Inhibiting manganese(Ⅱ)from catalyzing electrolyte decomposition in lithium-ion batteries

A once overlooked source of electrolyte degradation incurred by dissolved manganese(Ⅱ)species in lithium-ion batteries has been identified recently.In order to deactivate the catalytic activity of such manganese(II)ion,1-aza-12-crown-4-ether(A12C4)with cavity size well matched manganese(Ⅱ)ion is used in this work as electrolyte additive.Theoretical and experimental results show that stable complex forms between A12C4 and manganese(II)ions in the electrolyte,which does not affect the solvation of Li ions.The strong binding effect of A12C4 additive reduces the charge density of manganese(II)ion and inhibits its destruction of the PF_(6)^(-)structure in the electrolyte,leading to greatly improved thermal stability of manganese(II)ions-containing electrolyte.In addition to bulk electrolyte,A12C4 additive also shows capability in preventing Mn^(2+) from degrading SEI on graphite surface.Such bulk and interphasial stability introduced by A12C4 leads to significantly improved cycling performance of LIBs.

A Long Journey of Lithium: From the Big Bang to Our Smartphones

October 10,2019,Stockholm,Sweden.The Nobel Prize Committee announced that the Chemistry Award of that year was given to Goodenough,Whittingham,and Yoshino for their key roles in developing Liion battery technology.This long-overdue prize instantaneously electrified the research communities of electrochemistry and battery materials/chemistry.It is not only a long-overdue recognition for these chemists,but also marks a significant milestone to the saga of lithium that has become such an essential part of our daily life,in the hand-held smartphone that everyone walks with,in the increasing number of electric cars running on the roads,and in the grids that deliver electricity to millions of families[1].

Sulfolane-Graphite Incompatibility and Its Mitigation in Li-ion Batteries

The non-flammability and high oxidation stability of sulfolane(SL)make it an excellent electrolyte candidate for lithium-ion batteries(LIBs).However,its incompatibility with graphitic anode prevents the realization of these advantages.To understand how this incompatibility arises on molecular level so that it can be suppressed,we combined theoretical calculation and experimental characterization and reveal that the primary Li^(+) solvation sheath in SL is depleted of fluorine source.Upon reduction,SL in such fluorine-poor solvation sheath generates insoluble dimer with poor electronic insulation,hence leading to slow but sustained parasitic reactions.When fluorine content in Li^(+)-SL solvation sheath is increased via salt concentration,a high stability LiF-rich interphase on graphite can be formed.This new understanding of the failure mechanism of graphite in SL-based electrolyte is of great significance in unlocking many possible electrolyte solvent candidates for the high-voltage cathode materials for next-generation LIBs.

The Puzzles in Fast Charging of Li-Ion Batteries

Fast charging of Li-ion cells faces two aspects of challenges,1)accelerated capacity fade and 2)inferior charging capability.It is commonly believed that the former is due to Li plating and its resultant reactions with electrolyte at the graphite anode,which results in a loss in the inventory of Li+ions and an increase in the cell’s impedance.While the latter is ascribed to the high voltage polarization in relation to the slow transport of Li+ions between two electrodes.However,there are many other hidden facts that essentially affect the fast charging performances of Li-ion cells.This commentary intends,from the view of materials,to uncover these hidden factors,including failure of the solid electrolyte interphase and exfoliation of the graphite structure at the anode,structural degradation of the Ni-rich layered cathode materials,as well as the high solvation and desolvation activation energies of Li+ions in the electrolyte.Meanwhile,some solutions to the fast-charging problems of Li-ion cells are proposed based on the understanding of these hidden factors.

Design aspects of electrolytes for fast charge of Li-ion batteries

The electrolytes of Li-ion batteries consist mainly of a LiPF6 salt dissolved in a carbonate-based solvent mixture.Such electrolytes cannot support fast charge without detrimental impacts on performance and lifetime.Fast charge aggravates parasitic reactions of the electrolyte solvents and structural degradation of the lithium layered transition metal oxide cathode materials.This leads to not only the depletion of electrolyte solvents but also the loss of cyclable Li+ions,accompanied by impedance growth and volumetric swelling of the battery.In this perspective,the design aspects of the electrolytes for fast charge of Li-ion batteries are discussed and proposed.