Fluorinated electrolytes for 5-V Li-ion chemistry: Dramatic enhancement of LiNi0.5Mn1.5O4/graphite cell performance by a lithium reservoir

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: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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

Solchenbach

Wetjen

Pritzl

et al. 2018

“…The solvent F-EMC was chosen since it has shown relatively good stability against the LNMO cathode. 9,19 The floating tests were carried out at room temperature first, and the results are given in Figure 1a. Interestingly, all four cyclic carbonates showed similar levels of stability and overcharge tolerance at RT.…”

Section: Resultsmentioning

confidence: 99%

Fluorinated Electrolytes for 5-V Li-Ion Chemistry: Probing Voltage Stability of Electrolytes with Electrochemical Floating Test

Xue

et al. 2015

Self Cite

122

A series of electrolyte formulations containing fluorinated cyclic carbonates and fluorinated linear carbonates with LiPF 6 has been evaluated as electrolyte solvents for high-voltage Li-ion batteries. The anodic stability of the new electrolytes on fully charged spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode was examined by electrochemical floating tests. The effects of fluorine substitution on the cyclic and linear carbonate, ratio of cyclic vs. linear carbonate, and LiPF 6 concentration on the electrolyte oxidation stability were investigated. Based on this study, the floating test proved to be an effective tool for identification of stable electrolyte materials. have been proposed. Because of the large number of candidates for high-voltage electrolyte solvents, screening the voltage stability of each solvent would be very labor-intensive. Traditional methods of measuring the oxidation potential of organic solvents usually involves linear/cyclic voltammetry using an inert electrode such as platinum and glassy carbon. However, such measurements are in many cases misleading, because interactions of these organic solvents with actual electrode materials are usually more complicated and may happen at a much lower potential due to the catalytic effect of the cathode material lowering the kinetic barrier of oxidation. Unfortunately, using active cathode material to run voltammetry measurement has a drawback in that the material itself is redox active and can interfere with the observation of electrolyte oxidation. Thus, developing a fast and effective method to screen the voltage stability of electrolyte solvents on actual cathode materials is of vital importance. Herein, we report a method using constant potential electrolysis with a slightly overcharged LNMO cathode as the working electrode, abbreviated as an "electrochemical floating test", where the cell potential is allowed to "float" at different values to evaluate the voltage stability of the electrolyte. For an ideal electrolyte with no impurities and no oxidation at the working electrode, the only current observed when a potential is applied is the capacitance current, which should decline to zero when the equilibrium is reached. However, in reality, the electrolytes are oxidized, and the current intensity measured corresponds to the severity of oxidation. As a result, the leakage currents of each electrolyte at different potentials can be compared to produce a voltage stability profile of a given solvent. The effect of different ratios of mixed solvents and lithium salt concentrations can also be probed. * Electrochemical Society Active Member.z E-mail: zzhang@anl.gov Materials and MethodsTheoretical calculations.-The Gaussian 09 code was used for all calculations.14 Oxidation and reduction potentials were calculated by optimizing the geometries of the neutral and ionic species at the B3LYP/6-31G * level, followed by frequency calculations to determine gas-phase free energies. Solvation effects were taken into account by using a single-point B3LYP/6-31+G * PCM...

“…This work investigates some conclusions in the available literature of an additive molecule that decomposes on the charged cathode surface and passivates the surface against electrolyte oxidation. With the goal of increasing battery energy density for transportation applications, layered transition metal oxides cathodes of the form LiMO 2 (M = Ni, Co, Mn) have received significant attention.1-7 While these cathode materials have a high theoretical gravimetric capacity of ∼280 mAh/g, 8 achieving the high capacity of these materials requires very high charging voltages (>4.5V vs. Li + /Li), 2-4 above the stability range of conventional carbonate electrolytes.9,10 Enabling the additional high voltage capacity of the cathode materials requires new electrolytes that provide additional stability at high voltages at the charged cathode surface.The approach to creating compatible electrolytes has mainly fallen along two separate paths: either increasing the intrinsic anodic stability of the electrolyte, [11][12][13][14][15][16][17] or using a sacrificial component in small amounts (an additive) to "passivate" the cathode surface and inhibit continual electrolyte oxidation.18 20,32 have shown evidence of decomposition on various high voltage cathode surfaces through a variety of surface characterization techniques, including attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS).However, this cathode "passivation" is conventionally inferred through a combination of experimental results, such as increased capacity retention, voltammetric methods indicating a lower anodic stability attributed to additive oxidation, changes to the XPS and/or FTIR spectra of the cathode that include components of the additive, and decreased impedance after cycling. While the results provided by the previously mentioned characterization methods can be reconciled with the conclusion that cathode "passivation" led to the improvement, this mechanistic process can be more accurately evaluated by monitoring the electrolyte oxidation current at a charged cathode surface through potentiostatic holds.…”

mentioning

confidence: 99%

“…The approach to creating compatible electrolytes has mainly fallen along two separate paths: either increasing the intrinsic anodic stability of the electrolyte, [11][12][13][14][15][16][17] or using a sacrificial component in small amounts (an additive) to "passivate" the cathode surface and inhibit continual electrolyte oxidation.…”

mentioning

confidence: 99%

The Role of Additives in Improving Performance in High Voltage Lithium-Ion Batteries with Potentiostatic Holds

Tornheim

et al. 2017

Self Cite

In this work, various electrolyte additives designed for enhanced performance at high voltages were evaluated with elevated temperature potentiostatic holds with LiNi 0.5 Co 0.2 Mn 0.3 /Li 4 Ti 5 O 12 full cells to determine their effect on the high voltage stability. Of the additives investigated, many showed increased oxidation current through the 60 hour potentiostatic holds test, and adversely affected both the capacity retention and interfacial impedance. Improved high voltage performance was observed with two additives, vinylene carbonate (VC) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), which was attributed to two different mechanisms of improvement. This work investigates some conclusions in the available literature of an additive molecule that decomposes on the charged cathode surface and passivates the surface against electrolyte oxidation. With the goal of increasing battery energy density for transportation applications, layered transition metal oxides cathodes of the form LiMO 2 (M = Ni, Co, Mn) have received significant attention.1-7 While these cathode materials have a high theoretical gravimetric capacity of ∼280 mAh/g, 8 achieving the high capacity of these materials requires very high charging voltages (>4.5V vs. Li + /Li), 2-4 above the stability range of conventional carbonate electrolytes.9,10 Enabling the additional high voltage capacity of the cathode materials requires new electrolytes that provide additional stability at high voltages at the charged cathode surface.The approach to creating compatible electrolytes has mainly fallen along two separate paths: either increasing the intrinsic anodic stability of the electrolyte, [11][12][13][14][15][16][17] or using a sacrificial component in small amounts (an additive) to "passivate" the cathode surface and inhibit continual electrolyte oxidation.18 20,32 have shown evidence of decomposition on various high voltage cathode surfaces through a variety of surface characterization techniques, including attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS).However, this cathode "passivation" is conventionally inferred through a combination of experimental results, such as increased capacity retention, voltammetric methods indicating a lower anodic stability attributed to additive oxidation, changes to the XPS and/or FTIR spectra of the cathode that include components of the additive, and decreased impedance after cycling. While the results provided by the previously mentioned characterization methods can be reconciled with the conclusion that cathode "passivation" led to the improvement, this mechanistic process can be more accurately evaluated by monitoring the electrolyte oxidation current at a charged cathode surface through potentiostatic holds. A passivation layer will limit electron transfer from the electrolyte to the charged cathode surface. By this definition, this layer must have both a minimal electron conductivity as well as a minimal diffusio...