The concept of a hybrid nanostructured collector made of thin vertically aligned carbon nanotubes (CNTs) decorated with Si nanoparticles provides high power density anodes in lithium‐ion batteries. ...An impressive rate capability is achieved due to the efficient electronic conduction of CNTs combined with well defined electroactive Si nanoparticles: capacities of 3000 mAh g−1 at 1.3C and 800 mAh g−1 at 15C are achieved.
In this work, exhaustive characterizations of 3D geometries of LiNi1/3Mn1/3Co1/3O2 (NMC), LiFePO4 (LFP), and NMC/LFP blended electrodes are undertaken for rational interpretation of their measured ...electrical properties and electrochemical performance. X‐ray tomography and focused ion beam in combination with scanning electron microscopy tomography are used for a multiscale analysis of electrodes 3D geometries. Their multiscale electrical properties are measured by using broadband dielectric spectroscopy. Finally, discharge rate performance are measured and analyzed by simple, yet efficient methods. It allows us to discriminate between electronic and ionic wirings as the performance limiting factors, depending on the discharge rate. This approach is a unique exhaustive analysis of the experimental relationships between the electrochemical behavior, the transport properties within the electrode, and its 3D geometry.
Multiscale characterizations of 3D geometries and electrical properties of Li‐ion electrodes made of LiNi1/3Mn1/3Co1/3O2 (NMC), LiFePO4 (LFP), and NMC/LFP blend allow the rational interpretation of their electrochemical performance by discriminating between electronic and ionic wirings as the performance limiting factors, depending on the discharge rate.
► XPS and ToF-SIMS study of solid electrolyte interphase layer on Si thin film and Si nanowires electrodes is presented. ► Modification of Si electrode surface morphology is evidenced by SEM ...micrographs. ► Organic part of solid electrolyte interphase is present at the outer- and inorganic part in the innermost surface. ► Different thickness of solid electrolyte interphase is observed in a function of type of electrolytes.
The effect of the Si electrode morphology (amorphous hydrogenated silicon thin films – a-Si:H as a model electrode and Si nanowires – SiNWs electrode) on the interphase chemistry was thoroughly investigated by the surface science techniques: X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS analysis shows a strong attenuation and positive shift of the Si 2p peaks after a complete charge/discharge performed in PC- and EC:DMC-based electrolytes for both electrodes (a-Si:H and SiNW), confirming a formation of a passive film (called solid electrolyte interphase – SEI layer). As evidenced from the XPS analysis performed on the model electrode, the thicker SEI layer was formed after cycling in PC-based electrolyte as compared to EC:DMC electrolyte. XPS and ToF-SIMS investigations reveal the presence of organic carbonate species on the outer surface and inorganic salt decomposition species in the inner part of the SEI layer. Significant modification of the surface morphology for the both electrodes and a full surface coverage by the SEI layer was confirmed by the scanning electron microscopy (SEM) analysis.
The electrochemical behavior of new high potential electrolyte containing sebaconitrile in LiPF6/EC:DMC or LiBF4 was studied on glassy carbon and LiCoO2, LiCoPO4 as positive electrode materials. The ...increase of sebaconitrile concentration in EC:DMC electrolyte provides better electrolyte stability at higher potentials on glassy carbon as observed by cyclic voltammetry. Promising electrochemical results showing good reversibility and insertion/deinsertion efficiency have been also obtained on LiCoPO4 electrode cycled up to 5.3V vs Li+/Li as upper potential limit. However, the cycling of LiCoPO4 at higher potential (6V vs Li+/Li) shows lower reversibility and efficiency of insertion/deinsertion process due to the oxidative decomposition of the electrolyte at high potentials. The surface analysis performed by X-ray photoelectron spectroscopy confirms the formation of a surface layer induced by electrolyte degradation on both types of positive electrodes, which hinder the Li diffusion. The layer composition and morphology vary as a function of electrolyte composition and type of electrode.
