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• Recently, we have proposed the application of quaternary alkyl ammonium cations (QAA⁺) as the charge carries at carbon negative electrodes in the electric energy storage devices free of metals [1]. Since the risk of metal deposition at carbon negative electrodes is avoided and the thermal runaway hazard associated with the growth of metal dendrite can be completely prevented, the safety can be guaranteed. One of the key issues in these devices may be ascribed to the effective storage of QAA⁺ in carbon negative electrode materials. Although we have found out that activated mesophase microbeads appears a satisfactory host material to accommodate QAA⁺ [2], graphite might be a more promising candidate for this task by virtue of its low cost, environmental benignity, and abundance on the earth. Although there have been some reports on the preparation of QAA⁺-graphite intercalation compounds (GICs) via chemical routes [3-7], it is not so easy to make good use of graphite negative electrode in QAA⁺-based solutions because the influence of electrolyte solutions on its electrochemical performance has not been studied in depth. In our preliminary study [8], we have noticed that the solvent of propylene carbonate will co-intercalate into graphite electrode with a small QAA⁺. Therefore, the solvation structure of QAA⁺ may play a very important role in determining its storage behavior in graphite electrode.

  Unfortunately, we lack convincing measures to precisely quantify the solvation number around each QAA⁺ at present. However, we can flexibly tailor the solvation structure by changing the concentration of QAA⁺ in a solution. In this study, we pick up spiro-(1,1′)-bipyrrolidinium tetrafluoroborate (SBPBF₄) as the electrolyte salt and ethylene carbonate (EC) as the solvent mainly because the concentration of SBPBF₄ can reach as high as 4 mol/L in EC. The intercalation behavior of SBP⁺ into graphite electrode from SBPBF₄-EC solutions with varied concentrations is investigated by conventional electrochemical tests and in situ X-ray diffraction (XRD) measurements. The correlation between the electrochemical performance of graphite negative electrode and SBP⁺ concentration is clarified and the effect of graphite type is addressed. The physical properties of both graphite samples are compared in Table S1 (Supporting information).

  The storage behavior of SBP⁺ in graphite negative electrode is mainly accessed in graphite/activated carbon (AC) capacitors. Fig. 1 depicts the separate potential profiles of graphite negative or AC positive electrodes against the quasi-reference electrode (QRE) in the initial galvano-static charge-discharge curves of the capacitors using SBPBF₄-EC solutions. The potential plateaus of graphite electrodes occupy most capacities of the cation storage, corresponding to SBP⁺ intercalation into or de-intercalation from graphite electrodes. The potential plateaus of natural graphite (NG) electrodes become contracted with increasing of the salt concentration, especially in the case of 4 mol/L. On the other hand, the potential plateaus of the artificial graphite (AG) almost overlap with each other in all the solutions of SBPBF₄-EC. This dramatic contrast may hint that the storage mechanisms of SBP⁺ in both graphite negative electrodes in contact with these solutions are different.

  Figure 1

  

  Figure 1.  Initial galvano-static charge-discharge curves of graphite/AC capacitors using 1, 2, 3 and 4 mol/L SBPBF₄-EC solutions and the separate potential profiles of graphite negative electrodes and AC positive electrodes against AC-QRE, respectively.

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  Since the potential plateaus of graphite electrodes result from the transformations of SBP⁺-GICs, in situ XRD measurements are performed on the graphite electrodes in contact with these solutions to characterize the crystal structures of these GICs during the initial charge-discharge cycle of graphite/AC capacitors. As exhibited in Figs. 2 and 3, during the charge process with cell voltage rising, the (002) diffraction peak (26.5°) of original graphite gradually fades down. At the same time, new diffraction peaks standing for the formation of SBP⁺-GICs emerge at both higher and lower Bragg angles of 26.5°. On the contrary, during the discharge with the cell voltage dropping down, the diffraction peaks of SBP⁺-GICs gradually disappear, while the (002) peak of graphite will return. Therefore, the intercalation or de-intercalation of SBP⁺ into or from graphite electrodes correspond to the charge or discharge processes of graphite/AC capacitors, respectively. For both NG and AG in the solutions of 1–3 mol/L (Figs. 2 and 3), the in situ XRD patterns for SBP⁺-GICs generally possess a couple of peaks characteristic of the intercalated gallery heights (IGHs) near 0.952 nm. However, when the salt concentration increases to 4 mol/L, the in situ XRD patterns of the natural and artificial graphite electrodes are very different. There are two couples of SBP⁺-GICs' peaks in the case of NG, which belong to IGHs of 0.947 and 0.742 nm, respectively. The smaller IGH value (0.742 nm) may represent the SBP⁺-GICs with less EC solvents co-intercalated into graphite than those with the larger IGH values (0.947–0.952 nm). In contrast, there is only one couple of SBP⁺-GICs' peaks as to AG, which corresponds to the IGH values around 0.742 nm. This unique feature means that AG is more favorable for the formation of SBP⁺-GICs with milder EC co-intercalation.

