High Charge States Research Articles

Enhanced Electrochemical Performance of Disordered Rocksalt Cathodes Enabled by a Graphite Conductive Additive.

Cobalt-free cation-disordered rocksalt (DRX) cathodes are a promising class of materials for next-generation Li-ion batteries. Although they have high theoretical specific capacities (>300 mA h/g) and moderate operating voltages (∼3.5 V vs Li/Li+), DRX cathodes typically require a high carbon content (up to 30 wt %) to fully utilize the active material which has a detrimental impact on cell-level energy density. To assess pathways to reduce the electrode's carbon content, the present study investigates how the carbon's microstructure and loading (10-20 wt %) influence the performance of DRX cathodes with the nominal composition Li1.2Mn0.5Ti0.3O1.9F0.1. While electrodes prepared with conventional disordered carbon additives (C65 and ketjenblack) exhibit rapid capacity fade due to an unstable cathode/electrolyte interface, DRX cathodes containing 10 wt % graphite show superior cycling performance (e.g., reversible capacities ∼260 mA h/g with 85% capacity retention after 50 cycles) and rate capability (∼135 mA h/g at 1000 mA/g). A suite of characterization tools was employed to evaluate the performance differences among these composite electrodes. Overall, these results indicate that the superior performance of the graphite-based cathodes is largely attributed to the: (i) formation of a uniform graphitic coating on DRX particles which protects the surface from parasitic reactions at high states of charge and (ii) homogeneous dispersion of the active material and carbon throughout the composite cathode which provides a robust electronically conductive network that can withstand repeated charge-discharge cycles. Overall, this study provides key scientific insights on how the carbon microstructure and electrode processing influence the performance of DRX cathodes. Based on these results, exploration of alternative routes to apply graphitic coatings is recommended to further optimize the material performance.

Understanding the boundary and mechanism of gas-induced explosion for lithium-ion cells: Experimental and theoretical analysis

Thermal runaway (TR) of lithium-ion (Li-ion) batteries (LIBs) involves multiple forms of hazards, such as gas venting/jetting, fire, or even explosion. Explosion, as the most extreme case, is caused by the generated flammable gases, and a deflagration to detonation transition (DDT) may occur in this process. Here, overheat-to-TR tests and the corresponding outgas-induced explosion tests were conducted on 42 Ah Li-ion cells with Li[Ni1/3Co1/3Mn1/3]O2 cathode. The sum of CO2, H2, C2H4, CO, and CH4 accounted for more than 90% of the gases. Lower/upper explosion limits (LEL/UEL), laminar flame speed, and ideal stable detonation pressure were calculated to interpret the explosion characteristics and boundary. It turned out that shockwave was easily to be compressed and accelerated under higher state of charge (SOC) conditions. Thus, Li-ion cells explosion may evolve into unstable detonation in encapsulated battery pack and its evolution mechanism was explained, which provides a new idea for explosion-proof design of LIBs system. Additionally, a comprehensive assessment method was developed to intuitively characterize TR hazards. Severity of explosion presented an upward trend with the increase of SOC while the sensitivity was not the same. This study provides a further anatomy of TR, which is instructive to the safety of power battery systems.

Highlighting the Advantages of Operating NMC811 Cells to Voltages below 4.20 V Compared to NMC Grades with Lower Ni Content

Ni-rich Li-ion cells suffer from severe capacity fade at high states of charge (SOC) due to oxygen release and/or lattice volume collapse. We share long-term cycling results (12000 h) at 40 °C for NMC811, NMC532, and NMC640 cells cycled with different upper cut-off voltages (UCV), C-rates, and depths of discharge (DOD). We show that cycling to a greater UCV results in more capacity loss and resistance growth as expected. However, NMC811 suffered from 5–10 times greater resistance growth at 4.20 V compared to 4.06 V, while the resistance for NMC532 and NMC640 cycled to 4.30 V was roughly twice that of the 4.10 V UCV cells. This highlights a challenging problem with Ni-rich materials that can be mitigated if the UCV is limited to avoid NMC811 volume change. Finally, we used competitive graphite materials reported in previous work to examine the impact of UCV on the rate of SEI growth in NMC811 cells, which was found to increase with temperature and UCV, particularly at 4.08 V UCV. We hope this brief report will highlight the importance of lifetime energy output, rather than initial energy density, and encourage battery users to limit the UCV of Ni-rich cells when possible.

