875 research outputs found
Iron Intercalation in Covalent-Organic Frameworks: A Promising Approach for Semiconductors
Covalent-organic frameworks (COFs) are intriguing platforms for designing
functional molecular materials. Here, we present a computational study based on
van der Waals dispersion-corrected hybrid density functional theory (DFT-D) to
design boroxine-linked and triazine-linked COFs intercalated with Fe. Keeping
the original symmetry of the pristine COF (COF-Fe-0), we have
computationally designed seven new COFs by intercalating Fe atoms between two
organic layers. The equilibrium structures and electronic properties of both
the pristine and Fe-intercalated COF materials are investigated here. We
predict that the electronic properties of COFs can be fine tuned by adding Fe
atoms between two organic layers in their structures. Our calculations show
that these new intercalated-COFs are promising semiconductors. The effect of Fe
atoms on the electronic band structures and density of states (DOSs) has also
been investigated using the aforementioned DFT-D method. The contribution of
the -subshell electron density of the Fe atoms plays an important role in
improving the semiconductor properties of these new materials. These
intercalated-COFs provide a new strategy to create semi-conducting materials
within a rigid porous network in a highly controlled and predictable manner.Comment: 39 pages. arXiv admin note: text overlap with arXiv:1703.0261
Synergistic Performance of Lithium Difluoro(oxalato)borate and Fluoroethylene Carbonate in Carbonate Electrolytes for Lithium Metal Anodes
There is significant interest in the development of rechargeable high-energy density batteries which utilize lithium metal anodes. Recently, fluoroethylene carbonate (FEC) and lithium difluoro(oxalato)borate (LiDFOB) have been reported to significantly improve the electrochemical performance of lithium metal anodes. This investigation focuses on exploring the synergy between LiDFOB and FEC in carbonate electrolytes for lithium metal anodes. In ethylene carbonate (EC) electrolytes, LiDFOB is optimal when used in high salt concentrations, such as 1.0 M, to improve the electrochemistry of the lithium metal anode in Cu||LiFePO4 cells. However, in FEC electrolytes, LiDFOB is optimal when used in lower concentrations, such as 0.05–0.10 M. From surface analysis, LiDFOB is observed to favorably react on the surface of lithium metal to improve the performance of the lithium metal anode, in both EC and FEC-based electrolytes. This research demonstrates progress toward developing feasible high-energy density lithium-based batteries
Carbonate Free Electrolyte for Lithium Ion Batteries Containing γ-Butyrolactone and Methyl Butyrate
A novel carbonate free electrolyte, 1 M lithium difluoro(oxalato) borate (LiDFOB) in 1:1 gamma-butyrolactone (GBL)/methyl butyrate (MB), has been compared to a standard electrolyte, 1 M LiPF6 in 1:1:1 EC/DMC/DEC, and a 1 M LiDFOB in 1:1:1 EC/DMC/DEC electrolyte. The conductivity of 1 M LiDFOB in GBL/MB is higher at low temperature, but slightly lower at higher temperature compared to the standard electrolyte. The 1 M LiDFOB in GBL/MB electrolyte has comparable cycling performance to the standard electrolyte, and better cycling performance than the 1 M LiDFOB in EC/DMC/DEC electrolyte. The reversible cycling performance suggests that the LiDFOB in GBL/MB electrolyte forms a stable anode solid electrolyte interface (SEI) in the presence of GBL. Ex-situ surface analysis of the extracted electrodes has been conducted via a combination of XPS, FTIR-ATR and SEM which suggests that the stable anode SEI results is primarily composed of reduction products of LiDFOB. The widespread implementation of electric vehicles (EVs) requires further improvements in lithium ion batteries.1–3 Some of the biggest challenges for lithium ion batteries in EVs are cost, low temperature performance and battery lifetime.2,3 Improvements in the electrolyte can assist in the resolution of each of these problems.1,4,5 Most commercial electrolytes are composed of LiPF6 in a mixture of carbonate solvents.5 However, the high cost and poor thermal and hydrolytic stability of LiPF6 is problematic for the electrolyte.6–8 In addition, ethylene carbonate (EC) is typically a required component of the electrolyte due to the role of EC in the formation of the solid electrolyte interphase (SEI) on the anode.5,9–14 Since EC is a solid at room temperature, electrolytes containing EC frequently have poor performance at low