Lithium Alkyl Carbonates Research Articles

Organic-inorganic interlayer enabling the stability of PVDF-HFP modified Li metal for lithium-oxygen batteries

PVDF-HFP layer has been used in lithium (Li) metal-based batteries for its high Li-ion conductivity, excellent mechanical strength and water-proof property. Moreover, the layer reacts with Li metal and improves the performance of solid electrolyte interphase (SEI) by introducing the additional component of LiF. Herein, we further allow an organic molecular (11-mercaptoundecanoic acid, MUA) to involve in the reaction between the PVDF-HFP layer and Li metal, which results in an organic–inorganic SEI interlayer consisting of Li2S, LiF, lithium alkyl carbonate (RCOOLi), and MUA-Li etc. This interlayer shows the high Li+ transfer number (0.62) and Young’s modulus (6.17 GPa), which also serves to increase the adhesion between PVDF-HFP and Li metal. By using the organic–inorganic interlayer enhanced PVDF-HFP coated Li metal, the Li/Li symmetric cells can run for at least 2190 h steadily, and the Li-oxygen batteries can operate for 214 cycles, demonstrating the improved protection and ionic conductivity of Li metal anodes.

Utilizing Triethyl Phosphate to Increase the Solubility of Lithium Nitrate for Improved Silicon Anode Performance

An electrolyte consisting of lithium nitrate (LiNO3) and lithium difluoro(oxalto)borate (LiDFOB) in ethylene carbonate (EC), ethylmethyl carbonate (EMC), and triethyl phosphate (TEP) was used to improve the long-term cycling stability of silicon anodes. TEP was selected for its ability to dissolve LiNO3 in carbonates to a concentration of ~0.2 M. The large amount of LiNO3 combined with the LiDFOB salt led to a capacity retention of 87.1% after one hundred cycles due to the formation of a stable solid electrolyte interphase (SEI). Ex-situ surface analysis revealed that the SEI consists of oxalates, lithium alkyl carbonates, borates, and nitrate decomposition products. By selecting two components that preferentially reduce before the rest (LiNO3 and LiDFOB), the SEI formed was able to prevent significant solvent decomposition and allow for improved electrochemical cycling in pure silicon anodes.

Electrolytes Containing Triethyl Phosphate Solubilized Lithium Nitrate for Improved Silicon Anode Performance

An electrolyte consisting of lithium nitrate (LiNO3) and lithium difluoro(oxalato)borate (LiDFOB) in ethylene carbonate (EC), ethylmethyl carbonate (EMC), and triethyl phosphate (TEP) is used to improve the long-term cycling stability of silicon anodes. TEP was selected for its ability to dissolve LiNO3 in carbonates to a concentration of ∼0.2 M. The large amount of LiNO3 combined with the LiDFOB salt leads to a capacity retention of 87.1% after one hundred cycles due to the formation of a relatively stable solid electrolyte interphase (SEI). Ex-situ surface analysis reveals that the SEI consists of oxalates, lithium alkyl carbonates, borates, and nitrate reduction products. By selecting two components which are preferentially reduced (LiNO3 and LiDFOB), the SEI is able to inhibit continuous solvent decomposition and allows for improved electrochemical cycling for pure silicon anodes.

Exploring the redox decomposition of ethylene carbonate–propylene carbonate in Li-ion batteries

A small difference caused by a single methyl group between EC and PC could cause a large difference in lithium alkyl carbonates.

