1M LiPF6 Research Articles

Bismuth-titanium-silicon-based ternary oxide system: A comprehensive analysis and electrochemical utility

A novel TiO2-SiO2/Bi2O3 oxide material (TSB) was prepared for a first time via the sol-gel method. The synthesis was performed using titanium(IV) isopropoxide, tetraethoxysilane and bismuth nitrate pentahydrate as oxide precursors, ammonia as hydrolysis promoter and propan-2-ol as solvent. The properties of the synthesized material were comprehensively analyzed, including dispersion characteristics, morphology, crystalline structure, chemical composition and porous structure parameters. Additionally, to confirm the chemical structure and the presence of characteristic surface groups, Fourier transform infrared spectroscopy was applied. The electrochemical properties of a TiO2-SiO2/Bi2O3 anode, operating in 1M LiPF6 in EC/DMC (ethylene carbonate/dimethyl carbonate), were studied. The key point of the investigation was to demonstrate a potential application of TSB material in a Li-ion cell. Electrodes were analyzed with the use of cyclic voltammetry (CV), galvanostatic charging/discharging tests, and scanning electron microscopical (SEM) evaluations were performed. SEM images of both electrodes after charging/discharging processes showed that they to be covered with a film (electrochemical SEI formation). The reversible capacity of the TiO2-SiO2/Bi2O3 anode after 20 cycles at a low current rate was close to the theoretical value of 250 mAh g−1. However, at increasing current rates it decreased to ca. 200 mAh g−1. The reversibility of the process was close to 95%. Moreover after 100 cycle at 50 mA g−1 the value was 198 mAh g−1.

Probing the Nanoscale Heterogeneity of SEI on Silicon Anode Using Tip Enhanced Raman Spectroscopy (TERS)

The composition, morphology and thickness of solid electrolyte interphase (SEI) on silicon anodes are dictated by multiple factors, ranging from the chemistry of silicon surfaces, electrolyte solution structure and chemical passivation on Si surface. In addition, the SEI constantly evolves, as it is electrochemically lithiated (delithiated) due to the enormous volume changes. SEI on silicon typically is reported to be in the range of tens of nanometers and can be compositionally heterogeneous. Normal vibrational spectroscopy techniques such as confocal micro-Raman and FT-IR are useful, but they only provide chemical information at a relatively bulk scale, typically in micron-distance from the solid surface. This makes it extremely hard for getting information from the real SEI, which can be only on the order of 10 nm. Tip Enhanced Raman Spectroscopy (TERS) combines surface plasmon-enhanced Raman spectroscopy with atomic force microscopy, capable of providing the chemical vibrational information and topography of the sample in the nanometer spatial resolution simultaneously. In this report we demonstrate the first TERS study of SEI on cycled amorphous silicon. Amorphous silicon anode (20 nm in thickness on copper current collector) was cycled in a commercial lithium-ion battery electrolyte (1M LiPF6 in EC:DEC, 1:1 vol). TERS analysis on cycled amorphous silicon sample indicates that the nanometer scale SEI “islands” are unevenly distributed on the Si anode surface. Even for the same SEI “island”, the composition is different from point to point with inter-point distance smaller than 2 nm. Acknowledgment This work is funded by Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy

Cracks and Dendrites in Solid Electrolyte Interphases in Lithium-Ion Nanobatteries: Molecular Dynamics Simulations

Lithium-ion batteries are widely used as power sources for a variety of electronic devices; however, still further developments are expected for power applications in transportation and electrical grid backups, among several others. A passivating solid electrolyte interphase (SEI), naturally grow on the electrolyte-anode interface of a new battery or sometimes is artificially built to cover the anode. One way or the other, the SEI, once formed, protects the anode and electrolyte from mutually reacting each other, thus playing a critical role in the performance of batteries. For high-capacity intercalation anode materials, when they suffer a large volumetric increase, such as silicon, the SEI may crack and lead to the formation of dendrites, which are directly linked with the capacity fading of the battery over cycles, and even worse, dendrites may contact the cathode producing a short-circuit, one of the major concerns in lithium-ion batteries. A multiscale computational approach using molecular dynamics techniques will be presented for the simulation of full nanobattery test-beds that provides relevant information of silicon-based anode lithiation growth, including interfacial mechanical effects, and lithium-dendrite formation and growth.Therefore, we study the mechanical properties of LiF as a SEI layer of a SiLi nanoparticle (NP) expanded by a time-dependent expanding potential (TDEP) simulating the lithiation of Si and allowing to keep the number of Li and Si atoms of the NP constant. The stress on the SEI increases until reaching its maximum (tensile strength) at the LiF bond breaking and cracking onsets. As the expansion continues, the stress decreases and the number of bonds in the LiF SEI approaches to a minimum, confirming the SEI cracking. However, for the lowest currents, atoms have enough relaxation time that a surface reconstruction is observed with the stress reaching a second and higher maximum, then the SEI layer divides into pieces. The cracking mechanism is independent of the charging rate. The breaking of LiF bonds begins with the nearest ones to the SiLi-LiF interface, along a radial direction pointing to the outside of the NP, then forming larger and larger LiF rings until the SiLi alloy totally loses its protective shell.Then, we attempt to predict the mechanism of dendrite growth by simulating possible behaviors of charge distributions in the anode of an already cracked solid electrolyte interphase of a nanobattery under the application of an external field representing the charging of the battery; thus, elucidating the conditions for dendrite growth. Although a dendrite may take a few hours to grow, the simulation can be performed at totally different time scales because the extremely slow drift velocity of the Li-ions, ~1mm/hour in a typical commercial Li-ion battery and the so small time for the important chemistry to take place (~ps). The conditions before the growth are assumed and conditions that do not lead to the growth are ignored. This part of the work responds to the question: if dendrites grow at some point, what are the possible conditions that favored such a growth? We respond this question by using molecular dynamics simulations of a pre-lithiated silicon anode with a Li:Si ratio of 21:5, corresponding to a fully charged battery. The electrolyte is composed of 1M LiPF6 salt solvated in ethylene carbonate (C2H4CO3). We simulate the dendrite growth by testing a few charge distributions in a nanosized square representing a crack of the solid electrolyte interphase, which is where the electrolyte solution gets in direct contact with the LiSi alloy anode. Using selected charge distributions for such anode surface, the dendrites grow during the simulation when an external field is applied. We found that dendrites grow when strong deviations of charge distributions take place on the surface of the crack. In addition to our estimates of stress limits in the anode during charging, results from this work are important to find ways to constrain lithium dendrite growth using tailored coatings or pre-coatings covering the LiSi alloy anode.

