Lithium Counter Electrode Research Articles

Exploring the Impact of State of Charge and Aging on the Entropy Coefficient of Silicon–Carbon Anodes

Due to its improved capacity compared to graphite, silicon is a promising candidate to handle the demands of high-energy batteries. With the introduction of new materials, further aspects of the battery system must be reconsidered. One of those aspects is the heat generation during the charging and discharging of a cell, which delivers important information for the development of cooling systems, the battery management system and the overall performance of the cell. Since the reversible heat presents an important contribution to the overall heat generation during cycling, the entropy coefficient is the main value that needs to be determined. This study evaluates the entropy coefficient of custom-produced 2032 coin half-cells with lithium counter electrodes, containing 45 wt% nanosilicon and 45 wt% carbon black. The potentiometric method, utilizing VR and self-discharge curves, produced reliable results, yielding entropy coefficient values between 95% SoC and 10% SoC during delithiation. These values of the entropy coefficient are consistently negative. Furthermore, ICA measurements identified two phase transitions during delithiation, with these transitions shifting to lower SoC as SoH decreases, impacting the slope of the entropy coefficient.

Electrochemical impedance analysis of the charge transfer mechanism on Li4Ti5O12 surface in the presence or absence of the solid/electrolyte interface film

A novel three-electrode type cell is developed for highly reliable electrochemical impedance spectroscopy (EIS). A pair of Li4Ti5O12 (LTO) electrodes are placed at both end of the cell, and a lithium counter electrode is inserted between them to assemble a three-electrode-type LTO half-cell. The half-cell is initially discharged to either 0.3 or 1.4 V to prepare the pair of LTO electrodes with or without a solid/electrolyte interface (SEI) film, respectively. The half-cells are reassembled to LTO symmetric cells by removing the central Li electrode without damaging the LTO electrodes. EIS measurements of the symmetric cell indicate impedances associated with charge transfer (RCT) and the SEI film (RSEI) in distinctly different frequency ranges. The temperature dependences of RCT and RSEI are analyzed to clearly understand the influence of the SEI film on the charge transfer mechanism and to explicitly support the common understanding that lithium ions are desolvated at the film/electrolyte interface or inside the film through a different consideration than the conventional one.

Impact of Trace Nickel in the Electrolyte on the Electrochemistry of Positive Electrodes for Lithium-Ion Batteries

This study looks into the impact of Ni2+ ions as an electrolyte impurity on the electrochemical performance of NMC811 electrodes. The results indicate that the presence of Ni impurities in the electrolyte increases the polarization, primarily at the oxide-electrolyte interface. The influence on the metallic lithium counter electrode and the electrolyte conductivity can be neglected. The Ni impurities induce the formation of high impedance CEI layers on the oxide electrode, affected by both cations and anions of the impurity and impeding the current flow and cell performance. Ni impurities also enhance electrochemical side reactions, especially oxidation of the electrolyte, diminishing both, the Coulombic efficiency during cycling and the practical specific charge. The rates of these side reactions are related to the Ni2+ ion concentration. C-rate tests confirmed that a contaminated electrolyte reduces the practical specific charge due to the increased interfacial impedance of the CEI, especially at high discharge rates. The cycling performance of NMC811 electrodes in contaminated electrolytes showed that Ni impurities, especially NiCO3, promoted capacity fading during cycling and increased cell polarization. To optimize the performance of NMC-based oxide electrodes, controlling the concentration of Ni ions in the electrolyte is critical.

Two-Layer Structured Cathodes for Solid State Batteries

Designing a robust cathode is one of the major objectives in R&D of lithium-ion batteries. The challenges only get amplified in case of solid state battery cells as the methods for introduction of electrolyte into the cathode structure need to be developed. Ideally, a cathode should support high energy density of a cell which implies necessity for high thickness. This requirement is not trivial to fulfil, both from the standpoint of manufacturing and from the standpoint of overcoming transport limitations in thick cathodes. In this work, inspired by the previous effort on graded porosity cathode design [1], we present a two-layer cathode based on LiNi0.6Mn0.2Co0.2O2 (NMC622). The cathode is built using a densified “energy layer” as a support for the “power layer” in which high porosity and low tortuosity is achieved by the freeze tape casting technique. The distribution of the material in the two layers is guided by numerical simulations based on actual cathode microstructure. The performance of the structured cathodes containing Poly(ethylene oxide) (PEO) with lithium bis-trifluoromethanesulfonimide (LiTFSI) was experimentally investigated in cells containing metallic lithium counter electrode. The structured cathodes showed more than double capacity values and improved Coulombic efficiency compared to uniform cathodes of the same material loading.[1] Kalnaus, S., Livingston, K., Hawley, W.B., Wang, H., Li, J., 2021, “Design and processing for high performance Li ion battery electrodes with double-layer structure J Energy Storage 44 (2021) 103582. Acknowledgments: This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by Laboratory Directed Research and Development Program at ORNL.

