Capacity fade in Sn–C nanopowder anodes due to fracture

doi:10.1016/j.jpowsour.2011.09.025

Journal of Power Sources

Volume 197, 1 January 2012, Pages 246-252

https://doi.org/10.1016/j.jpowsour.2011.09.025 Get rights and content

Abstract

Sn based anodes allow for high initial capacities, which however cannot be retained due to the severe mechanical damage that occurs during Li-insertion and de-insertion. To better understand the fracture process during electrochemical cycling three different nanopowders comprised of Sn particles attached on artificial graphite, natural graphite or micro-carbon microbeads were examined. Although an initial capacity of 700 mAh g⁻¹ was obtained for all Sn–C nanopowders, a significant capacity fade took place with continuous electrochemical cycling. The microstructural changes in the electrodes corresponding to the changes in electrochemical behavior were studied by transmission and scanning electron microscopy. The fragmentation of Sn observed by microscopy correlates with the capacity fade, but this fragmentation and capacity fade can be controlled by controlling the initial microstructure. It was found that there is a dependence of the capacity fade on the Sn particle volume and surface area fraction of Sn on carbon.

Highlights

► Detailed microscopy was performed on Sn/C anodes before and after cycling. ► Significant fracture of the Sn was observed during cycling. ► The Sn volume average and surface area must be low to prevent fracture.

Introduction

Both Sn and Si are highly reactive with respect to Li, giving capacities of 996 mAh g⁻¹ and 4200 mAh g⁻¹ [1], respectively, upon maximum Li-insertion. They cannot be used, however, in their pure form as anodes due to the 300% volume expansion they experience upon the formation of Li alloys. This volume expansion and sub-sequent contraction, upon Li de-insertion, results in severe mechanical damage, which corresponds to capacity fading during cycling. After the first 20 cycles, therefore, it is possible for the capacity of Sn nanoparticles to drop to 210 mAh g⁻¹ [2], which is even lower than that of commercially used graphite (372 mAh g⁻¹ [3]).

Deformation is less severe at the nanoscale so keeping the particle size in the nm range is common practice for battery developers [4], [5]. Furthermore, mechanical damage of Si and Sn can be significantly reduced by embedding them in a matrix that can buffer/constrain their expansion during electrochemical cycling; an overview can be found in [6]

Particularly, embedding 25 nm Si nanoparticles in carbon allowed for capacities of 1000 mAh g⁻¹ to be retained for 30 cycles [7]. For Sn-based anodes, stable capacities of 500 mAh g⁻¹ have been achieved by embedding Sn nanoparticles in carbon [8]. In order to achieve such high capacities the Sn content had to be 50 wt%. A different type of Sn–C microstructure that can provide capacities that are greater than that of the C matrix is that of attaching Sn particles on carbon surfaces. In [9] the capacity of Vulcan C was increased 85% by attaching 8 wt%Sn as SnO₂ islands with diameters between 5 and 20 nm. Unfortunately, Vulcan is a low capacity carbon (180 mAh g⁻¹) and, therefore, it cannot provide high capacity anodes. In a previous study, in which Sn was attached on the surface of a higher capacity synthetic graphite, significant capacity fade was observed and after 20 cycles it had dropped below 400 mAh g⁻¹ [10].

Microstructural studies of cycled Sn-on-carbon electrode materials are very limited and there appears to be little quantitative data relating the microstructure of these materials to their capacity fade. This paper contributes to closing this gap in understanding, by using transmission and scanning electron microscopy (TEM & SEM) to examine the microstructural changes during electrochemical cycling and the dependence of capacity fade on the Sn particle configuration for three different Sn–C materials. Natural graphite, artificial graphite and microcarbon–microbeads with capacities of 350 mAh g⁻¹ (Shandong Carbon Materials Ltd., China) was employed as the base material onto which the Sn was attached. Sn deposition on all the carbons was carried out using the fabrication method described in [11]. TEM and SEM were performed after long term cycling to observe the mechanical damage that took place upon continuous Li-insertion and de-insertion through measurement of Sn particle sizes.

To further, document fracture upon Li-insertion, cells were interrupted at various stages of the first and second electrochemical cycles and post mortem TEM/SEM was performed, documenting “significant fracture” of the Sn islands during the first cycle.

Section snippets

Experimental procedure

The fabrication method employed was that described in [11], according to which SnCl₄ is reduced on the surface of carbon or graphite. Depending on the carbon used the microstructure differed significantly. Particularly, it is seen in the SEM and TEM images of Fig. 1 that when MCMB carbon (Fig. 1a,b) and artificial graphite (Fig. 1c,d) were employed the islands had a diameter in both the nanometer and micrometer scale (bimodel size distribution), whereas when natural graphite was used the Sn

Long term cycling

Fig. 2 shows the cyclability of the graphites prior to the attachment of Sn. Fig. 3 illustrates the capacity obtained by cycling the as prepared Sn–C materials shown in Fig. 1. The initial capacity for all Sn–C nanocomposites is approximately, between 700 and 800 mAh g⁻¹, for all cases. This high capacity is attributed to the Sn, which gives capacities of 900 mAh g⁻¹. With continuous cycling, though, a significant capacity fade occurred. This fade was anticipated to some extent for the MCMB and the

Discussion

Fig. 3 illustrates that the greatest capacity loss took place during the first cycle for all three materials. For Sn/SnO₂–MCMB this can be partially attributed to the fact that some Li-ions are lost in the irreversible formation of Li₂O during the first Li-insertion [13] since SnO₂ in addition to Sn metal were attached on the MCMB for this case. For Sn-artificial graphite and Sn-natural graphite, however, this high irreversible capacity loss is anticipated to be solely due to the fracture of

Conclusions

In the present study a detailed analysis was performed to study the fracture of Sn-based anodes that comprise of Sn or SnO₂ islands attached on artificial graphite, natural graphite and MCMB. SEM and TEM images illustrated that all Sn–C materials experienced severe fracture during long term cycling, while observing in more detail the first two electrochemical cycles it was seen that severe fracture occurs during the first lithiation. Particularly, it was shown that upon Li-insertion the Sn

Acknowledgment

This work was funded by KEA's European Research Starting Grant MINATRAN 211166.

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