Designing Potassium Battery Salts through a Solvent-in-Anion Concept for Concentrated Electrolytes and Mimicking Solvation Structures

Highly concentrated electrolytes
exhibit advantages in enhanced
stability, low solvent volatility, and superior battery safety. It
is therefore important to design salts that push the limit of solubility.
Herein, a solvent-in-anion design of potassium salts is proposed,
in which a solvent moiety is grafted onto a symmetric anion to form
a new asymmetric anion possessing the structural features of the solvent.
Guided by the like-dissolves-like rule, the resultant new salts exhibit
record high solubilities. Moreover, by solving the crystal structures
of these new salts, we suggest that the solvation structures around
K+ ions in these crystals may provide hints of the solvation
structures in solvent-lean concentrated electrolytes. In this work,
we have demonstrated this concept by grafting ether solvent moieties
onto trifluoromethylsulfonamide to form potassium asymmetric salts.
The solubility in dimethylethane reaches an unprecedent mole fraction
of 0.6. The resultant concentrated electrolyte increases the positive
electrochemical stability to >7 V vs K+/K and improves
the cycle life of potassium–oxygen batteries with reduced overpotentials.
Moreover, single-crystal X-ray diffraction of these salts reveals
a rich variety of coordination motifs with coordination numbers ranging
from 5 to 8 and extensive multidentate and bridging interactions from
all the possible coordinating atoms on the anion. Our solvent-in-anion
design represents a new approach that utilizes crystal structures
to probe the solvation structures in liquid and the revealed rich
variety of solvation structures are of use to the experimental and
theoretical modeling of concentrated electrolytes.