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[Hideka Ando](https://orcid.org/0009-0004-1487-4478), Yasuhiro Toyoda, Kenya Fujino, [Kenjiro Hashi](https://orcid.org/0000-0002-0320-4768), [Shinobu Ohki](https://orcid.org/0000-0002-7357-3833), Yuki Fujii, Daisuke Igarashi, Shinichi Komaba, Kazuma Gotoh

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[The structural stability and capacity increase of a phosphorus-doped hard carbon produced by zinc oxide templating used in sodium-ion batteries:](https://mdr.nims.go.jp/datasets/e6c15b5a-0ba5-4982-8505-1884cb7b155e)

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Carbon Reports 5(1): 40-47 (2026)40 H. Ando et al., Carbon Reports 5 (2026) 40–47.The structural stability and capacity increase of a phosphorus-doped hard carbon produced by zinc oxide templating used in sodium-ion batteries:Hideka Andoa),*,  Yasuhiro Toyodab),  Kenya Fujinob),  Kenjiro Hashia),  Shinobu Ohkia),  Yuki Fujiic),  Daisuke Igarashic),  Shinichi Komabac) and Kazuma Gotohb)a) National Institute for Materials Science: 3–13 Sakura, Tsukuba, Ibaraki 305–0003, Japanb) Center for Nano Materials and Technology, Japan Advanced Institute of Science and Technology: 1–1 Asahidai, Nomi, Ishikawa 923–1292, Japanc) Department of Applied Chemistry, Tokyo University of Science: 1–3 Kagurazaka, Shinjuku-ku, Tokyo 162–8601, JapanSodium (Na)-ion batteries (NIBs) are attracting increasing attention as next-generation energy storage systems because they do not rely on rare metals. Hard carbon (HC) is considered their most promising anode material. By tailoring the pore structure by templating methods, HC materials with a high energy density have been developed for NIBs. However, further improvements are required to achieve the desired properties without compromising the excellent characteristics already achieved. This study aims to further increase the battery capacity of zinc oxide (ZnO)–templated HC using a simple phospho-rus (P)-doping method. We investigated the effects of soaking in phosphoric acid and subsequent heat treatment on the carbon morphology and electrochemical properties. The results showed that P doping increased the battery capacity without altering the ZnO–templated HC morphology. Both the sloping and plateau regions of the capacity increased, suggesting that P doping promotes Na adsorption on the carbon surface and Na storage between the layers and in the pores. Furthermore, the types of P functional groups depended on the synthesis conditions and influence the battery performance. These findings show that sur-face modification with specific P functional groups can effectively increase the Na storage capability of HCs.KEYWORDS : Sodium-ion batteries, Hard carbon, Phosphorus doping, Anode, ZnO–templating1. IntroductionThe development of sustainable energy storage technologies is es-sential to meet the increasing global energy demand while mitigating environmental impacts. Lithium (Li)-ion batteries (LIBs) are widely used in diverse applications owing to their excellent performance, including high energy density and long cycle life [1]. However, LIBs depend on rare metals, such as Li and cobalt, as raw materials, raising concerns regarding resource shortages and price increases. Sodium (Na)-ion batteries (NIBs) are attracting attention as next-generation systems that do not require rare metals [2].The raw materials for NIBs are abundant in the Earth’s crust, and NIBs have operating principles and battery components similar to those of LIBs. However, their energy density is generally lower than that of LIBs because Na has a greater atomic mass and a higher stan-dard electrode potential (−2.71 V versus [vs.] the standard hydrogen electrode [SHE]) than Li (−3.04 V vs. SHE). To enable practical applications, it is essential to develop electrode materials with perfor-mance comparable to those used in LIBs.Graphite, the common anode material for LIBs, is unsuitable for NIBs owing to the instability of Na intercalation between graphene layers. While