XPS analysis of the SEI formed on carbonaceous materials

228

238

The performance degradation of graphite/LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) lithium ion cells, charged and discharged up to 300 cycles at different operating conditions of temperature and upper cutoff potential (4.2V/25 • C, 4.2V/60 • C, 4.6V/25 • C) was investigated. A combination of electrochemical methods with X-ray diffraction (XRD) both in situ and ex situ as well as neutron induced PromptGamma-Activation-Analysis (PGAA) allowed us to elucidate the main failure mechanisms of the investigated lithium ion cells. In situ XRD investigations of the NMC material revealed that the first cycle irreversible capacity is the cause of slow lithium diffusion kinetics. In full-cells, however, this "lost" lithium ions can be used to build up the SEI of the graphite electrode during the initial formation cycle. A new systematic approach to correlate the lithium content in NMC with its lattice parameters (c, a) allows a convenient quantification of the loss of active lithium in aged cells by determining the c/a ratio of harvested NMC cathodes in the discharged state using ex situ XRD. Besides loss of active lithium, transition metal dissolution/deposition on graphite and growth of cell impedance strongly effect cell aging, especially at elevated temperatures and high upper cutoff potentials. Besides their current use in portable power electronics, lithium ion batteries have recently been used for battery electric vehicles (BEV) and are envisioned for large-scale energy storage. For the latter applications, life times of >10 years are required so that it is essential to understand and quantify the mechanisms that contribute to battery failure. Among the commercially available lithium-ion battery chemistries, 1,2 the graphite/LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) system is one of the materials currently envisioned for automotive applications. 3 This cathode material demonstrates high capacity, good structural stability due to its small volume changes (<2%) during Li insertion and extraction, and high thermal stability in the charged state. [4][5][6] In addition, this material could theoretically be operated with high charge cutoff potentials up to 5.0 V, as its bulk structure is claimed to be stabilized by the presence of Mn 4+ , 7 even though other authors suggest that irreversible structural changes occur at these very high potentials and at high temperature.8 Due to its sloped potential profile, the capacity and also the average cell voltage increase with increasing charging potential. 7,9 Despite the improved safety and cycling performance of NMC material, operating NMC based cells (full-cells or half-cells) at elevated temperatures or at high charge potential leads to poor cycle life. [10][11][12][13] During cycling of graphite/NMC full-cells, transition metal dissolution from the NMC material is found to be a crucial factor controlling capacity fade. 11,12 In one of these studies, Zheng et al. demonstrated that upper cutoff potentials of >4.3 V lead to transition metal dissolution from NMC and thus compromise cycling performa...

Section: Ex Situ Xrd Analysis Of Aged Nmc Electrodes-mentioning

confidence: 99%

Aging Analysis of Graphite/LiNi_1/3Mn_1/3Co_1/3O₂Cells Using XRD, PGAA, and AC Impedance

Buchberger

Seidlmayer

Pokharel

et al. 2015

228

238

“…Li 1s spectrum shows minor compounds like lithium oxide (Li 2 O) 19 at the interface, organic (R-OCO 2 Li) and mineral (Li 2 CO 3 ) carbonates. 20 The most important feature in the Li 1s spectrum is a very high content in LiF 19,20 in the electrode cycled without SA. Nevertheless, when SA is added, the SEI becomes poorer in LiF as indicated by the decrease of Li 1s and F 1s signals and therefore lithium organic (R-OCO 2 Li) and mineral (Li 2 CO 3 ) compounds are present as minor species.…”

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 ...

“…Equation 3 was then used to calculate the steady-state transference number: [3] where I ss is the steady state current, R 0 is the initial interfacial resistance, and R ss is the interfacial resistance when I ss is reached. The transference number alludes to how polarizable an electrolyte is, thus a transference number near unity suggests that concentration gradients do not build up within the electrolyte.…”

mentioning

confidence: 99%

Effect of Anion Size on Conductivity and Transference Number of Perfluoroether Electrolytes with Lithium Salts

Shah

Olson

Karny

et al. 2017

Mixtures of perfluoropolyethers (PFPE) and lithium salts with fluorinated anions are a new class of electrolytes for lithium batteries. Unlike conventional electrolytes wherein electron-donating oxygen groups interact primarily with the lithium cations, the properties of PFPE-based electrolytes appear to be dependent on interactions between the fluorinated anions and the fluorinated backbones. We study these interactions by examining a family of lithium salts wherein the size of the fluorinated anion is systematically increased: lithium bis(fluorosulfonyl)imide (LiFSI), bis(trifluoromethanesulfonyl)imide (LiTFSI) salts and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). Two short chain perfluoroethers (PFE), one with three repeat units, C6-DMC, and another with four repeat units, C8-DMC were studied; both systems have dimethyl carbonate end groups. We find that LiFSI provides the highest conductivity in both C6-DMC and C8-DMC. These systems also present the lowest interfacial resistance against lithium metal electrodes. The steady-state transference number (t + ss ) was above 0.6 for all of the electrolytes and was an increasing function of anion size. The product of conductivity and the steady-state transference number, a convenient measure of the efficacy of the electrolytes for lithium battery applications, exhibited a maximum at about 20 wt% salt in all electrolytes. Amongst the systems studied, LiFSI/PFE mixtures were the most efficacious electrolytes.