Lithium-ion Cells Research Articles

Advanced Balancing of High-Energy Lithium Ion Cells Comprising Lithium-Rich Layered Oxide and a-Si/CuSi Nanowire Using a Cathode Pre-Lithiation Additive

Abstract Advances in lithium-ion battery (LIB) research have strived for high-energy, safe, and sustainable materials. Li-rich Li1.2Ni0.2Mn0.6O2 (LRNM) cathodes have shown great promise with high voltages and excellent specific capacities. Obstacles preventing the viability and commercialization of LRNM are the initial capacity loss of roughly 30% and rapid voltage decay upon cycling. Herein, lithium oxalate (Li2C2O4) is investigated as a pre-lithiation additive to utilize the first-cycle lithium re-intercalation losses of LRNM to compensate the irreversible lithium consumption in graphite and high-capacity a-Si on copper silicide nanowire (a-Si/CuSi NW) anodes. Specifically, the decomposition process of Li2C2O4 as well as the interaction between the electrode components inside the cell are comprehensively examined. This concept allows us to extend the cycle life of graphite||LRNM cells at 1C from less than 500 cycles (142 mAh g-1, 78%) to more than 900 cycles (143 mAh g-1, 82%) before reaching the 80% capacity retention threshold. Finally, LRNM electrodes with a precisely balanced concentration of Li2C2O4 are prepared in order to compensate the high irreversible losses of a-Si/CuSi NW anodes, achieving a capacity retention of almost 50% with a remaining specific capacity of 82 mAh g-1 after 400 cycles in high-energy a-Si/CuSi NW||LRNM lithium-ion cells.

Modeling Thermal Runaway Mechanisms and Pressure Dynamics in Prismatic Lithium-Ion Batteries

Lithium-ion batteries play a vital role in modern energy storage systems, being widely utilized in devices such as mobile phones, electric vehicles, and stationary energy units. One of the critical challenges with their use is the thermal runaway (TR), typically characterized by a sharp increase in internal pressure. A thorough understanding and accurate prediction of this behavior are crucial for improving the safety and reliability of these batteries. To achieve this, two new combined models were developed: one to simulate the thermal runaway and another to simulate the internal cell pressure. The thermal model tracks a chain of decomposition reactions that eventually lead to TR. At the same time, the pressure model simulates the proportional increase in pressure due to the evaporation of the electrolyte and the gases produced from the decomposition reactions. What sets this work apart is the validation of the pressure model through experimental data, specifically for prismatic lithium-ion cells using NMC chemistries with varying stoichiometries—NMC111 and NMC811. While the majority of the literature focuses on the simulation of temperature and pressure for cylindrical cells, studies addressing these aspects in prismatic cells are much less common. This article addresses this gap by conducting pressure validation experiments, which are hardly documented in the existing studies. Furthermore, the model’s accuracy and flexibility are tested through two experiments, conducted under diverse conditions to ensure robust and adaptive predictions of cell behavior during failure scenarios.

Comprehensive Analysis of Parametric Effects on the Specific Heat Capacity of Pristine and Aged Lithium-Ion Cells

The precise determination of the specific heat capacity of lithium-ion cells is essential for thermal management design. Though, varying influences and insufficient parametric analyses are found in the literature. Therefore, a simple and inexpensive measurement setup is utilized to measure the specific heat capacity of cells independent of their format and dimensions. A comprehensive parametric analysis is performed assessing the effect of cell casing, cell chemistry, temperature, state-of-charge, and state-of-health. For the first time ever, a predictive analysis on material level is conducted allowing for understanding the individual factors in detail. Thus, an analytical approach for calculating the specific heat capacity can be validated by comparing predictive values to experimental data for the first time. It is found that the cell format has a significant influence on the specific heat capacity due to varying mass fractions and housing materials. Furthermore, the cell chemistry and corresponding layer thicknesses are of high importance, too. By selecting specific heat capacities for individual materials from the general literature, the analytical prediction matches the experimental data and is thus validated for the first time ever. Moreover, temperature has a positive linear effect on the specific heat capacity which can increase by up to 15% over the operating range. Furthermore, the positive temperature dependency improves the charging performance. Finally, neither SOC nor SOH significantly affect the specific heat capacity of lithium-ion cells.

