Among the cathode materials of the formula Na3+2xMn1+xTi1-x(PO4)3, the compound with x =0.2 represent the best trade-off between electrochemical performances and phase purity. However, this material still suffers from important shortcomings, such as voltage hysteresis, incomplete utilization of manganese redox processes and unclear ageing mechanisms. Herein, we conduct an extensive electrochemical investigation of the Na3.4Mn1.2Ti0.8(PO4)3/C cathode, with particular emphasis on the interplay between structure, Na+transport, and reaction kinetics, trying also to clarify their role in determining the capacity fade. The effects of kinetic polarization are clearly visible in rate capability test, where redox peaks are shifted beyond the operational voltage window, inhibiting the Mn4+/Mn3+and Ti4+/Ti3+redox couples, and practically limiting the accessible capacity at high current densities. However, the kinetic polarization cannot explain the voltage hysteresis and the gap between practical and theoretical Na+ extracted/inserted encountered during the near-equilibrium condition GITT measurements. Therefore, these limitations can be also attributed to intrinsic structural factors, such as the anti- site disorder originated from manganese dislocation in Na vacancies. Finally, the ageing mechanism have been investigated through long-term EIS and GITT analysis, demonstrating that the capacity fade encountered for Mn- based NASICON cathodes is predominantly governed by a progressive slowdown of solid-state Na⁺ diffusion rather than interfacial degradation and charge-transfer limitations, with no structural transformations and electrode deterioration after 1500 cycles confirmed by post-mortem analysis. Overall, the obtained results provide a comprehensive understanding of electrochemical properties and degradation mechanisms in Mn-rich NASICON cathodes, thus paving the way to further improvements on electrochemical performances of phosphate-based cathodes for sodium-ion battery.
Unravelling sodium-ion transport, kinetic properties and ageing mechanisms in Mn-rich Na3.4Mn1.2Ti0.8(PO4)3/C NASICON cathode
Bottoni, L.
;Arslan, M.;Minnetti, L.;Lischio, F.;Pacetti, S.;Piechocki, M.;Nobili, F.
2026-01-01
Abstract
Among the cathode materials of the formula Na3+2xMn1+xTi1-x(PO4)3, the compound with x =0.2 represent the best trade-off between electrochemical performances and phase purity. However, this material still suffers from important shortcomings, such as voltage hysteresis, incomplete utilization of manganese redox processes and unclear ageing mechanisms. Herein, we conduct an extensive electrochemical investigation of the Na3.4Mn1.2Ti0.8(PO4)3/C cathode, with particular emphasis on the interplay between structure, Na+transport, and reaction kinetics, trying also to clarify their role in determining the capacity fade. The effects of kinetic polarization are clearly visible in rate capability test, where redox peaks are shifted beyond the operational voltage window, inhibiting the Mn4+/Mn3+and Ti4+/Ti3+redox couples, and practically limiting the accessible capacity at high current densities. However, the kinetic polarization cannot explain the voltage hysteresis and the gap between practical and theoretical Na+ extracted/inserted encountered during the near-equilibrium condition GITT measurements. Therefore, these limitations can be also attributed to intrinsic structural factors, such as the anti- site disorder originated from manganese dislocation in Na vacancies. Finally, the ageing mechanism have been investigated through long-term EIS and GITT analysis, demonstrating that the capacity fade encountered for Mn- based NASICON cathodes is predominantly governed by a progressive slowdown of solid-state Na⁺ diffusion rather than interfacial degradation and charge-transfer limitations, with no structural transformations and electrode deterioration after 1500 cycles confirmed by post-mortem analysis. Overall, the obtained results provide a comprehensive understanding of electrochemical properties and degradation mechanisms in Mn-rich NASICON cathodes, thus paving the way to further improvements on electrochemical performances of phosphate-based cathodes for sodium-ion battery.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
