Research On Key Materials Of Lithium Polymer Batteries

The rapid development of electric vehicles has led to higher demands for rechargeable batteries.Meanwhile,the energy density of conventional lithium-ion batteries has nearly reached the theoretical limit.Lithium metal anode is considered to be the most promising anode for next-generation high-energy batteries due to the high theoretical specific capacity?3860 mAh/g?and the most negative electrochemical potential?-3.04 V vs standard hydrogen electrode?.However,the commercialization of lithium metal batteries?LMBs?is still hindered by the high chemical reactivity,large volume changes and uncontrollable lithium dendrite growth of lithium metal anode.Replacement of organic liquid electrolyte with solid electrolyte may reduce the continuous side reactions between lithium and electrolyte,and suppressing lithium dendrite growth.Since solid electrolytes based on polymers are light,flexible and easy to process,Lithium polymer batteries are expected to become the best candidates for future batteries with high energy density and good safety performance.This work is mainly concerned with polymer electrolyte and lithium metal anode which are the two critical materials of lithium polymer batteries.Poly?ethylene oxide??PEO?has flexible segments and excellent solubility of lithium salts.However,PEO-based polymer electrolyte has poor ionic conductivity at room temperature.PEO is not fully thermodynamically stable with lithium metal.To improve the ionic conductivity and stability with lithium of PEO based electrolyte,lithium bis?trifluoromethanesulphonyl?imide?LiTFSI?and garnet-type LLZTO(Li_6.4La₃Zr_1.4Ta_0.6O₁₂)nanoparticles are chosen as lithium salt and inorganic fillers respectively,and the organic-inorganic composite electrolytes composed of PEO with different LiTFSI and LLZTO content are prepared through solution casting method.The composite electrolytes with mole ratio of EO/Li⁺=12 show higher conductivity at80?and 60?.The area-specific charge-transfer impedance between PEO and lithium metal can be decreased by 3⁴ times with appropriate amount of LLZTO fillers.The introduction of LLZTO fillers can reduce the overpotential of Li|Li symmetry cells and improve the cycling performance of Li|LiFePO₄ cells with PEO-based electrolyte.In addition,cross-linked PEO is synthesized by ultraviolet curing of poly?ethylene glycol?diacrylate?PEGDA?.The effect of adding inorganic fillers and mixing with non-crosslinked PEO are compared on the cross-linked PEO electrolyte.It is demonstrated that cross-linked PEO electrolyte mixed with non-crosslinked PEO can lead to improved mechanical properties,lower glass transition temperature,higher ionic conductivity and better cycling performance.Adding LLZTO fillers to cross-linked PEO electrolyte mixed with non-crosslinked PEO can improve the ionic conductivity to 2.3×10^-3 S/cm?80??.Through the failure analysis of single-layer Li|LiFePO₄ cell with as-obtained electrolyte,thick interphase layer?²?m?is observed between lithium metal and electrolyte.Because of the non-uniform volume change of lithium anode,there are also holes and gaps at the interface which will bring about the loss of electrical contact.This indicate that the interface between PEO-based electrolyte and lithium metal still need to be improved.In chapter 3,the effects of LiFSI?Lithium bis?fluorosulfonyl?imide?-carbonate electrolyte composition on the lithium anode electrochemical performance are studied.With variable organic solvent composition,LiFSI concentration,and cycling temperature controlled,Li|Cu,Li|Li and Li|LTO?Lithium Titanate?cells are assembled to evaluate the cycling performance of lithium metal anode.EC?ethylene carbonate?/DMC?dimethyl carbonate?/DEC?diethyl carbonate?ternary solvent system can improve the lithium cycling performance compared with EC/DMC binary solvent system and DMC unitary solvent.By comparing the LiFSI-EC/DMC/DEC ternary carbonate solvents with 9 different LiFSI concentrations,it is found that the cycle performance of the Li|Cu battery at room temperature increased with the increase of LiFSI concentration.At high lithium salt concentration,larger lithium deposits with smaller specific surface area.Elemental analysis of lithium surface by XPS revealed that the SEI film contains organic solvent decomposition products such as Li₂O₃,lithium alkyl carbonate,and lithium alkoxide,and FSI^-decomposition products such as LiF,Li₂S₂O₄,and Li₂S₃O₃.At high salt concentrations,SEI may contain more FSI^-decomposition products,and SEI film may be thicker and denser than low salt concentrations.At 45°C and 60°C,high salt concentration electrolytes provide higher coulombic efficiency and cycle stability.In chapter 4,the effect of polymer coatings as artificial solid electrolyte interphase?SEI?on electrodeposited lithium is studied.Various polymer coatings such as PEO-LiTFSI,SBR,PTFE,PAN-LiTFSI,are introduced on the surface of Cu current collector by spin coating.It is found that PEO-LiTFSI coating can significantly improve the cycle life of Li|Cu battery in ether electrolyte.In carbonate electrolyte,PAN-LiTFSI coatings with compact and uniform surface morphology promotes lithium deposition with larger particles,avoids the generation of lithium dendrites,and improves the coulombic efficiency of Li|Cu batteries through homogenizing ion flow and avoiding the direct contact between lithium and electrolyte.It is worth mentioning that the heat-treated PAN-LiTFSI coating promote the lithium deposition with larger particle size and further enhance the coulombic efficiency of Li|Cu cells.It is confirmed by scanning tunneling microscopy that the heat-treated PAN coatings had better compaction,higher mechanical strength,and better homogeneity of morphology and modulus distribution.It can be inferred by total reflection infrared spectroscopy that PAN undergoes a nucleophilic addition reaction during heat treatment to form a new molecular structure.In addition,this chapter also studies the effect of PAN coatings with different thickness on electrodeposited lithium.The properties of the SEI on the surface of lithium anode have a significant effect on its plating and stripping behavior.To improve the performance of Solid-state lithium metal batteries,designing and constructing an SEI film with good compaction and homogeneity between the solid electrolyte and lithium metal anode.