No Arabic abstract
Varying the amounts of silicon and carbon, different composites have been prepared by ball milling of Si, Ni$_{3.4}$Sn$_4$, Al and C. Silicon and carbon contents are varied from 10 to 30 wt.% Si, and 0 to 20 wt.% C. The microstructural and electrochemical properties of the composites have been investigated by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and electrochemical galvanostatic cycling up to 1000 cycles. Impact of silicon and carbon contents on the phase occurrence, electrochemical capacity and cycle-life are compared and discussed. For C-content comprised between 9 and 13 wt.% and Si-content >= 20 wt.%, Si nanoparticles are embedded in a Ni$_{3.4}$Sn$_4$-Al-C matrix which is chemically homogeneous at the micrometric scale. For other carbon contents and low Si-amount (10 wt.%), no homogeneous matrix is formed around Si nanoparticles. When homogenous matrix is formed, both Ni$_3$Sn$_4$ and Si participate to the reversible lithiation mechanism, whereas no reaction between Ni$_3$Sn$_4$ and Li is observed for no homogenous matrix. Moreover, best cycle-life performances are obtained when Si nanoparticles are embedded in a homogenous matrix and Si-content is moderate (<= 20 wt.%). Composites with carbon in the 9-13 wt.% range and 20 wt.% silicon lead to the best balance between capacity and life duration upon cycling. This work experimentally demonstrates that embedding Si in an intermetallic/carbon matrix allows to efficiently accommodate Si volume changes on cycling to ensure long cycle-life.
For a successful integration of silicon in high-capacity anodes of Li-ion batteries, its intrinsic capacity decay on cycling due to severe volume swelling should be minimized. In this work, Ni-Sn intermetallics are studied as buffering matrix during reversible lithiation of Si-based anodes. Si/Ni-Sn composites have been synthetized by mechanical milling using C and Al as process control agents. Ni3Sn4, Ni3Sn2 intermetallics and their bi-phasic mixture were used as constituents of the buffering matrix. The structure, composition and morphology of the composites have been analyzed by X-ray diffraction (XRD), 119Sn Transmission Mossbauer Spectroscopy (TMS) and scanning electron microscopy (SEM). They consist of ~ 150 nm Si nanoparticles embedded in a multi-phase matrix, the nanostructuration of which improves on increasing the Ni3Sn4 amount. The electrochemical properties of the composites were analyzed by galvanostatic cycling in half-cells. Best results for practical applications are found for the bi-phasic matrix Ni3Sn4-Ni3Sn2 in which Ni3Sn4 is electrochemically active while Ni3Sn2 is inactive. Low capacity loss, 0.04 %/cycle, and high coulombic efficiency, 99.6%, were obtained over 200 cycles while maintaining a high reversible capacity above 500 mAh/g at moderate regime C/5
Embedding silicon nanoparticles in an intermetallic matrix is a promising strategy to produce remarkable bulk anode materials for lithium-ion (Li-ion) batteries with low potential, high electrochemical capacity and good cycling stability. These composite materials can be synthetized at a large scale using mechanical milling. However, for Si-Ni3Sn4 composites, milling also induces a chemical reaction between the two components leading to the formation of free Sn and NiSi2, which is detrimental to the performance of the electrode. To prevent this reaction, a modification of the surface chemistry of the silicon has been undertaken. Si nanoparticles coated with a surface layer of either carbon or oxide were used instead of pure silicon. The influence of the coating on the composition, (micro)structure and electrochemical properties of Si-Ni3Sn4 composites is studied and compared with that of pure Si. Si coating strongly reduces the reaction between Si and Ni3Sn4 during milling. Moreover, contrary to pure silicon, Si-coated composites have a plate-like mor-phology in which the surface-modified silicon particles are surrounded by a nanostructured, Ni3Sn4-based matrix leading to smooth potential profiles during electrochemical cycling. The chemical homogeneity of the matrix is more uniform for carbon-coated than for oxygen-coated silicon. As a consequence, different electrochemical behaviors are obtained depending on the surface chemistry, with better lithiation properties for the carbon-covered silicon able to deliver over 500 mAh/g for at least 400 cycles.
Despite recent significant developments of Si composites, use of silicon with significance in the anodes for Li-ion batteries is still limited. In fact, nominal energy density is to be saturated around ~750 Wh/L regardless of cell-types under the current material strategies. Use of Si-rich anode can push the limit; however, the prolonged irreversible Li consumption becomes more prominent. We previously showed that repeating c-Li3.75(+{delta})Si formation/decomposition, typically recognized to degrade the anodes, can improve the irreversibility and accumulatively minimize the gross consumption. Utilizing the insights combined with prelithiation techniques, here we provide prototypic cell designs that can nonlinearly deplete the consumption.
The reversibility and cyclability of anionic redox in battery electrodes hold the key to its practical employments. Here, through mapping of resonant inelastic X-ray scattering (mRIXS), we have independently quantified the evolving redox states of both cations and anions in Na2/3Mg1/3Mn2/3O2. The bulk-Mn redox emerges from initial discharge and is quantified by inverse-partial fluorescence yield (iPFY) from Mn-L mRIXS. Bulk and surface Mn activities likely lead to the voltage fade. O-K super-partial fluorescence yield (sPFY) analysis of mRIXS shows 79% lattice oxygen-redox reversibility during initial cycle, with 87% capacity sustained after 100 cycles. In Li1.17Ni0.21Co0.08Mn0.54O2, lattice-oxygen redox is 76% initial-cycle reversible but with only 44% capacity retention after 500 cycles. These results unambiguously show the high reversibility of lattice-oxygen redox in both Li-ion and Na-ion systems. The contrast between Na2/3Mg1/3Mn2/3O2 and Li1.17Ni0.21Co0.08Mn0.54O2 systems suggests the importance of distinguishing lattice-oxygen redox from other oxygen activities for clarifying its intrinsic properties.
Lithium--sulfur (Li/S) batteries are regarded as one of the most promising energy storage devices beyond lithium-ion batteries because of their high energy density of 2600 Wh/kg and an affordable cost of sulfur. Meanwhile, some challenges inherent to Li/S batteries remain to be tackled, for instance, the polysulfide (PS) shuttle effect, the irreversible solidification of Li$_2$S, and the volume expansion of the cathode material during discharge. On the molecular level, these issues originate from the structural and solubility behavior of the PS species in bulk and in the electrode confinement. In this study, we use classical molecular dynamics (MD) simulations to develop a working model for PS of different chain lengths in applied electrolyte solutions of lithium bistriflimide (LiTFSI) in 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) mixtures. We investigate conductivities, diffusion coefficients, solvation structures, and clustering behavior and verify our simulation model with experimental measurements available in literature and newly performed by us. Our results show that diffusion coefficients and conductivities are significantly influenced by the chain length of PS. The conductivity contribution of the short chains, like Li$_2$S$_4$, is lower than of longer PS chains, such as Li$_2$S$_6$ or Li$_2$S$_8$, despite the fact that the diffusion coefficient of Li$_2$S$_4$ is higher than for longer PS chains. The low conductivity of Li$_2$S$_4$ can be attributed to its low degree of dissociation and even to a formation of large clusters in the solution. It is also found that an addition of 1 M LiTFSI into PS solutions considerably reduces the clustering behavior. Our simulation model enables future systematic studies in various solvating and confining systems for the rational design of Li/S electrolytes.