Correlating Electronic Transition With Electrical Output to Reveal Photon‐Mediated Energy Conversion Mechanism in Inner‐Shell Electron Battery

ABSTRACT

Inner‐shell electron batteries present a promising route to ultrahigh‐density energy storage by harnessing the potential energy confined within inner‐shell orbitals. While prior efforts have demonstrated the conversion of potential energy into electricity with photovoltaic (PV) modules, a direct correlation between inner‐shell electron transition and the resulting electrical output remains obscured by interfering photons. Here, by integrating systematic theoretical analysis of electron transition behaviors with a narrowband‐filter‐based experimental setup, we demonstrate a photon‐mediated energy‐conversion framework, exemplified by Mo¹⁵⁺ (370.81 nm), Mo¹⁶⁺ (370.85 nm), and La¹⁹⁺ (371.96 nm) as highly charged ions (HCIs) selected near the peak efficiency wavelength of the PV module, which directly correlates specific electronic transition with power output. After filtering out unrelated wavelengths, we achieve a current response of 2.3 × 10⁻¹² C by capturing 7 × 10⁷ photons emitted from 1 × 10⁶ La¹⁹⁺ ions. Furthermore, the replacement of the narrowband filter (365–375 nm) with an infrared filter to collect all emitted photons from HCIs yields an energy density of 1 × 10⁴ Wh kg⁻¹ based on the mass of 1 × 10⁶ La¹⁹⁺ ions, thereby holding great promise for future ultrahigh‐density energy storage applications.

By integrating an in‐house‐developed light‐emission detection system for highly charged ions (HCIs) with theoretical investigations of inner‐shell electron transition behavior, this study reveals the energy‐conversion mechanism of inner‐shell electron batteries under operational conditions, thereby paving the way for the advancement of high‐energy‐density batteries.

1. Introduction

Given the pivotal role of energy storage technology in sustainable social development, enhancing the device‐level energy density has become a critical objective [1, 2]. Current battery technologies [3, 4, 5, 6] rely on electrochemical redox reactions, which reversibly convert chemical potential into electrical energy through the transfer of outer‐shell electrons [7, 8]. Nevertheless, the limited number of valence electrons and low bond energies impose a fundamental upper limit on the energy density achievable via this mechanism, hindering its application in grid‐scale energy storage, and heavy‐duty transportation [9, 10, 11]. In contrast, inner‐shell electrons (ISEs) constitute the majority of electrons in medium‐ and high‐Z atoms and possess much larger binding energies than the outer‐shell electrons [12]. Exploiting ISEs for energy storage could theoretically yield an orders‐of‐magnitude improvement in energy density (potentially exceeding 10⁷ Wh kg⁻¹) [13]. Unfortunately, unlike the electron exchange in conventional redox reactions, ISEs remain relatively stable due to the Aufbau principle [14], which prevents the production of highly charged ions (HCIs) through standard chemical methods. Therefore, achieving efficient stripping of ISEs and high‐efficiency energy conversion represents a critical prerequisite for realizing high‐density storage based on ISE interactions.

Electron irradiation provides an effective approach to generating HCIs by stripping ISEs from atoms with a high‐energy electron beam [15]. Radial confinement of the HCIs is provided by the space‐charge potential barrier of the electron beam in conjunction with an axial magnetic field [16], while axial confinement is achieved via an electrostatic potential applied to the outer and central drift tubes (Figure S1) [17]. Following electron capture or electron‐impact excitation, HCIs populate excited states. Subsequent radiative transitions between these states give rise to photons with characteristic wavelengths determined by the energy differences between the involved orbitals [18, 19]. Afterwards, radiated photons are collected by a photovoltaic (PV) module, thereby converting the high potential energy stored in the HCI into electrical energy. Although previous studies have confirmed that substantial energy is released during the relaxation of HCIs to the ground state, PV modules indiscriminately absorb all ambient photons. As a result, a clear link between specific electronic transitions of HCIs and the net electrical power output remains absent.

