Some Fluorinated Carbonates as Electrolyte Additives for Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /Graphite Pouch Cells

In the present study, we explore the use of lithium oxalate as a "sacrificial salt" in combination with lithium nickel manganese spinel (LNMO) cathodes. By online electrochemical mass spectrometry (OEMS), we demonstrate that the oxidation of lithium oxalate to CO 2 (corresponding to 525 mAh/g) occurs quantitatively around 4.7 V vs. Li/Li + . LNMO/graphite cells containing 2.5 or 5 wt% lithium oxalate show an up to ∼11% higher initial discharge capacity and less capacity fade over 300 cycles (12% and 8% vs. 19%) compared to cells without lithium oxalate. In LNMO/SiG full-cells with an FEC-containing electrolyte solution, lithium oxalate leads to a better capacity retention (45% vs 20% after 250 cycles) and a higher coulombic efficiency throughout cycling (∼1%) compared to cells without lithium oxalate. When CO 2 from lithium oxalate oxidation is removed after formation, a similar capacity fading as in LNMO/SiG cells without lithium oxalate is observed. Hence, we attribute the improved cycling performance to the presence of CO 2 in the cells. Further analysis (e.g., FEC consumption by 19 F-NMR) indicate that CO 2 is an effective SEI-forming additive for SiG anodes, and that a combination of FEC and CO 2 has a synergistic effect on the lifetime of full-cells with SiG anodes. Lithium nickel manganese spinel (LiNi 0.5 Mn 1.5 O 4 , LNMO) is a promising cathode material for high energy lithium ion batteries due to its high operating potential around 4.7 V vs. Li/Li + , its high rate capability, structural stability and the absence of cobalt. However, its lower specific capacity (146 mAh/g LNMO ) compared to layered oxide materials (e.g. lithium nickel manganese cobalt oxide (NMC), specific capacity 150-250 mAh/g NMC ) 1 is regarded as a major drawback. In full-cells, the practically achievable capacity of LNMO is even lower, as the formation of the solid-electrolyte interphase (SEI) on the graphite anode consumes active lithium. For many layered oxide cathodes, however, the first cycle irreversible capacity of the cathode is similar to the capacity needed for SEI formation (∼20 mAh/g NMC ), and hence the practical discharge capacity of the cathode and the remaining active lithium are more or less balanced again. 2,3 In contrast, the first cycle irreversible capacity of LNMO (∼6 mAh/g LNMO ) is much lower than the capacity needed for SEI formation. This leads to a mismatch between active lithium and practical cathode capacity, i.e., there is not enough active lithium available to fully discharge the LNMO cathode during subsequent cycling.In cells with silicon-based anodes, active lithium losses on the anode are even higher compared to graphite, as the expansion of the silicon particles during the first lithiation leads to a continuous exposure of fresh, unpassivated silicon surface. 4 On this new surface, electrolyte reduction occurs instantaneously, which reduces the total lithium reservoir in the cell. Therefore, different ideas to increase the amount of active lithium in lithium ion full-cells have been suggested, ...

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Lithium Oxalate as Capacity and Cycle-Life Enhancer in LNMO/Graphite and LNMO/SiG Full Cells

Solchenbach

Wetjen

Pritzl

et al. 2018

“…[6][7][8] Recently, incorporation of the fluorinated compound either as solvent or as additive have been investigated for high-voltage LIB and positive effects have been reported. [9][10][11][12][13] Especially, utilization of fluoroethylene carbonate (FEC), which is well known to form solid-electrolyte interphase (SEI) on the anode surface with the small amounts addition, in high-voltage LIB has gathered attention. Based on 1 M LiPF 6 ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC/EMC = 3/7, wt/wt) electrolyte, FEC was used as a fluorinated solvent (20∼50%) to replace EC or as a fluorinated additive (>5%) in a base electrolyte solution.…”

mentioning

confidence: 99%

Fluorinated Carbonate-Based Electrolyte for High-Voltage Li(Ni_0.5Mn_0.3Co_0.2)O₂/Graphite Lithium-Ion Battery

