Elsevier

Journal of Power Sources

Volume 227, 1 April 2013, Pages 237-242
Journal of Power Sources

Nanoscale compositional changes during first delithiation of Si negative electrodes

https://doi.org/10.1016/j.jpowsour.2012.11.047Get rights and content

Abstract

The local composition of negative silicon electrodes is studied by ex situ electron energy-loss spectroscopy along the first delithiation in a lithium battery. By measuring dozens of sample areas for over a dozen compositions, the local and overall inhomogeneities in these practical electrodes are evaluated. The statistical treatment of the data highlights the existence of larger inhomogeneities at the beginning of the delithiation as well as at a 100 nm scale. It is also shown that, even if incremental capacity curves are different, the compositional changes during delithiation are identical for nano- and micro-Si. Namely, an initial Li15Si4 phase is replaced by a Li2±0.3Si amorphous phase in a biphasic process, the latter compound being further delithiated to amorphous silicon in single phase process. Results also show that the electrochemical irreversibility associated with the liquid electrolyte reduction/degradation is generated during the lithiation process, not the delithiation process.

Highlights

► A biphasic process for x > 2 followed by a solid solution is proven for delithiation. ► Both nano- and micro-Si present the same behaviour. ► In electrodes prepared with nano-silicon, nanoscale inhomogeneities can be large. ► These inhomogeneities decrease as delithiation increases. ► Inhomogeneities through the electrode thickness are relatively small.

Introduction

The considerable interest generated by the use of silicon as a negative electrode for Li-ion batteries has not diminished since the detailed studies conducted at the turn of the century [1], [2], [3]. Electrodes developed in this context, however, were not able to produce sufficiently reversible reactions beyond a few cycles due to the large volume change that occurs during the alloying/dealloying process. The design of the composite electrode architecture and the selection of suitable binders contributed to the production of current day electrodes displaying improved, if not entirely satisfactory, performance [4], [5].

A better knowledge of the processes of amorphisation and crystallisation of phases using techniques such as X-ray diffraction has since been achieved [6]. This information was particularly important due to the particular mechanism involved, i.e. the first lithiation of crystalline silicon is very different from the subsequent ones. After the first lithiation, cycling occurs on an amorphous phase [2], except at the end of discharge (full lithiation), where the formation of a Li15Si4 phase, not present in the Li–Si phase diagram, is observed [7]. The presence of such a crystalline phase is believed to decrease the reversibility of the charge–discharge processes [7], [8]. Apart from this Li15Si4 phase, complete cycling involves phases which are amorphous at the X-ray diffraction scale. If one is to make progress with respect to the determination of phases during the major part of cycling, as well as their local distribution and interaction in the composite electrode at the nanoscale, it has long been agreed that Transmission Electron Microscopy (TEM) is a technique to be privileged [1], [2]. The latest in situ experiments open up even more possibilities [9], [10]. Lithium is, however, a difficult atom to detect, especially in an amorphous environment [11]. Electron energy-loss spectroscopy has been shown to be well adapted to analyse lithium-containing compounds [12], [13]. In the case of LixSi alloys, particularly, it has recently been demonstrated that the plasmon region (around 20 eV in the energy-loss spectrum) is precise enough to determine local compositions [14]. Danet et al. showed that, during the first lithiation, the crystalline silicon is in equilibrium with a Li2.9Si amorphous phase. The database created on plasmon energies of LixSi alloys was used to arrive at this conclusion. This study seemed also to indicate a nucleation process, and an overlithiation was observed at the end of the discharge. At a nanoscale, these results complement others inferred from X-ray diffraction [6] and NMR techniques [15].

The first charge is a critical moment in the cycling process as it follows the creation of the initial SEI and therefore has to absorb for the first time the dramatic decrease in volume of the alloy. The purpose of this paper is to analyse the local compositional changes during the first charge and provide information on the initial irreversibility, by the use of the EELS technique already validated on the first lithiation. To this end, particular attention was paid to the statistical treatment of the EELS data. Our findings show that, at a nanoscale, a Li2Si phase is rapidly formed upon delithiation of a fully lithiated silicon electrode, regardless of which crystalline silicon is used (micrometric or nanometric). Further delithiation is shown to proceed via a single phase process.

