Electrochemical characterizations of Fe-substituted LiNiO2 synthesized in air by the combustion method

The phases that appear in the intermediate reaction steps for the formation of lithium nickel oxide were deduced from XRD and DTA analyses. XRD analysis and electrochemical measurements were performed for LiNi_1−yFe_yO₂ (0.000 ≤ y ≤ 0.300) samples calcined in air after preheating in air at 400 °C for 30 min. Rietveld refinement of the LiNi_1−yFe_yO₂ XRD patterns (0.000 < y ≤ 0.100) was carried out from a [Li,Ni]_3b[Li,Ni,Fe]_3a[O₂]_6c starting structure model. The samples of LiNi_1−yFe_yO₂ with y = 0.025 and 0.050 had higher first discharge capacities when compared with LiNiO₂ and exhibited better or similar cycling performance at a 0.1 C rate in the voltage range of 2.7–4.2 V. The LiNi_0.975Fe_0.025O₂ sample had the highest first discharge capacity of 176.5 mAh/g and a discharge capacity of 121.0 mAh/g at n = 100. With the exception of Co-substituted LiNiO₂, such a high first discharge capacity has not been previously reported.     

Effects of Zn or Ti substitution for Ni on the electrochemical properties of LiNiO2

LiNiO₂ and LiNi_1−yM_yO₂ (M = Zn and Ti, y = 0.005, 0.01, 0.025, 0.05, and 0.1) were synthesized with a solid-state reaction method by calcination at 750 °C for 30 h under oxygen stream after preheating at 450 °C for 5 h in air. LiNi_0.995Zn_0.005O₂ among the Zn-substituted samples and LiNi_0.995Ti_0.005O₂ among the Ti-substituted samples showed the best electrochemical properties. For similar values of y, LiNi_1−yTi_yO₂ had in general better electrochemical properties than LiNi_1−yZn_yO₂. Electrochemical properties seem to be closely related to R-factor but less related to I_0 0 3/I_1 0 4 value. In the FT-IR absorption spectra of LiNiO₂ and LiNi_1−yM_yO₂ (M = Zn and Ti, y = 0.005, 0.01, 0.025, 0.05 and 0.1), Li₂CO₃ was detected even if it is not observed from XRD pattern, with the samples LiNi_1−yZn_yO₂ (y = 0.05 and 0.1) showing Li₂ZnO₂ additionally. The smaller cation mixing of the Ti-substituted samples is considered to lead to their better electrochemical properties than the Zn-substituted samples.     

Variation of discharge capacities with C-rate for LiNi1−yMyO2 (M = Ni,Ga, Al and/or Ti) cathodes synthesized by the combustion method

LiNiO₂, LiNi_0.995Al_0.005O₂, LiNi_0.975Ga_0.025O₂, LiNi_0.990Ti_0.010O₂ and LiNi_0.990Al_0.005Ti_0.005O₂ specimens were synthesized by preheating at 400 °C for 30 min in air and calcination at 750 °C for 36 h in an O₂ stream. The variation of the discharge capacities with C-rate for the synthesized samples was investigated. LiNi_0.990Al_0.005Ti_0.005O₂ has the largest first discharge capacities at the 0.1 and 0.2 C rates. LiNi_0.990Ti_0.010O₂ has the largest first discharge capacity at the 0.5 C rate. In case of LiNiO₂ and LiNi_0.990Ti_0.010O₂, the first discharge capacity decreases slowly as the C-rate increases. LiNiO₂ has the largest discharge capacities at n = 10 (after stabilization of the cycling performance) at the 0.1, 0.2 and 0.5 C rates. This is considered to be related with the largest value of I_0 0 3/I_1 0 4 and the smallest value of R-factor (the least degree of cation mixing) among all the samples. LiNi_0.975Ga_0.025O₂ exhibits the lowest discharge capacity degradation rates at 0.1, 0.2 and 0.5 C rates.     

Electrochemical properties of LiNi<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>O<Subscript>2</Subscript> (M=Ni,Ga, Al and/or Ti) cathodes synthesized by the combustion method

LiNiO₂, LiNi_0.995Al_0.005O₂, LiNi_0.975Ga_0.025O₂, LiNi_0.990Ti_0.010O₂ and LiNi_0.990Al_0.005Ti_0.005O₂ were synthesized by preheating at 400 °C for 30 min in air and calcination at 750 °C for 36 h in an O₂ stream with excess lithium amount z = 0.10 in Li_1+z Ni_1−y M _y O₂. For these samples, the discharge capacities and discharge capacity degradation rate are compared. LiNiO₂ has the largest discharge capacity at the 20th cycle (n = 20) and the 50th cycle (n = 50). LiNiO₂ and LiNi_0.995Al_0.005O₂ have relatively good cycling performances and their discharge capacities at n = 50 are 134 and 123 mAh/g, respectively, at 0.1 C rate. The crystallite sizes and strains were calculated by the Williamson–Hall method with XRD patterns and compared for the samples as prepared and after 50 charge–discharge cycles.     

