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[Aranee Pleng Teepakakorn](https://orcid.org/0000-0002-8668-3513), Tomohiro Tanaka, [Nobuyuki Sakai](https://orcid.org/0000-0002-9395-6751), [Yasuo Ebina](https://orcid.org/0000-0003-3471-9825), [Takayuki Kikuchi](https://orcid.org/0000-0003-0588-2172), [Renzhi Ma](https://orcid.org/0000-0001-7126-2006), [Makoto Ogawa](https://orcid.org/0000-0002-3781-2016), [Takayoshi Sasaki](https://orcid.org/0000-0002-2872-0427)

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[Layered Alkali Metal Titanate with the Staging Structure and Superior Electrochemical Performance](https://mdr.nims.go.jp/datasets/33acc4db-43c2-4fd5-b229-c7f39d54e887)

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Layered Alkali Metal Titanate with the Staging Structure and Superior Electrochemical PerformanceLayered Alkali Metal Titanate with the Staging Structure andSuperior Electrochemical PerformanceAranee Pleng Teepakakorn, Tomohiro Tanaka, Nobuyuki Sakai, Yasuo Ebina, Takayuki Kikuchi,Renzhi Ma, Makoto Ogawa, and Takayoshi Sasaki*Cite This: Inorg. Chem. 2025, 64, 16289−16296 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: We systematically explored the formation range of α-NaFeO2-typelayered titanate and its ion-exchange behaviors and electrochemical performance.The layered sodium titanate of NaxTi1−x/3Lix/3O2 at x = 0.68−0.70 was synthesizedby the solid-state reaction at 900 °C. The titanate is composed of coplanar hostlayers with the α-NaFeO2-type structure. The interlayer Na+ ions underwent a facileexchange with other alkali metal ions in aqueous solutions at 80 °C, accompanied byconcurrent exchange with proton/oxonium ions. Powder X-ray diffraction data onthe products and their Rietveld refinement revealed the formation of a uniquestaging-structured titanate, in which the interlayer galleries are alternately occupiedby incoming alkali metal ions and oxonium ions. The electrochemical intercalation−deintercalation properties for Li+ and Na+ ionstorage were examined on the pristine sodium titanate and its ion-exchanged derivatives. The staging structure was found to providesuperior electrochemical performance, which may be due to the rather open nature of the interlayer galleries, providing abundantsites for Li+ and Na+ ions.1. INTRODUCTIONConsiderable attention has been paid to layered transitionmetal oxides for their useful functions, such as photocatalyticand dielectric properties.1−7 Layered alkali metal titanates withvarious structures are known, as illustrated in Figure 1. A seriesof titanates, such as Na2Ti3O7, K2Ti4O9, and Cs2Ti5O11, havecorrugated host layers of edge-shared TiO6 octahedra, whichare stepped via corner-sharing every three, four, and fiveoctahedra, respectively.8−10 A class of lepidocrocite-type (γ-FeOOH) layered titanates with the general formula ofAxTi2−yMyO4 (where A = K, Rb, Cs and M = vacancy, Li,Mg, Ni, Co, Zn, Cu, Fe(III), Mn(III))11−14 can be regarded astheir relatives with the infinite stepping width. The host layersof these compounds are negatively charged, and charge-compensating alkali metal ions are accommodated betweenthem. The interlayer cations undergo facile ion-exchangereactions under ambient conditions.15−18 This reactivityenables the intercalation of various organic and inorganicguest species to yield a range of nanocomposites and hybridmaterials. Furthermore, the titanates can be exfoliated intocolloidal individual layers via swelling, typically with someamines and organoammonium ions.19−21 The resulting 2Doxide nanosheets can be organized as a building block intoprecisely designed nanostructured materials. Functionalizationof the titanates through these processes has been studiedextensively.5,7,22There is another class of layered alkali metal titanates thatare characterized by coplanar host layers based on a hexagonallinkage of TiO6 octahedra (Figure 1). The titanates of the α-NaFeO2-type structure have been less investigated comparedwith the other compounds described above. The layeredtitanate based on the α-NaFeO2-type structure was firstobtained by the electrochemical deintercalation of Na+ fromNaTiO2 to form NaxTi4+1−xTi3+xO2.23,24 