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摘要: 近年來，提出了一種高效、環境友好的熔鹽電化學轉化方法，可將碳污染物直接轉化為高附加值的石墨化產物。本文綜述了熔鹽電化學石墨化的工藝流程、產物的結構特征與轉化機理。詳細介紹了碳納米材料在鋰離子電池和鋁離子電池等二次電池中的應用前景，突出了轉化和利用豐富的二次碳資源實現高附加值應用的高效策略。最后，對開發熔鹽電化學石墨化與規模化低能耗電解技術、構建先進高溫熔鹽電化學原位表征技術與定量化分析方法、深入研究電化學石墨化微觀轉化機理、推動石墨化產品的工程化應用進行了分析與展望。Abstract: In 2020, the Chinese government proposed the goals of “peaking carbon dioxide emissions” in 2030 and reaching “carbon neutrality” in 2060, with the expectation of enhancing the optimization of industrial structure and energy structures as well as promoting the development of control technologies and new energy technologies for pollution prevention. Carbon emissions lead to global warming, glacier melting, sea level rising, and other unexpected climate changes. It is highly significant to develop sustainable technologies for treating or converting carbon dioxide and low value-added solid carbon wastes and other carbon pollutants to achieve solid-state valuable carbon products. Carbon pollutants are also regarded as secondary carbon resources, which provide sufficient raw materials for developing carbon materials. Graphitization alters the chemical structure of carbonaceous materials. However, there are still some critical issues in the traditional graphitization processes, such as high processing temperature, insufficient graphitization, and emission of greenhouse gas. In recent years, an efficient and environmentally friendly method for electrochemical graphitization in molten salts has been established, which can be used to directly convert carbon pollutants into high graphitized products. In this review, there are three main topics: (1) process flow, (2) structure characteristics, (3) conversion mechanism of electrochemical graphitization. The use of carbon nanomaterials in secondary batteries such as lithium-ion batteries and aluminum-ion batteries has been discussed for a potential application. As a result, the efficient strategies of transforming and utilizing abundant secondary carbon resources to achieve the applications have also been analyzed. Finally, the ultimate goals for bridging the gap between molten salt electrochemical graphitization and engineering of graphitized products have been identified. Further efforts should be made to develop large-scale electrolytic technology with low energy consumption, build advanced in-situ characterization technology and quantitative analysis method for high-temperature molten salt electrochemistry, and understand the mechanism of electrochemical graphitization at the microscale.
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圖 1 （a）熔融鹽中無定形碳的電化學驅動石墨圖解；（b）炭黑(CB)在820 ℃的CaCl2中在2.6 V下電解1 h前后的拉曼光譜；（c）炭黑（CB）在820 ℃的CaCl2中在不同電壓下電解2 h的X射線衍射譜圖；（d）炭黑（CB）在不同溫度下的CaCl2中在2.4 V電壓下電解2 h的X射線衍射譜圖[11-12]
Figure 1. (a) Electrochemical driven graphite diagram of amorphous carbon in molten salt; (b) Raman spectra of carbon black (CB) before and after electrolysis in CaCl2 at 820 ℃ at 2.6 V for 1 h; (c) XRD spectra of carbon black (CB) electrolyzed in CaCl2 at 820 ℃ for 2 h under different voltages; (d) XRD spectra of carbon black (CB) electrolyzed in CaCl2 at different temperatures at 2.4 V for 2 h[11-12]
圖 2 （a）不同溫度下樣品的X射線衍射圖譜；（b）不同溫度下樣品的拉曼圖譜；不同溫度下電解所得樣品的SEM形貌：（c）原始石油焦；（d）800 ℃；（e）850 ℃；（f）900 ℃；（g）泡沫鎳包裹和鐵絲網包裹的石油焦恒壓電解脫硫時的電流?時間曲線；（h）不同包裹方式的產物X射線衍射圖譜[17]
Figure 2. (a) XRD patterns of samples at different temperatures; (b) Raman spectra of samples at different temperatures; SEM morphology of samples obtained by electrolysis at different temperatures: (c) original petroleum coke; (d) 800 ℃; (e) 850 ℃; (f) 900 ℃; (g) current?time curves of petroleum coke coated with nickel foam and barbed wire during constant pressure electrolytic desulfurization; (h) XRD patterns of products with different wrapping methods[17]