This study also suggests that, with respect to the nano-size silicon, the non-homogeneity of the electrode at a scale of a few hundred nanometres limits electrode performance.

Section snippets

Experimental

The composite electrodes prepared consist of 80% nanometric Si (98%, 50 nm, Alfa Aesar) or micrometric Si (99.9%, 1–5 μm, Alfa Aesar), 12% carbon black (super P, Timcal) and 8% carboxymethylcellulose (DS = 0.7, Mw = 90,000 Aldrich). They were prepared in an aqueous solution by ball milling at 500 rpm for 1 h using a Fritsch Pulverisette 7 apparatus [4]. Slurries were then tape casted onto a 25 μm copper foil, dried at room temperature for 12 h and then under vacuum at 100 °C for 2 h. The resulting active

Compositional homogeneity during delithiation

In this study, 14 compositions along the first charge (delithiation) of Si-based electrodes were investigated using EELS (Table 1). In Fig. 2, the different states of charge studied are marked as dots on typical 1st cycle curves for both nano- and micro-Si. The sizeable dispersion of potential observed for similar apparent compositions might well be explained by the fairly large range of mass loadings. In this same graph, it should be noted that the x values were calculated while taking in

Conclusions

Nanoscale experiments were performed using EELS to examine the variations of compositions occurring during the delithiation of Si practical electrodes. The effectiveness of the technique based on the measurement of alloy plasmons was further illustrated in this paper. Independently of the crystallisation state of the compound, local compositions could be obtained. Similar studies could be performed on conversion reaction systems where amorphous phases are often found. Inhomogeneities in

Acknowledgements

Dr. J. Gaubicher is greatly acknowledged for fruitful discussion on the electrochemical process in these electrodes. Financial funding from the Agence Nationale de la Recherche (ANR) of France (BASILIC project) and the Natural Science and Engineering Research Council (NSERC) of Canada is acknowledged.

References (38)

  • H. Li et al.

    Solid State Ionics

    (2000)
  • P. Limthongkul et al.

    J. Power Sources

    (2003)
  • L.Y. Beaulieu et al.

    Electrochem. Solid-State Lett.

    (2001)
  • D. Mazouzi et al.

    Electrochem. Solid-State Lett.

    (2009)
  • J.S. Bridel et al.

    Chem. Mater.

    (2010)
  • J. Li et al.

    J. Electrochem. Soc.

    (2007)
  • M.N. Obrovac et al.

    Electrochem. Solid-State Lett.

    (2004)
  • Y.-M. Kang et al.

    Electrochem. Commun.

    (2007)
  • S.-B. Son et al.

    Adv. Energy Mater.

    (2011)
  • X.H. Liu et al.

    Nano Lett.

    (2011)
  • Y. Oshima et al.

    J. Electron Microsc.

    (2010)
  • V. Mauchamp et al.

    J. Phys. Chem. C

    (2007)
  • F. Cosandey et al.

    Micron

    (2012)
  • J. Danet et al.

    Phys. Chem. Chem. Phys.

    (2010)
  • B. Key et al.

    J. Am. Chem. Soc.

    (2009)
  • P. Fallon, C.A. Walsh, University of Cambridge, England,...
  • R.F. Egerton

    Electron Energy-Loss Spectroscopy in the Electron Microscope

    (1996)
  • Y. Oumellal et al.

    J. Mater. Chem.

    (2011)
  • J.H. Ryu et al.

    Electrochem. Solid-State Lett.

