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

Maglia

et al. 2017

Self Cite

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: Discussionsupporting

confidence: 86%

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

Jung

Maglia

et al. 2017

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959

1,278

“…At the same time, the CO 2 concentration increases linearly at 60 • C and 80 • C to above 12000 ppm, in agreement with previous experiments on the water-driven hydrolysis of EC in LiClO 4 -based electrolytes. 38 Comparing the results of Figures 7b and 7c, it becomes apparent that only highly acidic species can trigger the formation of PF 5 at room temperature. Accordingly, the oxidation of an EC/LiPF 6 electrolyte (see Figure 6) must be leading to the formation of species which act as proton donors or Brønsted acids, which is consistent with the mechanism proposed by Borodin et al 24 and Li et al 25 (see Scheme 1).…”

Section: Pfmentioning

confidence: 84%

Quantification of PF₅ and POF₃ from Side Reactions of LiPF₆ in Li-Ion Batteries

Solchenbach

Egawa

et al. 2018

Self Cite

144

173

The electrochemical oxidation of LiPF 6-based electrolytes is reported to generate POF 3 gas. In order to enable a quantitative analysis of the LiPF 6 decomposition reactions, we aimed to establish calibration factors for POF 3 and PF 5 in on-line electrochemical mass spectrometry (OEMS). Thermal decomposition of dry LiPF 6 is expected to yield PF 5 , but instead all PF 5 is detected as POF 3 in our OEMS setup, rendering a differentiation of the two gases impossible and presenting an artefact which likely occurs with most on-line mass spectrometry systems due to the high reactivity of PF 5. However, we can still determine a cumulative calibration factor for POF 3 + PF 5 (referred to as "POF 3 "), which is then used to investigate the evolution of gases during the oxidation of an EC/LiPF 6 electrolyte on a carbon black electrode. Mechanistic experiments with protons or water added to EC/LiPF 6 electrolyte show that protons trigger the formation of PF 5 , while the kinetics for the hydrolysis of LiPF 6 with water at room temperature are too slow to be detectable. These findings let us conclude that the oxidation of EC generates highly acidic species, which cause the decomposition of PF 6 − to PF 5 and HF; the PF 5 is then detected as POF 3 in the OEMS.

“…When VEC was added to the electrolyte, the intensity of 1,3-butadiene was subtracted analogously from channel m/z = 26 to quantify C 2 H 4 : I 26 (C2H4) = I 26 -0.33 · I 39 .…”

Section: Gasmentioning

confidence: 99%

The Effect of CO₂on Alkyl Carbonate Trans-Esterification during Formation of Graphite Electrodes in Li-Ion Batteries

Strehle

Solchenbach

et al. 2017

Self Cite

103

Apart from the often-described formation of interphases between the electrodes and the electrolyte in Li-ion batteries, changes of the bulk electrolyte also occur during cycling. In this study, we use On-line Electrochemical Mass Spectrometry (OEMS) to measure the gas evolution associated with changes in the electrolyte during the initial cycles of graphite/lithium half-cells in an electrolyte composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and the conducting salt LiPF 6 . The reduction of the electrolyte at the graphite surface within the first cycle is accompanied by the release of lithium alkoxides (LiOR), which initiate the conversion of the co-solvent EMC into the linear carbonates dimethyl carbonate (DMC) and diethyl carbonate (DEC). This trans-esterification can be suppressed by the use of additives such as vinylene carbonate (VC) and vinyl ethylene carbonate (VEC). Upon reduction, VC generates CO 2 , while VEC generates 1,3-butadiene. The beneficial impact of the additives arises from these gases, which scavenge the highly reactive LiOR species by forming non-reactive products. Furthermore, our results demonstrate the positive effect of CO 2 on the cell chemistry and the importance of adjusting the electrolyte volume and additive concentration with respect to the active material mass in Li-ion batteries. Li-ion batteries (LIBs) have been successfully used in electronic devices during the past 25 years. Recently, the expanding market of hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs) powered by LIBs has pushed the requirements on this technology even further.1-4 However, the application of LIBs particularly in PHEVs and BEVs is still hampered by their limited safety, thermal stability, and cycle life. Although the applied electrolyte solutions are prone to thermal decomposition, mixtures of cyclic and linear organic carbonate solvents in combination with the conducting salt LiPF 6 are widely used for LIBs. They represent an optimum between low viscosity, low vapor pressure, good salt solubility, and high conductivity. The electrolyte composition plays an important role in terms of cycle life. During the initial cycles, the electrolyte is reduced at the anode and forms a passivating layer, the so-called solid electrolyte interphase (SEI). The SEI prevents the further reduction of the bulk electrolyte by blocking the electron transport while allowing Li-ions to pass through. Its stability is thus critical for attaining long LIB cycle and storage life, 5,6 whereby electrolyte additives have been shown to substantially alter the composition of the SEI and to improve its effectiveness. 7Besides the formation of electrode interphases, also changes of the bulk electrolyte can occur during the cycling of a cell. These processes can be induced either by thermal reactions or by the dissolution and/or further reaction of reduced or oxidized electrolyte species from the anode or cathode, respectively. For example, the thermal decomposi...