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[Hideka Ando](https://orcid.org/0009-0004-1487-4478), [Kenjiro Hashi](https://orcid.org/0000-0002-0320-4768), [Shinobu Ohki](https://orcid.org/0000-0002-7357-3833), [Yoshikiyo Hatakeyama](https://orcid.org/0000-0002-2938-8980), [Yuta Nishina](https://orcid.org/0000-0002-4958-1753), Norihiro Kowata, [Takahiro Ohkubo](https://orcid.org/0000-0001-5907-3683), [Kazuma Gotoh](https://orcid.org/0000-0002-8197-5701)

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[State change of Na clusters in hard carbon electrodes and increased capacity for Na-ion batteries achieved by heteroatom doping](https://mdr.nims.go.jp/datasets/b86c8c69-3c87-41cc-bce7-e9d3eb1c169e)

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State change of Na clusters in hard carbon electrodes and increased capacity for Na-ion batteries achieved by heteroatom dopingCarbon Trends 16 (2024) 100387Available online 27 July 20242667-0569/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).State change of Na clusters in hard carbon electrodes and increasedcapacity for Na-ion batteries achieved by heteroatom dopingHideka Ando a,d, Kenjiro Hashi b, Shinobu Ohki b, Yoshikiyo Hatakeyama c, Yuta Nishina a,Norihiro Kowata a, Takahiro Ohkubo a, Kazuma Gotoh d,*a Graduate School of Natural Science & Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japanb National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japanc Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjincho, Kiryu, Gunma 376-8515, Japand Center for Nano Materials and Technology (CNMT), Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, JapanA R T I C L E I N F OKeywords:Sodium-ion batteryAnode materialHard carbonHeteroatom dopingSolid-state nuclear magnetic resonanceSodium clusterA B S T R A C TAlthough heteroatom doping is an effective method to improve the capacity of hard carbon (HC) anodes in Na-ion batteries (NIBs), the complicated structure of HC leads to uncertainty when understanding the effects ofheteroatom doping on sodium storage. This study shows the effects of phosphorus and sulfur doping to HC onsodium storage using solid-state NMR to improve the capacity of HC prepared by the carbonization of resorcinolformaldehyde (RF) resin at 1100 ◦C. Heteroatom doping increased the battery capacity of the HC, especially theplateau capacity, but the interlayer distance of the carbon layers in the HC did not expand considerably. 23Nasolid-state NMR revealed that heteroatom doping facilitates the formation of quasi-metallic sodium clusters,thereby contributing to the plateau capacity increase. The metallicity of the sodium clusters in heteroatom-dopedHC samples was controlled by the amount of doped-phosphorous. XPS and 31P NMR detected various phosphorussites such as phosphine and phosphine oxide in the carbon structure.1. IntroductionNa-ion batteries (NIBs) are promising next-generation batteries thatare expected to replace or complement Li-ion batteries (LIBs) because ofthe abundance of sodium on Earth [1,2]. Moreover, NIBs obviate theneed for expensive materials such as cobalt and copper. Additionally,because of the similarity in working principles and components betweenNIBs and LIBs, the existing equipment and knowledge used for LIBs areapplicable to NIBs with minor modifications. The electrochemicalproperties of various carbon materials as NIB anodes have been evalu-ated. Although graphite, which is commonly used as an anode materialin LIBs, allows stoichiometric intercalation of sodium only up to NaC64,its potential as the NIB anode has been recognized via the use of a sol-vent co-intercalation mechanism [3,4]. Disordered carbon materials,especially hard carbon (HC), have also been investigated as new anodematerials for NIBs. Stevens and Dahn first reported a half cell usingglucose-derived HC exhibiting a reversible capacity of 300 mAh g− 1 in2000 [5]. Our group achieved stable cycle performance by a full cellcombining HC and NaNi0.5Mn0.5O2 operating in propylene carbonateelectrolyte solution [6]. Despite the proposal of various anode materialsfor NIBs, practical NIBs remain under development because of the needfor balanced performance.Synthesis conditions of HC play a fundamentally important role inNIB performance. Generally, HC is synthesized by carbonizing organicprecursors under an inert atmosphere. Earlier works have found arelation between the carbonization temperature and the performance ofHC as anodes [7–9]. They suggest that HC carbonized at 1400–1600 ◦Cis suitable for obtaining the highest capacity in NIBs because of itsbalanced structure, which includes a large interlayer distance andoptimal size of internal pores. Our group also reported thatsucrose-derived HC carbonized at 1600 ◦C showed the highest capacityof 302 mAh g− 1 [10].However, the optimal carbonization temperature of HC for NIBs,which is higher than that of HC for LIBs (1100–1200 ◦C), remainsproblematic. In fact, higher