Function Of Cycle Number Research Articles

A semi-empirical, electrochemistry-based model for Li-ion battery performance prediction over lifetime

• The semi-empirical model suggested has physically meaningful fitting parameters. • The model displays comparable accuracy and computational efficiency to state-of-the-art mathematical equations. • The model successfully predicts terminal voltage for both cycle testing and load testing protocols. Predicting the performance of Li-ion batteries over lifetime is necessary for design and optimal operation of integrated energy systems, as electric vehicles and energy grids. For prediction purposes, several models have been suggested in the literature, with different levels of complexity and predictability. In particular, electrochemical models suffer of high computational costs, while empirical models are deprived of physical meaning. In the present work, a semi-empirical model is suggested, holding the computational efficiency of empirical approaches (low number of fitting parameters, low-order algebraic equations), while providing insights on the processes occurring in the battery during operation. The proposed model is successfully validated on experimental battery cycles: specifically, in conditions of capacity fade >20%, and dynamic cycling at different temperatures. A comparable performance to up-to-date empirical models is achieved both in terms of computational time, and correlation coefficient R 2 . In addition, analyzing the evolution of fitting parameters as a function of cycle number allows to identify the limiting processes in the overall battery degradation for all the protocols considered. The model suggested is thus suitable for implementation in system modelling, and it can be employed as an informative tool for improved design and operational strategies.

Ratchetting in Cold-Drawn Pearlitic Steel Wires

The microscopic mechanisms that accommodate uniaxial ratchetting in cold-drawn pearlitic steel wires were explored. A two-stage evolution of ratchetting strain as a function of cycle numbers was observed. The initial sudden increase of plastic strain leads to a rapid decomposition of cementite, followed by a constant ratchetting strain rate with critical role of decomposed carbon atoms played in blocking dislocation motion. The dislocation configuration transforms from low-density lines and tangles to high-density cells and sub-grains with increasing strain. A possible mechanism of cementite decomposition is discussed in terms of carbon-dislocation interactions and an unfavorable cementite surface-to-volume ratio.

Incorporating Solvate and Solid Electrolytes for All‐Solid‐State Li2S Batteries with High Capacity and Long Cycle Life

AbstractThe development of all‐solid‐state lithium–sulfur batteries is hindered by the poor interfacial properties at solid electrolyte (SE)/electrode interfaces. The interface is modified by employing the highly concentrated solvate electrolyte, (MeCN)2−LiTFSI:TTE, as an interlayer material at the electrolyte/electrode interfaces. The incorporation of an interlayer significantly improves the cycling performance of solid‐state Li2S batteries compared to the bare counterpart, exhibiting a specific capacity of 760 mAh g−1 at cycle 100 (330 mAh g−1 for the bare cell). Electrochemical impedance spectroscopy shows that the interfacial resistance of the interlayer‐modified cell gradually decreases as a function of cycle number, while the impedance of the bare cell remains almost constant. Cross‐section scanning electron microscopy (SEM)/ energy dispersive X‐ray spectroscopy (EDS) measurements on the interlayer‐modified cell confirm the permeation of solvate into the cathode and the SE with electrochemical cycling, which is related to the decrease in cell impedance. In order to mimic the full permeation of the solvate across the entire cell, the solvate is directly mixed with the SE to form a “solvSEM” electrolyte. The hybrid Li2S cell using a solvSEM electrolyte exhibits superior cycling performance compared to the solid‐state cells, in terms of Li2S loading, Li2S utilization, and cycling stability. The improved performance is due to the favorable ionic contact at the battery interfaces.

Electrochemical-thermal modeling of lithium plating/stripping of Li(Ni0.6Mn0.2Co0.2)O2/Carbon lithium-ion batteries at subzero ambient temperatures

Electrochemical performance of lithium-ion batteries drops significantly at low temperatures, especially under charging, because the lithium ions are prone to deposit as lithium metal on instead of intercalating into the solid matrix of anode. At discharging, the existence of lithium stripping leads to the capacity reversed partially, whose amount is proportional to the width of extra voltage plateau displayed at the beginning. A physics-based electrochemical-thermal model considering effects of lithium plating/stripping is developed and validated to explore the degradation mechanism and behaviors of NMC/Carbon cells undergoing prolonged cycling. The temporal and spatial analyses of overpotential of lithium plating and surface concentration in solid phase at various operating conditions demonstrate that lithium plating starts to take place at the interface between composite anode and separator, and the degradation can be accelerated by the decreasing ambient temperatures and increasing charging current rates. The degradation effects including loss of recyclable lithium ions, loss of anode active material, growth of plated lithium and secondary solid electrolyte interphase (SEI) and consumption of electrolyte solvents are considered. The model is capable of estimating capacity as a function of cycle number with an overall accuracy of 3% if the capacity fade is less than 30%.