Lee

Ryou

et al. 2017

A stable electrolyte system at a charge voltage over 4.5 V is the key to successfully obtaining higher energy density by raising the charging cutoff voltage. We demonstrate a fluorinated electrolyte (1 M LiPF 6 fluoroethylene carbonate (FEC) and methyl (2,2,2-trifluoroethyl) carbonate (FEMC) (FEC/FEMC = 1/9, v/v)) for a high-voltage LiNi 0.5 Mn 0.3 Co 0.2 O 2 /graphite system. The stability of the fluorinated electrolyte for the LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) cathode was investigated using scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. The charge-discharge performance of the fluorinated electrolyte was superior to the corresponding non-fluorinated electrolyte system at a charging cutoff voltage of 4. Considerable efforts have been made for developing high-charge voltage platforms that provide large capacities, thereby helping improve the energy density of lithium-ion batteries (LIB). Because electrical energy is the product of the discharge capacity and the average discharge voltage, it is important to search for cathode materials with a high discharge voltage.1 To this end, many studies have been devoted to improving cathode materials for high-voltage applications, such as polyanion oxides (LiCoPO 4 ), spinel-type oxides (LiMn 2 O 4 ), and layered oxides (LiNi x Mn y Co 1-x-y O 2 ). Among cathode materials, LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) is one of the most promising for its potential practical application in high-voltage lithium-ion battery. 2-4However, there remains a critical obstacle that should be overcome for high-voltage lithium-ion batteries. Oxidative instability of the conventional non-aqueous electrolyte takes place at a charging voltage beyond 4.5 V, leading to severe decay of cycling performances.5 Per research, practical approaches to improving performance for highvoltage operation include use of a new solvent or introducing functional additives to the electrolyte. [6][7][8] Recently, incorporation of the fluorinated compound either as solvent or as additive have been investigated for high-voltage LIB and positive effects have been reported. [9][10][11][12][13] Especially, utilization of fluoroethylene carbonate (FEC), which is well known to form solid-electrolyte interphase (SEI) on the anode surface with the small amounts addition, in high-voltage LIB has gathered attention. Based on 1 M LiPF 6 ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC/EMC = 3/7, wt/wt) electrolyte, FEC was used as a fluorinated solvent (20∼50%) to replace EC or as a fluorinated additive (>5%) in a base electrolyte solution. However, there are a few studies reported on the electrolyte system consisting of wholly fluorinated solvents and additive. Here, we demonstrate a fully fluorinated electrolyte system composed of FEC and methyl (2,2,2-trifluoroethyl) carbonate (FEMC) at a volume ratio of 1:9 for high-voltage lithium-ion batteries. Amount of FEC is selected slightly higher than the case of FEC as additive in base solvent and much lower th...

“…This again is one of the advantages of the addition of such additive blends to EC-based electrolytes in NMC(442)/graphite cells cycled to high voltage. 29,43 Figure 8 shows that cells containing FEC:TFEC electrolyte had high impedance after storage or cycling, which may limit its high rate cycling performance. 30 Figure 8 shows that cells containing EMC:VC 98:2 electrolyte had low impedance after formation, storage or cycling tests.…”

Section: Resultsmentioning

confidence: 99%

Improving Linear Alkyl Carbonate Electrolytes with Electrolyte Additives

Xia

Glazier

Petibon

et al. 2017

Self Cite

The effectiveness of some selected electrolyte additives were studied in ethylene carbonate-free linear alkyl carbonate electrolyte mixtures containing 1 M LiPF 6 /ethylmethyl carbonate: vinylene carbonate (98:2 w:w). The electrolytes were used in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch type Li-ion cells tested to 4.5 V at elevated temperature. The results showed that cells with linear alkyl carbonate electrolytes outperformed cells with EC-based electrolytes having state-of-the-art additive blends and cells with fluorinated electrolytes during short-term and long-term cycling at elevated temperatures. The addition of triallyl phosphate or pyridine phosphorus pentafluoride as electrolyte additives to the EC-free electrolyte provided cells with better capacity retention during long-term cycling than cells containing vinylene carbonate alone. These results suggest that small quantities of electrolyte additives can be used to create EC-free, linear alkyl carbonate electrolytes that perform well at high voltages. Energy-dense, long-lasting lithium-ion cells are in high demand for electric vehicles, portable electronics and many other important technologies. The easiest way to increase the energy density of a conventional lithium-ion cell is to increase its upper charge cutoff potential when certain layered oxide positive electrode materials are used. However, increasing the upper cutoff potential normally decreases cell lifetime because the rate of unwanted parasitic reactions like electrolyte oxidation increase with potential. Parasitic reactions involving ethylene carbonate (EC)-a ubiquitous co-solvent in lithiumion electrolytes-at high potential has proven to be one of the most important factors to overcome. 1Until recently, EC has been regarded as an essential component of lithium-ion electrolytes and it has been used in nearly all commercial Li-ion cells produced today. EC was first considered as an electrolyte co-solvent by Elliot in 1964 2 and was used as an electrolyte component in cells with lithiated graphite electrodes by Fong et al. in 1990. 3 EC was found to preferentially react with Li + and electrons at the surface of the graphite electrode prior to lithium intercalation, forming a solid electrolyte interface (SEI) on the surface of the graphite electrode that then allowed the reversible intercalation and de-intercalation of lithium ions into the graphite electrode at very low potentials (<0.20 V).3-5 It was generally believed that the graphite electrode could not be electrochemically lithiated in most common solvents, including ethers, esters, butyrolactones and propylene carbonate, without the sacrificial reaction of EC to form the SEI. 6While the passivation of graphite is virtually complete during the first few cycles, the remaining EC in the electrolyte can be oxidized at high voltages, causing the performance of cells containing EC to diminish as the upper charge voltage increases above 4.4 V.7,8 Electrolyte oxidation at the positive electrode at high potentials can lead to salt consum...