•Original cycling conditions combining cut-off potential and involved capacity•Influence of capacity limitation is investigated for lithiation and delithiation•Importance of a reduced delithiation ...cut-off voltage is demonstrated•At C/5 rate, 1800 cycles are reached without electrode or electrolyte optimization•A 1C rate, 2000 cycles are reached with electrolyte containing VC additive
The cycling conditions of silicon nanowires anode are investigated and a set of conditions is proposed in order to improve the cycle life of these electrodes, without any structure or surface optimization. Limitation of lithiation or delithiation to an intermediate value of 900mAhg−1 allows to perform up to 1850 cycles at C/5 rate, which represents a significant increase of the electrode cycle life compared to that observed for standard cycling. This behavior is still attractive at higher current rate. At 1C rate, it is proved that combining the lithiation-limited cycling with an upper cut-off voltage of 0.8V improves the capacity retention by a factor 2. When using these optimized conditions, combined with a simple adding of fluoroethylene carbonate or vinylene carbonate in the electrolyte, a compact solid electrolyte interphase is formed upon cycling and an exceptional capacity retention is observed, reaching respectively 1500 and more than 2000 cycles.
► SiNWs as anode material for lithium-ion batteries. ► SiNWs diameter tailoring by tuning the thickness of gold film catalyst. ► SiNWs with the smallest diameters deliver the highest capacities. ► ...High rate anode material, with smallest diameters, is obtained. ► The best rate capability, up to 5
C, found for silicon nanowires.
For the first time, the effect of the nanowires diameter in terms of size and distribution on electrochemical properties of SiNWs grown by VLS was investigated. The diameter size was tuned by using three different gold catalyst film thicknesses. The crucial influence of this parameter is evidenced through comparison of the charge–discharge behavior and a study of the rate capability for the three samples. The rechargeable capacity as well as the rate capability is shown to be the best when the smallest diameters (<65
nm) are used compared to larger one (<210
nm and <490
nm). High capacity values of 3500
mAh
g
−1 are obtained for the smallest diameters at C/5 rate but still 2500, 1500 and 500
mAh
g
−1 are recovered at C, 2.5
C and 5
C. An excellent cycle life over 50 cycles is achieved at 1.3
C with a capacity of 2500
mAh
g
−1. This shows that by tailoring the diameter size and distribution, SiNWs can provide high power density anodes in lithium ion batteries.
Si thin films obtained by plasma enhanced chemical vapor deposition (PECVD) were used to investigate chemical and morphological modifications induced by lithiation potential and cycling. These ...modifications were thoughtfully analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling, which allows to distinguish the surface and bulk processes related to the formation of the solid electrolyte interphase (SEI) layer, and Li–Si alloying, respectively. The main results are a volume expansion/shrinkage and a dynamic behavior of the SEI layer during the single lithiation/delithiation process and multicycling. Trapping of lithium and other ions corresponding to products of electrolyte decomposition are the major reasons of electrode modifications. It is shown that the SEI layer contributes to 60% of the total volume variation of Si electrodes (100 nm). The apparent diffusion coefficient of lithium (D Li) calculated from the Fick’s second law directly from Li-ion ToF-SIMS profiles is of the order of ∼5.9 × 10–15 cm2.s–1. This quite low value can be explained by Li trapping in the bulk of electrode material, at the interfaces, continuous growth of the SEI layer and increase of SiO2 quantity. These modifications can result in limitation the ionic transport of Li.
