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Selection, testing and validation of electrolyte candidates for Li-ion cells are discussed, based on a 10-minute target for extreme fast charge (XFC). A combination of modeling and laboratory measurements create a timely and synergistic approach to identifying candidate electrolyte formulations. Multi-solvent systems provide a balanced set of properties, wherein lower molecular-weight solvents offer reduced viscosity, increased species diffusivity, and mitigation of concentration polarization at high charge rates. Carefully selected formulations can exhibit peak conductivity and usable conductivity range of two to three times that of the baseline EC-EMC (3:7, wt.) + LiPF6. Candidates are also chosen based on stability and longevity within the cell environment. Lab testing coincides with property predictions from the Advanced Electrolyte Model (AEM) and a macro-scale cell model. Cell testing utilized coin and pouch cells having NMC532 or NMC811 cathodes with graphite electrodes. Results indicate combinations of low-molecular weight solvents are key for fast-charge electrolytes as they extend the useful conductivity range to both low and higher salt concentrations, and possess higher self-diffusivities compared to conventional solvents. This reduces impacts from concentration polarization. The choice of electrolyte also influences the tendency for lithium metal deposition at the anode, as showcased by experimental and modeling results herein.

Enabling extreme fast charging (XFC, ≤10–15 min charging) requires a comprehensive understanding of its implications. While lithium plating is a key bottleneck for the anode, the full extent of limitations for the cathode are not well-understood, particularly in extended-cycle settings with well-defined battery designs and conditions. This article presents cycle-life implications of XFC on cathodes at multiple length scales, combining electrochemical analyses, degradation modeling, and post-test characterizations. The comprehensive test matrix includes 41 well-defined gr/NMC pouch cells under varied fast-charge rates (1–9C) and state-of-charges cycled up to 1000 times. Cathode issues remain minimal in early cycling, but begin to accelerate in later life, when distinct cracking is found and identified as a fatigue mechanism. The bulk structure of cathodes remains intact, but distinct particle surface reconstruction is observed; however, this shows less pronounced effect on cathode aging than does cracking.

A barrier to the widespread adoption of electric vehicles is enabling fast charging lithium-ion batteries. At normal charging rates, lithium ions intercalate into the graphite electrode. At high charging rates, lithiation is inhomogeneous, and metallic lithium can plate on the graphite particles, reducing capacity and causing safety concerns. We have built a cell for conducting high-resolution in situ X-ray microtomography experiments to quantify three-dimensional lithiation inhomogeneity and lithium plating. Our studies reveal an unexpected correlation between these two phenomena. During fast charging, a layer of mossy lithium metal plates at the graphite electrode-separator interface. The transport bottlenecks resulting from this layer lead to underlithiated graphite particles well-removed from the separator, near the current collector. These underlithiated particles lie directly underneath the mossy lithium, suggesting that lithium plating inhibits further lithiation of the underlying electrode.

Crosstalk between the cathode and the anode in Li-ion batteries has a great impact on performance, safety and cycle lifetime. However, a systematic investigation of crosstalk behavior in silicon (Si)-based cells with various cathode materials has not been reported. We investigated the crosstalk behavior of a Si anode coupled with one of the following cathodes—LiCoO2 (LCO), LiNi0.5Mn0.3Co0.2 (NMC532), and LiFePO4 (LFP)—in a full cell. For each electrochemical couple, we compared electrolyte decomposition products, solid electrolyte interphase (SEI) chemistry, and degradation mechanisms during cycling. From a very early stage of cycling, each couple showed different crosstalk behavior; different electrolyte decomposition products and SEI chemistry on the Si anodes were seen. Specifically, the formation and growth mechanism of Si SEI differ depending on cathode materials. For the LFP system, the Si SEI rich in LiF and inorganic species, which is stable and robust. It forms at an early stage of cycle. As a result, mechanical failure of the Si electrode and Li loss are well suppressed in the LFP system, resulting in stable cycle life.

Realization of extreme fast charging (XFC, ≤15 minutes) of lithium-ion batteries is imperative for the widespread adoption of electric vehicles. However, dramatic capacity fading is associated with XFC, limiting its implementation. To quantitatively elucidate the effects of irreversible lithium plating and other degradation mechanisms on the cell capacity, it is important to understand the links between lithium plating and cell degradation at both the local and global (over the full cell) scales. Here, we study the nature of local lithium plating after hundreds of XFC cycles (charging C-rates ranging from 4C to 9C) in industrially-relevant pouch cells using spatially resolved X-ray diffraction. Our results reveal a spatial correlation at the mm scale between irreversible lithium plating on the anode, inactive lithiated graphite phases, and local state-of-charge of the cathode. In regions of plated lithium, additional lithium is locally and irreversibly trapped as lithiated graphite, contributing to the loss of lithium inventory (LLI) and to a local loss of active anode material. The total LLI in the cell from irreversibly plated lithium is linearly correlated to the capacity loss in the batteries after XFC cycling, with a non-zero offset originating from other parasitic side reactions. Finally, at the global (cell) scale, LLI drives the capacity fade, rather than electrode degradation. We anticipate that the understanding of lithium plating and other degradation mechanisms during XFC gained in this work will help lead to new approaches towards designing high-rate batteries in which irreversible lithium plating is minimized.

Summary Broad use of global or spatially averaging measurements over a cell to characterize highly localized Li plating phenomena in lithium-ion batteries during fast charging has created a disconnect between measurements and the underlying causes. Consequently, the field is missing a clear path to implementing fast charging as well as to expand into extreme fast charging (XFC). Aiming to bridge these gaps, we present a detailed look into local detection of Li plating and the consequent cycle life implications for electrodes and cells under XFC by utilizing electrochemistry and high-energy X-ray diffraction. Significant heterogeneity in Li plating during XFC results in accelerated and non-uniform cycle life losses, in contrast to the prevailing acceptance that C rate is correlated to Li plating for XFC. This behavior is triggered by local electrode heterogeneity, which has yet to be identified and is not apparent in volume-averaged quantifications. A better understanding of these multiscale local electrode heterogeneities is crucial for identifying pathways to enable XFC.

