Research on the heterogeneous doping framework of the positive electrode of high-load large-rate lithium-sulfur batteries

Applications such as mobile electronic devices, low-emission electric vehicles, and smart grids place higher demands on the energy density of energy storage systems, prompting people to continue to explore new battery systems that can surpass existing lithium-ion batteries. Lithium-sulfur batteries based on multi-electron conversion reactions show extraordinary potential due to their high energy density (2567 Wh / kg) and resource richness. However, the shortcomings such as the insulation of elemental sulfur and its discharge product (Li2S2 / Li2S), the "shuttle effect" caused by soluble lithium polysulfide during charge and discharge, and the volume expansion of the electrode greatly hindered the commercialization of lithium-sulfur battery development of. Doping heteroatoms or metal nanoparticles as active sites into the shaped porous carbon framework can effectively solve the above problems, while ensuring uniform loading of high sulfur content and good conductivity, it can also chemically adsorb polysulfides, effectively Limit the "shuttle effect" and accelerate the catalytic polysulfide conversion reaction to improve rate performance (Journal of Energy Chemistry 2019, 38, 94-113). Recently, the team led by Li Chilin, a researcher at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, has made a series of research progress in the synthesis design of the hetero-doped framework of the positive electrode of high-load large-rate lithium-sulfur batteries, and related results have been published in Energy Storage Materials and ACS Nano. And other periodicals.

Hollow carbon spheres are ideal for high-load sulfur host materials due to good electrical conductivity, high specific surface area, adjustable pore structure, and low density. Currently, hollow carbon spheres are mainly made by the template method, the process is complicated and the reagents are harmful. The team provided a synthesis scheme for the synthesis of N / O heteroatom double-doped hollow carbon microspheres based on the self-sacrificing template method. Using micron-scale spherical carbon-nitrogen polymer (g-C3N4) as a self-sacrificial template and a nitrogen-rich precursor, the outer layer is coated with PDA, and the inner carbon-nitrogen polymer template is removed by a simple high-temperature carbonization method, while the external PDA Carbonized into porous nanosheets, resulting in N / O heteroatom double-doped hollow carbon microspheres (HCMs). HCMs have a shell composed of a large number of porous nanosheets, have high specific surface area (873 m2 / g) and pore volume (4.84 cm3 / g), and their N and O contents can reach 5.36 and 6.99 atom%. The obtained HCMs-S electrode has a reversible capacity of approximately 900 mAh / g at a large 2C rate and a retention capacity of approximately 530 mAh / g after 900 cycles due to its physical confinement effect, lithium affinity adsorption, and catalytic conversion. Even under the conditions of high sulfur content (90%) and loading (4.84 mg / cm2), an excellent reversible capacity of 700 mAh / g was obtained. The long-term high-rate cycle does not affect the uniform deposition of positive-end nanostructured Li2S, and no lithium is deposited on the negative terminal. This work developed a simple method that can achieve micron-level CS particles and high-capacity lithium-sulfur batteries. Its carbon host microstructure is characterized by a three-dimensional skeleton self-assembled by two-dimensional structural units, and has a rich quality of internal connectivity. Charge transfer channel. Related results were published in Energy Storage Materials (2019, DOI: 10.1016 / j.ensm.2019.06.009).

Two-dimensional layered porous carbon has the advantages of large specific surface area, good conductivity, and easy spreading control of catalytic active sites. It can effectively load sulfur molecules, slow down the volume expansion effect of sulfur electrodes, and accelerate electron and ion transport. The structure can also increase the contact between polysulfides and catalytically active sites. The team used MgCl2 · 6H2O as both a high-temperature solvent and a pore-forming agent, which acted as a cross-linking template when carbonizing nitrogen-rich biomass adenine at 900 ℃ at high temperature, and was supplemented with a small amount of CoCl2 · 6H2O. Finally, co-doped Co / N co-doped graphitized porous carbon (Co-CNCs), which is a two-dimensional layered structure of amphiphilic lithium-sulfur-philic sites, has a wrinkled overall morphology. Benefiting from its rich interlayer volume, stable graphitized structure, uniformly distributed chemical adsorption sites and electrocatalytic active sites, the initial capacity of S @ Co-CNCs at 0.2C is 1290.4 mAh / g, at a large magnification of 2C It can stably cycle for at least 600 cycles, and has a very small cycle decay rate (0.029%). Its rate performance test can tolerate a very high rate of 20C. In addition, even at an ultra-high sulfur content of 92.33 wt%, the HS @ Co-CNCs electrode can stabilize the rate cycle at a very low E / S (electrolyte volume / sulfur mass) ratio of 5 μl / mg; HS @ Co- CNCs can circulate at least 100 cycles with a surface capacity of 6 mAh / cm2 under a high area load of 9.7 mg / cm2. The composite positive electrode can maintain a complete initial morphology under a large-rate cycle for a long time, realize uniform S / Li2S electrodeposition, and the negative electrode of the lithium-sulfur battery has no obvious dendrite formation. Related results were published in ACS Nano (2019, 13, 9520-9532).

Relevant research work is supported and supported by the National Key R & D Program and the National Natural Science Foundation of China.

N / O heteroatom double-doped hollow carbon microspheres composed of porous nanosheet shells are used as large particle cathode host materials for high-load lithium-sulfur batteries

Adenine-derived, porous carbon with amphiphilic lithium-sulfur-philic sites and a two-dimensional layered structure interwoven as a positive host material for high-load lithium-sulfur batteries

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