Microcalorimetry electrothermal impedance spectroscopy (ETIS) informs entropy evolution at individual electrodes of PNb9O25 or TiNb2O7 battery cells

This study proposes a novel and fast microcalorimetry electrothermal impedance spectroscopy (ETIS) method based on heat generation rate measurements at each electrode of a lithium-ion battery cell. This new method is capable of retrieving the open-circuit voltage, the entropic potential, and the partial entropy changes at each electrode from measurements at a single temperature. It also shortens the measurement duration to a few hours compared to several days using the galvanostatic intermittent titration technique (GITT). The method consists of imposing a sinusoidal current on the cell assembled in an operando isothermal calorimeter. The induced sinusoidal potential response is used to calculate the open-circuit voltage of the cell as a function of the state of charge. The measured heat generation rates are analyzed by fast Fourier transform to determine not only the entropic potential of the cell but also the partial entropy changes at each electrode. This novel microcalorimetry ETIS method was first validated with numerical simulations. Then, it was experimentally demonstrated on battery cells consisting of PNb9O25 or TiNb2O7 working electrodes and metallic lithium counter electrodes in 1 M LiPF6 in EC:DMC 1:1 v/v electrolyte. The results of the open-circuit voltage and the normalized entropic potential matched those previously determined by potentiometric entropy measurements based on GITT measurements at different temperatures. Furthermore, while the normalized partial entropy changes exhibited notable features at the PNb9O25 or TiNb2O7 working electrodes, they varied little at the metallic lithium counter electrodes.

Decoupling Accurate Electrochemical Behaviors for High‐Capacity Electrodes via Reviving Three‐Electrode Vehicles

AbstractDeveloping high‐capacity electrodes requires the evaluation of electrochemical behaviors with an increasing current density. Currently, the current density for evaluation of high‐capacity electrodes has reached a new stage where the polarization at the lithium counter electrode has become a technical barrier for the accurate evaluation of battery electrodes, resulting in severe performance and mechanism mischaracterizations. Here, the accurate electrochemical behavior for high‐capacity electrodes via a single‐channel three‐electrode vehicle is decoupled, by which the impact of lithium counter electrode is minimized. The testing high‐capacity graphite electrode is capable of delivering an excellent rate capability with 81.7% capacity retention at 0.3 C, as well as stable cycling performance retaining 97.5% practical reversible capacity after 225 cycles, much higher than the graphite electrode tested with traditional half‐cell testing vehicle but in close agreement with the results obtained from a well‐matched full cell, reflecting accurate electrochemical performance evaluations of high‐capacity electrodes. Moreover, detailed electrochemical mechanisms of impedance and diffusion properties for working electrodes are also successfully decoupled individually. This work uncovers the mismatch between traditional evaluation configuration and increasing testing current density and provides a guideline for accurate electrochemical evaluation for ever‐increasing high‐capacity electrodes, which is of great significance for high‐energy lithium or other alkali‐metal ion batteries.

(Digital Presentation) Understanding the Impact of Process Conditions on Manufacturing Thick Cathodes By Systematically Reducing Inactive Material Percentages

With the lithium-ion battery industry moving to high energy density anode materials such as silicon and lithium metal as a substitute for lower specific capacity carbon-based materials like graphite, cathode energy density can become a bottleneck. Typical cathodes have less than 10% practical specific capacity compared to both pure lithium metal and silicon. For these anodes, cathodes with higher active material loadings per unit area would be needed to produce matching electrode pairs. However, cathode thickness cannot indefinitely be increased, as doing so leads to issues with electrolyte transportation, conductivity, and mechanical properties [1,2].Increasing the loading of the cathode could also be done by reducing the amounts of inactive components, such as the binder and the conductive additive, in the electrode. To achieve this, understanding the role of each component is important. The most important role of the binder is to act as an adhesive among particles and the current collector. But most binders are electronically insulating. The main role of the conductive additive, such as carbon black, is to improve the conductivity of the overall system [1].As the inactive material percentages are decreased, the distribution of these components in the electrode domain becomes more important to both the electrochemical and mechanical performance of the electrode. The distribution changes with the different process conditions used, such as mixing techniques, order, speeds, times, slurry viscosity, drying conditions, etc. This research consists of a systematic study to understand the effects of the aforementioned process conditions on the particle and polymer distribution as well as its contribution to the mechanical and electrochemical performance. With the use of the current findings, the active material percentage was increased up to 98% in electrodes with 5 mAh/cm2 loading, retaining similar performance to electrodes with 92.8% active material, as shown in figure 1. Some contribution of the capacity fade was found to be due to the lithium counter electrode used.The goal of this ongoing project is to produce a model that could be applied to different materials to identify the best process conditions for thick electrode manufacturing.References F. Ma, Y. Fu, V. Battaglia, R. Prasher, Microrheological modeling of lithium-ion battery anode slurry, J. Power Sources, 438 (2019), p. 226994Wen Yu, Nanping Deng, Lin Tang, Kewei Cheng, Bowen Cheng, Weimin Kang, Roles of metal element substitutions from the bimetallic solid-state electrolytes in lithium batteries, Particuology, 65 (2022), p. 51-71 Figure 1

