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Article

CO2 Pressure-Dependent Microstructure and Morphology of Carbon for Energy Storage: Unraveling the Role of CO2 in Green Synthesis of Carbon Materials

by
Peng Li
1,2,†,
Yun Chen
1,2,†,
Chu Liang
1,2,3,*,
Chengfu Zeng
2,
Xiaoyu (Baohua) Zhang
2,
Haichang Zhong
1,*,
Wenxian Zhang
1,
Xiaohua Zheng
2 and
Mingxia Gao
2,3
1
Fujian Provincial Key Laboratory of Functional Materials and Applications & School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, China
2
Zhejiang Carbon Neutral Innovation Institute & College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China
3
State Key Lab of Silicon Materials & School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Batteries 2023, 9(2), 130; https://doi.org/10.3390/batteries9020130
Submission received: 4 January 2023 / Revised: 6 February 2023 / Accepted: 10 February 2023 / Published: 12 February 2023
(This article belongs to the Special Issue Advanced Carbon-Based Materials for Batteries)
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Versions Notes

Abstract

:
Advanced carbon materials have played an important function in the field of energy conversion and storage. The green and low-carbon synthesis of elemental carbon with controllable morphology and microstructure is the main problem for carbon materials. Herein, we develop a green and low-carbon method to synthesize porous carbon by reacting CO2 with LiAlH4 at low temperatures. The starting reaction temperatures are as low as 142, 121, and 104 °C for LiAlH4 reacting with 1, 30, and 60 bar CO2, respectively. For the elemental carbon, the porosity of elemental carbon gradually decreased, whereas its graphitization degree increased as the CO2 pressure increased from 1 bar to 60 bar. CO2 serves as one of the two reactants and the CO2 pressure can adjust the thermodynamic and kinetic properties of the formation reaction for synthesizing elemental carbon. The mechanism for CO2 pressure-dependent microstructure and morphology of carbon is discussed on the basis of the formation reaction of elemental carbon and gas blowing effect of H2 and CO2. The elemental carbon with different morphology and microstructure exhibits distinct electrochemical lithium storage performance including reversible capacity, rate capability, cycling stability, and Coulombic efficiency, owing to their different lithium storage mechanism. The elemental carbon synthesized at 30 bar CO2 delivers the highest reversible capacity of 506 mAh g−1 after 1000 cycles even at 1.0 A g−1. Advanced energy storage technology based on the green and low-carbon synthesis of carbon materials is a requisite for providing a stable and sustainable energy supply to meet the ever-growing demand for energy.

1. Introduction

Carbon materials have been widely applied in the field of energy conversion and storage [1,2,3,4,5,6,7]. Energy conversion and storage technologies based on the green and low-carbon synthesis of carbon materials are the necessary premise for achieving carbon neutrality. Carbon materials play a key role in the commercial energy storage devices of lithium-ion batteries (LIBs). Various anode materials, including carbon [8], silicon [9], titanium [10], nitride [11], transition metal oxide [12,13], etc., have been explored. Among them, carbon is regarded as one of the most promising lithium storage materials for the commercial LIBs. Graphite has been widely selected as the commercial anode material for its rich source, low cost and relatively high lithium storage capacity (372 mAh g–1). However, graphite anode material cannot meet the demand of the next generation LIBs with high energy density and high power density [14,15].
Carbon materials are usually synthesized from the organic precursors of biomass [16], polymers [17], and fossil products [18] via a carbonization reaction. The synthesis process involves multiple steps and has the disadvantages of sluggish reaction rate and emission of greenhouse gases and harmful gases [19,20]. Thus, it is crucial to develop a facile, green and low-carbon technology to synthesize carbon materials in the long term. CO2, as the major contributor of greenhouse effect [21], is also a cheap and easily available precursor for synthesizing carbon materials. High temperature or gaseous pressure may be needed to synthesize elemental carbon from CO2 because of its high thermodynamic and kinetic stability [22]. CO2 reacting with magnesium metal and the electrolytic melting carbonate to produce carbon materials both need at least several hours at high temperature [23,24,25,26,27]. The conversion of CO2 into carbon using cation-excess magnetite is also reported to be required at 290 °C [28,29]. In our previous work, we have achieved green and efficient conversion of CO2 at low temperatures through various strategies, such as low-temperature heating [30] and mechanochemistry [31]. Moreover, these carbon materials derived from CO2 show excellent lithium storage performance.
In this work, we report a low-temperature and green method to synthesize elemental carbon with CO2 pressure dependent morphology and microstructure via reacting CO2 with LiAlH4. Porous carbon with the specific surface area of 522 m2 g–1 has been successfully synthesized by reacting CO2 with LiAlH4 at as low as 142 °C under 1.0 bar CO2. The function of CO2 is revealed in the green synthesis of elemental carbon. The mechanism for CO2 pressure-dependent morphology and microstructure of elemental carbon is studied on the formation reaction for synthesizing elemental carbon and gas-blowing effect of H2 and CO2.