Ageing of High Energy Density Automotive Li-Ion Batteries: The Effect of Temperature and State-of-Charge

Lithium ion batteries (LIB) have become a cornerstone of the shift to electric transportation. In an attempt to decrease the production load and prolong battery life, understanding different degradation mechanisms in state-of-the-art LIBs is essential. Here, we analyze how operational temperature and state-of-charge (SoC) range in cycling influence the ageing of automotive grade 21700 batteries, extracted from a Tesla 3 long Range 2018 battery pack with positive electrode containing LiNixCoyAlzO2 (NCA) and negative electrode containing SiOx-C. In the given study we use a combination of electrochemical and material analysis to understand degradation sources in the cell. Herein we show that loss of lithium inventory is the main degradation mode in the cells, with loss of material on the negative electrode as there is a significant contributor when cycled in the low SoC range. Degradation of NCA dominates at elevated temperatures with combination of cycling to high SoC (beyond 50%).

Phase behavior tuning enable high-safety and crack-free Ni-rich layered cathode for lithium-ion battery

Ni-rich layered cathodes suffer from rapid performance failure and structure degradation due to their complicated phase behavior during cycling. We demonstrate here a high-safety and crack-free Ni-rich cathode by tuning its phase behavior, through substituting a small-dose of Nb, which can induce an artificial surface cation mixing layer owing both high Ni migration barrier and stabilized oxygen framework. The irreversible phase transformation propagation is turned off after Nb doping, and the two-phase reaction during H2-H3 phase transition process transforms to a quasi-monophase reaction with minimal lattice mismatch and avoided interstress concentration, thus achieving a Nb doped LiNi0.9Co0.1O2 with outstanding capacity retention, a hysteretic onset exothermic temperature at high state of charge, and indistinguishable microcrack after cycling.

Improved Battery Balancing Control Strategy for Reconfigurable Converter Systems

In order to address the issue of battery cell disparity in lithium-ion battery systems, battery balancing techniques are required. This paper proposes an improved battery balancing strategy within a reconfigurable converter system. The strategy is based on the state of charge (SOC) of batteries, and utilizes the reconfigurable converter system to transfer energy from battery modules with high SOC to those with lower SOC. Additionally, it allows for battery module balancing while supplying power to loads. A MATLAB/Simulink simulation model with five batteries was built to validate the effectiveness of the proposed balancing strategy under unloaded and loaded conditions. The simulation results demonstrate that the proposed strategy achieves more efficient and accurate battery module balancing compared to the previous balancing modes.

The Role of Transition Metals on Chemo‐Mechanical Instabilities in Prussian Blue Analogues For K‐Ion Batteries: The Case Study on KNHCF Versus KMHCF

AbstractPrussian blue analogues (PBAs) cathodes can host diverse monovalent and multivalent metal ions due to their tunable structure. However, their electrochemical performance suffers from poor cycle life associated with chemo‐mechanical instabilities. This study investigates the driving forces behind chemo‐mechanical instabilities in Ni‐ and Mn‐based PBAs cathodes for K‐ion batteries by combining electrochemical analysis, digital image correlation, and spectroscopy techniques. Capacity retention in Ni‐based PBA is 96% whereas it is 91.5% for Mn‐based PBA after 100 cycles at C/5 rate. During charge, the potassium nickel hexacyanoferrate (KNHCF) electrode experiences a positive strain generation whereas the potassium manganese hexacyanoferrate (KMHCF) electrode undergoes initially positive strain generation followed by a reduction in strains at a higher state of charge. Overall, both cathodes undergo similar reversible electrochemical strains in each charge–discharge cycle. There is ~0.80% irreversible strain generation in both cathodes after 5 cycles. XPS studies indicated richer organic layer compounds in the cathode‐electrolyte interface (CEI) layer formed on KMHCF cathodes compared to the KNHCF ones. Faster capacity fades in Mn‐based PBA, compared to Ni‐based ones, is attributed to the formation of richer organic compounds in CEI layers, rather than mechanical deformations. Understanding the driving forces behind instabilities provides a guideline to develop material‐based strategies for better electrochemical performance.

