Increase In Cell Impedance Research Articles

In-situ analysis of cathode and anode impedances to probe the performance degradation of lithium–sulfur batteries

Improving the performance of Li-metal batteries with sulfur cathodes while maintaining high sulfur loading (>4 mg cm−2) and low electrolyte-to-sulfur ratio (<10 mg μL−1) is a significant challenge. These conditions often result in poor battery cycle life and sudden capacity failure during charge-discharge processes. To address this challenge, we investigate Li–S batteries in a three-electrode (3E) configuration, utilizing optimized materials and geometries for the reference electrode. This configuration allows us to independently track the in-situ variations in cathode and anode resistances, along with the electrode potential difference, unveiling the root causes of abrupt battery failure during cycling. Our 3E approach uncovers previously unnoticed distortion phenomena in the electrochemical impedance spectra of both the cathode and anode, providing essential indicators of imminent micro-short circuiting. Supported by simulations using the finite element method and equivalent circuit analysis software, these insights clarify that the primary reason for battery failure is the growth of Li dendrites and their subsequent contact with the cathode, rather than a sudden increase in cell impedance. By substituting traditional Li foil with a specially designed Li anode less susceptible to Li dendrite growth, we achieve reduced anode resistance and improved battery capacity and cycle life stability. This research reveals the potential of the 3E configuration for precise in-situ impedance monitoring, which is valuable for devising strategies to enhance cycle stability.

Detection of a Cobalt-Containing Interphase at the Li6PS5Cl-NMC111 Interface by In Situ μXANES and EIS

Sulfide electrolyte all-solid-state lithium batteries (ASLBs) with uncoated Li-NixMnyCo1−x−yO2 (NMC) cathodes suffer from a large capacity loss during initial cycling and an increase in cell impedance. Decomposition reactions are known to occur at the Li6PS5Cl-NMC111 interface due to incompatibility between the two materials. If a stabilizing coating is applied to the NMC, it delivers full capacity during initial charge. However, the loss in capacity during discharge still occurs. The interface was studied by μXANES and through EIS analysis. A chemically-formed interphase was detected by μXANES, evident from reduction of Co at an uncoated NMC particle surface. This interphase was produced by decomposition at rest. To study the effect of the interphase on electrochemically active surface area, piecewise in situ EIS was performed and the data was modeled using a transmission line model (TLM). The charge transfer resistance RCT was used to estimate the volume specific active surface area (aact). The median value for aact was 296 cm−1, a factor of 7.5 lower than the theoretical value of 2216 cm−1. This provided evidence of a lower electrochemically active surface area in the ASLB.

Lithium-ion battery thermal safety evolution during high-temperature nonlinear aging

The thermal safety performance of lithium-ion batteries is significantly affected by high-temperature conditions. This work deeply investigates the evolution and degradation mechanism of thermal safety for lithium-ion batteries during the nonlinear aging process at high temperature. Through a comprehensive analysis from multiple perspectives, it has been revealed that lithium plating and R-H+ reduction are the primary factors contributing to the notable deterioration for battery safety performance during high-temperature aging. Side reactions, such as lithium plating and R-H+ reduction, occurring with aging, not only result in the thickening of the interfacial film but also generate a significant amount of reductive gas. Consequently, these results in a significant increase in cell impedance and a notable decrease in cell thermal stability. These findings offer valuable insights for the development and optimization of safer cell designs.

Insight into Health Deterioration Induced by Multi-Cycle External Short Circuits in Commercial 18650 lithium-Ion Battery

External short circuit (ESC) can cause rapid temperature rise and even failure of lithium-ion batteries (LIBs) in a short time. To better identify the ESC-related hazards and analyze the influencing mechanisms, we have carried out repeated ESC tests for the first time and investigated the compound effects of multiple factors on ESC. Three ESC conditions of different strengths were set according to different levels of factors such as state of charge (SOC), environmental temperature, ESC duration and multi-cycle short circuits. The results showed that the initial SOC determines the discharge intensity of ESC, while the environmental temperature determines the ESC duration the cell could withstand. Furthermore, the duration and cycles of ESC determine the cumulative damage to the cell and promote the cell capacity attenuation or even disconnection. And cell disconnection is induced by current interrupt device activation. The thermal behavior of cell has hysteresis characteristics, and the initial low temperature can effectively reduce the temperature rise of cell caused by ESC. The electrochemical impedance results show that the increase of cell impedance is related to the increase of charge transfer resistance and solid electrolyte interface resistance.