(Invited) Visualizing Live-Formation of Interphase at Atomic Scale

The solid-electrolyte-interphase (SEI) in Li-ion battery has been considered one of the most important but least understood and elusive component. In this work, its live-formation was visualized quantitatively at atomistic level with unique combination of in-situ/operando techniques. For the first time, the precise weighing of graphitic anode during its initial lithiation process was realized with electrochemical quartz crystal microbalance, which unequivocally identifies lithium fluoride (LiF) and lithium alkylcarbonates as the main chemical components of the interphase at different potentials. In-situ gas analysis confirmed the preferential reduction of cyclic over acyclic carbonate molecules, making lithium ethylene dicarbonate (LEDC) the major product contributed from solvent. This lithium alkylcarbonate, at least in its nascent form, was revealed to be re-oxidizable, despite general belief that interphase is electrochemically inert and its formation irreversible. Meanwhile, atomic force microscope found that the presence of LEDC on graphitic surface started at edge sites, which does not seem to occur in an obvious staging manner as the intercalation of Li+ does. These atomistic and quantitative observations shed new light on the formation chemistry, mechanism and properties of SEI. Such in-depth understanding will serve as guiding principles for the tailor-design of ideal interphases in future battery chemistries.

Kinetic Selectivity of Lithium Alkyl Carbonate Formation from Combination Reactions of Ethylene Carbonate Radical Anions

The mechanism responsible for predominant alkyl carbonate formation over butylene dicarbonate from the reductive decomposition of ethylene carbonate has been elucidated using ab initio molecular dy...

Solid-electrolyte interphase formation process on Li2TiSiO5 anode in LiPF6-based carbonate electrolyte

Electrolyte decomposition in lithium-ion batteries results in the formation of a solid-electrolyte interphase (SEI), and this layer plays an essential role in determining the stability and cyclability of the battery. Besides electrolyte, electrode also affects the SEI products. In this work, the SEI formation processes on Li2TiSiO5 electrode is investigated and compared with nickel electrode using ex situ X-ray photoelectron spectroscopy. Li2TiSiO5 is a recently discovered Li-ion battery anode that works at an optimum voltage (~0.28 V vs. Li+/Li) but suffers from low coulombic efficiency. The electrolyte selected is 1 M LiPF6 in ethylene carbonate:diethyl carbonate:dimethyl carbonate (1:1:1, volumetric). The SEI layer formed on the surface of Li2TiSiO5 is shown to consist of lithium carbonate (Li2CO3), lithium alkyl carbonates (ROCOOLi), and lithium fluoride (LiF), with Li2CO3 being the dominant component. Small amounts of ROCOOLi and LiF are mostly formed in the deep lithiation stages. The SEI layer formed on nickel electrode, in comparison, only contains inorganic Li2CO3. These results suggest that Li2TiSiO5 either catalyzes or participates in the SEI formation reaction. A proper coating or a change in composition of Li2TiSiO5 can modify the property of SEI.

Thermogravimetric Evolved Gas Analysis and Microscopic Elemental Mapping of the Solid Electrolyte Interphase on Silicon Incorporated in Free-Standing Porous Carbon Electrodes.

Free-standing electrodes, which are free from additives (binders and conductive agents) and even current collectors, are useful in terms of both application research and fundamental study. Here, we demonstrate the preparation of binder-free monolithic carbon electrodes embracing Si nanoparticles in their well-defined porous scaffolds via the one-pot sol-gel reaction followed by carbonization. The free-standing electrodes with a thickness of 150 μm work out as a high-areal-density anode for Li-ion batteries, delivering up to ca. 7 mA h cm-2. As the Si content increases, the capacity decay on cycling becomes pronounced, which is likely to associate with the fracturing and pulverization of Si nanoparticles even with the size smaller than 100 nm after long-term cycles. The thermogravimetry-mass spectrometry profile of the cycled electrode corroborates the successive electrolyte decomposition to grow solid electrolyte interphase (SEI) mainly composed of lithium alkylcarbonates, polymeric species, and LiF, rendering the electrode mass nearly double of its original state after 200 cycles. The elemental mapping analysis reveals that LiF is generated inhomogeneously in the monolithic electrodes unlike the other SEI components, resulting in the concentration gradient depending on the distance from a Li counter electrode.

