Standard Graphite Research Articles

Making Room for Silicon: Including SiOx in a Graphite-Based Anode Formulation and Harmonization in 1 Ah Cells

Transitioning to more ambitious electrode formulations facilitates developing high-energy density cells, potentially fulfilling the demands of electric car manufacturers. In this context, the partial replacement of the prevailing anode active material in lithium-ion cells, graphite, with silicon-based materials enhances its capacity. Nevertheless, this requires adapting the rest of the components and harmonizing the electrode integration in the cell to enhance the performance of the resulting high-capacity anodes. Herein, starting from a replacement in the standard graphite anode recipe with 22% silicon suboxide at laboratory scale, the weight fraction of the electrochemically inactive materials was optimized to 2% carbon black/1% dispersant/3% binder combination before deriving an advantage from including single-wall carbon nanotubes in the formulation. In the second part, the recipe was upscaled to a semi-industrial electrode coating and cell assembly line. Then, 1 Ah lithium-ion pouch cells were filled and tested with different commercial electrolytes, aiming at studying the dependency of the Si-based electrodes on the additives included in the composition. Among all the electrolytes employed, the EL2 excelled in terms of capacity retention, obtaining a 48% increase in the number of cycles compared to the baseline electrolyte formulation above the threshold capacity retention value (80% state of health).

Graphene Materials from Coke-like Wastes as Proactive Support of Nickel-Iron Electro-Catalysts for Water Splitting.

Graphene materials, used as electrocatalyst support in green hydrogen production, contribute to increasing the efficiency and robustness of various systems. However, the preparation of a hybrid catalyst containing graphene materials from industrial wastes is still a challenge due to the heterogeneity of the waste. We report the synthesis of 3D electrodes using graphene oxides (GOs) from industrial waste (IW) prepared by immersion onto Toray carbon paper as a 3D support onto GO suspensions and electrodepositing NiFe layered double hydroxides (LDHs). Standard graphite was also used as the reference. The morphology of the two hybrid electrodes was determined by SEM, HRTEM, XPS. Although very similar in both, the sample containing graphene from IW (higher Csp3 hybridization in the graphene layer) has a NiFe phase with less crystallinity and larger presence of Fe2+ ions. These electrodes exhibited similar activity and stability as electrocatalysts of the oxygen evolution reaction (OER), demonstrating the proactive effect of the graphene into the 3D electrode even when this is prepared from heterogeneous industrial waste. Moreover, the defective graphenic structure of the waste GO enhances the reaction kinetics and improves the electron transfer rate, possibly due to the small differences in the electrodeposited NiFe LDH structure.

Porous MCM‐41 Silica Materials as Scaffolds for Silicon‐based Lithium‐ion Battery Anodes

AbstractAiming for specific energy improvements, lithium‐ion battery (LIB) research explores Si based materials as potential alternatives for the negative electrode/anode. Si exhibits a high specific capacity when lithiated, accompanied by a large volumetric expansion. To mitigate expansion induced failures, composite materials with finely distributed Si inside a scaffold have been established. Potential scaffolds to create such Si composites are nano porous SiO2 materials, such as MCM‐41. MCM‐41 exhibits a fine nanostructure, and the synthesis allows precise tuning of the pore properties and thus, after filling with Si, of Si morphology and Si content in the composite. In this work, insights into relevant MCM‐41 synthesis parameters are acquired, with special attention towards specific pore volumes and pore sizes of the SiO2 powders. Materials characterization is performed using nitrogen sorption analysis, X‐ray scattering, electron microscopy and thermogravimetric analysis. Selected MCM‐41 scaffolds are processed to Si composites and integrated into LIB electrodes. The specific capacity and the stability of the Si−SiO2 composites are evaluated by PITT and galvanostatic cycling. They show capacities above 800 mAh g−1, i. e. more than twice the specific capacity of industry standard graphite, and last over 200 charge/discharge cycles before their capacity loss exceeds 25 %.

