Lithium Cycling Efficiency Research Articles

Delineating the Lithium–Electrolyte Interfacial Chemistry and the Dynamics of Lithium Deposition in Lithium–Sulfur Batteries

AbstractStabilizing the metallic lithium anode is the major roadblock to realizing long cycle life lithium‐sulfur (Li–S) batteries with high energy density. The chemistry of the dynamically evolving solid‐electrolyte interphase layer on the lithium surface is critical to achieving reversible plating and stripping of lithium. Herein, electrolyte formulations are carefully varied and employed to modulate the composition, and thereby, the properties of the lithium‐electrolyte interface. The impact of these changes in the interfacial chemistry on the dynamics of lithium deposition is evaluated by tracking cyclability in an anode‐free full cell configuration. Critical insights are revealed on the role of different components of the electrolyte and their interplay, including various functional groups in the electrolyte salt, oxidizing effect of lithium nitrate, and the intrinsically stabilizing effect of polysulfide species, in determining the characteristics of lithium deposition and lithium cycling efficiency. The study illuminates the key elements underlying the unique chemistry of the lithium–electrolyte interface in Li–S batteries and it can spur further development of novel strategies toward stabilizing the lithium‐metal anode in the Li–S system.

Implications of in situ chalcogen substitutions in polysulfides for rechargeable batteries

Selenium and tellurium in Li–S batteries: formation of polyselenosulfides enables improved sulfur redox kinetics while formation of polytellurosulfides improves lithium cycling efficiency.

Tellurium-Polysulfide Chemistry Enables Stable Operation of Lithium-Sulfur Batteries with Limited Lithium Inventory and Electrolyte Supply

The practical realization of lithium-sulfur (Li-S) batteries with high energy density is currently limited by the poor cyclability of the lithium-metal anode. Lithium plating and stripping in the liquid ether-based electrolyte is irreversible over long-term cycling and this leads to rapid capacity fade. Due to the reducing nature of lithium, parasitic side reactions with the electrolyte also occur with considerable severity during cell operation. This eliminates a significant portion of the available lithium inventory and electrolyte supply, and the cell fails prematurely. These problems can be sidestepped by employing surplus lithium and electrolyte in the cell, but this compromises the stack-level energy density. Stabilizing lithium deposition and reducing electrolyte decomposition is of critical importance to enabling commercially viable Li-S batteries with high energy density and long cycle life.The chemistry of Li-S batteries is unique due to the generation of polysulfide intermediates at the cathode that are soluble in the electrolyte. The polysulfides migrate to the anode side, which fundamentally alters its interfacial chemistry by forming reduced sulfide species (Li2S/Li2S2) on the lithium surface. These species are known to have an intrinsically stabilizing effect on lithium deposition, although it is not sufficient to enable stable operation of Li-S batteries at reasonable current densities. Nevertheless, the modification of electrolyte-soluble polysulfide species offers a unique opportunity for tuning the interfacial chemistry of the lithium-metal anode towards stabilizing lithium deposition and reducing electrolyte decomposition.Tellurium, which sits two position below sulfur in Group 16 of the periodic table, is an excellent candidate element for modifying the composition of polysulfide intermediates in Li-S batteries and forming potentially favorable components in the lithium SEI layer. It was found that elemental tellurium (Te0) could be facilely assimilated into polysulfide species to form soluble mixed polytellurosulfide species when left in an ethereal solution of Li2S6. This was confirmed by the characteristic isotopic signature of tellurium using mass spectrometry. The polytellurosulfide species were found to reduce on the surface of lithium metal to form red-colored lithium thiotellurate (Li2TeS3), which was confirmed by X-ray techniques. In contrast to Li2S, Li2TeS3 is expected to be a significantly better ionic conductor due to the reduced electron density around the sulfur atoms. This would help engender a uniform lithium-ion flux on the lithium surface, enabling homogenous lithium deposition.Based on the results above, elemental tellurium was simply added in a 1 : 10 molar ratio to the sulfur/Li2S cathode to modify the chemistry of Li-S batteries. During cell operation, the generated polysulfides (Li2Sn) reacted with the Te0 additive in the cathode to form polytellurosulfide (Li2TexSy) species in-situ. The crossover of these tellurium-containing species to the anode side was confirmed by the detection of a significant amount of Li2TeS3 on the lithium surface. The favorable properties of Li2TeS3 as a lithium SEI component enabled the formation of dense and uniform lithium deposits and significantly improved lithium cycling efficiency. In the anode-free full cell configuration, which pairs a Li2S cathode with a bare anode current collector and contains zero excess lithium (N/P ratio = 1), the addition of tellurium improved cycle life by a factor of 7. The loss of the limited lithium inventory in the anode-free cell was reduced from 2% per cycle to 0.25% per cycle.Time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed the drastically reduced electrolyte decomposition on lithium surface due to tellurium-stabilized lithium deposition. The reduced electrolyte decomposition could allow a limited electrolyte supply to be retained for a longer number of cycles, thus enabling extended cyclability under lean-electrolyte conditions. This was confirmed using large-area pouch cells employing a high-loading sulfur cathode (5.2 mg cm-2), a lithium-foil anode, and a low electrolyte/sulfur (E/S) ratio of 4.5 µl mg-1. Under such stringent cell design conditions, the pouch cells could be cycled stably for nearly 100 cycles with the addition of tellurium. In contrast, the control cell failed within 25 cycles.This work demonstrates a novel, effective, and robust strategy of adding tellurium to the sulfur/Li2S cathode for stabilizing lithium deposition and reducing electrolyte decomposition in Li-S batteries. This improves cycle life in energy-dense cell configurations operating with a limited lithium inventory or electrolyte supply. Importantly, this approach is scalable as no expensive or complicated pre-treatments or coatings of the lithium-metal anode are necessary. The unique chemistry of tellurium in the polysulfide-rich environment of Li-S batteries opens a new frontier in research in Li-S batteries, bringing them one step closer to reality. Figure 1

