Thermally Activated P2‐O3 Mixed Layered Cathodes toward Synergistic Electrochemical Enhancement for Na Ion Batteries

Abstract

Layer‐structured oxide cathodes have a lot of phases, which can be varied depending on Na ion contents and finally determine their electrochemical properties. Therefore, the off‐stoichiometry of layer‐structured oxides with the Na ions may differentiate not only their capacities but also the cyclic stabilities, kinetics, and so on, highlighting the importance of Na ion content. However, Na2CO3 tends to be irreversibly formed on surface by making use of the Na ions lost from the lattice. Thereby, the O3 phase with stoichiometric Na content changes into the off‐stoichiometric P2 phase bringing about significant disadvantages. To address this issue, a thermal activation process is suggested to simultaneously decompose Na2CO3 into electrochemically active Na ions and modulate the off‐stoichiometric P2 phase into the stoichiometric O3 phase. This study indicates that minimizing the loss of Na ions and maintaining the lattice framework with higher contents of Na ions during the synthesis of Na‐incorporating layered cathodes should be a key toward attaining electrochemical superiority.

Understanding the Aging Mechanism of Na-Based Layered Oxide Cathodes with Different Stacking Structures

“…The weight loss in region II is related to the release of interlayer water molecules. 21 In this region, the weight loss of P2-Na 0.50 MnO 2 is about 0.44%, four times higher than 0.11% of O3-Na 0.85 MnO 2 , which is consistent with XRD results of hydrate phase appearance in aged P2 material. Furthermore, chemical titration was employed to determine the concentration of formed alkali species such as Na 2 CO 3 , NaOH, and Na 2 CO 3 •H 2 O, 55 and the results are shown in Figure 3c.…”

“…In region I, the weight loss of aged P2 and O3-Na x MnO 2 samples is 0.63% and 0.83%, respectively, which is mainly due to the evaporation of water molecules adsorbed on the material surface. The weight loss in region II is related to the release of interlayer water molecules . In this region, the weight loss of P2-Na 0.50 MnO 2 is about 0.44%, four times higher than 0.11% of O3-Na 0.85 MnO 2 , which is consistent with XRD results of hydrate phase appearance in aged P2 material.…”

“…Furthermore, the Na x MnO 2 materials, such as Na 0.67 MnO 2 , exhibit a high capacity and better redox reversibility compared with Mn-based LiMO 2 , which meets the need for low cost, environmentally friendly, and high-performance energy storage devices. However, the prospect of large-scale practical application of Mn-based Na x MnO 2 is hindered by their prominent air sensitivity because Mn 3+ is wildly recognized to be hygroscopic and air unstable. − In 1985, Mendiboure et al studied the electrochemical performance of Na x MnO 2 for the first time and only achieved the deintercalation of 0.3 mol Na ions . In 2011, however, Ceder et al found that its reversible specific capacity was as high as 185 mAh/g.…”

A High-Rate, Durable Cathode for Sodium-Ion Batteries: Sb-Doped O3-Type Ni/Mn-Based Layered Oxides

“…Besides, as more Sb is doped into the structure, the two preedge peaks of Mn K-edge spectra (Figure 2b, inset) shift to lower energies, together with a slight drop in intensity, due to a slight reduction of Mn 4+ ions. 57,58 The main Mn K-edge absorption edge exhibits a shift to a lower energy region when Sb is introduced, which implies that the average oxidation number of Mn reduces after Sb doping. The slight chemical shift is presumably owed to the change in the local structure of manganese coordination when some Mn ions are replaced by Sb ions.…”

Layered Oxide Cathode‐Electrolyte Interface towards Na‐Ion Batteries: Advances and Perspectives

“…Kang et al proposed a thermal activation process to decompose inactive Na 2 CO 3 layer on the surface and into electrochemically active Na ions and modulate the off-stoichiometric P2 phase into O3/ P2 composite phase. [145] During cooling process at air atmosphere, chemical extraction of Na + caused by CO 2 in the environment evolves the formation of Na 2 CO 3 on surface and Na + deficient P2 phase in the subsurface. A secondary calcining activation would re-incorporate Na into the bulk forming P2/O3 mixed layered Na x MnO 2 .…”