Studies On Lithium-Carbon Composite For Secondary Lithium Metal Battery Anode

In the past few decades,considerable demand for energy has been driven up by the fast-developing society.However,the gradual exhaustion of traditional fossil energy and the serious pollution to the environment have destroyed the ecological harmony between nature and humanbeings.Therefore,the development of an environmentally friendly and efficient energy system is crucial to our survival and development.Lithium-ion battery(LIB),as an efficient energy storage device,has been ubiquitous in our daily life and gone through tremendous changes since its invention more than 200 years ago.LIB with high energy density,long cycle life and excellent safety characteristics have been widely pursued by scientists and industrialists.Lithium(Li)metal is known as the"Holy Grail" for the anode in a LIB,because it has high energy density and low negative electrochemical potential.However,the practical applications of metallic Li anodes have been hindered by several serious challenges in the past few decades,such as instability of interface due to high reactivity of lithium itself,the tendency of infinite volume expansion,and lithium dendrite growth owing to uneven Li stripping/plating.Our group recently has prepared a lithium-carbon nanotube(Li-CNT)composite as an alternative to the Li metal anode via a facile and scalable molten impregnation method.The Li-CNT composite shows almost no volume expansion and lithium dendrite suppression during Li dissolution/deposition,improving the Coulombic efficiency(CE)of the Li metal significantly.In order to further improve the electrochemical performance of Li-CNT composite,we mainly focused on the following aspects in this thesis:(1)The wettability of molten lithium with four different kinds of carbon materials,acetylene black(AB),carbon nanotube(CNT),ketjen black(KB)and conductive carbon black(SP)was studied,with the main influencing factors discussed.(2)A CNT-AB framework consisting of lithiophilic CNTs skeleton and lithiophilic nano-sized AB particles as a stable host for lithium were designed and fabricated.When mixed with the molten lithium,the Li-CNT-AB composite exhibits a specific capacity as high as 2800 mAh g-1.The use of AB particles not only utilizes the internal free space of the CNTs framework,but also lowers the nucleation energy of lithium,which helps to regulate the deposition of Li metal.When paired the Li-CNT-AB with a commercial LiFePO4(LFP)cathode,a CE of?98.7%can be achieved,which is higher than that of Li anode(92.5%).In addition,there is negligible volume expansion on the Li-CNT-AB electrode during cycling by ex situ SEM examination.(3)A silicon loaded Li-CNT composite(Li-CNT-Si)by introducing Si nano particles into the Li-CNT composite was constructed.The introduction of Si nano particle not only increases the lithium content thus the specific capacity of the composite material(with 10 wt%addition of Si,the specific capacity of the Li-CNT increased from 2000 mAh/g to 2600 mAh/g),but also decreases the polarization for Li plating/stripping,resulting in an improved electrochemical performance.Meanwhile,the introduction of Si nano particles fill the pores inside the Li-CNT microspheres,reducing the infiltration of electrolyte into the composite microspheres to corrode the lithium metal inside the spheres and contributing to the enhanced CE.When the 10 wt%silicon-loaded Li-CNT-Si composite was prepared as an anode and coupled with a commercial LFP cathode,the resulted battery presented more than 900 stable cycles in an ether-based electrolyte at a charge/discharge rate of 1C,corresponding to a CE of 96.7%,which is much higher than that of the Li-CNT anode(90.1%)and the lithium metal foil anode(79.3%)obtained under the same condition(4)The nature of the SEI layer generated on the lithium metal electrode was regulated by adding a certain concentration of lithium hexafluorophosphate(LiPF6)in the ether-based electrolyte with 1 M lithium bis(trifluoromethane sulfonamide)(LiTFSI).The XPS results of the Li anodes with different cycle numbers suggest that the content of polyethers in the SEI increases significantly,which enhances the compactness and flexibility of the SEI and alleviates the side reactions effectively.The cycling behavior of O-Li-CNT || O-Li-CNT battery can remain stable for more than 450 cycles at a high current density of 10 mA cm-2.When paired the O-Li-CNT with a commercial LFP cathode in the LiPF6-added LiTFSI salt electrolyte,the battery can be stable for 300 cycles at a current density of 2.5 mA cm-2,corresponding to a high CE of?99.0%.(5)The Li-CNT composite was protected with a self-assembled OPA monolayer,giving rise to an excellent stability in dry air.Even in the oxygen environment,there is no dendrite formation and negligible volume expansion in the O-Li-CNT electrode during cycling.When the O-Li-CNT composite anode was coupled with another CNT microsphere(CNTm)-based O2 cathode,the reversibility and cycle life have been drastically improved for the resultant Li-O2 batteries.The obtained O-Li-CNT || CNTm battery shows a much enhanced electrochemical reversibility of?95.0%and stable cycling at a capacity of 500 mAh g-1carbon for more than 1800h.