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Lix(Al0.8Zn0.2) alloys as anode materials for rechargeable Li-ion batteries
Progress in Solid State Chemistry xxx (2014) 1e8
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Lix(Al0.8Zn0.2) alloys as anode materials for rechargeable Li-ion batteries Ihor Chumak a, Manuel Hinterstein a, b, Helmut Ehrenberg a, b, * a Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany b Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstr. 15, 76131 Karlsruhe, Germany
a b s t r a c t Keywords: Intermetallic compounds Negative electrodes in Li-ion batteries Ternary phase diagram LieAleZn Li-extraction mechanism
The influence of the lithium content in the starting composition, depth of discharge, binder and electrolyte on the cycle stability was investigated. The structural changes in Lix(Al0.8Zn0.2) electrodes during electrochemical lithium extraction and reinsertion were studied by in situ synchrotron diffraction. The crystal structure of the new compound Li4Al3.42Zn11.58 was determined by single-crystal X-ray diffraction and can be described as combination of the CaCu5 and MgFe6Ge6 structure types. The phase equilibria at 150 C in the LieAleZn system were investigated on six alloys, prepared along the lithium extractioneinsertion line. Ó 2014 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1. Phase equilibria at 150 C and the crystal structures of the ternary compounds R (Li26Al32.89Zn21.11) and N (Li4Al3.42Zn11.58) . . . . . . . . . . . . . . . 00 3.2. In situ synchrotron investigation of the structural changes in the Li1.0(Al0.8Zn0.2) electrode during electrochemical lithium extractioneinsertion 00 3.3. Approaches for improving electrode performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3.1. Li content in the starting composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3.2. The role of the Li4Al3.42Zn11.58 phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3.3. Depth of discharge (DOD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3.4. Electrolyte and binder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
1. Introduction Lithium alloys have been intensively studied in recent years as alternatives to graphite-based anode materials in rechargeable Liion batteries due to their high theoretical capacities and safety characteristics. The electrochemical performance of alloy anodes * Corresponding author. Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 EggensteinLeopoldshafen, Germany. E-mail address: [email protected] (H. Ehrenberg).
for lithium-ion batteries was reviewed by Park et al. and Zhang [1,2]. The most serious disadvantage of alloy anodes is their poor cycle stability. The main causes of the capacity fading are: Substantial volume changes during insertionedeinsertion of lithium cause large mechanical strain, which in combination with the brittleness of these materials results in a disintegration of the electrode and therefore reduces the performance parameters and electrode lifetime. The passivation of active material by a continuous increase of the solideelectrolyte interface (SEI) film.
http://dx.doi.org/10.1016/j.progsolidstchem.2014.04.008 0079-6786/Ó 2014 Elsevier Ltd. All rights reserved.
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In attempt to reduce the capacity fading several strategies are proposed, based on nanostructured and porous materials, the incorporation of active material in an inactive matrix or the use of multiphase instead of single-phase materials [3,4]. An essential impact on the electrochemical performance of the alloy anodes has the binder chemistry and the electrolyte composition. The commonly used binder for anode materials is polyvinylidene fluoride (PVDF). The improved cycle stability of the Si/C electrode with the carboxymethyl cellulose (CMC) binder can be explained by the formation of a strong chemical bond between the binder and the active particles [5]. The electrochemical performance of Sn30Co30C40 electrodes using the lithium polyacrylate (PAA-Li) binder was characterized by Dahn et al. [6]. The PAA-Li binder performed better than PVDF and CMC, the electrodes using PAA-Li binder retain over 450 mAh/g for 100 cycles. Wachtler et al. distinguished the positive influence on cycle stability of the electrolyte additives as filming agents (ethylene carbonate) or surfactant additives (trans-decalin) [7]. The addition of small amounts (3% w/w) of fluoroethylene carbonate or succinic anhydride into the electrolyte improves the electrochemical performance of amorphous Si thin film electrode during cycling [8,9]. A new approach for intermetallic anodes, based on the LiAle LiZn continuous solid solution with NaTl-type structure, has been proposed [10]. The best electrochemical performance shows the alloy Li(Al0.8Zn0.2). The significant increase in the capacity is observed for the Li(Al0.8Zn0.2) electrode from cycle number 2 to 6, accompanied by an anomaly in the voltage profile for cycle number 2, which can be interpreted as a second plateau at 0.76 V, fading away in the next successive cycles until the intermediate maximum in capacity is reached (Fig. 1). The capacity fading after 10 cycles can be explained by additional SEI formation and loss of mobile lithium. The phase diagram of the ternary system LieAleZn has not yet been studied comprehensively. There are only literature data about crystal structures of the ternary compounds Li1.63(Al,Zn)3.37
Table 1 Crystal data and structure refinement for Li26Al32.89Zn21.11 and Li4Al3.42Zn11.58. Empirical formula
Li26Al32.89Zn21.11
Li4Al3.42Zn11.58
Diffractometer/radiation
Bruker Kappa APEXII/Mo-Ka Cubic, Im-3 Na13(Cd,Tl)27 cI160 293 (2)
IPDS-2T/Mo-Ka
Symmetry, space group Structure type Pearson symbol Temperature, K Unit cell dimensions: a, A c, A V, A3 Z Calculated density, g/sm3 Absorption coefficient, mm1 q range for data collection F(000) Reflections collected/unique Data/restraints/parameters Goodness-of-fit on F2 R indices [I > 2s(I)]
Hexagonal, P-62m Own hP19 293 (2)
13.949 (4)
R indices (all data) Largest diff. peak and hole, e/ A3
5.0027 (7) 13.781 (3) 238.66 (8) 1 4.876 22.876 4.44e29.14 404 5531/335 335/0/26 1.082 R1 ¼ 0.0212 wR2 ¼ 0.0468 R1 ¼ 0.0228 wR2 ¼ 0.0476 0.578 and 0.951
2714 (1) 2 2.995 9.675 2.06e30.03 2427 20,784/745 745/0/43 1.189 R1 ¼ 0.0198 wR2 ¼ 0.0432 R1 ¼ 0.0230 wR2 ¼ 0.0452 0.609 and 0.463
(R-phase) [11] and Li6.05Al12.96Zn1.90 (t) [12], determined by single crystal X-ray diffraction. The binary LieAl system was thermodynamically assessed by Hallstedt and Kim [13] and revised in the Lirich region by Puhakainen et al. [14]. The LieZn phase diagram was reviewed by Pelton [15] and AleZn was assessed by Murray [16]. In the current work the crystal structure of the new ternary compound Li4Al3.42Zn11.58 with a new structure type (space group P-62m) is reported. The structural changes in the Lix(Al0.8Zn0.2) electrode during electrochemical lithium extraction and reinsertion were investigated by in situ synchrotron diffraction and compared with the phase equilibria at 150 C. The effect of different electrolytes and binders on the cycling behavior of the Lix(Al0.8Zn0.2) electrode material is reported.
Table 2 Atomic coordinates and displacement parameters ( A2 103) for Li26Al32.89Zn21.11 and Li4Al3.42Zn11.58. Atom
Fig. 1. Voltage profiles and cycle stability of the Li(Al0.8Zn0.2) electrode against a Li counter electrode. Closed symbols refer to charge capacities, open symbols to discharge capacities (extracted from Figs. 2c and 3b in Ref. [10]).