The structure and electrochemical performance of LiFeBO3 as a novel Li-battery cathode material

LiFeBO₃ cathode material has been synthesized successfully by solid-state reaction using Li₂CO₃, H₃BO₃ and FeC₂O₄·2H₂O as starting materials. The crystal structure has been determined by the X-ray diffraction. Electrochemical tests show that an initial discharge capacity of about 125.8 mAh/g can be obtained at the discharge current density of 5 mA/g. When the discharge current density is increased to 50 mA/g, the specific capacity of 88.6 mAh/g can still be held. In order to further improve the electrochemical properties, the carbon-coated LiFeBO₃, C-LiFeBO₃, are also prepared. The amount of carbon coated on LiFeBO₃ particles was determined to be around 5% by TG analysis. In comparison with the pure LiFeBO₃, a higher discharge capacity, 158.3 mAh/g at 5 mA/g and 122.9 mAh/g at 50 mA/g, was obtained for C-LiFeBO₃. Based on its low cost and reasonable electrochemical properties obtained in this work, LiFeBO₃ may be an attractive cathode for lithium-ion batteries.     

A study on electrochemical characteristics of LiCoO2/LiNi1/3Mn1/3Co1/3O2 mixed cathode for Li secondary battery

In this study, the LiCoO₂/LiNi_1/3Mn_1/3Co_1/3O₂ mixed cathode electrodes were prepared and their electrochemical performances were measured in a high cut-off voltage. As the contents of LiNi_1/3Mn_1/3Co_1/3O₂ in the mixed cathode increases, the reversible specific capacity and cycleability of the electrode enhanced, but the rate capability deteriorated. On the contrary, the rate capability of the cathode enhanced but the reversible specific capacity and cycleability deteriorated, according to increasing the contents of LiCoO₂ in the mixed cathode. The cell of LiCoO₂/LiNi_1/3Mn_1/3Co_1/3O₂ (50:50, wt.%) mixed cathode delivers a discharge capacity of ca. 168 mAh/g at a 0.2 C rate. The capacity of the cell decreased with the current rate and a useful capacity of ca. 152 mAh/g was obtained at a 2.0 C rate. However, the cell shows very stable cycleability: the discharge capacity of the cell after 20th charge/discharge cycling maintains ca. 163 mAh/g.     

Electrochemical characteristics of cobalt-substituted lithium nickel oxides synthesized from lithium hydro-oxide and nickel and cobalt oxides

LiNi_1−yCo_yO₂ (y=0.1, 0.3 and 0.5) were synthesized by solid state reaction method at 800 °C and 850 °C from LiOH·H₂O, NiO and Co₃O₄ as starting materials. The electrochemical properties of the synthesized LiNi_1−yCo_yO₂ were investigated. As the content of Co decreases, particle size decreases rapidly and particle size distribution gets more homogeneous. When the particle size is compared at the same composition, the particles synthesized at 850 °C are larger than those synthesized at 800 °C. LiNi_0.7Co_0.3O₂ synthesized at 850 °C has the largest intercalated and deintercalated Li quantity Δx among LiNi_1−yCo_yO₂ (y=0.1, 0.3 and 0.5). LiNi_0.7Co_0.3O₂ synthesized at 850 °C has the largest first discharge capacity (178 mAh/g), followed by LiNi_0.7Co_0.3O₂ (162 mAh/g) synthesized at 800 °C. LiNi_0.7Co_0.3O₂ synthesized at 800 °C has discharge capacities of 162 and 125 mAh/g at n=1 and n=5, respectively.     

Electrochemical properties of LiNiO2 cathode after TiO2 or ZnO addition

LiNiO₂ was prepared by solid state reaction, and LiNiO₂ was mixed with 1-, 2-, or 5 wt% TiO₂ or ZnO for the preparation of cathodes for a lithium ion battery. The electrochemical properties of the cathodes were investigated and the effects of the addition of TiO₂ or ZnO were discussed. The voltage vs. capacity curves for charge and discharge at different numbers of cycles for LiNiO₂, 2 wt% TiO₂-added LiNiO₂, and 2 wt% ZnO₂-added LiNiO₂ showed that in all the samples the first discharge capacity is much smaller than the first charge capacity. The addition of TiO₂ or ZnO decreased the discharge capacities, but improved the cycling performance. The discharge capacities of LiNiO₂ and 2 wt% TiO₂-added LiNiO₂ decreased as the number of cycles increased. However, the discharge capacity of 2 wt% ZnO-added LiNiO₂ increased overall as the number of cycles increased. The −dx/|dV| vs. voltage curves for the 1st and 2nd cycles of 0, 1-, 2-, or 5 wt% TiO₂ or ZnO-added LiNiO₂ showed that all the samples underwent four phase transitions during charging and discharging.     

