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As one of the landmark technologies, Li-ion batteries (LIBs) have reshaped our life in the 21stcentury, but molecular-level understanding about the mechanism underneath this young chemistry is still insufficient. Despite their deceptively simple appearances with just three active components (cathode and anode separated by electrolyte), the actual processes in LIBs involve complexities at all length-scales, from Li migration within electrode lattices or across crystalline boundaries and interfaces to the Li accommodation and dislocation at potentials far away from the thermodynamic equilibria of electrolytes. Among all, the interphases situated between electrodes and electrolytes remain the most elusive component in LIBs. Interphases form because no electrolyte component (salt anion, solvent molecules) could remain thermodynamically stable at the extreme potentials where electrodes in modern LIBs operate, and their chemical ingredients come from the sacrificial decompositions of electrolyte components. The presence of an interphase on electrodes ensures reversibility of Li intercalation chemistry in anode and cathode at extreme potentials and defines the cycle life, power and energy densities, and even safety of the eventual LIBs device. Despite such importance and numerous investigations dedicated in the past two decades, we still cannot explain why, nor predict whether, certain electrolyte solvents can form a protective interphase to support the reversible Li intercalation chemistries while others destroy the electrode structure. The most representative example is the long-standing "EC-PC Disparity" and the two interphasial extremities induced therefrom: differing by only one methyl substituent, ethylene carbonate (EC) forms almost ideal interphases on the graphitic anode, thus becoming the indispensable solvent in all LIBs manufactured today, while propylene carbonate (PC) does not form any protective interphase, leading to catastrophic exfoliation of the graphitic structure. With one after another hypotheses proposed but none satisfactorily rationalizing this disparity on the molecular level, this mystery has been puzzling the battery and electrochemistry community for decades. In this Account, we attempted to decipher this mystery by reviewing the key factors that govern the interaction between the graphitic structure and the solvated Li right before interphase formation. Combining DFT calculation and experiments, we identified the partial desolvation of the solvated Li at graphite edge sites as a critical step, in which the competitive solvation of Li by anion and solvent molecules dictates whether an electrolyte is destined to form a protective interphase. Applying this model to the knowledge of relative Li solvation energy and frontier molecular orbital energy gap, it becomes theoretically possible now to predict whether a new solvent or anion would form a complex with Li leading to desirable interphases. Such molecular-level understanding of interphasial processes provides guiding principles to the effort...

The oxidative decomposition mechanism of the lithium battery electrolyte solvent propylene carbonate (PC) with and without PF(6)(-) and ClO(4)(-) anions has been investigated using the density functional theory at the B3LYP/6-311++G(d) level. Calculations were performed in the gas phase (dielectric constant ε = 1) and employing the polarized continuum model with a dielectric constant ε = 20.5 to implicitly account for solvent effects. It has been found that the presence of PF(6)(-) and ClO(4)(-) anions significantly reduces PC oxidation stability, stabilizes the PC-anion oxidation decomposition products, and changes the order of the oxidation decomposition paths. The primary oxidative decomposition products of PC-PF(6)(-) and PC-ClO(4)(-) were CO(2) and acetone radical. Formation of HF and PF(5) was observed upon the initial step of PC-PF(6)(-) oxidation while HClO(4) formed during initial oxidation of PC-ClO(4)(-). The products from the less likely reaction paths included propanal, a polymer with fluorine and fluoro-alkanols for PC-PF(6)(-) decomposition, while acetic acid, carboxylic acid anhydrides, and Cl(-) were found among the decomposition products of PC-ClO(4)(-). The decomposition pathways with the lowest barrier for the oxidized PC-PF(6)(-) and PC-ClO(4)(-) complexes did not result in the incorporation of the fluorine from PF(6)(-) or ClO(4)(-) into the most probable reaction products despite anions and HF being involved in the decomposition mechanism; however, the pathway with the second lowest barrier for the PC-PF(6)(-) oxidative ring-opening resulted in a formation of fluoro-organic compounds, suggesting that these toxic compounds could form at elevated temperatures under oxidizing conditions.

