Gas Evolution at Graphite Anodes Depending on Electrolyte Water Content and SEI Quality Studied by On-Line Electrochemical Mass Spectrometry

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959

1,278

Layered LiNi x Mn y Co z O 2 (NMC) is a widely used class of cathode materials with LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC111) being the most common representative. However, Ni-rich NMCs are more and more in the focus of current research due to their higher specific capacity and energy. In this work we will compare LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC111), LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622), and LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) with respect to their cycling stability in NMC-graphite full-cells at different end-of-charge potentials. It will be shown that stable cycling is possible up to 4.4 V for NMC111 and NMC622 and only up to 4.0 V for NMC811. At higher potentials, significant capacity fading was observed, which was traced back to an increase in the polarization of the NMC electrode, contrary to the nearly constant polarization of the graphite electrode. Furthermore, we show that the increase in the polarization occurs when the NMC materials are cycled up to a high-voltage feature in the dq/dV plot, which occurs at ∼4.7 V vs. Li/Li + for NMC111 and NMC622 and at ∼4.3 V vs. Li/Li + for NMC811. For the latter material, this feature corresponds to the H2 → H3 phase transition. Contrary to the common understanding that the electrochemical oxidation of carbonate electrolytes causes the CO 2 and CO evolution at potentials above 4.7 V vs. Li/Li + , we believe that the observed CO 2 and CO are mainly due to the chemical reaction of reactive lattice oxygen with the electrolyte. This hypothesis is based on gas analysis using On-line Electrochemical Mass Spectrometry (OEMS), by which we prove that all three materials release oxygen from the particle surface and that the oxygen evolution coincides with the onset of CO 2 and CO evolution. Interestingly, the onsets of oxygen evolution for the different NMCs correlate well with the high-voltage redox feature at ∼4.7 V vs. Li/Li + for NMC111 and NMC622 as well as at ∼4.3 V vs. Li/Li + for NMC811. To support this hypothesis, we show that no CO 2 or CO is evolved for the LiNi 0.43 Mn 1.57 O 4 (LNMO) spinel up to 5 V vs. Li/Li + , consistent with the absence of oxygen release. Lastly, we demonstrate by the use of 13 C labeled conductive carbon that it is the electrolyte rather than the conductive carbon which is oxidized by the released lattice oxygen. Taking these findings into consideration, a mechanism is proposed for the reaction of released lattice oxygen with ethylene carbonate yielding CO 2 , CO, and Li-Ion batteries have recently been used as power supply for electric vehicles (EVs). In order to penetrate the mass market, a significant reduction in costs and further performance improvements have to be achieved to realize a longer driving range of EVs.1 The latter highly depends on the choice of the cathode active material, for which several potential materials exist, 2 of which layered lithium nickel manganese cobalt oxide (LiNi x Mn y Co z O 2 , NMC) is one of the most promising class of cathode materials.3 This is due to the high specific capacity and good stability of the lay...

Section: Discussionmentioning

confidence: 62%

Section: Gas Analysis Of Nmc-li and Lnmo-li Half-cells By Oems-mentioning

confidence: 99%

Oxygen Release and Its Effect on the Cycling Stability of LiNi_xMn_yCo_zO₂(NMC) Cathode Materials for Li-Ion Batteries

Jung

Metzger

Maglia

et al. 2017

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959

1,278

“…Previous publications contain detailed descriptions of the Online Electrochemical Mass Spectrometer 34 the two-compartment cell, 35 and the Al-sealed laminated Ohara glass 36 that were used for LSV-OEMS experiments, demonstrating that it provides a gastight barrier between anode and cathode compartment of the OEMS cell. 37 Cathodes were separated from Li foil anodes by an Al-sealed laminated Ohara glass and one glass fiber separator on each side of the Ohara glass. Each glass fiber separator was soaked with 200 μL of pure LP30 or LP30 + 0.5 wt% LiBFEP electrolyte.…”

Section: Methodsmentioning

confidence: 99%

Lithium Bis(2,2,2-trifluoroethyl)phosphate Li[O₂P(OCH₂CF₃)₂]: A High Voltage Additive for LNMO/Graphite Cells

Milien

Beyer

Beichel

et al. 2018

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The fluorinated phosphate lithium bis (2,2,2-trifluoroethyl) phosphate (LiBFEP) has been investigated as a film-forming additive employed to passivate the cathode and hinder continuous oxidation of the electrolyte. Cyclic voltammetry (CV) and linear sweep voltammetry coupled with online electrochemical mass spectrometry (LSV-OEMS) on a conductive carbon electrode (i.e., a C65/PVDF composite) showed that LiBFEP decreases electrolyte oxidation (CV and LSV) and LiPF 6 decomposition at high potentials. Incorporation of LiBFEP (0.1 and 0.5 wt%) into LiPF 6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7 wt) results in improved coulombic efficiency and capacity retention for LNMO/graphite cells. Ex-situ surface analysis of the electrodes suggests that incorporation of LiBFEP results in the formation of a cathode electrolyte interface (CEI) and modification of the solid electrolyte interface (SEI) on the anode. The formation of the CEI mitigates electrolyte oxidation and prevents the decomposition of LiPF 6 , which in turn prevents HF-induced manganese dissolution from the cathode and destabilization of the SEI. The passivation of the cathode and stabilization of the SEI is responsible for the increased coulombic efficiency and capacity retention. Since their debut in 1991, lithium ion batteries (LIB) have become the universal power source for consumer electronics.1 Larger format LIBs such as those needed to power electric vehicles (EVs), an important future market, have amassed considerable interest; however higher specific energy densities are required for larger format LIBs.1,2 The practical way to increase energy density is to employ cathode materials with increased theoretical capacities and/or high discharge plateaus, and thus high energy (HE) or high voltage (HV) cathodes are required in order for LIBs to meet the demands of the EV market.3 While both HE and HV cathodes have been implemented, current research efforts are focused on overcoming the caveats associated with these materials. The oxidative instability of carbonate-based electrolytes is a central limitation for cells with various cathode chemistries operated above 4.4 V. [3][4][5][6][7] In addition to the instability of the electrolyte, cathodes such as nickel-rich layered oxides (LiNi x Mn y Co z O 2 ), lithium-rich layered oxides (0.6 Li 2 MnO 3 • 0.4 Li(Ni 1/3 Co 1/3 Mn 1/3 )O 2 ), and HV spinel (LiNi 0.5 Mn 1.5 O 4 ) (LNMO) all suffer from structural instability when operated at high potentials.5-11 While the layered oxides are capable of delivering higher practical energy densities, the lack of cobalt in LNMO alleviates the issues of cost and resource limitations.7 As the higher energy densities associated with HE materials can only be obtained at higher cutoff potentials, oxidation of the electrolyte is a universal problem to both HE and HV cathodes. This work focuses on improving the performance of LNMO/Graphite cells.The capacity fading observed in LNMO/Graphite cells is due to continuous oxidation of the electrolyte and transition...