(Invited) High Coulombic Efficiency Electrolyte Enables Li||Span Batteries with Superior Cycling Performance

Lithium metal is being extensively studied as an anode to replace graphite due to its high capacity and the low electrochemical potential.Current research focus is to mitigate lithium dendrite growth and to increase coulomb efficiency during electrochemical cycling by using various surface coatings and electrolyte additives.1 , 2 In addition, a 3D host can be beneficial due to the reduced volume change.3 While sulfur or metal oxide intercalation cathodes can both in principle achieve full-cell energy densities of 500 Wh kg-1, sulfur-based cathodes are particularly appealing due to their low cost, abundant supply, and high storage capacity. Among these, the SPAN (sulfurized polyacrylonitrile) cathode is particularly promising due to the physical confinement of the small molecular sulfur in the conductive polymer network which provides a high specific capacity of > 550 mAh g-1 and an average discharge potential of ~ 1.8 V. We have recently focused on designing novel electrolyte solution chemistry that enables Li||SPAN batteries with superior cycling performance.In the first example, we designed a high-concentration, ether-based electrolyte with LiTFSI and LiNO3 as the co-salts. In addition to providing excellent protection for lithium metal anodes by forming the solid electrolyte interface (SEI), the electrolyte promotes the formation of a crystalline cathode electrolyte interface (CEI) on the SPAN surface composed of LiF and LiNO2. The CEI effectively prevents the formation of soluble polysulfide species and enables stable cycling of the Li||SPAN battery. The benefit of having effective CEI and SEI layers is also demonstrated in Li||SPAN cells with limited lithium supply, which exhibit lithium cycling efficiency values consistent with or exceeding Li||Cu test results, reaching 99.5%. The development of electrolyte chemistries to enable the formation of effective CEI layers is a promising approach to long-life lithium metal batteries.In another example, we formulated a highly concentrated salt/ether electrolyte diluted in a fluorinated ether, which realized an average lithium coulombic efficiency of 99.37% at 0.5 mA cm−2 and 1 mAh cm−2 for more than 900 cycles. This electrolyte also maintained a record coulombic efficiency of 98.7% at 10 mA cm−2, indicative of its ability to provide fast-charging with high cathode loadings. Morphological studies reveal dense, dendrite free Li depositions after prolonged cycling, while surface analyses confirmed the formation of a robust LiF-rich SEI layer on the cycled Li surface. Moreover, this ether-based electrolyte is highly compatible with the SPAN cathode, where the constructed Li||SPAN cell exhibited reversible cathode capacity of 579 mAh g-1 and no capacity decay after 1200 cycles. A cell where a high areal loading SPAN electrode (3.5 mAh cm−2) is paired with only one-fold excess Li was constructed and cycled at 1.75 mA cm−2, maintaining a coulombic efficiency of 99.30% for the lithium metal. This study provides a path to enable high energy density Li||SPAN battery with stable cycling. Acknowledgements This work was supported by the Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research Program (Battery 500 Consortium) under Contract DE-EE0007764. Part of the work used the UCSD-MTI Battery Fabrication Facility and the UCSD-Arbin Battery Testing Facility. H. Liu, X. Wang, H. Zhou, H.-D. Lim, X. Xing, Q. Yan, Y. S. Meng and P. Liu, ACS Applied Energy Materials, 2018, DOI: 10.1021/acsaem.8b00348.H. Liu, H. Zhou, B.-S. Lee, X. Xing, M. Gonzalez and P. Liu, Acs Applied Materials & Interfaces, 2017, 9, 30635-30642.H. Liu, X. Yue, X. Xing, Q. Yan, J. Huang, V. Petrova, H. Zhou and P. Liu, Energy Storage Materials, 2018, DOI: https://doi.org/10.1016/j.ensm.2018.09.021.

