Publications

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A stable solid electrolyte interphase (SEI) layer is key to high performing lithium ion and lithium metal batteries for metrics such as calendar and cycle life. The SEI must be mechanically robust to withstand large volumetric changes in anode materials such as lithium and silicon, so understanding the mechanical properties and behavior of the SEI is essential for the rational design of artificial SEI and anode form factors. The mechanical properties and mechanical failure of the SEI are challenging to study, because the SEI is thin at only ∼10-200 nm thick and is air sensitive. Furthermore, the SEI changes as a function of electrode material, electrolyte and additives, temperature, potential, and formation protocols. A variety of in situ and ex situ techniques have been used to study the mechanics of the SEI on a variety of lithium ion battery anode candidates; however, there has not been a succinct review of the findings thus far. Because of the difficulty of isolating the true SEI and its mechanical properties, there have been a limited number of studies that can fully de-convolute the SEI from the anode it forms on. A review of past research will be helpful for culminating current knowledge and helping to inspire new innovations to better quantify and understand the mechanical behavior of the SEI. This review will summarize the different experimental and theoretical techniques used to study the mechanics of SEI on common lithium battery anodes and their strengths and weaknesses.

Silicon is a promising candidate as a next generation anode to replace or complement graphite electrodes due to its high energy density and low lithiation potential. When silicon is lithiated, it experiences over 300% expansion which stresses the silicon as well as its solid electrolyte interphase (SEI) leading to poor performance. The use of nano-sized silicon has helped to mitigate volume expansion and stress in the silicon, yet the silicon SEI is still both mechanically and chemically unstable. Identifying the mechanical failure mechanism of the SEI will help enhance calendar and cycle life performance through improved SEI design. In situ moiré interferometry was investigated to try and track the in-plane strain in the SEI and silicon electrode for this purpose. Moiré can detect on the order of 10 nm changes in displacement and is therefore a useful tool in the measurement of strain. As the sample undergoes small deformations, large changes in the moiré fringe allow for measurements of displacement below the diffraction limit of light. Figure 1a shows how the moiré fringe changes as the sample grating deforms. As the sample contracts or expands, the frequency of the moiré fringe changes, and this change is proportional to the strain in the sample.

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The traditional design approach for product development is to develop a design based on customer requirements and technical knowledge, build the product according to the detail requirements provided, and then test the product to validate that it works as intended. The team typically starts by identifying a single design approach and spends their time validating a single design during testing. Design teams often encounter issues in the course of development with performance, manufacturability, interfaces and more. To compensate for test failures, teams often build in time into the schedule for additional design loopbacks. Further, when the loopbacks lead to change, the late changes are costly and risky, causing the team to focuses on "fixing the bare minimum" to meet cost and schedule expectations.

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This quarter, we have focused on characterizing the electrochemical of native oxide and "pristine' silicon surfaces by electrochemical cycling for various conditions, starting with either a freshly etched Si surface, or varying amounts of oxide on the surface (either native grown or deposited). These changes can be used to determine if the pristine surface evolves differently than those that have been modified (Q1 milestone). We are also developing new diagnostics (microcalorimetry and stress measurement in-situ) to determine how the nature of the silicon surface affects the composition, function, and thickness of the SEI (Q2 milestone).

This quarter, we have focused on characterizing the electrochemical response, both through cyclic voltammetry and through constant current charge/discharge characterization of the silicon samples coated with silicates containing varying amounts of Li in the SiOx layer. These studies were performed using a standard Gen-2 electrolyte without FEC. We also performed electrochemical impedance spectroscopy on samples exposed to the Gen-2 electrolyte continually, and collected EIS spectra as a function of time and temperature.

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In situ Raman microscopy was used to study polysulfide speciation in the bulk ether electrolyte during the discharge and charge of a Li-S electrochemical cell to assess the complex interplay between chemical and electrochemical reactions in solution. During discharge, long chain polysulfides and the S3− radical appear in the electrolyte at 2.4 V indicating a rapid equilibrium of the dissociation reaction to form S3−. When charging, however, an increase in the concentration of all polysulfide species was observed. This highlights the importance of the electrolyte to sulfur ratio and suggests a loss in the useful sulfur inventory from the cathode to the electrolyte.

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This report summarizes the activities that Sandia National Laboratories undertook in support of the Si anode Fundamentals program managed by the Vehicle Technology Office of the Department of Energy. The program is led by the National Renewable Energy Laboratory, and Sandia is one of four laboratories (including Oak Ridge National Laboratories and Berkeley National Laboratories) included in the program. The initial set of activities included establishing the baseline protocols for cell assembly and testing, and executing a number of round robin style tests to compare data collected under nominally identical conditions at each of the participating laboratories to ensure that similar results were obtained and that no extraneous secondary factors were affecting the results. Because the nature of the interface between electrode and electrolyte was in question, as well as how the interface evolved over time and electrochemical cycling, an effort to build “model” interfaces based upon previously observed lithium silicate structures within the native film.

In this study, the structure and composition of lithium silicate thin films deposited by RF magnetron co-sputtering is investigated. Five compositions ranging from Li2Si2O5 to Li8SiO6 were confirmed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) and structure analysis on the evolution of non-bridging oxygens in the thin films was conducted with fourier transform infrared (FTIR) spectroscopy. It was found that non-bridging oxygens (NBOs) increased as the silicate network breaks apart with increasing lithium content which agrees with previous studies on lithium silicates. Thin film impurities were examined with x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (TOFSIMS) and traced back to target synthesis. This study utilizes a unique synthesis technique for lithium silicate thin films and can be referred to in future studies on the ionic conductivity of lithium silicates formed on the surface of silicon anodes in lithium ion batteries.

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Dissolved polysulfides, formed during Li-S battery operation, freely migrate and react with both the Li anode and the sulfur cathode. These soluble polysulfides shuttle between the anode and cathode – the so-called shuttle effect – resulting in an infinite recharge process and poor Columbic efficiency. In this study, water present as an additive in the Li-S battery electrolyte is found to reduce the shuttle effect in Li-S batteries. Batteries where water content was below 50 ppm exhibited a substantial shuttle effect and low charge capacity. Alternatively, addition of 250 ppm water led to stable charge/discharge behavior with high Coulombic efficiency. XPS results show that H2O addition results in the formation of solid electrolyte interphase (SEI) film with more LiOH on Li anode which protects the Li anode from the polysulfides. Batteries cycled without water result in a SEI film with more Li2CO3 likely formed by direct contact between the Li metal and the solvent. Intermediate quantities of H2O in the electrolyte result in high cycle efficiency for the first few cycles which then rapidly decays. This suggests that H2O is consumed during battery cycling, likely by interaction with freshly exposed Li metal formed during Li deposition.

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