The potential for operating automotive battery packs at high upper cutoff voltages (UCV) has been explored using preliminary data on eight cathode materials. The pack level energy density, specific energy, and battery cost are calculated using the spreadsheet tool BatPaC. The tool used experimental data for some cathode materials such as the lithiated oxides of nickel manganese cobalt (NMC), nickel cobalt aluminum (NCA), layered lithium- and manganese-rich nickel manganese cobalt (LMRNMC). The half-cell data were obtained at UCVs between 4.2 and 4.7 V vs. Li/Li+. The experimental data showed LMRNMC with the highest lithiation capacity gain, increasing from 176 mAh·g−1 at 4.2 V to 260 mAh·g−1 at 4.7 V; this advantage is partly offset by its lower average voltage. Assuming optimized cell materials/design and an area-specific impedance of 12 Ω⋅cm2 for all the materials, the BatPaC results indicate that the specific energies or energy densities of the battery electric vehicle (BEV) and plug-in hybrid electric vehicle (PHEV) battery packs with the LMRNMC and NMC cathodes can exceed 180 (BEV) and 160 (PHEV) Wh·kg−1 at UCV>4.6 V vs. Li/Li+. The costs of these battery packs are lowest at UCV = 4.7 V (vs. Li/Li+); estimated at 135–145 and 210–220 $·kWh−1 for BEV and PHEV packs, respectively.

The capacity fading phenomenon of high energy lithium-ion batteries (LIBs) using a silicon monoxide (SiO) anode and a nickel-rich transition metal oxide cathode were investigated during life test. The capacity loss of this electrode couple was found to increase not only with cycles (cycle life), but also with rest time (calendar life). The capacity fading rate for this type of LIB using SiO as the anode was found to be time-dependent, rather than cycle-count-dependent. Further detailed investigation revealed that the capacity loss of this electrode couple during rest was caused by the parasitic reactions on the anode, which consumed the lithium ions and lead to less cyclable lithium in the battery system.

Heterogeneous battery performance is a critical issue for maximization of cell lifetime capacity and safety. Using high energy synchrotron X-ray diffraction, the influence of charge rate, voltage limit, uneven stack pressure, and gas generation on the lithium transport properties was quantified in single-layer graphite/LiNi0.5Mn0.3Co0.2O2 pouch cells. A freshly formatted cell tracked in operando during initial fast charge cycles indicated variable position-dependent performances, while lateral mapping showed a significant fast charge (6C) heterogeneity compared to slow charge (C/2). Pressure effects were non-dominant compared to charge rate. Maps of previously aged and rested cells indicate that lateral heterogeneity slowly equilibrates at rest, but regenerates upon further cycling at fast charge rate. Furthermore, an unformatted cell was mapped at charge and discharge during its first formation cycle to analyze the effect of byproduct gases on the heterogeneous lithium transport. Gas was observed as randomly interspersed “bubbles” which locally hindered lithium intercalation and caused significant heterogeneity. Electrode architectures and charging protocols that promote homogeneous intercalation are critical for predictable high-performance and long-life batteries.

Silicon has been extensively studied as an anode material in lithium-ion batteries due to its extremely high theoretical specific capacity of 3578 mAh/g (assuming Li15Si4 as intermediate product). However, silicon undergoes huge volume variations during repeated discharge and charge, leading to poor electrode mechanical integrity, continual electrolyte decomposition and fast capacity decay. Developing high strength binders in silicon anodes have been considered as an efficient pathway to alleviate various capacity decay pathways. Currently, lithiated poly acrylic acid (LiPAA) is mostly used as the binder for silicon electrodes due to its strong binding capability and (electro)chemical compatibility with Si and electrolyte. However, cracks at the electrode level can still be observed for the cycled electrodes, which suggests that a stronger binder is required to mitigate volume expansion of Si electrode and hold the particles together. Herein we study the polyimide (PI) materials as the binders for Si anode in an attempt to achieve stable cycling performance. With PI binder, the silicon electrode exhibits a higher tensile strength than that of conventional PAA binder. The strong adhesion of the PI binder suppresses the structural collapse of the Si negative electrode during lithiation/delithiation, enabling high capacity retention and stable cycle life. However, the PI crosslinking process requires high temperature and inert atmosphere, which is a challenge for its practical application. In this work, we explore various PI crosslinking conditions and their effects on electrochemical performance. This work offers us an alternative binder material with high tensile strength for the Si electrode. Acknowledgement We gratefully acknowledge the support from the U.S. Department of Energy's Vehicle Technologies Office. This work is conducted under the Cell Analysis, Modeling, and Prototyping (CAMP) Facility at Argonne National Laboratory. Argonne National Laboratory is operated for DOE office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357.

Energy density of full cells containing layered-oxide positive electrodes can be increased by raising the upper cutoff voltage above the present 4.2 V limit. In this article we examine aging behavior of cells, containing LiNi0.5Co0.2Mn0.3O2 (NCM523)-based positive and graphite-based negative electrodes, which underwent up to ~400 cycles in the 3–4.4 V range. Electrochemistry results from electrodes harvested from the cycled cells were obtained to identify causes of cell performance loss; these results were complemented with data from X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) measurements. Our experiments indicate that the full cell capacity fade increases linearly with cycle number and results from irreversible lithium loss in the negative electrode solid electrolyte interphase (SEI) layer. The accompanying electrode potential shift reduces utilization of active material in both electrodes and causes the positive electrode to cycle at higher states-of-charge. Full cell impedance rise on aging arises primarily at the positive electrode and results mainly from changes at the electrode-electrolyte interface; the small growth in negative electrode impedance reflects changes in the SEI layer. Our results indicate that cell performance loss could be mitigated by modifying the electrode-electrolyte interfaces through use of appropriate electrode coatings and/or electrolyte additives.

