Effect of Lewis Acids on Graphite-Electrode Properties in EC-Based Electrolyte Solutions with Organophosphorus Compounds

Abstract

Compatibility of self-extinguishing property and electrochemical stability of electrolyte solutions containing organophosphorus compounds as flame retardants has been required for practical application of electrolyte solutions to lithium-ion batteries. By adding a Lewis acid, Ca 2+ , Mg 2+ , or Na + , to ethylene carbonate (EC)-based electrolyte solutions containing 50 vol.% trimethylphosphate or dimethyl methylphosphonate, the electrolyte solutions are able to suppress the co-intercalation of the organophosphorus compounds with Li + into graphite. However, the coulombic efficiency in the first charge-discharge in a graphite/Li metal half-cell with an electrolyte solution depends on the species of the added Lewis acid and organophosphorus compound. The difference between the chemical-shift changes of the oxygen of the C=O bond in EC and the oxygen of the P=O bond in organophosphorus compounds in the oxygen-17 nuclear magnetic resonance ( 17 O NMR) spectra of the electrolyte solutions was investigated. The investigation revealed that the difference in the acidities of the Lewis acids, electron-donating abilities of the organophosphorus compounds, and solvated structures with a Lewis acid affect the electrochemical stability of self-extinguishing electrolyte solutions. Safety, specifically concerning flammability, is still an important concern with lithium-ion batteries, especially as they are now widely used in high-power and -energy applications such as electric-load leveling and electric-vehicle systems. Organophosphorus compounds, such as phosphate, 1-5 phosphonate, 6,7 and phosphazene, 8,9 are widely known as excellent flame retardants. These organophosphorus compounds are used either as co-solvents or additives in the conventional alkyl-carbonate-based electrolyte solution used in lithium-ion batteries. Trimethylphosphate (TMP) suppresses the flammability of electrolyte solutions because of its self-extinguishing property; that is, it can selectively scavenge the active hydrogen and oxygen radicals that contribute to the combustible chain reaction.1,10,11 However, TMP, which has a higher electron-donating ability than ethylene carbonate (EC), 12 tends to co-intercalate with Li + into the graphite negative electrode, resulting in continuous decomposition of the electrolyte solution and large, irreversible capacity loss during the initial chargedischarge of a lithium-ion battery. 1,[13][14][15][16]20 A solution to this co-intercalation problem was devised by Takeuchi et al. That is, the compatibility of TMP with graphite was improved by introducing a salt composed of stronger Lewis acids than Li + , such as Ca 2+ . 16 Moreover, the charge-discharge efficiency in the case of TMP in graphite was improved by adding calcium bis(trifluoromethanesulfonyl) amide (Ca(TFSA) 2 ) to an electrolyte containing TMP. This suppression of co-intercalation is derived from the solvation structure of Li + being altered by the stronger Lewis acid, namely, Ca 2+ . Fluorinated alkyl phosphates, such as tris(2,2,2-trifluoroethyl) pho...

Unraveling the New Role of an Ethylene Carbonate Solvation Shell in Rechargeable Metal Ion Batteries

“…Ethylene carbonate (EC) is the most commonly used cosolvent in the electrolyte and its discovery is one of the greatest achievements contributing to the commercialization of lithium-ion batteries (LIBs). − This is because the high dielectric constant of EC can help dissociate metal salts (e.g., LiPF 6 ) efficiently, − while the reduction of EC can also form a requested solid electrolyte interphase (SEI) on the electrode surface for higher stability. − Particularly, the EC-based electrolyte is effective at suppressing the graphite anode exfoliation (i.e., caused by the Li + –solvent coinsertion) compared to that of the PC-based electrolyte reported earlier. − This is because a robust and/or durable SEI layer can be formed on the electrode surface as a result of EC-based electrolytes decomposition. − These effects result in better battery stability and lifetime. − These two commonly believed roles of EC are popular in the LIBs community and have been invoked in the electrolyte studies of current sodium and potassium (ion) batteries. − …”

Molecular-Scale Interfacial Model for Predicting Electrode Performance in Rechargeable Batteries

“…Solid–electrolyte interphase (SEI) formation on the electrode interface has become an omnipotent explanation for the improved performance of electrodes in rechargeable batteries. − There is a consensus that the SEI is formed by electrolyte decomposition in the initial cycles and thus reduces the contact area of the electrode and electrolyte for mitigated side reactions. − This belief is particularly popular regarding the graphite anode in commercial lithium-ion batteries (LIBs). − This is because SEI formation is considered to be a positive effect that improves the first Coulombic efficiency (i.e., mitigating electrolyte decomposition) and enhances the compatibility of graphite with electrolyte, achieving highly reversible Li + (de)­intercalation. − This SEI formation is additionally believed to suppress Li + –solvent co-insertion into graphite in propylene carbonate (PC)-based electrolyte, thereby avoiding the well-known graphite exfoliation process. − In addition, the positive effect of SEI for the improved stability of the cathode (e.g., LiNi 1– x – y Co x Mn y O 2 , NCM) has also been widely reported. − …”

Electrochemical Stabilization of Self-Extinguishing Electrolyte Solutions with Trimethyl Phosphate by Adding Potassium Salts

“…In this study, we found potassium bis­(trifluoromethanesulfonyl)­amide (KTFSA) with fewer Lewis acid K + than Na + demonstrated a superior Coulombic efficiency in the first charge-discharging process. However, the improvement in Coulombic efficiency by adding KTFSA has not been known yet, and we could not simply interpret improvement based on our previous study on salts with strong Lewis acids . Therefore, we investigated the dominant factor to improve the Coulombic efficiency by adding KTFSA.…”

“…However, conventional alkyl carbonate-based electrolyte solutions in lithium-ion batteries can easily catch fire in air. Recently, a flame retardant electrolyte solution has been achieved by including organophosphorus compounds, such as phosphates, − phosphonates, , and phosphazenes. , In particular, trimethylphosphate (TMP) suppresses the flammability of electrolyte solutions when the TMP content is 30% or more in an ethylene carbonate (EC) and ethylmethyl carbonate (EMC) mixed solution . However, TMP tends to cointercalate with lithium ions into the graphite anode because its donor number is higher than that of EC, resulting in continuous decomposition and a large irreversible capacity loss during the initial cycling. − …”

“…In our previous study, the cointercalation was suppressed by adding calcium bis­(trifluoromethanesulfonyl)­amide (Ca­(TFSA) 2 ), magnesium bis­(trifluoromethanesulfonyl)­amide (Mg­(TFSA) 2 ), or sodium bis­(trifluoromethanesulfonyl)­amide (NaTFSA) to these solutions. Moreover, the Coulombic efficiencies of the electrolyte solutions with different Lewis acids were different in the first charge-discharging process.…”