High‐Resolution Mass Spectroscopy for Revealing the Charge Storage Mechanism in Batteries: Oxamide Materials as an Example

The pursuit of high‐performance electrode materials is highly desired to meet the demand of batteries with high energy and power density. However, a deep understanding of the charge storage mechanism is always challenging, which limits the development of advanced electrode materials. Herein, high‐resolution mass spectroscopy (HR‐MS) is employed to detect the evolution of organic electrode materials during the redox process and reveal the charge storage mechanism, by using small molecular oxamides as an example, which have ortho‐carbonyls and are therefore potential electrochemical active materials for batteries. The HR‐MS results adequately proved that the oxamides could reversibly store lithium ions in the voltage window of 1.5–3.8 V. Upon deeper reduction, the oxamides would decompose due to the cleavage of the C–N bonds in oxamide structures, which could be proved by the fragments detected by HR‐MS, 1H NMR, and the generation of NH3 after the reduction of oxamide by Li. This work provides a strategy to deeply understand the charge storage mechanism of organic electrode materials and will stimulate the further development of characterization techniques to reveal the charge storage mechanism for developing high‐performance electrode materials.


Introduction
Innovative electrode materials can endow secondary batteries with high energy density, high power density, high safety, and long life cycle, which accelerates the progress of the next-generation large-scale applications. [1][2][3][4] Among the extensive attempts, organic electrode materials have attracted considerable attention in view of their potentially high capacity, low cost from vast natural resources, flexibility, and insignificant volume variation during cycling etc. [5][6][7][8][9][10][11] However, deep understanding of the charge storage mechanism is always challenging, which limits the development of advanced electrode materials, including not only inorganic but also organic materials, [12][13][14][15][16][17] although various characterization methods have been exploited to detect the component and structural variation of the electrodes. [18][19][20] As for organic electrodes, taking carbonyl compounds as an example, it is normally believed that the conjugated carbonyl groups are indispensable to be electrochemical active; [21][22][23][24][25][26] however, it was reported that non-conjugated carbonyl groups could also be redox active. [27,28] On the other hand, it has been found that the practical capacity does not always proportionally increase with the increase active sites. Most of the carbonyl compounds with more than two carbonyl groups showed a low utilization of active materials, which was even lower than 50% (e.g., tetraketopiperazine derivatives, [29] aromatic imides, [30][31][32] etc.). On the contrary, some organic electrode materials showed a superlithiation phenomenon under low operation voltage. [33] Therefore, compared with the often used spectroscopies that could only qualitatively deduce the change in components and structures, the development of analytical methods that could precisely identify the component and structural variation during cycling is highly required for deep understanding the charge storage mechanism.
Herein, high-resolution mass spectroscopy (HR-MS) is employed to detect the structure evolution during the redox process and reveal the charge storage mechanism. Small molecular oxamides were adopted as an example, which have ortho-carbonyls and are therefore potential electrochemical active materials for batteries. The small molecular structure makes it possible to be detected in both charged and discharged states by MS. The HR-MS results adequately proved that the oxamides materials could reversibly store lithium ions in the voltage window of 1.5-3.8 V. The HR-MS also indicated that the initial stored Li ions were located at between two molecules. Upon deeper reduction, the oxamides would decompose due to the cleavage of the C-N bonds in oxamide structures, which could be proved by the fragments detected by HR-MS, 1 H NMR, and the generation of NH 3 after the reduction of oxamides by Li. This work provides a strategy to deeply understand the charge storage mechanism of organic electrode The pursuit of high-performance electrode materials is highly desired to meet the demand of batteries with high energy and power density. However, a deep understanding of the charge storage mechanism is always challenging, which limits the development of advanced electrode materials. Herein, highresolution mass spectroscopy (HR-MS) is employed to detect the evolution of organic electrode materials during the redox process and reveal the charge storage mechanism, by using small molecular oxamides as an example, which have ortho-carbonyls and are therefore potential electrochemical active materials for batteries. The HR-MS results adequately proved that the oxamides could reversibly store lithium ions in the voltage window of 1.5-3.8 V. Upon deeper reduction, the oxamides would decompose due to the cleavage of the C-N bonds in oxamide structures, which could be proved by the fragments detected by HR-MS, 1 H NMR, and the generation of NH 3 after the reduction of oxamide by Li. This work provides a strategy to deeply understand the charge storage mechanism of organic electrode materials and will stimulate the further development of characterization techniques to reveal the charge storage mechanism for developing high-performance electrode materials. materials and will stimulate the further development of characterization techniques to reveal the charge storage mechanism for developing high-performance electrode materials.