Effect of lithium salt type on silicon anode for lithium-ion batteries

Silicon (Si) is one of the most promising anode materials in the next generation of Li-ion batteries. However, the application of Si is restricted by its unsatisfactory electrochemical performance. This work investigates the influence of lithium salt type on the electrochemical performance of Si anode, and shows that lithium salt type plays an importing role in determining the electrochemical performance of Si anode. Through investigation and optimization, lithium hexafluorophosphate (LiPF6)-lithium bis(fluorosulfonyl)imide (LiFSI)-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) ternary composite lithium salt with a mole ratio of 7:1:2 exhibits the best cycling stability and rate capability. It retains a high capacity of 1879.8 mAh g−1 after 400 cycles at 0.5 C charge and discharge, and still maintains 1851.4 mAh g−1 at 30 C; which are significantly higher than the 583.3 and 316.4 mAh g−1 of the LiPF6-based electrolyte under the same conditions. The underlying mechanism is investigated, and the results indicate that the lithium type has an effect on both the Si anode and the lithium counter electrode, and the synergistic effect of LiPF6-LiFSI-LiTFSI enables high-rate and long-cycle Si anodes.

EIS Analysis of Sulfur Cathodes with Water-Soluble Binder NV-1A for Lithium-Sulfur Batteries

The paper discusses the electrochemical behavior of a Li-S battery with a new water-soluble binder NV-1A. It is shown that the main contribution is made by the interface, which is formed on the lithium counter electrode. It is noteworthy that the nonlinear growth of the resistance of SEI layer during the discharge process correlates with the change in the resistance of charge transfer through the interface.

Asymmetric Li-Ion Diffusion Profiles during Lithium Plating and Stripping in Ionic Liquid Electrolytes

The electrochemical behavior of lithium metal in ionic-liquid based Li-electrolytes is typically characterized by an anodic peak due to the formation of an inhibition layer. In symmetric Li-Li cells, this shows up as a symmetric set of peaks in the current-cell voltage curves (Figure left), as here the current is limited by the slowest process, in this case the anodic dissolution side. Indeed, for 3-electrode measurements with a Li reference electrode and a lithium counter electrode with much larger surface area as the lithium working electrode, an asymmetric cathodic and anodic current-potential behavior is found (Figure right). Peculiar is that this behavior is reversible and thus not due to a single formation of an irreversible electrolyte decomposition layer, often referred to as a solid-electrolyte interphase or SEI layer even though the dense electronically isolating character and even ionic conductance of these decomposition layers are often doubtful. Our group has reported previously on the peculiar electrochemical behavior of ionic-liquid based electrolytes against lithium metal [1],[2]. We attributed the asymmetric behavior in the 3 electrode measurements to the formation of a temporary passivation layer on top of the usual SEI layer (dead lithium) which diffuses away when stopping the lithium dissolution, i.e., explaining the reversible behavior. As the focus of these papers was on the electrolyte material and cell performance, respectively, we did not go into more detail on the mechanism. In this paper, we will discuss the changes at the interface explaining the time dependent change in kinetics.[1] “Silica gel solid nanocomposite electrolytes with interfacial conductivity promotion exceeding the bulk Li-ion conductivity of the ionic liquid electrolyte filler” Xubin Chen, Brecht Put, Akihiko Sagara, Knut Gandrud, Mitsuhiro Murata, Julian A. Steele, Hiroki Yabe, Thomas Hantschel, Maarten Roeffaers, Morio Tomiyama, Hidekazu Arase, Yukihiro Kaneko, Mikinari Shimada, Maarten Mees and Philippe M. Vereecken, Science Advances, 6, 2, eaav3400 (2020). [2] “ High-Rate Performance Solid-State Lithium Batteries with Silica-Gel Solid Nanocomposite Electrolytes using Bis (fluorosulfonyl) imide-Based Ionic Liquid” Akihiko Sagara, Xubin Chen, Knut B Gandrud, Mitsuhiro Murata, Maarten Mees, Yukihiro Kaneko, Hidekazu Arase, Philippe M Vereecken, Journal Electrochemical Society 167 (7), 070549 (2020). Figure 1