Surface Area of Lithium-Metal Electrodes Measured by Argon Adsorption

Rechargeable cells that rely on the stripping and plating of lithium during discharge and charge, respectively, must maintain smooth and flat lithium morphology to attain long cycle life. Higher surface area lithium metal, associated with dendrites or porous deposits, will increase the rate of reaction with liquid electrolyte, accelerate capacity loss, and decrease safety. Here we use argon BET to measure the specific surface area of lithium-metal electrodes as a function of cycle number. For “anode-free” cells with 1M LiPF6 in FEC:DEC electrolyte, the specific surface area of the lithium electrode increases by more than three times after 20 charge-discharge cycles. This measurement provides an assessment of degradation that can be linked to capacity loss and safety, which averages over the entire electrode and is therefore an excellent quantitative complement to SEM images used to look at lithium-morphology.

Electrochemical-thermal P2D aging model of a LiCoO2/graphite cell: Capacity fade simulations

Abstract We simulate a simple model of aging in a lithium-ion cell that accounts for growth of a passivation film on the negative electrode, on top of an electrochemical-thermal pseudo-two-dimensional (P2D) description of “ideal” (i.e., ignoring aging effects) cell dynamics, using a finite-volume Matlab code. Capacity fade is assessed by evaluating at each instant in time the loss of cyclable lithium ions, which follows as a consequence of a single irreversible electrolyte decomposition reaction in the aging model. Assuming reasonable values for the side reaction rate and SEI film conductivity, we show an estimated 2.5% capacity loss after 1600 cycles at 1C. We also find a growth rate of about 0.64 nm/hr at 1C for the SEI film on the negative electrode, in a linear growth regime obtained by neglecting any diffusive transport limitations during new SEI formation. In addition, we show that for an “ideal” cell under cyclic loading conditions its energy dissipation (per cycle) is constant as a function of cycle number, while that same quantity keeps increasing with cycle number for an aging cell. Finally, we discuss the influence of the aging model as it affects surface cell temperature as a function of a (convective) heat transfer coefficient. Specifically, for values of h larger than about 1 W/(m2 K) we find essentially no effect of the aging model on surface cell temperature, which remains equal for all time to the surrounding ambient temperature. For smaller values of h, however, we find surface cell temperatures that are systematically larger than their “ideal” cell counterparts, with a strictly monotonic (increasing) behavior of the steady-state surface cell temperature as a function of the (reciprocal) heat transfer coefficient.

Multicycle sorption enhanced steam methane reforming with different sorbent regeneration conditions: Experimental and modelling study

Replacing coal and oil with natural gas reduces greenhouse gas emissions without weakening economic development. Sorption enhanced steam methane reforming (SESMR) is an emerging process for an affordable transition towards cleaner energy systems, as it exploits natural gas coupled with in-situ CO2 capture to produce hydrogen. SESMR is performed on a steam reforming catalyst and a sorbent (cyclically regenerated by calcination) or, alternatively and more efficiently, on bi-functional Combined Sorbent-Catalyst Materials (CSCM). A double fluidized bed (reformer and oxyfuel combustor for calcination) with circulation of particulate material is anticipated for continuous SESMR at commercial scale. In this work, a Ni-CaO-mayenite CSCM was investigated in multicycle experimental tests under mild (pure N2, 850 °C) and severe (pure CO2, 925 °C) regeneration conditions, in thermogravimetric analysis for CO2-capture/regeneration and in an automated packed bed test rig for SESMR/calcination: the evolution of Ni and CaO functionalities as functions of cycle number was highlighted, also with the aid of scanning electron microscopy. Moreover, experimental results obtained with the hybrid material, collected under mild regeneration conditions, were used to simulate both CO2 sorption in TGA and packed bed SESMR multicycle tests: a Particle Grain Model (PGM) and an Axial Dispersion Plug Flow Reactor (ADPFR) model were integrated numerically, respectively. The same parameter values needed to characterize the material behaviour allowed a good fitting of both types of multicycle tests, thus demonstrating the possibility to use CO2-capture TGA data as a predictive tool for the CSCM performance in SESMR, provided that the catalytic activity is preserved.

