Showing papers in "Journal of The Electrochemical Society in 2018"

Review—Organic-Inorganic Hybrid Functional Materials: An Integrated Platform for Applied Technologies

Abstract: 1Department of Materials System Science, Graduate School of Nanobioscience, Yokohama City University, Kanazawa-ku, Yokohama 236-0012, Japan 2Advanced Materials and BioEngineering Research Centre (AMBER) & CRANN, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland 3Department of Chemical and Biomolecular Engineering, Whiting School of Engineering, Johns Hopkins University, Baltimore, Maryland 21218, USA 4Chemical and Biological Engineering, University of Buffalo, New York 14260, USA 5Department of Mechanical System Science, Graduate School of Science and Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan

Factors Limiting Li+ Charge Transfer Kinetics in Li-Ion Batteries

Abstract: Understanding the factors limiting Li+ charge transfer kinetics in Li-ion batteries is essential in improving the rate performance, especially at lower temperatures. The Li+ charge transfer process involved in the lithium intercalation of graphite anode includes the step of de-solvation of the solvated Li+ in the liquid electrolyte and the step of transport of Li+ in the preformed solid electrolyte interphase (SEI) on electrodes until the Li+ accepts an electron at the electrode and becomes a Li in the electrode. Whether the de-solvation process or the Li+ transport through the SEI is a limiting step depends on the nature of the interphases at the electrode and electrolyte interfaces. Several examples involving the electrode materials such as graphite, lithium titanate (LTO), lithium iron phosphate (LFP), lithium nickel cobalt aluminum oxide (NCA) and solid Li+ conductor such as lithium lanthanum titanate or Li-Al-Ti-phosphate are reviewed and discussed to clarify the conditions at which either the de-solvation or the transport of Li+ in SEI is dominating and how the electrolyte components affect the activation energy of Li+ charge transfer kinetics. How the electrolyte additives impact the Li+ charge transfer kinetics at both the anode and the cathode has been examined at the same time in 3-electrode full cells. The resulting impact on Li+ charge transfer resistance, Rct, and activation energy, Ea, at both electrodes are reported and discussed.

Synthesis of Single Crystal LiNi0.6Mn0.2Co0.2O2 with Enhanced Electrochemical Performance for Lithium Ion Batteries

Abstract: Single crystal LiNi0.5Mn0.3Co0.2O2 (NMC532) was shown to have superior stability at high voltages and elevated temperatures compared to conventional polycrystalline NMC532 by the authors. Conventional LiNi0.6Mn0.2Co0.2O2 (NMC622) usually offers more capacity than NMC532 when charged to the same upper cutoff voltage so NMC622 is attractive. It is expected that single crystal NMC622 could also provide better performance than typical polycrystalline NMC622 materials. This work explores the synthesis of single crystal LiNi0.6Mn0.2Co0.2O2 and preferred synthesis conditions were found. A washing and reheating method was used to remove residual lithium carbonate after sintering. The synthesized single crystal NMC622 material worked poorly after the washing-heating treatment without the use of electrolyte additives in the electrolyte. However, with selected additives, single crystal cells outperformed the polycrystalline reference cells in cycling tests. It is our opinion that single crystal NMC622 has a bright future in the Li-ion battery field.

Updating the Structure and Electrochemistry of LixNiO2 for 0 ≤ x ≤ 1

Abstract: Ni-rich transition metal layered oxide materials are of great interest as positive electrode materials for lithium ion batteries. As the popular electrode materials NMC (LiNi1-x-yMnxCoyO2) and NCA (LiNi1-x-yCoxAlyO2) become more and more Ni-rich, they approach LiNiO2. Therefore it is important to benchmark the structure and electrochemistry of state of the art LixNiO2 for the convenience of researchers in the field. In this work, LiNiO2 synthesized from a commercial Ni(OH)2 precursor and modern synthesis methods shows a specific capacity close to the theoretical specific capacity of 274 mAh/g. In-situ X-ray diffraction (XRD) measurements were conducted to obtain accurate structural information versus lithium content, x. The known multiple phase transitions of LixNiO2 during charge and discharge were clearly observed, and the variation in unit cell lattice constants and volume was measured. Differential capacity versus voltage (dQ/dV vs. V) studies were used to investigate the electrochemical properties including regions of composition that show very slow kinetics. It is hoped that this work will be a useful reference for those working on Ni-rich positive electrode materials for Li-ion cells.

Lithium Metal Penetration Induced by Electrodeposition through Solid Electrolytes: Example in Single-Crystal Li6La3ZrTaO12 Garnet

Abstract: Solid electrolytes potentially enable rechargeable batteries with lithium metal anodes possessing higher energy densities than today's lithium ion batteries. To do so the solid electrolyte must suppress instabilities that lead to poor coulombic efficiency and short circuits. In this work, lithium electrodeposition was performed on single-crystal Li6La3ZrTaO12 garnets to investigate factors governing lithium penetration through brittle electrolytes. In single crystals, grain boundaries are excluded as paths for lithium metal propagation. Vickers microindentation was used to introduce surface flaws of known size. However, operando optical microscopy revealed that lithium metal penetration propagates preferentially from a different, second class of flaws. At the perimeter of surface current collectors smaller in size than the lithium source electrode, an enhanced electrodeposition current density causes lithium filled cracks to initiate and grow to penetration, even when large Vickers defects are in proximity. Modeling the electric field distribution in the experimental cell revealed that a 5-fold enhancement in field occurs within 10 micrometers of the electrode edge and generates high local electrochemomechanical stress. This may determine the initiation sites for lithium propagation, overriding the presence of larger defects elsewhere.

Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods

Abstract: We present an unbalanced compositionally-symmetric flow cell method for revealing and quantifying different mechanisms for capacity fade in redox flow batteries that are based on molecular energy storage. We utilize it, accompanied in some cases by a corresponding static-cell cycling method, to study capacity fade in cells comprising anthraquinone di-sulfonate, di-hydroxy anthraquinone, iron hexacyanide, methyl viologen, and bis-trimethylammoniopropyl viologen. In all cases the cycling capacity decay is reasonably consistent with exponential in time and is independent of the number of charge-discharge cycles imposed. By introducing pauses at various states of charge of the capacity-limiting side during cycling, we show that in some cases the temporal fade time constant is dependent on the state of charge. These observations suggest that molecular lifetime is dominated by chemical rather than electrochemical mechanisms. These mechanisms include irrecoverable chemical decomposition and recoverable interactions with cell materials. We conclude with recommendations for cell cycling protocols for evaluating stability of single electrolytes.

A Study of the Physical Properties of Li-Ion Battery Electrolytes Containing Esters

Abstract: Adding esters as co-solvents to Li-ion battery electrolytes can improve low-temperature performance and rate capability of cells. This work uses viscosity and electrolytic conductivity measurements to evaluate electrolytes containing various ester co-solvents, and their suitability for use in high-rate applications is probed. Among the esters studied, methyl acetate (MA) outperforms other esters in its impact on the conductivity and viscosity of the electrolyte. Therefore, viscosity and conductivity were measured as a function of temperature and LiPF6 concentration for electrolytes ethylene carbonate (EC): linear carbonate: MA in the ratio 30:(70-x):x, where linear carbonate = {ethyl methyl carbonate (EMC), dimethyl carbonate (DMC)}, and x = {0, 10, 20, 30}. Adding MA leads to an increase in conductivity and decrease in viscosity over all conditions. Calculations of electrolyte properties from a model based on a statistical-mechanical framework, the Advanced Electrolyte Model (AEM), are compared to all measurements and excellent agreement is found. All electrolytes studied roughly agree with a Stokes' Law model of conductivity. A Walden analysis shows that the ionicity of the electrolyte is not significantly impacted by either MA content or LiPF6 concentration. Li[Ni0.5Mn0.3Co0.2]O2/graphite cells containing MA were cycled at charging rates up to 2C and showed improved cycling performance.

Resolving the discrepancy in tortuosity factor estimation for Li-Ion battery electrodes through micro-macro modeling and experiment

Abstract: Resolving the Discrepancy in Tortuosity Factor Estimation for Li-Ion Battery Electrodes through Micro-Macro Modeling and Experiment Francois L. E. Usseglio-Viretta,1,∗ Andrew Colclasure,1 Aashutosh N. Mistry, 2,∗∗ Koffi Pierre Yao Claver, 3 Fezzeh Pouraghajan, 4,∗∗ Donal P. Finegan, 1 Thomas M. M. Heenan,5 Daniel Abraham, 3,∗ Partha P. Mukherjee, 2,∗ Dean Wheeler, 4,∗ Paul Shearing, 5 Samuel J. Cooper,6 and Kandler Smith 1,∗,z

Comprehensive Modeling of Temperature-Dependent Degradation Mechanisms in Lithium Iron Phosphate Batteries

Abstract: For reliable lifetime predictions of lithium-ion batteries, models for cell degradation are required. A comprehensive semi-empirical model based on a reduced set of internal cell parameters and physically justified degradation functions for the capacity loss is developed and presented for a commercial lithium iron phosphate/graphite cell. One calendar and several cycle aging effects are modeled separately. Emphasis is placed on the varying degradation at different temperatures. Degradation mechanisms for cycle aging at high and low temperatures as well as the increased cycling degradation at high state of charge are calculated separately. For parameterization, a lifetime test study is conducted including storage and cycle tests. Additionally, the model is validated through a dynamic current profile based on real-world application in a stationary energy storage system revealing the accuracy. Tests for validation are continued for up to 114 days after the longest parametrization tests. The model error for the cell capacity loss in the application-based tests is at the end of testing below 1% of the original cell capacity and the maximum relative model error is below 21%. © The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.1181714jes]

Perspective—Commercializing Lithium Sulfur Batteries: Are We Doing the Right Research?

