Ethylene bis-carbonates as telltales of SEI and electrolyte health, role of carbonate type and new additives

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%

mentioning

confidence: 99%

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

Thompson

Stone

Eldesoky

et al. 2018

103

“…When Gr/Gr cells are cycled in the electrolyte without SA, three new peaks located at 11.8 minutes, 12.9 minutes and 13.9 minutes arise which are assigned respectively to ethylene glycol bis-(methyl carbonate), ethylene glycol ethylmethyl bis-(carbonate), ethylene glycol bis-(ethyl carbonate). 28 These bis-(alkyl carbonates) are depicted (molar masses and chemical structure) in Table IV. These compounds have been earlier identified as electrolyte degradation byproducts, 29 and form on the graphite electrode according to the reduction mechanism 30,31 reported in Fig. 13.…”

Section: Xps Analysis Of Lithiated Graphite Interfaces-symmetricmentioning

confidence: 99%

Reactivity of Succinic Anhydride at Lithium and Graphite Electrodes

Charton

Santos-Peña

Biller

et al. 2017

Succinic anhydride (SA) is an useful electrolyte additive for high voltage cathodes but has also a negative impact on graphite (Gr) or Li anodes. For this reason, the Li/electrolyte and Gr/electrolyte interfaces were investigated at 20 • C and 45 • C using half or symmetrical cells. When SA is added at 1% by weight to the electrolyte (alkylcarbonate mixture + LiPF 6 ), an increase in the Li/Li cell impedance at 20 • C occurs owing to the formation of a resistive solid electrolyte interphase (SEI), composed of an inorganic inner layer and an organic outer layer, whereas at 45 • C or in absence of SA, the interphase is more inorganic in nature. The reversible capacity of graphite, cycled in Gr/Li half-cells in the presence of SA, is very low at 20 • C but almost ten times larger at 45 • C. Cycling symmetrical Gr/Gr cells at 45 • C indicates that Gr capacity is lower in the presence of SA in connection with the presence of an organic and polymer rich SEI. GC-MS analysis of the electrolyte after cycling shows that ethylene glycol bis-(alkylcarbonate) derivatives disappear when SA is present, owing to the ability of SA to react with lithium alkoxides to yield oligopolyester. Lithium ion batteries (LIBs) are a reliable way to store electric energy, with high gravimetric and volumetric energy density, which allows their application in nomad systems including electric and/or hybrid vehicles. However, electric vehicles range is unsuitable for most of the consumers and, therefore, energy density should be improved. In the past twenty years, many researches have been performed on electrode materials and electrolytes with the aim to enhance battery performance. Use of 5V class positive electrodes (cathodes) is a convenient way to increase LIBs energy density. It explains the development of promising "high voltage" materials like Li 3 V 2 (PO 4 ) 3 , 1 Li 2 CoPO 4 F 2 or the LiNi 0.5 Mn 1.5 O 4 (LNMO) 3 spinel. Nevertheless, the use of new cathodes operating at high voltage leads to the degradation of the battery electrolyte, usually based on a mixture of cyclic and linear alkylcarbonates containing LiPF 6 as salt and functional additives. Unfortunately, standard electrolytes are oxidized on 5 V cathodes and this significantly limits the battery cycling life. From the first cycles, electrolyte components degrade forming by-products, which in turn, can interact with electrodes surface. As an example, a passive layer is formed on graphite (Gr)/electrolyte interface at the first charge. This layer, known as the Solid Electrolyte Interphase (SEI), has a strong impact on the LIBs performances. Electrolyte formulation influences the chemical nature and structure of the SEI. With the aim to improve SEI quality and stability, SEI-forming additives like vinylene carbonate (VC) are generally added to the electrolyte in low amount (less than 5% by weight). These additives are designed to generate a protective passive layer on the active material surface preventing continuous electrolyte decomposition reactions.Whereas additives can be ...

“…4 showcases the presence of two peaks between the normal end-of-charge potential of 1.35 V and the end-of-overcharge at 0.66 V: ❶ around 1 V, ❷ around 0.7 V. The shapes and potentials associated with both peaks are reminiscent of the peaks observed by other researchers during known occurrences of carbonate solvent based electrolyte decomposition. [22][23][24] From this electrochemical analysis, one can conclude that the overcharged LTO || NMC pouch cells encountered reductive conditions compatible with decomposition of carbonate solvent based electrolyte under the 3.6 V overcharge conditions. As a result of this electrolyte decomposition, generation of gaseous products is a likely outcome based on the available literature.…”

Section: Resultsmentioning

confidence: 99%

Overcharge Study in Li₄Ti₅O₁₂Based Lithium-Ion Pouch Cell

Devie

Dubarry

Wu³

et al. 2016

An Li4Ti5O12 || LiNi1/3Mn1/3Co1/3O2 lithium-ion pouch cell has been subjected to an overcharge early in its cycle-life and kept cycling it up to 1500 cycles afterwards. We report on the non-invasive experimental verifications we conducted to corroborate our initial findings obtained via incremental capacity analysis. First, we used incremental capacity analysis on the negative electrode during overcharge to bring evidence of electrolyte reduction below 1 V vs. Li/Li+. Second, we used X-ray computerized tomography (CT scan) to demonstrate that gas bubbles were trapped between layers of positive and negative electrodes, thereby disabling the ionic conduction pathway between the two and causing capacity fade. Third, we administered a massage to the pouch cell to establish that the gas bubbles could be displaced, thereby recovering about 60% of the faded capacity. Finally, we proposed a new approach based on the study of the open-circuit voltage to describe quantitatively the gains of active materials that led to the partial capacity recovery.