temperatures are undesirable because of thehigh production costs associated with heating processes. Doping ofheteroatoms (e.g., nitrogen, sulfur, and phosphorus) into HC is apromising method to increase capacity without high-temperature* Corresponding author.E-mail address: kgotoh@jaist.ac.jp (K. Gotoh).Contents lists available at ScienceDirectCarbon Trendsjournal homepage: www.elsevier.com/locate/cartrehttps://doi.org/10.1016/j.cartre.2024.100387Received 22 July 2024; Accepted 26 July 2024mailto:kgotoh@jaist.ac.jpwww.sciencedirect.com/science/journal/26670569https://www.elsevier.com/locate/cartrehttps://doi.org/10.1016/j.cartre.2024.100387https://doi.org/10.1016/j.cartre.2024.100387https://doi.org/10.1016/j.cartre.2024.100387http://creativecommons.org/licenses/by-nc/4.0/http://creativecommons.org/licenses/by-nc/4.0/Carbon Trends 16 (2024) 1003872carbonization [11–25]. In some carbons, heteroatom doping induceschanges in the HC structure such as the interlayer distance, pore volume,and the number of defect sites. Phosphorus-doped carbons reduce thediffusion barrier of sodium because of expansion of the interlayer spacebetween the carbon layers [26–29]. Sulfur doping also enlarges theinterlayer distance of carbon because of the large radius of sulfur atoms,thereby increasing the battery capacity [29–32]. Biomass materials haveoften been preferred as precursors for heteroatom-doped HC because ofthe efficient use of natural resources [33–44]. Also, the mechanismsunderlying the increase of capacity by heteroatom doping of these ma-terials have been investigated. However, the complicated and inhomo-geneous chemical composition of the carbon materials produced frombiomass, which contains plural heteroatoms and impurities, makes itdifficult to clarify the effects of the heteroatoms. A systematic analysis ofthe effect of heteroatoms must be conducted.Usually, HC exhibits two processes during electrochemical sodiationand desodiation: a primal slope region above 0.1 V and a followingplateau region below 0.1 V for sodiation [45–48]. The high plateaucapacity is beneficial for achieving high energy density. Sodium storagemechanisms in the slope region and the plateau region have been dis-cussed in reports of numerous studies [6,29,49–58]. Some groups suchas Stratford et al. [59–61] and our group [62,63] have used 23Nasolid-state NMR to observe the formation of quasi-metallic sodiumcluster in pores near 0 V, at the last phase of the plateau region.Recently, the sodium cluster has been also investigated by Ramanspectroscopy, electron paramagnetic resonance, and calculation[64–66]. Iglesias et al. have provided insight into the sodium pore-fillingprocess for different HC microstructures such as the pore size and defectconcentration by multimodal approach [67]. Controlling the formationof the closed pores suitable for cluster formation is expected to increasethe plateau capacity. In fact, pore-size-controlled HC synthesized usingthe MgO-template method exhibits high plateau capacity and an intense23Na NMR signal, which is assigned to quasi-metallic sodium [68]. Theincrease in the plateau capacity has also been observed by doping het-eroatoms [27,69,70]. The doped heteroatoms likely affect sodiumstorage in pores, especially cluster formation, although the effects ofdoped heteroatoms have not been evaluated in detail. Direct observationof sodium in heteroatom-doped HC can contribute to elucidation of theeffects of heteroatoms on enhancing the plateau capacity.For this study using solid-state NMR, we try to ascertain the effects ofheteroatom doping into HC on sodium storage. To investigate the effectsof phosphorus doping, phosphorus-doped HC samples with differentdopant amounts were prepared from resorcinol-formaldehyde (RF) resinusing carbonization at 1100 ◦C. The prepared HC samples were analyzedusing techniques such as X-ray photoelectron spectroscopy (XPS),powder X-ray diffraction (PXRD), and small-angle X-ray scattering(SAXS). Additionally, 31P MAS NMR was applied to reveal the structureof phosphorus doping sites. The storage state of sodium in phosphorus-doped HC samples was evaluated using 23Na MAS NMR. Sulfur-dopedHC samples were also evaluated for comparison with phosphorous-doped HC samples.2. Methods2.1. Synthesis of HC samplesAn undoped HC sample was synthesized by carbonizing RF resin. In atypical preparation, 4.15 g of resorcinol was dissolved in 1.0 mL ofethanol and 6.8 mL of 0.01M hydrochloric acid. Subsequently, 5.0 mL offormalin was added to the solution at 0 ◦C in an ice-water bath. Afterthorough mixing, the solution was placed at 40 ◦C in an oil bath for 24 h,followed by a temperature increase to 80 ◦C. It was kept at that tem-perature for 24 h. After the obtained product was washed three timeswith ethanol at 60 ◦C every 4 h, it was dried. The obtained RF resin wasmilled with a mortar. Subsequently, it was carbonized at 1100 ◦C with aheating rate of 10 ◦C min− 1, maintaining the