“…The consumption of EC is calculated from the CO 2 release measured by OEMS, under the assumption that 1 mol CO 2 originates from the decomposition of 1 mol EC, which is based on the reaction mechanism proposed in the Discussion section (see Scheme 1 in the Discussion section). While CO 2 can also be consumed by an oligomerization reaction between CO 2 and initially formed di-alcoholates, [29][30][31] the roughly constant CO 2 evolution rates in Figure 1 (i.e., reasonably constant slopes toward the end of each temperature step) suggest that this reaction is slow compared to the ethylene carbonate decomposition reaction. The thus estimated consumption of EC (in μmol) by the end of each temperature step is given in Table I and may be compared to the total moles of EC in the OEMS cell of 3680 μmol (≡ 1.50 g/cm 3 · 240 μl · 0.9 / (88 g/mol), using the density of the base electrolyte, the mass fraction of EC in the base electrolyte and the molecular weight of EC).…”

Section: Co 2 Evolution From Ec Hydrolysis With H 2 O and Ohmentioning

confidence: 99%

Hydrolysis of Ethylene Carbonate with Water and Hydroxide under Battery Operating Conditions

Metzger

Strehle

Solchenbach

et al. 2016

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117

150

This study deals with the decomposition of ethylene carbonate (EC) by H 2 O in the absence and presence of catalytically active hydroxide ions (OH − ) at reaction conditions close to lithium-ion battery operation. We use On-line Electrochemical Mass Spectrometry (OEMS) to quantify the CO 2 evolved by these reactions, referred to as H 2 O-driven and OH − -driven EC hydrolysis. By examining both reactions at various temperatures (10 -80 • C) and water concentrations (<20 ppm or 200, 1000, and 5000 ppm H 2 O) with or without catalytically active OH − ions in EC with 1.5 M LiClO 4 , we determine an Arrhenius relationship between the CO 2 evolution rate and the cell temperature. While the apparent activation energy for the base electrolyte (<20 ppm H 2 O) is very large (app. E a ≈153 kJ/mol), substantially lower values are obtained in the presence of H 2 O (app. E a ≈99 ± 3 kJ/mol), which are even further decreased in the presence of catalytically active OH − (app. E a ≈43 ± 5 kJ/mol). Our data show that OH − -driven EC hydrolysis is relevant already at room temperature, whereas H 2 O-driven EC hydrolysis (i.e., without catalytically active OH − ) is only relevant at elevated temperature (≥40 • C), as is the case for the base electrolyte. Thus, catalytic quantities of OH − , e.g., from hydroxide contaminants on the surface of transition metal oxide based active materials, would be expected to lead to considerable CO 2 gassing in lithium-ion cells. Lithium-ion battery electrolytes typically rely on the cyclic carbonate ethylene carbonate (EC) as a co-solvent, mixed with linear carbonates like diethyl carbonate (DEC), dimethyl carbonate (DMC), and/or ethyl methyl carbonate (EMC) and a lithium salt, usually lithium hexafluorophosphate (LiPF 6 ). EC has unique properties in terms of the formation of a solid-electrolyte interphase (SEI) at the negative electrode (typically graphite) in lithium-ion batteries (LIBs). Upon the electrochemical reduction of EC, ethylene gas is released and lithium ethylene dicarbonate (LEDC) is deposited on the surface of the negative electrode, forming an SEI that prevents further electrolyte reduction but at the same time still allows for Li + -ion conduction. [1][2][3][4] Compared to the numerous articles dealing with the electrochemical decomposition of EC, it is rarely discussed in battery literature that EC can also undergo chemical decomposition reactions.In principle, all cyclic monomers can undergo ring-opening reactions followed by polymerization. It has been shown in the polymer literature that the five-membered aliphatic cyclic carbonates, such as ethylene carbonate and propylene carbonate, can be polymerized using Lewis acids, transesterification catalysts, or bases as initiators. [5][6][7][8][9][10][11][12] There is a consensus among most authors that CO 2 is lost during the polymerization of EC and that the repeat units of the resultant polymers are a mixture of carbonate units and the corresponding oxide units. When Lewis acids (e.g., Al(acac) 3 or Ti(OBu) 4 ) 5,6 or transester...