2. Experimental Section

2.1. Materials

Lithium aluminium hydride (LiAlH4, 95%), carbon dioxide (CO2, 99.995%), ethanol (99.7%), and hydrochloric acid (HCl, 37 wt.% in water) were purchased from Alfa Aesar, Jinggong, Ante, and Xilong, respectively. Deionized (DI) water was produced in our lab. All chemicals were used as received.

2.2. Synthesis Procedurer5f4

First, 0.38 g of LiAlH4 were weighted and loaded into a homemade reactor with temperature and gas pressure monitors. These operations were carried out in a glovebox (O2 < 0.01 ppm, H2O < 0.01 ppm) filled with argon to prevent LiAlH4 from contacting with water and air. About 1, 30, and 60 bar CO2 were then introduced into the homemade reactor via a gas injection valve, respectively. The LiAlH4-CO2 mixture was heated from room temperature to the preset temperature at a rate of 2 °C/min. After reacting LiAlH4 with CO2, the solid products were collected for washing with excess hydrochloric acid at 200 °C to remove the byproducts. Finally, the suspension of fine particles was collected and washed with DI water and ethanol several times, subsequently dried at 80 °C for 6 h. The elemental carbons synthesized by reacting with 1, 30, and 60 bar CO2, are referred to as Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar, respectively, in this work.

2.3. Characterization

X-ray diffraction (XRD) patterns of carbon samples were measured in the 2θ range of 10–80° on an X-ray diffractometer (X’Pert Pro diffractometer) with Cu Kα radiation at 40 kV and 40 mA. Raman spectra of carbons were measured on a Renishaw Raman spectrophotometer (Invia plus) with the excitation wavelength of 532 nm. Specific surface area and pore size distribution were analyzed using the Brunauer–Emmett–Teller (BET) model. Field emission scanning electron microscopy (FESEM, Hitachi S-4700) images and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F30) images were employed to observe the microstructures and morphologies of electrode materials.

2.4. Electrochemical Measurements

CR2032-typed coin cell was used to evaluate the electrochemical performance of as-synthesized carbons. The working electrode consisted of as-synthesized carbon and polyvinylidene fluoride (PVDF) in a mass ratio of 85:15. The active material (carbon) was mixed with PVDF in the NMP solvent to form a slurry via stirring. The slurries were evenly coated on copper foils and then drying at 80 °C for 20 h in vacuum. Lithium foil (99.9%, China Energy Lithium) was used as the counter electrode, Celgard 2400 membrane as the separator and a solution of 1 M LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) in a 1:1 by volume was used as electrolyte. CR2032 coin cells were assembled in an argon-filled glove-box. The electrochemical measurements including galvanostatic discharge/charge test and cyclic voltammetry (CV) measurement were conducted using a Neware battery test system and a CHI650B electrochemical workstation, respectively, in a voltage range of 0.01–3.0 V. All electrochemical performances were tested at room temperature.