Site
x/a
y/b
z/c
SOF
Uiso/Ueq
Li26Al32.89Zn21.11 Zn/Al1 24g 0
0.3122 (1)
0.1770 (1)
13 (1)
Zn/Al2
48h
0.0939 (1)
0.3094 (1)
0.3423 (1)
Zn/Al3
24g
0
0.1540 (1)
0.0940 (1)
Al4 Li5 Li6 Li7
12e 16f 12e 24g
0.0953 (1) 0.1880 (2) 0.3008 (5) 0
0 0.1880 (2) 0 0.1179 (4)
1/2 0.1880 (2) 1/2 0.3035 (3)
Zn: 0.53 (1), Al: 0.47 (1) Zn: 0.46 (1), Al: 0.54 (1) Zn: 0.32 (1), Al: 0.68 (1) 1 1 1 1
Li4Al3.42Zn11.58 Zn1 6i 0.4969 (2) Zn/Al2 4h 1/3
0 2/3
0.1824 (1) 0.3492 (1)
Zn/Al3
3g
0.3216 (3)
0
1/2
Zn/Al4
2e
0
0
0.0952 (1)
Li5 Li6
2e 2c
0 1/3
0 2/3
0.314 (1) 0
1 Zn: 0.90 (1), Al: 0.10 (1) Zn: 0.50 (1), Al: 0.50 (1) Zn: 0.24 (1), Al: 0.76 (1) 1 1
13 (1) 11 (1) 12 29 28 28
(1) (1) (2) (1)
15 (1) 17 (1) 19 (1) 14 (1) 21 (3) 32 (4)
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Fig. 2. Structural relationship of Li4Al3.42Zn11.58 with CaCu5- and MgFe6Ge6-type structures.
2. Experimental The alloys were prepared from lithium rod (99.9%, Alfa Aesar, Karlsruhe, Germany), aluminum slug (99.999%, Alfa Aesar), and zinc shot (99.99%, Alfa Aesar) in a welded tantalum crucible. All preparation steps were performed in a glove box under dried argon atmosphere. The reaction between the metals was carried out in an induction furnace at 1100 C. After 15 min, the samples were rapidly cooled down to room temperature by removing the crucible from the furnace. For equilibration alloys were then annealed at 150 C for one month. After this treatment, the samples could easily
be separated from the tantalum container. No side reaction of the alloys with the crucible was detected. Initial sample characterization was performed by X-ray powder diffraction in Debye-Scherrer mode using a STOE STADI P powder diffractometer (Mo-Ka1 radiation). The single crystals were selected under dried paraffin and sealed in glass capillaries. The single-crystal X-ray diffraction data were
Table 3 The nominal compositions and lattice parameters of identified phases at 150 C. Sample composition
Phase
Lattice parameters, A a
Li50Al40Zn10 (Li1.0Al0.8Zn0.2) Li40Al48Zn12 (Li0.67Al0.8Zn0.2)
Li30Al56Zn14 (Li0.43Al0.8Zn0.2)
Li20Al64Zn16 (Li0.25Al0.8Zn0.2) Li10Al72Zn18 (Li0.11Al0.8Zn0.2)
Li5Al76Zn19 (Li0.05Al0.8Zn0.2)
Li(Al,Zn) Li(Al,Zn) R “t” R Al “t” R Al R Al Li4Al3.42Zn11.58 Al Li4Al3.42Zn11.58 Zn LiZn4
c
6.3410 (1) 6.3483 (1) 13.9703 (3) 13.9703 (3) 4.0469 (1) 14.009 4.0482 13.9718 4.0508 5.0059 4.0475 5.000 2.6651 2.7850
(1) (1) (9) (1) (3) (2) (4) (7) (6)
13.787 (1) 13.783 (2) 4.944 (4) 4.408 (2)
Fig. 3. Phae equilibria at 150 C in the LieAleZn system along the Li-extraction/ insertion line through the composition Li(Al0.8Zn0.2).
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carbon black and 10% w/w PVDF. This electrode mixture was pressed as a pellet (about 20 mg, 8 mm diameter). The cell was charged and discharged in galvanostatic mode at a rate close to C/ 10 with metallic Li as counter electrode and standard electrolyte. A stainless steel foil was used as the current collector. In situ diffraction during the first cycle was performed at beamline B2 [18,19] and at P02.1 [17] during the second cycle (DESY, Hamburg, Germany). Structure refinement was performed by the Rietveld method using the Winplotr software package [20]. 3. Results and discussion 3.1. Phase equilibria at 150 C and the crystal structures of the ternary compounds R (Li26Al32.89Zn21.11) and N (Li4Al3.42Zn11.58) Fig. 4. Observed and calculated patterns together with their difference for the electrode material after the second charge to 2.0 V. The identified phases are listed in the figure together with the phase ratios in % w/w and lattice parameters. Reflection marks are shown in the same sequence from top to bottom.
collected on a Bruker Kappa APEXII CCD area detector and an IPDS2T diffractometer at 293 K. The electrochemical tests were performed in Swagelok-type cells assembled in an argon-filled dry box. The electrode materials were prepared by grinding the powders of 80% w/w Li1.0(Al0.8Zn0.2) or Li0.11(Al0.8Zn0.2) as the active materials, 10% w/w carbon black as an electronically conductive additive and 10% w/w binder in an agate mortar. The resulting mixture was pressed onto a stainless steel grid. The standard electrolyte was the 1 M LiPF6 in a 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), the standard binder polyvinylidene fluoride (PVDF). Electrodes with PAA-Li binder were prepared from mixtures of Li0.11(Al0.8Zn0.2), carbon black and binder in water, and this slurry was cast on a stainless steel grid. PAA-Li binder was made from polyacrylic acid, neutralized with lithium hydroxide. The Lix(Al0.8Zn0.2) alloys were cycled in galvanostatic mode against counter electrodes of metallic lithium. A Swagelok-type electrochemical cell was used for in situ synchrotron diffraction [17]. In this case, the electrode material was prepared by grinding powders of 80% w/w Li1.0(Al0.8Zn0.2), 10% w/w
The existence of the R-phase ternary compound (composition obtained after structure refinement was Li26Al32.89Zn21.11) and of a new ternary compound Li4Al3.42Zn11.58 (“N”) was confirmed by single crystal X-ray diffraction. The structures were solved by direct methods followed by difference Fourier synthesis, and refinements were performed by least-squares methods on F2. All calculations were carried out with the SHELX-97 package [21]. Final difference Fourier syntheses revealed no significant residual peaks. Crystal structural data and details of the refinements for both compounds are listed in Table 1, atomic coordinates and displacement parameters in Table 2. The crystal structure of Li4Al3.42Zn11.58 was first refined in the centrosymmetric space group P6/mmm. However, the distance between the statistical mixture (Zn,Al)3 on the site 6k was unreasonably short, ca. 1.60 A, suggesting a partial occupancy of this site. The centrosymmetric spage group P6/mmm is the translationsgleiche supergroup of the non-centrosymmetric space group P-62m, and the Wyckoff site 6k in P6/mmm splits into two 3g sites in P-62m [22]. 3g sites in P-62m are fully occupied by the statistical mixtures (0.50 (1) Zn þ 0.50 (1) Al), but the (Zn,Al)3e(Zn,Al)3 distance becomes reasonable: 2.786 (3) A. Therefore, the noncentrosymmetric space group P-62m was preferred. The structure of Li4Al3.42Zn11.58 can be described as a stacking sequence of the MgFe6Ge6- and hypothetical CaCu50 - (derivative of CaCu5 in a non-centrosymmetric setting) type structures (Fig. 2).
Fig. 5. The charge/discharge profiles of the Lix(Al0.8Zn0.2) electrode with phase ratios during Li-extraction/insertion (first charge (a), first discharge (b), second charge (c), second discharge (d)). “a” denotes the Al-type phase with small amounts of Li (up to 15 at.%).
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Fig. 6. Lattice parameters of the phases observed during the first charge/discharge of the Lix(Al0.8Zn0.2) electrode.
In order to compare the equilibrium phase diagram at 150 C with the phases obtained during the electrochemical lithium extraction (insertion) from (into) the Lix(Al0.8Zn0.2) electrode material, six alloys with different Li contents along the lithium extractioneinsertion line (fixed Al:Zn ratio 4:1) were prepared. The results of XRD phase analyses are summarized in Table 3, a graphical representation of phase equilibria is given in Fig. 3. The powder patterns of the alloys Li40Al48Zn12 and Li30Al56Zn14 show additional reflections from an unidentified phase, probably a new
ternary phase with a composition close to that of the t-phase, or the reported crystal structure of the t-phase is not fully correct. Therefore, the t-phase is indicated in parentheses. The four phases detected in the alloy Li5Al76Zn19 gives evidence for an invariant four-phase equilibrium close to 150 C or thermodynamical equilibrium was not reached. According to the ex situ X-ray experiments the reflections from an unidentified phase are detected after the second full charge of the Li1.0(Al0.8Zn0.2) electrode [10]. These reflections could be very
Fig. 7. Lattice parameters of the phases observed during the second charge/discharge of the Lix(Al0.8Zn0.2) electrode.
