Anode Active Material Research Articles

Room-temperature ionic liquid electrolytes for carbon fiber anodes in structural batteries

Structural batteries require thermally stable electrolytes paired with carbon fibers (CFs), which offer advantages of lightweight, high mechanical strength, and good electrical conductivity. This work evaluated various room-temperature ionic-liquids (RTILs) as compatible electrolytes for CF anodes and LiFePO4 (LFP) cathodes on CFs. This LFP/CF full-cell design eliminates Cu and Al current-collectors, potentially enhancing gravimetric energy and reducing costs. Among various RTILs, LiTFSI in N-propyl-N-methylpyrrolidinium (PYR13) – bis(fluorosulfonyl)imide (FSI) offered promising LFP/CF full-cell performances, attributed to the formation of solid electrolyte interphase (SEI) layer on the CF anode with components such as Li2Sx, Li2S–SO3, LiF, LixFy and F–SO2, identified through X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analyses further confirmed the electrochemical stability of the SEI layer on CF anodes. The LFP/CF cell delivered an initial capacity of 119 mAh/g and relatively high Coulombic efficiency when using the 1 M LiTFSI in PYR13-FSI. CF cycled in different electrolytes exhibit varying mechanical properties with up to 10.08 % loss in tensile strength, which may be related to CF surface degradation during cycling. The 1 M LiTFSI in PYR13-FSI is non-flammable, offering a significant thermal safety. This work successfully demonstrated the significant potential of 1 M LiTFSI in PYR13-FSI RTILs, which enables the use of CF as both an anode active material and cathode current collector for structural battery applications.

Metal‐Organic Framework Hosted Silicon for Long‐Cycling, Low‐Cost, and Flexible Batteries

AbstractDeveloping biodegradable electrodes is a significant step toward environmental sustainability and cost reduction in battery technology. This paper presents a new approach that utilizes metal‐organic framework (MOF)‐encapsulated silicon nanoparticles (SiNPs) as the active anode material within a cellulose‐based electrode. The electrode is free‐standing, flexible, biodegradable, and significantly reduces the strain experienced by SiNPs during volume expansion, resulting in improved capacity and cycle life stability of SiNPs. The high porosity of the electrode creates efficient lithium (Li+) ion transport channels and enhances electrolyte absorption, thus promoting effective contact between the electrolyte and active material. The outcome is rapid electrode kinetics and a highly reversible discharge capacity of 1744.6 mAh g−1 at 0.5 A g−1 versus Li/Li+ over 1000 cycles. Further, full cell tests are conducted that demonstrate a stable cycle performance exceeding 3400 cycles, with an initial discharge capacity retention of 80.5% at 0.5 A g−1. By employing cellulose and avoiding toxic organic solvents and binders, the proposed approach is promising for a substantial reduction of the production costs of Li‐ion batteries. The scalability of the proposed method further enhances its practical viability, offering intriguing economic implications for the future of battery technology.

Analysis of the interfacial reaction between Si-based anodes and electrolytes in Li-ion batteries.

To ensure optimal operation of electrochemical energy storage devices, a precise design of the interfaces formed between the electrodes and the electrolyte is essential. Si is a promising anode active material for Li-ion batteries owing to its high theoretical capacity; however, the large volume change associated with Li absorption and release hinders its practical applications. The development of high-performance storage batteries depends on the construction of an electrode-electrolyte interface that facilitates Li+ movement. To this aim, the authors have developed Si-specific interface observation methods. In this feature article, we review and discuss recent advances in the reaction behavior of Si-based anodes, particularly at the Si electrode-electrolyte interface.

Direct recycling of anode active material from Li-ion batteries using TiNb2O7 anode

As the demand for large lithium-ion batteries (LIBs) for automotive and other applications rapidly grows, recycling of electrode materials has become essential owing to the large amount of waste material generated by battery manufacturing (e.g., battery scrap) and end-of-life (EOL) batteries. While there have been many studies on recycling of cathode materials, we propose a direct recycling process for anode material using TiNb2O7 (TNO) as the active material, as it offers high capacity and long life. Calcination was introduced to separate the anode active material from the current-collecting foil, and a regeneration method was investigated for TNO anodes from scrap and EOL battery waste. After calcination, the active material can be easily separated from the aluminum current-collecting foil owing to the binder disappearing. The structure of the active material was investigated using X-ray diffraction and scanning electron microscopy. TNO active material recycled from scrap and EOL was found to maintain the structure of virgin material. A cell was fabricated using recycled TNO and cell performance was evaluated. The cell capacity and rate capacity were found to be almost equivalent to those of virgin material. In terms of the carbon footprints of products (CFP), CO2 emissions during the recycling process were compared between recycled material and virgin material, and it was found that CO2 emissions were reduced to 0.7-CO2eq/kg for scrap material and 3.8-CO2eq/kg for EOL waste material, compared with 4.8-CO2eq/kg for virgin material. Thus, the feasibility of low CO2 emissions during TNO anode direct recycling was confirmed in principle. A simple and low CO2 emission recycling loop can thus be constructed by employing a stable TNO active material structure and direct-recycling process with calcination.

