High energy and power density Li–O₂ battery cathodes based on amorphous RuO₂ loaded carbon free and binderless nickel nanofoam architectures

Abstract

Herein, amorphous RuO₂ nanoflakes deposited on Ni nanofoam (NF) with diameters of ca. 30–100 nm are utilized as an innovative cathode in Li–O₂ batteries for the first time. The stability of the RuO₂/Ni NF cathode is shown to possess ca. 87.7% capacity retention after 75 cycles with minute alteration of the charge–discharge profiles. A capacity as high as 6537.8 mA h g⁻¹ based on RuO₂ weight can be reached at 0.02 mA cm⁻² with a low charge potential of 3.78 V leading to a high voltaic efficiency of 70.11%. Energy densities range from 2702.97 W h kg⁻¹ at a power density of 29.22 W kg⁻¹ to 1746.32 W h kg⁻¹ at 822.20 W kg⁻¹. The superior performance of the RuO₂/Ni NF results from the intimate contact between catalysts and current collector, and the porous nanostructure providing sufficient space for deposition of lithium oxides, and short lithium ion and oxygen diffusion pathways, as evidenced by the impedance analysis. The binder-less and carbon-free nature of the electrode prevent binder, electrode and excessive electrolyte decomposition, rendering it a prospective candidate for rechargeable Li–O₂ batteries.

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1. Introduction

Energy storage is indispensable in various applications, such as portable electronics, electric vehicles, and complementary power leveling of unstable wind or solar energies.^1,2 Li-ion batteries are the most widespread energy storage device on account of their good cyclability and adequate energy density. Nonetheless, the capacities of Li-ion batteries are still mainly limited by the intercalation nature of the cathodes, such as LiCoO₂ with a theoretical value of 274 mA h g⁻¹. Accordingly, new cathode materials utilizing conversion reactions, such as O₂ forming Li₂O₂ with ca. 1168 mA h g⁻¹, are promising to raise the capacities and energy densities of Li batteries.¹ Considering the active materials of the cathode only, the theoretical gravimetric energy densities of LiCoO₂ and O₂ (Li₂O₂) are 1014 and 3457 W h kg⁻¹, respectively, indicating that ca. 3.4 times higher energy density can be achieved with an O₂ cathode.

However, Li–O₂ battery still suffers from several issues, such as low cyclability and low round-trip efficiency.^1,3 Low cyclability results from the decomposition of electrolyte, positive carbon electrode and binder.^1,3,4 Insoluble Li₂CO₃ accumulation from carbon electrode decomposition when charging voltage is larger than 3.5 V_Li, and electrolyte decomposition catalyzed by carbon electrode cause electrode passivation and capacity fading.⁴ Binders, such as polyvinylidene fluoride, can be decomposed by superoxide O₂⁻,⁵ which is an intermediate reactant of oxygen reduction reaction (ORR).³ Round-trip efficiency (=V_discharge/V_charge) is low due to high overpotentials of ORR and lithium oxides decomposition, leading to below 70% for Li–O₂ batteries compared to the 95% for Li-ion batteries.¹ Formation of insulating Li₂CO₃ and lithium carboxylates⁶ upon cycling is responsible for high charge potentials, normally 1–1.5 V higher than the theoretical 2.96 V_Li of Li₂O₂.^1,4,7 Accordingly, non-carbon electrodes with catalysts active for both ORR and Li₂O₂ decomposition are necessary to reduce the overpotentials. Carbon nitride⁸ and transition metal oxides, such as MnO₂,^9,10 NiCo₂O₄,¹¹ CoO,¹² Co₃O₄ (ref. 13–15) and RuO₂ (ref. 16–20) have been utilized as effective catalysts in Li–O₂ batteries. Compared to other catalysts, RuO₂ demonstrates lower charge potentials mostly below 4 V_Li,^16–20 and reversible reactions between Li₂O₂ and Li with minute electrolyte decomposition and performance fading.^17–19 Non-carbon structures, such as Ni nanowire frameworks, can be alternative candidates to replace carbon current collector.^21–23 Commercial Ni nanofoam consisting of interconnected Ni nanowires decorated with SnO₂ demonstrates good stability, high capacity and rate capability for Li-ion anode.²¹ Ni nanowires can be synthesized by glycerol and Ni acetate.^22,23 Surface area can be further increased by etching Ni into Ni oxalate nanostructures²⁴ followed by reduction back into Ni under reducing atmosphere.^25,26

In this work, Ni nanofoam (NF) decorated with amorphous RuO₂ nanoflakes without any carbon additives and binders is utilized as a novel cathode for Li–O₂ battery. Highest areal and specific capacities are 1.85 mA h cm⁻² and 6537.84 mA h g⁻¹ at 0.02 mA cm⁻², respectively. Even at higher current density 0.2 mA cm⁻², the RuO₂/Ni NF cathode demonstrates 87.7% capacity retention after 75 cycles with average discharge and charge potentials ca. 2.5 and 3.5 V, respectively. These results manifest the superior performances in capacities, stability and round-trip efficiency of the RuO₂/Ni NF electrode.

