Improved electrochemical performances of lithium–oxygen batteries with tungsten carbide-coated cathode

A cathode material was prepared by mixing Ketjen black (KB) carbon (Ketjen Black EC600-JD, specific surface area: 1270 m² g⁻¹) and the binder (PVDF) dissolved in N-methyl-2-pyrrolidone, in the weight ratio of $80:20$. Following that, the O₂ cathode was prepared by uniformly casting the slurry onto Ni foam. In order to retain the porosity of the fabricated cathode, the slurry coated nickel foam was slightly blown with an air gun. The O₂ cathode thus obtained was dried at 80 °C for 24 h. Subsequently, the O₂ cathode was coated with WC by PVD (Dongwoo Surface Tech) (Fig. 1). In order to generate plasma, C₂H₂ gas was injected in a chamber. Thus, after the plasma tore off the tungsten carbide material (target), it was deposited on a substrate (electrode). The deposition was carried out for the duration of 5 min at a temperature of 100 °C. The thickness of the resulting WC coating was approximately 20 nm. The morphology of the O₂ cathode before and after cycling was observed by using scanning electron microscopy (SEM; Hitachi S-4300). Furthermore, the WC-coated O₂ cathode was analyzed by field-emission transmission electron microscopy (FETEM), energy-dispersive X-ray spectroscopy (EDX) and electron probe microanalysis (EPMA). The sample for TEM analysis was prepared using a focused ion beam (FIB).

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The electrochemical properties of the WC-coated O₂ cathode were analyzed by assembling a cell using a HS flat cell (Fig. 2). In the typical process, the electrochemical cell was assembled with Li metal as the anode, a glass microfiber filter paper (Whatman GF/C) soaked in the electrolyte consisting of 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME), and the WC-coated O₂ cathode prepared by casting the active material onto Ni foam, with the HS flat cell in a dry room. The cell was assembled completely gastight, except that the WC-coated O₂ cathode was exposed to oxygen atmosphere through a pipe. Cell testing was performed under 1 atm of O₂. The performance of the cell with a WC-coated electrode was compared with that of the cell with an uncoated KB electrode assembled under the same experimental condition. The electrochemical performance of the cell was analyzed by a galvanostatic discharge/charge method (WonATech WBCS 3000) at constant current densities of 100 and 200 mA g⁻¹_carbon for a limited duration of 10 h. The impedance measurements (Solatron SI1280B) were performed in the frequency range of 10⁻² to 10⁵ Hz at an AC amplitude of 5 mV. The corresponding impedance data was displayed by using Z-view software (Scribner Associates).

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Figure 3 shows the SEM images of the bare KB electrode (uncoated O₂ cathode) [Fig. 3(a)] and WC-coated O₂ cathode [Fig. 3(b)]. As is seen, no significant changes in the morphology can be observed between the KB electrode and WC-coated electrodes. Therefore, further studies on the uniformity of the WC coating are performed by EPMA (Fig. 4). The EPMA mapping of the WC-coated O₂ cathode indicates uniform distributions of elemental tungsten and carbon, suggesting the uniform coating of WC onto the O₂ cathode. This is further confirmed by TEM analysis, which reveals that the WC-coating layer thickness is approximately 20 nm (Fig. 5). The image represents that the WC-coating layer is uniformly deposited on the electrode. This indicates that the WC has good adhesion onto the active materials in the cathode.²³⁾ The presence of tungsten and carbon on the surface of the electrode is further confirmed by EDX analysis (Fig. 5). Furthermore, the carbon in the active material is distinguished from the carbon in WC via point profiling by EDX analysis. Results indicate the existence of carbon in the coating layer, confirming the uniform coating of WC onto the O₂ cathode.