Isomorphous sub-stitution of Ti4+ in the sheet with Co2+, Ni2+, and Li+ has beenreported to yield Na2/3Co1/3Ti2/3O2, NaxNix/2Ti1−x/2O2 (0.6 <x < 0.66), and Na0.66Li0.22Ti0.78O2 through solid-state syn-thesis.25−27 Their chemical reactivities, such as ion exchangeand redox intercalation, have not been explored in depth,except for electrochemical reactivities toward possibleapplication as a negative electrode for sodium-ion batteries.In the present study, the α-NaFeO2-type layered alkali metaltitanate of NaxTi1−x/3Lix/3O2 (x ∼ 0.68) was synthesized bysolid-state calcination of starting reagents Na2CO3, Li2CO3,and TiO2. The ion-exchange behaviors of NaxTi1−x/3Lix/3O2with the alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) fromaqueous solutions were examined at 80 °C. We found that theion exchange proceeded by forming a unique staging structurecomposed of alternately occupied and unoccupied interlayergalleries. The titanate of NaxTi1−x/3Lix/3O2 and its ion-Received: April 17, 2025Revised: July 23, 2025Accepted: July 28, 2025Published: August 4, 2025Featured Articlepubs.acs.org/IC© 2025 The Authors. Published byAmerican Chemical Society16289https://doi.org/10.1021/acs.inorgchem.5c01712Inorg. Chem. 2025, 64, 16289−16296This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on August 19, 2025 at 07:24:39 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Aranee+Pleng+Teepakakorn"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tomohiro+Tanaka"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Nobuyuki+Sakai"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yasuo+Ebina"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takayuki+Kikuchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Renzhi+Ma"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Renzhi+Ma"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Makoto+Ogawa"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takayoshi+Sasaki"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.inorgchem.5c01712&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/inocaj/64/32?ref=pdfhttps://pubs.acs.org/toc/inocaj/64/32?ref=pdfhttps://pubs.acs.org/toc/inocaj/64/32?ref=pdfhttps://pubs.acs.org/toc/inocaj/64/32?ref=pdfpubs.acs.org/IC?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.inorgchem.5c01712?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/IC?ref=pdfhttps://pubs.acs.org/IC?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/exchanged phases showed electrochemical intercalation/deintercalation properties for the storage of Li+ and Na+ ions.2. EXPERIMENTAL SECTION2.1. Chemicals and Materials. Titanium dioxide (rutile,99.99%), lithium carbonate (99.99%), sodium carbonate (99.99%),rubidium chloride (99.9%), and cesium chloride (99.99%) wereobtained from Rare Metallic, Japan, and used as received. Sodiumchloride (99.5%) and potassium chloride (99.5%) were obtained fromKanto Chemical, Japan. Lithium chloride (99.0%) was purchasedfrom Fujifilm Wako Pure Chemical, Japan. Acetylene black (HS100)was purchased from Denka, Japan. Polyvinylidene fluoride suspension(12 wt %, PVDF) was obtained from Kureha, Japan. N-Methyl-2-pyrrolidone (NMP, 99.5%) was purchased from Kishida Chemical,Japan. A Whatman glass fiber filter (thickness 200−300 μm) wasobtained from Merck.2.2. Synthesis of Layered Sodium Titanate. Reagents ofNa2CO3, Li2CO3, and TiO2 were intimately mixed at a molar ratio ofx/2:x/6:(1−x/3) for the composition NaxTi1−x/3Lix/3O2. The ex-plored x value was 0.60 ≤ x ≤ 0.80. The mixture was placed in a Ptcrucible and heated at 900 °C for 30 min for decarbonation. Aftercooling, the powder was ground and then heated at 900 °C for 24 h.The obtained sample, Na0.68Ti0.77Li0.23O2, referred to as NTLO, wasused in the following experiments to explore its chemical reactivities.2.3. Ion Exchange of Layered Sodium Titanate with AlkaliMetal Ions. Ion-exchange experiments were carried out byequilibrating 0.2 g of NTLO with 20 cm3 of an aqueous solution ofalkali metal chloride (0.5 M) at 80 °C for 24 h. The treatment wasrepeated 2 times by decanting the solution with the fresh one. Theproducts were collected by filtration, washed with deionized waterseveral