Nowadays conversion-type electrode materials definitively lie as the core of any research programs related to Li-ion batteries. Requirements are high capacity, good rate capability, and a long cycle ...life. Indeed, the goal of much lithium battery research is to achieve the highest energy density battery as possible. In the case of pnictide materials, such performances are the results of the following conversion reaction: M x X y + 2yLi ↔ xM0 + yLi3X (X = P, Sb; M = Fe, Ni, Co, ...). However, these materials are still suffering from serious issues such as (i) low Coulombic efficiency, (ii) high polarization, (iii) poor cycle life (volume expansion), and (iv) limited rate capability that unfortunately still prevent them for any close commercial viability. In this article, the most recent research developments of our group and through collaborations in this specific field will be reported. In the interest of overcoming the limitations listed above, a cautious and rigorous scrutinizing of the electrochemical behavior of any studied materials is necessary. In our research group, we have extensive experience in the use of sophisticated in and ex situ characterization tools, in the aim to probe bulk pnictide in the Li batteries and the electrolyte/electrode surface as well. Indeed, thanks to these methods, we could unambiguously show that electrochemical conversion reactions are leading to some unstable phases, which cannot be synthesized via common chemical reaction paths. One can observe the key role of the solid/solid Li3X/M0 interfaces in the reversibility of the conversion mechanism. Contrarily, during the process, the solid/liquid electrode/electrolyte interfaces are subject to continuous parasitic reactions which drastically limit the cycle life of the battery. Fortunately, both nanostructuration of the pnictide electrodes as well as the confinement of pnictide into a porous carbon matrix play a great role in improving the performance of the cell mainly due (i) to the shortening of the distance over which Li+ diffuses or (ii) to the buffer effect of the carbon matrix against the local volume change during the charge and discharge process.
An innovative concept of activation of electrode materials for Li-ion batteries is proposed through the preparation of carbon-phosphorus (P/C) composites. Carbon-encapsulated phosphorus composites ...can be successfully prepared via a simple route by the vaporization–condensation of red phosphorus onto mesoporous carbon. Surface area measurements and Raman spectroscopy were used for the characterization of the P/C composites, which were then tested as anode materials in Li-ion batteries showing enhanced electrochemical properties. In contrast to what is observed for pure unsupported phosphorus, Li storage in P/C composite occurs through the reversible formation of Li
3P during the discharge process, as clearly evidenced by
in situ XRD, leading to capacities greater than 900
mAh
g
−1 after 20
cycles.
Niobium-doped nanofibers elaborated by facile, single-step electrospinning present a higher rate capability in electrochemical cycling experiments than nondoped materials. This is attributed to the ...reduction of Li+ diffusion path lengths and enhanced intimate interparticle contact, in combination with improved intraparticle conductivity. Niobium doping has a significant effect on the electronic structure and provokes a substantial decrease in particle size.
•Evoked spikes preceded the depression in Local Field Potential.•Evoked LFP is the result of spikes convoluted by a kernel.•Evoked PSTH can be inferred from LFP using a probability density function ...of spikes.
Recording spontaneous and evoked activities by means of unitary extracellular recordings and local field potential (LFP) are key understanding the mechanisms of neural coding. The LFP is one of the most popular and easy methods to measure the activity of a population of neurons. LFP is also a composite signal known to be difficult to interpret and model. There is a growing need to highlight the relationship between spiking activity and LFP. Here, we hypothesized that LFP could be inferred from spikes under evoked noxious conditions.
Recording was performed from the medullary dorsal horn (MDH) in deeply anesthetized rats. We detail a process to highlight the C-fiber (nociceptive) evoked activity, by removing the A-fiber evoked activity using a model-based approach. Then, we applied the convolution kernel theory and optimization algorithms to infer the C-fiber LFP from the single cell spikes. Finally, we used a probability density function and an optimization algorithm to infer the spikes distribution from the LFP.
We successfully extracted C-fiber LFP in all data recordings. We observed that C-fibers spikes preceded the C-fiber LFP and were rather correlated to the LFP derivative. Finally, we inferred LFP from spikes with excellent correlation coefficient (r = 0.9) and reverse generated the spikes distribution from LFP with good correlation coefficients (r = 0.7) on spikes number.
We introduced the kernel convolution theory to successfully infer the LFP from spikes, and we demonstrated that we could generate the spikes distribution from the LFP.
