In Situ Formation of Protective Coatings on Sulfur Cathodes in Lithium Batteries with LiFSI‐Based Organic Electrolytes

Over the last decade, vast improvements have been made in the field of lithium-sulfur batteries bringing it a step closer to reality. In this field of research, deep understanding of the polysulfide shuttle phenomenon and their affinity with carbons, polymers and other hosts have enabled the design of superior cathodes with prolonged life. However, the anode side has undergone comparatively less transformation. In this work, we have developed a new electrolyte based on 1,2-dimethoxyethane (DME) solvent that enables reversible intercalation of lithium ions in graphite. A novel method to introduce solid lithium polysulfide into a carbon current collector as the cathode has been demonstrated and the electrode shows stable cycling with the new electrolyte. A full cell consisting of a lithiated graphitic anode and lithium polysulfide cathode is constructed, which exhibits an initial capacity as high as 1,500 mAh g −1 (based on the sulfur in the cathode) and a reversible capacity of 700 mAh g −1 for 100 cycles. This full cell is capable of delivering over 460 mAh g −1 at rates as high as 2C. The cell degradation over prolonged cycles could be due to the polysulfide shuttle which results in instability of the SEI layer on the graphitic anode. The demand for energy consumption by mankind is ever increasing due to rapid growth and accessibility of technology by the masses. This has led to our dependence on fossil fuels like coal and petroleum. Fortunately, sulfur, one of the promising cathode materials for inexpensive high energy density lithium batteries arises as a by-product of petroleum refining.1 Its abundant, benign nature combined with the ability of lithium-sulfur (Li-S) cells to provide a theoretical specific capacity of 1,672 mAh g −1 and specific energy of ∼2,600 Wh kgmakes it an attractive cathode material. 2 With high promises come significant challenges in utilizing this material effectively toward commercialization. The significant ones being the low conductivity of sulfur and lithium sulfide, the shuttle effect caused by the mobile intermediate polysulfides, and the volume changes upon cycling in the cathode. [2][3][4] In recent years, most of the research efforts have been focused to tackle issues at the cathode side. The pure lithium metal used in the cell also poses crucial challenges in the development of Li-S systems. Chief among them being the formation of Li dendrites and mossy deposits on the Li anode, 5,6 presence of excess lithium which assists the shuttle effect, 7 and low Coulombic efficiency associated with Li metal deposition and stripping which leads to short cycle life.5 To overcome the shortcomings on the anode side different nonLi metal anodes have been tested such as graphite, [8][9][10][11] hard carbon, 12 silicon, 13,14 tin, 15 and other alloys. 8 Although non-lithium anodes prevent Li-dendrite formation and increase Coulombic efficiency at the anode side, it is imperative that a compatible electrolyte that can form a stable solid electrolyte interphase (SEI) and promote high anode ca...

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

A Graphite-Polysulfide Full Cell with DME-Based Electrolyte

Bhargav

2016

“…For example, in addition to providing mechanistic explanations for electrolyte transport properties, 3 it may be important to understand the solvate species present and their populations (i.e., extent of ionic aggregation) as a function of electrolyte salt (i.e., anion) and salt concentration because anion reduction at an anode may dominate over solvent reduction in the highly aggregated regime, 41,42 whereas solvent reduction may instead dominate when most of the Li + cations are fully solvated. MD simulations of (AN) n -LiX mixtures with LiBF 4 , LiCF 3 SO 3 and LiCF 3 CO 2 have been analyzed in detail.…”

Section: Discussionmentioning

confidence: 99%

Electrolyte Solvation and Ionic Association

Borodin

Han

Daubert

et al. 2015

Self Cite

Molecular dynamics (MD) simulations of acetonitrile (AN) mixtures with LiBF 4 , LiCF 3 SO 3 and LiCF 3 CO 2 provide extensive details about the molecular-and mesoscale-level solution interactions and thus explanations as to why these electrolytes have very different thermal phase behavior and electrochemical/physicochemical properties. The simulation results are in full accord with a previous experimental study of these (AN) n -LiX electrolytes. This computational study reveals how the structure of the anions strongly influences the ionic association tendency of the ions, the manner in which the aggregate solvates assemble in solution and the length of time in which the anions remain coordinated to the Li + cations in the solvates which result in dramatic variations in the transport properties of the electrolytes. Understanding the origin of the widely varying transport properties (e.g., viscosity, ionic conductivity and diffusion coefficients) of battery electrolytes remains a key challenge. The present work continues a detailed study into the solution structure of electrolytes-ion solvation and ionic association interactions-to provide mechanistic explanations for electrolyte properties. Previous manuscripts have examined in detail acetonitrile electrolytes with numerous lithium salts: (AN) n -LiX. [1][2][3][4][5][6] Electrolytes with the most strongly associated anions studied (i.e., CF 3 SO 3 − and CF 3 CO 2 − ), in which anion. . . Li + cation coordination (i.e., ionic association interactions) is a prominent feature of solvate formation, were found to have a much lower ionic conductivity than electrolytes with more weakly coordinating anions (e.g., PF 6 − and ClO 4 − ). 3 This is as one might expect once the ionic association tendency of the salts is well understood, but the details of why the conductivity values are low are missing.Two particularly notable points from the previous studies are that:(1) The average solvation numbers, obtained from a Raman spectroscopic analysis, for the (AN) n -LiCF 3 CO 2 solutions were found to be almost constant near a value of 1 (i.e., one AN molecule per Li + cation) over a wide concentration range, even for very dilute solutions. It remains unclear, however, as to why such electrolytes have such a low degree of solvation and why highly concentrated (>5 M) liquid electrolytes-perhaps contrary to expectations-can be prepared with LiCF 3 CO 2 with this being the case.(2) Experimental measurements of the solution structure and transport properties of (AN) n -LiCF 3 SO 3 mixtures were unobtainable due to the rapid crystallization of a solid solvate phase (i.e., (AN) 1 :LiCF 3 SO 3 ) from the electrolytes. 1 Liquid electrolytes with this salt can, however, be readily studied using molecular dynamics (MD) simulations to better understand how and why solution interactions differ for differing anions.The present work therefore delves deeper into the molecular-and mesoscale-level interactions using MD simulations applied to the characterization of (AN) n -LiCF 3 SO 3 and -LiCF ...