The electrochemical oxidative stability of solvent molecules used for lithium ion battery, ethylene carbonate (EC), propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate in the forms of simple molecule and coordination with anion PF(6)(-), is compared by using density functional theory at the level of B3LYP/6-311++G (d, p) in gas phase. EC is found to be the most stable against oxidation in its simple molecule. However, due to its highest dielectric constant among all the solvent molecules, EC coordinates with PF(6)(-) most strongly and reaches cathode most easily, resulting in its preferential oxidation on cathode. Detailed oxidative decomposition mechanism of EC is investigated using the same level. Radical cation EC(*+) is generated after one electron oxidation reaction of EC and there are five possible pathways for the decomposition of EC(*+) forming CO(2), CO, and various radical cations. The formation of CO is more difficult than CO(2) during the initial decomposition of EC(*+) due to the high activation energy. The radical cations are reduced and terminated by gaining one electron from anode or solvent molecules, forming aldehyde and oligomers of alkyl carbonates including 2-methyl-1,3-dioxolane, 1,3,6-trioxocan-2-one, 1,4,6,9-tetraoxaspiro[4.4]nonane, and 1,4,6,8,11-pentaoxaspiro[4.6]undecan-7-one. The calculation in this paper gives a detailed explanation on the experimental findings that have been reported in literatures and clarifies the mechanism on the oxidative decomposition of EC.

headlines, [3][4][5] for which the highly flammable nonaqueous electrolytes used in LIBs are mainly responsible. It is under this context that aqueous LIBs (ALIBs) are revisited as a fundamental solution to safety, despite their low energy densities due to the narrow electrochemical stability window of water. [2,[6][7][8][9][10] Recently, a new class of high-voltage aqueous electrolyte was discovered by dissolving 21 molality (mol) lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) in 1 kg of water. Such a "water-in-salt" electrolyte (WiSE) expands the electrochemical stability window from 1.23 to 3.0 V, which supports a 2.5 V chemistry using LiMn 2 O 4 (LMO) cathode and Mo 6 S 8 anode to stably deliver ≈100 Wh kg −1 for thousand cycles. The significantly improved electrochemical stability therein mainly comes from the depletion of free water molecules and the formation of an anion-derived solid-electrolyte interphase (SEI) on the anode surface. [9] However, because of the intrinsic repulsion of anions and adsorption of the Li + -4(H 2 O) solvates by the negatively polarized anode surface, [4] the formation of such an anion-derived SEI has been impossible below 1.9 V versus Li + /Li. [9] This "cathodic challenge" has essentially excluded many desired energy-dense anodes that operate at low potentials such as Li-metal, graphite, or silicon. Even Li 4 Ti 5 O 12 (LTO) that operates at mild potential (≈1.70 V Li + /Li in WiSE) suffers from irreversibility, because it sits right on the edge of the cathodic limit in WiSE. Efforts aiming to resolve the "cathodic challenge" with additional lithium salts such as lithium trifluoromethane sulfonate (LiOTf) [11] or lithium bis(pentafluoroethane sulfonyl) imide (LiBETI) [10] achieved limited success, because solubility limits of the salts impose restrictions on how high their concentration can go, while the effectiveness of added anions still faces intrinsic resistance from anode surface against their accumulation at inner-Helmholtz layer, not to mention that these additional salts further worsen the already problematic viscosity and ionic conductivity of WiSE. Introducing a nonaqueous solvent, dimethyl carbonate (DMC), [12] into WiSE expands the electro chemical window of the hybrid electrolyte to 4.1 V, because the neutral solvent is less sensitive to anode repulsion and hence participates in interphasial chemistry more easily than anions. The additional protection from an SEI consisting of both anion-and solvent-derived products enables LTO operation Although the "water-in-salt" electrolyte has significantly expanded the electrochemical stability window of aqueous electrolytes from 1.23 to 3 V, its inevitable hydrogen evolution under 1.9 V versus Li + /Li prevents the practical use of many energy-dense anodes. Meanwhile, its liquidus temperature at 17 °C restricts its application below ambient temperatures. An advanced hybrid electrolyte is proposed in this work by introducing acetonitrile (AN) as co-solvent, which minimizes the presence of interfacial water at the nega...