The Modified Cathode/Anode Materials And Structural Design For Li-S Batteries

Nowadays,in spite that Li–ion batteries have been popularly applied in practice,they cannot satisfy the demand for long-lasting and high energy storage due to the relatively low theory energy density.Li–S batteries are considered as one of the most promising choices,because of high specific capacity?1675 mAh/g?and energy density?2600mWh/g?,far exceeding those of prevailing Li–ion batteries.And sulfur is abundant,nontoxic,environmentally benign and low cost.However,the commercialization of Li–S batteries is confronted with challenges due to the poor conductivity of sulfur,volume expansion of about 79.2%from sulfur to Li₂S₂/Li₂S during discharge,and especially the shuttle effect between the sulfur cathode and lithium anode induced by soluble lithium polysulfide Li₂S_x?4?x?8?.Aimed at these problems,this dissertation researched the whole Li–S battery system,including the cathode,interlayer,separator and anode.The relation between the pore properties of micro-/meso-/macroporous carbon materials and their electrichemical performances was explored.Based on porous carbon materials,the compounds combined with Fe₂O₃,Al₂O₃ and TiN were designed to separately modify the cathode,interlayer and separator.And new preparation technologies were exploited,such as the techniques of electron beam evaporation and magnetron sputtering.In addition,an innovative strategy was presented to protect the Li metal anode and suppress Li dendrites by a copper coating.The major work and achievements are listed below:Three kinds of micro-/meso-/macroporous carbon materials were synthesized with different pore properties by facile hard-template method.Characterization of the carbon materials and electrochemical performances tests of the S/C cathodes were conducted in detail to study the effect of porocity on the electrochemical performances.It was found that in the case of identically high sulfur content,the pore size distribution substantially influences the performances of Li–S batteries.In the assembly of micro-/meso-/macropores,the micropore volume ratio to the total pore volume is dominant to the capabilities of batteries.Among the samples,the porous carbon C950prepared with the precursor of sucrose at 950?C presented the highest initial discharge specific capacity of 1327 mAh/g and retention of 630 mAh/g over 100 cycles at 0.2C rate along with the best rate capability,showing the best performances.This sample possesses the largest micropore volume ratio of 47.54%while a moderate specific surface area of 1217 m²/g and inferior total pore volume of 0.54 cm³/g.In situ Fe₂O₃ nanoparticles into pores were synthesized by solution impregnation to modify the C950 material.The effect of Fe₂O₃ on the performances of the S/C cathode was studied.And the compound host composed of C950 directly mixed with Fe₂O₃ was also investigated.The results show that the introduction of Fe₂O₃ can slow down the reduction of capacity.And the composite cathode with Fe₂O₃ into pores?Fe₂O₃@C/S?presents the best performances.It delivered an initial specific capacity of 1125 mAh/g and retention of 601 mAh/g over 100 cycles at 0.5C rate.A dual–faced carbon paper was prepared by depositing Al₂O₃ on one side of the carbonized filter paper via the technique of electron beam evaporation,serving as the interlayer.The influence of this interlayer on the preformances of the cathode of 70%sulfur content was studied.The results show that the optimized depositing time is 200 s and the corresponding interlayer battery delivered an initial capacity of 1253 mAh/g and retention of 700 mAh/g over 120 cycles at a current rate of 0.5C,presenting conspicuous electrochemical performances better than the single carbon interlayer battery and the traditional battery.Besides,the characterization of the cycled interlayer was performed to prove the absorption to polysulfides.A kind of micro-/mesoporous carbon was prepared with magnesium citrate tetradecahydrate as a precursor,which possesses high specific surface area of 1571 m²/g and total pore volume of 1.56 cm³/g.The micro-/mesoporous carbon mixed with highly conductive TiN nanoparticles was coated on the commercial Celgard separator for modifying.The perfomances of the modified separator in Li–S batteries were studied.The results show that the cathode of 60%sulfur content with the modified separator delivered a high initial capacity of 1130 mAh/g at 1 C and maintained 500 mAh/g after400 cycles.Further characterization and analysis indicate that the superior cycling stability is ascribed to the N-S bond formed between TiN and polysulfides.The Li anode was modified by a Cu coating via the technique of magnetron sputtering.The effects of Cu coating on protecting Li anode was investigated.And the performances of the full Li–S battery coupled with the S/C cathode was studied.The results show that the optimized depositing time is 300 s.The full Li–S battery delivered an initial discharge specific capacity of 1148 mAh/g and maintained 526 mAh/g after300 cycles with high Coulombic efficiency of 99.6%at 0.5 C,indicating enhancement of performances compared with the traditional Li–S battery.The characterization of the modified Li anode after cycling proves the Cu coating can suppresss growth of Li dendrites.