Combinatorial Methods for Improving Lithium Metal Cycling Efficiency

An efficient combinatorial approach for the design and evaluation of surface coatings to improve lithium metal cycling efficiency is demonstrated. The reliability of a 64 electrode combinatorial cell was verified through lithium metal cycling in a range of electrolytes on unmodified Ni and Cu electrode plates with parallel verification using conventional Cu|| Li coin cells. A 1M LiPF6 FEC:TFEC fluorinated electrolyte demonstrated the best performance with coulombic efficiencies of 98.3% and 97.6% in the combinatorial cell (cyclic voltammetry) and coin cell (galvanostatic cycling) respectively, compared to only 91.0% (CV) and 90.3% (galvanostatic) for a 1M LiPF6 EC:DEC control electrolyte. High throughput sputtering was used to deposit different thin film coatings of varying thickness on the 64-channel cell plates to probe the lithium cycling performance as a function of electrolyte and surface coating. Zn surface coatings with thickness between 400–900 nm proved most beneficial; significantly reducing the lithium nucleation potential and leading to more stable cycling at high coulombic efficiency (99.0% in the combinatorial cell with FEC:TFEC electrolyte). These findings not only highlight some useful strategies for improving lithium metal cycling efficiency, but also show the potential for more advanced combinatorial analysis to design superior anodes for lithium metal cells.

In Situ Measurement of the Effect of Stress on the Chemical Diffusion Coefficient of Li in High-Energy-Density Electrodes

Sputter deposited germanium thin films were assembled in a half-cell configuration with lithium foil as counter/reference electrode and 1M LiPF6 in EC, DEC, DMC solution (1:1:1, wt%) as electrolyte. The Ge films were subjected to potentiostatic intermittent titration technique (PITT) and galvanostatic intermittent technique (GITT) conditions while simultaneously measuring the stress evolution in the electrodes. It was observed that the electrode stresses varied significantly in a single titration step during a GITT experiment, which violates the assumptions of simple Fickian transport model where the electrode stresses are usually neglected. Therefore, only the PITT data was analyzed to obtain the chemical diffusion coefficient of Li in Ge. As expected, the diffusion coefficient value increased considerably with Li concentration; however, the values obtained during delithiation are at least two times greater than those obtained during lithiation at any given Li concentration, with the difference becoming significantly higher at higher Li concentration. This difference is attributed to the stress state, i.e., tensile stress during delithiation leads to higher values compared to the compressive stresses during lithiation. The data and observations presented here will be helpful in developing and using electrochemomechanical models in producing optimized electrode microstructures.

Evaluating the effect of solid electrolyte interphase formers on lithium depth profiles of the solid electrolyte interphase layer and bulk electrode material in LiNi0.4Mn0.4Co0.2O2/graphite pouch cells obtained with lithium nuclear reaction analysis

Lithium loss resulting from positive electrode deterioration and lithium accumulation within the solid electrolyte interphase (SEI) at the negative electrode leads to charge capacity loss in lithium-ion batteries (LIBs). We present a novel method to quantify origins of capacity loss resulting from positive electrode degradation and lithium immobilization within the SEI by comparative lithium depth profiles using Lithium Nuclear Reaction Analysis (Li-NRA). Li(Ni0.4Mn0.4Co0.2)O2/artificial graphite (NMC442/AG) pouch cells were cycled with electrolytes containing 1M LiPF6 in either 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC) or DMC with 3% vinylene carbonate (VC) SEI former. Lithium accumulated rapidly within the SEI in cells containing EC as compared to cells containing VC. Lithium loss from the positive electrode occurred at a linear rate, after formation, which was independent of electrolyte composition tested. The technique and methods provide quantitative insight into the impact of materials on capacity loss.