Using X-ray Microscopy to Probe Failure Mechanisms in Anode-free Cells: An Industry Perspective

To meet the energy demands of future electric vehicle technologies, batteries with ever-increasing energy densities are desired. One promising technology is an anode-free lithium metal battery (AFLMB) cell, where lithium ions are deposited directly on the anode current collector, resulting in more energy dense cells relative to the current state-of-the-art lithium-ion battery cell. Nevertheless, anode-free cells are prone to early capacity degradation and cell failure. To better understand the degradation mechanisms in these devices, we present a methodology for assessing microstructural changes in battery cells that can be easily implemented within existing battery manufacturing facilities. We employed X-ray tomographic imaging and analyses on small format, AFLMB pouch cells. Anode thickness variations were characterized non-destructively by housing the pouch cells in fabricated pressurized jigs during both cycling and tomographic imaging. Additionally, we present a technique to measure cathode porosities and tortuosities at the end-of-life (EOL) with higher resolution X-ray imaging. The proposed methodology is able to accurately reproduce known microstructural behaviors in AFLMBs. At the anode, significant thickness changes are observed because of continuous electrolyte degradation and solid electrolyte interphase growth. At the cathode, large porosity changes are detected at the EOL, potentially owing to NCM (LiNixCoyMnzO2) particle cracking.

Analysis of cell balancing of Li-ion batteries with dissipative and non-dissipative systems for electric vehicle applications

This article describes the essential components of contemporary battery management systems (BMS), such as power electronics bidirectional charging and discharging, reverse protection against constant current and voltage, and Li-ion battery cell balancing, which is the process of introducing Li-ion The majority of domestic electrical applications, including e-cars, are powered by batteries. These qualities are crucial for ensuring the safety, longevity, and effectiveness of batteries when it comes to renewable energy sources and electric vehicles (EVs). Two other crucial BMS characteristics that protect against reverse current and overcharging destroying batteries are constant current and voltage reverse protection. Ensuring that every single cell in a battery is in balance is known as evenly charging and discharging every single cell. In-depth discussions of these characteristics' advantages and operation, as well as their uses in EVs, renewable energy systems, and other contexts, are provided in this article. It is seen from the analysis that the non-dissipative lithium-ion battery cell balancing strategy, which significantly enhances safety and efficiency, provides greater benefits than the dissipative balancing approach. The modelling of an SoC charge-controlled Li-Ion battery with an optimum battery voltage of 3.6 V. Its rated capacity of 4 Ah is considered a test cell that has contrasted dissipative and non-dissipative balancing in MATLAB/Simulink with five cells in the battery bulk. It is seen from the analysis that the non-dissipative lithium-ion battery cell balancing strategy provides greater benefits than the dissipative balancing approach.

Assessment of Internal Short-Circuit Risks in Overdischarged Lithium Batteries Under Various Charging Conditions

This study analyzes the internal short-circuit behavior of an overdischarged secondary lithium-ion cell (OD-cell) under subsequent charging conditions. The results revealed that Cu ions dissolved from the anode Cu substrate during overdischarge were initially deposited on the cathode, migrated back to the anode, and deposited there during subsequent charging. These Cu deposits on the anode maintained their shape throughout normal charge–discharge cycles. Post-charging destructive analysis of the OD-cells revealed that the morphology of the Cu deposits on the anode surface varied significantly with the charging current. Cu formed coarse spherical particles at low currents (< 0.2 C-rate). As the current increased, a mixture of spherical and dendritic structures appeared. Only dendritic structures were observed above a certain threshold (> 3 C-rate). The leakage currents in OD-cells charged at various rates were measured to calculate their internal short-circuit resistance (ISR), and the relationship between Cu morphology and self-discharge was investigated. The “ISR decrease rate” was defined to compare the calculated ISR with the ISR of a normal cell, which was used as an indicator of the internal short circuit of the cell. The OD-cells charged at low currents (< 0.2 C-rate) maintained an ISR similar to that of normal cells (ISR decrease rate < 10%) with minimal self-discharge. However, high-current charging (> 3 C-rate) significantly reduced the ISR (decrease rate of > 97%) and introduced severe self-discharge. These findings suggest that high-rate charging immediately after overdischarge may exacerbate the internal short circuit and self-discharge owing to Cu-bridge formation.