Li can be intercalated into graphite up to the composi-tion LiC6, the maximum Na intercalation without solvent cointercala-tion is limited to the composition NaC64 [3, 4].Hard carbon (HC) is considered the most promising anode material for NIBs [5]. HC provides suitable Na storage sites, including en-larged interlayer spacing between graphene layers as well as numer-ous defects and pores. Charge–discharge profiles of half-cells with HC anodes exhibit two regions: a sloping region above 0.1 V and a plateau region below 0.1 V. Na storage mechanisms have been exten-sively investigated [6–18].The “adsorption–intercalation–pore filling” model has recently  *  Corresponding Author, E-mail: ANDO.Hideka@nims.go.jp  (Received October 21, 2025, Accepted December 5, 2025)https://doi.org/10.7209/carbon.050106  (J-STAGE Advance published December 12, 2025) Carbon Reports Research PaperVol. 5 No. 1 2026 40–47Research Paper41H. Ando et al., Carbon Reports 5 (2026) 40–47.been regarded as a plausible mechanism [7, 15, 19, 20]. This model proposes three processes: (1) adsorption at edges and defects of carbon layers in the initial sloping region, (2) intercalation into inter-layer spacing from the end of the sloping region to the plateau region, and (3) pore filling toward the end of the plateau region. Addition-ally, Na stored in pores has been observed to form quasimetallic Na clusters near 0 V at the end of the plateau region [8, 16, 21–24]. The slope capacity supports rapid charging and discharging, whereas the plateau capacity contributes to a high energy density. Therefore, de-veloping electrode materials tailored to specific NIB applications is necessary.To develop carbon anode materials with increased performance, various approaches have been explored, including changing precur-sors, adjusting synthesis conditions, and modifying surfaces. Among these approaches, HCs synthesized using magnesium oxide (MgO)– or zinc oxide (ZnO)–templating techniques have been reported [25, 26]. These methods create numerous closed pores suitable for Na storage, particularly facilitating the formation of Na clusters that con-tribute to high energy density. MgO– and ZnO–templated HCs have demonstrated high capacities of 478 and 464 mAh g−1, respectively, with substantial plateau capacities [25, 26]. Heteroatom doping (for example, nitrogen [N], phosphorus [P], and sulfur [S]) in HCs is an-other effective strategy for enhancing performance [27, 28].P doping is particularly effective at increasing capacity [9, 29–33]. Heteroatom doping alters the morphology and physicochemical prop-erties of HCs, including interlayer spacing, surface wettability, and electronic conductivity [34, 35]. The expansion of the interlayer spac-ing facilitates Na intercalation, and the introduced defects serve as Na adsorption sites. Typically, heteroatom-doped HCs are synthesized by calcining precursors containing the desired heteroatoms, such as thie-nyl-based polymers, N-doped carbon nanofibers, and biomass materi-als [36–38]. However, available heteroatom-containing precursors are limited unless specific handling procedures are employed, restricting the development of advanced materials. Another heteroatom-doping approach is the calcination of mixtures of precursors and dopants (for example, urea for N-doping, sulfur powder or sulfuric acid for S-doping, and phosphoric acid or sodium dihydrogen phosphate for P-doping) [31, 39, 40]. However, this process often impairs the inher-ent characteristics of undoped carbon materials. Therefore, improving the desired properties while preserving the excellent characteristics of the developed materials remains necessary.This study proposes a simple P-doping method to increase the capacity of HCs synthesized via template methods. ZnO–templated HCs were soaked in phosphoric acid and subsequently heat-treated (220 °C and 600 °C) to introduce P dopants. Although P was pre-sumed to be incorporated only on the surface without altering the bulk structure, slope and plateau capacities