Article pubs.acs.org/JPCC
Operando X‑ray Studies of Crystalline Ge Anodes with Different Conductive Additives Linda Y. Lim,†,‡,∥ Shufen Fan,§,∥ Huey Hoon Hng,*,§ and Michael F. Toney*,‡ †
Department of Materials Science and Engineering, Stanford University, 496 Lomita Mall, Stanford, California 94305, United States Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States § School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore ‡
S Supporting Information *
ABSTRACT: There is limited understanding of the lithiation reaction mechanisms for crystalline Ge and the effects of various battery components, including conductive additive type, on Ge transformations into amorphous and crystalline phases during electrochemical charge and discharge processes. In this work, we study the dependence of the phase transformations of crystalline Ge anodes on two common carbon-based conductive additives used in lithium-ion battery electrodes through operando X-ray diffraction and X-ray absorption spectroscopy. We find that Ge electrodes using carbon nanotubes as conductive additives exhibit higher structural and electrochemical reversibility compared with those with carbon black additives as well as better stability in cycling. On the basis of this, a proposed strategy to prolong the cycle life of crystalline Ge anodes is presented. Our operando XRD and XAS results show how the reaction pathways (phase transformations and local structural changes) of Ge anodes depend on conductive additive and impact the battery cycling performance.
1. INTRODUCTION In recent years, there is much effort devoted to developing rechargeable lithium-ion batteries (LIBs) for large-scale applications, such as electric vehicles and grid energy storage.1 To achieve those goals, new electrode materials with high energy density have been widely studied to address the challenging requirements (such as battery life and specific energy density) for LIBs. Among the potential alternative LIB anode materials, germanium (Ge) has attracted considerable attention recently for high capacity applications because its ability to alloy with as many as 4.4 Li atoms per Ge atom2−5 enables it to possess high theoretical capacity of 1620 mAh g−1. Additionally, the fast lithium diffusivity in Ge (400 times faster than in Si)6 and higher electrical conductivity in Ge compared with Si7 further attracts attention for fast-charging and highcapacity LIBs applications.4,8−10 Moreover, because Ge has similar properties and crystal structure to Si (another highcapacity Li-alloying anode), Ge is a model for Si anodes. In addition, Ge is abundant in the Earth’s crust, and hence the price of Ge may drop in the future, making this a viable anode material.11 Thus, it is important and interesting to investigate, operando, the reaction mechanisms of Ge as a lithium-ion battery anode. There have been a number of recent operando reports on the reaction mechanisms in Ge electrodes.3,12−16 In our previous work, we investigated the electrochemical reaction mechanism of crystalline Ge (c-Ge) electrodes cycled at C/15 using © 2015 American Chemical Society
operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) and proposed a reaction mechanism for the system. We showed that c-Ge lithiates inhomogeneously, first forming an amorphous Li9Ge4-like structure, followed by the conversion of the remaining c-Ge to amorphous Ge (a-Ge). Lithiation of a-Ge forms amorphous LixGe structures (aLixGe), which were then further lithiated and converted into crystalline Li15Ge4 (c-Li15Ge4). During delithiation, c-Li15Ge4 transforms into a-LixGe and eventually a-Ge, although this transformation was incomplete with Ge−Li bonds still present, which is part of the reason for the capacity fade.15 The inhomogeneous lithiation of Ge nanowires was also reported in a recent work using operando X-ray diffraction and spectroscopy and suggested that limiting the cell discharge depth to avoid full lithiation will help maintain some degree of crystallinity in the nanowires.17 Most recently, we explored the relationship between cycling rate (C-rate), the structure of Ge anode, and battery cycling stability. We find that cycling at a slow C-rate (such as C/20) led to complete conversion into c-Li15Ge4 compared with C/10 (where a-LixGe was also formed) and resulted in steady cycling of 1800 mAh g−1 for up to 100 cycles. In contrast, an increase in rate to 1C rate suppressed the formation of c-Li15Ge4 with a Received: June 18, 2015 Revised: September 22, 2015 Published: September 22, 2015 22772
DOI: 10.1021/acs.jpcc.5b05857 J. Phys. Chem. C 2015, 119, 22772−22777
Article
The Journal of Physical Chemistry C
Figure 1. Real-time XRD patterns showing the (a) 1st lithiation and (b) 1st delithiation for Ge−CB electrode cycled at C/10. The identified peaks and their corresponding markers are c-Li15Ge4 (⧫), c-Ge (●), and Li (*). Unidentified peaks are either from the polymer pouch or the polymer separator. The real-time XRD patterns for Ge−CB electrode are qualitatively similar to Ge−CNT electrode, and the latter is shown in Figure S1.
phase transformation and cycling performance in Ge anodes. We show that carbon nanotubes (CNTs) are a better additive compared with carbon black (CB) for the following: There is a higher level of structural reversibility (recovery of the local or long-range structure upon delithiation) in the c-Ge anode when CNT is used as compared with CB, and as a result, the capacity fading of Ge electrode is less severe when CNT is used. This understanding on the effects of conductive additives allows us to develop insight needed to further improve the performance of Ge and Ge-based materials as anodes for LIBs.
