Chemical Preinsertion of Lithium: An Approach to Improve the Intrinsic

Letter pubs.acs.org/JPCL

Chemical Preinsertion of Lithium: An Approach to Improve the Intrinsic Capacity Retention of Bulk Si Anodes for Li-ion Batteries Ruijun Ma, Yongfeng Liu,* Yanping He, Mingxia Gao, and Hongge Pan* State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: Silicon represents one of the most promising anodes for next-generation Li-ion batteries due to its very high capacity and low electrochemical potential. However, the extremely poor cycling stability caused by the huge volume change during charge/ discharge prevents it from the commercial use. In this work, we propose a strategy to decrease the intrinsic volume change of bulk Si-based anodes by preinsertion Li into Si with a chemical reaction. Amorphous Li12Si7 was successfully synthesized by a hydrogendriven reaction between LiH and Si associated with subsequent energetic ball milling. The as-prepared amorphous Li12Si7 anode exhibits significantly improved lithium storage ability as ∼70.7% of the initial charge capacity is retained after 20 cycles. This finding opens up the possibility to develop bulk Si-based anodes with high capacity, long cycling life and low fabrication cost for Li-ion batteries. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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311%, respectively, in comparison with pure Si. However, if taking Li12Si7 as the starting active material, the volume expansion rate would be reduced to ∼70% with further lithiating to form Li15Si4 (the fully lithiated phase of Si at room temperature),20 and it is only ∼54% for the volume contraction while delithiating to generate Si. Thus, the relative volume change is significantly lower than that of Si (∼270%).21 Such a change is expected to be very helpful for reducing the fracture and pulverization of bulk Si-based anodes. However, little is known about the electrochemical lithium storage behavior of Li12Si7 because it is rather difficult to prepare the single-phase Li−Si alloys by conventional melting techniques due to the big discrepancy between the melting points of Li and Si (differing by ∼1230 °C). Here we develop an approach of hydrogen-driven chemical reaction to prepare the high-purity Li−Si alloys and propose a new strategy to mitigate the pulverization of bulk Si anode by using the amorphous Li12Si7 as the starting active material. Upon forming Li12Si7, Li is chemically preinserted into Si, which induces a pre-expansion of volume of bulk Si. Such a phenomenon decreases the relative volume change of Si-based anode during charge/discharge. More importantly, the amorphous structure of the as-milled Li12Si7 also alleviates the anisotropic expansion/contraction in the lithiation/ delithiation process. As a result, a significantly improved capacity retention is achieved.

ilicon has been regarded as one of the most promising anode candidates for next-generation Li-ion batteries because it possesses a maximum theoretical capacity of 4200 mAh/g, a highest known Li insertion capacity, with the formation of the Li22Si5 compound.1 This is more than 10 times larger than that of commercialized graphite (372 mAh/g).2 However, the huge volume change (>300%) during charge/ discharge leads to severe electrode pulverization and subsequent electrical disconnection from current collectors, consequently resulting in poor reversibility and rapid capacity degradation, which prevent silicon from the commercial use in Li-ion batteries.3,4 Considerable efforts have been devoted to improving the electrochemical Li storage properties, in particular, the cycling stability over the last two decades, including (1) decreasing particle size,5,6 (2) alloying with other active/inactive elements,7,8 (3) building specially designed nanostructured electrodes,9−14 and (4) adding buffer components or coating with carbon.15−19 Unfortunately, the overall properties of Si-based anodes are not satisfactory for practical applications by taking into comprehensive consideration gravimetric and volumetric energy densities, rate capacity, cycling stability, and fabrication cost. From the point of view of the volumetric energy density and fabrication cost, the bulk Si should be the most favorable structure for large-scale applications. Therefore, it is still of both scientific interest and practical importance to develop the new strategies for mitigating the intrinsic pulverization of bulk Si upon charging/ discharging. It is well known that there are four crystalline phases of Li12Si7, Li7Si3, Li13Si4, and Li22Si5 in the Li−Si phase diagram,1 and their volume expansion rates are ∼117, 157, 236, and © 2012 American Chemical Society

Received: October 31, 2012 Accepted: November 20, 2012 Published: November 21, 2012 3555

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The Li12Si7 alloy was successfully synthesized by reacting LiH and Si stoichiometrically at 600 °C according to reaction 1. Such a reaction was first reported on the destabilized LiH-Si hydrogen storage system.22 12LiH(s) + 7Si(s) → Li12Si 7(s) + 6H 2(g)