A Study of Vinylene Carbonate and Prop-1-ene-1,3 Sultone Electrolyte Additives Using Polycrystalline Li[Ni0.6Mn0.2Co0.2]O2 in Positive/Positive Symmetric Cells

Vinylene carbonate (VC) and prop-1-ene-1,3 sultone (PES) have been reported to have beneficial effects on both negative and positive electrodes of Li-ion cells. To simplify evaluation of these two additives with LiNi0.6Mn0.2Co0.2O2 (NMC622), positive/positive symmetric cells were used to exclude the influence from any negative electrode. The compatibility of electrolytes containing 2%VC or 2%PES with NMC622 in 1M LiPF6 EC:DEC (v/v 1:2) electrolyte was evaluated by multiple voltage hold periods at 55 °C to accelerate the capacity fade. EIS spectra showed 2%PES is superior for impedance control compared to 2%VC. dV/dQ vs V fitting results showed that active mass loss is worse at high voltage than low voltage regardless of electrolyte used. Cross-sectional SEM images showed more microcracking of NMC622 particles at high voltage than at low voltage, which was more severe at the end of testing than at the beginning of testing. Symmetric cell storage at 0 V (ca. 3.8 V vs Li/Li+) and 0 °C showed a significant increase in cell impedance for cells stored after the end of testing and a moderate impedance increase for cells stored at the beginning of testing suggesting the accumulation of deleterious reaction products in the cells during testing.

Long-Term Stable Lithium Metal Anode in Highly Concentrated Sulfolane-Based Electrolytes with Ultrafine Porous Polyimide Separator.

Highly concentrated solutions composed of lithium bis(fluorosulfonyl)imide (LiFSI) and sulfolane (SL) are promising liquid electrolytes for lithium metal batteries because of their high anodic stability, low flammability, and high compatibility with lithium metal anodes. However, it is still challenging to obtain the stable lithium metal anodes in the concentrated electrolytes due to their poor wettability to the conventional polyolefin separators. Here, we report that the highly concentrated 1:2.5 LiFSI/SL electrolyte coupled with a three-dimensionally ordered macroporous polyimide (3DOM PI) separator enables the stable lithium plating/stripping cycling with an average Coulombic efficiency of ca. 98% for over 400 cycles at 1.0 mA cm-2. The 3DOM PI separator shows good electrolyte wettability and large electrolyte uptake due to its high porosity and polar constituent of the imide structure, allowing superior cycling performance in the highly concentrated solution, compared with the polyolefin separators. Electrochemical and spectroscopic analyses reveal that the superior cycling stability in the concentrated electrolyte is attributed to the formation of highly stable and Li+ ion conductive solid electrolyte interphase (SEI) layer derived from FSI- anions, which reduces the side reactions of SL with lithium metal, prevents the growth of lithium dendrites, and suppresses the increase in cell impedance over long-term cycling. Our findings demonstrate that polar and porous separators could effectively improve the affinity to the concentrated electrolytes and allow the formation of the anion-derived SEI layer by increasing the salt concentration of the electrolytes, achieving the long-term stable lithium metal anode.

Enhanced Solid Electrolyte Interface (SEI) on Li Metal Surfaces through Catalytic Electrolyte Decomposition

