Accepted Article
Title: Elucidating the Impact of Cobalt-Doping on the Lithium Storage Mechanism in Conversion/Alloying-type Zinc Oxide Anodes
Authors: Franziska Mueller; Dorin Geiger; Ute Kaiser; Stefano Passerini; Dominic Bresser
This manuscript has been accepted after peer review and the authors have elected to post their Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is pu- blished to ensure accuracy of information. The authors are responsible for the content of this Accepted Article.
To be cited as: ChemElectroChem 10.1002/celc.201600179
Link to VoR: http://dx.doi.org/10.1002/celc.201600179
A Journal of
www.chemelectrochem.org ChemElectroChem 10.1002/celc.201600179
1 E lucidating the Impact of Cobalt-Doping on the Lithium Storage Mechanism
2 in Conversion/Alloying-type Zinc Oxide Anodes
3 4 Franziska Mueller,a,b,c Dorin Geiger,d Ute Kaiser,d Stefano Passerini,a,b,* Dominic Bresser a,b,e,*
5 6 a Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081, Ulm, Germany
7 b Karlsruher Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany
8 c Institute of Physical Chemistry, University of Muenster, Corrensstrasse 28/30, 48149 Muenster, Germany
9 d Central Facility for Electron Microscopy, Group of Electron Microscopy of Materials Science,
10 University of Ulm, 89081 Ulm, Germany
11 e DRF/INAC/SYMMES/PCI, CEA, UMR-5819, CEA-CNRS-UJF, 17 Rue des Martyrs, 38054 Grenoble, France
12
13 Abstract
14 Herein, an in-depth investigation on the influence of transition metal doping on the structural and
15 electrochemical characteristics of a hybrid conversion/alloying-type lithium-ion anode material is
16 presented. Therefore, pure zinc oxide (ZnO) and cobalt-doped ZnO (Zn0.9Co0.1O) were comparatively
17 investigated. The characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM), and
18 transmission electron microscopy (TEM) confirmed the successful incorporation of the cobalt (Co) dopant
19 into the wurtzite ZnO structure, leading to a decreased particle size for the doped compound. The
20 electrochemical in situ XRD analysis of the first de-/lithiation of ZnO and Zn0.9Co0.1O revealed the highly
21 beneficial impact of the transition metal doping on the reversible degradation of lithium oxide (Li2O) and
22 suppression of the zinc crystallite growth upon lithiation; both effects being essential for the greatly
23 improved electrochemical performance. As a result, the Co-doping lead to a substantially increased
-1 -1 24 specific capacity from 326 mAh g for pure ZnO to 789 mAh g for Zn0.9Co0.1O after 75 full dis-/charge
25 cycles.
26
27 Keywords: zinc oxide nanoparticles, cobalt doping, conversion/alloying, lithium-ion anode, battery
28 29 * Corresponding authors: [email protected]; [email protected]
30
1
ChemElectroChem 10.1002/celc.201600179
1 1. Introduction 2 Despite great technological interest in nanostructured zinc oxide (ZnO) for a large variety of
3 applications, including, for instance, light-emitting diodes,[1] gas sensors,[2] or dye-sensitized solar cells,[3]
4 their investigation as lithium-ion anode materials was reported rather scarcely so far. In fact, although a
5 theoretical specific capacity of about 988 mAh g-1 (considering the reaction ZnO + 3 Li+ + 3 e- ⟷ LiZn +
[4] 6 Li2O as fully reversible) certainly arouses interest, apart from a few outstanding exceptions most
7 electrochemical studies revealed poor performance and rapid capacity fading.[5] In order to explain the
8 origin of this inferior electrochemical performance of ZnO-based electrodes, Pelliccione and co-workers[6]
9 performed very recently an in situ X-ray absorption fine structure (XAFS) spectroscopy study. It was
10 found that this substantial capacity decay mainly originates from the formation of relatively large metallic
11 zinc particles upon continuous de-/lithiation. Once their size exceeds a certain limit, the formation of ZnO
12 accompanied by the degradation of Li2O, i.e., the conversion reaction, shows no reversibility anymore and
13 only the alloying reaction of zinc with lithium takes place reversibly – which, in turn, is associated with
14 the characteristic issues of alloying materials.[5f]
15 Also, the large volume changes upon alloying-dealloying lead to the pulverization of the active
16 material, ongoing electrolyte decomposition, and finally the loss of electrical contact within the composite
17 electrode.[5f] Additionally, the decrease in electronic conductivity within the electrode owing to the
[6] 18 insulating nature of Li2O presumably contributes to the observed rapid capacity fading. We have
19 recently reported that these challenges and in particular the reversibility of the conversion reaction can be
20 substantially enhanced by introducing a transition metal dopant into the metal oxide structure.[7] This new
21 concept follows the general approach of introducing a metallic element that does not alloy with lithium
22 once reduced to the metallic state,[8] thus, ensuring a sufficient electron supply throughout the initial
23 particles to enable the degradation of Li2O. However, little is known so far about the detailed reaction
24 mechanism of this new class of conversion/alloying materials.
25 In this study, we present an in-depth structural and electrochemical characterization of pure ZnO and
26 Co-doped ZnO nanoparticles, employing XRD, SEM, TEM, cyclic voltammetry (CV), and galvanostatic
27 cycling. Moreover, we performed a comparative in situ XRD analysis of Co-doped and pure ZnO
28 nanoparticles. The obtained results, discussed also in regard to our previous findings for Fe-doped ZnO,[7a]
2
ChemElectroChem 10.1002/celc.201600179
1 provide an enhanced understanding of the advantageous impact of the transition metal dopant on the
2 reversibility of the de-/lithiation mechanism in general and the conversion reaction in particular. In fact,
3 doping ZnO with cobalt results in a substantially enhanced specific capacity and cycling stability of
4 nanoparticulate ZnO-based lithium-ion anodes.
5
6 2. Experimental Section
7 2.1 Materials Synthesis
8 For the synthesis of Co-doped ZnO nanoparticles stoichiometric amounts of zinc (II) gluconate hydrate
9 and cobalt (II) gluconate hydrate (both from ABCR) were dissolved in deionized water. The resulting
10 solution, having a total metal ion concentration of 0.2M, was added dropwise under vigorous stirring to an
11 aqueous 1.2M solution of sucrose (ACROS ORGANICS). After continuous stirring for additional 15 min
12 at room temperature, the solution was heated up to about 160 °C to remove the water. Concurrently, the
13 sucrose started to be thermally decomposed. The remaining solid precursor was further dried at around
14 300 °C, afterwards ground and calcined for 4 h under air at 400 °C (heating rate: 3 °C min-1). Pure ZnO
15 nanoparticles were prepared following an analogous synthesis protocol in absence of the Co precursor and
16 applying an annealing temperature of 450 °C.
