Lithium-ion batteries (LIBs) have been widely used for electric vehicles and consumer
electronics, but the further application is limited by long charging time and poor cycle
performance. The kinetics of Li
+ ions and electrons and the stability of electrode materials need to be significantly improved in order to achieve high rate and long lifetime LIBs.
In the first research project, the calcination strategy for synthesizing LiNi
0.5Mn
0.3Co
0.2O
2 (NMC532) cathode material is first studied. The formation of Li
2O during calcination is found to be detrimental to the material quality of NMC532. A three-step calcination strategy is devised to prepare layered NMC532 without the formation of Li
2O. The as-synthesized NMC532 possesses a highly ordered layered structure with minimized Ni
2+/Li
+ c...[
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Lithium-ion batteries (LIBs) have been widely used for electric vehicles and consumer
electronics, but the further application is limited by long charging time and poor cycle
performance. The kinetics of Li
+ ions and electrons and the stability of electrode materials need to be significantly improved in order to achieve high rate and long lifetime LIBs.
In the first research project, the calcination strategy for synthesizing LiNi
0.5Mn
0.3Co
0.2O
2 (NMC532) cathode material is first studied. The formation of Li
2O during calcination is found to be detrimental to the material quality of NMC532. A three-step calcination strategy is devised to prepare layered NMC532 without the formation of Li
2O. The as-synthesized NMC532 possesses a highly ordered layered structure with minimized Ni
2+/Li
+ cation mixing and oxygen vacancy, and also excellent electrochemical performance. This rational three-step calcination strategy is applicable to not only LiOH∙H
2O but also other Li precursors like LiNO
3 and Li
2CO
3.
Second, the electrochemical performance of NMC532 are further enhanced through
constructing hierarchical spherical morphology, improving elemental distribution, and
exposing {010} planes using N-methyl-2-pyrrolidone (NMP)- and urea-based solvothermal
method. The heating treatment in H
2 atmosphere for quasi melting Ni and Co elements results in much more homogeneous elemental distribution. The use of NMP could induce the exposure of {010} planes, and also tackles the issue of Ni deficiency when using polyvinylpyrrolidone
(PVP) as capping agent. Owing to the greatly facilitated Li
+ diffusion and enhanced structural stability, the NMC532 cathode shows excellent rate capability and good cycle performance.
Third, high-quality LiNi
0.8Co
0.15Al
0.05O
2 (NCA) with exposed {010} planes, desired stoichiometric ratio, and few structural defects are obtained by using NMP- and urea-based
solvothermal method. Excellent rate capability and good cycle performance are achieved due to the facilitated Li
+ diffusion and enhanced structural stability. Using urea as precipitation
agent has been demonstrated to be highly suitable for preparing high-quality NCA precursors
with ideal stoichiometric ratio and enhanced elemental distribution. Compared with the
commonly used NaOH, urea does not result in Al deficiency.
Fourth, transition metal oxides (MO
x) microparticles (i.e., NiO microspheres, binary NiO/CuO
microspheres, and MnCo
2O
4 tetradecahedrons) with diverse interior structures including solid yolk-shelled structure, porous yolk-shelled structure, and hollow structure are obtained through controlling the weight percent of the coordinated metal in the metal-benzyl alcohol precursors. In particular, no surfactant or expensive organic metal salt is used. The as-prepared MO
x microparticles, especially the microparticles with the well-defined porous yolk-shelled structure, i.e., porous yolk@void@shell structure, exhibit excellent electrochemical performance due to the much enhanced kinetics and structural stability.
Fifth, a facile solvent-directed self-assembly method is applied to prepare N-doped carbon-encapsulated nickel selenides (NiSe) with varied morphologies. Morphology is found to have
an important impact on electrochemical performance due to different kinetics, structural
stability, and solid electrolyte interphase (SEI) growth. By constructing the well-defined
hierarchical porous flower-like morphology and introducing N-doped carbon encapsulation,
the rate capability, cycle performance, structural stability, and chemical stability of NiSe can
be effectively enhanced.
This devised strategies for boosting the rate and cycle performance of electrode materials are
believed to be effective for fast and long life-time lithium storage.
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