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We determine the effects of the catalytic property and high electrical conductivity of the WC on the capacity of the cell. The Li–O₂ batteries are discharged at a current density of 100 mA g⁻¹_carbon, as shown in Fig. 6. Discharged to the limiting voltage of 2.0 V, the cell with the WC-coated electrode presents a higher capacity of 7000 mA h g⁻¹_carbon when compared to the cell with the KB electrode (3200 mA h g⁻¹_carbon). Indeed, a capacity of more than double of the KB cell is obtained. The observed increase in the initial capacity of the cell with the WC-coated electrode can be attributed to its catalytic property and high electrical conductivity.^18–22)

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That is, it is observed that Li₂O₂ is efficiently generated with the help of the WC catalytic effect on the ORR during discharge, and the evenly coated WC mitigates the accumulation of Li₂O₂ that uniformly induces the generation of Li₂O₂ on the electrode. Consequently, the quantity of Li₂O₂ increases, leading to capacity enhancement.

Figure 7 shows the cycling behavior of batteries with bare KB and WC-coated cells for the first cycle and 10th cycles at a current densities of 100 and 200 mA g⁻¹_carbon. The voltage profile of the KB cell is compared with that of the WC-coated cell at 100 mA g⁻¹_carbon. The first discharge and charge voltages of the KB cell are observed to be 2.65 and 4.23 V, respectively, while the first discharge and charge voltages of the WC-coated cell are observed to be 2.75 and 3.63 V, respectively [Fig. 7(a)]. The 10th discharge–charge voltages of the uncoated cell (KB) are observed to be 2.3 and 4.4 V, respectively, while the corresponding values of the WC-coated cell are observed to be 2.74 and 3.64 V, respectively. [Fig. 7(b)]. The first discharge–charge voltage gap ("overpotentials") of the cell with the WC-coated electrode is estimated to be 0.88 V which is 700 mV smaller than that of the cell with the KB electrode at a current density of 100 mA g⁻¹_carbon. Also, the 10th discharge–charge overpotentials of the cell with the WC-coated electrode is estimated to be 0.9 V, which is 1200 mV smaller than that of the cell with the KB electrode at a current density of 100 mA g⁻¹_carbon. The KB cell shows a gradual increase in discharge–charge overpotentials during the 10th cycles, while the WC-coated cell does not exhibit any voltage change during the 10th cycles. Moreover, the KB cell leads to the complete failure of the cell after approximately the 14th cycles. The decreases in overpotentials during the charge and discharge can be explained on the basis of the catalytic effect of WC, which is similar to that of the platinum.^18,20,24) In other words, the overpotentials respectively decreases and increases during the charge and discharge cycling processes owing to the catalytic effect of WC that favors the rapid creation and decomposition of the reaction product (Li₂O₂).

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It should be noted that the overpotentials of both the ORR and OER gradually increased, probably owing to the accumulation of incompletely decomposed Li₂O₂ and side reaction products, which might have resulted from the decomposition of the TEGDME solvent even below 4.2V_Li^28–30) and the TEGDME–O₂ reaction. The accumulation of reaction products causes the slow performance and capacity degradation along with an increase in the number of cycles. A very improved cyclability can be achieved by using the WC to limit the discharge–charge overpotentials.

Figure 7(c) shows that the WC-coated cell can considerably enhance the cycle life with a current density of 1000 mA g⁻¹_carbon. After the 12th cycles, the cell with the KB electrode exhibits capacity decrease, unsuitable for further use. On the other hand, the cell with the WC-coated electrode efficiently operates beyond the 36th cycles, albeit with a slight increase in overpotentials. The observed increases in cycle stability and round-trip efficiency can be attributed to the improvement in reaction rate and decrease in discharge or charge voltage gap,²⁵⁾ as a result of the high electrical conductivity and catalytic activity of WC.

We further substantiate this observation by performing cell tests at a high current density. The voltage profiles of the KB and WC-coated cells are compared at a high current density of 200 mA g⁻¹_carbon. As is seen, the first discharge and charge voltages of the KB cell are 2.61 and 4.25 V, respectively [Fig. 7(d)]. However, the WC-coated cell shows a decreased overpotentials, as indicated by the first discharge and charge voltages of 2.66 and 3.94 V, respectively [Fig. 7(d)]. This can be attributed to the catalytic properties and the high electrical conductivity of WC, which results in a higher rate capability performance.²⁷⁾ These results once again confirm the benefits of the coating of WC onto the O₂ cathode.

