A silicon nanoparticle/reduced graphene oxide composite anode with excellent nanoparticle dispersion to improve lithium ion battery performance
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- de Guzman, R.C., Yang, J., Cheng, M.M. et al. J Mater Sci (2013) 48: 4823. doi:10.1007/s10853-012-7094-7
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Composite anodes of Si nanoparticles (SiNPs) and reduced graphene oxide (RGO) sheets with highly dispersed SiNPs were synthesized to investigate the performance-related improvements that particle dispersion can impart. Three composites with varying degrees of particle dispersions were prepared using different ultrasonication, and a combination of ultrasonication and surfactant. With more dispersed SiNPs, the capacity retention and rate performance as evaluated by galvanostatic cycling using increasing current density rates (500–2500 mA/g) also improved compared with anodes that have poor particle dispersion. These results demonstrate that better nanoparticle dispersion (small clusters to mono-dispersed particles) between the stable and the highly conducting RGO layers, allows the carbonaceous matrix material to complement the SiNP-Li+ electrochemistry by becoming highly involved in the charge–discharge reaction mechanisms as indicated by chronopotentiometry and cyclic voltammetry (CV). Particle dispersion improvement was confirmed to be a key component in a composite anode design to maximize Si for high-performance lithium ion battery (LIB) application.
The promise of increasing the anode-specific capacity of lithium ion batteries (LIBs) by as much as ten-fold through the substitution of graphite (theoretical capacity of 372 mAh/g ) with Si (theoretical capacity of 3,572 mAh/g ) has tremendously influenced the direction of recent scientific efforts. Utilization of Si promotes a high-capacity Li alloying reaction which produces a Li-rich phase (Li15Si4) compared with the intercalation reaction with graphite (LiC6). However, the increased accommodation of Li+ ions during charge–discharge cycles induces large volume variations (as much as 370 %) and stress on the bulk anode matrix that ultimately leads to failure. In view of this hindrance, different strategic schemes have been pursued to alleviate the effect of volume expansion including: amorphous thin films [3–6], nanowires [7–9], nanotubes , and porous morphologies [11–13]. Despite these advances, capacity degradation during cycling is still problematic, suggesting electrode fracturing and eventual electrical contact losses. The most promising recent advances have employed Si particles limited to within few nanometers in dimension coupled with carbonaceous material [14–22] as a form of support. Here, we focus on composites from SiNP dispersed within graphene sheets to display performance improvements via particle dispersion.
A recent state-of-the art material, graphene, is such a carbonaceous support material. The highly organized sp2-bonded carbon atoms in graphene provide outstanding mechanical and electrical properties compared with other known materials [23, 24]. It has the potential to provide limit/control of Si volume expansion while assisting in electron conductivity within composite anodes. It also has excellent chemical stability that is crucial in minimizing intensive side reactions between the SiNPs and the electrolyte which forms an unstable non-conducting solid–electrolyte interphase (SEI)  that degrades electrochemical performance.
Effect of preparative condition on SiNP dispersion in RGO matrix
Graphite oxide synthesis
The RGO precursor, graphite oxide was prepared using a two-stage Hummers’ method according to Kovtyukhova . Graphite (2 g, Dixon Microfyne, Ashbury, NJ) was pre-oxidized in an oil bath at 80 °C for 4.5 h using 30-mL H2SO4 (95 %) with pre-dissolved K2S2O8 (1 g) and P2O5 (1 g). After cooling, the solution was diluted with 1 L of deionized (DI) water, and then filtered and washed until the filtrate was pH neutral. The pre-oxidized graphite was mixed with 80-mL H2SO4 in an ice bath. While maintaining stirring, KMnO4 (10 g) was added stepwise for the reaction to proceed just below room temperature for 2 h then followed by careful dilution using 150-mL DI water. During this process, the solution temperature was kept below 50 °C. After additional stirring for 2 h, further dilution with 500-mL DI water was then followed by slow addition of H2O2 (30 %, 8.3 mL). The mixture was then allowed to settle overnight and then decanted. The product was purified using repeated rinsing and centrifugation with 5 % HCl and DI water. The resulting graphite oxide suspension (about 10 mg/mL) was then stored in an amber bottle at room temperature.