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Fig. 8. Observed and calculated patterns together with their difference curve for the electrode material after the second charge to 2.0 V. The positions of the reflections are shown from top to bottom: b-type Li(Al,Zn), a-type Al, Cu and bcc-Fe phases from current collectors, Li.
well explained by the new hexagonal phase Li4Al3.42Zn11.58 (Fig. 4). The additional plateau at 0.76 V in the voltage profile during the second lithium extraction from Li1.0(Al0.8Zn0.2) can therefore be explained by the formation of the phase Li4Al3.42Zn11.58. 3.2. In situ synchrotron investigation of the structural changes in the Li1.0(Al0.8Zn0.2) electrode during electrochemical lithium extractioneinsertion The chargeedischarge profiles of the electrochemical lithium extractioneinsertion taken during an in situ experiment are shown in Fig. 5aec together with the phase ratios. The phase analyses of all diffraction patterns reveal the coexistence of the Li(Al,Zn) and Altype phases with different ratios. The reflections from Al can be detected already in the diffraction pattern of the freshly prepared cell (open cell voltage), although the Li1.0(Al0.8Zn0.2) active material was a single phase Li(Al,Zn). This shifting into the two-phase region can be explained by the formation of an SEI film at the contact of the electrode material with the electrolyte and some lithium consumption from the electrode for this SEI formation, even before the potential sweeps. During the first Li-extraction the amount of Li(Al,Zn) continuously decreases from 94 to 20% w/w, the amount of the a-phase “Al” (Al with some Li) increases from 6 to 80% w/w. The lattice parameters of Li(Al,Zn) and “Al” decrease almost linearly up to x(Li) w 0.65, and remain constant after this value up to the end of charge (Fig. 6a and b). According to the LieAl phase diagram [17], up to about 15 at. % of Li can be dissolved in Al (a-phase), the homogeneity range from 45 to 55 at. % of Li for the LiAl (b-phase) is
Fig. 10. Cycle stability of the Lix(Al0.8Zn0.2) electrode, cycled up to 0.7 V and 0.8 V.
reported. The decreasing of the lattice parameters of Li(Al,Zn) and Al-type LiyAl1y (a, y < 0.15) corresponds to the decreasing of the lithium amount in these phases. Therefore, the electrochemical Liextraction up to x(Li) ¼ 0.65 proceeds with two distinct mechanisms: extraction of lithium from Li(Al,Zn) and from a plus a changing of the Li(Al,Zn)/a phase ratio. From x(Li) ¼ 0.65 to the end of charge only the Li(Al,Zn)/a phase ratio changes. During the following Li-insertion a continuous change of the Li(Al,Zn)/a phase ratio from 21:79 to 92:8% w/w is observed, and the lattice parameters of both phases increase slightly (Fig. 6). The decrease of lattice parameters with a constant Li(Al,Zn)/a phase ratio during the second charging up to x(Li) ¼ 0.93 correlates very well with the notably rise of the voltage profile from 0.39 V to 0.57 V (Fig. 7). Below x(Li) ¼ 0.93 lattice parameters of both phases remain almost constant, while the phase ratio continuously changes. The additional plateau at 0.76 V is clearly visible in the voltage profile during the second Li-extraction, but in contrast to the ex situ experiment the new hexagonal phase Li4Al3.42Zn11.58 cannot be detected. The regions with broad reflections (marked by the arrows in Fig. 8) become visible in the diffraction patterns during the charge in the range 0.76e2.0 V. The positions of these two reflections correspond to the most intensive 113 and 203 reflections of the Li4Al3.42Zn11.58 phase. The presence of this compound in a state with a low degree of crystallinity during the in situ experiment and its well crystallized state in the ex situ measurement can be explained by the time limitations e the measuring time of one diffraction pattern at P02.1 is about 3 min and therefore too fast to reach the final equilibrium state; the ex situ XRD was done after electrochemical charging at least 24 h later (the Swagelok cells was disassembled in an argon-filled dry box, the electrode material was washed with dimethyl carbonate (DMC), dried in the vacuum, filled into the capillary). The second Li-insertion is
Fig. 9. Cycle stability and rate capability of electrodes with the initial compositions Li1.0(Al0.8Zn0.2) and Li0.11(Al0.8Zn0.2), respectively.
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brittle, while those alloys with lower Li content are more and more ductile, like for example Li0.11(Al0.8Zn0.2). The composition Li0.11(Al0.8Zn0.2) provides much higher capacities and much better rate capability than Li1.0(Al0.8Zn0.2) (Fig. 9). The ranges of stable cycling are followed by a fast degradation process between cycle numbers 10 and 15 for both compositions.
Fig. 11. Discharge capacity of the Li0.11(Al0.8Zn0.2) vs. cycle number for the different states of DOD.
Fig. 12. Cycle stability of the Li0.11(Al0.8Zn0.2) electrode with fresh Li counter electrodes or fresh electrolyte after 8 cycles.
characterized by constant phase ratios and an increase of the lattice parameters of Li(Al,Zn) up to x(Li) ¼ 0.43, followed by a continuous change of the phase ratios and almost constant lattice parameters during the subsequent discharging (Figs. 5c and 7).
3.3. Approaches for improving electrode performance 3.3.1. Li content in the starting composition In order to check the influence of the initial Li content in the Lix(Al0.8Zn0.2) electrode on the electrochemical properties, the compositions Li1.0(Al0.8Zn0.2) and Li0.11(Al0.8Zn0.2) were chosen for further investigations. Note that a brittle material is easier handled during electrode preparation by grinding the powder of the active material together with carbon black and binder. Li1.0(Al0.8Zn0.2) is
3.3.2. The role of the Li4Al3.42Zn11.58 phase The formation of the Li4Al3.42Zn11.58 compound at 0.76 V during the second electrochemical Li-extraction from the Li1.0(Al0.8Zn0.2) electrode was confirmed by ex situ XRD. The Li1.0(Al0.8Zn0.2) electrode cycled up to 0.7 V, below the formation potential of the Li4Al3.42Zn11.58, shows a rapid capacity fading during the first few cycles (similar to LiAl [10]). However, the Li1.0(Al0.8Zn0.2) electrode cycled up to 0.8 V provides good capacity retention up to ten cycles. In the Li0.11(Al0.8Zn0.2) alloy the Li4Al3.42Zn11.58 phase exists already after solid state synthesis (Table 3), and the Li0.11(Al0.8Zn0.2) electrode shows good capacity retention up to ten cycles even during cycling up to 0.7 V only (Fig. 10). This is strong hint that the Li4Al3.42Zn11.58 phase acts as a buffer to alleviate the volume changes of the active particles. On the other hand, the Li4Al3.42Zn11.58 phase may act as a catalyst to promote the formation of a dense SEI. This SEI seems to be growing continuously, and a critical thickness for Li transfer is reached after about ten cycles [10]. 3.3.3. Depth of discharge (DOD) During the first discharge of the Li0.11(Al0.8Zn0.2) electrode up to 0.05 V at the C/20 rate the amount of the stored lithium Dx(Li) is almost 0.9 (full discharge DOD ¼ 1.0 Q). In order to minimize the volume changes during the Li-insertion/extraction, the Li0.11(Al0.8Zn0.2) electrode was discharged up to a DOD ¼ 0.5 Q (Dx(Li) ¼ 0.45) and to a DOD ¼ 0.25 Q (Dx(Li) ¼ 0.225). The electrode cycled to the 0.5 Q shows the good cycle stability up to 15 cycles, followed by the strong capacity fading in the next cycles. The electrode cycled to the 0.25 Q provides excellent capacity retention over the 40 cycles (Fig. 11). These results can be explained by less electrolyte reduction with the decrease of the DOD, and consequently, the deceleration (or even stop) of the SEI growth. 3.3.4. Electrolyte and binder Electrolyte decomposition and the passivation of the metallic Li counter electrode could also be reasons for the pronounced capacity losses after 10 cycles. In order to check these contributions, the Li0.11(Al0.8Zn0.2) electrode was cycled up to 8 cycles in three different cells. After the cycling the used Li counter electrode was replaced by a fresh one in the first cell. The second cell was assembled with fresh Li and fresh electrolyte. In the third cell the electrode material was washed several times with DMC to remove
Fig. 13. Discharge capacity vs. cycle number for the Lix(Al0.8Zn0.2) electrode with different binders (a) and different electrolytes (b).
