Design And Properties Of Composite Solid Electrolyte Based On Two-dimensional Nanosheets

Lithium metal batteries(LMBs)are considered as one of the important aspects for future energy storage devices developing due to their significantly high energy density.However,the wide use of flammable liquid electrolytes and the growth of lithium dendrites are prone to cause safety problems during cycling of LMBs.The replacement of liquid electrolytes with solid electrolytes offers a perfect way to improve the safety of LMBs.Compared with others,solid polymer electrolyte(SPE)is more suitable for conventional thin-film battery technology and promises a great prospect.But existing SPEs with many problems such as the trade-off between ionic conduction and mechanical strength,difficulties in thinning,etc.,are unable to meet the requirements for high-energy-density LMBs.Herein,this study focused on constructing Li⁺transport channels in the electrolyte,improving chemical environment for Li⁺transport,and designing the structure of SPE.Functionalized nanosheets were introduced into composite solid electrolytes(CSEs)to enhance ionic conduction and mechanical strength simultaneously.The impact of distribution and surface chemical environment of nanosheets on the ionic conduction and mechanical properties of SPEs was explored in order to support the design of high-performance thin SPE.The main research contents and results were summarized as follows:(1)Design of CSE based on lithiated nanosheets.Lithiated nanosheets(Li DGO nanosheets)with high concentration of free Li⁺was synthesized through lithiation of the anchored Ar-OH groups on polydopamine-modified graphene oxide,followed by incorporated into PEO matrix to fabricate CSE.Despite the decreased crystallinity of PEO,well-dispersed nanosheets combined with the intrinsic advantage of organic-inorganic hybridization afford CSE highly improved mechanical stability.The tensile strength and elongation at break of CSE are,respectively,237%and 133%higher than those of blank PEO.The constructed long-range conduction pathways with locally concentrated Li⁺at PEO-Li DGO interfaces impart CSE highly enhanced ionic conductivity.The ionic conductivity reaches 3.4×10^-5 S cm^-1 at 30 ^oC,10 times high than that of blank PEO.These then bring excellent battery performances.The assembled LMB exhibits excellent cycling stability of?156 mAh g^-1 after 200 cycles at 0.5 C with an ultra-high capacity retention of 98.7%,and the discharge capacity can reach?125 mAh g^-1 at a high rate of 2 C.(2)Design of laminar composite solid electrolyte(LCSE)based on oxygen-vacancy-rich nanosheets.As-prepared Co doped Ce O₂ nanosheets were incorporated into PEO matrix to fabricate LSPE by a filtration method.PEO is used as a binder to bond the nanosheet framework and then a layer-by-layer structure is formed.Taking advantage of the structure,LCSE exhibits thin thickness(?32?m)and acceptable mechanical strength.The abundant oxygen vacancies on the surface of the nanosheets promote the dissociation of lithium salt.The formed double active layers at nanosheet-polymer-nanosheet interfaces concentrate free Li⁺into the PEO phase.The constructed continuous conduction network imparts LCSE enhanced ionic conductivity of 5.81×10^-5 S cm^-1 at 30 ^oC,and Li⁺transference number of 0.59.Benefiting from the improvement of ion conduction and mechanical properties,LSPE exhibits excellent interface stability to Li metal anode,and the assembled lithium symmetrical battery still operates stably at a current density of 0.7 mA cm^-2.The resultant LMBs also display good cycle and rate performance.The capacity retention of 83.6%is achieved over 200 cycles,and the discharge capacity reaches 120.7 mAh g^-1 under 2 C rate.