Received Date:
12 January 2020 Available Online:
15 May 2020
Fund Project:
Project supported by the jointly financial support from the National Key Research and Development Program of China (2018YFA0209100, 2017YFA0206500) and the National Natural Science Foundation of China (21773111, 21972061, 21832003, 21573107, 51571110)
Abstract:
Rechargeable magnesium batteries (rMBs) are promising next-generation secondary batteries owing to the low-cost, high safety and dendrite-free property of Mg metal. The key of rMBs technology is to develop high-performance cathode materials. Usually, the intercalation-type cathodes such as Mo6S8, MoS2 and Ti3C2Tx suffer from the inferior rate performance owing to the sluggish Mg2+ ions solid-diffusion kinetics, and the conversion-type cathodes such as S and CuS are beset with the poor cycling stability owing to the pulverization and loss of active species. Recently, sp2 carbon materials exhibited considerable magnesium storage performance through an interfacial charge storage/release. Ideal carbon-based cathodes for rMBs should possess the features of high specific surface area and abundant active sites for magnesium storage, high conductivity and porous structure for facilitating charge transfer, as well as high mechanical stability. Herein, we employed the hierarchical nitrogen-doped carbon nanocages (hNCNC) featuring large surface area, abundant surface defects, coexisting micro-meso-macropores and high conductivity as the rMBs cathode for the first time, which exhibited high discharge capacity of 71 mAh·g-1 at 100 mA·g-1, excellent rate performance (60 mAh·g-1 at 2000 mA·g-1) and ultra-high cycling stability (83% capacity retention after 1000 cycles at 1000 mA·g-1). The capacitive magnesium storage mechanism is predominant in the charging-discharging process. Theoretical studies reveal that magnesium ions are adsorbed on the carbon, pyridinic-nitrogen or pyrrolic-nitrogen atoms at the edge of micropores. The excellent magnesium storage performance of hNCNC is attributed to the following reasons:(i) the hNCNC with large surface area (1590 m2·g-1), abundant micropore defects and high content of pyridinic and pyrrolic nitrogen (4.49 at.%) provides sufficient active sites for magnesium storage, resulting in the high discharge capacity; (ii) the coexisting micro-meso-macropores structure, good conductivity and improved wettability via N-doping facilitate the charge transfer kinetics, and decrease the equivalent series resistance of rMBs, thereby leading to the improved rate capability; (iii) the robust scaffold of hNCNC and the capacitive-dominated magnesium storage mechanism ensure the high cycling stability. This study demonstrates the high-rate and durable performance of hNCNC in rMBs, and suggests a promising strategy to improve the rMBs performance by increasing edges and suitable dopants of nanocarbons.
Figure 1.
Cell structure of rMBs and the morphology and structure characterizations of hNCNC. (a) Schematic illustration of hNCNC||APC||Mg battery. (b) SEM and (c, d) TEM images of hNCNC. (e) N2 adsorption/desorption isotherms of hNCNC, hCNC and CNT. Inset is the corresponding pore size distributions. (f) N1s XPS spectrum of hNCNC
Figure 2.
Electrochemical performances of rMBs. (a) CV curves of hNCNC, hCNC and CNT. Scan rate: 0.2 mV·s－1. (b) Cycling performances at 100 mA·g－1. (c) Discharge/charge profiles of hNCNC at typical current densities. (d) Rate capabilities of hNCNC and hCNC. (e) Cycling stability of hNCNC at 1000 mA·g－1
Figure 3.
Quantitative kinetic analysis of hNCNC electrode. (a) CV curves at the scan rates of 0.2, 0.3, 0.5, 0.7, 0.9 and 1.0 mV·s－1. (b) Calculated b-values as a function of potential. Inset is the examples of linear fitting of log i versus log v, where the b-values are the slopes. (c) Extracted capacitive contribution (shaded area) of CV curve at 0.5 mV·s－1 as an example. (d) Ratios of the capacitive-controlled capacity versus scan rates
Figure 4.
DFT calculations of the adsorption energy of solvated Mg2+ ions on the five typical configurations: graphite plane with micropore (ca. 0.6 nm), micropore edge decorated by pyridinic N, micropore edge decorated by pyrrolic N, graphitic N embedded in graphite plane and pristine graphite plane
Figure 5.
Wettability measurement of cathodes and electrochemical impedance spectroscopies (EIS) of rMBs. (a) Dynamic contact angle measurements for hNCNC and hCNC with the electrolyte (0.4 mol·L－1 PhMgCl-AlCl3-THF). (b) The Nyquist plots of hNCNC- and hCNC-based rMBs after 500 cycles at 1000 mA·g－1. Frequency region: 100 kHz~0.1 Hz. The upper inset is the equivalent circuit and the lower inset is the enlargements in the high-frequency region. (c) Z'-ω－1/2 curves in the low-frequency region. The slopes are corresponding to the Warburg coefficient σ
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Figure 1
Cell structure of rMBs and the morphology and structure characterizations of hNCNC. (a) Schematic illustration of hNCNC||APC||Mg battery. (b) SEM and (c, d) TEM images of hNCNC. (e) N2 adsorption/desorption isotherms of hNCNC, hCNC and CNT. Inset is the corresponding pore size distributions. (f) N1s XPS spectrum of hNCNC
Figure 2
Electrochemical performances of rMBs. (a) CV curves of hNCNC, hCNC and CNT. Scan rate: 0.2 mV·s－1. (b) Cycling performances at 100 mA·g－1. (c) Discharge/charge profiles of hNCNC at typical current densities. (d) Rate capabilities of hNCNC and hCNC. (e) Cycling stability of hNCNC at 1000 mA·g－1
Figure 3
Quantitative kinetic analysis of hNCNC electrode. (a) CV curves at the scan rates of 0.2, 0.3, 0.5, 0.7, 0.9 and 1.0 mV·s－1. (b) Calculated b-values as a function of potential. Inset is the examples of linear fitting of log i versus log v, where the b-values are the slopes. (c) Extracted capacitive contribution (shaded area) of CV curve at 0.5 mV·s－1 as an example. (d) Ratios of the capacitive-controlled capacity versus scan rates
Figure 4
DFT calculations of the adsorption energy of solvated Mg2+ ions on the five typical configurations: graphite plane with micropore (ca. 0.6 nm), micropore edge decorated by pyridinic N, micropore edge decorated by pyrrolic N, graphitic N embedded in graphite plane and pristine graphite plane
Figure 5
Wettability measurement of cathodes and electrochemical impedance spectroscopies (EIS) of rMBs. (a) Dynamic contact angle measurements for hNCNC and hCNC with the electrolyte (0.4 mol·L－1 PhMgCl-AlCl3-THF). (b) The Nyquist plots of hNCNC- and hCNC-based rMBs after 500 cycles at 1000 mA·g－1. Frequency region: 100 kHz~0.1 Hz. The upper inset is the equivalent circuit and the lower inset is the enlargements in the high-frequency region. (c) Z'-ω－1/2 curves in the low-frequency region. The slopes are corresponding to the Warburg coefficient σ
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