Sulfolane-Based Electrolyte for High Voltage Li(Ni 0.42 Mn 0.42 Co 0.16 )O 2 (NMC442)/Graphite Pouch Cells

Self Cite

Electrolyte reacts at the surfaces of charging electrodes during the first cycle of a Li-ion cell (formation). This creates the initial solid electrolyte interphases (SEIs), which passivate the electrodes against further reactions with electrolyte. The quality of the SEIs improve over time as a cell matures. The coulombic efficiencies (CEs) of fresh cells (measured by high-precision coulometry, (HPC)) stabilize after a certain period of time (up to 600 hours), after which different cell chemistries can then be ranked for their quality. The purpose of this work is to accelerate the maturation of SEIs during the first cycle, allowing for stabilized CE measurements to be taken after less time. The effects of formation cycle temperature, voltage and duration on subsequent CE are explored. Undergoing formation at 60 • C accelerates the maturity of cells. XPS analysis suggests that organic SEI species are replaced by inorganic SEI species as a cell matures. These results give insight into the mechanisms of cell maturity. Application of the formation protocol described in this work may decrease the time needed for HPC experiments. The first cycle of a lithium-ion cell is when the initial solid electrolyte interphases (SEIs) form from the decomposition of electrolyte at the surfaces of the charging electrodes. If an SEI is ionically conductive and electronically insulating, it passivates the electrode against further reactions with the electrolyte. SEI quality is related to both the SEI thickness 1-4 and the SEI chemistry. [5][6][7] The SEI layers formed in the first cycle are not completely passivating but become more and more passivating over time. Over the past years, the Dahn lab has cycled thousands of new Li-ion cells on its ultra high precision chargers (UHPCs), used to evaluate cell chemistries and rank them in order of coulombic efficiency. [8][9][10][11][12][13] In every case, the CE of a fresh cell increases toward the ideal value of 1.0000. . . during testing but never actually reaches 1.0000. . . After about 15-20 cycles at C/20 (600-800 hours) and 40• C the coulombic efficiencies reach relatively stable values where reliable comparisons between cells can be made.12,14-18 One must wait a sufficient amount of time in an UHPC experiment to rank cells in order of their CEs. Storing cells for extended periods of time before UHPC measurements can also impact the CEs measured in early charge-discharge cycles as the SEI layers can mature to varying degrees and lithium can be incorporated within the anode overhang region, which impacts early CE measurements. 19Ideally in a search for the optimum electrolyte chemistry for a particular cell type, a number of cells differing only in electrolyte composition should undergo formation under the same conditions and be mounted for UHPC testing after the same time interval after formation. This is the way experiments are carried out in our laboratory where we are fortunate to be able to fill pouch cells with electrolyte and perform the formation steps ourselves. Even with th...

Section: Resultssupporting

confidence: 60%

“…A photo of the clamping method has been published in a previous paper from our group. 29 The importance of clamping cells during formation and cycling has also been discussed in a paper from our group. 30 If pouch cells are not clamped, gas evolution, if it occurs, can decrease the stack pressure between the electrodes.…”

Section: Methodsmentioning

confidence: 99%

High-Precision Coulometry Studies of the Impact of Temperature and Time on SEI Formation in Li-Ion Cells

Ellis

Allen

Hill

et al. 2018

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“…Sun et al 4 and Hu et al 5 showed that AlF 3 or ZrO 2 -coatings improved the rate capability, cycling stability and interface stability for Li [ Alternative solvents are another way of extending the potential operating range for LIB. Fluorinated solvents, 8,9 dinitriles 10 or sulfones 11,12 are of interest because their oxidation potentials are higher than that of traditional alkyl carbonate solvents. 13 However these alternative solvents have disadvantages such as high viscosity and/or high cost and can lead to Li-ion cells with high gas production and/or high impedance.…”