Shutdown potential adjustment of modified carbene adducts as additives for lithium ion battery electrolytes

To improve the intrinsic safety of lithium ion batteries (LIBs) by preventing cells from a thermal runaway, we studied two carbene adduct electrolyte additives. The recently synthesized compounds (1,3-dimethylimidazolidin-2-μm-trifluoroborate (NHC-BF3) and 1,3-dimethylimidazolidin-2-μm-tetrafluorotrifluoromethylphosphate (NHC-PF4CF3)) were investigated on LiNi1/3Co1/3Mn1/3O2 (NMC111) electrodes in Li metal and Li-ion cell setups as overcharge protection shutdown additives in 1M LiPF6 in EC:DEC (3:7, by wt.) electrolyte. By varying the NHC-ligand (-BF3, -PF5, -PF4CF3) in the molecule, the shutdown potential of the investigated carbene adduct electrolyte additives can be tailored for specific applications with different cut-off potentials. NHC-BF3 was identified as a promising candidate for the application with NMC111 electrodes up to 4.4 V vs. Li/Li+, whereas the carbene adduct NHC-PF4CF3 is ideal for the high-voltage application with the NMC-based electrode up to 4.6 V vs. Li/Li+. Next to electrochemical investigations in NMC111/Li and NMC111/graphite cells, Atomic Force Microscopy (AFM) and X-Ray Photoelectron Spectroscopy (XPS) were performed to verify the presence of a decomposition layer on the cathode, responsible for the shutdown effect. Furthermore, it has been proven that the investigated electrolyte additives have no influence on the cell performance under normal conditions in both, Li metal and Li-ion cell setups.

Surface Reactivity of a Tin Electrode in Organic Carbonate Electrolytes

The development of the next generation Li-ion batteries (LIBs) requires a fundamental understanding of the phenomena occurring at the electrode/electrolyte interface during operation. The investigation on the mechanisms governing the function and operation of battery materials, interfaces and interphases, is indissolubly linked to the successful obtainment of long life and high performance LIB systems. Interfacial reactions, which include lithium ion insertion/extraction, decomposition of the electrolyte and formation of a solid electrolyte interphase (SEI) layer determine the irreversible capacity, cycling ability and lifetime of LIBs [1,2]. Among the high energy density anode materials candidates, tin has attracted significant attention as a Li-alloying anode due to its high specific capacity (993 mAh g-1) as compared to graphite electrodes (372 mAh g-1) and high volumetric capacity (2020 mAh cm-3) comparable with the one of silicon (2400 mAh cm-3) [3]. However, large irreversible capacity observed during the first cycle, volumetric changes of Sn particles during the (de-)alloying process, particle disintegration and loss of electronic contact are responsible for the rapid degradation of Sn-based anodes in LIB systems. [3-5]. In addition, tin presents a strong catalytic activity toward the organic-based electrolyte being responsible of a poor electrode passivation leading to continuous electrolyte reduction upon cycling [5-7]. In this scenario, it is fundamental the understanding of the kinetic processes occurring at the interphase to undertake a rational and basic study on the SEI components properties. To gain insights into the mechanism of the electrolyte decomposition and SEI layer (re)formation upon cycling we investigated the role of single solvents and additives into the electrolyte. The role of LiF and Li2CO3 in the SEI has been investigated through electrochemical tests, such as cyclic voltammetry and galvanostatic polarization and FTIR surface characterization. Furthermore, a comparative study to investigate the effect of oxide species on tin surface is shown. The presence of an oxide layer plays an important role in the surface reactivity toward solvent decomposition. Galvanostatic polarization of a tin electrode at different current densities (Fig.1) shows that the electrode potential tends to stabilize at around 1.6V at low current densities, i.e. 20 µA cm-2, which indicates a continuous electrolyte reduction process. While at higher current densities a fast voltage drop to the alloying region (0.6-0.4 V) is observed. Interestingly, the presence of an oxide layer on the tin surface limits the electrolyte reduction and induces an improved stability of the SEI layer. The results confirm our earlier observations that the surface film on Sn does not effectively prevent continuous electrolyte reduction and that a large fraction of the reaction products dissolve in the electrolyte. In addition, we found that these detrimental electrocatalytic processes could be suppressed by changes of the electrolyte composition (solvent and salt additives), or by using surface protective oxide layers. Figure 1. Galvanostatic polarization of different tin electrodes in 1M LiPF6 in EC:DEC at different current densities. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, under the Applied Battery Research for Transportation (ABR) Program and Award Number DE-EE0006443.

Nanocrystalline LiCo0.8Fe0.2PO4 / Multi-Walled Carbon Nanotubes Positive Electrode for 5-Volts Class Lithium Ion Batteries

