LiPF6 In EC Research Articles

Effect of the succinonitrile additive, electrode processing, and N/P ratios in the performance of high‐voltage lithium‐ion batteries using LiNi0.5Mn1.5O4 cathode

AbstractThe progressive improvement in the gravimetric energy density of lithium‐ion batteries (LIBs) leads to electrolyte design, positive electrode, and full‐cell optimization processes. The spinel lithium nickel manganese oxide (LiNi0.5Mn1.5O4, LNMO) is one of the potential candidates for the next generation of LIBs applied for electric vehicles due to high working potential (4.75 V vs. Li+/Li), affordable price, and environmental friendliness. Nevertheless, the degradation of cycling performance at high potential induces massive challenges for commercializing LNMO‐based batteries. This study investigated the impact of succinonitrile (SN), electrode processing, and N/P ratios to strengthen the LNMO half‐cell and graphite||LNMO full‐cell performance. According to the performance in half‐cell, the sample contained 85 wt% LNMO: 7.5 wt% C65: 7.5 wt% PVDF/NMP combining with the electrolyte 1.5 M LiPF6 in EC:EMC:DMC (2:1:7 – v/v) at 0.5 wt% SN seems to be the optimal condition for further full‐cell. In addition, the full‐cell with N/P = 1.3 displays a remarkable initial capacity of 118.75 mAh g−1, a Coulombic efficiency of 91.64%, and a superior gravimetric energy density of 537.71 mWh g−1. Moreover, it maintains a capacity retention of around 58% at the current density of 0.1C after 100 cycles. As a result, this study presents an optimal solution to augment the material's potential for commercialization in the immediate future.

Shape Matters: Understanding the Effect of Electrode Geometry on Cell Resistance and Chemo-Mechanical Stress

Rechargeable batteries that incorporate shaped three-dimensional electrodes have been shown to have increased power and energy densities when compared to a conventional geometry, i.e. a planar cathode and anode that sandwich an electrolyte. Electrodes can be shaped to enable a higher active material loading, while keeping ion transport distances small. However, the relationship between electrical and mechanical performance of shaped electrodes remains poorly understood. Many electrode designs have been explored, where the electrodes are individually shaped or intertwined, and advances in manufacturing and shape/topology optimization have made such designs a reality. Here, we explore sinusoidal half cells and interdigitated full cells. First, we use a simple electrostatics model to understand the cell resistance as a function of shape. We focus on low-temperature conditions, where the electrolyte conductivity decreases relative to that of the electrode; here, LiPF6 EC:DMC electrolyte and MnO2 electrode are considered. Next, we use a chemo-mechanics model to examine the stress that arises due to intercalation-driven volume expansion. We show that shaped electrodes provide a significant reduction in resistance in low-temperature conditions, however, they exhibit unfavorable stress concentrations. Overall, we find that the fully interdigitated electrodes may provide the best balance with respect to this resistance-stress trade-off.

Enabling fast discharge of Li-ion batteries via electrolyte formulations for urban air mobility applications

High-power discharge requirements are critical for lithium-ion batteries (LIBs) used in electric Vertical Takeoff and Landing (eVTOL) vehicles that are increasingly considered in urban mobility. This investigation places a particular emphasis on understanding the impact of electrolytes on discharge processes and rate capability. We aim to compare the discharge behavior of LiBs using a conventional electrolyte (Gen 2: 1.2 M LiPF6 in EC:EMC) and the dual salt LiTFSI-LiBOB-based electrolyte. We carefully examine the profiles of charging and discharging, the behavior during extended cycles, impedance spectroscopy results, and the characteristics of the electrode surface. Our research findings demonstrate the complex relationship between the composition of electrolytes and the specific high-power discharge requirements of eVTOL systems. This research highlights the importance of customizing electrolyte compositions to optimize energy storage density while simultaneously enabling higher power extraction to enhance performance in short-range electric aviation.

Optimize Electrolytes and Additives for Lithium-Ion Batteries Using C/SiO2 Synthesized from Rice Husk As an Anode Material

