Preparation and electrochemical performance of ultra-thin reduced graphene oxide/lithium metal composite foils
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摘要: 超薄锂金属(≤50 μm)是下一代高比能锂金属电池负极选择。然而纯锂质软、易脆,机械加工性较差,导致超薄锂箔的制备工艺复杂、成本高昂;此外相比于较厚的锂金属负极,超薄锂金属负极常呈现更差的电化学循环性能。本文提出一种“自下而上”的策略制备10~50 μm厚度可控的超薄还原氧化石墨烯/锂金属(rGO/Li)复合箔材,其结构由大量无序随机的rGO片层非平行排列并均匀分散在锂金属内。首先将还原氧化石墨烯(rGO)粉片与熔融锂金属在200 °C下搅拌复合,获得微米级的还原氧化石墨烯/锂复合粉片,之后将复合粉片作为原材料进一步通过反复辊压制备出结构均匀、超薄的复合箔材,该方法具有一定的规模化潜力。不同于其他所报道的rGO层状薄膜结构,在复合箔材中rGO片层随机无序分散形成三维网络,有利于实现锂的均匀沉积/剥离。所制50 μm超薄无序结构rGO/Li复合箔材负极在对称电池中以1 mA cm−2、1 mAh cm−2条件在醚基电解液中可稳定循环1600 h以上,在与硫化聚丙烯腈(SPAN)正极组配全电池以0.2 C倍率循环220次后比容量高达~675 mAh g−1,优于使用同厚度纯锂负极的电池。Abstract: Ultra-thin (≤50 μm) lithium metal anodes (LMAs) are highly desirable for high energy density lithium metal batteries (LMBs). However, their fabrication is complicated and costly due to the sticky and brittle nature of metallic Li, and they have a worse cycling stability than their thick counterparts. We report the fabrication of ultra-thin reduced graphene oxide/Li metal (rGO/Li) composite foils with thicknesses ranging from 10 to 50 μm. During the fabrication, disordered rGO sheets and molten metallic Li were stirred at 200 ºC to produce micrometer-size rGO/Li particles, which were rolled to form an ultra-thin uniform composite foil. The rGO sheets were randomly distributed in the composite to form a three-dimensional network, which is different from the laminated rGO structure previously reported, and supported stable Li plating/stripping behavior. As expected, a superior electrochemical performance was achieved using this composite sheet for the anode. A 50 μm-thick rGO/Li composite foil electrode showed stable cycling for > 1 600 h at 1 mA cm−2 and 1 mAh cm−2 in symmetrical cells in an ether-based electrolyte. A full cell consisting of a 50 μm-thick rGO/Li composite foil anode and a sulfurized polyacrylonitrile cathode had a high capacity retention of 675 mAh g−1 after 220 cycles at 0.2 C.
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图 2 (a)rGO粉片;(b)rGO/Li复合粉片;(c)超薄无序结构rGO/Li复合箔材;(d)超薄无序结构rGO/Li复合箔材极片的光学照片;(e)缠绕实验;(f)扭曲实验;(g)剪切实验;(h)复合箔材的XRD谱图;(i)50 μm复合箔材的平面SEM照片;(j)10 μm和(k)50 μm复合箔材的截面SEM图像;对应EDS(l)元素O和(m)元素C的分布图像
Figure 2. Optical images of (a) rGO sheets, (b) rGO/Li composite sheets, (c) ultra-thin rGO/Li composite foil, (d) ultra-thin rGO/Li composite foil electrode. Optical images of (e) winding test, (f) twisting test, and (g) shearing test for 50 μm-thick rGO/Li composite foil. (h) XRD pattern and (i) top-view SEM image of a 50 μm-thick rGO/Li composite foil. Cross-section SEM image of (j) 10 μm-thick and (k) 50 μm-thick rGO/Li composite foil. Corresponding EDS images of (l) O and (m) C elements
图 3 50 μm超薄无序结构rGO/Li复合箔材在醚类电解液中(a)1 mA cm−2、1 mAh cm−2、(b)3 mA cm−2、1 mAh cm−2、(c)1 mA cm−2、3 mAh cm−2时和(d)在酯类电解液中1 mA cm−2、1 mAh cm−2时在对称电池中的电化学性能;内嵌图为标注的不同循环时的时间-电压曲线
Figure 3. Electrochemical cycling of 50 μm-thick rGO/Li composite electrode in symmetric cells at (a) 1 mA cm−2 with a fixed capacity of 1 mAh cm−2, (b) 3 mA cm−2 with a fixed capacity of 1 mAh cm−2, and (c) 1 mA cm−2 with a fixed capacity of 3 mAh cm−2 in ether-based electrolyte, and (d) 1 mA cm−2 with a fixed capacity of 1 mAh cm−2 in carbonate-based electrolyte. Inset figures show the potential curves for selected cycles
图 4 50 μm超薄无序结构rGO/Li复合箔材在(a)未循环前和(b)循环50次时对称电池的电化学阻抗谱(酯类电解液);(c)超薄无序结构rGO/Li复合箔材和(d)纯锂在首次锂剥离后的SEM图像;(e)超薄无序结构rGO/Li复合箔材和(f)纯锂在首次锂沉积后的SEM图像
Figure 4. EIS of symmetric cells with 50 μm-thick rGO/Li composite foil electrodes (a) before cycling, (b) after 50 cycles in carbonate-based electrolyte. SEM images of (c) 50 μm-thick rGO/Li composite foils and (d) pure lithium foils after initial lithium stripping. SEM images of (e) 50 μm-thick rGO/Li composite foils and (f) pure lithium foils after initial lithium plating
图 5 SPAN // 超薄rGO/Li复合箔材(50 μm)的全电池电化学性能:(a)循环-比容量和循环-库伦效率曲线;(b)1st、(c)5th、(d)100th、(e)150th循环的比容量-电压曲线
Figure 5. Electrochemical performance of SPAN // rGO/Li (50 μm) cell. (a) Specific capacity-cycle number plot and the corresponding CE on cycling. Corresponding voltage-specific capacity plots for the (b) 1st, (c) 5th, (d) 100th, and (e) 150th cycle
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