With the deployment of more and more large batteries on the grid addressing different applications, it is essential to better understand their degradation pattern and to accurately forecast their durability. In this work, the electrochemical data from a laboratory testing experiment replicating real usage was extensively investigated using incremental capacity and variance analyses. Results showed that the degradation of these cells is much more complex than what the capacity loss and the resistance increase indicated. A significant amount of "silent" degradation was uncovered. This silent degradation was quantified and forecasted to induce, in some cases, an acceleration of the capacity loss.

With the deployment of more and more large batteries on the grid addressing different applications, it is essential to better understand their degradation pattern and to accurately forecast their durability. In this work, the electrochemical data from a laboratory testing experiment replicating real usage was extensively investigated using incremental capacity and variance analyses. Results showed that the degradation of these cells is much more complex than what the capacity loss and the resistance increase indicated. A significant amount of "silent" degradation was uncovered. This silent degradation was quantified and forecasted to induce, in some cases, an acceleration of the capacity loss. (c) The Author(s) 2019. Published by ECS.

Vehicle-to-grid (V2G) and Grid-to-vehicle (G2V) strategies are often cited as promising approaches to mitigate the intermittency of renewable energy on electric power grids. However, their impact on vehicle battery degradation have yet to be investigated in detail. Since battery degradation is path dependent, i.e. different usage schedules lead to different degradation mechanisms, it is essential to investigate batteries under realistic V2G and G2V scenarios. The aim of this work is to understand the effect of bidirectional charging on the degradation mechanisms of commercial Li-ion cells used in electric vehicles today. Results showed that an extra V2G step during cycle-aging accelerated capacity loss and degraded the kinetics at the negative electrode. Moreover, for all cycling duty cycles, the loss of active material at the negative electrode was higher than the loss of lithium inventory. This condition could trigger lithium plating and shorten cell lifetimes. In the calendar-aging experiments, state of charge was shown to be an important factor and interacted with temperature to accelerate the loss of active material at the positive electrode and the loss of lithium. It was also found that high state of charge values caused loss of active material at the negative electrode and kinetic limitations.

The use of lithium batteries for power and energy-hungry applications has risen drastically in recent years. For such applications, it is necessary to connect the batteries in large assemblies of cells in series and parallel. With a large number of cells operating together, it is necessary to understand their intrinsic variabilities, not only at the initial stage but also upon aging. In this study, we studied a batch of commercial cells to address their initial cell-to-cell variations and also the variations induced by cycling. To do so, we not only tracked several metrics associated with cell performance, the maximum capacity, the resistance, and the rate capability but also the degradation mechanism via a non-invasive quantification of the loss of lithium inventory (LLI), the loss of active material (LAM) and the kinetic degradation on both electrodes. We found that, even with small initial cell-to-cell variations, significant variations will be observed upon aging because the cells degrade at a different pace. We also observed that these variations were not correlated with the initial variations.

Battery Energy Storage Systems (BESSs) can facilitate renewable energy sources integration onto the grid. For this application, these systems are expected to last for a decade or more, but the actual battery degradation under different real-world conditions is still largely unknown. In this work, three years of lithium titanate BESS usage in Hawai'i were analyzed. In addition, the representative usage was subjected on individual cells under controlled laboratory conditions to study the degradation mechanisms and enable life prognosis. The BESS was found to be operational 90% of the time and it stored a cumulative 1.5 GWh of energy, which represented more than 5000 equivalent full cycles on the cells. From this BESS usage data, an initial estimate of BESS degradation was provided and a representative duty cycle was developed. The analysis of the maintenance cycles indicated that these 5000 equivalent cycles induced an estimated 5-10% degradation of the single cells. The battery duty cycle was characterized based on 5 parameters: pulses duration, pulses intensity (current), state of charge (SOC) swing range, SOC event ramp rate, and temperature. The average usage consisted of several 9-second C/2 charge and discharge pulses organized to generate 5% SOC swings with a 0.75% SOC/min ramp rate at 35°C. However, extreme values such as currents up to 4C, swings of 100% SOC, and temperatures above 50°C were also recorded. Laboratory testing and analysis, in conjunction with a more thorough SOH estimation protocol, resulted in a detailed description of degradation that improved the predictions of the remaining useful battery life. Based on the BESS representative usage profile, cycle-aging and calendar-aging experiments were designed to test the degradation of the associated Li-ion cells in a controlled fashion. It was proven that the cell temperature history had the strongest impact on battery degradation followed by the C-rate and the state of charge. Interestingly, the impact of SOC, both on the cycle-aging and the calendar-aging experiments, was revealed to be counterintuitive. During cycle aging, small SOC swings were more detrimental than larger ones. During calendar aging, batteries lost capacity faster at low SOC than at high SOC. The associated degradation mechanisms and their path dependency were analyzed using incremental capacity analysis.