3. Results and Discussion

3.1. Synthesis of Elemental Carbon via Reacting CO2 with LiAlH4

The elemental carbon was synthesized by reacting gaseous CO2 with solid LiAlH4 in the heating process. Figure 1a shows the time dependence of sample temperatures and gas pressures in the reactor during heating from room temperature to a preset temperature. Three initial CO2 pressures (1, 30, and 60 bar) were applied to react with LiAlH4 to produce elemental carbon. Abrupt changes in both sample temperatures and gaseous pressures are observed in all the heating process of LiAlH4 reacting with 1, 30, and 60 bar CO2, implying an exothermic reaction between CO2 and LiAlH4. The gaseous pressure change is mainly caused by the heat of external heating and internal reaction induced temperature change. As shown in Figure 1a, the starting temperature of the exothermic reaction is found to strongly depend on the initial CO2 pressure. It is noteworthy that when the sample temperature was elevated to the starting temperature of the exothermic reaction, the external heating was stopped. The starting temperature is found to decrease with increasing the initial CO2 pressure. The starting temperatures are 142, 121, and 104 °C, respectively, for the LiAlH4 reacting with 1, 30, and 60 bar CO2. For the exothermic reaction between CO2 and LiAlH4, the starting temperature increases to the heat of reaction induced maximum temperature in less than 10 s. Interestingly, the maximum temperature increases with increasing the initial CO2 pressure, in which the maximum temperatures are 165, 330, and 810 °C, respectively, for the LiAlH4 reacting with 1, 30, and 60 bar CO2.
The elemental carbon was successfully separated from the three abovementioned solid-state products by reacting with hydrochloric acid, confirmed by the energy dispersive X-ray spectroscopies (EDS) as shown in Figure 1d. It can be seen that only element C, O, and a small amount of Al were detected in the three as-synthesized carbon samples. The homogeneity of C, O, and Al can be seen in the elemental distribution mapping of the as-synthesized carbons (Supplementary Figure S1). Taking the Carbon−1 bar as an example, the content of element O is less than 9.74 wt.%, in which the oxygen absorbed may be responsible for the very small amount of element O. Carbon−1 bar exhibits a low graphitization degree as the broad peaks around 25° are seen in the XRD pattern (Figure 1b). As shown in the Raman spectrum of Carbon−1 bar (Figure 1c), the D band centered at 1355 cm−1 and G band centered at 1588 cm−1 are observed, demonstrating the formation of elemental carbon.

3.2. Effect of CO2 Pressure on the Morphology and Microstructure of Elemental Carbon

The XRD patterns of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar are shown in Figure 1b. Two diffraction peaks centered at 25.9° and 42.4°, assigned to the (002) and (100) planes of hexagonal graphite, are observed in the XRD patterns of both Carbon−30 bar and Carbon−60 bar, which is different from the Carbon−1 bar with low graphitization degree. These results suggest that the graphitization degree of elemental carbon may be related to the CO2 pressure applied in the synthesis process. The effect of CO2 pressure on the graphitization degree of elemental carbon was further characterized by Raman spectra (Figure 1c). The area ratio of G band centered at 1588 cm−1 to D band centered at 1355 cm−1 is used to evaluate the graphitization degree of elemental carbon. The area ratios are 0.20, 0.65, and 0.96, respectively, for Carbon−1 bar, Carbon−30 bar and Carbon−60 bar, implying that the graphitization degree of elemental carbon increases with the CO2 pressure. The Raman results are consistent with the XRD results.
The porosity of elemental carbon was assessed via N2 adsorption–desorption isotherms as shown in Figure 2. The BET surface areas are calculated to be 522 m2 g–1 for Carbon−1 bar, 217 m2 g–1 for Carbon−30 bar and 37 m2 g–1 for Carbon−60 bar, respectively, suggesting that the porosity may be distinct for the elemental carbons synthesized under different CO2 pressures. As shown Figure 2b, three elemental carbons exhibit a continuous mesopore and macro-pore size distribution, in which the pore diameter of elemental carbon increases with the CO2 pressure applied in its synthesis process.
To verify the effect of CO2 pressure on the porosity of elemental carbon, the morphology and microstructure of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar were observed using SEM and TEM (Figure 3). Carbon−1 bar shows a honeycomb morphology with obvious porous structure (Figure 3a,d). When the CO2 pressure increased to 30 bar, the pore diameter of Carbon−30 bar becomes greater than that of Carbon−1 bar (Figure 3b,e), in good agreement with the BET results. For the Carbon−60 bar, a flaky morphology with unobvious porous structure is observed in the SEM and TEM images (Figure 3c,f). Furthermore, the graphitization degree of three elemental carbons can be characterized using high-resolution TEM (HRTEM) images (Figure 3g–i). For the Carbon−60 bar, distinct lattice fringes with a distance of 0.36 nm can be seen in Figure 3i, corresponding to the (002) plane of graphite [32,33]. As the CO2 pressure decreased to 30 bar, lattice is defective and graphite plane cannot be calculated accurately (Figure 3h), supporting the decreased graphitization degree compared with Carbon−60 bar. For the Carbon−1 bar (Figure 3g), almost no lattice fringes were detected, resulting from the amorphous nature. The HRTEM results are consistent with the conclusion on graphitization degree drawn from XRD results.
Based on the XRD, BET, TEM, and HRTEM analysis, it has been demonstrated that the morphology and microstructure of elemental carbon synthesized by reacting CO2 with LiAlH4 is strongly dependent on the CO2 pressure.