mixture of a-LixGe and unlithiated c-Ge structures. The cycling stability at 1C was better than at C/10, which we attribute to the better reversibility (but lower capacity) of partly converted, amorphous structures.18 In situ X-ray transmission microscopy was used to image in two and three dimensions the morphological changes in Ge particles and revealed a sizedependent behavior of c-Ge particles during cycling at C/5 with carbon black as the conductive additive. Smaller particles experienced volume expansion before the larger ones, and small particles become inactive after one cycle.16 Recent work using ex situ XRD, pair distribution function (PDF) analyses, and in/ex situ high-resolution 7Li solid-state nuclear magnetic resonance investigated the first lithiation cycle of c-Ge with carbon black cycled at C/50 and elucidated a proposed reaction mechanism.19 It was reported that a mixture of amorphous and crystalline Li7Ge3 was present after the lithiation of crystalline Ge; further lithiation results in the transformation of Li7Ge3 to Li7Ge2, and toward the end of discharge, crystalline Li15Ge4 and an overlithiated phase Li15+δGe4 were formed. The differences between the results from ref 19 and our previous work are likely due to different cycling rates and experimental conditions. All of the past reports have explored how the fundamental reaction mechanisms and reasons for failure in Ge electrodes are dependent on Ge morphologies and C-rates. To date, studies of Ge anodes have not investigated the effect of different type of conductive additives on phase transformation and cyclability. In this work, we study crystalline micron-size Ge particles in operando, varying the type of conductive additive, because the conductive additive affects the charge transport within the Ge electrode. To the best of our knowledge, there have been no reports on utilizing operando Xray studies on micron-size c-Ge particles to study and understand the dependence of the phase transformation, Li storage capacity, and cycling stability on the types of conductive additives used. With synchrotron-based operando XRD and XAS, we specifically investigate how different types of commonly used carbon-based conductive additives affect the
2. EXPERIMENTAL DETAILS Similar to our previous report,18 Ge electrode was fabricated by mixing Ge powder (99.999%, Alfa Aesar, particle size ∼20 μm after grinding) with carbon black (Super P, TIMCAL, Switzerland) or carbon nanotubes (CNT, P2-SWNT, Carbon Solutions) and polyvinylidene difluoride binder (PVDF, Kynar HSV 900) with a mass ratio of 8:1:1 and stirred in Nmethylpyrrolidone (NMP) overnight. Because the CNTs used are of low functionality and are not chemically bonded to Ge, the amount of capacity contributed by CNT is ∼20 mAh g−1.18 The slurry was then casted on Cu foil (copper, >99.999%, MTI) and baked in a vacuum oven at 50 °C for 24 h before cell assembly. The operando battery cells were fabricated out of a Ge electrode, Li metal as the counter electrode, and a Celgard separator soaked in electrolyte, similar to previous report.15 The cells are cycled galvanostatically in the voltage range of 0.01 to 1.20 V versus Li/Li+ unless otherwise stated. Details of the cell fabrication, operando XRD, and operando XAS measurements can be found in the Supporting Information. 3. RESULTS AND DISCUSSION 3.1. Carbon Black versus Carbon Nanotubes as Conductive Additives. Traditionally, carbon black (CB) is chosen as the conductive additive, but micron-size Ge generally exhibits very poor electrochemical reversibility when paired with CB.15 This motivates us to explore other conductive additives for better performance. A conductive additive that 22773
DOI: 10.1021/acs.jpcc.5b05857 J. Phys. Chem. C 2015, 119, 22772−22777
Article
The Journal of Physical Chemistry C
Figure 2. Crystalline phase fraction of Ge plotted against (a) voltage and (b) zoomed-in portion of panel a showing the remaining c-Ge fraction at the end of the first lithiation cycle at constant current cycling.
Figure 3. Evolution of Fourier transforms of the Ge K-edge EXAFS spectra during lithiation (black) and delithiation (red) for (a) Ge−CNT electrode and (b) Ge−CB electrode cycled at C/10. For Ge−CNT, the pristine electrode has a first shell amplitude of 2.47, and with subsequent lithiation, the first shell amplitude at the end of first discharge has dropped to 1.02. During charging, the first shell amplitude increased and has a value of 1.48 at the end of first charge. For Ge−CB, the pristine electrode has an amplitude of 2.50, which decreases to 1.46 at the end of first discharge and slightly increases to 1.55 at the end of first charge. For both electrodes throughout the first discharge/charge cycle, the position of the first shell remains unchanged (to within experimental error) at 2.14 Å.
and their eventual disappearance, followed by the emergence of crystalline Li15Ge4 peaks, which persist until the end of lithiation. During delithiation, crystalline Li15Ge4 peaks decrease in intensity and eventually disappear, and there is no recovery of crystalline Ge peaks at the end of the first delithiation cycle. Unlike ref 19, we did not observe an overlithiated phase Li15+δGe4 or crystalline Li7Ge3. Possible reasons for this are that the C-rate in ref 19 (C/50) is 5 times slower than the C-rate used in this work (C/10) or both the XRD and PDF measurements illustrated in ref 19 were carried out ex situ. We have previously shown18 that the C-rate plays a very important factor in influencing the types of phases and structures formed during battery cycling and that ex situ measurements may create experimental artifacts compared with in situ/operando measurements.15 The crystalline phase fraction of Ge is calculated for both electrodes (see Supporting Information) and plotted against voltage and is shown in Figure 2 to obtain a quantitative analysis of Figure 1 and Figure S1. Figure 2a shows that the overall trend for both plots is similar. However, Figure 2b, which presents a zoomed-in section of Figure 2a in the voltage range of 0 V < V < 0.25 V, shows that there exists c-Ge at the end of first lithiation for the Ge−CB (∼5%), whereas no c-Ge can be detected for
