ARTICLE pubs.acs.org/JPCC
Study on Microstructural Deformation of Working Sn and SnSb Anode Particles for Li-Ion Batteries by in Situ Transmission X-ray Microscopy Sung-Chieh Chao,† Yen-Fang Song,*,‡ Chun-Chieh Wang,‡ Hwo-Shuenn Sheu,‡ Hung-Chun Wu,§ and Nae-Lih Wu*,† †
Department of Chemical Engineering, National Taiwan University Taipei 106, Taiwan, R.O.C. National Synchrotron Radiation Research Center, Hsinchu 30077, Taiwan, R.O.C. § Materials Research Laboratories, Industrial Technology Research Institute Chutung, Hsinchu 310, Taiwan, R.O.C. ‡
bS Supporting Information ABSTRACT: Sn-containing compounds are potential highcapacity anode materials for Li-ion batteries. They, however, suffer from significant dimensional variations during electrochemical lithiation and delithiation, causing cycling instability. Understanding the dynamics of these deformation processes may provide valuable information in the establishment of viable high-energy anodes. In this paper, the evolution of interior microstructures of two types of Sn-containing particles, including Sn and SnSb, during initial cycles of electrochemical lithiation/delithation has been revealed by in situ synchrotron transmission X-ray microscopy, complemented by in situ synchrotron X-ray diffraction to provide phase information. The microstructures and deformation rates are shown to depend on particle composition, size, and alloy stoichiometry with Li. During first lithiation, both particles exhibit core (metal)shell (lithiated compounds) interior structures. Initial formation of a dense surface layer containing LixSn phases of low Li-stoichiometry on the Sn particle hinders further lithiation kinetics, resulting in delayed expansion of large particles. In contrast, Sb in SnSb is readily lithiated to form a porous Li-rich (Li3Sb) surface layer at higher potential than Sn, which enables the acceleration of lithiation and removal of the size dependence of the lithiation process. Both lithiated particles only partially contract upon delithiation, and their interiors evolve into porous structures due to metal recrystallization. Such porous structures allow for fast lithiation and mitigated dimensional variations upon subsequent cycles. Neither of the two anode particles pulverize upon cycling.
1. INTRODUCTION Lithium-ion batteries, which have already been ubiquitous in high-end consumer electronic products, are widely regarded as the choice power source for future electric vehicle applications. To meet this demand, the specific capacities of both cathode and anode have to be significantly increased from the state-of-the-art materials. The currently predominant anode material is graphite, which has a theoretical specific capacity of 370 mAh/g. Several elements, such as Sn,1,2 which form alloys with Li are potential anode materials with far greater theoretical lithiation capacities. These Li-alloying materials are known to be subject to significant volumetric deformations, namely expansion and contraction, during electrochemical lithiation and delithiation due to change in density, and these cyclic dimensional variations tend to cause structural instability of the electrode, leading to fast capacity fading. Understanding the dynamics of these deformation processes can provide valuable information to researchers for establishing viable high-energy anodes based on these materials. Devising an analytic tool that is capable of in situ monitoring of the behaviors of active materials within a working Li-ion battery is very challenging, considering the extreme sensitivity to r 2011 American Chemical Society
humidity and oxygen of the various components within the battery. Outstanding works in revealing the dynamics of dimensional variation of working Li-alloying anodes have been demonstrated in earlier years with a few in situ techniques, including dilatometry,3 atomic force microscopy,4,5 and atmospheric scanning electron microscopy.6 These analyses, however, detect only the dimension of the active materials but are not capable of revealing the microstructural evolution concurrently taking place within the bulk of the anode particles during the deformation processes. Recent progress in X-ray microscopy (TXM)7 and transmission electron microscopy (TEM)8,9 for battery research has offered the opportunity to directly image the interior microstructures of electrode active materials during the course of electrochemical lithiation/delithiation (L/D). In our previous TXM study,7 Sn particles are dispersed within the matrix of a free-standing graphite electrode, which is subsequently assembled with a Li counter electrode into a “half-cell”. Received: July 18, 2011 Revised: September 17, 2011 Published: September 27, 2011 22040
dx.doi.org/10.1021/jp206829q | J. Phys. Chem. C 2011, 115, 22040–22047
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Schematics of the experimental setup of in situ TXM.
High-energy X-rays were allowed to pass through the entire cell as well as the individual Sn particles so as to give transmission images of their interior microstructures with a resolution down to a few tenths of a micrometer. In this work, we have combined in situ TXM with in situ synchrotron X-ray diffraction (XRD) to study the evolution of the interior microstructures, complemented with information of phase compositions, of two Sn-based anode particles, including Sn and SnSb, during electrochemical L/D. These particles exhibit different classes of electrochemical behaviors. An in-depth review on the electrochemical aspects of Sn-based anodes can be found in ref 10. In brief, for Sn, the maximum theoretical lithiation stoichiometry in crystalline form is Li22Sn5, which gives a theoretical capacity of 990 mAh/g-Sn and a theoretical volume expansion by ca. 350% when compared with Sn. For SnSb, both constituent elements are capable of undergoing reversible lithiation reactions but at different potentials, giving a maximum capacity of 825 mAh/g-SnSb. The results obtained in this work provide valuable information for the understanding of the microstructure-cycling behaviors of these anode materials and may pave the way to better design of Li-alloying anode materials against capacity fading.
2. EXPERIMENTAL SECTION 2.1. Samples and Electrode Preparations. Sn powder was used as received (Aldrich) without any treatment. For preparing SnSb, powders of Sn (with 99%