Quantification and modeling of mechanical degradation in lithium-ion batteries based on nanoscale imaging

Capacity fade in lithium-ion battery electrodes can result from a degradation mechanism in which the carbon black-binder network detaches from the active material. Here we present two approaches to visualize and quantify this detachment and use the experimental results to develop and validate a model that considers how the active particle size, the viscoelastic parameters of the composite electrode, the adhesion between the active particle and the carbon black-binder domain, and the solid electrolyte interphase growth rate impact detachment and capacity fade. Using carbon-silicon composite electrodes as a model system, we demonstrate X-ray nano-tomography and backscatter scanning electron microscopy with sufficient resolution and contrast to segment the pore space, active particles, and carbon black-binder domain and quantify delamination as a function of cycle number. The validated model is further used to discuss how detachment and capacity fade in high-capacity materials can be minimized through materials engineering.

Interface Engineering with Ionic Liquid Composite Materials for Efficient and Durable Electrocatalysis

Polymer electrolyte membrane fuel cells (PEMFCs) are among the most promising candidates in carbon neutral renewable energy systems because of their superior efficiency, environmentally friendly principles, as well as a wide range of potential applications 1. The development of catalytic materials for electrochemical oxygen reduction has received significant attention as the sluggish kinetics of cathodic oxygen reduction reaction (ORR) is the major source of efficiency loss in current PEMFCs. To this end, significant achievements have been made in the past decade to improve both intrinsic and mass-based activity through the evolution of nanoscale morphology and the compositional profile 2-4. However, these adjustments on geometric and compositional profiles remain insufficient for the widespread integration of fuel cells into industry applications 5. It is necessary to look for new strategies, such as shifting focus away from the catalyst and move toward the optimization of electrocatalytic interfaces. Electrocatalyst development for reactions involving multiple charge transfer steps and more than one adsorbed intermediate is severely limited by the scaling relations that govern the free energy of adsorption of those intermediates. Adsorbed intermediate scaling relations create a thermodynamic minimum overpotential of 0.37 V due to the fixed difference in free energy of adsorption (ΔΔGad) for *OH and *OOH of ~3.2 eV which is independent of the identity of the catalyst 1,6. It is found that by decreasing the degree of intermediate solvation, ΔΔGad can approach 2.46 eV which is the value required for ORR to take place at zero overpotential 7. Here we present our progress in addressing intermediate scaling for the ORR through the manipulation of metal/electrolyte interface with chemically tailored ionic liquids (ILs). Figure 1 demonstrates the impact of IL on the ORR on Pt(111). A significant increase in the ORR current is observed at both low overpotential and the potential region at which an adsorbed monolayer of hydrogen is formed on Pt. At low overpotential we attribute the enhanced activity to the disruption of *OH-H2O hydrogen bonded structure due to the control of the interaction of water with the catalytic surface in the presence of the IL. We argue that it is this change in water interaction with the surface, potentially in combination with other properties of the IL including reactant solubility, that results in the enhanced ORR activity. We also studied the manner by which ILs impact the stability of 3-dimensional, nanoporous nanoparticle electrocatalysts, such as nanoporous PtNi nanoparticles (np-NiPt). In addition to the standard mechanisms of catalyst degradation including Pt dissolution/Ostwald ripening and coalescence/aggregation, nanoporous materials can lose electrochemically active surface area (ECSA) through surface smoothening driven coarsening 8. With a better understanding of the interplay between nanoporous structure coarsening and transition metal loss, we have developed strategies to mitigate coarsening through incorporating ILs at the metal/electrolyte interface to directly mitigate electrochemically enhanced surface diffusion, which leads to ECSA loss. The hydrophobicity of these ILs is known to change the interaction of water with the surface, positively shifting the onset potential of Pt oxidation. The interfacial IL acts to limit the charge dependent formation of a surface metal/electrolyte anion complex which is responsible for the potential dependent enhancement in surface diffusion. The oxygen reduction reaction activity, ECSA of these complex composite nanoporous electrocatalysts, as shown in Figure 2, was tracked as a function of cycle number during accelerated durability testing (ADT) providing insight into the mechanism of catalyst degradation and the effect of ILs on catalyst stability. Nørskov, J. K. et al., Phys. Chem. B 108 (2004) 17886–17892.Stamenkovic, V. R. et al., Nature Material 6 (2007) 241–247.Chen, C. et al., Science 343 (2014) 1339-1343.Snyder, J. et al. Am. Chem. Soc. 134 (2012) 8633-8645.Gasterger, H. A. et al. Applied Catalysis B: Environmental 56 (2005) 9-35.Hansen, H. A., et al., Phys. Chem. C 118 (2014) 6706–6718.Viswanathan, V., et al., Catal., 27 (2014) 215-221.Li, Y. et al., ACS Catal., 7 (2017) 7995-8005. Figure 1

