Deciphering the Ethylene Carbonate–Propylene Carbonate Mystery in Li-Ion Batteries

Abstract

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...

Facilitating Tetrafluoroborate Intercalation into Graphite Electrodes from Ethylmethyl Carbonate‐Based Solutions

“…[8] Compared with EMC-solvated PF 6 À (4.5 V vs. Li/Li + ), EMC-solvated BF 4 -(4.7 V vs. Li/Li + ) appears more difficult to intercalate into graphite. Actually, ion-pair effect plays some role in the formation of SEI film at the graphite negative electrode in terms of lithium intercalation as well.…”

Deciphering the Interface of a High‐Voltage (5 V‐Class) Li‐Ion Battery Containing Additive‐Assisted Sulfolane‐Based Electrolyte

“…The energy gap between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy can predict the chemical stability of compounds . As the sole solvent, SL demonstrates the highest energy gap of 7.522 eV (Table S1, Supporting Information), indicating the highest chemical stability.…”

“…The energy gap between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy can predict the chemical stability of compounds. [79,80] As the sole solvent, SL demonstrates the highest energy gap of 7.522 eV (Table S1, Supporting Information), indicating the highest chemical stability. After 39 days storage at 80 °C, the unchanged -CH 2 -peaks (51.2 and 22.8 ppm in 13 C NMR spectra, Figure 7a; 3.04 and 2.23 ppm in 1 H NMR spectra, Figure 7b) confirm the excellent chemical stability of SL solvent.…”

Recent research progresses in ether‐ and ester‐based electrolytes for sodium‐ion batteries

“…PC, with the desired wide liquid temperature range but a relatively lower dielectric constant (64.92) than EC, is another attractive solvent for SIBs . PC is also compatible with hard carbon (HC), the predominant carbonaceous anode in SIBs, unlike its unappealing nature in LIBs, where the structural destruction of graphite anodes is caused by PC solvent cointercalation . Nevertheless, some studies also reported that the degradation of a SIB is strongly correlated with the continuous decomposition of PC .…”