Cycling Aging in Different State of Charge Windows in Lithium-Ion Batteries with Silicon-Dominant Anodes

Reducing the capacity utilization of silicon-containing anodes and choosing the optimal full-cell voltage window improve the lifetime significantly. In this study, we investigate how different voltage windows affect the aging modes with a common 50% cycling depth. First, the cyclic stability, the anode potentials, and the polarization increase are analyzed for the different voltage windows using 70 wt% microscale silicon anodes and NCA cathodes with a lithium metal reference electrode to investigate the electrode-specific characteristics. Further, the underlying aging modes are quantified in the post-mortem analysis. Finally, the anode thickness increase is quantified using a dilatometer setup for different anode lithiations. In contrast to the literature, the highest voltage window is most beneficial for the lifetime since high anode delithiation potentials and high surface increases are avoided. The anode potential at the end-of-discharge, the charge-averaged full-cell potentials, and the resistance increase are a function of the state of health (SoH). The common underlying main aging mechanism is the loss of lithium inventory, followed by the loss of anode active material. In contrast, the loss of cathode active materials only plays a minor role.

Progressive degradation behavior and mechanism of lithium-ion batteries subjected to minor deformation damage

Minor deformation damage poses a concealed threat to battery performance and safety. This study delves into the progressive degradation behavior and mechanisms of lithium-ion batteries under minor deformation damage induced by out-of-plane compression. The effects of varying initial states of charge and loading speed on battery degradation are also analyzed. It has been observed that a deformation damage degree as low as 3.1 % can cause a significant transient capacity reduction up to 6.3 %. More transient capacity reduction can occur at higher initial state of charges and loading speed. Additionally, the investigation reveals that the capacity decay rate between the normal cell and cells experiencing deformation damage degree of <4.7 % is almost similar, with a maximum difference of merely 1.887 mAh/cycle and a minimum difference of 0.001 mAh/cycle, both observed over 3000 cycles. Non-destructive and destructive analysis methods are used to investigate degradation behaviors of the cells experiencing minor deformation dagame. It is found that such progressive degradation is primarily driven by the loss of active lithium ions, followed by the loss of cathode and anode active materials. It is anticipated that the findings will offer theoretical underpinnings for advancements in battery damage assessment, early failure prediction, and facilitation of loss adjustment and compensation by insurance agencies.

Flotation separation of lithium–ion battery electrodes predicted by a long short-term memory network using data from physicochemical kinetic simulations and experiments

Anode and cathode active materials from spent lithium–ion batteries may be recovered and potentially used in new batteries to promote recycling and resource circulation. Froth flotation was applied to pristine active materials and the black mass obtained from pretreated spent batteries. Flotation kinetics was simulated with the use of computational fluid dynamics and surface chemistry. Bubble surface coverage and entrainment in the flotation kinetics model were selected and optimized by systematic fitting to experimental data. Entrainment influences the recovery and grade of the active materials. The optimized flotation kinetics model was used for generating additional data that, along with the fitted data, were used to train a deep learning neural network. The trained network was validated using anode–cathode and black mass flotation experiments, and its predictions showed a maximum residual error of 0.18 ± 0.11 recovery. The simulation framework was developed into a desktop application that predicts the flotation behavior of active materials. It provides information for estimating results following operational and physicochemical changes and for optimizing flotation processes. Finally, recovered anode active materials from black mass were selected for coin cell tests. The coulombic efficiency of these coin cells was initially lower (86.8 %) than that of cells made with pristine graphite particles (98.4 %).

Optimizing anode materials for lithium-ion batteries: The role of lithium iron phosphate/graphite composites

ABSTRACT In this study, a novel composite anode material for lithium-ion batteries has been developed, targeting advancements in energy storage technology. The study is centered on integrating various percentages of LiFePO4 known for its high thermal stability and high capacity when used as an anode active material with graphite to increase capacity and thermal durability of anode electrode. This approach has led to the creation of LiFePO4/graphite composite anodes, utilizing a slurry mixing technique to combine the beneficial properties of both materials. Thermogravimetric analysis indicate LiFePO4/graphite’s higher thermal stability than its graphite counterpart, which implies that the addition of LiFePO4 alters the thermal decomposition profile of the anode electrode. Electrochemical assessments show that particularly LiFePO4: graphite = 6:94 wt% composite anode electrode delivers the highest discharge capacity of 437 mAh g−1 with Coulombic efficiency of 95%; the highest discharge capacity of 385 mAh g−1 after 200 cycles at 70 mA g−1 with superior capacity retention and rate performance of 311.5 mAh g−1 at 700 mA g−1 compared to the graphite anode electrode. These findings highlight the potential of the LiFePO4/graphite composite as an anode material in improving lithium-ion battery performance, making it a viable option for future energy storage solutions.

