•facile preparation by polyacrylamide assisted sol-gel route.•greatly improved electro-chemical performance of the material.•The role Cobalt doping played was clarified.Zn1-xCoxO (0 ≤ x ≤ 0.15) anode material was prepared by an easy polyacrylamide assisted sol-gel route. The successful replacement of Zinc by Cobalt within Cobalt content x ≤ 0.09 was confirmed by structural characterization. The introduction of Cobalt element greatly improved the electro-chemical performances of the matrix Zinc oxide. Without carbon coating, at the 20th cycle, Zn0.91Co0.09O anode still preserved a capacity a little bit more than 1000  mA h g−1 and a capacity more than 600  mA h g−1 was retained at the end of the 50th cycle. Better rate capability was also witnessed. The SEM, EIS at OCV, CV and in situ XRD were further carried out to elucidate the lithiation mechanism. The role Cobalt doping played can be summarized as follows: the stabilization of the Li2Zn phase, the minimization of charge transfer resistance and the enhanced reversibility of the reduction from metal oxide to metal.The lithiation/delithiation mechanism of Zn0.91Co0.09O revealed by in-situ XRD measurement.Download high-res image (317KB)Download full-size image

In this paper, a series of Na-doped Li2Na2Ti6O14 samples are synthesized by a simple solid-state reaction method through Li-site substitution with Na. Morphology observation shows that all five materials are well crystallized with a particle size in the range of 150–300 nm. Electrochemical analysis shows that Li1.95Na2.05Ti6O14 exhibits lower charge–discharge polarization (0.05 V) than that (0.11 V) of other Li2−xNa2+xTi6O14 samples (x = 0.00, 0.10, 0.15, 0.20). As a result, Li1.95Na2.05Ti6O14 has the highest initial charge capacity of 243.6 mA h g−1, and maintains a reversible capacity of 210.7 mA h g−1 after 79 cycles. For comparison, Li2−xNa2+xTi6O14 (x = 0.00, 0.10, 0.15 and 0.20) samples only hold a reversible capacity of 159.1, 203.5, 190.1 and 156.7 mA h g−1, respectively. Moreover, Li1.95Na2.05Ti6O14 also delivers the best rate performance compared with the other four samples, with a charge capacity of 221.1 mA h g−1 at 200 mA g−1, 211.9 mA h g−1 at 300 mA g−1, and 198.7 mA h g−1 at 400 mA g−1. Besides, the reversible in situ structural evolution proves that Li1.95Na2.05Ti6O14 is a stable host for lithium storage. All the improved electrochemical properties of Na-doped Li2Na2Ti6O14 should be attributed to the Na-doping with low content, which reduces the charge–discharge polarization and improves the ionic conductivity.

To further study the lithium ion transportation behavior of cathode material FeF3 · 0.33H2O/C synthesized by a simple one-step chemico-mechanical method, the Electrochemical impedance spectrum (EIS) measured at series of open-circuit voltages were investigated in detail. The results showed that the EIS profiles of FeF3 · 0.33H2O/C materials were strongly potential dependent. The equivalent circuit parameters obtained by fitting the experimental data as a function of open-circuit voltage (OCV) level were depicted. The ohmic resistance R0, solid electrolyte inter-phase resistance RSEI, electronic conduction resistance RE, charge transfer resistance RR, and Q parameter of CPE circuit characteristic of Li+ diffusion Qdiff all showed a sudden change at the OCV level 2.5 V. Ohmic resistance R0 had a relatively lower resistance of ca. 10 Ω above OCV level 2.5 V and a higher resistance of about 40 Ω below 2.5 V. Similar situation was also observed for RSEI, which was around 20 Ω above 2.5 V and soared up quickly when the equilibrium potential fell below 2.5 V. Similar variations were also observed for RE and RR. A high resistance of ca. 410 and 520 Ω was obtained at OCV level 2.05 V, respectively. Qdiff showed a convex profile, which matched the variation of Li+ diffusion coefficient well.

Time-dependent elementary polarizations of FeF3·3H2O/C cathode material were quantitatively investigated in dc polarization in order to determine the key factors that comprise the total polarization. The measurement of electrochemical impedance spectrum at a given state of charge and the subsequent least square fitting of its equivalent circuit allow the calculation of elementary contributions of individual kinetic step to the total polarization. The profiles of the calculations were well consistent with those of experiments based on the same states of charge, and the elementary contributions could be differentiated successfully which reveal that the solid-state diffusion process makes the largest contribution to the total polarization after 2.5 s discharge beginning with open-circuit voltage (OCV) level 3.5 V. The results may be helpful for the design of batteries of better performance with FeF3 cathode.

