Fluoroethylene Carbonate Electrolyte Research Articles

Positive film-forming effect of fluoroethylene carbonate (FEC) on high-voltage cycling with three-electrode LiCoO2/Graphite pouch cell

Three-electrode LiCoO2/graphite pouch cells have been used to investigate the film formation mechanism of Fluoroethylene carbonate (FEC) for high-voltage performance. It is found that the cells exhibit improved cycling performance in 5% FEC containing electrolyte in the voltage range of 3.0–4.4 V. In situ electrochemical impedance spectroscopy (EIS) results reveal that the gradual film formation process in FEC electrolyte is not a negative lithium consumption process, different from that of the base electrolyte. The characterizations from scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) demonstrate that FEC facilitates the formation of stable solid electrolyte interfaces (SEI) simultaneously on anode and cathode of the LiCoO2/graphite pouch cells with different decomposition mechanisms, which can effectively protect electrodes and prevent electrolyte decomposition on both electrodes.

A novel aluminum dual-ion battery

The development of new rechargeable safe battery with high energy density and low cost is one of the most desirable goals for personal electronics and grid storage. Aluminum based rechargeable ion batteries offer the possibilities for safe, high energy density and low cost. Here, we developed a novel aluminum based high-rate capability dual-ion battery with an aluminum anode and a 3D graphene cathode. The battery operated through the electrochemical deposition and dissolution of aluminum at the anode, and intercalation/de-intercalation of ClO4- anions in the graphene based cathode with a new Al(ClO4)3/Propylene carbonate - Fluoroethylene carbonate electrolyte. The battery exhibited high discharge voltage plateaus (about 1V), high-rate capacity (101mAhg-1 at 2000mAg-1) and long cycle life (more than 400 cycles). More importantly, the battery also showed superior electrochemical properties with fast charge and slow discharge (the battery could be fully charged in 13min and discharged for more than 73min). And the reaction mechanisms of the aluminum based dual-ion battery is also proposed and discussed in detail.

In Situ Analysis of Solid Electrolyte Interface over Si Based Anodes Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy

In lithium ion battery (LIB), silicon is hopeful electrode having a high theoretical capacity of 3579 mAh/g, which is ten times higher than the currently used graphite anode (1). On the other hand, Si-based electrodes suffer from the poor capacity retention due to the large volume changes during the lithium lithiation and delithiation process. The consequence of the volume change will lead to continuous decomposition of the electrolyte, which results in the formation of solid electrolyte interphase (SEI). Herein, the SEI formed based on fluoroethylene carbonate (FEC) electrolyte additive on a Si-based anode was investigated using in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), which have been also used to analyze SEI species on different electrodes (2, 3). The SEI generated on Si-based anode using 1 M LiPF6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC), 1/2 (v/v)/ vinylene carbonate (VC), (2 wt%) was started to observe at the onset of 1.1 V in the potential region of 1.0 – 1.3 V due to the decomposition of VC, which preferentially reduce prior to EC and EMC (4). Formation of poly (VC), polycarbonates, and Li2CO3 were identified during first cycle discharging. The incorporation of 10 wt% FEC results with the formation of poly (FEC)/(VC), polycarbonates, Li2CO3, alkoxy species and alkyl phosphorous fluorides. Using FEC as an additive with the onset potetial of 1.4 V the SEI formation started at a higher potential region of 1.3 – 2.5 V. In the course of second discharging, incorporation of FEC also impedes the decomposition of both solvent and salt anion reduction. Notable enough, in-situ DRIFTS spectrum due to the impact of FEC additive after five cycles also show the formation of SEI-film species is in fact increase in the lithiation process. DRIFT spectrum of the SEI-film formed over Si-based anode with and without FEC electrolyte at the onset potential is shown in Fig. 1. Fig. 1 (a) The first cycle CV results of silicon-based electrodes with EC:EMC:VC/LiPF6 (black) and EC:EMC:VC + FEC/LiPF6 (blue). (b) DRIFT difference spectra obtained at the onset potential of silicon-based electrodes using (I) EC:EMC:VC/LiPF6 and (II) EC:EMC:VC + FEC/LiPF6based electrolytes.