Elucidating effects of form factors on thermal and aging behavior of cylindrical lithium-ion batteries

Cylindrical lithium-ion cells are commercially available in different form factors, e.g., 18650, 21700, 26650, 32700 and 4680. The larger 4680 cells are gaining popularity, especially after the patent filing by Tesla. This paper aims to perform a comparative analysis of the thermal and aging behavior of different form factors using NMC–graphite chemistry during prolonged cycling at moderate operating conditions. A physics-based aging model for lithium-ion batteries is introduced, incorporating side reactions such as solid electrolyte interface (SEI) formation, SEI reformation due to graphite layer cracking, and lithium plating. During cycling, the cell temperature increases with time for all form factors due to internal heat generation. Notably, larger form factor cells experience higher temperatures, attributed to their inferior surface-to-volume ratio for heat dissipation, thereby accelerating both SEI formation and triggering lithium plating in subsequent cycles. This accelerated aging manifests in increased heat generation and prolonged charging for larger cells. The 4680 cell showed the highest rate of capacity loss with a 20% reduction over 480 cycles as compared to the 600 cycles of the 18650 cell. Highlighting accelerated aging in the larger cells due to suboptimal heat dissipation, this study underscores the pressing need for advanced thermal management strategies tailored to the unique challenges posed by larger cells.

Mechanical Characterization and Modeling of Large-Format Lithium-Ion Battery Cell Electrodes and Separators for Real Operating Scenarios

This study presents a novel application-oriented approach to the mechanical characterization and subsequent modeling of porous electrodes and separators in lithium-ion cells to gain a better understanding of their real mechanical operating behavior. An experimental study was conducted on the non-linear stiffness of LiNi0.8Co0.15Al0.05O2 and graphite electrodes as well as PE separators, harvested from large-format lithium-ion cells, using compression tests. The mechanical response of the components was determined for different operating conditions, including nominal stress levels, mechanical loading rates, and mechanical cycles. The presented work describes the test procedure, the experimental setup, and an objective evaluation method, allowing for a detailed summary of the observed mechanical behavior. A distinct nominal stress level and mechanical cycle dependency of the non-linear stiffnesses of the porous materials were found. However, no clear dependency on compression rate was observed. Based on the experimental data, a poroelastic mechanical model was utilized to predict the non-linear behavior of these porous materials under real mechanical operating scenarios with a normalized root-mean-squared error less than 5.5%. The results provide essential new insights into the mechanical behavior of porous electrodes and separators in lithium-ion cells under real operating conditions, enabling the accelerated development of high-performing and safe batteries for various applications.

State of health estimation of lithium-ion battery cell based on optical thermometry with physics-informed machine learning

Effective thermal management and accurate state of health (SOH) estimation of lithium-ion batteries is crucial for ensuring their safety, reliability, and longevity. This study presents three innovative physics-informed machine learning-based SOH estimation techniques trained and demonstrated using experimental temperature data. Temperature distribution measurements were obtained using optical frequency domain reflectometry with optical fibers embedded in a cylindrical lithium-ion battery cell under various SOH. One of the trained model accurately predicted the SOH of a cell within 2% with only a 10-minute measurement. This technique also enables the estimation of SOH for individual cells connected in series or parallel within a battery module or pack simultaneously, thereby reducing the overall SOH estimation uncertainty without the need for disassembly. Furthermore, this not only highlights the necessity of precise thermal management in maintaining battery health but also offers a practical and efficient solution for real-time SOH monitoring in battery systems.

Analysis of the Thermal Runaway Mitigation Performances of Dielectric Fluids During Overcharge Abuse Tests of Lithium-Ion Cells with Lithium Titanate Oxide Anodes

