Study On Cathode Material And Structure Design Of Li-S Battery

With the pursuit of energy density of battery system,lithium-sulfur battery and lithium-air battery system have attracted more and more attention.The lithium-sulfur battery system is considered to be the next generation of industrial battery system.Despite a great deal of work on lithium-sulfur battery research has also made some breakthroughs,but the sulfur cathode with low electronic conductivity,shuttle effect,low rate performance and active material utilization is still a thorny problem.In addition,the high cost of complex preparation methods is gradually emerging.In addition,most of the current research focuses on button batteries,high capacity flexible Li-S batteries lack of research.In view of the above problems,we have carried on the preliminary exploration from the Li-S battery cathode design and development,diaphragm modification and improvement,finally to the preparation of flexible battery and its application in wearable electrical equipment.The thesis is divided into four parts as follows:1.The treatise outlined working principle,advantages and defect of the lithium-sulfur.Recent research in cathode,Li anode and diaphragm of lithium-sulfur batteries were summarized.And these problems and solutions were proposed.2.Porous Molybdenum Carbide Nanorods as Novel“Bifunctional”Cathode Material for Li-S Batteries.The shuttle effect of polysulfide is the primary obstacle in Li-S batteries.Although some effort has been made to suppress the shuttle process,more highly conductive materials with stronger adsorption ability to polysulfides are still in urgent demand.Another important problem is the deteriorating performance of Li-S batteries with prolonged cycling owing to irreversible deposition of lithium sulfide?Li2S?.However,a host material for the effective conversion of insoluble Li2S has been elusive up to now.Herein,porous molybdenum carbide nanorods?Mo2C NRs?,with high catalytic activity for Li2S and ultrastrong adsorption for polysulfides,are used as a“bifunctional”host material and incorporated into sulfur cathodes for the first time.The“bifunctional”Mo₂C NRs have the advantage of immobilizing polysulfides over the conventional host,with adsorption energies from-4.89 to-8.20 eV for Li2Sx?x=1,2,4,6,and 8?.The electrochemical analysis shows that Mo2C NRs,more than an anchor,can activate Li2S by greatly reducing the charging overpotential.Therefore,the irreversible deposition of polysulfides is effectively restrained and the utilization of Li2S is clearly enhanced.The Mo2C NRs-sulfur composites?Mo2C NRs-S?cathodes provide a capacity of 1298 mAhg/1 at 0.1 C in the Li-S batteries.More importantly,the battery with Mo2C NRs-S cathodes exhibits a much lower capacity decay of 0.062%per cycle at 1 C over500 cycles.3.Anode Red Phosphorus of Lithium-ion Battery as Adsorbent of Polysulfides for Lithium-Sulfur Battery.The shuttle effect is a major obstacle for the practical application of lithium–sulfur?Li-S?batteries with low cost and high energy density.Some compounds were reported as the polysulfide adsorbents to effectively inhibit the lithium polysulfide?LiPSs?diffusion.However,not only efficiency but also cost need to be taken into account in practical applications of Li-S batteries.Here,red phosphorus?RP?anode of a lithium-ion?Li-ion?battery is used as a cheap and new adsorbent for Li-S batteries.We introduce RP nanoparticles to decorate the conventional carbon-coated separator and achieve long cell lifetime.We theoretically elucidate and experimentally verified that the inexpensive RP ensures strong chemisorption towards LiPSs by both a Lewis acid-base interaction and sulfur-chain catenation.Moreover,the high ionic conductive by-products Li3PO4 in oxidation provide effective ion transportation channels for the modified separator.The cells using a carbon-sulfur electrode with this modified separator showed good rate capability,delivering a capacity of 809 mA h/g at 2 C against 1287 mA h/g at 0.1 C,and excellent cycling performance?729 mAh/g after 500 cycles at 1 C with 82%capacity retention?.This simple and low-cost adsorbent can thus promote the development of Li-S batteries at the industrial level.This work may shed a light upon the new path to solve the shuttle effect of Li-S batteries.4.Hierarchical and Highly Stable Conductive Network Cathode for Ultraflexible Li-S Batteries.Flexible Li-S batteries have great potential for next-generation energy storage which can meet the rising demand of rollable displays and wearable electronic devices because of the high theoretical energy density and competitive price.Here,we design and fabricate an integrated electrode with hierarchical structure and interconnected 3D conductive networks as a cathode of flexible Li-S batteries.The composite cathode exhibits high electrochemical performance and cycling stability.The initial reversible discharge capacity is 1312 mAh/g at0.2 C with sulfur load 2.0 mg/cm²,and the capacity decay rate is 0.09%per cycle within 500cycles at current of 1 C.Notably,the composite electrode can sustain 15.2 MPa stress with 10%strain and retain structural integrity after 200 000 bending cycles,the highest number of bending cycles found to date for any flexible S cathodes.The soft package batteries with different sizes and shapes are fabricated,and they exhibit extraordinary flexibility and stability after bending and flattening over 2100 times.Moreover,their potential applications in rollable displays,flexible lighting,and wearable electronic devices are also investigated.