Ethyl Methyl Carbonate Research Articles

Transition Metal Dissolution in Lithium-Ion Cells: A Piece of the Puzzle

Static leaching tests were performed using the chemically delithiated positive electrode materials, LiFePO4, LiCoO2, LiMn2O4, LiNiO2, and LiNi0.8Mn0.1Co0.1O2. Instead of the common electrolyte, which contains LiPF6, the solvent consisted of only ethylene carbonate (EC) and ethyl methyl carbonate (EMC), limiting the possible reactions to only those that depend on the solvent. The product liquids from these experiments showed that there were lithium-bearing species in common, such as Li(EC)+ and Li(EMC)+. Interestingly, we found evidence of electrolyte degradation products in both the positive- and negative-ion mass spectral results. The positive-ion results showed that the products tended to coordinate to lithium. The negative-ion results showed that most of the products tended to complex with transition metals. It was difficult to discern which positive ion was associated with which negative ion.

Improved SiGr/NCM523 Cycling via Triethyl Phosphate-Solubilized Lithium Nitrate Electrolyte

The performance of silicon–graphite (SiGr)/Li and SiGr/LiNi0.5Co0.2Mn0.3O2 (NCM523) cells was investigated in electrolytes containing lithium difluoro(oxalato)borate (LiDFOB) and lithium nitrate (LiNO3) in ethylene carbonate (EC), ethylmethyl carbonate (EMC), and different proportions of triethyl phosphate (TEP). While the LiDFOB and LiNO3-containing electrolytes both showed improved cycling stability over industry standards, the 10% TEP electrolyte demonstrated the best capacity retention in SiGr/Li cells and the best overall cycling performance in SiGr/NCM523 cells. Ex-situ surface analysis reveals that the LiDFOB and LiNO3 preferentially decompose on both the anode and cathode surface during the first cycle to generate films consisting of oxalates, fluoroborates, and nitrate reduction products. Therefore, incorporation of two different film-forming additives into the electrolyte is significantly more beneficial in formation of a robust SEI and improved cycle life in silicon-containing anodes.

Tunable Transesterification of Dimethyl Carbonate with Ethanol on K2CO3/Al2O3 Catalysts: Kinetic Modeling

The light alkyl carbonates are indispensable co-solvents of electrolytes for lithium-ion batteries. In this work, the tunable transesterification of dimethyl carbonate (DMC) with ethanol to co-produce ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) was investigated based on an efficient catalytic system with the solid alkaline catalyst. It was found that the γ-Al2O3-supported K2CO3 catalyst exhibited high activity for the transesterification reaction, with a tunable feature for the selectivities of EMC and DEC by changing the space velocity and feed ratio of DMC to ethanol at the temperature between 120 and 150 °C. According to the results of characterizations and activity tests, a synergistic mechanism was proposed. The reaction kinetic model was also established, which displayed good agreement with the experimental results.

Effective SEI Formation via Phosphazene‐Based Electrolyte Additives for Stabilizing Silicon‐Based Lithium‐Ion Batteries

AbstractSilicon, as potential next‐generation anode material for high‐energy lithium‐ion batteries (LIBs), suffers from substantial volume changes during (dis)charging, resulting in continuous breakage and (re‐)formation of the solid electrolyte interphase (SEI), as well as from consumption of electrolyte and active lithium, which negatively impacts long‐term performance and prevents silicon‐rich anodes from practical application. In this work, fluorinated phosphazene compounds are investigated as electrolyte additives concerning their SEI‐forming ability for boosting the performance of silicon oxide (SiOx)‐based LIB cells. In detail, the electrochemical performance of NCM523 || SiOx/C pouch cells is studied, in combination with analyses regarding gas evolution properties, post‐mortem morphological changes of the anode electrode and the SEI, as well as possible electrolyte degradation. Introducing the dual‐additive approach in state‐of‐the‐art electrolytes leads to synergistic effects between fluoroethylene carbonate and hexafluorocyclotriphosphazene‐derivatives (HFPN), as well as enhanced electrochemical performance. The formation of a more effective SEI and increased electrolyte stabilization improves lifetime and results in an overall lower cell impedance. Furthermore, gas chromatography‐mass spectrometry measurements of the aged electrolyte with HFPN‐derivatives as an additive compound show suppressed ethylene carbonate and ethyl methyl carbonate decomposition, as well as reduced trans‐esterification and oligomerization products in the aged electrolyte.

