Influence of Ion Diffusion on the Lithium-Oxygen Electrochemical Process and Battery Application Using Carbon Nanotubes-Graphene Substrate

Abstract

Lithium-oxygen (Li-O₂) batteries are nowadays among the most appealing next-generation energy storage systems in view of a high theoretical capacity and the use of transition-metal-free cathodes. Nevertheless, the practical application of these batteries is still hindered by limited understanding of the relationships between cell components and performances. In this work, we investigate a Li-O₂ battery by originally screening different gas diffusion layers (GDLs) characterized by low specific surface area (<40 m² g^-1) with relatively large pores (absence of micropores), graphitic character, and the presence of a fraction of the hydrophobic PTFE polymer on their surface (<20 wt %). The electrochemical characterization of Li-O₂ cells using bare GDLs as the support indicates that the oxygen reduction reaction (ORR) occurs at potentials below 2.8 V vs Li⁺/Li, while the oxygen evolution reaction (OER) takes place at potentials higher than 3.6 V vs Li⁺/Li. Furthermore, the relatively high impedance of the Li-O₂ cells at the pristine state remarkably decreases upon electrochemical activation achieved by voltammetry. The Li-O₂ cells deliver high reversible capacities, ranging from ∼6 to ∼8 mA h cm^-2 (referred to the geometric area of the GDLs). The Li-O₂ battery performances are rationalized by the investigation of a practical Li⁺ diffusion coefficient (D) within the cell configuration adopted herein. The study reveals that D is higher during ORR than during OER, with values depending on the characteristics of the GDL and on the cell state of charge. Overall, D values range from ∼10^-10 to ∼10^-8 cm² s^-1 during the ORR and ∼10^-17 to ∼10^-11 cm² s^-1 during the OER. The most performing GDL is used as the support for the deposition of a substrate formed by few-layer graphene and multiwalled carbon nanotubes to improve the reaction in a Li-O₂ cell operating with a maximum specific capacity of 1250 mA h g^-1 (1 mA h cm^-2) at a current density of 0.33 mA cm^-2. XPS on the electrode tested in our Li-O₂ cell setup suggests the formation of a stable solid electrolyte interphase at the surface which extends the cycle life.

Keywords: Li−O2 battery; MWCNTs; cycle life; diffusion; energy storage; few-layer graphene.

Figure 1

(a–h) SEM images of the (a,b) 22BB, (c,d) 28BC, (e,f) 36BB, and (g,h) 39BB GDLs acquired in either (a,c,e,g) back-scattered electrons or (b,d,f,h) secondary electrons mode; insets in panels (b,d,f,h) show the corresponding EDS elemental maps for C and F. (i) XRD patterns measured for 22BB (green), 28BC (cyan), 36BB (blue), and 39BB (red). The reference pattern for graphite (black, ICSD #230104) is also reported for comparison.

Figure 2

(a) TGA curves measured under N₂ flow in the 25–1000 °C temperature range for 22BB (green), 28BC (cyan), 36BB (blue), and 39BB (red); (b–e) N₂ absorption/desorption isotherms for (b) 22BB, (c) 28BC, (d) 36BB, and (e) 39BB, used to estimate the specific surface area and pore size characterization (see Table 1).

Figure 3

(a,c,e,g) CV curves and (b,d,f,h) Nyquist plots recorded before and after each CV cycle [see insets in panels (b,d,f,h) for OCV condition] measured for Li–O₂ cells using (a,b) 22BB, (c,d) 28BC, (e,f) 36BB, or (g,h) 39BB as cathodes. CV potential range: 2.5–4.2 V vs Li⁺/Li; scan rate: 0.05 mV s^–1. EIS frequency range: 500 kHz–100 mHz; alternate voltage signal: 10 mV.

Figure 4

Voltage profiles during galvanostatic charge/discharge cycling measured for the Li–O₂ cells using (a) 22BB, (b) 28BC, (c) 36BB, or (d) 39BB as cathodes, at a constant current of 0.2 mA and limiting the cell capacity to 2 mA h (1 mA h cm^–2 considering the GDL geometric area of 2.0 cm²). Bottom x-axes report the geometrical areal capacity (mA h cm^–2), while top x-axes show the cell capacity (mA h). Maximum voltage range: 1.5–4.8 V.

Figure 5

(a–d) CV curves, (e–h) Nyquist plots recorded by EIS, and (i–l) voltage profiles during galvanostatic charge/discharge cycling measured for the Li–O₂ cells using (a,e,i) 22BB, (b,f,j) 28BC, (c,g,k) 36BB, or (d,h,l) 39BB as cathodes. CV potential range 1.5–4.3 V vs Li⁺/Li; scan rate: 0.05 mV s^–1. EIS carried out after each CV scan in the 500 kHz to 100 mHz frequency range. Voltage profile measured for the investigated cells during galvanostatic charge/discharge cycling at 0.2 mA and voltage between 1.5 and 4.5 V with no cell capacity limitation; in panels (i–l), bottom x-axes report the geometrical areal capacity (mA h cm^–2), while top x-axes show the cell capacity (mA h).

Figure 6

(a,c,e,g) GITT curves reporting the potential vs x and (b,d,f,h) D trends calculated by GITT equation (eq 1)^,, at various SOCs for the Li–O₂ cells using (a,b) 22BB, (c,d) 28BC, (e,f) 36BB, or (g,h) 39BB as cathodes (see maximum and minimum D values in in Table S1 in Supporting Information and the potential vs time GITT curves in Figure S7). Square current pulse: 0.4 mA; time of pulse: 1 h; potential relaxation step time: 1 h; and potential range: 1.5–4.5 V vs Li⁺/Li.

Figure 7

(a–c) SEM images at various magnifications of the electrode using the MWCNTs/FLG mixture coated onto the 39BB GDL (see the Experimental Section); (d,f) voltage profiles; and (e,g) corresponding specific capacity with Coulombic efficiency trends measured for Li–O₂ cells using the 39BB GDL coated with MWCNTs/FLG as the cathode [MWCNTs/FLG loading of either (d,e) 0.8 mg cm^–2 or (f,g) 1.0 mg cm^–2 considering the GDL geometric area of 2.0 cm²]. The batteries are cycled at a constant current of 0.66 mA by limiting the capacity to either (d,e) 2 mA h (1 mA h cm^–2) or (f,g) 1 mA h (0.5 mA h cm^–2). In panels (d,f), bottom x-axes report the geometrical areal capacity (mA h cm^–2), while top x-axes show the specific capacity (mA h g^–1); in panels (e,g), right y-axis displays the Coulombic efficiency. Voltage range: 1.5–4.8 V.

Figure 8

XPS measurements of the 39BB GDL coated with MWCNTs/FLG at the pristine state and after three cycles in Li–O₂ cells at a constant current of 0.66 mA and capacity limited to 2 mA h (see the Experimental Section for details). In particular: (a) survey spectra and (b–f) high-resolution signals acquired in the (b) C 1s, (c) O 1s, (d) F 1s, (e) S 2p, and (f) Li 1s regions.