Lithium Solid State Electrolyte Based On Three-Dimensional Carbon Materials

With the continuous improvement of people’s living standards,the energy density of lithium-ion batteries required by people is also increasing.The use of lithium-ion batteries is a bit of an anxiety.Therefore,a large number of researchers hope that replacing liquid electrolytes with solid polymer electrolytes is an effective way to quickly solve safety problems and improve energy density.However,the solid polymer electrolyte itself still has problems such as low ionic conductivity,narrow electrochemical window,low ion migration number,and poor interfacial compatibility with electrodes,making it difficult to maximize the performance of solid polymer lithium batteries.Therefore,in this thesis,two three-dimensional porous carbon materials and composite electrolytes were designed and synthesized to solve the above problems,and they were applied to lithium batteries for performance testing.The main contents of this thesis are as follows:（1）we report a three-dimensional（3D）holey graphene（hG）cross-linked with PEO composite solid electrolyte with Li NO₃ as addition,as shown in Figure 1a.As can be seen,the hG with plenty of holey structures and edges is more favorable for the formation of tightly cross-linked PEO chains perpendicular to（002）plane of hG,which facilitate Li⁺with fast diffusion rate in one direction.Moreover,such 3D cross-linked PEO-hG（PE-hG）structure not only significantly reduce the crystallinity of PEO but also favor the uniform ionization of Li salts through the whole structures,leading to the increased transfer number and rapid diffusion of Li⁺.As a result,the ionic conductivity and transfer number of PE-hG electrolyte can reach2.1?10^-4 S cm^-1 and 0.87 at 80 ^oC,respectively.For a symmetrical cell application,the Li||PE-hG||Li cell delivers a high average Coulombic efficiency（CE）of～99.9%over 1200 h at a current density of 0.05mA cm^-2 with a controlled capacity of 0.05mA hcm^-2.At a higher capacity of 0.1m Acm^-2 with a controlled capacity of 0.05m Ahcm^-2,the cell run over 1000h with a high average CE of～99.9%.Moreover,using PE-hG as the electrolyte together with Li Fe PO₄ as the cathode and Li metal as the anode,the symmetric Li Fe PO4||PE-hG||Li cell exhibits outstanding performance with a high initial discharge capacity of 135.8 m Ah g^-1 at 1C and an initial capacity retention of 71%over 350 cycles.（2）Considering carbon based materials with 3D networks and heteroatom-doping,as well as highly conductive inorganic components（e.g.,Li₃N,Li₃P and Li₂S₆）,promoting the SSEs with enhanced electrochemical performance,herein,we designed and synthesized a N,P,S tri-doped holey carbon（NPS-HC）incorporated with a poly（ethylene oxide）/（PEO）polyvinyl pyrrolidone（PVP）polymer electrolyte and the Li NO₃ salt as electrolyte additive.By optimizing the contents（x=1,2 and 3 wt.%）of NPS-HC in the electrolyte（NPS-HC/PEO-x）,the NPS-HC/PEO-1 at 80℃presents a high ionic conductivity of 2.7×10^-4 S cm^-1 and a low electronic conductivity of 2.08×10^-10 S cm^-1.The symmetric Li||NPS-HC/PEO-1||Li cell using the NPS-HC/PEO-1 as electrolyte at 80℃exhibits the remarkably stable Li plating/striping over 2220 h at a current density of 0.05 m Acm^-2.When the NPS-HC/PEO-1 electrolyte was assembled in the asymmetric Li||NPS-HC/PEO-1||Li Fe PO₄ cell,the cell displayed a high reversible capacity of 159.2 m Ah g^-1at 0.1 C.At 1 C,the cell also retained a high reversible capacity of 90.0 m Ah g^-1 after 700 cycles with an initial capacity retention of 74.3%.