3.3. Mechanism for CO2 Pressure-Dependent Morphology and Microstructure of Elemental Carbon

Figure 4 exhibits the XRD patterns of the solid products of LiAlH4 reacting with 1, 30, and 60 bar CO2. LiAlO2, Li2CO3, and Al are the main crystal phases in the solid products of LiAlH4 reacting with 1, 30, and 60 bar CO2, but the relative intensity of three crystal phases is strongly dependent on the CO2 pressure. As the CO2 pressure increased from 1 bar to 60 bar, the intensity of LiAlO2 significantly increased, whereas the intensity of Al gradually weakened, as shown in Figure 4, supporting the key role of CO2 in the chemical interaction of CO2 with LiAlH4. These results imply that the contents of LiAlO2, Li2CO3, and Al phases in the solid products are related to the CO2 pressure applied in the synthesis process of elemental carbon due to the different thermodynamic properties of the reaction between LiAlH4 and CO2. According to the abovementioned discussion and our previous work [34], the chemical interaction of CO2 with LiAlH4 can be described as follows:
CO2+LiAlH4→LiAlO2+Li2CO3+Al+C (elemental carbon)+H2
where the equation of chemical reaction of CO2 with LiAlH4 is not balanced owing to the CO2 pressure-related phase contents of LiAlO2, Li2CO3, and Al in the solid products. The exothermic nature of chemical reaction (1) can be concluded from the reaction characteristics of CO2 with LiAlH4 as shown in Figure 1a. It should be noted that the morphology and microstructure of elemental carbon synthesized via Equation (1) have been demonstrated to depend on the CO2 pressure as well.
CO2 is one of the two reactants of exothermic reaction (1) in which the thermodynamic and kinetic properties of reaction (1) can be tuned using the CO2 pressure. It also is the carbon precursor in the green synthesis of elemental carbon (Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar) via the chemical reaction (1). For the exothermic reaction (1), thermodynamic performance is a crucial factor for the reaction heat released during synthesizing elemental carbon. As shown in Figure 1a, it can be known that the most reaction heat was released in the synthesis process of Carbon−60 bar and the least reaction heat was released in the synthesis process of Carbon−1 bar, according to their difference value between maximum temperature and starting temperature. Moreover, the content of LiAlO2 in the solid products increases with the CO2 pressure (Figure 4). These results support that more CO2 was reacted with LiAlH4 to produce more reaction heat under the high CO2 pressure because the mass of LiAlH4 used in each experiment is a fixed value in this work. As a result, the thermodynamic performance of CO2 reacting with LiAlH4 can be tuned via the CO2 pressure. Kinetic performance is the key factor for determining the starting temperature of CO2 reacting with LiAlH4. The concentration and gaseous pressure of reactants has been demonstrated to be the key factors for the kinetic performance of a gas-solid reaction [35,36,37]. Compared with Carbon−1 bar, both concentration and gaseous pressure of CO2 in the synthesis process of Carbon−60 bar are much higher than that of Carbon−1 bar, resulting in the starting reaction temperature decreased with the CO2 pressure (Figure 1a). The CO2 pressure-tuned thermodynamic and kinetic performance of chemical reactions for synthesizing elemental carbon may be responsible for the CO2 pressure-dependent microstructure and morphology of elemental carbon.
For the gas–solid reaction (1), gaseous CO2 and solid LiAlH4 as reactants are consumed to produce gaseous H2, solid elemental carbon, and other solid products. Temperature and gaseous pressure are the rate-determining factors of the gas–solid reaction (1). The gaseous H2 and CO2 blowing serves the function of adjusting the morphology in the synthesis process of elemental carbons. Gas blowing velocity is found to positively relate to the pore size formed in elemental carbon. No pores were found in the elemental carbon when the gas blowing velocities exceed a certain value. As shown in Figure 1a, a continuous and fast temperature and gaseous pressure distribution, resulting from the exothermic reactions between CO2 and LiAlH4, is observed in the synthesis process of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar, supporting the formation of a continuous gas-blowing velocity distribution. Therefore, the elemental carbon with a continuous mesopore and macro-pore size distribution is formed in the synthesis of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar (Figure 2 and Figure 3). For the Carbon−60 bar, the pore size is greater than that of Carbon−1 bar and Carbon−30 bar due to a higher gas blowing velocity distribution resulted from higher CO2 pressure and more reaction heat released from CO2 reacting with LiAlH4. Moreover, the reaction heat and CO2 pressure are the main factors for tuning the graphitization degree of elemental carbon via the thermodynamic and kinetic performance of reaction (1).

