Effects of Upper Cutoff Potential on LaPO 4 -Coated and Uncoated Li[Ni 0.42 Mn 0.42 Co 0.16 ]O 2 /Graphite Pouch Cells

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286

Single-crystal LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) with a grain size of 2-3 μm was compared to conventional polycrystalline uncoated NMC532 and polycrystalline Al 2 O 3 -coated materials in this work. Studies were made to determine how single crystal NMC532 material with large grain size could be synthesized. Ultra high precision coulometry (UHPC), in-situ gas measurements and isothermal microcalorimetry were used to make comparative studies of the three materials in Li-ion pouch cells. All the diagnostic measurements suggested that the single crystal material should yield Li-ion cells with longer lifetime. Long-term cycling tests verified these predictions and showed that cells with single crystal NMC532 exhibited much better capacity retention than cells with the polycrystalline materials at both 40 • C and 55 • C when tested to an upper cutoff potential of 4.4 V. The reasons for the superior performance of the single crystal cells were explored using thermogravimetric analysis/mass spectrometry experiments on the charged electrode materials. The single crystal materials were extremely resistant to oxygen loss below 100 • C compared to the polycrystalline materials. The major drawback of the single crystal material is its slightly lower specific capacity compared to the polycrystalline materials. However, this may not be an issue for Li-ion cells designed for long lifetime applications. Lithium ion batteries with high energy density, long lifetime and low cost need to be developed for applications in electric vehicles and stationary energy storage. The family of Li(Ni x Mn y Co z )O 2 (x + y + z = 1) (NMC) materials with high nickel and low cobalt are used as positive electrode materials in lithium ion cells.1,2 One simple way to increase the energy density of NMC lithium ion cells is to increase their upper cutoff voltage which gives access to higher specific capacity from the positive electrode.3,4 However, increasing the upper cutoff voltage usually decreases the lifetime of cells due to an acceleration of 'unwanted' parasitic reactions between the electrolyte and the delithiated positive electrode surface at high voltages. Such reactions include oxidation of species found in the electrolyte, transition metal dissolution, etc. [5][6][7] In addition, structural reconstruction of the positive electrode surface can occur which can contribute to impedance growth and capacity loss. 3,4 The by-products of oxidation at the positive electrode can migrate to the negative electrode surface and be reduced there. 8,9 Such reactions can lead to the consumption of lithium ions from the electrolyte, (to maintain charge neutrality in the electrolyte), a reduction in lithium inventory, as well as a thickening of the negative electrode solid electrolyte interface (SEI) which together ultimately cause cell-failure.10,11 These processes are accelerated by higher charging potentials and higher temperatures.Methods such as modification of the positive electrode surface with coatings or dopants 12,13 and/or modification of electr...

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confidence: 99%

Comparison of Single Crystal and Polycrystalline LiNi_0.5Mn_0.3Co_0.2O₂Positive Electrode Materials for High Voltage Li-Ion Cells

Cameron

et al. 2017

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309

286

“…[12][13][14][15][16][17][18][19][20][21][22] Ma et al and Nelson et al found that a ternary additive combination can greatly improve NMC442/graphite cell performance at high voltage. [11][12][13] Xia et al reported that fluorinated electrolyte and ECfree electrolyte also enhance NMC/graphite cell performance. [16][17][18][19] Another approach to improve cell stability is the application of an inorganic coating onto the NMC surface.…”

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confidence: 99%

Measuring Oxygen Release from Delithiated LiNi_xMn_yCo_1-x-yO₂and Its Effects on the Performance of High Voltage Li-Ion Cells

Xiong

Ellis

et al. 2017

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There can be a trade-off between the lifetime and energy density of LiNi x Mn y Co 1-x-y O 2 (NMC)-containing cells that depends on their upper cutoff voltage. This work applies thermogravimetric analysis coupled with mass spectrometry (TGA-MS) to measure the release of oxygen from delithiated NMC electrode materials at high electrode potentials, i.e., low lithium content. This release is observed at relatively mild temperatures as low as 40 • C. The amount of oxygen released is greatly limited using single crystal NMC particles. Electrochemical measurements demonstrate a correlation between TGA-MS results and the cycling performance of NMC/graphite cells. X-ray photoelectron spectroscopy complements the TGA-MS results and provides evidence of the solid electrolyte interphase decomposition. The results in this work offer strong support that the release of oxygen from NMC can cause oxidative decomposition of the electrolyte and is a major reason why high voltage cells can generate gas and can have poor capacity retention. There is a need for longer-lasting and higher energy density lithium-ion cells for electrified vehicles and grid energy storage applications.1-3 Layered LiNi x Mn y Co 1-x-y O 2 (NMC) materials make good positive electrodes on account of their well-balanced properties of high specific capacity, moderate cost, and good safety.4-9 As a result, NMC/graphite cells have become a ubiquitous part of our society. However, to avoid electrolyte and material degradation, commercially available NMC-containing cells normally operate at a much lower voltage (typically ≤ 4.2 V) than their theoretical limit. This is because trade-offs exist between the lifetime and the energy density of a NMC cell that depends on its upper cutoff voltage. 10,11 One method to improve existing NMC technology is the development of new electrolyte solution chemistries. The use of new solvent blends and the introduction of electrolyte additives are two practical ways to improve the lifetime of high voltage NMC Li-ion cells. 12-22Ma et al. and Nelson et al. found that a ternary additive combination can greatly improve NMC442/graphite cell performance at high voltage.11-13 Xia et al. reported that fluorinated electrolyte and ECfree electrolyte also enhance NMC/graphite cell performance. [16][17][18][19] Another approach to improve cell stability is the application of an inorganic coating onto the NMC surface. [23][24][25][26][27][28][29] For example, Arumugam et al. reported that a surface coating of Al 2 O 3 greatly improves NMC622/graphite cell performance. 24 Whereas both of these approaches have proven to be quite valuable, further improvements to the lifetime of NMC-containing cells at high voltages are still desired.Great effort has been put forward to understand the chemical mechanisms behind the degradation of high voltage NMC cells. Analytical approaches have included monitoring impedance growth, 12,13,30-38 the volume and composition of gases produced, 39-43 the heat flow [44][45][46][47][48] and the leakage current 49,50 du...

