Preparation and electrochemical properties of ZnO/conductive-ceramic nanocomposite as anode material for Ni/Zn rechargeable battery

A novel ZnO/conductive-ceramic nanocomposite was prepared by a homogeneous precipitation between Zn(NO₃)₂ and CO(NH₂)₂ with conductive ceramic powders as the nucleation sites. The conductive ceramic powders contained zinc oxide, bismuth oxide, cobalt oxide and rare earth oxide, and the nominal chemical composition (mole fraction) was represented by (ZnO)_0.92(Bi₂O₃)_0.054(Co₂O₃)_0.025(Nb₂O₅)_0.00075(Y₂O₃)_0.00025. The phase composition and surface morphology of the as-synthesized materials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). ZnO nanorods with the length of about 200 nm were dispersed homogeneously on the surface of the conductive ceramic, and identified as a well-defined single-phase hexagonal structure. The electrochemical properties of the ZnO/conductive-ceramic nanocomposite as anode material of Ni/Zn battery were investigated by charge/discharge cycling test, slow rate cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Compared with pure nanosized ZnO, the ZnO/conductive-ceramic nanocomposite showed better cycling stability, higher discharge capacity and utilization ratio. The initial discharge capacity of ZnO/conductive-ceramic nanocomposite reached about 644 mAh g⁻¹. The discharge capacity hardly declined over 50 cycling tests, and the average utilization ratio could reach 99.5%. EIS revealed that the charge-transfer resistance was lower than that of pure nanosized ZnO.     

Effects of the addition of ZnO and Y2O3 on the electrochemical characteristics of a Ni(OH)2 electrode in nickel–metal hydride secondary batteries

This study examined the effects of the addition of ZnO and Y₂O₃ on the electrochemical characteristics of a Ni(OH)₂ electrode in nickel–metal hydride (Ni–MH) secondary batteries. The discharge capacity of the electrode was less affected by the addition of ZnO and Y₂O₃ at a 0.2 C-rate and 25 °C. However, the addition of Y₂O₃ deteriorated the discharge capacity and the cycle life of the electrode by increasing the charge transfer resistance of the electrode at an increased C-rate of 1 C and 25 °C. Under severer conditions at 1 C-rate and 60 °C, the electrode materials were separated from the current collector and, accordingly, the discharge capacity was abruptly degraded with cycling for the electrodes comprising only 4 wt% ZnO or 4 wt% Y₂O₃. In contrast, the electrodes containing both 2 wt% ZnO and 2 wt% Y₂O₃ exhibited stable discharge capacity with cycling and excellent cycle life due to the high overvoltage for oxygen evolution. The present results indicate that the addition of ZnO and Y₂O₃ with an optimum composition suppresses oxygen evolution in the interfaces between active materials and the current collector and improves the cycle life of the electrode.     

Influence of surface modification with Sn6O4(OH)4 on electrochemical performance of ZnO in Zn/Ni secondary cells

The surface-modified ZnO by Sn₆O₄(OH)₄ was prepared by a simple hydrolyzation process and the influence of Sn₆O₄(OH)₄ on electrochemical performance of ZnO was investigated by charge/discharge cycling test, slow rate cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Compared with the unmodified ZnO, the Sn₆O₄(OH)₄-modified ZnO showed improved electrochemical properties, such as superior electrochemical cycle stability, higher discharge capacity and utilization ratio. The surface modification could suppress the dissolution of ZnO in the alkaline electrolyte and maintain the electrochemical activity of ZnO. When the Sn₆O₄(OH)₄ content reached 27 wt.%, the discharge capacity of the modified ZnO hardly declined over 80 cycling test, the average utilization ratio could reach 98.5%, and the modified ZnO electrodes had no obvious weight loss after the cycling tests. However, the charge/discharge plateau voltage with the Sn₆O₄(OH)₄-modified ZnO slightly decreased. For the modified ZnO electrodes, two anodic peaks occurred in the CV curves, and the charge transfer resistance increased from the EIS results, both of which were ascribed to the suppressive effect of surface modification on the electrochemical reactions.     