Partially fluorinated electrolyte for high-voltage cathode with sulfurized carbon anode from polyacrylonitrile for lithium-ion battery

A new lithium-ion battery is configured by coupling sulfurized carbon anode and high voltage LiNi0.5Mn1.5O4 (LNMO) cathode. The anode is derived from sulfurized polyacrylonitrile (S-C(PAN)). Severe capacity fading usually becomes unavoidable due to the oxidative decomposition of solvents, primarily when a conventional carbonate electrolyte with 1 M lithium hexafluorophosphate (LiPF6) is employed. Fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and 1, 1, 2, 2-Tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether (TTE) are formulated as the best electrolyte (3:2:5 in vol. ratio) for this new high-voltage lithium-ion battery to mitigate this capacity fading and improve the adaptability of the S-C(PAN) and LNMO. The discharge capacity of full cell made with 1 M lithium hexafluorophosphate (LiPF6) in FEC/EMC/TTE (3:2:5) electrolyte reaches 688 mAh g−1 at a rate of 2 C, while 19 mAh g−1 for the control electrolyte. X-ray photoelectron spectroscopy (XPS) results confirm that the fluorinated electrolyte stabilizes both surfaces of S-C(PAN) and LNMO in the full cell effectively. Compared to the control electrolyte, the developed electrolyte enhances the cyclic stability and rate capability of both half cells (Li//S-C(PAN and Li//LiNi0.5Mn1.5O4) and S-C(PAN)//LiNi0.5Mn1.5O4 full cells.

Strain Compensation Methods for Fiber Bragg Grating Temperature Sensors Suitable for Integration into Lithium-Ion Battery Electrolyte

Temperature is a crucial factor for the safe operation of lithium-ion batteries. During operation, the internal temperature rises above the external temperature due to poor inner thermal conductivity. Various sensors have been proposed to detect the internal temperature, including fiber Bragg grating sensors. However, to the authors’ knowledge, there is no detailed description of the encapsulation of the fiber Bragg grating sensor in the literature to shield it from strain. In this study, different encapsulation methods for strain compensation were compared to find the encapsulation material most compatible with the electrolyte. For this, we stored the proposed sensors with different encapsulation methods in ethylene carbonate:ethyl methyl carbonate (EC:EMC) 3:7 with LiPF6 (lithium hexafluorophosphate) electrolyte and applied temperature changes. After evaluating the sensor encapsulation methods in terms of handling, diameter, uncertainty, usability, and hysteresis behavior, the most suitable sensor encapsulation was found to be a fused silica capillary with polyimide coating.

Mechanical Evolution of Solid Electrolyte Interphase on Metallic Lithium Studied by in situ Atomic Force Microscopy

Metallic lithium is a promising candidate as anode material in lithium batteries due to its high specific capacity and cell voltage. However, the high reactivity of metallic lithium leads to a rapid formation of the solid electrolyte interphase (SEI), even without an applied voltage. Unfortunately, the formation mechanism of the SEI is not yet fully understood. An exact understanding of the SEI includes mechanical properties, such as stiffness. Here, the mechanical properties during SEI formation are studied in an electrolyte consisting of 1.2 M LiPF6 in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) without applied potential using in situ atomic force microscopy (AFM). The formation of the SEI is investigated by mapping the mechanical evolution of the surface. Thereby, changes of surface composition are visualised over time. Moreover, for the addition of vinylene carbonate as an additive to the electrolyte, the impact on the morphology as well as the stiffness is demonstrated.

Mechanistic understanding of the correlation between structure and dynamics of liquid carbonate electrolytes: impact of polarization.

