Si Mole Research Articles

Investigation and Improvement of the Pressure Control in Si Anode Based Solid-State Batteries

Solid state batteries (SSBs) are promising next generation propulsion for electric vehicles due to their improved safety, high gravimetric and volumetric energy densities, and increased reliability. Silicon is one of the limited choices for the anode material for the SSBs due to its high applicable capacity, low operating potential and natural abundance. However, significant challenges remain for the commercialization of Si anode based SSBs, including the insufficient operational and calendar life, requirement of high stack pressure for operation, and inadequate fast charge capability.Almost each of the technical challenges associated with Si anode is entangled with the volume change of Si during charging and discharging. Si possesses high lithium storage capacity because it can form various LixSiy compounds upon lithiation. For example, when 3.75 moles of Li+ is added to 1 mole of Si, the compound Li15Si4 is formed, which gives a capacity of 3578 mAh/gSi. What accompanies this is a considerable volume expansion (e.g., 280% increase for Li15Si4), resulting from the reorganization of the compound crystals. Assuming an anode that initially composes of perfectly compacted Si particles, this volume expansion translates to a 56% increase in the thickness of the Si anode. In a practical SSB, the real thickness variation in the Si anode is expected to be smaller because of the architecture design of the anode. Nonetheless, this volume change in Si creates many problems, including the Si particle pulverization, delamination of electrode from the current collector and electrolyte separator, loss of electronic and ion conductions in the electrode, among others. To maintain the electric contact in Si anode based SSBs, high stack pressures, from several MPa to several tens of MPa, are normally applied.In this work, we carefully measure the real-time stack pressure in a specially designed setup. The stress-strain characteristics of Si anode based SSBs during operation, including the stack pressure spike and the degradation of the average stack pressure, is studied and correlated with the Si anode structure changes. Furthermore, different measures, including external and internal ones, are explored to improve the cell pressure control. The external measures include selecting an appropriate spring and using a mechanical buffer layer, while the internal measure refers to an elastic current collector design. As a result, a better SSB pressure control is obtained. More importantly, a reduction of the stack pressure significantly below 10 MPa without compromising the cell performance is achieved.

High Performance Lithium Silicide Electrode Enable By Molecular Layer Deposition

The energy demand of our society is increasing without precedents. Lithium-Ion Batteries (LIBs) are and have been an enabler of the present communication revolution; LIBs are the powerhouse of portable communication devices. Consumers are starting to demand more sustainable and more environmentally friendly energy sources, energy production and distribution. To satisfy these demands, the next generation of rechargeable batteries will need higher specific energy and power (per mass) and energy and power density (per volume) than the current generation of LIBs. The current technology uses Lithium Metal Oxide (LMO) as positive electrodes and graphite as a negative electrode. This system has an energy density value ca. 250 WhL-1. To improve the power and energy density of the LIBs devices; new electrode material needs to be developed.Silicon (Si) has become one of the most investigated materials for LIB negative electrodes because of its ability to accommodate 3.75 moles of Li per mole of Si (Li15Si4), leading to a theoretical capacity of 3,579 mA h/g at room temperature1. While 372 mA h/g is the theoretical capacity for graphite2, this material can only accommodate 1 mole of Li per 6 mol of C.Despite Si advantages, progress towards a commercially available Si negative electrode has been impeded by their rapid capacity fade, poor rate capability, and low coulombic efficiency. The cause of electrode degradation is the Si volume change of ~300% upon lithium insertion and extraction3, presenting a major problem for electrochemical performance. The volume fluctuation damages the electrical contact between Si and the current collector, being too large to be controlled by currently developed coating technologies.One of the leading strategies established for the realization of such an approach is coating nano-Si particles with flexible materials to attempt to accommodate the volumetric changes of the particles. In this context, we propose to carry out a surface modification, in which Molecular Layer Deposition (MLD) is utilized to grow a mechanically robust, flexible coating, to undertake the Si expansion and contraction. For it, we have used TiCl4 molecule and as it is a relatively small molecule, when combined with the larger organic reactants, has high growth rates that are mainly limited by steric hindrance caused by the organic reactants. For these reasons, the recipe here developed is based on Glycerol and TiCl4 as the titanium precursor.The composition of the films was studied using Fourier-Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope-Energy-dispersive X-ray spectroscopy (SEM-EDX). Multiple characteristic absorption modes for both carbon and titanium related groups can be observed in the FTIR spectrum (Figure 1). These techniques confirm that the combination of both the carbon-related groups and the Ti absorptions providing evidence for the successful deposition of a hybrid organic-inorganic film showing that we are able to detect Ti in the surface of the electrode.Cyclic Voltammetry was used to characterize the electrochemical performance of the LixSi negative electrode coated with titanicone. We have successfully shown the deposition of titanicone on LixSi electrode and an initial electrochemical assessment of this electrode. These electrodes exhibit high capacity, around 2,000 mAhg-1 at 0.10C, and 270 mAhg-1 at 20C corroborating the goodness of the proposed methodology.Typical voltammogram shows the formation of the solid electrolyte interface, evident in the first cycle. In the subsequent scans, these peaks disappear (Figure 2). The sample exhibit the regular waves are corresponding to the Si redox reaction. Prosini PP, Cento C, Rufoloni A, Rondino F, Santoni A. A lithium-ion battery based on LiFePO4 and silicon nanowires. . 2015;269:93-97.Ikonen T, Nissinen T, Pohjalainen E, Sorsa O, Kallio T, Lehto VP. Electrochemically anodized porous silicon: Towards simple and affordable anode material for li-ion batteries. . 2017;7(1):7880.Zhao J, Lu Z, Liu N, Lee H, McDowell MT, Cui Y. Dry-air-stable lithium silicide and lithium oxide shell nanoparticles as high-capacity prelithiation reagents. . 2014;5:5088. Figure 1