3.4. Electrochemical Properties of Elemental Carbon

The electrochemical lithium storage performance of the three abovementioned elemental carbons were investigated to further confirm the different morphology and microstructure of Carbon−1bar, Carbon−30bar, and Carbon−60bar. Figure 5 displays the discharge/charge and CV curves of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar. In the first cycle, the elemental carbon anode delivers a discharge/charge capacities of 2415/960 mAh g−1 for Carbon−1 bar, 1256/535 mAh g−1 for Carbon−30 bar and 711/364 mAh g−1 for Carbon−60 bar, corresponding to the initial coulombic efficiency of 39.8, 46.6 and 51.2%, respectively. The irreversible capacity, which can be ascribed to the formation of solid electrolyte interface (SEI)layers on the surface of elemental carbon and the irreversible reaction of lithium ion with functional groups on the surface of elemental carbon [38,39,40], is found to decrease with the increasing CO2 pressure, owing to the different specific surface area of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar. For the Carbon−60 bar, the least SEI layer was formed on its surfaces because of its lowest specific surface area in three elemental carbons. As shown in the CV curves (Figure 5), the lithium storage behaviors of elemental carbon are in good agreement with their galvanostatic discharge/charge results. Three elemental carbon anodes show similar CV curves (Figure 4d–f). In the first cycle, there are two broad cathodic peaks around 0.8 and 1.3 V, corresponding to the formation of SEI layers and the irreversible reactions between elemental carbon anodes and liquid electrolyte. In addition, a sharp peak around 0.01 V was observed for all the three elemental carbon anodes in the following scans. Meanwhile, an anodic peak emerged at around 0.1 V, which can be ascribed to the deintercalation of Li+ cations from the graphite [41,42,43]. As the graphitization degree of elemental carbon increased from Carbon−1 bar to Carbon−60 bar, the intensity of anodic peak around 0.1 V become more and more sharp and manifest.
Figure 6a presents the cycling properties of the Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar at a current density of 100 mA g−1. It can be clearly seen that the electrochemical performance of elemental carbon, including lithium storage capacity, Coulombic efficiency and cycling stability, is found to be related to the CO2 pressure applied in the synthesis of elemental carbon. For Carbon−1 bar and Carbon−30 bar, the capacity deterioration is only observed in the first 10 cycles. On the contrary, Carbon−60 bar exhibits a good cycling stability with the minimum reversible capacities of ~370 mAh g−1 over 100 cycles. Carbon−1 bar delivers the highest reversible lithium storage capacity of 738 mAh g−1 over 100 cycles. The corresponding reversible capacity is ~570 mAh g−1 for Carbon−30 bar.
The rate capability of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar was tested at the current densities of 0.1, 0.2, 0.5, 1.0 and 2.0 A g−1 (Figure 6b). The Carbon−1 bar exhibits the maximum reversible lithium storage capacity at all the current densities. The charge capacity of the Carbon−1 bar is 795, 589, 430, 348, and 285 mAh g−1, respectively, at 0.1, 0.2, 0.5, 1.0, and 2.0 A g−1. However, the Carbon−1 bar has the lowest capacity retention, compared with Carbon−30 bar and Carbon−60 bar. Carbon−60 bar exhibits the highest capacity retention, in which the capacity retention is about 92%, 83%, 71%, and 62%, respectively, at the current densities of 0.2, 0.5, 1.0, and 2.0 A g−1. To evaluate the electrochemical stability of obtained carbon derived from CO2, long-term cycling performance of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar was measured at a current density of 100 mA g−1 for first 10 cycles, and then at 1.0 A g−1 for the following 1000 cycles (Figure 6c). At the current density of 1.0 A g−1, elemental carbon reaches the minimum capacity of 299 mAh g−1 for Carbon−1 bar after 10 cycles, 246 mAh g−1 for Carbon−30 bar after 2 cycles, and 242 mAh g−1 for Carbon−60 bar after 1 cycle. It can be seen that the charge capacity measured at 1.0 A g−1 increases gradually from a minimum value to a stable value at 500th cycle during repeated cycling. The increase rate is found to be related to the CO2 pressure applied in the synthesis of elemental carbon. Carbon−30 bar exhibits the highest increase rate as shown in Figure 6c. After 500 cycles, they deliver stable reversible capacities of 443 mAh g−1 for Carbon−1 bar, 486 mAh g−1 for Carbon−30 bar and 390 mAh g−1 for Carbon−60 bar, respectively. After 1000 cycles, Carbon−30 bar delivers the highest reversible capacity of 506 mAh g−1 at 1.0 A g−1. Compared with the recent work on carbon materials [44,45,46,47], the elemental carbons synthesized from CO2 in this work show higher lithium storage capacity and longer cycle life. Moreover, the electrochemical properties of the elemental carbon can be tuned by CO2 pressure in the synthesis process, in order to match different application scenarios.
As discussed above, it is known that the electrochemical performance is dependent on the CO2 pressure applied in the synthesis of elemental carbon. For Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar, the difference in electrochemical performance including lithium storage capacity, rate capability, and cycling stability can be ascribed to their different microstructures and morphologies, resulting from the distinct lithium storage mechanisms. As shown in Figure 5, Carbon−1 bar and Carbon−30 bar exhibit remarkable capacitance-dominated lithium storage behaviors, but a very small proportion of the graphite-dominated lithium storage behaviors is observed in the Carbon−30 bar. For the Carbon−60 bar, the remarkable graphite-dominated and capacitance-dominated lithium storage behaviors are observed in Figure 5c,f. The capacitance-dominated lithium storage mechanism is responsible for the high lithium storage capacity and the superior rate capability due to more lithium storage sites on the surfaces and in the interfaces of elemental carbon with porous structure. In contrast, the graphite-dominated lithium storage mechanism is responsible for relatively low lithium storage capacity and the high Coulombic efficiency, especially for the initial efficiency, and the excellent cycling stability. As predicted, Carbon−1bar, Carbon−30 bar, and Carbon−60 bar exhibit the distinct electrochemical lithium storage properties, supporting that the CO2 pressure-tuned microstructure and morphology of elemental carbons.

