| In recent years,lithium-ion batteries have been widely used in electronic products,electric vehicles,and grid energy storage.It is increasingly important for developing new-type lithium batteries with high specific energy density,long cycle life,and high safety.Li-S batteries are considered as the most promising candidate for next-generation lithium battery systems due to their high theoretical specific capacity(1675 mAh g-1)and energy density(2600 Wh kg-1 or 2800 Wh L-1).Selenium(Se)is in the same main group(group VIA)of the periodic table as sulfur(S),which has many desirable merits similar to S or even better.For instance,the electronic conductivity of Se(1×10-5 S cm-1)is almost 25 orders higher than S(5×10-30 S cm1).In addition,the bulk energy density of Se(3270 mAh cm-3)is highly comparable to S(3467 mAh cm-3).Therefore,Se is also considered as the most promising candidate for cathode material for high-performance Li batteries.However,the widespread use of Li-S and Li-Se batteries is hindered by the shuttle effect of soluble polysulfides and polyselenides,the poor conductivity and the large volume change of sulfur and selenium,and the slow redox reaction kinetic of sulfur and selenium.In this thesis,three research works were carried out to mitigate the above issues in Li-S and Li-Se batteries,which were summarized in the followed three parts:(1)To address the problem of the "shuttle effect" in Li-S batteries using liquid electrolytes,Li6PS5Cl argyrodite solid-state electrolyte was employed to avoid the issues of the dissolution of intermediate product.Nitrogen-doped hierarchical porous carbon(NHPC)was synthesized by using silica as a template and polyaniline as a carbon source.The as-prepared NHPC has a high specific surface area of 628.8 m2 g-1 and a large pore volume of 1.62 cm3 g-1,which was the optimal host for S.The mass loading of S in S/NHPC composites can reach up to 70.9%through a simple melting diffusion method.The assembled all-solid-state Li-S batteries deliver a large specific capacity of 1328 mAh g-1 at a current density of 50 mA g-1.This Li-S battery still maintains a specific capacity of 268 mAh g-1 with a capacity retention of 44.73%after 150 cycles at a current density of 200 mA g-1.(2)To address the "shuttle effect" and volume change of the cathode in the Li-Se batteries,NHPC and Li6PS5Cl were introduced to the fabrication of high-performance Li-Se batteries.The as-prepared Se/NHPC composites with 80.99%Se loading were obtained by a simple melting diffusion method.The assembled all-solid-state Li-Se cells deliver a specific capacity of 256 mAh g-1 at current densities of 200 mA g-1.The capacity retention rate was 84.8%after 70 cycles at a current density of 67.5 mA g-1,indicating the good cycling performance of the Se/NHPC cathode based all-solid-state Li-Se battery.According to ex-situ Raman,XPS,and SEM analysis,we found that the trigonal Se was transformed into Se chains in the carbon matrix as Se was compounded with NHPC.Se chains were then transformed into Se-rings after the discharging/charging process.After long-term cycling,Se will separate from the carbon matrix and then aggregate,which may be associated with the weak binding of Se rings to the carbon matrix(3)The above studies show that amorphous Se is formed when it is combined with the porous carbon matrix.It’s worth noting that the electron conductivity of amorphous Se is lower than crystalline Se,which is not conducive to electrochemical reactions,especially high-rate performance.To address this issue,we propose to use MXene,a metal carbide with high electrical conductivity,as a host for Se.The accordion-like multilayer Ti3C2 MXene was obtained by HF etching of Ti3AIC2 MAX,which has a hexagonal crystal structure.The Se@MXene composites were synthesized by the melting diffusion method.The gaps between the layers provide the space for subsequently accommodating Se and alleviating the volume change during the electrochemical process.SEM and TEM results showed that Se nanocrystals were homogeneously distributed into the interlayer of MXene.Thermogravimetric tests revealed that the Se mass loading in Se@MXene composite could reach up to 63.3%.An allsolid-state Li-Se cell was assembled by using Se@MXene composite as cathode,Li6PS5Cl as the electrolyte,and a Li-In alloy as the anode.All-solid-state Li-Se cell delivers a high specific capacity of 638 mAh g-1 at the current density of 20 mA g-1.Even at a high current density of 200 mA g-1,this cell maintains a specific capacity of 429 mAh g-1.Such good electrochemical performance of this cell may be attributed to the high electrical conductivity of MXene and the stability of Se nanocrystals in the composite. |
