Mechanisms for overcharging of carbon electrodes in lithium-ion/sodium-ion batteries analysed by operando solid-state NMR

A precise understanding of the mechanism for metal (Li and Na) plating on negative electrodes that occurs with overcharging is critical to managing the safety of lithium- and sodium-ion batteries. In this work, an in-depth investigation of the overlithiation/oversodiation and subsequent delithiation/desodiation of graphite and hard carbon electrodes in the first cycle was conducted using operando7Li/23Na solid-state NMR. In the 7Li NMR spectra of half cells of carbon electrodes and metal counter electrodes, three types of signals corresponding to Li dendrites that formed on the surface of graphite, hard carbon, and the counter electrode were distinguished from the signal of Li metal foil of the counter electrode by applying an appropriate orientation of the testing cell. For graphite overlithiation, the deposition of Li dendrites started immediately or soon after the minimum electric potential in the lithiation curve. In contrast, the deposition of Li dendrites in hard carbon started after the end of quasimetallic lithium formation for overlithiation at rates below 3.0C. Similar behaviour was also observed for the oversodiation of hard carbon. The formation of quasimetallic Li or Na in the pores of hard carbon serves as a buffer for the metal plating that occurs with overcharging of the batteries. Furthermore, some of the deposited Li/Na dendrites contribute to reversible capacities. A mechanism for the inhomogeneous disappearance of quasimetallic Li during delithiation of hard carbon is also proposed

Optical pumping NMR in the compensated semiconductor InP:Fe

The optical pumping NMR effect in the compensated semiconductor InP:Fe has been investigated in terms of the dependences of photon energy (E_p), helicity (sigma+-), and exposure time (tau_L) of infrared lights. The {31}P and {115}In signal enhancements show large sigma+- asymmetries and anomalous oscillations as a function of E_p. We find that (i) the oscillation period as a function of E_p is similar for {31}P and {115}In and almost field independent in spite of significant reduction of the enhancement in higher fields. (ii) A characteristic time for buildup of the {31}P polarization under the light exposure shows strong E_p-dependence, but is almost independent of sigma+-. (iii) The buildup times for {31}P and {115}In are of the same order (10^3 s), although the spin-lattice relaxation times (T_1) are different by more than three orders of magnitude between them. The results are discussed in terms of (1) discrete energy spectra due to donor-acceptor pairs (DAPs) in compensated semiconductors, and (2) interplay between {31}P and dipolar ordered indium nuclei, which are optically induced.Comment: 8 pages, 6 figures, submitted to Physical Review

Properties of a novel hard-carbon optimized to large size Lion secondary battery studied by 7Li NMR

The state of lithium in a novel hard-carbon optimized to the anode of large size Li ion secondary battery, which has been recently commercialized, was investigated and compared with other existing hard-carbon samples by 7Li NMR method. The new carbon material showed a peak at 85 ppm with a shoulder signal at 7 ppm at room temperature in static NMR spectrum, and the former shifted to 210 ppm at 180 K. The latter at room temperature was attributed to Li doped in small particles contained in the sample. The new carbon sample showed weaker intensity of cluster-lithium signal than the other hard-carbon samples in NMR, which corresponded to a tendency of less "Constant Voltage" (CV) capacity in charge-discharge curves of electrochemical evaluation. Smaller CV capacity and initial irreversible capacity, which are the features of the novel hard-carbon, are considered to correspond to a blockade of the diffusion of Li into pore of carbon.</p

Surface-sensitive NMR in optically pumped semiconductors

We present a scheme of surface-sensitive nuclear magnetic resonance in optically pumped semiconductors, where an NMR signal from a part of the surface of a bulk compound semiconductor is detected apart from the bulk signal. It utilizes optically oriented nuclei with a long spin-lattice relaxation time as a polarization reservoir for the second (target) nuclei to be detected. It provides a basis for the nuclear spin polarizer [IEEE Trans. Appl. Supercond. 14, 1635 (2004)], which is a polarization reservoir at a surface of the optically pumped semiconductor that polarizes nuclear spins in a target material in contact through the nanostructured interfaces.Comment: 4 pages, 5 figure