Molecular Layer Deposition of Titanicone over Lithium Silicide Anode

The increasing demands for more advanced consumer electronics, electric vehicles, and stationary facilities call for the next generation of rechargeable batteries with higher specific energy (energy per mass) and energy density (energy per volume) than the current generation of lithium ion batteries (LIBs), which largely use lithium metal oxide (LMO) positive electrodes and graphite negative electrode.Si has become one of the most highly investigated materials for LIB negative electrodes because of its ability to accommodate 3.75 moles of Li per mole of Si (Li15Si4) for a theoretical capacity of 3579 mA h g−1 at room temperature. Despite Si inherent advantages, progress towards a commercially viable Si anode has been impeded by Si rapid capacity fade, poor rate capability, and low coulombic efficiency (CE). Si exhibits volume changes of ~300% upon lithium alloying and de-alloying, leading to material degradation and presenting a major problem for electrochemical performance. This high volume expansion and contraction is too large to be controlled by currently developed coating technologies.As the electrode matrix fractures, the continuous exposure of fresh nano-Si surface to the liquid electrolyte causes parasitic formation of a solid-electrolyte interphase (SEI) leading to irreversible charge loss and low CE. The volume fluctuation, also causes a damage of the electrical contact between Si and the current collector.One of the main front-end strategies established for the realization of such an approach is coating nano-Si particles with flexible materials in order to attempt to accommodate the volumetric changes of the particles. In order to address the challenge of Si dramatic volumetric change, we carried out a surface modification, in which Molecular layer deposition (MLD) was utilized to grow a mechanically robust, flexible coating, producing high-capacity Si nanocomposite anodes.MLD is a thin film deposition technique where hybrid organic–inorganic films are grown by exposing the substrate to subsequent, self-limiting, metal–organic precursor gases. The processes used here is based on TiCl4 as the titanium precursor. TiCl4 is a relatively small molecule, and when combined with the larger organic reactants it leads to high growth rates which are mainly limited by steric hindrance caused by the organic reactants.The composition of the films was studied using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscope-Energy-dispersive X-ray spectroscopy (SEM-EDX). Multiple characteristic absorption modes for both carbon and titanium related groups can be observed in the FTIR spectrum, Figure 1. The combination of both the carbon related groups and the Ti absorptions provides evidence for the successful deposition of a hybrid organic–inorganic film containing titanium.The electrochemical performance of the LixSi coated with titanicone used as an anode was investigated by Cyclic Voltammetry (CV). Typical voltammograms of the half cell in the first scan show the formation of the SEI. In the subsequent scans these peaks disappears.We successfully show the deposition of titanicone on LixSi electrode and an initial electrochemical assessment of this electrode. Figure 1