Synthesis and Characterization
Scalable small molecular oxamide derivative (i.e., quinoxaline-2,3 (1H,4H)-dione (POxa)) was easily synthesized by a condensation reaction between o-diaminobenzene and oxalic acid (Figures S1 and S2, Supporting Information). Colorless crystals were obtained directly. The as-prepared POxa crystals exhibited a regular rod-like shape with lengths of several hundred micrometers ( Figure S3, Supporting Information), and the C, O, and N elements were homogeneously distributed in the crystals ( Figure S4, Supporting Information). Subsequently, the elemental analyses verified the proposed formula (Table S1, Supporting Information). Additionally, the 1 H nuclear magnetic resonance (NMR) spectra ( Figure 1a) clearly showed three peaks at 7.09, 7.12, and 11.92 ppm with an integral area ratio of 1:1:1, which was consistent with the three types of H atoms in POxa. At the same time, four signature peaks (115.56, 123.44, 126.05, and 155.61 ppm) in the 13 C NMR spectra agreed well with the four types of C atoms in POxa (Figure 1b). More importantly, the found molecular weight of deprotonated POxa [C 8 H 6 N 2 O 2 -H + ] − was 161.03461 in HR-MS, which was precisely coincident with the theoretical value of 161.03565 (Figure 1c). The isotopic peak also matched perfectly. Finally, the signals of the characteristic functional groups (C=O, N-H, and C=C bonds) were clearly observed in the Fourier transformed infrared (FT-IR) and Raman spectra of POxa ( Figures S5 and S6, Supporting Information). All the above results convincingly certified the formation of pure POxa.
The molecular arrangement of POxa crystals was confirmed by utilizing powder X-ray diffraction (XRD). The XRD patterns of POxa crystals or POxa crystals after adequately grinding matched well with the simulated XRD patterns by using the previously reported POxa single crystal structure (No. CCDC 227509), [34] demonstrating the same molecular arrangement with the reported single crystal structure (Figure 1d). [35] In the crystal lattice, two types of intermolecular hydrogen bonds ( Figure 1e) and π-π stacking interactions exist. [34] Benefiting from the strong intermolecular interactions, the POxa could maintain stable up to a temperature near 300°C that is unusually high for small molecules (Figure 1f). [36,37]

Electrochemical Performance of POxa
POxa was then studied as active material for Li storage in coin cells with Li metal as the counter electrode and 1 M LiTFSI in DME/DOL (v/ v = 1:1) as the electrolyte (see Experimental Section). As displayed in the charge-discharge curves at different discharge cut-off potentials, the cells always showed a discharge plateau located at about 1.6 V for the first cycle with capacity of about 100 mAh g −1 (Figure 2), which corresponded to a storage of 0.6 Li ions per POxa molecule. However, the following cycling performance was highly dependent on the discharge cut-off potentials: 1) when the discharge cut-off potential was set at 1.5 V, reversible charge-discharge curves could be observed, in which the recharge plateau was located at approximately 3.4 V and the following discharge plateau was located at about 2.0 V (Figure 2a). The different potential of the first and following discharge plateau could be regarded as the relaxation of strain/stress in the first lithiation due to the large-sized rod-like POxa crystals that transformed into particles with micron sizes during the first cycle ( Figure S7, Supporting Information). [38][39][40] The voltage difference between the charge and discharge plateau was about 1.4 V and such large polarization was abnormal, which will be further discussed in the following. The almost zero charge capacity of cells that were firstly charged to 3.8 V from open circuit voltage also indicated the charge plateau and capacity contribution were indeed derived from redox of POxa rather than some side reactions ( Figure S8, Supporting Information). It should be noted that the gradual decrease in the capacity should be ascribed to the dissolution of POxa in the electrolytes ( Figure S9, Supporting Information). [41] 2) However, interestingly, when the discharge cut-off potential was set at 1.0 V or below (e.g., 0.5 V), the cells became irreversible, although the first discharge gave higher capacity. When the discharge cut-off voltage was set at 1.0 or 0.5 V, the discharge capacity of the first cycle corresponded to a reaction of approximately 1.4electron and 2.2-electron per POxa molecule (after deducting the capacity contribution of super P, Figure S10, Supporting Information), respectively (Figure 2b,c), which could be attributed to the redox reaction of ortho-carbonyl groups. [42,43] However, the discharge plateau above 1.5 V completely disappeared after the first cycle and the total discharge capacity also dramatically decreased