17
18 2.2 Morphological and Structural Characterization
19 Powder X-ray diffraction (XRD) patterns were recorded with a BRUKER D8 Advance employing a
20 Cu-Kα1 radiation (λ = 154 pm) and setting the step size to 0.0092°. Scanning electron microscopy (SEM)
21 was carried out using a ZEISS LEO 1550VP field emission electron microscope. All samples were
22 sputtered with platinum to prevent charging effects. High-resolution transmission electron microscopy
23 (HRTEM) of Zn0.9Co0.1O nanoparticles was performed on samples deposited on a copper grid coated by a
24 holey carbon film utilizing a Philips CM 20 at 200 kV.
25
26 2.3 Electrochemical Characterization
27 Electrodes were prepared by dissolving sodium carboxymethyl cellulose (CMC, WALOCELTM CRT
28 2000 PPA 12, Dow Wolff Cellulosics) in deionized water (1.25 wt.% of CMC in solution). The 3
ChemElectroChem 10.1002/celc.201600179
® 1 conductive carbon (Super C65 , IMERYS Graphite and Carbon) and ZnO or Zn0.9Co0.1O nanoparticles
2 were added, resulting in a relative dry material composition of 75 wt.% active material, 20 wt.%
3 conductive carbon, and 5 wt.% CMC. Dispersion of the obtained mixture was carried out by means of
4 planetary ball-milling for 2 h. The resulting slurry was cast on dendritic copper foil (SCHLENK, 99.9%),
5 serving as current collector, utilizing a laboratory doctor blade (wet film thickness: 120 μm). After an
6 initial drying at room temperature overnight, disk electrodes (ø = 12 mm) were punched and vacuum-dried
7 for 24 h at 120 °C. The average active material mass loading was around 2.0-2.5 mg cm-2.
8 The electrochemical characterization was performed in three-electrode Swagelok™-type cells,
9 assembled in an MBraun MB-200B eco glove box (O2 and H2O content < 0.1 ppm), using lithium metal
10 foil (Rockwood Lithium, battery grade) as counter and reference electrodes. The electrodes were separated
11 by a stack of polypropylene fleeces (Freudenberg FS 2190), drenched with a 1M solution of LiPF6 in a
12 3:7 wt/wt mixture of ethylene carbonate and diethyl carbonate (UBE). As metallic lithium foil was used as
13 counter and reference electrodes, all potential values given herein refer to the Li/Li+ redox couple. Cyclic
14 voltammetry and galvanostatic cycling were performed using a VMP3 potentiostat (BioLogic) and a
15 Maccor Battery Tester 4300, respectively. All electrochemical studies were conducted at 20 °C ± 2 °C in a
16 potential range from 0.01 to 3.0 V.
17
18 2.4 In situ XRD Analysis
19 In situ XRD analysis of Co-doped and pure ZnO nanoparticles upon galvanostatic de-/lithiation was
20 conducted using a self-designed two-electrode in situ cell.[9] Electrodes were prepared analogously to
21 those used for the electrochemical characterization, having an overall composition of 65 wt.% active
22 material, 25 wt.% conductive carbon, and 10 wt.% CMC. The resulting mixture was dispersed by means
23 of planetary ball-milling and cast onto a beryllium (Be) disk (thickness: 250 µ, Brush Wellman), serving
24 concurrently as current collector and “window” for the X-ray beam. The coated Be disk was dried for
25 30 minutes at 80 °C under ambient atmosphere and subsequently at 40 °C under vacuum overnight.
26 Metallic lithium foil served as counter electrode. Two sheets of Whatman glass fiber, drenched with
27 500 µl of a 1M solution of LiPF6 in a 3:7 wt/wt mixture of ethylene carbonate and diethyl carbonate
28 (UBE), were used as separator. The readily assembled cell was allowed to rest for 10 h prior to the in situ
4
ChemElectroChem 10.1002/celc.201600179
1 XRD measurements. Simultaneous galvanostatic cycling was performed by means of a VSP
-1 2 potentiostat/galvanostat (BioLogic), applying a specific current of around 55 mA g (Zn0.9Co0.1O) or
3 60 mA g-1 (ZnO) and setting the cut-off potentials to 0.01 V and 3.0 V. The XRD analysis was carried out
4 in a 2θ range from 25 to 80° with a step size of 0.02758° and a time per step of 0.65 s. Consequently,
5 every scan lasted exactly 30 min, including an initial rest step of 419 s.
6
7 3. Results and Discussion
8 3.1 Structural and Morphological Characterization
9 The X-ray diffraction patterns of pure ZnO and Zn0,9Co0,1O, presented in Figure 1, show the
10 characteristic pattern of hexagonal wurtzite structured ZnO with the P63mc space group (JCPDS File Card
11 No. 01-071-6424), indicating the synthesis of phase-pure materials and the successful doping of ZnO with
12 cobalt. The additional low intensity reflections at about 28.7°, 31.1°, and 32.7° (marked by asterisks)
13 originate from the diffraction of the Cu-Kβ1 radiation at the (100), (002), and (101) lattice planes. The
14 Co-doped sample reveals slightly broader full width at half maximum values and lower reflection
15 intensities compared to the pure ZnO, which indicates a reduced crystallite size. These results are in
16 excellent agreement with a recent in-depth crystallographic analysis, reported by Giuli et al.[10]
17
* reflections from K radiation 1
ZnO
* * *
Zn Co O
Intensity / a.u. / Intensity 0.90 0.10 * * *
JCPDS File Card No. 01-071-6424
(101)
(100)
(002)
(110)
(103)
(102)
(112)
(201)
(200) (004) 20 30 40 50 60 70
18 2deg.
19 Figure 1. XRD patterns of as-synthesized ZnO nanoparticles (upper pattern in black) and Co-doped
20 ZnO nanoparticles (lower pattern in green); the JCPDS reference file card no. 01-071-
21 6424 (wurtzite structure, hexagonal symmetry, P63mc space group) is given below.
5
ChemElectroChem 10.1002/celc.201600179
1 Figure 2 shows the SEM images of the pristine powdery samples. The SEM image of ZnO (Figure 2a)
2 reveals a particle diameter of about 40 nm, while the introduction of Co into the ZnO lattice results in a
3 decreased particle size of around 20 to 30 nm. These findings are in good agreement with the XRD
4 analysis and previous studies.[7a, 10] The TEM micrographs of Co-doped ZnO depicted in Figure 3 confirm
5 this particle size, while revealing the additional presence of slightly smaller (~10 nm) and larger (~35 nm)
6 particles. Moreover, the HRTEM micrograph presented in Figure 3b does not show any significant lattice
7 distortion, further confirming the successful incorporation of the cobalt dopant in the wurtzite ZnO
8 structure.