In this study, we systematically investigate the electronic orbital transitions and photon emission behaviors of HCIs with specific charge states. Using a platform consisting of a photomultiplier tube (PMT), a narrowband filter, and an electron beam ion trap (EBIT; Figure 1), we establish a direct link between orbital potential energy and the electrical power output. To maximize the signal response and suppress background noise, we selected three ionic species based on the peak external quantum efficiency (EQE) response of the PV module: Mo¹⁵⁺ (370.81 nm based on 3d ⁹² D _3/2 → ² D _5/2 transition), Mo¹⁶⁺ (370.85 nm based on 3d ⁸³ P ₂ → ¹ D ₂ transition; 373.70 nm based on 3d ⁸¹ D ₂ → ³ F ₃ transition) [20], and La¹⁹⁺ (371.96 nm based on 4d ²³ F ₃ → ³ F ₂ transition) [21]. These ions emit strong spectral lines in the 370–373 nm region, which are well separated from the broadband thermal radiation of the electron gun. After filtering out unrelated wavelengths, we successfully achieved a current response of 2.3 × 10⁻¹² C by capturing 7 × 10⁷ photons at 371.96 nm emitted from 1 × 10⁶ La¹⁹⁺ ions. Furthermore, when the narrowband filter (365–375 nm) was replaced with an infrared filter to collect all HCIs‐emitted photons, the ISEs transition process yielded an energy density of 1 × 10⁴ Wh kg⁻¹ based on the mass of 1 × 10⁶ La¹⁹⁺ ions.

FIGURE 1.

Schematic illustration of the experiment setup. The specific spectral lines emitted by HCIs are directed either into the PV module for photon‐to‐electricity conversion or into the PMT for photon detection.

2. Results and Discussion

Investigating photon radiation from electron transitions and the subsequent energy conversion requires determining the emission wavelengths for specific orbital transitions of selected ions and examining the corresponding photoelectric conversion behavior. Meanwhile, since the electron gun emits long‐wavelength red/infrared radiation during heating, minimizing interference with the HCI‐emitted photon signal favors short‐wavelength photons and PV materials with high‐efficiency in the same range. As shown in Figure S2, the PV module exhibited high EQE values in the wavelength range of 340–500 nm. Therefore, to optimize the coupling between HCI photon radiation and the PV module energy conversion, we first analyzed the relationship between electron transition behavior and radiation wavelength across different elements and orbitals. Figure 2a presents the characteristic optical magnetic‐dipole (M1) emission lines from highly charged Mo ions. According to collisional‐radiative spectral modelling, Mo¹⁵⁺ (3d ⁹) exhibits one characteristic optical M1 line, whereas Mo¹⁶⁺ (3d ⁸) exhibits six characteristics optical M1 lines. The three Mo emission lines that fall within the 370–373 nm spectral region were therefore selected for subsequent investigations based on the EQE values of the PV module. These lines correspond to M1 transitions ² D _3/2 → ² D _5/2, ³ P ₂ → ¹ D ₂, and ¹ D ₂ → ³ F ₃, located at 370.81, 370.85, and 373.70 nm, respectively, which were calculated from relativistic many‐body perturbation theory [22] and spectral modelling performed with the Flexible Atomic Code [23]. For La¹⁹⁺, seven optical M1 lines are produced by fine‐structure transitions within the Sr‐like 4d ² ground configuration. The 4d ² level structure and transition rates were calculated from the multi‐configuration Dirac–Hartree–Fock method [24] followed by relativistic configuration interaction calculations [25], among which the ³ F ₃ → ³ F ₂ transition at 371.96 nm was selected as the target (Figure 2b). The estimation of ion density was also conducted under the predefined experimental conditions, and the Mo¹⁵⁺ (7.2 × 10⁵ cm⁻³), Mo¹⁶⁺ (7.5 × 10⁵ cm⁻³), and La¹⁹⁺ (1.0 × 10⁶ cm⁻³) exhibited markedly higher ion densities than the other charge states (Figure 2c,d).

FIGURE 2.

Energy level diagrams of the ground state configurations of (a) Mo¹⁵⁺/Mo¹⁶⁺ ions and (b) La¹⁹⁺ ions. The simulated ion density and detected Wien intensity of different (c) Mo ions and (d) La ions. The spectra of (e) Mo ions and (f) La ions within the range of 360–380 nm. The insets show the characteristic emission peaks fitted with Gaussian functions. (g) Schematic illustration of the charge state detector and spectrum measurement with the EBIT equipment.