increased. Moreover, the doping conditions influenced battery performance, suggesting that the type of incorporated P is crucial in determining the electrochemical properties of HCs as NIB anode materials. This study provides novel insights into a simple P-doping method that further improves the capacity of ZnO–templated HCs without compromising the carbon morphology optimized for NIBs. The proposed method could also apply to other carbon materials for advanced NIB applications.2. Experimental2.1 Synthesis of ZnO–templated HC samplesZnO–templated HC samples were prepared following a previ-ously reported method [26]. Zinc gluconate was preheated at 600 °C at a rate of 10 °C min−1 and held for 1 h under N2 gas flow (500 mL min−1). The resulting product was ground using an agate mortar and sonicated in 1 M hydrochloric acid for 1 h. The material was then washed with deionized water and vacuum-dried overnight at 110 °C. The washed precursor was calcined at 1,400 °C at a rate of 10 °C min−1 and maintained at this temperature for 1 h under N2 gas flow (500 mL min−1). The resulting undoped ZnO–templated HC sample was designated as HC–ZnO.P-doped HC–ZnO samples were prepared by sonicating the syn-thesized HC–ZnO in 1 and 2 M phosphoric acid for 1 h, followed by heat treatment. The sonicated samples were collected by vacuum filtration and heated at 220 °C overnight in air; these samples were denoted as 1M-P220 and 2M-P220, respectively. To evaluate the ef-fect of heating temperature, the 1M-P220 sample was further heated at 600 °C for 1 h under N2 gas flow (500 mL min−1) and designated as 1M-P600.2.2 Characterization of pristine HC samplesThe morphology and elemental distribution of each sample were examined using scanning electron microscopy (SEM; TM3030Plus, Hitachi High-Tech Corp.) equipped with an energy-dispersive X-ray spectrometer (EDS). Elemental analysis of P was conducted at the Center for Organic Elemental Analysis, Kyoto University. Powder X-ray diffraction (XRD) patterns were obtained using a MiniFlex600 diffractometer (Rigaku Corp.) with copper (Cu) Kα radiation at 40 kV and 10 mA. X-ray photoelectron spectroscopy (XPS) profiles were recorded using a KRATOS AXIS-ULTRA DLD instrument (Shimad-zu Corp.) with an aluminum (Al) Kα X-ray source operated at 15 kV and a 10 mA emission current.Solid-state 31P nuclear magnetic resonance (NMR) measure-ments were performed on an NMR system (JNM-ECZ500R, JEOL) equipped with an 11.7 T magnet at the National Institute for Materials Science. An aqueous solution of 85% phosphoric acid was used as a reference at 0 ppm. A single-pulse sequence was applied with a pulse length of 2.9 µs and a recycle delay of 30 s. The magic angle spinning (MAS) rate was 10 kHz.2.3 Electrochemical measurementsNa half-cells were assembled using CR2032 coin cells in an argon-Vol. 5 No. 1 2026Research Paper42 H. Ando et al., Carbon Reports 5 (2026) 40–47.Carbon Reportsfilled glove box. The anodes comprised HC samples as the active material, conductive carbon black (EQ-Lib-SuperP, MTI Corp.), and a poly(sodium acrylate) (Kishida Chemical Co., Ltd.) as a binder at a weight ratio of 85 : 10 : 5. Deionized water was added as the disper-sion medium to form a slurry. The slurry was applied to a 100 µm thickness on Cu foil using the doctor blade method and vacuum dried at 110 °C for 1 h. After drying, the electrodes were punched to a 15.95 mm diameter and vacuum-dried overnight at 150 °C. Na metal (≧99.8%; Sigma-Aldrich Co. LLC) served as the counter electrode, and the electrolyte was 200 µL of 1.0 M NaPF6 in ethylene carbon-ate (EC)/diethylene carbonate (DEC) (1 : 1 v/v%). A glass fiber filter (GF/D grade; Whatman) was used as the separator.Galvanostatic charge–discharge tests (HJ1020mSD8, Hokuto Denko Corp.) were conducted to evaluate the electrochemical prop-erties of the HC–ZnO and P-doped HC–ZnO samples. The potential range was 0.002–2.0 V (vs. Na+/Na) at a 25 mA g−1 current density. During discharge, the potential was maintained at 0.002 V for 8 h. Cyclic voltammetry (CV) measurements (HZ-Pro S2A, Meiden Ho-kuto Corp.) were performed between 0.0 and 2.0 V (vs. Na+/Na) at a 2 mV s−1 scan rate using cells after 10 galvanostatic cycles.2.4 31P NMR measurements of sodiated and desodiated samplesThe state of doped P after charge–discharge cycling was evaluated by solid-state 31P MAS NMR. To eliminate P signals originating from the electrolyte, samples for NMR analysis were obtained from cells assembled with NaClO4 in EC/DEC (1 : 1 v/v%) as the electrolyte rather than NaPF6. Coin cells were disassembled in an argon-filled glove box after charging or discharging. The anodes were washed with dimethyl carbonate, and the anode materials were collected by scraping the electrode surface. The recovered samples were packed into NMR tubes with polyvinylidene fluoride powder to reduce con-ductivity effects. The experimental conditions were identical to those used for the initial HCs.3. Results and Discussion3.1 CharacterizationThe morphologies of HC–ZnO, 1M-P220, and 1M-P600 were examined using SEM. As shown in Fig. 1, all samples exhibited ir-regular particles with sizes ranging from 20 to 50 µm. P doping did not produce considerable changes in the morphology of the ZnO–templated carbon.EDS mapping and elemental analysis of 1M-P220 and 1M-P600 are presented in Fig. 1(b), (c) and Table 1. In the EDS maps of 1M-P220, a small region with concentrated P was observed. The P con-centration in the bright region was 0.81 at%, compared to 0.13 at% in the dark region (Fig. S1 and Table 1). The overall P content in 1M-P220 was 0.17 at%, indicating a surface-wide P distribution. Regions of 1M-P220 with high P concentrations may contain residual phosphoric acid. In contrast, 1M-P600 showed no regions with a high P concentration.The P content in 1M-P600, determined via EDS, decreased to 0.05 at%, likely owing to phosphoric acid pyrolysis. During heating at 400–500 °C, phosphoric acid undergoes dehydration and condensa-tion, reacts with carbon, and is released as gaseous products [30]. Organic elemental analysis also indicated a decrease in P content from 0.25 wt% in 1M-P220 to 0.12 wt% in 1M-P600, consistent with EDS results.The chemical states of the doped P were evaluated using XPS and 31P NMR, as depicted in Figs. 2, 3, respectively. The high-resolution Fig. 1 (a) SEM image of HC–ZnO, (b) SEM image and EDS mapping of 1M-P220, and (c) SEM image and EDS mapping of 1M-P600.Table 1 EDS elemental analysis results of undoped and P-doped ZnO–template HC samples.Elemental Analysis/at%C O PHC–ZnO 96.48 3.51 —1M-P220 (average) 94.21 5.62 0.171M-P220 (P accumulation area) 84.29 14.90 0.811M-P220 (excluding P accumulation area) 94.72 5.15 0.131M-P600 96.90 3.05 0.05Research Paper43H. Ando et al., Carbon Reports 5 (2026) 40–47.P 2p spectrum of 1M-P220 exhibited a single peak at 134.2 eV, cor-responding to the phosphate groups (R–O–PO3) [41, 42]. Addition-ally, the 31P NMR spectra of 1M-P220 showed two types of signals: a sharp peak at 0 ppm and a broad peak at approximately −20 ppm. The sharp signal was assigned to residual phosphoric acid, consistent with EDS mapping, whereas the broad signal was attributed to R–O–PO3 species as the chemical shift of PO(OC6H5)3 is −18 ppm [43]. These observations align with the XPS results. The broadening of the signal reflects the structural complexity of the carbon framework bonded to phosphate species.The XPS profile of sample 1M-P600 was deconvoluted into two peaks at 133.4 and 134.3 eV. The 134.3 eV peak corresponds to R–O–PO3 species observed in 1M-P220, and the 133.4 eV peak was attributed to R–PO3 and R2–PO2 species [44, 45]. The appearance of this additional peak indicates that high-temperature treatment pro-motes direct P–carbon bond formation.The 31P NMR spectrum of 1M-P600 exhibited two peaks at −4 and −18 ppm: one relatively sharp and one broad. The broad −18 ppm peak corresponds to phosphate species (R–O–PO3), as observed in 1M-P220. The −4 ppm peak was broader and shifted to a lower frequency side than the signal at 0 ppm