We report the electrochemical study of cubic and monoclinic NiP2 polymorphs toward Li, as a candidate for anodic applications for Li-ion batteries. We found that the monoclinic form is the most ...attractive one performance-wise. Monoclinic NiP2 can reversibly uptake 5 lithium per formula unit, leading to reversible capacities of 1000 mAh/g at an average potential of 0.9 V vs Li+/Li°. From complementary X-ray diffraction (XRD) and HRTEM (high-resolution transmission electron microscopy) measurements, it was shown that, during the first discharge, the cubic phase undergoes a pure conversion process (NiP2 + 6 Li+ + 6e- → Ni° + 2Li3P) as opposed to a sequential insertion−conversion process for monoclinic NiP2. Such a different behavior rooted in subtle structural changes was explained through electronic structure calculations. Once the first discharged is achieved, both phases were shown to react with Li through a classical conversion process. More importantly, we report a novel way to design NiP2 electrodes with enhanced capacity retention and rate capabilities. It consists in growing the monoclinic NiP2 phase, through a vapor-phase transport process, on a commercial Ni-foam commonly used in Ni-based alkaline batteries. These new self-supported electrodes, based on chemically made interfaces, offer new opportunities to fully exploit the capacity gains provided by conversion reactions.
The charge and discharge of a Li-ion battery based on conversion type electrode material are investigated
operando by acoustic emission (AE). The AE technique gives a direct evidence of both ...structural and morphological impacts of the electrochemical conversion reaction on the electrode. During the first discharge a huge AE energy is measured not only during the biphasic conversion process, but also during the SEI reaction. On first charge, the cumulated AE energy (CAEE) shows a significant increase, during the back conversion process, while upon further cycling the CAEE fluctuation is smoothed out, but very much reproducible. This demonstrates that a conversion reaction creates an “earthquake” in the electrode during conversion, which is correlated to a strong polarization of the electrochemical curve in the first discharge. More importantly, this study demonstrates that AE is a powerful tool to survey the real-time morphological changes and to discriminate the nature of electrochemical process in the electrode.
The Li reactivity of NiP2 is investigated by means of electrochemical tests, in situ XRD, and 31P NMR characterizations as well as first principles DFT calculations. A two-step insertion/conversion ...reaction is shown to transform the NiP2 starting electrode into an intermediate Li2NiP2 single phase and then to convert into the Li3P/Ni° nanocomposite. The ternary phase is fully characterized and is shown to be structurally very close to the starting NiP2 regarding the Ni ions environment. This demonstrates that its formation results from a P-redox insertion mechanism associated with a very good reversibility. However, its nucleation upon delithiation from the fully converted Li3P/Ni composite is shown to be kinetically limited (poor structural relationship) which strongly suggests that restricted lithiation is required for best cycleability of the NiP2/Li cell.
Solid polymer electrolytes (SPEs) are faced with many specifications to replace today's liquid organic electrolyte in Li-ion batteries. In this paper, a branched polyethyleneimine (BPEI) is proposed ...as a new SPE. Infrared and
13
C NMR spectroscopy and DFT modelling were combined to clarify the ionic conduction mechanisms and performance of this new kind of polymer electrolyte. To improve its mechanical properties, the BPEI was semi-interpenetrated in a PEO-based network. Afterwards to provide mechanical integrity and efficient ionic transport, inorganic fillers of different natures were added into the membranes: Al
2
O
3
, SiO
2
, ZrO
2
and LAGP. For instance, the addition of 10 wt% SiO
2
resulted in an enhancement in ionic conductivity, reaching 2.2 × 10
−4
S cm
−1
at 80 °C. Furthermore, in a symmetric cell, improved cycling facing lithium metal, with a stable response over hundreds of hours without polarization, regardless of filler nature was observed.
Solid polymer electrolytes (SPEs) are faced with many specifications to replace today's liquid organic electrolyte in Li-ion batteries, here BPEI based polymer is proposed as a new SPE.
Titanium dioxide mixed phases containing TiO2(b) and anatase phase in different ratios were prepared via a facile, template free, low temperature hydrothermal synthesis. Morphology and phase ...composition were tuned by adequately adjusting the main synthesis parameters, i.e., temperature, powder/liquid ratio and basicity of the mother solution. The effect of different phase compositions on the lithium insertion and de-insertion properties was tested by electrochemical cycling at increasing cycling rates. Results indicate a superiority of monoclinic TiO2(b) phase over the tetragonal anatase phase especially at elevated cycling rates. Further analysis shows that internal resistance is one of the major limitations for electrochemical cycling of anatase.
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► Wide range of morphology and phase composition by adjusting two synthesis parameters. ► Tailoring of phase composition by adjusting alkalinity of hydrothermal synthesis. ► Tailoring of morphology in hydrothermal synthesis by adjusting powder concentration. ► Monoclinic TiO2(b) phase shows higher rate capability than anatase phase. ► Higher internal resistance of anatase compared to TiO2(b) is detected.
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