Silicon-based anodes derived from agricultural waste, particularly rice husks, have recently been focused on replacing conventional graphite due to its abundance, low cost, fast charge ability, and remarkable theoretical capacity (~ 3600 mAh.g-1). SiO2 has several drawbacks, including low electrical conductivity and short cycling life associated with high-volume expansion (~ 300%) during the charge-discharge process. Hence, a hybrid composite C/SiO2 was an ideal solution to overcome these challenges.To strengthen the electrochemical properties of LIBs, the compatibility between electrodes and electrolytes plays a crucial role. Lithium hexafluorophosphate (LiPF6) and Lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) are promising candidates for investigation due to their effectiveness cost. Regarding LiPF6, widely used in LIBs, offers high solubility and stability, while LiTFSI provides enhanced ionic mobility. However, LiTFSI is corrosive to aluminum current collectors, and LiPF6 unexpectedly dissociates to form HF acid and PF5, leading to cycling degradation. To mitigate these issues, additives such as Fluoroethylene-carbonate (FEC) and Vinylene Carbonate (VC) are introduced to enhance battery performance. This study employed FEC and VC, adding in the optimal electrolyte to stabilize the SEI layer, prevent lithium loss during cycling due to the instability of the interfacial layer, as well as to enhance high-rate performance. In detail, FEC could effectively prevent SEI degradation by enhancing the F-based component of this layer, stabilizing the SEI layers, and protecting electrodes. Simultaneously, VC offers a synergistic effect on the interphase modulation by reinforcing the inorganic/organic components. Thus, we investigate various electrolyte systems to evaluate half-cell performance of C/SiO2 synthesized from rice husk as the anode. Then, the optimal-chosen electrolytes were used to assemble full-cell LFP||C/SiO2 and investigated at high-rate performance.According to the experimental sections, there are two main lithium-salts comprised of LiPF6 and LiTFSI, which dissolved in three solvent systems, including EC: DEC (1: 1-v/v); EC: DMC (1:1-v/v), and EC: DMC: DEC (1:1:1 – v/v/v). After optimizing the solvents, the system of EC: DEC (1: 1-v/v) shows the highest performance, which was chosen to investigate the impact of various ratios of FEC and VC additive. The 1M LiPF6 in EC: DEC (1:1-v/v) + 5 wt% FEC has the highest specific capacity compared to the LiPF6 samples, reaching the value of 220.0 mAh.g-1. Meanwhile, the 1M LiTFSI in EC: DEC (1:1-v/v) 2.5 wt% FEC + 2.5 wt% VC is the best electrolyte to offer a stable capacity of 223.4. mAh.g-1 after 50 cycles in comparison to LiTFSI samples. This phenomenon is plausible with the conductivity of 6.16 and 7.45 mS.cm-1, respectively. Furthermore, they undergo remarkable oxidation at 4.05 and 4.23 V at a current density of 0.002 mA, demonstrating they have a good stability potential window. Since there is no significant difference in half-cell performance, they were used to evaluate full-cell properties. According to battery performance, the full-cell LFP||C/SiO2 using LiPF6 + 5 wt% FEC shows that the initial capacity reaches 111.6 mAh.g-1 and is maintained 40% after 50 cycles with a stable CE above 96%. Full cell LiTFSI +2.5 wt% FEC + 2.5 wt% VC shows an initial discharge capacity of 159.9 mAh.g-1. However, this capacity value is significantly reduced to only 28.5 mAh.g-1 after 50 cycles and CE ~ 94%. Regarding rate capability results, full cell 1M LiPF6 in EC: DEC (1:1-v/v) + 5 wt% FEC illustrates better performance in all discharged rates than full cells using LiTFSI salt. Moreover, this full-cell shows a discharge capacity of ~ 50 mAh.g-1 at high-rate performance while full-cell using LiTFSI-based electrolyte displays a negligible capacity value. To explain, the total resistance of 25 Ohm is obtained for full cell LiPF6 + 5 wt%. FEC is three times lower than full-cell 1M LiTFSI + 2.5 wt% FEC + 2.5wt% VC. Furthermore, the HR-TEM ex-situ indicates that the stability of SEI layer formation is consistent in thickness but exhibits uneven distribution when using different electrolytes. Besides, the SEI components analyzed by XPS data indicate the presence of carbonate groups and LixPFyOz-based compounds due to the irreversible reactions on the SEI layer after the initial discharge cycle. This research brings some remarkable information to study the electrolyte compatibility for full-cell performance using C/SiO2 as an anode material. Figure 1

A Coupled NMR and Differential Capacity Study of the Consumption of Electrolyte and Additive Components during the Formation Cycle of Li-Ion Pouch Cells

Understanding the side reactions occurring inside lithium-ion cells remain a significant challenge, presenting opportunities for innovation. In this context, we developed a coupled 1H NMR (nuclear magnetic resonance) and dQ/dV (differential capacity) analysis method aimed at enhancing our understanding of these reactions during the formation cycle. Using deuterated acetonitrile as an extraction solvent, we measured the relative consumption of electrolyte components inside different electrolyte, including in 1.2M LiPF6 EC:EMC 3:7 2% VC and 1.2M LiPF6 EC:EMC 3:7 2% VC + 1% DTD electrolytes at different voltage. Our NMR observations were subsequently compared with electrochemical data to better understand and validate the results. These findings offer valuable insights into lithium-ion cell dynamics and mark a significant step towards the advancement of diagnostic techniques.