Liquid electrolyte design and modelling is an essential part of the development of improved lithium ion batteries. For mixed organic carbonates (ethylene carbonate (EC) and ethyl-methyl carbonate (EMC) mixtures)-based electrolytes with LiPF6 as salt, we have compared a polarizable force field with the standard non-polarizable force field with and without charge rescaling to model the structural and dynamic properties. The result of our molecular dynamics simulations shows that both polarizable and non-polarizable force fields have similar structural factors, which are also in agreement with X-ray diffraction experimental results. In contrast, structural differences are observed for the lithium neighborhood, while the lithium-anion neighbourhood is much more pronounced for the polarizable force field. Comparison of EC/EMC coordination statistics with Fourier transformed infrared spectroscopy (FTIR) shows the best agreement for the polarizable force field. Also for transport quantities such as ionic conductivities, transference numbers, and viscosities, the agreement with the polarizable force field is by far better for a large range of salt concentrations and EC : EMC ratios. In contrast, for the non-polarizable variants, the dynamics are largely underestimated. The excellent performance of the polarizable force field is explored in different ways to pave the way to a realistic description of the structure-dynamics relationships for a wide range of salt and solvent compositions for this standard electrolyte. In particular, we can characterize the distinct correlation terms between like and unlike ions, relate them to structural properties, and explore to which degree the transport in this electrolyte is mass or charge limited.

A Low‐Concentration Electrolyte for High‐Voltage Lithium‐Metal Batteries: Fluorinated Solvation Shell and Low Salt Concentration Effect

AbstractConcentration of electrolyte has significant effects on performances of rechargeable batteries. Previous studies mainly focused on concentrated electrolytes. So far, only several recipes on low‐concentration electrolytes were studied, performing enhanced performance in advanced rechargeable batteries. Here, based on common electrolyte components, a low‐concentration electrolyte composed of 0.2 M lithium hexafluorophosphate (LiPF6) solvated in fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) is employed for high‐voltage Li metal battery. The synergistic working mechanisms of introducing fluorine‐containing solvent in the solvated structure and low salt concentration effect are revealed, resulting in LiF‐rich, uniform, and robust solid electrolyte interphase layer and fewer unfavorable decomposition products. As a result, this low‐concentration electrolyte significantly enhances electrochemical performances of Li||Li symmetric cells and high‐voltage LiCoO2||Li batteries.

A Low-Concentration Electrolyte for High-Voltage Lithium-Metal Batteries: Fluorinated Solvation Shell and Low Salt Concentration Effect.

Concentration of electrolyte has significant effects on performances of rechargeable batteries. Previous studies mainly focused on concentrated electrolytes. So far, only several recipes on low-concentration electrolytes were studied, performing enhanced performance in advanced rechargeable batteries. Here, based on common electrolyte components, a low-concentration electrolyte composed of 0.2 M lithium hexafluorophosphate (LiPF6 ) solvated in fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) is employed for high-voltage Li metal battery. The synergistic working mechanisms of introducing fluorine-containing solvent in the solvated structure and low salt concentration effect are revealed, resulting in LiF-rich, uniform, and robust solid electrolyte interphase layer and fewer unfavorable decomposition products. As a result, this low-concentration electrolyte significantly enhances electrochemical performances of Li||Li symmetric cells and high-voltage LiCoO2 ||Li batteries.

Electrolyte recovery from spent Lithium-Ion batteries using a low temperature thermal treatment process

Electrolyte recovery is seldomly considered in state-of-art lithium-ion battery recycling methods but rather evaporates and decomposes uncontrolled during the pre-treatment steps. However, controlled and safe removal of the electrolyte is inevitable and of high importance to the recycling industry to minimize the environmental impact of the recycling processes by preventing severe threats produced by the inflammable, toxic and hazardous components of the electrolyte. This study investigated the effects of temperature and process time of a low temperature thermal treatment process on electrolyte recovery. The process exhaust gases and recovered products were analyzed by In-Situ Fourier-transform infrared spectroscopy (FT-IR) and gas chromatography-mass spectrometry (GC–MS) to determine the effectiveness of the significant process parameters. The results show that the electrolyte solvents, which are dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC), were successfully recovered for 80 minutes of processing time at 130 °C. The LiPF6 decomposition products hydrogen fluoride (HF) and phosphoryl fluoride (POF3) were detected in the exhaust gas stream and recovered as acidic solutions. Thermal treatment below 150 °C is a promising approach for the recovery of the electrolyte solvents prior to the metal recycling stage due to its simplicity, feasibility, and environmental benefit.

