School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of MOE and Jiangsu Provincial Laboratory for Nanotechnology, Nanjing University, Nanjing 210023
Received Date:
24 March 2020 Available Online:
15 June 2020
Fund Project:
Project supported by the jointly financial support from the National Key Research and Development Program of China (Nos. 2018YFA0209100, 2017YFA0206500) and the National Natural Science Foundation of China (Nos. 21773111, 21972061, 21832003, 21573107 and 51571110)
Abstract:
Li-O2 batteries have received much attention due to the high theoretical energy density. However, they still suffer from many challenges such as unsatisfactory practical specific capacity, cycle stability, and relatively high overpotential. The electrochemical performance of Li-O2 batteries is closely related to the reversibility of the discharge (oxygen reduction reaction, ORR) and charge (oxygen evolution reaction, OER) processes. During the discharge process, non-conductive Li2O2 product is formed and gradually covers on the surface of the positive electrode material, leading to the deactivation of battery. The charging process is accompanied by the electrochemical decomposition of Li2O2 products. Therefore, how to achieve the highly reversible formation and decomposition of the Li2O2 product is the key to improve the electrochemical performance of Li-O2 batteries. To date, two strategies have been developed:(ⅰ) sp2 carbon materials with large specific surface area, suitable pore structure and high conductivity are used as cathode materials to disperse/accommodate the Li2O2 product and promote the electron transfer; (ⅱ) the soluble redox mediators with ORR/OER bifunctionally catalytic activities are adopted as the electrolyte additive to promote the formation and decomposition of the Li2O2 product and lower the overpotentials. Recently, we reported a novel 3D hierarchical carbon nanocages (hCNC) featuring on the ultrahigh specific surface area, multiscale pore structure (micro-meso-macropore coexistence), high conductivity, and abundant defects, which demonstrated the excellent electrochemical performances in energy conversion and storage. Herein, taking advantages of hCNC, the high performances of Li-O2 batteries were fabricated, showing high full discharge specific capacity (14080 mAh·g-1) and good cyclability. After adding acetylacetone ferrous (Fe(acac)2) as the redox mediator to electrolyte, the electrochemical performances are further promoted. Namely, the discharge capacity reaches to 23560 mAh·g-1 at the current density of 0.1 A·g-1 (7.82 times of XC-72), and the cycle numbers are up to 138 cycles at the current density of 0.5 A·g-1 and the discharge/charge depth of 800 mAh·g-1 (far higher than 68 cycles of hCNC without Fe(acac)2 and 13 cycles of XC-72). Especially, at the high current density of 5.0 A·g-1, the cycle numbers still reach to 63 cycles, far higher than 21 cycles of hCNC without Fe(acac)2. Such excellent electrochemical performances can be ascribed to:the unique structure of hCNC facilitating electron transfer, reversible conversion of 2Li++O2+2e-⇆Li2O2(s), and dispersion/accommodation of the insulating Li2O2 product; the soluble redox mediator of Fe(acac)2 effectively catalyzing the discharge products of Li2O2 to form uniformly dispersed small-sized particles and decompose completely during the charge process. This provides a promising strategy for improving the performance of Li-O2 batteries via designing novel carbon-based positive electrode materials and adding efficient soluble redox mediators.
Figure 1.
Morphology and structure characterizations of hCNC. (a, b) Typical low- and high-magnification SEM images. (c, d) Typical TEM and HRTEM images. (e, f) N2 adsorption/desorption isotherms and pore size distribution. As contrast, the curves of XC-72 are plotted in (e, f)
图 2.
以hCNC和XC-72为正极材料和是否添加Fe(acac)2的Li-O2电池的完全放电性能图.电流密度0.1 A·g－1, 放电截止电压2.0 V (vs. Li+/Li)
Figure 2.
Discharge curves of Li-O2 batteries for hCNC and XC-72 with and without Fe(acac)2 redox mediator at a current density of 0.1 A·g－1 and a terminated discharge voltage of 2.0 V (vs. Li+/Li)
Figure 3.
Cyclic stability of hCNC-based Li-O2 batteries with and without Fe(acac)2 redox mediator at four typical current densities. Terminated discharge/charge capacity: 800 mAh·g－1. Terminated discharge/charge voltage: 2.0 and 5.0 V (vs. Li+/Li), respectively. Current densities: 0.5, 1.0, 2.0 and 5.0 A·g－1
Figure 4.
Morphological and structural evolution of hCNC-based Li-O2 batteries with and without Fe(acac)2 redox mediator at the first discharge/charge cycle. (a~c) SEM images and XRD patterns of the batteries with Fe(acac)2 after discharging and charging. XRD pattern of pristine hCNC is plotted in (c). (d, e) SEM images of the batteries without Fe(acac)2 after discharging and charging. Current density: 0.1 A·g－1. Discharge depth: 10000 mAh·g－1
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Figure 1
Morphology and structure characterizations of hCNC. (a, b) Typical low- and high-magnification SEM images. (c, d) Typical TEM and HRTEM images. (e, f) N2 adsorption/desorption isotherms and pore size distribution. As contrast, the curves of XC-72 are plotted in (e, f)
图 2
以hCNC和XC-72为正极材料和是否添加Fe(acac)2的Li-O2电池的完全放电性能图.电流密度0.1 A·g－1, 放电截止电压2.0 V (vs. Li+/Li)
Figure 2
Discharge curves of Li-O2 batteries for hCNC and XC-72 with and without Fe(acac)2 redox mediator at a current density of 0.1 A·g－1 and a terminated discharge voltage of 2.0 V (vs. Li+/Li)
Figure 3
Cyclic stability of hCNC-based Li-O2 batteries with and without Fe(acac)2 redox mediator at four typical current densities. Terminated discharge/charge capacity: 800 mAh·g－1. Terminated discharge/charge voltage: 2.0 and 5.0 V (vs. Li+/Li), respectively. Current densities: 0.5, 1.0, 2.0 and 5.0 A·g－1
Figure 4
Morphological and structural evolution of hCNC-based Li-O2 batteries with and without Fe(acac)2 redox mediator at the first discharge/charge cycle. (a~c) SEM images and XRD patterns of the batteries with Fe(acac)2 after discharging and charging. XRD pattern of pristine hCNC is plotted in (c). (d, e) SEM images of the batteries without Fe(acac)2 after discharging and charging. Current density: 0.1 A·g－1. Discharge depth: 10000 mAh·g－1
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