Summary
A nitrogen‐doped carbon coated subglobose Na3V2(PO4)2F3@C (NVPF) cathode for sodium‐ion batteries was synthesized by using hexadecyl trimethyl ammonium bromide (CTAB) as soft template and ...polyvinylidene fluoride (PVDF) as carbon source. CTAB plays a significant role on the formation of sphere micelles. Precursor ions are self‐assembled on the surface at appropriate concentration and its mechanism is investigated in subglobose NVPF@C‐4. CTAB also increases the conductivity of carbon layer as −(CH3)3N+ in CTAB is combined with residual carbon from PVDF to form partially N‐doped carbon. Meanwhile, the carbon source PVDF contributes to prevent the generation of impurity Na3V2(PO4)3 by compensating the evaporative fluorine. Generally, CTAB and PVDF play multifunctional roles in regulating Na3V2(PO4)2F3@C cathode with well‐developed crystallite, high rate performance, good conductivity, and ultra‐long cycle life. The specific capacity of NVPF@C‐4 cathode at 0.1 C and 10 C is as high as 121.5 mAh·g−1 and 99.2 mAh·g−1 with high capacity retention of 90.1% even after 1000 cycles at 10 C. The excellent rate performance is also attributed to the high diffusion coefficient of Na+ and high exchange current according to the kinetic analysis. The enhanced electrochemical performances reveal the special regulation in this paper is feasible to obtain excellent structural stability of NVPF materials.
Spherical micelles are formed through the regular arrangement of hydrophobic long carbon chain (C16H33−) and hydrophilic trimethylamine (−(CH3)3N+) of CTAB. The hydrophobic PVDF are homogeneously dispersed in the aqueous solution under the dispersing role of CTAB. Through the self‐assembled of precursor anions (PO43−, F−) and cations (Na+, V3+) on the surface of micelle, subglobose Na3V2(PO4)3F3 cathode for sodium‐ion batteries with well‐developed crystallite, ultra‐long cycle life and high rate performance are constructed after the sintering process.
The graphene sheets (GNs) and toluene-soluble component of pitch (TS) are used as the mixed carbon source in the synthesis of LiMn
0.8
Fe
0.2
PO
4
@C@GNs through different carbon-coating methods. The ...results show that the ball–milling (BM) method is more favorable for the uniform dispersion of GNs and reduction of agglomeration compared to mechanical agitation (MA). The bridging effect of GNs and the pyrolytic carbon of TS with a high graphitization degree constructs an intact carbon skeleton on the surface of LiMn
0.8
Fe
0.2
PO
4
crystal, thus greatly improving the structural stability and electronic conductivity. Sample BM–1.5 containing 1.5 wt.% GNs prepared by ball–milling has a small (111) grain size, large specific surface area (48.32 m
2
·g
−1
), and mesoporous volume (0.099 cm
3
·g
−1
). Cathode BM–1.5 delivers the discharge-specific capacity of 150.9, 150.4, and 115.7 mAh·g
−1
at the rate of 0.1 , 1, and 5 C, respectively, which are the optimal values of all samples. Moreover, the capacity in BM–1.5 at 1 C is well maintained at the retention of 95.7% after 100 cycles. And no obvious crack appears in the cathode film, showing excellent structural stability.
Graphical abstract
LiMn
0.8
Fe
0.2
PO
4
@C@GNs prepared by ball-milling with the toluene-soluble component of pitch and graphene sheet as the mixed carbon source has excellent electrochemical performance.
Summary
The nanocrystalline material LiMn0.8Fe0.2PO4/C was prepared by the solvothermal method using the toluene‐soluble component of pitch or sucrose as a carbon source. The effects of carbon source ...types and contents, and the carbonization‐heating rate between 350°C and 500°C on the physicochemical and electrochemical properties of the obtained materials were investigated. When the carbonization heating rate was between 350°C and 500°C is 0.5°C·min−1, cathode TS‐200‐0.5 coated by 200 mg toluene‐soluble component of pitch showed the optimum electrochemical performance with discharge specific capacity of 156.4, 149.2, and 116.4 mAh·g−1 at 0.1, 1, and 5 C, respectively. After 150 cycles at 1 C, the specific discharge capacity was still 135.1 mAh·g−1, and the capacity retention rate was 92.7%. Compared with sucrose, the toluene‐soluble component of pitch is an aromatic compound, and its pyrolytic carbon has streamlined structure with high graphitization. Moreover, small amounts of sulfur and nitrogen doped exist in pyrolysis carbon of the toluene‐soluble component of pitch. Cathode TS‐200‐0.5 exhibits high ionic and electronic conductivity and stability.
The chemical structure of the toluene‐soluble component of pitch is composed of aromatic compounds and aliphatic compounds with many side chains or side groups. Its pyrolytic carbon exhibits streamlined structure with the characteristics of short‐range ordered and long‐range disordered, which is constructed through further condensation, movement, orientation, and orderly stacked of aromatic hydrocarbon molecules. LiMn0.8Fe0.2PO4/C cathode for lithium‐ion batteries with well‐developed crystallite, excellent conductivity, high capacity, and rate performance are constructed after the sintering process.
