Deciphering the Ethylene Carbonate–Propylene Carbonate Mystery in Li-Ion Batteries (2018)

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<sup>+</sup> migration within electrode lattices or across crystalline boundaries and interfaces to the Li<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> right before interphase formation. Combining DFT calculation and experiments, we identified the partial desolvation of the solvated Li<sup>+</sup> at graphite edge sites as a critical step, in which the competitive solvation of Li<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> leading to desirable interphases. Such molecular-level understanding of interphasial processes provides guiding principles to the effort of tailor-designing new electrolyte systems for more aggressive battery chemistries beyond Li-ion.