in 1M-P220, suggest-ing that it does not originate from residual phosphoric acid. Although its exact assignment remains uncertain, the correlation with XPS results suggests that this peak may originate from phosphonate func-tional groups containing C–P bonds. Hereinafter, this peak is referred to as the R–PO3 species, consistent with the XPS results. These re-sults indicate that P introduced via low-temperature treatment primar-ily remains as surface phosphate and residual phosphoric acid, where-as high-temperature treatment promotes direct C–P bond formation.XRD patterns of HC–ZnO, 1M-P220, and 1M-P600 are shown in Fig. 4. All patterns exhibited two broad diffraction peaks at 2θ= 15–30° and 41–47°, corresponding to the 002 and 10 Bragg diffrac-tions, respectively. Although ZnO signals were observed before acid treatment (Fig. S2), they disappeared in the final samples, confirm-ing complete ZnO removal. None of the samples exhibited notable changes in peak positions or peak widths. Based on the diffraction patterns, the calculated average interlayer distance for all samples was 0.388 nm. As P doping was performed after high-temperature carbonization, no substantial structural changes were observed.3.2 Electrochemical measurementGalvanostatic charge–discharge testing was performed on each HC sample. In addition to ZnO–templated HCs, MgO–templated HCs Fig. 2 High-resolution XPS profiles of the P 2p region of (a) 1M-P220 and (b) 1M-P600.Fig. 3 31P MAS NMR spectra of pristine P-doped HC samples. Fig. 4 XRD patterns of undoped and P-doped HC samples.Vol. 5 No. 1 2026Research Paper44 H. Ando et al., Carbon Reports 5 (2026) 40–47.Carbon Reportswere also studied following a previously reported method [25]. Al-though MgO–templated HCs demonstrated high capacity, consistent synthesis of samples with stable performance was sometimes chal-lenging owing to subtle differences in the prefreezing state prior to freeze-drying and slight variations in synthesis environments. This variability prompted the selection of ZnO–templated HCs, which exhibited improved reproducibility, despite still showing some variation in reversible capacity. To evaluate the effects of P doping, the electrochemical performance of undoped and P-doped samples with the same lot numbers was compared. For clarity and trace-ability, each sample was labeled with its lot number appended to the sample name (for example, HC–ZnO_Lot.1). Fig. 5(a) presents the galvanostatic charge–discharge curves of HC–ZnO_Lot.1 and its P-doped derivatives (1M-P220_Lot.1 and 1M-P600_Lot.1). The initial charge (desodiation) capacities of HC–ZnO_Lot.1, 1M-P220_Lot.1, and 1M-P600_Lot.1 were 310, 432, and 369 mAh g−1, respectively. The P-doped HC samples exhibited higher capacities than those of the undoped HC, with 1M-P220_Lot.1 showing a 39% increase. In addition, the initial coulombic efficiencies of HC–ZnO_Lot.1, 1M-P220_Lot.1, and 1M-P600_Lot.1 were 89%, 88%, and 85%, respec-tively. HC–ZnO_Lot.1 and 1M-P220_Lot.1 exhibited similar cycling stability during the initial five cycles (Fig. S3). The capacity enhance-ment in 1M-P220 was consistently observed across different lot num-bers (Fig. S4). As the sample morphology remained unchanged after P doping, the improved performance is attributed to the P functional groups.To evaluate the effect of phosphoric acid concentration on battery performance, a sample designated 2M-P220 was synthesized using 2 M phosphoric acid during doping. The initial charge capacity of 2M-P220_Lot.1 was 285 mAh g−1, lower than that of HC–ZnO_Lot.1 (Fig. S5). HC–ZnO is likely to contain a limited number of reaction sites due to the high carbonization temperatures. Excess phosphoric acid may have remained unreacted, thereby inhibiting Na storage in HCs. Optimization of treatment conditions will be required in future studies to maximize electrochemical performance.To analyze the changes in battery capacity, the initial charge ca-pacities of each HC sample were separated into slope and plateau capacities at 0.1 V. The