A Guide to Ethylene Carbonate-Free Electrolyte Making for Li-Ion Cells

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103

A set of LiNi 0.5 Mn 0.3 Co 0.2 O 2 /graphite lithium-ion cells underwent 750 charge-discharge cycles during about 8 months at 55 • C to upper cutoff potentials of 4.0, 4.1, 4.2, 4.3, and 4.4 V. The electrolyte in these cells was extracted using a centrifuge method and studied using gas chromatography/mass spectrometry to determine the changes to the solvents and by inductively coupled plasma-mass spectrometry to determine the changes to the salt content in the electrolyte. The negative electrodes from the cells were harvested and studied by micro-X-ray fluorescence to quantify the amount of transition metals which migrated from the positive electrode to the negative electrode during the testing. Emphasis is given to a detailed description of the quantitative methods used in the hope that others will adopt them in similar studies of different types of aged lithium-ion cells. The cells studied here initially had 1.1 molal LiPF 6 ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7 by weight) electrolyte. The aged cells showed increasing amounts of dimethyl carbonate and diethyl carbonate created by transesterification as the upper cutoff potential increased. Only extremely small amounts of Mn, less than 0.1% of the total Mn in the positive electrode, were found on the negative electrode after this aggressive testing. Lithium-ion batteries are currently used in a wide range of applications: cell phones, power tools, vehicles and even grid energy storage.

Section: Resultsmentioning

confidence: 99%

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confidence: 99%

Quantifying Changes to the Electrolyte and Negative Electrode in Aged NMC532/Graphite Lithium-Ion Cells

Thompson

Stone

Eldesoky

et al. 2018

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103

“…Some of these strategies will be used in the work here. For example, Petibon et al and Ma et al 13,14 showed that ethylene carbonate (EC)-free electrolytes, like 1M LiPF 6 in 95% ethyl methyl carbonate with 5% fluoroethylene carbonate was very effective in allowing NMC/graphite cells to operate to 4.4 V. EC-free electrolytes will be explored in this preliminary work.Electrolyte additives like pyridine boron trifluoride (PBF) and pyridine phosphorus pentafluoride (PPF) have been reported to provide * Electrochemical Society Fellow.z E-mail: jeff.dahn@dal.ca; huangl@xmu.edu.cn better capacity retention during high-voltage and high temperature cycling of NMC/graphite cells. 15,16 Their use will be explored in this preliminary work.…”

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confidence: 99%

Using the Charge-Discharge Cycling of Positive Electrode Symmetric Cells to Find Electrolyte/Electrode Combinations with Minimum Reactivity

Shen

Xiong

Ellis

et al. 2017

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The effects of solvents, salts, electrolyte additives and surface coatings on LiNi 0.4 Mn 0.4 Co 0.2 O 2 (NMC442) or LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) have been probed using positive electrode Li-ion symmetric cells coupled with dV/dQ analysis. A robust symmetric cell design is presented which prevents hardware corrosion for over at least 800 hours of testing to 4.5 V vs. Li/Li + at 40 • C. Positive electrode symmetric cells using uncoated positive electrode materials and 1M LiPF 6 EC:EMC 3:7 or 1M LiPF 6 EMC electrolyte rapidly developed high impedance and showed poor capacity retention. However, if 1% pyridine boron trifluoride (PBF) was added to these electrolytes, cell performance was dramatically improved. Replacing LiPF 6 by LiBF 4 in the electrolytes above, with or without PBF, yielded positive electrode symmetric cells with good capacity retention. Two types of surface coatings were explored on NMC622 positive electrodes. Cells using surface-coated positive electrodes demonstrated better capacity retention for all electrolytes compared to cells without surface coatings. This work can be used as a guide by those attempting to find electrolyte/electrode pairs suitable for use in NMC/graphite cells that can operate with long lifetime to 4. 1-4 The energy density of NMC/graphite cells can be easily improved if the cells can be charged to higher potential, for example to 4.4 or 4.5 V at which point the positive electrode is at about 4.48 or 4.58 V vs. Li/Li + . However, charging NMC/graphite cells to such potentials normally leads to impedance growth and a reduction in cycle and calendar life. 5,6 Increasing the charging cutoff voltage to 4.40 V can cause some problems: 1) Parasitic reactions like electrolyte oxidation can create harmful products and damage the surface of the electrodes. 7,8 In addition, in some cases, thick SEI layers on the positive electrode can be created.2) The layered surface structure of NMC (R3m) can change into a spinel-like framework (Fd3m) and then to the disordered rocksalt structure (Fm3m). 9,10 This is thought to be one cause of impedance growth at the positive electrode.3) The unit cell volume of most NMC materials begins to contract rapidly, by about 4%, when the potential is raised above 4.4 V vs Li/Li + as the lithium content is decreased below about z = 0.25 in Li x Ni 1-y-z Mn y Co z O 2 . This could induce a detachment between binder and active particles along with cracks forming inside the secondary particles. 11,12A number of strategies have been used to protect the positive electrode surface from parasitic reactions. These include the use of electrolyte additives, different salts, different solvents and electrode material coatings. Some of these strategies will be used in the work here. For example, Petibon et al. and Ma et al. 13,14 showed that ethylene carbonate (EC)-free electrolytes, like 1M LiPF 6 in 95% ethyl methyl carbonate with 5% fluoroethylene carbonate was very effective in allowing NMC/graphite cells to operate to 4.4 V. EC-free electrolytes will be expl...

“…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][31][32] However, only a salt concentration of 1 M LiPF 6 in this novel electrolyte was investigated.…”

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confidence: 99%

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

Xiong¹,

Hynes²,

Dahn³

2017

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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...