2. Experimental

2.1 Materials synthesis

Detailed synthesis of RuO₂/Ni NF can be found elsewhere.²⁷ Briefly, Ni foam was etched in HCl before Ni nanowire growth. HCl-etched Ni foam was immersed in nickel acetate/glycerol at 370 °C.²³ Oxalic acid/ethanol with 5 wt% water was utilized to etch Ni nanowire at 120 °C into Ni oxalate,²⁴ which was reduced to Ni with H₂ at 350 °C. Amorphous RuO₂ was prepared with a modified sol–gel method.^27–29 NaOH was dripped dropwise into RuCl₃ solution to reach pH = 7. The mixed solution was centrifuged and washed with deionized water. RuO₂ was dropped onto Ni nanofoam until its loading reached 0.28 mg cm⁻² (0.5 mg on 15 mm diameter Ni foam disk). A balance (Explorer Pro, OHAUS EP64) with 0.1 mg accuracy was utilized to measure the weight difference before and after RuO₂ deposition. RuO₂/Ni NF composite was annealed at 150 °C for 2 h under vacuum before electrochemical measurement.

2.2 Materials characterization

Scanning electron microscopy (SEM, NovaNanoSEM 450) with energy dispersive X-ray spectroscopic (EDX) detector was used to characterize morphology and elemental analysis. Crystallinity was examined by X-ray diffraction (XRD, PANalytical Empyrean). Transmission electron microscopy (TEM, Philips CM300) was used to investigate nanostructure and crystalline phase of RuO₂. The Fourier transform infrared (FTIR) spectroscopic data were acquired using a Bruker Tensor 27 FTIR interferometer in attenuated total reflection (ATR) mode. X-ray photoelectron spectroscopy (XPS) characterization was carried out by using a Kratos AXIS Ultra DLD XPS system equipped with an Al monochromatic X-ray source and an electron energy hemispherical analyzer (165 mm of mean radius). The vacuum pressure was kept below 3 × 10⁻⁹ torr, and a charge neutralizer was applied during the data acquisition.

2.3 Electrochemical characterization

Electrochemical performance of the RuO₂/Ni NF was evaluated by Biologic VMP3 with Li foil (MTI Corp.) negative electrode in a split cell (MTI Corp., EQ-STC-LI-AIR) using electrolyte comprising 1 M bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, Sigma-Aldrich) in tetraethylene glycol dimethyl ether (TEGDME, Sigma-Aldrich, ≥99%). The cell was assembled in an Ar filled glovebox (VAC Omni-lab). Celgard 2400 was used as the separator. Cyclic voltammetry (CV) was scanned at 0.1 mV s⁻¹ in 2.0 to 4.0 V (vs. Li/Li⁺). Galvanostatic charge–discharge tests were measured with various current densities. Electrochemical impedance spectroscopy (EIS) after charge with capacity limit 0.11 mA h cm⁻² at E_we = 0 V vs. OCV between 0.1 to 1 MHz with amplitude 10 mV were performed.