Abstract: A picture of the challenges faced by the lithium-sulfur technology and the activities pursued by the research community to solve them is synthesized based on 1992 scientific articles. It is shown that, against its own advice of adopting a balanced approach to development, the community has instead focused work on the cathode. To help direct future work, key areas of neglected research are highlighted, including cell operation studies, modelling, anode, electrolyte and production methods, as well as development goals for real world target applications such as high altitude unmanned aerial vehicles. © The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0071801jes] All rights reserved.

Energy Density of Cylindrical Li-Ion Cells: A Comparison of Commercial 18650 to the 21700 Cells

Abstract: The standard format for cylindrical Li-ion cells is about to change from 18650-type cells (18mm diameter, 65mm height) to 21700-type cells (21mm diameter, 70mm height). We investigated the properties of five 18650 cells, three of the first commercially available 21700, and three types of the similar 20700 cells in detail. In particular, the (i) specific energy/energy density and electrode thickness, (ii) electrode area and cell resistance, (iii) specific energy as a function of discharge C-rate, as well as (iv) heating behavior due to current flow are analyzed. Finally, the production effort for cells and packs are roughly estimated for 21700 cells compared to 18650 cells.

“Water-in-Salt” for Supercapacitors: A Compromise between Voltage, Power Density, Energy Density and Stability

Abstract: We report here electrochemical capacitors using an aqueous electrolyte based on the concept of "water-in-salt" with the aim to improve the energy density by increasing the voltage of the cell. A "water-in-salt" consists of a highly concentrated aqueous LiTFSI solution in which both volume and mass of LiTFSI are greater than those of water. With activated carbon supercapacitor electrodes (PICA) and 31 m "water-in-salt" electrolytes (m stands for molality), we were able to reach a cell voltage of 2.4 V whereas it is difficult to exceed 1.6 V in conventional aqueous devices because of water splitting. Moreover, it was observed that the specific capacitance of the cell is improved using "water-in-salt" electrolytes. In these conditions, an energy density of 30 Wh kg−1 was obtained which is at least three times greater than for conventional aqueous devices and in the same order of magnitude than for redox enhanced capacitors. Interestingly, fair stability, over 2000 cycles, was obtained for the 7 m electrolyte. Up to 90 sec charging-discharging rate, this latter electrolyte offers the best compromise between voltage, power and energy densities and stability. This study demonstrates the feasibility of water-in-salt as an electrolyte for supercapacitors and points out the most suited compositions for these electrolytes.

XPS-Surface Analysis of SEI Layers on Li-Ion Cathodes: Part II. SEI-Composition and Formation inside Composite Electrodes

Abstract: In this contribution, we investigate the solid electrolyte interface (SEI) layers’ composition depending on the spatial location within LiCoO2 composite cathode of a commercial Li-Ion battery. The surface chemistry is analyzed by X-ray photoelectron spectroscopy (XPS), and possible SEI morphology and the differences in the SEI composition are discussed in detail. Finally, related SEI formation reactions and the controlling processes are characterized as a function of the depth in the composite cathode. The SEI is assumed to be a multi-component, layered system. The inorganic inner SEI layer consists of LiF and degraded LiCoO2, confirmed as Co(II,III)xOy(OH)z. The much thicker outer SEI layer is mainly composed of a poly-organic network with a significantly smaller portion of, presumably, randomly distributed macroscopic LixPOyFz/LixPOy-1Fz+1 and LixPOy domains. A higher content of Co(II,III)xOy(OH)z, and especially of the poly-organic deposit, was found on the outer cathode surface compared to the analysis position near the current collector, resulting in a 4 nm thicker SEI and indicating a stronger decomposition of LiCoO2 and solvents. These differences in SEI composition and thickness are attributed to a significantly higher cathode polarization at the outer electrode surface during cell operation leading to a higher rate of electrochemically induced decomposition reactions.

Depth-dependent redox behavior of LiNi0.6Mn0.2Co0.2O2

Abstract: Author(s): Tian, C; Nordlund, D; Xin, HL; Xu, Y; Liu, Y; Sokaras, D; Lin, F; Doeff, MM | Abstract: Nickel-rich layered materials are emerging as cathodes of choice for next-generation high energy density lithium ion batteries intended for electric vehicles. This is because of their higher practical capacities compared to compositions with lower Ni content, as well as the potential for lower raw materials cost. The higher practical capacity of these materials comes at the expense of shorter cycle life, however, due to undesirable structure and chemical transformations, especially at particle surfaces. To understand these changes more fully, the charge compensation mechanism and bulk and surface structural changes of LiNi0.6Mn0.2Co0.2O2 were probed using synchrotron techniques and electron energy loss spectroscopy in this study. In the bulk, both the crystal and electronic structure changes are reversible upon cycling to high voltages, whereas particle surfaces undergo significant reduction and structural reconstruction. While Ni is the major contributor to charge compensation, Co and O (through transition metal-oxygen hybridization) are also redox active. An important finding from depth-dependent transition metal L-edge and O K-edge X-ray spectroscopy is that oxygen redox activity exhibits depth-dependent characteristics. This likely drives the structural and chemical transformations observed at particle surfaces in Ni-rich materials.