times, and then dried overnight at room temperature. Thesamples after the ion exchange of NTLO with alkali metal ions, Li+,Na+, K+, Rb+, and Cs+, are designated as Li-NTLO, Na-NTLO, K-NTLO, Rb-NTLO, and Cs-NTLO, respectively. The ion exchange forLi+ and Na+ was also examined under the same conditions with K-NTLO, and the obtained samples are referred to as Li-KNTLO andNa-KNTLO, respectively.2.4. Electrochemical Studies. The electrode materials (NTLOand Na-KNTLO), acetylene black, and PVDF were mixed in a weightratio of 88:8:4 in NMP using a mixer (AR-100, Thinky). Theobtained slurry was spread onto a Cu foil using blade coating anddried at 80 °C for NTLO and at 50 °C for Na-KNTLO under N2 gasovernight, and it was then pressed in order to closely pack theelectrode composite on the Cu foil and dried at 110 °C for NTLOand at 50 °C for Na-KNTLO under vacuum for 15 h. The lowerdrying temperature applied to the Na-KNTLO electrodes wasintended to suppress collapse of the staging structure. The loadingamount of the electrode materials was 7−12 mg/cm2. The resultingelectrodes were used to assemble a CR2032 coin-type cell with Li orNa foil as the counter electrode and a glass fiber filter (thickness 200−300 μm) as the separator in a dry atmosphere (<0.2 ppm of H2O).LiPF6 (1 M) in ethylene carbonate (EC)/diethyl carbonate (DEC)(3:7 in v/v) and 1 M sodium bis(trifluoromethanesulfonyl)imide(NaTFSI) in EC/dimethyl carbonate (DMC) (1:1 in v/v) were usedas electrolytes for Li+- and Na+-ion batteries, respectively. Theintercalation/deintercalation studies were carried out using a charge/discharge unit (HJ1001SD8, Hokuto Denko, Japan) at roomtemperature.2.5. Characterizations. Powder X-ray diffraction (XRD) datawere recorded by using a Rigaku ULTIMA IV diffractometer withmonochromatic Cu Kα radiation (λ = 0.15405 nm). The high-resolution data for NTLO and K-NTLO were collected at BL02B1 ofthe SPring-8 synchrotron radiation facility (Hyogo, Japan) under theagreement from the Japan Synchrotron Radiation Research Institute(JASRI) and analyzed by Rietveld refinement with the programRIETAN-2000.28 Scanning electron microscopy (SEM) observationswere performed with JEOL, JSM-6010LA. For chemical analysis ofthe titanates, a weighed amount (∼0.1 g) of the sample was dissolvedwith a mixed acid solution (H2SO4 + HF) at 135 °C for 16 h. Then,metal ion contents were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, Hitachi, SPS3520UV-DD)after appropriate dilution with water. Thermogravimetric differentialthermal analysis (TG-DTA) was performed with the instrument ofRigaku, TG-8120, at a heating rate of 10 °C/min in the temperaturerange of 25−1000 °C.3. RESULTS AND DISCUSSION3.1. Synthesis and Structural Characterizations. Anintimate mixture of Na2CO3, Li2CO3, and TiO2 for thecomposition of NaxTi1−x/3Lix/3O2 with the x value of 0.6−0.8was heated at 900 °C. The obtained samples were composed ofseveral tens of micrometer-sized particles with a granular toplaty shape (Figure S1). Powder XRD data on the products inthe range 0.68 ≤ x ≤ 0.70 could be indexed in terms of thehexagonal structure with unit cell dimensions of a ∼ 0.296 andc ∼ 1.11 nm (Figure 2), confirming the single-phase formationof the α-NaFeO2-type layered titanate. On the other hand,impurity phases, such as Na2Ti6O13 and Li4Ti5O12, coexistedwith the target compound outside this range.Crystal structure of NTLO was refined on its high-resolutionsynchrotron XRD data based on a model with the α-NaFeO2-type layered structure (space group: P63/mmc (No. 194)). Asshown in Figure S2, the refinement led to satisfactory fittingwith the residual indices Rwp = 0.0798, Rp = 0.0620, RI =0.0392, RF = 0.027, and s = 1.952. The atomic positionalparameters are listed in Table S1. The results are comparableto previous reports.27 As illustrated in Figure 1, MO6 octahedraare joined via edge-sharing to form the coplanar host layer ofTi0.77Li0.23O2. It is known that oxide layers in the α-NaFeO2-type architecture are stacked in various sequences to providesuitable coordination environments for interlayer cations.29,30Two of the most typical stacking modes are P2 and O3, whereP and O represent the trigonal