Solvothermal synthesis of hierarchical LiFePO4 microflowers as cathode materials for lithium ion batteries

Hierarchical LiFePO₄ microflowers have been successfully synthesized via a solvothermal reaction in ethanol solvent with the self-prepared ammonium iron phosphate rectangular nanoplates as a precursor, which is obtained by a simple water evaporation method beforehand. The hierarchical LiFePO₄ microflowers are self-assemblies of a number of stacked rectangular nanoplates with length of 6-8 μm, width of 1-2 μm and thickness of around 50 nm. When ethanol is replaced with the water-ethanol mixed solvent in the solvothermal reaction, LiFePO₄ micro-octahedrons instead of hierarchical microflowers can be prepared. Then both of them are respectively modified with carbon coating through a post-heat treatment and their morphologies are retained. As a cathode material for rechargeable lithium ion batteries, the carbon-coated hierarchical LiFePO₄ microflowers deliver high initial discharge capacity (162 mAh g⁻¹ at 0.1 C), excellent high-rate discharge capability (101 mAh g⁻¹ at 10 C), and cycling stability, which exhibits better electrochemical performances than carbon-coated LiFePO₄ micro-octahedrons. These enhanced electrochemical properties can be attributed to the hierarchical micro/nanostructures, which can take advantage of structure stability of micromaterials for long-term cycling. Furthermore the rectangular nanoplates as the building blocks can improve the electrochemical reaction kinetics and finally promote the rate performance.     

Effects of addition of different carbon materials on the electrochemical performance of nickel hydroxide electrode

Nickel hydroxide is used as an active material in positive electrodes of rechargeable alkaline batteries. The capacity of nickel-metal hydride (Ni-MH) batteries depends on the specific capacity of the positive electrode and utilization of the active material because of the Ni(OH)₂/NiOOH electrode capacity limitation. The practical capacity of the positive nickel electrode depends on the efficiency of the conductive network connecting the Ni(OH)₂ particle with the current collector. As β-Ni(OH)₂ is a kind of semiconductor, the additives are necessary to improve the conductivity between the active material and the current collector. In this study the effect of adding different carbon materials (flake graphite, multi-walled carbon nanotubes (MWNT)) on the electrochemical performance of pasted nickel-foam electrode was established. A method of production of MWNT special type of catalysts had an influence on the performance of the nickel electrodes. The electrochemical tests showed that the electrode with added MWNT (110-170 nm diameter) exhibited better electrochemical properties in the chargeability, specific discharge capacity, active material utilization, discharge voltage and cycling stability. The nickel electrodes with MWNT addition (110-170 nm diameter) have exhibited a specific capacity close to 280 mAh g⁻¹ of Ni(OH)₂, and the degree of active material utilization was ∼96%.     

High-rate discharge properties of nickel hydroxide/carbon composite as positive electrode for Ni/MH batteries

A chemical co-precipitation method was attempted to synthesize nickel hydroxide/carbon composite material for high-power Ni/MH batteries. The XRD analysis showed that there were a large amount of defects among the crystal lattice of the Ni(OH)₂/C composite, and the SEM investigation revealed that the as-synthesized spherical particles were composed of hundreds of nanometer crystals with a unique three-dimensional petal shape. Compared with pure Ni(OH)₂, the Ni(OH)₂/C composite showed improved electrochemical properties such as superior cycling stability, higher discharge capacity and higher mean voltage of discharge under high-rate discharge conditions, the discharge capacity and the mean discharge voltage of the Ni(OH)₂/C composite were about 281 mAh g⁻¹ and 0.303 V (vs. Hg/HgO) at 1 C-rate, 273 mAh g⁻¹ and 0.296 V at 5 C-rate, 250 mAh g⁻¹ and 0.292 V at 10 C-rate, respectively. The cyclic voltammetry (CV) tests showed that the Ni(OH)₂/C composite exhibited good electrochemical reversibility and the formation of γ-NiOOH during the charge–discharge processes was prevented. The existence of carbon in the Ni(OH)₂/C composite contributed great effect on the improvement of high-rate discharge performance.     

Nano-NiOOH prepared by splitting method as super high-speed charge/discharge cathode material for rechargeable alkaline batteries

The present paper introduces a new method to prepare nano-NiOOH by oxidizing and cracking spherical Ni(OH)₂ of nano-structure in NaClO–NaOH solution. The prepared samples were characterized by X-ray powder diffraction (XRD), field emission scan electronic microscope (FESEM) and transmission electron microscopy (TEM). Results indicate that the synthesized sample is nano-NiOOH rod of 60–150 nm in width. The charge/discharge tests show that the nano-NiOOH cathode shows good cycling reversibility at high current density of 10,000 mA g⁻¹, provides a high capacity of 276 mAh g⁻¹ and reduces the charge time to as short as 1.83 min. Furthermore, the nano-NiOOH still keeps a reversible capacity of 93.7% after 120 cycles at a super high charge/discharge current of 10,000 mA g⁻¹, showing a good charge/discharge property.     