Non-Flammable Electrolyte for Large-Scale Ni-Rich Li-Ion Batteries: Reducing Thermal Runaway Risks

This research explores the properties and applications of flame-retardant esters (FREs), specifically triethyl phosphate (TEP) and tributyl phosphate (TBP), within electrolyte systems. The investigation assesses their roles as additives (30%v/v) and primary solvents (80%v/v) in conjunction with carbonate-based electrolytes (1.0 M LiPF6 in EC:DEC:EMC) and a novel non-flammable electrolyte formulation (1.2 M LiPF6 in 70%v/v TEP with 30%v/v fluoroethylene carbonate, FEC). Results indicate that incorporating FREs as additives adversely affects cell performance and maintain flammability risks, necessitating alternative strategies for achieving nonflammability. However, utilizing FREs as primary solvents alongside carbonate solvents shows promise for developing non-flammable electrolytes, despite concerns such as reductive decomposition that may limit suitability for certain battery cells. Conversely, the incorporation of 70%v/v FREs with 30%v/v FEC in the new electrolyte formulation yields a stable solid electrolyte interface, inhibiting FRE decomposition and sustaining battery cycling performance. This combination offers a potential solution for producing non-flammable electrolytes, serving as a viable alternative to solid-state electrolytes. The compatibility of the 1.2 M LiPF6 in TEP:FEC, 70:30 %v/v formulation with existing battery manufacturing processes further enhances its applicability. Figure 1

Unraveling Morphology and Chemistry Dynamics in Fluoroethylene Carbonate Generated Silicon Anode Solid Electrolyte Interphase across Delithiated and Lithiated States

The silicon (Si) solid electrolyte interphase (SEI) faces cyclical cracking and reconstruction due to the ~350% volume expansion. Understanding the SEI dynamic morphology and chemistry evolution from delithiated to lithiated states is thereby paramount to engineering a stable Si anode. Fluoroethylene carbonate (FEC) is a preferred additive with widely demonstrated enhancement of the Si cycling. Thus, insights into the dynamics of the FEC-SEI may provide hints toward engineering the Si interface. Herein, complementary ATR-FTIR, AFM, tip IR, and XPS probing reveal the presence of an elastomeric polycarbonate-like matrix in the FEC-generated SEI which is absent from the FEC-free SEI. Adding FEC to the baseline 1 M LiPF6 in EC:EMC (1:1) electrolyte promotes formation of a thinner and more conformal SEI, and subdues morphology and chemistry changes between consecutive half-cycles. From AFM, morphological stabilization of the FEC-SEI occurs earlier. Furthermore, conventional SEI biproducts such as Li2CO3 and LiEDC appear in reduced quantities in the FEC-SEI implying a reduced quantity of Li-consuming species. The thin polymeric FEC-SEI enables deeper (de)lithiation of silicon. In conclusion, the enhanced mechanical compliance, chemical invariance, and reduced Li inventory consumption of the FEC-SEI are logically the key features underlying the Si cycling enhancement by FEC.

Formation and Cycling of Microscale Silicon Anodes at Elevated Temperatures with FEC-Based and EC-Based Electrolytes

Silicon is a promising candidate to replace graphite (372 mAh g-1) as anode material in lithium-ion batteries (LIBs) due to its high theoretical gravimetric capacity of 3589 mAh g-1.1 Graphite is already partially substituted by silicon in commercial battery applications in order to increase the energy density of LIBs.2 However, a complete transition to silicon is still hindered by the extreme volume expansion of up to 300% upon complete electrochemical lithiation to Li15Si4.3 This causes particle cracking, electrical contact loss of active material, and the continuous (re-)formation of the solid electrolyte interphase (SEI) on the newly formed surface, leading to a continuous loss of cyclable lithium and a fast capacity fading of the cell.One approach to mitigate Si particle cracking is the partial lithiation of microscale silicon, limiting the capacity to ~1200 mAh g-1, i.e., one-third of the theoretical capacity, enabling a cycle-life of more than 200 cycles before reaching 80% state-of-health (SOH).4 While electronic disconnection of lithiated silicon contributes to the overall cyclable lithium losses, the continuous formation of the SEI remains one of the major challenges that limit the lifetime.5 Fluoroethylene carbonate (FEC) has been shown to be one of the most suitable additives for silicon-containing cells to ensure the formation of a suitable SEI and comparably stable cycling performance.6,7 However, especially at elevated temperatures, FEC decomposes to form vinylene carbonate and hydrogen fluoride, which is detrimental to the SEI.8 This study investigated temperature-dependent cycle-life stability of microscale-silicon dominant anodes with EC-based and FEC-based electrolytes at room and elevated temperature. The electrodes consist of 70% silicon (Wacker Chemie AG) with an LiPAA binder (8%) and conductive carbon (2%). Additionally, graphite (8% KS6L, Imerys) was added to enhance the electronic conductivity, whereby within the operating voltage range of the anode, only a minor capacity of the graphite is used. As electrolytes, either 1 M LiPF6 in EC:DEC 1:2 v:v (EC-based) or 1 M LiPF6 in FEC:DEC 2:8 v:v (FEC-based) were used. Full-cells were cycled with a nickel-rich transition metal oxide cathode active material (LiNi0.83Co0.12Mn0.05O2 (NCM831205), BASF SE) based cathode at 25 °C and 45 °C. As shown in Figure 1, cycling in coin cells revealed the expected decreased cycle-life of cells with the FEC-based electrolyte at 45 °C (light blue) compared to 25 °C (dark blue).8 However, cycling cells with the FEC-free, EC-based electrolyte (green) at elevated temperatures, showed the opposite trend. The cycle-life until 80% SOH increased by 110 cycles (+100%), surpassing that of the cells with the FEC-based electrolyte at 45 °C. The charge-averaged mean voltages for both charge and discharge were investigated to identify the main failure mechanisms. This allowed us to differentiate between the loss of cyclable lithium and internal cell resistance build-up.To better understand the contributions of anode and cathode to capacity fading, we assembled Swagelok® T-Cells with the same cell chemistry and cycled them analogously. While the resistance of the cathode remained nearly constant over the whole course of cycling, the resistance of the anode showed an increase with decreasing cell capacity. Furthermore, online electrochemical mass spectrometry (OEMS) was performed to determine the gases evolved during formation with the EC-based electrolyte at 25 °C and 45 °C. This allowed us to gain insights into the overall SEI formation mechanism and to differentiate the formation of suitable SEI components.