1. Introduction Lithium cobalt phosphate (LiCoPO4) with olivine structure has attracted much attention as a new positive electrode material for lithium ion batteries because of its high potential (4.8V vs. Li/Li+), theoretical capacity (167 mAh g-1) and thermal stability due to the P-O covalent bonding. However, the practical use of LiCoPO4 has been limited by the capacity fading during cycling due to the electrolyte oxidation and irreversible structure changes such as amorphization and anti-site defects of the material during charge and discharge1). Besides, it suffers from its poor rate performance related to the inherently low electron conductivity (<10-9 S cm-1)2) and Li+ diffusivity (<10-13 cm2 s-1)2) due to its b-axis oriented diffusion path. To overcome these limitations, we have synthesized Fe-substituted LiCoPO4 (LiCo0.8Fe0.2PO4) nanoparticles which are highly dispersed within a multi-walled carbon nanotube (MWCNT) matrix via an original in-situ material synthesis method called ultracentrifugation (UC) treatment3,4). Here, we also report about the influence of Fe substitution on cycle and rate performances of LiCo0.8Fe0.2PO4. 2. Experimental UC treatment was carried out on the prepared mixture of Co(CH3COO)2・4H2O, Fe(CH3COO)2, CH3COOLi, citric acid, MWCNT and H3PO4 aqueous solution. Obtained composites were electrochemically characterized using a 2032 coin half-cell with Li metal in 1M LiPF6 / EC:PC:DMC (1:1:3, volume ratio). Physicochemical characterizations of the LiCo0.8Fe0.2PO4/ MWCNT composite were conducted by HRTEM, XRD, XPS, XAFS measurements and Mössbauer spectroscopy. 3. Results and Discussion The XRD pattern of LiCo0.8Fe0.2PO4 / MWCNT composite indicates an olivine structure, without impurity phases. The combination of XRD, XPS, XAFS and Mössbauer analysis suggests that substituted Fe3+ into LiCoPO4 crystals was not placed in the Co2+ site, but in the Li+ site. The result of HRTEM observation indicates that LiCo0.8Fe0.2PO4 nanocrystals (ca. 100 nm) were highly dispersed within a MWCNT matrix. In addition, HRTEM images of LiCo0.8Fe0.2PO4 / MWCNT composite showed that a substitution of Fe3+ into LiCoPO4 crystals prevents degradations such as amorphization of LiCoPO4and electrolyte decomposition during charge and discharge processes. Then, LiCo0.8Fe0.2PO4 / MWCNT composite showed a higher discharge capacity of 130 mAh g-1 than LiCoPO4 / MWCNT composite and enabled a 100C (36 seconds) rate discharge with 45 mAh g-1. Furthermore, the obtained composite showed stable cycle performance with 81 % of capacity retention after 100 cycles at 0.2C. These enhancements were achieved by the combination of an entanglement between nanocrystalline LiCo0.8Fe0.2PO4 and MWCNT matrix and Fe3+ substitution in the Li+ site of LiCoPO4. The Fe3+substitution gives a “pillar effect” which effectively prevents an irreversible crystal structure change and electrolyte decomposition during charge and discharge processes, resulting in the stable cycle performance. References 1) A. Boulineau et al., Chem. Mater., 27, 802 (2015). 2) S. Brutti et al., ACS Symposium Series, (2013). 3) K. Naoi et al., Acc. Chem. Res., 46, 1075 (2013). 4) K. Naoi et al., Energy Environ. Sci., 5, 9363 (2012).

Electrochemical Properties of PDTT and PETDP As Cathode Materials for Rechargeable Lithium Battery

Recently, Organodisulfide compounds have attracted extensive research interests as cathode materials due to the advantages of higher theoretical capacity, environmental friendness, lightweight and abundant resources for high specific energy lithium secondary batteries. The key moiety of high specific capacity in organodisulfide compounds is the reversible two-electron redox reaction of the disulfide bond (S-S). However, the exiting problem of organodisulfide compounds as electrodes materials in LIBs is the large capacity fading caused by the high solubility of polysulfides in liquid electrolytes. An Important strategy to solve this problem is to polymerize the small molecules into insoluble polymeric chains in order to improve the cyclic stability of the electrodes. Based on this strategy, the electrochemical performance and energy storage mechanism of Poly [1, 2-dithiole-3-thion-4(5)-thio] (PDTT) and Poly [2-(1, 2-ethylenediamino)-1, 6, 6a, Δ4-trithia-1, 6-diaza-pentalen-5-yl] (PETDP) were designed, synthesized, and investigated. As shown in figure 1, based on the two-electron reaction, PDTT can provide theoretical capacity of 326 mAh g-1, and the theoretical capacity of PETDP is calculated to be as high as 371 mAh g-1 based on the three-electron reaction. Electrochemical performances of PDTT and PETDP were evaluated by cyclic voltammogram and galvanostatic discharge/charge measurements. CV results indicated that PDTT electrode has two pairs of redox peaks at 2.09/2.38 and 2.29/2.63 V. PETDP cathode also presents two pairs of well-defined redox peaks at 2.11/2.26 and 2.37/2.42 V, respectively. Cyclic performance of PDTT and PETDP was compared using two type of electrolytes, the ether based (1M LiTFSI in DME/DOL) and carbonate based (1M LiPF6 in EC/DMC) electrolytes. Results show that both of them deliver best cyclic stability in ether based electrolyte. In addition, the discharge and charge profiles of PDTT electrode are consistent with its CV curves and having specific capacity of 321 mAh g-1 at a current density of 0.1 C (1C=33 mA g-1), this value is as high as 98.4% of the theoretical capacity. Even at 0.5C rate, high initial capacity of 318 mAh g-1 (97.5% of theoretical value) was obtained, confirming the almost fully utilization of active materials. When cycled over 50 cycles, PDTT electrodes can still deliver high capacities of 291 mAh g-1with capacity retentions of 90.6%, showing a high potential to be used as organodisulfide cathode materials for high capacity Li-S batteries.More details about the charge storage mechanism studies using synchrotron based X-ray absorption spectroscopy will also be presented at the meeting. Acknowledgement The work at Brookhaven National Lab. was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies (BMR and VTO Battery500 projects) under Contract Number DE-AC02-98CH10886. Figure 1