prismatic and octahedralcoordination, respectively, while 2 and 3 indicate the numberof oxide layers in the unit cell. The NTLO in this study adoptsthe P2 structure. The interlayer gallery is 0.556 nm high (= c/2), accommodating Na+ ions at trigonal prismatic sites (2b and2d). Their distribution in these sites was refined, revealing thepreferred occupancy at the 2d site. This tendency has beenreported for other isomorphous-layered metal oxides, e.g.,Figure 1. Crystal structures of various types of layered titanates. The unit cell is indicated by the broken line.Inorganic Chemistry pubs.acs.org/IC Featured Articlehttps://doi.org/10.1021/acs.inorgchem.5c01712Inorg. Chem. 2025, 64, 16289−1629616290https://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig1&ref=pdfpubs.acs.org/IC?ref=pdfhttps://doi.org/10.1021/acs.inorgchem.5c01712?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asNa0.74CoO2.31 The interatomic distance between the metal sitein the host layer and the interlayer 2b site is closer, leading to aless stable accommodation.3.2. Ion-Exchange Behaviors. Ion-exchange behaviorswere examined by treating NTLO with aqueous solutions ofalkali metal chlorides at 80 °C. SEM observations indicate thatthe particle size and platelet morphology remained virtuallyunchanged after the treatment, suggesting a topotactic reactionprocess (Figure S3). The chemical composition of the samplesafter the ion-exchange reaction is given in Table 1. Na+ ionswere nearly absent when NTLO was treated with aqueoussolutions of KCl, RbCl, and CsCl. It should be pointed out thatthe amount of incoming K+, Rb+, and Cs+ ions is much lessthan that of Na+ ions originally accommodated, indicating thatstoichiometric ion exchange between alkali metal ions did nottake place. This difference may be explained by concurrentproton exchange. Based on ignition loss results, the protoncontents were estimated, as indicated in Table 1. The chemicalformulas of K-NTLO, Rb-NTLO, and Cs-NTLO indicate thatapproximately half of the protons is accommodated in the formof oxonium ions, and another half is in the form of H+. TheFT-IR spectrum of ion-exchanged phases (Figure S4) isconsistent with the assignment above. A broad absorptionband at 3600−2700 cm−1 and a rather sharp peak at 1600cm−1 are attributable to stretching and bending modes of watermolecules, respectively, indicating the presence of oxoniumions. On the other hand, the strong absorption at 930 cm−1 ischaracteristic of the hydroxyl group, which may be formed viabonding of a proton to the oxygen atom on the host layer. Asimilar feature was observed in protonated layered titanates,such as H2Ti4O9·1.3H2O and H2Ti5O11·3H2O,16,17,32 whichcontain oxonium ions and hydroxyl groups. Another note-worthy point is that the Li content remained virtuallyunchanged, indicating that Li+ ions in the host layer werenot involved in the ion-exchange reaction. Figure 3 showspowder XRD data of the ion-exchanged products. All the peakscould be indexed in terms of a hexagonal structure, and therefined unit cell parameters are summarized in Table 1.Different from the parent compound (NTLO) ofNa0.68Ti0.77Li0.23O2, basal peaks having odd indices appeared,and their intensity is relatively weak compared with those witheven indices. These features suggest that the layered titanate ofFigure 2. XRD patterns of calcined samples for NaxTi1−x/3Lix/3O2with the x value of 0.65−0.75. The peaks with orange circles areattributable to the titanate with the α-NaFeO2-type structure, whilethose with green triangles are from impurity phases.Table 1. Chemical Composition and Unit Cell Dimensions of Ion-Exchanged Phasessample compositionlattice constants (nm)a cNTLO Na0.68Ti0.77Li0.23O2 0.29611(4) 1.1113(2)Li-KNTLOa Li0.13H0.56Ti0.77Li0.23O2·0.39H2O 0.29879(4) 1.1674(2)Na-KNTLO Na0.17H0.58Ti0.77Li0.17O2·0.36H2O 0.29938(4) 1.1811(2)K-NTLO K0.15H0.56Ti0.77Li0.19O2·0.27H2O 0.29877(5) 1.1818(3)Rb-NTLO Rb0.13H0.55Na0.02Ti0.77Li0.21O2·0.25H2O 0.29875(4) 1.1928(2)Cs-NTLO Cs0.12H0.56Na0.02Ti0.77Li0.21 O2·0.25H2O 0.29894(4) 1.2034(3)aThe Li content in the host layer is assumed to be maintained during the ion-exchange process.Figure 3. XRD patterns of NTLO and its ion-exchanged phases withalkali metal ions. Values in blue represent the d-spacing of the 002peak.Inorganic Chemistry pubs.acs.org/IC Featured Articlehttps://doi.org/10.1021/acs.inorgchem.5c01712Inorg. Chem. 2025, 64, 16289−1629616291https://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig3&ref=pdfpubs.acs.org/IC?ref=pdfhttps://doi.org/10.1021/acs.inorgchem.5c01712?