4. Conclusions

In summary, we have demonstrated a green and low-carbon method to synthesize carbon materials with controllable morphology and microstructure from CO2 by reacting LiAlH4 with different CO2 pressure. As the CO2 pressure increased from 1 bar to 30 and 60 bar, the starting reaction temperature decreased from 142 °C to 121 and 104 °C, resulting in CO2 pressure-dependent morphology and microstructure of carbon materials. As an example, the BET surface areas are 522 m2 g−1 for Carbon−1 bar, 217 m2 g−1 for Carbon−30 bar and 37 m2 g−1 for Carbon−60 bar, respectively. CO2 can react with LiAlH4 to produce LiAlO2, Li2CO3, Al, C (elemental carbon), and H2 in the synthesis of carbon materials. The reaction temperature and products, which are the key factors for determining morphology and microstructure of carbon materials, are demonstrated to be strongly associated with CO2 pressure. More CO2 was reacted with LiAlH4 to produce more reaction heat under the high CO2. The CO2 pressure-tuned thermodynamic and kinetic performance of chemical reactions for synthesizing elemental carbon is responsible for the CO2 pressure-dependent microstructure and morphology of elemental carbon. The elemental carbon exhibits CO2 pressure-related electrochemical performance. Carbon−1 bar delivers a high reversible capacity of 738 mAh g−1 in the current density of 100 mA g−1 after 100 cycles. Carbon−30 bar shows a high-rate capability of 506 mAh g−1 even at 1 A g−1 over 1000 cycles. Carbon−60 bar exhibits high capacity retention at various current densities. The difference in electrochemical performance can be ascribed to their different microstructures and morphologies, resulting from the distinct lithium storage mechanisms.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/batteries9020130/s1, Supplementary Figure S1: EDS mapping of (a) Carbon−1 bar, (b) Carbon−30 bar and (c) Carbon−60 bar.