temperature for 1 h in aflowing nitrogen atmosphere (500 mL min− 1). The resultant undopedHC was designated as RF11. (Scheme 1)Phosphorus-doped HC samples were synthesized using a similarmethod to that for RF11 with the addition of phosphoric acid. Forphosphorus doping, 4.0 mL of 1, 3, 5, or 7 M phosphoric acid was addedto 6.8 mL of 0.01 M hydrochloric acid solution, which contains 4.15 g ofresorcinol and 1.0 mL of ethanol. The molar ratios of phosphoric acidand resorcinol in the respective solutions correspond to 0.1:1, 0.3:1,0.5:1, and 0.7:1. The resulting mixtures were polymerized and carbon-ized using the same process as RF11. Additionally, sulfur-doped HCsamples of two types were synthesized using sulfuric acid instead ofphosphoric acid, with the molar ratios of 0.1:1, and 0.3:1. Thephosphorous-doped HC samples are denoted as 0.1PRF11, 0.3PRF11,0.5PRF11, and 0.7PRF11, whereas the sulfur-doped HC samples are0.1SRF11 and 0.3SRF11, arranged in ascending order of the dopantaddition amount. 0.5SRF11 and 0.7SRF11 were also prepared, but thesesamples exhibited similar morphology and battery performance as0.3SRF11. Therefore, only 0.1SRF11 and 0.3SRF11 samples are dis-cussed for the sulfur-doped HCs in the following.2.2. CharacterizationThe morphology and elemental mapping of undoped andphosphorus-doped resins were obtained using a scanning electron mi-croscope (SEM, TM3030Plus; Hitachi High-Tech Corp.) equipped withan energy dispersive X-ray Spectrometer (EDS). The chemical compo-sitions of the HC samples were measured using elemental analysis forhydrogen, carbon, phosphorus, and sulfur. Because the rawmaterials forthe RF resins contain only hydrogen, carbon, phosphorus or sulfur, ox-ygen, and a trace amount of chlorine, the value of 100 minus the amountof hydrogen, carbon, phosphorus, and sulfur is considered to be theamount of oxygen. The Brunauer–Emmett–Teller (BET) specific surfacearea (SSA) of each sample was calculated using the nitrogen adsorp-tion–desorption isotherms with a multi-point method (Belsorp-mini II;MicrotracBEL Corp.). Before BET SSA measurement, the samples wereheated to 300 ◦C for 5 h to remove physisorbed gases. The distribution ofthe micropores in the undoped and phosphorus-doped HC samples wasanalyzed by the nonlocal density functional theory (NLDFT). The state ofdoped phosphorus in HC samples was evaluated using XPS and solid-state 31P MAS NMR. The XPS measurements were taken using a device(JPS-9030; JEOL) with an X-ray gun operating at 12 kV with emissioncurrent of 25 mA. The 31P MAS NMR spectra were recorded using aspectrometer (11.7 T magnet, JNM-ECZ500R; JEOL/Oxford In-struments) at a MAS speed of 7 kHz. We used 85 % H3PO4 aqueoussolution as a reference at 0 ppm. The PXRD patterns were collected usinga diffractometer (MiniFlex600; Rigaku Corp.) with Cu Kα radiation (λ =0.15418 nm) at 40 kV and 10 mA, using a scan rate of 5◦ min− 1. Also,SAXS experiments were performed using the apparatus at the BL-6Astation of the Photon Factory (PF) operated at 2.5 GeV and 450 mA inthe Institute of Materials Structure Science, High Energy AcceleratorResearch Organization (KEK), Tsukuba, Japan [71,72]. The X-ray beamgenerated by bending magnet was monochromatized to λ = 0.15 nm andwas focused to 0.5 × 0.3 mm2 at the sample. Software developed at PF,SAngler, was used to convert 2D scattering data into 1D [73]. The X-rayprofiles from the empty sample holder and glass-like carbon SRM 3600were used to eliminate the background intensity and to normalize theSAXS intensity to absolute intensity [74,75]. In SAXS analysis, curvefitting using theoretical scattering curves was performed, assuming thatthe pores are spherical and that their size distribution is described by agamma distribution. In the case of powder samples, the interstitialspaces within the sample also act as scatterers. For this reason, theanalysis was conducted in a region where the scattering parameter ex-ceeds 2 nm− 1, where contributions from scatterings other than micro-pores are minimized.H. Ando et al.Carbon Trends 16 (2024) 10038732.3. Electrochemical measurementsThe electrochemical properties of the undoped and heteroatom-doped HC samples were evaluated using CR2032 coin cells assembledin an argon-filled glove box. The anodes were composed of HC as activematerials, carbon black (VALCAN XC72R; Cabot Corp.), and polyimidebinder (Dreambond; Industrial Summit Technology Corp.) in a weightratio of 8:1:1. Slurry, formed bymixing these components and N-methyl-2-pyrrolidone (NMP), was coated to 100 µm thickness using the doctorblade method on aluminum or copper foil. The working electrodes weredried at 120 ◦C for over 15 min under vacuum and were then punchedout with 15.95 mm diameter. The counter electrode was sodium metal.The electrolyte solution consisted of 1.0 M NaPF6 in ethylene carbonate(EC) / diethyl carbonate (DEC) (1:1 v/v%). Galvanostatic char-ge–discharge tests were conducted (HJ1001-SD8; Hokuto Denko Corp.)in