Electrochemical properties of carbon-mixed LiFePO4 cathode material synthesized by the ceramic granulation method

LiFePO₄/carbon composite cathode material was prepared using polyvinyl alcohol (PVA) as carbon source by pelleting and subsequent pyrolysis in N₂. The samples were characterized by XRD, SEM and TGA. Their electrochemical performance was investigated in terms of charge–discharge cycling behavior. It consists of a single LiFePO₄ phase and amorphous carbon. The special micro-morphology via the process is favorable for electrochemical properties. The discharge capacity of the LiFePO₄/C composite was 145 mAh/g, closer to the theoretical specific capacity of 170 mAh/g at 0.1 C low current density. At 3 C modest current density, the specific capacity was about 80 mAh/g, which can satisfy for transportation applications if having a more planar discharge flat.     

Electrochemical and ex situ XRD investigations on (1−x)LiNiO2·xLi2TiO3 (0.05≤x≤0.5)

An attempt to understand the unusual electrochemical behaviors in (1−x)LiNiO₂·xLi₂TiO₃ (0.05≤x≤0.5), an excess initial charge capacity exceeding the oxidation of transitional metal to +4 accompanying the appearance of an irreversible initial charge plateau when x reached 0.075, was performed. The decreased charge-discharge polarization after charging to 4.6 and 4.8 V and increased columbic reversibility after charging to 4.6 V typically for x=0.1 and 0.2, in contrast to charging to 4.4 V, suggested that the excess initial charge capacity possibly did not come mainly from electrolyte decomposition; while ex situ XRD results in the sample with x=0.2 confirmed that Li⁺ were really extracted at the stage of the charge plateau, ruling out the possibility that electrolyte decomposition mainly accounted for the unusual electrochemical behaviors. It was inferred that the species responsible for charge compensation for the excess charge capacity must be oxygen ions in these materials, considering that Ni⁴⁺ and Ti⁴⁺ are generally impossible to be oxidized to a higher valence. Various electrochemical cycling experiments demonstrated that the sample for x=0.05 with high resistant ability to high voltage and temperature is very promising cathode material in view of observed capacity and cycleability from a viewpoint of application.     

Cathodic performance of LiMn1−xMxPO4 (M = Ti, Mg and Zr) annealed in an inert atmosphere

To improve the cathodic performance of olivine-type LiMnPO₄, we investigated the optimal annealing conditions for a composite of carbon with cation doping. Nanocrystalline and the cation-doped LiMn_1−xM_xPO₄ (M = Ti, Mg, Zr and x = 0, 0.01, 0.05 and 0.10) was synthesized in aqueous solution using a planetary ball mill. The synthesis was performed at the fairly low temperature of 350 °C to limit particle size. The obtained samples except for the Zr doped one consisted of uniform and nano-sized particles. The performance of LiMnPO₄ was much improved by an annealing treatment between 500 and 550 °C with carbon in an inert atmosphere. A small amount of metal-rich phosphide (Mn₂P) was detected in the sample annealed at 900 °C. In addition, 1 at.% Mg doping for Fe enhanced the rate capability in our doped samples. The discharge capacity of LiMn_0.99Mg_0.01PO₄/C was 146 mAh/g at 0.1 mA/cm² and 125 mAh/g even at 2.0 mA/cm².     

Zr-doped Li[Ni0.5−xMn0.5−xZr2x]O2 (x = 0, 0.025) as cathode material for lithium ion batteries

Layered Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) have been prepared by the mixed hydroxide and molten-salt synthesis method. The individual particles of synthesized materials have a sub-microsize range of 200-500 nm, and LiNi_0.475Mn_0.475Zr_0.05O₂ has a rougher surface than that of LiNi_0.5Mn_0.5O₂. The Li/Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) electrodes were cycled between 4.5 and 2.0 V at a current density of 15 mA/g, the discharge capacity of both cells increased during the first ten cycles. The discharge capacity of the Li/LiNi_0.475Mn_0.475Zr_0.05O₂ cell increased from 150 to 220 mAh/g, which is 50 mAh/g larger than that of the Li/LiNi_0.5Mn_0.5O₂ cell. We found that the oxidation of oxygen and the Mn³⁺ ion concerned this phenomenon from the cyclic voltammetry (CV). Thermal stability of the charged Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) cathode was improved by Zr doping.     