Microfluidic Shape Analysis of Non-spherical Graphite for Li-Ion Batteries via Viscoelastic Particle Focusing.

The size and shape of graphite, which is a popular active anode material for lithium-ion batteries (LIBs), significantly affect the electrochemical performance of LIBs and the rheological properties of the electrode slurries used in battery manufacturing. However, the accurate characterization of its size and shape remains challenging. In this study, the edge plane of graphite in a cross-slot microchannel via viscoelastic particle focusing is characterized. It is reported that the graphite particles are aligned in a direction that shows the edge plane by a planar extensional flow field at the stagnation point of the cross-slot region. Accurate quantification of the edge size and shape for both spheroidized natural and ball-milled graphite is achieved when aligned in this manner. Ball-milled graphite has a smaller circularity and higher aspect ratio than natural graphite, indicating a more plate-like shape. The effects of these differences in graphite shape and size on the rheological properties of the electrode slurry, the structure of the coated electrodes, and electrochemical performance are investigated. This method can contribute to the quality control of graphite for the mass production of LIBs and enhance the electrochemical performance of LIBs.

Characterization of Lithium-Ion Batteries from Recycling Perspective towards Circular Economy

Recycling processes of lithium-ion batteries used in electric and hybrid vehicles are widely studied today. To perform such recycling routes, it is necessary to know the composition of these batteries and their components. In this work, three pouch and three cylindrical LIBs were discharged, dismantled, and characterized, having their compositions known and quantified. The dismantling was performed using scissors, pliers, and a precision cutter equipment. The organic liquid electrolyte was quantified via mass loss after it evaporated at 60 °C for 24 h. The separators were analyzed using Fourier-transform infrared spectroscopy (FTIR), and the cathode and anode active materials were analyzed using a scanning electronic microscope coupled to an energy-dispersive spectroscope (SEM-EDS), X-ray diffraction (XDR), and energy-dispersive X-ray fluorescence spectrometry (EDXRF). All LIBs were identified by type (NCA, NMC 442, NMC 811, LCO, and two LFP batteries), and a preliminary economic evaluation was conducted to understand their potential economic value (in USD/t). Both results (characterization and preliminary economic evaluation) were considered to discuss the perspective of recycling towards a circular economy for end-of-life LIBs.

Physical Discharge of Spent Lithium-Ion Batteries Induced Copper Dissolution and Deposition.

Complete discharge of spent lithium-ion batteries (LIBs) is a crucial step in LIB recycling, with the physical discharge method being particularly noted for its high discharge efficiency and environmental friendliness. However, previous studies and standards have focused on the performances of the discharge methods, neglecting the battery materials changes caused by discharge. Here we demonstrate that although prolonged discharge of spent batteries keeps the voltage around 0 V, an obvious current flow can be still observed, resulting from the dissolution and subsequent deposition of the copper foil. The deposited copper, primarily in the forms of Cu, Cu2O, and CuO, shows a gradient distribution on the surface of the anode and cathode active materials. This copper deposition significantly compromises the electrochemical performance of the discharged battery, with evident deterioration observed in the first charge-discharge capacity, cycling performance, and coulombic efficiency when compared to the original battery. This study provides guidance for the discharge methods and offers new insights into the materials failure mechanisms during discharge of spent batteries.

Unraveling the impact of pore length on the conversion reaction of Fe3O4/carbon anodes in lithium-ion batteries

Despite numerous studies investigating Fe3O4/porous carbon anode materials for high-performance lithium-ion batteries (LIBs), the inherent limitations of the Fe3O4 conversion reaction, such as low reversibility and sluggish kinetics, continue to pose significant challenges. While modifying the porous structure of Fe3O4 anodes has demonstrated considerable promise in overcoming these limitations, the precise relationship between the porous structure and Fe3O4 conversion behavior still remains unclear. Here, we explore the impact of the pore length of the anode active material on the electrochemical performance of LIBs. Using two different composites of Fe3O4 and porous carbon with varying pore lengths as model materials, we conduct various in-depth electrochemical and ex-situ analyses. Our findings confirm that a shorter pore length facilitates higher Li+ ion diffusivity compared to longer pore length. Consequently, the conversion reaction of Fe3O4 to Fe and Li2O during lithiation process can be expedited, leading to a decrease in the resistance at the solid electrolyte interphase. As a result, we demonstrate that shortening the pore length of the porous anode composite materials can enhance the initial Coulombic efficiency (CE), reversible capacity, and rate capability of LIBs.