9
10
11 Figure 2. SEM micrographs of (a) pure ZnO and (b) Co-doped ZnO nanoparticles at a
12 magnification of 100 kx.
13
14 Figure 3. (HR)TEM micrographs of Co-doped ZnO nanoparticles at different magnifications.
15
16 6
ChemElectroChem 10.1002/celc.201600179
1 3.2 Cyclic Voltammetry
2 In Figure 4 the cyclic voltammograms of ZnO- and Zn0.9Co0.1O-based electrodes, studied between
3 0.01 and 3.0 V, are presented. In the first cathodic sweep, ZnO (Figure 4a) reveals three reduction peaks at
4 potentials of (a) 0.70 V, (b) 0.38 V, and (c) 0.13 V. Subsequently, i.e., below 0.07 V, a continuously
5 decreasing specific current was recorded, which is assigned to charge accumulation at the surface and in
6 the carbon black, employed as conductive additive. Peak (a) at 0.70 V is commonly ascribed to the
7 reduction of electrolyte components at the anode/electrolyte interface resulting in the formation of the
8 solid electrolyte interphase (SEI).[11] The main peak (b) at 0.38 V is mainly related to the reduction of ZnO
9 to form metallic Zn and Li2O. Subsequently, at potentials below 0.2 V, the metallic Zn alloys with Li.
10 However, the slope of the baseline is rather steep suggesting that the lithiation processes might partially
11 overlap. Upon delithiation various peaks in region (d) at 0.19 V, 0.29 V, 0.35 V, 0.54 V, and 0.68 V, one
12 broad peak (e) at 1.32 V, probably consisting of two overlapping peaks, and a low-intensity peak (f) at
13 2.55 V were detected. The peaks in the low potential region, i.e., below 0.75 V, are related to the stepwise
14 de-alloying of LiZn as previously reported in literature.[5e, 12] The underlying reaction corresponding to the
15 broad, anodic peak (e) at about 1.32 V remained unexplained for a long time.[5e] Huang et al.[13] recently
16 ascribed this feature to the de-alloying of LixZn. Moreover, they correlated the low-intensity peak (f) at
[13] 17 2.55 V to the partially reversible degradation of Li2O accompanied by the formation of ZnO.
18 Nevertheless, considering studies on tin (oxide)-based anodes, it appears also conceivable to assign
[7b] 19 peak (e) to the partially reversible formation of Li2O and peak (f) to a partial degradation of SEI
20 components,[14] in particular, with respect to its absence in case of carbon-coated ZnO.[15] However, we
21 will discuss this assignment of the peaks to specific electrochemical reactions in more detail in
22 consideration of the in situ XRD analysis, presented hereafter. During the following sweeps, shown in
23 Figure 4b, new cathodic peaks at (g) 0.96 V and (h) 0.78 V arise, whereas the anodic peak (f) at 2.55 V
24 completely vanishes; the latter is, in fact, in good agreement with the aforementioned studies on tin-based
25 anodes, which assigned this peak to a partial SEI degradation.[14] In addition, the peaks (g), (h), (b), (c),
26 and (e) show a shift towards slightly lower potentials and a considerable intensity decrease. Peak (g)
27 completely disappears after a few cycles. Moreover, peak (b) is split into two peaks, which are assigned to
28 the alloying reaction of Zn with Li, thus, the stepwise alloying becomes more apparent. In contrast, the
7
ChemElectroChem 10.1002/celc.201600179
1 peak s in region (d) related to the dealloying processes remain rather unchanged, although revealing a
2 slightly decreasing intensity. In fact, these results evince the often-reported irreversibility of the Li2O
3 degradation and the limited reversibility of the Li-alloying process of Zn.[5a-e, 6]
4
5
nd th 6 Figure 4. Cyclic voltammograms of ZnO ((a) first sweep, (b) 2 to 10 sweep) and Zn0.9Co0.1O
7 ((c) first sweep, (d) 2nd to 10th sweep). Sweep rate: 50 µV s-1; reversing potentials: 0.01 V
8 and 3.0 V vs. Li/Li+.
9 In Figure 4c, the first potentiodynamic cyclic sweep for Zn0.9Co0.1O is presented. Co-doped ZnO
10 exhibits similar characteristics as the undoped ZnO. However, the features are much less pronounced,
11 which might be assigned to the smaller particle and crystallite size as revealed by SEM (Figure 2) and
12 XRD (Figure 1), and slightly shifted to higher potentials, presumably resulting from an enhanced
13 electronic conductivity.[16] Analogous to ZnO, as described above, the SEI formation occurs at about
14 0.72 V (peak (a)), followed by the reduction of ZnO at (b) 0.46 V. Below 0.25 V the electrochemical
15 alloying of Zn with Li, lithium insertion into the conductive carbon, and charge storage at the surface take
8
ChemElectroChem 10.1002/celc.201600179
1 place. According to the standard redox potential, Co is nobler than Zn, i.e., Co is reduced at higher
2 potentials. However, no clear feature, related to the reduction of Co, was observed by performing CV -
3 which is expected considering previous results for zinc and cobalt ferrite.[17] Nonetheless, the influence of
4 the Co-dopant is indicated by a slightly higher potential of peak (a) compared to pure ZnO. During the
5 anodic sweep, the de-alloying of Zn occurs in the potential range between (d) 0.16 V and 0.74 V, followed
6 by the degradation of Li2O (e) at 1.35 V, as mentioned before. In the subsequent cycles, the peaks (a) and
7 (b) vanish and new features (g) at 0.93 V and (h) at 0.81 V appear (Figure 4d). Upon further cycling, the
8 peak intensities of these new features decrease and their maxima are shifted to slightly lower potentials.
9 Additionally, a broad feature characterized by a constantly increasing specific current at a potential of
10 about 0.5 V arises. Nonetheless, compared to ZnO, Zn0.9Co0.1O shows a remarkably improved
11 reversibility, in particular for peak (e).
