Understanding the Multiple Effects of TiO₂ Coating on NaMn_0.33Fe_0.33Ni_0.33O₂ Cathode Material for Na-Ion Batteries

Abstract

O3-type NaTMO2 (TM = transition metal) as cathodes for Na-ion batteries have aroused much interest. But the structural instability during charge–discharge cycles and the inferior rate capability restricts their application. In this study, we report a synergetic modification method to simultaneously increase the rate capacity and cycling stability of O3-type NaMn0.33Fe0.33Ni0.33O2(MFN) cathode material by integrating TiO2 coating and Ti4+ doping. Moreover, the synergetic mechanism has been put forward. First, the TiO2-coating layer prevents the side reactions on the surface, which can retain the structural integrity and stability. Second, TiO2-coating induces Ti4+ doping which enlarges Na–O and increases the interslab spacing d. This raises the Na+ diffusion coefficient and improves the rate performance. Third, because of the large Ti–O bond energy, the TM–O bond shrinks when Ti4+ is doped into the transition metal site. O–O is also shortened due to the Ti doping. TMO2 slabs are compressed, which benefits the structural stability and the cyclic property. Fourth, Ti doping accompanying TiO2 coating decreases Mn3+/Mn4+ and mitigates the Jahn–Teller effect. This increases the stability of the layered structure. The understanding of the multiroles of TiO2 coating is equally instructive for the exploration of other cathode materials for Na-ion or Li-ion batteries.

Probing the charged state of layered positive electrodes in sodium-ion batteries: reaction pathways, stability and opportunities

“…189 The synergy of Ti 4+ doping and TiO 2 coating has also been reported for the O3-NaMn 0.33 Fe 0.33 Ni 0.33 O 2 composition. 213 The TiO 2 layer mitigates side reactions at the surface, while Ti 4+ doping enlarges the interslab spacing, facilitating sodium ion diffusion.…”

Research progress in O3-type phase Fe/Mn/Cu-based layered cathode materials for sodium ion batteries

“…The Al 2 O 3 coated Na[Ni 0.6 Co 0.2 Mn 0.2 ]O 2 cathode exhibited a high specific capacity of 151 mA h g −1 at 15 mA g −1 within the voltage range of 1.5–4.1 V, and delivered a high discharge capacity of approximately 115 mA h g −1 at a 1500 mA g −1 . The pouch-type full cell assembled from Al 2 O 3 coated Na[Ni 0.6 Co 0.2 Mn 0.2 ]O 2 /hard carbon exhibited a high energy density of 130 W h kg −1 and capacity retention of 75% after 300 cycles in the voltage range of 1.0–4.1 V. Yu et al 44 proposed that a synergetic modification method of TiO 2 coating and Ti 4+ doping not only suppressed the side reaction on the cathode surface, but also improved the structure stability and cycling performance of the layered structure. In addition, other research studies about AlF, 46 and AlPO 4, 147 for the surface coating on the layer oxide are reported; more effective coating strategies need to be explored in the future.…”

“…In addition, the surface of the cathode material is modified to inhibit side reactions with the electrolyte and effectively delay the dissolution of the active material, which effectively improves the cycle stability, rate performance, and thermal stability of the material. 44–47…”

Understanding the (De)Sodiation Mechanisms in Na‐Based Batteries through Operando X‐Ray Methods

“…There are several good examples where the results from operando XRD has been used to improve the cyclability of a material by implementing small changes. For example by controlling the potential window or coating the materials [70,74,90] …”