Impact of Graphitization Degree on the Electrochemical and Thermal Properties of Coal.

Coal-based cryptocrystalline graphite is an intermediate phase formed during the transformation of highly metamorphic anthracite into crystalline graphite. In order to explore the relationship between the graphitization degree of coal-based cryptocrystalline graphite and its physical properties from macromolecular structure to provide a theoretical basis for industrial application, samples were tested by X-ray diffraction, electrochemistry, and thermal conductivity and compared with standard graphite (SG) and artificial thermal simulation graphitized samples. The results show that with the increase of graphitization degree and the growth of microcrystalline structure, the electrical impedance of cryptocrystalline graphite decreases, the conductivity increases, the specific capacity of initial discharge increases, and the thermal conductivity increases, which gradually approach the electrical and thermal properties of crystalline graphite. The linear equations between impedance and La and Lc are y = -0.42x + 70.44 and y = -1.87x + 70.62, and the correlation coefficients are 0.93 and 0.88. The linear equations between thermal conductivity and the horizontal extension length (La) and vertical stacking thickness (Lc) are y = 0.09x + 1.36 and y = 0.4x + 0.76, the correlation coefficients are 0.82 and 0.84., and the reduction of microcrystalline parameters d002 and the increase of La and Lc lead to a direct improvement of physical properties. Artificial thermal simulation samples also show the same regularity, but their physical properties are lower than those of natural evolution samples. Short-term high-temperature simulation is different from long-term magma heat and pressure, and the growth of graphite microcrystals is more complete under long-term geological conditions, resulting in better physical properties.

A portable primary-standard level graphite calorimeter for absolute dosimetry in clinical pencil beam scanning proton beams

Objective. To report the use of a portable primary standard level graphite calorimeter for direct dose determination in clinical pencil beam scanning proton beams, which forms part of the recommendations of the proposed Institute of Physics and Engineering in Medicine (IPEM) Code of Practice (CoP) for proton therapy dosimetry. Approach. The primary standard proton calorimeter (PSPC) was developed at the National Physical Laboratory (NPL) and measurements were performed at four clinical proton therapy facilities that use pencil beam scanning for beam delivery. Correction factors for the presence of impurities and vacuum gaps were calculated and applied, as well as dose conversion factors to obtain dose to water. Measurements were performed in the middle of 10 × 10 × 10 cm3 homogeneous dose volumes, centred at 10.0, 15.0 and 25.0 g·cm−2 depth in water. The absorbed dose to water determined with the calorimeter was compared to the dose obtained using PTW Roos-type ionisation chambers calibrated in terms of absorbed dose to water in 60Co applying the recommendations in the IAEA TRS-398 CoP. Main results. The relative dose difference between the two protocols varied between 0.4% and 2.1% depending on the facility. The reported overall uncertainty in the determination of absorbed dose to water using the calorimeter is 0.9% (k = 1), which corresponds to a significant reduction of uncertainty in comparison with the TRS-398 CoP (currently with an uncertainty equal or larger than 2.0% (k = 1) for proton beams). Significance. The establishment of a purpose-built primary standard and associated CoP will considerably reduce the uncertainty of the absorbed dose to water determination and ensure improved accuracy and consistency in the dose delivered to patients treated with proton therapy and bring proton reference dosimetry uncertainty in line with megavoltage photon radiotherapy.

Lithium‐Metal Free Sulfur Battery Based on Waste Biomass Anode and Nano‐Sized Li2S Cathode