DBU-assisted expeditious synthesis of highly stable IL@ZIFs intergrowth composite: A nanocatalyst for EMC production

Methyl ethyl carbonate (EMC) is the primary raw material for the electrolyte of lithium-ion batteries. It is mainly prepared by the transesterification reaction of dimethyl carbonate (DMC) and EtOH. However, homogeneous alkaline catalysts such as sodium alcohol commonly used in industry are easy to deactivate and challenging to separate and recover.Therefore, the alkaline ionic liquid decorated zeolitic imidazolate frameworks (IL@ZIFs) heterogeneous catalysts were prepared using a simple method with DBU as an inducer. In the synthetic route of IL@ZIFs, the ligand of ZIFs can form an ionic liquid with DBU, and deprotonated ligands will rapidly coordinate with metal ions at room temperature. The DBU can also play the structural guidance and stabilizer, which is conducive to the rapid and effective preparation of ZIFs materials. Meanwhile, the DBU, which has a large molecular diameter, will be trapped in the pore cage and can combine with other protic materials (imidazole, triazole, 2-methylimidazole, phenol, etc.) to form alkaline ionic liquids. The prepared DBU-IL@ZIFs were used as heterogeneous catalysts for synthesizing EMC by transesterification reaction. The DBU-PO@ZIF-8 composite with phenol as anion has better catalytic activity. Under the appropriate reaction conditions, the yield of EMC can reach 55.7% with high selectivity of 94.7%.The catalytic mechanism of DBU-PO@ZIF-8 in the transesterification reaction was also explored and revealed.

Chemometric and Machine Learning Analysis of Lithium Concentration and Solvation Behavior in Li-Ion Battery Electrolytes

The demand for batteries is rapidly growing across a range of technologies. The increasingly diverse use cases for batteries require various capabilities, particularly requirements for high energy densities, that are currently unmet by traditional Li-ion batteries. Electrolyte stability proves to be a bottleneck for battery advancement towards energy dense chemistries beyond Li-ion, including metal anodes. In situ spectroscopy tools, such as Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray spectroscopy, have provided insight into critical molecular-level interactions in batteries during cycling. These in situ tools have yielded continuous improvement of electrolyte properties. Spectroscopy datasets, however, contain many nuances that challenge meaningful human understanding. Artificial intelligence and chemometric tools can be coupled with in situ spectroscopy to find relevant interpretations of spectral datasets and elucidate complex molecular phenomena. In this research, an analysis was performed on FTIR spectroscopy data from an electrolyte composed of LiPF6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) to discern solvation behavior using principal component analysis (PCA) and a convolutional neural network (CNN). PCA pinpointed exact band locations of solvation shifting behavior in the IR spectra and improved understanding of the relationship between lithium concentrations and peak changes. The CNN was trained with spectral datasets of electrolytes with known lithium concentrations and then could predict lithium concentrations from spectral datasets with extraordinarily high accuracy. Additionally, the CNN interpreted FTIR spectral datasets from a graphite half-cell with EC/EMC/LiPF6 electrolyte and accurately determined the lithium concentration in the bulk electrolyte. The CNN also observed lithium depletion events (up to 10% lithium depletion) in the graphite anode during fast-charging cycles of the galvanostatic intermittent titration technique. This research breaks new ground on using advanced computational tools for in situ spectroscopic analysis of battery electrolytes and demonstrates an improved understanding of complex molecular-level phenomena in electrolytes.

In Situ Multi-Modal Approach for Electrode-Electrolyte Interfacial Chemistryand Electrode and Electrolyte Aging Behavior Studies

Increasing demands for cost-effective electric vehicles and electrochemical energy storage systems require advanced secondary batteries with higher energy density, longer lifetime, and enhanced safety. Increase of the battery operating voltage is one realistic strategy to extend the energy density, but it is accompanied by irreversible structural changes to the electrodes and parasitic reactions within an electrode-electrolyte interphase resulting in capacity fading and subsequent battery failure. Therefore, a fundamental understanding of underlying electrochemical mechanisms in the electrodes during battery cycling is critical for the development of next-generation secondary batteries.Based on our previous studies utilizing in situ surface-enhanced Raman spectroscopy to monitor the evolution of the Si-electrolyte interphase1 and in situ ATR-FTIR to investigate the voltage dependent electrolyte solution structure changes at the interface, transition metal redox chemistry, and cathode/electrolyte interfacial layer evolution,2 recently we updated the in situATR-FTIR with a newly designed ‘full-cell’ that enables us to analyze the interfacial reactions and interactions on the surfaces of both electrodes that directly influence battery performance, lifetime, and safety. Specifically, we focus on transition metal complex formation and its effect on solid-electrolyte interphase (SEI) formation and evolution on the anode due to transition metal dissolution and crosstalk of Mn-rich cathodes at high voltages (≥4.4 V).In addition, using our multi-modal characterization—a combination of in situ gas chromatography with flame ionization detection (GC-FID) and in situ ATR-FTIR—(electro)chemical degradation of the electrolyte is correlated with gas evolution in the battery. For example, GC-FID measures increasing amounts of ethylene gas until the end the first charging cycle of a LiNiO2//Graphite cell with Gen2 electrolyte (LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC), 3:7 wt.%). Concurrently, in situ ATR-FTIR shows decreasing concentration of EC solvent, as calculated from the FTIR intensity of EC vibrational absorption. Ethylene is a known by-product of ethylene carbonate electrochemical reduction and is produced during SEI formation.3 Quantifying multiphase reactions occurring during battery operation is critical to understanding and mitigating battery degradation pathways, and developing next-generation battery materials systems.