Polypeptide-based batteries toward sustainable and cyclic manufacturing

Polypeptide-based batteries toward sustainable and cyclic manufacturing

Investigations on the Improved Electrochemical Performance of Layered Fe-V-O Kazakhstanite Phase By Using Superconcentrated Electrolytes

In order to develop high energy-density lithium-ion batteries, multi-electron redox reaction based cathode materials are being actively researched across the globe, such as those observed in vanadium and molybdenum based compounds. The vanadium based cathode materials provide significantly higher specific capacity in comparison to the other state of the art materials such as over-lithiated layered oxides (OLO), NMC, NCA etc. However, their drawback of possessing a lower nominal voltage of operation brings their energy density values at the same level as those observed for OLO.In this presentation, we investigate the electrochemical performance of one of the vanadium containing compounds, namely the Kazakhstanite phase (Fe5V15O39(OH)9.9H2O), as a cathode material for lithium-ion batteries. Kazakhstanite is a naturally occuring mineral found in Kazakhstan and United States. Preliminary structural characterization is performed to identify the microstructural, compositional and structural features of the as-synthesized material. Electrochemical investigation with the conventional electrolyte (1M LiPF6 in EC:DMC = 1:1 v:v, Code named: PL) reveals that the material delivers a specific capacity of ~350mAhg-1 between 1.5-3.8V (Li+/Li). However, the cycleability of the material is observed to be extremely poor with the conventional electrolyte. The cause is identified to be elemental dissolution of Fe and V from the active material with the help of EDX analysis of the cycled separators. It is observed that the active material dissolution is arrested to a great extent by changing the electrolyte system to a solvent-in-salt based one, without any additional engineering done on the material. By reducing the amount of solvent in the electrolyte, a solubility limit can be attained which prevents dissolution of active material. The electrochemical performance of the Kazakhstanite phase is evaluated with a series of electrolytes containing LiTFSI as the salt and DOL:DME = 1:1 v:v as the solvent, with varying concentration of the LiTFSI salt ranging from 1M to 7M. The cycleability of the Kazakhstanite phase with 7M LiTFSI in DOL:DME=1:1 v:v (Code named: OL) is found to be the best amongst all the other electrolyte compositions tested. Electrochemical impedance spectroscopy reveals that there should be a bilayer diffusion mechanism operational over the Lithium counter electrode. This indicates that the dissolved products form a passivation layer over the Lithium electrode apart from the SEI. The lithium-ion diffusion across the bilayer of the SEI and the passivation layer is observed to follow equation (1).Zmt ∝ Δc1(0,s)/ΔJ1(0,s) = [Λtanh(λu)+tanh(u)]/[mu(Λtanh(u)tanh(λu)+1)] (1)where:D1, L1 : Diffusion coefficient of Li-ion through the passivation layer and thickness of the passivation layerD2, L2 : Diffusion coefficient of Li-ion through the SEI layer and thickness of the SEI layerλ = √(D1/D2) (L2/L1)Λ = √(D1/D2)m = -D1/L1 u = √(s/D1) L1 with s being the complex variable jωThe proposed bilayer diffusion model, along with the experimental results, explains the superiority of superconcentrated electrolytes over conventional organic electrolyte in tuning the electrochemical characteristics of the SEI and the passivation layer.Molecular Dynamics Simulations are carried out to understand the co-ordination environment of the dissolved vanadium ions in the superconcentrated and conventional electrolytes. The simulated physico-chemical properties such as density, diffusion coefficient of the ions, and ionic conductivity match well with the values reported in the literature. For the superconcentrated electrolyte OL, it is observed from the simulations that the fraction of TFSI- is higher in the first solvation shell of vanadium ions in comparison to the one observed for PL case. The magnitude of the solvation energy of the vanadium ions, calculated using Free Energy Perturbation (FEP) and Finite-Difference Thermodynamic Integration (FDTI) methods, is found to be lower in superconcentrated electrolytes. This indicates that the dissolution of the active material in superconcentrated electrolyte is unfavorable in comparison to the conventional organic electrolyte. Additional EMD simulations are carried out to correlate the motion of the vanadium ions with respect to Li+, TFSI- and solvent molecules.Figure Caption: (a) Co-ordination environment of Vanadium ions (0.16M) inside PL electrolyte. (b) Co-ordination environment of Vanadium ions (0.16M) inside OL electrolyte Figure 1