The realization of a stable lithium‐metal free (LiMF) sulfur battery based on amorphous carbon anode and lithium sulfide (Li2S) cathode is here reported. In particular, a biomass waste originating full‐cell combining a carbonized brewer's spent grain (CBSG) biochar anode with a Li2S‐graphene composite cathode (Li2S70Gr30) is proposed. This design is particularly attractive for applying a cost‐effective, high performance, environment friendly, and safe anode material, as an alternative to standard graphite and metallic lithium in emerging battery technologies. The anodic and cathodic materials are characterized in terms of structure, morphology and composition through X‐ray diffraction, scanning and transmission electron microscopy, X‐ray photoelectron and Raman spectroscopies. Furthermore, an electrochemical characterization comprising galvanostatic cycling, rate capability and cyclic voltammetry tests were carried out both in half‐cell and full‐cell configurations. The systematic investigation reveals that unlike graphite, the biochar electrode displays good compatibility with the electrolyte typically employed in sulfur batteries. The CBSG/Li2S70Gr30 full‐cell demonstrates an initial charge and discharge capacities of 726 and 537 mAh g−1, respectively, at 0.05C with a coulombic efficiency of 74%. Moreover, it discloses a reversible capacity of 330 mAh g−1 (0.1C) after over 300 cycles. Based on these achievements, the CBSG/Li2S70Gr30 battery system can be considered as a promising energy storage solution for electric vehicles (EVs), especially when taking into account its easy scalability to an industrial level.

Catalyzed oxidation of nuclear graphite by simulated fission products Sr, Eu, and I

The influence of three fission products Sr, Eu, and I on the oxidation of IG-110 nuclear graphite was studied in the temperature range of 400 to 1000 °C. Sr and Eu were introduced as chlorides, and I was introduced as NaI. The temperature dependence of both CO2 and CO production during the graphite oxidation measured with mass spectroscopy and infrared spectrometry shows that the introduction of these three compounds to graphite significantly decreases the onset temperature for the oxidation of graphite. Among the three compounds, NaI is the most active towards the oxidation reaction, characterized by a significant decrease of the onset temperature from approximately 650 to 400 °C before and after its introduction to graphite. Separate measurements of CO2 and CO concentration at varying temperatures enable the calculation of the activation energy for the formation of CO2 and CO. The activation energies for the oxidation of pure and fission product-impregnated graphite samples decrease in the following order: standard IG-110 graphite, EuCl3-impregnated IG-110, SrCl2-impregnated IG-110, and NaI-impregnated IG-110. This trend indicates that the three compounds catalyze the oxidation of graphite at temperatures relevant to the operation of high-temperature gas-cooled reactors. Furthermore, it is found that the three compounds can also affect the molar ratio of reaction products CO2 and CO, and the rates of the graphite oxidation. At temperatures higher than about 850 °C, the impregnated samples exhibit lower CO2: CO ratios than the pure graphite. Different from EuCl3 and NaI, the introduction of SrCl2 decreases the graphite oxidation rates at temperatures higher than about 770 °C. Their catalytic mechanism can be understood based on a redox cycle of the intermediate active species, promoting the dissociation of molecular oxygen and transfer to the carbon.

Solution processable Si/Ge heterostructure NWs enabling anode mass reduction for practical full-cell Li-ion batteries.

Here, we report the solution phase synthesis of axial heterostructure Si and Ge (hSG) nanowires (NWs). The NWs were grown in a high boiling point solvent from a low-cost Sn powder to achieve a powder form product which represents an attractive route from lab-scale to commercial application. Slurry processed anodes of the NWs were investigated in half-cell (versus Li-foil) and full-cell (versus NMC811) configurations of a lithium ion battery (LIB). The hSG NW anodes yielded capacities of 1040 mA h g-1 after 150 cycles which corresponds to a 2.8 times increase compared to a standard graphite (372 mA h g-1) anode. Given the impressive specific and areal capacities of the hSG anodes, a full-cell test against a high areal capacity NMC811 cathode was examined. In full-cell configuration, use of the hSG anode resulted in a massive anode mass reduction of 50.7% compared to a standard graphite anode. The structural evolution of the hSG NW anodes into an alloyed SiGe porous mesh network was also investigated using STEM, EDX and Raman spectroscopy as a function of cycle number to fully elucidate the lithiation/delithiation mechanism of the promising anode material.

