Preparation And Performances Of Ultrathin Solid Polymer Electrolytes For All-solid-state Lithium Metal Batteries

All-solid-state lithium metal batteries are considered as one of the most promising next-generation energy storage devices for its excellent cycling performance and safety performance.Solid polymer electrolyte has attracted extensive attention due to its good flexibility,low cost and easy industrial production.Solid polymer electrolytes initiated by in-situ thermal curing small molecule polymer monomer generally possess high ionic conductivity and good electrochemical performance,but there is lack of the thickness,physical and chemical stability,mechanical properties,electrochemical stability and interface stability.Introduction an ultrathin polymer skeleton with controllable thickness and good mechanical strengthis a promising strategy because it improves the electrochemical and mechanical properties of the polymer electrolyte without sacrificing its flexibility.For solid polymer electrolytes,thickness,ionic conductivity,mechanical properties and electrochemical stability are key technical parameters.In addition,there are many problems of all-solid-state lithium metal batteries assembled with solid polymer electrolytes operating at high temperature,low temperature,and high rates or matching with high voltage electrode materials,which limits their further commercial applications.In view of this,this paper starts from greatly reducing the thickness of solid polymer electrolytes,selects different supports,and then fills the support skeleton with ionic conduction medium that can efficiently conduct lithium ions,so as to improve the comprehensive performances of solid polymer electrolytes.The main research contents are as follows:（1）Firstly,polyvinylidene fluoride（PVDF）porous membrane was prepared by phase inversion method as the support skeleton.After filling with the polyethylene glycol（PEG）electrolytes,the dense PVDF@PEG solid polymer electrolyte was fabricated via an in-situ thermocuring method,in which that the filled PEG electrolyte formed fast lithium ion transport channels.After the introduction of the support skeleton,the thickness of the PVDF@PEG electrolyte is controlled to about 20μm（±5μm）,which reduces the transport path of lithium ions.The ionic conductivity and electrochemical stability window of the solid polymer electrolyte are high up to 8.03×10^-5 S cm^-1 at 30^oC and 5.2 V,respectively.In addition,three-dimensional structure of PVDF can effectively improve the mechanical properties and maintain the structure integrity of SPE.More importantly,PEG electrolyte is embedded uniformly into the three-dimensional network to fix anions and form a uniformly distributed space charge layer,and thus effectively avoiding excessive local current density during lithium deposition/stripping process and guiding the uniform deposition of lithium.The Li/PVDF@PEG/Li symmetric cell can cycle continuously at 0.1 m A cm^-2 for more than3200 h without short-circuiting at both 60 and 100 ^oC.Based on these outstanding advantages,the capacity retention of the Li Fe PO₄/PVDF@PEG/Li pouch cell is 86.36%and 79.16%under 1 C after 1000 cycles at 60 ^oC and 100 ^oC,respectively.（2）In order to improve the high rate performance of PVDF@PEG electrolyte assembled all-solid-state battery,a flexible PVDF-PEI solid composite supporting membrane was designed and prepared by blending PVDF and polyetherimide（PEI）.The amorphous phase of the homogeneous PVDF-PEI composite porous membrane was increased by optimizing the PVDF and PEI weight ratio.The optimized PVDF-PEI@PEG solid polymer electrolyte（26-μm-thick）exhibits high ionic conductivity of 2.36×10^-4 S cm^-1at 60 ^oC.In addition,PEI contains rich carbonyl groups and excellent electrochemical stability,which can fix a certain number of anions to avoid concentration polarization.The optimized PVDF-PEI@PEG solid polymer electrolyte possesses high lithium ion transference number of 0.578 as well as excellent electrochemical stability window of5.5 V.In particular,the Li Fe PO₄//Li pouch cell delivers high discharge capacities of132.4 m Ah g^-1 and 111.5 m Ah g^-1 with a capacity retention of 86.6%and 85.9%after200 cycles at 2 C and 100 cycles at 3 C rate,respectively.The results show that PVDF-PEI@PEG electrolyte can be used in high-rates energy storage devices.（3）To further reduce the thickness and improve the mechanical properties of the electrolyte,an 10-μm-thick solid polymer electrolyte with a modified polyethylene（PE）film as a host was prepared,in which that the porous polymethyl methacrylate-polystyrene（PMMA-PS）interface layers（1-μm-thick）closely attached on both sides of the PE effectively improves the interface compatibility among electrolytes and electrodes.The resultant solid polymer electrolytes possesses an ultrahigh ionic conductance of 34.84 m S at room temperature and excellent mechanical properties of103.0 MPa with the elongation is up to 142.3%.Moreover,the Li Fe PO₄//Li pouch cell can stable cycle over 1000 cycles at 1 C rate and with a capacity retention of 76.4%from 148.9 m Ah g^-1 to 113.7 m Ah g^-1 at 60 ^oC.The Li Co O₂//Li pouch cell can stable operate at 0.1 C and 0.2 C rate for each 100 cycles.Furthermore,the excellent flexibility and safety of m-PPL electrolyte were demonstrated by the stable operation of Li Fe PO₄/m-PPL/Li pouch cell after crimping and folding,providing a promising strategy for solid polymer electrolytes with ultrathin thickness,excellent compatibility,high strength as well as safety.（4）Low ionic conductivity and poor mechanical properties are the main challenges for low temperature polymer electrolyte based solid state batteries.Herein,ultrathin and high-strength solid polymer electrolyte（PPLD）was achieved by employing PE film as the support skeleton and poly（vinylidene fluoride）/quasi-ionic liquid for rapid lithium ion conduction medium.The PPLD electrolyte shows a high ionic conductivity of 1.04×10^-4,1.82×10^-5 and 9.5×10^-6 S cm^-1 at 30 ^oC,0 ^oC and-10 ^oC,respectively.Moreover,the tensile stress reaches 116.5 MPa with the strain elongation rate of 134.8%.The PPLD based Li Fe PO₄//Li solid state pouch cell exhibits excellent cyclic performances with a capacity retention of 94.7%at 0.5 C rate over 700 cycles at room temperature.Even at low temperatures of 0 ^oC and-10 ^oC,79.3%and 48.6%of its room-temperature capacity can be remains at 0.1 C,respectively.In addition,no capacity decay is observed under low temperature after each 100 cycles,indicating great potential of the PPLD electrolyte for practical applications in low temperature solid state lithium battery.