    (2004)
  • Cited by (24)

    • Sea Sand-Derived Magnesium Silicide as a Reactive Precursor for Silicon-Based Composite Electrodes of Lithium-Ion Battery

      2017, Electrochimica Acta
      Citation Excerpt :

      In the charging (delithiation) profiles of both materials, two distinguishable plateaus at ∼0.3 V and ∼0.4 V were observed (Fig. 5b,c). The relative length of the two plateaus is related to the particle size of Si [59]. The plateau at ∼0.3 V can be lengthened when the particle size of Si is reduced, whereas the plateau at ∼0.4 V is lengthened when the Si particle size is increased.

    • Influence of the Si particle size on the mechanical stability of Si-based electrodes evaluated by in-operando dilatometry and acoustic emission

      2016, Journal of Power Sources
      Citation Excerpt :

      By comparing the potential curves of the Si 85 nm (Fig. 4a) and Si 230 nm (Fig. 4b) electrodes, one can see that the plateau at about 0.45 V, which corresponds to the delithiation of the crystalline Li15Si4 phase [42], is well discernible for the Si 230 nm electrode in contrast to the Si 85 nm electrode displaying a sloping delithiation curve. Such a difference has been commonly observed in other studies comparing the lithiation/delithiation behavior of nano Si versus micro Si (e.g. Refs. [43–45]). A possible explanation is that for smaller particles, crystallization is thermodynamically unfavorable because the required interface energy increase from amorphous to crystalline Li15Si4 particles cannot be compensated by the volume-energy decrease due to the large surface areas of small particles [45].

    • Threshold-like dependence of silicon-based electrode performance on active mass loading and nature of carbon conductive additive

      2016, Electrochimica Acta
      Citation Excerpt :

      In order to examine whether the capacity fade is rather due to mechanical/electrical disconnection of the active mass or/and irreversible electrons consummation associated with formation of the SEI layer through liquid electrolyte degradation, we inspired from Gauthier et al. work [8]. This calculation considers that the mechanical/electrical disconnections are mostly created as a consequence of the Si particles deflation upon dealloying during the charge through the formation of cracks [39,47] and the obstruction of pores by accumulated electrolyte degradation products [46], whereas the electrolyte degradation and the SEI formation mostly occur when the Si particles inflate upon alloying at low potential during the discharge [48]. The relative irreversible capacity associated with the disconnection (RICDISCONN.)

    • High yield and low-cost ball milling synthesis of nano-flake Si@SiO<inf>2</inf> with small crystalline grains and abundant grain boundaries as a superior anode for Li-ion batteries

      2015, Journal of Alloys and Compounds
      Citation Excerpt :

      For the anodic process of the first cycle, as an overall, there are two broad peaks in the potential range of 0.35–0.54 V. The peak at low potential is assigned to the amorphous Li15Si4 phase transforming into the amorphous Li2Si phase [64], and the peak at high potential corresponds to the transformation of the Li2Si phase to pure amorphous Si [65]. However, the position of the peaks is somewhat different for the different microstructures of three samples.

    • Exfoliated graphite as a flexible and conductive support for Si-based Li-ion battery anodes

      2014, Carbon
      Citation Excerpt :

      Before the first discharge, Si-NPs dispersed on the surface, embedded in the GNs or stored in the pores. After that, the lithium ions intercalated in the GNs and Si-NPs, which could form the graphite intercalation compounds (GIC) [30,31] and LixSi phase [32–35], respectively. GNs expanded from dozens to almost one hundred nanometers due to the formation of blossom-like GIC (Fig. 6b and c).

    • Enhanced cycling performance of Si/C composite prepared by spray-drying as anode for Li-ion batteries

      2013, Powder Technology
      Citation Excerpt :

      In particular, Si-based composites have been regarded as one of the most promising candidate material for the next generation of LIBs owing to its high theoretical capacity (4200 mAh g− 1), natural abundance and low charge potential [7–9]. Nevertheless, the poor electronic conductivity and huge volume change of Si (~ 320%) during lithiation/delithiation process severely limited its practical application [10–12]. In recent years, many attempts have been carried out to improve electrochemical performance of Si-based anodes.

    View all citing articles on Scopus
    View full text