Lithium titanate oxide cells are gaining attention in electric vehicle applications due to their ability to support high-current charging and their enhanced thermal stability. However, despite these advantages, safety concerns, particularly thermal runaway, pose significant challenges during abuse conditions such as overcharging. In this study, we investigated the effectiveness of various dielectric fluids in mitigating thermal runaway during overcharge abuse tests of cylindrical LTO cells with a capacity of 10 Ah. The experimental campaign focused on overcharging fully charged cells (starting at 100% State of Charge) at a current of 40A (4C). The tests were conducted under two conditions: the first benchmark test involved a cell exposed to air, while the subsequent tests involved cells submerged in different dielectric fluids. These fluids included two perfluoropolyether fluorinated fluids (PFPEs) with boiling points of 170 °C and 270 °C, respectively, a synthetic ester, and a silicone oil. The results were analyzed to determine the fluids’ ability to delay possible thermal runaway and prevent catastrophic failures. The findings demonstrate that some dielectric fluids can delay thermal runaway, with one fluid showing superior performance with respect to the others in preventing fire during thermal runaway. The top-performing fluid was further evaluated in a simulated battery pack environment, confirming its ability to mitigate thermal runaway risks. These results provide important insights for improving the safety of battery systems in electric vehicles.

Operando visualisation of lithium plating by ultrasound imaging of battery cells.

While developing battery cells, the achievement of fast-charging capability is heavily dependent on avoiding metallic plating on the anode surface (i.e., lithium plating in lithium-ion cells). However, this objective hinges on the effectiveness of plating detection. Currently, measurement techniques are either inadequate in providing spatial, temporal, or causal information, incur high costs when employing, e.g., neutron imaging, or are lengthy due to destructive post-mortem examinations that additionally lack operando data. In this work, we demonstrate an ultrasound imaging method for operando visualization of the interior of a multi-layer pouch battery cell. Here we show that this method can non-invasively visualize the formation and stripping of lithium plating during cycling. Extensive reference electrode studies and ex-situ analysis verify the effectiveness of our method for plating detection. Ultimately, this work enables researchers and industry to significantly accelerate the development of new cell technologies and their optimized utilization.

Multiscale approaches for optimizing the impact of strain on Na-ion battery cycle life

Abstract The high costs and geopolitical challenges inherent to the lithium-ion (Li-ion) battery supply chain have driven a rising interest in the development of sodium-ion (Na-ion) batteries as a potential alternative. Unfortunately, the larger ionic radius of Na limits the reversibility of cycling because of the extensive atomic rearrangements that accompany Na-ion insertion, which in turn limit diffusion and charging speed, and lead to rapid degradation of the electrodes. The Center for Strain Optimization for Renewable Energy (STORE) was established to address these challenges and develop new electrode materials for Na-ion cells. This article discusses the current state-of-the-art materials used in Na-ion cells and several directions that STORE believes are critical to understand and control the structural and volumetric changes during the reversible (de)insertion of large cations. Graphical abstract Highlights Understanding the fundamental way materials respond to localized strains at the atomic length-scale is a critical first step in the development of highly reversible, long cycle life, Na-ion insertion hosts. This perspective explores a variety of methods that can be employed to mitigate the detrimental effects of large strain. The insights gained from these investigations should help lay the foundation for the creation of more economical and sustainable batteries that could have immediate impact on global energy infrastructure. Discussion Although there is near universal agreement that electrochemical energy storage must be an integral part of a green-energy future, there is less agreement about how to reduce the cost of energy storage. Replacing high-cost lithium-ion cells with lower-cost sodium-ion batteries is one option frequently considered in future energy models, but the details of what can be achieve with optimized sodium cell performance remains unclear. Here we posit that developing methods to mitigating strain on the electrode particle length scale is a key factor for achieving long-cycle-life sodium-ion batteries. Mitigating strain on the atomic scale suppress electrode-level volume change. Allowing for fast cycling in materials without the problems of electrode cracking or delamination. We further posit that understanding volume change in sodium-ion electrodes at a fundamental level will lead to the designing new sodium-ion electrode materials that will allow for efficient, stable, lower-cost energy storage.

A deterministic approach to achieving repeatable sidewall rupture failure modes within lithium-ion thermal abuse studies

Sidewall rupture is among the most violent and destructive ways a lithium-ion cell can fail during a thermal runaway event. When individual cells are aggregated into a complete system there is a high risk of catastrophic propagation. Developing methods to reliably initiate this failure mode will allow deterministic study of appropriate mitigation materials to contain or prevent it. Comprehensive results identified sidewall ruptures were reliably initiated by three primary factors: heating location across a central band of the cell, high heating rates, and restricted positive cell vents. Application of all three factors allowed consistent massive sidewall tears in all thermal abuse experiments. Additionally, for the first time we show a means of directing the failure at a specified position on the cell casing via a simple scoring approach down the sidewall. These results are critical for informing the repeatability of results and the quality of data gathered by allowing data gathering sources to be focused on the point of rupture.