a range of potentials of 0.0–2.0 V (vs. Na+/Na) at a current rate of 25mA g− 1. At least three cells were assembled for each HC. Their char-ge–discharge capacities were evaluated.2.4. NMR evaluation of the sodium state stored in hard carbonThe state of sodium in undoped and heteroatom-doped HC sampleswas analyzed using 23Na MAS NMR and 31P MAS NMR. Initially, sodi-ated HC samples were prepared electrochemically. The coin cells wereassembled and subjected to a galvanostatic discharge–charge cycleranging from 0.0 to 2.0 V at 25 mA g− 1. Subsequently, they were dis-charged to 0.2 mV at 25 mA g− 1 and maintained at 0.2 mV until thecurrent reached 0.25 mA g− 1. The discharged coin cells were dis-assembled in an argon-filled glove box. The anode materials, afterwashing with dimethyl carbonate (DMC), were scraped from the nega-tive electrode collector, and were placed into ϕ3.2 or 4 mmNMR samplerotors with polyvinylidene fluoride (PVDF) powder. Then, using aspectrometer (11.7 T magnet, DD2 NMR; Agilent Technologies Inc.),23Na MAS NMR spectra of the undoped and phosphorous-doped HCsamples were recorded. Those of the sulfur-doped HC samples weremeasured using a spectrometer (11.7 T magnet, AVANCE III NMR;Bruker Biospin Corp.). The RIDE pulse sequence with pulse length of 3.0µs, a recycle delay of 0.1 s, 5000 scans, and MAS speed of 12 kHz wasapplied. 1MNaCl aqueous solution was used as a reference at 0 ppm. 31PMAS NMR spectra of 0.1PRF11 samples discharged to 0 V and charged to2 V were measured using a spectrometer (11.7 T magnet, AVANCE IIINMR; Bruker Biospin Corp.). The single pulse sequence with pulselength of 3.75 µs, a recycle delay of 30 s, and MAS speed of 7 kHz wasapplied. 85 % H3PO4 aqueous solution was used as a reference at 0 ppm.3. Results and discussion3.1. CharacterizationThe morphology of prepared resins is shown in Fig. S1. The undopedresin sample had irregularly shaped particles, but the 0.1PRF resin had amorphology resembling a series of spherical particles of approximately10 µm in diameter, while the 0.3PRF, 0.5PRF, and 0.7PRF resins hadalmost completely spherical particles of 2–7 µm diameter. As shown inFig. S2, the EDS mapping of the phosphorus-doped resins confirmed thatphosphorus was uniformly present inside the resins. These results indi-cate that phosphorus is incorporated into the resins during the synthesisprocess and that the doped phosphorus modifies the structure of theresins. The percentage of carbon, oxygen, and phosphorus in thephosphorus-doped resins evaluated by EDS elemental analysis is shownin Table S1. Aluminum was also observed in these resins, possiblyoriginating from the carbon tape (background). Therefore, the contentof each element may not be the correct absolute value, but there is nodoubt that phosphorus is present in the resins and that the phosphoruscontent increases with the amount of phosphoric acid added.The percentage of elements and the results of the nitrogen adsorp-tion–desorption method of the undoped and phosphorus-doped HCsamples are presented in Table 1 and Fig. S3. Elemental analysis showedthat all phosphorus-doped HC samples contained phosphorous. FromRF11 to 0.3PRF11, the percentage of phosphorus increased to 1.8 wt.%with the increase in phosphoric acid. However, further addition ofphosphoric acid caused little change in the doped phosphorus. 0.1PRF11had the lowest BET SSA and open pore volume among undoped andphosphorus-doped HC samples. 0.3PRF11 and 0.5PRF11 showed almostthe same BET SSA as RF11, and the pore distribution of 0.3PRF11 and0.5PRF11 was also almost the same as RF11 except for the 2.5–3 μmsized pores. 0.7PRF11 had the highest BET SSA and open pore volume.In the case of sulfur-doped HC samples, 0.1SRF11 and 0.3SRF11 showedsimilar sulfur doping levels (Table S2). These results suggest that the HCsynthesized using this method has an upper limit for the number ofheteroatom doping sites. The excess heteroatoms are emitted as gasesduring carbonization, thereby forming many defects and open pores [20,28].To evaluate the state of phosphorus in HC samples, XPS and 31P MASNMR measurements were taken. The XPS spectra of the P 2p region ofthe phosphorus-doped HC samples and the 31P MAS NMR spectra ofphosphorus-doped HC samples are presented respectively in Fig. 1(a–d)and Fig. 1(e). The P 2p peaks of all samples (Fig. 1(a–d)) were decon-voluted into three peaks at 131.4, 133.0, and 134.8 eV, and wereassigned respectively to phosphine (PR3) or alkoxy phosphine (R2POR’),phosphine oxide (R3P = O) or phosphinic acid (R2POOH), and phos-phate (ROP(=O)(OH)2) or phosphonic acid (RP(=O)(OH)2) [76]. TheScheme 1. Schematic illustration of the synthesis of the undoped and heteroatom-doped HC samples.Table 1Results of elemental analysis and BET SSA measurements of the undoped andphosphorus-doped HC samples. *) The wt.% of oxygen was estimated by sub-tracting the amount of hydrogen, carbon, and phosphorus from 100 %.Elemental analysis (wt.