There is an ever-increasing demand for rechargeable batteries for electric vehicles, which primarily use lithium-ion batteries (LIB). One of the major proposed improvements in using LIBs for vehicles is in reducing the long recharging times (order of hours), to make it comparable to the 5-10 minute refueling times in gasoline-powered vehicles. Therefore, extreme fast charging (XFC) has been defined for LIBs, with a target to bring the charging time down to 15 minutes or less [1]. However, XFC of LIB is associated with several problems, the most prominent of which are a large loss of battery capacity over cycling and safety issues [2]. Therefore, an understanding of the how local, irreversibly plated lithium affects the local SOC of the cathode and anode, as well as tying the contributions from individual loss mechanisms quantitatively over the cell to the overall capacity fade of the cell during fast charging is necessary to design the battery for better safety and consistent performance. Towards addressing this issue, high energy X-ray diffraction (XRD) is employed, which helps build on the existing understanding of lithium plating in two ways. Firstly, XRD provides a way to quantify the amount of Li plating, as well as other loss mechanisms, and tie them to the global cell performance after XFC cycling. Second, XRD is an in-situ technique, allowing to characterize the entire battery in the fully assembled condition. Thus, local heterogeneities in the cathode and anode can be studied and correlated to heterogeneities in Li plating. In this work, sub-mm-scale XRD is used to quantify Li plating across different single layer pouch cells (3 mAh/cm2 specic capacity, with graphite anode and NMC cathode), where the charging rate and protocol are systematically varied (4C to 9C). The cells are cycled through hundreds of XFC cycles and studied in the discharged state. At the local level, the characteristics of plated lithium crystallites such as the preferred crystallographic orientations and size of plated Li on graphite are studied. Additionally, the regions with local lithium plating are correlated with the local SOC (lithium occupancy) and loss of active surface area in the cathode and anode, in the discharged state. Finally, the capacity fade of the cycled cells is correlated to the amount of dead Li, with separated contributions from irreversibly plated Li, Li trapped in graphite as LixC (which cannot be reversibly extracted from the anode) and reaction of plated Li with the electrolyte. Based on this knowledge of the properties of lithium plating and the conditions that favor it, as well as it's effect on overall battery performance, new approaches towards designing batteries can be realized, such that irreversible Li plating is minimized. This step will in turn help to guide the rational design of the next generation of XFC capable LIBs with a consistent and safe performance. References [1] T. R. Tanim, E. J. Dufek, M. Evans, C. Dickerson, A. N. Jansen, B. J. Polzin, A. R. Dunlop, S. E. Trask, R. Jackman, I. Bloom, et al. Extreme fast charge challenges for lithium-ion battery: Variability and positive electrode issues. Journal of The Electrochemical Society, 166(10):A1926{A1938, 2019. [2] A. Tomaszewska, Z. Chu, X. Feng, S. O'Kane, X. Liu, J. Chen, C. Ji, E. Endler, R. Li, L. Liu, et al. Lithium-ion battery fast charging: A review. eTransportation, 1:100011, 2019.

Silicon monoxide (SiO) is a very promising anode material for the next generation high energy lithium ion batteries due to its high theoretical specific capacity of 1710 mAh/g and volumetric capacity of 1547 Ah/L.[1] Compared to commercially used graphite, SiO can offer 18% cell stack level improvement in volumetric energy density and 11% in gravimetric energy density (calculated based on the same cell stack model), enabling utilization of smaller and lighter batteries. It also offers more stable cycle performance compared to Si due to less volume change (134% initial volume expansion and 117% reversible volume expansion), making it a more practical choice for lithium ion batteries in the near future. SiO is composed of Si nanodomains in a SiO2 matrix. An interphase region of transitional stoichiometry (SiOx, 0 < x < 2) is also present between Si and SiO2 and takes up 20-25 at.% of the entire composition.[2] During the lithiation and delithiation, the Si nanodomains react reversibly with lithium, similar to amorphous silicon, to give the reversible capacity. The SiO2 matrix reacts irreversibly with lithium to form lithium silicates and lithium oxide. Therefore, the Si domain size and interphase suboxide between Si and SiO2 have effect on the electrochemical performance of SiO. In this study, we gradually changed the microstructure of SiO via annealing treatment and thoroughly studied the change of the SiO microstructure and its impact on the electrochemical performance of the SiO. Together with electrode formulation optimization, up to 99% capacity retention during 50 cycles was obtained in full cells when using pure SiO as anode and LiNi0.5Mn0.3Co0.2O2 as cathode. Acknowledgement We gratefully acknowledge the support from Peter Faguy at the U.S. Department of Energy’s (DOE) office of Energy Efficiency & Renewable Energy (EERE) - Vehicle Technologies Office. This work is conducted under the Cell Analysis, Modeling, and Prototyping (CAMP) Facility at Argonne National Laboratory. Argonne National Laboratory is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357. Reference: Obrovac, M. N.; Chevrier, V. L., Alloy Negative Electrodes for Li-Ion Batteries. Chemical Reviews 2014, 114 (23), 11444-11502. Hirata, A.; Kohara, S.; Asada, T.; Arao, M.; Yogi, C.; Imai, H.; Tan, Y.; Fujita, T.; Chen, M., Atomic-scale disproportionation in amorphous silicon monoxide. Nature Communications 2016, 7, 11591.