Machine learning-based lifelong estimation of lithium plating potential: A path to health-aware fastest battery charging

To enable a shift from fossil fuels to renewable and sustainable transport, batteries must allow fast charging and exhibit extended lifetimes—objectives that traditionally conflict. Current charging technologies often compromise one attribute for the other, leading to either inconvenience or diminished resource efficiency in battery-powered vehicles. For lithium-ion batteries, the way to meet both objectives is for the lithium plating potential at the anode surface to remain positive. In this study, we address this challenge by introducing a novel method that involves real-time monitoring and control of the plating potential in lithium-ion battery cells throughout their lifespan. Our experimental results on three-electrode cells reveal that our approach can enable batteries to charge at least 30% faster while almost doubling their lifetime. To facilitate the adoption of these findings in commercial applications, we propose a machine learning-based framework for lifelong plating potential estimation, utilizing readily available battery data from electric vehicles. The resulting model demonstrates high fidelity and robustness under diverse operating conditions, achieving a mean absolute error of merely 3.37 mV. This research outlines a practical methodology to prevent lithium plating and enable the fastest health-conscious battery charging.

Characterization of element distribution and thickness change in a high-capacity lithium-ion battery unit cell during charging via laser-induced plasma spectroscopy

Characterization of element distribution and thickness change in a high-capacity lithium-ion battery unit cell during charging via laser-induced plasma spectroscopy

Internal Temperature Estimation for Lithium-Ion Cells Based on a Layered Electro-Thermal Equivalent Circuit Model

In the domain of Battery Management System (BMS) research, the precise acquisition and estimation of internal temperature distribution within lithium-ion cells is a significant challenge. The commercial viability precludes the use of internal temperature sensors, and existing methodologies for online estimation of internal temperatures under various electrical loads are constrained by computational limitations and model accuracy. This study presents a layered electro-thermal equivalent circuit model (LETECM), developed by integrating a layered second-order fractional equivalent circuit model with a layered thermal equivalent circuit model. A lithium-ion battery divided into three layers was employed to illustrate the development of this LETECM. The model’s precision was validated against a 3D Newman Finite Element Model (3DNFEM), constructed using actual battery parameters. Given that the thermal gradient inside the battery is usually more pronounced under high load conditions, a 10C direct current discharge for 60 s followed by a rest period of 240 s was adopted as the test condition in the simulation. The results indicate that at the end of the DC discharge, the temperature difference between the inner layer and the surface of the battery was the largest and the maximum temperature difference predicted by the LETECM was 3.58 °C, while the 3DNFEM exhibited a temperature difference of 3.74 °C. The trends in each layer temperature and battery surface temperature obtained by the two models are highly consistent. The proposed model offers computational efficiency and maintains notable accuracy, suggesting its potential integration into BMS for real-time online applications. This advancement could provide critical internal temperature data for refining battery charging and discharging performance assessments and lifespan predictions, thereby optimizing battery management strategies.

An ethylene carbonate/propylene carbonate electrolyte for improved cycle life and safety of silicon-graphite/NMC (Ni = 80–83 %) high-energy lithium-ion battery cells

High energy Silicon-Carbon (Si-C) electrodes have been paired with ‘high Nickel’ NMC cathodes and used to optimize a ‘linear alkyl carbonate-free’ electrolyte. The optimization was conducted via the screening of 23 electrolyte formulations using two sets of silicone carbon composite electrodes in coin cells. The best electrolyte formulation was then ‘validated’ in a setting closer to their final application, using single layer pouch cells and a commercial surfactant coated separator to solve the wetting issue met with this family of electrolytes with both conventional and ceramic coated separators. Despite its lower rate capability above 1C, this electrolyte allows a 45 % increase of the capacity retention in pouch cell with a 9 % Si-C-based anode and a NMC83 cathode. The safety of the electrolyte is also improved markedly. During cycling up to 4.3V and 5.0V, the total volume of hydrogen evolved was reduced more than 3.6 times over 3 cycles for the electrolyte free of linear carbonates. In fine, accelerating rate calorimetry shows that the use of these electrolytes improves the overall cell safety for this electrolyte, which shows that, for high energy applications, EC/PC electrolytes with the right additive combination can allow both longer cycle life and improved safety.

