Research On Poly(Ethylene Oxide)-Based Solid Electrolyte And Solid-state Battery

High energy density lithium secondary batteries are an important development direction in the field of energy storage.At present,the energy density development of lithium ion batteries has encountered a bottleneck,and it may bring safety problems when increasing battery energy density.All solid-state batteries with non-flammable and high Young's modulus solid electrolytes instead of flammable organic electrolytes is expected to use metal lithium anodes to improve energy density,while abating,even avoiding the safety problems caused by dendrite penetration and other factors.So it become the focus of scientific and industrial research.After decades of research,the lithium ion conductivity of solid electrolytes has been greatly improved,and some solid electrolyte have even exceeded liquid electrolytes.Because PEO solid polymer electrolyte has the advantages of high lithium ion conductivity at elevated temperature,good film formation,easy processing and good contact with electrodes,a stable-cycled PEO-based solid polymer battery is commercialized by French Bollorécompany and applied in electric vehicle.However,PEO electrolyte still has the disadvantage of low room-temperature conductivity so must be used at 60-80?.At the same time,due to the low oxidation potential of PEO,it can only be used with low-voltage positive electrode materials such as V₂O₅ or LiFePO₄,which severely limits the increase in the energy density of PEO-based solid-state batteries.In order to continuously improve the application prospects of PEO-based solid-state batteries,this work conducts research on the disadvantage of PEO electrolytes.First,lithium dodecylbenzenesulfonate?LiDBS?was compounded in the PEO electrolyte to increase the room-temperature conductivity.Then we explored the oxidation phenomenon and the property changes of PEO electrolyte when the voltage veyond the oxidation voltage,as well as the problems existing when PEO was used with the high-voltage cathode are investigated.Finally,according to the failure mechanism of PEO/high-voltage cathode interface,a method to improve the stability of the interface between the PEO electrolyte and the high-voltage cathode was proposed.In this paper,the surfactant lithium dodecylbenzenesulfonate is first used as an additive to the PEO electrolyte.Because of the different interaction between the hydrophilic or hydrophobic ends of the additive with PEO,The LiDBS will be in the form of micelles in the PEO electrolyte,similar to in the water.This new additive is cheap to use,easy to be disperse in the PEO matrix,and simple to be prepared.By observing the surface morphology and diffraction signal of the prepared electrolyte membrane,it was confirmed that the crystallinity of the PEO electrolyte membrane decreased after adding the additive,and the crystallinity was the lowest when 2%was added.The variable temperature conductivity test shows that the PEO electrolyte membrane has the highest room temperature lithium ion conductivity when the same addition amount is 2%.But due to the instability of the additive and the lithium metal,the interface resistance between the PEO electrolyte membrane containing the additive and the lithium metal increases,exceeding the polarization of the electrolyte body phase.This leads to an increase in the polarization of the assembled battery,which cannot be operated at room temperature.An interfacial modification method should be adopted between electrolyte and lithium anode.This work proposes the method of using the intermolecular interaction to make the additives form a specific structure in the PEO electrolyte to improve the conductivity at room temperature,which is easy and cheap to preparation and have great improvement.Secondly,the oxidation behavior of PEO at high voltage was studied.When the PEO polymer electrolyte was tested in a SP|PEO-LiTFSI|Libattery using a high specific surface area carbon inert electrode without positive electrode material,the voltage linear scanning test found that the PEO solid electrolyte began to oxidize and decompose at 3.9 V,which is the same as reported in the literature.When further conducting an impedance test after applying 4.2 V constant potential for different times,it was found that the impedance increased slowly with time,which indicating that the voltage-driven PEO oxidation reaction had a slower kinetic rate and the product was a substance with lithium ion conductivity.However,when the PEO-based battery cycled with LiCoO₂cathode at 4.2 V,the resistance of cathode/electrolyte interface increased and the capacity decayed rapidly.By applying