A Novel Si/C Composite as a High Capacity Anode Material for Lithium-ion Batteries

Silicon based electrode materials for Li-ion battery have attracted great attention due to the high theoretical capacity of 4212 mAh/g (3.75 moles of Li per mole of Si) compared to graphite (1 mol Li per 6 mol C) used in commercial LIBs. Unfortunately, practical applications of silicon as an anode material exhibit the huge volume change (>300%) during the alloying / dealloying process with Li, which would result in the loss of electrical conductivity, dramatic pulverization of silicon, and consequently a rapid capacity fading in the cycling. To alleviate this problem, silicon nanosized or nanostructures materials have been suggested, which can increase the transport of electron and lithium ion and reduce mechanical stress during cycling, resulting in improved electrochemical performance. Among these nanostructures, the combination with carbon is becoming attractive and important material because it improves the electric conductivity and plays a role of a buffer of volume expansion. In this research, a novel composite electrode material consisting of nanocrystalline silicon and porous graphite has been developed as an anode candidate for LIB. Nanocrystalline silicon was coated onto porous graphite via chemical vapor decomposition of SiH4 gas and subsequently pitch coating was carried out, which is known for a good method for preventing the natural oxidation of silicon layer and improving capacity. We analyzed electrochemical properties of blended anode with graphite and nanocrystalline silicon­-porous graphite composite material. The capacity retention of the blending anode for nanocrystalline silicon­-porous graphite is 96% after 50 cycles with high coulombic efficiency of 90.1%, which is improved electrochemical performance compared with silicon non-porous graphite composite anode. Especially, the nanocrystalline silicon­-porous graphite composite material has lower expansion of electrode, comparing with silicon non-porous graphite composite material. The expansion of the nanocrystalline silicon­-porous graphite composite electrode is 42% after 1 cycle, while the expansion of the silicon non-porous graphite composite is 62% after 1 cycle. These improvements are attributed to the porous carbon shell, which reduces the cell expansion, prevents rupture of electrical path, stabilize the cycle performance.

EPR and Structural Characterization of Water-Soluble Mn2+-Doped Si Nanoparticles.

Water-soluble poly(allylamine) Mn2+-doped Si (SiMn) nanoparticles (NPs) were prepared and show promise for biologically related applications. The nanoparticles show both strong photoluminescence and good magnetic resonance contrast imaging. The morphology and average diameter were obtained through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM); spherical crystalline Si NPs with an average diameter of 4.2 ± 0.7 nm were observed. The doping maximum obtained through this process was an average concentration of 0.4 ± 0.3% Mn per mole of Si. The water-soluble SiMn NPs showed a strong photoluminescence with a quantum yield up to 13%. The SiMn NPs had significant T1 contrast with an r1 relaxivity of 11.1 ± 1.5 mM–1 s–1 and r2 relaxivity of 32.7 ± 4.7 mM–1 s–1 where the concentration is in mM of Mn2+. Dextran-coated poly(allylamine) SiMn NPs produced NPs with T1 and T2 contrast with a r1 relaxivity of 27.1 ± 2.8 mM–1 s–1 and r2 relaxivity of 1078.5 ± 1.9 mM–1 s–1. X-band electron paramagnetic resonance spectra are fit with a two-site model demonstrating that there are two types of Mn2+ in these NP’s. The fits yield hyperfine splittings (A) of 265 and 238 MHz with significant zero field splitting (D and E terms). This is consistent with Mn in sites of symmetry lower than tetrahedral due to the small size of the NP’s.

A study of silicon tetrafluoride reduction with hydrogen in radiofrequency discharge

The process of plasma-chemical conversion of silicon tetrafluoride in a radiofrequency (13.56 MHz) discharge has been studied. The dependence of the yield of silicon on the specific energy input and the H2/SiF4 molar ratio has been examined in the pressure range of 0.1–0.3 Tort. The maximum yield of silicon is 60%, and the optimal specific energy consumption is 44.6 MJ per mole of Si. A mechanism has been proposed for the plasma-assisted reduction of silicon tetrafluoride under the given experimental conditions.