Calcium as an innovative and effective catalyst for the synthesis of graphene-like materials from cellulose

Pyrolysis of lignocellulosic biomass (hard carbon) produces poorly graphitic biochar. In this study, nano-structured biochars were produced from microcrystalline cellulose using calcium as a non-conventional catalyst. Calcium is abundant, environmental-friendly and widely accessible. Graphitization of calcium-impregnated cellulose was carried out at 1800 °C, a temperature below 2000 °C where the graphitization usually occurs. XRD, Raman spectroscopy, high-resolution TEM together with the in-house numerical tool developed enable the quantification of the graphene fringes in the biochars. The non-impregnated cellulose biochar was composed of short and poorly stacked graphene fringes. The impregnation with 2 wt.% of calcium led to the conversion of the initial structure into a well-organized and less defective graphene-like one. The graphene-like structures obtained were composed of tens of stacked graphene fringes with a crystallite size up to 20 nm and an average interlayer spacing equal to 0.345 nm, close to the reference value of standard hexagonal graphite (0.3354 nm). The increase of the calcium concentration did not significantly improve the crystallite sizes of the graphene-like materials but rather drastically improved their rate. Our results propose a mechanism and provide new insights on the synthesis of graphene-like materials from bio-feedstocks using calcium where the literature is focused on transition metals such as iron and nickel among others. The decrease of the graphitization temperature below 2000 °C should lower the production cost as well as the environmental impact of the thermal graphene-like materials synthesis using biomass. This finding should stimulate further research in the field and broaden the application perspectives.

Direct emission spectrometry determination of fluorine in carbon materials: Argon dc arc and spark radiation sources

In this work, the procedure for direct atomic emission spectrometry (AES) determination of fluorine impurities in carbon materials was proposed. Two types of simple radiation sources with direct solid (powdered) sample introduction were studied: direct current arc (25A) and high-power low voltage spark (300 V, of 2348 μF). For each source, standard graphite extra high purity electrodes and argon discharge atmosphere were used. Red spectral region approach to non-metal AES determination was employed with analytical line F I 685.603 nm. The spectra were registered by compact tunable fiber-optic spectrometer with linear complementary metal oxide semiconductor array (CMOS) sensor and short-focus lens. Special measures were taken to counter red spectral region interferences, such as CN molecular bands and thermal radiation. The excitation of F in both types of radiation source was studied. A set of artificial graphite and NaF based standards was prepared for calibration and evaluated by two independent approaches: indirect dc arc AES and water extraction photometry. Limit of detection was determined for dc arc source was 5 μg/g, and for spark 20 μg/g of F. The whole cycle of measurement with each type of radiation source takes 10 min. As the power of dc arc is limited by generator capabilities, it is possible that even lower limit of detection is achievable with higher discharge current.

Application of a portable primary standard level graphite calorimeter for absolute dosimetry in a clinical low-energy passively scattered proton beam

Objective. A calibration service based on a primary standard calorimeter for the direct determination of absorbed dose for proton beams does not exist. A new Code of Practice (CoP) for reference dosimetry of proton beams is being developed by a working party of the UK Institute of Physics and Engineering in Medicine (IPEM), which will recommend that ionisation chambers are calibrated directly in their clinical beams against the proposed Primary Standard Proton Calorimeter (PSPC) developed at the National Physical Laboratory (NPL). The aim of this work is to report on the use of the NPL PSPC to directly calibrate ionisation chambers in a low-energy passively scattered proton beam following recommendations of the upcoming IPEM CoP. Approach. A comparison between the dose derived using the proposed IPEM CoP and the IAEA TRS-398 protocol was performed, and values were determined experimentally for three types of chambers. In total, 9 plane-parallel and 3 cylindrical chambers were calibrated using the two protocols for two separate visits. Main results. The ratio of absorbed dose to water obtained with the PSPC and with ionisation chambers applying TRS-398 varied between 0.98 and 1.00, depending on the chamber type. The new procedure based on the PSPC provides a significant improvement in uncertainty where absorbed dose to water measured with a user chamber is reported with an uncertainty of 0.9% (1), whereas the TRS-398 protocol reports an uncertainty of 2.0% and 2.3% (1) for cylindrical and plane-parallel chambers, respectively. The values found agree within uncertainties with those from TRS-398 and Monte Carlo calculations. Significance. The establishment of a primary standard calorimeter for the determination of absorbed dose in proton beams combined with the introduction of the associated calibration service following the IPEM recommendations will reduce the uncertainty and improve consistency in the dose delivered to patients.