12
13 3.3 In Situ XRD
14 In order to further study the structural changes and phase transitions occurring during the
15 electrochemical lithiation and subsequent delithiation reaction and to investigate the influence of the
16 cobalt dopant, in situ XRD analysis was performed on electrodes based on pure ZnO and Co-doped ZnO
17 nanoparticles. In both cases, the electrodes were subjected to a constant current lithiation (discharge) from
18 OCV to 0.01 V and, subsequently, delithiation (charge) from 0.01 V to 3.0 V. The results for the pure ZnO
19 nanoparticles are presented in Figure 5. Figure 5a shows the corresponding potential profile, which can be
20 roughly divided in five different regions (A, B, and C for the discharge as well as D and E for the
21 subsequent charge). To provide an overview on the evolution of the simultaneously recorded XRD
22 patterns, a waterfall diagram showing all XRD patterns is presented in Figure 5b, within which these
23 regions are indicated by the scans highlighted in red. A detailed discussion of the therein-occurring
24 processes, however, is based on the following panels. Region A covers the initial potential decrease until
25 the onset of the voltage plateau at about 0.55 V. For this region (scans 1-5), a decrease in intensity of the
26 main wurtzite-related reflections is observed (Figure 5c; marked by the black arrows), indicating an
27 incipient reduction of the long-range order within the wurtzite structure. This structural change is
28 accompanied by some electrolyte decomposition, indicated by the slight change in slope of the discharge
9
ChemElectroChem 10.1002/celc.201600179
1 curve, which is in good agreement with the CV results (Figure 4a). The initial reduction of the oxide is -
2 though somehow expected - of particular interest, as it is in contrast to our earlier finding for Fe-doped
3 ZnO,[7a] for which we did not observe any initial decrease in intensity during the first scans. In fact, this
4 remarkable difference regarding the structural evolution may support our previous assumption that for Fe-
5 doped ZnO prior to the onset of the voltage plateau an insertion of lithium ions into the wurtzite structure
[10] 6 may occur, favored by the presence of cationic vacancies in Zn0.9Fe0.1O as proposed by Giuli et al.
7 A careful analysis of the different XRD patterns reveals also the appearance of a very broad and yet tiny
8 reflection at about 43.0° (marked by the red arrow). Considering the indicated reduction of ZnO
[5e, 6-7, 18] 9 accompanied by the formation of amorphous Li2O, it appears reasonable to assign this feature to
10 the main reflection of metallic zinc (JCPDS Card File No. 00-004-0831), which is also in good agreement
11 with the onset of the following voltage plateau (Figure 5a) assigned to the occurrence of an equilibrium
12 between different, but simultaneously present phases.
13 Indeed, for the following scans (Figure 5d, Region B, scans 6-12), the continuous decrease and
14 increase in intensity of the reflections corresponding to ZnO and Zn0, respectively, indicates that the oxide
15 is further reduced and more metallic zinc is formed. Starting from scan 13/14, three new reflections appear
16 at about 37.0°, 41.0°, and 42.7° in the selected 2θ range; the latter one is initially present as shoulder of the
17 Zn0-related reflection and subsequently continuously increasing in intensity, while the Zn0-related
18 reflection itself vanishes simultaneously. Their positions and relative intensity are in very good agreement
19 with the JCPDS Card File No. 01-071-9525 reported for Li0.105Zn0.895, indicating that along the voltage
20 plateau the previously formed metallic zinc also starts to alloy electrochemically with lithium, thus,
21 forming a LixZn alloy phase with a rather low lithium content, i.e., x << 1. In fact, this finding is further
22 supported by the concurrent appearance of two additional reflections at around 56.5° and 67.0° - see
23 Figure 5b. Apparently, the reduction of the oxide and the initial alloying reaction occur partially
24 simultaneously, i.e., at the same voltage. At the end of the voltage plateau, in scan 19, the reflections
25 corresponding to the wurtzite-structured oxide completely vanished and only the LixZn-related reflections
26 remain.
27
10
ChemElectroChem 10.1002/celc.201600179
1
2
3 Figure 5. In situ XRD analysis of ZnO: (a) Galvanostatic dis-/charge profile of the first cycle,
4 (b) corresponding XRD patterns, (c) to (g) close-ups of selected XRD patterns.
5
6 In Region C, no significant changes are observed for the scans 19-22, i.e., along the initial rather steep
7 voltage decrease. Subsequently, however, the intensity of the two reflections at about 37.0° and 42.7°
8 corresponding to the LixZn (x << 1) phase decrease in intensity, while the one at around 41.0°
9 continuously increases, initially shifting to slightly larger 2θ values and then back to slightly lower 2θ
10 values. Indeed, analyzing also the extended 2θ range (up to 80.0°) reveals that this shifting and increase in
11 intensity is related to the appearance of the fully lithiated LiZn alloy phase, as, starting from scan 23, also
12 at around 48.5°, 60.0°, and 75.0° new reflections appear. This is in excellent agreement with JCPDS Card 11
ChemElectroChem 10.1002/celc.201600179
1 File No. 03-065-3016 reported for LiZn. In addition, the further lithiation, i.e., the formation of the fully
2 lithiated binary LiZn alloy at potentials below 0.2 V along a sloped voltage plateau is in good agreement
3 with previous, purely electrochemical studies on the lithium-zinc alloying reaction mechanism.[5e, 12, 19]
4 Finally, in scan 28, only reflections corresponding to the LiZn phase are present, indicating that the
5 lithiation reaction is completed at the end of the discharge step (0.01 V).