Elucidating the Effect of Pre-Lithiation of Silicon Dominant Anodes on the Full Cell Performance

Over the last few years, there has been a high interest in increasing the energy density of lithium-ion batteries in order to increase the driving range of battery electric vehicles. On a material level, silicon is a promising anode active material candidate due to its high theoretical capacity of 3579 mAh g-1, which is approximately ten-fold higher than that of the currently used graphite anodes with a theoretical capacity of 372 mAh g-1.[1] However, the main drawbacks of silicon are its intrinsic volume expansion of +300 % upon lithiation.[2] This leads to rupturing of the passivating solid electrolyte interphase (SEI), particle cracking, and loss of electronic contact, limiting the lifetime of batteries with silicon based anodes, thus hindering their transition into application.As a possible solution, Jantke et al. proposed the concept of partial utilization of µm-sized crystalline silicon, where only one-third of the theoretical capacity is used for (de)lithiation. Thereby, a crystalline core remains, and the volume expansion on the particle-level is reduced to approximately + 100%.[3] In addition, it was shown that irreversible losses during the formation and upon cycling can be compensated by pre-lithiation of the anode, which increases the lifetime significantly.[4,5] In this study, we investigated a promising cell chemistry for high-energy applications, containing a Si-dominant anode and a cathode based on a Co-free lithium- and manganese-rich NMC. The here used anode consists of 70 wt% silicon, 20 wt% graphite, 2 wt% conductive C65, and 8 wt% LiPAA-binder. Note that graphite mainly acts as a conductive additive within the operating voltage window, with a negligible contribution to the overall anode capacity. The electrochemical testing was conducted at 25 °C in Swagelok® T-cells containing two glass fiber separators, a 1M LiPF6 in FEC:DEC (2:8, v:v) electrolyte and a lithium metal reference.Without pre-lithiation, these full-cells suffered from rapid capacity fading, reaching the 80 % state-of-health (SOH) criterion after ~100 cycles. As will be discussed, our analysis shows that the primary mode of failure in these cells is the loss of cyclable lithium, evidenced by an increase in the end-of-discharge potential at the anode. It was found that with increasing end-of-discharge potential at the anode, the anode resistance at low state-of-charge rises significantly.Therefore, silicon anodes were lithiated in coin half-cells to 25, 50, and 75 % of the reversible cathode capacity (corresponding to 300, 600, and 900 mAh gSi -1). Then, the anodes were harvested in the lithiated state and assembled in a Swagelok® T-cell with the same cathode, separator, and electrolyte as the non-pre-lithiated reference cells. Pre-lithiation leads to similar reversible charge/discharge capacities, but generates a cyclable lithium reservoir in the anode.Successively increasing the pre-lithiation of the anode increases the lifetime from 100 to 550 cycles. The onset of the above-described aging phenomena is delayed by 150 cycles for every additional 25 % pre-lithiation. Further, the influence of aging and lithium inventory on anode and cathode resistance will be discussed, showing that the increase in anode resistance is mainly a function of (de)lithiation rather than cycle number.Acknowledgments:The authors gratefully acknowledge the funding from the BMBF (Federal Ministry of Education and Research, Germany) in the ExZellTUM III project (grant number 03XP0255). We also kindly thank Wacker Chemie AG and BASF SE for providing the anode and cathode active materials.

Evaluating the Influence of SiOx on the Calendar Lifetime of Li Ion Batteries across Varied Temperatures and State of Charge Conditions

Lithium-ion batteries (LIBs) have a wide range of application in various fields, such as automotive and stationary applications. The incorporation of silicon (Si) in the anode is an attractive strategy to increase energy density and enable fast charging. This is due to the excellent theoretical specific capacity of Si, which exceeds 3000 mAh/g. However, due to the alloying behavior of Si, a significant volume change can be observed by Si based anodes when cycled. In literature the rapid cyclic degradation is related to these volume changes. A practical approach is to use silicon oxide (SiOx) as anode material, which undergoes lower volume changes during cycling but still exhibits a specific capacity >2000 mAh/g. Studies on the cycle life of SiOx-containing cells are frequently published in literature, but calendar life studies are scarce.Here, the calendar life of two different 1 Ah pouch cell configurations, possessing different anode chemistries, were investigated. One cell contained artificial graphite and 10% SiOx (SiO-cell) while the other cell only used artificial graphite as anode material (G-cell). Both cell configurations were built with an NMC811 cathode. 1 M LiPF6 in EC: EMC (3:7 (wt.)) + 2%wt. VC was used as electrolyte. Temperatures of 20 °C, 40 °C and 60 °C and state of charge (SoC) levels of 20%, 40%, 60%, 80% and 100% were investigated in this study. It has been observed that for 20 °C and 40 °C, the degradation rate of both cell types intensifies when stored at 100% SoC. At 60 °C, however, the capacity fade already starts intensifying drastically at around 60% SoC. The capacity fading during storage experiments of the SiO- cells, depending on the parameters tested, is higher by a factor of around 1.1 to 4.1 than the capacity fading of the G- cells. Interestingly, the capacity fade ratio of the SiO-cell to G-cell is at maximum when cells were stored below 40% SoC at 20 °C. When stored at low SoC, SiOx is the anode active material being lithiated majorly. These findings indicate that SiOx indeed causes stronger calendaric degradation compared to graphite. Hence, in future studies on SiOx based anodes calendaric degradation is an interesting topic to address additionally to cyclic degradation.