Effect of Solid Electrolyte Interphase of Amorphous Si Flake Anode on Cycle Performance in Solvate Ionic Liquid Electrolytes

Introduction Si is expected to apply for lithium ion batteries as an anode material with a high capacity. However, Si anodes show poor cycleability caused by a pulverization of the Si particles due to large volume changes during charge and discharge cycles. Hence, we designed a nano-flake Si called Si LeafPowder® (Si-LP), which can relieve the physical stress caused by expansion and shrinkage of Si during alloying/dealloying reactions with lithium. A good cycleability of Si-LP with a thickness of 100 nm has been reported.1) On the other hand, S is a promising cathode material with a high capacity. Therefore, Si-S batteries are expected as a next generation electrochemical energy storage device. However, when S cathode is charged and discharged in a conventional ethylene carbonate (EC) based electrolyte, polysulfide anions dissolve into the electrolyte resulting in a poor cycleability. Recently, the dissolution of lithium polysulfide anions was successfully suppressed by using a solvate ionic liquid electrolyte, tri- or tetra-glyme and lithium salt (1:1) complex electrolyte2). We recently reported that cycleability of the Si-LP electrodes in the glyme-based electrolyte was improved by the addition of fluoroethylene carbonate (FEC), which has been recognized as an effective additive that forms a stable solid electrolyte interphase (SEI) on Si electrodes. 3). However, the capacity retention was inferior to the case of using EC based electrolyte. Therefore, we tried to form a pre-film on the Si-LP electrode with the FEC electrolyte prior to cycling to improve the cycleability in the glyme based electrolyte. Experimental The Si-LP composite electrodes consisted of 83.3 wt.% Si-LP, 5.6 wt.% Ketjen black, and 11.1 wt.% carboxymethyl cellulose sodium salt. The loading of the Si-LP composite on a Cu foil with the thickness of 20 mm was approximately 0.3 mg/cm2. The electrolyte was prepared by mixing a purified tetraglyme (G4) and lithium bis(trifluoromethansulfonyl)amide (Li[TFSA]) at the equimolar ratio in an Ar-filled glovebox (referred to as [Li(G4)][TFSA]). [Li(G4)][TFSA] was mixed with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) in a molar ratio of 1:4 to reduce the viscosity of the electrolyte (referred to as [Li(G4)][TFSA]/HFE). 10 wt.% FEC-added [Li(G4)][TFSA]/HFE was also prepared. The pre-film on the Si-LP electrode was formed by a charge and discharge pre-cycle in the 1 M LiPF6/FEC electrolyte with a coin-type two electrodes half-cell at C/6 rate (1C = 4200 mAh g-1), 30°C. The working and counter electrodes were the Si-LP electrode and a Li metal, respectively. Charge and discharge properties of the Si-LP electrodes with the pre-film were investigated in the additive-free [Li(G4)][TFSA]/HFE electrolytes. For comparison, the pristine Si-LP electrodes were cycled in the [Li(G4)][TFSA]/HFE with and without FEC at C/2 rate. The surface of the Si-LP electrodes after cycling was evaluated by SEM/EDX. Results and discussion Figure 1(a) and 1(b) show charge and discharge curves of the Si-LP electrodes in [Li(G4)][TFSA]/HFE without and with FEC, respectively. By the addition of FEC, the capacity retention at the 100th cycle was improved and polarization was reduced. However, the discharge capacity of the Si-LP electrode with FEC-addition decreased quickly after 100th cycle and the capacity retention at the 200th cycle was 38.4%. On the other hand, the Si-LP electrodes with the pre-film achieved a superior cycleability in [Li(G4)][TFSA]/HFE as shown in Figure 1(c); the capacity retention at the 200th cycle was 72.8%. This result implies that a stable SEI was formed by the pre-cycle in 1 M LiPF6/FEC electrolyte. From SEM/EDX analysis, the reductive decomposition of the electrolyte was suppressed by the presence of the pre-film, whereas on the Si-LP electrodes without the pre-film, SEI film with a large amount of F content was confirmed, indicating the decomposition of HFE. Formation of stable SEI is necessary to achieve a long cycle-life of Si anodes in the glyme-based electrolytes. References 1) M.Saito et al., Solid State Ionics, 225, 506 (2012). 2) K. Dokko et al., J. Electrochem. Soc., 160,A1304 (2013). 3) M. Haruta et al., Electrochemistry, 83,837 (2015). Acknowledgment This study was supported by JST-ALCA SPRING and JSPS-KAKENHI Grant Number 16H04649. Figure 1