3. Results and discussion

Hierarchical Ni NF can serve as electrical conducting support for the RuO₂ nanocatalysts, and the open-channel structure can provide short pathways of electrolyte and oxygen conduction. SEM images of the RuO₂/Ni NF are shown in Fig. 1 and S1.† To synthesize Ni NF, HCl-treated Ni foam is heated at 370 °C with Ni(Ac)₂/glycerol solution (Fig. 1a and S1a†). Ni²⁺ ions reduced with glycerol²³ nucleate into nanoparticles and develop into nanowires (dia. ca. 200–700 nm) along the magnetic field of magnetic stir rotor of the hotplate. Self-assembled magnetic alignment of ferromagnetic materials has been demonstrated by Ni nanoparticles growing into nanowires in three-dimensional nonwoven clothes.³⁰ XRD peaks at 45.1°, 52.5° and 76.9° indicates cubic Ni metal phase (ref. code: 01-070-0989, Fig. 2a) of the Ni nanowires. To further increase the surface area, etching Ni nanowires in oxalic acid/ethanol solution gives Ni oxalate leaf-like nanostructure (Fig. 1b and S1b†) with XRD peaks at 18.9°, 23.0°, 30.4°, 35.8°, and 41.2° (NiC₂O₄·2H₂O, ref. code 00-014-0742). Ni NF with diameters ca. 30–100 nm (Fig. 1c and S1c†) can be obtained with Ni oxalate reduced by H₂ at 350 °C. The XRD pattern shows only Ni metallic phases without the existence of Ni oxalate. RuO₂/Ni NF electrode is manufactured by coating Ni NF with RuO₂ nanoflakes followed by annealing at 150 °C (Fig. 1d and S1d†). EDX mapping (Fig. S1e†) demonstrates that RuO₂ is uniformly dispersed on Ni NF. The RuO₂/Ni NF electrode shows only Ni metal phase (Fig. 2a), indicating the low reflection intensity and amorphous nature of the RuO₂ nanoflakes. The RuO₂ nanoflakes consist of nanoparticles with diameters ranging from 2–3 nm (Fig. 2b). Weak selected area electron diffraction rings further confirm that the RuO₂ is amorphous.

Fig. 1 SEM images of (a) Ni nanowires, (b) Ni oxalate nanowires, (c) Ni nanofoam and (d) RuO₂/Ni NF.

Fig. 2 (a) XRD patterns of Ni nanowires (NW), Ni oxalate nanowires (Ni Oxa), Ni nanofoam (NF), and RuO₂/Ni NF. (b) TEM image of amorphous RuO₂ with the inset of its selected area electron diffraction pattern.

RuO₂/Ni NF is tested in a two-electrode configuration with Li foil as the counter electrode. CV profiles are measured in 2.0–4.0 V at 0.1 mV s⁻¹ (Fig. 3a). Without RuO₂ coating, Ni NF under O₂ atmosphere shows negligible current, demonstrating that Ni NF neither decompose the electrolyte nor contribute to the charge–discharge capacities. For RuO₂/Ni NF in 1 atm Ar, the CV curve shows rectangular shape without obvious redox peaks, which can be attributed to the lithiation effects^31–33 and pseudocapacitive behaviors^28,29,34 of amorphous RuO₂. Galvanostatic charge–discharge curves of RuO₂/Ni NF in 1 atm Ar (Fig. 3b) showing linear-like zigzag curves without clear plateaus further exhibit capacitive and lithiation characteristics, corresponding to Li_1.57RuO₂ with capacity ca. 316 mA h g⁻¹ based on RuO₂ weight. Once oxygen is introduced, higher current density starting from ca. 2.75 V and reaching maximum at ca. 2.20 V is observed in the cathodic scan, suggesting the formation of lithium oxides, including Li₂O₂ and LiO₂-like species.^35–39 In the anodic scan, the sharp peak at ca. 3.22 V and a broad peak at ca. 3.70 V can be attributed to the oxidation of lithium oxides or hydroxide.^{35,36,38–40} Introduction of O₂ results in the charge–discharge plateaus and capacity improvements for the RuO₂/Ni NF electrode (Fig. 3c), indicating the decomposition and formation of lithium oxides. Charge–discharge capacities at 0.02, 0.05, 0.1 and 0.2 mA cm⁻² are 1.85, 1.00, 0.76 and 0.42 mA h cm⁻², corresponding to 6537.8, 3540.4, 2682.5 and 1501.2 mA h g⁻¹ based on RuO₂ weight, respectively. Round-trip or voltaic efficiencies at 0.02, 0.05, 0.1 and 0.2 mA cm⁻² are 70.11% (2.65 V/3.78 V), 66.58% (2.53 V/3.80 V), 63.28% (2.43 V/3.84 V), and 61.70% (2.40 V/3.89 V), respectively. Lower voltaic efficiencies and higher overpotentials at increased current densities are due to electrode kinetic effects.⁴¹ Stability of the RuO₂/Ni NF electrode is demonstrated in 2.0–4.05 V at 0.2 mA cm⁻² with minute change of the charge–discharge profiles in 75 cycles (Fig. 4a). The capacity retention is ca. 87.7% after 75 cycles (Fig. 4b) with average coulombic efficiency 102.65%, which results from the lower charge capacity attributed to the insufficiently high charge potential and incomplete removal of discharge byproducts. The lower charge voltage is set to prevent excessive electrolyte decomposition at high potentials and to improve electrode stability.⁴ The periodic capacity variation results from the temperature fluctuation^42,43 during a day. The morphology of fully discharged electrode (Fig. S2a and b†) demonstrates that the void space between Ni NF is covered with discharge products. XRD pattern (Fig. S2c†) with peaks at 20.7°, 32.7° and 36.0° demonstrates LiOH (ref. code 01-085-0777)⁴⁰ is the principal discharge product. The existence of LiOH is further confirmed by FTIR measurement (Fig. S2d†).⁴⁴ Recent studies reveal that discharge product of LiO₂ (ref. 45) and Li₂O₂ (ref. 40) can chemically react with H₂O to form LiOH, and low charge potentials are observed for the decomposition of LiOH either by LiI redox mediator⁴⁵ or Ru catalyst.⁴⁰ In this study, the hydrous nature of the RuO₂ nanoflakes is shown in the XPS measurement (Fig. S2e†).²⁷ It is proposed that the LiO₂-like species and Li₂O₂ formed during discharge react with the water of hydrous RuO₂ following similar reaction pathways: 4LiO₂ + 2H₂O → 4LiOH + 3O₂;⁴⁵ Li₂O₂ + 2H₂O → 2LiOH + H₂O₂,⁴⁰ while the RuO₂ might catalyze the LiOH decomposition during charge: 2LiOH → 2Li⁺ + ½O₂ + H₂O + 2e⁻,⁴⁰ which can explain the low charge potentials (mostly below 4 V) of the RuO₂/Ni NF cathode.