Battery energy storage systems (BESS) are often viewed as solution to mitigate the intermittency of renewable energies in electric grids. However, battery degradation associated with grid-tied BESS usage has never been investigated in detail. This work was aimed at understanding the impact of a BESS representative usage profile on the degradation of commercial Li-ion cells. It was found that the cell temperature history had the strongest impact on battery degradation followed by the C-rate and the state of charge (SOC). Also, batteries lost capacity faster at low SOCs during calendar aging and under small SOC swings while cycling.

The utility of a single-point impedance-based technique to monitor the state-of-health of a pack of four 18650 lithium-ion cells wired in series (4S) was demonstrated in a previous publication. This work broadens the applicability of the single-point monitoring technique to identify temperature induced faults within 4S packs at 0 °C by two distinct discharge cut-off thresholds: individual cell cut-off and pack voltage cut-off. The results show how the single-point technique applied to a 4S pack can identify cell faults induced by low temperature degradation when plotted on a unique state-of-health map. Cell degradation is validated through an extensive incremental capacity technique to quantify capacity loss due to low temperature cycling and investigate the underpinnings of cell failure.

This paper presents a novel approach for automated state of health estimation that offers similar advantages to the adaptive methods without being computation intensive. The onboard diagnosis uses a look-up table compiling the evolution of selected features of interest under any possible degradation paths. The look-up table is built from simulations of the impact of degradation on the cell electrochemical behavior. This multi-step method only requires intensive calculations prior to deployment. This approach is validated by experimental data from cells that underwent normal aging as well as plating and overcharge. Additional validation via modeling showed that the method is able to diagnose cells undergoing any degradation scenario automatically in close to 90% of cases.

Vehicle-to-grid and Grid-to-vehicle strategies are often cited as promising to mitigate the intermittency of renewable energy on electric power grids. However, their impact on the vehicle battery degradation has not been investigated in detail. The aim of this work is to understand the impact of bidirectional charging on commercial Li-ion cells used in electric vehicles today. Results show that additional cycling to discharge vehicle batteries to the power grid, even at constant power, is detrimental to cell performance. This additional use of the battery packs could shorten the lifetime for vehicle use to less than five years. By contrast, the impact of delaying the charge in order to reduce the impact on the power grid is found to be negligible at room temperature, but could be significant in warmer climates.

An Li4Ti5O12 parallel to LiNi1/3Mn1/3Co1/3O2 lithium-ion pouch cell has been subjected to an overcharge early in its cycle-life and kept cycling it up to 1500 cycles afterwards. We report on the non-invasive experimental verifications we conducted to corroborate our initial findings obtained via incremental capacity analysis. First, we used incremental capacity analysis on the negative electrode during overcharge to bring evidence of electrolyte reduction below 1 V vs. Li/Li+. Second, we used X-ray computerized tomography (CT scan) to demonstrate that gas bubbles were trapped between layers of positive and negative electrodes, thereby disabling the ionic conduction pathway between the two and causing capacity fade. Third, we administered a massage to the pouch cell to establish that the gas bubbles could be displaced, thereby recovering about 60% of the faded capacity. Finally, we proposed a new approach based on the study of the open-circuit voltage to describe quantitatively the gains of active materials that led to the partial capacity recovery. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.

