Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for ...electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization, and electrochemical performance in the oxygen evolution reaction of Fe-doped NiO nanocrystals. The facile solvothermal synthesis in tert-butanol leads to the formation of ultrasmall crystalline and highly dispersible Fe x Ni1–x O nanoparticles with dopant concentrations of up to 20%. The increase in Fe content is accompanied by a decrease in particle size, resulting in nonagglomerated nanocrystals of 1.5–3.8 nm in size. The Fe content and composition of the nanoparticles are determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy measurements, while Mössbauer and extended X-ray absorption fine structure analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous ca. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance. Fe0.1Ni0.9O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 s–1 at the overpotential of 300 mV. The current density of 10 mA cm–2 is reached at an overpotential of 297 mV with a Tafel slope of 37 mV dec–1. The extremely high catalytic activity, facile preparation, and low cost of the single crystalline Fe x Ni1–x O nanoparticles make them very promising catalysts for the oxygen evolution reaction.
The performance degradation of graphite/LiNi1/3Mn1/3Co1/3O2 (NMC) lithium ion cells, charged and discharged up to 300 cycles at different operating conditions of temperature and upper cutoff ...potential (4.2V/25°C, 4.2V/60°C, 4.6V/25°C) was investigated. A combination of electrochemical methods with X-ray diffraction (XRD) both in situ and ex situ as well as neutron induced Prompt-Gamma-Activation-Analysis (PGAA) allowed us to elucidate the main failure mechanisms of the investigated lithium ion cells. In situ XRD investigations of the NMC material revealed that the first cycle irreversible capacity is the cause of slow lithium diffusion kinetics. In full-cells, however, this "lost" lithium ions can be used to build up the SEI of the graphite electrode during the initial formation cycle. A new systematic approach to correlate the lithium content in NMC with its lattice parameters (c, a) allows a convenient quantification of the loss of active lithium in aged cells by determining the c/a ratio of harvested NMC cathodes in the discharged state using ex situ XRD. Besides loss of active lithium, transition metal dissolution/deposition on graphite and growth of cell impedance strongly effect cell aging, especially at elevated temperatures and high upper cutoff potentials.
Although lithium ion batteries (LiBs) are being used successfully in portable electronics, automobiles, and stationary power storages, safety and degradation issues are still impeding their long term ...application. The main LiB aging mechanisms that have been reported include structural changes of the cathode, loss of active material, loss of cycleable lithium due to SEI growth, and impedance rise of the cell 1.
A study by Dubarry et al. and Burns et al. on LiNi
1/3
Mn
1/3
Co
1/3
O
2
(NMC) based cells showed that during early cycles, the loss of lithium inventory is the main cause of capacity fade and that it follows a linear relationship. A rapid capacity roll-over failure, however, was attributed to deterioration of the interfacial kinetics 2,3. Further studies on the graphite/NMC system showed that also transition metal dissolution has to be considered when cycling to high charging potential limits 4 or at elevated temperatures 5. In both cases, the capacity loss which can be contributed to the loss of cyclable lithium was not quantified.
In this work, we focus on the performance degradation of graphite/NMC cells charged and discharged for 300 cycles at different temperatures (25 °C and 60 °C) and with different charging voltage limits (4.2 V, 4.6 V). Cycling data clearly suggest that two degradation mechanisms can be distinguished: (i) a linear capacity fade at a 4.2 V limit (at both 25 and 60 °C) and, (ii) a rapid capacity roll-over failure at the more positive 4.6 V limit. To analyze the major contribution to cell failure, different diagnostic techniques comprising structural investigations via powder X-ray diffraction (XRD) and neutron based elemental analysis via prompt gamma activation analysis (PGAA) to determine transition metal deposition on graphite are applied.
As NMC shows no severe structural changes like phase transition after cycling (ex-situ XRD), the material exhibits good crystal structure stability at the different tested operating conditions, which is consistent with Zheng et al. 4. Slight differences observed in peak shifts are directly associated with changes in the lattice constants
a
and
c
.
Figure 1 shows an in-situ XRD analysis of the Li/NMC system. XRD pattern collection during delithiation and lithiation of NMC demonstrates a clear correlation between the lattice ratio
c
/
a
and the corresponding lithium content of the cathode. We will show that this correlation allows for the quantification of the loss of cyclable lithium via post-mortem ex-situ XRD measurements. In addition, Fig. 1 also shows that NMC exhibits an irreversible capacity loss (ICL) in the initial cycle which goes along with structural irreversibility. As this is important for the in-situ XRD calibration, this issue will be discussed in more detail.
In conclusion, loss of lithium inventory is most detrimental for NMC/graphite cells cycled at different temperatures, whereas transition metal dissolution and active material inactivation is most severe when cycling with a charge limit of 4.6 V.
Figure 1:
In-situ XRD calibration: Potential profile of Li/NMC (top) and the corresponding ratio of the lattice parameters
a
and
c
(bottom) determined by in-situ XRD during OCV steps (indicated by spikes) as a linear function of lithium content.
References:
1
J. Vetter, P. Novák, M. Wagner, C. Veit, K-C. Möller, J. Besenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Volger, and A. Hammouche, J. Power Sources, 147, 269 (2005).
2
M. Dubarry, C. Truchot, B. Y. Liaw, K. Gering, S. Sazhin, D. Jamison, and C. Michelbacher, J. Power Sources, 196, 10336 (2011).
3
J. C. Burns, A. Kassam, N. N. Sinha, L. E. Downie, L. Solnickova, B. M. Way, and J. R. Dahn, J. Electrochem. Soc., 160, A1451 (2013).
4
H. Zheng, Q. Sun, G. Liu, X. Song, and V.S. Battaglia, J. Power Sources, 207, 134 (2012).
5
K. Amine, Z. Chen, Z. Zhang, J. Liu, W. Lu, Y. Qin, J. Lu, L. Curtis, and Y-K. Sun, J. Mater. Chem. 21, 17754 (2011).
Acknowledgement:
Funding was provided by the BMBF (Federal ministry of Education and research, Germany) of the ExZellTUM project, grant number 03X4633A.
Figure 1