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.

Investigation of an eco‐friendly polyacrylic acid binder system on LiFePO4 cathode electrode processing to enhance the performance of coin‐cell and pouch‐cell graphite||LiFePO4 batteries

AbstractThis study investigates the influence of two types of binders (aqueous and nonaqueous) on the LiFePO4 (LFP) electrode processing and its electrochemical properties. Specifically, polyvinylidene fluoride (PVDF) and polyacrylic acid (PAA) were dissolved in NMP (N‐methyl‐2‐pyrrolidone) or the aqueous solvent (H2O) at varying mass ratios of 5%, 10%, and 15%. Binder durability and inertness were assessed by immersing prepared LFP electrodes in an electrolyte comprising 1.0 M LiPF6 in EC:DEC:DMC (1:1:1 in vol%). Notably, PVDF/NMP 10% and PAA/H2O 10%‐based electrodes displayed good durability without peeling. Electrochemical characteristics were evaluated through cycling voltammetry and galvanostatic cycling with potential limitation. The PAA/H2O 10%‐based‐LFP electrode exhibited a specific capacity of ~148.9 mAh g−1 with a Coulombic efficiency (CE) of around 97.27%, surpassing PVDF/NMP 10%. The graphite||PAA/H2O 10%‐based‐LFP electrode in a full cell demonstrated higher capacity and superior retention after 30 cycles. In a pouch cell (6 cm × 4 cm), utilizing graphite||LFP with PAA/H2O 10%, a capacity of 25.5 mAh was achieved, maintaining 93% capacity with a CE of about 99% after 30 cycles at a rate of 0.1C.

Tris(pentafluoro)phenylborane electrolyte additive regulates the highly stable and uniform CEI membrane components to improve the high-voltage behaviors of NCM811 lithium-ion batteries

Broadening the charging and discharging voltage window of high nickel cathode material NCM811 is the most expected method to improve the high specific energy density of batteries currently, yet the cathode-electrolyte interface (CEI) formed by the oxidized and decomposed products of carbonate-based electrolyte under high voltage are always so unsatisfied. Therefore, a voltage-stabilizer, TPFPB (Tris(pentafluoro)phenylborane), added into baseline electrolyte (1 M LiPF6 in EC:EMC:DMC=1:1:1 vol%) to promote the electrochemical performance of the battery at 4.5 V. The results interpret that the TPFPB-contained NCM811-Li half-cells exhibit high specific capacity (167.10 mAh/g), excellent capacity retention rate (CRR) (75.37 %), and high rate performance (173.3 mAh/g at 5C) during 4.5 V. Meanwhile, through the analysis of the physical characterization techniques. the B- and F-rich interfacial layer, named as CEI film, existing at the interface between the cathode and the electrolyte, produced under 4.5 V, is superior, resulting in impeding the structural collapse of the cathode material and the continued dissolution of transition metal ions (TMn+) from the cathode material, as well as, ameliorate the electrochemical polarization of the battery, ultimately, it can stabilize the electrochemical performance of the battery under high voltage. Therein, the present work elucidate a new and substantial approach to enhance the high-voltage performances of rich-Ni cathode materials.