Lithium metal batteries have seen a rise in attention as the next generation battery is sought. Li metal is considered the “Holy grail” anode material with a high theoretical capacity of 3860 mAh g-1.[1] However, Li metal is unstable during cycling leading to issues with dendrite formation leading to cell short or “dead” Li, and increase in cell impedance due to the excess formation of SEI on the “dead” Li..[2–4] The SEI plays an important role in altering the ion transport which alters the structure evolution of the Li metal anode during repeated cycling. While the understanding of the fundamental properties of the SEI has advanced continuously in recent years, effectively tuning the SEI components, especially the inorganic constituents, is still challenging. In this work, tungsten oxide, WO 3, is found to promote the formation of inorganic salts e.g., LiF/Li2CO3 in SEI layers and thus enhancing the SEI properties allowing for the smoother plating of Li metal. Additionally, WO3 is reduced to W during electrochemical process, the addition of W within the SEI increases the electronic conductivity, reducing the formation of “dead” Li which otherwise is completely wrapped by insulating SEI layers. The possibility of WO3 to catalyze electrolyte decomposition through favored reaction pathway to produce more robust SEI layers is discussed, providing new insights to control the SEI formation on Li metal. Xu, W., Wang, J., Ding, F., Chen, X., Nasybulin, E., Zhang, Y. and Zhang, J.G., 2014. Energy & Environmental Science, 7(2), pp.513-537.Pei, A.; Zheng, G.; Shi, F.; Li, Y.; Cui, Y. Nano Lett 2017, 17, 1132–1139.Wu, B.; Lochala, J.; Taverne, T.; Xiao, J. Nano Energy 2017, 40, 34–41.Aurbach, D.; Zinigrad, E.; Cohen, Y.; Teller, H. J. Solid State Ionics 2002, 148, 405–416.

Effect of Mechanical Stress Factors on Large Format Li-Ion Cell Thermal Runaway Characteristics

Expansion and contraction of Li-ion cell electrodes during cycling is known to exert mechanical stress and strain within cell components. During cell charge, Li-ion intercalation into the graphite anode structure increases anode swelling and thickness. Upon cell discharge, Li-ions de-intercalate from the carbon anode structure causing cell-level relaxation and a proportional reduction in cell swelling. Li-ion cell swelling has been shown to be a function of cell discharge rate, state-of-charge (SoC), and age. The impact of mechanical stress-strain swelling on Li-ion cells creates high contact pressure between electrode layers, degrades conductive carbon particle contact resistance, and irreversibly deforms the cell case structure. Mechanical stress loading also causes fatigue within internal cell components leading to an increase in cell impedance and capacity loss which may reduce cell cycle life performance. Li-ion battery (LIB) manufacturers have long recognized that the effects of cell-stack level stress fatigue on battery module architectures must be considered in LIB packaging designs requiring long cycle life performance. In LIB designs utilizing prismatic or elliptical-cylindrical cell geometries, cells are commonly packaged into modules by restraining the module stack by compression loading. In this manner, the adverse effects of cell stack deformation during charge-discharge cycling can be mitigated. In space applications, compression loading of cell modules also enables LIB’s to survive launch vehicle shock and vibration environments at high SoC’s. More importantly, mechanical abuse of LIB’s can result in an energetic catastrophic failure event. At high SoC’s, the hazards of a LIB safety incident may include venting, smoke, and fire indicative of catastrophic thermal runaway. Recently, the NASA has investigated the severity and consequences of catastrophic thermal runaway of large format Li-ion cell-based LIBs for human space flight applications. Thermal runaway safety testing of certain space-qualified Li-ion cell designs has shown that there is a correlation between applied cell compression forces and thermal runaway severity. This work focuses on the relationship between compression force and SoC on large format Li-ion cells qualified for human space flight applications. Empirical results from simulated internal short circuit trigger testing designed to induce catastrophic thermal runaway will be discussed.

Multifunctional Energy Storage Composites: Analysis and Optimization of Current Density Distribution