•Li2Na2Ti6O14 is prepared by a solid state reaction.•Cu/C layer is coated on Li2Na2Ti6O14 by a thermal decomposition.•Cu/C coating layer improves the electrochemical properties of Li2Na2Ti6O14.•Core–shell Li2Na2Ti6O14@Cu/C shows a reversible capacity of 120.3 mAh g−1.Core–shell Li2Na2Ti6O14@Cu/C is prepared by a preliminary formation of Li2Na2Ti6O14 by solid state reaction and a following coating process with Cu/C layer by thermal decomposition. The amorphous Cu/C coating layer reveals a thickness of 5 nm on the surface of Li2Na2Ti6O14, which improves the electronic conductivity and charge transfer rate of active materials. As a result, Li2Na2Ti6O14@Cu/C shows lower electrochemical polarization and quicker kinetic behavior compared to bare Li2Na2Ti6O14. Cycled at 50 mA g−1, Li2Na2Ti6O14@Cu/C can deliver a reversible capacity of 120.3 mAh g−1 after 50 cycles, which is much higher than the value of 96.8 mAh g−1 obtained by Li2Na2Ti6O14. Even kept at 400 mA g−1, a reversible lithium storage capacity of 76.3 mAh g−1 can be delivered by Li2Na2Ti6O14@Cu/C. The improved electrochemical properties of Li2Na2Ti6O14 are attributed to the electronic conductive Cu/C coating layer on the surface.

•PbSbO2Cl and PbCl2/Sb4O5Cl2 are prepared by a solution method.•PbCl2/Sb4O5Cl2 shows higher lithium storage capacity than PbSbO2Cl.•PbCl2/Sb4O5Cl2 shows better cycling stability than PbSbO2Cl.•PbCl2/Sb4O5Cl2 shows a reversible capacity of 339.7 mA h g−1 after 30 cycles.In this work, PbCl2/Sb4O5Cl2 and PbSbO2Cl are prepared by a simple solution route, and compared by using as lithium storage materials. PbCl2/Sb4O5Cl2 is the intermediate precursor for hydrothermal preparing PbSbO2Cl. Morphology analysis shows that PbCl2/Sb4O5Cl2 is composed of irregular particles in the range of 50–100 nm and PbSbO2Cl consists of well-dispersed bulks with the particle size of 200–500 nm. Similar chemical composition between PbCl2/Sb4O5Cl2 and PbSbO2Cl make they show similar electrochemical behaviors. However, different morphology and structure also make PbCl2/Sb4O5Cl2 and PbSbO2Cl exhibit different lithium storage capacity and cycling performance. Charge/discharge tests reveal that nanocomposite PbCl2/Sb4O5Cl2 can deliver a higher initial lithiation capacity (1036.7 mA h g−1) than PbSbO2Cl (993.8 mA h g−1). Upon repeated cycles, PbCl2/Sb4O5Cl2 also shows better electrochemical properties than PbSbO2Cl, which may be contributed to the maintaining of structural stability by nanocomposite structure. As a result, PbCl2/Sb4O5Cl2 delivers a reversible capacity of 339.7 mA h g−1 after 30 cycles.

•Si@C@CNT@C is prepared by forming precursors under vacuum condition.•Si@C@CNT@C reveals spherical shape as lithium storage material.•Si@C@CNT@C delivers a reversible capacity of 563.5 mA h g−1 after 60 cycles.Si@C@CNT@C composite is prepared by preliminary low-pressure forming Si@C and Si@C@CNT precursors from Si powder. After pyrolysis from glucose, acetylene and pitch, Si@C@CNT@C shows a spherical multi-phase composite structure. By using as lithium storage material, Si@C@CNT@C shows an initial discharge capacity of 620.5 mA h g−1 with an initial coulombic efficiency of 82.2%. After 60 cycles, this spherical sample can maintain a reversible capacity of 563.5 mA h g−1 at 100 mA g−1, corresponding to a capacity retention of 90.8%. For comparison, the reversible capacities for Si powder, Si@C and Si@C@CNT are 10.9, 380.3 and 494.6 mA h g−1, respectively. Even cycled at 400 mA g−1, Si@C@CNT@C can deliver a reversible lithium storage capacity of 389.2 mA h g−1. It indicates that spherical Si@C@CNT@C can be used as a high performance anode material for lithium-ion batteries.Graphical abstract

