Moisture exposed layered oxide electrodes as Na-ion battery cathodes

Abstract

Mn-rich layered oxides of P2 Na2/3Mn0.8Fe0.1Ti0.1O2 have been shown to exhibit a remarkably stable electrochemical performance even after exposure to moisture for extended periods of time.

The stability of P2-layered sodium transition metal oxides in ambient atmospheres

“…1a . The commonly reported hydration impurity phase in the Na x TmO 2 is birnessite 27 , 30 , 37 , whose structure is very similar to that of P2-Na 0.67 MnO 2 , except the presence of extra water molecules in the sodium layers and a broader interlayer distance of ~7.1 Å. With further H 2 O insertion, the buserite phase with layer spacing of ~9.1 Å can be identified (Fig.…”

“…The undesired structural transformations in charge/discharge processes, such as P2-O2 in Na 0.67 Ni 0.33 Mn 0.67 O 2 16 – 20 , P2-P2’ in Na 0.67 MnO 2 10 , 21 – 23 and O3-P3 in NaNi 0.5 Mn 0.5 O 2 24 , can be suppressed or delayed by element substitution, thus resulting in improved cycling stability and rate capability. The contact between air-instable Na x TmO 2 and moisture-air usually produces cracks 25 , electrical insulation species 26 , and hydration phases 27 , which result in a shorter lifetime and poorer rate capability of the exposed layered oxides 28 – 30 . Therefore, air-stability is considered an important factor to evaluate a qualified Na x TmO 2 electrode 31 – 33 .…”

“…Unlike degradation, H 2 O insertion is widely encountered in P2-Na x TmO 2 electrodes and can be easily distinguished by X-ray diffraction (XRD), because the intercalation of H 2 O expands the interlayer distance of the Na + layers from ~5 to ~7 Å and even up to ~9 Å 36 – 38 . Kubota et al proposed the Na + /H + exchange mechanisms in O3-type NaMeO 2 oxides 39 and Rojo’s group further identified the presence of H + in the Na + layers using a neutron powder diffraction technique 30 . In addition, in the presence of H 2 O, CO 2 was believed to get inserted into the transition metal layers of P2-Na 2/3 Fe 0.5 Mn 0.5 O 2 28 .…”

A Water Stable, Near‐Zero‐Strain O3‐Layered Titanium‐Based Anode for Long Cycle Sodium‐Ion Battery

“…In situ X‐ray diffraction (XRD) reveals that this material does not experience any phase transition upon charge/discharge and exhibits nearly zero volumetric strain during Na + insertion/de‐insertion, which ensures an exceptionally high cycle stability over 6000 cycles. More importantly, we find this O3‐Na 0.73 Li 0.36 Ti 0.73 O 2 shows superior moisture stability, with no capacity loss in water soaking tests, which is rarely reported for conventional layered transition metal (TM) oxides in SIBs . Our discovery provides new possibilities and understanding in practical SIBs materials development …”

“…According to the different stacking modes of the edge‐shared MO 6 octahedrons ( Figure a), these Na x MO 2 compounds can be categorized into two types: the P2‐type with ABBA stacking, where Na + resides in the interlayer triangular prism (P) position, and the O3‐type with ABCABC stacking, where Na + resides in the octahedral (O) position. However, their sensitivity to moisture greatly hinders large‐scale manufacturing, regardless of their crystal structure as P2 or O3 …”