Structure and Properties of Sulfonated Pentablock Terpolymer Films as a Function of Wet–Dry Cycles

The structure and properties of poly(tert-butylstyrene-b-hydrogenated isoprene-b-sulfonated styrene-b-hydrogenated isoprene-b-tert-butylstyrene) (tBS-HI-SS-HI-tBS) films were investigated as a function of “wet–dry cycles”, where one “cycle” is defined as a 24 h soak in deionized water followed by a 24 h drying period in air. Films were characterized with a variety of complementary measurements that include X-ray scattering, infrared spectroscopy, water uptake, impedance spectroscopy, and tensile tests. We find that cycling drives a structural transition toward increasingly interconnected SS domains, which is favorable for water and ion transport. However, cycling can also induce mechanical deformations that reduce ductility, swelling, and water uptake. The significance of this trade-off is illustrated by comparing the properties for two film thicknesses as a function of cycle number: The ductility of thinner films (15 μm) is lost after four cycles, an effect that is correlated with the appearance of macro...

A single particle model with chemical/mechanical degradation physics for lithium ion battery State of Health (SOH) estimation

State of Health (SOH) estimation of lithium ion batteries is critical for Battery Management Systems (BMSs) in Electric Vehicles (EVs). Many estimation techniques utilize a battery model; however, the model must have high accuracy and high computational efficiency. Conventional electrochemical full-order models can accurately capture battery states, but they are too complex and computationally expensive to be used in a BMS. A Single Particle (SP) model is a good alternative that addresses this issue; however, existing SP models do not consider degradation physics. In this work, an SP-based degradation model is developed by including Solid Electrolyte Interface (SEI) layer formation, coupled with crack propagation due to the stress generated by the volume expansion of the particles in the active materials. A model of lithium ion loss from SEI layer formation is integrated with an advanced SP model that includes electrolytic physics. This low-order model quickly predicts capacity fade and voltage profile changes as a function of cycle number and temperature with high accuracy, allowing for the use of online estimation techniques. Lithium ion loss due to SEI layer formation, increase in battery resistance, and changes in the electrodes' open circuit potential operating windows are examined to account for capacity fade and power loss. In addition to the low-order implementation to facilitate on-line estimation, the model proposed in this paper provides quantitative information regarding SEI layer formation and crack propagation, as well as the resulting battery capacity fade and power dissipation, which are essential for SOH estimation in a BMS.

The Effect of Catalyst Layer Coating Irregularities on Fuel Cell Performance Induced By Load Cycling

The polymer electrolyte membrane fuel cell (PEMFC) is capable of high power outputs at low operating temperatures, making it an attractive alternative to the petroleum-based internal combustion engine (ICE) common in vehicular applications [1]. PEMFC commercialization is contingent on future cost reductions and durability improvements. According to the Department of Energy (DOE), fuel cell systems (FCS) will have to surpass annual production levels of 500,000 units at $40/kW in order to compete with current ICEs [2-3]. Roll-to-roll (R2R) methods can achieve high yields at decreased per-area costs, enabling FCS manufacturing and cost targets to be met. Unfortunately coating irregularities that harm the performance and/or durability of PEMFC membrane electrode assemblies (MEAs) can occur during R2R fabrication processing [4-5]. This work studies the effect of a subset of such irregularities on performance when operating the New European Drive Cycle (NEDC). If the MEA experiences statistically significant lower performance as a result of a coating irregularity, then the irregularity should be classified as a defect. Classification of irregularities as defects will ultimately assist the industry by developing threshold detection limits for in-line quality control diagnostics. Samples were made in-house by creating Pt/C electrodes of 5 cm2 area and 0.2/0.2 mg Pt/cm2 loading. These samples were either pristine or contained an intentionally introduced 100% catalyst reduction spot centered on the cathode. Electrode coating irregularities ranged from 0.0625 to 0.125 cm2, or 1.25% and 2.5% of the MEA active area, respectively. Membrane materials were NRE-211 (25 µm) and NRE-212 (50 µm). Operating conditions for all samples were Tcell: 80°C, anode/cathode relative humidity (RH) at 100/100%, 105/350 sccm H2/Air, and 150/150 kPa backpressure. The MEAs were conditioned and subsequently subjected to 1008 repetitions of the NEDC drive cycle. The single drive cycle profile of the NEDC is shown in Figure A. The NEDC combines urban driving conditions, i.e. low engine load and lower speeds similar to busy city driving, and extra-urban driving conditions, i.e. more transient operation and higher speeds similar to highway driving. At the beginning of life and every 72 cycles onwards, a current controlled polarization (VI) curve was performed. Figure B displays polarization data of two MEAs made with NRE-211 membrane: one pristine and one having a 0.0625 cm2 coating irregularity. The figure shows, for each sample, one VI curve taken at beginning of life and one at the end of test. While the performance at beginning of life was comparable for both samples, the sample with the coating irregularity showed a reduced performance after 1008 cycles when compared to the pristine sample. Figure C shows the cell open circuit voltage (OCV) as a function of cycle number. The four samples were made with NRE-211 and NRE-212 membranes: two pristine and two having a 0.125 cm2 coating irregularity. The MEA made with the 25 µm membrane experienced a sharp decline in OCV starting at around 500 cycles. Conversely, the MEA with the same sized irregularity but made with 50 µm membrane, retained its OCV throughout the entirety of the experiment. Figure D displays the performance normalized to the beginning of life at 1 A cm-2 for three MEAs made with 50 µm membrane. One was pristine and the other two had 0.0625 cm2 and 0.125 cm2 coating irregularities. The data indicate that throughout the entirety of the experiment the coating irregularities may have caused increased performance degradation. In this presentation, we will discuss how irregularity location and size can impact PEMFC performance, and how the NEDC drive cycle paired with electrochemical measurements can be potentially used as a defect classification method. References 1) Barbir, Frano. PEM Fuel Cells: Theory and Practice. Academic Press, 2013. 2) Marcinkoski, Jason et al. “Fuel Cell System Cost – 2015”. September 30, 2015. 3) James, Brian et al. “Mass Production Cost Estimation of Direct H2 PEM Fuel Cell Systems for Transportation Applications: 2015 Update”. December 2015. 4) Kundu, S., et al. Journal of Power Sources157.2 (2006): 650-656. 5) Benchmarking and Best Practices Center of Excellence. Manufacturing Fuel Cell Manhattan Project. November 2011. Figure 1