NEW SAMPLE PREPARATION LINE FOR RADIOCARBON MEASUREMENTS AT THE GXNU LABORATORY

ABSTRACTA new system for preparing 14C samples was established for a compact accelerator mass spectrometer (GXNU-AMS) at Guangxi Normal University. This sample preparation system consists of three units: a vacuum maintenance unit, a CO2 purification unit, and a CO2 reduction unit, all of which were made of quartz glass. A series of radiocarbon (14C) preparation experiments were conducted to verify the reliability of the system. The recovery rate of graphite obtained was more than 80%. The carbon content in the commercial toner and wood sample was linearly fitted to the CO2 pressure in the measurement unit of the system. The results showed a good linear relationship, indicating that the reliability of the sample preparation system. AMS measurements were conducted on a batch of standard, wood, and dead graphite samples prepared using this system. The results showed that the beam current of 12C- for each sample was more than 40 μA, the carbon contamination introduced during the sample preparation process was ∼ 2 × 10–15, and that the new sample preparation system is compact, low-contamination, and efficient and meets the GXNU-AMS requirements for 14C samples.

Nuclear heating measurements for fusion and fission relevant materials in the JSI TRIGA reactor

The knowledge of nuclear heating inside nuclear (fission or fusion) reactor is essential for safe reactor operation, to know and meet limits expressed in terms of cooling requirements, and it represents critical input information for the design of any experimental irradiation device. In the framework of a research collaboration project between the Atomic and Alternative Energy Commission (CEA) in France, and the Jožef Stefan Institute (JSI) in Slovenia, a new project was launched in 2018 with the objective to measure the total heating rates in different materials of interest inside the JSI TRIGA Mark II reactor using the calorimetry technique. Three standard materials, graphite, aluminum and tungsten, and a fusion-relevant material, Eurofer-97, have been selected. The design of the calorimeters to host the four selected material samples is based on the CALMOS and CARMEN differential calorimeters, previously developed at the CEA, which were used to perform nuclear heating measurements in the OSIRIS material testing reactor (MTR) at the Saclay research center.The design of these specific calorimeters, and the dimensioning of the material samples located within, necessitated preliminary particle transport calculations using the MCNP V6.1 Monte Carlo code in order to estimate the expected heating levels in the experimental Central Channel of the core, and to avoid excessive perturbation of the measured radiation field. On the other hand, a parametric study was carried out using the COMSOL multiphysics code, to evaluate the expected temperature differences that could be measured using calorimeters designed specifically for low heating rates in the JSI TRIGA reactor core. A dedicated measurement campaign took place in July 2021 in which heating measurements were carried out at 100 kW reactor power and at nominal reactor power - 250 kW. This paper presents the design and preparation of the calorimeters, their preliminary calibration, the measurements carried out and the analysis of the obtained experimental results.

Spruce Hard Carbon Anodes for Lithium‐Ion Batteries

AbstractIn this work, lithium‐ion battery full‐cells based on spruce‐derived hard carbon anodes and an electrochemical pre‐lithiation method are presented in combination with a detailed analysis of full‐cell operation and the lithiation state. The physical and electrochemical properties agree well with those of previous biomass‐derived hard carbon anodes. However, low initial coulombic efficiencies of 65 % represent one of the major challenges of the developed anodes with respect to full‐cell operation. To counteract the initial lithium loss, in‐situ electrochemical pre‐lithiation was conducted, allowing battery operation in the same cell setup without reassembly. Consequently, significantly increased capacities, cycle life, and first cycle coulombic efficiency were obtained in comparison to untreated anodes (195 mAh/g versus 150 mAh/g, state of health (SOH) 80 after 150 cycles versus 70 cycles, and 90 % versus 65 %). In summary, spruce‐based hard carbon has the potential to be an environmentally friendly alternative to standard graphite.