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asNTLO underwent some structural change upon ion exchange,as will be discussed below in depth.On the other hand, treatment with the LiCl solution did notproduce a material comparable to ion-exchanged phases withK+, Rb+, and Cs+ ions but yielded a poorly crystalline sample,showing rather broad diffraction peaks (Figure S5a). Chemicalanalysis revealed that ∼2/3 of Na+ ions were replaced with Li+ions, and the proton exchange was not significant. The sampletreated with the NaCl solution showed the XRD pattern forpristine NTLO accompanied by some minor byproducts(Figure S5b). In contrast, when K-NTLO was treated withaqueous solutions of LiCl and NaCl, an ion-exchange reactiontook place in a similar way to the process of K+, Rb+, and Cs+ions from NTLO. Powder XRD data after the reaction showthe formation of such a unique phase (Figure 3). The chemicalcompositions were comparable to those for K-, Rb-, and Cs-exchanged phases, except for higher degrees of hydration(Table 1).3.3. Staging Structure of Ion-Exchanged Materials. Asdescribed above, after the ion-exchange process, 00l basaldiffraction peaks (l = 2n + 1) appeared, suggesting a change inthe stacking periodicity along the c-axis. There are twopossibilities to account for this feature: (i) differentoccupancies of guests in adjacent interlayer galleries and (ii)noneven displacement of the host layers along the c-axis.Because no systematic extinction of diffraction peaks wasrecognized, five space groups, such as P m6 2, P m62 , P6mm,P622, and P6/mmm, are possible. After considerations, wefound that a reasonable structure model can be constructedaccording to the space group P m6 2. Then, the structureanalysis was conducted for the K+ ion-exchanged phase, K-NTLO, as the representative sample. The refinement yielded areasonable convergence with the residual indices of Rwp =0.0597, Rp = 0.0442, RI = 0.0286, RF = 0.0159, and s = 1.590(Figure 4). The structural parameters are summarized in Table2, and the refined structure is illustrated in Figure 5. The hostlayers are stacked along the c-axis with alternate repeatingspacings of 0.700 and 0.482 nm. The neighboring layers glidewith respect to each other along the a-axis by a/2, generating atrigonal prismatic site for interlayer guest species. The widerinterlayer galleries accommodate guest species, such as K+ andH3O+ ions, while the narrower galleries are empty. The largethermal parameter may reflect from widely distributed positionof these guest species, in addition to expressing their thermalvibration. As discussed above, this phase contains protons, aswell as H3O+ ions. The position of protons is not availablebecause the structure refinement is based on the XRD data.The K+ ion-exchanged phase (K-NTLO) can be charac-terized by the so-called staging structure. It is well-known thatgraphite forms the various staging structures upon intercalationof guests, typically alkali metals.33−36 Apart from graphiteintercalation compounds, some layered compounds, such aslayered double hydroxides37−40 and interstratified clayminerals,41−43 were reported to evolve such a unique structure.However, the formation of the staging structure in highcrystallinity, which allows full structure refinement, is rare.Although there have been a number of studies for a range oflayered titanates and their derivatives as described in theSection 1, to the best of our knowledge, this is the first exampleof the staging-structured titanate. The layer architecture of theα-NaFeO2 type is rather thin and flexible, which cannotadequately screen the electrostatic repulsion between guestcations located in neighboring interlayer space. Thus, thestaging structure was produced, avoiding the occupancy of K+ions at every interlayer space. The