Internal Hybrid Li-Ion Battery and Li-Ion Capacitor Energy Storage Cells

Lithium (Li)-ion battery (LIB) and electric double-layer capacitor (EDLC) are two widely used electrochemical energy storage devices. LIB is made with Li intercalated graphite anode and Li metal oxide cathode and EDLC is made with high surface activated carbon for both anode and cathode. To compare two energy storage devices, LIB has a higher specific energy (or energy density) than EDLC, but has low specific power (or power density) and poor cycle life than EDLC as shown in Fig. 1. In high energy and low power applications, the LIB is the best choice; in high power and long cycle applications, the EDLC is used; however, in many applications, the power requirement is not a constant but rather spans over a range of power levels. It has been demonstrated that when batteries and supercapacitors were externally connected to form a hybrid power source, the voltage drop during the pulsed current for the hybrid power source was less than that for the battery, and operating time of the hybrid power source was significantly extended [1,2]. However, because the maximum cell voltage of single cell EDLC is less than that of LIB; therefore, hybrid power sources could only be made with two separate battery and EDLC cells and electrically connect them with or without electric circuits. The Li-ion capacitor (LIC) is a new energy storage device which consists of an EDLC cathode and a LIB anode, between which the ions shuttle during charge and discharge processes. The LIC not only retained all the advantages of EDLC such as specific power >5 kW/kg and cycle life > 100,000 cycles; but also had higher specific energy of 15-30 Wh/kg and higher maximum cell voltage of 4.1 V than that of EDLC [3,4]. Because the potentials of anode and cathode as well as the maximum cell voltage of LIC is comparable to that of LIB, it allows the LIC and LIB to be assembled in one package as a monolithic LIB and LIC hybrid cell. Very recently, we have demonstrated a new hybrid energy storage device that combines the advantages of both the LIB and the LIC, thereby avoiding their inherent defects, while bridging the gap between the high energy densities offered by batteries and the high power densities seen in LIC as shown in Fig. 1. The energy density and power density of the hybrid cell can be designed to meet the requirements by a reasonable distribution of the ratio between LIB and LIC electrode materials in the internal hybrid cell. For example, in a LIB/LIC internal hybrid cell, made with a 20 wt.% LiFePO4 (LFP) and 80 wt.% activated carbon (AC) mixed cathode and a pre-lithiated hard carbon anode, the specific energy increased by 40% to compare with LIC. As can be seen in Fig. 2(a), the discharge profile can be divided into two parts: at cell voltage range of 2.2-3.0 V and 3.3-3.8 V, the capacity was contributed by capacity material, while at 3.0-3.3 V, it was contributed by battery material. It can also be seen from Fig. 2, that the capacity contribution by capacitance material was less sensitive than battery material when the discharge rate increased from 0.1C to 53C. Fig. 2(b) shows the charge-discharge voltage profiles measured at an interruption rate of 0.2C as a function of cycle number after the hybrid cell was cycled at 43C rate. It must be notified that cycle life of the hybrid energy storage cell was found to be prolonged significantly as shown in Fig. 2(c). After 20,000 cycles, the degradation of specific capacity for both capacitance and battery materials inside hybrid cell was less than 5%. As a comparison, the specific capacity of a battery cell using LFP cathode, as a control experiment, decreased by 40% after 2,000 cycles. Reference: [1] J.P. Zheng, S.P. Ding, and T.R. Jow, “Hybrid Power Sources for Pulse Current Applications”, IEEE Transactions on Aerospace and Electronic Systems, 37, 288 (2001). [2] D. Shin, Y. Kim, Y. Wang, N Chang, and M. Pedram, “Constant-current regulator-based battery-supercapacitor hybrid architecture for high-rate pulsed load applications”, J. Power Sources, 205, 516 (2012). [3] W.J. Cao and J.P. Zheng, “Li-ion Capacitors with Carbon Cathode and Hard Carbon/SLMP Anode Electrodes”, J. Power Sources, 213, 180 (2012). [4] W.J. Cao, J. Shih, J.P. Zheng, and T. Doung, “Development and characterization of Li-ion capacitor pouch cells”, J. Power Sources, 257, 388 (2014). Figure 1