Experimental investigations were subsequently carried out on the low‐energy EBIT device [26, 27] to validate the above simulations. Typically, HCIs are produced by electron‐impact ionization driven by an electron beam and are subsequently confined within the drift tube [28]. The ionic valence state is controlled by the energy of the electron beam, which is defined by the potential difference between the electron gun and the target [29]. To accurately obtain the valence state distribution and characterize the spectral emission of HCIs, a charge state distribution (CSD) analysis system and a spectroscopic detection system were equipped in the EBIT. In the CSD analysis system, the Wien filter separates ions according to specific charge‐state‐dependent trajectories under crossed electric and magnetic fields, allowing selected charge states to pass through. The microchannel plate (MCP) then detects the transmitted ions and converts their impact signals into measurable CSD information. The spectroscopic detection system was employed outside the side window of the trap region, which contains a focus lens and the spectrograph, utilizing the charge‐coupled device (CCD) array to detect photons of different wavelengths (Figure 2g). The whole system is placed in a dark room, so the photons can pass through the window, be focused by the lens, and finally enter the spectrograph.

To successfully introduce the target elements into the chamber, suitable molecular precursors must be screened based on sufficient volatility, delivery stability, low background contamination, and controllable injection flux. Conventional metal compounds, such as metal oxides, halides, or inorganic salts, are generally unsuitable because of their high lattice energies, low vapor pressures, strong ionic character, and potential introduction of additional O‐ or Cl‐related backgrounds or pipeline deposition. Consequently, Mo(CO)₆ and tris(i‐propylcyclopentadienyl) lanthanum (La(iPrCp)₃) were selected as the volatile metal‐complex precursors. Whereas Mo(CO)₆ was chosen because its simple carbonyl ligands afford relatively clean dissociation fragments, La(iPrCp)₃ serves to stabilize the La³⁺ center through its Cp‐based ligands, thereby enhancing gas‐phase delivery. Based on the preceding theoretical simulation results, the experimental validation was first carried out for the element Mo. Typically, Mo(CO)₆ was introduced into the chamber via thermal evaporation, where it was then irradiated by the electron beam and ionized to highly charged states. Moreover, the wavelengths of photons radiated from Mo¹⁵⁺ and Mo¹⁶⁺ ions were measured by the Czerny–Turner spectrometer (Andor Shamrock SR‐303i). As shown in the M1 line spectra (Figure 2e), two distinct spectral lines located at 370.85 and 373.70 nm were observed, corresponding well to the theoretical simulation results (Figure 2a). The emission at 373.70 nm can be attributed to the transition in Mo¹⁶⁺ from ¹ D ₂ to ³ F ₃, while the emission at 370.85 nm, with an apparent enhancement in the measured intensity, can be attributed to the M1 transitions from ² D _3/2 to ² D _5/2 (Mo¹⁵⁺) and ³ P ₂ to ¹ D ₂ (Mo¹⁶⁺). The overlapping peak was assigned by selecting electron beam energies for spectral acquisition based on the CSD results. In the case of La¹⁹⁺, La(iPrCp)₃ was used for HCI production. Additionally, a strong emission line at 371.96 nm was observed (Figure 2f), which can be attributed to the transition of ³ F ₃ → ³ F ₂, originated from the 4d ² open‐shell configuration.

To investigate the conversion of HCI potential energy into electrical power and establish a direct correlation between photon radiation and electrical output, we examined the electrical output of the device under varying ion densities. To regulate the ion charge state, we first analyzed the valence state distribution of Mo ions under various beam energies ranging from 540 to 620 eV. As shown in Figure S3, at an electron beam energy of 580 eV, Mo¹⁵⁺, and Mo¹⁶⁺ yielded the highest signal intensities (Figure 2c), and hence, the electron beam energy was set to 580 eV for subsequent studies. Moreover, by modification of the trap potential, which is determined by the applied voltage difference between the drift tube 2 and drift tube 3 (Figure 3a), the axial confinement strength of HCIs can be modulated. As the trap potential ramps from 10 to 90 V, the total Mo ion signal intensity increases, with Mo¹⁵⁺/Mo¹⁶⁺ showing the highest ion density among other valence states (Figure 3c). Specifically, the Mo¹⁵⁺/Mo¹⁶⁺ ion density in the trapping region exhibited a clear increasing trend from 4 × 10⁵ to 8 × 10⁵ cm⁻³ as the trap potential increased from 10 to 90 V (Figure 3d). Additionally, to isolate the characteristic emission signals of Mo¹⁵⁺/Mo¹⁶⁺, a narrowband optical filter with a transmission window centered at 370 nm and a full width at half maximum (FWHM) of 10 nm was mounted on the side window of the trap region (Figure S4). Therefore, this filter selectively transmitted the characteristic HCI emission lines near 370–373 nm while blocking unrelated photons, especially the long‐wavelength red/infrared radiation from the electron gun. With the narrowband filter in place, as the trap potential increases from 10 to 90 V, the current output of the PV module rises from 900.1 to 903.4 pA (Figures 3e and S5). The increasing trend is consistent with the growth in Mo¹⁵⁺/Mo¹⁶⁺ ion density (Figure 3d), demonstrating that the specific 370.8 and 373.7 nm photons radiated by HCIs were efficiently converted into electrical energy. To further consolidate the above observation, we investigated the effect of the sample injection rate on the electrical output behavior because the injection rate also affects the ion density within the trap region. The implementation of the differential gas injection system allows the modification of the sample injection rate via tuning the heating temperature (Figure 3b). Upon raising the injection pressure from 2.0 × 10⁻⁶ to 7.5 × 10⁻⁶ Pa, a clear increase in the Mo ion density within the trapping region was observed (Figure 3f), corresponding to an increase of Mo¹⁵⁺/Mo¹⁶⁺ ion density from 6 × 10⁵ to 7 × 10⁵ cm⁻³ (Figure 3g). More importantly, this increase directly affected the photon emission behavior (Figure S6), consequently raising the current output from 911.7 to 914.0 pA (Figure 3h).

