrated that a small amount of MWNTs greatly improved the electrochemical performance of the Si-core/C-shell nanofibers due to the enhanced participation of Si through the additional conductive paths formed between the Si nanoparticles.

Electrochemical

Effects of a diamond-like carbon (DLC) coating on a silicon monoxide-graphite composite electrode are studied in order to improve the electrochemical characteristics of silicon monoxide anodes. The discharge capacity of the coated cell is 523 mAh/g at the first cycle and 409 mAh/g at the 100th cycle at a 0.5 C rate. The 100-cycle capacity retention is 78.2%, which is greater than that of the bare cell (52%).

low cost approach to fabricate hybrid silicon nanowire (SiNW)/graphene nanostructures was reported. It exhibit enhanced cycle performance with the capability of retaining more than 90% of their initial capacity after 50 cycles. They also demonstrate the use of hot-pressing in the absence of any common polymer binder such as PVDF to bind the hybrid structure to the current collector.