Cathode performance of LiMnPO4/C nanocomposites prepared by a combination of spray pyrolysis and wet ball-milling followed by heat treatment

LiMnPO₄/C nanocomposites could be prepared by a combination of spray pyrolysis and wet ball-milling followed by heat treatment in the range of spray pyrolysis temperature from 200 to 500 °C. The ordered LiMnPO₄ olivine structure without any impurity phase could be identified by X-ray diffraction analysis for all samples. It could be also confirmed from scanning electron microscopy and transmission electron microscopy observations that the final samples were the LiMnPO₄/C nanocomposites with approximately 100 nm in primary particles size. The LiMnPO₄/C nanocomposite samples were used as cathode active materials for lithium batteries, and the electrochemical tests were carried out for the cell Li|1 M LiPF₆ in EC:DMC = 1:1|LiMnPO₄/C at various charge/discharge rates in three charge modes. As a result, the final sample which was synthesized at 300 °C by spray pyrolysis showed the best electrochemical performance due to the largest specific surface area, the smallest primary particle size and a well distribution of carbon. At galvanostatic charge/discharge rates of 0.05 C, the cell delivered first discharge capacities of 123 and 165 mAh g⁻¹ in correspondence to charge cutoff voltages of 4.4 and 5.0 V, respectively. Furthermore, in a constant current-constant voltage charge mode at 4.4 V, the cells also exhibited initial discharge capacities of 147 mAh g⁻¹ at 0.05 C, 145 mAh g⁻¹ at 0.1 C, 123 mAh g⁻¹ at 1 C and 65 mAh g⁻¹ at 10 C. Moreover, the cells showed fair good cycleability over 100 cycles.     

Preparation and characterization of 18650 Li(Ni1/3Co1/3Mn1/3)O2/graphite high power batteries

The commercial 18650 Li(Ni_1/3Co_1/3Mn_1/3)O₂/graphite high power batteries were prepared and their electrochemical performance at temperatures of 25 and 50 °C was extensively investigated. The results showed that the charge-transfer resistance (R_ct) and solid electrolyte interface resistance (R_sei) of the high power batteries at 25 °C decreased as states of charge (SOC) increased from 0 to 60%, whereas R_ct and R_sei increased as SOC increased from 60 to 100%. The discharge plateau voltage of batteries reduced greatly with the increase in discharge rate at both 25 and 50 °C. The high power batteries could be discharged at a very wide current range to deliver most of their capacity and also showed excellent power cycling performance with discharge rate of as high as 10 C at 25 °C. The elevated working temperature did not influence the battery discharge capacity and cycling performance at lower discharge rates (e.g. 0.5, 1, and 5 C), while it resulted in lower discharge capacity at higher discharge rates (e.g. 10 and 15 C) and bad cycling performance at discharge rate of 10 C. The batteries also exhibited excellent cycle performance at charge rate of as high as 8 C and discharge rate of 10 C.     

Characterizing capacity loss of lithium oxygen batteries by impedance spectroscopy

Polymer based carbon aerogels were prepared by synthesis of a resorcinol formaldehyde gel followed by pyrolysis at 1073 K under Ar and activation of the resultant carbon under CO₂ at different temperatures. The prepared carbon aerogels were used as active materials in the preparation of cathode electrodes for lithium oxygen cells and the electrochemical performance of the cells was evaluated by galvanostatic charge/discharge cycling and electrochemical impedance measurements. It was shown that the storage capacity and discharge voltage of a Li/O₂ cell strongly depend on the porous structure of the carbon used in cathode. EIS results also showed that the shape and value of the resistance in the impedance spectrum of a Li/O₂ cell are strongly affected by the porosity of carbon used in the cathode. Porosity changes due to the build up of discharge products hinder the oxygen and lithium ion transfer into the electrode, resulting in a gradual increase in the cell impedance with cycling. The discharge capacity and cycle life of the battery decrease significantly as its internal resistance increases with charge/discharge cycling.     