6 For the subsequent charge (scans 29-44), the LiZn-related reflections are first slightly shifted to larger
7 2θ values, indicating that the lattice parameters slightly decrease, presumably due to a decreasing lithium
8 content in the alloy (Figure 5f, scans 29-32, first slope). Beginning from scan 33 (i.e., after the change in
9 slope), the reflections corresponding to the LixZn (x << 1) phase reappear and steadily increase in
10 intensity, while the reflections related to the LiZn phase keep decreasing. In scan 35, at the end of
11 Region D only the reflections corresponding to the LixZn phase remain. These reflections, however,
12 abruptly vanish for the next scan (scan 36) as shown in Figure 5g (Region E), in which scan 35 is included
13 again for the sake of clarity. Instead, four new reflections are observed at about 36.2°, 38.8°, 43.0°, and
14 54.0°, which are in very good agreement with the JCPDS reference reported for metallic zinc (Card File
15 No. 00-004-0831), indicating that the dealloying reaction is completed at around 0.7 V upon charge. This
16 finding is, in fact, also in good agreement with the recorded specific capacity (Figure 5a), as the alloying
17 contribution for ZnO theoretically corresponds to 329 mAh g-1, and our assignment of the different peaks
18 in the CV profile (Figure 4a and b). The subsequent decrease in intensity observed for the two reflections
19 at around 38.8° and 43.0° may be ascribed to the partial re-oxidation of the metallic zinc at potentials
20 higher than 0.7 V.[6] This is further supported by the appearance of a very broad reflection in the range
21 from 31.0° to 35.0° as well as the limited decrease in intensity of the reflection at about 36.2°, as it
22 overlaps with the main reflection of the wurtzite-structured oxide. Nevertheless, this re-oxidation process
23 appears to be limited (see also Figure 4b), as very intense reflections corresponding to metallic zinc
24 remain even at the end of the charge process at 3.0 V. Additionally, the specific capacity obtained in this
25 voltage range is substantially lower than theoretically expected, considering the fully reversible
-1 -1 26 conversion of Li2O (ca. 416 mAh g rather than 658 mAh g ). Based on the above-described findings, we
27 may qualitatively describe the multistep process as follows:
28
12
ChemElectroChem 10.1002/celc.201600179
+ - 0 1 (A) ZnO + Li + e → ZnO + Li2O + Zn
2 0 + - 3 (B) ZnO + Zn + Li + e → LixZn + Li2O
4 + - 5 (C) LixZn + (1-x) Li + (1-x) e → LiZn
6 + - 0 + - 7 (D) LiZn → LixZn + (1-x) Li + (1-x) e → Zn + Li + e
8 0 0 + - 9 (E) Zn + Li2O → Zn + ZnO + Li + e
10
11 In sum:
+ - -1 12 ZnO + 3 Li + 3 e ⥂ LiZn + Li2O (≙ 988 mAh g )
13
14 Please note that the equations for the single regions are not balanced stoichiometrically as an
15 adjustment by unknown general variables would certainly only complicate the understanding and reading
16 substantially, while not adding any beneficial further information. Also, the electrode chemistry presented
17 for each step focuses only on the reactions taking place in the corresponding potential range. We may note
18 at this point that our findings obtained by performing in situ XRD are in excellent agreement with the very
19 recent in situ XAFS study by Pelliccione et al.,[6] who also observed a fully reversible LiZn formation in
20 the first cycle, but the incompleteness of the subsequent re-oxidation of metallic zinc upon delithiation,
21 presumably related to the formation of relatively large metallic zinc particles (approximately 10-20 nm)
22 and the corresponding extensive structural reorganization in the electrode, which was further supported by
23 a very recent in situ TEM study.[18]
24 To investigate the effect of introducing the cobalt dopant in the wurtzite structure, we carried out an in
25 situ XRD analysis for Zn0.9Co0.1O-based electrodes as well (Figure 6). The corresponding potential profile
26 is presented in Figure 6a while the overview on the evolution of the recorded XRD patterns is given in
27 Figure 6b.
13
ChemElectroChem 10.1002/celc.201600179
1
2
3 Figure 6. In situ XRD analysis of Zn0.9Co0.1O: (a) Galvanostatic dis-/charge profile of the first cycle,
4 (b) the corresponding XRD patterns, (c) to (g) close-ups of selected XRD patterns.
5
6 Again, the potential profile can be subdivided in five different regions (indicated by the red highlighted
7 scans), following a similar approach as for ZnO. While the larger number of scans is also a result of the
8 slightly lower specific current applied (55 mA g-1 vs. 60 mA g-1 for ZnO), two major differences are
9 evident. The potential profile is generally smoother, i.e., it shows less remarkable changes in slope, which
10 may originate partially from the smaller initial crystallite size[16a, 16b] and is in good agreement with the
11 cyclic voltammetry results shown in Figure 4 as well as the slightly higher potential of the main voltage
12 plateau upon discharge. The latter may be assigned to the higher reduction potential of cobalt compared to
14
ChemElectroChem 10.1002/celc.201600179
1 zinc .[20] Nevertheless, for the initial reaction of the oxide upon lithiation (Figure 6c, Region A), the same
2 structural evolution is observed: A decrease in intensity of the oxide-related reflections, indicating that
3 also in the case of Co-doped ZnO the oxide is immediately reduced and that no substantial initial lithium
4 ion insertion into the wurtzite lattice takes place. This finding is, again, in good agreement with the in-
5 depth structural analysis by Giuli et al.,[10] revealing that, in contrast to iron, cobalt is purely divalent, thus,
6 not requiring the presence of, e.g., cationic vacancies to balance trivalent cations in the oxide lattice. In
7 addition, starting from scan 6, i.e., at the onset of the subsequent voltage plateau the appearance of a very
8 broad yet tiny reflection in the range from 41.0° to 45.0° is observed. Its intensity is further increasing
9 during the following scans (Figure 6d, Region B), while the oxide-related reflections are continuously
10 decreasing until they completely vanish at the end of the voltage plateau (scan 22). Remarkably, however,
11 in contrast to ZnO, no well pronounced reflections are observed for the newly appearing reflection in the
12 range from 41.0° to 45.0° and the intensity remains substantially lower in general. In fact, it appears that
13 this rather broad new feature may be composed of two reflections, presumably corresponding to the
0 14 concurrent formation of metallic Zn and LixZn (at around 43.0° and 42.0°, respectively), which largely
15 overlap. These findings provide two important information: First, the crystallinity of the formed phases is
16 substantially lower than in case of ZnO (similarly to the findings for Fe-doped ZnO[7a]), indicating that –
17 apart from the slightly smaller crystallite size of the pristine oxides (see Figure 2) - the presence of cobalt
18 as dopant prevents the formation of large crystalline particles upon lithiation, i.e., favors the
19 nanocrystallinity of the sample. Second, the introduction of the cobalt dopant appears to kinetically favor
[21] 20 the formation of the LixZn alloy. These two findings are, in fact, of particular interest, as Su et al. very
21 recently reported that the growth of Zn particles and the alloying reaction are two competing processes,
22 depending on the applied current density. We may thus assume that the formation of a continuous
23 percolating network of metallic cobalt leads to an enhanced electron transport and distribution within the
24 initial nanoparticles, favoring the alloying reaction rather than the zinc crystal growth.
25 Upon further lithiation (Figure 6e, Region C), beginning from scan 31, a new reflection appears at
26 about 41.0°, which is in excellent agreement with the results obtained for ZnO, indicating the formation of
27 the LiZn alloy phase at a potential of less than 0.2 V (see also Figure 6a).