The stability of silicon-containing anodes can, in principle, be extended by constraining these electrodes to limited states of lithiation. Partial utilization of the anode lessens the volume changes experienced by silicon particles during cycling, which can mitigate mechanisms of performance degradation. In full-cells, anode utilization can be modulated by adjusting the relative capacities of the negative and positive electrodes – the N/P ratio. Here, we examine how the N/P ratio affects the long-term stability of Si-based full-cells, and investigate how this parameter would impact the cost and energy of realistic Li-ion cells. We show that, for some configurations, cell failure due to rapid anode degradation can only by avoided at higher N/P ratios. The price of this enhanced stability is accelerated impedance rise at the cathode. Surprisingly, when electrode expansion is taken into account, increasing N/P ratio can actually increase the specific energy and energy density of silicon-rich Li-ion cells.

A combination of cell testing and electrochemical-thermal modeling is used to investigate extreme fast charging (XFC) performance for cells with a low loading of 1.5 mAh.cm−2 and moderate loading of 2.5 mAh.cm−2. Cells with a low loading of 1.5 mAh.cm−2 withstand XFC performance remarkably well even up to 9C constant current (CC) charging with high charge capacity, high coulombic efficiency and very little apparent lithium plating. For a moderate loading of 2.5 mAh.cm−2, the 6C CC charge capacity is poor with significant amounts of visually observed lithium plating. Simulated electrolyte transport properties are revealed to be insufficient and majorly set limitations for XFC performance in case of the moderate and the only simulated higher loadings (>2.5 mAh.cm-2). Charging at elevated temperature is shown to be an effective strategy for moderate loading cells enabling good 10-min charge capacity, high coulombic efficiency, and mitigating lithium plating. Lastly, an electrochemical model is used to investigate strategies for enabling 4–6C CC charging for cells incorporating loading beyond 3 mAh.cm−2. As a result, the combination of an increased cell temperature, reduced electrode tortuosity, and enhanced ion-transport in the electrolyte are most likely required to facilitate XFC for state of the art and future high energy lithium-ion batteries.

Lithium-alloying materials are of great interest to improve the gravimetric and volumetric energy density of lithium-ion batteries, though their associated volume fluctuation with cycling often leads to poor cycling performance. Active–inactive alloys and blending alloys with carbon materials are common strategies to accommodate volume fluctuation. Herein we set out to optimize graphite–SiO blend electrode formulations to eliminate rapid capacity fade. Electrodes with highly stable cycling were prepared by simple planetary mixing procedures, enabled by the use of just a fraction of a weight percent of commercial SWCNTs as the only conductive additive, and by the appropriate choice of binder/stabilizing agent. In fact, the use of SWCNTs allowed for graphite-free SiO electrodes with approximately 74% higher volumetric energy density relative to traditional graphite electrodes, and superior capacity retention in coin-type full-cell testing versus NMC532 cathodes.

Increasing the areal capacity or electrode thickness in lithium ion batteries is one possible means to increase pack level energy density while simultaneously lowering cost. The physics that limit use of high areal capacity as a function of battery power to energy ratio are poorly understood and thus most currently produced automotive lithium ion cells utilize modest loadings to ensure long life over the vehicle battery operation. Here we show electrolyte transport limits the utilization of the positive electrode at critical C-rates during discharge; whereas, a combination of electrolyte transport and polarization lead to lithium plating in the graphite electrode during charge. Experimental measurements are compared with theoretical predictions based on concentrated solution and porous electrode theories. An analytical expression is derived to provide design criteria for long lived operation based on the physical properties of the electrode and electrolyte. Finally, a guideline is proposed that graphite cells should avoid charge current densities near or above 4 mA/cm2 unless additional precautions have been made to avoid deleterious side reaction.

The impedance of a lithium- and manganese-rich layered transition-metal oxide (LMR-NMC) positive electrode, specifically Li1.2Ni0.15Mn0.55Co0.1O2, is compared to two other transition-metal layered oxide materials, specifically LiNi0.8Co0.15Al0.05O2 (NCA) and Li1.05(Ni1/3Co1/3Mn1/3)0.95O2 (NMC). A more detailed electrochemical impedance spectroscopy (EIS) study is conducted on the LMR-NMC electrode, which includes a range of states-of-charge (SOCs) for both current directions (i.e. charge and discharge) and two relaxation times (i.e. hours and one hundred hours) before the EIS sweep. The LMR-NMC electrode EIS studies are supported by half-cell constant current and galvanostatic intermittent titration technique (GITT) studies. Two types of electrochemical models are utilized to examine the results. The first type is a lithium ion cell electrochemical model for intercalation active material electrodes that includes a complex active material/electrolyte interfacial structure. The other is a lithium ion half-cell electrochemical model that focuses on the unique composite structure of the bulk LMR-NMC materials. The LMR-NMC electrode impedance is much higher than the impedance of the NCA and NMC electrodes. The higher impedance of the LMR-NMC electrode suggests that it would be better utilized in a high energy battery rather than a high power battery. The intercalation active material electrochemical model and supporting studies indicate the source of the higher impedance can be associated with a much higher electronic contact resistance between the oxide active material and the conducting carbon additive, as well as the material’s interfacial and bulk transport and kinetic characteristics being significantly worse than the other oxides, as typified by the active material lithium diffusion coefficients for the surface layer and bulk material (DSi and DSb) and the kinetic exchange current density (io). As typical of layered oxide active materials the LMR-NMC electrode impedance increases at low and high SOCs, corresponding to DSi, DSb, and io all decreasing. Further, the rate of decrease in the parameters with SOC (i.e increase in impedance) is higher than observed for the other layered oxides. When the LMR-NMC electrode is allowed to relax for a very long time (i.e. approximately 100 hours) the impedance is observed to be significantly higher. Also, the resulting DSi, DSb, and io parameters determined by the intercalation electrochemical model from the EIS studies during charge and discharge where found to correlate with electrode voltage. This unique behavior for the LMR-NMC electrode is explained by the LMR-NMC bulk reaction and transport electrochemical model, which accounts for the individual domains in the active material. Specifically, the Li2MnO3 domains when allowed to relax are either nearly full or empty depending on the SOC. Because of the large characteristic time constant for the Li2MnO3 domains’ transition, when the LMR-NMC material is used in transportation applications the Li2MnO3 domains typically never relax and the electrode exhibits a lower impedance. The LMR-NMC bulk reaction and transport electrochemical model describes much of the observed behavior that cannot be accounted for using a standard intercalation active material electrochemical model. This includes the voltage hysteresis as shown in Figure 1, as well as the slow relaxation phenomena. However, the complexity of this model makes determining the parameters for the individual domains quite a challenge. ACKNOWLEDGMENT Support from the Vehicle Technologies Program, Hybrid and Electric Systems, David Howell (Team Lead) and Peter Faguy, at the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, is gratefully acknowledged. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Figure 1. Electrochemical model simulation of LMR-NMC standard electrode half-cell charge and discharge curves at a C/300 rate. Figure 1