Water-mediated structural tunability of an alkyl/siloxane hybrid: from amorphous material to lamellar structure or bilamellar superstructure

Mono-amide cross-linked alkyl/siloxane hybrids (classed as mono-amidosils and represented by the notation m-A(x), where m and A stand for mono and amide, respectively, and x is the number of methylene repeat units) with water-mediated tunable structures have been prepared by means of sol–gel chemistry and self-direct assembly routes from the organosilanes CH3(CH2)x–C(O)NH–(CH2)3–Si–(OCH2CH3)3. The amorphous sample m-A(8) has been produced under the stoichiometric conditions (molar ratio Si : ethanol : water = 1 : 4 : 2) used previously to obtain the lamellar bilayer highly organized m-A(14) hybrid material, showing that the length of the pendant alkyl chains affects the degree of order of the materials. Three structurally ordered samples identified as AC-m-A(8) (1 : y) (where AC represents acid catalysis and y is the number of moles of water per mole of Si) have been obtained using the acid catalyzed hydrolytic route and different water contents (y = 600, 300 and 100). Water plays a unique role in the organized mono-amidosils: it not only reverses the natural tendency of the precursor molecule with x = 8 to yield a disordered material, but it also allows the induction of order, leading to the formation of a lamellar structure exclusively in AC-m-A(8) (1 : 100). The presence of a higher water content promotes extra ordering in AC-m-A(8) (1 : 300) and AC-m-A(8) (1 : 600), where the lamellar structure coexists with a bilamellar superstructure. Upon heating the AC-m-A(8) (1 : 600) sample to 120 °C and then cooling it to room temperature, the lamellar structure remained unaffected, while the superstructure was destroyed. The occurrence of the superstructure, although basically associated with the preferential entrapment of water molecules every two lamellae at the siliceous nanodomains, is also correlated with the amide–amide hydrogen bonded array. The m-A(8) and AC-m-A(8) (1 : 600) hybrids are multi-wavelength emitters under UV/VIS excitation. The emission spectra exhibit a broad band (300–650 nm) resulting from two contributions: a “blue” band, due to electron–hole recombinations in the NH/CO groups of the amide cross-links, and a “purplish-blue” band, due to oxygen-related defects •O–O–Si≡(CO2) in the siliceous nanoclusters. The absolute emission quantum yields determined for m-A(8) and AC-m-A(8) (1 : 600) in the 360–380 nm excitation wavelength range were 0.10 ± 0.01 and 0.15 ± 0.01, respectively. While the lifetime values of the high and low wavelength components of the siliceous-related emission are practically the same for both samples (∼3.5 ms), the lifetime value of the NH/CO component for m-A(8) is higher than that of AC-m-A(8) (1 : 600) (∼292 and ∼156 ms, respectively), suggesting a lower non-radiative transition probability for the former hybrid.

Study on Synthesis of TPA-Silicalite-1 from Initially Clear Solutions of Various Base Concentrations by in Situ Calorimetry, Potentiometry, and SAXS

The synthesis process of TPA-silicalite-1 from a series of initially clear solutions (with a general formula based on mole ratios of the components x:25:480:100 TPAOH:SiO2:H2O:C2H5OH, where TPA = tetrapropylammonium, x = 3−13) was investigated using small-angle X-ray scattering (SAXS), in situ calorimetry, and in situ pH measurement. The size of the nanoparticles detected in the initially clear solutions decreases from 8.5 to 2.5 nm as x increases from 3 to 13. Crystal growth from these solutions at 95 °C is first exothermic and then endothermic. The exo−endo thermal switch coincides with a jump in the solution alkalinity. With increasing x the integral heat of exothermic crystal growth decreases monotonically from −0.61 kJ/mol (per mole of Si in the mixture) at x = 4 and approaches zero at x = 12. The integral heat of endothermic crystal growth increases to approach a plateau (0.26 kJ/mol of Si) at x = 7. Crystal growth is fastest at x = 6−7 and relatively slower at x either higher than 7 or lower than 6...