Artificial graphite scrap prepared from petroleum coke with low degree of graphitization was further graphitized under various conditions. Different categories of coke were also treated with the ...optimum technology. The prepared samples were characterized with X-ray diffraction, ash content determination, morphology observation, and galvanostatic charge and discharge. It was shown in the experiments that the heat treatment temperature should be increased to 2800 °C to remove impurities. Slow heating rate and evacuation technology were beneficial to the growth of graphite crystallite and the improvement of discharge capacity. And the latter condition possessed the larger influences, especially on the growth of crystallite dimension in the b axis direction, degree of graphitization, and discharge capacity. The sample D-3000 prepared from pure needle coke possessed the maximum discharge capacity of 342.1 mAhg
−1
among all prepared samples. The linear regression equations between the volume of graphite crystallite and discharge capacity were established.
Pitch is constituted of aromatic hydrocarbon, which is one kind of organic liquid with high viscosity. The soft coal pitch was divided into different group components by solvent extraction, and the ...different fractions of soft coal pitch were carbonized separately. The relationship between the microstructures of carbonization products and the chemical compositions of group components was investigated in this paper. Results show that the structure and morphology of the carbonization products from pitch were significantly influenced by the chemical composition of the group components. The main microstructure of the toluene soluble (TS) fraction-derived carbonization products is streamline and partial regional type. The microstructure of products from toluene insoluble (TI) fraction is mosaic and regional type. The structure of toluene insoluble-quinoline soluble (TI-QS) fraction-derived carbonization product is mainly streamline type. Nevertheless, the carbonization product from quinoline insoluble (QI) fraction is constituted with isotropy micron grade isotropic particles.
LiMn0.8Fe0.2PO4 nanocrystal composite cathode material for lithium ion batteries was prepared via a simple solvothermal method. The quinoline soluble substance from coal pitch (QS) was used as the ...coated carbon source to prepare LiMn0.8Fe0.2PO4/C. The structural performance of QS and QS-C, and the effects of QS-C on the physical, chemical and electrochemical properties of LiMn0.8Fe0.2PO4/C were investigated. The research shows that QS is an aromatic compound containing C–N and C–S bonds, and the structure of QS-C is a streamlined structure with high graphitization degree. Hence, the N, S-dual doped carbon conductive network can be effectively established to enhance charge transfer and the diffusion and migration rate of lithium ion. The prepared LiMn0.8Fe0.2PO4/C delivery excellent specific discharge capacity of 159.2, 145.7, and 117.3 mAh·g-1 at 0.1, 1, and 5 C, respectively. After 150 cycles at 1 C, the specific discharge capacity was still 133.5 mAh·g-1, and the capacity retention is 93%. This work provides a new insight into the search for coated carbon sources.
•High-performance LiMn0.8Fe0.2PO4/C is constructed by using quinoline soluble substance from coal pitch (QS) as carbon source.•QS contains C-S and C-N bonds, and its pyrolysis carbon has a streamlined structure with a high degree of graphitization.•Nitrogen and sulfur co-doped carbon conductive network can be successfully prepared.•The capacity of QS-6.4–670 is 159.2 and 145.7 mAh·g-1 at 0.1 and 1 C, showing the retention of 93% after 150 cycles at 1 C.
●LiMn0.8Fe0.2PO4/C nanocrystal is synthesized by a facile solvothermal reaction.●Melamine is used as a nitrogen source of N-doped carbon coated LiMn0.8Fe0.2PO4/C.●The transformation law of morphology ...from nanosheet to nanoparticle is studied.●The positive effect of N atoms on the electrochemical performance is illustrated.●Particle-like LMFP-7 delivers excellent cycling performance and chemical stability.
Display omitted
N-doped carbon coated LiMn0.8Fe0.2PO4 nanocrystal for lithium-ion batteries was prepared by a facile solvothermal method. The doping effect of nitrogen is regulated by altering the addition amount of nitrogen source melamine. Result shows that the introduction of N atoms does not change the crystal structure of LiMn0.8Fe0.2PO4/C. The ‒NH2 functional groups in melamine can react with the ‒OH groups in pyrolytic carbon generated by the pre-sintering of sucrose. Then, multiple nanosheets in LMFP-0 are combined together and the morphology of LiMn0.8Fe0.2PO4/C is transformed to nano particle-like in LMFP-7. The doped nitrogen in the forms of pyridinic, pyrrolic and graphitic N are derived from the combination of pyrolytic carbon and melamine, which can generate active defective sites and improve the electronic conductivity and diffusion rate of lithium ions. Sample LMFP-7 delivers the best electrochemical performance with a capacity of 154.7, 144.2 and 110.0 mA h g−1 at 0.1, 1 and 5 C, respectively. The LiMn0.8Fe0.2PO4/C cathode exhibits good electrochemical reversibility, low charge transfer resistance (46.9 Ω) and high diffusion coefficient (1.35 × 10−13 cm2 s−1). It also delivers excellent cyclic performance, structural stability and chemical stability.