Three-Dimensional Mapping of Cycling Changes in Silicon-Graphite Composite Anodes Via Scanning Probe Microscopy

Silicon (Si) anodes have the potential to greatly improve the energy density of lithium-ion batteries due to the greater specific capacity of Si relative to standard graphite (Gr) anodes (1). However, Si anode implementation is limited by issues such as volumetric expansion during cycling and an unstable solid-electrolyte interphase (SEI). Composite anodes with Si and Gr particles can increase capacity while mitigating Si anode degradation. However, these electrodes can be challenging to characterize (and therefore optimize) due to their multiple components, heterogeneous surfaces, and the complex processes occurring there. Scanning spreading resistance microscopy (SSRM) is a form of scanning probe microscopy that measures local resistivity in a sample. By removing material with the probe tip while measuring resistivity, three-dimensional resistivity maps can be determined (2). For Si-Gr composite anodes, the different resistivities of the electrode components (e.g., Si, Gr, carbon, and lithium polyacrylate binder) allows direct identification through SSRM. 3D nanoscale volumes can then be created for both the whole electrode and the individual components.This presentation will demonstrate 3D SSRM mapping of pristine and cycled anodes with three compositions – Si, Gr, and Si-Gr. By comparing changes in the morphology, thickness, and resistivity in the active material and SEI in each single-component electrode, the component distribution and evolution of the Si-Gr composite anode is better understood. SSRM results will be compared to chemical analysis techniques such as scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and electron energy loss spectroscopy to verify the component identification and to correlate chemical composition to resistivity. These data will then be correlated to electrochemical performance and used to further the development of Si-Gr electrodes with enhanced performance. W.-J. Zhang, Journal of Power Sources, 196, 13 (2011). C. Stetson, Z. Huey, A. Downard, Z. Li, B. To, A. Zakutayev, C. S. Jiang, M. M. Al-Jassim, D. P. Finegan, S. D. Han and S. C. DeCaluwe, Nano Lett (2020). Figure 1

Study on pore and chemical structure characteristics of atmospheric circulating fluidized bed coal gasification fly ash

The pore and chemical structure characteristics of the raw coal and the coal gasification fly ash (CGFA) in an atmospheric pressure circulating fluidized bed gasifier with a gas production capacity of 20000 and 40000 Nm3 h−1 were studied. The results indicated the presence of developed pore structure and high carbon content of the CGFA. The comprehensive fractal dimension method was proposed to analyze the pore structure of the pore section. The comprehensive fractal dimension was found to be > 2.6. The surface of CGFA is rough and porous. When compared to raw coal, the relative content of the CGFA carbon functional group is lower than 20%, and that of aliphatic carbon gets reduced. The interlamellar spacing (0.3489 nm, 0.3501 nm) of the A-filter cake ash (FCAA), A-dry ash (DAA), B-filter cake ash (FCAB), and B-dry ash (DAB) carbon is higher than that of the standard graphite (0.3354 nm), lean coal (0.3468 nm), and anthracite (0.344 nm). Moreover, the carbon structure is loose. The comprehensive results of physical and chemical structure revealed that CGFA showed a good chemical reaction activity and also the value of secondary combustion and gasification as fuel. This study exhibits significance and importance in the field of waste valorization towards a circular economy.