A Single Particle-Based Battery Degradation Model Including Chemical and Mechanical Degradation Physics

Accurate and rapid prediction of the status of a lithium-ion battery is an important process in Battery Management System (BMS). In this work, a single particle model is developed by focusing on crack propagation coupled with Solid Electrolyte Interface (SEI) layer formation and its evolution. The lithium ion loss due to the SEI layer evolution is integrated with our previously developed advanced single particle model that includes electrolytic physics. This model is fairly well predictive of capacity fade and voltage change as a function of cycle number and temperature. Despite its implementation in a single particle model, the results provide quantitative information on the role of SEI layer growth and crack propagation, and corresponding capacity fade and power loss.

(Invited) Durability of Electrochromic Films: Aging Kinetics and Rejuvenation

A major challenge for energy-efficient smart window technology is to ensure the durability of electrochromic (EC) devices over a service life of more than 20 years. In this paper, we report recent results from a fundamental study of the aging kinetics of EC tungsten oxide and nickel oxide thin films and describe electrochemical rejuvenation mechanisms that are able to restore the films to their initial state. The aging kinetics displays an approximate power-law decrease of the charge capacity as a function of cycle number. This decay of charge capacity can be understood in terms of models built on so-called dispersive chemical kinetics. Tungsten oxide and nickel oxide EC films can be rejuvenated by applying a high electrochemical potential or a small constant current. Trapped ions in the bulk or at the surface of the films can be released by these procedures.

A Reduced-Order Battery Degradation Model for Battery Management Systems

An accurate and prompt prediction of the State of Health (SOH) of lithium ion batteries is a critical process for a Battery Management System (BMS). In this work, a single particle-based degradation model is developed by focusing on Solid Electrolyte Interface (SEI) layer formation and its evolution, which is coupled with the crack propagation due to the developed stress inside particles. The lithium ion loss estimated from the SEI layer evolution is incorporated with an advanced single particle model, where the electrolyte physics is included, for prompt calculation that is critical for online estimation. This model well predicts the measured capacity fade and voltage change as a function of cycle number and temperature. The lithium ion loss due to SEI layer formation, resistance rise, and operating voltage window are examined to explain the capacity fade. Besides its lower-order implementation in a single particle, the results provide quantitative information about the role of SEI layer growth and crack propagation, and corresponding capacity fade and power loss, which are critical for SOH and BMS.

Self-supported Porous Silicon Nanotube Arrays As Anode Material for Li-ion Batteries