Battery Energy Storage Systems (BESSs) show promise to help renewable energy sources integration onto the grid. These systems are expected to last for a decade or more, but the actual battery degradation under different real world conditions is still largely unknown. In this paper we analyze 3 years of usage of a lithium titanate BESS installed and in operation on an island power system in Hawaii. The BESS was found to be operational 90% of the time and stored a cumulative 1.5 GWh of energy, which represents more than 5000 equivalent full cycles on the cells. This paper presents a statistical analysis of the BESS usage, develops a representative duty cycle, and provides an initial estimate of BESS degradation. The battery duty cycle was characterized based on 5 parameters: pulses duration, pulses intensity (current), SOC swing range, SOC event ramp rate, and temperature. (C) 2016 Elsevier B.V. All rights reserved.

Metallic lithium plating (Li plating) is considered one of the most detrimental phenomenon in lithium ion batteries (LIB), as it not only leads to further aging but also to safety deterioration [1], [2]. Li plating occurs during charge, when Li ions deposit on the carbonaceous anode, in place of Li intercalation. Because metallic Li is highly reactive with the electrolyte, it further reacts consuming more lithium and inducing degradation on the electrode surface. These phenomena results in loss of lithium inventory (LLI) and loss of active material (LAM), leading to capacity loss and power fade. Safety deterioration occurs when the metallic Li results in the formation of moss-like deposits and dendrites [3]. Dendrites may eventually grow and pierce the separator, leading to short-circuits than could potentially trigger thermal runaway. Due to the critical impact of Li plating on LIB’s performance and safety, several studies have focused on this topic [4], [5], aiming to further elucidate its effects and provide a proper method for its detection. Despite the recent improvements in this field, to our best knowledge, an advanced, in situ and cost-effective technique to detect and quantify Li plating still remains to be presented. In this study, we will show the analysis to operando estimate and quantify Li plating on a commercial Graphite‖LiFePO4 cell. The cells were tested at ambient temperature (23 ºC) using a stressful – yet realistic – long term cycling testing scheme. First, we coupled incremental capacity (IC) and peak area (PA) analyses to identify and quantify the presence of reversible Li plating from a new IC peak, 0, that eventually emerged after cycle 600 (see Fig. 1). Then, we studied the nature of Li plating origins, to observe that gradual cell degradation, and not a sporadic event, lead to Li plating. To conclude, mechanistic model simulations (‘alawa toolbox with harvested half-cell data) allowed us to identify the ongoing aging modes, estimate the reversible amount of Li plating and project half-cell degradation on each individual electrode throughout cycling (see Fig. 2). The results showed that large LAM on delithiated negative electrode (i.e., LAMdeNE) eventually caused cell imbalance, leading to overcharge the NE subsequently inducing Li plating. The prospect of obtaining these parameters online during cell monitoring in a battery system operation creates remarkable benefits to improve battery management system (BMS) function for battery diagnosis performance. References [1] J. Vetter, P. Novák, M. R. Wagner, C. Veit, K.-C. Möller, J. O. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler, and A. Hammouche, “Ageing mechanisms in lithium-ion batteries,” J. Power Sources, vol. 147, no. 1–2, pp. 269–281, Sep. 2005. [2] M. C. Smart and B. V. Ratnakumar, “Effects of electrolyte composition on lithium plating in lithium-ion cells,” J. Electrochem. Soc., vol. 158, no. 4, p. A379, 2011. [3] M. Dollé, L. Sannier, B. Beaudoin, M. Trentin, and J.-M. Tarascon, “Live scanning electron microscope observations of dendritic growth in lithium/polymer cells,” Electrochem. Solid-State Lett., vol. 5, no. 12, p. A286, 2002. [4] J. C. Burns, D. a. Stevens, and J. R. Dahn, “In-Situ Detection of Lithium Plating Using High Precision Coulometry,” J. Electrochem. Soc., vol. 162, no. 6, pp. A959–A964, 2015. [5] M. Petzl and M. A. Danzer, “Nondestructive detection, characterization, and quantification of lithium plating in commercial lithium-ion batteries,” J. Power Sources, vol. 254, pp. 80–87, May 2014. Fig. 1. Incremental capacity evolution of the test cells at C/25 Fig. 2. Schematic representation of the Graphite‖LFP test cell, showing the evolution with cycle aging of the simulated results Figure 1