圖 3 （a）通過熱萃取和熔鹽電解將低階煤轉化為石墨的典型合成路線；（b）原煤（AC）、超精煤（HPC）和合成石墨樣品的石墨化程度；（c）HPC和合成石墨的ID/IG值；（d）950 ℃、2.8 V、6 h電解條件下產物的掃描電鏡圖[18]
Figure 3. (a) A typical synthetic route for converting low-rank coal into graphite by thermal extraction and molten salt electrolysis; (b) graphitization degree of raw coal (AC), ultra-clean coal (HPC) and synthetic graphite samples; (c) ID/IG values of HPC and synthetic graphite; (d) SEM image of the product at 950 ℃, 2.8 V and 6 h[18]
圖 4 （a）纖維素資源的電化學石墨化過程；（b） 纖維素資源在不同溫度下電解X射線衍射圖譜；（c） 纖維素資源在不同溫度下電解Raman圖譜；（d）纖維素材料的掃描電鏡圖；（e）含碳前驅體的掃描電鏡圖；（f）950 ℃、2.8 V、8 h電解條件下的掃描電鏡圖[20]
Figure 4. (a) Electrochemical graphitization process of cellulose resources; (b) electrolysis XRD patterns of cellulose resources at different temperatures; (c) electrolysis Raman spectra of cellulose resources at different temperatures; (d) SEM images of cellulosic materials; (e) SEM images of carbon-containing precursors; (f) SEM images under electrolysis conditions of 950 ℃, 2.8 V and 8 h[20]
圖 5 電化學產生的納米結構石墨（EGN）和商用石墨納米片，石墨粉在室溫下的電化學性能.（a）第二圈 CV （10 mV·s?1）；（b）第 150 圈 CV（10 mV·s?1）；（c）比放電容量和200 次循環的庫侖效率（1800 mA·g-1）；（d）倍率性能；（e）1800 mA 時的比放電容量和庫侖效率，而倍率性能測試后，上限充電電位提高到5.25 V；（f）在 2.25 和 5.25 V 之間的充電/放電曲線[11]
Figure 5. Electrochemical performance of electrochemically generated nanostructured graphite (EGN graphite) and commercial graphite nanosheet and graphite powder in Pyr14TFSI at room temperature: (a) 2nd CVs (10 mV·s?1); (b) 150th CVs (10 mV·s?1); (c) specific discharge capacity and coulombic efficiency over 200 cycles (1800 mA·g?1); (d) rate performances; (e) specific discharge capacities and coulombic efficiencies at 1800 mA while the upper limit charging potential is increased to 5.25 V after the rate performance tests; (f) charge and discharge curves for (e) between 2.25 and 5.25 V[11]
圖 6 合成石墨材料的電化學性能。（a）950 ℃、2.8 V、6 h電解條件下產物在1 mV·s?1掃描速率下的 CV 曲線；（b）950 ℃、2.8 V、6 h電解條件下產物從 0.1C~1C范圍的充放電曲線（GCD）；（c）當前速率為 2 C 時的循環性能和庫侖效率；（d）950 ℃、2.8 V、6 h電解條件下產物在各種條件下的速率能力速率范圍從0.1C到5C；（e）950 ℃、2.8 V、6 h電解條件下產物的奈奎斯特圖（插圖：所研究系統的等效電路模型）；（f）比較文獻中報道的比容量與本研究中報道的比容量[18]
Figure 6. Electrochemical properties of the synthesized graphite materials: (a) CV curves of the products at a scanning rate of 1 mV·s?1 at 950 ℃, 2.8 V and 6 h; (b) GCD curve of the product in the range of 0.1C to 1C under the electrolysis conditions of 950 ℃, 2.8 V and 6 h; (c) cyclic performance and coulomb efficiency at the current rate of 2C; (d) rate capability of the product at 950 ℃, 2.8 V and 6 h under various conditions ranges from 0.1C to 5C; (e) Nyquist diagram of the product under electrolysis conditions of 950 ℃, 2.8 V and 6 h (illustration: equivalent circuit model of the studied system); (f) compare the specific capacity reported in literature with that reported in this study[18]
圖 7 鋁離子電池用轉化片狀石墨的電化學性能。（a）5 mV·s?1條件下樣品的CV曲線；（b）950 ℃、2.8 V、6 h電解條件下產物前3圈的充放電曲線；（c）950 ℃、2.8 V、6 h電解條件下產物正極在25、50和第 75 次循環充電/放電曲線；（d）樣品陰極在 50 到 400 mA·g?1的不同電流密度下的倍率能力；（e）電流密度為 100 mA·g?1時的循環性能[20]
Figure 7. Electrochemical properties of converted flake graphite for aluminum ion battery: (a) CV curves of the samples at 5 mV s?1; (b) charge and discharge curves of the first three cycles of the product at 950 ℃, 2.8 V and 6 h; (c) charge/discharge curves of the product anode in the 25th, 50th and 75th cycles under the electrolysis conditions of 950 ℃, 2.8 V and 6 h; (d) rate capability of the sample cathode at different current densities of 50 to 400 mA·g?1; (e) cycle performance at a current density of 100 mA·g?1[20]