Fig. 3 Electrochemical measurements of RuO₂/Ni NF cathode. (a) CV curves of Ni NF in O₂, RuO₂/Ni NF in Ar or O₂. Charge–discharge curves of RuO₂/Ni NF in (b) Ar at 0.02 mA cm⁻² and in (c) O₂ at 0.02–0.2 mA cm⁻².

Fig. 4 (a) Charge–discharge curves of RuO₂/Ni NF in O₂ at 0.2 mA cm⁻² in 2.0–4.05 V for 75 cycles with (b) capacity and coulombic efficiency vs. cycle numbers.

It is noted that in Fig. 3c the discharge curves polarize (rapid voltage drop) at lower current densities (0.02 and 0.05 mA cm⁻²) with larger capacities at the end of the discharge, while polarization does not obviously occur at higher current densities (0.1 and 0.2 mA cm⁻²) with lower capacities. The non-polarized curves and lower capacities at higher current densities are commonly observed in various Li-ion anodes^41,46 and cathodes^47,48 owing to slow ionic diffusion in the active materials.⁴⁹ In addition, deep discharge results in large amount of discharge products, which could cause pore blockage or loss of electrical contacts due to the volume expansion.⁵⁰ Cycling stability could be improved if the deep discharge is prevented. Compared to cycling without deep discharge (Fig. 4a), cycling of the RuO₂/Ni NF electrode at 0.1 mA cm⁻² after 3 full CV cycles in 2.0–4.0 V at 0.1 mV s⁻¹, which are analogous to deep discharge–charge cycles, with capacity limit (0.28 mA h cm⁻²) demonstrates rapid performance fading in terms of end discharge potentials diminishing from 2.21 V to 1.96 V (11.3% loss) in only 13 cycles (Fig. S2f†). Accordingly, cycling without deep discharge promotes the stability of the electrode.

Performance of the carbon-free RuO₂/Ni NF cathode are compared with other non-carbon RuO₂ electrodes and Li-ion cathodes in a Ragone plot (Fig. 5) with energy and power densities based on the total weight of cathode, including RuO₂ catalysts, lithiated lithium and lithium oxides. Energy density 2702.97 W h kg⁻¹ can be reached at 29.22 W kg⁻¹, and it is 1746.32 W h kg⁻¹ when power density increases to 822.20 W kg⁻¹. RuO₂/Ni NF cathode performs better than RuO₂ hollow sphere (HS)¹⁷ and nanosheet (NS),¹⁸ RuO₂ on nanoporous gold (RuO₂/NPG),¹⁶ and traditional Li-ion cathodes, such as LiNi_0.5Mn_0.5O₂,^1,51 LiCoO₂ and LiFePO₄.^1,52 The superior performance can be credited to the intimate electrical contact between Ni NF and RuO₂, and the porous framework allowing access for both the discharge product deposition and short electrolyte pathway for lithium ions and oxygen diffusion.

Fig. 5 Ragone plot of RuO₂/Ni NF, RuO₂ hollow sphere (HS) and nanosheet (NS), RuO₂ on nanoporous gold (RuO₂/NPG), and Li-ion cathodes.