Cathode electrolyte interface enabling stable Li–S batteries

A high-concentration, ether-based electrolyte with LiTFSI and LiNO3 as the co-salts is proposed, which enables stable cycling of a lithium metal battery using sulfurized polyacrylonitrile (SPAN) as the cathode material. In addition to providing excellent protection for lithium metal anodes by forming the solid electrolyte interface (SEI), the electrolyte promotes the formation of a crystalline cathode electrolyte interface (CEI) on the SPAN surface composed of LiF and LiNO2. The CEI effectively prevents the formation of soluble polysulfide species and enables stable cycling of the Li-SPAN battery. The benefit of having effective CEI and SEI layers is also demonstrated in Li-SPAN cells with limited lithium supply, which exhibit lithium cycling efficiency values consistent with or exceeding Li/Cu test results, reaching 99.5%. The development of electrolyte chemistries to enable the formation of effective CEI layers is a promising approach to long-life lithium metal batteries.

Polyethylene oxide film coating enhances lithium cycling efficiency of an anode-free lithium-metal battery.

The practical implementation of an anode-free lithium-metal battery with promising high capacity is hampered by dendrite formation and low coulombic efficiency. Most notably, these challenges stem from non-uniform lithium plating and unstable SEI layer formation on the bare copper electrode. Herein, we revealed the homogeneous deposition of lithium and effective suppression of dendrite formation using a copper electrode coated with a polyethylene oxide (PEO) film in an electrolyte comprising 1 M LiTFSI, DME/DOL (1/1, v/v) and 2 wt% LiNO3. More importantly, the PEO film coating promoted the formation of a thin and robust SEI layer film by hosting lithium and regulating the inevitable reaction of lithium with the electrolyte. The modified electrode exhibited stable cycling of lithium with an average coulombic efficiency of ∼100% over 200 cycles and low voltage hysteresis (∼30 mV) at a current density of 0.5 mA cm-2. Moreover, we tested the anode-free battery experimentally by integrating it with an LiFePO4 cathode into a full-cell configuration (Cu@PEO/LiFePO4). The new cell demonstrated stable cycling with an average coulombic efficiency of 98.6% and capacity retention of 30% in the 200th cycle at a rate of 0.2C. These impressive enhancements in cycle life and capacity retention result from the synergy of the PEO film coating, high electrode-electrolyte interface compatibility, stable polar oligomer formation from the reduction of 1,3-dioxolane and the generation of SEI-stabilizing nitrite and nitride upon lithium nitrate reduction. Our result opens up a new route to realize anode-free batteries by modifying the copper anode with PEO to achieve ever more demanding yet safe interfacial chemistry and control of dendrite formation.