Hard Carbon Anodes and Novel Electrolytes for Long‐Cycle‐Life Room Temperature Sodium‐Sulfur Full Cell Batteries

A novel combination of hard carbon anode sodium pre‐loading and a tailored electrolyte is used to prepare room temperature sodium‐sulfur full cell batteries. The electrochemical loading with sodium ions is realized in a specific mixture of diethyl carbonate, ethylene carbonate, and fluoroethylene carbonate electrolyte in order to create a first solid electrolyte interface (SEI) on the anode surface. Combining such anodes with a porous carbon/sulfur composite cathode results in full cells with a significantly decreased polysulfide shuttle when compared to half cells combined with metallic sodium anodes. Further optimization involves the use of Na2S/P2S5 doped tetraethylene glycol dimethyl ether based electrolyte in the full cell for the formation of a second SEI, reducing polysulfide shuttle even further. More importantly, the electrochemical discharge processes in the cell are improved by adding this dissolved complexation agent to the electrolyte. As a result of this combination sodium‐sulfur cells with tailored cathode materials and electrolytes can achieve high discharge capacities up to 980 mAh g−1sulfur and 1000 cycles with 200 mAh g−1sulfur remaining capacity, at room temperature.

Enhanced Lithiation Cycle Stability of ALD-Coated Confined a-Si Microstructures Determined Using In Situ AFM.

Microfabricated amorphous silicon (a-Si) pits ∼4 μm in diameter and 100 nm thick were fabricated to be partially confined in a nickel (Ni) current collector. Corresponding unconfined pillars were also fabricated. The samples were coated with 1.5, 3, or 6 nm of Al2O3 ALD. These samples were tested in electrolytes of 3:7 by weight ethylene carbonate:ethyl methyl carbonate (EC:EMC) with 1.2 M LiPF6 salt with and without 2% fluoroethylene carbonate (FEC) and in a pure FEC electrolyte with 10 wt % LiPF6. The samples were imaged with an atomic force microscope during electrochemical cycling to evaluate morphology evolution and solid electrolyte interphase (SEI) formation. The partially confined a-Si structures had superior cycle efficiency relative to the unconfined a-Si pillars. Additionally, samples with 3 nm of ALD achieved higher charge capacity and enhanced cycle life compared to samples without ALD, demonstrated thinner SEI formation, and after 10 cycles at a 1 C rate remained mostly intact and had actually decreased in diameter. Finally, the samples with 3 nm of ALD had better capacity retention in the baseline 3:7 EC:EMC than in either of the FEC containing electrolytes.

Hard X-ray Photoelectron Spectroscopy (HAXPES) Investigation of the Silicon Solid Electrolyte Interphase (SEI) in Lithium-Ion Batteries.