Fragment Ion Abundance Reveals Information about Structure and Charge Localization in Highly Charged Proteins.

Top-down mass spectrometry (MS) is a versatile tool that has been employed to investigate both protein sequence and structure. Although a variety of different fragmentation methods are available in top-down MS that can potentially yield structural information, quantifying differences between spectra remains challenging. Herein, we show that subtle differences in spectra produced by a variety of fragmentation methods are surprisingly sensitive to protein structure and/or charge localization, even in highly unfolded proteins observed in high charge states. In addition to exposing information about the protein structure, differences in fragmentation also reveal insight into the mechanisms underlying the dissociation methods themselves. The results further reveal that small changes in experimental parameters (such as the addition of methanol instead of acetonitrile) lead to changes in structure that are reflected in statistically reproducible differences in dissociation. Collisional annealing of structurally dissimilar ions in the gas phase eventually leads to dissociation spectra that are indistinguishable, suggesting that structural differences can be erased by sufficient thermal activation. Additional experiments illustrate that identical charge states of the same protein can be distinguished if those produced directly by electrospray are compared to ions manipulated by in vacuo proton-transfer charge reduction. Overall, the results show that subtle differences in both three-dimensional structure and charge-site localization can influence the abundance of fragment ions produced by top-down MS, including dissociation methods not typically thought to be structurally sensitive.

Effect of Regenerative Braking on Battery Life

It is a well-known fact that automotive industries in every country are shifting towards electric vehicles (EVs) and in the days to come it is expected that the industry will become dominated by them, along with hybrid electric vehicles (HEVs). Unfortunately, the acceptance of EVs for mobility is affected by its poor range per charge. Thus, energy optimization and waste energy recuperation are currently in need. A promising method to recover energy that is lost during vehicle deceleration is regenerative braking, which extends the range of a vehicle by recovering the kinetic energy from braking and using it to recharge the battery. However, the intensity of the charging–discharging rate and the operating temperature of lithium–ion (Li–ion) batteries make them vulnerable to failure, making the rate of current delivered to the battery by regenerative braking a serious concern. Therefore, the focus of this review article is on how regenerative braking affects battery life and the precautions being taken to safeguard the battery against increased charge during regenerative braking. In this review paper, various research articles are referred to in order to examine how regenerative braking affects battery life. It is concluded that charging current obtained from long-term regenerative braking is the prominent factor in battery deterioration, regardless of the current intensity. Additionally, the rate of lithium plating is increased if the temperature and state of charge (SOC) are outside of the ideal range. By lowering the depth of discharge (DOD) and using shorter recharging times, higher levels of regenerative braking will extend a battery’s lifecycle even at high SOC and temperature.

In operando detection of Lithium plating via electrochemical impedance spectroscopy for automotive batteries

Reducing the charging time is one of the main goals towards increasing the acceptance of electric vehicles. One of the limits of fast charging lithium-ion batteries with graphite-based anodes is lithium plating, a process which occurs mainly at high charging rates, low temperatures and high state of charge. Detecting the onset of lithium plating during a charging procedure is essential in order to optimize fast-charging profiles. This paper introduces a new method that allows the detection of the onset of lithium plating in automotive lithium-ion batteries using on-line electrochemical impedance spectroscopy. Compared to similar approaches, the measurement itself does not result in a significant increase in the charging time. The detection is based on observing the real part of the impedance across the state of charge during the charging procedure. The onset of lithium plating can be detected by a deviation from the nominal behavior in the form of a drop towards lower impedance. The behavior is both explained theoretically using electrochemical equivalent circuits and shown experimentally. The impedance measurements are carried out using an integrated small-scale device to allow on-line impedance analysis in an automotive environment and an integration into future battery management systems.