temperature.15 Despite the shortcomings of LiPF6 / EC based electrolytes, these formulations have proven very difficult to replace. While there have been significant efforts to develop novel electrolytes with superior performance to LiPF6 in carbonates, there has been limited success. The development of novel solvent systems has been more limited and frequently targeted toward specific problems such as high voltage cathodes, salt solubility, or reactivity issues.16–21 The development of novel salts has encountered problems related to salt solubility and corrosion of the aluminum current collector on the cathode.21,22 One of the more interesting and promising alternative salts is lithium difluoro(oxalato) borate (LiDFOB).1,4,23,24 LiDFOB is promising due to good solubility, thermal stability, passivation of the aluminum current collector, stable SEI formation, and potentially lower cost. While there have been a limited number of investigations of LiDFOB as the conducting salt in the electrolyte,4,16,25 there have been several reports of the use of LiDFOB and the related salts lithium bis(oxalato) borate (LiBOB) and lithium tetrafluoro(oxalato) phosphate (LiTFOP) as additives to LiPF6 based electrolytes to form a more stable SEI.1,14,23,26–30 There have also been reports of the use of oxalate salts enabling the cycling of PC based electrolytes due to better SEI formation.30 The presence of the oxalate group in the anode thus may enable the use of EC free electrolytes and electrolytes with non-carbonate solvents. The investigation of LiDFOB has been expanded to include carbonate free electrolyte formulations. Esters and lactones are an interesting alternative to carbonate solvents. Linear esters have been studied as co-solvents due to the high dielectric constants and low freezing points which have been reported to improve the low temperature performance of lithium ion batteries,15 while lactones such as γ-butyrolactone (GBL) have high dielectric constants5 and a very wide liquid temperature range (−43.5 to 204°C). However, the use of GBL as a primary solvent in lithium ion battery electrolytes has been plagued by problems with the stability of the anode SEI.5 Despite the issues with GBL as a solvent in carbonate based electrolytes, GBL has been studied with LiBOB based electrolytes due to the limited solubility of LiBOB in carbonates.18,31 In order to investigate the use of novel electrolyte formulations for lithium ion batteries, a comparative study of three electrolytes has been conducted; a standard LiPF6 electrolyte in 1:1:1 EC/ dimethyl carbonate (DMC)/ diethyl carbonate (DEC) was tested against 1 M LiDFOB in 1:1:1 EC/DMC/DEC and 1 M LiDFOB in 1:1 GBL/MB (MB is methyl butyrate)
Fluorinated Acetic Anhydrides as Electrolyte Additives to Improve Cycling Performance of the Lithium Metal Anode
The investigation of novel fluorinated electrolyte additives for lithium metal anodes has been conducted. Two acetic anhydride derivatives, difluoroacetic anhydride (DFAA) and trifluoroacetic anhydride (TFAA), were investigated in electrolytes composed of LiPF6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The addition of either DFAA or TFAA results in a significant improvement in capacity retention and reversibility of lithium plating. Ex situ surface analysis (XPS, IR-ATR) suggests that incorporation of either TFAA or DFAA results in a lithium carboxylate rich SEI which in turn inhibits SEI degradation resulting in superior cycling performance
Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries: LiPF6, LiBF4, LiDFOB, LiBOB, and LiTFSI
The reduction products of common lithium salts for lithium ion battery electrolytes, LiPF6, LiBF4, lithium bisoxalato borate (LiBOB), lithium difluorooxalato borate (LiDFOB), and lithium trifluorosulfonylimide (LiTFSI), have been investigated. The solution phase reduction of different lithium salts via reaction with the one electron reducing agent, lithium naphthalenide, results in near quantitative reactions. Analysis of the solution phase and head space gasses suggests that all of the reduction products are precipitated as insoluble solids. The solids obtained through reduction were analyzed with solution NMR, IR-ATR and XPS. All fluorine containing salts generate LiF upon reduction while all oxalate containing salts generate lithium oxalate. In addition, depending upon the salt other species including, LixPFyOz, LixBFy, oligomeric borates, and lithium bis[N-(trifluoromethylsulfonylimino)] trifluoromethanesulfonate are observed
