Abstract
This work elucidated the structure and electrochemical performance of the layered cathode material NaxMn0.5Co0.5O2(NMC) with x~1 calcined at 650, 800 and 900oC. XRD diffraction indicated that the NMC material possessed a phase transition from P3- to P2-type layered structure with bi-phasic P3/P2 at medium temperature. The sodium storage performance was evaluated by constant current charging/discharging. Specifically, P2-NMC exhibits the highest initial capacity of 156.9 mAh.g-1 with capacity retention of 76.2% after 100 cycles, which is superior to the initial discharge capacity is only 149.3 mAh.g-1 and severe capacity fading per cycle of P3-NMC, indicating high robust structure stability by calcined at the higher temperature. Additionally, the best performance of P2-NMC also demonstrated by the high-rate capability that delivered a capacity of 50 mAh.g-1 at charge/discharge rate up to 10C. The sodium diffusion coefficient of Na+ ions into the P2-type layered structure obtained by Cyclic Voltammetry (CV) was in the range of 10−13–10−12 cm2.s−1, which is inferior to the others. However, the structure stability of P2 phase contributed to the best prerformance in sodium-ion half-cell.
Keywords: calcinated temperature, electrochemical performance, sodium-ion batteries, NaxMn0.5Co0.5O2, P-type layered structure.
INTRODUCTION
The planet-warming due to the greenhouse gases exhausted from burning fossil fuels is the primary motivation for switching to renewable energy sources and electricity. In this context, electrochemical energy storage technologies like rechargeable batteries which are large storage capacity, easy to install and low cost, are crucial to adapt to the interruption of energy sources. For e long time, lithium-ion batteries (LIBs) have been the dominant power sources for various electronic and electric devices serving every aspect of modern life. Recently, the expansion of the electric vehicles market requires more lithium-ion cells to produce enormous energy to ensure proper travel distance. Despite the large number of lithium-ion cells consumed worldwide and growing steadily every year, the predicted shortfall of lithium resources is still a massive challenge for the stability of LIBs uses and the downfall of its price 1. Therefore, alternative power sources that lower costs and use more abundant and sustainable elements are ever needed. Many researchers are interested in LIB analogs using alkaline and earth elements like sodium, potassium, aluminum and magnesium… Among them, sodium-ion batteries (SIBs) are the ones that have gained high maturity and are poised to commercial. Sodium-ion batteries’ price will be lower than their lithium counterparts because of the abundance of raw materials, including sodium and metal elements 2. However, the batteries’ currently low energy density, about half of LIBs, limits their applications primarily to household and stationary markets.
Improvement in component engineering, especially cathode electrodes, is expected to improve the energy density of SIB. The high energy density SIBs could be gained using layered structure oxides that are conventionally high in the specific capacity. Sodium transition oxides typically crystallize in O3-type and P2-type layered structures, as named by Delmas and co-workers 3, in which the sodium ions are accommodated at octahedral and prismatic sites, respectively. P2-type materials give good reversible capacity, rate performance and longer cycle life compared to O3-type ones due to higher sodium diffusibility 4–10. The unitary compounds like NaxMnO2 and NaxCoO2 containing only one transition metal in the repeating (MO2)n sheets were intensively investigated 11,12, but their poor performance prevented them from being considered as cathode materials for SIBs. Introducing extra ions through substitution and doping is verified to improve the electrochemical properties via synergistic effect. Up to now, abundant multicomponent materials with P2-type layered structures have been evaluated during the development of cathode materials for SIBs 4–10.
Among several combinations built for sodium insertion cathode such as Fe-Mn, Ni-Mn, Ni-Mn-Co, Fe-Ni-Mn… Mn-Co incorporation is attractive for its specific capacity and rate capability13–21. For instance, P2-Na2/3[Mn0.8Co0.2]O2illustrated excellent electrochemical performance with a capacity of about 175 mAh.g-1 at 0.1C, and over 90% of its initial capacity remains after 300 cycles at 0.1C and 10C22. For sodium-deficient layered cathodes, P3 and P2 are low and high-temperature phases 23. Typically, Chen and co-workers reported the preparation of P2-Na0.67Co0.5Mn0.5O2and P3/P2-Na0.66Co0.5Mn0.5O2at 950 oC and 800 oC, respectively, through a facile and straightforward sol-gel route19,24. The P2-phase material delivered a high discharge capacity of 147 mAh.g-1 at 0.1C rate and excellent cyclic stability with nearly 100% capacity retention over at least 100 cycles at 1C 19. The P2/P3-phase material exhibited an impressively higher discharge capacity of 156.1 mAh.g-1 at 1C in the voltage range of 1.5-4.3 V24. It was noticed that synthesis temperatures change affects the macro and nanostructure of the cathode materials related to the electrochemical properties. The two phases intergrowth cathodes showed the improvement in electrochemical performance but the proper ratio of the two phases have not been considered.