The introduction of electric vehicles (EVs) has posed many technical challenges to the automotive industry; in particular, the state-of-the-art lithium-ion (Li-ion) batteries add massive weight to vehicles both in the form of battery weight and supporting systems, immensely hindering vehicle performance and efficiency. A primary source of this challenge is that the battery packs serve only one purpose – energy storage, as current EV batteries do not carry loads or absorb collision impact energy. Recently introduced by the authors, a hybrid energy storage design, called the Multifunctional Energy Storage Composites (MESC), possesses the unique feature of simultaneously carrying tremendous structural loads and providing energy-storage capabilities (1-3). It has been demonstrated that the concept potentially leads to significant weight and volume savings in electric vehicle and system designs. In brief, the MESC involves a unique integration technique for embedding Li-ion battery materials in structural carbon-fiber-reinforced-polymers (CFRP). The crux of the multifunctional design is the incorporation of three-dimensionally interlocking “rivets” (1-3). The electrode layers, which make up the battery stack, contain a rectangular array of perforations (with diameter on the order of millimeters), instead of being solid, continuous, planar films. The perforations in each layer of the stack line up to fit the interlocking rivets, which three-dimensionally interlock the electrodes and rigidly anchor onto the stiff CFRP faceplates. The interlocking rivets exploit the intrinsic mechanical properties of the Li-ion battery electrodes, allowing them to carry mechanical loads. Despite the non-conventional cell construction, the MESC has demonstrated cycling and cycle-life performance tantamount to commercial pouch cells, with similar mechanical load carrying capabilities to automotive structural materials (2). Even though the rivets are indispensable in the MESC design and offer immense mechanical benefits, they present a slight electrochemical challenge. The presence of the perforations influences the current flow path, current density distribution, and depth of discharge (DoD) uniformity. Therefore, in this work, special attention is given to the numerical modelling and optimization of current density distribution in the electrodes, with the presence of a perforation array. Standard non-perforated graphite/NMC pouch cells are used to determine the DoD dependency of the intrinsic open-circuit voltage, electrochemical conductance, and resistance. The governing equations for charge conservation, polarization, and current density are solved numerically in COMSOL simulation (4-6). The perforations are included as boundary conditions on the electrode domains. The model is parameterized to capture various geometrically feasible perforation patterns, array densities, and diameters. Under different constant-current profiles, the non-uniformity of current density distribution and DoD is quantified. The increase in cell impedance, due to the local constriction and spreading of electrical current, is also evaluated. Complete functional relationships between perforation parameters (e.g. pattern, density, and hole diameter) and the electrochemical performance (current density distribution, DoD distribution, cell impedance, etc.) are established. Counter-intuitively, our results confirm minimal electrochemical impact in MESC cells with current path disruption sufficient for mechanical stabilization. Compared to unperforated cells of the same dimensions, less than a 10% increase in non-uniformity of current distribution and less than a 5% increase in cell impedance are observed. Beyond these indicative results, the model provides meaningful physical insights into the current flow path around the holes, as well as near the current collector tabs. Together, these allow the MSEC electrode design to be optimized in the electrochemical-mechanical tradeoff. Additionally, towards a broader context, it is believed this study will shed light upon electrode design and the optimization of conventional high-energy, high-power Li-ion batteries. To this end, a first step has already been taken to generalize the model and employ advanced optimization schemes e.g. genetic algorithms (GA). References P. Ladpli, R. Nardari and F.-K. Chang, Multifunctional Energy Storage Composites, WO patent application 2016127122, published 2016-08-11.P. Ladpli, R. Nardari, R. Rewari, H. Liu, M. Slater, K. Kepler, Y. Wang, F. Kopsaftopoulos and F.-K. Chang, Multifunctional Energy Storage Composites: Design, Fabrication, and Experimental Characterization, in ASME 2016 Energy Storage Forum, Charlotte, NC (2016).P. Ladpli, R. Nardari, Y. Wang, P. A. Hernandez-Gallegos, R. Rewari, H. T. Kuo, F. Kopsaftopoulos, K. D. Kepler, H. A. Lopez and F. Chang, Multifunctional Energy Storage Composites for SHM Distributed Sensor Networks, in International Workshop on Structural Health Monitoring 2015, Stanford, CA (2015).U. S. Kim, C. B. Shin and C.-S. Kim, Journal of Power Sources, 189, 841 (2009).K. H. Kwon, C. B. Shin, T. H. Kang and C.-S. Kim, Journal of Power Sources, 163, 151 (2006).J. Newman and W. Tiedemann, Journal of The Electrochemical Society, 140, 1961 (1993). Figure 1

Effects of Additives in Dual-Salt/Carbonate Electrolytes on Performances of Lithium Metal Batteries