“…65,66,68 The S 2p spectrum of the electrode shows peaks of two types of sulfur atoms in the surface films: a doublet at 168.2 eV and a non-resolved doublet at about 169.5 eV. These signals may relate to the products of the polymerization reaction of LiFSI and Li 2 S 2 proposed by Kim et al 68 The two doublets, may be probably, assigned to sulfonyl amide groups in the resulted products, namely, F-S * (O 2 )-N-S(O 2 )-with sulfur atoms connected to one F and one N atom, and F-S(O 2 )-N-S * (O 2 )-with sulfur atoms bound to one S and one N atom (Fig. 12d).…”

Section: Direct Evidences Of Sei Formation On S/c Composite Cathodes mentioning

confidence: 99%

Review—On the Mechanism of Quasi-Solid-State Lithiation of Sulfur Encapsulated in Microporous Carbons: Is the Existence of Small Sulfur Molecules Necessary?

Markevich

Salitra

Talyosef

et al. 2016

102

In this work we analyzed the phenomenon of quasi solid state (QSS) lithiation of sulfur-carbon (S/C) composite electrodes with sulfur confined in the micropores of carbon matrices based on our recent studies and data published in literature. We demonstrated that the existence of sulfur in the form of small molecules is not a necessary condition for the realization of QSS mechanism. QSS operation behavior was demonstrated both for carbons with small up to 1nm micropores and for carbons with larger pore size up to 2-3 nm. A key role in the operation of S/C electrodes via a QSS mechanism plays surface electrolyte interphase (SEI) which is formed on the surface of S/C composite during the initial discharge. The formation of SEI was supported by X-ray photoelectron spectroscopy and by scanning electron microscopy. Small pore size (up to 1 nm) of the carbon matrices has a positive effect on the cycling of S/C electrodes. A superior cycling performance for more than 3500 charge-discharge cycles was demonstrated for S/C composite electrodes based on carbons synthesized by carbonization of polyvinylidene dichloride (PVDC) resin. In recent years lithium-sulfur batteries have been the subject of very intensive research due to the high theoretical specific capacity of sulfur cathodes (1672 mA h g −1 ) which is an order of magnitude higher than that of lithiated transition-metal oxides and phosphates cathode materials used in commercial Li-ion batteries (140-200 mA h g −1 ). [1][2][3][4][5][6] This high capacity relates to the ability of sulfur atoms to accept two electrons resulting in the conversion of elemental sulfur to lithium sulfide (Li 2 S). Furthermore, sulfur is naturally abundant, environmentally friendly and relatively cheap.Due to the low electrical conductivity of elementary sulfur the addition of conductive additives or the use of conductive host materials it is necessary to ensure good performance of sulfur electrodes. Besides, during the discharge process elemental sulfur S 8 accepts electrons to give a chain of electroactive Li-polysulfides (Fig. 1a). The long-chain polysulfides Li 2 S n (4 ≤ n ≤ 8) are soluble in commonly used ethereal solvents and diffuse freely throughout the cell to the anode side where they are chemically reduced. This phenomenon known as the shuttle effect presents one of the main problems of Li-S cells. 7 The shuttle reactions prevent the possibility of extracting the full capacity of sulfur cathodes and lead to low Coulombic efficiency. Encapsulation of sulfur within activated carbons with high pores volume is one of the most effective approaches to mitigate the detrimental shuttle mechanism and stabilize composite sulfur cathodes during prolong cycling. 8-10The typical cyclic voltammetry and galvanostatic charge-discharge curves of the Li-S cell with C/S encapsulated cathode are shown in Fig. 1a. Two peaks observed in the cathodic voltammetric response of sulfur electrodes correspond to two plateaus observed in the voltage profiles of the discharge processes of these cathodes upon ...