Lithium Difluorophosphate (LiPO2F2): An Electrolyte Additive to Help Boost Low-Temperature Behaviors for Lithium-Ion Batteries

A promising lithium salt of Li difluorophosphate (LiPO2F2) is introduced and added to the basic electrolyte (1 M LiPF6 + dimethyl carbonate (DMC)/ethyl methyl carbonate (EMC)/propylene carbonate (PC)/fluoroethylene carbonate (FEC)) to enhance the electrochemical performance of lithium-ion batteries by changing its concentration at low temperatures. Experiments show that LiPO2F2 can obviously improve the electrolyte’s ionic conductivity, especially. Compared with the baseline (6.29, 5.14, 4.62, 3.77, and 3.15 mS cm–1) under the same conditions, the ionic conductivities of the 2 wt % LiPO2F2-containing electrolyte (2 LiPO2F2) are 7.18, 5.32, 5.23, 4.04, and 3.57 mS cm–1. From this, it is clear that the integrated low-temperature electrochemical performances have been significantly improved. The specific discharge capacities of the 2 wt % LiPO2F2-added electrolyte at 0.2C are 161.4, 157.2, 148.9, and 137.5 mAh g–1 in the range from 25 to −40 °C, which are significantly much higher compared with the basic electrolyte (145.3, 149.4, 135.1, and 107.0 mAh g–1, respectively). Additionally, the addition of LiPO2F2 can promote profoundly the composition and morphology characteristics of the cathode–electrolyte interface (CEI) film; namely, it is a uneven lithium alkyl carboxylate before the addition of LiPO2F2; after that, it becomes an even LiF film, and meanwhile, the new film will facilitate Li+ transport in the electrolyte, further boosting the eletrochemical performance of the cells under low temperatures. At −40 °C under the same current density, the specific discharge capacity is 81.97 mAh g–1 after 50 cycles, while at the same temperature, the specific discharge capacity of the basic electrolyte battery is only 33.37 mAh g–1; that is, the discharge capacity has a staggering 145.6% improvement at −40 °C. In conclusion, the lithium-ion batteries containing LiPO2F2 may meet the application requirements of ultra-low-temperature conditions.

Current-driven solvent segregation in lithium-ion electrolytes

Liquid lithium-battery electrolytes universally incorporate at least two solvents to balance conductivity and viscosity. Almost all continuum models treat cosolvent systems such as ethylene carbonate:ethyl-methyl carbonate (EC:EMC) as single entities whose constituents travel with identical velocities. We test this “single-solvent approximation” by subjecting LiPF 6 :EC:EMC blends to constant-current polarization in Hittorf experiments. A Gaussian process regression model trained on physicochemical properties quantifies changes in composition across the Hittorf cell. EC and EMC are found to migrate at noticeably different rates under applied current, demonstrating conclusively that the single-solvent approximation is violated and that polarization of salt concentration is anticorrelated with that of EC. Simulations show extreme solvent segregation near electrode/liquid interfaces: a 5% change in EC:EMC ratio, post-Hittorf polarization, implies more than a 50% change adjacent to the interface during the current pulse. Understanding how lithium-ion flux induces local cosolvent or additive imbalances suggests new approaches to electrolyte design. • Applied current rearranges neutral cosolvents in lithium-ion electrolytes • A solution-property machine-learning model pinpoints electrolyte composition changes • This polarization effect is expected to be most severe near electrode interfaces While it is understood that electrolyte solvents are consumed throughout the lifetime of battery operation, less is known about intra-cycle variation in cosolvent composition. Wang et al. pair polarization experiments with machine learning to measure solvent segregation occurring under applied current. This effect is especially prominent near electrode surfaces and may illuminate guiding principles for electrolyte design.