Functional Ionic Liquid Based Electrolytes

NOHMs Technologies is developing functional ionic liquid based hybrid electrolytes for high energy density, high voltage, and high power batteries for EV and military applications. The composition of electrolytes greatly affects the cell performance from rate capability, lithium cycling efficiency, and capacity retention at various temperatures to tolerance against electrical, mechanical and thermal abuses. Ionic liquids offer a host of attractive properties, including ultralow vapor pressure, high thermal stability, high ionic conductivity and wide electrochemical stability, which make them attractive as electrolytes. However, there are some physical property limitations that to-date has made them unattractive for lithium battery applications. High viscosity and the fraction of the ionic conductivity of the electrolyte arising from mobile lithium ions (i.e. the so-called lithium transference numbers) are typically low for these materials, which make cells assembled using ionic liquid electrolytes prone to large polarization during operation of the cell. These limitations can be easily overcome by blending ionic liquids with appropriate co-solvents. These hybrid electrolytes simultaneously overcome the poor thermal & electrochemical stability and safety problems that have plagued lithium battery electrolytes for years while still maintaining high conductivity. They provide a platform for engineering electrolytes with both chemical and interfacial tunability that beyond improving safety, expand the range of available battery form factors. During the presentation, we will show how the ionic liquid functionality affects the SEI formation on the graphite anode and high voltage stability. We intend to answer the following questions: 1) functional ionic liquid and co-solvent formulations to achieve a >4.5V electrolyte, 2) is the capacity fade dependent on the upper voltage cut-off, and 3) how the electrolyte formulations improve the capacity retention of at high voltages and elevated temperatures using 200 mAh pouch cells. Figure 1

Effects of Pulse Plating on lithium electrodeposition, morphology and cycling efficiency

In order to address lithium dendrite formation and low cycling efficiency issues, Pulse Plating (PP) and Reverse Pulse Plating (RPP) have been systematically investigated for lithium electrodeposition with a modified button cell device. Compared with Direct Current (DC) electrodeposition, PP waveforms with short and widely spaced pulses improve lithium deposition morphology and cycling efficiency under diffusion-controlled conditions. While RPP waveforms with high current density anodic pulses further improve lithium cycling efficiency, no obvious improvement in morphology was seen under the conditions tested. This study suggests that PP and RPP could be powerful tools for utilizing lithium metal anodes in high energy density rechargeable battery systems, especially when high instant power is required.

Stabilized Lithium-Metal Surface in a Polysulfide-Rich Environment of Lithium-Sulfur Batteries.

Lithium-metal anode degradation is one of the major challenges of lithium-sulfur (Li-S) batteries, hindering their practical utility as next-generation rechargeable battery chemistry. The polysulfide migration and shuttling associated with Li-S batteries can induce heterogeneities of the lithium-metal surface because it causes passivation by bulk insulating Li2S particles/electrolyte decomposition products on a lithium-metal surface. This promotes lithium dendrite formation and leads to poor lithium cycling efficiency with complicated lithium surface chemistry. Here, we show copper acetate as a surface stabilizer for lithium metal in a polysulfide-rich environment of Li-S batteries. The lithium surface is protected from parasitic reactions with the organic electrolyte and the migrating polysulfides by an in situ chemical formation of a passivation film consisting of mainly Li2S/Li2S2/CuS/Cu2S and electrolyte decomposition products. This passivation film also suppresses lithium dendrite formation by controlling the lithium deposition sites, leading to a stabilized lithium surface characterized by a dendrite-free morphology and improved surface chemistry.

Effects of organic silicon compounds as additives on charge–discharge cycling efficiencies of lithium in nonaqueous electrolytes for rechargeable lithium cells

Influence of mixing organic silicon compounds into 1 M (M: mol L−1) LiPF6-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) mixed solvent electrolytes on charge–discharge cycling efficiencies of lithium metal negative electrodes is examined. As organic silicon compounds, polyether-modified siloxanes with polyethylene oxide chains, chlorotrimethylsilane, tetraethoxysilane, cis-tetra [isobutyl (dimethylsiloxy)] cyclotetrasiloxane and cage-type silsesquioxane are investigated. Charge–discharge cycling tests of lithium are galvanostatically carried out using stainless steel working electrodes. Charge–discharge cycling efficiencies of lithium tend to improve by mixing organic silicon compounds. A cage-type silsesquioxane, octaphenyloctasilsesquioxane (Ph8T8) exhibits the highest cycling efficiency of approximately 80% with small mixing amount of 0.02 M Ph8T8. Mechanism of enhancement of lithium cycling efficiencies by mixing organic silicon compounds is considered to be due to the suppression of excess reduction of LiPF6-EC/EMC by lithium and the growth of surface film on lithium.