Silicon anode start-ups attract new investment

Sila Nanotechnologies and Amprius Technologies, two firms developing silicon-rich materials as alternatives to standard graphite for lithium-ion battery anodes, have secured new private investments to help push their technologies into the marketplace. Sila has received $45 million from the Canada Pension Plan Investment Board in exchange for equity. The investment takes the California-based company’s funds raised this year to $210 million. Sila says it will use the new cash to ramp up production. It aims to start supplying its silicon nanocomposite anode material to makers of batteries for consumer electronics starting next year. Meanwhile, Stanford University spin-off Amprius says it has received an undisclosed “strategic investment” from the aerospace firm Airbus. Amprius’s silicon anodes are used in batteries that power Airbus’s Zephyr, a stratosphere-traveling satellite known as a pseudosatellite. Lux Research analyst Chloe Holzinger says she expects Amprius to use the new funding “to expand production capabilities and develop a

Characterisation of a megavolt X-Ray therapeutic beam using non-orthogonal strip detectors for future enhanced primary standard dosimetry

The National Physical Laboratory (NPL) provides traceable high accuracy measurements for absorbed dose via primary standard graphite calorimeters. Patients undergoing radiotherapy for cancer treatments in the U.K. receive a dose that is traceable to these devices. In the case of hadron radiotherapy, reducing the uncertainty of these measurements requires the addition of spatial information to understand not only the integral dose delivered, but where the energy is deposited locally. The PRaVDA consortium developed strip detectors able to track individual protons at low rates for proton computed tomography. For calorimetry at clinical dose rates only real-time spot location is required, which could be projected to reconstruct a 2D visualisation from which the beam centre can be determined. This work reports on a proof-of-concept trial carried out using 6 MV X-rays using an Elekta Synergy linear accelerator. We show how the beam positions can be determined within ± 0.5 mm of the expected radiation beam position, which is similar to the tolerances of the field defining multileaf collimator within the linear accelerator treatment head. The trial will be extended to perform measurements in a clinical proton beam alongside the NPL graphite proton calorimeter.

(Invited) Microcalorimetry of Silicon Anodes for Lithium-Ion Batteries

Isothermal microcalorimetry is a promising characterization tool to both enhance understanding of degradation processes of silicon anodes in lithium-ion batteries, as well as to measure the impacts of efforts to improve the performance of these electrode materials. Isothermal microcalorimetry is a thermal characterization technique in which a sample is held in isothermal conditions while heat flow into / out of the sample to maintain a constant temperature is measured. This heat flow is determined by endothermic or exothermic reactions within the prepared sample. Isothermal titration calorimetry is a similar technique in which controlled amounts of a reactant are introduced to the sample during measurement, allowing capture of the reaction energies between materials. Both techniques are employed in our study of silicon anodes. Silicon has long been pursued as a next generation anode material for lithium-ion batteries due to its very high theoretical capacity relative to the currently employed graphite anode. So far, however, the practical realization of silicon anodes has been limited by both mechanical degradation caused by volume change during lithiation / delithiation as well as electrode surface instability that combine to shorten battery life and reduce capacity. Mitigation of these issues is being pursued through advanced binder materials, electrolyte additives, nano-scale architectures, and composite active materials.1 Previous microcalorimetry work on graphite anodes has demonstrated the ability of the technique to discern different lithiation reactions, measure the severity of parasitic losses, and identify the points of electrolyte degradation.2 Sandia National Laboratories is employing isothermal microcalorimetry to study new composite anodes of 15 wt.% silicon combined with standard graphite active material. Our microcalorimetry characterization approach uses a TA Instruments TAM IV with resolution to ± 200 nW to analyze samples of silicon-composite electrodes and compare their thermal signatures to baseline graphite samples to offer an understanding of the impact of silicon during electrode processing and fabrication, initial active material passivation upon electrolyte exposure, and electrochemical cycling of the battery. Our initial work focuses on the impacts of processing various silicon materials using a commonly employed LiPAA binder in an aqueous solvent. While silicon has a naturally passivating surface oxide layer, heat and gas generation can be observed during slurry mixing with the sub-micron sized silicon materials used in lithium-ion batteries. This reactivity can serve to rob capacity and increase material impedance before the electrode is even built into a battery. Later work will focus on characterizing the composite silicon electrodes during electrochemical cycling. When cycled in situ, the microcalorimeter can capture heat flow related to both surface electrolyte reactions and lithiation reactions of the silicon electrodes as well as the impacts of new binders, additives, or surface passivation approaches to mitigate these effects. References Yang J., Wang B. F., Wang K., Liu Y., Xie J. Y., Wen Z. S., Solid-State Lett., 6, A154 (2003).Krause L. J., Jensen L. D., Dahn J. R., Electrochem. Soc., 159, A937 (2012). Acknowledgment: Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Lithiation of Oxidised Silicon