Femtosecond multiphoton ionization of dialkyl carbonate green solvents at 400 nm using photoelectron-photoion coincidence imaging

Femtosecond laser based multiphoton ionization experiments have been carried out for the first time on dialkyl carbonate green solvent molecules i.e. dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate (DMC, DEC, EMC). The dissociation products produced consequent to ionization have been studied by detecting electrons and ions in coincidence mode using velocity map imaging technique. Based on detailed comparative analysis of the mass spectra of DMC, DEC and EMC, we propose possible mechanisms involving a combination of direct bond cleavage as well as isomerization and reorganization to generate the final dissociation products. Further, photoelectron kinetic energy release distributions measured in coincidence with various fragment ions correlated with the possible fragmentation pathways and compared for all three molecules offers a validation of the proposed mechanisms. As an offshoot we obtain experimental estimates for the ionization potentials of DEC and EMC which are hitherto unreported.

Optimization and model-based control of sustainable ethyl-methyl carbonate and diethyl carbonate synthesis through reactive distillation

As excellent solvents of lithum-ion electrolyte, ethyl-methyl carbonate (EMC) and diethyl carbonate (DEC) are synthesized through transesterification of dimethly carbonate (DMC) and ethanol. Process intensification, optimization and the corresponding control scheme of the synthesized process are essential when sustainability is becoming a necessity. In the present work, the intensified flowsheet for EMC and DEC production process was firstly proposed and optimized with conventional reactive distillation (RD) and reactive dividing wall column (RDWC), respectively. Compared with RD process, RDWC could achieve 25.3% and 8.2% total annualized cost reduction for EMC and DEC production process, respectively. And carbon dioxide emission could be decreased by 29.4% and 11.7% by further integrating RD and separation column into RDWC arrangement for DEC and EMC production. Then multi-loop PI control schemes were developed as benchmark and linear model predictive controller was designed to handle the persistent feed disturbance. The superiorities of model-based controllers for the strongly interactive and coupled system are demonstrated by the dynamic response comparisons in terms of settling time, overshoot and integral of squared error (ISE), especially for the controlled variables with poor control performance under conventional multi-loop PI control structure. To summarize, the comprehensive control performance index-ISE with MPC is reduced by 55.9%–99.1%, as compared to those under PI control strategy.

Temperature Dependent Formation of the Graphite SEI with Vinylene Carbonate Electrolyte Additive

The charge/discharge efficiency and the cycle life of lithium-ion batteries (LiBs) depends on a large number of competing interfacial processes. Probably the best-known interface in LiBs is the solid-electrolyte-interphase (SEI), which is a passivating layer formed at the negative electrode by electrolyte decomposition during the first cycles.[1–3] Since the SEI stability is a deciding factor with regards to cycle life, understanding and improving the SEI formation process is crucial. In order to yield an effective SEI, commercial battery cells undergo an extensive so-called formation procedure after cell assembly, which generally consist of multiple voltage-holds and current-steps at various temperatures.[4,5] During the first cycles of a LiB, the SEI is generated by the reduction products of the commonly used carbonate-based electrolyte components, such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).[6,7] Additionally, electrolyte additives are commonly added to form an SEI layer with increased cycling stability and reduced irreversible capacity loss during the first cycle.[6,8–10] Among a variety of additives, vinylene carbonate (VC) is the most prominent. Since the reduction of VC initiates at higher reduction potentials compared to alkyl carbonate solvents (e.g., EC), it is reduced preferentially during the first charge and thus strongly affects the SEI composition.[6,11] In the present study we investigated the effect of the formation temperature in a range between 10-60 °C on the performance characteristics of full-cells with a graphite anode (MAGE) and an NCM831205 cathode as well as of MAGE/Li half-cells, both with LP572 (1 M LiPF6 in EC:EMC 3:7wt/wt + 2wt% VC) electrolyte. These include the first-cycle irreversible capacity loss, the negative electrode impedance build-up, the gas evolution during formation, the rate capability, and the cycle life.Employing electrochemical impedance spectroscopy using a µ-reference electrode allowed us to determine the graphite intercalation resistance (R Int), representing the sum of the SEI resistance (R SEI) and of the charge transfer resistance (R CT), after two 0.1C formation cycles at a given temperature. The impedance spectrum was recorded at 25 °C and 40% SOC referenced to 190 mAh/gNCM reversible capacity. As depicted in Figure 1, both R Int of the graphite electrode in the MAGE/NCM83125 full-cell and the first-cycle irreversible Li-loss determined a MAGE/Li half-cell increase with increasing formation temperature.Additionally, on-line electrochemical mass spectrometry (OEMS) was applied to identify and quantify the gaseous reduction products for LP572 in a MAGE/NCM831205 full-cell during one 0.1C formation cycle at different temperatures. As displayed in Figure 1, the relative increase of the total gas evolved, which is the sum of hydrogen, ethylene, carbon monoxide, and carbon dioxide, follows the relative increase of the intercalation resistance. Next to the overall gas amounts, changes in the quantity of the individual gases give insights into the highly temperature dependent reduction mechanisms of VC. Acknowledgement: The authors thank BMW AG for their financial support.

