Cathode materials made of
x
LiVPO
4
F·
y
Li
3
V
2
(PO
4
)
3
/C (
x:y
= 1:0, 2:1, 0:1) are synthesized via a feasible sol-gel method for high-performance lithium-ion batteries. The structures, ...morphology, and electrochemical properties of the composites are thoroughly investigated. The results show that LiVPO
4
F/C, Li
3
V
2
(PO
4
)
3
/C, and 2LiVPO
4
F·Li
3
V
2
(PO
4
)
3
/C can be synthesized under 750°C without the formation of impurities. Meanwhile, the unique morphology of the 2LiVPO
4
F·Li
3
V
2
(PO
4
)
3
/C composite, which is porous, with nanoflakes adhering to the surface, is revealed. This composite integrates the advantages of LiVPO
4
F and Li
3
V
2
(PO
4
)
3
. There are four discharge plateaus near 4.2, 4.1, 3.7, and 3.6 V, and the cathode material delivers high capacities of 143.4, 141.6, 133.2, 124.1, and 117.6 mAh g
−1
at rates of 0.1, 0.2, 0.5, 1, and 2 C, respectively. More importantly, the discharge capacity can be almost fully recovered when the discharge rate returns to 0.1 C. The study is highly promising for the development of cathode material for LIBs.
Compositing with metal oxides is proved to be an efficient strategy to improve electrochemical performance of anode material Li
4
Ti
5
O
12
for lithium-ion batteries. Herein, spherical Li
4
Ti
5
O
12
.../NiO composite powders have been successfully prepared via a spray drying method. X-ray diffraction and high-resolution transmission electron microscopy results demonstrate that crystal structure of the powders is spinel. Scanning electron microscopy results show that NiO uniformly distributes throughout Li
4
Ti
5
O
12
matrix. It is found that compositing with NiO increases both discharge platform capacity and rate stability of Li
4
Ti
5
O
12
. The as-prepared Li
4
Ti
5
O
12
/NiO (5%) exhibits a high initial discharge capacity of 381.3 mAh g
−1
at 0.1 C, and a discharge capacity of 194.7 mAh g
−1
at an ultrahigh rate of 20 C.
As a promising high-capacity anode material for Li-ion batteries, NiMn2O4 always suffers from the poor intrinsic conductivity and the architectural collapse originating from the volume expansion ...during cycle. Herein, a combined structure and architecture modulation is proposed to tackle concurrently the two handicaps, via a facile and well-controlled solvothermal approach to synthesize NiMn2O4/NiCo2O4 mesocrystals with superlattice structure and hollow multi-porous architecture. It is demonstrated that the obtained NiCo1.5Mn0.5O4 sample is made up of a new mixed-phase NiMn2O4/NiCo2O4 compound system, with a high charge capacity of 532.2 mAh g-1 with 90.4% capacity retention after 100 cycles at a current density of 1 A g-1. The enhanced electrochemical performance can be attributed to the synergistic effects of the superlattice structure and the hollow multi-porous architecture of the NiMn2O4/NiCo2O4 compound. The superlattice structure can improve ionic conductivity to enhance charge transport kinetics of the bulk material, while the hollow multi-porous architecture can provide enough void spaces to alleviate the architectural change during cycling, and shorten the lithium ions diffusion and electron-transportation distances.
In this paper, we investigate the secure transmission of wireless relaying systems with multidestinations and an eavesdropper (Eve) in Nakagami-m fading channels. To protect the confidential message ...from leaking to the Eve, an intentional interference is sent from a selected destination. We propose two jammer selection schemes under two typical communication scenarios: the channel state information (CSI) of the Eve is unknown, where ajammer is selected based on the CSI between the selected legitimate receiver and the other destination users and the CSI of the Eve is available, where a jammer is selected based on the CSI between the Eve and the other destination users. To assess the performance of the two proposed schemes, we first derive the exact closed-form expressions of intercept probability and outage probability to evaluate the security and the reliability of the system, respectively. In addition, to evaluate the security and reliability as a whole, we propose a new definition of the security-reliability tradeoff, which allows us to easily optimize the power allocation and improve the performance. Finally, simulation results are provided to verify the availability of the proposed schemes.