%) BET surface area (m2 g-1)C H O PRF11 87.2 1.1 11.7 – 6090.1PRF11 79.7 1.6 17.6 1.1 4210.3PRF11 76.0 1.9 20.3 1.8 6180.5PRF11 74.9 2.0 21.2 1.9 6730.7PRF11 72.8 2.4 22.9 1.9 855H. Ando et al.Carbon Trends 16 (2024) 1003874NMR spectra of all P-doped HC samples showed a major peak at − 9 ppmand a small peak at − 20 ppm. Although accurate assignment of thesepeaks is difficult because of the complicated structure of the HC, theelectron density around phosphorus is expected because of a tendencyfor chemical shifts in organic compounds. For trivalent phosphorus, thechemical shift of phosphorus in P(C6H5)3 is − 6 ppm, whereas thechemical shift of P(OC6H5)3 appears at the high-frequency side at 127ppm. For pentavalent phosphorus, the chemical shift of PO(OC6H5)3 is− 18 ppm, whereas the chemical shift of (C6H5)3P = O is 27 ppm [77].Therefore, the NMR peaks of the HC samples at − 9 and − 20 ppm arelikely to be assignable respectively to PR3 and PO(OR)3. Additionally,the 31P NMR spectra of some phosphorus-doped HC samples showed avery broad and negligible signal near 30 ppm. This signal might repre-sent an overlapping of various phosphorus signals with positive chem-ical shifts such as R3P = O and R2P = O(OR), indicating the presence ofslight phosphorus in various states. Although XPS has been used widelyfor evaluating phosphorus-doped HC in many studies, including ourexperiment, the results of NMR apparently differ from those of XPS. Thisdiscrepancy is explainable by the fact that each method observesdifferent parts of the samples: XPS can only observe the surface of HCparticles, whereas the NMR signal is obtained from the entire sampleincluding the surface and internal structure. Therefore, the NMR mea-surements indicate the presence of many PR3 species in bulk, whichdiffers from the phosphorus species present on the surface, as revealedby XPS. Hasegawa et al. reported that most phosphorus atoms doped inRF resin-derived activated carbon are oxidized to P(V) with time byreacting with oxygen and moisture [76]. However, for this study, a largeamount of PR3 species remained unoxidized, suggesting that manyphosphorus sites exist between interlayers and in pore structures thatare not exposed to oxygen and moisture.PXRD patterns of the phosphorus-doped HC samples are presented inFig. 2(a). The PXRD patterns of all samples showed two broad diffractionpeaks at 2θ = 15–27◦ and 40–48◦, which were assigned respectively tothe 002 and the 10 Bragg diffractions of graphite. The interlayer dis-tances calculated from 002 peaks were 0.394, 0.397, 0.398, 0.398, and0.402 nm, respectively, for RF11, 0.1PRF11, 0.3PRF11, 0.5PRF11, and0.7PRF11. In fact, RF11 already had large interlayer spacing. Phos-phorus doping only expanded it slightly. In addition, the pore sizeFig. 1. High-resolution XPS spectra of the P 2p region of (a) 0.1PRF11, (b) 0.3PRF11, (c) 0.5PRF11, and (d) 0.7PRF11, and (e) 31P MAS NMR spectra of phosphorus-doped HC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)H. Ando et al.Carbon Trends 16 (2024) 1003875distribution in the HC samples was estimated using SAXS measurementsto evaluate the closed pores, which cannot be assessed based on a ni-trogen adsorption–desorption isotherm. Curve fitting analysis of theSAXS patterns, assuming the pores as spherical, revealed that themaximum pore size increased with added phosphorus, suggesting thatphosphorus doping forms larger pores (Fig. 2(b)). Based on SAXS andnitrogen adsorption–desorption results, the number of openpores in0.7PRF11 is higher in 0.1PRF11. The interlayer distance and porediameter of sulfur-doped HC samples evaluated using XRD and SAXS areshown in Fig. S4. The interlayer distances for both 0.1SRF11 and0.3SRF11 are similar to that of RF11 (Fig. S4(a)). Moreover, the poresizes for both HC samples are almost identical (Fig. S4(b)). However, thesulfur-doped HC samples exhibited broader pore distribution peaks thanthe phosphorus-doped HC samples did. Several reports of earlier studieshave described the increase in interlayer distance caused by heteroatomdoping as a primary factor contributing to increased battery capacity[16,19,21,26–28,30,31]. However, in this study, RF11 already had alarge interlayer distance. Therefore, this study’s increase in battery ca-pacity resulting from heteroatom doping should be attributed to factorsother than the interlayer distance, as discussed later.3.2. Electrochemical measurementsThe electrochemical performances of the HC samples as the anodematerials are shown in Fig. 3. Fig. 3 (a) shows the initial char-ge–discharge curves of the HC samples exhibiting capacities closest tothe average value. The averaged initial charge capacities of RF11,0.1RFF11, 0.3PRF11, 0.5PRF11, and 0.7PRF11 were, respectively, 255,328, 310, 286, and 100mAh g− 1. The capacities of 0.1PRF11, 0.3PRF11,and 0.5PRF11 show improvement compared to that of RF11. Amongthem, 0.1PRF11 exhibits the greatest capacity: 29 % higher than that ofRF11. By