LiMn0·8Fe0·2PO4/C nanocrystal was synthesized by a facile solvothermal reaction. The pH and concentration of lithium ion are changing with the increase of LiOH. The deposition law of precursor ions ...is investigated, in which Li+ exceeds the necessary stoichiometric ratio even in the lowest amount of LiOH. Mn2+ and Fe2+ possess the similar fixation tendency, and 87.88% Mn2+ are deposited at the pH of 3.30. However, nearly all Fe2+ are precipitated in a wide pH range (2.96–3.85). The morphology changes from nanosheet to nanoellipsoid under the cooperation of pH and precursor ions. The components of LiMnPO4 and LiFePO4 in LiMn0·8Fe0·2PO4/C are predicted and their contributions to capacity are close to the actual results. Sample S-2.6 delivers the optimum electrochemical performance with a capacity of 150.9, 134.6 and 107.5 mA h·g−1 at 0.05, 1 and 5 C, respectively. It also exhibits high reversibility, low charge transfer resistance (41.2 Ω) and excellent diffusion coefficient (5.38 × 10−11 cm2·s−1). The capacity retention of sample S-2.6 reaches 96.03% after 200 cycles and it maintains original structure without obvious change according to the ex-situ XRD results. The morphology of the cycled cathode film also maintains its integrity without evident cracks. The low dissolution of Mn2+ and Fe2+ from LiMn0·8Fe0·2PO4/C shows the enhanced chemical stability.
Display omitted
•LiMn0·8Fe0·2PO4/C nanocrystal is synthesized by a facile solvothermal reaction.•The deposition law of precursor ions under the driving of pH is researched.•The transformation law of morphology from nanosheet to nanoellipsoid is studied.•The component of LiMnPO4 and LiFePO4 and contribution to capacity are predicted.•Nanoellipsoid S-2.6 delivers excellent cycling performance and chemical stability.
Poly(vinyl alcohol), whose pyrolysis carbon possesses high conductivity of 8.88 × 10−1 S/cm, was used to synthesize xLiFePO4/C·(1 − x)Li3V2(PO4)3/C cathode. It was characterized by X‐ray diffraction, ...scanning electron microscopy, conductivity, cyclic voltammetry, and galvanostatic charge and discharge experiments. Results show that LiFePO4/C and Li3V2(PO4)3/C coexists in the cathode. The particles sizes of 0.75LiFePO4/C·0.25Li3V2(PO4)3/C (x = 0.75) are much smaller than 100 nm due to the role of poly(vinyl alcohol). Its conductivity is 8.79 × 10−2 S/cm. The oxidative and reductive peaks in cyclic voltammetry are sharp and symmetrical. Their low potential gaps indicate that the extractions and insertions of lithium ion possess excellent reversibility. Its discharge capacities at 1 and 5 C are 141.1 and 100.1 mAh/g. The more Li3V2(PO4)3/C in cathode results in the deterioration of electrochemical performances due to its low theoretical capacity. It is concluded that poly(vinyl alcohol) is an effective carbon source in the preparation of xLiFePO4/C·(1 − x)Li3V2(PO4)3/C composite cathode with excellent performances.
•High performance N-doped Na3V2(PO4)2F3/C is constructed by a PECVD technology.•Pyridinic, pyrrolic and graphitic nitrogen are generated at high energy condition.•The formation of defects on the ...carbon can accelerate the diffusion of sodium ions.•NVPF@N-3 delivers excellent rate, cyclic performance and structural stability.•The capacity retention of cathode NVPF@N-3 is 91.4% at 10 c after 1000 cycles.
One N-doped carbon coated Na3V2(PO4)2F3/C cathode for sodium-ion batteries was successfully prepared by a Plasma enhanced chemical vapor deposition (PECVD) method. N2 gas is used as the nitrogen source and it is ionized to form chemically active nitrogen ion. It can easily react with the carbon atoms in sample Na3V2(PO4)2F3/C. Research results show that the introduction of N does not affect the crystal structure of the material. The N-doped carbon layer in sample Na3V2(PO4)2F3/C becomes more and more homogeneous. The formation of the pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen in the carbon layer can generate additional active sites to shorten the transmission distance of sodium ions, which can improve the electrochemical performance of Na3V2(PO4)2F3/C cathode. The NVPF@N-3 cathode treated by PECVD for 30 min displays excellent rate performance (109.8 mAh•g−1 at 5 C) and cyclic performance, in which the capacity retention after 1000 cycles at 10 C is 91.4% and the crystal structure of NVPF@N-3 is well maintained. This research demonstrates that the pyrolytic carbon in Na3V2(PO4)2F3/C cathode for sodium-ion batteries can be effectively doped by Nitrogen through a novel PECVD technology.
Display omitted
PDF