from the second cycle after deducting the capacity contribution of super P (Figure 2b,c), indicating an irreversible redox reaction after deeper reduction. [29,44] In addition, as shown in cyclic voltammetry (CV) curves, the POxa electrodes displayed large reduction peaks at around 1.5 V (vs Li/Li + ) during the first discharge ( Figure S11, Supporting Information). However, the following electrochemical properties of POxa electrodes were significantly different in various potential windows. In the narrow potential window of 1.5-3.8 V, an obvious reduction peak at about 2.0 V appeared in the second cycle and the reason of the difference between the first and following cycles has been discussed above (Figure S11a, Supporting Information); while, the reduction peaks above 1.5 V were not observed when the potential window was set at 1.0-3.8 V or 0.5-3.8 V ( Figure S11b,c, Supporting Information). These results again demonstrated that the POxa electrodes would undergo an irreversible redox reaction upon deeper reduction. The special signal of (POxa + Li + + POxa) + seemed to be the reason of the large polarization in the charge-discharge curves. Similar to coordination polymers, [45][46][47] the ortho-carbonyl groups in POxa probably formed a chelation of Li-O 4 coordination unit during the discharge process. The strong chelation effect of the Li-O 4 coordination unit immensely hindered the extraction of Li + ions, leading to a higher charge overpotential.
Although the HR-MS already could strongly evidence the storage of Li + ions in POxa, to further gain insight into the component and structure variation of POxa, ex situ electron paramagnetic resonance (EPR) spectroscopy was carried out. Compared with the negligible EPR signal of the pristine POxa electrode, the storage of Li ions in POxa electrodes led to a strong signal centered at g = 2.001, implying radical species present in the POxa molecule (corresponding to (POxa − + Li + ) 0 , Figure 3f). Upon removal of Li ions, the radical signals weakened, further confirming the reversible lithiation/delithiation process. To ascertain the redox centers of POxa in the discharge/ charge process, a series of ex situ characterizations were conducted. The characteristic peak of the C=O bond in FT-IR spectra apparently weakened while the C-O bond strengthened upon storage of Li + ions and recovered back after charge (Figure 3g), indicating the typical redox reaction of carbonyl groups. [24,35] The intensity variation of the C=O bond in the ex situ Raman spectra ( Figure S13, Supporting Information) and O 1 s XPS spectra ( Figure S14, Supporting Information) displayed a similar tendency. These results were in good agreement with the charge storage mechanism of carbonyl groups. [26,48,49] The structural stability of the POxa during the discharge/charge process was investigated by ex situ powder XRD. Strikingly, in the  (Figure 3h), implying a reversible process of POxa, which could also be inferred from the stable morphology of POxa electrode during cycling ( Figure S7, Supporting Information).

Component and Structure Variation of POxa with Deeper Reduction
Encouraged by the validity of HR-MS for precisely identify the component and structure variation of POxa in the voltage range of 1.5-3.8 V, HR-MS was further performed to monitor the molecular structure variation after deeper reduction. However, when the cells were further discharged to 1.0 V, the peaks of (POxa + Li + ) + , (POxa + Li + + DMSO) + , and (POxa + Li + + POxa) + almost disappeared (Figure 4a). These weak signals could only be observed in regionally amplified HR-MS (by 200-700 times, Figure 4c-e). After further discharging to 0.5 V (Figure 4b), only the extremely weak signals of (POxa + Li + ) + and (POxa + Li + + DMSO) + were found in regionally amplified HR-MS (by 200-1000 times, Figure 4f-h), and their isotope signals as well as the signal of (POxa + Li + + POxa) + thoroughly disappeared. The disappearance of (POxa + Li + + POxa) + peak could be ascribed to the further storage of Li + ions. However, the almost disappearance of other peaks and the isotopic signals suggested that the POxa molecules probably decomposed after storing more Li ions, leading to high initial capacity but inferior cycle stability. At the same time, as shown in the negative mode of HR-MS ( Figure S15, Supporting Information), in contrast with the obvious signals of (POxa-H + ) − in the voltage window of 1.5-3.8 V, negligible (POxa-H + ) − signals were observed when the cells were discharged to 1.0 V or 0.5 V, further demonstrating POxa molecules probably decomposed after storing more Li ions. The results agreed well with the results of positive mode. As expected, the electrodes became amorphous, which should be contributed to the decomposition of POxa ( Figure S16, Supporting Information).
