We report on results of a comprehensive investigation on reaction mechanisms occurring during Li uptake and release of the composite NiFe
2
O
4
/CNT.
Operando
X-ray diffraction (XRD) and X-ray ...absorption spectroscopy (XAS) data collected simultaneously using one
in situ
cell allowed thorough elucidation of structural and electronic alterations happening during Li uptake. From the beginning of Li uptake, the Bragg intensity of the spinel reflections decreases which can be explained by reduction of Fe
3+
ions and simultaneous movement of the Fe
2+
cations from tetrahedral 8a to empty octahedral 16c sites. The reduction of Fe
3+
is clearly evidenced by XAS. The occupation of tetrahedral sites by Li
+
can be excluded based on results of density functional theory calculations. Increasing the Li content leads to formation of a new crystalline phase resembling a monoxide with a NaCl-like structure. The appearance of the new phase is accompanied by a steady decrease of the sizes of coherently scattering domains of the spinel and a growth of the domains of the monoxide phase. After uptake of about 2.5 Li per NiFe
2
O
4
, all Fe
3+
cations are reduced to Fe
2+
and the tetrahedral 8a sites are empty (XAS spectra). Careful Rietveld refinements of X-ray powder patterns demonstrate that the tetrahedral 8a site is successively depleted with increasing Li content. Interestingly, the occupancy of the octahedral 16d site is also slightly reduced. Increasing the Li content beyond 2.5 Li/NiFe
2
O
4
leads to successive reduction of the cations to very small metal particles embedded in a Li
2
O matrix (as evidenced by
7
Li MAS NMR investigations). During Li release metallic Ni and Fe are reoxidized to Ni
2+
resp. Fe
3+
. The cycling stability of NiFe
2
O
4
/CNT is significantly improved compared to pure NiFe
2
O
4
or a mechanical mixture of NiFe
2
O
4
and CNTs.
Transition metal cations on the move: simultaneous
operando
XAS and XRD investigations during Li uptake and release of a NiFe
2
O
4
/CNT composite.
Both the chemical shift and quadrupole coupling tensors for 14 N and 27 Al in the wurtzite structure of aluminum nitride have been determined to high precision by single-crystal NMR spectroscopy. A ...homoepitaxially grown AlN single crystal with known morphology was used, which allowed for optical alignment of the crystal on the goniometer axis. From the analysis of the rotation patterns of 14 N ( I = 1 ) and 27 Al ( I = 5 / 2 ), the quadrupolar coupling constants were determined to χ ( 14 N ) = ( 8.19 ± 0.02 ) kHz, and χ ( 27 Al ) = ( 1.914 ± 0.001 ) MHz. The chemical shift parameters obtained from the data fit were δ i s o = − ( 292.6 ± 0.6 ) ppm and δ Δ = − ( 1.9 ± 1.1 ) ppm for 14 N, and (after correcting for the second-order quadrupolar shift) δ i s o = ( 113.6 ± 0.3 ) ppm and δ Δ = ( 12.7 ± 0.6 ) ppm for 27 Al. DFT calculations of the NMR parameters for non-optimized crystal geometries of AlN generally did not match the experimental values, whereas optimized geometries came close for 27 Al with χ ¯ calc = ( 1.791 ± 0.003 ) MHz, but not for 14 N with χ ¯ calc = − ( 19.5 ± 3.3 ) kHz.
We studied Li/Li4Ti5O12 and Li/LiCoO2 half-cells as well as Li4Ti5O12/LiCoO2 coin-type battery cells at different state-of-charge over several cycles by means of electrochemical impedance ...spectroscopy. A simple equivalent-circuit model has been applied to probe changes at the internal interfaces of these batteries. The parameters of this model are used to follow the state-of-charge and state-of-health of the cells. Moreover plain empirical relations were introduced to accurately delineate those courses.
► Electrochemical impedance spectroscopy on complete lithium-ion battery cells ► Full cells with Li4Ti5O12 and LiCoO2 electrodes ► Degradation at internal interfaces ► Determination of state-of-charge and state-of health.