Influence of Cycling in Different State-of Charge Windows on Lifetime and Electrode Expansion of Microscale Silicon-Dominant Lithium-Ion Batteries

Silicon (Si) is a promising active anode material for next-generation lithium-ion batteries due to its high theoretical electrochemical capacity of 3579 mAh/gSi (Li15Si4) and low costs using microscale Si particles.1,2 However, the wide usage of Si as an active anode material is hindered due to its lower lifetime compared to graphite anodes. The degradation in silicon-containing batteries can be suppressed by partial lithiation strategies,2,3,4 with applications for automotive cells leading to improved lifetime.5 However, the lifetime problem still exists in partially lithiated microscale Si particles with lower volume expansion2,3 with ≈200 cycles for one-third silicon utilization, i.e., 1200 mAh/gSi 2. Moreover, we confirmed in our previous study6 as reported by Wetjen et al.4 and Haufe et al.2 that lower cell discharge cut-off voltages and thus higher silicon delithiation potentials versus Li+/Li led to accerlated aging and should be avoided.Thus, the motivation for this study is to avoid these high anode delithiation potentials by operating the cells in a high full cell state of charge (SoC) and to investigate the influence of different SoC windows on lifetime with laboratory 182.1 ± 0.7 mAh/gNCA Swagelok T-cells comprising a microscale silicon-dominant anode and a NCA cathode. The electrode-specific aging is monitored with the lithium metal reference electrode based on the anode and cathode potentials and pulse tests. To investigate the aging in different SOC regimes, three cycling conditions were chosen in the low (LV), middle (MV) and high voltage (HV) window and compared to cells cycled between 2.8 and 4.2 V as full voltage6. The voltage window of LV, MV and HV were chosen to represent cycling between 0-50%, 25-75%, and 50-100% SOC, respectively. Formation and checkups were the same for all cells with 2.80 V to 4.20 V. After reaching the end-of-life criterion of 55% state of health (SoH), the cells were disassembled and anode and cathodes were re-assembled in half-cells for a post-mortem analysis to reveal loss of lithium inventory (LLI) and loss of active material of the anode LAMAn, and the cathode LAMCat of the respective full-cells. Complementary to the electrochemical aging, dilatometer measurements7 were conducted to quantify the electrode expansion for the different silicon anode SoC windows since particle decoupling is expected to be one of the major aging mechanisms for microscale silicon2, which is expected for high electrode expansions.For the electrochemical results, the lifetime can be drastically increased from ≈230 to ≈560 equivalent full cycles (EFCs) for operating the full cell in the HV compared to the FV window, as shown in Fig. 1a. One EFCs is defined as the total charge throughput for charging and discharging one cycle normalized to the capacity of the initial checkup cycle. Over aging, the full cell degradation correlates very well with the end of discharge potential of the silicon anodes. Moreover, the electrode-specific pulse results show strong anode degradation while the cathode remains intact. The post-mortem results reveal LLI as most dominenat aging mode, which was very similar for all voltage windows. The HV cells showed both the highest LAMNE and LAMPE, whereas LAMNE was more dominant compared to LAMPE. The silicon amorphization was the lowest for the LV anodes and the highest for the HV anodes at the same SoH. The dilatometer results show that the silicon electrode expansion is the lowest for the low lithiation, i.e., LV, and the highest for the highest lithiation, i.e., HV. From the dilatometry results, we conclude that the mechanical electrode stability does not limit lifetime.Our results demonstrate that lifetime can be significantly improved by a factor of ≈2.4 based on the EFCs if operating cells with silicon dominant anodes in high SoCs and that low SoCs should be avoided. Acknowledgments: S. F. gratefully acknowledges the financial support from the BMBF (Federal Ministry of Education and Research, Germany) under the auspices of the ExZellTUM III project (grant number 03XP0255). The authors want to thank our student worker, Maximilian Reichl, for the fabrication of the NCA coatings. We also thank Wacker Chemie AG for kindly providing the microscale silicon material.