External Short-Circuit Scenarios under Different Mechanical Pressures Applied to Lithium-Ion Cells with Silicon-Dominant Anodes

Abstract The safety behavior of lithium-ion cells using high contents of silicon as new anode active material was barely investigated. This work investigates the short-circuit behavior of self-manufactured single-layered pouch cells using 70%wt microscale silicon and NCA cathodes in a novel dual reference cell setup consisting of a gold wire and a lithium metal reference electrode. Six cells were externally shortened at three different compression levels using a novel developed quasi-isothermal calorimetric shortcircuit test bench. The same current plateaus and transition zones were observed for all pressure settings, as reported in the literature for graphite-containing anodes. In addition, a direct dependency of the short-circuit intensity and the heat generation is measured for the differently compressed cells. This higher short-circuit intensity was quantitatively explained by the decreasing impedance at increased pressure levels. The built-in reference electrodes allow for monitoring of the negative terminal voltage, revealing potentials of up to 3:508 V versus Li+/Li and thus explaining the measured over-discharge in the range of ≈25% SoC. The post-mortem analysis reveals copper deposition on both electrodes and the separator, confirming the electrochemically measured over-discharge. Linear scans and potential trajectories reveal 3.379 V versus Li+/Li as oxidation potential for copper dissolution using copper||lithium-metal coin cells.

Novel SoC-Based FBG Calibration Method for Decoupled Temperature and Strain Analysis within LIB Cells

Abstract Internal temperature monitoring of battery cells can be very useful, as the core temperature can deviate significantly from that of the housing, especially in case of cells with a thick electrode stack. Conventional resistance temperature detectors can accurately measure temperature, but are limited to the outer surface of the cell due to induction effects. They are therefore not suitable for internal in situ measurements. Fiber Bragg grating (FBG) sensors are unaffected by the electric field as they operate by reflecting light. However, a specific difficulty is the distinction of temperature vs. strain effects as the grating is sensitive to both. In this work a calibration routine to separate the influences of temperature and strain in a lithium-ion battery cell is presented and examined for two multi-layer stack pouch cells (10 and 20 Ah). The obtained in situ temperature data reveal a difference of up to 2°C between center and cell housing at elevated discharge rate (4C) and a delay in detection of temperature peaks by the external sensor by 12 s. Strain data correlate with numbers of electrode layers in the stack and yield a stress of up to 27.3 MPa in the center of the 20 Ah cell.

Assessment of a Top and Bottom Cooling Strategy for Prismatic Lithium-Ion Cells Intended for Automotive Use

In contemporary vehicle applications, lithium-ion batteries have become a leading option among the diverse array of battery technologies available. This preference is attributed to their advantageous properties, which include low self-discharge rates and no memory effect. Despite these benefits, lithium-ion batteries are not without their challenges. The key issues include a restricted driving range, concerns regarding longevity, safety risks, and prolonged charging durations. Efforts aimed at minimizing the charging duration frequently entail the introduction of elevated currents into the battery, a practice that can significantly elevate its temperature and, in turn, diminish its operational lifespan. Generally, battery packs in electric vehicles are equipped with flat cooling plates located on their side or bottom surfaces, which also serve the dual purpose of providing heating in colder conditions. Nevertheless, this cooling configuration faces difficulties during fast charging and may not efficiently heat or cool the batteries. In this work, a novel thermal management approach is proposed, in which a battery module is cooled not only with a bottom cooling plate but also using another cooling plate in contact with the busbars, located on the top of the battery module. The simulations and experimental tests show that this new configuration demonstrates significant improvements. The thermal time constant is reduced by 47%, enabling faster cooling of the module. Additionally, the maximum temperature reached by the battery during charging with dual cooling is lowered by 6 °C compared to the conventional approach. In this configuration, the top cooling plate acts as a thermal bridge. This is a key advantage that promotes temperature homogenization within the battery module. As a result, it supports an even aging process of batteries, ensuring their longevity and optimal performance.