Effects of Cell Geometry on Rate Performance of Secondary Batteries

3D geometries for microbattery applications have been explored to increase capacity per footprint area, relevant for small footprint area micro devices [1] [2]. Simulations of interdigitated Li-ion batteries suggest 4 orders of magnitude increase in areal capacity at 330 C-rate from 0.58 Ah/m2in the conventional geometry to 318 Ah/m2 for the 3D geometry [3]. Realization of practical 3D battery designs can potentially decouple their power and energy [4], properties that are historically coupled in conventional battery design, which might have significant implications for high power devices. Using commonly found commercially available materials and tools, we designed and conducted a simple experiment to elucidate how battery performance can be improved by varying the relative arrangements of the active and inactive components. Examples include using Li/LiCoO2 2032-type coin cells with 1M LiPF6 and carbonate containing electrolytes. The maximum active ion diffusion length required to fully delithiate the LiCoO2 electrodes during cycling was increased by 400-6500 μm compared with standard reference cells, and the effect on rate performance was measured between C/24 and C/3. Results suggest that extending the maximum active ion diffusion path length by >1600 μm still allows for attaining specific capacity that matches that of the reference cell at C/24. We discuss possible new geometries and optimized designs in the framework of battery performance. [1] C. Liu, E.I. Gillette, X. Chen, A.J. Pearse, A.C. Kozen, M.A. Schroeder, K.E. Gregorczyk, S.B. Lee, G.W. Rubloff. Nature Nanotechnology 9 (2014) 1031-1039. [2] T.S. Arthur, D.J. Bates, N. Cirigliano, D.C. Johnson, P. Malati, J.M. Mosby, E. Perre, M.T. Rawls, A.L. Prieto, B.Dunn. MRS Bulletin 36 (2011) 523-531. [3] D. Miranda, C.M. Costa, A.M. Almeida, S. Lanceros-Méndez. Applied Energy 165 (2016) 318-328. [4] N. Cirigliano, G. Sun, D. Membreno, P. Malati, C.J. Kim, B. Dunn. Energy Technology 2 (2014) 362-369.

Lithium Dendrite Suppression By Polyethylene Oxide Film Coating on Copper Electrode Enable Lithium Metal Anode

Efforts to realize lithium metal as anode limited by dendrite formation and low Coulombic efficiency. These challenges stem from extensive reaction of lithium with electrolyte, deposition of more dead lithium and formation of unstable solid electrolyte interface. Herein, we revealed a homogeneous lithium deposition and dendrite suppression via regulating lithium flux with polyethylene oxide (PEO) film coated copper electrode in 1M LiTFSI, DME/DOL (v/v 1/1) and 2 wt% LiNO3 electrolyte. PEO coating reinforces a thin and robust SEI layer film formation by hosting lithium and regulating the inevitable reaction of lithium with electrolyte. The modified electrode showed a stable and an extremely excellent Coulombic efficiency over 200 cycles with low voltage hysteresis (~30 mV) at rate of 1 C. In comparison the modified electrode tested with 1M LiPF6, EC/DEC (v/v 1/1) electrolyte and showed a rapid capacity fading due to PEO film damage with fluoride ion interaction. This enhanced cycle life and capacity retention results from the synergy of PEO coating, high electrode-electrolyte interface compatibility, stable polar oligomer formation from reduction of 1, 3-dioxolane and generation of SEI stabilizing nitrite and nitride upon lithium nitrate reduction in an ether electrolyte.

Anodes for Lithium Ion Batteries Produced from Recovered Silicon Powders

Lithium Ion Batteries (LIB) offers the highest energy density among rechargeable batteries technologies. However, improvement in materials are needed for LIB in order to increase energy density, safety, durability, decrease cost and allow the further deployment of technologies like electric vehicles and energy storage. One way to improve these batteries is to develop new electrodes, both cathode and anode, resulting in higher energy density. Graphite is today used as anode materials, but Silicon has been extensive studied due to their approximately 10 times higher gravimetrical capacity. However, it is well known that silicon has challenges related to volume expansion during (de)lithiation upon cycling. Several proposed solutions, like nanostructuring, nanowires, encapsulation etc. have been proposed. Here we present initial work, using Silicon kerf (recovered Silicon powders from cutting of silicon ingots). Silicon kerf was delivered by the ECOSOLAR partner Garbo S.r.l. and the materials were cleaned to remove most of the impurities. The electrochemical performance of Silicon kerf was investigated in half cells. The silicon was mixed with Super C 65 carbon black as conducting agent and Na CMC as binder. Na CMC was dissolved in an aqueous solution containing KOH and citric acid at pH 3 to facilitate chemical bonding between the binder and the active material. Electrodes were fabricated by tape casting on plain Cu foil as current collector. Casts with varying thickness were produced to investigate the effect of electrode loading. C2016 coin cells was prepared using electrolyte consisted of 1M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by vol) added 5 wt% FEC (fluoroethylene carbonate). Initial studies for using silicon kerf as a source for high capacity anode materials in lithium ion batteries has demonstrated that kerf should be evaluated further to determine the long time cycling stability of this materials. The materials were evaluated both to determine the maximum capacity and stability using capacity limited cycling. The electrodes using Si kerf were stable for 150 cycles during capacity limited cycling using a loading of 0.6 mg/cm3 and a bit lower cycle stability for electrodes with higher loading. It must be highlighted that the electrode structures and composition, electrolyte composition etc. has not been optimized. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 679692.

