Development of safe aqueous batteries and supercapacitors critically relies on expanding the electrolyte electrochemical stability window. A novel mechanism responsible for widening the ...electrochemical stability window of water-in-salt electrolytes (WiSEs) compared to conventional salt-in-water electrolytes is suggested based on molecular dynamics (MD) simulations of the electrolyte–electrode interface. Water exclusion from the interfacial layer at the positive electrode provided additional kinetic protection that delayed the onset of the oxygen evolution reactions. The interfacial structure of a WiSE at negative electrodes near the potential of zero charge clarified why the recently discovered passivation layers formed in WiSEs are robust. The onset of water accumulation at potentials below 1.5 V vs Li/Li+ leads to formation of water-rich nanodomains at the negative electrode, limiting the robustness of the WiSE. Unexpectedly, the bis(trifluoromethanesulfonyl)imide anion adsorbed and trifluoromethanesulfonate desorbed with positive electrode polarization, demonstrating selective anion partitioning in the double layer.
Capacitive energy storage devices are receiving increasing experimental and theoretical attention due to their enormous potential for energy applications. Current research in this field is focused on ...the improvement of both the energy and the power density of supercapacitors by optimizing the nanostructure of porous electrodes and the chemical structure/composition of the electrolytes. However, the understanding of the underlying correlations and the mechanisms of electric double layer formation near charged surfaces and inside nanoporous electrodes is complicated by the complex interplay of several molecular scale phenomena. This Perspective presents several aspects regarding the experimental and theoretical research in the field, discusses the current atomistic and molecular scale understanding of the mechanisms of energy and charge storage, and provides a brief outlook to the future developments and applications of these devices.
Transition-metal dissolution from cathode materials, manganese in particular, has been held responsible for severe capacity fading in lithium-ion batteries, with the deposition of the ...transition-metal cations on anode surface, in elemental form or as chelated-complexes, as the main contributor for such degradations. In this work we demonstrate with diverse experiments and calculations that, besides interfacial manganese species on anode, manganese(II) in bulk electrolyte also significantly destabilizes electrolyte components with its unique solvation-sheath structure, where the decompositions of carbonate molecules and hexafluorophosphate anion are catalyzed via their interactions with manganese(II). The manganese(II)-species eventually deposited on anode surface resists reduction to its elemental form because of its lower electrophilicity than carbonate molecule or anion, whose destabilization leads to sustained consumption. The reveal understanding of the once-overlooked role of manganese-dissolution in electrolytes provides fresh insight into the failure mechanism of manganese-based cathode chemistries, which serves as better guideline to electrolyte design for future batteries.
Metallic zinc is an ideal anode due to its high theoretical capacity (820 mAh g
), low redox potential (-0.762 V versus the standard hydrogen electrode), high abundance and low toxicity. When used in ...aqueous electrolyte, it also brings intrinsic safety, but suffers from severe irreversibility. This is best exemplified by low coulombic efficiency, dendrite growth and water consumption. This is thought to be due to severe hydrogen evolution during zinc plating and stripping, hitherto making the in-situ formation of a solid-electrolyte interphase (SEI) impossible. Here, we report an aqueous zinc battery in which a dilute and acidic aqueous electrolyte with an alkylammonium salt additive assists the formation of a robust, Zn
-conducting and waterproof SEI. The presence of this SEI enables excellent performance: dendrite-free zinc plating/stripping at 99.9% coulombic efficiency in a Ti||Zn asymmetric cell for 1,000 cycles; steady charge-discharge in a Zn||Zn symmetric cell for 6,000 cycles (6,000 h); and high energy densities (136 Wh kg
in a Zn||VOPO
full battery with 88.7% retention for >6,000 cycles, 325 Wh kg
in a Zn||O
full battery for >300 cycles and 218 Wh kg
in a Zn||MnO
full battery with 88.5% retention for 1,000 cycles) using limited zinc. The SEI-forming electrolyte also allows the reversible operation of an anode-free pouch cell of Ti||Zn
VOPO
at 100% depth of discharge for 100 cycles, thus establishing aqueous zinc batteries as viable cell systems for practical applications.
Molecular dynamics (MD) simulations of an electrolyte comprised of ethylene carbonate (EC), dimethyl carbonate (DMC), and LiPF6 salt near the basal face of graphite electrodes have been performed as ...a function of electrode potential. Upon charging of the electrodes, the less polar DMC molecule is partially replaced in the interfacial electrolyte layer by the more polar EC. At negative potentials, the carbonyl groups from the carbonate molecules are repelled from the surface, while at positive potentials, we find a substantial enrichment of the surface with carbonyl groups. PF6 – rapidly accumulates at the positive electrode with increasing potential and vacates the negative electrode with increasing negative potential. In contrast, Li+ concentration in the interfacial layer is found to be only weakly dependent on potential except at very large negative potentials. Hence, both composition of the electrolyte at the electrode surface and solvent environment around Li+ are observed to vary dramatically with the applied potential with important implications for oxidation/reduction of the electrolyte and the process of Li+ intercalation/deintercation.