A salt-derived solid electrolyte interphase by electroreduction of water-in-salt electrolyte for uniform lithium deposition

Notorious growth of dendritic lithium with low Coulombic efficiency causes safety and stability issues, which hinder practical applications. To control the lithium deposition so that one has a dendrite-free lithium metal anode with high Coulombic efficiency is highly desirable but challenging. Here, this work reports a copper substrate covered by a salt-derived solid electrolyte interphase that produced by electroreduction of a highly concentrated water-in-salt electrolyte to realize stable lithium deposition. Quite distinct from the resistive layer comprised principally of solvent-derived organic species (such as lithium alkyl carbonates) that produced in conventional dilute electrolyte, this salt-derived solid electrolyte interphase is rich in inorganic components (mainly Li2SxOy and LiF), which effectively reduce the overpotential and facilitate fast Li+ transport. In addition, a small number of reduced fluorine organic anions -CFx stabilize the space charge to give a uniform distribution of Li+. Such a solid electrolyte interphase on the copper substrate is able to modulate the lithium plating/stripping to produce uniform spherical lithium deposition with no dendrites, and a high Coulombic efficiency (≈98.2%) is achieved. This work provides a unique strategy to enlarge the functions of the lithium salt decompositions on the protection of lithium metal anodes.

Dynamic evolution of Cathode−Electrolyte interface of LiNi0.6Co0.2Mn0.2O2 during the initial Charge−Discharge process

The cathode−electrolyte interface (CEI) is a key factor that affects the electrochemical performance of lithium-ion batteries. However, considerable ambiguity still exists with regard to the composition and properties of the CEI at different potentials during cycling. In this study, we investigate the dynamic evolution behavior of the CEI between the LiNi0.6Co0.2Mn0.2O2 cathode and organic carbonate electrolytes by electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and synchrotron soft X-ray absorption spectroscopy (soft XAS). XPS results reveal the continuous formation and decomposition of the CEI components during the initial charge and discharge process. Lithium alkyl carbonates and lithium fluoride (LiF) form during charging at low voltages, decompose at high voltages, and regenerate during discharging. In addition, soft XAS highlights the formation of electrochemically inactive species on the cathode surface, which generates extra barrier for Li+ diffusion and also decreases active sites for Li+ intercalation. The surface Ni undergoes incomplete oxidation during the charging process and exhibits partial reduction after discharging. These results are consistent with impedance variation in the first cycle and morphology changes on the cathode surface. These findings underscore the value of investigating and regulating the interfacial properties of lithium-ion batteries.

(Invited) Generation and Evolution of Materials in the Anode Solid Electrolyte Interphase (SEI) of Lithium Ion Batteries

A solid electrolyte interphase is generated on the anode of lithium ion batteries during the first few charging cycles. The presence and stability of the solid electrolyte interphase (SEI) is critical to the performance of the battery. Despite thorough investigation of the SEI for over 20 years, the mechanism of formation and function are still relatively poorly understood. We have investigated the structure of the initial SEI on graphite and silicon electrodes along with changes which occur to the SEI upon additional cycling via a combination of ex-situ surface analysis techniques. The investigation of SEI provides significant insight into the structure and properties of the anode SEI. The one electron reducing agent, lithium napthalenide has been used to independently prepare the reduction products which constitute the SEI. The initial reduction product of ethylene carbonate is lithium ethylene dicarbonate (LEDC). However, the thermal decomposition of LEDC and related lithium alkyl carbonates in the presence of LiPF6 generates a intricate mixture of compounds which greatly complicates the structure of the SEI. The reduction products and their subsequent thermal decomposition products have been thoroughly investigated via a combination of NMR, XPS, IR-ATR, TGA, and GC-MS. Mechanisms for the generation of the complicated mixture of products will be discussed. Thermal behavior of the SEI on a silicon electrode for Lithium ion batteries has been investigated by TGA. In order to provide a better understanding of the thermal decomposition of the SEI on silicon, the thermal decomposition behavior of independently synthesized lithium ethylene dicarbonate (LEDC) was investigated as a model SEI. The model SEI (LEDC) has three stages of thermal decomposition. Over the temperature range of 50~300oC, LEDC decomposes to evolve CO2 and C2H4 gasses leaving lithium propanoate (CH3CH2CO2Li) and Li2CO3 as solid residues. The lithium propanoate decomposes over the temperature range of 300~600oC to evolve pentanone leaving Li2CO3 as a residual solid. Finally, the Li2CO3 decomposes over 600oC to evolve CO2 leaving Li2O as a residual solid. A very similar thermal decomposition process is observed for the SEI generated on cycled silicon electrodes. However, two additional thermal decomposition reactions were observed characteristic of LixPOyFz at 300oC and the polyimide binder at 550oC. TGA measurements of Si electrodes after various numbers of cycles suggest that the LEDC on Si electrodes thermally decomposes during cycling to form lithium propanoate and Li2CO3, resulting in increased complexity of the SEI.