Li-ion batteries (LIBs) have been used as a power source ranging from microelectronics to electrical vehicles. However, the use of graphite as anode material for next generation LIBs is unsuitable because of the low Li+ storage capacity (372 mAh.g-1). Various alternative, such as Si, Sn, Ge, has been considered as alternative due to their superior specific gravimetric capacity (for example Si = 4200 mAh.g-1) resulting in high power/energy densities1. However, these materials are intrinsically prone to large volume change during alloying/dealloying mechanism resulting in pulverization, thick SEI formation, loss of electrical contact and active material2. The use of nanostructure has been widely studied to address this problem. Recently, our group has developed techniques to fabricate ultrathin crystalline silicon nanotube arrays (Si NTAs). In comparison to previous works which focus on sealed SiNTs3 and larger size SiNTs (> 400 nm)4, our Si NTAs are ultrathin (10 nm) with porous side walls5. Fig.1 a & b, shows the SEM and TEM images of the Si NTAs fabricated by three step template directed method. The first step is the formation of sacrificial ZnO into stainless steel substrates followed by the deposition of Si onto the ZnO nanowires. The last step is the removal of the sacrificial ZnO nanowire templates to obtain the Si NTAs with porous side wall structure. Fig. 1 (a) SEM and (b) TEM image of Si NTAs. Fig. 2 shows the gravimetric capacity of Si NTAs as a function of cycle number. The electrochemical tests were performed in two electrode Swagelok cell. The half-cells were assembled by using Si NTAs as a working electrode, Li foil as a counter electrode, and a Whatman glass microfiber soaked in 1 M LiPF6 in EC:DEC electrolyte as separator. The cell was cycled at C/20 in a potential window of 0.01 – 1.75 V vs Li/Li+. It delivers a specific capacity of 1670 mAh.g-1 after 30 cycles. The high capacity values are attributed to the 3D porous structure of the Si NTAs. During this work, the improved electrochemical performance of Si NTAs will be discussed. Fig. 2. Gravimetric capacity vs cycle number for Si NTAs at C/20. Reference 1. Liang, B.; Liu, Y.; Xu, Y., Silicon-based materials as high capacity anodes for next generation lithium ion batteries. Journal of Power Sources 2014, 267, 469-490. 2. Kasavajjula, U.; Wang, C.; Appleby, A. J., Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells. Journal of Power Sources 2007, 163 (2), 1003-1039. 3. Song, T.; Xia, J.; Lee, J.-H.; Lee, D. H.; Kwon, M.-S.; Choi, J.-M.; Wu, J.; Doo, S. K.; Chang, H.; Park, W. I., Arrays of sealed silicon nanotubes as anodes for lithium ion batteries. Nano letters 2010, 10 (5), 1710-1716. 4. Wu, H.; Chan, G.; Choi, J. W.; Yao, Y.; McDowell, M. T.; Lee, S. W.; Jackson, A.; Yang, Y.; Hu, L.; Cui, Y., Stable cycling of double-walled silicon nanotube battery anodes through solid-electrolyte interphase control. Nature nanotechnology 2012, 7 (5), 310-315. 5. Huang, X.; Gonzalez-Rodriguez, R.; Rich, R.; Gryczynski, Z.; Coffer, J. L., Fabrication and size dependent properties of porous silicon nanotube arrays. Chemical Communications 2013, 49 (51), 5760-5762. Figure 1

Investigating the Three-Dimensional Microstructural Characteristics of Lithium-Sulfur Electrodes with X-Ray Micro-Tomography

Lithium sulfur (Li-S) batteries offer a theoretical specific capacity of 1675 mAh g-1, an order of magnitude greater than current Li-ion battery technology, along with lower cost and improved safety. However, their commercialization has been hindered by poor cycle life and low coulombic efficiency. As opposed to the relatively simple ‘rocking chair’ intercalation-based mechanism underpinning Li-ion battery technology, the reaction mechanism within Li-S conversion-type cathodes involves multiple stages and phase changes. During the charge process, solid lithium sulfide is delithiated by a series of electrochemical reactions involving soluble polysulfide intermediates before being converted into elemental sulfur, and during the discharge process, the reverse occurs. The development of Li-S batteries has been hindered by complex phenomenon including large volume change (ca. 80%) between elemental sulfur (S) and lithium sulfide (Li2S), active material loss resulting from polysulfide dissolution, poor electrical conductivity of both S and Li2S, significant self-discharge, and the polysulfide shuttle mechanism causing poor coulombic efficiency and anode corrosion.Whilst recent advances have been made in contained cathode materials for Li-S batteries to prevent polysulfide dissolution into the bulk electrolyte phase and suppress active material loss and the polysulfide shuttle effect, a more comprehensive understanding of the underlying mechanisms contributing to active material loss to the electrolyte phase is essential for further optimization of cycle life and capacity, particularly so for high mass loading sulfur cathodes. However, the roles that the electrode morphology and the microstructure of the carbon binder domain (CBD) play in contributing to these failure mechanisms during cycling remain poorly understood. Lab-scale micro- and nano-tomography was performed on a Li-S battery with the ZEISS Xradia Versa 520 and Ultra 810 X-ray microscopes, achieving voxel dimensions of ca. 126 nm and 30 nm respectively. These instruments enabled a multi length-scale, three-dimensional X-ray tomography study to be carried out, with the extraction of various metrics as a function of cycle life and state of charge on the sulfur cathode. The metrics included morphological parameters, such as sulfur mass loading and particle size distribution, and microstructural characteristics, such as electrode porosity, tortuosity and contact area between phases. Nano-tomography of the CBD revealed the resolution dependent nature of certain parameters (i.e. contact area between phases), and highlighted the significance of multi length-scale characterization. Digital volume correlation (DVC) was applied to track the evolution of sulfur particles as a function of cycle number. The 3D visualization of Li-S electrodes with this technique provides a wealth of information, improving understanding of the evolution of morphological and microstructural parameters of the different phases; as X-ray imaging is inherently non-destructive, it enables these parameters to be observed in-situ as a function of cycle life and state of charge (SoC). The inherently anisotropic nature of active material dissolution, recrystallization and other degradation processes within the electrode necessitates a three-dimensional approach to enable their quantification, and X-ray tomography is a useful tool in informing the development of more optimal electrode designs. Figure 1