A 5–10-nm distribution of SiNP was acquired from Meliorum Nanotechnology (Rochester, NY). The particles were stored in an Ar-filled glovebox with O2 and moisture content both <1 ppm. Prior to use, the particles were exposed to air overnight to develop ample surface hydrophilic oxide layer for proper dispersion.
Composite anode formation
The oxidized SiNPs were weighed and then dispersed in three different ways: (1) sonication (Branson Model 2510, Danbury, CT, 100 W, 50/60 Hz) for 1 h (S1) or (2) sonic probing (Misonix, Ultrasonic Cell Disruptor, Farmingdale, NY, 100 W, 22.5 kHz) for 1 h (S2). Dispersion (3) was prepared by dispersing the SiNP in methanol with a 1 % (v/v) content of n-octyl alcohol (99 %) by sonic probing under the conditions described for S2 (S3). After the dispersion step, the standalone composite anode was produced without the need for binders and conductive diluents following the procedures as outlined in Lee et al.’s  work. An appropriate amount of graphite oxide suspension was added to the dispersion to make a 1:1 weight ratio (SiNP:RGO). It was then subjected to the same sonication technique (from prior step) for 2 h. Sonics exfoliate the expanded structure of graphite oxide to form the GO platelets while at the same time dispersing the SiNPs. All sonics-based steps were performed in room temperature conditions and were closely monitored to minimize temperature increase. After this, the sample was vacuum-filtered (setup: Millipore 47 mm all-glass vacuum filter holder: funnel and flask; filter: 0.2 μm pore, Whatman Anodisc) forming a solid composite. During filtration, SiNP cross-over was minimized due to the initial deposition of GO platelets on the filter surface. The resulting anode was then air-dried followed by thermal reduction. The reduction was done using 10 % H2 (balance Ar, 100 mL/min) at 700 °C for 1.25 h. After the reduction, the standalone SiNP/RGO composite material was sampled, weighed, and prepared for testing.
The cells were assembled in a Swagelok-type cell , loaded typically with ~1 mg of the composite (1.5 mg/cm2, 5–10 μm thick) as the working electrode and Li metal (99.9 %, 0.75 mm thick, Alfa-Aesar) acting as both counter and reference electrode (half-cell configuration). A solution of 1.0 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) 1:1(v/v) solution was used as the electrolyte. The cells were fabricated and cycled in an Ar-filled glove box.
Scanning electron microscopy (SEM) images were taken using a JEOL (Peabody, MA) Model JSM-6510LV-LGS at 25 kV. Chemical composition analysis was done using the equipped Energy Dispersive X-ray Spectrometer (EDS). Thermogravimetric analysis (TGA) was carried out on a Model TA Instruments 2960 (New Castle, DE) at a heating rate of 10 °C/min in air, from which the contents of Si in the composites were determined considering the adjustments for Si oxidation. GO platelet imaging was acquired by atomic force microscopy (AFM) (Digital Instruments Dimension 3100, Plainview, NY) by immobilizing a representative area onto a freshly cleaved mica surface at room temperature under N2 purge. A normal tapping mode silicon cantilever (300 kHz, 40 N/m, T300, nanoScience Instruments/Vista Probes, Phoenix, AZ) was utilized for optimum resolution.
The composite morphology was investigated using a transmission electron microscope (TEM). TEM micrographs were acquired on a JEOL-2010 FasTEM at 200 kV. The TEM samples were prepared by dispersing a small area of dry anode in ethanol with sonication for 1 h. Then a drop of sample solution was casted on 300-mesh copper TEM grids covered with thin amorphous carbon films.
Electrochemical cycling of the assembled cells was performed galvanostatically with a cut-off voltage range of 0.02–2.0 V while maintaining a constant current density of 500 mA/g for 40 cycles. This current density was used to determine the highest (optimal) capacity of the composite material. After this step, varying densities of 900, 1,500, and 2,500 mA/g were used after every 10 cycles to assess the rate performance of the cells. The cells were then cycled back to a current density of 500 mA/g to measure the changes in capacity following previous high current density cycles. Cyclic voltammetry (CV) measurement was performed using a single scan rate of 0.04 mV/s over a range of 0.01–2.0 V to gain a better understanding of the reaction mechanisms. The baseline performance was evaluated using around 1 mg of RGO (0.7 mg/cm2, 25 μm thick) without any SiNP loading. In all the electrochemical tests, a Gamry (Warminster, PA) Reference 3000 and series G 300 potentiostat/galvanostat was used.