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residual electrolyte and dried under vacuum. The third cell was assembled also with fresh Li and fresh electrolyte. The strong capacity fading between cycle numbers 10 and 15 for all three cells was observed (Fig. 12). This allows concluding that only changes at the LieAleZn electrode are responsible for the poor cycle stability. The cycle stability of the Lix(Al0.8Zn0.2) electrode was evaluated for the following different binders: polyvinylidene fluoride (PVDF), polyacrylic acid (PAA-H), lithium polyacrylate (PAA-Li) and carboxymethyl cellulose (CMC) (Fig. 13a). Li0.11(Al0.8Zn0.2) was cycled at the rate C/10 in the 0.05e2.0 V voltage window with the PVDF, PAA-H, PAA-Li. The Li1.0(Al0.8Zn0.2) electrode was cycled at the rate C/20 in the voltage window 0.15e0.8 V with CMC and PVDF, marked with the star. The standard electrolyte with LiPF6 salt was used. PVDF provides the best discharge capacities, but a pronounced capacity fading between cycle numbers 10 and 20 is observed for all binders. The cycle stability of the Li1.0(Al0.8Zn0.2) electrode was also compared for the two different salts LiPF6 and LiBOB, lithium bis(oxalato) borate. In the case of LiBOB, a continuous capacity fading with the cycle number was observed, similar as for LiAl [10]. In this work the cycle stability of Li0.11(Al0.8Zn0.2) electrodes (mixed with PVDF binder) was tested for three different electrolytes: 1 M LiBF4 in a 1:1 mixture of adiponitril and g-butyrolacton (LiBF4/ AND-BL), 1 M LiBF4 in a 1:1 mixture of adiponitril and ethylene carbonate (LiBF4/AND-EC) and 1 M LiClO4 in a 1:1 mixture of propylene carbonate and 1,2 dimethoxyethane (LiClO4/PC-DME) (Fig. 13b). The highest capacities were obtained for the LiBF4-based electrolyte, but strong capacity losses between cycle numbers 10 and 20 are observed for all investigated electrolyte combinations. 4. Conclusions The Lix(Al0.8Zn0.2) provides high capacity and good capacity retention up to 10 cycles. The replacement of some Al by Zn is therefore a promising approach to a composite anode material with enhanced performance properties. The Li4Al3.42Zn11.58 phase protects the electrode from particle cracking and contact loss during the first ten cycles by the higher tolerance against the accompanied volume changes. The continuous increase of the SEI during cycling leads to the passivation of the active material and strong capacity fading between cycles 10 and 15. It seems that the limitations caused by the dramatic volume changes could be overcome for Li(Al,Zn)-based electrodes, but no sufficiently stable electrolyte was established so far. Acknowledgments Financial support from the Deutsche Forschungsgemeinschaft (DFG, EH183/7) is gratefully acknowledged. This work has benefited
from beamtime allocation at P02.1 (HASYLAB) and a corresponding grant for the enhancement of this synchrotron beamline by the Bundesministerium für Bildung und Forschung (BMBF, 05K13VK1). References [1] Park C-M, Kim J-H, Kim H, Sohn H-J. Li-alloy based anode materials for Li secondary batteries. Chem Soc Rev 2010;39:3115e41. [2] Zhang W-J. A review of the electrochemical performance of alloy anodes for lithium-ion batteries. J Power Sources 2011;196:13e24. [3] Winter M, Besenhard JO. Electrochemical lithiation of tin and tin-based intermetallics and composites. Electrochim Acta 1999;45:31e50. [4] Yang J, Wachtler M, Winter M, Besenhard JO. Submicrocrystalline Sn and Sne SnSb powders as lithium storage materials for lithium-ion batteries. Electrochem Solid-State Lett 1999;2:161e3. [5] Hochgatterer NS, Schweiger MR, Koller S, Raimann PR, Wohrle T, Wurm C, et al. Silicon/graphite composite electrodes for high-capacity anodes: influence of binder chemistry on cycling stability. Electrochem Solid-State Lett 2008;11(5):A76e80. [6] Li J, Le D-B, Ferguson PP, Dahn JR. Lithium polyacrylate as a binder for tine cobaltecarbon negative electrodes in lithium-ion batteries. Electrochim Acta 2010;55:2991e5. [7] Wachtler M, Besenhard JO, Winter M. Tin and tin-based intermetallics as new anode materials for lithium-ion cells. J Power Sources 2001;94:189e93. [8] Choi NS, Yew KH, Lee KY, Sung M, Kim H, Kim SS. Effect of fluoroethylene carbonate additive on interfacial properties of silicon thin-film electrode. J Power Sources 2006;161(2):1254e9. [9] Han G-B, Ryou M-H, Cho KY, Lee YM, Park J-K. Effect of succinic anhydride as an electrolyte additive on electrochemical characteristics of silicon thin-film electrode. J Power Sources 2010;195(11):3709e14. [10] Chumak I, Dmytriv G, Pavlyuk V, Oswald S, Eckert J, Trill H, et al. Li(Al1zZnz) alloys as anode materials for rechargeable Li-ion batteries. J Mater Res 2010;25(8):1492e9. [11] Lee C-S, Miller GJ. Experimental and theoretical studies of elemental site preferences in quasicrystalline approximants (R-phases) within the LieMge ZneAl system. Inorg Chem 2001;40:338e45. [12] Leblanc M, Le Bail A, Audier M. Crystalline phases related to the icosahedral AleLieCu phase. A single crystal X-ray diffraction study of the tetragonal tauAl56(Cu,Zn)11Li33 phase. Physica B 1991;173(3):329e55. [13] Hallstedt B, Kim O. Thermodynamic assessment of the AleLi system. Int J Mater Res 2007;98:961e9. [14] Puhakainen K, Boström M, Groy T-L, Häussermann U. A new phase in the system lithiumealuminum: characterization of orthorhombic Li2Al. J Solid State Chem 2010;183:2528e33. [15] Pelton AD. The LieZn system. J Phase Equilib 1991;12(1):42e5. [16] Murray JL. The AleZn system. Bull Alloy Phase Diagrams 1983;4(1):55e73. [17] Herklotz M, Scheiba F, Hinterstein M, Nikolowski K, Knapp M, Dippel A-C, et al. Advances in in situ powder diffraction of battery materials e a case study of the new beamline P02.1 at DESY Hamburg. J Appl Cryst 2013;46:1117e27. [18] Knapp M, Baehtz C, Ehrenberg H, Fuess H. The synchrotron powder diffractometer at beamline B2 at HASYLAB/DESY: status and capabilities. J Synchrotron Radiat 2004;11:328e34. [19] Knapp M, Joko V, Baehtz C, Brecht H, Berghauser A, Ehrenberg H, et al. Position-sensitive detector system OBI for high resolution X-ray powder diffraction using on-site readable image plates. Nucl Instrum Methods Phys Res Sect A 2004;521(2e3):565e70. [20] Roisnel T, Rodriguez-Carvajal J. WinPLOTR: a Windows tool for powder diffraction pattern analysis. Mater Sci Forum 2001;378:118e23. [21] Sheldrick GM. SHELXL-97. Program for crystal structure refinement. University of Goöttingen; 1997. [22] Bärnighausen H. Group-subgroup relations between space groups: a useful tool in crystal chemistry. Commun Math Chem 1980;9:139e75.