15
ChemElectroChem 10.1002/celc.201600179
1 This reaction is subsequently reversed upon delithiation (i.e., charge; Figure 6f, Region D), just like for
2 ZnO. Nevertheless, the dealloying reaction is apparently completed at relatively higher potentials
3 (ca. 1.2 V compared to about 0.7 V for ZnO; see Figure 5a and Figure 6a, respectively). Also, it appears
4 that for Zn0.9Co0.1O in Region E (Figure 6g) some LixZn (x << 1) phase remains, as indicated by the
5 shoulder of the reflection at about 43.0°; the latter assigned to the presence of metallic zinc, though the
6 rather broad nature of the reflection makes it difficult to clearly differentiate between the two phases. This
7 is especially true as no significant additional reflections are observed for this sample. In either case, this
8 rather broad reflection continuously decreases in intensity upon further delithiation until it completely
9 vanishes at the end of the charge process at 3.0 V. Concurrently, a new broad reflection appears within
10 30.0-37.0° and steadily increases, accompanied by the appearance of an even broader reflection at around
11 57.0°. Considering the substantially reduced crystallinity of the sample after the first cycle, due to the
12 extensive structural reorganization processes that occur, it appears conceivable that these two broad
13 reflections indicate the reappearance of a wurtzite-structured oxide phase. Moreover, the complete
14 disappearance of the metallic zinc phase in combination with the obtained specific capacity of 970 mAh g–
1 15 , i.e., roughly the theoretical capacity expected for the fully reversible formation of LiZn and Li2O
16 (≙ 962 mAh g-1), further support the formation of a metal oxide phase at the end of charge - including
17 both zinc and/or cobalt. The slight excess in capacity may be well explained by the lithium storage
18 capability of the comprised conductive carbon, as also observed previously.[9] Based on these results, we
19 propose the following multistep process for the first lithiation and delithiation process:
20
+ - 0 0 21 (A) Zn0.9Co0.1O + Li + e → Zn0.9Co0.1O + Co + Zn + Li2O
22 0 + - 0 0 23 (B) Zn0.9Co0.1O + Zn + Li + e → LixZn + Co + Zn + Li2O (x << 1)
24 0 + - 25 (C) LixZn + Zn + Li + e → LiZn
26 0 + - 27 (D) LiZn → LixZn + Zn + Li + e
28 0 0 + - 29 (E) LixZn + Zn + Co + Li2O → Zn1-yCoyO* + Li + e (y ≈ 0.1)
30
16
ChemElectroChem 10.1002/celc.201600179
1 In sum:
+ - 0 -1 2 Zn0.9Co0.1O + 2.9 Li + 2.9 e ⇄ 0.9 LiZn + Co + Li2O (≙ 962 mAh g )
3
4 Please note that also in this case, the equations for the single regions are not balanced
5 stoichiometrically in order to facilitate the understanding and reading of the overall reaction mechanism.
6 We may also note that we do not have any direct evidence at this point for the reversible formation of Co-
7 doped ZnO (marked by asterisks in the above equations), not to mention the stoichiometry of such a
8 reversibly formed phase. In fact, it may well be that zinc oxide and cobalt oxide are formed instead, as
[17b, 22] 9 reported for spinel-structured ZnCo2O4. Further complementary studies will have to be performed to
10 clarify this aspect.
11 We may briefly summarize at this point the effect of introducing the cobalt dopant into the wurtzite
12 zinc oxide structure. Most importantly, in particular regarding the application of such materials as
13 alternative anode material for lithium-ion batteries, the presence of evenly distributed cobalt cations within
[10] 14 the ZnO lattice enhances the reversibility of the Li2O formation and thus the reversible specific
15 capacity. This beneficial effect presumably results from the presence of a percolating network of ultrafine
16 metallic cobalt nanoparticles formed upon lithiation,[8a, 23] which ensures the required electron supply
17 throughout the Li2O matrix. Additionally, the Co-doping leads to an enhanced nanocrystallinity of the
18 lithiated sample, thus constricting the formation of rather large LiZn and Zn0 crystals. The latter aspect, in
19 fact, appears to be of major importance for alloying materials in general, as previously reported for the
20 commercial Sn-Co-C anode material composite,[24] employed in the ‘Nexelion’ battery (SONY), and also
21 silicon-based lithium-ion anodes.[25]
22
23 3.4 Galvanostatic Cycling and Rate Capability
24 The impact of the Co-doping of ZnO on the long-term galvanostatic cycling performance is presented
25 in Figure 7. In the specific capacity vs. cycle number plot (Figure 7a) the positive effect of the doping in
26 terms of achievable specific capacity and first cycle irreversible capacity loss, i.e., 42.6% for ZnO and
27 29.5% for Zn0.9Co0.1O, is immediately apparent. After some formation cycles, pure ZnO shows a stabilized
28 reversible capacity of about 332 mAh g-1 in the 30th cycle, which is close to the theoretical capacity of
17
ChemElectroChem 10.1002/celc.201600179
1 ZnO considering only the reversible formation of the LiZn-alloy (≙ 329 mAh g-1). The small fraction of
2 extra capacity may be assigned to a partially reversible electrolyte decomposition as already reported for
3 other transition metal oxides as well as the contribution of the conductive carbon.[26] In fact, the potential
4 profiles of selected cycles (Figure 7b) show a stepped shape related to the gradual de-/alloying process and
5 no significant capacity in the high potential regime, in which the degradation of Li2O occurs. Likewise,
6 Co-doped ZnO shows a slight capacity fading during the initial cycles. However, after 40 cycles,
-1 7 Zn0.9Co0.1O reveals a stable performance providing a specific capacity of 762 mAh g .
8
9
10
11 Figure 7. Constant current cycling of ZnO- (black) and Zn0.9Co0.1O-based (green) electrodes cycled
12 at a specific current of 48 mA g-1 (C/20); 1st cycle: 24 mA g-1 (C/40). (a) Capacity vs. cycle
13 number plot, (b) and (c) corresponding potential profiles of ZnO- and Zn0.9Co0.1O-based
14 electrodes for selected cycles (2nd, 10th, 20th, 30th, 40th, 50th, 60th, and 70th).
18
ChemElectroChem 10.1002/celc.201600179
1 The corresponding potential profiles, depicted in Figure 7c, exhibit a sloped profile as already stated
2 above. Upon lithiation no particular features can be correlated with specific electrochemical reactions
3 suggesting a catalytic effect of Co resulting in the promoted reduction of ZnO. However, in the low
4 potential range of the charge profile similar features as the pure ZnO are observed. More importantly, for
5 the delithiation process, at potentials above 1.5 V, a significant capacity is obtained, which, in accordance
6 with the in situ XRD analysis presented herein, is ascribed to the degradation of Li2O.