Results and discussion
SiNP/RGO composite anode
Kim et al.  reported that ~10 nm is the optimal size for SiNPs as a LIB anode material. This particle diameter was found to be the most stable when cycling at 900 mA/g, showing no appreciable changes in particle size observed after 40 cycles. Likewise, modeling by Dimitrijevic et al.  predicted that cracking damages during lithiation are negligible for Si (and Sn) particles that are 20 nm and smaller. Thus, the SiNP employed in this study was chosen considering both works: distribution of 5–10 nm.
The next 14 cycles of S1 (Fig. 5a) showed a very steep drop in discharge capacity from about 2000 to 1000 mAh/g (~50 % of initial) with an average efficiency of 98 %. At this same point Si alone displayed similar capacity (32 % of initial). However, considering the first discharge, S1 shows improved capacity retention. Continuing until cycle number 40, a much slower rate of capacity decay was observed, with a capacity value of 650 mAh/g at 98 % efficiency. In relation to the results of the pure graphene anode, where only the initial run demonstrated very low Coulombic efficiency (afterward the reversible capacity is stabilized), this observed early cycle decay can be attributed to the SiNP component stability. This predominant observation in the literature has led some to conclude that pristine Si particles have poor cycling stability [17, 38]. Based on the morphology of S1, it seems as the agglomerated SiNPs are acting as macro-scale Si (together with its detriments such as severe volume expansion, structure destruction, etc.) that masks the stabilizing effect of the RGO matrix. With this, it is presumed that even using graphene material that exhibits improved cycling stability such as in  would bring minimal improvements unless proper focus to the SiNPs is given.
For composite S2, the capacity retention presents a slight improvement compared to S1. At cycle 15, the reversible capacity is 1,200 mAh/g (98 % efficiency) which translates to a 20 % improvement compared with both S1 and Si. It is also evident that the decay slope is less steep in S2 compared with S1, and at the end of cycle 40 there is still an observed capacity of 850 mAh/g (98 % efficiency). These improvements are likely due to the better dispersion of the SiNPs, as that is the only differentiating trait between the two. As particles are less agglomerated, the SiNP behaves better in cycling the Li+ ions.
Further improvement of the dispersion with the surfactant approach of S3 resulted in improvements in capacity retention as demonstrated in Fig. 5. After 15 cycles, the discharge capacity output is ~1700 mAh/g, a 70 and 42 % improvement from S1 and S2, respectively. The key for this improved performance can be attributed to the capacity retention at early runs. A closer observation of the composites’ relative capacity (vs. the first discharge) after the first 10 cycles as indicated in Fig. 5b distinctly illustrates that significant capacity degradation occurs during the early cycles. At cycle 5, S3 showed substantial capacity retention (96 %) compared with S1 (74 %), S2 (80 %), and Si (60 %). Beyond this point, S3 displays <1 % capacity drop every cycle, strongly correlated with the extent of SiNP dispersion. With this improved capacity retention, S3 has retained a capacity of 1200 mAh/g (57 % of initial) after 40 cycles, 85 and 41 % above from S1 and S2, respectively.
In summary, RGO has been demonstrated as a stable matrix material that supports high-capacity SiNPs. Likewise, improvement of the SiNP dispersion within the composite results in improved capacity retention as well as rate capability. As the anode preparation was mainly through simple sonics-based techniques, future design aspects such as improvements in anchoring/adhesion of SiNPs to the matrix and/or employment of particle protection means to isolate the active particles from unstable SEI can be considered to improve the electrochemical performance.
The combination of high power/energy sonics and surfactants can produce improved particle dispersions containing non-agglomerated SiNPs (10–20 nm) to small particle clusters (<100 nm) within the composite. With this attribute, charge–discharge profile and CV curves of the anode suggest that the mechanism of Li+ accommodation/release highly involves the stable and conducting RGO matrix. Together with the electrochemical benefits of having SiNPs, the composite anode material displays higher capacity retention and better rate performance in contrast with other composites with poor particle dispersion. Particle dispersion improvement is a step forward in composite anode design to maximize Si for high-performance LIB application.
Financial support from the Department of Energy (Grant DE-EE0002106) for this research is gratefully acknowledged.