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Lix(Al0.8Zn0.2) alloys as anode materials for rechargeable Li-ion batteries Ihor Chumak a, Manuel Hinterstein a, b, Helmut Ehrenberg a, b, * a Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany b Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstr. 15, 76131 Karlsruhe, Germany
a b s t r a c t Keywords: Intermetallic compounds Negative electrodes in Li-ion batteries Ternary phase diagram LieAleZn Li-extraction mechanism
The influence of the lithium content in the starting composition, depth of discharge, binder and electrolyte on the cycle stability was investigated. The structural changes in Lix(Al0.8Zn0.2) electrodes during electrochemical lithium extraction and reinsertion were studied by in situ synchrotron diffraction. The crystal structure of the new compound Li4Al3.42Zn11.58 was determined by single-crystal X-ray diffraction and can be described as combination of the CaCu5 and MgFe6Ge6 structure types. The phase equilibria at 150 C in the LieAleZn system were investigated on six alloys, prepared along the lithium extractioneinsertion line. Ó 2014 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1. Phase equilibria at 150 C and the crystal structures of the ternary compounds R (Li26Al32.89Zn21.11) and N (Li4Al3.42Zn11.58) . . . . . . . . . . . . . . . 00 3.2. In situ synchrotron investigation of the structural changes in the Li1.0(Al0.8Zn0.2) electrode during electrochemical lithium extractioneinsertion 00 3.3. Approaches for improving electrode performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3.1. Li content in the starting composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3.2. The role of the Li4Al3.42Zn11.58 phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3.3. Depth of discharge (DOD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3.4. Electrolyte and binder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
1. Introduction Lithium alloys have been intensively studied in recent years as alternatives to graphite-based anode materials in rechargeable Liion batteries due to their high theoretical capacities and safety characteristics. The electrochemical performance of alloy anodes * Corresponding author. Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 EggensteinLeopoldshafen, Germany. E-mail address: [email protected] (H. Ehrenberg).
for lithium-ion batteries was reviewed by Park et al. and Zhang [1,2]. The most serious disadvantage of alloy anodes is their poor cycle stability. The main causes of the capacity fading are: Substantial volume changes during insertionedeinsertion of lithium cause large mechanical strain, which in combination with the brittleness of these materials results in a disintegration of the electrode and therefore reduces the performance parameters and electrode lifetime. The passivation of active material by a continuous increase of the solideelectrolyte interface (SEI) film.
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In attempt to reduce the capacity fading several strategies are proposed, based on nanostructured and porous materials, the incorporation of active material in an inactive matrix or the use of multiphase instead of single-phase materials [3,4]. An essential impact on the electrochemical performance of the alloy anodes has the binder chemistry and the electrolyte composition. The commonly used binder for anode materials is polyvinylidene fluoride (PVDF). The improved cycle stability of the Si/C electrode with the carboxymethyl cellulose (CMC) binder can be explained by the formation of a strong chemical bond between the binder and the active particles [5]. The electrochemical performance of Sn30Co30C40 electrodes using the lithium polyacrylate (PAA-Li) binder was characterized by Dahn et al. [6]. The PAA-Li binder performed better than PVDF and CMC, the electrodes using PAA-Li binder retain over 450 mAh/g for 100 cycles. Wachtler et al. distinguished the positive influence on cycle stability of the electrolyte additives as filming agents (ethylene carbonate) or surfactant additives (trans-decalin) [7]. The addition of small amounts (3% w/w) of fluoroethylene carbonate or succinic anhydride into the electrolyte improves the electrochemical performance of amorphous Si thin film electrode during cycling [8,9]. A new approach for intermetallic anodes, based on the LiAle LiZn continuous solid solution with NaTl-type structure, has been proposed [10]. The best electrochemical performance shows the alloy Li(Al0.8Zn0.2). The significant increase in the capacity is observed for the Li(Al0.8Zn0.2) electrode from cycle number 2 to 6, accompanied by an anomaly in the voltage profile for cycle number 2, which can be interpreted as a second plateau at 0.76 V, fading away in the next successive cycles until the intermediate maximum in capacity is reached (Fig. 1). The capacity fading after 10 cycles can be explained by additional SEI formation and loss of mobile lithium. The phase diagram of the ternary system LieAleZn has not yet been studied comprehensively. There are only literature data about crystal structures of the ternary compounds Li1.63(Al,Zn)3.37
Table 1 Crystal data and structure refinement for Li26Al32.89Zn21.11 and Li4Al3.42Zn11.58. Empirical formula
Li26Al32.89Zn21.11
Li4Al3.42Zn11.58
Diffractometer/radiation
Bruker Kappa APEXII/Mo-Ka Cubic, Im-3 Na13(Cd,Tl)27 cI160 293 (2)
IPDS-2T/Mo-Ka
Symmetry, space group Structure type Pearson symbol Temperature, K Unit cell dimensions: a, A c, A V, A3 Z Calculated density, g/sm3 Absorption coefficient, mm1 q range for data collection F(000) Reflections collected/unique Data/restraints/parameters Goodness-of-fit on F2 R indices [I > 2s(I)]
Hexagonal, P-62m Own hP19 293 (2)
13.949 (4)
R indices (all data) Largest diff. peak and hole, e/ A3
5.0027 (7) 13.781 (3) 238.66 (8) 1 4.876 22.876 4.44e29.14 404 5531/335 335/0/26 1.082 R1 ¼ 0.0212 wR2 ¼ 0.0468 R1 ¼ 0.0228 wR2 ¼ 0.0476 0.578 and 0.951
2714 (1) 2 2.995 9.675 2.06e30.03 2427 20,784/745 745/0/43 1.189 R1 ¼ 0.0198 wR2 ¼ 0.0432 R1 ¼ 0.0230 wR2 ¼ 0.0452 0.609 and 0.463
(R-phase) [11] and Li6.05Al12.96Zn1.90 (t) [12], determined by single crystal X-ray diffraction. The binary LieAl system was thermodynamically assessed by Hallstedt and Kim [13] and revised in the Lirich region by Puhakainen et al. [14]. The LieZn phase diagram was reviewed by Pelton [15] and AleZn was assessed by Murray [16]. In the current work the crystal structure of the new ternary compound Li4Al3.42Zn11.58 with a new structure type (space group P-62m) is reported. The structural changes in the Lix(Al0.8Zn0.2) electrode during electrochemical lithium extraction and reinsertion were investigated by in situ synchrotron diffraction and compared with the phase equilibria at 150 C. The effect of different electrolytes and binders on the cycling behavior of the Lix(Al0.8Zn0.2) electrode material is reported.
Table 2 Atomic coordinates and displacement parameters ( A2 103) for Li26Al32.89Zn21.11 and Li4Al3.42Zn11.58. Atom
Fig. 1. Voltage profiles and cycle stability of the Li(Al0.8Zn0.2) electrode against a Li counter electrode. Closed symbols refer to charge capacities, open symbols to discharge capacities (extracted from Figs. 2c and 3b in Ref. [10]).
Site
x/a
y/b
z/c
SOF
Uiso/Ueq
Li26Al32.89Zn21.11 Zn/Al1 24g 0
0.3122 (1)
0.1770 (1)
13 (1)
Zn/Al2
48h
0.0939 (1)
0.3094 (1)
0.3423 (1)
Zn/Al3
24g
0
0.1540 (1)
0.0940 (1)
Al4 Li5 Li6 Li7
12e 16f 12e 24g
0.0953 (1) 0.1880 (2) 0.3008 (5) 0
0 0.1880 (2) 0 0.1179 (4)
1/2 0.1880 (2) 1/2 0.3035 (3)
Zn: 0.53 (1), Al: 0.47 (1) Zn: 0.46 (1), Al: 0.54 (1) Zn: 0.32 (1), Al: 0.68 (1) 1 1 1 1
Li4Al3.42Zn11.58 Zn1 6i 0.4969 (2) Zn/Al2 4h 1/3
0 2/3
0.1824 (1) 0.3492 (1)
Zn/Al3
3g
0.3216 (3)
0
1/2
Zn/Al4
2e
0
0
0.0952 (1)
Li5 Li6
2e 2c
0 1/3
0 2/3
0.314 (1) 0
1 Zn: 0.90 (1), Al: 0.10 (1) Zn: 0.50 (1), Al: 0.50 (1) Zn: 0.24 (1), Al: 0.76 (1) 1 1
13 (1) 11 (1) 12 29 28 28
(1) (1) (2) (1)
15 (1) 17 (1) 19 (1) 14 (1) 21 (3) 32 (4)
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3
Fig. 2. Structural relationship of Li4Al3.42Zn11.58 with CaCu5- and MgFe6Ge6-type structures.
