P2-Na0.6[Cr0.6Ti0.4]O2 cation-disordered electrode for high-rate symmetric rechargeable sodium-ion batteries

Abstract

Most P2-type layered oxides exhibit Na+/vacancy-ordered superstructures because of strong Na+–Na+ interaction in the alkali metal layer and charge ordering in the transition metal layer. These superstructures evidenced by voltage plateaus in the electrochemical curves limit the Na+ ion transport kinetics and cycle performance in rechargeable batteries. Here we show that such Na+/vacancy ordering can be avoided by choosing the transition metal ions with similar ionic radii and different redox potentials, for example, Cr3+ and Ti4+. The designed P2-Na0.6[Cr0.6Ti0.4]O2 is completely Na+/vacancy-disordered at any sodium content and displays excellent rate capability and long cycle life. A symmetric sodium-ion battery using the same P2-Na0.6[Cr0.6Ti0.4]O2 electrode delivers 75% of the initial capacity at 12C rate. Our contribution demonstrates that the approach of preventing Na+/vacancy ordering by breaking charge ordering in the transition metal layer opens a simple way to design disordered electrode materials with high power density and long cycle life.

Phosphate Framework Electrode Materials for Sodium Ion Batteries

“…A large variety of compounds, such as transition metal oxides,6, 7, 8, 9 phosphates,10, 11, 12 ferrocyanide,13, 14, 15 hard carbon,16, 17, 18, 19 metal alloys,20, 21, 22 and organic materials,23, 24, 25 have demonstrated considerable Na‐storage capacities for SIBs. However, most of them suffer from structural instability during Na‐insertion/extraction reactions.…”

Utilizing Co2+/Co3+ Redox Couple in P2‐Layered Na0.66Co0.22Mn0.44Ti0.34O2 Cathode for Sodium‐Ion Batteries

“…They can be assigned to the complex redox processes of Co and Mn with mixed valence states,12, 44 since the redox potential of Ti ions is usually below 1.5 V 35. The polarization of charge and discharge is relatively low compared with many other P2‐layered cathode materials for SIBs 35, 44, 45. After the initial charge process, the CV curves are overlapped very well, indicating high reversibility.…”

“…It has been reported by Hu et al that there are three types of ordering in the structure of P2‐layered Na x [M1,M2]O 2 oxides which could have negative effects on the electrochemical performance: transition metal ordering (different transition metal ions prefer to occupy different atomic sites, rather than indistinguishable occupation), charge ordering (preferred site occupations of ions with different charge, such as M 3+ /M 4+ ordering), and Na + /vacancy ordering 35. In a large number of P2‐type layered oxides,12, 22, 23, 24, 36, 37, 38 transition metal ordering is mainly determined by the difference between the ionic radii of M1 and M2, in which small difference (usually less than 15%) favors to form disordered arrangement if M1 and M2 content is close to a rational rate, while a larger difference (greater than 15%) favors an ordered arrangement.…”

Symmetric Electrodes for Electrochemical Energy‐Storage Devices

“…Furthermore, Zhou's group developed a symmetric SIB system based on this P2‐Na 0.6 [Cr 0.6 Ti 0.4 ]O 2 electrode. 75% of the initial capacity at 12C rate could be retained in this cell 65. Although the research history of using titanate‐based symmetric electrodes in SIBs is slightly shorter than that of LIBs, a broader and deeper study has been carried out in SIBs.…”

“…64 This cell exhibited a high average voltage of 2.8 V, a reversible discharge capacity of 85 mAh g –1 , 75% capacity retention after 150 cycles, and good rate capability. In addition, Cr 3+ and Ti 4+ , with the similar ionic radii and different redox potentials, were employed in a P2‐Na x [M1,M2]O 2 ‐layered oxide system, leading to a completely Na + vacancy‐disordered P2‐Na 0.6 [Cr 0.6 Ti 0.4 ]O 2 composite 65. Furthermore, Zhou's group developed a symmetric SIB system based on this P2‐Na 0.6 [Cr 0.6 Ti 0.4 ]O 2 electrode.…”