FIGURE 3.

Schematic diagram showing the regulation of (a) trap potential and (b) injection pressure. (c) The CSD of the Mo ion under different trap potentials. (d) The estimate ion density of Mo¹⁵⁺ under different trap potentials from 10 to 90 V. (e) The current output under different trap potentials with the narrowband optical filter centered at 370 nm. (f) The CSD of the Mo ion under different injection pressures. (g) The ion density under different injection pressures. (h) PV current output measured under different injection pressures.

To verify the generality of the above findings, we further analyzed the La element and replaced the injected sample with La(iPrCp)₃ for La HCI generation. The investigation began with the modulation of charge‐state distribution and ion density modification. The electron beam energy was first optimized (Figures 4a and S7), and an energy of 600 eV produced the maximum La¹⁹⁺ signal intensity. Meanwhile, deepening the ion trap voltage progressively enhances the La¹⁹⁺ signal (from 0 to 40 V), corresponding to an increased ion population (from 1.93 × 10⁵ to 1.02 × 10⁶ cm⁻³) of La¹⁹⁺ (Figure 4b). Therefore, the radiative emission associated with the characteristic transition of La¹⁹⁺ at 371.96 nm becomes more intense. To enable quantitative measurement of the 371.96 nm photon emission, a PMT was installed at the side window of the trap region, in combination with a narrowband optical filter. The PMT operates by converting incident photons into photoelectrons at the photocathode, which are then amplified via a dynode cascade. It should be noted that because the collected electrons flow toward the anode under the applied bias, the output was recorded as a negative current in the measurement circuit. As the trap potential increased from 0 to 40 V, the PMT signal showed progressively enhanced fluctuations, with the output current changing from approximately −233 to −299 pA (Figure 4c). At the same side window position, a PV module was further employed to convert the emitted photons into an electrical output. As the trap potential increases from 0 to 40 V, the PV array output current rises from 598.2 to 600.5 pA (Figure 4d), corresponding to a net photocurrent of approximately 2.3 pA. Based on the PMT response at a trap potential of 40 V, the photon emission rate from HCIs is estimated to be approximately 7 × 10⁷ photons·s⁻¹. Such a photon flux is expected to generate a photocurrent of approximately 5 pA in the PV module based on Equation S2. The experimentally measured photocurrent of approximately 2.3 pA is lower than this estimated value, which can be attributed to limited photon collection efficiency and optical transmission losses. Moreover, the injection pressure played a critical role in regulating the La¹⁹⁺ ion population in the trap region. With increasing injection pressures, the intensity of La¹⁹⁺ ions was enhanced, and the corresponding ion density increased from 1.22 × 10⁶ to 1.42 × 10⁶ cm⁻³ (Figure 4e). The larger La¹⁹⁺ population subsequently gave rise to stronger characteristic photon emissions at specific wavelengths. These photons were then converted into an electrical signal by the PV device, producing a pronounced photocurrent of approximately 3.62 pA (Figure 4f).

FIGURE 4.

