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

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

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

Reversible Graphite Anode Cycling with PC-Based Electrolytes Enabled by Added Sulfur Trioxide Complexes

Pyridine sulfur trioxide (PyrSO3), trimethyl amine sulfur trioxide (Me3NSO3), and triethyl amine sulfur trioxide (Et3NSO3) complexes have been investigated as electrolyte additives for lithium ion batteries. Incorporation of 0.5 to 2.0% of the SO3 complexes into a PC/EMC (1:1 v/v) 1 M LiPF6 baseline electrolyte affords reversible cycling of graphite anodes confirming generation of a stable Solid Electrolyte Interphase (SEI). Good cycling performance is observed for graphite/LiNi0.5Mn1.5O4 cells cycled to high potential (4.8 V vs Li) containing PC based electrolyte with added SO3 complexes. Ex-situ surface analysis via X-ray Photoelectron Spectroscopy (XPS) of the anodes reveals SO3 complex reduction on the surface of the graphite anode generates a sulfur-based SEI containing sulfites, sulfide, and sulfate species. The presence of the sulfur containing species is likely critical for the stability of the SEI. Ex-situ XPS analyses of the LiNi0.5Mn1.5O4 cathodes suggest that reaction of Me3NSO3 or Et3NSO3 complexes at high potential result in the generation of a stable passivation layer which affords good capacity retention and coulombic efficiency

Perspective—Surface Reactions of Electrolyte with LiNixCoyMnzO2 Cathodes for Lithium Ion Batteries

Layered metal oxides with high nickel content are commonly used cathode materials in commercial lithium ion batteries due to high capacity and lower cost resulting from higher nickel content and lower cobalt content. Cathodes with increased nickel content suffer from rapid capacity fade due to a combination of thickening of the anode solid electrolyte interphase (SEI) and impedance growth on the cathode after extended cycling. While transition metal catalyzed degradation of the anode SEI has been widely proposed as a primary source of capacity loss, we propose that a related acid induced degradation of the anode SEI also occurs