Exploring Iron Oxides Anodes By Tuning Their Structure and Morphology for Lithium-Ion Batteries

Demand for lithium-ion batteries is increasing with the increase in portal electronic, electric vehicle and renewable energy. High energy dense electrode materials are required to meet this demand. In this study we are exploring an alternative anode material to graphite electrodes. We chose Iron Oxides due their abundancy and high capacity1 .We altered the structure and morphology of iron oxide nanoparticles (IONP) and prepared electrodes from the following nanoparticles (NP): Commercial iron (III) oxide NP (Fe2O3), iron oxide NP + succinic acid, iron oxide NP + citric acid, silica-encapsulated Fe3O4 NP and iron oxide NP + perchloric acid. Slurries were made using a conductive carbon (Super P) carboxymethyl cellulose and ultra-pure water in a weight ratio of 8:1:1 (w/w). Lithium perchlorate (1 M) was dissolved in ethylene carbonate and dimethyl carbonate and used as the electrolyte. Using an ARBIN battery test unit, cyclic voltammetry was performed between 0.05 and 3.0 V and compared to a lithium counter electrode at a scan rate of 0.1 mV/s. Figure 1 compares the electrochemical performance of these iron oxide anodes with different structure and morphology.This was supported by Oklahoma State University’s CEAT undergraduate research scholarship.[1] Wang, X. et al (2016). Uniform Fe3O4 microflowers hierarchical structures assembled with porous nanoplates as superior anode materials for lithium-ion batteries. Elsevier Applied Surface Science vol (389) 240-246. Doi: 10.1016/j.apsusc.2016.07.105 Figure 1

Imaging XAFS Study on Reaction Distribution of Composite Electrode Used As Commercial Lithium-Ion Battery

Demands for large-scale lithium-ion batteries for electric vehicles and grids have increased in order to reduce CO2 emissions in consideration of the global environment. To improve the reliability for practical large-scale batteries, further improvements in safety, rate capability, and cycle life are required. One of the unique behaviors taking place in lithium-ion battery composite electrodes is an inhomogeneous reaction of active materials.1 Since the heterogeneous reaction behavior induces overcharge and degradation of local active materials, the homogeneous charge-discharge reaction should be required. Ouvrard et al. reported that an inhomogeneous electrode reaction was caused during the charge-discharge process in LiFePO4 lithium-ion battery cathode, which was detected by using X-ray absorption spectroscopy.2 The study concluded that the ionic and electronic connectivity influenced by applied pressure is a crucial parameter for reaction distribution in composite electrodes.In this study, we examine the commercially utilized composite electrodes where enough electronic conduction path is formed. We compare the electrochemical characteristics using the commercial electrodes with self-made composite electrodes prepared in the laboratory and then measured the reaction distribution by imaging XAS.Two types of composite electrodes using LiFePO4 as the active material were used, and their thickness and mass loading were almost similar. Two electrode cells were assembled using 1 M LiFP6 in a solvent mixture of ethylene carbonate (EC) and ethyl metal carbonate (EMC) at a volume ratio of 3:7, and lithium counter electrode. Charge-discharge measurements were performed with an upper limit voltage of 3.6 V and a lower limit voltage of 2.0 V at rates of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10, 20, 30, 40 and 50 C. Imaging XAS measurements were performed at beamline BL-4 in Ritsumeikan University SR Center. Fe K-edge XAS was measured in a transmission mode using a two-dimensional detector.As a result of the discharge rate capability test, the capacity retention of 5 C was 92% for the commercial electrode and 5.4% for the self-made electrode, compared with the discharge capacity at 0.1 C, and the self-made electrode showed a remarkable decrease in capacity at high rates. The imaging XAS of the commercial electrodes implies a lack of inhomogeneous reaction after discharging at 5C. The previous reports have shown that the reaction in the planar direction of LiFePO4 proceeds from island-shaped points.3 We assume that the commercial composite electrodes are developed to be improved ionic and electronic connectivity.Reference: J. Liu, M. Kunz, K. Chen, N. Tamura, J. Richardson, J. Phys. Chem. Lett., 1, 2120-2123 (2010).G. Ouvrard, M. Zerrouki, P. Soudan, B. Lestrez, C. Masquelier, M. Morcrette, S. Hamelet, S. Belin, A. M. Flank, F. Baudelet, J. Power Sources, 229, 16-21 (2013).M. Katayama, K. Sumiwaka, R. Miyahara, H. Yamashige, H. Arai, Y. Uchimoto, T. Ohta, Y. Inada, Z. Ogumi, J. Power Sources, 269, 994-999 (2014).