Post-test analyses of cells consisting of a layered-layered cathode, a graphite anode and an organic carbonate-based electrolyte were performed. Many changes in the cell components were visible. Gas analysis revealed possible evidence of oxygen evolution from cathode and electrolyte degradation, including likely reaction products between electrolyte solvents and salt or binder. The anode surface was highly inhomogeneous and was comprised of a surface film of varying thickness, white precipitates, and dendritic growth of metallic Li and/or deliquescent salts. Manganese was incorporated into the anode and P, C, and O were preferentially incorporated into some areas of the anode surface. Analysis of the composition of the SEI layer shows that, unsurprisingly, it was complex.

In this article we describe efforts to improve performance and cycle life of cells containing Li1.2Ni0.15Mn0.55Co0.1O2-based positive and graphite-based negative electrodes. Initial work to identify high-performing materials, compositions, fabrication variables, and cycling conditions is conducted in coin cells. The resulting information is then used for the preparation of double-sided electrodes, assembly of pouch cells, and electrochemical testing. We report the cycling performance of cells with electrodes prepared under various conditions. Our data indicate that cells with positive electrodes containing 92 wt.% Li1.2Ni0.15Mn0.55Co0.1O2, 4 wt.% carbons (no graphite), and 4 wt.% PVdF (92–4–4) show ∼20% capacity fade after 1000 cycles in the 2.5–4.4 V range, significantly better than our baseline cells that show the same fade after only 450 cycles. Our analyses indicate that the major contributors to cell energy fade are capacity loss and impedance rise. Therefore incorporating approaches that minimize capacity fade and impedance rise, such as electrode coatings and electrolyte additives, can significantly enhance calendar and cycle life of this promising cell chemistry.

Athree-dimensionalelectrochemicalmodelwasdevelopedtoexaminetheperformance(i.e.impedancedeterminedbyapulsechangeinpotential)ofalithium-ionintercalationpositiveelectrodesingleparticle.Acomprehensiveparameterstudywasconducted,basedona layered nickel oxide parameter set previously developed. The dominance of the interfacial impedance on the particle performanceaccentuates the impact of the secondary particle porosity and electrolyte wetting of the porosity. Electronic distribution effects,includingconductivityandparticlecontacts,alsobecomesignificantwhenthesecondaryparticle’seffectiveconductivityislessthanabout 10

Lithium metal plating on graphite anodes of Li-ion batteries is one of the causes of capacity fading and failure. An overcharge experiment on the graphite anode was conducted and the morphology and structure of graphite were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the failure mechanism of graphite during overcharge is similar to that of the lithium metal electrode. The deposited lithium will react with the electrolyte to form a new solid electrolyte interface (SEI) layer, which will prevent further accessibility during the following cycles. According to SEM and TEM images, this SEI layer shares the same morphologies of the lithium metal electrode during cycling.

Although a variety of cathode and electrolyte materials have been studied and commercialized over the past two decades, nearly all commercial cells have used a graphitic carbon anode. Several reasons exist for this choice—including cost, low insertion voltage, and ease of use in the cell manufacturing process. However as uses for lithium-ion batteries expand, alternative anodes that may offer better energy and power capability are being explored. For transportation-oriented purposes, anodes based on simple lithiated Zintl compounds, e.g. Li17Sn4, or intermetallic insertion anodes offer significant advantages in capacity (volumetric and gravimetric) and stability in the cell environment that make them attractive candidates for future cell chemistries. Within this context, little however is known about how these alternative anode materials perform as a function of temperature, which is important for applications where operation at temperatures as low as −30 °C can be expected. In this study we evaluated a series of intermetallic insertion anodes that operate by a simple metal displacement mechanism. We have found that for Cu6Sn5, Ag/Cu6Sn5, and Cu2Sb, the drop-off in performance with temperature is in line with that observed for a commercial graphite-based anode and indicates that additional variables such as cation diffusion through the electrode passivation film or the electrochemical double layer may be playing an important role that is independent of the underlying anode material. We additionally characterized the NiAs-type mineral Sorosite (CuSn0.9Sb0.1), as various literature reports had indicated that substitution of antimony for tin eliminated the need for interstitial copper, however powder X-ray diffraction studies of samples made by annealing or high energy ball milling indicated mixed phase samples.

High energy and power density lithium iron phosphate was studied for hybrid electric vehicle applications. This work addresses the effects of porosity in a composite electrode using a four-point probe resistivity analyzer, galvanostatic cycling, and electrochemical impedance spectroscopy (EIS). The four-point probe result indicates that the porosity of composite electrode affects the electronic conductivity significantly. This effect is also observed from the cell's pulse current discharge performance. Compared to the direct current (dc) methods used, the EIS data are more sensitive to electrode porosity, especially for electrodes with low porosity values.