Atomic Layer Vs. Sol-Gel Deposited Coatings for Long Cycle-Life Li-Ion Battery Positive Electrodes

High nickel content layered oxide positive electrodes for Li-ion batteries suffer from capacity fade due to, among other factors, particle cracking induced by lithiation/delithiation, transition metal dissolution into the electrolyte, and attack from HF. Surface modification has been shown to reduce the severity of these degradation mechanisms by blocking transition metal ions from dissolving, and protecting the surface from the electrolyte decomposition products, as well as physically restraining the particles to reduce cracking. Atomic layer deposition (ALD) is a coating method that promises good uniformity and a high level of thickness control with a wide range of possible coating materials. ALD is a derivation of chemical vapor deposition in which chemical precursors are injected separately into a low pressure heated reaction vessel. Precursors are selected to create a self-limiting reaction at each step, allowing monolayers of precursor to be deposited on each cycle.1 With a large chemical space to explore, the challenge remains to find a coating material that offers substantial protection and lifetime extension, while allowing facile Li+ diffusion that does not substantially impede the capacity and rate capability of the cell.Alumina, Al2O3, being the archetypal ALD coating material has been frequently explored to improve the stability of known Li-ion positive electrode materials.2 Recently Wang et al. found marked stability improvement of Al2O3 coated LiNi0.6Mn0.2Co0.2O2 over even as few as 40 charge/discharge cycles.3 In this work, we present a comparison study of alumina coatings on LiNi0.6Mn0.2Co0.2O2 deposited by ALD versus coatings prepared through a sol-gel method tested to 4.5 V vs Li+/Li (Figure 1). Depositing a range of Al2O3 thicknesses by ALD we see a general trend of increased capacity retention with increased coating thickness. Multiple Al-containing precursors have been investigated for sol-gel coatings and we have found varying results, some giving comparable performances to the ALD coated samples, whereas other sol-gel precursors resulted in poorer performance than the uncoated material. Both types of films have been characterised by low energy ion scattering and x-ray photoelectron spectroscopy to detect the presence of the coatings, as well as electron microscopy with energy dispersive x-ray spectroscopy to characterise the uniformity and thickness of the coatings. R. L. Puurunen, J. Appl. Phys., 97, 121301 (2005).L. A. Riley, S. V. Atta, A. S. Cavanagh, Y. Yan, S. M. George, P. Liu, A. C. Dillon, and S. Lee, J. Power Sources, 196, 3317–3324 (2011).X. Wang J. Cai, Y. Liu, X. Han, Y. Ren, J. Li, Y. Liu, and X. Meng, Nanotechnology, 32, 115401 (2020). Figure 1: Capacity retention after 3 formation cycles of LiNi0.6Mn0.2Co0.2O2 positive electrodes uncoated (black), Al2O3 coating by 50 ALD cycles (blue) and 2 wt% Aluminium tri sec-butoxide sol-gel (red). Cells were cycled at 2.5-4.5 V vs Li+/L at 25 °C at 1C after 3 formation cycles at C/10. The negative electrode was Li metal and the electrolyte was 1.0 M LiPF6 in ethylene carbonate : ethyl methyl carbonate 3:7 v/v with 1 wt% vinyl carbonate. Figure 1