2. Experimental The alloys were prepared from lithium rod (99.9%, Alfa Aesar, Karlsruhe, Germany), aluminum slug (99.999%, Alfa Aesar), and zinc shot (99.99%, Alfa Aesar) in a welded tantalum crucible. All preparation steps were performed in a glove box under dried argon atmosphere. The reaction between the metals was carried out in an induction furnace at 1100 C. After 15 min, the samples were rapidly cooled down to room temperature by removing the crucible from the furnace. For equilibration alloys were then annealed at 150 C for one month. After this treatment, the samples could easily
be separated from the tantalum container. No side reaction of the alloys with the crucible was detected. Initial sample characterization was performed by X-ray powder diffraction in Debye-Scherrer mode using a STOE STADI P powder diffractometer (Mo-Ka1 radiation). The single crystals were selected under dried paraffin and sealed in glass capillaries. The single-crystal X-ray diffraction data were
Table 3 The nominal compositions and lattice parameters of identified phases at 150 C. Sample composition
Phase
Lattice parameters, A a
Li50Al40Zn10 (Li1.0Al0.8Zn0.2) Li40Al48Zn12 (Li0.67Al0.8Zn0.2)
Li30Al56Zn14 (Li0.43Al0.8Zn0.2)
Li20Al64Zn16 (Li0.25Al0.8Zn0.2) Li10Al72Zn18 (Li0.11Al0.8Zn0.2)
Li5Al76Zn19 (Li0.05Al0.8Zn0.2)
Li(Al,Zn) Li(Al,Zn) R “t” R Al “t” R Al R Al Li4Al3.42Zn11.58 Al Li4Al3.42Zn11.58 Zn LiZn4
c
6.3410 (1) 6.3483 (1) 13.9703 (3) 13.9703 (3) 4.0469 (1) 14.009 4.0482 13.9718 4.0508 5.0059 4.0475 5.000 2.6651 2.7850
(1) (1) (9) (1) (3) (2) (4) (7) (6)
13.787 (1) 13.783 (2) 4.944 (4) 4.408 (2)
Fig. 3. Phae equilibria at 150 C in the LieAleZn system along the Li-extraction/ insertion line through the composition Li(Al0.8Zn0.2).
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carbon black and 10% w/w PVDF. This electrode mixture was pressed as a pellet (about 20 mg, 8 mm diameter). The cell was charged and discharged in galvanostatic mode at a rate close to C/ 10 with metallic Li as counter electrode and standard electrolyte. A stainless steel foil was used as the current collector. In situ diffraction during the first cycle was performed at beamline B2 [18,19] and at P02.1 [17] during the second cycle (DESY, Hamburg, Germany). Structure refinement was performed by the Rietveld method using the Winplotr software package [20]. 3. Results and discussion 3.1. Phase equilibria at 150 C and the crystal structures of the ternary compounds R (Li26Al32.89Zn21.11) and N (Li4Al3.42Zn11.58) Fig. 4. Observed and calculated patterns together with their difference for the electrode material after the second charge to 2.0 V. The identified phases are listed in the figure together with the phase ratios in % w/w and lattice parameters. Reflection marks are shown in the same sequence from top to bottom.
collected on a Bruker Kappa APEXII CCD area detector and an IPDS2T diffractometer at 293 K. The electrochemical tests were performed in Swagelok-type cells assembled in an argon-filled dry box. The electrode materials were prepared by grinding the powders of 80% w/w Li1.0(Al0.8Zn0.2) or Li0.11(Al0.8Zn0.2) as the active materials, 10% w/w carbon black as an electronically conductive additive and 10% w/w binder in an agate mortar. The resulting mixture was pressed onto a stainless steel grid. The standard electrolyte was the 1 M LiPF6 in a 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), the standard binder polyvinylidene fluoride (PVDF). Electrodes with PAA-Li binder were prepared from mixtures of Li0.11(Al0.8Zn0.2), carbon black and binder in water, and this slurry was cast on a stainless steel grid. PAA-Li binder was made from polyacrylic acid, neutralized with lithium hydroxide. The Lix(Al0.8Zn0.2) alloys were cycled in galvanostatic mode against counter electrodes of metallic lithium. A Swagelok-type electrochemical cell was used for in situ synchrotron diffraction [17]. In this case, the electrode material was prepared by grinding powders of 80% w/w Li1.0(Al0.8Zn0.2), 10% w/w
The existence of the R-phase ternary compound (composition obtained after structure refinement was Li26Al32.89Zn21.11) and of a new ternary compound Li4Al3.42Zn11.58 (“N”) was confirmed by single crystal X-ray diffraction. The structures were solved by direct methods followed by difference Fourier synthesis, and refinements were performed by least-squares methods on F2. All calculations were carried out with the SHELX-97 package [21]. Final difference Fourier syntheses revealed no significant residual peaks. Crystal structural data and details of the refinements for both compounds are listed in Table 1, atomic coordinates and displacement parameters in Table 2. The crystal structure of Li4Al3.42Zn11.58 was first refined in the centrosymmetric space group P6/mmm. However, the distance between the statistical mixture (Zn,Al)3 on the site 6k was unreasonably short, ca. 1.60 A, suggesting a partial occupancy of this site. The centrosymmetric spage group P6/mmm is the translationsgleiche supergroup of the non-centrosymmetric space group P-62m, and the Wyckoff site 6k in P6/mmm splits into two 3g sites in P-62m [22]. 3g sites in P-62m are fully occupied by the statistical mixtures (0.50 (1) Zn þ 0.50 (1) Al), but the (Zn,Al)3e(Zn,Al)3 distance becomes reasonable: 2.786 (3) A. Therefore, the noncentrosymmetric space group P-62m was preferred. The structure of Li4Al3.42Zn11.58 can be described as a stacking sequence of the MgFe6Ge6- and hypothetical CaCu50 - (derivative of CaCu5 in a non-centrosymmetric setting) type structures (Fig. 2).
Fig. 5. The charge/discharge profiles of the Lix(Al0.8Zn0.2) electrode with phase ratios during Li-extraction/insertion (first charge (a), first discharge (b), second charge (c), second discharge (d)). “a” denotes the Al-type phase with small amounts of Li (up to 15 at.%).
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Fig. 6. Lattice parameters of the phases observed during the first charge/discharge of the Lix(Al0.8Zn0.2) electrode.
In order to compare the equilibrium phase diagram at 150 C with the phases obtained during the electrochemical lithium extraction (insertion) from (into) the Lix(Al0.8Zn0.2) electrode material, six alloys with different Li contents along the lithium extractioneinsertion line (fixed Al:Zn ratio 4:1) were prepared. The results of XRD phase analyses are summarized in Table 3, a graphical representation of phase equilibria is given in Fig. 3. The powder patterns of the alloys Li40Al48Zn12 and Li30Al56Zn14 show additional reflections from an unidentified phase, probably a new
ternary phase with a composition close to that of the t-phase, or the reported crystal structure of the t-phase is not fully correct. Therefore, the t-phase is indicated in parentheses. The four phases detected in the alloy Li5Al76Zn19 gives evidence for an invariant four-phase equilibrium close to 150 C or thermodynamical equilibrium was not reached. According to the ex situ X-ray experiments the reflections from an unidentified phase are detected after the second full charge of the Li1.0(Al0.8Zn0.2) electrode [10]. These reflections could be very
Fig. 7. Lattice parameters of the phases observed during the second charge/discharge of the Lix(Al0.8Zn0.2) electrode.