EIS analysis with equivalent circuit model (Fig. 6a) is performed to investigate the RuO₂/Ni NF cathode after cycling (Fig. 6b). Experimental results are fitted by straight lines with fitting parameters (Table 1). Constant phase elements (CPEs) are non-ideal capacitances with Q analogous to capacitance and the ideality factor n. R_s is the equivalent series resistance (ESR) for the electrolyte, current collectors and electrode materials.^53–57 The first parallel branch (R_INT + CPE_INT) accounts for the interfacial contact between RuO₂ catalyst and Ni NF. The second branch describes the solid electrolyte interface (SEI) layers (R_SEI + CPE_SEI) and lithium ion and oxygen diffusion in liquid phase near the electrode surface (CPE_LD). The third branch is responsible for double-layer impedance (CPE_DL), charge transfer resistance (R_CT) at the interface of electrolyte and active materials,⁵⁸ and lithium ion diffusion within RuO₂ (CPE_SD). The first depressed semicircle is attributed to the SEI layers and RuO₂/Ni NF interfaces.^58,59 The second semicircle results from charge transfer impedance.^58,59 The low frequency tail is ascribed to lithium ion diffusion in active materials,⁵⁷ and lithium ion and oxygen diffusion in the electrolyte, represented by CPE_SD and CPE_LD, respectively. CPE for diffusion processes has been successfully utilized in the other systems.^55,60,61

Fig. 6 EIS analysis with (a) equivalent circuit model and (b) Nyquist plots of experimental (symbols) and fitted results (lines).

Table 1 EIS fitting parameters of RuO₂/Ni NF

Cycle	Rs (Ω)	RINT (Ω)	RSEI (Ω)	RCT (Ω)	CPEINT		CPESEI		CPELD		CPEDL		CPESD
					Q (μF s^n−1)	n	Q (μF s^n−1)	n	Q (mF s^n−1)	n	Q (μF s^n−1)	n	Q (mF s^n−1)	n
1^st	12	10	90	950	0.9	0.95	0.8	0.85	6	0.4	7	0.85	9	0.6
2^nd	12.5	10	68	350	0.9	0.95	0.85	0.85	9	0.45	15	0.86	25	0.6
4^th	15	10	73	165	0.9	0.95	0.9	0.85	10	0.5	30	0.86	20	0.6
6^th	15	10	84	110	0.9	0.95	0.8	0.88	15	0.45	50	0.86	25	0.6
9^th	17.5	10	63	109	0.9	0.95	1.5	0.83	20	0.5	60	0.86	30	0.7

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The impedance data fitting demonstrates the electrode improvement after cycling. In our Li–O₂ battery system, O₂ could be reduced into O₂⁻ or O₂²⁻ by the reaction: O₂ + ne⁻ → O₂ⁿ⁻, n = 1 or 2. With higher O₂ concentration, charge transfer reaction is faster, leading to lower R_CT. R_CT decreases from 950 Ω after 1^st cycle and stabilized after 6^th cycles at ca. 110 Ω, indicating facilitated oxygen diffusion pathway^62,63 and less agglomeration of the cathode. R_s slightly increasing from 12 to 17.5 Ω is ascribed to minute accumulation of reaction byproducts. R_SEI and CPE_SEI fluctuation implies the variation of the SEI layer thickness and composition. Constant R_INT and CPE_INT indicate stable contact between RuO₂ and Ni NF. These results show the RuO₂/Ni NF electrode is relatively stable during cycling, and provide evidences for the superior performance of RuO₂/Ni NF cathode due to intimate contact and effective lithium ions and oxygen diffusion in the porous electrode.

4. Conclusion

In conclusion, we have demonstrated the utilization of RuO₂/Ni NF as the cathode for Li–oxygen battery. Charge–discharge capacity as high as 6537.8 mA h g⁻¹ and 1.85 mA h cm⁻² can be reached at 0.02 mA cm⁻² with 70.11% voltaic efficiency. Superior energy and power densities resulting from intimate contact between catalysts and substrate, and the porous nanostructure for oxides deposition and short diffusion pathway, are demonstrated in the Ragone plot and evidenced in the EIS analysis. Stability of the RuO₂/Ni NF cathode is shown in 75 cycles with ca. 87.7% capacity retention, which further renders the RuO₂/Ni NF electrode a promising candidate in Li–air batteries.

Acknowledgements

XPS measurements were recorded at UCR with support from the National Science Foundation under Grant No. DMR-0958796.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13007k

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