The suitability of multication doping to stabilize the disordered Fd3̅m structure in a spinel is reported here. In this work, LiNi0.3Cu0.1Fe0.2Mn1.4O4 was synthesized via a sol–gel route at a ...calcination temperature of 850 °C. LiNi0.3Cu0.1Fe0.2Mn1.4O4 is evaluated as positive electrode material in a voltage range between 3.5 and 5.3 V (vs Li+/Li) with an initial specific discharge capacity of 126 mAh g–1 at a rate of C/2. This material shows good cycling stability with a capacity retention of 89% after 200 cycles and an excellent rate capability with the discharge capacity reaching 78 mAh g–1 at a rate of 20C. In operando X-ray diffraction (XRD) measurements with a laboratory X-ray source between 3.5 and 5.3 V at a rate of C/10 reveal that the (de)lithiation occurs via a solid-solution mechanism where a local variation of lithium content is observed. A simplified estimation based on the in operando XRD analysis suggests that around 17–31 mAh g–1 of discharge capacity in the first cycle is used for a reductive parasitic reaction, hindering a full lithiation of the positive electrode at the end of the first discharge.
Developing cost-effective and reliable solid-state sodium batteries with superior performance is crucial for stationary energy storage. A key component in facilitating their application is a ...solid-state electrolyte with high conductivity and stability. Herein, we employed aliovalent cation substitution to enhance ionic conductivity while preserving the crystal structure. Optimized substitution of Y3+ with Zr4+ in Na5YSi4O12 introduced Na+ ion vacancies, resulting in high bulk and total conductivities of up to 6.5 and 3.3 mS cm−1, respectively, at room temperature with the composition Na4.92Y0.92Zr0.08Si4O12 (NYZS). NYZS shows exceptional electrochemical stability (up to 10 V vs. Na+/Na), favorable interfacial compatibility with Na, and an excellent critical current density of 2.4 mA cm−2. The enhanced conductivity of Na+ ions in NYZS was elucidated using solid-state nuclear magnetic resonance techniques and theoretical simulations, revealing two migration routes facilitated by the synergistic effect of increased Na+ ion vacancies and improved chemical environment due to Zr4+ substitution. NYZS extends the list of suitable solid-state electrolytes and enables the facile synthesis of stable, low-cost Na+ ion silicate electrolytes.
The local structure and oxygen ion dynamics in pure CeO2 and CeO2 doped with Ta and Gd is studied by 17O nuclear magnetic resonance (NMR) spectroscopy, Raman spectroscopy, and conductivity ...measurements. 17O magic-angle spinning nuclear magnetic resonance spectra of CeO2 doped with Ta show two oxygen environments. A major peak at 876 ppm represents the oxygen ions on the regular crystal lattice site with four Ce4+ neighbors. A second small peak at 751 ppm (1–2% area fraction) represents point defects that can be ascribed to interstitial oxygen ions. The concentration of these point defects increases with the dopant concentration. Oxygen ion dynamics was studied by variable-temperature spin–lattice relaxation time measurements on pure and Ta-doped CeO2 powders. The obtained results reveal a fast local jump process in Ta-doped CeO2 with a jump rate of about 2.5·108 s–1 at 350 K. This local hopping process is assigned to interstitial oxygen ions trapped by tantalum dopand ions.
Spinel-type Li2NiF4 has been synthesized by a novel sol-gel synthesis process without HF, LiF or F2 as fluorine source. In an easily scalable process a solution of water containing Ni(CH3COO)2·4H2O, ...Li(CH3COO)·2H2O, and CF3COOH was spray-dried to form a precursor powder. It was pyrolyzed to directly result in Li2NiF4. The Li2NiF4 powder was ball-milled with carbon and a binder to produce an Li2NiF4/carbon/binder composite suitable for electrochemical investigation as a positive electrode in a lithium-ion battery. During cycling, the discharge capacity confirmed that at least 2 eq. of lithium had been inserted into the initial Li2NiF4/carbon/binder composite. The electrochemical behavior of this composite cathode and the associated structural changes of the host material during cycling against lithium metal were investigated by in situ X-ray powder diffraction, X-ray photoelectron spectroscopy, and 7Li nuclear magnetic resonance spectroscopy.
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