| 其他摘要 | Currently, lithium ion batteries (LIB) are widely used in various portable electronic devices, because they have the following merits, such as high voltage, large energy density, long lifespan, light weight, low self-discharge rate, and no memory effect. With the continuous expansion of their application fields, the performance requirements of LIB are constantly enhanced. To achieve high-performance LIB, it is definitely necessary to implement R&D in battery materials (e.g. cathode, anode, and electrolytes), and on the other hand, it is also imperative to deepen the understanding of related electrochemical reaction processes and phenomena, which is helpful to guide the R&D in modifying traditional materials and designing novel materials.
In this dissertation, we first review the recent development of LIB and related battery materials, and then introduce what we have done in the investigation of electrochemical interphases and in the modification and design of anode materials.
(1) With the aid of focused ion beam (FIB) technique, we investigated the formation and evolution of solid electrolyte interphase (SEI) film on the surface of natural graphite (NG) spheres that experienced different electrochemical cycles. More importantly, we discovered that the SEI film can also be formed inside the NG spheres, and thus proposed a new concept of “internal SEI film”. Finally, we proposed capacity fading mechanisms of NG spheres on the basis of our understanding of the SEI film.
(2) According to the overall results from FIB, XPS, Raman, XRD, and TG-DSC, we clarified the debate on interfacial interaction between propylene carbonate (PC)-based electrolytes and graphite anode: PC does have the ability of co-intercalation; The intercalation site is around the crystal boundary; The main wire-puller for the bad interfacial compatibility is the methyl group in PC molecule, which makes decomposition products of PC puffed and thus incapable of inhibiting the co-intercalation. And the co-intercalation subsequently triggers the occurrence of a chain reaction.
(3) In view of the capacity fading mechanisms of NG, we adopted a surface-coating strategy to modify the NG: (i) Using a fluidized bed reactor, the NG spheres were coated in one step by a uniform layer of pyrolytic carbon with a thickness of ~250 nm. The coated sample displayed excellent cyclability, with the capacity above 320 mAh/g at the 15th cycle in comparison with less than 200 mAh/g for the pristine NG. Meanwhile, the initial Coulombic efficiency was also improved to 88% from original 80%. (ii) Through adding isothermal heating steps and properly adjusting heating rates in a carbonization procedure, we also achieved a homogeneous coating of PVC-carbonized products on the surface of NG spheres. Furthermore, the influence of different carbonization procedures on the structure, morphology, specific surface area, pore size distribution, and hydrogen content of coated samples was systematically studied. Electrochemical performance measurements indicated that the coated sample under the particular carbonization procedure including isothermal heating steps at 280 and 450 ℃ was better than a commercial MCMB in terms of reversible capacity and rate capability.
(4) Inspired by a special biological structure in nature, we proposed a kind of urchin-like nano/micro hybrid design to modify conventional electrode materials. Using catalytic chemical vapor deposition to in situ grow carbon nanofibers (CNF) on the surface of micrometer-sized NG spheres (or Cr2O3 particles), we fabricated the nano/micro hybrid composite with an urchin-like structure. The typical characteristics of the as-grown CNF were ~100 nm in diameter and several micrometers in length, having a hollow core throughout the length and a herringbone-type graphene structure. It was found that this kind of design was especially helpful for improving electrochemical cyclability and had a wide generality.
(5) On the aspect of exploring high-capacity anode materials, we for the first time synthesized nickel (Ni) silicide nanobelts and nanosheets based on the chemical reaction of Ni substrate with SiHCl3 under H2 atmosphere. Their morphological, structural, and compositional features were detailedly characterized, and their possible growth mechanisms were discussed. The Ni silicide nanosheets (comprising Ni3Si and Ni31Si12), subjected to electrochemical measurements, demonstrated a capacity of above 540 mAh/g even at the 20th cycle.
In conclusion, the meaningful results obtained in the dissertation not only deepen the understanding and knowledge of electrochemical interphases between electrodes and electrolytes, but also are helpful and revelatory for the modification of conventional electrode materials and for the design of novel materials that can be used in next-generation high-performance LIB. |
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