Binder-free silicon (BF-Si) nanoparticle anodes were cycled with 1.2 M LiPF6 in ethylene carbonate (EC), fluoroethylene carbonate (FEC), or EC with 15% FEC (EC:FEC), extracted from cells and analyzed by Hard X-ray Photoelectron Spectroscopy (HAXPES). All of the electrolytes generate an SEI which is integrated with Si containing species. The EC and EC:FEC electrolytes result in the generation of LixSiOy after the first cycle while LixSiOy is only observed after five cycles for the FEC electrolyte. The SEI initially generated from the EC electrolyte is primarily composed of lithium ethylene dicarbonate (LEDC) and LiF. However, after five cycles, the composition changes, especially near the surface of silicon because of decomposition of the LEDC. The SEI generated from the EC:FEC electrolytes contains LEDC, LiF, and poly(FEC) and small changes are observed upon additional cycling. The SEI generated with the FEC electrolyte contains LiF and poly(FEC) and small changes are observed upon additional cycling. The stability of the SEI correlates with the observed capacity retention of the cells.

Flame-retardant co-solvent incorporation into lithium-ion coin cells with Si-nanoparticle anodes

The cycling performance of Si-nanoparticle/Li cells with different electrolytes has been investigated. Cells containing standard binary LiPF6/ethylene carbonate/ethyl methyl carbonate electrolytes have poor capacity retention (46 %) after 50 cycles. Cells cycled with fluoroethylene carbonate (FEC)-based electrolyte have much better capacity retention (74 %). The effect of incorporation of flame-retardant co-solvents triphenyl phosphate and dime- thyl methylphosphonate was investigated with both the standard and FEC electrolytes. The incorporation of the FR co-solvents did not significantly alter the performance of either electrolyte. Ex situ analysis via scanning electron microscopy, attenuated total reflectance infrared spec- troscopy, and X-ray photoelectron spectroscopy was con- ducted to gain a better understanding of the role of electrolyte in solid electrolyte interphase structure and stability.

Storage Studies on Li/Graphite Cells and the Impact of So-Called SEI-Forming Electrolyte Additives

The growth of the solid electrolyte interphase on graphite electrodes during storage at different temperatures was studied using the High Precision Automated Storage System at Dalhousie University. The Li/graphite cells were precycled at 30°C before storage. The open circuit potential was automatically measured every six hours during storage and the capacity before and after storage was measured with high accuracy. The mechanism responsible for the capacity loss during storage is the growth and reformation of solid electrolyte interphase (SEI) on the graphite surface. The experimental results show that the addition of the electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) increase the capacity loss during storage compared to the control electrolyte at 30, 40, 50 and 60°C. A beneficial effect of VC was not even observed at 60°C and cells with the FEC electrolyte additive showed substantially higher capacity loss at 60°C than control cells. Storage experiments on negative electrodes can lead to a better understanding of the solid electrolyte interphase and also can be used to screen for better SEI-forming electrolyte additives.

Low-Temperature Performance of Li-Ion Battery with Fluoroethylene Carbonate Electrolyte

Low-Temperature Performance of Li-Ion Battery with Fluoroethylene Carbonate Electrolyte

00/01375 Fluoroethylene carbonate electrolyte and its use in lithium ion batteries with graphite anodes

00/01375 Fluoroethylene carbonate electrolyte and its use in lithium ion batteries with graphite anodes

Fluoroethylene carbonate electrolyte and its use in lithium ion batteries with graphite anodes

The electrolyte decomposition during the first lithiation of graphite is reduced to 85 mA h/g in an electrolyte containing equal volumes of fluoroethylene carbonate (Fluoro-EC) and of a co-solvent propylene carbonate (PC). The volume fraction of Fluoro-EC can be further reduced to 0.05 in a tri-solvent system with a co-solvent containing equal volumes of ethylene carbonate (EC) and PC. A lithium ion cell containing Fluoro-EC PC and EC shows a long cycle life. The capacity decreases by 37% from the initial value in over 200 cycles. Cell current efficiency is 100%, thus solving the poor cell current efficiency when chloroethylene carbonate (Chloro-EC) is used in place of Fluoro-EC.