To obtain further insights into the discharge and charge processes of Li–O₂ batteries with the uncoated (KB) and WC-coated cells, a SEM technique is employed to analyze the surface morphology at discharge–charge stages, as presented in Fig. 8. Figures 8(a) and 8(b) show the initial morphologies of the KB and WC-coated electrodes, demonstrating that both electrodes are composed of carbon and microspheres. The WC coating layer is too thin to be distinguished. After discharge, the carbon and microspheres are covered by massive Li₂O₂, as shown in Figs. 8(c) and 8(d). Based on previous report,³¹⁾ it can be considered that Li₂O₂ (discharge product) gradually blocks microspheres. The morphologies of the KB and WC-coated electrodes after subsequent charge are shown in Figs. 8(e) and 8(f). For the KB electrode, Li₂O₂ is not completely decomposed. After charge, the remaining Li₂O₂ is observed as a Li₂O₂ film, blocking microspheres.^11,32) Li₂O₂ with low electronic conductivity³³⁾ covers the surfaces of microspheres and would further block the pathways for oxygen diffusion and electron transfer. Thus, these results lead to the low cycling performance of Li–O₂ batteries.³⁴⁾ As for the WC-coated electrode, Li₂O₂ disappears after charge. This demonstrates that the WC coating layer can facilitate the effective decomposition of discharge products.

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Electrochemical impedance spectroscopy measurements of the uncoated (KB) and WC-coated cells are performed before and after the 10th charge cycles (Fig. 9). As is seen, the WC-coated cell obviously improves its impedance results. Before the charge cycling, the radius of the semicircle in the Nyquist plot of the cell with the WC-coated electrode is smaller than that of the cell with the uncoated electrode (KB). This demonstrates that the high electrical conductivity of the WC coating reduces charge transfer resistance.²⁶⁾ Likewise, after the 10th cycles, the radius of the semicircle in the Nyquist plot of the cell with the WC-coated electrode is smaller than that of the cell with the uncoated electrode. The catalytic effect of WC also affects charge transfer resistance during discharge–charge. As the cycling proceeds for the KB electrode, the product decomposition is not completely finished [Fig. 8(e)] on charge, that is, Li₂O₂ of low electronic conductivity accumulates with charge transfer resistance increase. However, in the case of the WC-coated cell, the charge transfer resistance increment is reduced by the excellent catalytic effect on the OER, on which the reaction product is effectively decomposed. This could be attributed to the excellent catalytic effect of WC, which decreases the charge transfer resistance at the interface.

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These results suggest that the WC coating improves the electrochemical properties of the electrode. When compared to Li–air batteries using MnO₂ nanowires as catalysts (3000 mA h g⁻¹ at 70 mA g⁻¹)¹⁵⁾ and Li–air batteries using PtAu nanoparticles as bifunctional catalysts (2200 mA h g⁻¹ at 50 mA g⁻¹),¹⁶⁾ we also observed good catalytic activity of WC from the rate capacity (7000 mA h g⁻¹_carbon at 100 mA g⁻¹) and the decrease in overpotentials. That is, WC as a catalyst is suitable as an effective coating material. However, the performance of the WC coated cell gradually terminates after the 37th cycles at 100 mA g⁻¹_carbon since the WC coating could only act as a catalytic source but could not effectively suppress the side reactions (Li₂CO₃, HCO₂Li) during discharge. Also, the oxidation of the lithium metal anode is not suppressed on discharge–charge. To compensate for the defects, we propose research studies on a dual-coated electrode with WC and other stable materials that can suppress the formation of byproducts or the fabrication of an electrochemically stable electrolyte that can minimize the side reactions. Finally, in order to stop the oxidation of lithium and enhance ionic conductivity, we suggest ionic conductivity polymer coating on the lithium metal anode.