Stress-Relieving Carboxylated Polythiophene/Single-Walled Carbon Nanotube Conductive Layer for Stable Silicon Microparticle Lithium-Ion Battery Anodes

Silicon is one of the most promising anode materials for next generation Lithium ion batteries (LIBs) on account of its ultrahigh specific capacity (4200 mAh g-1) and relatively low working potential of 0.3 V (vs. Li+/Li). However, practical commercial implementation of silicon poses a great challenge due to technical difficulties such as low conductivity (10-5-10-3 S cm-1), low initial Coulombic efficiency (50-80%) and stress-induced fracture, along with SEI accumulation due to severe volume changes (~400%) inherent in silicon-based anodes.To address these challenges and advance the next generation of LIBs, we present poly[3-(potassium-4-butanoate)thiophene] (PPBT) and single-walled carbon nanotube (SWNT) coated silicon microparticles (Si MPs) (PPBT/SWNT@Si MPs) as active materials for highly stable Si-based anodes for LIB systems. The water-soluble conjugated polymer, PPBT, was selected for its ability to serve as an ion and electron conducting physical/chemical linker (10-5 S cm-1 vs. PVDF; ~ 10-8 S cm-1) to the surface of various high capacity anode active materials systems. In consideration of current practical requirements for Si-based anode materials, Si MPs were selected as starting materials due to their lower surface area than the nanoscale analogs, a characteristic that can further improve LIB bulk capacity. In addition, Si MPs are known to have high initial Coulombic efficiency and industrial compatibility. With the help of PPBT, which facilitates unraveling of entangled single-walled carbon nanotubes, SWNTs were successfully attached to the surface of the active materials. PPBT carboxylate substituents were found to be chemically bound to the Si MP surface, providing an electrochemically conductive environment in intimate contact with the 1D SWNTs and Si MPs.Due to facilitated ion/electron transport kinetics, PPBT/SWNT@SiMP anodes exhibited superior capacity retention and higher reversible capacities, even at a high current density of 8 A g-1, compared to bare Si MP anodes. Improved initial Coulombic efficiency and stable cycling performance over 300 cycles were achieved. In situ Raman spectroscopy was employed to investigate the evolution of stress on the SWNTs during lithiation/delithiation, which elucidated the mechanism underlying the enhancement in PPBT/SWNT@Si MP anode cycling performance. The single-walled carbon nanotubes within the PPBT/SWNT@Si MP anodes consistently exhibited reversible stress recovery and 45 % less tensile stress variation after the 10th cycle than SWNT@Si MP control anodes fabricated by direct mixing of the microparticles with SWNTs to create SWNT@Si MPs.Combined, the results suggest that a well-developed conductive interface can enhance the rate performance, and improve electrode stability and Coulombic efficiency. Furthermore, the PPBT/SWNT coating directly bound to the Si MP surface provides for efficient stress relaxation of Si-based electrodes, resulting in reversible lithiation/delithiation, low electrode resistance, and reduced SEI layer formation.

Seeding and Growing Plain Li-Metal Surfaces - New Approaches to Increase the Coulombic Efficiency of Anodeless Lithium-Metal Batteries