Safety enhanced novel polymer electrolytes for lithium-ion battery: Anomalous output performance with long term cycling stability by doping and polymer blending

Electrolyte with better safety and stability are the primary concern in the present lithium-ion battery (LIB) technology. Herein, a unique polymer blend electrolyte of polyvinylidene difluoride (PVdF)/poly (acrylonitrile butadiene styrene) (ABS) (75: 25) is fabricated by the non-solvent induced phase inversion method. The structural, morphological, and electrochemical performance of the PVdF/ABS blend is enhanced by incorporating the silica nanoparticles as the ceramic filler by the in-situ method. The gel polymer electrolyte (GPE) constitutes 6 wt % of in-situ synthesised silica in PVdF/ABS blend (PASI-06) exhibits better microporous structure with 80 % porosity. The thermal shrinkage, thermogravimetric analysis (TGA) and flammability test safeguards the exceptional thermal stability of the polymer membrane in LIBs. The polymer membrane activated in the liquid electrolyte (1 M LiPF6 in EC:EMC:DMC (1: 1: 1 v/v)) reveals an electrolyte uptake of 400 %, high electrolyte retention (0.82) and a room temperature ionic conductivity of 5.7 mS cm−1. The better oxidative stability exhibited by the PASI-06 (4.5 V) ensures its applicability with high voltage cathodes. Moreover, the electrochemical performance of the coin cell constitutes of Li/GPE/LFP able to exhibits an initial discharge capacity of 143 mAh g−1 and retained a capacity of 115 mAh g−1 even after 200 cycles at 0.1 C-rate at 25 °C. The long term cycling stability evaluated over 500 cycles at 1 C shows that the cell can exhibit an initial discharge capacity 120 mAh g−1 with 77 % capacity retention over 500 cycles that corresponds to 99 % of coulombic efficiency. These excellent performances exhibited by the PASI-06 GPE guarantee its applicability in thermally stable LIBs with better safety and reliability.

Battery Electrolyte Design for Electric Vertical Takeoff and Landing (eVTOL) Platforms

AbstractThe development of robust and high‐performance battery systems is crucial for the advancement of Electric Vertical Takeoff and Landing (eVTOL) vehicles for urban air mobility. This study evaluates the performance of different lithium‐ion battery chemistries under Electric Vertical Takeoff and Landing (eVTOL) load profiles. The actual flight data coupled with physical models is used to create discharge profiles for testing on developed lithium‐ion cells for eVTOLs. The performance of a standard liquid electrolyte (1.2 M LiPF6 in EC:EMC), labeled Gen‐2, is benchmarked and compared with a fast‐charging electrolyte (1.2 M LiFSI in EC:EMC), labeled XFC. Cell analysis involves the use of various techniques, such as impedance spectroscopy, polarization curves, and capacity retention measurements. Capacity retention is stable for both systems over 500 cycles, but unique discharge capacity trends are observed for different mission segments. During the initial takeoff hover stages, Gen‐2 electrolytes experience substantial voltage fade, while XFC electrolytes maintain consistent behavior. In general, the Gen‐2 electrolyte demonstrated lower discharge overpotentials and higher decay during cycling compared to the XFC electrolyte. This work highlights the complexity of eVTOL battery behavior and provides insights into battery system design, contributing to the advancement of battery energy storage solutions for urban air mobility.

Methods–Temperature-Dependent Gassing Analysis by On-Line Electrochemical Mass Spectrometry of Lithium-Ion Battery Cells with Commercial Electrolytes

The evolution of gases is often associated with the decomposition of the electrolyte or active materials. Thus, its detection can be powerful for understanding degradation mechanisms in Li-ion batteries (LIBs). Here, we present an evaluation method for gas detection and quantification by on-line electrochemical mass spectrometry (OEMS) when using volatile electrolytes (e.g., electrolytes with linear alkyl carbonates) and a new OEMS cell design for improved leak tightness. With a significant fraction of the gases in the cell head-space being electrolyte vapor, we observe a pressure/time-dependency of the electrolyte background in the mass spectrometer, for which we here developed a correction method. We apply this method for the temperature-dependent gas analysis of a graphite/NCM831205 full-cell with an LP57 (1 M LiPF6 in EC:EMC 3:7 wt:wt) electrolyte. We conclude that the activation energy of the gas evolution associated with the formation of the solid-electrolyte interphase (SEI) is ∼15–20 kJ mol−1. Furthermore, we identify a significant temperature dependence of the lithium alkoxide triggered trans-esterification of EMC with an activation energy of ∼70 kJ mol−1. Lastly, the temperature-dependent analysis reveals the relation between the evolution of hydrogen related to water and HF impurities during the initial SEI formation and in situ generated protons.

