3.1. Optimization of Process Conditions of Bi2O3-Li2O-ZnO Glass
To study the influence of different crucibles and temperatures on the glass formation process, the solidified glasses were analytically examined after the melting process and the most suitable melting process was selected. Figure 2
shows the XRD analysis of the molten glass after melting in (a) an Al2
crucible at 1050 °C, (b) in a Pt crucible at 1050 °C, and (c) in a Pt crucible at 1200 °C.
Using an Al2
crucible at 1050 °C results in a partly crystalline structure with portions of Bi2
and ZnO (Figure 2
curve a). Using a Pt crucible results in a mainly amorphous structure with only few ZnO crystals (Figure 2
curve b), and at 1200 °C, no ZnO crystals can be found (Figure 2
curve c). These results are confirmed by SEM images of cross sections of the glasses as shown in Figure 3
The glass that has been melted in the Al2
crucible at 1050 °C shows a high number of ZnO crystals with aluminum impurities as shown by spot S1 in Figure 3
a. In a Pt crucible the number of ZnO crystals is reduced significantly (Figure 3
b). The use of an Al2
crucible at such high temperatures causes an alumina leaching into the glass melt. The alumina impurities may serve as nuclei that cause a change in the nucleation rate and an increased crystallization in the subsequent quenching step.
After increasing the melting temperature to 1200 °C, only a few small ZnO crystals (EDX-analysis of spot S2 in Figure 3
d) can be detected (Figure 3
c) due to the almost complete dissolution of ZnO in the melt. Based on this, ZnO cannot act as a nucleus for crystallization. The latter procedure likely prevents an enhanced nucleation and crystal growth at those ZnO centers during discharge. Therefore, this synthesis procedure was selected for further coating experiments.
3.2. Optimization of the Coating Parameters for Zinc–Glass Composite Material
In order to optimize the coating quality of the zinc particles with the glass described in Section 3.1
, a parameter variation of the mechanical coating process was applied. Figure 4
shows SEM micrographs of coated zinc particles (250 μm) and the corresponding layer thicknesses in cross section after 15 min and after 30 min coating time at either 500 rpm, 750 rpm or 1000 rpm, respectively.
An increase of rotational speed from 500 rpm to 750 rpm and 1000 rpm (Figure 4
a–c) results in a more homogeneous and compact glass coating. The glass layer thickness is reduced from (5.1 ± 1.3) μm to (3.0 ± 1.3) μm at 15 min. (Figure 4
g). Probably a stronger adhesion of the glass layer to the zinc particles is achieved due to the increased kinetic energy during milling. If the coating time is raised from 15 to 30 min (Figure 4
d–f), a similar behavior can be seen. By increasing the speed of the rotor, the layer thickness decreases from (7.8 ± 2.1) μm at 500 rpm to (7.5 ± 2.1) μm at 750 rpm to (4.4 ± 0.5) μm at 1000 rpm (Figure 4
Doubling the coating time leads to an increase of the coating thickness of about 30–50% as is exemplarily confirmed by determination of the amount of glass (analyzed by ICP-OES) in Table 2
for sample 1 (1.61 wt %) and sample 3 (4.83 wt %) after coating. Due to the longer coating process and the increased number of collisions between grinding balls, zinc particles, and glass, more energy can be transferred into the system. This yields a higher coating content.
On the other hand, rotational speed increases compaction of the glass layer, as indicated, e.g., by sample 2 and sample 3. The amount of glass does not increase while the coating thickness decreases. By applying 1000 rpm for 30 min to fine zinc particles with d50
= 30 μm (see Table 2
; sample 4), a strongly bonded glass layer with uniform thickness was achieved. The resulting content of glass is 10.72 wt % at an average layer thickness of 2 μm. The thinner glass layer of the coated fine zinc particles compared to the coated coarse zinc particles can be explained by the increased number of impact processes of the smaller zinc particles. Thus, a uniform thin layer is formed. The higher glass content (10.72 wt % instead of 4.83 wt %) of the fine zinc particles is attributed to the higher surface area of the fine zinc particles. This means that a higher glass coating content can be applied with approximately the same zinc surface coverage with glass (Table 2
3.3. Electrochemical Performance
In order to investigate the influence of different coating contents, glass layer thicknesses, and particle sizes, selected zinc–glass composites (Table 2
) were electrochemically investigated and compared with uncoated zinc powder. Figure 5
shows the results of the galvanostatic cycling of glass-coated 250 μm and 30 μm zinc particles (samples 1 to 4 listed in Table 2
Uncoated zinc achieved a specific capacity of 596 mAh g−1
a) at the first discharge, while zinc–glass composites (sample 1 to 3) show a reduced specific capacity of 440 mAh g−1
, 483 mAh g−1
, and 462 mAh g−1
, respectively (Figure 5
a). The reduced discharge capacity of the zinc–glass composites is caused on the one hand by the mass reduction of active material itself, as the coating does not directly contribute to electrochemical discharge reactions. On the other hand, the accessibility of the hydroxide ions to the zinc surface is limited by the glass coating, thus the cut-off voltage is reached earlier during discharge.