► Nano-sized 9LiFePO4·Li3V2(PO4)3/C powders are prepared by a sol–gel method. ► Mutual doping in 9LiFePO4·Li3V2(PO4)3/C can improve its electronic conductivity. ► The addition of Li3V2(PO4)3 can ...improve the ionic diffusivity of LiFePO4. ► LiFePO4, Li3V2(PO4)3 and LiFePO4–Li3V2(PO4)3 unit cells coexist in the composite.
9LiFePO4·Li3V2(PO4)3/C composite cathode material is prepared by a sol–gel method, using ferric citrate, V2O5, Li2CO3, NH4H2PO4 and citric acid as raw materials. The composite material is composed of the olivine LiFePO4 and monoclinic Li3V2(PO4)3 phases. XRD results indicate that most of the iron and vanadium in the raw materials tend to form the LiFePO4 and Li3V2(PO4)3 phases, and only small amounts of Fe and V as the dopants enter into the lattice of Li3V2(PO4)3 and LiFePO4, respectively. The electronic conductivity and Li+ diffusion coefficient of 9LiFePO4·Li3V2(PO4)3/C are 6.615×10−3Scm−1 and ∼10−10cm2s−1, which are three orders of magnitude and one order of magnitude larger than those of the LiFePO4/C, respectively. The composite material shows a first discharge specific capacity of 131.3mAhg−1 and capacity retention of 95.1% after 200 cycles at 10C rate. Compared with the LiFePO4/C, its rate capability and cycle performance are both remarkably improved.
Hollow sphere structure Na2MnPO4F/C composite is synthesized through spray drying, following in-situ pyrolytic carbon coating process. XRD results indicate that the well crystallized composite can be ...successfully synthesized, and no other impurity phases are detected. SEM and TEM results reveal that the Na2MnPO4F/C samples show intact hollow spherical architecture, and the hollow spherical shells with an average thickness of 150 nm–250 nm are composed of nanosized primary particles. Furthermore, the amorphous carbon layer is uniformly coated on the surface of the hollow sphere, and the nanosized Na2MnPO4F particles are well embedded in the carbon networks. Consequently, the hollow sphere structure Na2MnPO4F/C shows enhanced electrochemical performance. Especially, it is the first time that the obvious potential platforms (∼3.6 V) are observed during the charge and discharge process at room temperature.
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•Na2MnPO4F/C hollow sphere composite is synthesized by spray drying method.•The hollow sphere shell is composed of nanosized primary particles of Na2MnPO4F/C.•Na2MnPO4F particles are well embedded and interconnected by carbon networks.•The obvious platform ∼3.6 V can be observed at room temperature for the first time.
We designed and engineered the construction of conductive PPy-decorated hydrangea-type 1T-MoS2 as a cathode host material for high-energy-density and long-life Li-S batteries.
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...•PPy-encapsulated hydrangea-type 1 T-MoS2 sulfur host was successfully prepared.•1 T-MoS2@PPy shows favorable catalytic activities to LiPSs/Li2S redox reactions.•The synergistic effect of 1 T-MoS2 and PPy promoted the conversion of LiPSs.•Hierarchical architecture of 1 T-MoS2@PPy is benefit to electron/ion transfer.•1 T-MoS2-S@PPy cathode exhibits high-rate and ultra-stable cycling performance.
Lithium-sulfur batteries are regarded as promising candidates for next-generation energy storage applications owing to their high theoretical specific capacity, low cost, and eco-friendliness. However, the poor conductivity, large volume variation of sulfur species during the charging/discharging process, complicated conversion reactions of sulfur species, shuttle effect of lithium polysulfide, and low sulfur loading greatly hinder the practical application of lithium-sulfur batteries. In this study, we propose an efficient approach to design polypyrrole (PPy)-encapsulated 1T-MoS2 microspheres with a hydrangea-like structure as catalytic sulfur hosts for lithium-sulfur batteries. The catalytic effect of 1T-MoS2 and the high conductivity of the PPy layer accelerated the adsorption/conversion of lithium polysulfides. The hydrangea-like structure of the 1T-MoS2 microspheres provided adequate number of active sites and sufficient space for sulfur loading. Meanwhile, the specific inter-porous/outer-coating layer structure could also restrain the expansion of sulfur electrode during the electrochemical reaction process. The obtained 1T-MoS2-S@PPy cathode material exhibited outstanding electrochemical performance with an excellent reversible capacity of 1434 mAh g−1 at 0.1C rate, a considerable capacity of 1023 mAh g−1 at 1C rate, and excellent cycling stability for over 800 cycles with a low cycling decay rate of 0.051% per cycle.
