Understanding the Critical Role of Carbon Nanotube Surface Chemistry Towards the Electrochemical Behavior in Li-O₂ Cells

The aprotic lithium-oxygen (Li-O2) battery stands alone owing to its highest theoretical specific energy (3.5 kWh/kg) of all battery chemistries.1 The electrochemistry of metallic lithium and gaseous oxygen in the negative and positive electrodes respectively gives the ideal reversible reaction (2Li + O2 ↔ Li2O2, E=2.96 V vs Li/Li+). However, translating the promise of Li-O2 batteries into reality has been stifled by the lack of thorough understanding of the Li2O2 formation and decomposition processes. Additional well-documented challenges relating to electrolyte and electrode instability, low round-trip efficiency, poor reversibility and cycleability, and lower-than-theoretical capacity will need to be tackled before practical Li-O2can be realized. Of note, to achieve the high energy densities desired, it is critical that lightweight materials be used for the positive electrode. Carbon, is the natural choice owing to its numerous advantages including abundance, cost, porosity, conductivity, and weight. Moreover, carbon has been found to be sufficiently active with low kinetic overpotentials for the formation and decomposition of Li2O2 the use of ‘catalysts’ may not be warranted.2 However, several studies have shown side product formation including lithium carbonate and lithium carboxylates originating from carbon-based electrodes, prompting researchers to explore non-carbon electrode materials. The use of non-carbon materials has been met with mixed results with McCloskey et al. demonstrating similar to worse reversibility of nanoporous Au and TiC in comparison to carbon-based electrodes.1 Therefore, provided the management and mitigation of issues relating to instability, carbon remains a promising electrode material. However, surprisingly the influence of the surface characteristics of carbon towards the behavior of Li-O2cells has not been thoroughly investigated. Carbon surfaces contain surface functional groups and vary in morphology with respect to edges, defects and degree of graphitization all of which can have influence on the electrochemical behavior. Here, we highlight the critical role of carbon nanotube surface chemistry towards the behavior of lithium-oxygen (Li-O2) cells by systematically modifying the surface morphology and surface functional groups of multi-walled carbon nanotubes (MWNT). MWNTs are ideal for this study due to their ease of surface modification and ability to fabricate binder-free electrodes. Our study utilizes a variety of techniques including scanning electron microscopy (SEM), x-ray diffraction (XRD), rotating ring disk electrode (RRDE), temperature programmed desorption (TPD) and in situ quantitative gas analysis to understand the differences in discharge capacity, recharge overpotential and reversibility (Figure 1). As seen in Figure 1 discharge capacity is dependent on the degree of disorder of the MWNT surfaces which is found to affect the discharge mechanism related to the adsorption affinity of O2 and soluble Li2O2 intermediates with the MWNT surface. Moreover, the recharge process is found to be dependent on the preceding discharge process, where O-functional groups acts as a promoter in the formation of amorphous Li2O2 which is in intimate contact with the MWNT electrode, enabling the low Li2O2 decomposition potential of 3.0-3.2 V. Our findings have broader implications that can be extended to include the general role of carbon towards capacity, recharge overpotential, and reversibility. Figure 1 – (a) Galvanostatic discharge-recharge profiles performed with 0.5M LiClO4 in tetraglyme at current density 50 mA/g of multi-walled carbon nanotubes with defective edges decorated with O-functional groups (Oxidized CNT), defective CNTs with O-functional groups removed through Ar annealing at 900oC (Oxidized CNT-900), as-received carbon nanotubes (pristine CNT), and lastly pristine CNTs annealed at 2800oC (graphitized CNT) (b) capacity normalized profiles showing trend in recharge overpotential (c) Raman (Id/Ig) ratio versus discharge capacity (d-g) Scanning electron micrographs (SEM) of discharge product morphology following discharge to 2 V. Figure 1

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