The Fundamental Studies On Water-in-salt Electrolytes

Batteries based on aqueous electrolytes offer significant advantages owing to the intrinsic safety of water,especially for energy storage systems.Notably,the introduction of Water-in-salt(WIS)electrolytes successfully expanded the stable window of the aqueous electrolyte from 2V to over 3V.In the high-concentration electrolytes,the thermodynamic stability of water is greatly improved,and the formation of the solid electrolyte interphase(SEI)also plays a dynamic protection role on the anode side.The high-concentration electrolytes with expanded stable window iare of great potential on realizing stable and high-voltage aqueous batteries.Undoubtedly,high-concentration WIS electrolytes have great advantages,but there are also some scientific problems to be explored.On the one hand,the SEI would be formed along the negative electrode surface in WIS electrolyte,which plays an important role in improving the output voltage and cycle stability of the aqueous batteries,but the corresponding formation mechanism and source of reactants are still controversial;In addition,WIS electrolytes can effectively inhibit the dissolution of electrode materials and further reduce the loss of active materials,but the mechanism of suppression is not clear;On the other hand,compared with dilute solution,there is a significant ionic association in high-concentration WIS electrolytes,while the traditional ion transport mechanism is no longer applicable,so the transport mechanism of ions in high-concentration electrolytes needs further systematic research.To make a more comprehensive understanding of WIS electrolytes as well as their corresponding working mechanism,we have designed the following three works:Firstly,we have explored how the dissolved gases(CO₂,O₂,Air,Ar)affect the formation of the solid electrolyte intherphase(SEI)in the WIS electrolyte.Impressively,we found that there was a hitherto unknown association between CO₂ and TFSI anion in an aqueous LiTFSI solution,whose interaction is concentration-dependent and reaches maximum strength at 5m LiTFSI.This new TFSI-CO₂ complex and its reduction chemistry allow us to decouple the interphsial responsibility of an aqueous electrolyte from its bulk composition,and hence trailblazing a new direction to make high voltage aqueous Li-ion batteries practical as compared with the super-concentration approach.Leveraging this discovery,we designed a dilute Salt-in-Water(SIW)electrolyte with CO₂ as a chemical source for interphase,which not only inherits the wide electrochemical stability window and safety from its WIS predecessor but also successfully circumvents the numerous disadvantages induced by excessive salt concentration,such as high viscosity,slow kinetics of mass transport,narrow service temperature range,and high cost.The prototype aqueous battery constructed on such dilute aqueous electrolyte displayed comparable voltage tolerance as its WIS predecessor while delivering excellent rate performance,superior low-temperature performance(-40?)as well as the capability of accessing high capacity from a high mass loading thickness electrode that its WIS predecessor could not.This work represents an important correction to the WIS pathway,which not only directly benefits the development of practical aqueous batteries but also opens a new horizon to understand the complex interactions among the electrolyte components and harness them to manipulate interphasial chemistry.Secondly,we have made investigations on the dissolution suppressing mechanism in the high-concentration electrolytes.As we all known,mass dissolution is one main problems for cathodes in aqueous electrolytes due to the strong polarity of water molecules.In principle,mass dissolution is a thermodynamically favorable process as determined by the Gibbs free energy.However,in real situations,dissolution kinetics,which include viscosity,dissolving mass mobility,and interface properties,are also a critical factor influencing the dissolution rate.Both thermodynamic and kinetic dissolving factors can be regulated by the ratio of salt to solvent in the electrolyte.In this work,by introducing three different concentrated electrolytes(1m NaOTf,9m TEAOT,9m NaOTf+22m TFAOTf),the concentration-controlled cathode dissolution is investigated in a susceptible Na₃V₂(PO₄)₃ cathode whose time-,cycle-,and state-of-charge-dependent dissolubility are evaluated by multiple electrochemical and chemical methods.It is verified that the super-highly concentrated Water-in-salt electrolyte has a high viscosity,low vanadium ion diffusion,low polarity of solvated water,and scarce solute-water dissolving surfaces.These factors significantly lower the thermodynamic-controlled solubility and the dissolving kinetics via time and physical space local mass interfacial confinement,thereby inducing a new mechanism of interface concentrated-confinement which improves the cycling stability in real aqueous rechargeable sodium-ion batteries.Thirdly,a deep study on the ionic transportation in the high-concentrated solution has been carried out.Generally,the ion transport is more complex in high concentration electrolyte due to the complex of strong ion-ion/ion-solvent interaction resulting in the invalidation of most ionic conducting theories based on the dilute solution.Therefore,it is challenging to give a full picture of ionic transportation in the concentrated electrolyte.Here,the concentrated electrolyte(Water-in-salt)is investigated by the multi-experiments,including advanced tools(NMR,Synchrotron X-ray diffraction,and Spallation Neutron Scattering)combining with MD simulation to draw out the unique microstructure and uncover its intrinsic relationship with the ionic transportation.Based on the results,we firstly proposed the ionic transport model of Water-in-salt electrolyte where the solid-like nano anion clusters constructs a super-fluid ordered framework and lithium-ion is able to move freely like the ionic atmosphere.Our model gives a unified explanation to understand the unique phenomenons previously discovered in Water-in-salt electrolytes,including the decoupling of conductivity-viscosity and the nano-phase separation between anion and water.Our model on the basis of the super-high concentrated electrolyte gives a new structural model with the unique Li conducting mechanism that can fill in the missing puzzle between the solid-state conductor and dilute liquid electrolytic solution.