The lithium (Li) metal battery (LMB) is a promising high-energy-density energy storage system. However, the uncontrollable dendrite formation and low Coulombic efficiency (CE) associated with the Li metal anode have largely hindered the practical application of LMBs [1]. During recent years, extensive efforts have been made to address the main problems of Li metal anode and to improve the electrochemical performance of the related LMBs, and plenty of promising results have been achieved. Among the reported approaches, the reformulation of electrolytes with different Li salts and additives has been regarded as an easy and cost-effective method. The conventional LiPF6/carbonate electrolytes have been found to be detrimental to Li metal anode especially at high charge current densities, which is mainly due to the serious corrosion of Li metal anode, the consumption of electrolyte, the continuous formation of solid electrolyte interphase, and the increase of cell impedance. The highly concentrated electrolytes of LiTFSI or LiFSI in ethers have been reported to be effective to suppress Li dendrites growth and to improve the Li CE [2]. However, the ether-based electrolytes normally are not electrochemically stable at the voltages around or above 4 V. In our previous works, we have systematically investigated the dual-salt electrolytes of LiTFSI-LiBOB in carbonate solvent mixture and their effects on the performances of Li||LiFePO4 [3], Li||LiNi0.80Co0.15Al0.05O2 [4] battery systems. It has been found that the dual-salt electrolytes are favorable to protect Li metal anode and to improve the rate capability of the LMBs. Especially, our recent work discovered that further significant improvements in fast charging capability and long-term cycling stability of the Li||LiNi0.4Mn0.4Co0.2O2 batteries can be achieved by adding an optimal additive amount of LiPF6 to the LiTFSI-LiBOB/carbonate electrolyte [5]. Nevertheless, this electrolyte still leads to relatively low Li CE, which is about 90.6%. In order to further improve the Li CE of the dual-salt/carbonate electrolyte, we systematically investigated different additives such as VC, FEC, VEC, etc. By using certain additives the Li CE in the electrolytes of LiTFSI-LiBOB dual-salt in carbonate solvents can be improved from 90.6% to 98.1%. Moreover, the electrochemical performances of the LMBs with a 4-V Li-ion cathode using the newly developed dual-salt electrolytes were studied, and enhanced battery performances have been obtained. The promising results will be reported at the meeting.

The Impact of Electrolyte Additives and Upper Cut-off Voltage on the Formation of a Rocksalt Surface Layer in LiNi0.8Mn0.1Co0.1O2 Electrodes

Many literature reports show that layered Li-Ni-Mn-Co oxides (NMC) have a surface reconstruction to a rocksalt (Fm3m) structure which is claimed to be responsible for the increase in cell impedance during high voltage cycling. It is important to determine if appropriate electrolyte additives can suppress the surface reconstructions of NMC materials. LiNi0.8Mn0.1Co0.1O2 (NMC811)/Graphite pouch cells with different electrolyte additives and different upper cutoff potentials were charge-discharge cycled and the electrodes were recovered for z-contrast scanning transmission electron microscope (STEM) studies. It was found that there was no significant surface layer growth for cells cycled between 2.8 and 4.1 V. For cells with an upper cutoff voltage of 4.3 V, the electrodes from cells with control electrolyte (no additives) showed the thickest surface layer. The electrolyte additives vinylene carbonate (VC) and prop-1-ene-1,3-sultone (PES) were found to suppress the growth of the surface layer. However, cells with PES showed a more rapid capacity fade than control cells or cells with 2% VC showing that, at least for NMC811/graphite cells with PES or VC additives, failure cannot only be solely ascribed to a growing rocksalt surface layer. Other processes, for example associated with electrolyte oxidation, are believed to be responsible for failure.

The Effect of Binders on the Performance and Degradation of the Lithium/Sulfur Battery Assembled in the Discharged State

Capacity fading on cycling of lithium/sulfur batteries may result from at least four processes: increase of SEI thickness resistance, loss of cathode capacity (precipitation of sulfur species outside the cathode), agglomeration and thickening of sulfur species and increase in cell impedance as a result of reduction of the electrolyte. A very important issue that has not been properly addressed up to now is the influence of the type and content of the cathode binder on the cell parameters and on the electrochemical performance of lithium/sulfur batteries. We present here a detailed analysis and discussion of the electrochemical behavior, during prolonged cycling, of Li2S-based cathodes containing five different binders. The binders under investigation are: poly(vinylidene fluoride) (PVDF-HFP), polyvinylpyrrolidone (PVP), mix of PVP with polyethylene imine (PEI), polyaniline (PANI) and lithium polyacrylate (LiPAA). Sulfur utilization in the cathode follows the order of LiPAA > PVP:PEI > PVP > PVDF-HFP > PANI. Depending on the type of binder, cells provide 500 to 1400 mAh/g (S), 94.6–98.0% faradaic efficiency and enable more than 500 reversible cycles.