In Situ Observation of Lithium Dendrite Growth and SEI By Optical Microscopy and Raman Spectroscopy

Li metal negative electrode is expected for the next-generation Li battery electrode material due to its extremely high theoretical specific capacity (3,860 mAhg-1), low density (0.534 g cm-3) and the lowest negative electrochemical potential (-3.040 vs standard hydrogen electrode). However, the critical challenges of Li metal anodes (poor cycle life and safety) remain unsolved. This is attributed to electrolyte decomposition and the Li dendrite growth during electrochemical cycling. In this study, we developed a transparent two-electrode cell to carry out an in situ observation of the Li dendrite growth on Cu surface. Galvanostatic plating and stripping cycle(s) are applied at same current density on each cell. Meanwhile, the Raman spectroscopy is employed to take a mapping image along with the grown lithium surface. The morphology evolution of the Li dendrite growth and nucleation mechanism is observed in different electrolytes and additives. The behavior of lithium dendrite and SEI were analyzed comparatively with 1M LiPF6 EC/ DEC (v: v=1:1), 1M LiTFSI, DME/DOL (v : v=1:1), and 1M LiClO4, PC electrolyte at the same current density. We have clearly observed that after a few cycles of platting and stripping lithium dendrite grew dramatically on Cu surface with carbonate electrolyte and the Cu surface still remained some lithium dendrite due to the dead lithium formation, which causes low Coulombic efficiency. In addition, Raman spectroscopy is used to probe SEI composition. The correlation of lithium growth and its surface composition are established in this work, which can help to further understand the mechanism of lithium growth.

Influence of electrochemical cycling on the rheo-impedance of anolytes for Li-based Semi Solid Flow Batteries

The recently launched concept of Semi-Solid Flow Batteries (SSFBs) shows a strong potential for flexible energy storage, but the liquid-dispersed state of the electrode materials introduces several aspects of which a scientific understanding is lacking. We studied the effect of electrochemical cycling on the rheological and electrical properties of a SSFB anolyte containing Li4Ti5O12 (LTO) and Ketjen Black (KB) particles in EC:DMC solvent with 1M LiPF6, using an adapted rheometer that allows in situ electrochemical cycling and electrical impedance spectroscopy. Charging (lithiation) caused a reduction in the electronic conductivity, yield stress and high shear viscosity of the fluid electrode. For mildly reducing voltages (1.4 V), these changes were partially reversed on discharging. For more reducing voltages these changes were stronger and persistent. The finding of comparable trends for a fluid electrode without the LTO, lends support to a simplistic interpretation, in which all trends are ascribed to the formation of a surface layer around the conductive KB nanoparticles. This Solid Electrolyte Interphase (SEI) insulates particles and reduces the van der Waals attractions between them. SEI layers formed at less reducing voltages, partially dissolve during the subsequent discharge. Those formed at more reducing voltages, are thicker and permanent. As these layers increase the electronic resistance of the fluid electrode by (more than) an order of magnitude, our findings highlight significant challenges due to SEI formation that still need to be overcome to realize SSFBs.

A high energy 3V lithium-ion capacitor synthesized via electrostatic spray deposition

Reduced graphene oxide-carbon nanotube (rGO-CNT) and anatase TiO2-Li4Ti5O12 (ATO-LTO) composite electrodes were synthesized via electrostatic spray deposition (ESD) and analyzed as cathode and anode vs. lithium, respectively. The rGO-CNT and ATO-LTO electrodes were able to deliver discharge capacities of ca. 63 mAhg -1 and 95 mAhg -1 , respectively for a current rate of 0.1 Ag -1 with superior rate capability and cycle stability. Post electrode analyses, lithium-ion hybrid electrochemical capacitors (Li-HEC) were constructed comprising a prelithiated ATO-LTO anode and an activated rGO-CNT cathode in a carbonate based 1M LiPF6 salt electrolyte. The Li-HEC cells were stable for a cell potential of 0.05-3V and were able to deliver a maximum gravimetric energy density of 33.35 Whkg -1 and a maximum power density of 1207.4 Wkg -1 , where the cell parameters were normalized with the total mass of the anode and cathode active materials. Furthermore the Li-HEC cells were able to retain ~77% of the initial capacity after 100 cycles. The superior Li-HEC performance is attributed to the utilization of a prelithiated lithium-intercalating anode and a double layer cathode in an asymmetric configuration. The feasibility of using a low-cost, facile process like ESD was therefore shown to produce high performance Li-HECs.