Study the effect of conductive fillers on a secondary Zn electrode based on ball-milled ZnO and Ca(OH)2 mixture powders

The active materials of the secondary Zn electrode containing a mixture powder of zinc oxide (ZnO) and calcium hydroxide (Ca(OH)₂) powders were prepared by a ball-milled method. The characteristic properties of active materials of ball-milled ZnO + Ca(OH)₂ mixture powders were examined by scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) system, X-ray diffraction (XRD) analysis, and micro-Raman spectroscopy. The prepared Zn powder electrodes were by using the ball-milled active materials powder +2 wt.% highly electronic conductive fillers, i.e., nano-copper or carbon nanotubes (CNTs) powder. The electrochemical properties of the secondary Zn electrodes without and with the conductive fillers were studied by using cyclic voltammetry (CV) and galvanostatic charge/discharge tests. It was found that the charge/discharge properties of the secondary Zn electrode could be improved when the nano-sized conductive fillers were added into the electrode. In fact, it may be due to the formation of a better electronic conduction path in the electrode matrix. In particular, it was found that the best electrochemical properties were the secondary Zn electrode with 2 wt.% nano-copper fillers. According to the results, it is demonstrated here that the CV method is a quick technique to effectively evaluate the performance of a secondary Zn electrode.     

Preparation and electrochemical properties of TiO2 hollow spheres as an anode material for lithium-ion batteries

TiO₂ hollow spheres are fabricated by a sol-gel process using carbon spheres as template. The diameter and the shell thickness of the TiO₂ hollow spheres are about 400-600 nm and 60-80 nm, respectively. The electrochemical properties of the hollow spheres are investigated by galvanostatic cycling and cyclic voltammetry (CV) measurements. The initial discharge capacity reaches 291.2 mAh g⁻¹ at a current density of 60 mA g⁻¹. The average discharge capacity loss is about 1.72 mAh g⁻¹ per cycle from the 2nd to the 40th cycles and the coulombic efficiency is approximately 98% after 40 cycles, indicating excellent cycling stability and reversibility.     

Preparation and electrochemical properties of Li[Ni1/3Co1/3Mn1−x/3Zrx/3]O2 cathode materials for Li-ion batteries

Layer-structured Zr doped Li[Ni_1/3Co_1/3Mn_1−x/3Zr_x/3]O₂ (0 ≤ x ≤ 0.05) were synthesized via slurry spray drying method. The powders were characterized by XRD, SEM and galvanostatic charge/discharge tests. The products remained single-phase within the range of 0 ≤ x ≤ 0.03. The charge and discharge cycling of the cells showed that Zr doping enhanced cycle life compared to the bare one, while did not cause the reduction of the discharge capacity of Li[Ni_1/3Co_1/3Mn_1/3]O₂. The unchanged peak shape in the differential capacity versus voltage curve suggested that the Zr had the effect to stabilize the structure during cycling. More interestingly, the rate capability was greatly improved. The sample with x = 0.01 presented a capacity of 160.2 mAh g⁻¹ at current density of 640 mA g⁻¹(4 C), corresponding to 92.4% of its capacity at 32 mA g⁻¹(0.2 C). The favorable performance of the doped sample could be attributed to its increased lattice parameter.     

Preparation and characterization of Ni-based positive electrodes for use in aqueous electrochemical capacitors

We have prepared NiO particles on Ni sheet and Ni foam substrates by chemical bath deposition and the following heat-treatment, and assembled a hybrid capacitor (HC) cell with the NiO-loaded Ni sheet or Ni foam positive electrode and activated carbon negative electrode. The deposited NiO particles had flower-like porous morphology which was composed of aggregated nanosheets. The maximum operating voltage of both HC cells was 1.5 V, which was much higher than theoretical decomposition voltage of water (1.23 V). The HC cell with NiO/Ni foam (HC_foam) had higher discharge capacitance and high-rate dischargeability and lower IR drop than the HC cell with NiO/Ni sheet (HC_sheet) because of the increase in the utilization of NiO active material. Both energy and power densities per mass of active materials, were much higher than those for the HC_sheet. Both HC_foam and HC_sheet showed excellent cycle stability for 2000 cycles.     

