Locally Concentrated LiPF₆ in a Carbonate-Based Electrolyte with Fluoroethylene Carbonate as a Diluent for Anode-Free Lithium Metal Batteries

Abstract

Currently, concentrated electrolyte solutions are attracting special attention because of their unique characteristics such as unusually improved oxidative stability on both the cathode and anode sides, the absence of free solvent, the presence of more anion content, and the improved availability of Li + ions. Most of the concentrated electrolytes reported are lithium bis(fluorosulfonyl)imide (LiFSI) salt with ether-based solvents because of the high solubility of salts in ether-based solvents. However, their poor anti-oxidation capability hindered their application especially with high potential cathode materials (>4.0 V). In addition, the salt is very costly, so it is not feasible from the cost analysis point of view. Therefore, here we report a locally concentrated electrolyte, 2 M LiPF 6 , in ethylene carbonate/diethyl carbonate (1:1 v/v ratio) diluted with fluoroethylene carbonate (FEC), which is stable within a wide potential range (2.5−4.5 V). It shows significant improvement in cycling stability of lithium with an average Coulombic efficiency (ACE) of ∼98% and small voltage hysteresis (∼30 mV) with a current density of 0.2 mA/cm 2 for over 1066 h in Li||Cu cells. Furthermore, we ascertained the compatibility of the electrolyte for anode-free Li−metal batteries (AFLMBs) using Cu||LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC, ∼2 mA h/ cm 2 ) with a current density of 0.2 mA/cm 2 . It shows stable cyclic performance with ACE of 97.8 and 40% retention capacity at the 50th cycle, which is the best result reported for carbonate-based solvents with AFLMBs. However, the commercial carbonate-based electrolyte has <90% ACE and even cannot proceed more than 15 cycles with retention capacity >40%. The enhanced cycle life and well retained in capacity of the locally concentrated electrolyte is mainly because of the synergetic effect of FEC as the diluent to increase the ionic conductivity and form stable anion-derived solid electrolyte interphase. The locally concentrated electrolyte also shows high robustness to the effect of upper limit cutoff voltage.

Inhibition of transition metals dissolution in cobalt-free cathode with ultrathin robust interphase in concentrated electrolyte

“…Three major bonding modes approximately at 717, 894, and 917 cm −1 are assigned to the vibration of free carbonate-based solvent (black line). The peak at ~743 cm −1 corresponds to the symmetric vibration of the LiPF 6 (PF 6 − ) in the solvent, whose intensity increases with the increase of concentration 40 . Similarly, the intensity of peak at ~905 cm −1 increases remarkably with increasing salt concentration, which indicates this bonding mode belongs to the coordinated solvent with Li + .…”

Unfolding the Hidden Message of Anode-Free Lithium Metal Battery

“…3g. The Li//Cu cell is a useful protocol to provide reliable information for the study of electrolyte 23,24,27 , and surface engineering approaches 25,31,32 for mitigating the irreversible Coulombic efficiency ascribed to dead Li formation and reductive electrolyte decomposition.…”

“…Meanwhile, several key factors would still affect the cycling performance of LMB and are crucial in achieving high specific energy of 500 Wh kg -1 demanded by electric vehicle energy-storage market such as electrolyte amount 17 , temperature 18 , pressure 19 , amount of Li or cahode 20,21 , and current density applied 9 , etc. Recently, Anode-free lithium metal batteries (AFLMBs) are considered as phenomenal energy storage systems due to higher energy density than that of LMB which with excess Li in the system, and greatly reduced safety risks since no Li metal is used during cell manufacturing, which remarkably increases the simplicity of cell fabrication and reduces the cost of cell assembly, too 22,23,24,25,26,27 .…”

Anode‐Free Full Cells: A Pathway to High‐Energy Density Lithium‐Metal Batteries

“…Hagos et al investigated the use of carbonate electrolytes, employing an electrolyte consisting of 2 m LiPF 6 in EC/diethyl carbonate (DEC) (1:1, v/v) diluted by 50% with fluoroethylene carbonate (FEC). [ 44 ] This is notable in that the use of carbonate electrolytes enables the use of higher voltage cathodes such as high‐nickel layered oxides, as carbonate‐based electrolytes generally possess greater electrochemical stability windows than ether‐based electrolytes. The large volume percent of the fluorinated solvent FEC stabilizes lithium deposition due to the formation of a LiF‐rich SEI layer, while reducing the overall salt amount and viscosity of the electrolyte compared to high concentration electrolytes.…”