Post-Mortem Investigations of Fluorinated Flame Retardants for Lithium Ion Battery Electrolytes by Gas Chromatography with Chemical Ionization

Using flame retardants (FRs) in lithium ion battery (LIB) electrolytes is usually a tradeoff between electrochemical performance and electrolyte flammability. Fluorinated FRs are a promising class of FRs which are currently under investigation. During this work, three FRs originating from triethyl phosphate with varying degree of fluorination were investigated regarding their electrochemical stability on cathode (LiNi0.33Co0.33Mn0.33O2, NCM) and anode (graphite) in half cells. During long-term cycling, changes in performance were observed. Especially on the anode side the FR addition showed a decrease in performance in comparison to the standard electrolyte (DEC/EC 1:1, 1M LiPF6). The electrolytes containing the three FRs were extracted from the cells and analyzed regarding their changes in composition and structural degradation. The decomposition products were investigated by gas chromatography (GC) with electron impact (EI) ionization and mass selective (MS) detection. To obtain more information with regard to the identification of unknown decomposition products further GC‐MS experiments with positive chemical ionization (PCI) and negative chemical ionization (NCI) were performed. Twelve different volatile organic decomposition products were identified. These decomposition products can be subdivided regarding their basic structure. Ether based, carbonate based and phosphate based fluorinated and non-fluorinated decomposition products were identified. Furthermore, possible formation pathways for all groups of decomposition products were postulated taking existing literature into account.

Improved high-voltage performance of LiNi1/3Co1/3Mn1/3O2 cathode with Tris(2,2,2-trifluoroethyl) phosphite as electrolyte additive

Tris(2,2,2-trifluoroethyl) phosphite (TTFEP) is investigated as an electrolyte additive to improve the electrochemical performance of LiNi1/3Co1/3Mn1/3O2 cathode at high operating voltage (4.6V). Charge/discharge measurements demonstrate that TTFEP is effective to improve the cycling stability and rate capability of LiNi1/3Co1/3Mn1/3O2 cathode. The capacity retention of LiNi1/3Co1/3Mn1/3O2/Li cell with 1% TTFEP-containing electrolyte reaches up to 85.4% after 100 cycles at 0.5C (1C=160mAg−1), while that of the cell with the baseline electrolyte (1M LiPF6 in EC/DMC electrolyte) only remains 74.2%. Moreover, the discharge capacity of the cathode with 1% TTFEP-containing electrolyte could maintain around 112.0mAhg−1 at 4C. Based on the characterization of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), a protective interphase film formed on the cathode surface can be found due to the preferential oxidation of TTFEP, which inhibits the electrolyte decomposition and mitigates the cathode structural destruction, leading to the improved electrochemical performance of LiNi1/3Co1/3Mn1/3O2 cathode at high voltage.

Quantifying Contributions to Reversible and Irreversible Capacities of Silicon Electrodes

Silicon has received much attention as a next-generation anode material for lithium-ion battery (LIB) because it can give a theoretical capacity of up to 4000 mAh/g. Typically, silicon electrodes show poor cycle performance, which is attributed to large volume expansion during lithiation, pulverization of the active material, continuous solid electrolyte interphase (SEI) layer growth etc. Though, there are hardly any studies that can tell how much of the measured capacity comes from each process. The aim of this study is to establish a method to understand and quantify the contributions to the reversible and irreversible capacities of silicon anode in order to develop strategies to improve the stability of the electrodes. Here, we report the use of an electrochemical approach – depth of discharge test – to separate the charge-discharge capacities of crystalline silicon electrodes into four contributions: (1) SEI formation and electrolyte decomposition, (2) lithium accommodation in carbon and binder, (3) lithiation and delithiation of the Si active material, and (4) capacity loss associated with particle cracking and detachment. Silicon electrodes using bulk silicon particles (Sigma Aldrich), acetylene black (AB) and carboxymethyl cellulose (CMC) binder were made with a ratio of 4:3:3, 6:2:2 and 8:1:1. The electrodes are assembled into 2032-type coin cells with Celgard separator and Li metal as the counter electrode. 1M LiPF6 in fluorinated ethylene carbonate (FEC) and diethyl carbonate (DEC) = 1:1 by volume is used as the electrolyte. For the depth of discharge test, multiple cells were made, and each of them is initially discharged to different cutoff capacity (250, 500, 1000, 1500, 2000, 2500, 3000 and 3500mAh/g) and charged back to 1V. Figure 1a shows a summary of the initial discharge-charge curves of different cells with a composition of Si:AB:CMC = 4:3:3. The respective first charge capacity vs. first discharge capacity is summarized in Figure 1b, which shows a linear behavior that is shifted from the origin. The slope of the line corresponds to the intrinsic first cycle efficiency of Si, which is about 90% and is independent on the degree of lithiation. This suggests that no matter how much lithium is initially inserted, 10% of them remains in the lattice and cannot be removed. This is attributed to partial irreversibility of the crystalline-to-amorphous transition of Si during lithiation, and is consistent with a core-shell model of lithiation during first discharge. The x-intercept of Figure 1b is non-zero, which corresponds to the irreversible capacity of the electrode. The x-intercept depends on the composition of the electrode (see inset of Figure 1b) – larger x-intercept for large amount of carbon and binder, which is due to irreversibility from these two components. Accounting for all of these, we estimate the capacity from SEI formation and electrolyte decomposition to be about 16 mAh/(g Si) for bulk Si (with BET = 1.6 m2/g). SEI formation is expected to depend on particle size and surface area. We verify this by performing the depth of discharge test with ballmilled Si (in Ar at 200rpm for 24 hours) with BET = 6.6 m2/g. The capacity due to SEI formation during 1st discharge is increased to about 68 mAh/(g Si), and scales with BET surface area. We estimate that the SEI formation in our system to be about 10 mAh per square metres of active material surface. This value would depend on the type of electrolyte used in the system. Mechanical issues and particle isolations are observed in fully discharged electrode when the amount of binder is less than 20%. This is represented in the bend in the depth of discharge curve for the CMC-811 electrode, as in Figure 1b. Our results suggest that there is a threshold to mechanical breakdown. In the case of the CMC-811, high reversibility is still obtained when the discharge capacity is below 2000 mAh/g. With larger discharge capacity, volume expansion is larger, leading to severe capacity fading. In subsequent cycles, cycle stability is governed by the coulombic efficiency (CE) of the electrode - the larger the coulombic efficiency, the better the stability. The irreversible capacity is not only due to continuous SEI formation, but also to Li trapping in the active material and mechanical breakdown of the electrode. CE can be increased by limiting capacity, reducing particle size, or changing to a stronger binder such as polyimide. Some of results can be found in our recent publication1. More results will be shown during the meeting. [1] P.-K. Lee, Y. Li, D. Y. W. Yu, J. Electrochem. Soc. 164, A6206 (2017). Figure 1

