Design And Protection Of The Lithium Metal Anodes And New Electrolyte For The Lithium Metal Batteries

Lithium ion batteries have been widely applied as the energy strorage system for the portable electronic devices.Traditional graphite anode for LIBs suffers low specific capacity of only 372 mAh g-1 and greatly hinders the further improvement of the energy density of the LIBs.Lithium metal anodes(LMBs)has become one of the most promising anode material for the high-enery-density Li-based batteries owing to its ultra-high specific capacity and lowest redox potential.Unfortunately,the uncontrolled formation of the Li dendrites severely threats the safty of the Li-metal batteries.Beside,the Li dendrites would lead to formation of the "dead Li”,which results in the irreversible Li loss and decrease the cycling life of the Li metal batteries.In this thesis,agarose(AG)polymer film serve as a functional protective layer for Li-metal anode attributed to its stiff,but flexible properties.Results show that the Li coulombic efficiency can be enhanced by employing the agarose film as the protective film of the Li metal anodes.The agarose protective Li-metal anode(Li@AG-Cu)exhibits high Li coulombic efficiency,stable cycling performace and long lifespan.Besides,enhaced electrochemical performace of both LiFePO4 and Li-S full cells also demonstrates that the agarose film can improve the Li-utilization.The enhancing performance of the AG-Cu electrodes can be attributed to the AG film which suppresses the ever-growth of the dendrites Li,and stabilizes the interface between Li and electrolyte by physical separation.This conclusion has also been supported by the SEM and AFM characterization,the Li@AG-Cu electrode exhibits a smooth surface without dendritic Li growth,which can greatly improve the cycling performace of Li-metal anodes.Besides,Al2O3 loading porous carbon(Al2O3@MOF-C)derived from metal-organic frameworks is used as the host matrix for the Li metal anodes.The high specific surface area and wide pore distribution enable the uniform initial Li nucleation and buffer the volume expansion during the Li plating and stripping processes.Otherwise,the Al2O3 in the host matrix can serve as the nano seeds for the Li deposition and decrease the overpotential of the Li deposition.Hence,an enhace Li coulombic efficiency and stable cycling performace can be achieved by employing the Al2O3@MOF-C host matrix.The SEM images of the surface indicates that the Li metal can be well spreaded bu the Al2O3@MOF-C particles,and lead to smooth surface.Even after 50 cycles at 2 mA cm-2,1 mAh cm-2,the dendrite-free surface can still be maintained.In addition,coupling high-voltage cathode material like(LiCo02?MNC etc.)with Li metal anode can significantly improve the power density of the Li batteries.Tranditional carbonate-based electrolytes have wide potential window,but the high reactivity between Li anode and electrolyte would lead to serious Li loss,resulting in limited cycling life of the Li metal batteries.Ester has been proved that they are stable with the Li metal anodes.Tetraethylene glycol dimethyl ether(TEGDME)is a kind of ester solvent which has a potential window as high as 4.5 V.From our results,the TEGDME-based electrolyte suffers a low coulombic efficiency,which maybe results in its high viscosity and low ionization with Li salt.To address this issue,ethylene carbonate(EC)was adopted to increase the ionic conductivity of the TEGDME-based electrolyte.Results show that the EC can significantly improve the Li coulombic efficiency,and when EC ratio up to 50%,the EC/TEGDME-55 electrolyte can exhibit the highest coulombic efficiency of 96.5%.In addition,by adding 0.1 M LiNO3 in the EC/TEGDME electrolyte can further improve the Li coulombic efficiency to over 97.5%.Moreover,EC/TEGDME-55 with 0.1 M LiNO3 was used as the electrolyte of LiCoO2/Li half cells can obviously improve the cycling stability and lengthen the cycling life of the Li metal batteries.The 20 mg cm-2-loading 4.25 V LiCoO2/Li cell can exhibits nealy 90%capacity retention at both 0.2 and 0.5 C rate after 100 cycles by using EC/TEGDME-55 with 0.1 M LiNO3 as electrolyte,which is much better than that of with EC/DMC/EMC electrolyte.Similar electrochemical performance also can be observed in 15 mg cm-2-loading 4.5 V LiCoO2/Li batteries.