圖 8 非晶碳材料和850 ℃、2.8 V電解2 h下樣品正極電化學性能.（a）5 mV s?1掃速時，第二圈CV曲線;（b）100 mA·g?1時的充放電曲線；（c）0.2 mV·s?1時，GNF7樣品正極分離電容和擴散控制貢獻；（d）在不同掃速下，GNF7樣品電容和擴散的貢獻比;（e）不同電流密度下的倍率能力；（f）該樣品陰極在200 mA·g?1下的循環性能[16]
Figure 8. Amorphous carbon material and electrochemical performance of positive electrode of sample at 850 ℃ and 2.8 V for 2 h: (a) CV curve of the second cycle at the scanning speed of 5 mV·s?1; (b) charge-discharge curve at 100 mA·g?1; (c) the positive electrode separation capacitance and diffusion control contribution of GNF7 sample at a scan rate of 0.2 mV·s?1; (d) under different scanning rates, the contribution ratio of capacitance and diffusion of GNF7 samples; (e) contribution rate of capacitance and diffusion under different scanning rates; (f) cyclic performance of the cathode of the sample at 200 mA·g?1[16]
表 1 不同電解溫度下電解得到樣品的石墨化度[17]
Table 1. Graphitization degree of carbon products under different temperatures[17]
Electrolytic temperature/℃ d002/nm Graphitization degree/% Original coke 0.348 ?47.46 800 0.3389 60.05 850 0.3373 77.64 900 0.3358 95.07 
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Electrolytic temperature/℃ ID IG ID/IG 800 81.6969 136.262 0.60 850 44.9753 152.417 0.30 900 20.2230 123.252 0.16
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表 3 不同電解條件下得到樣品的拉曼及石墨化度數據[18]
Table 3. Raman and graphitization data of samples under different electrolysis conditions[18]
Electrolytic condition Samples Graphitization degree/% ID/IG KL RAW coal RC — 0.91 HyperCoal HP — 1.6 850 ℃?2.6 V?10 h EG1 13 1.03 900 ℃?2.6 V?10 h EG2 46 0.41 950 ℃?2.2 V?10 h EG3 37 0.76 950 ℃?2.4 V?10 h EG4 40 0.68 950 ℃?2.6 V?2 h EG5 10 1.05 950 ℃?2.6 V?4 h EG6 27 0.9 950 ℃?2.6 V?6 h EG7 49 0.3
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Electrolytic condition D peak position/cm?1 G peak position/cm?1 ID/IG 800 ℃?2.8 V?8 h 1343 1596 2.068 850 ℃?2.8 V?8 h 1347 1590 1.055 900 ℃?2.8 V?8 h 1347 1576 0.531 950 ℃?2.8 V?8 h 1347 1577 0.197
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表 5 電化學制備的石墨與商業化石墨性能對比
Table 5. Performance comparison between electrochemically prepared graphite and commercialized graphite
Graphite
materialParticle size/μm ID/IG Graphitization degree/% Compaction density/
（g·cm?3）First cycle capacity/
（mA·h·g?1）Specific surface area/(m2·g?1) D10 D50 D90 Commercialized graphite 6.07 11.04 20.14 — 93.4 1.17–1.7 353.4 1.545 Commercialized graphite 7–9 18–19.5 <36 ≤0.5 ≥94 ≥1.5 360 3–4 Hard carbon 12.0–15.0 0.96 — 1.3–1.60 350–400 <5 Graphitized sample [11] — — — ≤0.5 ≥50 — 116 — Graphitized sample [18] — — — 0.3 49 — 510 131.180 Graphitized sample [16] — — — 0.112 46 — — 165.7 Graphitized sample [20] — — — 0.197 42 — — 121
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表 6 電化學制備的碳納米管與商業化碳納米管性能對比
Table 6. Performance comparison between electrochemically prepared carbon nanotubes and commercialized carbon nanotubes
Materials Graphite number
of pliesDiameter/
mmLength/
μmSpecific surface area/
(m2·g?1)Ash mass fraction/% Purity/% IG/ID Use Commercialized single-walled carbon nanotubes Monolayer <2 >5 600–800 <60 >90 >50 Conductive agent Commercial double-walled carbon nanotubes Double 1.5–2 >15 — <10 >90 >100 Conductive agent Commercialized multi-walled carbon nanotubes Multilayer 10–20 5–15 100~160 <3 >97 — Conductive agent Graphitized multi-walled carbon nanotubes Multilayer 15 10 140 — — 177 Conductive agent
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