“…Useful electrolyte additives include triallyl phosphate (TAP), 21 pyridine boron trifluoride (PBF) [22][23][24] and a mixture of additives containing prop-1-ene-1,3-sultone (PES), tris-(trimethylsilyl) phosphite (TTSPi) and 1,3,2-dioxathiolane-2,2-dioxide (DTD) or methylene methyldisulfonate (MMDS). [16][17][18]25,26 Mixtures with 2 wt% PES + 2 wt% TTSPi + 2 wt% DTD or 2 wt% PES + 1 wt% TTSPi + 1 wt% MMDS in the control electrolyte (1 M LiPF 6 ethylene carbonate (EC): ethyl methyl carbonate (EMC) 3:7 w:w electrolyte) are called PES222 or PES211in this report, respectively. The use of surface coatings at the positive electrode is another effective way to enhance high voltage cell performance.…”

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confidence: 99%

“…Nelson et al found that LaPO 4 -coated NMC442/graphite pouch cells with PES222 had worse cell performance than uncoated NMC442/graphite pouch cells with PES222. 17 Even though the effects of surface coatings have been extensively studied, the mechanism of function of positive electrode surface coat- * Electrochemical Society Fellow.…”

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confidence: 99%

Effects of Surface Coating on Gas Evolution and Impedance Growth at Li[Ni_xMn_yCo_1-x-y]O₂Positive Electrodes in Li-Ion Cells

Xiong

Hynes

Ellis

et al. 2017

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The effects of surface coatings on Li[Ni x Mn y Co z ]O 2 (NMC, x+y+z = 1, x:y:z = 4:4:2 (NMC442), x:y:z = 5:3:2 (NMC532), x:y:z = 6:2:2 (NMC622)) electrodes in pouch cells and pouch bags, containing electrolyte with a certain additive blend, were systematically studied. Ex-situ gas measurements, gas chromatography coupled with a thermal conductivity detector and electrochemical impedance spectra were used to study the reactions that occurred. The results obtained from pouch bag experiments at elevated temperature indicate that the LaPO 4 surface coating did not impede impedance growth at the NMC442 surface and did not reduce gas generation at 60 • C while the Al 2 O 3 surface coating effectively prevented impedance growth at the NMC622 surface at 60 • C. The coating procedures and the underlying NMC materials were different so it is difficult to conclude that the Al 2 O 3 coating is more effective than the LaPO 4 coating in any situation. Hydrogen was only detected in pouch cells rather than pouch bags suggesting a crosstalk between the lithiated graphite and delithiated NMC electrodes as has been proposed before by the Gasteiger group. That is, species created at the positive electrode migrate to the negative, and react there to produce hydrogen. An increase in energy density of Li-ion cells is needed to extend the range and/or decrease the cost of electrified vehicles. The use of high potential positive electrode materials (vs Li/Li + ) is a simple and effective approach to improve cell energy density.1-3 Layered Li[Ni x Mn y Co z ]O 2 (NMC x+y+z = 1, x;y:z = 4:4:2, (NMC442), 5:3:2, (NMC532) and 6:2:2, (NMC622)) materials have been extensively investigated because they are cheaper than conventional LiCoO 2 (LCO) and can operate at high potentials up to 4.5 V (vs Li/Li + ) without significant material degradation. [4][5][6][7][8][9][10][11][12][13][14][15] It is difficult to cycle NMC/graphite cells repeatedly to a high voltage (above 4.3 V) without capacity loss. Reactions between the electrolyte and the NMC electrode at high potential can cause rapid charge-transfer impedance growth, gas generation, depletion of liquid electrolyte, and eventual cell failure. [16][17][18][19][20] The addition of electrolyte additives is an effective way to improve the performance of high voltage NMC/graphite cells. Useful electrolyte additives include triallyl phosphate (TAP), 21 pyridine boron trifluoride (PBF) 22-24 and a mixture of additives containing prop-1-ene-1,3-sultone (PES), tris-(trimethylsilyl) phosphite (TTSPi) and 1,3,2-dioxathiolane-2,2-dioxide (DTD) or methylene methyldisulfonate (MMDS). [16][17][18]25,26 Mixtures with 2 wt% PES + 2 wt% TTSPi + 2 wt% DTD or 2 wt% PES + 1 wt% TTSPi + 1 wt% MMDS in the control electrolyte (1 M LiPF 6 ethylene carbonate (EC): ethyl methyl carbonate (EMC) 3:7 w:w electrolyte) are called PES222 or PES211in this report, respectively. The use of surface coatings at the positive electrode is another effective way to enhance high voltage cell performance. Surface coatings and electroly...