Prof. Chunsheng Wang, University of Maryland, ACS Energy Lett .: 63 m Ultra-Concentrated Electrolyte for Lithium Ion Batteries

【introduction】
With people‘s dependence on lithium-ion batteries in daily life, people‘s attention to the safety of lithium-ion batteries has grown rapidly. Among them, highly flammable organic electrolytes bear most of the safety responsibility. A safer aqueous electrolyte instead of an organic electrolyte will greatly improve battery safety, but the narrow electrochemical stability window of the aqueous electrolyte severely limits the energy density of the aqueous lithium-ion battery. In order to solve this problem, it is necessary to reduce the water molecule content and electrochemical activity in the Li + solvation structure of the aqueous electrolyte. Recently, a new water-based electrolyte, water-in-salt (WiS) electrolyte, has increased the electrochemical stability window of the water-based electrolyte to about 3.0 V. This is because the average number of water molecules in each Li + solvated structure is much lower than the number of water molecules in Li + solvated structures in ordinary dilute solution electrolytes. Li + solvation structure and electrolyte Bulk structure have undergone significant changes, leading to changes in the interface chemical reaction of the electrolyte on the electrode surface, thereby widening the stable electrochemical window of the aqueous electrolyte. In order to further broaden the stable electrochemical window of WIS electrolyte, the lithium salt concentration can be further increased. However, the further increase of the lithium salt concentration is usually limited by the solubility of the lithium salt, and the accompanying increase in viscosity and ion conductivity Low problem.

[Achievement Profile]
Recently, Dr. Chen Long and others from Professor Wang Chunsheng‘s group at the University of Maryland and others used ethyl trimethylammonium salt (Me3EtN + · TFSI) to increase the solubility of LiTFSI in water by two times. The new water-in-hybrid-salt (WIHS) aqueous electrolyte salt concentration reached an unprecedented 63 m, and the salt / water molar ratio reached 1.13 for the first time. The anomalous phenomenon is that the ammonium salt alone cannot be dissolved in water, but it can be dissolved in an aqueous solution with LiTFSI and can double the solubility of LiTFSI. Despite the high salt concentration, the 63 m WIHS aqueous electrolyte maintains a relatively high ionic conductivity (0.91 mS cm-1) and a relatively low viscosity (407 mPa s), while having a wider 3.25 V electrochemical Stability window. It can be seen from Figure 1C that in the WIHS aqueous electrolyte, the number of water molecules in the Li + solvated structure and in the bulk are greatly reduced. The TFSI- anion ratio in the Li + solvated structure increases, which is more conducive to the growth of SEI. The electrolyte can support a 2.5 V water-based lithium-ion (LiMn2O4 // Li4Ti5O12) full cell with stable cycling at a rate of 1 C and 0.2 C, and the battery energy density can reach 145Wh kg-1. The above results were published in the international top energy journal ACS Energy Letter.

[Picture and text guide]

Figure 1. FTIR diagram and molecular dynamics simulation of liquid structure

(A) FTIR images of 21 m WiSE, 42 m WIHS, and 63 m WIHS electrolytes;
(B) molecular dynamics simulation;
(C) Coordination numbers of O (TFSI) and O (water) for Li + in 21 m WiSE, 42 m WIHS, and 63 m WIHS electrolytes and the number of free water molecules in Bulk.


figure 2.
(A) Self-diffusion coefficients of 21 m WiSE, 42 m WIHS, 63 m WIHS, and LiTFSI-Pyr14 · TFSI electrolytes;
(B) The composition of 63 m WIHS electrolyte inside the double layer on the electrode surface;
(C) Internal view of 63 m WIHS electrolyte double laye on the electrode surface.


Figure 3. Electrochemical window of WIHS electrolyte
(A) Electrochemical windows of different aqueous electrolytes at a scan rate of 5 mV s-1;
(B) Schematic diagram of the redox potential energy transfer of 63 m (42 m LiTFSI + 21 m Me3EtN · TFSI) WIHS electrolyte and LiMn2O4 positive electrode and Li4Ti5O12 negative electrode caused by high concentration of lithium salt.


Figure 4. High-energy water-based lithium-ion battery
(A) Charge and discharge curve of a LiMn2O4 // Li4Ti5O12 full cell containing 63 m WIHS aqueous electrolyte (42 m LiTFSI + 21 m Me3EtN · TFSI) at a rate of 1 C;
(B) Cycle stability and coulomb efficiency of a LiMn2O4 // Li4Ti5O12 full cell with 63 m WIHS aqueous electrolyte (42 m LiTFSI + 21 m Me3EtN · TFSI) at a rate of 1 C;
(C) Cycle stability and coulomb efficiency of a LiMn2O4 // Li4Ti5O12 full cell with 63 m WIHS aqueous electrolyte (42 m LiTFSI + 21 m Pyr14 · TFSI) at a rate of 1 C;
(D) Cycle stability and Coulomb efficiency of a LiMn2O4 // passivated Li4Ti5O12 full cell containing 63 m WIHS aqueous gel electrolyte (42 m LiTFSI + 21 m Pyr14 · TFSI) at a rate of 0.2C.


【summary】
Adding an inert cation to the WiS electrolyte doubles the solubility of LiTFSI in water and the salt / water molar ratio reaches 1.13. The extremely high salt concentration regulates the Li + solvation structure and the electrolyte bulk structure, forming a new type of aqueous electrolyte based on mixed salts. Despite the salt concentration up to 63 m, the obtained electrolyte maintained satisfactory viscosity and ionic conductivity. These changes further widen the electrochemical stability window of aqueous electrolytes.
Literature link: A 63 m Super-concentrated Aqueous Electrolyte for High Energy Li-ion Batteries (ACS Energy Lett., 2020, DOI: 10.1021 / acsenergylett.0c00348)
Source of information: material source