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Fig. 8. Observed and calculated patterns together with their difference curve for the electrode material after the second charge to 2.0 V. The positions of the reflections are shown from top to bottom: b-type Li(Al,Zn), a-type Al, Cu and bcc-Fe phases from current collectors, Li.
well explained by the new hexagonal phase Li4Al3.42Zn11.58 (Fig. 4). The additional plateau at 0.76 V in the voltage profile during the second lithium extraction from Li1.0(Al0.8Zn0.2) can therefore be explained by the formation of the phase Li4Al3.42Zn11.58. 3.2. In situ synchrotron investigation of the structural changes in the Li1.0(Al0.8Zn0.2) electrode during electrochemical lithium extractioneinsertion The chargeedischarge profiles of the electrochemical lithium extractioneinsertion taken during an in situ experiment are shown in Fig. 5aec together with the phase ratios. The phase analyses of all diffraction patterns reveal the coexistence of the Li(Al,Zn) and Altype phases with different ratios. The reflections from Al can be detected already in the diffraction pattern of the freshly prepared cell (open cell voltage), although the Li1.0(Al0.8Zn0.2) active material was a single phase Li(Al,Zn). This shifting into the two-phase region can be explained by the formation of an SEI film at the contact of the electrode material with the electrolyte and some lithium consumption from the electrode for this SEI formation, even before the potential sweeps. During the first Li-extraction the amount of Li(Al,Zn) continuously decreases from 94 to 20% w/w, the amount of the a-phase “Al” (Al with some Li) increases from 6 to 80% w/w. The lattice parameters of Li(Al,Zn) and “Al” decrease almost linearly up to x(Li) w 0.65, and remain constant after this value up to the end of charge (Fig. 6a and b). According to the LieAl phase diagram [17], up to about 15 at. % of Li can be dissolved in Al (a-phase), the homogeneity range from 45 to 55 at. % of Li for the LiAl (b-phase) is
Fig. 10. Cycle stability of the Lix(Al0.8Zn0.2) electrode, cycled up to 0.7 V and 0.8 V.
reported. The decreasing of the lattice parameters of Li(Al,Zn) and Al-type LiyAl1y (a, y < 0.15) corresponds to the decreasing of the lithium amount in these phases. Therefore, the electrochemical Liextraction up to x(Li) ¼ 0.65 proceeds with two distinct mechanisms: extraction of lithium from Li(Al,Zn) and from a plus a changing of the Li(Al,Zn)/a phase ratio. From x(Li) ¼ 0.65 to the end of charge only the Li(Al,Zn)/a phase ratio changes. During the following Li-insertion a continuous change of the Li(Al,Zn)/a phase ratio from 21:79 to 92:8% w/w is observed, and the lattice parameters of both phases increase slightly (Fig. 6). The decrease of lattice parameters with a constant Li(Al,Zn)/a phase ratio during the second charging up to x(Li) ¼ 0.93 correlates very well with the notably rise of the voltage profile from 0.39 V to 0.57 V (Fig. 7). Below x(Li) ¼ 0.93 lattice parameters of both phases remain almost constant, while the phase ratio continuously changes. The additional plateau at 0.76 V is clearly visible in the voltage profile during the second Li-extraction, but in contrast to the ex situ experiment the new hexagonal phase Li4Al3.42Zn11.58 cannot be detected. The regions with broad reflections (marked by the arrows in Fig. 8) become visible in the diffraction patterns during the charge in the range 0.76e2.0 V. The positions of these two reflections correspond to the most intensive 113 and 203 reflections of the Li4Al3.42Zn11.58 phase. The presence of this compound in a state with a low degree of crystallinity during the in situ experiment and its well crystallized state in the ex situ measurement can be explained by the time limitations e the measuring time of one diffraction pattern at P02.1 is about 3 min and therefore too fast to reach the final equilibrium state; the ex situ XRD was done after electrochemical charging at least 24 h later (the Swagelok cells was disassembled in an argon-filled dry box, the electrode material was washed with dimethyl carbonate (DMC), dried in the vacuum, filled into the capillary). The second Li-insertion is
Fig. 9. Cycle stability and rate capability of electrodes with the initial compositions Li1.0(Al0.8Zn0.2) and Li0.11(Al0.8Zn0.2), respectively.
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brittle, while those alloys with lower Li content are more and more ductile, like for example Li0.11(Al0.8Zn0.2). The composition Li0.11(Al0.8Zn0.2) provides much higher capacities and much better rate capability than Li1.0(Al0.8Zn0.2) (Fig. 9). The ranges of stable cycling are followed by a fast degradation process between cycle numbers 10 and 15 for both compositions.
Fig. 11. Discharge capacity of the Li0.11(Al0.8Zn0.2) vs. cycle number for the different states of DOD.
Fig. 12. Cycle stability of the Li0.11(Al0.8Zn0.2) electrode with fresh Li counter electrodes or fresh electrolyte after 8 cycles.
characterized by constant phase ratios and an increase of the lattice parameters of Li(Al,Zn) up to x(Li) ¼ 0.43, followed by a continuous change of the phase ratios and almost constant lattice parameters during the subsequent discharging (Figs. 5c and 7).
3.3. Approaches for improving electrode performance 3.3.1. Li content in the starting composition In order to check the influence of the initial Li content in the Lix(Al0.8Zn0.2) electrode on the electrochemical properties, the compositions Li1.0(Al0.8Zn0.2) and Li0.11(Al0.8Zn0.2) were chosen for further investigations. Note that a brittle material is easier handled during electrode preparation by grinding the powder of the active material together with carbon black and binder. Li1.0(Al0.8Zn0.2) is
3.3.2. The role of the Li4Al3.42Zn11.58 phase The formation of the Li4Al3.42Zn11.58 compound at 0.76 V during the second electrochemical Li-extraction from the Li1.0(Al0.8Zn0.2) electrode was confirmed by ex situ XRD. The Li1.0(Al0.8Zn0.2) electrode cycled up to 0.7 V, below the formation potential of the Li4Al3.42Zn11.58, shows a rapid capacity fading during the first few cycles (similar to LiAl [10]). However, the Li1.0(Al0.8Zn0.2) electrode cycled up to 0.8 V provides good capacity retention up to ten cycles. In the Li0.11(Al0.8Zn0.2) alloy the Li4Al3.42Zn11.58 phase exists already after solid state synthesis (Table 3), and the Li0.11(Al0.8Zn0.2) electrode shows good capacity retention up to ten cycles even during cycling up to 0.7 V only (Fig. 10). This is strong hint that the Li4Al3.42Zn11.58 phase acts as a buffer to alleviate the volume changes of the active particles. On the other hand, the Li4Al3.42Zn11.58 phase may act as a catalyst to promote the formation of a dense SEI. This SEI seems to be growing continuously, and a critical thickness for Li transfer is reached after about ten cycles [10]. 3.3.3. Depth of discharge (DOD) During the first discharge of the Li0.11(Al0.8Zn0.2) electrode up to 0.05 V at the C/20 rate the amount of the stored lithium Dx(Li) is almost 0.9 (full discharge DOD ¼ 1.0 Q). In order to minimize the volume changes during the Li-insertion/extraction, the Li0.11(Al0.8Zn0.2) electrode was discharged up to a DOD ¼ 0.5 Q (Dx(Li) ¼ 0.45) and to a DOD ¼ 0.25 Q (Dx(Li) ¼ 0.225). The electrode cycled to the 0.5 Q shows the good cycle stability up to 15 cycles, followed by the strong capacity fading in the next cycles. The electrode cycled to the 0.25 Q provides excellent capacity retention over the 40 cycles (Fig. 11). These results can be explained by less electrolyte reduction with the decrease of the DOD, and consequently, the deceleration (or even stop) of the SEI growth. 3.3.4. Electrolyte and binder Electrolyte decomposition and the passivation of the metallic Li counter electrode could also be reasons for the pronounced capacity losses after 10 cycles. In order to check these contributions, the Li0.11(Al0.8Zn0.2) electrode was cycled up to 8 cycles in three different cells. After the cycling the used Li counter electrode was replaced by a fresh one in the first cell. The second cell was assembled with fresh Li and fresh electrolyte. In the third cell the electrode material was washed several times with DMC to remove
Fig. 13. Discharge capacity vs. cycle number for the Lix(Al0.8Zn0.2) electrode with different binders (a) and different electrolytes (b).
