Lithium-ion batteries (LIBs) have received considerable attention in portable electronics. Uneven plating/stripping and uncontrolled dendrite growth of Li that could induce internal short circuits and explosion of the battery are the major limitations. Researchers at CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, Chinese Academy of Sciences (CAS), School of Chemistry and Chemical Engineering University of Chinese Academy of Sciences (CAS) and Beijing Institute of Nanoenergy and Nanosystems, China have developed a novel porous Cu current collector with vertically aligned microchannels (VAMCs), which regulates the current density distributions to prevents dendrite growth of Li metal anodes.
Vertically aligned Cu micro-channels with varying pore radius, pore depth and pore spacing were fabricated using a laser micro-processing system (Fig. 1(a)). The sample of porous Cu with a pore radius of 5 μm, a pore depth of 50 μm, and a pore spacing of 12 μm is designated as porous Cu-5-50-12. The porous Cu current collector with VAMCs due to its large specific surface area and low local current density successfully prevents the dendrite growth of Li. The large surface area of VAMCs enables uniform deposition of Li not only on their surface but also in the microchannels.
For VAMCs with a fixed pore radius of 5 μm, for a pore spacing of 10 μm the current efficiency is lower at locations away from the channels (Fig. 1(b)). When the pore spacing is decreased to 6 and 2 μm, the current efficiency at these locations is increased but the extent of increase is much lower than those experienced in the mouth of the channels (Figs. 1(c) and 1(d)). Since the current density within the microchannels is much larger than that on the upper surface of the porous Cu, preferential nucleation of Li occurs inside the mouth of channels (Fig. 1(e)).
Fig. 1 (a) Schematic of the porous Cu current collectors; (b–d) current density distribution on the surface of porous Cu collectors obtained from COSMOL simulation: (b) Cu-5-50-20; (c) Cu-5-50-16; (d) Cu-5-50-12; and (e) schematic diagram depicting preferential deposition of Li inside the mouth of channels.
For VAMCs with a fixed pore radius of 5 μm, an increase in pore depth increases the current density around the entire mouth of the channel. The current density distribution gradient is inevitable, which helps to accommodate most of the Li inside the channels. Hence, systems having a highest current efficiency in the pores and lowest current efficiency at the locations away from the pores is expected to effectively suppress the Li dendrite formation.
The morphology of Li deposits formed using 1 M lithium bis(trifluoromethane-sulfonyl)imide in 1:1 1,3-dioxolane/1,2-dimethoxyethane as the electrolyte containing 1 wt% LiNO3 on porous Cu with different pore radii as well as those formed on planar Cu at 3 mA h/cm2 is compared (Figs. 2(a)–2(e)). Li deposits on porous Cu-5-50-12 indicate enrichment of Li in the VAMCs (Fig. 2(a)) while an increase in pore radius decreased the ability of VAMCs to restrict Li deposition within the microchannels (Figs 2(b)-2(d)). Formation of Li deposits with a spherical shape could not be observed on planar Cu (Fig. 2e). The voltage profiles of Li deposition on porous and planar Cu current collectors (Fig. 2(f)) indicate a lower overpotential of ≈144 mV for porous 5-50-12 whereas for planar Cu, the overpotential is 280 mV under similar conditions. The lower overpotential values obtained for porous Cu points out a decrease in local current density due to the larger specific surface area of the VAMCs.
Fig. 2 Morphology of Li deposits formed on porous and planar Cu current collectors: (a–d) SEM images of Li deposits formed on the porous Cu with varying pore radii: (a) 5 μm; (b) 7.5 μm; (c) 10 μm; (d) 15 μm; (e) SEM image of Li deposits form on the planar Cu; and (f) Voltage profiles of Li deposition on Cu current collectors.
Galvanostatic cycling measurements performed using symmetrical cells indicate that porous Cu-7.5-50-17 possesses a low voltage hysteresis of ≈20 mV and improved cycling stability even after 300 h. In contrast, under similar conditions, planar Cu exhibits a gradual rise in voltage hysteresis after 50 h. The cycling stability and CE of porous Cu collectors are also ascertained by cells assembled using commercial lithium foil as the counter electrodes. For porous Cu-5-50-12, the CE of the cell remains stable for 200 cycles with an average CE of 98.5%. In contrast, for planar copper, the CE of the cell reaches 98.2% after 13 cycles and exhibits a rapid decline to 68.1% after 81 cycles, with an average CE of 94%.
Full cell galvanostatic cycling of Li/LFP cells performed using planar Cu as well as porous Cu-5-50-12 anodes indicate that cells with porous Cu anode exhibits good cycling behavior, delivering a capacity of 134 mA h/g (Fig. 3(a) with a capacity retention of ≈90% after 100 cycles (Fig. 3(b). The cell with planar Cu anode exhibits a larger polarization effect of 177 mV (Fig. 3(a)) and a poor capacity retention of 80% after 100 cycles (Fig. 3(b)).
Fig. 3 (a, b) Electrochemical performance of Li/LiFePO4 cells with (a) porous Cu-5-50-12 anode; and (b) planar Cu anode; and (c) cycling performance of the Li/LFP cells
The ability of porous Cu current collector with VAMCs to control the dendrite growth of Li is mainly due to the larger specific surface area, lower local current density, restriction in lithium volume change, lower charge transfer resistance, and high Li+ transport properties in the cell. The porous structure served as a cage for Li, thus accommodating large amounts of Li in the pores.
T.SN. Sankara Narayanan
For more information, the reader may kindly refer: Shu-Hua Wang et al., Stable Li Metal Anodes via Regulating Lithium Plating/Stripping in Vertically Aligned Microchannels, Adv. Mater. 2017, 1703729, DOI: 10.1002/adma.201703729
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