The Impact of Electrolyte Additives and Cycling Voltage on the Formation of a Rocksalt Surface Layer in LiNi0.8Mn0.1Co0.1 Electrodes

High energy density lithium ion batteries that are cheaper, safer, and with longer lifetimes need to be developed in order to meet the increasing demand for applications such as electric vehicles. LiNi0.8Mn0.1Co0.1O2 (NMC811) can deliver a high capacity of ∼200 mAh/g with an average discharge potential of ∼3.8 V (vs. Li+/Li), making it a promising positive electrode material for high energy density lithium ion batteries. However, electrochemical tests of NMC811 from half cells and full cells show poor cycling performance when charged to potentials above 4.2 V. In our previous report1, it was shown the there are no significant structural changes in the bulk of the material during charge-discharge cycling. Instead, the parasitic reactions between the electrolyte and the highly reactive delithiated cathode surface at high potentials were suggested as the main reason for the failure of cells cycled above 4.2 V. Recently, Lin et al 2 showed that the surface of LiNi0.42Mn0.42Co0.16O2 went through a structural reconstruction from layered (Rm) to rocksalt (Fmm), which caused a significant increase in cell impedance under high voltage cycling conditions. Takamatsu et al 3 also reported that Co3+ at the surface of LiCoO2 was reduced to Co2+ after soaking in the electrolyte, however, the reduction of Co was suppressed with the presence of vinylene carbonate (VC) additives. This suggests that appropriate electrolyte additives might be able to suppress surface reconstructions of NMC materials. In this work, the impact of electrolyte additives and cell upper cut-off potential on the formation of a rocksalt surface layer in NMC811 cells was studied. NMC811/graphite pouch cells (220 mAh) were cycled for 100 cycles between 2.8 to 4.1 or 2.8 to 4.3 V. The rate used was C/5 for 5 cycles and followed by one C/20 cycle. The control electrolyte was 1M LiPF6in 3:7 v:v EC:EMC. The electrolyte additives studied in this work were 2% VC and “PES211”. PES211 is a blend of 2% prop-1-ene-1,3-sultone (PES) + 1% methylene methane disulfonate + 1% tris(trimethylsilyl) phosphate in control electrolyte. The cycled cells were discharged to 3.0 V and held for 24 h before disassembly in an argon-filled glovebox. The recovered positive electrodes were then washed with diethyl carbonate (DEC). Thick layers of carbon (~3 µm) and tungsten (~10 µm) were first deposited on the surface of the electrode to avoid beam damage during the FIB process. The samples were then analyzed with HAADF, EELS and NBD in aberration-corrected STEM mode. Figure 1A shows the HAADF-STEM images of the pristine NMC811 electrode near the surface before contacting any electrolyte. Every other column of the transition metal atoms, as indicated by the blue arrows, observed at the surface disappeared when moving into the bulk region, indicating a reconstructed rocksalt surface layer. This is likely due to the reaction between the electrode surface, that has high nickel content, with moisture in the air. The thickness of the rocksalt layer is ~2 nm. Figures 1B, 1C and 1D show the HAADF-STEM images of the electrodes after 100 cycles between 2.8 – 4.3 V with control, control plus 2% VC and control plus PES211 electrolyte in the cells respectively. The thickness of the surface layer on the control electrode was ~4 nm, while the thickness of surface layer in the electrodes in cells with 2% VC and PES 211 was about ~2 nm, almost the same as the pristine electrode. This suggests that both the VC and PES211 additives can suppress the formation of the rocksalt surface in NMC811 electrodes. Our previous report1showed that NMC811/graphite pouch cells with PES211 additives had worse cycling performance than the cells with only control electrolyte. Hence, at least for NMC811 cells, failure cannot only be ascribed to a growing rocksalt surface layer. Instead, other processes, for example associated with electrolyte oxidation, are believed to be responsible for failure. Figures 1E and 1F show the nano beam diffraction (NBD) from the surface and bulk of the control electrode shown in Figure 1B, respectively. The red dashed lines showed the remaining diffraction spots while the blue dashed lines showed the diffraction spots which disappeared when the beam moved from the bulk to the surface. This result confirms the that the surface was reconstructed. More results about EELS results will be discussed.