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residual electrolyte and dried under vacuum. The third cell was assembled also with fresh Li and fresh electrolyte. The strong capacity fading between cycle numbers 10 and 15 for all three cells was observed (Fig. 12). This allows concluding that only changes at the LieAleZn electrode are responsible for the poor cycle stability. The cycle stability of the Lix(Al0.8Zn0.2) electrode was evaluated for the following different binders: polyvinylidene fluoride (PVDF), polyacrylic acid (PAA-H), lithium polyacrylate (PAA-Li) and carboxymethyl cellulose (CMC) (Fig. 13a). Li0.11(Al0.8Zn0.2) was cycled at the rate C/10 in the 0.05e2.0 V voltage window with the PVDF, PAA-H, PAA-Li. The Li1.0(Al0.8Zn0.2) electrode was cycled at the rate C/20 in the voltage window 0.15e0.8 V with CMC and PVDF, marked with the star. The standard electrolyte with LiPF6 salt was used. PVDF provides the best discharge capacities, but a pronounced capacity fading between cycle numbers 10 and 20 is observed for all binders. The cycle stability of the Li1.0(Al0.8Zn0.2) electrode was also compared for the two different salts LiPF6 and LiBOB, lithium bis(oxalato) borate. In the case of LiBOB, a continuous capacity fading with the cycle number was observed, similar as for LiAl [10]. In this work the cycle stability of Li0.11(Al0.8Zn0.2) electrodes (mixed with PVDF binder) was tested for three different electrolytes: 1 M LiBF4 in a 1:1 mixture of adiponitril and g-butyrolacton (LiBF4/ AND-BL), 1 M LiBF4 in a 1:1 mixture of adiponitril and ethylene carbonate (LiBF4/AND-EC) and 1 M LiClO4 in a 1:1 mixture of propylene carbonate and 1,2 dimethoxyethane (LiClO4/PC-DME) (Fig. 13b). The highest capacities were obtained for the LiBF4-based electrolyte, but strong capacity losses between cycle numbers 10 and 20 are observed for all investigated electrolyte combinations. 4. Conclusions The Lix(Al0.8Zn0.2) provides high capacity and good capacity retention up to 10 cycles. The replacement of some Al by Zn is therefore a promising approach to a composite anode material with enhanced performance properties. The Li4Al3.42Zn11.58 phase protects the electrode from particle cracking and contact loss during the first ten cycles by the higher tolerance against the accompanied volume changes. The continuous increase of the SEI during cycling leads to the passivation of the active material and strong capacity fading between cycles 10 and 15. It seems that the limitations caused by the dramatic volume changes could be overcome for Li(Al,Zn)-based electrodes, but no sufficiently stable electrolyte was established so far. Acknowledgments Financial support from the Deutsche Forschungsgemeinschaft (DFG, EH183/7) is gratefully acknowledged. This work has benefited
from beamtime allocation at P02.1 (HASYLAB) and a corresponding grant for the enhancement of this synchrotron beamline by the Bundesministerium für Bildung und Forschung (BMBF, 05K13VK1). References [1] Park C-M, Kim J-H, Kim H, Sohn H-J. Li-alloy based anode materials for Li secondary batteries. Chem Soc Rev 2010;39:3115e41. [2] Zhang W-J. A review of the electrochemical performance of alloy anodes for lithium-ion batteries. J Power Sources 2011;196:13e24. [3] Winter M, Besenhard JO. Electrochemical lithiation of tin and tin-based intermetallics and composites. Electrochim Acta 1999;45:31e50. [4] Yang J, Wachtler M, Winter M, Besenhard JO. Submicrocrystalline Sn and Sne SnSb powders as lithium storage materials for lithium-ion batteries. Electrochem Solid-State Lett 1999;2:161e3. [5] Hochgatterer NS, Schweiger MR, Koller S, Raimann PR, Wohrle T, Wurm C, et al. Silicon/graphite composite electrodes for high-capacity anodes: influence of binder chemistry on cycling stability. Electrochem Solid-State Lett 2008;11(5):A76e80. [6] Li J, Le D-B, Ferguson PP, Dahn JR. Lithium polyacrylate as a binder for tine cobaltecarbon negative electrodes in lithium-ion batteries. Electrochim Acta 2010;55:2991e5. [7] Wachtler M, Besenhard JO, Winter M. Tin and tin-based intermetallics as new anode materials for lithium-ion cells. J Power Sources 2001;94:189e93. [8] Choi NS, Yew KH, Lee KY, Sung M, Kim H, Kim SS. Effect of fluoroethylene carbonate additive on interfacial properties of silicon thin-film electrode. J Power Sources 2006;161(2):1254e9. [9] Han G-B, Ryou M-H, Cho KY, Lee YM, Park J-K. Effect of succinic anhydride as an electrolyte additive on electrochemical characteristics of silicon thin-film electrode. J Power Sources 2010;195(11):3709e14. [10] Chumak I, Dmytriv G, Pavlyuk V, Oswald S, Eckert J, Trill H, et al. Li(Al1zZnz) alloys as anode materials for rechargeable Li-ion batteries. J Mater Res 2010;25(8):1492e9. [11] Lee C-S, Miller GJ. Experimental and theoretical studies of elemental site preferences in quasicrystalline approximants (R-phases) within the LieMge ZneAl system. Inorg Chem 2001;40:338e45. [12] Leblanc M, Le Bail A, Audier M. Crystalline phases related to the icosahedral AleLieCu phase. A single crystal X-ray diffraction study of the tetragonal tauAl56(Cu,Zn)11Li33 phase. Physica B 1991;173(3):329e55. [13] Hallstedt B, Kim O. Thermodynamic assessment of the AleLi system. Int J Mater Res 2007;98:961e9. [14] Puhakainen K, Boström M, Groy T-L, Häussermann U. A new phase in the system lithiumealuminum: characterization of orthorhombic Li2Al. J Solid State Chem 2010;183:2528e33. [15] Pelton AD. The LieZn system. J Phase Equilib 1991;12(1):42e5. [16] Murray JL. The AleZn system. Bull Alloy Phase Diagrams 1983;4(1):55e73. [17] Herklotz M, Scheiba F, Hinterstein M, Nikolowski K, Knapp M, Dippel A-C, et al. Advances in in situ powder diffraction of battery materials e a case study of the new beamline P02.1 at DESY Hamburg. J Appl Cryst 2013;46:1117e27. [18] Knapp M, Baehtz C, Ehrenberg H, Fuess H. The synchrotron powder diffractometer at beamline B2 at HASYLAB/DESY: status and capabilities. J Synchrotron Radiat 2004;11:328e34. [19] Knapp M, Joko V, Baehtz C, Brecht H, Berghauser A, Ehrenberg H, et al. Position-sensitive detector system OBI for high resolution X-ray powder diffraction using on-site readable image plates. Nucl Instrum Methods Phys Res Sect A 2004;521(2e3):565e70. [20] Roisnel T, Rodriguez-Carvajal J. WinPLOTR: a Windows tool for powder diffraction pattern analysis. Mater Sci Forum 2001;378:118e23. [21] Sheldrick GM. SHELXL-97. Program for crystal structure refinement. University of Goöttingen; 1997. [22] Bärnighausen H. Group-subgroup relations between space groups: a useful tool in crystal chemistry. Commun Math Chem 1980;9:139e75.
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