7
8
9 Figure 8. Comparison of the rate capability of electrodes based on ZnO (black, diamonds) and
10 Co-doped ZnO (green, circles) nanoparticles.
11
12 To evaluate the rate capability of ZnO in comparison with Zn0.9Co0.1O, a multi-rate cycling test was
13 conducted (Figure 8). The plot of the specific charge capacity vs. the applied current density demonstrates
14 once more the superior specific capacity provided by the Co-doped ZnO, in particular compared to pure
15 ZnO. Nevertheless, for very high current densities the delivered capacity decreases dramatically,
16 indicating the need for enhanced electrode composite architectures as, for instance, the incorporation of
17 secondary carbon nanostructures like graphene or carbonaceous coatings.[27]
18
19
19
ChemElectroChem 10.1002/celc.201600179
1 4. Conclusions
2 Pure and Co-doped ZnO were synthesized and structurally, morphologically, and electrochemically
3 characterized. XRD, SEM, and TEM analyses revealed a reduction of the particle size, due to the
4 introduction of the dopant, which is also evident from the smoother and broadened current peaks recorded
5 by cyclic voltammetry. In order to elucidate the impact of the dopant on the lithium storage mechanism
6 electrochemical in situ XRD analysis of pure ZnO and Co-doped ZnO was performed. The results reveal a
7 beneficial impact of the transition metal dopant on the electrochemical performance of ZnO by preventing
8 the formation of large crystals and enhancing the reversibility of the Li2O formation. Additionally, the
9 reduction of the metal dopant presumably leads to an improved electronic conductivity within the lithiated
10 particles. As a result, the reversible specific capacity initially approaches the theoretical value of
11 962 mAh g-1. Long-term galvanostatic cycling further demonstrates the significant improvement of the
12 achievable specific capacity. After 75 full dis-/charge cycles, electrodes based on Zn0.9Co0.1O show more
13 than twice the capacity of pure ZnO, i.e., 789 mAh g-1 vs. 326 mAh g-1.
14
15
16 Acknowledgments
17 Financial support by BMW AG within the ABILE (Air Batteries with Ionic Liquid Electrolytes)
18 project is gratefully acknowledged. D.B. would like to thank the CEA NanoSciences Programme within
19 the ELLIPSE project (14.4.1) and the CEA/EU Enhanced Eurotalents Fellowship for financial support.
20
ChemElectroChem 10.1002/celc.201600179
1
2 References
3 [1] a) A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S. F. 4 Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 2005, 4, 42-46; 5 b) D. I. Son, B. W. Kwon, D. H. Park, W. S. Seo, Y. Yi, B. Angadi, C. L. Lee, W. K. Choi, Nat. 6 Nanotechnol. 2012, 7, 465-471. 7 [2] a) N. Han, L. Y. Chai, Q. Wang, Y. J. Tian, P. Y. Deng, Y. F. Chen, Sensor Actuat. B-Chem. 8 2010, 147, 525-530; b) J. X. Wang, X. W. Sun, Y. Yang, H. Huang, Y. C. Lee, O. K. Tan, L. 9 Vayssieres, Nanotechnology 2006, 17, 4995-4998. 10 [3] a) M. Gratzel, Nature 2001, 414, 338-344; b) M. Law, L. E. Greene, J. C. Johnson, R. Saykally, P. 11 D. Yang, Nat. Mater. 2005, 4, 455-459. 12 [4] a) G. Z. Yang, H. W. Song, H. Cui, Y. C. Liu, C. X. Wang, Nano Energy 2013, 2, 579-585; b) N. 13 Li, S. X. Jin, Q. Y. Liao, C. X. Wang, Acs Appl. Mater. Inter. 2014, 6, 20590-20596; c) Y. Sun, G. 14 Z. Yang, H. Cui, J. Wang, C. X. Wang, Acs Appl. Mater. Inter. 2015, 7, 15230-15239; d) O. B. 15 Chae, S. Park, J. H. Ryu, S. M. Oh, J. Electrochem. Soc. 2013, 160, A11-A14; e) G. L. Xu, Y. Li, 16 T. Y. Ma, Y. Ren, H. H. Wang, L. F. Wang, J. G. Wen, D. Miller, K. Amine, Z. H. Chen, Nano 17 Energy 2015, 18, 253-264; f) B. Oschmann, M. N. Tahir, F. Mueller, D. Bresser, I. Lieberwirth, 18 W. Tremel, S. Passerini, R. Zentel, Macromol. Rapid Comm. 2015, 36, 1075-1082. 19 [5] a) F. Belliard, P. A. Connor, J. T. S. Irvine, Ionics 1999, 5, 450-454; b) M. Ahmad, Y. Y. Shi, A. 20 Nisar, H. Y. Sun, W. C. Shen, M. Wei, J. Zhu, J. Mater. Chem. 2011, 21, 7723-7729; c) H. B. 21 Wang, Q. M. Pan, Y. X. Cheng, J. W. Zhao, G. P. Yin, Electrochim. Acta 2009, 54, 2851-2855; d) 22 K. T. Park, F. Xia, S. W. Kim, S. B. Kim, T. Song, U. Paik, W. I. Park, J. Phys. Chem. C 2013, 23 117, 1037-1043; e) F. Belliard, J. T. S. Irvine, J. Power Sources 2001, 97-8, 219-222; f) W. J. 24 Zhang, J. Power Sources 2011, 196, 13-24. 25 [6] C. J. Pelliccione, Y. J. Ding, E. V. Timofeeva, C. U. Segre, J. Electrochem. Soc.2015, 162, 26 A1935-A1939. 27 [7] a) D. Bresser, F. Mueller, M. Fiedler, S. Krueger, R. Kloepsch, D. Baither, M. Winter, E. Paillard, 28 S. Passerini, Chem. Mater. 2013, 25, 4977-4985; b) F. Mueller, D. Bresser, V. S. K. 29 Chakravadhanula, S. Passerini, J. Power Sources 2015, 299, 398-402. 