TY - JOUR

T1 - Why has phosphate bronze Na2 +xNb6P4O26 never been investigated as a material for energy storage?

AU - Skachilova, Maria G.

AU - Podgornova, Olga A.

AU - Tsydypylov, Dmitry Z.

AU - Korotaev, Evgeniy V.

AU - Fedorenko, Anastasiya D.

AU - Shapovalova, Alexandra A.

AU - Shindrov, Alexander A.

N1 - This work was supported by the Russian Science Foundation, (Project #25–73–00012, https://rscf.ru/en/project/25–73−00012/). XPS measurements were supported by the Ministry of Science and Higher Education of the Russian Federation within the governmental order for SRF SKIF Boreskov Institute of Catalysis (FWUR−2024–0040). The authors are grateful to the Dr. B. A. Kolesov (NIIC SB RAS) for the registration of the Raman spectra. The A.A.S. expresses gratefulness to Yelizaveta A. Morkhova (Samara State Technical University) for constructive advice and comments on the article and Artem S. Ulihin (ISSCM SB RAS) for carrying out EIS measurements. Authors would like to acknowledge the Multi-Access Chemical Research Center SB RAS for spectral and analytical measurements.

PY - 2026/6/15

Y1 - 2026/6/15

N2 - For the first time, the optimization of synthesis conditions, the study of Na+ mobility, and the evaluation of the electrochemical properties of sodium-niobium phosphate bronze Na2+xNb6P4O26 were carried out. The synthesis of phase-pure Na2+xNb6P4O26 was conducted at 900°C for 2.5 h with 15 mol% and 5 mol% excess of Na and P sources, respectively. A detailed study of the structure of Na2+xNb6P4O26 showed the presence of four-, five- and six-coordinated bottlenecks for possible Na+ movements. The energy barriers to Na+ migration and ionic conductivity were evaluated using the bond valence site energy analysis, and kinetic Monte Carlo modeling implemented in the softBV program. 1D, 2D, and 3D pathways with energy barriers from 0.086 eV to 1.044 eV were detected. The theoretical and experimental ionic conductivity values are consistent with each other and are on the order of ∼10−7 S cm−1. Galvanostatic cycling of the Na2+xNb6P4O26/C composite was performed in Na cell. During the first discharge/charge, the capacity was ∼330 mAh g−1/161 mAh g−1 (∼14.0 Na+/7.0 Na+), and during subsequent discharges/charges, the capacity did not exceed 175/162 mAh g−1 (7.0–8.0 Na+). The contributions from surface-controlled capacitive reactions and diffusion-controlled processes were analyzed. According to the obtained data, the capacitive contribution predominates and accounts for ∼75% at 0.1 mV s⁻¹, remaining nearly unchanged with increasing scan rate (∼79% at 1.0 mV s⁻¹). Ex situ XRD and XPS analyses revealed that the discharge/charge process proceeds via both (de)intercalation and conversion mechanisms.

AB - For the first time, the optimization of synthesis conditions, the study of Na+ mobility, and the evaluation of the electrochemical properties of sodium-niobium phosphate bronze Na2+xNb6P4O26 were carried out. The synthesis of phase-pure Na2+xNb6P4O26 was conducted at 900°C for 2.5 h with 15 mol% and 5 mol% excess of Na and P sources, respectively. A detailed study of the structure of Na2+xNb6P4O26 showed the presence of four-, five- and six-coordinated bottlenecks for possible Na+ movements. The energy barriers to Na+ migration and ionic conductivity were evaluated using the bond valence site energy analysis, and kinetic Monte Carlo modeling implemented in the softBV program. 1D, 2D, and 3D pathways with energy barriers from 0.086 eV to 1.044 eV were detected. The theoretical and experimental ionic conductivity values are consistent with each other and are on the order of ∼10−7 S cm−1. Galvanostatic cycling of the Na2+xNb6P4O26/C composite was performed in Na cell. During the first discharge/charge, the capacity was ∼330 mAh g−1/161 mAh g−1 (∼14.0 Na+/7.0 Na+), and during subsequent discharges/charges, the capacity did not exceed 175/162 mAh g−1 (7.0–8.0 Na+). The contributions from surface-controlled capacitive reactions and diffusion-controlled processes were analyzed. According to the obtained data, the capacitive contribution predominates and accounts for ∼75% at 0.1 mV s⁻¹, remaining nearly unchanged with increasing scan rate (∼79% at 1.0 mV s⁻¹). Ex situ XRD and XPS analyses revealed that the discharge/charge process proceeds via both (de)intercalation and conversion mechanisms.

KW - Discharge/charge capacity

KW - Na+ migration

KW - Na2+xNb6P4O26

KW - Phosphate bronze

KW - Synthesis conditions

UR - https://www.scopus.com/pages/publications/105040682011

UR - https://www.mendeley.com/catalogue/6374f0a9-351e-392a-8441-12a40621f448/

U2 - 10.1016/j.jallcom.2026.188948

DO - 10.1016/j.jallcom.2026.188948

M3 - Article

VL - 1073

JO - Journal of Alloys and Compounds

JF - Journal of Alloys and Compounds

SN - 0925-8388

M1 - 188948

ER -