The electrochemical energy storage market is growing rapidly, and is currently dominated by lithium ion batteries. Replacing the standard graphite anode with silicon, whose theoretical gravimetric capacity is roughly ten times higher, promises a step-change improvement in capacity. Unfortunately, the use of silicon in lithium ion battery anodes frequently leads to poor cycle life, related at least in part to the challenges in creating a stable solid-electrolyte interphase (SEI). This issue is made more complex by the fact that the silicon used in composite anodes is typically nanostructured, with a poorly defined and often oxidized surface, especially when the silicon was prepared at elevated temperature. SiOx has been proposed as an alternative anode material to Si, whereas SiO2 is an electronic insulator. It is therefore valuable to understand the effect of surface oxides in more detail. In this work, we take a fundamental approach to understanding how an oxide on a silicon surface affects SEI formation and lithiation. Silicon wafers with precisely controlled thermal SiO2 thicknesses were prepared by thinning thicker oxides down to 1-30nm SiO2 using buffered hydrofluoric acid solution. These wafers were then cycled against a Li counter and reference electrode in Gen2 electrolyte (1.2M LiPF6 in EC:EMC (3:7 by wt.)) using custom three-electrode cells, which use an O-ring seal on the silicon to prevent contact between the electrolyte and the edges or rear of the wafers. Galvanostatic cycling at 20µAcm-2 indicates that while Si with 1nm SiO2 will lithiate above 0V vs Li, Si with 3nm SiO2 will only lithiate after a potential spike to -0.4V vs Li. Thereafter, it actually cycles with higher Coulombic efficiency (CE) than Si with 1nm SiO2. However, Si with 5-30nm SiO2 initially requires voltages below -1.0V vs Li to sustain 20µAcm-2, resulting in localized Li plating at pinholes in the SiO2 rather than lithiation. The voltages required to induce this pinhole plating will be compared to the dielectric breakdown strength of the SiO2 layers. To understand these phenomena in more detail, cyclic voltammetry (CV) was performed on Si with 1-5nm SiO2 (1mV/s, see abstract figure). Lithiation only occurred in the 1.0nm sample in the first cycle, but by the fourth cycle, lithiation occurred in samples with up to 2.9nm SiO2. Samples with 3.9nm and 4.8nm SiO2 on the other hand exhibit no lithiation or reversible electrochemistry during ten CV cycles. Two distinct delithiation peaks are observed for 1.0 and 1.4nm SiO2, but only the higher-voltage peak is observed for the 2.9nm SiO2, suggesting a unique delithiation chemistry that may be correlated to the high CE during galvanostatic cycling, and which will be studied by XPS. The peak currents (0.05V for lithiation, 0.5V for delithiation) decrease exponentially with SiO2 thickness, indicating that tunneling electron transfer across the SiO2 may play a key role in the reaction kinetics. Overall, we find that dense thermal oxides of 5nm or more severely impede lithiation, whereas 1nm SiO2 allows lithiation to proceed unimpeded. Intermediate thicknesses of ~3nm alter the delithiation chemistry while yielding improved CE during galvanostatic cycling. These effects will be discussed in relation to the bonding of the SiO2 network in thermal oxides, as compared to native or sputtered oxides. Figure 1