Unraveling Morphology and Chemistry Dynamics in Fluoroethylene Carbonate Generated Silicon Anode Solid Electrolyte Interphase Across Delithiated and Lithiated States: Relative Cycling Stability Enabled by an Elastomeric Polymer Matrix

The silicon solid electrolyte interphase (SEI) faces cyclical cracking and reconstruction due to the ∼350% volume expansion. Understanding the SEI dynamic morphology and chemistry evolution from delithiated to lithiated states is thereby paramount to engineering a stable Si anode. Fluoroethylene carbonate (FEC) is a preferred additive with widely demonstrated enhancement of the Si cycling. Thus, insights into the dynamics of the FEC-SEI may provide hints toward engineering the Si interface. Herein, complementary ATR-FTIR, AFM, tip IR, and XPS probing reveal the presence of an elastomeric polycarbonate-like matrix in the FEC-generated SEI which is absent from the FEC-free SEI. Adding FEC to the baseline 1 M LiPF6 in EC:EMC (1:1) electrolyte promotes formation of a thinner and more conformal SEI, and subdues morphology and chemistry changes between consecutive half-cycles. From AFM, morphological stabilization of the FEC-SEI occurs earlier. Furthermore, conventional SEI biproducts such as Li2CO3 and LiEDC appear in reduced quantities in the FEC-SEI implying a reduced quantity of Li-consuming species. The thin polymeric FEC-SEI enables deeper (de)lithiation of silicon. In conclusion, the enhanced mechanical compliance, chemical invariance, and reduced Li inventory consumption of the FEC-SEI are logically the key features underlying the Si cycling enhancement by FEC.

Lithium-ion battery functionality over broad operating conditions via local high concentration fluorinated ester electrolytes

Delivered capacities of non-flammable LHCE vs. flammable LiPF6 EC:DMC electrolyte at low temperature, high voltage charge, and fast charge.

Experimental Validation of Newman Model Analysis for Modern Li-Ion Battery Cathode Materials

The Butler-Volmer equation is a fundamental equation extensively used to describe electrochemical kinetics and relates the reaction current at an electrode interface to the voltage. Newman and coworkers suggested an equation (Fig. 1a) for the exchange current density, which includes the state of charge (SoC) dependence of Li-ion batteries as well as theory-based anodic and cathodic transfer coefficients. This description of interface kinetics, part of the commonly used Newman model, is widely used for battery modeling.1,2 Because of the lack of experimental data, several assumptions were made in the derivations,3 such as assuming the intercalation reaction to be a first order reaction, and assuming the transfer coefficients to be 0.5. As new battery materials emerge, it is important to validate whether these assumptions are applicable, or whether the model needs to be extended.In this work, we compare experimentally obtained interface resistances from different cathode materials with the theoretical behavior of the interface kinetics predicted from the Newman model as well as experimental characterizations published in the research literature. The experimental data for the interface resistance is determined using electrochemical impedance spectroscopy,4 where the electrodes are built into a three-electrode cell setup, using a gold wire reference electrode, which allows studying the cathode impedance separately.5 As anode, we use graphite that is prelithiated to ~20% to make sure there is enough lithium in the cell to be able to relithiate the cathode completely. The loading of the electrodes is low to ensure negligible pore resistance contributions, so that the semicircle in the Nyquist-plot originates from kinetics only. The obtained charge transfer resistance is analyzed as a function of Li-content to show how the kinetics of the cathode depend on the degree of lithiation.From the Newman model, it is expected that the interface resistance shows a U-shape over the degree of lithiation with a minimum at 50% SoC with a drastic increase in charge transfer resistance for the first and last 10%. This is because the kinetics are slower when the cathode is completely empty and completely full, which can be seen from the equation in Fig. 1a. When c=c max or c=0, i 0 becomes 0. Fig. 1b. shows the theoretical behavior based on the Newman model, the experimental results found in this study, and experimental results found in the literature for NMC1116.Figure 1: a) Exchange current density suggested by Newman and coworkers. i 0 is the exchange current density, F is the Faraday constant, k a and k c are reaction rate constants for the anodic and cathodic reactions, α a and α c are the anodic and cathodic transfer coefficients, c max is the maximum lithium concentration in the material, c is the lithium concentration in the material, and c l is the salt concentration in the electrolyte. b) Charge transfer resistance vs. Li-content for ~0.5 mAh/cm² NMC111 (Gelon Lib) with prelithiated graphite (Sigma Aldrich) anode, gold wire reference electrode with a core diameter of 50 µm coated with a 7 µm polyimide insulation (Goodfellow), 80 µl 1 M LiPF6 in EC:EMC (3:7) (Gotion), two glass fiber separators (260 µm, VWR) at 25 °C determined from semi-circle width of cathode impedance (100 kHz to 10 mHz, 10 mV amplitude) using a potentiostat (VMP3, BioLogic) recorded in three electrode Swagelok cell (average of 3 measurements) is represented by dark grey points. The red points show theory according to Eq. 1 with, and the light brown points show NMC111 from Ref. 6. The lines between the points are drawn to enhance the features of the figure.Comparison of experimental data and the theoretical framework shows pronounced discrepancies, which cannot be resolved if the transfer coefficients are kept at 0.5. Similar measurements are conducted for a wide range of active materials and changed environmental conditions to further elaborate validity and/or necessity to modify the commonly used description of the interface kinetics relation. This will help to better understand interface resistance in battery cells and thereby allow optimization of material properties by, e.g., surface modification. References M. Ecker et al., J. Electrochem. Soc., 162, A1836 (2015).T. F. Fuller, M. Doyle, and J. Newman, J. Electrochem. Soc., 141, 1–10 (1994).M. Doyle, T. F. Fuller, and J. Newman, J. Electrochem. Soc., 140, 1526–1533 (1993).A. Lasia, in Electrochemical Impedance Spectroscopy and its Applications, A. Lasia, Editor, p. 203–250, Springer, New York, NY (2014).S. Solchenbach, D. Pritzl, E. J. Y. Kong, J. Landesfeind, and H. A. Gasteiger, J. Electrochem. Soc., 163, A2265–A2272 (2016).R. Morasch, H. A. Gasteiger, and B. Suthar, J. Electrochem. Soc., 170, 080522 (2023). Figure 1