contrast, the capacity of 0.7PRF11 decreased compared to thatof RF11. The initial Coulombic efficiencies (ICE) of RF11, 0.1PRF11,0.3PRF11, 0.5PRF11, and 0.7PRF11 were, respectively, 63, 71, 66, 67,and 33 %. A certain increase in the ICE of 0.1PRF11 compared to RF11was observed, whereas 0.7PRF11 was found to have a marked decrease(Fig. 3). The lower ICE of 0.7PRF11 is mainly ascribable to the increaseof specific surface area due to the increase of the number of open pores.To elucidate changes in battery capacity, the initial charge capacitiesof these HC samples were divided into a slope region and a plateau re-gion at 0.12 V, as shown in Fig. 3(b). Regarding the slope capacity,0.1PRF11 exhibited the largest, reaching 183 mAh g− 1. The best elec-trochemical performance of 0.1PRF11 suggests that phosphorus dopinghas an advantage over the disadvantage caused by increased specificsurface area. The slope capacities decreased with the amount of addedphosphorus. The slope capacity of 0.3PRF11 was almost identical to thatof RF11 (0.3PRF11:146 mAh g− 1, RF11:144 mAh g− 1). Also, both0.5PRF11 and 0.7PRF11 showed lower slope capacities than RF11(0.5PRF11:132 mAh g− 1, 0.7PRF11:78 mAh g− 1). The slope capacityand ICE decrease are attributable to their excessive defect sites and highspecific surface area [27,78]. The plateau capacities of 0.1PRF11,0.3PRF11, and 0.5PRF11 all improved (RF11:110 mAh g− 1,0.1PRF11:145 mAh g− 1, 0.3PRF11:164 mAh g− 1, 0.5PRF11:155 mAhg− 1). The increased plateau capacity is attributable to their increasedinserted sodium into interlayer spaces and sodium stored in pores [50,54,61,79]. It is noteworthy that the interlayer distance of RF11 wasalready large. The effect of heteroatom doping on the change in theinterlayer spacing was small. The increased plateau capacity with noincrease in interlayer distance indicates that sodium stored in the porescontributes to the increased plateau capacity. The plateau capacity of0.7PRF11 decreased considerably (22 mAh g− 1), whereas the irrevers-ible capacity of 0.7PRF11 increased compared to that of RF11. Thisresult suggests that the excessive phosphorus addition engenders dra-matic changes in the carbon morphology, leading to a decrease inFig. 2. (a) XRD patterns and (b) pore size distributions estimated from SAXS patterns of the undoped and phosphorus-doped HC samples. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)Fig. 3. (a) Initial charge–discharge profiles and (b) initial charge capacities of the slope and plateau region of the undoped and phosphorus-doped HC samples. (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)H. Ando et al.Carbon Trends 16 (2024) 1003876sodium storage sites. It also facilitates the formation of a large amount ofsolid electrolyte interface (SEI) because of the increased surface area.Regarding the sulfur-doped HC samples, both HC samples (0.1SRF11and 0.3SRF11) also exhibited improved capacities compared to RF11.The slope capacities of 0.1SRF11 and 0.3SRF11 were, respectively, 187and 168mAh g− 1 (Fig. S5). Improvement in the slope capacity was likelyattributable to the increase of sulfur doping sites acting as sodiumstorage sites, similar to the case of phosphorus doping. The plateau ca-pacities of 0.1SRF11 and 0.3SRF11 were, respectively, 140 and 143mAhg− 1. The plateau capacities were also improved compared to that ofRF11, resulting from increased amounts of stored sodium in the pores, asobserved for phosphorus doping.3.3. Evaluation of the sodium state in hard carbonThe sodium in the HC samples was observed using 23Na MAS NMR,as shown in Fig. 4. Each NMR spectrum showed a peak at − 8 ppm, whichwas assigned to ionic sodium species such as residual electrolyte, SEI,and sodium inserted between layers [54,62]. Additionally, threephosphorus-doped HC samples exhibited a broad peak at 600–1000ppm: 0.1PRF11, 0.3PRF11, and 0.5PRF11 (Fig. 4(a)). This signal, shiftedto a higher frequency side because of the Knight shift, is assigned to thequasi-metallic sodium cluster, which has been reported for several HCsamples prepared especially using higher carbonization temperatures[59,61,62]. Similarly, the NMR spectra of sulfur-doped HC samples,0.1SRF11 and 0.3SRF11, exhibited a broad peak attributed toquasi-metallic sodium at 600–1100 ppm (Fig. 4(b)). However, becauseof the formation of many open pores resulting from the emission ofexcess heteroatoms as gases during the carbonization of 0.7PRF11, theNMR spectrum of 0.7PRF11 showed no distinct quasi-metalliccomponent.The signals of quasi-metallic sodium clusters in the phosphorus-doped HC samples shifted to a higher frequency side with more addedphosphorus. In addition, the signal intensities increased with the addedphosphorous (except for 0.7PRF11). However, the chemical shifts ofquasi-metallic sodium formed in sulfur-doped HC samples (0.1SRF11and 0.3SRF11) are almost constant. Fundamentally, larger closed poresin HC allow for the forming of larger sodium