Generation and Evolution of Materials in the Anode Solid Electrolyte Interphase (SEI) of Lithium Ion Batteries

A solid electrolyte interphase is generated on the anode of lithium ion batteries during the first few charging cycles. The presence and stability of the solid electrolyte interphase (SEI) is critical to the performance of the battery. Despite thorough investigation of the SEI for over 20 years, the mechanism of formation and function are still relatively poorly understood. We have investigated the structure of the initial SEI on graphite and silicon electrodes along with changes which occur to the SEI upon additional cycling via a combination of ex-situ surface analysis techniques. The investigation of SEI provides significant insight into the structure and properties of the anode SEI. The one electron reducing agent, lithium napthalenide has been used to independently prepare the reduction products which constitute the SEI. The initial reduction product of ethylene carbonate is lithium ethylene dicarbonate (LEDC). However, the thermal decomposition of LEDC and related lithium alkyl carbonates in the presence of LiPF6 generates a intricate mixture of compounds which greatly complicates the structure of the SEI. The reduction products and their subsequent thermal decomposition products have been thoroughly investigated via a combination of NMR, XPS, IR-ATR, TGA, and GC-MS. Mechanisms for the generation of the complicated mixture of products will be discussed. Thermal behavior of the SEI on a silicon electrode for Lithium ion batteries has been investigated by TGA. In order to provide a better understanding of the thermal decomposition of the SEI on silicon, the thermal decomposition behavior of independently synthesized lithium ethylene dicarbonate (LEDC) was investigated as a model SEI. The model SEI (LEDC) has three stages of thermal decomposition. Over the temperature range of 50~300oC, LEDC decomposes to evolve CO2 and C2H4 gasses leaving lithium propanoate (CH3CH2CO2Li) and Li2CO3 as solid residues. The lithium propanoate decomposes over the temperature range of 300~600oC to evolve pentanone leaving Li2CO3 as a residual solid. Finally, the Li2CO3 decomposes over 600oC to evolve CO2 leaving Li2O as a residual solid. A very similar thermal decomposition process is observed for the SEI generated on cycled silicon electrodes. However, two additional thermal decomposition reactions were observed characteristic of LixPOyFz at 300oC and the polyimide binder at 550oC. TGA measurements of Si electrodes after various numbers of cycles suggest that the LEDC on Si electrodes thermally decomposes during cycling to form lithium propanoate and Li2CO3, resulting in increased complexity of the SEI.

