Studies of the Effect of High Voltage on the Impedance and Cycling Performance of Li[Ni0.4Mn0.4Co0.2]O2/Graphite Lithium-Ion Pouch 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: 83%

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

“…[18][19][20] The additive triallyl phosphate (TAP) is reported to improve the high-voltage performance.…”

Section: 2mentioning

confidence: 99%

“…[15][16][17] Ternary additives of 2% propene sultone (PES), 1% ethylene sulfate (DTD) and 1% tris(-trimethylsilyl)-phosphite (TTSPi) (called PES211) improved the cycle and calendar life of NMC/graphite Li-ion cells under normal (i.e., 4.2 V) and high voltage (i.e., 4.4 V) operation. [18][19][20] The additive triallyl phosphate (TAP) is reported to improve the high-voltage performance. 21 Many electrolyte additives have been proposed and shown in the literature to improve cell performance and lifetime.…”

mentioning

confidence: 99%

Effects of Electrolyte Additives and Solvents on Unwanted Lithium Plating in Lithium-Ion Cells

Liu

Petibon

et al. 2017

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Unwanted lithium plating on the graphite anode of lithium ion batteries can reduce the cycle life and safety of lithium ion batteries. Increased charging rates, lower temperatures, thicker electrodes, lower Li-ion diffusion constant and larger graphite particles all increase the propensity for unwanted lithium plating. In this work, a variety of electrolyte additives and electrolytes, which extend lifetime during low rate cycling, were used in Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 /graphite (NMC111/graphite) pouch cells subjected to high rate charging at 20 • C. It was found that additives and electrolytes which increased the negative electrode area-specific resistance, R negative , decreased the onset current, I u , for unwanted lithium plating. Here, the processes of ion desolvation, electron and ion transport through the solid electrolyte interphase and contact resistance are lumped into the R negative . Under conditions where R negative is the dominant factor determining when unwanted Li plating occurs, the onset current for lithium plating could be well predicted by the expression: I u = 0.080 V x S/R negative , where S is the geometric electrode surface area. R negative is easily determined using negative electrode coin-type symmetric cells. This simple rule-of-thumb relation will help guide researchers seeking to select electrolyte additives that simultaneously increase lifetime and also allow fast charging. Unwanted lithium plating can occur in Li-ion cells at high charge rates due to the close working potential of the graphite anode to metallic lithium. If the lithium ions and corresponding electrons cannot intercalate within the graphite particles fast enough, then metallic lithium can deposit on the surface of the graphite anode. Unwanted lithium plating is one mechanism that can limit the lifetime of lithiumion cells. Even worse, plated Li can form dendrites which may pierce the separator and affect the safety of lithium-ion cells. 1,2The propensity for unwanted lithium plating depends on many factors such as electrolyte components, 3 cell design (e.g., anode/cathode ratios, thickness and porosity of the graphite anode), 4 cell history and age (e.g., thickening of the negative electrode solid electrolyte interphase (SEI)) 5 and harsh charging conditions (e.g., high charge rates, low temperature and overcharge).6-8 Among these factors, the electrolyte used in lithium-ion cells not only determines the ionic conductivity but also plays a significant role in determining the properties of the SEI films.9,10 Therefore, the electrolyte components (i.e., Li salt, solvents and additives) can strongly affect unwanted lithium plating.The choice of appropriate electrolyte additives is an efficient and simple way to improve the lifetime of Li ion batteries by modifying the SEI properties.11 Vinylene carbonate (VC) has been commonly used as an additive for forming a robust negative electrode SEI and has been shown to improve the cycle life and calendar life of Li-ion cells.12-14 Ethylene sulfite (ES) was reported to form a...

“…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. This can cause inhomogeneous lithiation of the negative electrode, which can lead to lithium plating.…”

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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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...