Author Contributions

Conceptualization and experimental method, P.L., Y.C., C.L. and H.Z.; materials synthesis and characterization, P.L., Y.C., C.Z. and X.Z.; electrochemical tests and data analysis, P.L., Y.C., X.Z., W.Z. and M.G.; writing—original draft preparation, P.L. and Y.C.; writing—review and editing, C.L., H.Z. and M.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Upon reasonable request, the corresponding authors will provide the data that supports the findings of this study.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 52072342 and 51677170), National Key Research and Development Program of China (No. 2022YFB2502000) and open fund of Fujian Provincial Key Laboratory of Functional Materials and Applications (No. fma2020003).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Time dependence of sample temperatures and gaseous pressures in the reactor during the synthesis of elemental carbon. (b) XRD patterns and (c) Raman spectra of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar. (d) Content of elements in the as-obtained carbon.
Figure 1. (a) Time dependence of sample temperatures and gaseous pressures in the reactor during the synthesis of elemental carbon. (b) XRD patterns and (c) Raman spectra of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar. (d) Content of elements in the as-obtained carbon.
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Figure 2. (a) Nitrogen adsorption–desorption isotherms and (b) the corresponding pore diameter distribution of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar, respectively.
Figure 2. (a) Nitrogen adsorption–desorption isotherms and (b) the corresponding pore diameter distribution of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar, respectively.
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Figure 3. SEM and TEM images and high-resolution TEM images of Carbon−1 bar (a,d,g), Carbon−30 bar (b,e,h), and Carbon−60 bar (c,f,i).
Figure 3. SEM and TEM images and high-resolution TEM images of Carbon−1 bar (a,d,g), Carbon−30 bar (b,e,h), and Carbon−60 bar (c,f,i).
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Figure 4. XRD patterns of the solid products of LiAlH4 reacting with 1, 30, and 60 bar CO2.
Figure 4. XRD patterns of the solid products of LiAlH4 reacting with 1, 30, and 60 bar CO2.
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Figure 5. The galvanostatic charge/discharge profiles (a–c) at a current density of 100 mA g−1 and CV curves (d–f) of the elemental carbon anodes in a voltage range of 0.01–3.0 V.
Figure 5. The galvanostatic charge/discharge profiles (a–c) at a current density of 100 mA g−1 and CV curves (d–f) of the elemental carbon anodes in a voltage range of 0.01–3.0 V.
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Figure 6. Electrochemical performance of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar. (a) Cycling properties and Coulombic efficiency at a current density of 100 mA g−1, (b) rate performance and (c) ultra-long cycling performance at a current density of 1000 mA g−1.
Figure 6. Electrochemical performance of Carbon−1 bar, Carbon−30 bar, and Carbon−60 bar. (a) Cycling properties and Coulombic efficiency at a current density of 100 mA g−1, (b) rate performance and (c) ultra-long cycling performance at a current density of 1000 mA g−1.
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Li, P.; Chen, Y.; Liang, C.; Zeng, C.; Zhang, X.; Zhong, H.; Zhang, W.; Zheng, X.; Gao, M. CO2 Pressure-Dependent Microstructure and Morphology of Carbon for Energy Storage: Unraveling the Role of CO2 in Green Synthesis of Carbon Materials. Batteries 2023, 9, 130. https://doi.org/10.3390/batteries9020130

AMA Style

Li P, Chen Y, Liang C, Zeng C, Zhang X, Zhong H, Zhang W, Zheng X, Gao M. CO2 Pressure-Dependent Microstructure and Morphology of Carbon for Energy Storage: Unraveling the Role of CO2 in Green Synthesis of Carbon Materials. Batteries. 2023; 9(2):130. https://doi.org/10.3390/batteries9020130

Chicago/Turabian Style

Li, Peng, Yun Chen, Chu Liang, Chengfu Zeng, Xiaoyu (Baohua) Zhang, Haichang Zhong, Wenxian Zhang, Xiaohua Zheng, and Mingxia Gao. 2023. "CO2 Pressure-Dependent Microstructure and Morphology of Carbon for Energy Storage: Unraveling the Role of CO2 in Green Synthesis of Carbon Materials" Batteries 9, no. 2: 130. https://doi.org/10.3390/batteries9020130

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