Li-ion half-cells studied operando during cycling by small-angle neutron scattering

Small-angle neutron scattering (SANS) was recently applied to the in situ and operando study of the charge/discharge process in Li-ion battery full-cells based on a pouch cell design. Here, this work is continued in a half-cell with a graphite electrode cycled versus a metallic lithium counter electrode, in a study conducted on the SANS-1 instrument of the neutron source FRM II at the Heinz Maier-Leibnitz Zentrum in Garching, Germany. It is confirmed that the SANS integrated intensity signal varies as a function of graphite lithiation, and this variation can be explained by changes in the squared difference in scattering length density between graphite and the electrolyte. The scattering contrast change upon graphite lithiation/delithiation calculated from a multi-phase neutron scattering model is in good agreement with the experimentally measured values. Due to the finite coherence length, the observed SANS contrast, which mostly stems from scattering between the (lithiated) graphite and the electrolyte phase, contains local information on the mesoscopic scale, which allows the development of lithiated phases in the graphite to be followed. The shape of the SANS signal curve can be explained by a core-shell model with step-wise (de)lithiation from the surface. Here, for the first time, X-ray diffraction, SANS and theory are combined to give a full picture of graphite lithiation in a half-cell. The goal of this contribution is to confirm the correlation between the integrated SANS data obtained during operando measurements of an Li-ion half-cell and the electrochemical processes of lithiation/delithiation in micro-scaled graphite particles. For a deeper understanding of this correlation, modelling and experimental data for SANS and results from X-ray diffraction were taken into account.

Separation of predominant processes in electrochemical impedance spectra of lithium-ion batteries with nickel-manganese-cobalt cathodes

Impedance spectra of a high energy lithium-ion battery (graphite vs. NMC) are investigated in detail, using the distribution of relaxation times (DRT) and complementary methods. Two predominant processes are found, one at lower frequencies and one at moderate frequencies. By investigating impedance spectra of corresponding half-cells (graphite vs. Li and NMC vs. Li), it is found that the process at lower frequencies can clearly be assigned to the NMC's charge transfer. Further, it is assumed that the process at moderate frequencies is correlated to the graphite's solid electrolyte interphase (SEI). However, the relevant frequency-range of the anode half-cell is seemingly overshadowed by the lithium counter-electrode and thus cannot be investigated properly. To investigate this issue, symmetrical lithium cells are build and their impedance spectra are investigated as well. Indeed, the lithium's main-process occurs at the presumed frequencies. Eventually, a combination of several measurements and analyses show that the process at moderate frequencies is correlated to the graphite's SEI. Among these are i) an investigation of a calendric aged commercial cell and its corresponding half-cells and ii) a differential regression analysis (DRA), which is established in this work and proved to be a powerful analysis-tool.

<i>In situ</i> observation of lithiation mechanism of SnO<sub>2</sub> nanoparticles

Tin oxide (SnO<sub>2</sub>) has attracted a lot of attention among lithium ion battery anode materials due to its rich reserves, high theoretical capacity, and safe potential. However, the mechanism of the SnO<sub>2</sub> nano materials in the lithiation-delithiation reaction, especially whether the first-step conversion reaction is reversible, is still controversial. In this paper, SnO<sub>2</sub> nanoparticles with an average particle size of 4.4 nm are successfully prepared via a simple hydrothermal method. A nanosized lithium ion battery that enables the <i>in situ</i> electrochemical experiments of SnO<sub>2</sub> nanoparticles is constructed to investigate the electrochemical behavior of SnO<sub>2</sub> in lithiation-delithiation process. Briefly, the nanosized electrochemical cell consists of a SnO<sub>2</sub> working electrode, a metal lithium (Li) counter electrode on a sharp tungsten probe, and a solid electrolyte of lithium oxide (Li<sub>2</sub>O) layer naturally grown on the surface of metal Li. Then, the whole lithiation-delithiation process of SnO<sub>2</sub> nanocrystals is tracked in real time. When a constant potential of –2 V is applied to the SnO<sub>2</sub> with respect to lithium, lithium ions begin to diffuse from one side of the nanoparticles, which is in contact with the Li/Li<sub>2</sub>O layer, and gradually propagate to the other side. Upon the lithiation, a two-step conversion reaction mechanism is revealed: SnO<sub>2</sub> is first converted into intermediate phase of Sn with an average diameter of 4.2 nm which is then further converted into Li<sub>22</sub>Sn<sub>5</sub>. Upon the delithiation, a potential of 2 V is applied and Li<sub>22</sub>Sn<sub>5</sub> phase can be reconverted into SnO<sub>2</sub> phase when completely delithiated. It is because the interfaces and grain boundaries of nano-sized SnO<sub>2</sub> may impede the Sn diffusing from one grain into another during lithiation/delithiation and then suppress the coarsening of Sn, and enable the Li<sub>2</sub>O and Sn to be sufficiently contacted with each other and then converted into SnO<sub>2</sub>. This work provides a valuable insight into an understanding of phase evolution in the lithiation-delithiation process of SnO<sub>2</sub> and the results are of great significance for improving the reversible capacity and cycle performance of lithium ion batteries with SnO<sub>2</sub> electrodes.