(1)

XRD examination (Figure 1) shows that the diffraction peaks of the resultant product match quite well with those of Li12Si7

Figure 2. (a) Voltage profiles of as-milled Si, (b) voltage profiles of asmilled Li12Si7, (c) comparison of voltage profiles of as-milled Si and Li12Si7, and (d) cycling charge capacities of as-milled Si and Li12Si7.

preinserting Li to form Li12Si7. However, the low initial discharge capacity of the as-milled Li12Si7 (∼1823 mAh/g) can be attributed to the chemically preinserted Li because its initial charge capacity is still as high as ∼2942 mAh/g, as shown in Figure 2b. In addition, it is noteworthy that the cycling stability of the Li12Si7 anode is much better than that of the as-milled bulk Si. As shown in Figure 2c, the charge capacity of the Li12Si7 anode still stays at ∼2079 mAh/g after 20 cycles, which is even higher than that of the second cycle for the as-milled bulk Si anode (∼1804 mAh/g). The capacity retention of the Li12Si7 anode was calculated to be ∼70.7%, whereas it is only ∼18.6% for the bulk Si anode (Figure 2d). The significantly improved cycling stability of the Li12Si7 anode is possibly attributed to the limited fracture and pulverization of active material particles and the decreased irreversible capacity loss due to the reduced relative volume change rate during charge/ discharge. In general, the reduced relative volume change ratio can lower the internal stress of the active materials of the Li-ion batteries that causes the microcracks and fracture in the electrode.24 Figure 3 presents SEM images of the as-milled bulk

Figure 1. XRD patterns of the as-purchased Si and as-prepared Li12Si7 before and after ball milling.

in PDF database of JCPDS-ICDD with the disappearance of LiH and Si, indicating the formation of a single phase Li12Si7. The total amount of hydrogen released is ∼4.10 wt %, as shown in Figure S1 (Supporting Information), which corresponds to a purity of 98.8% because the theoretical dehydrogenation amount is 4.15 wt % for reaction 1. Thus, the high-purity Li12Si7 was readily synthesized at reasonable temperatures by chemically reacting Si with LiH instead of metallic Li, which avoids the big difference in the melting points of Li and Si. The as-synthesized Li12Si7 was then subjected to ball milling to produce the amorphous Li12Si7 because the crystalline Si becomes the amorphous form after 1 discharge/charge cycle.20,21,23 After ball milling for 48 h, the typical diffraction peaks of crystalline Li12Si7 disappear, and only two broad bumps at around 20−30 and 36−50° are discernible, as shown in Figure 1, indicating the amorphization of Li12Si7. SEM observations (Figure S2, Supporting Information) revealed that the amorphous Li12Si7 exhibited similar particle size distribution as the as-purchased Si at 0.5 to 1 μm. For the purpose of comparison, the as-purchased crystalline Si was also ball-milled under the same conditions. The postmilled Si still exhibited good crystallinity (Figure 1), and the particle sizes were in the range of 100−500 nm (Figure S2, Supporting Information), smaller than that of the as-milled Li12Si7. Figure 2a−c shows the galvanostatic charge/discharge voltage profiles of the as-milled bulk Si and Li12Si7 samples at a current density of 100 mA/g. Significantly, the as-prepared Li12Si7 exhibits superior lithium storage properties as it delivers a maximum discharge capacity of ∼2988 mAh/g and the corresponding charge capacity of ∼2841 mAh/g with a rather high Coulombic efficiency of ∼95.1%. However, the initial Coulomic efficiency of the as-milled bulk Si is only ∼72.7%, although its first discharge capacity is as high as ∼3009 mAh/g. This fact indicates that the electrochemical lithium storage reversibility of the bulk Si is remarkably improved by chemically

Figure 3. SEM images of electrode surfaces of (a) as-milled Si and (b) as-milled Li12Si7 after 10 cycles.

