Effect of LiPF6 concentration in Li[Ni0.4Mn0.4Co0.2]O2/graphite pouch cells operated at 4.5 V

978

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: Resultssupporting

confidence: 89%

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

978

1,278

“…R ct of the NMC positive electrodes cycled in LiBF 4 and LiPF 6 electrolytes are more or less equivalent, except for an increase in the high frequency semi-circle (labelled with * ), which is attributed to the contact resistance of the positive electrode with the Al current collector. 29 Overall, Figure 6 indicates that the increased R ct of control cells with LiBF 4 , "barn" cycled to 4.5 V, is a result of increased graphite electrode R ct and increased contact resistance of the positive electrode with the aluminum current collector. This is in agreement with other Electrolyte reactions during storage.- Figure 7A shows the voltage drop during the 500 hours of storage.…”

Section: Resultsmentioning

confidence: 93%

Effect of Substituting LiBF₄for LiPF₆in High Voltage Lithium-Ion Cells Containing Electrolyte Additives

Ellis

Xia

Louli

et al. 2016

This work evaluates the performance of LiBF4-containing electrolytes in lithium-ion cells charged to high voltages (>4.4 V) and explores the compatibility of LiBF4 with additives known to improve the performance of LiPF6-based electrolytes. It was found that the benefit imparted by LiBF4 is lost when cells are charged beyond 4.4 V, at which point electrolyte oxidation causes gas evolution and impedance growth. In the case of LiBF4, impedance growth is shown to be due to increased charge transfer resistance at the negative electrode, indicating poor passivation of the negative electrode. Of the additives tested, only those containing prop-1-ene-1,3-sultone benefit cells with LiBF4 electrolyte at high voltage.

“…[33][34][35][36] Wang et al found that different concentrations of LiPF 6 in EC-based electrolyte can affect LiCoO 2 /graphite cell cycle life and calendar life.…”

mentioning

confidence: 99%

“…[33][34][35][36] Wang et al found that different concentrations of LiPF 6 in EC-based electrolyte can affect LiCoO 2 /graphite cell cycle life and calendar life. 33 Petibon et al observed that high concentrations of LiPF 6 (more than 2 M) in EC-based electrolyte hinder impedance growth at the positive electrode when cells were tested to 4.5 V. 34 In this report, a study of different concentrations of LiPF 6 in EMC and EC-based electrolytes (0.3-2 M) was carried out. The electrolytes were used in NMC442/graphite pouch cells that were subjected to elevated temperature storage, charge-discharge and impedance spectroscopy studies.…”

mentioning

confidence: 99%

Dramatic Effects of Low Salt Concentrations on Li-Ion Cells Containing EC-Free Electrolytes

Xiong¹,

Hynes²,

Dahn³

2017

Different concentrations of LiPF 6 (0.3 M-2 M) in ethyl methyl carbonate (EMC) electrolyte and ethylene carbonate (EC)-based electrolyte were studied in LiNi 0.4 Mn 0.4 Co 0.2 O 2 (NMC442)/graphite pouch cells. Fresh cells containing 0.3 M LiPF 6 in EMC electrolyte showed extremely large charge transfer resistance while those with 0.3 M LiPF 6 in EC/EMC electrolyte did not. Impedance spectra taken on symmetric cells and ionic conductivity measurements suggest this difference is due to difficulty in dissociating and desolvating Li + ions from the EMC-based electrolyte to intercalate into both the electrodes. After elevated temperature storage experiments at 4.5 V, cells with 0.3 M LiPF 6 in EC/EMC showed a large increase in positive electrode charge transfer impedance, presumably caused by electrolyte oxidation. With salt concentrations greater than 1 M, charge transfer resistance was much smaller in EMC-based electrolytes and was stable during storage for both electrolyte types. Conductivity and cycle testing measurements suggest that 1.5 M LiPF 6 should be used in EC-free EMC-based electrolytes to optimize cell performance. Using high potential positive electrodes can increase the energy density of Li-ion cells. [1][2][3][4] Layered LiNi x Mn y Co z O 2 (x + y + z = 1) (NMC) electrodes have been extensively studied because they are cheaper materials than LiCoO 2 (LCO) and can operate at high working potentials.5-15 LiNi 0.4 Mn 0.4 Co 0.2 O 2 (NMC442) is a possible choice for high voltage application due to its high working potential, high specific capacity, moderate cost and excellent safety. [16][17][18][19] NMC442 has been shown to be structurally stable up to 4.7 V 16,17 and its specific capacity increases almost linearly as the upper cutoff voltage increases from 4.1 V to 4.7 V. At 4.7 V, NMC442 can deliver a reversible specific capacity of ∼207 mAh/g. NMC442 shows attractive safety features compared to other NMC grade materials as well. 19It is difficult to create NMC442/graphite cells that show long lifetime when operated continually to an upper cutoff potential above 4.4 V. Electrolyte additives such as pyridine boron fluoride (PBF), 20,21 triallyl phosphate (TAP) 22 and additive blends such as prop-1-ene-1,3-sultone (PES) + 1,3,2-dioxathiolane-2,2-dioxide (DTD) + tris-(trimethyl-silyl) phosphite (TTSPi) [23][24][25][26][27] in EC-based electrolyte have been used to improve charge-discharge cycle life and calendar life of cells operated to 4.4 V. Sulfone-based and fluorinated electrolyte systems have also shown to be of value in getting NMC442/graphite cells to operate beyond 4.4 V. 28,29 In spite of these improvements, further improvement is required. Recently, it was found that EC-free electrolyte using only ethyl methyl carbonate as the solvent can enhance high voltage operation and lifetime of NMC442/graphite pouch cells compared to EC-based electrolyte.30-32 However, only a salt concentration of 1 M LiPF 6 in this novel electrolyte was investigated.The concentration of LiPF 6 in EC-based electrolyte...