flexible layer is alsofavorable, stabilizing the staging structure by forming domainssimilar to graphite known as the Daumas and Heŕold model.44Recently, the staging structure was reported in the electro-chemical intercalation process of LiCoO2, the layer of which isalso thin and flexible.45 On the other hand, the other titanatesare composed of relatively thicker corrugated layers, which canfully screen the electrostatic interaction. The interlayer cationsmay be considered to be isolated from those in the neighboringgallery.The narrower interlayer spacing of 0.482 nm can be taken asthe thickness of the titanate layers. This value is comparable tothe layer thickness of LiCoO2 with a similar layerarchitecture.46 The difference of 0.218 nm (= 0.700−0.482)in the gallery heights should be related to the size of K+ andH3O+ ions. It is to be noted that a change in the c-parameterfor the ion-exchanged phases is rather modest in comparisonwith a variation of the ionic size for Li+ to Cs+ (Figure 3). Thismay indicate that H3O+ ions play a main role in opening thewider interlayer gallery.3.4. Electrochemical Intercalation/Deintercalation.Previous study showed that NTLO can work as a superioranode material because of negligible volume change uponelectrochemical cycling of the intercalation/deintercalationprocess.27 Thus, it is of interest to explore the electrochemicalperformance of the ion-exchanged derivatives obtained in thisstudy.The compounds of Na-KNTLO with the staging structureand the pristine NTLO were employed as working electrodesin the coin-type cell filled with 1 M LiPF6 in EC/DEC, andtheir electrochemical intercalation/deintercalation behaviors ofLi+ ions in lithium half-cells were examined at 50 mA/g in thevoltage range of 0.3−3.0 V vs Li counter electrode. As shownin Figure 6, the capacity delivered in the initial Li+-ionintercalation process for Na-KNTLO reached ∼210 mAh/gdue to the formation of a solid electrolyte interphase (SEI) andthen decreased in the following cycles. The relatively largecapacity observed for Na-KNTLO may be ascribed to thedecomposition of water persisted in the material. After 10cycles, the reversible capacity of 82 mAh/g was obtained,corresponding to 0.23 Li+ insertion per formula unit, which isslightly smaller than the available unoccupied sites (0.25 (= 1− 0.17 − 0.58)) in Na0.17H0.58Ti0.77Li0.17O2, assuming that theFigure 4. Rietveld fitting of synchrotron X-ray diffraction data for K-NTLO. Observed and calculated profiles are denoted by dotted andsolid lines, respectively. The differences between them and locationsof reflections are indicated at the bottom.Inorganic Chemistry pubs.acs.org/IC Featured Articlehttps://doi.org/10.1021/acs.inorgchem.5c01712Inorg. Chem. 2025, 64, 16289−1629616292https://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig4&ref=pdfpubs.acs.org/IC?ref=pdfhttps://doi.org/10.1021/acs.inorgchem.5c01712?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aswater has been decomposed. In contrast, the pristine NTLOshowed a reversible capacity of 67 mAh/g, which correspondsto 0.22 Li+ insertion per formula unit. This value is muchsma l l e r than the ava i l ab l e vacancy (0 .32) inNa0.68Ti0.77Li0.23O2. The crystal structure and morphology ofthe electrode materials that experienced the intercalation/deintercalation of Li+ ions were examined. Upon 50 cycles, the002 reflection shifted to a higher angle (d = 0.492 nm) in theXRD pattern for Na-KNTLO, and the peaks due to 001 and003 reflections disappeared (Figure 7). These changes can beascribed to the ion exchange of Na+ ions with Li+ ions in theirrepeated intercalation/deintercalation processes because of thelarge excess of Li+ in the system. The disappearance of 001 and003 peaks from Na-KNTLO suggests the intercalation of Li+ions into the empty interlayer galleries during the process,leading to a loss of the staging structure. The shift of the 002reflection was also observed for the pristine NTLO afterrepeated intercalation/deintercalation processes, and its d-spacing decreased to 0.496 nm, which is close to that of Na-KNTLO after the cycles. On the other hand, the platymorphology of Na-KNTLO and NTLO did