Si and Li12Si7 anodes after 10 cycles. For the as-milled bulk Si anode, a large number of cracks and interstices can be clearly seen on the surface of electrode after 10 cycles. These cracks and interstices induce the pulverization and disintegration of the electrode, consequently losing active materials and decreasing electrical contact, which should be the most important reason for the rapid fading of the electrochemical lithium storage capacity of the as-milled Si anode. However, the electrode surface of the Li12Si7 anode is very smooth, and no obvious crack is observed after 10 cycles. As previously mentioned, the total relative volume change rate is decreased 3556

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to ∼124% while taking Li12Si7 as the starting material, a more than 50% reduction in comparison with the direct lithiation of Si to form Li15Si4, which may retard the fracture and pulverization of the Li12Si7 anode during charge/discharge. As a result, the reduced volume change rate effectively prevents the fracture and pulverization of the electrode, which is reasonably responsible for the good cycle durability of the as-prepared Li12Si7 anode. In addition, the amorphous structure of the asmilled Li12Si7 is also an important reason for the improved cycling stability because the anisotropic expansion/contraction in the crystalline Si is quite harmful for the stability of the electrode structure during charge/discharge process.25,26 The rate capabilities of the as-milled bulk Si and Li12Si7 anodes were also investigated by measuring the discharge/ charge curves at different current densities, as shown in Figure 4a−c. The maximum discharge capacities of the as-milled

to the quality of electricity participating in the electrochemical reaction.27 As a result, the almost identical peak areas of the cathodic and anodic peaks indicate a good lithium storage reversibility during discharge/charge, and the increased peak intensity suggests an enhanced reaction rate for lithiation and delithiation. The improved lithium storage reversibility can be attributed to the improved electrical contact due to the alleviated fracture and pulverization of the Li12Si7 anode, as shown in Figure 3, which decreases the Li trapping in the active materials. Moreover, such an improvement should also be an important factor for the enhanced lithiation/delithiation kinetics. Further investigations on the improved Li storage properties of the as-milled Li12Si7 are in progress in our laboratory. In summary, we have successfully demonstrated a new concept for improving the intrinsic cycling stability of the bulk Si anode by reducing the relative volume change during lithiation and delithiation. The amorphous Li12Si7 alloy was prepared with a facile hydrogen-driven chemical reaction, followed by ball milling. By using Li12Si7 as the starting material rather than Si, the relative volume change was remarkably reduced during charge/discharge because the unit volume of Li12Si7 has been expanded by 117% in comparison with Si. Therefore, the resultant material exhibits a significantly improved lithium-storage performance in terms of good cycling stability and reasonable rate capacity. This strategy opens up a new possibility for developing advanced bulk Si-based anodes with long cycling lifetime, high rate capacity and low fabrication cost by using the Li−Si alloys. Moreover, the facile and low cost hydrogen-driven chemical reaction developed in this work has the potential to be extended to other intermetallic compounds composed of elements with quite different melting points.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 4. (a) Voltage profiles of as-milled Si at various current densities, (b) voltage profiles of as-milled Li12Si7 at various current densities, (c) comparison of voltage profiles of the as-milled Si and Li12Si7 at 1.6 A/g, and (d) CV curves of the as-milled Si and Li12Si7 at the second cycle.

Experimental details, dehydrogenation curve of the postmilled 12LiH-7Si mixture, SEM images of Si, as-milled Si and Li12Si7, and discharge capacities at various current densities with cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

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Li12Si7 anodes are 2455, 1673, and 998 mAh/g at the current densities of 400, 800, and 1600 mA/g, respectively. However, they are only 2273, 1224, and 389 mAh/g for the as-milled Si anodes. Apparently, the maximum discharge capacities of the Li12Si7 anode are much higher than those of the Si anode at a given current density, especially at the higher current density. As shown in Figure 4c, the discharge/charge capacities of asmilled Si anode are even much lower than half those of Li12Si7 at 1600 mA/g. These facts indicate that the rate discharge/ charge capability is also remarkably improved by forming a Li− Si alloy. In other words, an enhanced lithiation/delithiation kinetics is achieved for the Li12Si7 anode. More interestingly, the improved lithium storage kinetics persists in the subsequent cycling process, as shown in Figure S3 (Supporting Information). Further cycling voltammetry (CV) measurements confirmed the improved lithium storage reversibility and enhanced lithium storage kinetics for the Li12Si7 anode. As seen in Figure 4d, the cathodic and anodic peaks of the CV curve are distinctly intensified with more symmetric shape for the Li12Si7 anode in comparison with the as-milled bulk Si anode. It is well known that the peak intensity of the CV curve represents the electrochemical reaction rate, and the peak area corresponds

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.L.); [email protected] (H.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support received from National NSFC (51025102, 51171170, 51222101), and STD of Zhejiang Province (2011R10017 and 2010R50013).

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