initial charge capacities of HC–ZnO_Lot.1, 1M-P220_Lot.1, and 1M-P600_Lot.1, divided into slope and plateau regions at 0.1 V, are shown in Fig. 5(b). The results showed that 1M-P220 exhibited a considerable increase in plateau capacity and a slight increase in slope capacity, whereas 1M-P600 showed increases in slope and plateau capacities, though to a lesser extent than 1M-P220.Among the proposed Na storage mechanisms in HCs, slope capac-ity is currently considered to originate from Na adsorption on the car-bon surface [7, 15, 19]. Therefore, the observed change in slope ca-pacity upon P doping likely results from the addition of P functional groups on the carbon surface, which act as additional Na adsorption sites. Additionally, the P-doped samples exhibited increased plateau capacities.In the plateau region, Na intercalates into the interlayer spacing of graphene layers and subsequently occupies pores near 0 V [7, 8, 15]. Thus, the observed increase in plateau capacity suggests that P dop-ing facilitates Na storage in interlayer spacing and pores. Although P doping has been reported to alter carbon morphology and improve capacity [9, 29], in this study, the internal structure of the samples, in-cluding interlayers and pores, likely remained unaltered. It is possible that P was doped at the surfaces and entrances of carbon layers with relatively wide interlayer spacing, likely contributing to the observed capacity increase. These results indicate that surface modification is crucial in promoting Na storage within interlayer spacing and pores without directly altering the internal structure.Another factor potentially contributing to the plateau capacity in-crease is enhanced pore filling. In the CV results, the cathodic peak near 0 V was not fully developed (Fig. S6), indicating incomplete Na storage in the pores even at 0 V [19]. HC–ZnO samples were de-signed with optimized pore structures for Na storage; however, some Fig. 5 (a) Initial charge–discharge profiles and (b) initial charge capacities of the slope and plateau regions for the HC–ZnO_Lot.1, 1M-P220_Lot.1, and 1M-P600_Lot.1 samples. Values within each bar indicate the corresponding capacity.Research Paper45H. Ando et al., Carbon Reports 5 (2026) 40–47.pores remained partially occupied. This suggests that P functional groups likely facilitated more complete pore filling rather than creat-ing new Na storage sites. Although the specific mechanism by which P doping facilitates pore filling remains unclear, it is hypothesized that surface P functional groups reduce the energy barrier for Na ac-cess into the bulk structure.The capacity increase owing to P doping was smaller for 1M-P600 than for 1M-P220, which was attributed to the number and types of P functional groups on the carbon surface. The P content of 1M-P600 was lower than that of 1M-P220, and XPS and 31P NMR analyses identified distinct P functional groups in 1M-P220 and 1M-P600. Specifically, 1M-P220 contained only R–O–PO3 species, whereas 1M-P600 exhibited both R–O–PO3 and R–PO3 species. These find-ings suggest that R–O–PO3 species may be more crucial in improv-ing battery performance than R–PO3 species. Xie et al. [46] reported that PO4 groups exhibit more favorable Na adsorption energies than pure carbon, whereas the substantially higher adsorption energy of PO3− groups impedes Na desorption. Additionally, they noted that C–O groups lead to irreversible capacity, whereas C= O groups enhance reversible Na adsorption. These results imply that oxygen functional groups may also be crucial in determining anodic perfor-mance. Therefore, the functional group type is likely a key factor in optimizing performance. Systematic identification and quantification of surface functional groups and their respective roles in Na storage will be addressed in future studies.To investigate the state of doped P during charge–discharge cy-cling, 31P NMR measurements were performed on sodiated and de-sodiated 1M-P220 and 1M-P600 samples. The resulting spectra are shown in Fig. 6. For