Synergistic Impact of Electrolyte Components: Interplay between the Novel Phosphorus-Based Additives and Conducting Salt Advancing NMC811||SiC Cell Performance

A lithium-ion battery (LIB) typically consists of an anode, a lithiated transition metal oxide cathode, and an electrolyte formulation containing inorganic lithium conducting salt in aprotic organic solvents/co-solvents, and functional additives. In-depth research is being conducted in order to develop novel chemistries for the next-generation LIBs. Within the line, silicon-based anodes and nickel-rich cathodes are recognized as two feasible next-generation electrode materials. The long-term galvanostatic cycling performance of resulting cell chemistries relies on the effectivity and robustness of solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) formed during the initial cycles and originate from the galvanostatic cycling conditions, as well as the complex interplay between electrolyte and corresponding electrodes .[1] To enhance the SEI and CEI properties, implementing functional electrolyte additives into baseline electrolyte has been shown to be one of the most effective and cost-favorable strategies. Film-forming additives sacrificially reduce and/or oxidize at corresponding electrodes to form protective layers (SEI and CEI).Previous research has shown that phosphorus-based additives improve electrode |electrolyte interfaces stability and simultaneously enhance cell flame-retardant capability advancing the overall cell performance.[2,3] In this report, we indicated part of the above-mentioned properties originate from the synergy between lithium hexafluorophosphate (LiPF6) which is the state-of-the-art (SOTA) electrolyte conducting salt in LIBs and considered functional additives. Here, we demonstrated the functionality of 2-phenoxy-1,3,2-dioxaphospholane (PhEPi) as a novel film-forming additive[4] and 2,2,4,4,6-pentafluoro-6-(2,2,2-trifluoroethoxy)-1,3,5,2λ5,4λ5,6λ5-triazatriphosphinine (CF3PFFPN) as a new flame-retardant additive, synthesized for the first time by our group. The presence of PhEPi and CF3PFPN at the optimum concentrations in the baseline electrolyte (1 M LiPF6 in EC: EMC 3:7 + 8 % VC) enhances the performance and safety of the resulting NMC811||Si-graphite (20% Si) cell chemistry. Impact of phospholane-based additive was confirmed by substituting LiPF6 with LiBF4 conducting salt and complementary post mortem analysis. This study along with improved cell chemistry, provided a better understanding and deeper insights into the synergistic interplay and impact of considered electrolyte components on the resulting NMC811||Si-graphite cell chemistry. [1] C. Wölke, B. A. Sadeghi, G. G. Eshetu, E. Figgemeier, M. Winter, I. Cekic-Laskovic, Advanced Materials Interfaces 2022, 9, DOI 10.1002/admi.202101898.[2] N. Von Aspern, D. Diddens, T. Kobayashi, M. Börner, O. Stubbmann-Kazakova, V. Kozel, G. V. Röschenthaler, J. Smiatek, M. Winter, I. Cekic-Laskovic, ACS Applied Materials and Interfaces 2019, 11, 16605–16618, DOI 10.1021/acsami.9b03359.[3] N. von Aspern, C. Wölke, M. Börner, M. Winter, I. Cekic-Laskovic, Journal of Solid State Electrochemistry 2020, 24, 3145–3156, DOI 10.1007/s10008-020-04781-1.[4] B. A. Sadeghi, C. Wölke, F. Pfeiffer, M. Baghernejad, M. Winter, I. Cekic-Laskovic, Journal of Power Sources 2023, 557, 232570, DOI 10.1016/j.jpowsour.2022.232570.

