Two major strategies are currently pursued to improve the energy density of lithium-ion batteries using LiNi x Co y Mn z O2 (NCM) cathode materials. One is to increase the fraction of redox active Ni ...(≥80%), which allows larger amounts of Li to be extracted at a given cutoff voltage (U max). The other is to increase U max, in particular for medium-Ni content NCM materials. However, the accompanying lattice changes ultimately lead to capacity fading in both cases. Here the structural changes occurring in Li1.02Ni x Co y Mn z O2 (with x = 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) during cycling operation in the voltage range between 3.0 and 4.6 V vs Li are quantified by means of operando X-ray diffraction combined with detailed Rietveld analysis. All samples show a large decrease in unit cell volume upon charging, ranging from 2.4% for NCM111 (33% Ni) to 8.0% for NCM851005 (85% Ni). To make a fair comparison of the structural stability of the different NCM materials, energy densities as a function of U max are estimated and correlated with X-ray diffraction results. It is shown that NCMs with a lower Ni content allow for specific energies similar to that of, e.g., Ni-rich NCM811 (80% Ni) when operated at sufficiently high U max, but still undergo less pronounced changes in structure. Nevertheless, as indicated by charge/discharge tests, the capacity retention of low- and medium-Ni content NCMs cycled to high U max is also strongly affected by factors other than stability of the layered crystal lattice (electrolyte decomposition etc.). Overall, it is demonstrated that the complexity of the degradation processes needs to be better understood to identify optimal cycling conditions for specific cathode compositions.
In the near future, the targets for lithium-ion batteries concerning specific energy and cost can advantageously be met by introducing layered LiNi x Co y Mn z O2 (NCM) cathode materials with a high ...Ni content (x ≥ 0.6). Increasing the Ni content allows for the utilization of more lithium at a given cell voltage, thereby improving the specific capacity but at the expense of cycle life. Here, the capacity-fading mechanisms of both typical low-Ni NCM (x = 0.33, NCM111) and high-Ni NCM (x = 0.8, NCM811) cathodes are investigated and compared from crystallographic and microstructural viewpoints. In situ X-ray diffraction reveals that the unit cells undergo different volumetric changes of around 1.2 and 5.1% for NCM111 and NCM811, respectively, when cycled between 3.0 and 4.3 V vs Li/Li+. Volume changes for NCM811 are largest for x(Li) < 0.5 because of the severe decrease in interlayer lattice parameter c from 14.467(1) to 14.030(1) Å. In agreement, in situ light microscopy reveals that delithiation leads to different volume contractions of the secondary particles of (3.3 ± 2.4) and (7.8 ± 1.5)% for NCM111 and NCM811, respectively. And postmortem cross-sectional scanning electron microscopy analysis indicates more significant microcracking in the case of NCM811. Overall, the results establish that the accelerated aging of NCM811 is related to the disintegration of secondary particles caused by intergranular fracture, which is driven by mechanical stress at the interfaces between the primary crystallites.
Ni-rich LiNi x Co y Mn z O2 (NCM) cathode materials have great potential for application in next-generation lithium-ion batteries owing to their high specific capacity. However, they are subjected to ...severe structural changes upon (de)lithiation, which adversely affects the cycling stability. Herein, we investigate changes in crystal and electronic structure of NCM811 (80% Ni) at high states of charge by a combination of operando X-ray diffraction (XRD), operando hard X-ray absorption spectroscopy (hXAS), ex situ soft X-ray absorption spectroscopy (sXAS), and density functional theory (DFT) calculations and correlate the results with data from galvanostatic cycling in coin cells. XRD reveals a large decrease in unit cell volume from 101.38(1) to 94.26(2) Å3 due to collapse of the interlayer spacing when x(Li) < 0.5 (decrease in c-axis from 14.469(1) Å at x(Li) = 0.6 to 13.732(2) Å at x(Li) = 0.25). hXAS shows that the shrinkage of the transition metal–oxygen layer mainly originates from nickel oxidation. sXAS, together with DFT-based Bader charge analysis, indicates that the shrinkage of the interlayer, which is occupied by lithium, is induced by charge transfer between O 2p and partially filled Ni eg orbitals (resulting in decrease of oxygen–oxygen repulsion). Overall, the results demonstrate that high-voltage operation of NCM811 cathodes is inevitably accompanied by charge-transfer-induced lattice collapse.
LiNi0.5Mn1.5O4 (LNMO) cathode active materials for lithium‐ion batteries have been investigated for over 20 years. Despite all this effort, it has not been possible to transfer their favorable ...properties into applicable, stable battery cells. To make further progress, the research perspective on these spinel type materials needs to be updated and a number of persisting misconceptions on LNMO have to be overcome. Therefore, the current knowledge on LNMO is summarized and controversial points are addressed by detailed considerations on the composition and crystallography of LNMO. The findings are supported by an in‐situ high temperature X‐ray diffraction study and the investigation of four different types of LNMO materials, including Mn(III) rich ordered LNMO, and disordered LNMO with low Mn(III) content. It is shown that the importance of cation order is limited to a small composition range. Furthermore, new evidence contradicting the idea of oxygen defects in LNMO is presented and an enhanced classification of LNMO based on the Ni content of the spinel phase is proposed. Moreover, a balanced chemical equation for the formation of LNMO is presented, allowing for comprehensive calculations of key properties of LNMO materials. Finally, suitable target compositions and calcination programs are suggested to obtain better LNMO materials.