Study on the rapid synthesis of LiNi1−xCoxO2 cathode material for lithium secondary battery

LiNi_1−xCo_xO₂ (x = 0, 0.1, 0.2) cathode materials were successfully synthesized by a rheological phase reaction method with calcination time of 0.5 h at 800 °C. All obtained powders are pure phase with α-NaFeO₂ structure (R-3m space group). The samples deliver an initial discharge capacity of 182, 199 and 189 mAh g⁻¹ (25 mA g⁻¹, 4.35-3.0 V), respectively. The reaction mechanism was also discussed, which consists of a series of defect reactions. As a result of these defect reactions, the reaction of forming LiNi_1−xCo_xO₂ takes place in high speed.     

Microstructure effect on the electrochemical property of Li4Ti5O12 as an anode material for lithium-ion batteries

Porous (P-) and dense (D-) lithium titanate (Li₄Ti₅O₁₂) powders as an anode material for lithium-ion batteries have been synthesized by spray drying followed by solid-state calcination. Electrochemical testing results showed that the discharge capacities of P-Li₄Ti₅O₁₂ are 144 mAh/g, 128 mAh/g and 73 mAh/g at the discharging rate of 2C, 5C and 20C, respectively (cut-off voltages: 0.5-2.5 V). The corresponding values for D-Li₄Ti₅O₁₂ are 108 mAh/g, 25 mAh/g and 17 mAh/g. The higher capacity of the P-Li₄Ti₅O₁₂ at high charge/discharge rates was attributed to the shorter transport path of Li ions and higher electronic conductivity in the P-Li₄Ti₅O₁₂ as a result of its smaller primary particle size and higher surface area compared with those of the D-Li₄Ti₅O₁₂.     

Preparation and characterization of ramsdellite Li2Ti3O7 as an anode material for asymmetric supercapacitors

Ramsdellite Li₂Ti₃O₇ was first synthesized via sol-gel process with good crystallity of an average particle size of 0.175 μm. The product was thoroughly investigated as a lithium intercalation compound, and as an active anode material in asymmetric supercapacitors coupling with activated carbon as cathode. Lithium intercalation reactions were found occurring at 1.32 and 1.62 V versus Li/Li⁺, respectively. A reversible specific capacity of 150 mA h g⁻¹ at 1C was obtained on Li₂Ti₃O₇ electrode in a nonaqueous electrolyte. The charge current was found to strongly influence the anodic discharge capacity in the asymmetric cell. The capacity retention at 10C charge-discharge rate was found to be 75.9% in comparison with that at 1C.     

Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

Comparative study of Li[Co1−zAlz]O2 prepared by solid-state and co-precipitation methods

Li[Co_1−zAl_z]O₂ (0 ≤ z ≤ 0.5) samples were prepared by co-precipitation and solid-state methods. The lattice constants varied smoothly with z for the co-precipitated samples but deviated for the solid-state samples above z = 0.2. The solid-state method may not produce materials with a uniform cation distribution when the aluminum content is large or when the duration of heating is too brief. Non-stoichiometric Li_x[Co_0.9Al_0.1]O₂ samples were synthesized by the co-precipitation method at various nominal compositions x = Li/(Co + Al) = 0.95, 1.0, 1.1, 1.2, 1.3. XRD patterns of the Li_x[Co_0.9Al_0.1]O₂ samples suggest the solid solution limit is between Li/(Co + Al) = 1.1 and 1.2. Electrochemical studies of the Li[Co_1−zAl_z]O₂ samples were used to measure the rate of capacity reduction with Al content, found to be about −250 ± 30 (mAh/g)/(z = 1). Literature work on Li[Ni_1/3Mn_1/3Co_1/3−zAl_z]O₂, Li[Ni_1−zAl_z]O₂ and Li[Mn_2−yAl_y]O₄ demonstrates the same rate of capacity reduction with Al/(Al + M) ratio. These studies serve as baseline characterization of samples to be used to determine the impact of Al content on the thermal stability of delithiated Li[Co_1−zAl_z]O₂ in electrolyte.     

Structure and electrochemical properties of nanocarbon-coated Li3V2(PO4)3 prepared by sol-gel method

A liquid-based sol-gel method was developed to synthesize nanocarbon-coated Li₃V₂(PO₄)₃. The products were characterized by XRD, SEM and electrochemical measurements. The results of Rietveld refinement analysis indicate that single-phase Li₃V₂(PO₄)₃ with monoclinic structure can be obtained in our experimental process. The discharge capacity of carbon-coated Li₃V₂(PO₄)₃ was 152.6 mAh/g at the 50th cycle under 1C rate, with 95.4% retention rate of initial capacity. A high discharge capacity of 184.1 mAh/g can be obtained under 0.12C rate, and a capacity of 140.0 mAh/g can still be held at 3C rate. The cyclic voltammetric measurements indicate that the electrode reaction reversibility is enhanced due to the carbon-coating. SEM images show that the reduced particle size and well-dispersed carbon-coating can be responsible for the good electrochemical performance obtained in our experiments.