The CSD of the La ion under different (a) beam energy and (b) trap potential values. (c) PMT current–time curves measured for the La¹⁹⁺ characteristic emission at 371.96 nm under different trap potentials ranging from 0 to 40 V. (d) The current output under different trap potentials. (e) The CSD of the La ion under different injection pressures. (f) The current output under different injection pressures.

To evaluate the energy conversion capability of photons across a broader spectrum, the narrowband filter was replaced by a broadband filter. Based on CSD analysis system in Figure 4b, La ion accounts for approximately 90%. As shown in Figure S8, when photons over a broader wavelength range were allowed to pass through, the PMT exhibited an enhanced current response of 112 pA (Figure S9), which is 67.2% higher than that with the narrowband filter. Meanwhile, when the trap voltage was gradually increased from 0 to 80 V, the device current output also increased accordingly (Figure 5a). By comparing the difference in ion count and current output between the trap voltages of 0 and 80 V, it can be found that a difference of 1 × 10⁶ La¹⁹⁺ ions in the 0.5 cm³ drift tube 2 results in a current output difference of 58.8 pA. (Figure 5b). In addition, the device voltage output likewise exhibited an increasing trend with trap voltage (from 1.17 to 1.21 mV; Figure 5c). A direct comparison of the voltage output differences under varying trap voltage conditions reveals that 1 × 10⁶ La¹⁹⁺ ions contribute a voltage output difference of 40 µV (Figure 5d). Integrating the above findings on current and voltage output, as well as the ion count, the device thus yielded an energy density of 1 × 10⁴ Wh kg⁻¹ based on the mass of La¹⁹⁺ ions.

FIGURE 5.

(a) The current output of the La ions under different trap potentials ranging from 0 to 80 V. (b) The relationship between La ions current output and trap potential. (c) The voltage output of the La ions under different trap potentials ranging from 0 to 80 V. (d) The relationship between voltage output and trap potential for La ions.

3. Conclusion

In summary, we have demonstrated a photon‐mediated energy‐conversion pathway based on the radiative relaxation of HCIs. Through systematic theoretical analysis of the valence state distributions and characteristic photon emission wavelengths during ISE transitions, Mo¹⁵⁺ (370.81 nm; 3d ⁹² D _3/2 → ² D _5/2), Mo¹⁶⁺ (370.85 nm; 3d ⁸³ P ₂ → ¹ D ₂), and La¹⁹⁺ (371.96 nm 4d ²³ F ₃ → ³ F ₂) ions were selected as the research subjects. Therefore, guided by theoretical simulation results, an in‐house experimental setup comprising charge‐state control, spectroscopic identification, PMT photon detection, PV modules, and the EBIT was established to experimentally investigate the photon emission behavior and the subsequent energy conversion processes. Typically, after filtering out irrelated photons using a narrow band filter, we successfully achieved a current response of 2.3 × 10⁻¹² C by capturing 7 × 10⁷ photons at 371.96 nm emitted from 1 × 10⁶ La¹⁹⁺ ions, building the direct link of the ISE transition−photon radiation−energy conversion. More importantly, a filter with an extended wavelength range was employed to amplify the photon transmission, yielding an estimated energy density of 1 × 10⁴ Wh kg⁻¹ based on the weight of HCIs. These results collectively provide theoretical and experimental evidences for the ion–photon–electron energy conversion process and establish a framework linking HCI charge states, radiative transitions, photon flux, and electrical output.

Author Contributions

Kai Mo: writing – review and editing, writing – original draft, conceptualization, methodology, validation, investigation, formal analysis, visualization, software. Yuhang Ge: resources, writing – review and editing, conceptualization, investigation, methodology, validation, formal analysis, software. Chen Zhao: project administration, writing – review and editing, resources, methodology, supervision, formal analysis. Jinghao Zhang: writing – review and editing, methodology. Yi Jiang: writing – review and editing, methodology. Jiamin Chen: writing – review and editing, visualization. Xin Liu: writing – review and editing, validation. Ke Yao: writing – review and editing, resources. Meng Liao: writing – review and editing, validation. Peining Chen: validation, writing – review and editing. Bingjie Wang: validation and writing – review and editing. Bingsheng Tu: writing – review and editing, methodology, supervision, resources, project administration, data curation. Huisheng Peng: conceptualization, writing – review and editing, funding acquisition, resources, project administration, supervision, data curation, formal analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie73829‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.