Cathode Dissolution as a Battery Failure Mechanism in Silver Vanadium Oxide (SVO) and Structurally-Stabilized SVO-Analogue Materials Using Phosphate (PO4 3-)

Battery failure manifested as an increase in cell impedance as a result of cathode dissolution is discussed. Analogous oxide and phosphate based materials exhibit similar electrochemical performance yet the phosphate based materials display increased structural stability and resistance to solvation formation in non-aqueous electrolytes. Structural modifications could optimize existing battery systems and address one of the main causes of increased cell impedance observed over time.

Effect of Rapid Charging or Discharging on Performance of High Energy Li- and Mn-Rich NMC Full Cells

It is commonly accepted that rapid charging shortens the cycle life of full cells much more than rapid discharging when equal magnitudes of cell current are applied. The reason for this behavior is the low operating voltage of the graphite anode once it is partially lithiated during charging. Polarization below the redox voltage of Li metal triggers Li plating on the anode surface; the Li deposits then react with the electrolyte irreversibly leading to capacity loss and impedance rise. The goal of this presentation is to show how high charge and discharge currents affect the cycle life of Li-ion full cells that contain a Li- and Mn-rich transition metal oxide (LMR-NMC) [1]. Asymmetric cycle life ageing using different charge and discharge currents is the key experimental procedure: all cells were subjected asymmetrically to a low charge and a high discharge current - or vice versa. The low current corresponds to approximately a C/14 rate for a symmetrically cycled cell. Three different current levels were chosen for the corresponding rapid charge or discharge, corresponding to a rate of C/3, 1C, and 3C. All tests were conducted in 2032-sized coin cells (1.6 cm2 area electrodes) at 30°C. The positive electrodes contained Li1.2Mn0.55Ni0.15Co0.1O2, and the negative electrodes contained graphite as the active component. Data on full cells containing Li4Ti5O12 (LTO)-based negative electrodes were also obtained during our study.Figures 1A and 1B show the asymmetric cycle-life performance of six different tests: rapid charging and rapid discharging, each with three different current levels during the high current half-cycles. After every eight rapid cycles, two low current cycles are implemented to track the mobile Li inventory of the cells. Significant differences between the three current levels are seen for the fast discharge tests (bottom), while relatively small differences are seen for the fast charge tests (top). Regardless of fast charging or discharging, the low current capacity decreases continuously, almost linearly, as all cells continue to age – indicating a continuous loss of mobile Li in the cells, which is believed to result from Li trapping in the negative electrode SEI. The decrease in discharge capacity with increasing current is related to the slow Li-intercalation kinetics and the hysteretic behavior intrinsic to the LMR-NMC. The abrupt drop in the high-current capacity that is encountered during fast discharging after ~30 cycles is due to a significant increase in cell impedance, and not due to excessive Li trapping, as the low current capacity is nearly identical for the two cases. In summary, the findings show that, contrary to common belief, the effect of rapid discharging on the cycle life can be more detrimental than that of rapid charging for some cycling conditions and cell chemistries, such as the ones described in this study. Reference: [1] Understanding Long-Term Cycling Performance of Li1.2Ni0.15Mn0.55Co0.1O2–Graphite Lithium-Ion Cells, Y. Li, M. Bettge, B. Polzin, Y. Zhu, M. Balasubramanian, D. P. Abraham, J. Electrochem. Soc. 160 (5) A3006-A3019 (2013) Figure 1

Characterizing Solid Electrolyte Interphase on Sn Anode in Lithium Ion Battery

Tin (Sn) nanoparticle electrodes have been prepared and battery cycling performance has been investigated with 1.2 M LiPF6 in ethylene carbonate (EC) / diethyl carbonate (DEC) electrolyte (1:1, w/w) with and without added vinylene carbonate (VC) or fluoroethylene carbonate (FEC). Incorporation of either VC or FEC improves the capacity retention of Sn nanoparticle electrodes although incorporation of VC also results in a significant increase in cell impedance. The best electrochemical performance was observed with electrolyte containing 10% of added FEC. In order to develop a better understanding of the role of the electrolyte in capacity retention and solid electrolyte interface (SEI) structure, ex-situ surface analysis has been performed on cycled electrodes with infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Hard XPS (HAXPES). The ex-situ analysis reveals a correlation between electrochemical performance, electrolyte composition, and SEI structure.