Recharge of pure zinc powder is impossible (Figure 5
b) due to the electrical passivation by the ZnO formation. The existence of an insulating ZnO layer can be confirmed by the increase in the internal resistance after the first discharge, which is shown in Table 3
The internal resistance of pure Zn increases from 1.16 Ω to 9.08 Ω and remains at the same level after attempted charging. The same effect occurs in sample 1. We attribute it to the incomplete glass layer formation, which probably cannot prevent the formation of a compact insulating ZnO layer, as post-mortem characterizations of zinc particle cross sections indicate (Figure 6
a; analyzed spot of sample 1). The internal resistance increases from 1.03 Ω to 11.29 Ω after the first discharge. Due to the low glass content, Ri
decreases slightly to 6.30 Ω after the first incomplete recharge. Nevertheless, a passivated ZnO layer is formed and charging is not possible.
In contrast, a glass coating content of 4.39 wt % (Table 2
; sample 2) allows recharging at a low level with a charge capacity of 32 mAh g−1
at the first cycle (Figure 5
increases from 1.18 Ω to 2.97 Ω after the first discharge. Due to the increased glass coating content, this increase is lower compared to sample 1, indicating decreased ZnO layer formation. It is assumed that zincate ions can be immobilized during the first discharge in the formed gel-like glass layer [27
]. As a result, locally bound zincate ions can be utilized for charging and hence for subsequent discharge (Figure 5
a). After the first recharge, Ri
decreases to 1.32 Ω due to the reduced ZnO layer. However, zinc particles are completely passivated after three cycles and the rechargeability is lost. The SEM cross section in Figure 6
b indicates that ZnO forms a dense layer under the glass coating (Figure 6
b; analyzed spot of sample 2) and passivates the coated zinc particle.
In case of sample 3 with a thin and homogeneous glass layer, recharging is possible eight times with an improved charge capacity of 74 mAh g−1
at the first cycle (Figure 5
b). Furthermore, sample 3 shows a low increase in internal resistance after the first discharge (Table 3
; 1.14 Ω to 1.68 Ω) and only a slightly increased internal resistance after the first recharge (Table 3
; 1.35 Ω) compared to the initial state. Due to the uniform, thin and compact glass layer of sample 3, a homogeneous gel coating is formed. It entraps the zincate ions close to the electrode, enabling improved cycle stability. Due to the progressive irreversible passivation of the electrode, recharging is only possible eight times. Nevertheless, in each cycle, the discharge capacity is higher than the previous charge capacity. This indicates that the formation of passivation layers on the glass-coated zinc particles is reduced and subsequent oxidation of zinc is maintained, as illustrated after charging step 9 where recharge stops, but discharging in cycle 10 is performed successfully (Figure 5
). Then, a higher utilization of the zinc is achieved, which is confirmed by post-mortem analysis in Figure 6
c. Improved recharging creates a sponge-like ZnO layer with an embedded Bi2
-based glass layer (Figure 6
c; analyzed spot of sample 3).
X-ray diffraction patterns of the charged cycled zinc–glass composites in Figure 7
indicate the existence of zinc oxide, metallic zinc, and rhombohedral metallic bismuth.
These Bi metal reflections have already been observed and discussed in the literature on zinc anodes with bismuth oxide [18
]. Metallic bismuth enhances the electrical conductivity through the gel matrix and enables the zinc–glass composite (sample 2, sample 3, and sample 4) to be recharged.
In addition, Figure 5
illustrates the results of galvanostatic cycling of the coated fine zinc particles (sample 4) in comparison to sample 3. Sample 4 with a specific first discharge capacity of 561 mAh g−1
shows an increased first zinc utilization compared to sample 3 with 462 mAh g−1
a). This can be explained as follows: The fine zinc particle fraction has an increased specific surface area of 0.0407 m2
compared to 0.0197 m2
of the coarse zinc particle fraction. This allows more active material to participate in the electrochemical reactions, and thus considerably increases the first discharge capacity.
Recharging of sample 4 is possible with 82 mAh g−1
after the first discharge. The increase of the charging capacity in the following cycles up to 140 mAh g−1
in the fourth charging step is remarkable (Figure 5
b). This can be justified by the enhanced formation of electrically conductive metallic bismuth paths (Figure 7
d) owing to the increased glass content on fine zinc particles. (10.72 wt % compared to 4.83 wt % of coarse zinc particles). Due to the higher glass content and the compact Bi2
-based glass layer of sample 4, a more stable glass-gel matrix with increased retention for zincate ions close to the electrode is proposed. In addition, smaller particles provide more contact points amongst each other and thus improve the electrical conductivity through the electrode/electrolyte matrix. Thus, passivation of the zinc particles is reduced and a discharge capacity of 52 mAh g−1
is achieved even in the 20th cycle (Figure 5
a). The average charge capacity over 20 cycles is 113 mAh g−1