sodiated 1M-P220, an NMR signal appeared at 15 ppm. This signal shifted to an elevated frequency and decreased in intensity relative to the signals of pristine 1M-P220. In desodiated 1M-P220, a broad signal appeared from −30 to 15 ppm, similar to the signal assigned to R–O–PO3 species in pristine 1M-P220. These findings suggest that R–O–PO3 species function as reversible Na storage sites. The signal corresponding to residual phosphoric acid, observed in the NMR spectra of pristine 1M-P220, disappeared after cycling, indicating its removal during the washing step prior to NMR sampling. For 1M-P600, similar signal shifts and intensity decreases were observed upon sodiation, comparable to those in 1M-P220, demonstrating that P functional groups substantially contribute to Na storage.The observed signal shifts are likely caused by reduced electron shielding around P atoms, whereas the decreased signal intensity may result from line broadening, potentially induced by dipolar interac-tions with Na nuclei near P atoms. However, in desodiated 1M-P600, only a broad signal attributed to R–O–PO3 species remained, whereas the R–PO3 signal present in the pristine sample disappeared. This suggests that R–O–PO3 species function as reversible Na storage sites, while R–PO3 species are likely irreversible. The difference in functional group types may explain why the capacity of 1M-P600 is lower than that of 1M-P220.4. ConclusionThis study investigated the effect of P doping on ZnO–templated HCs for NIB performance. P doping was performed under mild con-ditions by soaking HC–ZnO in phosphoric acid, followed by heating at 220 °C or 600 °C. Although the ZnO–templated HC had already been carbonized at 1,400 °C, the resulting P-doped HC samples ex-hibited increased battery capacities. These results demonstrate that P doping contributes to capacity enhancement without altering the car-bon structure. The increased slope capacity suggests that P functional groups act as additional Na adsorption sites, while the increased pla-teau capacity indicates that these groups facilitate Na storage within interlayer spacing and pores. The presence of P functional groups on the carbon surface may also enhance Na accessibility to the bulk carbon matrix.By varying the heating temperature during P doping, HC samples treated at 220 °C demonstrated greater capacity improvements than those treated at 600 °C. XPS and 31P NMR analyses identified R–O–PO3 species on the carbon surface of 1M-P220, whereas R–O–PO3 Fig. 6 31P MAS NMR spectra of (a) 1M-P220 and (b) 1M-P600 in their sodiated (bottom) and desodiated (top) states.Vol. 5 No. 1 2026Research Paper46 H. Ando et al., Carbon Reports 5 (2026) 40–47.Carbon Reportsand R–PO3 species were detected in 1M-P600. 31P NMR spectra further demonstrated strong interactions between P and Na during so-diation. Additionally, the NMR signals of R–O–PO3 species showed high reversibility during charge–discharge cycling, whereas those of R–PO3 species disappeared after cycling. These findings demonstrate that controlling the types of P functional groups is critical for opti-mizing battery performance. Overall, this study provides insights into the capacity improvement of HCs through simple P doping and sup-ports the development of high-energy-density carbon anode materials for NIBs. This surface modification technique is not limited to P and may be extended to other heteroatom dopants introduced using differ-ent dopants, such as sulfuric and nitric acids. Systematic investigation into the effects of dopant types, base carbon structures, and treatment conditions on electrochemical performance will be a vital direction for future research.AcknowledgementsThis study was partly supported by the “Advanced Research In-frastructure for Materials and Nanotechnology in Japan (ARIM)” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Proposal Number: JPMXP1224NM0135. Additional sup-port was provided by JST GteX (Grant Number: JPMJGX23S4). 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