To get further insights into the component and structure variation of POxa upon deeper reduction, the POxa electrode was discharged to 0.5 V and then the entire cell was studied by HR-MS. Two interesting species were observed on the Li metal anode: besides the main peaks of  Figure S17, Supporting Information), the signals of 103.04008 and 117.05553 were observed, which can be assigned to the C 6 H 3 N 2 + and C 7 H 5 N 2 + , respectively (Figure 5a). These results indicated that the C-N bond in the oxamide structure of POxa molecule probably cleaved after deeper reduction (Figure 5b). The 1 H NMR spectra were further performed. When the cell was discharged to 1.5 V, three signature peaks with an integral area ratio of about 1:1:1 ( Figure S18, Supporting Information) were still clearly observed, demonstrating the POxa structure maintained stable after shallow reduction. It should be noted that these signature peaks underwent a upfield shift compared with the pristine POxa electrode (Figure 5c), which could be ascribed to the electrondonating effect after the carbonyl groups accepted electrons. However, when the cell was further discharged to 0.5 V, the signal of N-H at approximately 12 ppm completely disappeared, again proving the decomposition of POxa. Moreover, the signals of the two types of H atoms in benzene ring shifted more to upfields due to the conjugated effect of electron-donating groups (-NH 2 ), which behaved much alike with those of o-Phenylenediamine (o-PDA), confirming the proposed decomposition process of POxa.
These results inspired us to study the component and structure variation of oxamide in LIBs, which has a similar structure to POxa. Interestingly, the charge-discharge curves of the oxamide electrode act quite similarly with those of POxa: the electrodes could be reversibly charged/discharged when the discharge cut-off voltage was set at 1.3 V (Figure 5d), although the overpotential of oxamide was even higher than POxa that could be ascribed to the smaller conjugated system of oxamide and hence poorer conductivity. [39,50] However, the discharge/charge plateaus vanished when the cells were further discharged to a lower voltage of 0.5 V (Figure 5e). Moreover, the structure of oxamide could maintain stable in the electrochemical window of 1.3-3.8 V, while the structure would become amorphous upon discharged to 0.5 V ( Figure S19, Supporting Information). These results confirmed the similar component and structure variation of materials with oxamide structure during cycling.
Based on our previous analyses, the deeper reduction of oxamide should result in the formation of ammonia (Figure 5b), which could be visually observed by the photo images of ammonia detection when oxamide was reduced by Li metal. The color changes in the red litmus test paper and pH test paper ( Figure S20, Supporting Information),

Conclusion
In summary, high-resolution mass spectroscopy (HR-MS) was reported to detect the structure evolution during the redox process and reveal the charge storage mechanism. The consistency between the experimental results and the theoretical results was in two aspects: i) the exact molecular weights and ii) the isotopic peaks. The HR-MS results adequately proved that the oxamides materials could reversibly store lithium ions in the voltage window of 1.5-3.8 V. The HR-MS also indicated that the initial stored Li ions were located at between two molecules. Upon deeper reduction, the oxamides would decompose due to the cleavage of the C-N bonds in oxamide structures, which could be proved by the fragments detected by HR-MS, 1 H NMR, and the generation of NH 3 after the reduction of oxamide by Li. It should be noted that not only the HR-MS here but also other mass spectroscopy techniques could be adopted for detecting the structure evolution. This work provides a strategy to deeply understand the charge storage mechanism of organic electrode materials and will stimulate the further development of characterization techniques to reveal the charge storage mechanism for developing high-performance electrode materials.

Experimental Section
All chemicals were purchased from Energy Chemical or Sinopharm Chemical Reagent Co., Ltd and used without further purification.