A dual-layered artificial solid electrolyte interphase formed by controlled electrochemical reduction of LiTFSI/DME-LiNO3 for dendrite-free lithium metal anode

Dendrite growth and low coulombic efficiency (CE) are the key issues to hinder the lithium (Li) metal anode in practical battery application. Naturally formed fragile and unstable solid electrolyte interphase (SEI) film between Li anode and electrolyte is one of the main reasons for them. Here we create a dual-layered artificial SEI (ASEI) film by controlled electrochemical method, which can simultaneously suppress dendrite formation and improve electrochemical performance of Li metal anode. The ASEI film with an average thickness of 40 nm shows a dual-layered structure with organic species (−(CH2CH2O)n− and lithium alkylcarbonate) in the outer layer and abundant inorganic species (LiF, Li2O, Li3N and Li2S) in the inner layer, providing enough flexibility, high mechanical modulus, high kinetic and thermodynamic stability. The ASEI film demonstrates extremely superior Li+ ion conductivity and low interface impedance, enabling a compact and dendrite-free Li deposition during cycling, and remarkably improving the CE to an average value of 97.5% for over 350 cycles. The superior electrochemical performance of lithium−sulfur (Li−S) full cells with the protection of ASEI film on Li anode further proves that the ASEI film has optimistic potential in Li metal batteries. This work provides a simple and feasible strategy for preparing ASEI film for dendrite-free Li metal anode, which has great prospects in high specific energy Li metal batteries.

Effect of Anion Species in Early Stage of SEI Formation Process

We investigate the early stage of the solid electrolyte interphase (SEI) layer formation process in four electrolyte solutions, using electrochemical quartz crystal microbalance (EQCM) and X-ray photoelectron spectroscopy (XPS). The EQCM result proves that the LiBH4 in THF solution form a negligible amount of SEI layer. The SEI formation process of the LiPF6 solution occurs at 2.3 V vs. Li, and the SEI layer continuously grows even during the anodic scan. The electrolyte solutions containing LiTFSA form SEI layer around 1.5 V vs. Li. The XPS study shows that the SEI layer formed at high electrode potential, in the electrolyte containing LiPF6 forms a dense LiF layer. The LiTFSA solution forms a thick SEI layer containing organic components such as lithium alkyl carbonate or polyether.

In situ quantification of interphasial chemistry in Li-ion battery.

The solid-electrolyte interphase (SEI) is probably the least understood component in Li-ion batteries. Considerable effort has been put into understanding its formation and electrochemistry under realistic battery conditions, but mechanistic insights have mostly been inferred indirectly. Here we show the formation of the SEI between a graphite anode and a carbonate electrolyte through combined atomic-scale microscopy and in situ and operando techniques. In particular, we weigh the graphitic anode during its initial lithiation process with an electrochemical quartz crystal microbalance, which unequivocally identifies lithium fluoride and lithium alkylcarbonates as the main chemical components at different potentials. In situ gas analysis confirms the preferential reduction of cyclic over acyclic carbonate molecules, making its reduction product the major component in the SEI. We find that SEI formation starts at graphite edge sites with dimerization of solvated Li+ intercalation between graphite layers. We also show that this lithium salt, at least in its nascent form, can be re-oxidized, despite the general belief that an SEI is electrochemically inert and its formation irreversible.

In‐Situ Electrochemical SHINERS Investigation of SEI Composition on Carbon‐Coated Zn0.9Fe0.1O Anode for Lithium‐Ion Batteries

AbstractCarbon‐coated Zn0.9Fe0.1O is a promising anode material for lithium‐ion batteries with good cycling performance and a theoretical specific capacity of 966 mAh g−1, as a result of the combined conversion‐alloying reaction during lithiation. The solid electrolyte interphase (SEI) formed on this electrode was investigated by in‐situ Raman spectroscopy and in‐situ shell‐isolated nanoparticles for enhanced Raman spectroscopy (SHINERS) during the first discharge/charge cycle. The spectra collected via in‐situ Raman spectroscopy showed that the carbon coating is also (de)lithiated and it remains mechanically intact after the first complete cycle. There was no evidence of peaks related to the SEI due to the absence of surface enhancement of the Raman effect in this material, as was previously observed for carbon‐coated ZnFe2O4. However, bands assigned to polyethylene oxide species (PEO) and different lithium alkyl carbonate compounds (i. e., ROCO2Li, ROLi and RCOOLi) from the SEI were observed via SHINERS. The enhancement of the Raman effect by Au−SiO2 core‐shell nanoparticles allows the detection of surface films at potentials at which the SEI is formed and their chemical composition, which is not possible otherwise due to the intrinsically weak scattering process. Therefore, these results show that the SHINERS technique is a powerful method to investigate the structural evolution of the electrode material with potential and interfacial reactions on lithium‐ion batteries.