The continuous and uniform Na3V2(PO4)3/C nanofibers are successfully synthesized through room-temperature pre-reduction with oxalic acid assisting electrospinning method for the first time. With the ...reduction and chelation of oxalic acid in the spinning solution, the V5+ can be reduced to V3+, and the viscosity and conductivity of the solution are well modified, so that the precursor and the final product nanofibers have more uniform, surface-smooth morphologies without spindle-like protrusions. The sample shows the well-crystallized rhombohedral Na3V2(PO4)3 phase and excellent electrochemical performances. The initial discharge specific capacity of the sample at 0.05C reaches up to 114.0 mAh g−1 and still remains 78.1 mAh g−1 at 10C rate. The capacity retention after 100 cycles at 0.05C still holds 97.0%, and the coulombic efficiency is always close to 100%. The causes that the Na3V2(PO4)3/C nanofibers exhibit good rate performances, cyclic ability and structural reversibility can be mostly ascribed to the perfect nanofibers. The uniform and smooth Na3V2(PO4)3/C nanofiber consists of Na3V2(PO4)3/C nanoparticles uniformly embed in the perfect 1D carbon nanowire, which builds up a continuous network with highly efficient electronic and ionic channels, thus the impressive properties can be realized.
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•Na3V2(PO4)3/C nanofibers are prepared by a pre-reduction electrospinning method.•Pre-reduction can decrease the synthesis temperature of Na3V2(PO4)3/C nanofibers.•Pre-reduction can adjust the viscosity and conductivity of the spinning solution.•Pre-reduction can make the Na3V2(PO4)3/C nanofibers uniform, smooth and continuous.•The performances of Na3V2(PO4)3/C are obviously improved by the pre-reduction.
Y-doped Li3V2(PO4)3 cathode materials were prepared by a carbothermal reduction(CTR) process. The properties of the Y-doped Li3V2(PO4)3 were investigated by X-ray diffraction (XRD) and ...electrochemical measurements. XRD studies showed that the Y-doped Li3V2(PO4)3 had the same monoclinic structure as the undoped Li3V2(PO4)3. The Y-doped Li3V2(PO4)3 samples were investigated on the Li extraction/insertion performances through charge/discharge, cyclic voltammogram (CV), and electrochemical impedance spectra (EIS). The optimal doping content of Y was x=0.03 in Li3V2-xYx(PO4)3 system. The Y-doped Li3V2(PO4)3 samples showed a better cyclic ability. The electrode reaction reversibility was enhanced, and the charge transfer resistance was decreased through the Y-doping. The improved electrochemical performances of the Y-doped Li3V2(PO4)3 cathode materials were attributed to the addition of Y3+ ion by stabilizing the monoclinic structure.
The 9LiFePO4·Li3V2(PO4)3/C composite cathode material is synthesized by spray-drying and post-calcining method based on citrate. The composite is well crystallized, and contains olivine-type LiFePO4 ...and monoclinic Li3V2(PO4)3 phases. The composite material exhibits spherical particles in the size of 0.5–5μm, and shows a high tap-density of 1.64gcm−3. The electrochemical performance of the material is excellent. At 5C and 10C rates, the sample exhibits the initial discharge capacities of 135.3 and 109.6mAhg−1 and capacity retentions of 96.2% and 93.7% after 100cycles, respectively. The homogenous mixing of the LiFePO4 and fast ion conductor additive Li3V2(PO4)3, which is resulted from spray-drying, can be the reason why the composite has good rate capability.
The 9LiFePO4·Li3V2(PO4)3/C composite cathode material exhibits spherical particles in the size of 0.5–5μm. The small particles fill in the gaps between large particles, which are helpful to improve the tap-density of material. The tap-density of the as-prepared powders is as high as 1.64gcm−3, which is remarkably higher than those of irregularly-shaped or nano-sized LiFePO4, Li3V2(PO4)3 and xLiFePO4·yLi3V2(PO4)3. Display omitted
► Spherical 9LiFePO4·Li3V2(PO4)3/C is prepared by spray-drying and calcining method. ► The composite shows a high tap-density of 1.64gcm−3. ► The composite shows excellent rate capability and cycle performance.
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