Mechanism of action of electrochemically active carbons on the processes that take place at the negative plates of lead-acid batteries

It is known that negative plates of lead-acid batteries have low charge acceptance when cycled at high rates and progressively accumulate lead sulphate on high-rate partial-state-of-charge (HRPSoC) operation in hybrid-electric vehicle (HEV) applications. Addition of some carbon or graphite forms to the negative paste mix improves the charge efficiency and slows down sulfation of the negative plates. The present investigation aims to elucidate the contribution of electrochemically active carbon (EAC) additives to the mechanism of the electrochemical reactions of charge of the negative plates. Test cells are assembled with four types of EAC added to the negative paste mix in five different concentrations. Through analysis of the structure of NAM (including specific surface and pore radius measurements) and of the electrochemical parameters of the test cells on HRPSoC cycling, it is established that the electrochemical reaction of charge Pb²⁺ + 2e⁻ → Pb proceeds at 300-400 mV lower over-potentials on negative plates doped with EAC additives as compared to the charge potentials of cells with no carbon additives. Hence, electrochemically active carbons have a highly catalytic effect on the charge reaction and are directly involved in it. Consequently, the reversibility of the charge/discharge processes is improved, which eventually leads to longer battery cycle life. Thus, charging of the negative plates proceeds via a parallel mechanism on the surfaces of both Pb and EAC particles, at a higher rate on the EAC phase. Cells with EAC in NAM have the longest cycle life when their NAM specific surface is up to 4 m² g⁻¹ against 0.5 m² g⁻¹ for the lead surface. The proposed parallel mechanism of charge is verified experimentally on model Pb/EAC/PbSO₄ and Pb/EAC electrodes. During the charge and discharge cycles of the HRPSoC test, the EAC particles are involved in dynamic adsorption/desorption on the lead sulfate and lead surfaces. Another effect of electrochemically active carbons is also evidenced namely that, above a definite concentration, some EAC forms reduce the mean pore radius of NAM. When it diminishes to less than 1.5 μm, access of sulfuric acid into the pores is impeded and PbO forms instead of PbSO₄ in the pores of NAM during discharge. Thus, it may be presumed that electrochemically active carbons change the overall electrochemical reaction of charge and discharge of lead-acid cells when operated under HRPSoC cycling conditions.     

Preparation and electrochemical characterization of gel polymer electrolyte based on electrospun polyacrylonitrile nonwoven membranes for lithium batteries

Electrospun membranes of polyacrylonitrile are prepared, and the electrospinning parameters are optimized to get fibrous membranes with uniform bead-free morphology. The polymer solution of 16 wt.% in N,N-dimethylformamide at an applied voltage of 20 kV results in the nanofibrous membrane with average fiber diameter of 350 nm and narrow fiber diameter distribution. Gel polymer electrolytes are prepared by activating the nonwoven membranes with different liquid electrolytes. The nanometer level fiber diameter and fully interconnected pore structure of the host polymer membranes facilitate easy penetration of the liquid electrolyte. The gel polymer electrolytes show high electrolyte uptake (>390%) and high ionic conductivity (>2 × 10⁻³ S cm⁻¹). The cell fabricated with the gel polymer electrolytes shows good interfacial stability and oxidation stability >4.7 V. Prototype coin cells with gel polymer electrolytes based on a membrane activated with 1 M LiPF₆ in ethylene carbonate/dimethyl carbonate or propylene carbonate are evaluated for discharge capacity and cycle property in Li/LiFePO₄ cells at room temperature. The cells show remarkably good cycle performance with high initial discharge properties and low capacity fade under continuous cycling.     

Temperature-controlled microwave solid-state synthesis of Li3V2(PO4)3 as cathode materials for lithium batteries

Monoclinic Li₃V₂(PO₄)₃ can be rapidly synthesized at 750 °C for 5 min (MW5m) by using temperature-controlled microwave solid-state synthesis method (TCMS). The carbon-free sample MW5m presents well electrochemical properties. In the cut-off voltage 3.0-4.3, MW5m presents a charge capacity 132 mAh g⁻¹, almost equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹), and discharge capacity 126.4 mAh g⁻¹. In the cut-off voltage 3.0-4.8 V, MW5m shows an initial discharge capacity of 183.4 mAh g⁻¹, near to the theoretical discharge capacity. In the cycle performance, the capacity fade of Li₃V₂(PO₄)₃ is dependent on the cut-off voltage and the preparation method.     

Si thin platelets as high-capacity negative electrode for Li-ion batteries

Pure Si platelets and Ni or Cu layer-laminated Si platelets with difference thickness were prepared, and their charge/discharge properties were examined in 1 M LiClO₄/EC + DEC (1:1 by volume) as alternative negative electrode materials to graphite for Li-ion batteries. The shape of thin platelets and lamination with Ni layer are significantly effective to improve the cycleability in Li-Si alloy system by relieving the stress during the alloying/de-alloying processes, reinforcing the mechanical strength and reducing the Li⁺ ion diffusion length. Moreover, the first irreversible capacity is minimized by reduction of the amount of Ketjen Black (KB) in the composite electrode because of electrolyte decomposition on the surface of KB. Consequently, the Si/Ni/Si-LP30 (30/30/30 nm) composite electrode with 5 wt% KB also exhibits over 700 mAh g⁻¹ even after 50 cycles in 1 M LiPF₆/EC + DEC (1:1).     

