| Lithium metal cathodes have received widespread attention because of their high specific capacity and low potential.However,when conventional liquid electrolyte systems are combined with lithium metal cathodes,there are problems such as flammability,chemical instability,short life span and even lithium dendrite generation during battery cycling,which can lead to short circuits and further explosions.These problems have seriously hindered the commercial application of lithium metal batteries.The use of solid-state electrolytes instead of traditional liquid electrolytes is widely considered as a promising direction to inhibit the growth of lithium dendrite and address the security issue for next-generation high-energy-density lithium metal batteries（LMBs）.Polymer electrolytes are considered to be one of the most promising solid-state electrolytes for the next generation of LMBs due to their high safety,high mechanical flexibility,viscoelasticity and film-forming properties.To date,research on polymer electrolytes has focused on ionic conductivity and interfacial stability.However,the thickness of the electrolyte plays an equally important role in the energy density and electrochemical performance of all-solid-state batteries.In general,ultra-thin solid-state electrolytes have lower impedance,shorter ion transport paths and low mass and volume,which result in higher energy densities.Based on this,this thesis conducts research on ultra-thin polymer electrolytes for application in all-solid-state batteries.Based on PEO electrolytes,the thesis significantly improves the electrochemical properties and mechanical strength of the polymer electrolytes by introducing PTFE porous membranes.And furthermore,PIM-1 was introduced as a nano-filler to improve the performance of PEO matrix,and the mechanism that how PIM-1 improves ion transport properties and electrochemical performance of the ultra-thin polymer electrolyte was investigated.The main research ideas are summarized as follows.（1）Firstly,for PEO electrolytes its own poor mechanical strength prevents the preparation of thin electrolytes.In this study,the mechanical strength of PEO electrolytes is enhanced by PTFE fiber membranes and combined with a hot-pressing process to produce dense and ultra-thin solid electrolytes.Compared to other reinforcements,PTFE fiber membranes improve the mechanical properties and thermal stability of the composite polymer electrolyte while their higher porosity and interconnected pores reduce the effect on ion conduction.The addition of the hot-pressing process effectively reduces porosity defects due to solvent volatilisation and increases the amorphous zone of the PEO matrix,thereby improving the overall ionic conductivity of the ultra-thin electrolyte film.In this system,PEO/Li TFSI was injected into the PTFE fibre membrane and thermocompressed to form an ultra-thin PTFE/PEO/Li TFSI-HP/PTFE（PLP-HP）SPE with a thickness of only 14.5μm.The deformation at 200°C was only 2.43%,indicating that PLP-HP has excellent thermal dimensional stability and avoids thermal runaway of the cell at high temperatures.This indicates that PLP-HP has excellent thermal dimensional stability and avoids thermal runaway of the battery at high temperatures.It is worth noting that the all-solid-state LFP/PLP-HP/Li battery can be stable for up to 500cycles at 0.5 C with an area capacity of 0.955 m Ah cm-2 at high content positive electrodes when the test temperature is 60°C.（2）Based on the above studies,PIM-1 was introduced into PEO to enhance various properties of the ultrathin electrolyte by targeting the lower lithium ion mobility,ionic conductivity and electrochemical window of the ion-conducting matrix.Highly reinforced ultrathin composite polymer electrolytes（PLPP）were successfully prepared on a large scale by densely filling a porous polytetrafluoroethylene（PTFE）matrix with a mixture of well-dispersed polyoxide（PEO）,lithium salt（Li TFSI）and Ben’s microporous polymer（PIM-1）.It was shown that the prepared PEO/Li TFSI/PIM-1/PTFE（PLPP）exhibited a high lithium ion transfer number of 0.56,an elevated electrochemical stability window of 4.5 V and an ionic conductivity of 9.13×10-5 S cm-1 at 60°C,under the synergistic enhancement of a PTFE matrix with excellent mechanical properties and a soluble PIM-1 molecule with suitable functional groups.Thus,Li/Li symmetric cells with this highly enhanced PLPP electrolyte exhibit excellent cycling stability of over 2000 h at 0.1 m A cm-2 and over 600 h at 0.2 m A cm-2,and LFP/PLPP/Li full cells can also provide excellent multiplier performance,even stable cycling at high multipliers of 10 C.Furthermore,at 0.1 C and 60°C,full cells with a high LFP loading of 8.41 mg cm-2can exhibit a very stable reversible capacity of up to 147.0 m Ah g-1,corresponding to a high area capacity of 1.236 m Ah cm-2.When assembled with a high-voltage cathode（NCM523）,the PLPP full cell can operate stably for 50 cycles at 0.1 C and 60°C with an initial capacity of154.1 m Ah g-1 and a capacity retention rate of 76.4%. |
PDF Full Text Request