mentioning

confidence: 99%

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

Glazier

Petibon

et al. 2016

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120

119

Li [Ni 0.42 Mn 0.42 Co 0.16 ]O 2 (NMC442)/graphite pouch cells demonstrate superb performance at high voltage when ethylene carbonate (EC)-free electrolytes, using a solvent mixture that is >95% ethyl methyl carbonate (EMC) and between 2 and 5% of an "enabler", are used. The "enablers", required to passivate graphite during formation, can be vinylene carbonate (VC), methylene-ethylene carbonate (MEC), fluoroethylene carbonate (FEC) or difluoro ethylene carbonate (DiFEC), among others. In order to optimize the amount of "enabler" added to EMC, gas chromatography coupled with mass spectrometry (GC-MS) was used to track the consumption of "enabler" during the formation step. Storage tests, electrochemical impedance spectroscopy (EIS), ultrahigh precision coulometry (UHPC), long-term cycling, differential voltage analysis and isothermal microcalorimetry were used to determine the optimum amount of enabler to add to the cells. It was found that the graphite negative electrode cannot be fully passivated when the amount of "enabler" is too low resulting in gas production and capacity fade. Using excess "enabler" can cause large impedance and gas production in most cases. The choice of "enabler" also impacts cell performance. A solvent blend of 5% FEC with 95% EMC (by weight) provides the best combination of properties in NMC442/graphite cells operated to 4.4 V. It is our opinion that the experiments and their interpretation presented here represent a primer for the design of EC-free electrolytes. Lithium-ion batteries (LIB) are now widely used in electrified vehicles and energy storage systems.1 These applications require longer calendar and cycle lifetime as well as higher energy density. In order to increase the energy density of LIB, researchers focus on developing electrode materials with high specific capacity that may involve charging to increased upper cutoff potentials.2,3

“…The discharge capacity fade rate, charge end point capacity slippage and the increase in V/cycle were calculated from the slope of a best fit line to the final five points (cycles [11][12][13][14][15] of the data given in Figures S3 and S4. A CIE closer to 0 (CE is closer to 1.0000) and a decrease in charge end point capacity slippage are indications of a reduction in parasitic reactions at both the positive and negative electrode which normally correlates with longer lifetime.…”

Section: Resultsmentioning

confidence: 99%

Succinic Anhydride as an Enabler in Ethylene Carbonate-Free Linear Alkyl Carbonate Electrolytes for High Voltage Li-Ion Cells

Xia

Liu

Hebert

et al. 2017

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Ethylene carbonate-free electrolytes containing 1 M LiPF 6 in ethyl methyl carbonate with succinic anhydride as an enabler exhibited promising cycling and storage performance in Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 /graphite pouch type Li-ion cells tested to 4.5 V. Although cells using 1 M LiPF 6 in EMC can barely function due to the poor passivation of the graphite electrode, the addition of 1% succinic anhydride allows cells to operate well. Compared to other enablers such as vinylene carbonate, succinic anhydride provides similar cycling and storage performance at 40 • C and improved storage or cycling performance at 60 • C. Succinic anhydride also limits gas evolution, especially at high temperatures where cells with vinylene carbonate normally produces large amounts of gas. Symmetric cell studies showed that adding succinic anhydride to cells greatly increased the impedance of the graphite electrode after formation. However, the impedance of the graphite electrode decreased substantially during cycling, leading to a significant decrease of impedance in the full pouch cells. With succinic anhydride as an enabler, ethylene carbonate-free linear alkyl carbonate electrolytes may be suitable for high temperature applications in some Li-ion cells. Long life and energy dense Li-ion cells are attractive to battery manufacturers and electric vehicle (EV) makers. Cycling a Li-ion cell to higher voltage increases its energy density but normally decreases cell lifetime. This is because electrolyte solvents are unstable at high voltages and the positive electrode/electrolyte interface becomes more reactive as the potential increases.1,2 Recent results have shown that ethylene carbonate (EC) itself is responsible for many issues associated with the operation of NMC/graphite cells to high potentials. 3,4 Reactions involving EC at the positive electrode cause gas evolution and impedance growth, leading to cell failure. Such reactions become very problematic above 4.5 V vs. Li/Li + regardless of the electrolyte additives used. 5The removal of EC has been shown to enhance high voltage performance of NMC(442)/graphite cells at both room temperature and high temperature.4 When used at small loadings, several cyclic carbonates including vinylene carbonate (VC), fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DiFEC), can enable the use of linear alkyl carbonate-based electrolytes without ethylene carbonate.3 The cycling performance of these EC-free electrolytes was determined to be dependent upon the initial loading of passivating agent added, which must be high enough to passivate the graphite electrode, yet low enough to ensure low impedance (for VC and DiFEC) and low gassing during cell use. The addition of co-additives helps lower polarization growth during high voltage cycling. 4 Problems with EC-free ethyl methyl carbonate (EMC)-based electrolytes include gas evolution during both cycling and storage at high temperatures. Figure S1 (supporting information) shows open circuit voltage (OCV) versus time as well as gas ...