Development of Long Life and Low Cost Li-Rich Layered Oxide Positive Active Material for xEVs

Lithium-rich layered oxides have drawn much attention as a positive active material in the future automotive applications because of a high gravimetric capacity of more than 250 mAh g-1,1 which make the high energy density LiBs. However, this positive active material suffer from several problems such as low initial coulombic efficiency, poor capacity retention and voltage fading upon charge/discharge cycling.2 While a lot of efforts have been made to mitigate these problems, the problems have not sufficiently improved. This situation makes the industrial application difficult. According to previous works, a transition metal composition (Ni/Co/Mn ratio) and a Li/Metal ratio have much influence on electrochemical properties of this positive active material.3-5 However the specific chemical composition of this positive active material has not been appeared to show the high discharge capacity, the high initial coulombic efficiency and good charge/discharge cycle life performance. In this work, the various compositions of lithium-rich layered oxide positive active materials were prepared with a co-precipitation process and a calcination process. The electrochemical properties of these materials were investigated. The relation between compositions and electrochemical properties of these positive active materials were discussed. The twenty different kinds of transition metal compositions carbonate precursors were prepared by a co-precipitation method. The lithium-rich layered oxide positive active materials were made from these carbonate precursors with the different Li/Me ratio and the different calcination temperature. The materials were characterized by means of X-ray diffraction and inductively coupled atomic plasma emission spectroscopy measurements. The materials showing the high discharge capacity were chosen in each transition metal composition. The relation between the composition and electrochemical properties of these materials was organized. Then several compositions were picked up to evaluate the charge/discharge cycle life performance by means of the half-cell with a metallic lithium counter electrode. The material showing the high energy density and good charge/discharge cycle life performance was evaluated the long-term charge/discharge cycle life performance for the cell with graphite anode. Various compositions of lithium-rich layered oxide positive active materials successfully synthesized. The chemicla compositions showed the specific relation with the electrochemical properties. Discharge capacities and energy densities showed the local maximum value in the relation to the x Li2MnO3 - (1-x) LiTMO2 (TM = transition metal). On the other hand, the average potentials showed gradually decreased with the increasing the x in x Li2MnO3 - (1-x) LiTMO2. There was the threshold value of x that discharge capacity retentions suddenly decreased in the relation to the x Li2MnO3 - (1-x) LiTMO2. The average potentials retention showed gradually decreased with the increasing the x in x Li2MnO3 - (1-x) LiTMO2. These results suggest that the chemical composition is extremely important for electrochemical properties of this positive active material. The critical chemical composition has been also found out to show the high discharge capacity (> 260 mAh g-1), the high initial coulombic efficiency (> 90 %) and excellent charge/discharge cycle life performance as well as high energy density (Figure 1). Energy density retention is 78 % after 1000 cycles with 4.6-2.0 V at 1 I tA. This critical composition is based on the hypothesis that valences of Ni and Mn should be only 2+ and 4+, and the ratio of Li2MnO3 and LiTMO2 is predetermined value in order to show stable crystal structure. Moreover, this is composed of less cobalt, so that this has to be one of the promising candidates for xEV-use LiBs. Reference M. S. Whittingham, Chemical Reviews, 104, 4271-4301 (2004).J. R. Croy et al., J. Phys. Chem. C, 117, 6525–6536 (2013).Z. Lu et al., J. Electrochem. Soc., 149, A778-A791 (2002).G. Y. Kim et al., Materials Research Bulletin, 43, 3543-3552 (2008).Q. C. Zhang et al., J. Power Sources, 250, 40-49 (2014). Caption (a) The initial charge/discharge profile of the critical chemical composition lithium-rich layered oxide positive active material for the cell with metallic lithium counter electrode. Charge: CC-CV 0.1 I tA 4.7 V (vs. Li/Li+), 0.02 I tA cut-off at 25 °C. Discharge: CC 0.1 I tA 2.0 V (vs. Li/Li+) at 25 °C. (b) The cycle performance of the critical chemical composition lithium-rich layered oxide positive active material for the cell with graphite anode. The energy density means the value that calculated the cell energy in a gram of positive active material. Cycling condition Charge: CC-CV 0.5 I tA 4.6 V, 0.1 I tA cut-off at 25 °C. Discharge: CC 1 I tA 2.0 V at 25 °C. Capacity check every 50 cycles Charge: CC-CV 0.1 I tA 4.6 V, 0.05 I tA cut-off at 25 °C. Discharge: CC 0.1 I tA 2.0 V at 25 °C. Figure 1