not changesignificantly after 50 cycles of the process, as observed by SEM(Figure S6).Table 2. Structural Parameters for K-NTLOaatom position occupancyd x y z Beq (×10−2 nm2)G1b 1b 0.120(1) 0 0 1/2 18.7(1)G2b 1f 0.244 2/3 1/3 1/2 15.9Mc 2g 1 0 0 0.20393(5) 0.60(1)O1 2h 1 1/3 2/3 0.2864(1) 0.98(4)O2 2i 1 2/3 1/3 0.1128(1) 1.02(3)aHexagonal, P m6 2 (No. 187), a = 0.299035(1) nm, c = 1.18259(1) nm. bG = 0.15 K+ + 0.27 H3O+. cM = 0.77 Ti4+ + 0.19 Li+. dThe occupancyfactor and the atomic displacement parameter were refined independently avoiding their strong correlation. The large atomic displacementparameter may be partly due to wide positional distribution of these guest species.Figure 5. Staging structure of the K+ ion-exchanged phase.Figure 6. Intercalation/deintercalation curves (first, second, and 20th cycles) of Li+-ion batteries using Na-KNTLO (a,b) and pristine NTLO (c,d)and their specific capacity cycle performance at 50 mA/g in the voltage range of 0.3−3.0 V, starting from the intercalation process.Inorganic Chemistry pubs.acs.org/IC Featured Articlehttps://doi.org/10.1021/acs.inorgchem.5c01712Inorg. Chem. 2025, 64, 16289−1629616293https://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig6&ref=pdfpubs.acs.org/IC?ref=pdfhttps://doi.org/10.1021/acs.inorgchem.5c01712?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asIntercalation/deintercalation of Na+ ions was also studiedfor Na-KNTLO and NTLO (Figure S7). The reversiblecapacity obtained after 10-cycle intercalation/deintercalationwas 77 mAh/g for Na-KNTLO, corresponding to 0.21 Na+insertion per formula unit, which is slightly smaller than theavailable vacant sites (0.25) in Na0.17H0.58Ti0.77Li0.17O2. Incontrast, the pristine NTLO showed a reversible capacity of 56mAh/g, which corresponds to 0.18 Na+ insertion per formulaunit. This value is much smaller than the available vacancy(0.32) in Na0.68Ti0.77Li0.23O2. As shown in Figure 7, the peaksdue to 001 and 003 reflections in the XRD pattern for Na-KNTLO disappeared, indicating the loss of the stagingstructure, as is the case in the intercalation/deintercalation ofLi+ ions. The 002 reflection from Na-KNTLO was shifted fromd = 0.577 to d = 0.546 nm after the cycles, the latter of which isclose to the 002 reflection from NTLO before (d = 0.552 nm)and after (d = 0.548 nm) intercalation/deintercalation of Na+ions. The repeated intercalation/deintercalation processesmight promote the equivalent insertion of Na+ ions in eachgallery of Na-KNTLO, transforming its crystal structure similarto that of NTLO.Although the staging structure of Na-KNTLO was lostduring cycling, Na-KNTLO showed more efficient utilization(91% and 86%) of available vacant sites compared to thepristine NTLO (68% and 56%) in the reversible intercalation/deintercalation of Li+ and Na+ ions, respectively. Repeatedintercalation/deintercalation into the empty interlayer spacesof the staging structure may facilitate a more efficientutilization of the interlayer galleries, resulting in a higherreversible capacity.4. CONCLUSIONSThe α-NaFeO2-type layered sodium lithium titanate wassynthesized by solid-state calcination for the stoichiometry ofNaxTi1−x/3Lix/3O2 of 0.68 ≤ x ≤ 0.70. Li+, as well as Ti4+ ionsoccupy the octahedral site in the host layer, while Na+ ions areaccommodated in the trigonal prismatic cavity of the interlayerspace. The interlayer Na+ ions could be exchanged with alkalimetal ions (Li+, Na+, K+, Rb+, and Cs+) and protons/oxoniumions when brought into contact with aqueous solutions of thecorresponding alkali metal salts. The ion-exchange processproduced the staging structure, in which the occupied andunoccupied interlayer galleries alternate. The staging-struc-tured alkali titanates were applied as a host material forelectrochemical intercalation/deintercalation of Li+ and Na+ions, and we found that the specific capacities of Na-KNTLOwere higher than the pristine NTLO for both Li+ and Na+ ions,suggesting superior performance of the staging structure forenergy storage.