Titanium oxides are an important class of lithium–ion battery electrodes owing to their good capacity and stability within the cell environment. Although most Ti(IV) oxides are poor electronic conductors, new methods developed to synthesize nanometer scale primary particles have achieved the higher rate capability needed for modern commercial applications. In this report, the anionic water stable titanium oxalate anion [TiO(C 2 O 4 ) 2 ] 2− was isolated in high yield as the insoluble DABCO (1,4-diazabicyclo[2.2.2]octane) salt. Powder X-ray diffraction studies show that the titanium dioxide material isolated after annealing in air is initially amorphous, converts to N-doped anatase above 400 °C, then to rutile above 600 °C. Electrochemical studies indicate that the amorphous titanium dioxide phase within a carbon matrix has a stable cycling capacity of ∼350 mAh g −1 . On crystallizing at 400 °C to a carbon-coated anatase the capacity drops to 210 mAh g −1 , and finally upon carbon burn-off to 50 mAh g −1 . Mixtures of the amorphous titanium dioxide and Li 4 Ti 5 O 12 showed a similar electrochemical profile and capacity to Li 4 Ti 5 O 12 but with the addition of a sloping region to the end of the discharge curve that could be advantageous for determining state-of-charge in systems using Li 4 Ti 5 O 12 .

Lithium-ion cells being designed for transportation applications must sustain high current pulses under rapid discharge and rapid charge conditions without significant degradation of cell performance. In this article we examine the pulse discharge and charge performance of cells, containing a LiNi 0.8 Co 0.15 Al 0.05 O 2 -based cathode, a graphite-based anode, and a LiPF 6 -bearing EC:EMC electrolyte, at current rates ranging from 3 to 25 C. Impedance data indicate that 18650-type cells containing this chemistry can withstand 18s discharge pulses at rates up to 17 C in the 3.7-4.0V voltage window. Data from cells containing a LiSn micro-reference electrode show that the positive electrode impedance increases, whereas the negative electrode impedance decreases, with increasing magnitude of the discharge current pulse. The discharge pulse-current that can be sustained by the cell is limited by lithium diffusion into oxide particles of the positive electrode.

Hybrid-electric vehicles require lithium-battery electrolytes that form stable, low impedance passivation layers to protect the electrodes, while allowing rapid lithium-ion transport under high current charge/discharge pulses. In this article, we describe data acquired on cells containing LiNi{sub 0.8}Co{sub 0.15}Al{sub 0.05}O{sub 2}-based positive electrodes, graphite-based negative electrodes, and electrolytes with lithium hexafluorophosphate (LiPF{sub 6}), lithium tetrafluoroborate (LiBF{sub 4}), lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato) borate (LiF{sub 2}OB) salts. The impedance data were collected in cells containing a Li-Sn reference electrode to determine effect of electrolyte composition and testing temperature on individual electrode impedance. The full cell impedance data showed the following trend: LiBOB > LiBF{sub 4} > LiF{sub 2}OB > LiPF{sub 6}. The negative electrode impedance showed a trend similar to that of the full cell; this electrode was the main contributor to impedance in the LiBOB and LiBF{sub 4} cells. The positive electrode impedance values for the LiBF{sub 4}, LiF{sub 2}OB, and LiPF{sub 6} cells were comparable; the values were somewhat higher for the LiBOB cell. Cycling and impedance data were also obtained for cells containing additions of LiBF{sub 4}, LiBOB, LiF{sub 2}OB, and vinylene carbonate (VC) to the EC:EMC (3:7 by wt.) + 1.2 M LiPF{sub 6} electrolyte. Ourmore » data indicate that the composition and morphology of the graphite SEI formed during the first lithiation cycle is an important determinant of the negative electrode impedance, and hence full cell impedance.« less

An electrochemical model was developed to compare the use of internal and external reference electrodes for the study of lithium-ion cells. The impact of reference electrode placement on electrode measurements was examined using idealized reference electrodes in cells undergoing short-duration pulse currents. The effectiveness of internal reference electrodes with finite size and impedance was also examined.

Lithium-ion batteries are considered to be the next battery system for hybrid electric vehicles (HEVs) due to their high power density. However, their power is severely limited at −30 °C and the concern exists that lithium metal could plate on the negative electrode during regen (charge) pulses. The goal of this work is to determine the reason for this poor low-temperature performance using an in situ Li y Sn micro reference electrode (RE) over a wide temperature range of 30 °C to −30 °C. A variety of negative and positive electrode materials with unique morphologies was used in this work to help elucidate the dominant low-temperature mechanism. In this work, it was observed that the potential of graphite negative electrodes does dip below lithium potentials not only during charge pulses, but also under normal charging if the cell cutoff voltage is not reduced from its room-temperature setting of 4.1 V, whereas hard carbon electrodes do not because they operate further from lithium potential. The most surprising finding from this work was that a second impedance mechanism dominates below 0 °C that affects the positive and negative electrodes almost equally. This suggests that the responsible phenomenon is independent of the active material and is most likely a pure electrolyte-interface effect.

The C /25 discharge data from 18650-size cells containing LiNi 0.8 Co 0.1 Al 0.1 O 2 cathode and graphite anode laminates were analyzed through the use of the differential voltage, d V /d Q , curves. Using half-cell data, the peaks in the d V /d Q curve of the full cell data were assigned. Analysis of the relative peak shifts allowed for the determination of the source of capacity fade. For cells formed and aged at 45 °C for 40 weeks (capacity fade = 7.5%), the analysis indicated negligible loss of accessible material at the anode and at the cathode. Capacity loss of the cell could be accounted for, largely, by side reactions at the anode. This type of analysis can be used when the introduction of a reference electrode is difficult or impractical.