Aging Property of Halide Solid Electrolyte at the Cathode Interface.

Halide solid electrolytes have recently emerged as a promising option for cathode-compatible catholytes in solid-state batteries (SSBs), owing to their superior oxidation stability at high voltage and their interfacial stability. However, their day- to month-scale aging at the cathode interface has remained unexplored until now, while its elucidation is indispensable for practical deployment. Herein, the stability of halide solid electrolytes (e.g., Li3 InCl6 ) when used with conventional layered oxide cathodes during extended calendar aging is investigated. It is found that, contrary to their well-known oxidation stability, halide solid electrolytes can be vulnerable to reductive side reactions with oxide cathodes (e.g., LiNi0.8 Co0.1 Mn0.1 O2 ) in the long term. More importantly, the calendar aging at a low state of charge or as-fabricated state causes more significant degradation than at a high state of charge, in contrast to typical lithium-ion batteries, which are more susceptible to high-state-of-charge calendar aging. This unique characteristic of halide-based SSBs is related to the reduction propensity of metal ions in halide solid electrolytes and correlated to the formation of an interphase due to the reductive decomposition triggered by the oxide cathode in a lithiated state. This understanding of the long-term aging properties provides new guidelines for the development of cathode-compatible halide solid electrolytes.

Multielectron coincidence spectroscopy of the Ar2+(2p−2) double-core-hole decay

The dominant decay pathways of argon $2{p}^{\ensuremath{-}2}$ double-core-hole states have been investigated using synchrotron radiation and a magnetic-bottle-type spectrometer coupled with an ion time-of-flight spectrometer. This experiment allows for efficient multi-electron-ion coincidence measurements, and thus for following the Auger cascade step by step in detail. Dominant decay pathways leading to ${\mathrm{Ar}}^{4+}$ final states via ${\mathrm{Ar}}^{3+}$ intermediate states have been assigned with the help of theoretical ab initio calculations. The weak correlated decay of the two core holes by emission of a single Auger electron, leading to ${\mathrm{Ar}}^{3+}$ final states, has been observed at 458.5-eV kinetic energy. Compared to the total decay of the $2{p}^{\ensuremath{-}2}$ double core vacancies, this two-electron--one-electron process was measured to have a branching ratio of $1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ifmmode\pm\else\textpm\fi{}1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. Furthermore, the remaining decay paths of the ${\mathrm{Ar}}^{1+}\phantom{\rule{0.28em}{0ex}}(1{s}^{\ensuremath{-}1})$ core hole to higher charge states and their respective contributions to the total yield have been analyzed and show very good agreement with theoretical results.

Model predictive control of a microgrid with energy-stored quasi-Z-source cascaded H-bridge multilevel inverter and PV systems

This paper presents a new energy management system (EMS) based on model predictive control (MPC) for a microgrid with solar photovoltaic (PV) power plants and a quasi-Z-source cascaded H-bridge multilevel inverter that integrates an energy storage system (ES-qZS-CHBMLI). The system comprises three modules, each with a PV power plant, quasi-impedance network, battery energy storage system (BESS), and voltage source inverter (VSI). Traditional EMS methods focus on distributing the power among the BESSs to balance their state of charge (SOC), operating in charging or discharging mode. The proposed MPC-EMS carries out a multi-objective control for an ES-qZS-CHBMLI topology, which allows an optimized BESS power distribution while meeting the system operator requirements. It prioritizes the charge of the BESS with the lowest SOC and the discharge of the BESS with the highest SOC. Thus, both modes can coexist simultaneously, while ensuring decoupled power control. The MPC-EMS proposed herein is compared with a proportional sharing algorithm based on SOC (SOC-EMS) that pursues the same objectives. The simulation results show an improvement in the control of the power delivered to the grid. The Integral Time Absolute Error, ITAE, achieved with the MPC-EMS for the active and reactive power is 20 % and 4 %, respectively, lower than that obtained with the SOC-EMS. A 1,3 % higher charge for the BESS with the lowest SOC is also registered. Furthermore, an experimental setup based on an OPAL RT-4510 unit and a dSPACE MicroLabBox prototyping unit is implemented to validate the simulation results.