30 [8] a) P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. M. Tarascon, Nature 2000, 407, 496-499; b) 31 G. X. Wang, Y. Chen, K. Konstantinov, M. Lindsay, H. K. Liu, S. X. Dou, J. Power Sources 32 2002, 109, 142-147; c) D. Bresser, E. Paillard, P. Niehoff, S. Krueger, F. Mueller, M. Winter, S. 33 Passerini, ChemPhysChem 2014, 15, 2177-2185; d) A. Brandt, A. Balducci, J. Power Sources 34 2013, 230, 44-49; e) C. M. Ban, Z. C. Wu, D. T. Gillaspie, L. Chen, Y. F. Yan, J. L. Blackburn, 35 A. C. Dillon, Adv. Mater. 2010, 22, E145-E149; f) Y. J. Zhang, Y. Yan, X. Y. Wang, G. Li, D. R. 36 Deng, L. Jiang, C. Y. Shu, C. R. Wang, Chem.-Eur. J. 2014, 20, 6126-6130; g) F. Lin, D. 37 Nordlund, T. C. Weng, Y. Zhu, C. M. Ban, R. M. Richards, H. L. Xin, Nat. Commun. 2014, 5. 38 [9] D. Bresser, E. Paillard, R. Kloepsch, S. Krueger, M. Fiedler, R. Schmitz, D. Baither, M. Winter, 39 S. Passerini, Adv. Energy Mater. 2013, 3, 513-523. 40 [10] G. Giuli, A. Trapananti, F. Mueller, D. Bresser, F. d'Acapito, S. Passerini, Inorg. Chem. 2015, 54, 41 9393-9400. 42 [11] K. Xu, Chem. Rev. 2004, 104, 4303-4417. 43 [12] T. Fujieda, S. Takahashi, S. Higuchi, J. Power Sources 1992, 40, 283-289. 44 [13] X. H. Huang, X. H. Xia, Y. F. Yuan, F. Zhou, Electrochim. Acta 2011, 56, 4960-4965. 45 [14] a) B. K. Guo, J. Shu, K. Tang, Y. Bai, Z. X. Wang, L. Q. Chen, J. Power Sources 2008, 177, 205- 46 210; b) X. W. Guo, X. P. Fang, Y. Sun, L. Y. Shen, Z. X. Wang, L. Q. Chen, J. Power Sources 47 2013, 226, 75-81; c) D. Bresser, F. Mueller, D. Buchholz, E. Paillard, S. Passerini, Electrochim. 48 Acta 2014, 128, 163-171. 49 [15] X. Y. Shen, D. B. Mu, S. Chen, B. R. Wu, F. Wu, Acs Appl. Mater. Inter. 2013, 5, 3118-3125. 50 [16] a) H. Li, Y. Wei, Y. Zhang, F. Yin, C. Zhang, G. Wang, Z. Bakenov, Ionics 2016, 1-7; b) J. 51 Hassoun, G. Derrien, S. Panero, B. Scrosati, Adv. Mater. 2008, 20, 3169-3175; c) I. A. Courtney, 52 J. R. Dahn, J. Electrochem. Soc. 1997, 144, 2943-2948.
21
ChemElectroChem 10.1002/celc.201600179
1 [17] a) Y. Ding, Y. F. Yang, H. X. Shao, Electrochim. Acta 2011, 56, 9433-9438; b) N. Du, Y. F. Xu, 2 H. Zhang, J. X. Yu, C. X. Zhai, D. R. Yang, Inorg. Chem. 2011, 50, 3320-3324. 3 [18] Y. F. Zhang, Z. Y. Wang, Y. J. Li, K. J. Zhao, Mech. Mater. 2015, 91, 313-322. 4 [19] J. Q. Wang, P. King, R. A. Huggins, Solid State Ionics 1986, 20, 185-189. 5 [20] J. Allen, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, Marcel Dekker Inc., New 6 York & Basel, 1985. 7 [21] Q. M. Su, Z. M. Dong, J. Zhang, G. H. Du, B. S. Xu, Nanotechnology 2013, 24, 255705. 8 [22] Y. Sharma, N. Sharma, G. V. S. Rao, B. V. R. Chowdari, Adv. Funct. Mater. 2007, 17, 2855- 9 2861. 10 [23] a) F. Wang, R. Robert, N. A. Chernova, N. Pereira, F. Omenya, F. Badway, X. Hua, M. Ruotolo, 11 R. G. Zhang, L. J. Wu, V. Volkov, D. Su, B. Key, M. S. Whittingharn, C. P. Grey, G. G. 12 Amatucci, Y. M. Zhu, J. Graetz, J. Am. Chem. Soc. 2011, 133, 18828-18836; b) F. Wang, H. C. 13 Yu, M. H. Chen, L. J. Wu, N. Pereira, K. Thornton, A. Van der Ven, Y. M. Zhu, G. G. Amatucci, 14 J. Graetz, Nat. Commun. 2012, 3, 1201. 15 [24] a) A. D. W. Todd, R. E. Mar, J. R. Dahn, J. Electrochem. Soc. 2006, 153, A1998-A2005; b) J. R. 16 Dahn, R. E. Mar, A. Abouzeid, J. Electrochem. Soc. 2006, 153, A361-A365; c) A. D. W. Todd, R. 17 E. Mar, J. R. Dahn, J. Electrochem. Soc. 2007, 154, A597-A604; d) Q. Fan, P. J. Chupas, M. S. 18 Whittingham, Electrochem. Solid St. 2007, 10, A274-A278. 19 [25] a) M. N. Obrovac, L. Christensen, Electrochem. Solid St. 2004, 7, A93-A96; b) B. Key, M. 20 Morcrette, J. M. Tarascon, C. P. Grey, J. Am. Chem. Soc. 2011, 133, 503-512. 21 [26] a) F. Mueller, D. Bresser, N. Minderjahn, J. Kalhoff, S. Menne, S. Krueger, M. Winter, S. 22 Passerini, Dalton T. 2014, 43, 15013-15021; b) B. Liu, J. Zhang, X. F. Wang, G. Chen, D. Chen, 23 C. W. Zhou, G. Z. Shen, Nano Lett. 2012, 12, 3005-3011; c) S. Laruelle, S. Grugeon, P. Poizot, 24 M. Dolle, L. Dupont, J. M. Tarascon, J. Electrochem. Soc. 2002, 149, A627-A634; d) S. Grugeon, 25 S. Laruelle, L. Dupont, J. M. Tarascon, Solid State Sci. 2003, 5, 895-904; e) A. Ponrouch, P. L. 26 Taberna, P. Simon, M. R. Palacin, Electrochim. Acta 2012, 61, 13-18. 27 [27] a) R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, Nat. Mater. 2015, 14, 271-279; b) Y. R. 28 Zhong, M. Yang, X. L. Zhou, Z. Zhou, Mater. Horiz. 2015, 2, 553-566. 29
22
ChemElectroChem 10.1002/celc.201600179
1 2 ToC Graphic
3
4 5 Reversible conversion: The introduction of Co into the ZnO wurtzite structure results in a twice to thrice 6 enhanced specific capacity by inhibiting zinc crystal growth upon (de-)lithiation, thereby, enabling the 7 long-term reversible cycling of hybrid conversion/alloying-type lithium-ion anodes.
23