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...

Phenyl vinyl sulfone (PVS) as a novel electrolyte additive is used to construct a protective interface film on layered lithium-rich cathode. Charge-discharge cycling demonstrates that the capacity retention of Li(LiMnNiCo)O after 240 cycles at 0.5 C between 2.0 and 4.8 V (vs Li/Li) reaches about 80% by adding 1 wt % PVS into a standard (STD) electrolyte, 1.0 M LiPF in EC/EMC/DEC (3/5/2 in weight). This excellent performance is attributed to the special molecular structure of PVS, compared to the additives that have been reported in the literature. The double bond in the molecule endows PVS with preferential oxidizability, the aromatic ring ensures the chemical stability of the interface film, and the sulfur provides the interface film with ionic conductivity. These contributions have been confirmed by further electrochemical measurements, theoretical calculations, and detailed physical characterizations.

Nitriles have received extensive attention for their unique ability in stabilizing electrolytes against oxidation at high voltages. It was generally believed that their anodic stability originates from a monolayer of chemisorbed nitrile molecules on transition-metal oxide surface, which physically expels carbonate molecules and prevents their oxidative decomposition. We overturn this belief based on calculation and experimental results and demonstrate that, like many high voltage film-forming electrolyte additives, nitriles also experience an oxidative decomposition at high voltages, and the high oxidation stability of nitrile-containing electrolytes is merely the consequence of a new interphasial chemistry. This important mechanistic correction would be of high significance in guiding the design of new electrolytes and interphases for the future battery chemistries.

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