Effects of temperature on the structures of silicate liquids: 29Si NMR results

The effect of temperature on the structures of three silicate liquids has been studied by 29Si NMR spectroscopy on glass samples prepared with different glass transition temperatures. The compositions studied are (Na 2O) .34(SiO 2) .66 and (CaO) .10(Na 2O) .15(SiO 2) .75 and (CaO) .25(MgO) .25(SiO 2) .50. In the first two, the abundance of SiO 4 tetrahedra with four bridging oxygens ( Q 4 ) increases significantly with temperature. This change results from the disproportionation reaction 2Q 3 a ́ i Q 2 + Q 4 ( Q n is a SiO 4 tetrahedron with n bridging oxygens), with the observed increase in disproportionation at higher temperatures implying increased randomization of the anionic structure. In the (Na 2O) .34(SiO 2) .66 composition, the temperature dependence of Q 4 abundance enables us to estimate ΔH 0 for this reaction to be 30 ± 15 kJ per mole of Si. Although this reaction thus makes only a minor contribution to the conngurational heat capacity of each liquid, it probably has a major influence on the thermodynamic activity of SiO 2. Widening of the (CaO) .25(MgO) .25(SiO 2) .50 spectral peak with increasing temperature likewise implies increasing randomization of the anionic structure, although specific structural contributions to this widening are not identified. Models of melt structures at liquidus temperatures will be limited in accuracy if they assume that the speciation observed in glasses is unaffected by temperature change.

Hydrolysis-Condensation of Zr-O-Si Alkoxides

A Zr-O-Si alkoxide complex was synthesized and its hydrolysis-condensation reaction was studied using small angle X-ray scattering. Transparent and homogeneous Zr-O-Si alkoxide solutions were formed by reacting Zr(OC3H7)4 with Si(OC2H5)4 hydrolyzed with one mole of water per one mole of Si(OC2H5)4 and HCl catalyst, whereas the solutions hydrolyzed with NH4OH catalyst or without any catalyst resulted in precipitation of ZrO2 crystals. Homogeneous Zr-O-Si alkoxide solutions were hydrolyzed with 0 to 8 moles of water to metal alkoxide, whose X-ray scattering curves were analyzed in both power law and Guinier law. The synthesized Zr-O-Si alkoxide consisted of chain like polymers, which were aggregated into three dimensional branched polymers with water content up to 4 moles per 1 mole metal alkoxide and their size increased up to 65A in the Guinier radius. However, no further changes in size and shape of aggregated polymers were observed with more than 4 moles of water.

The influence of organic matter extracted from humified clover on the properties of amorphous aluminosilicates.II. Phosphate retention

Freshly formed water soluble organic matter extracted from humified clover leaves was used to treat a range of amorphous aluminosilicates and hydrous alumina. Phosphate retention isotherms were determined on the resulting organomineral complexes. These isotherms revealed the influence of the organic matter on phosphate retention by amorphous aluminosilicates. The organic treatment was found to reduce phosphate retention by hydrous alumina and amorphous synthetic aluminosilicates with high and low Al/(Al+Si) mole ratios. There was no depression of phosphate retention with amorphous aluminosilicates of intermediate composition. Similarly, the phosphate retention capacity of an allophanic soil clay of intermediate Al/(Al+Si) mole ratio was not reduced by organic treatment. These results appear to be related to the degree of polymerization of charge balancing hydroxy-aluminium in the aluminosilicates.

A thermodynamic study of the crosslinking of methyl silicone rubber

The crosslinking reaction in a two component methyl silicone rubber has been studied by thermomechanical analysis (TMA) and differential scanning calorimetry (DSC). The rubber was formed from two methyl silicone prepolymers; one containing reactive hydrogens every 50 to 100 groups and the other polymer containing pendant vinyl groups at the same frequency. In the presence of a platinum catalyst above 60°C crosslinking proceeds without a loss or gain in weight. The heat of reaction, energy of activation (calculated by two methods) crosslink density and elastic modulus (Young's) were studied as a function of prepolymer concentration, dilution and swelling. A preliminary value for the heat of reaction per mole of SiH and SiCHCH 2 has been calculated. From crosslink density measurements both by hexane swelling and TMA and DSC heats of reaction a qualitative picture has been obtained of the role of entangled chains in producing effective crosslinks.