While lithium hexafluorophosphate (LiPF6) still prevails as the main conducting salt in commercial lithium-ion batteries, its prominent disadvantage is high sensitivity toward water, which produces ...highly corrosive HF that degrades battery performance. The hydrolysis mechanism and its correlation with high voltage in the battery environment remain poorly understood, despite the wide application of high voltage cathode. In this work, combining theoretical and experimental approaches, we identified the direct reaction between H2O and PF6 – as main source of HF based on the preferential solvation of PF6 – anion by water and the low energy barrier for the decomposition of PF6 ––H2O complex. Such a hydrolysis process would be accelerated by high voltage the electrolytes face at the cathode side. This important clarification of electrolyte failure mechanism points us to design more effective mitigation strategies with the purpose of stabilizing LiPF6-based electrolytes for high voltage LIBs.
The capacitance enhancement experimentally observed in electrodes with complex morphology of random subnanometer wide pores is an intriguing phenomena, yet the mechanisms for such enhancement are not ...completely understood. Our atomistic molecular dynamics simulations demonstrate that in subnanometer slit-geometry nanopores, a factor of 2 capacitance enhancement (compared to a flat electrode) is possible for the 1-ethyl-3-methylimidazolium (EMIM)–bis(trifluoro-methylsulfonyl)imide (TFSI) ionic liquid electrolyte. This capacitance enhancement is a result of a fast charge separation inside the nanopore due to abrupt expulsion of co-ions from the pore while maintaining an elevated counterion density due to strong screening of electrostatic repulsive interactions by the conductive pore. Importantly, we find that the capacitance enhancement can be very asymmetric. For the negatively charged 7.5 Å wide pore, the integral capacitance is 100% larger than on a flat surface; however, on the positive electrode, almost no enhancement is observed. Detailed analysis of structure and composition of electrolyte inside nanopores shows that the capacitance enhancement and the shape of differential capacitance strongly depend on the details of the ion chemical structure and a delicate balance of ion–surface and ion–ion interactions.
Molecular dynamics simulations were performed on N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (pyr13FSI) room temperature ionic liquid (RTIL) confined between graphite electrodes as a ...function of applied potential at 393 and 453 K using an accurate force field developed in this work. The electric double layer (EDL) structure and differential capacitance (DC) of pyr13FSI was compared with the results of the previous study of a similar RTIL pyr13bis(trifluoromethanesulfonyl)imide (pyr13TFSI) with a significantly larger anion Vatamanu J. ; Borodin O. ; Smith G. D. J. Am. Chem. Soc. 2010, 132, 14825 . Intriguingly, the smaller size of the FSI anion compared to TFSI did not result in a significant increase of the DC on the positive electrode. Instead, a 30% higher DC was observed on the negative electrode for pyr13FSI compared to pyr13TFSI. The larger DC observed on the negative electrode for pyr13FSI compared to pyr13TFSI was associated with two structural features of the EDL: (a) a closer approach of FSI compared to TFSI to the electrode surface and (b) a faster rate (vs potential decrease) of anion desorption from the electrode surface for FSI compared to TFSI. Additionally, the limiting behavior of DC at large applied potentials was investigated. Finally, we show that constant potential simulations indicate time scales of hundreds of picoseconds required for electrode charge/discharge and EDL formation.
The dependence on electrode potential of the interfacial structure and differential capacitance (DC) for 1-alkyl-3-methyimidazolium bis(trifluoromethanesulfonyl)imide (C n mimTFSI, n = 2, 4, 6, and ...8) ionic liquids (IL) near basal (flat) and prismatic edge face (rough) graphite electrodes was investigated here with atomistic simulations. Overall camel-shaped DCs were observed for both surfaces. The prismatic graphite generated systematically larger capacitances than the atomically flat basal face. Although on the flat electrodes the DC is almost constant at electrode potential bellow saturation (i.e., roughly within ±2 V), on the prismatic edge face the DC showed large amplitude changes between minima and maxima. This trend in DC was explained from the dependence versus potential of the structure and composition of the interfacial electrolyte layer; specifically, faster counterions accumulation and ion segregation in the interfacial layer are observed for atomically corrugated electrode surfaces as compared to the flat ones. Surprisingly, the increase of the charge-neutral alkyl tail length of the cation resulted only in a small reduction in DC, indicating ions ability to rearrange/reorient charge-caring groups such that it maximizes the counterions charge near the surface. This finding shows a promising route for optimization of ions structure to achieve the desired/optimal properties of electrolyte (e.g., low melting point and viscosity) without significant reduction of energy density storage capabilities.
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