clusters, which are highlymetallic and which give a high Knight shift value [61–63]. However, theNMR signals of the quasi-metallic sodium cluster formed in thesulfur-doped HC samples and 0.1PRF11 showed a nearly constant shift,although the sulfur-doped HC samples had larger pores than 0.1PRF11showed. Indeed, the doping elements can also affect the sodium clustermetallicity. Stratford et al. have reported that the number of defects onthe carbon surface affects the sodium cluster metallicity [60]. Theysuggest that carbon with defects has higher trapped electron densitynear the defects and a larger partial charge on the sodium ions, whichleads to a smaller Knight shift. By contrast, in this study, the metallicityof sodium clusters formed in phosphorus-doped HC samples increasedwith added phosphorus. This phenomenon might be caused by a dif-ference in the ease of transfer of electron density from sodium to thedefect sites and from sodium to the doped heteroatoms. Because theelectronegativities of phosphorus and sulfur are respectively moreminorthan and similar to those of carbon (C:2.55, P:2.19, and S:2.58), thedefect sites with phosphorus might be favorable for the electron transferand the formation of sodium clusters with high metallicity, whereasthose with sulfur might behave similarly to defect sites without het-eroatoms. We have observed that a 31P MAS NMR signal of the sodiated0.1PRF11 sample shifted to a higher frequency side compared to that ofthe desodiated 0.1PRF11 sample, which may be due to the decreasedshielding from electrons around phosphorus (Fig. S6).The increase in quasi-metallic Na cluster signal intensity by additionof phosphorous (Fig. 4(a)) can be attributed to an increase in the amountof quasi-metallic clusters. It is possible that the number of defect sites onthe graphene surface, which are the starting point for cluster formation[66], has also increased.3.4. Effects of heteroatom doping on sodium storageOur experiments showed that phosphorus and sulfur doping into HCimproved the electrochemical performance, especially the extension ofplateau capacities. The heteroatom-doped HC samples (except for0.7PRF11) showed more quasi-metallic sodium clusters. This observa-tion demonstrates that the facilitation of forming quasi-metallic sodiumclusters by heteroatom doping plays an important role in the increase ofthe plateau capacity. This facilitation of the formation of quasi-metallicsodium clusters might be attributed to the increased closed pore size andthe increased number of heteroatoms doping sites in the pores, wheresodium can be absorbed easily. Based on the results, a schematic dia-gram of the effect of heteroatom doping sites on sodium storage is shownin Fig. 5. In the sodiation process, sodium ions are inserted into in-terlayers and adsorbed around defects, including heteroatom dopingsites, as shown in Fig. 5(b). Because the 31P NMR measurements takenfor this study suggest that large amounts of PR3 species are present in thebulk of phosphorus-doped HC samples, the PR3 species might contributeas a major sodium adsorption site in the pores. In fact, some studies havecalculated the binding energies of sodium ions to phosphorus dopingsites. The results demonstrate that sodium ions adsorb more easily toPR3 species than to graphene [20,22,29]. Unfortunately, the state ofsulfur in the bulk of sulfur-doped HC could not be evaluated using 33SNMR because of its low sensitivity. However, the 23Na NMR signal of thesodium cluster was observed clearly in sulfur-doped HC samples, sug-gesting that sulfur doping has some positive effects on cluster formation.Then, sodium ions are likely to gather around the heteroatom dopingsites. They might grow into quasi-metallic sodium clusters with that siteas a core around 0 V, as shown in Fig. 5(c). In the desodiation process,most sodium is desorbed from the HC, but some sodium remains in theHC as irreversible sodium, as shown in Fig. 5(d). In particular, 0.7PRF11Fig. 4. 23Na MAS NMR spectra of (a) undoped and phosphorus-doped HC samples and (b) sulfur-doped HC samples.H. Ando et al.Carbon Trends 16 (2024) 1003877contained a large amount of irreversible sodium. The XPS spectrum of0.7PRF11 showed more ROP(=O)(OH)2 and RP(=O)(OH)2, suggestingthat highly oxidized phosphorus on the edge is likely to contribute toirreversible capacity. In fact, Li et al. reported that edge-doped PC(OH)2and PCO(OH) exhibit extremely high binding energy with sodium [29].Our results suggest that HC containing closed pores with adequateheteroatom sites can contribute to NIBs with higher energy density.4. ConclusionWe tried to increase the plateau capacity of HC anodes by hetero-atom doping without high carbonization temperatures and to investi-gate details of heteroatom doping effects on sodium storage.Phosphorus-doped and sulfur-doped HC samples were synthesized at1100 ◦C from RF resins with different dopant amounts. Despite the lowcarbonization temperature of 1100 ◦C, which is known to be insufficientfor achieving high plateau capacity, the phosphorus-doped and sulfur-doped HC samples exhibited higher plateau capacity than that foundfor undoped HC. The increased capacity can be attributed to the facili-tation of sodium cluster formation provided by the change in pore sizeand the heteroatom doping sites, as evidenced by the appearance of the23Na NMR signal of the quasi-metallic sodium cluster. No markedchange in the interlayer distance by heteroatom doping, which isconsidered the main factor for increasing capacity, was observed in theHC samples used for this study. The 23Na NMR chemical shifts of thesodium cluster signals in heteroatom-doped HC samples shifteddepending on the heteroatom type and amount, suggesting that theelectronegativity of the doped heteroatoms influences metallicity inaddition to the pore size. Regarding the phosphorus-doped HC samples,phosphorus doping sites in carbon structures such as PR3, observedusing 31P NMR, might play an important role in facilitating sodiumcluster formation and changing the state of the sodium cluster. The stateof sulfur in the bulk of sulfur-doped HC samples has not yet been clar-ified, but it might exert similar positive effects on cluster formation. Thisstudy provides new insights into the effects of heteroatom-doping onHC, which are expected to be helpful for realizing high-energy anodematerials at low carbonization temperatures.CRediT authorship contribution statementHideka Ando: Conceptualization, Data curation, Formal analysis,Methodology, Writing – original draft. Kenjiro Hashi: Formal analysis,Methodology, Resources. Shinobu Ohki: Methodology, Resources.Yoshikiyo Hatakeyama: Data curation, Methodology, Writing – review& editing. Yuta Nishina: Methodology, Resources, Writing – review &editing. Norihiro Kowata: Data curation, Formal analysis, Methodol-ogy. Takahiro Ohkubo: Formal analysis, Methodology, Writing – re-view & editing. Kazuma Gotoh: Conceptualization, Fundingacquisition, Investigation, Project administration, Supervision, Writing– review & editing.Declaration of competing interestThe authors declare the following financial interests/personal re-lationships which may be considered as potential competing interests:Kazuma Gotoh reports financial support was provided by JapanScience and Technology Agency. Kazuma Gotoh reports financial sup-port was provided by Japan Society for the Promotion of Science.Kazuma Gotoh reports financial support was provided by AcquisitionTechnology and Logistics Agency. Hideka Ando reports financial sup-port was provided by Japan Science and Technology Agency. If there areother authors, they declare that they have no known competing financialinterests or personal relationships that could have appeared to influencethe work reported in this paper.Data availabilityData will be made available on request.AcknowledgementsThis work was supported by JST SPRING Grant No. JPMJSP2126,JST GteX JPMJGX23S4, JSPS KAKENHI Grant No. 23K04535, and ATLAGrant No. JPJ004596.We are grateful to the PF Advisory Committee of KEK for approvingthe SAXS measurements under Proposal No. 2022G101.The authors thank Dr. Zhou Yang (Okayama University) and Dr.Shun Nishimura (JAIST) for support with XPS measurement and nitro-gen adsorption–desorption measurement, respectively.Supplementary materialsSupplementary material associated with this article can be found, inFig. 5. Schematic diagram of sodium storage state in heteroatom-doped HC (a) before sodium storage, (b) during sodium storage, (c) during quasi-metallic sodiumcluster formation at 0 V, and (d) after desodiation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version ofthis article.)H. Ando et al.Carbon Trends 16 (2024) 1003878the online version, at doi:10.1016/j.cartre.2024.100387.References[1] N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Research development on sodium-ion batteries, Chem. Rev. 114 (2014) 11636–11682.[2] J.-Y. Hwang, S.-T. Myung, Y.-K. Sun, Sodium-ion batteries: present and future,Chem. Soc. Rev. 46 (2017) 3529–3614.[3] Z.T. Gossage, T. Hosaka, R. Tatara, S. Komaba, Branched diamine electrolytes forNa+-solvent cointercalation into graphite, ACS Appl. Energy Mater. 7 (2024)845–849.[4] I. Escher, A.I. Freytag, J.M. López del Amo, P. 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State change of Na clusters in hard carbon electrodes and increased capacity for Na-ion batteries achieved by heteroatom doping 1 Introduction 2 Methods 2.1 Synthesis of HC samples 2.2 Characterization 2.3 Electrochemical measurements 2.4 NMR evaluation of the sodium state stored in hard carbon 3 Results and discussion 3.1 Characterization 3.2 Electrochemical measurements 3.3 Evaluation of the sodium state in hard carbon 3.4 Effects of heteroatom doping on sodium storage 4 Conclusion CRediT authorship contribution statement Declaration of competing interest Data availability Acknowledgements Supplementary materials References