•Li4Ti5O12@C is prepared by a facile carbon coating strategy.•Amorphous carbon coating layer shows a thickness of 4–6 nm.•Li4Ti5O12@C delivers a capacity of 160.6 mA h g−1 at 20 mA g−1 in 1.0–2.0 V.•Li4Ti5O12@C displays a capacity of 205.1 mA h g−1 at 1500 mA g−1 in 0.0–2.0 V.In this study, carbon-encapsulated Li4Ti5O12 particles are prepared by a facile carbon formation strategy. Under the thermal hydrolysis of acetate in Ar atmosphere, Li4Ti5O12 particles are uniformly coated by amorphous carbon layer with the thickness of 4–6 nm. The electrochemical properties of Li4Ti5O12@C are characterized as high rate anode material for lithium ion batteries. Electrochemical results show that Li4Ti5O12@C can respectively deliver the reversible lithium storage capacities of 155.0, 134.7 and 60.3 mA h g−1 at 20, 600 and 1500 mA g−1 in a narrow working window of 1.0–2.0 V. Cycled in a broad potential range of 0.5–2.0 V, Li4Ti5O12@C remains the reversible charge capacities of 158.2, 134.5 and 114.7 mA h g−1 at 20, 600 and 1500 mA g−1 with capacity retentions of 91.5%, 97.5% and 100%, respectively. Even working between 0.0 and 2.0 V, Li4Ti5O12@C can still display outstanding electrochemical properties with the reversible capacities of 220.2, 200.6 and 205.1 mA h g−1 at 20, 600 and 1500 mA g−1.

•In situ growth of CNTs on LiFePO4 is prepared by hydrolysis of acetylene.•The existence of P element in catalysts results in the formation of coiled CNTs.•Crosslinked CNTs provide electronic conductive pathways.•LiFePO4@CNTs shows a reversible capacity of 108.0 mA h g−1 at 10 C.In this paper, LiFePO4@CNTs is fabricated by in situ growth of CNTs on the surface of LiFePO4 particles. The use of Ni–P alloy as catalyst results in the formation of coiled CNTs. The crosslinked CNTs provide good electronic conductive pathways interconnecting LiFePO4 particles. By using as cathode material for lithium ion battery, LiFePO4@CNTs shows higher electronic conductivity and charge transfer rate than those of the pristine LiFePO4 before coating. As a result, LiFePO4@CNTs displays outstanding lithium storage capacity and rate property. Electrochemical results reveal that LiFePO4@CNTs can deliver the reversible capacities of 161.3 mA h g−1 at 0.2 C and 108.0 mA h g−1 at 10 C. For comparison, bare LiFePO4 only reveals reversible capacities of 144.1 mA h g−1 at 0.2 C and 90.9 mA h g−1 at 10 C.Graphical abstract

•Correlate the Ddiff and the sLi in phase transition region.•An inverted V-shaped Ddiff vs. OCV profile was observed.•Each kinetic step of ion transportation as a function of OCV was scrutinized.LiFePO4 cathode material prepared by solid state carbo-thermal reduction exhibited pretty good electro-chemical performance. It showed an initial discharge capacity of 152.6 mAh·g− 1 and only 2.3% capacity fading was observed after 50 cycles. The coulomb efficiencies were between 98 and 100%. The electrochemical impedance spectroscopy measured at series of equilibrium potentials within the voltage range 2.5–4.5 V showed that they were highly voltage dependent. This indicated the different kinetics steps of charge carrier transportation through the whole battery components. The coefficients of lithium ions and polarizations of charge carrier transportation as a function of equilibrium potentials were determined from the EIS profiles. An inverted V shaped curve was observed as to the relation between the lithium diffusion coefficient and equilibrium potential. However, contrary to the previous reports, based on Einstein equation, the lithium ion diffusion in the phase transition region from LiFePO4 to FePO4 was calculated to be in the range 10− 11–10− 12 cm·s− 1, which was about 1–2 order of magnitude larger than that of other region.