Synthesis of Quinoxaline-2,3(1H,4H)-Dione (POxa): POxa was readily prepared according to a previous report with some modifications ( Figure S1, Supporting Information). [51] Briefly, a aqueous HCl solution (20 mL, 4 mmol mL −1 ) of o-diaminobenzene (2.16 g, 20 mmol) was added to another aqueous HCl solution (20 mL, 4 mmol mL −1 ) of oxalic acid (1.80 g, 20 mmol), and then the mixture was heated at 90°C for 5 h. Finally, after cooling down to room temperature naturally, the large-scale colorless POxa crystals (3.07 g, 94.8%) were collected by filtering and washing several times with distilled water ( Figure S2, Supporting Information). 1  Materials characterizations: The 1 H NMR and 13 C NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer by using DMSO-d 6 as solvent. High-resolution mass spectrometric (HR-MS) analysis results were acquired from Agilent 1100 LC/MSD Trap Spectrometer. Thermogravimetric analysis (TGA) was performed on a Pyris1 thermogravimetric analyzer by raising the temperature from room temperature to 600°C at a heating rate of 10°C min −1 under an N 2 atmosphere. Powder X-ray diffraction patterns were collected on an X'Pert3 powder instrument with Cu Kα X-ray radiation. The FT-IR spectra were measured by Bruker ALPHA spectrometer (KBr pellets). The Raman spectra were obtained from LabRAM HR800. The morphology characteristics were observed by scanning electron microscope (ZEISS Gemini 300). X-ray photoelectron spectroscopy (XPS) measurements were conducted by Thermo Scientific K-Alpha at Shiyanjia lab (www. Shiyanjia.com). The EPR spectra were carried out on Bruker A300 with a microwave frequency of 9.854 GHz and microwave power of 20.23 mW. Elemental analyses for C, H, and N were obtained with a Vario Micro Cube Elemental Analyzer.
Electrochemical measurements: The POxa electrode was prepared by casting the mixture of POxa crystals, super P conductive additive, and polyacrylic acid binder with a weight ratio of 6:3:1 in deionized water on Al foil. The film was dried at 80°C for 12 h under vacuum and the average areal mass loading of electrochemically active materials was 1.5-3.0 mg cm −2 . The 2032 type coin cells were assembled in an Ar-filled glove box with a low level of H 2 O (<0.1 ppm) and O 2 (<1.0 ppm) by using 1 M LiTFSI in 1:1 v/v 1,2-dimethoxyethane (DME)/1,3dioxolane (DOL), Glass fiber membrane (Whatman, GF/B) and polypropylene Energy Environ. Mater. 2023, 6, e12557 6 of 8 membrane (Celgard2500), and Li metal as electrolyte, separator, and anode, respectively. The preparation of Super P electrode was similar to the POxa electrode. Super P and polyacrylic acid binder were mixed uniformly with the ratio of 9:1 and coated on the surface of Al foil. Galvanostatic charge-discharge tests were evaluated on the LAND CT2001A battery testing system (Wuhan, China) at room temperature. Cyclic voltammetry (CV) tests were conducted on a BioLogic VMP3 potentiostat. For all the ex situ characterizations, these electrodes were prepared by disassembling the cells in an Ar-filled glove box for further tests.
Capacity calculation: The theoretical capacity of the POxa compound was calculated based on the mass of active material (POxa), according to the following Equation 1: where n is the number of gaining or losing electrons, and M is the relative molecular mass of the POxa compound. Therefore, the theoretical specific capacity of the POxa compound is 165.4 mAh g −1 if one electron can be accepted by per POxa molecule. The specific capacity of the Super P electrode was calculated according to the active mass of Super P. Therefore, in the POxa electrodes, when the discharge cut-off voltages were set at 1.5, 1.0, and 0.5 V, the specific capacities of the Super P were 3.5, 38.0, and 194.1 mAh g −1 , respectively. The capacity contribution of the POxa in the electrode was calculated according to the active mass of only POxa and the following Equation 2: where the C measured is the measured capacity of electrodes. When the discharge cut-off voltages were set at 1.5, 1.0, and 0.5 V, the C measured were 98.3, 257.3, and 462.6 mAh g −1 , respectively. Therefore, the specific capacities of the POxa (C POxa ) were 96.6, 238.3, and 365.6 mAh g −1 , which corresponded to the reactions of approximately 0.