In the pursuit of higher energy densities, lithium-ion batteries with lithium metal anode appear as one particular promising technology. Especially the option to plate non-dendritic, superficially even Li-layers [1] directly onto the current collector of the negative electrode without the use of an anode active material (“anode-less battery design”) should enable gravimetric energy densities of up to 400 Wh/kg [2] and 1200 Wh/l [3] in future cell systems [4].In the presented work, Swagelok-cells in “anode-less” configuration, comprising a NMC 811-cathode and an ether-based electrolyte containing 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) and 1,2-dimethoxyethane (DME) in 1.2:3 molar ratio, 1.5 M LiFSI [2] were cycled under the application of pressure (up to 400 kPa) onto the cell stack.As the proper formation of Li-nuclei (number, morphology, etc.) and the even growth of lithium-layers onto the Cu-based anode (Figure 1) seems from great importance to plate and stripe Li-metal at high coulombic efficiencies, several charging programs with presumably Li-nucleation promoting effects were tested in a cycling experiment along 50 full cycles (Figure 2). As a promising result and as clear improvement compared to standard charging processes (e.g. 0.1 C), average coulombic efficiencies as high as 99.2 % together with low capacity fading were observed.Due to the design-related limitation of Swagelok-based cells and a noticeable diffusion of air fractions (N2, O2, H2O) across the PFA-seals into the cell, in a second experimental part the influence of ambient air on the cycling performance was tested under three conditions (Figure 2, left, middle and right diagram): a) Cells cycling in common (humid), laboratory air, b) under dry air (P2O5) in sealed steel-containers and c) inside an argon filled glovebox (< 0.1 ppm H2O, < 0.1 ppm O2). As a result, a strong sensitivity of operating Li-metal cells not solely versus traces of ambient, humid air, but also versus P2O5-dried nitrogen and oxygen was obvious. Reference s [1] Fang, C.; Lu, B.; Pawar, G.; Zhang, M.; Cheng, D.; Chen, S.; Ceja, M.; Doux, J.-M.; Musrock, H.; Cai, M.; Liaw, B.; Meng, Y. S., Pressure-tailored lithium deposition and dissolution in lithium metal batteries. Nature Energy 2021, 6, (10), 987-994.[2] Niu, C.; Liu, D.; Lochala, J. A.; Anderson, C. S.; Cao, X.; Gross, M. E.; Xu, W.; Zhang, J.-G.; Whittingham, M. S.; Xiao, J.; Liu, J., Balancing interfacial reactions to achieve long cycle life in high-energy lithium metal batteries. Nature Energy 2021, 6, (7), 723-732.[3] Louli, A. J.; Eldesoky, A.; Weber, R.; Genovese, M.; Coon, M.; deGooyer, J.; Deng, Z.; White, R. T.; Lee, J.; Rodgers, T.; Petibon, R.; Hy, S.; Cheng, S. J. H.; Dahn, J. R., Diagnosing and correcting anode-free cell failure via electrolyte and morphological analysis. Nature Energy 2020, 5, (9), 693-702.[4] Li, Q.; Yang, Y.; Yu, X.; Li, H., A 700 W⋅h⋅kg−1 Rechargeable Pouch Type Lithium Battery. Chinese Physics Letters 2023, 40, (4), 048201. Figure 1

Decomposition Mechanism of Lithium Methyl Carbonate with Electrolyte in the Solid Electrolyte Interphase to Elucidate the Early Stages of Thermal Runaway in Lithium-Ion Batteries

With the growing demand for electric vehicles, the safety of lithium-ion batteries has been greatly emphasized. Thermal runaway caused by the continuous side reactions inside the battery makes it difficult to extinguish fires caused by abnormal activity. The temperature at which a typical thermal runaway begins is around 100°C, the temperature at which several elements in the solid electrolyte interphase (SEI) layer on the anode surface experience thermal decomposition reactions (causing gas generation and heat production). Therefore, to understand the first stage of thermal runaway and increase battery safety, it is necessary to describe the thermal decomposition process of the SEI layer.To investigate the thermal decomposition process of the SEI layer, it is necessary to understand the various organic components. During the initial charging phase, the reduction reaction between the lithium ions and the electrolyte produces a series of components that make up the SEI layer. On the surface of the anode active material, the SEI layer is a thin layer with an overall thickness of 10-50 nm. It is composed of various organic (inner layer) and inorganic (outer layer) elements. Under thermal stress, the unstable organic SEI layer decomposes into an inorganic SEI layer due to its low stability in the presence of heat and gas generation, which is mostly caused by the decomposition of lithium carbonate (ROCOOLi). It is important to note that the most used electrolytes in batteries today are ethylene carbonate and ethyl methyl carbonate (EMC) dissolved in salts based on lithium hexafluorophosphate. Due to the application of EMC, lithium methyl carbonate (LMC) accounts for more than 50% of the SEI layer in the ROCOOLi compounds.This study used both theoretical and experimental techniques to fully elucidate the pyrolysis process of high-purity synthesized LMC with the electrolyte. First, LMC was chemically synthesized using a nucleophilic substitution reaction. Then, in-situ thermogravimetry analysis (TGA)-mass spectrometer(MS), attenuated total reflectance (ATR)- fourier transform infrared (FTIR), and nuclear magnetic resonance (NMR) results along with bond dissociation energy (BDE)/spectral analysis/energy diagrams obtained by density functional theory (DFT) calculations were analyzed to propose the elementary reaction steps involved in the pyrolysis reaction. We propose six elementary reaction steps for the pyrolysis reaction of LMC. To verify the validity of the proposed basic reaction mechanisms, chain reactions involving radicals generated during pyrolysis were finally calculated using GRI-MECH 3.0, and the calculated results were compared with the composition of the actual gas. We hope to utilize the techniques used in this study to obtain the reaction steps of additional SEI components (LEMC, Li2CO3, etc.). The precise mechanisms obtained through modeling studies can be used to predict the self-heating phase of thermal runaway and to develop materials that can stop the SEI decomposition reaction.