A new cathode material for lithium ion battery FeF3 · 0.33H2O/C was synthesized successfully by a simple one-step chemico-mechanical method. It showed a noticeable initial discharge capacity of 233.9 mAh g−1 and corresponding charge capacity of 186.4 mAh g−1. A reversible capacity of ca.157.4 mAh g−1 at 20 mA g−1 can be obtained after 50 charge/discharge cycles. To elucidate the lithium ion transportation in the cathode material, the methods of electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) were applied to obtain the lithium diffusion coefficients of the material. Within the voltage level of 2.05–3.18 V, the method of EIS showed that \( {D}_{{\mathrm{Li}}^{+}} \) varied in the range of 1.2 × 10−13 ~ 3.6 × 10−14 cm2 s−1 with a maximum of 1.2 × 10−13 cm2 s−1 at 2.5 V. The method of GITT gave a result of 8.1 × 10−14 ~ 1.2 × 10−15 cm2 s−1. The way and the range of the variation for lithium ion diffusion coefficients measured by the GITT method show close similarity with those obtained by the EIS method. Besides, they both reached their maximum at a voltage level of 2.5 V.

A cathode material, 0.5Li2MnO3 0.5LiNi0.5Mn0.5O2, was prepared by citric acid-assisted sol–gel method and its electrochemical performance was investigated. It delivered a charge capacity of 270 mAh g−1 and a discharge capacity of 189 mAh g−1 in the first cycle. With the increase of current density from 14 to 28 mA g−1, the discharge capacity dropped severely to 130 mA g−1. Obviously, the rate capability of the material was inferior to most of the oxide cathode materials. The diffusion coefficient of this material was calculated to be 6.04 × 10−12 cm2 s−1 from the results of cyclic voltammetry measurements. Moreover, diffusion coefficients between 3.13 × 10−12 and 1.22 × 10−10 cm2 s−1 in the voltage range of 3.8–4.7 V were obtained by capacity intermittent titration technique. This, together with the localized Li2MnO3 domains in the crystal structure, may validate the poor rate capability.

Nasicon-type solid electrolyte Li1.3Al0.1Zn0.1Ti1.8P3O12 was prepared by citric acid-assisted acrylamide polymerisation gel method. X-ray diffraction pattern showed that the introduction of Zn2+ in the parent matrix Li1+xAlxTi2−xP3O12 made it easier to get high-purity rhombohedral structure (space group \( R\overline 3 C \)) Li1.3Al0.1Zn0.1Ti1.8P3O12 without the evidence of impurity secondary phase. The Li+ kinetics were investigated by complex impedance in bulk pellet and ionic conductivity in battery-type composite cathode, respectively. Grain-interior resistance measured by galvanostatic intermittent titration technique, potential step chronoamperometry, and AC impedance spectroscopy at 20 °C varies in the range 1.2–1.95 × 10−4 S cm−1, which is in good agreement with that obtained by complex impedance method 1.5 × 10−4 S cm−1.

Layered lithium ion battery cathode material LiNi1/3Co1/3Mn1/3O2 with a uniform particle size of about 6 μm was synthesized by a spray pyrolysis method. The lithium ion diffusion kinetics in LiNi1/3Co1/3Mn1/3O2 composite cathode were systematically studied by the ratio of potentio-charge capacity to galvano-charge capacity method, galvanostatic intermittent titration technique, electrochemical impedance spectroscopy, and potential step chronoamperometry methods. The variations of lithium ion diffusion coefficients obtained by the four methods show a close similarity. They vary in the range of 10−8 to 10−10 cm2 s−1, with a maximum at 4.1- to 4.2-V voltage level.

Layered lithium ion battery cathode material LiNi1/3Co1/3Mn1/3O2 with uniform particle size of about 6 μm was synthesized by a spray pyrolysis method. Infrared and X-ray diffraction analyses show that the pyrolysis at 1,000 °C for 2 s in the tube furnace eliminates nearly all the organic components but is still not enough for the complete crystallization of LiNi1/3Co1/3Mn1/3O2 materials. Therefore, further annealing at 850 °C is needed. The prepared LiNi1/3Co1/3Mn1/3O2 cathode materials show excellent electrochemical performances. By increasing the C-rates, the cell shows discharge capacities of 159.3, 148.2, 133.7, and 125.7 mAh g−1 at 0.1, 0.2, 0.5, and 1C rates, respectively. Only 2.1 mAh g−1 capacity loss is observed when back to 0.1C rate. Moreover, LiNi1/3Co1/3Mn1/3O2 cathode retains 96, 97.7, 97.1, 94.5, and 97.1 % of its initial discharge capacities after 20 cycles at 0.1, 0.2, 0.5, 1, and back to 0.1C rates, respectively. More than 97 % coulombic efficiencies are observed at all the current densities in 20 cycles.