Increased Cycling Efficiency of Lithium Anodes in Dimethyl Sulfoxide Electrolytes For Use in Li-O2 Batteries

High lithium cycling efficiencies are required if a metal anode system is to be considered for use in Li-O2 batteries. In this work electrolyte additives (0.3 M LiNO3 and 0.14 M VC) were used to increase the efficiency from 25 to 82.5% in the topical DMSO based electrolyte. Furthermore, we show that oxygen also acts to improve the cycling efficiency to 87%. This work highlights the importance of anode considerations in the development of metal O2 batteries in alternative solvents (DMSO, Acetonitrile and DMA) and suggests realistic strategies for performance improvements. © The Author(s) 2014. 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

Effects of nonaqueous electrolyte solutions mixed with carbonate-modified siloxane on charge–discharge performance of negative electrodes for secondary lithium batteries

Influence of mixing carbonate-modified siloxane into 1 M (M: mol L−1) LiPF6-ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (mixing volume ratio = 3:7) mixed solvent electrolytes on charge–discharge cycling properties of lithium is examined. As siloxane, 4-(2-bis(trimethylsilyloxy)methylsilylethyl)-1,3-dioxolan-2-one is investigated. Charge–discharge cycling efficiencies of lithium metal anodes are improved and impedance of anode/electrolyte interface decreases by mixing siloxane, compared with those in 1 M LiPF6-EC/EMC alone. Mechanism of enhancement of lithium cycling efficiency is considered to be due to the suppression of excess reduction of LiPF6-EC/EMC by lithium and growth of surface film (SEI, solid electrolyte interphase) on lithium. Infrared study indicates that SEI formed in LiPF6-EC/EMC with siloxane contains more inorganic compounds than that in LiPF6-EC/EMC alone. Better cycling behavior of α-Fe2O3 and Si–SiO2–carbon composite anodes is obtained by mixing siloxane. Thermal behavior of electrolyte solutions toward graphite-lithium anodes is evaluated with a differential scanning calorimeter. Heat-output of graphite–lithium anodes with 1 M LiPF6-EC/EMC electrolyte solutions tends to decrease by mixing siloxane.

Spatially heterogeneous carbon-fiber papers as surface dendrite-free current collectors for lithium deposition

Summary Despite the high energy density, the lithium metal electrode has been plagued for decades with a dendrite growth problem that can result in a battery thermal runaway. Here, we introduce anisotropic spatially heterogeneous three dimensional (3D) current collectors that prevent lithium deposition on the insulating electrolyte-facing surface and accommodate lithium deposition inside the spacious voids. The anisotropic spatial heterogeneity was introduced by a line-of-sight deposition of a thin SiO2 layer onto a carbon-fiber paper. The deposited SiO2 was later converted into SiC by a magnesiothermic reaction. The SiO2 and SiC decorated 3D current collectors were confirmed dendrite-free by ex situ SEM observation after a deep lithium deposition of 28.8 C cm−2 at a high current density of 4 mA cm−2. A high lithium cycling efficiency of 94% was achieved over deep deposition (14.4 C cm−2) and stripping cycles in a carbonate based organic electrolyte, demonstrating the superiority of the novel current collector for dendrite control and lithium cycling. This strategy opens new avenues to address the dendrite problem by rationally designed current collectors and for the creation of a high energy density electrode.

Effects of Tetrahydrofuran Pretreatment on the Performance of Pyrrole Modified Li Metal Electrode

To get rid of impurities on a Li surface and avoid hydrogen gas generation during the preparation of a lithium pyrrolide film,tetrahydrofuran(THF) was used to pretreat the surface of a Li electrode.We found that THF pretreatment can obviously refresh the Li surface as well as improve the uniformity and density of the lithium pyrrolide film.The interface resistance effectively decreases and the interfacial stability improves.Additionally,the reversibility of the lithium deposition/dissolution process increases.All the 'sponge-shaped' lithium deposits are homogeneously distributed on the electrode surface and the lithium cycling efficiency is enhanced by THF pretreatment.