  Figure 2

  

  Figure 2.  In situ XRD patterns of NG negative electrodes in the 1, 2, 3 and 4 mol/L SBPBF₄-EC solutions during the initial galvano-static charge-discharge cycle of NG/AC capacitors.

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  Figure 3

  

  Figure 3.  In situ XRD patterns of AG negative electrodes in the 1, 2, 3 and 4 mol/L SBPBF₄-EC solutions during the initial galvano-static charge-discharge cycle of AG/AC capacitors.

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  From each in situ XRD pattern demonstrating the formation SBP⁺-GICs in Figs. 2 and 3, the key parameters of IGH and stage number (SN) can be calculated according to the procedures introduced [9, 10]. They are plotted against the cell voltage in Fig. 4. Both NG and AG electrodes experience the decrease of SN in the charge process but the increase of SN in the discharge process. On one hand, in the solutions of 1–3 mol/L, nearly all the IGH values fluctuate around 0.95 nm. On the other hand, when the salt concentration amounts to 4 mol/L, NG electrode can develop the SBP⁺-GICs with two sets IGHs, about 0.947 and 0.742 nm, respectively. By contrast, in the case of AG, there are merely the IGH values near 0.742 nm. Why AG appears more selective than NG towards the intercalation of EC-solvated SBP⁺? The previous studies have discovered that there are considerable structural defects in AG [11], which may be helpful for stripping off the surplus EC solvent bond to the cation during its intercalation into the graphite.

  Figure 4

  

  Figure 4.  Evolvements of the stage number (SN) and intercalated gallery height (IGH) of SBP⁺-GICs developed in Figs. 2 and 3.

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  The cycle performance of graphite/AC capacitors using SBPBF₄-EC solutions is compared in Fig. 5. The capacity fades drastically in NG/AC capacitors using the 4 mol/L solution. By a big contrast, the AG/AC capacitors display very satisfactory cycle-ability, especially in the cases of highly concentrated solutions, which might be in a great part ascribed to the formations of SBP⁺-GICs with neat solvation structures.

  Figure 5

  

  Figure 5.  Cycle performance of graphite/AC capacitors using SBPBF₄-EC solutions.

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  In conclusion, the intercalation of SBP⁺into graphite negative electrodes from "diluted" SBPBF₄-EC solutions give rise to the SBP⁺-GICs with the IGH values around 0.95 nm. Once the salt concentration rises to 4 mol/L, a new kind of SBP⁺-GICs with the IGH values about 0.75 nm will form, implying that less EC solvent co-intercalated into graphite electrode with SBP⁺ cation. In the case of NG in the 4 mol/L solution, both kinds of SBP⁺-GICs coexist. By contrast, AG negative electrode in the 4 mol/L solution only displays neat IGH values around 0.75 nm. This indicates that the abundant defects in AG help filter out SBP⁺ heavily solvated by EC during its interaction into graphite.

  Declaration of competing interest

  All of the authors declare there is no interest conflict.

  Acknowledgment

  This work was financially supported by National Natural Science Foundation of China (No. 21975251).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.04.073.

  
  
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  Figure 1  Initial galvano-static charge-discharge curves of graphite/AC capacitors using 1, 2, 3 and 4 mol/L SBPBF₄-EC solutions and the separate potential profiles of graphite negative electrodes and AC positive electrodes against AC-QRE, respectively.

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  Figure 2  In situ XRD patterns of NG negative electrodes in the 1, 2, 3 and 4 mol/L SBPBF₄-EC solutions during the initial galvano-static charge-discharge cycle of NG/AC capacitors.

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  Figure 3  In situ XRD patterns of AG negative electrodes in the 1, 2, 3 and 4 mol/L SBPBF₄-EC solutions during the initial galvano-static charge-discharge cycle of AG/AC capacitors.

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  Figure 4  Evolvements of the stage number (SN) and intercalated gallery height (IGH) of SBP⁺-GICs developed in Figs. 2 and 3.

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  Figure 5  Cycle performance of graphite/AC capacitors using SBPBF₄-EC solutions.

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