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712.SEM images of as-synthesized NTLO, its ion-exchangedphases, and Na-KNTLO and NTLO before and after Li+ion intercalation/deintercalation; Rietveld fitting ofXRD data for NTLO and its structural parameters;XRD data for NTLO treated with LiCl and NaCl; andNa+-ion intercalation/deintercalation behaviors for Na-KNTLO and NTLO (PDF)■ AUTHOR INFORMATIONCorresponding AuthorTakayoshi Sasaki − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; orcid.org/0000-0002-2872-0427;Email: sasaki.takayoshi@nims.go.jpAuthorsAranee Pleng Teepakakorn − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; School of Energy Science and Engineering,Vidyasirimedhi Institute of Science and Technology(VISTEC), Rayong 21210, Thailand; orcid.org/0000-0002-8668-3513Tomohiro Tanaka − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,JapanNobuyuki Sakai − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; orcid.org/0000-0002-9395-6751Yasuo Ebina − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; orcid.org/0000-0003-3471-9825Takayuki Kikuchi − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forFigure 7. XRD patterns of Na-KNTLO (a) and pristine NTLO (b)before and after intercalation/deintercalation of Li+ ions or Na+ ionsat 50 mA/g in the voltage range of 0.3−3.0 V. The peak designatedwith an asterisk (d = 0.516 nm) may be ascribed to dried Na-KNTLOformed during the electrode preparation.Inorganic Chemistry pubs.acs.org/IC Featured Articlehttps://doi.org/10.1021/acs.inorgchem.5c01712Inorg. Chem. 2025, 64, 16289−1629616294https://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5c01712/suppl_file/ic5c01712_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takayoshi+Sasaki"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-2872-0427mailto:sasaki.takayoshi@nims.go.jphttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Aranee+Pleng+Teepakakorn"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-8668-3513https://orcid.org/0000-0002-8668-3513https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tomohiro+Tanaka"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Nobuyuki+Sakai"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-9395-6751https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yasuo+Ebina"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0003-3471-9825https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takayuki+Kikuchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig7&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01712?fig=fig7&ref=pdfpubs.acs.org/IC?ref=pdfhttps://doi.org/10.1021/acs.inorgchem.5c01712?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; orcid.org/0000-0003-0588-2172Renzhi Ma − Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Tsukuba, Ibaraki 305-0044,Japan; orcid.org/0000-0001-7126-2006Makoto Ogawa − School of Energy Science and Engineering,Vidyasirimedhi Institute of Science and Technology(VISTEC), Rayong 21210, Thailand; orcid.org/0000-0002-3781-2016Complete contact information is available at:https://pubs.acs.org/10.1021/acs.inorgchem.5c01712NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThe authors are grateful for a Research Chair Grant 2017(Grant number FDA-CO-2560-5655) from the NationalScience and Technology Development Agency (NSTDA),Thailand. This work was supported by the World PremierInternational Research Center Initiative (WPI), MEXT, Japan,and CREST of the Japan Science and Technology Agency(JST) (Grant No. JPMJCR22B1), Japan. The authors aregrateful to Dr. Shoji Yamaguchi and Mr. Shin Kimura of theNIMS Battery Research Platform for technical assistance withthe electrode preparation. We also acknowledge technicalsupport by the Surface and Bulk Analysis Unit in NIMS forICP-OES and XRD measurements. A.P.T. acknowledgesVISTEC for the scholarship to her Ph.D. study and 1-yearresearch exchange in MANA, NIMS, Japan.■ REFERENCES(1) Clearfield, A. Role of Ion Exchange in Solid-state Chemistry.Chem. Rev. 1988, 88, 125−148.(2) Liu, G.; Wang, L.; Sun, C.; Yan, X.; Wang, X.; Chen, Z.; Smith,S. C.; Cheng, H. M.; Lu, G. Q. Band-to-band Visible-Light PhotonExcitation and Photoactivity Induced by Homogeneous NitrogenDoping in Layered Titanates. Chem. Mater. 2009, 21, 1266−1274.(3) Kalantar-zadeh, K.; Ou, J. 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