Electrochemical properties and lithium ion solvation behavior of sulfone–ester mixed electrolytes for high-voltage rechargeable lithium cells

Sulfone–ester mixed solvent electrolytes were examined for 5V-class high-voltage rechargeable lithium cells. As the base-electrolyte, sulfolane (SL)–ethyl acetate (EA) (1:1 mixing volume ratio) containing 1M LiBF4 solute was investigated. Electrolyte conductivity, electrochemical stability, Li+ ion solvation behavior and cycleability of lithium electrode were evaluated. 13C NMR measurement results suggest that Li+ ions are solvated with both SL and EA. Charge–discharge cycling efficiency of lithium anode in SL–EA electrolytes was poor, being due to its poor tolerance for reduction. To improve lithium charge–discharge cycling efficiency in SL–EA electrolytes, following three trials were carried out: (i) improvement of the cathodic stability of electrolyte solutions by change in polarization through modification of solvent structure; isopropyl methyl sulfone and methyl isobutyrate were investigated as alternative SL and EA, respectively, (ii) suppression of the reaction between lithium and electrolyte solutions by addition of low reactivity surfactants of cycloalkanes (decalin and adamantane) or triethylene glycol derivatives (triglyme, 1,8-bis(tert-butyldimethylsilyloxy)-3,6-dioxaoctane and triethylene glycol di(methanesulfonate)) into SL–EA electrolytes, and (iii) change in surface film by addition of surface film formation agent of vinylene carbonate (VC) into SL–EA electrolytes. These trials made lithium cycling behavior better. Lithium cycling efficiency tended to increase with a decrease in overpotential. VC addition was most effective for improvement of lithium cycling efficiency among these additives. Stable surface film is formed on lithium anode by adding VC and the resistance between anode/electrolyte interfaces showed a constant value with an increase in cycle number. When the electrolyte solutions without VC, the interfacial resistance increased with an increase in cycle number. VC addition to SL–EA was effective not only for Li/LiCoO2 cell with charge cut-off voltage of 4.5V but also for Li/LiNi0.5Mn1.5O4 cells even with high charge cut-off voltage of 5V in Li/LiNi0.5Mn1.5O4 cells.

Gel electrolytes based on lithium modified silica nano-particles

In this work lithium modified silica (Li-SiO2) nano-particles were synthesized and used as a single ion lithium conductor source in gel electrolytes. It was found that Li-SiO2 exhibited good compatibility with DMSO, DMA/EC (a mixture of N,N-dimethyl acetamide and ethylene carbonate) and the ionic liquid, N-methyl-N-propyl pyrrolidinium bis(trifluoromethylsulfonyl) amide ([C3mpyr][NTf2]). Several gel electrolytes based on Li-SiO2 were obtained. These gel electrolytes were investigated by DSC, solid state NMR, conductivity measurements and cyclic voltammetry. Conductivities as high as 10−3 S/cm at room temperature were observed in these nano-particle gel electrolytes. The results of electrochemical tests showed that some of these materials were promising for using as lithium conductive electrolytes in electrochemical devices, with high lithium cycling efficiency evident.

Lithium metal stripping/plating mechanisms studies: A metallurgical approach

In the hope of meeting the requirements of long term cyclability for polymer electrolyte metallic lithium battery (LMP) (>600 cycles) and widening the scope of their commercial application (low temperature operation), numerous studies have been devoted to the understanding of the intrinsic limitations of these systems. Dendritic lithium growth is known to result in short circuits upon cycling. Before considering ways to master lithium deposits morphologies, we have embarked on a study aimed at characterizing by mean of Scanning Electron Microscopy (SEM): (1) the texture of metallic lithium (2) the influence of cycling parameters, such as current densities and liquid electrolyte formulation, on lithium stripping mechanisms and (3) the lithium deposits morphologies on various lithium surface states ( in and ex situ prepared through electrochemical means, physical procedures such as polishing and Pulsed Laser Deposition technique). Different lithium stripping processes have been revealed depending on current densities. They lead to various surface defects from which lithium dendrites will preferentially grow upon following Li plating. Furthermore, cycling experiments have highlighted, for the first time, the internal lithium micro-texture. Pursuing this study, we finally shed light on the crucial role of the electrolytic mix formulation and cell pressure on lithium cycling efficiency.