Modeling the Impact of Microstructure Variation on Charging Capability in Lithium-Ion Batteries

Battery models can be used to predict conditions ripe for lithium metal plating in the anode during rapid charging. One common procedure is to simulate a rapid charge using a pseudo-two-dimensional (P2D) model and determine if and when the minimum local potential reaches 0.0 V with respect to metallic lithium in the anode. This normally occurs at higher states of charge during a DC fast charging or rapid regenerative charging event. However, we have recently determined that three-dimensional (3D) models, while broadly agreeing with P2D results, demonstrate much variation in the in-plane and through plane direction, which P2D models cannot resolve. Because P2D models employ effective medium approximations, any “local” value in the P2D space along the through-plane coordinate represents an average across a slice of the domain. Therefore, it may be insufficient to use minimum anode potentials from P2D models as indicators of local plating and may only be representative of global lithium plating onset.The results of rapid-charge 3D microstructure simulations are compared to P2D results. This allows us to verify whether rapid charging induces negative local potentials at the anode active material surface that are left undetected in a P2D model. The expected result implies that the 3D microstructure model is necessary to predict the onset of lithium plating. We then extend our previous study to look at the impact of design changes on the microstructure performance, and then link the microstructure performance changes to performance metrics at the battery cell, module, pack, and vehicle level. Finally, the impact of variation in the microstructure domain considering particle size variation is quantified to understand tolerances that may be necessary when developing charging schemes.

Effect of Mechanical Pressure on Rate Capability, Lifetime, and Expansion of Multilayer Pouch Cells with Silicon-Dominant Anodes

Silicon-dominant (Si) anodes with microscale silicon particles meet the requirements of high energy density and low costs due to its ten times higher theoretical electrochemical capacity of 3579 mAh/gSi (Li15Si4) compared to graphite,1 lifetime extension due to partial lithiation,1-2 and high abundance and economic availability due to existing industrial infrastructure.1-2 The major drawback of silicon is the large volume expansion of nearly 300% upon full lithiation. This hinders the broad application of silicon as an active anode material due to continuous SEI (re-) formation and electrochemical milling of particles. Additionally, the volume expansion impacts the overall electrode stability, leading to thickness changes, the disruption of the electronic pathways, and the decoupling of particles. Although the approach of partial lithiation of microscale1 Si eases these effects by lowering the overall volume expansion, these problems still exist.2,3 In this work, the effect of different mechanical cell pressures on lithium-ion batteries with partially lithiated silicon-dominant anodes is investigated with 5.4 Ah multilayer pouch cells as a follow-up study of our previous work with laboratory T-Cells.3 Methodologically, a novel cell holder and pressure device were developed and validated to apply different mechanical cell pressures. Our investigation covers a pressure range from uncompressed as zero (ZP, 0.00 MPa) to high (HP, 0.50 MPa) external cell pressure to determine the optimal pressure for high rate capacity, cyclic lifetime, energy density, and low thickness gain. The cells were tested by checkup cycles at C/10, rate tests up to 3C, and cycled at C/2 until 70% state of health (SoH). Electrochemical impedance spectroscopy was conducted at different SoHs to quantify the effect of mechanical pressures on the impedance response.4 The post-mortem analysis after reaching 70% SoH and operando thickness measurements in a compression test5 bench give insights into the electrode and cell thickness gain.In the electrochemical results, the discharge capacity Q DCH was immediately reduced by decreasing the mechanical pressure from 0.20 MPa to 0.00 MPa or 0.05 MPa, both for the cells in the cell holder (see inset in Figure 1a) and the compression test bench (not shown). This pressure change was also quantified by an increased charge-averaged full cell potential during charging and a decreased charge-averaged full cell potential during discharging. The impedance was reduced for higher mechanical pressure, especially the cathode contact resistance initially and over aging, resulting in lower temperature increases during the rate tests. The capacity retention Q DCH and the total charge throughput were increased to 360 cycles until 70% SoH for higher mechanical pressures at NP (normal pressure, 0.20 MPa) and HP (see Figure 1) compared to 181 cycles for ZP. Overall, applying higher mechanical pressures reduced the thickness increase, as shown by operando thickness measurements and the post-mortem analysis. A dependency of the reversible thickness on the charged capacity was found. Moreover, the porosity decreased with increasing pressure, which was measured with mercury intrusion porosimetry (MIP) and pycnometry. The anode mass increase correlates to the total charge throughput, which is pressure-dependent and the highest for NP.Our findings demonstrate that applying the optimal mechanical cell pressure is significant when using microscale silicon-dominant anodes. We recommend a mechanical pressure of 0.50 MPa for increased rate capacity, minimizing impedance, heat evolution, and thickness gain, and 0.20 MPa for increased energy density. Acknowledgments: S.F. gratefully acknowledges the financial support from the BMBF (Federal Ministry of Education and Research, Germany) under the auspices of the ExZellTUM III project (grant number 03XP0255). The authors want to thank the research battery production team of iwb at TUM for the multi-layer pouch cell production. We also thank Wacker Chemie for kindly providing the microscale silicon material.