Design,fabrication and Operando Characterization of High-performance Lithium Sulfur Batteries Based on 3D Nanocarbon Architectures

Author:Wang Jian

Supervisor:zhang yue gang


Degree Year:2019





Lithium/sulfur(Li/S)battery is a promising next-generation energy storage system owing to the advantages of high energy density.However,its commercialization has been restricted by several problems such as the loss of active materials,rapid capacity decay,and unsatisfactory cycling stability.During the charge/discharge process,these problems mainly result from the depressed interconversion reaction kinetics among sulfur species,sluggish lithium ion kinetics,the huge delithiation barrier of Li2S,large volume changes and polysulfide shuttle effect.Although the hierarchical porous nanostructure and chemical doping solve a part of the polysulfides shuttling problem,the effect is still limited,especially when a Li/S cell reaches a higher areal sulfur loading(>3 mg cm-2)and works at a higher current density.Therefore,it is of great urgency to look for ideal matrix to help to realize higher performance.Herein,we have designed and fabricated a variety of highly active catalytic nanocomposites based on carbon materials to reduce the energy barriers of the reactions,improving the electrochemical behaviors at high areal loading.In addition,operando characterizations have been developed to investigate the electrochemical behaviors of lithium polysulfide in the electrolyte or on the electrode/electrolyte interface.The main achievements are listed as follows:1.We report a facile one-pot method to prepare porous three-dimensional nitrogen,sulfur-codoped graphene foam through hydrothermal reduction of graphene oxide with multi-ion mixture modulation.We show solid evidences that the multi-ion mixture modulation can not only improve the surface affinity of the nanocarbons to polysulfides,but also alter their assembling manner and render a more favorable three-dimensional porous morphology for accommodating and confining sulfur.The as-prepared sulfur cathode also has an excellent rate performance and cycling stability,showing 613 mA h g-1 at 5 C and maintaining a capacity fading as low as 0.028%per cycle at 2 C for 1500 cycles.Moreover,a high areal capacity of 5.1 mA h cm-2 at 0.2 C is achieved at an areal sulfur loading of 6.3 mg cm-2,which is the best value reported at that time for dual-doped sulfur cathodes.2.In-situ polymerization has been applied to improve the structural stability of the material.The partially carbonized polymer forms a functionalized carbon shell along with sulfur illusion loading,which helps to enhance the adsorption ability to lithium polysulfide.Furthermore,the lithium electrode coupled with high area loading has been studied,and the morphology and composition of surface SEI are preliminarily observed.The prepared large soft pack(77×50 mm2)battery can work for dozens of cycles and exhibits a high area capacity of 9.04 mA h cm-2 at 0.1 C.This result will provide direction for the development of practical high performance Li/S batteries.3.The highly active oxygen defect-rich nanoclusters catalyst has been proposed to promote the electrochemical conversion reaction kinetics of sulfur species.X-ray spectroscopies and electrochemical measurements confirm the existence and the roles of oxygen defects,which can significantly improve the activity of the matrix.As a consequence,the as-fabricated sulfur cathode exhibits excellent rate performance of 644 mA h g-1 at 10 C and superior cycling stability with an average capacity fading rate as low as 0.055%per cycle under ultrahigh rate of 10 C.More importantly,up to 11.02 mg cm-2,the Li/S cell can still deliver an areal capacity of 8.7 mA h cm-2 at 0.5 C(9.23 mA cm-2).Such performance is the highest among the metal oxide-catalyzed sulfur cathodes,opening a new route to boosting conversion reaction kinetics for various emerging ultra-fast batteries.4.For the first time,we show that the single atom catalyst can play a compelling role in boosting the electrochemical conversion kinetics and achieving previously-unreachable ultra-high-rate performance.Both experimental and theoretical results prove that atomically-dispersed single iron atoms promote the delithiation of relatively-inert Li2S cathode and accelerate the reversible electrochemical conversion reactions during its long-term cycling.The work demonstrates the effectiveness of single atom catalysis in improving the performance of Li2S/Li batteries(588 mA h g-1 at 12 C and capacity fading rate of 0.06%per cycle for 1000 cycles at 5 C)and opens a new route to breaking the rate limit for other future conversion-based high energy batteries.5.We report single metal atom catalyst to render superior performances of high converted-Li2S loading electrodes from an inexpensive chemical source(Li2SO4).Electrochemical measurements and theoretical simulations reveal that the kinetics of lithium ion and transformed conversions is significantly enhanced in the aid of the SACo catalyst.As a result,the as-prepared converted-Li2S electrode exhibits high rate capacity(441 mA h g-1 based on Li2S at 10 C)and long lifespan with low average capacity fading of 0.04%(for 1500 cycles at 2 C).More importantly,the cell with high as-converted Li2S loading displays high rate capacity(340 mA h g-1 based on Li2S at 23.4 mA cm-2)and long life(2.26 mA h cm-2 at 6.77 mA cm-2 after 200 cycles),which are among the highest level in achieving high Li2S loading cathode in overall assessments.6.Distinct from the previous studies on monitoring sulfur species in either solid or liquid phase,operando sum frequency generation(SFG)is applied here to selectively probe the cathode/electrolyte interface in a working Li/S cell.We have observed that the affinity of lithium polysulfides on a sulfur cathode surface is much higher than that of the LiTFSI molecules,as also confirmed by theoretical simulations.We have found that,unlike the electrochemically generated polysulfides that only yield an unstable adsorption layer,the pre-added lithium polysulfides can derive a stable passivation interphase layer between the cathode and electrolyte,which alleviates further polysulfide shuttling and diminishes the interfacial energy barrier.Our work provides much-needed knowledge regarding the role of polysulfide species at electrode/electrolyte interfaces in Li/S cells,and opens a more practical route for in-situ probing of electrochemical interfaces using the SFG spectroscopy.