Study On G-C₃N₄ And Fluoride Based Solid Electrolytes To Inhibit Lithium Dendrite And To Achieve All-solid Lithium Metal Batteries

Lithium metal anode has an ultrahigh theoretical specific capacity?3,860 mAh/g?and very low redox potential?-3.040 V versus standard hydrogen electrode?,especially when it is paired with the conversion type cathodes,the lithium metal battery even reaches an energy density of 900 Wh/kg which is considered as a prospect candidate for the next generation of electrochemical energy storage devices.However,the uneven lithium stripping/stripping process and the unstable structure of the SEI?solid electrolyte interface?often leads to dendrite.Lithium dendrite can deteriorate the electrochemical performance of the battery and even penetrate the separator,causing a safety hazard to the battery.Recently,various strategies have been proposed to suppress lithium dendrite and smooth the surface of the lithium anode,such as electrolyte optimization by additives or using solid electrolytes.This paper mainly focuses on inhibiting lithium dendrites by modifying traditional electrolytes and developing several new electrolytes or realizing all-solid lithium metal batteries.The main work includes:?1?We firstly used the three-dimensional g-C₃N₄ mesoporous microspheres as the dimethyl ether electrolyte?LiTFSI/DGM?filler to obtain a lightweight polymer-reinforced nanostructured slurrylike electrolyte?denoted as g-C₃N₄-LiTFSI-DGM?.The electrolyte shows good interfacial properties with a small interface resistance to lithium anode as 115?·cm².Thanks to the high mechanical strength,nanosheet-built hierarchical structure with surface polarity of g-C₃N₄,The slurrylike electrolyte can effectively suppress the lithium dendrite.The electrolyte can enable a lithium symmetric battery undergo a stable plating/stripping process with reduced voltage polarization difference and enhanced battery life,even at a high current density of2mA/cm²,6 mAh/cm²,after 100 cycles,the voltage polarization is still around 100 mV.When the electrolyte is used in a Li/FeS₂ battery,it can ensure a long battery life of more than 400 cycles,benefiting from the smoothing lithium anode and weakened shuttle effect of the polysulfide.The two-dimensional structure O-g-C₃N₄ which was exfoliated by oxygen doping was inferior to the three-dimensional structure of g-C₃N₄.?2?The composite polymer electrolyte PEO-LiTFSI-C₃N₄?CN-CSPE?with excellent comprehensive performance was prepared by combining three-dimensional g-C₃N₄ mesoporous microspheres with polyethylene oxide?PEO?.Nanosheet-built hierarchical structured g-C₃N₄ and PEO are strongly cross-linked with each other.PEO is tightly entangled in micron-scaled g-C₃N₄ microspheres,which reduces the crystallization the polymer and improves the ionic conductivity.The CN-CSPE PEO-LiTFSI-0.2C₃N₄ obtains a high room temperature conductivity of 3.06×10^-5 S/cm,which is one order of magnitude higher than PEO based SPE?2.32 x 10^-66 S/cm?and gives a high lithium ion migration number of 0.69.The composite polymer solid electrolyte exhibits excellent interfacial properties and maintains a stable interface resistance of about 480?·cm² for a long period of time at 60°C.The lithium symmetric batteries based on PEO-LiTFSI-0.2C₃N₄ can achieve a stable plating/stripping process for 6600 hours,benefiting from the addition of g-C₃N₄ and the ultrastable interface,while the longtime cycled membrane remains good shaped and the lithium surface is also dense and smooth.The Li/LiFePO₄ cells based on the PEO-LiTFSI-0.2C₃N₄maintains a capacity of above 130 mAh/g after 200 cycles whether with a current of 50?A or 100?A at 60°C.At the same time,it also exhibits excellent rate performance,and still endure a large capacity of 110 mAh/g at 3 C or 6 C.?3?The addition of functional additives to the electrolyte is an effective method for suppressing dendrites,particularly when the LiF-rich SEI layer is formed by decomposition or deposition of an additive.However,LiF is still hindered by its poor bulk ionic conductivity and difficulty in obtaining tailored nanostructure.In the absence of the required structural prototype or mineral phase,the exploration of new lithium-based fluoride with high Li-ion conductivity as a SEI component remains a significant challenge.In view of this,we propose a nanostructured lithium-rich phase Li₃AlF₆derivative from cryolite as a solid electrolyte additive.Li₃AlF₆ synthesized from ionic liquid shows a textured nonporous structure with ionic liquid coating on the surface.Its room temperature ionic conductivity is as high as 2×10^-5 S/cm,with low activation energy of 0.29 eV,which is the best level in fluorine-based solid electrolyte.These features can ensure uniform ion flux at the bulk and grain boundary of Li₃AlF₆-enriched SEI layer,and enhance the inhibitory effect on Li dendrites.The lithium symmetric batteries based on Li₃AlF₆ modified electrolyte can achieve a stable plating an d striping process for least 100 cycles?with a duration of 600 hours?even at a high area capacity of 3 mAh/cm²;The capacity retention of various lithium metal batteries based on LiFePO₄,FeS₂ and S is significantly improved,which is attributed to the stability of the lithium anode.?4?Based on the synthesis of fluoride solid electrolyte Li₃AlF₆ from the ionic liquid BmimBF₄,a new Li-rich Li₃GaF₆ derivative from cryolite phase is obtained by thermal fluorination using hydrated gallium nitrate as the precursor from C₁₀mimBF₄ with a longer side chain.The synthesized Li₃GaF₆ is characterized by a cleaner ionic liquid coating and discrete distribution of square particles compared to the texture morphology of Li₃AlF₆.Li₃GaF₆ obtained a higher room temperature conductivity of 8.8×10^-5 S/cm and a activation energy of 0.41 eV.After the nanostructured Li₃AlF₆ and Li₃GaF₆ are pressed into pellets,the in-situ bonding of the ionic liquid thin layer at the grain boundaries is observed from the cross section,which greatly promotes the ion transport in the solid electrolyte.The Li/LiFePO₄ batteries based on a fluoride based solid electrolytes are successfully realized with a novel interface wetting agent.For Li₃GaF₆,it can achieve a capacity of 90 mAh/g for at least 150 cycles at 0.5 C,and the capacity can maintain well at a large rate of 1.5 C.For Li₃AlF₆,the solid-state battery can achieve200 cycles at 0.5 C with a capacity of not less than 80 mAh/g.Li₃GaF₆-based lithium symmetric battery can achieve a stable plating/stripping process for at least 220 hours.