This study summarizes and extends the knowledge of the solid‐state chemistry of LiNi0.5Mn1.5O4 (LNMO) cathode active materials for lithium‐ion batteries. A balanced equation for the formation of LNMO is presented, allowing for comprehensive calculations of LNMO material properties. A coherent understanding of LNMO emerges, suggesting that Mn(III) rich disordered LNMO materials are not necessarily the best option for energy storage applications.
High-performance Ag–Se-based n-type printed thermoelectric (TE) materials suitable for room-temperature applications have been developed through a new and facile synthesis approach. A high magnitude ...of the Seebeck coefficient up to 220 μV K–1 and a TE power factor larger than 500 μW m–1 K–2 for an n-type printed film are achieved. A high figure-of-merit ZT ∼0.6 for a printed material has been found in the film with a low in-plane thermal conductivity κF of ∼0.30 W m–1 K–1. Using this material for n-type legs, a flexible folded TE generator (flexTEG) of 13 thermocouples has been fabricated. The open-circuit voltage of the flexTEG for temperature differences of ΔT = 30 and 110 K is found to be 71.1 and 181.4 mV, respectively. Consequently, very high maximum output power densities p max of 6.6 and 321 μW cm–2 are estimated for the temperature difference of ΔT = 30 K and ΔT = 110 K, respectively. The flexTEG has been demonstrated by wearing it on the lower wrist, which resulted in an output voltage of ∼72.2 mV for ΔT ≈ 30 K. Our results pave the way for widespread use in wearable devices.
LiNi0.5Mn1.5O4 (LNMO) based spinel cathode materials for lithium-ion batteries are promising alternatives to widely used mixed transition-metal layered Li(Ni,Co,Mn)O2 (NCM) oxides. LNMO is cobalt ...free and thus cost efficient, while providing a high operating voltage of 4.7 V (vs. Li/Li+) and remarkable energy density of ∼650 W h kg−1. Commercialization and large-scale application however are still hindered, as short cycle life remains a main issue. To help overcome this problem, we present a comprehensive investigation into Fe–Ti doped LNMO materials with the formal composition LiNi0.5Mn1.37Fe0.1Ti0.03O3.95 (LNMFTO). Within this study, samples were calcined at temperatures between 460 °C and 940 °C and were cooled down to room temperatures rapidly or slowly. Small changes in the crystal structures were tracked by using a high-precision powder X-ray diffraction (PXRD) setup, while changes in cation order were investigated with Raman spectroscopy. It is shown that carefully elaborated calcination programs allow to maintain the optimized morphological features such as particle size, shape and specific surface, while crystallographic properties, such as the amounts of Mn(iii) or (partial) cation order, can be adjusted independently. We provide experimental evidence that calcination at high temperatures leads to nickel loss in the spinel phase, but not to the formation of additional oxygen defects. LNMFTO samples show good cycling stabilities and over 98% capacity retention after 100 cycles with capacities larger than 90 mA h g−1 at discharge rates of 10C when cycled vs. lithium metal. These results are almost fully transferable to cathodes with high active material loadings cycled vs. graphite anodes. A capacity retention of > 89% for 500 cycles and residual capacities of >100 mA h g−1 are observed, which makes LNMFTO a suitable candidate for industrial applications.
Abstract
Owing to electric-field screening, the modification of magnetic properties in ferromagnetic metals by applying small voltages is restricted to a few atomic layers at the surface of metals. ...Bulk metallic systems usually do not exhibit any magneto-electric effect. Here, we report that the magnetic properties of micron-scale ferromagnetic metals can be modulated substantially through electrochemically-controlled insertion and extraction of hydrogen atoms in metal structure. By applying voltages of only ~ 1 V, we show that the coercivity of micrometer-sized SmCo
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, as a bulk model material, can be reversibly adjusted by ~ 1 T, two orders of magnitudes larger than previously reported. Moreover, voltage-assisted magnetization reversal is demonstrated at room temperature. Our study opens up a way to control the magnetic properties in ferromagnetic metals beyond the electric-field screening length, paving its way towards practical use in magneto-electric actuation and voltage-assisted magnetic storage.
Li-rich disordered rock-salt oxides such as Li1.2Ni1/3Ti1/3Mo2/15O2 are receiving increasing attention as high-capacity cathodes due to their potential as high-energy materials with variable ...elemental composition. However, the first-cycle oxygen release lowers the cycling performance due to cation densification and structural reconstruction on the surface region. This work explores the influence of lithium excess on the charge compensation mechanism and the effect of surface modification with LiNbO3 on the cycling performance. Moving from a stoichiometric LiNi0.5Ti0.5O2 composition toward Li-rich Li1.2Ni1/3Ti1/3Mo2/15O2, oxygen redox is accompanied by oxygen release. Thereby, cationic charge compensation is governed by the Ni2+/3+ and Mo3+/6+ redox reaction. Contrary to the bulk oxidation state of Mo6+ in the charged state, a mixed Mo valence on the surface is found by XPS. Furthermore, it is observed that smaller particle sizes result in higher specific capacities. Tailoring the surface properties of Li1.2Ni1/3Ti1/3Mo2/15O2 with a solid electrolyte layer of LiNbO3 altered the voltage profile, resulting in a higher average discharge voltage as compared to the unmodified material. The results hint at the interdiffusion of cations from the metal oxide surface coating into the electrode material, leading to bulk composition changes (doping) and a segregated Nb-rich surface. The main finding of this work is the enhanced cycling stability and lower impedance of the surface-modified compound. We argue that surface densification is mitigated by the Nb doping/surface modification.
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