Synthesis of an Al2O3-coated polyimide nanofiber mat and its electrochemical characteristics as a separator for lithium ion batteries

Polyimide (PI) nanofibers with an average diameter of 300 nm, possessing superior electrolyte wettability and thermal stability, are synthesized by electrospinning a poly(amic acid) (PAA) solution followed by an imidization process. The large pore volume of the PI nanofiber mat can facilitate faster Li+-ion transport and greater rate capability, but it also cause an irreversible increase in cell impedance during long term cycling. To overcome these problems, thin Al2O3 over-layers are coated on both sides of a PI separator via a dip-coating process. The Al2O3-coated PI separators exhibit enhanced capacity, cyclability (95.53% retention after 200 cycles at 1 C), and rate capabilities (78.91% at 10 C) compared to the bare PI separator (68.65% at 10 C) and a commercial polypropylene separator (18.25% at 10 C) with a limited increase of cell impedance.

Investigation of solid electrolyte interfacial layer development during continuous cycling using ac impedance spectra and micro-structural analysis

The formation of passivating surface films on the electrodes of a lithium-ion polymer battery was investigated at various cycling state using ac impedance spectroscopy and scanning electron microscopy (SEM). A sealed commercial cell (Sony Co.) with a nominal capacity of 840 mAh was used for the experiment. An equivalent circuit used to model the impedance spectra show that, with continuous cycling there is a relatively large increase in the interfacial impedance and charge transfer resistances after a few hundred charge–discharge cycles. It was observed that the cell capacity decrease with increase cell impedance. SEM analysis on the electrodes shows that during continuous charge–discharge cycling, the deposition of sub-micro-size particles and dissolution of surface films on the graphite surface. This observation is consistent with increase in cell impedance as a function of charge/discharge cycling.

Label-Free and Real-Time Cell-Based Kinase Assay for Screening Selective and Potent Receptor Tyrosine Kinase Inhibitors Using Microelectronic Sensor Array

Kinases are the 2nd largest group of therapeutic targets in the human genome. In this article, a label-free and real-time cell-based receptor tyrosine kinase (RTK) assay that addresses limitation of existing kinase assays and can be used for high-throughput screening and lead optimization studies was validated and characterized. Using impedance, growth factor-induced morphological changes were quantitatively assessed in real time and used as a measure of RTK activity. COS7 cells treated with epidermal growth factor (EGF) and insulin results in a rapid increase in cell impedance. Assessment of these growth factor-induced morphological changes and levels of receptor autophosphorylation using fluorescent microscopy and enzyme-linked immunosorbent assay, respectively, demonstrates that these changes correlate with changes in impedance. This assay was used to screen, identify, and characterize a potent EGF receptor inhibitor from a compound library. This report describes an assay that is simple in that it does not require intensive optimization or special reagents such as peptides, antibodies, or probes. More important, because the assay is cell based, the studies are done in a physiologically relevant environment, allowing for concurrent assessment of a compound's solubility, stability, membrane permeability, cytotoxicity, and off-target interaction effects.

Effect of an Alkyl Dicarbonate on Li-Ion Cell Performance

The effect of an alkyl dicarbonate, diethyl 2,5-dioxahexane dicarboxylate (DEDOHC, ), which is formed by an ester exchange reaction in a mix solvent system of ethylene carbonate (EC) and diethyl carbonate (DEC), on the performance of lithium-ion cells was investigated. In the presence of 10 vol % DEDOHC in , discharge capacity decreased, in particular at a high charge/discharge rate. Capacity retention at a high charge/discharge rate also decreased during repeated charge and discharge cycles. These deteriorations in performance were attributed to an increase in cell impedance and deficiency of mobile lithium ion, which is caused by continuous consumption of lithium ion on graphite negative electrode. The results of half-cell tests revealed that the addition of DEDOHC causes serious degradation for the graphite half-cell. From ac impedance measurements it was found that surface film resistance on graphite electrode was fourfold higher in the solution containing DEDOHC, whereas charge-transfer resistance scarcely changed. Atomic force microscopy observation revealed the formation of dense and highly adhesive surface precipitate layer on graphite electrode in the presence of DEDOHC. Consequently, it was concluded that the increase in cell impedance, which leads to deterioration of charge and discharge performance, is mainly due to the increase in the resistance of the surface film formed from the reduction of DEDOHC on the graphite negative electrode.