A Hierarchical Identification Method for Lithium-Ion Battery SOC Based on the Hammerstein Model

Two-input one-output Hammerstein model consists of two parallel nonlinear static blocks followed by a linear dynamic part. By using Hammerstein structure to map relation between a battery State of Charge (SOC) and its terminal voltage/current, a hierarchical stochastic gradient algorithm is studied to estimate parameters of Hammerstein SOC model, so as to predict battery SOC. Firstly, the Hammerstein model is transformed into a bilinear parameter system with the least number of required parameters. Then, a hierarchical stochastic gradient algorithm with a forgetting factor is used to update the two sets of model parameters of the bilinear parameter system, so as to realize SOC estimation. Furthermore, the experiment platform of lithium-ion battery was built and the data of the urban dynamometer driving schedule (UDDS) profile and the Los Angeles 92 (LA92) profile were collected. Finally, the MATLAB simulation results show that the proposed parameter optimized method based Hammerstein model has the advantages of fast convergence speed and high SOC estimation accuracy.

Three-body fragmentation dynamics of BrCNq+ (q=3–6) induced by 1-keV electron impact

The three-body fragmentation dynamics of $\mathrm{BrC}{\mathrm{N}}^{q+} (q=3--6)$ produced by 1-keV electron impact has been investigated using the ion momentum imaging technique. Up to 11 three-body Coulomb explosion channels were identified and analyzed. The corresponding kinetic energy release (KER) distributions were obtained and compared with the predictions of the Coulomb explosion model. By means of the Dalitz plot, Newton diagram, and native frame method, we have studied the concerted and sequential fragmentation mechanisms for channels leading to ${\mathrm{Br}}^{l+}+{\mathrm{C}}^{+}+{\mathrm{N}}^{+} (l=1--3)$. The KER for the intermediate dications $\mathrm{Br}{\mathrm{C}}^{2+}$ and $\mathrm{C}{\mathrm{N}}^{2+}$ from the sequential mechanism were determined and the electronic states of the intermediate molecular ion $\mathrm{C}{\mathrm{N}}^{2+}$ were discussed. For the channels leading to higher charge states of the carbon and nitrogen ions, only the concerted fragmentation mechanism was observed.

Characterization of fire behaviors associated with a thermal runaway in large-scale commercial LiNi0.8Co0.1Mn0.1O2/graphite cells under external ignition

The fire behaviors of large-format commercial batteries at varying states of charge (SOCs) under an external ignition source were experimentally studied. The thermal runaway of the battery mainly starts in the temperature interval of 470.1–526.5 K. The trigger time and temperature of the thermal runaway present a relatively stronger decreasing tendency with an increase in SOC. This implies that the thermal stability of the battery decreases with SOC. The maximum flame temperature is found in the temperature range of 1062.5–1492.0 K, which depends on the combustion efficiency and flame shape. Two peaks of the observed heat release rate (HRR) show that the battery undergoes two intense burning processes at a low SOC. There is no monotonically increasing relationship between total heat release (THR) and SOC. However, the remaining mass of the battery decreases as SOC increases. This indicates that the materials eject fiercely so that some of them are extremely fast to be ignited, resulting in insufficient combustion, particularly for a battery at a high SOC. The gas emission and physicochemical analysis of residue can support this point. The fire hazards involving thermal risk and gas toxicity are also evaluated and discussed.