High-power, 18,650 lithium-ion cells have been designed and fabricated in order to understand the factors limiting the calendar life of the lithium-ion system. Each cell consisted of a LiNi0.8Co0.2O2 positive electrode, a blend of MCMB-6 and SFG-6 carbon negative electrode, and a LiPF6 in EC:DEC (1:1) electrolyte. These cells, which initially meet the power requirement set by the partnership for a new generation of vehicles (PNGV), were subjected to accelerated calendar life and cycle life testing. After testing at elevated temperatures, the cells experienced a significant impedance rise and loss of power. The fade rate of power in these cells was dependent of the state of charge and the temperature of testing. Micro-reference electrode and ac-impedance studies on symmetrical cells have confirmed that the interfacial resistance at the positive electrode was the main reason behind the impedance rise in the high power cell.

Dissolution and migration of manganese from cathode lead to severe capacity fading of lithium manganate-carbon cells. Overcoming this major problem requires a better understanding of the mechanisms of manganese dissolution, migration and deposition. Here we apply a variety of advanced analytical methods to study lithium manganate cathodes that are cycled with different anodes. We show that the oxidation state of manganese deposited on the anodes is +2, which differs from the results reported earlier. Our results also indicate that a metathesis reaction between Mn(II) and some species on the solid-electrolyte interphase takes place during the deposition of Mn(II) on the anodes, rather than a reduction reaction that leads to the formation of metallic Mn, as speculated in earlier studies. The concentration of Mn deposited on the anode gradually increases with cycles; this trend is well correlated with the anodes rising impedance and capacity fading of the cell.

Long-life and safe lithium-ion batteries have been long pursued to enable electrification of the transportation system and for grid applications. However, the poor safety characteristics of lithium-ion batteries have been the major bottleneck for the widespread deployment of this promising technology. Here, we report a novel nonaqueous Li(2)B(12)F(12-x)H(x) electrolyte, using lithium difluoro(oxalato)borate as an electrolyte additive, that has superior performance to the conventional LiPF(6)-based electrolyte with regard to cycle life and safety, including tolerance to both overcharge and thermal abuse. Cells tested with the Li(2)B(12)F(9)H(3)-based electrolyte maintained about 70% initial capacity when cycled at 55 °C for 1,200 cycles, and the intrinsic overcharge protection mechanism was active up to 450 overcharge abuse cycles. Results from in situ high-energy X-ray diffraction showed that the thermal decomposition of the delithiated Li(1-x)[Ni(1/3)Mn(1/3)Co(1/3)](0.9)O(2) cathode was delayed by about 20 °C when using the Li(2)B(12)F(12)-based electrolyte.

Compound refractive lenses (CRLs) are arrays of concave lenses whose simple design and ease in implementation and alignment make them an attractive optic to focus x‐rays. Factors considered in designing CRLs include lens material, fabrication, and assembly. Lithium is a desirable material because it provides the largest index of refraction decrement per unit absorption length of any solid elements. Lithium is a difficult material to handle and fabricate because it is rather malleable and more importantly, it reacts with moisture, and to a lesser extent, with oxygen and nitrogen in air. It also tends to adhere to molds and dies.We report on the fabrication and performance of a parabolic lithium lens consisting of 32 lenslets. Lenslets are fabricated in a precision press using an indenter with a parabolic profile and a 100 μm tip radius. The indenter is made of stainless steel and is figured using a computer numerically controlled (CNC) machine. The lens is designed to have a 1.7 m focal length at 10 keV en...

Lithium- and manganese-rich layered transition-metal oxide (LMR-NMC) intercalation electrodes are projected to enable batteries with high energy density and low costs for energy. However, implementation of LMR-NMC materials are challenged by life limiting mechanisms as well as less than desired rate performance. Here-in, we use electrochemical characterization of LMR-NMC electrodes to examine the large magnitude of impedance and the asymmetric polarization between charge and discharge at low states of charge (SOC). The lithium metal oxide cathode materials examined were LMR-NMC and standard NMC both from Toda Kogyo (Japan). The LMR-NMC material, referred to as HE5050, had the composition 0.5Li2MnO3•0.5LiMn0.375Ni0.375Co0.25O2 (or in layered notation Li1.2Ni0.15Mn0.55Co0.10O2). The NMC material had composition of Li1+x(Ni1/3Mn1/3Co1/3)1-xO2 and can be considered to be the commercial standard stoichiometry for Li-ion transportation cells at the time of this publication. Full cells used A12 graphite (Conoco Phillips) and half cells used lithium metal (FMC Lithium) both with standard LiPF6based carbonate electrolyte. A three electrode pouch cell was utilized for the asymmetry polarization experiments in the study, with a composite LMR-NMC electrode as the working electrode, lithium metal was used as the reference and counter electrodes The area-specific impedance (ASI) of LMR-NMC in Fig. 1 displays a similar dependency as standard layered lithium metal oxides when compared as a function of voltage rather than SOC. The increasing and asymmetric behavior of the polarization in LMR-NMC measured at low SOC is hypothesized to be the result of the differing lithium contents and lithium mobilities in the heterogeneous, nano-composite metal oxide material. Transport of lithium within LMR-NMC is suggested to be governed by the relatively facile nickel- and cobalt-rich domains even as this domain appears to reach full lithiation. As the majority of capacity at low SOC in LMR-NMC is associated with the lithium- and manganese-rich domains, the observed high impedance presents a significant hurdle to utilizing this capacity in moderate power-to-energy ratio applications. Much work is needed to clarify the transport and structural mechanism in LMR-NMC. Other studies have shown the structural transformation known as voltage fade is less severe with lower lithium content. However, lowering the lithium stoichiometry also tends to lower the specific energy of the material at beginning of life. Optimizing the stoichiometry of the lithium and manganese to achieve a maximum specific energy over life at relevant discharge rates is a potentially viable path forward. Figure 1