Origin for the enhanced copper spin echo decay rate in the pseudogap regime of the multilayer high-T_c cuprates

We report measurements of the anisotropy of the spin echo decay for the inner layer Cu site of the triple layer cuprate, Hg_0.8Re_0.2Ba_2Ca_2Cu_3O_8 (T_c=126 K) in the pseudogap T regime below T_pg ~ 170 K and the corresponding analysis for their interpretation. As the field alignment is varied, the shape of the decay curve changes from Gaussian (H_0 \parallel c) to single exponential (H_0 \perp c). The latter characterizes the decay caused by the fluctuations of adjacent Cu nuclear spins caused by their interactions with electron spins. The angular dependence of the second moment (T_{2M}^{-2} \equiv ) deduced from the decay curves indicates that T_{2M}^{-2} for H_0 \parallel c, which is identical to T_{2G}^{-2} (T_{2G} is the Gaussian component), is substantially enhanced, as seen in the pseudogap regime of the bilayer systems. Comparison of T_{2M}^{-2} between H_0 \parallel c and H_0 \perp c indicates that this enhancement is caused by electron spin correlations between the inner and the outer CuO_2 layers. These results provide the answer to the long-standing controversy regarding the opposite T dependences of (T_1T)^{-1} and T_{2G}^{-2} in the pseudogap regime of bi- and trilayer systems.Comment: 4 pages, 4 figure

Advanced Biopolymer-Based Nanocomposites and Hybrid Materials

The exploitation of naturally occurring polymers to engineer advanced nanocomposites and hybrid materials is the focus of increasing scientific activity, explained by growing environmental concerns and interest in the peculiar features and multiple functionalities of these macromolecules. Natural polymers, such as polysaccharides and proteins, present a remarkable potential for the design of all kinds of materials for application in a multitude of domains. This Special Issue collected the work of scientists on the current developments in the field of multifunctional biopolymer-based nanocomposites and hybrid materials with a particular emphasis on their production methodologies, properties, and prominent applications. Thus, materials related to bio-based nanocomposites and hybrid materials manufactured with different partners, namely natural polymers, bioactive compounds, and inorganic nanoparticles, are reported in the Special Issue Advanced Biopolymer-Based Nanocomposites and Hybrid Materials

Analysis of bis(trifluoromethylsulfonyl)imide-doped paramagnetic graphite intercalation compound using F-19 very fast magic angle spinning nuclear magnetic resonance

F atoms bonding to paramagnetic/conductive graphene layers in accepter-type graphite intercalation compounds (GICs) are analyzed using very fast magic angle spinning nuclear magnetic resonance, which is applied for the first time on F-19 nuclei to investigate paramagnetic materials. In the bis(trifluoromethylsulfonyl)imide(TFSI)-doped GIC, C-F bonds between fluorine atoms and graphene layers conform to a weak bonding of F to the graphene sheets. TFSI anions intercalated in the GIC do not show overall molecular motion; even at room temperature only the CF3 groups rotate

NMR study for electrochemically inserted Na in hard carbon electrode of sodium ion battery

The state of sodium inserted in the hard carbon electrode of a sodium ion battery having practical cyclability was investigated using solid state 23Na NMR. The spectra of carbon samples charged (reduced) above 50 mAh g−1 showed clear three components. Two peaks at 9.9 ppm and 5.2 ppm were ascribed to reversible sodium stored between disordered graphene sheets in hard carbon because the shift of the peaks was invariable with changing strength of external magnetic field. One broad signal at about −9 to −16 ppm was assigned to sodium in heterogeneously distributed closed nanopores in hard carbon. Low temperature 23Na static and magic angle spinning NMR spectra didn't split or shift whereas the spectral pattern of 7Li NMR for lithium-inserted hard carbon changes depending on the temperature. This strongly suggests that the exchange of sodium atoms between different sites in hard carbon is slow. These studies show that sodium doesn't form quasi-metallic clusters in closed nanopores of hard carbon although sodium assembles at nanopores while the cell is electrochemically charged