Scalable in-Situ Solid State Reductive Approach to Reversible Silicon/Metal Oxide/Carbon Based Nanocomposite Li-Ion Anodes

Silicon based anode has attracted considerable attention to replace graphite as a high energy density anode material for lithium ion batteries due to the high theoretical specific capacity of silicon (~4200 mAh/g). However, bulk crystalline silicon exhibit poor performance due to colossal volumetric expansion (~400%) on reaction with lithium. The past decade has witnessed different approaches to address and counter the issue of mechanical degradation of silicon anodes. These approaches involve the use of silicon nanoparticles [1], composite structures of amorphous silicon [2], active-inactive matrix [3], electrodeposited thin films and VACNT-Si heterostructures [4-6]. However, the systems developed are either based on high cost synthesis approaches or utilize pre synthesized silicon nano particles and still suffer from poor capacity retention. Silica and silicon oxides are commercially cheap sources to synthesize silicon and high energy mechanical milling (HEMM) is also a well-known commercial processing technology to generate nano particles/nano-composites. In the present work, nanocrystalline Si, nc-Si/metal oxide/carbon composite (nc-Si/MO/C) has been synthesized and studied as a promising anode material for Li-ion batteries. Reduction of commercially available silicon monoxide has been carried out with suitable metal based reducing agents using an in-situ high energy mechanochemical reduction (HEMR) followed by low temperature heat treatment to enable completion of the reduction reaction. This directly generates silicon and metal oxide based nanocomposite which acts as an electrochemically active/inactive composite for use in lithium ion batteries. The obtained nanocomposite material was embedded in different carbon (C) based matrices to further improve the performance of the system. Slurry based electrodes of these nanocomposites mixed with binder and conductive additives were then fabricated and tested in a half cell configuration within the voltage range of 0.01V – 1V vs. Li/Li+ in 1M LiPF6 (dissolved in EC:DEC:FEC=45:45:10) electrolyte. XRD pattern (Fig 1a) shows the evolution of the nanocrystalline silicon (nc-Si) and decrease in intensity of SiO amorphous broad peak with the increase in milling time from 20h to 40h, hence, indicating the reduction of SiO by the metal based reducing agent to form nc-Si. The heat treatment of the material obtained after 40h HEMR (Fig 1a) increases the crystalline nature of silicon and improves the formation of Si by completing the reaction. The XRD pattern (Fig 1a) after heat treatment shows no unreacted SiO phase indicating the completion of reduction process. FTIR, Raman spectroscopy, SEM and TEM analysis has been conducted on the material at different stages of reduction and heat treatment process to study the evolution of the phases in the nanocomposite. The nanocomposite embedded in carbon (nc-Si-MO/C) system shows a first and second cycle discharge capacity ~1500mAh/g and ~1340mAh/g at a current rate of ~50mA/g with a first cycle irreversible (FIR) loss of ~25%-35% (Fig 1b). The obtained capacities are in agreement with theoretical calcluated specific capacity of the composite. Following long term cycling, the system shows a stable specific capacity of ~730mAh/g after 120 cycles at a charge/discharge rate of ~500mA/g with a columbic efficiency of ~99.65-99.82% and a fade rate of ~0.15% capacity loss per cycle. Retention of the MO matrix in the HEMR generated composite coupled with the interconnected carbon acts as an efficient buffer to relieve the stresses generated during alloying/dealloying and also maintains the electrical continuity during expansion of the nc-Si. Results of these studies combined with the extensive electrochemical characterization including electrochemical impedance, rate capability and SEM analysis of the electrodes before and after electrochemical cycling will be presented and discussed. Acknowledgement: The authors gratefully acknowledge financial support of the DOE-BATT (DE-AC02-05CHl1231), DOE-PNNL and NSF (CBET-0933141) programs. The authors also acknowledge the Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) for support of this research.