Compound refractive lenses (CRLs) are arrays of concave lenslets used to focus X-rays. For a given incident X-ray beam energy, the focal length of a CRL depends on the material and shape of the individual lenslets, and in particular is inversely related to the number of lenslets in the array. The throughput of a lens array is heavily affected by absorption of the X-rays in the lens. For this reason, it is necessary to employ low-atomic-number materials and fabricate the lenses as thin as possible, especially for low to moderate X-ray energy range (~ 5 - 20 keV) photons. Lithium and beryllium are two of the best candidate materials for X-ray lenses due to their relatively high (real decrement) index of refraction and low X-ray absorption. Lithium is very malleable, however, and reacts strongly with moisture in the air, requiring a special fabrication environment and housing. Beryllium, on the other hand, is a solid metal and is easy to machine and handle. This paper summarizes the recent work at the Advanced Photon Source (APS) on parabolic lithium and cylindrical beryllium lenses. These lenses have been tested on APS X-ray beamlines. Their performance in terms of the focal size and gain is described and further improvements including tighter manufacturing tolerances and thinner lens walls are discussed.

Magnesium-substituted Li{sub 4-x}Mg{sub x}Ti{sub 5}O{sub 12} spinel electrodes (0

A novel functionalized electrolyte based on Li2B12F9H3 was investigated to improve the electrochemical and safety performance of lithium-ion cells using a lithium manganese oxide spinel positive electrode and a mesocarbon microbeads (MCMB) negative electrode. The test results showed that Li2B12F9H3 can act both as the lithium salt, like the conventional LiPF6, and as the redox shuttle for overcharge protection of lithium-ion cells. In addition, the performance of the lithium-ion cells was dramatically improved by the addition of lithium bis(oxalato)borate and tris(pentafluorophenyl)borane as electrolyte additives. With the help of the proposed additives, MCMB/Li1.156Mn1.844O4lithium ion cells maintained more than 85% capacity after 1200 cycles.

Redox shuttle is a promising mechanism for intrinsic overcharge protection in lithium-ion cells and batteries. Two lithium borate cluster salts are reported to function as both the main salt for a nonaqueous electrolyte and the redox shuttle for overcharge protection. Lithium borate cluster salts with a tunable redox potential are promising candidates for overcharge protection for most positive electrodes in state-of-the-art lithium-ion cells.

Low-temperature performance loss is a significant barrier to commercialization of lithium-ion cells in hybrid electric vehicles. Increased impedance, especially at temperatures below 0 C, reduces the cell pulse power performance required for cold engine starts, quick acceleration, or regenerative braking. Here we detail electrochemical impedance spectroscopy data on binder- and carbon-free layered-oxide and spinel-oxide electrodes, obtained over the +30 to ?30 C temperature range, in coin cells containing a lithium-preloaded Li{sub 4/3}Ti{sub 5/3}O{sub 4} composite (LTOc) counter electrode and a LiPF{sub 6}-bearing ethylene carbonate/ethyl methyl carbonate electrolyte. For all electrodes studied, the impedance increased with decreasing cell temperature; the increases observed in the midfrequency arc dwarfed the increases in ohmic resistance and diffusional impedance. Our data suggest that the movement of lithium ions across the electrochemical interface on the active material may have been increasingly hindered at lower temperatures, especially below 0 C. Low-temperature performance may be improved by modifying the electrolyte-active material interface (for example, through electrolyte composition changes). Increasing surface area of active particles (for example, through nanoparticle use) can lower the initial electrode impedance and lead to lower cell impedances at -30 C.

An electrochemical model was developed to examine hybrid pulsed power characterization (HPPC) tests on the positive electrode of lithium-ion cells. By utilizing the same fundamental equations as in previous electrochemical impedance spectroscopy studies, this investigation serves as an extension of the earlier work and a comparison of the two techniques. The electrochemical model was used to examine performance characteristics and limitations for the positive electrode during HPPC tests. Parametric studies using the electrochemical model and focusing on the positive electrode thickness were employed to examine methods of slowing electrode aging and improving performance.

Lithium-ion cells for hybrid electric vehicle applications must deliver and accept current pulses at relatively high rates and over a range of temperatures. We examined the effect of test temperature from room temperature to 55°C on the performance of three electrode cells (LiNi 0.8 Co 015 Al 0.05 O 2 -based positive electrode, Li 4/3 Ti 5/3 O 4 -based negative electrode, and Li-Sn alloy reference electrode) by galvanostatic cycling and electrochemical impedance spectroscopy (EIS) measurements. Increasing the test temperature reduced cell impedance, which improved its power delivery capability. The higher test temperatures reduced the width of the midfrequency impedance arc in the positive-electrode EIS data but did not affect the low-frequency Warburg impedance portion. We discuss the impedance reductions and capacity gains from the higher temperature measurements in terms of the electrochemical processes occurring at the electrode/electrolyte interfaces.

Department of Chemical Engineering, Illinois Institute of Technology, Chicago, Illinois, USAAn electrochemical model was developed to describe alternating current ~ac! impedance experimental studies conducted onlithium-ion positive electrodes. The model includes differential mass and current balances for the positive electrode’s compositestructure, as well as details of the oxide-electrolyte interface. A number of specialized experiments were conducted to help definethe parameter set for the model. The electrochemical ac impedance model was used to examine aging effects associated with thepositive electrode.© 2005 The Electrochemical Society. @DOI: 10.1149/1.1928169# All rights reserved.Manuscript submitted November 18, 2004; revised manuscript received January 19, 2005. Available electronically June 10, 2005.