Resonance inelastic X-ray spectroscopy?RIXS?analysis,it was found that the lattice oxygen ions in LiCoO₂ at the 4.2 V charged state have the valence-change effect,which can promote the oxidative decomposition of PEO.At the same time,the surface of LiCoO₂ tended to loses oxygen to form spines or salt-rock structure,which is a poor conductor of lithium ions,resulting in a significant increase in the resistance of cathode/electrolyte interface.Experimentally,we observed the presence of Co₃O₄ and CoO structures on the surface of LiCoO₂ in contact with PEO through high-resolution TEM,and confirmed the intense decomposition of the solid electrolyte of PEO through FTIR spectroscopy.This shows that in addition to the electrochemical stability of PEO itself,the charged state of LiCoO₂ with oxidizing ability will also promote the rapid oxidative decomposition of PEO and is accompanied by the restruction of the surface structure of LiCoO₂,resulting in a rapid increase in the interface resistance of the cathode/electrolyte interface and causing the failure of battery.The results of this study suggest that if the surface oxidizability of the positive electrode material can be suppressed or a cathode material that does not have a strong oxidizing surface is used,the PEO polymer electrolyte may work stably at a higher voltage than the intrinsic electrochemical oxidation potential.Guided by this idea,PEO-based all-solid-state batteries with LiCoO₂coated by solid electrolyte LATP and LiMn_0.7Fe_0.3PO₄ cathode were assembled,which can achieve a stable cycle at a 4.2 V charge cut-off voltage.In this work,by studying the oxidation process of PEO at a high potential and the reason for the rapid failure of the interface when LiCoO₂ is used as cathode,the problem of applying PEO electrolyte to the high-voltage cathode is clarified.Based on this,a PEO-based all-solid-state battery which can stably cycle at 4.2 V cut-off voltage is proposed and realized.Through this study,it can also be seen that the stability of the electrolyte in the battery system is not only related to the properties of the electrolyte itself,but also closely related to the interface characteristics between electrolyte and electrode.The design of the interface is crucial to the realization of a practical all-solid-state battery.Finally,this work proposes a simple electrodeposition method to form a modified layer on the surface of cathode to protect PEO from oxidation of cathode during cycling.According to the requirements of the anti-oxidation and high lithium ion conductivity properties of the interface layer,three kind of electrodeposition monomers were selected,which are acrylonitrile,sulfolane and vinylene carbonate,respectively.The cathode and litium salt were filted,too.After comparing the performance of PEO solid-state batteries with different electrodeposited layers,acrylonitrile was selected as the electrodeposition monomer,NCM523 and LiDFOB were selected as cathode and lithium salt respectively.The precursor solution undergoes an electrodeposition reaction at3.3 V,forming a protective layer with oxidation potential at 4.5 V.Characterization of the morphology and composition of the electrodeposited layer revealed that the deposited layer completely covered the surface of the positive electrode,and it mainly contained PAN,which is polymerized from AN,and LiB_xO_yF_z,which is the decomposited from LiDFOB,indicating that there may be a synergistic effect between AN and LiDFOB during electrodeposition process.The comparison of PEO batteries with different modification layers,which are bare NCM523 and with modification layer that deposited by only acrylonitrile,only LiDFOB and both acrylonitrile&LiDFOB respectively,proves the synergistic effect between acrylonitrile and LiDFOB.But the specific mechanism still needs further explored.The electrodeposition conditions such as current density,cut-off voltage and concentration of acrylonitrile were optimized for a best parameter for electrodeposition.It was found that the larger current density and the higher voltage during electrodeposition results to the greater polarization of battery.The addition concentration of 10%acrylonitrile resulted to minimum polarization.The assembled PEO solid-state batteries with optimized modification layer possessed 150.9 m Ah/g discharge capacity,92.9%capacity retaintion and 99.8%coulombic efficiency after 120 cycles with 4.3 V cut-off voltage,meaning the formation of stable cathode/PEO interface.This work proposes a new electrodeposition interface modification method,which can be used in PEO solid-state batteries to protect the electrolyte from being oxidized by the cathode.