Electrolyte Design And Interface Regulation For High Energy Density Rechargeable Lithium Batteries

Rechargeable lithium(Li)batteries have been popularized in modern society due to their high energy density and long lifespan.Recently,with the upgrading of 3C electronic products and the popularization of electric vehicles,batteries with higher energy density are pursuited relentlessly.Application of high-voltage cathode or lithum metal anode can increase the operating potential or capacity of the battery.Increasing the operating potential is the most effective method to evaluate the energy density.However,the solid-electrolyte interface issues faced by conventional carbonate electrolytes toward high-voltage cathode and Limetal anode restrict the practical application of high-voltage cathode and lithum metal anode.Conventional carbonate electrolytes suffer severe oxidative decomposition on the high-voltage cathode surface,resulting in decreased Coulombic efficiency and lifespan.The dramatic electrolyte decomposition also generates a lot of gas and heat,inducing safety problems such as thermal runaway or even explosion.As for the Limetal anode,the main interface issues are serious parasitic reactions and dendrites growth.Conventional carbonate electrolytes are thermodynamically and electrochemically unstable on Limetal anodes.Once fresh Limetal is exposed,the electrolytes decompose on Limetal immediately to form a solid-electrolyte interface(SEI)layer.This proceeded reaction during charge/discharge progress consumes both electrolytes and active Limetal.Severe side reactions also increase the internal impedance and shorten lifespan of the battery.In the charge progress,Li⁺ions tend to deposite on the surface of Limetal anode in the dendritic form.On the one hand,the grown dendrites may pierce the separator and bring about internal short circuit.On the other hand,dendritic Liraises surface area and aggravates the side reactions between the electrolyte and the fresh Limetal.Moreover,Lidendrites may be isolated from the bulk to form"dead Li",further reducing cycle life and energy density of the battery.In this paper,functional electrolytes for 5 V-class LiNi_0.5Mn_1.5O₄ cathodes and Limetal anodes are developed by solvents,Lisalts,and additives regulation.Theoretical calculations are also involved to predict the molecular properties.In terms of LiNi_0.5Mn_1.5O₄ cathode,the effect of fluorinated ether co-solvent on the electrolyte-separator and electrolyte-cathode interface is investigated.Nonflammable high-voltage electrolyte incorporating fluorinated ether and phosphate is also developed.With respect to Limetal anode,low concentration electrolytes and film-forming additive containing azide group are developed to construct a stable solid-electrolyte interface(SEI)on the surface of Limetal anode.Followings are the main sections and conclusions of this paper:(1)1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether(HFE)is involved in the LiPF₆–propylene carbonate(PC)electrolyte to improve the electrolyte-polyolefin separator interface and electrolyte-cathode interface.The dipole moment and polarity of HFE is low.Adding HFE as co-solvent reduces the contact angle of electrolyte-separator interface and enhances the wettability of the PC-based electrolyte towards non-polar polyethylene(PE)separator.Theoretical calculations indicat that HFE possesses lower highest occupied molecular orbital energy and higher theoretical oxidative potential than PC.Electrochemical floating test further proves that parasitic reactions in electrolyte containing HFE are much less than that in HFE-free electrolyte even at 5 V.The Li||LiNi_0.5Mn_1.5O₄ and Li₄Ti₅O₁₂||LiNi_0.5Mn_1.5O₄ cells exhibit capacity retention of 88.2%and 92.5%respectively after 100 cycles with electrolyte containing 20%HFE.(2)HFE is nonflammable because most of the hydrogen atoms in HFE have been replaced by fluorine atoms.Therefore,HFE is introduced as a co-solvent into the LiPF₆–trimethyl phosphate(TMP)electrolyte to construct a totally nonflammable electrolyte while improving interface issues.Lower polarization,higher Coulombic efficiency and capacity retention of Li₄Ti₅O₁₂||LiNi_0.5Mn_1.5O₄ cells are realized after adding 20%HFE co-solvent.Flammability tests reveal that both TMP and TMP+HFE electrolyte are nonflammable.The microcalorimetry tests were conducted ranging from room temperature to 300?.The heat generation of the electrolyte decreases from-110.2 J g^-1 to-69.7 J g^-1,and the heat generation of the electrolyte with LiNi_0.5Mn_1.5O₄ powder decreases from-896.0 J g^-1 to-538.8 J g^-1 after adding 20%HFE.(3)Dual-salt electrolytes conbining 0.1 M LiDFP and 0.4 M lithium bis(oxalate)borate(LiBOB)/lithium bis(fluorosulfonyl)imide(LiFSI)/lithium bis(trifluoromethanesulfonyl)imide(LiTFSI)are developed to form a robust and conductive SEI on Limetal anode.Theoretical calculations reveal that BOB^-and LiDFP have lower lowest unoccupied orbital energy and chemical hardness,and are easier to be reduced and decomposed on the surface of Limetal.The SEI formed in 0.1 M LiDFP+0.4 M LiBOB electrolyte is rich in LiF,P-O compounds and Li₂BO_x.LiF and P-O are beneficial for the Li⁺diffusion in SEI and Li₂BO_x can inhibit the dissolution of organic compounds in the outer layer of SEI.With this electrolyte,the Coulombic efficiency of Li||Cu cell reaches 97.6%,Li||Lisymmetric cell maintains low polarization for more than280 h at 1 m A cm^-2 and the capacity retention of Li||LiFe PO₄ cell reaches 95.4%after300 cycles at 2 m A cm^-2.In addition,the salt cost of the low concentration dual-salt electrolyte is only 60%of that of the conventional electrolyte with 1 M LiPF₆.(4)Two PC based multi-salt electrolytes containing 0.1 M LiDFBOP+0.4 M LiBOB or 0.1 M LiDFBOP+0.2 M LiBOB+0.2 M LiPF₆ are designed.The dual-salt electrolyte has higher thermal stability,while the triple-salt electrolyte maintains a high conductivity.These electrolytes not only exhibit good compatibility with Limetal anode,but also form a uniform cathode-electrolyte interface on high nickel cathode LiNi_0.7Co_0.1Mn_0.2O₂ to improve the cathode stability.The Li||LiNi_0.7Co_0.1Mn_0.2O₂ cells exhibit higher capacity and cycling stability at both-25? and 70? due to the lower melting point of solvents and higher thermal stability of Lisalts.(5)Diphenylphosphoryl azide(DPPA)is used as an additive to form a high conductive SEI layer rich in Li₃N on Lianode.With 1%DPPA,the deposited Liexhibits a nodular morphology on copper foil.The electrolyte decomposition and Lidendrite growth are also inhibited effectively.The Li||Lisymmetric cells remain stable for more than 500 h at1 m A cm^-2 with much higher exchange current density.The Li||LiNi_0.8Co_0.1Mn_0.1O₂ cell still possesses 83.5%capacity after 180 cycles at 2.6 m A cm^-2.Theoretical calculation and molecular dynamics simulation show that DPPA decomposes on the surface of Limetal prior to electrolyte.The azide group in DPPA decomposes and generates N₂.The N₂ released then reacts with Limetal and forms Li₃N.