Electrochemical properties and lithium ion solvation behavior of sulfone–ester mixed electrolytes for high-voltage rechargeable lithium cells

Sulfone–ester mixed solvent electrolytes were examined for 5 V-class high-voltage rechargeable lithium cells. As the base-electrolyte, sulfolane (SL)–ethyl acetate (EA) (1:1 mixing volume ratio) containing 1 M LiBF₄ solute was investigated. Electrolyte conductivity, electrochemical stability, Li⁺ ion solvation behavior and cycleability of lithium electrode were evaluated. ¹³C NMR measurement results suggest that Li⁺ ions are solvated with both SL and EA. Charge–discharge cycling efficiency of lithium anode in SL–EA electrolytes was poor, being due to its poor tolerance for reduction. To improve lithium charge–discharge cycling efficiency in SL–EA electrolytes, following three trials were carried out: (i) improvement of the cathodic stability of electrolyte solutions by change in polarization through modification of solvent structure; isopropyl methyl sulfone and methyl isobutyrate were investigated as alternative SL and EA, respectively, (ii) suppression of the reaction between lithium and electrolyte solutions by addition of low reactivity surfactants of cycloalkanes (decalin and adamantane) or triethylene glycol derivatives (triglyme, 1,8-bis(tert-butyldimethylsilyloxy)-3,6-dioxaoctane and triethylene glycol di(methanesulfonate)) into SL–EA electrolytes, and (iii) change in surface film by addition of surface film formation agent of vinylene carbonate (VC) into SL–EA electrolytes. These trials made lithium cycling behavior better. Lithium cycling efficiency tended to increase with a decrease in overpotential. VC addition was most effective for improvement of lithium cycling efficiency among these additives. Stable surface film is formed on lithium anode by adding VC and the resistance between anode/electrolyte interfaces showed a constant value with an increase in cycle number. When the electrolyte solutions without VC, the interfacial resistance increased with an increase in cycle number. VC addition to SL–EA was effective not only for Li/LiCoO₂ cell with charge cut-off voltage of 4.5 V but also for Li/LiNi_0.5Mn_1.5O₄ cells even with high charge cut-off voltage of 5 V in Li/LiNi_0.5Mn_1.5O₄ cells.     

Thermal stability of Vale Inco nanonometric nickel as a catalytic additive for magnesium hydride (MgH2)

The structure stability of nanometric-Ni (n-Ni) produced by Vale Inco Ltd. Canada as a catalytic additive for MgH₂ has been investigated. Each n-Ni filament is composed of nearly spherical interconnected particles having a mean diameter of 42 ± 16 nm. After ball milling of the MgH₂ + 5 wt.%n-Ni mixture for 15 min the n-Ni particles are found to be uniformly embedded within the particles of MgH₂ and at their surfaces. Neither during ball milling of the MgH₂ + 5 wt.%n-Ni mixture nor its first decomposition at temperatures of 300, 325, 350 and 375 ^°C the elemental n-Ni reacts with the elemental Mg to form the Mg₂Ni intermetallic phase (and eventually the Mg₂NiH₄ hydride). The n-Ni additive acts as a strong catalyst accelerating the kinetics of desorption. From the Arrhenius and Johnson–Mehl–Avrami–Kolmogorov theory the activation energy for the first desorption is determined to be ∼94 kJ/mol. After cycling at 300 ^°C the activation energy for desorption is determined to be ∼99 kJ/mol. This is much lower than ∼160 kJ/mol observed for the undoped and ball milled MgH₂. During cycling at 275 and 300 ^°C the n-Ni additive is converted into Mg₂Ni (Mg₂NiH₄). The newly formed Mg₂NiH₄ has a nanosized grain on the order of 20 nm. Its catalytic potency seems to be similar to its n-Ni precursor. The formation of Mg₂Ni (Mg₂NiH₄) may be one of the factors responsible for the systematic decrease of hydrogen capacity observed upon cycling at 275 and 300 ^°C.