Symmetric Lithium Sulfide – Sulfur Cells: A Method to Study Degradation Mechanisms of Cathode, Separator and Electrolyte Concepts for Lithium-Sulfur Batteries

Novel cathode, electrolyte and separator concepts are expected to improve the capacity retention and cycle life of lithium-sulfur batteries by avoiding the dissolution of polysulfide-intermediates into the electrolyte or preventing their diffusion to and reaction with the lithium metal anode. However, their intrinsic evaluation is obscured by ill-defined degradation mechanisms of the lithium counter electrode due to electrolyte and polysulfide consumption during reformation of the solid electrolyte interface. Herein, we introduce symmetric lithium sulfide – sulfur cells to study the “cathode" specific degradation mechanisms of the cathode/electrolyte/separator-system in order to differentiate cathode-, separator, and electrolyte and lithium metal anode degradation. Two identical carbon/sulfur-composite cathodes, either of them one at first lithiated versus lithium, are assembled in a coin cell versus each other. The virtue of this test cell design is demonstrated evaluating prime shuttle-suppression concepts, namely a Nafion-coated polyolefin separator and a sparingly polysulfide-solvating electrolyte. We conclude that symmetric lithium sulfide – sulfur cells provide a valuable methodology to analyze the intrinsic capacity decay and cycle life of cathodes for lithium-sulfur cells with limited solvation and/or blocking of polysulfides, and it is an essential strategy to distinguish cathode from lithium anode degradation.

Reducing Inhomogeneous Current Density Distribution in Graphite Electrodes By Design Optimization

Future battery applications require higher energy densities as e.g. higher range and longer lifetime in automotive applications are demanded by the customers and will help to increase the acceptance of electric vehicles. To increase energy density in lithium-ion cells, the ratio between passive components and active material needs to be improved e.g. by thicker electrodes [1]. Unfortunately, thicker electrodes can lead to an even more pronounced inhomogeneous utilization of the active material as already seen in normal sized electrodes [2, 3]. Inhomogeneous utilization of electrodes and consequent limitations in the operating conditions have recently been brought to attention [4]. Relaxation measurements after an inhomogeneous current distribution through the thickness of an electrode have not been addressed so far. By using a previously developed laboratory cell setup [5] that can be seen in Figure 1, we are able to show an inhomogeneous retrieval of lithium-ions from a graphite electrode through the layer with spatial resolution for two different graphites. After provoking inhomogeneities via constant current operations, equilibration processes are recorded and can be assigned to two different effects [6, 7]. One effect is an equilibration inside the particles (intra-particle) from surface to bulk and vice versa. The second effect is an equalization between the particles (inter-particle) to reach a homogeneous degree of lithiation in each particle throughout the electrode layer. With the recorded data, we implemented a P2D model with multiple particle sizes and considered the electrode thickness in several separate domains. Using the relaxation data of intra- and inter-particle relaxation (which are about 4 and 40 h respectively) for parameterizing the model, we performed a design study varying particle size and distribution spatially through a graphite electrode to achieve a more homogeneous utilization of the electrode. Figure caption Figure 1: Scheme of cell setup showing (a) the cross section of a typical half-cell measurement versus a lithium-metal counter electrode (CE). The dashed lines represent the exemplary sectioning of this electrode which is implemented in (b) with each layer being connected to a single potentiostat – called slave channels. A separate master channel is used for applying the "cell current" between the lithium counter electrode and the three working electrodes (WE).