Organic electrode materials offer obvious opportunities to promote cost-effective and environmentally friendly rechargeable batteries. Over the last decade, tremendous progress has been made thanks ...to the use of molecular engineering focused on the tailoring of redox-active organic moieties. However, the electrochemical performance of organic host structures relies also on the crystal packing, like the inorganic counterparts, which calls for further efforts in terms of crystal chemistry to make a robust redox-active organic center electrochemically efficient in the solid state. Following our ongoing research aiming at elaborating lithiated organic cathode materials, we report herein on the impact of polymorphism on the electrochemical behavior of dilithium (2,3-dilithium-oxy-)terephthalate vs. Li. Having isolated dilithium (3-hydroxy-2-lithium-oxy)terephthalate through an incomplete acid-base neutralization reaction, its subsequent thermally induced decarboxylation mechanism led to the formation of a new polymorph of dilithium (2,3-dilithium-oxy-)terephthalate referred to as Li4-o-DHT (β-phase). This new phase is able to operate at 3.1 V vs. Li+/Li, which corresponds to a positive potential shift of +250 mV compared to the other polymorph formerly reported. Nevertheless, the overall electrochemical process characterized by a sluggish biphasic transition is impeded by a large polarization value limiting the recovered capacity upon cycling.
One of the weaknesses of the LiPF6-based electrolyte is its poor thermal stability. It can result in an accelerated battery capacity fading and CO2 gas generation due to the formation of PF5 which ...are highly reactive towards the electrolyte solvents. As the formation of PF5 is inevitable, efforts are dedicated to inhibiting its deleterious impact by adding a Lewis base to form a complex. However, no study investigates the PF5-complexation efficiency through gas analysis. Here, gas analysis shows that N-containing additives are effective in reducing gas generation upon electrolyte storage at 55 °C. Out of our expectations, the trend is reversed when it comes to thermal storage of NMC-graphite batteries, involving competitive chemisorption processes on the NMC acid and basic sites. It turned out that, NMC surface can be more effective than additives in mitigating the thermal degradation of the electrolyte. Furthermore, the gas level thermally generated does not decrease while replacing the culprit LiPF6 salt by mixtures of LiFSI + LiPF6 or LiFSI + LiODFB. Especially in presence of LiODFB, water triggers hydrolysis reactions that also lead to gas evolution.
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•CO2 gas analysis as a very sensitive tool to study electrolyte thermal degradation.•N-containing additives are effective for PF5-complexing in electrolyte alone.•PF5 adsorbs onto the NMC surface, preventing electrolyte solvents degradation.•LiFSI hydrolysis leads to LiODFB decomposition, generating gas.
Storage of nickel-rich LiNi1-x-yMnxCoyO2 positive active materials under improper environmental conditions brings about undesirable surface reactions causing poor Li-ion cell electrochemical ...performances and gas generation. In this paper, a detailed stepwise investigation of the increasing reactivity of 532, 622, 811 and 901 NMC materials towards a specific H2O/CO2 atmosphere is reported. Water-soluble salts as lithium carbonate, hydroxide and sulfates are accurately quantified thanks to acid-base titration and inductively coupled plasma atomic emission spectroscopy techniques. On the other hand, the presence of insoluble species as transition metal oxyhydroxides and oxides is unveiled through a thermal decomposition study using thermogravimetric mass spectrometer coupling technique. Their formation and thermal decomposition mechanisms are proposed after careful analysis of the characteristic mass loss and O2, CO2 and H2O evolution profile in the 50–500 °C region. Interestingly, the results also highlight an in situ lithiated layered oxide material reformation reaction. To assess their chemical reactivity towards LiPF6-based electrolyte, the latter was analyzed through 19F nuclear magnetic resonance after storage with various reference salts and 811 NMC. LiPO2F2 is detected from storage tests with Li2CO3, Li2O and Li2SO4, and fluorophosphate-type molecules are detected from LiOH; this experiment can help discriminate between LiOH and Li2O as NMC surface species.
•NMCs surface species composition before and after H2O/CO2 exposure is elucidated.•Warder's titration method allows for an accurate quantification of OH− and CO32−.•ICP-AES, IR and SEM/EDX data indicate the presence Li or Na sulfates.•The amount of basic species increases in Ni-rich NMCs surfaces.•Li2O is postulated instead of LiOH through 19F NMR analysis of the electrolyte.
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