Research On Preparation And Properties Of Li-Metal Composite Anode

The pursuit of high-energy-density secondary batteries has become a hotspot of current society.On this basis,lithium metal anode is considered as the “Holy Grail” of energy storage materials because of the extremely high specific capacity(3860 m Ah/g)and the lowest redox potential(-3.040 V vs.the standard hydrogen electrode).On the other hand,Li metal anode can be used in the high-energy-density Li-free cathode batteries,such as Li-S and Li-O2 batteries.However,the dendrite growth,infinite volume change and low coulombic efficiency(CE)caused by unstable SEI hinder the application of lithium metal anode.Furthermore,because of the extremely high chemical reactivity of Li metal,the process and storage of Li metal anode must be conducted with the protection of inert gas atmosphere,which increases the cost of Li metal anode.In this thesis,dendrite-free Li metal anode has been prepared via induced homogeneous deposition method.Moreover,based on existed Li-C composite,we have developed a series of Li metal anode with high CE and high environmental stability.The results are listed as follows:Firstly,we have assembled benzotriazole(BTA)onto surface of copper foil via N-Cu coordination bond namely BTA-Cu.The results of DFT simulation showed that N atom of BTA molecular has a better affinity compared to Cu atom,which enables BTA as the nucleator of Li+.With the induced nucleation effect of N atom,Li+ deposited on the surface of BTA-Cu homogeneously,which suppresses the growth of dendrite.Besides,the average CE of BTA-Cu at the current density of 1 m A/cm2 and capacity density of 1 m Ah/cm2 could be maintained at 99.0% for more than 200 cycles.On the other hand,BTA-Cu@Li anode was prepared via electroplating method and then used in BTA-Cu@Li‖LFP full battery exhibiting 98.5% of CE under a 33% depth of discharge(DOD)at a 1C rate.Secondly,we have presented the passivation of the Li metal-carbon nanotube(CNT)composite with molecular self-assembly of a long chain aliphatic phosphonic acid.The dynamics of the self-assembly process was investigated via sum-frequency generation spectroscopy(SFG).The results of SFG showed that the aliphatic phosphonic acid molecules self-assemble on the Li metal surface via the lithium phosphate bonding,while the well-aligned long chains of the molecules help to prevent corrosion of lithium by oxygen and water in the air.As a result,the self-assembled monolayer(SAM)passivated Li-CNT composite displays excellent stability in dry or even humid air,and could be slurry-coated with organic solvents.The resulting slurry-coated Li anode exhibits a high CE of 98.8% under a 33% depth of discharge(DOD)at a 1 C rate in full battery cycling.The concept of molecular self-assembly on Li metal and the stability of the resulting SAM layer open vast possibilities of designed reagents for surface passivation of Li,which might eventually pave the way for practical application of Li metal in secondary batteries.Thirdly,utilizing the excellent processability of OPA-Li-CNT,we have adopted it as pre-lithiated agent and mixed with graphite via slurry-coating process.The results of XPS and SEM showed that SEI on graphite has formed before the battery cycling.With the 5 wt% of OPA-Li-CNT added,the initial CE of graphite was promoted from 87.5% to 100.1% and presented more than 80 stable cycles.Finally,OPA-Li-Ag-CNT composite was prepared for secondary battery anode.The doping of Ag nanowires enhances the lithiophilic of CNT particles,thus resulting in the increase of Li metal loading(with 30 wt% addition of Ag nanowires,the specific capacity of the OPA-Li-CNT increasing from 1890 to 2552 m Ah/g).On the other hand,the introduction of Ag nanowires could decrease the overpotential for Li+ plating/stripping,resulting in the excellent stability of materials.It is noteworthy that OPA-Li-Ag-CNT maintained the good processability of OPA-Li-CNT material.OPA-Li-Ag-CNT ‖ OPA-Li-Ag-CNT symmetry battery presented more than 150 stable cycles at a high current density of 5 m Ah/cm2.In addition,when the OPA-Li-Ag-CNT electrode was paired with an LFP cathode at a 1:2 cathode to anode capacity ratio,over 330 stable cycles were observed,which corresponds to a CE of 99.1%.