(Digital Presentation) Unraveling of the Morphology and Chemistry Dynamics in the FEC-Generated Silicon Anode SEI across Delithiated and Lithiated States

The silicon solid electrolyte interphase (SEI) faces cyclical cracking and reconstruction due to the ~350% volume expansion of Si which leads to shortened cell life during electrochemical cycling. Understanding the SEI morphology/chemistry and more importantly its dynamic evolution from delithiated and lithiated states is paramount to engineering a stable Si anode. Fluoroethylene carbonate (FEC) is a preferred additive with widely demonstrated enhancement of the Si cycling. Thus, insights into the effects of FEC on the dynamics of the resulting SEI may provide hints toward engineering the Si interface. Herein, ATR-FTIR, AFM, tip IR, and XPS probing all show pronounced relative invariance of the FEC-generated SEI compared to the FEC-free SEI between adjacent lithiated and delithiated states beyond the formation cycles. The SEI of Si thin film model surfaces in the baseline 1 M LiPF6 in EC:EMC (1:1) undergoes major morphological and chemical speciation swings between half-cycles while comparatively the SEI upon addition of FEC displays far less dynamic evolution. This morphology and chemistry stability of the FEC-SEI supports the enhanced cycling stability of silicon anodes in FEC-containing electrolytes. The experimental evidence gathered suggests that the FEC-SEI invariance is enabled by an elastomeric polycarbonate matrix that preserves the SEI integrity against the expansion of silicon upon lithiation. In turn, less electrolyte-consuming reconstruction occurs which manifests as and high LiF content from one half-cycle to the next. This work provides critical insights to enhance the silicon anode stability via targeted SEI engineering, namely that LiF protected by an elastomeric protective matrix may be key to buffering the unavoidable mechanical disruption. Figure 1

Investigating the Change in the Microporous Structure of the Graphite Electrode during Formation

Till today, the graphite-based electrode is the most widely used negative electrode in commercially available lithium-ion batteries (LIBs).[1] While extended research has been conducted in the investigation of the graphite electrode’s properties, especially the formation of the solid-electrolyte interphase (SEI) within the first cycles of LIBs remains a field of ongoing research.[2] The SEI forms during the first cycle(s) on the anode surface at potentials below the stability window of the electrolyte components, due to the reductive decomposition of the respective solvents, additives, and conductive salts.[2,3] The initially formed SEI has an influence both on the long-term cycling stability of the cell, depending on how good the passivation, i.e., electronic insulation, of the graphite surface is,[4] and on the rate performance of the anode, depending on the thickness of the SEI, its chemical composition, and its lithium-ion conductivity, referred to as SEI resistance.[5] Additionally, with a thickness of 10-20 nm when employing additives such as vinylene carbonate (VC), the SEI takes up parts of the void volume of the pristine electrode and may change the pores and the electrolyte conduction pathways within the electrode (see Figure 1).[6] In this study, we investigated the change of the microporous structure of an artificial graphite (MAGE-5, Hitachi, Japan) with an LP572 electrolyte (1 M LiPF6 in EC:EMC 3:7 w:w with 2 wt% VC, Gotion, USA) during formation at 45 °C. The electrode’s microstructure is often connected to the macroscopic properties using the McMullin number N M in the form of:[7] N M = τ / ε = R Ion ·A · κ / d, (1)correlating the porosity ε, electrodes thickness d, ionic pore resistance R Ion, area of the electrode A, conductivity of the electrolyte κ, and the tortuosity τ (see Figure 1).Here, we analyzed graphite electrodes with varying initial porosities from 20 to 50 %, employing electrochemical impedance spectroscopy (EIS) in a MAGE//Li half-cell setup with a micro-reference electrode and a free-standing graphite electrode.[8,9] Measuring EIS at temperatures of -5 °C allowed the reliable determination of the ionic resistance before and after formation. Due to the formed SEI covering the graphite particles and partially filling the void of the porous electrode, we discovered an increase of the ionic pore resistance throughout the whole range of initial porosities.Next, we determined a porosity dependent increase in thickness of the electrodes from electrodes cycled at the same conditions in coin cells that were subsequently harvested from the cells and analyzed for thickness changes. Additionally, mercury intrusion porosimetry (MIP) was used to determine the porosity of formed graphite electrodes, revealing a decrease in porosity after formation for all samples. The combination of these findings - using Equation 1 as described above with a constant electrode area A and assuming a constant electrolyte conductivity κ - allowed us to evaluate the change in the tortuosity of the graphite electrodes, giving insights into the overall changes in the graphite anode properties due to SEI formation.