Vehicle-to-grid (V2G) and grid-to-vehicle (G2V) strategies are considered to help stabilize the electric grid but their true impact on battery degradation is still unknown. The intention of this study is to test the impact of such strategies on the degradation of commercial Li-ion batteries. This first part looks into the preliminary testing performed prior to the start of degradation studies to ensure that the selected cells are compatible. Both the thermodynamic and kinetic cell-to-cell variation within the selected batch and the diagnostic-ability of the cells were investigated. The cells were found to have low cell-to-cell variations and are thus consistent. Moreover, the emulation of the full cell from the half-cell data prepared from harvested electrodes was successful and the degradation forecast showed that the main degradation modes can be differentiated.

The Hawai‘i Natural Energy Institute (HNEI) is leading a team engaged in the research, development, deployment, and analysis of grid-scale battery energy storage systems (BESS) that are designed for system control and power quality support at the generation, transmission, and distribution levels. The program aims to identify high value BESS applications at various system levels, develop control algorithms that maximize the benefit to the grid/customer and the lifetime of the BESS, and evaluate and optimize those algorithms under real world operating conditions. The focus is to deploy, operate, and validate the performance of four grid-scale BESS for various ancillary service applications on grid systems across the state. Large scale battery energy storage systems will become an important part of the electric grid in the near future and it will be essential to ensure their reliability. The objective of this project is to understand the degradation of the individual batteries to anticipate failures. Laboratory testing of advanced Li-ion battery cells is performed to support life-time analysis of technologies targeted for large-scale grid energy storage applications. HNEI’s battery testing efforts have focused on grid scale deployment lithium ion titanate battery technology which is successfully being used for a variety of purposes including frequency and power regulation of large scale wind and solar energy generation. There is currently a lack of understanding of long term performance of battery technology in general, and specifically lithium ion titanate, used under large scale, grid conditions. This work aims to fill that gap in knowledge through laboratory scale battery testing and the development predictive lifetime models of performance of grid battery technology. Accelerated testing of identical lithium ion titanate battery technology was performed in the laboratory and those results will be used to develop predictive performance models. This model will combine existing modules [1-4] that account for single cell asymmetric degradation, single cell heat generation, string imbalance, and cell paralleling. As real world data is collected from the grid batteries, the predictive models will be compared and assessed for accuracy and ability to predict performance. This work will present preliminary results of the cycle and calendar aging long-term studies. [1] M. Dubarry, N. Vuillaume and B. Y. Liaw, J. Power Sources 186(2), (2009) 500-507. [2] M. Dubarry, C. Truchot and B.Y. Liaw, J. Power Sources 219 (2012) 204-216. [3] https://www.soest.hawaii.edu/HNEI/alawa/ [4] M. Dubarry, C. Truchot, A. Devie and B. Y. Liaw, J. Electrochem. Soc 162(6), (2015) A877-A884.

Lithium-ion batteries are attractive for vehicle electrification or grid modernization applications. In these applications, battery packs are required to have multiple-cell configurations and battery management system to operate properly and safely. Here, a useful equivalent circuit model was developed to simulate the spontaneous transient balancing currents among parallel strings in a battery system. The simulation results were validated with experimental data to illustrate the accuracy and validity of the model predictions. Understanding the transient behavior of such cell and string balancing in a parallel circuit configuration is very important to assess the impacts of current fluctuation and cell variability on a battery system's performance, regarding durability, reliability, safety, abuse tolerance and failure prevention, including possible short circuit or open circuit conditions. Additional features and advantages, including the ability to assessing impacts on the performance of the string assemblies from string swapping or cell/module replacement in the strings, could be realized to aid battery management, maintenance and repair.