Poly-ether modified siloxanes as electrolyte additives for rechargeable lithium cells

Influence of four poly-ether modified siloxanes as electrolyte additives on charge–discharge cycling properties of lithium was examined. As siloxanes, diethylene glycol methyl-(3-dimethyl(trimethylsiloxy)silyl propyl)ether (sample A), diethylene glycol methyl-(3-dinethyl(trimethylsiloxy)silyl propyl)-2-methylpropyl ether (sample B), diethylene glycol methyl-(3-bis(trimethylsiloxy)silyl propyl)ether (sample C) and diethylene glycol-(3-methyl-bis(trimethylsiloxy)silyl-2-methylpropyl)ether (sample D) were investigated. As a base electrolyte solution, 1 M (M, mol L −1) LiPF 6-ethylene carbonate (EC)/methylethyl carbonate (MEC) (mixing volume ratio = 3:7) was used. As the anodes, lithium metal, natural graphite carbon and silicon–SiO 2–carbon (Si–C) composite electrodes were used. Lithium cycling efficiencies of these three anodes improved and an impedance of anode/electrolyte interface decreased by adding poly-ether modified siloxanes. Graphite/LiCoO 2 and Si–C/LiCoO 2 cells exhibited better anode utilization and good cycling performance by using 1 M LiPF 6–EC/MEC + siloxane electrolytes. It was also found that thermal stability of the electrolyte solutions improved by adding siloxanes. Thermal decomposition temperature of 1 M LiPF 6–EC/MEC shifted to higher temperature by adding siloxanes. Amount of heat-output of graphite–lithium anodes with M LiPF 6–EC/MEC electrolyte solutions decreased and the temperature starting the heat-output shifted to higher temperature by adding siloxanes. Among siloxanes examined here, samples B and D exhibited much better performance.

Electrochemical properties of aryladamantanes as new overcharge protection compounds for lithium cells

The development of new overcharge protection compounds for 4 V-class lithium cells with LiCoO 2 cathodes was attempted in order to protect the cells from the hazards of ignition and/or explosion when overcharged. At least two requirements must be met as regards overcharge protection compounds. They must have an oxidation potential ( E ox) in the 4.6–4.8 V range versus Li/Li + and they must not adversely influence the lithium cycling efficiency (Eff). The overcharge protection compounds were used by adding them to the electrolyte solution, which was LiClO 4/propylene carbonate. First, this study involved a basic examination of the relationships among the E ox values of arenes including polyacenes, naphthalene derivatives and methylbenzenes, their chemical structure, the aromatic ring size and the effects of substituent functional groups. Next, the E ox and Eff values of cycloalkanes including cyclohexane, decaline, adamantane and norbornane were examined. Based on these examination results, various adamantane derivatives were synthesized and their electrochemical properties were investigated. Finally, four aryladamantanes (1-( p-tolyl)adamantane, 1-( m-tolyl)adamantane, 1-( o-tolyl)adamantane, 1-(4-ethylphenyl)adamantane) were found to exhibit better E ox and Eff values than those of biphenyl, which is already used in commercial cells as an overcharge protection additive.

Structural and Functional Analysis of Surface Film on Li Anode in Vinylene Carbonate-Containing Electrolyte

The lithium cycling efficiencies of the lithium anode in the ethylene carbonate (EC)-based electrolytes were improved by adding vinylene carbonate (VC) to the electrolyte. We analyzed the surface films of deposited lithium on a nickel substrate in a VC-containing electrolyte with scanning electron microscopy, Fourier transform infrared spectroscopy, two-dimensional nuclear magnetic resonance, gel permeation chromatography, and X-ray photoelectron spectroscopy. The corresponding surface films comprise various polymeric species including poly-(vinylene carbonate) [poly-(VC)], oligomeric VC, and a ring-opened polymer of VC. Furthermore, the surface film of carbon double bonds and lithium carboxylate (RCOOLi) as reduction products of VC were formed on deposited lithium. These structures of the surface film on the lithium anode were similar to those on the graphite anode. At elevated temperatures, the VC-containing electrolyte led to the formation of surface films comprising poly-(VC). The VC-derived polymeric surface film, which exhibited gel-like morphology, could prevent the deleterious reaction which occurs between deposited lithium and the electrolyte, resulting in an enhanced lithium cycling efficiency. © 2004 The Electrochemical Society. All rights reserved.