EIS and XPS Investigation on SEI Layer Formation during First Discharge on Graphite Electrode with a Vinylene Carbonate Doped Imidazolium Based Ionic Liquid Electrolyte

Adding organic carbonates (e.g., vinylene carbonates, VC) into ionic liquid based electrolyte generally improves the electrochemical performances of graphite electrode (Cgr) in lithium ion batteries. In this study, step-by-step electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) were performed during the first reduction cycle to follow the formation of the solid electrolyte interphase (SEI) in lithium/graphite (Li/Cgr) cell using (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) [C1C6Im][NTf2] doped with vinylene carbonate (VC). The EIS spectra evolution, recorded at different cutoff voltages, indicated a two-step SEI thin film formation. These results were supported by XPS measurement at the same cutoff voltage. Hence, we pointed out that the first film results from the VC decomposition at 0.8 V to form an organic layer constituted of lithium alkylcarbonates. This film was developed up to 0.2 V. From 0.8 to 0.2 V, we detected a slight decomposition of IL ...

Investigation of CO2 Reduction Products in Lithium Ion Batteries Using Model Systems

Lithium ion batteries (LIBs) are essential to renewable energy development and are presently being utilized in more numerous applications including but not limited to consumer electronics and larger scale energy storage for backup residential power as well as electric vehicles. It is known that during cycling and especially at elevated temperatures, LIBs form gaseous reduction and degradation products which cause performance fade as well as unsafe intracellular pressure which can contribute to cell rupture. With cycling, carbonate based electrolytes decompose into CO2 and various other SEI components and vary in concentration dependent on the construction of the cell. Here, we present preliminary NMR, IR, and GC-MS results of studies performed on model systems consisting of lithium napthalenide, and solid sodium and lithium reacted with CO2 gas. We confirm here that the fate of evolved CO2 is to react with metallic lithium to form simple lithium carbonate and not more complex products such as lithium oxalate or lithium alkyl carbonates.

Online Digital Holographic Method for Interface Reaction Monitoring in Lithium-Ion Batteries

Understanding the reaction mechanisms at the interface of electrode and electrolyte is both of fundamental interest and essential to improve lithium-ion battery (LIB) performance. Herein, we report an online digital holographic method to in situ observe the entire interface change between electrode and electrolyte in lithium-ions batteries. The accuracy of this technology is well verified in LiFePO4/graphite full-cell systems, graphite/Li half-cell systems in EC-based and PC-based electrolyte, respectively, and supported by the characterized results of conventional instruments, including scanning electron microscopy and X-ray photoelectron spectroscopy. In particular, the time resolution of the digital holographic method is 0.04 s and fast enough to distinguish detail reduction process of ethylene carbonate (EC), for which EC will be first reduced to generate lithium alkyl carbonates, and then the reduction product is Li2CO3 to form a stable SEI films. To our best of knowledge, this is the first report on...

A stable organic–inorganic hybrid layer protected lithium metal anode for long-cycle lithium-oxygen batteries

A stable organic–inorganic hybrid layer (OIHL) is direct fabricated on lithium metal surface by the interfacial reaction of lithium metal foil with 1-chlorodecane and oxygen/carbon dioxide mixed gas. This favorable OIHL is approximately 30 μm thick and consists of lithium alkyl carbonate and lithium chloride. The lithium-oxygen batteries with OIHL protected lithium metal anode exhibit longer cycle life (340 cycles) than those with bare lithium metal anode (50 cycles). This desirable performance can be ascribed to the robust OIHL which prevents the growth of lithium dendrites and the corrosion of lithium metal.