Battery degradation is extremely sensitive to usage and chemistry. Some batteries may be markedly sensitive to temperature, to state of charge, to both, or to some other factors. This raises concerns over battery durability in the rollout of electric vehicles (EVs) in hot climate. Additionally, with the integration of more and more intermittent renewable energy power plants on the grid, there is a push to use EV batteries as energy storage systems which may stress the batteries even more. In most studies on the techno-economical impact of EVs on the grid, the battery is often viewed as a black box and therefore there is no real understanding on the actual long term impact of climate and/or vehicle-to-grid (V2G) and grid-to-vehicle (G2V) charging profiles on batteries. This work aims at assessing such impact. Daily vehicle usage can be broken down into driving, charging and idling periods. In most cases, car batteries spend most of their time idling and it is therefore essential to understand the degradation associated with time, temperature and state of charge on top of the impact of the duty cycles. Herein, we studied the impact of driving on the cell degradation with or without V2G and G2V episodes by looking at both the calendar-induced and cycling-induced degradation. All the data was analyzed using HNEI’s unique diagnosis and prognosis tools [1-3] to forecast the degradation over the battery warranty period and beyond. This research supports the goals of the Electric Vehicle Transportation Center. [1] M. Dubarry, C. Truchot and B.Y. Liaw, J. Power Sources 219 (2012) 204-216 [2] M. Dubarry, A. Devie and B.Y. Liaw, J. Energy Power Sources 1(5) (2014) 242-249 [3] https://www.soest.hawaii.edu/HNEI/alawa/

The Hawai‘i Natural Energy Institute (HNEI) is leading research efforts to understand the degradation of lithium-ion batteries under two distinct projects. The first research project is focused on grid-scale battery energy storage systems (BESS) while the second project is targeting electric vehicles (EVs) and their synergy with the grid. Both applications require a combination of long cycle-life (1000 cycles or more) and long shelf-life (10 to 20 years in operation) to meet the expectations of the customers. To determine whether these durability goals are realistic or not, we performed accelerated testing of different lithium-ion battery technologies in the laboratory. For each technology, we conducted a series of cycle and calendar aging experiments. The results of these studies will be used to develop predictive performance models. The concept of accelerated aging is only valid if the degradation the cell underwent is the same than of the one it experienced in real life. In this presentation, we want to share our preliminary observations regarding the differences between cycle aging and calendar aging across the different chemistries and across the industries (EVs, BESS). Our primary objective is to showcase the different degradation pathways, whether calendar-driven or cycle-driven, which can lead to identical capacity losses. This concept is better known as path dependence and this talk will highlight its implications for the design of long-life battery systems, the laboratory testing it requires and the validity of accelerated testing strategies. [1] M. Dubarry, N. Vuillaume and B. Y. Liaw, J. Power Sources 186(2), (2009) 500-507. [2] M. Dubarry, C. Truchot and B.Y. Liaw, J. Power Sources 219 (2012) 204-216. [3] https://www.soest.hawaii.edu/HNEI/alawa/ [4] M. Dubarry, C. Truchot, A. Devie and B. Y. Liaw, J. Electrochem. Soc 162(6), (2015) A877-A884.

One of the major issues hampering the acceptance of electric vehicles (EVs) is the anxiety associated with long charging time. Hence, the ability to fast charging lithium-ion battery (LIB) systems is gaining notable interest. However, fast charging is not tolerated by all LIB chemistries because it affects battery functionality and accelerates its aging processes. Here, we investigate the long-term effects of multistage fast charging on a commercial high power LiFePO4-based cell and compare it to another cell tested under standard charging. Coupling incremental capacity (IC) and IC peak area analysis together with mechanistic model simulations (‘Alawa’ toolbox with harvested half-cell data), we quantify the degradation modes that cause aging of the tested cells. The results show that the proposed fast charging technique caused similar aging effects as standard charging. The degradation is caused by a linear loss of lithium inventory, coupled with a less degree of linear loss of active material on the negative electrode. This study validates fast charging as a feasible mean of operation for this particular LIB chemistry and cell architecture. It also illustrates the benefits of a mechanistic approach to understand cell degradation on commercial cells.