In this work, we evaluated the structure and the electrochemical properties relationship of NaxMn0.5Co0.5O2(NMC55) obtained at 650, 800 and 900 oC. We started from a sodium/transition element ratio of unity that was the same as the O3-type phases to lower the mixture’s temperature. The results suggested that high temperature is better for this material to maximize performance in SIBs due to its own structure stability. In oposite, the phase integrated P3/P2 does not effectively enhance the material’s specific capacity.
EXPERIMENTAL
NaMn0.5Co0.5O2 (NMC) was prepared by the conventional solid-state reaction. The starting precursor mixture including Na2CO3(Merck, >99%), MnCO3 (Sigma-Aldrich, >99%) and Co(NO3)2.6H2O (Sigma-Aldrich, >99%) in a Na:Mn:Co ratio of 2.1:1:1 respectively was mixed with 1 mL distilled to make a homogeneous slurry. The slurry was stored for about30 mins to stable and then heated at 500oC for 12 hours in the air to decompose and ground into precursor powder. The next heat treatment step is calcinating the precursor bronze between 650 oC to 900oC for 12 hours in the Ar atmosphere to gain the final material. In this step, the materials were removed from the furnace at calcinated temperature to cool rapidly in the air and then transferred to an Ar-filled glovebox (GP-Campus, Jacomex). The final samples were named after the calcined temperatures, which are NMC-650, NMC-800 and NMC-900.
The crystal structure of the synthesized NMC samples was examed using X-ray diffraction performed on D8 Advanced (Bruker) diffractometer equipped with a Cu X-ray source (λ = 0.15418 nm). The measurement was conducted in a 2θ angle range from 10o to 70 o with 0.02o/step/0.25s scan rate. In addition, the lattice parameters were obtained from Rietveld refinement using Material Studio (version 2017).
The Raman spectrum was collected on Xplora One Raman Spectrometer (Horiba) equipped with Ar+ laser with a wavelength of 532 nm. The morphologies of NMC materials were evaluated using FE-SEM S-4800 (Hitachi) scanning electron microscope with energy-dispersive X-ray spectroscopy (EDS) coupled optionally to explore the chemical composition and elements distribution.
The cathode electrode was prepared in the Ar-filled glovebox. The as-prepared NMC material, conducting carbon C65 and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) binder in a weight ratio of 80:15:5 respectively, was mixed with N-methy1-2-pyrrolidone (NMP) solvent under magnetic stirring for several hours to make a slurry. Then, the slurry was coated onto technical Al foil and dried in a vacuum oven at 100oC for 12 hours. Finally, the foil was punched into round pieces with a diameter of 12 mm and mass loading of 2-3 mg.cm-2.
Coin cells CR2032 were used in electrochemical properties characterization. The cells were assembled in the Ar-filled glovebox with the cathode film, a circular Na foil as reference and counter electrodes, two Whatman glass fibers (GF/F) as separator, and the electrolyte was a solution of 1 M NaClO4 in propylene carbonate (PC) with 2% fluoroethylene carbonate (FEC) additive.
Electrochemical testing was controlled by multi-channel MPG-2 and VSP battery cycler (Biologic, France). Cyclic voltammetry (CV) was conducted with a scan rate of 20-100 µV.s-1 between 1.5 and 4.5 V. Charge-discharge performance was processed in galvanostatic mode with different C-rate (1C corresponding to a nominal specific capacity of 150 mA.g-1). Electrochemical impedance spectroscopy (EIS) was measured by applying an alternating potential with an amplitude of 10 mV in the frequency range from 1 MHz to 100 mHz at equilibrium potential.
For ex-situ XRD to evaluate the structure during charging, the cells were charged by a current density of C/20 to the target voltage and then rested for at least 3 hours before being opened in the glovebox to obtain the cathode films. The cathode films were washed in DMC at least three times then dried in a hot plate at 100 oC and sealed before being subjected to XRD analysis on D8 Advance (Bruker) coupled with a CuKα source (λ = 1.5814 Å) and the scan rate was 0.02°/step/0.25s. The cathode films for XRD analysis after cycling were prepared the same way as above from the washing step.
RESULTS AND DISCUSSION
Figure 1 shows the thermal behavior of the precursor between room temperature to 1000 oC in the dry air. It can be seen in the TGA curve that the precursor exhibits several mass losses starting from about 80 oC and lasting until the end of the investigated temperature range. The DSC curve displays a sloping curve demonstrated to endothermal reactions. The first mass loss of about 2% with a slope at roughly 83 oC in the TGA curve is of water desorption, corresponding to the first peak in the DSC curve. Additionally, the acceleration of mass loss is observed starting from roughly 200, 550 and 750 oC corresponding to the continued decomposition of residual anions/metal salts, oxygen removal, and sodium evaporation under an oxidation atmosphere. During this time, the solid-state reaction between the components also activated, and the reconstruction of the precursor happened. It could be noticed in the TGA that the decomposition seems to finish at around 800oC where no significant mass loss could be observed.