Development of Electrolytes for Si-Graphite Composite Electrodes
The performance of Si-graphite/Li cells and Si-graphite/NMC111 cells has been investigated in 1.2 M LiPF6 /EC:DEC (1/1, w/w) with different electrolyte additives including LiNO3, FEC, and MEC. The addition of additives into electrolytes result in a significant improvement in capacity retention compared to the standard electrolyte for Si-graphite/Li cells. The cells cycled with electrolyte containing 0.5 wt% LiNO3, 5–10 wt% MEC or 10 wt% FEC have high capacity retention, at least 88%, while the cells cycled with standard electrolyte have lower capacity retention, 64%, after 100 cycles. Investigation of Si-graphite/NCM111 cells reveals that the cells cycled in electrolyte containing 0.5 wt% LiNO3 have better capacity retention than cells cycled with 10 wt% FEC, 57.9% vs. 44.6%, respectively. The combination of 10% MEC and LiNO3 further improves the capacity retention of the Si-graphite/NCM111 full cells to 79.9% after 100 cycles which is highest among the electrolytes investigated. Ex-situ surface analyses by XPS and IR-ATR have been conducted to provide a fundamental understanding the composition of the solid-electrolyte interphase (SEI) and its correlation to cycling performance
Using Triethyl Phosphate to Increase the Solubility of LiNO\u3csub\u3e3\u3c/sub\u3e in Carbonate Electrolytes for Improving the Performance of the Lithium Metal Anode
The investigation of novel electrolytes for lithium metal anodes has been conducted. Incorporation of LiNO3 into lithium difluoro(oxalato) borate (LiDFOB) in ethylene carbonate (EC) and dimethyl carbonate (DMC) electrolytes results in a significant improvement in capacity retention and Coulombic efficiency. While the solubility of LiNO3 is very low in common carbonate solvents (~0.03 M), the use of triethyl phosphate (TEP) significantly increases the solubility of LiNO3 and improves the capacity retention and Coluombic efficiency of lithium metal anodes. Ex-situ surface analysis of the cycled electrodes suggests that incorporation of LiNO3 results in nitrogen containing species (NO3−, NO2−, and N3−) in the solid electrolyte interphase (SEI) which is likely responsible for the performance enhancement
Investigation of the Lithium Solid Electrolyte Interphase in Vinylene Carbonate Electrolytes Using Cu||LiFePO\u3csub\u3e4\u3c/sub\u3e Cells
The influence of vinylene carbonate (VC) on the plating/stripping of lithium was investigated using Cu||LiFePO4 cells. These cells allow for easy fabrication and in-situ generation of lithium, with no excess lithium to influence performance. Addition of VC to the electrolyte improves both capacity retention and efficiency. IR and XPS spectroscopy of the surface of the plated lithium suggests the presence of a significant amount of poly(VC) when the electrolyte (1.2 M LiPF6 in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7, vol)) contains 5% of added VC. This suggests employing additives that generate polymeric species on the surface of lithium improves plating/stripping performance in carbonate electrolytes
Interfacial Issues and Modification of Solid Electrolyte Interphase for Li Metal Anode in Liquid and Solid Electrolytes
The high energy density required for the next generation of lithium batteries will likely be enabled by a shift toward lithium metal anode from the conventional intercalation-based anode such as graphite. However, several critical challenges for Li metal originate from its highly reactive nature and the hostless reaction of deposition and stripping impede the practical use of Li metal as an anode. The role of the solid electrolyte interphase (SEI) is very important for the Li metal anode where the SEI must protect the dynamically changing surface of the Li metal. Since the SEI-generating reaction mechanisms for the two different electrolyte systems, liquid and solid, are considerably different, the SEI layers formed between the Li metal and the electrolytes in the two electrolyte systems have substantially different properties, causing different interfacial issues. Inhibition of the interfacial problems requires different strategies to reinforce the SEI layer for each of the electrolyte systems. However, the differences in the two electrolyte systems have not been clearly compared in the prior literature. In this report, the interfacial issues for the two different electrolyte systems are compared and different strategies for SEI modification are provided to overcome the issues
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