Capacity Degradation and Aging Mechanisms Evolution of Lithium-Ion Batteries under Different Operation Conditions

Since lithium-ion batteries are rarely utilized in their full state-of-charge (SOC) range (0–100%); therefore, in practice, understanding the performance degradation with different SOC swing ranges is critical for optimizing battery usage. We modeled battery aging under different depths of discharge (DODs), SOC swing ranges and temperatures by coupling four aging mechanisms, including the solid–electrolyte interface (SEI) layer growth, lithium (li) plating, particle cracking, and loss of active material (LAM) with a P2D model. Additionally, the mechanisms causing accelerated capacity to drop near a battery’s end of life (EOL) were investigated systematically. The results indicated that when the battery operated with a high SOC range, the capacity was more prone to accelerated degradation near the EOL. Among the four degradation mechanisms, li plating was mainly sensitive to the operation temperature and SOC swing ranges, while the SEI growth was mainly sensitive to temperature. Furthermore, there was an inhibitory interaction between li plating and SEI growth, as well as positive feedback between LAM and particle cracking during battery aging. Additionally, we discovered that the extremely low local porosity around the anode separator could cause the ‘knee point’ of capacity degradation.

On Proton Defects and the Phase Transformation of NiO2

One approach to high-capacity Li-ion battery cathodes is to increase the Ni content. Unfortunately, Ni-rich materials undergo a phase transition and volume collapse at a high state of charge (SOC), which degrades battery performance. At a high SOC, NiO2 undergoes a phase transition from the O3 to the O1 phase. The O1 phase appears to be more thermodynamically stable than the O3 phase. However, density functional theory (DFT) often predicts higher stability for O3, depending on the DFT approximation. The energy difference between the O1 and O3 phases is on the order of 10 meV/atom. We find that proton defects are more stable in the O1 phase than in the O3 phase by about 90 meV per proton. Defects in NiO2 energetically favor the O3–O1 phase transition observed in a high SOC in LixNiO2.

A novel combined online method for SOC estimation of a Li-Ion battery with practical and industrial considerations

Estimation of the state of charge (SOC) in the battery management systems (BMS) is very crucial. However, the BMS consistently suffers from inaccuracy of SOC estimation and high computational burden in industrial applications. Herein, we propose a novel combined SOC estimation approach with high accuracy, simplicity in implementation and, robustness that can be implemented on the low-cost microcontrollers. The basis of the method is to online identification the open circuit voltage (OCV) value of the battery simultaneously with other unknown parameters of the battery equivalent circuit model (ECM). Using this identified OCV, SOC is extracted from the OCV-SOC curve, directly. This direct method is an open-loop estimation method and tends to get lower SOC accuracy compared with known methods. To overcome this disadvantage, a combined method is proposed. Hence, to estimate SOC in this method, in the middle part of the OCV-SOC curve (linear part) and after identifying OCV from recursive least square algorithm (RLS) method with a forgetting factor, SOC is estimated directly via OCV-SOC curve. Furthermore, in the other parts of the curve especially non-linear parts, SOC is estimated with the EKF algorithm. In summary, not only estimation accuracy will increase, but also the computational burden of the EKF will decrease. In this paper, voltage and current samples of the battery at each sample time are the inputs of the algorithms and gained from the known “LA-92 dynamic driving cycle” test. To validate the efficiency of the presented method, results are compared with the EKF algorithm and direct method. The experimental results show that the proposed method reduces the EKF calculations by almost 15 % and guarantee high accuracy SOC estimation. The maximum error of the proposed method is <2.5 %.

A direct method to quantify lithium plating on graphite negative electrode of commercial Li-ion cells

The deposition of metallic lithium is a degradation mechanism, also known as lithium plating, that might occur at the negative electrode surface of Li-ion battery cell, especially during fast charging, low temperature charging and high state of charge. The loss of cyclable lithium may lead to irreversible capacity fade and might also bring significant safety hazards. In this context, the experimental quantification of plated lithium on the negative electrode surface could be used to accurately validate electrochemical and physical models that might be used to predict plated lithium without cell opening. Also, the determination of plated Li may help in the evaluation of the safety and aging of commercial cells from different manufacturers for a specific application. This work shows a fast and cost-effective method that can be used to quantify plated lithium on commercial Li-ion battery graphite electrodes.