High-Performance P2-Phase Na_2/3Mn_0.8Fe_0.1Ti_0.1O₂ Cathode Material for Ambient-Temperature Sodium-Ion Batteries

Abstract

High-performance Mn-rich P2-phase Na2/3Mn0.8Fe0.1Ti0.1O2 is synthesized by a ceramic method, and its stable electrochemical performance is demonstrated. 23Na solid-state NMR confirms the substitution of Ti4+ ions in the transition metal oxide layer and very fast Na+ mobility in the interlayer space. The pristine electrode delivers a second charge/discharge capacity of 146.57/144.16 mA·h·g–1 and retains 95.09% of discharge capacity at the 50th cycle within the voltage range 4.0–2.0 V at C/10. At 1C, the reversible specific capacity still reaches 99.40 mA·h·g–1, and capacity retention of 87.70% is achieved from second to 300th cycle. In addition, the moisture-exposed electrode reaches reversible capacities of more than 130 and 80 mA·h·g–1 for C/10 and 1C, respectively, with excellent capacity retention. The correlation between overall electrochemical performance of both electrodes and crystal structural characteristics are investigated by neutron powder diffraction. The stability of pristine electrode’s crystallographic structure during the charge/discharge process has been investigated by in situ X-ray diffraction, where only a solid solution reaction occurs within the given voltage range except for a small biphasic mechanism occurring at or below 2.2 V during the discharge process. The relatively small substitution (20%) at the transition metal site leads to stable electrochemical performance, which is in part derived from the structural stability during electrochemical cycling. Therefore, the small cosubstitution (e.g., with Ti and Fe) route suggests a possible new scope for the design of sodium-ion battery electrodes that are suitable for long-term cycling.

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition, Air Stability, and Performance

“…Ti-doped P2-phase Na 2/3 Mn 0.8 Fe 0.1 Ti 0.1 O 2 also shows improved electrochemical performance and very fast Na + mobility in the interlayer space. At 1 C, the reversible specific capacity of pristine electrode reaches 99.40 mA h g −1 , and a capacity retention of 87.70% is achieved from second to 300th cycle within the voltage range 2.0-4.0 V. [41] The problems of hygroscopic character and the sodium deficiency in this system can be migrated by partial Cu substitution and addition of NaN 3 according to the newest reports. The other challenges for Fe/Mn-based oxides may still lie in (1) low average voltage based on the Mn 2+ /Mn 3+ /Mn 4+ redox reaction; (2) a large volume change during Na + extraction/insertion; (3) possibility of Mn ions dissolution in the electrolyte.…”

“…The interlayer spacing becomes larger due to increased repulsion between adjacent oxide layers because the shielding effect by the positively charged Na ions decreases. [38][39][40][41][42] (iii) Layered oxides can also uptake CO 2 on exposure to air, involving the unprecedented room temperature insertion of CO 3 2− within the TM layers of the Na-based metal oxide, which is simultaneously balanced by the oxidation of TM in the lattice to higher valence. [43] The mechanisms proposed above all result in the formation of electrochemically inactive NaOH or Na 2 CO 3 on the surface of active materials, leading to the deteriorated battery performance.…”

Electrode Materials for Sodium-Ion Batteries: Considerations on Crystal Structures and Sodium Storage Mechanisms

“…These compounds are layered metal oxides with the sequence OMOAOMOA, where O = oxygen, M = Co and A = Na. Within the composition range 0.5 < x < 1, up to four phases have been identified, in which the sodium coordination is either octahedral or trigonal prismatic [2][3][4][5]. O3, O'3, P3 and P2 phases are comprised in the mentioned x range.…”

“…Recently, sodium-ion (Na-ion) batteries have been emerging as an attractive system for renewable energy storage. Na-ion batteries were originally investigated in the 1970s and 1980s [2][3][4][5], but became less attractive due to the great success of lithium-ion batteries. Ambient temperature Na-ion batteries are preferred for large-scale applications in terms of safety, operating cost and ease of maintenance.…”

“…(2) Phosphate polyanions. The strong inductive effect of the PO 4 3− polyanion modulates the energetics of the transition metal redox couple to generate relatively high operating potentials (Fig. 3b).…”