(Invited) Durability of Electrochromic Films: Ageing Kinetics and Rejuvenation

A major challenge for energy-efficient smart window technology is to ensure the durability of electrochromic (EC) devices over a service life of more than 20 years. Hence, their degradation under operating conditions must be better understood and preferably avoided. In this paper we review our recent results from a fundamental study of the ageing kinetics of EC tungsten oxide and nickel oxide thin films, as well as electrochemical rejuvenation mechanisms that are able to restore the films to their initial state. The ageing of tungsten oxide films upon cycling in the range 2-4 V vs. Li/Li+ displays a power-law decrease of the charge capacity as a function of cycle number. In the case of severe ageing at potentials below 2.0 V there is evidence for a superposition of two processes. In the case of nickel oxide the ageing process seems to be more complex and it can be described by either a power-law or a stretched exponential function. The ageing of nickel oxide was also observed to decrease and even stop after a few thousand cycles just prior to final breakdown of the film. The decay of charge capacity can be understood in terms of models built on so-called dispersive chemical kinetics. The term “rejuvenation” describes electrochemical processes that may restore aged films to full EC functionality. During severe ageing of tungsten oxide coatings, Li ions are trapped in the film, and subsequently these ions can be released by application of a high electrochemical potential for a few hours. The trapping/detrapping of Li has been verified by Elastic Recoil Detection Analysis (ERDA) measurements. Very recently we have found that a similar process can be applied to EC nickel oxide films, although using a lower potential than for the case of tungsten oxide. For nickel oxide, the rejuvenation mechanism is not yet clear, but ERDA measurements are in progress and are expected to give detailed information. An increased understanding of ageing and rejuvenation will facilitate large-scale practical application of electrochromic materials. Our phenomenological models of ageing may make it possible to predict the service life of EC devices using empirical relationships. An understanding of rejuvenation processes will be of major importance in order to prevent or minimize performance decrease under extended cycling. The next step will be to investigate whether rejuvenation treatments can be applied successfully to full EC devices.

The influence of microstructure on the cyclic deformation and damage of copper and an oxide dispersion strengthened steel studied via in-situ micro-beam bending

Service materials are often designed for strength, ductility, or toughness, but neglect the effects of cyclic time-variable loads ultimately leading to macroscopic mechanical failure. Fatigue originates as local plasticity that can first only be observed on the micro scale at defects serving as stress concentrators such as inclusions or grain boundaries. Thus, a recently developed technique to perform in-situ observation of micro scale bending fatigue experiments was applied. Micro-beams fabricated from copper, single grained and ultrafine grained (ufg), and an oxide dispersion strengthened (ODS) steel were subject to cyclic deformation and subsequent damage. The elastic stiffness, yield strength, dissipated energy, and maximum stress were measured as a function of cycle number and plastic strain amplitude. From these properties, cyclic stress-strain curves were developed. Initial pronounced monotonic hardening and an increasing Bauschinger effect were observed in all samples with increasing strain amplitude. Cyclic stability was maintained until plastic strain amplitudes reached a critical value. At this point, dramatic cyclic softening and microcracking occurred. The critical strain amplitude was found to be approximately 5.4×10−3 for the copper with a refined grain structure and 1.2×10−2 for the steel specimen. Grain rotation and noticeable changes in sub-grain structure were evident in the ufg copper after a critical strain amplitude of εa=8.3×10−3. In-situ micro fatigue bending couples the cyclic evolution of bulk mechanical properties measurements with real-time electron microscopy analysis techniques of damage and failure mechanisms, which renders it a powerful method for developing novel fatigue resistant materials.