Understanding the Capacity Fading Mechanisms of O3‐Type Na[Ni_0.5Mn_0.5]O₂ Cathode for Sodium‐Ion Batteries

Abstract

numerous types of compounds have been suggested as cathode materials for SIBs, including polyanion structures, organic compounds, P2-type, and O3-type layered oxides. [10-12] Among them, the O3-type sodium layered cathodes, Na x MeO 2 (Me = transition metal, 0.7 < x ≤ 1) which is isostructural with LiCoO 2 , have attracted great interest as one of the most suitable candidates owing to their higher theoretical capacity and synthesis processes similar to commercial Li[Ni x Co y (Mn or Al) 1-x-y ]O 2 cathodes for LIBs. [13-15] Moreover, sufficient Na + ion content in the host structure allows the fabrication of practical full-cells using a hard carbon anode with high Coulombic efficiency. [16] In addition, spherical O3-type cathode particles with hierarchical structure can be synthesized by the coprecipitation method so that the high tap density of the cathode ensures increased volumetric energy density for energy storage applications. [17,18] However, unlike the analogous compounds in LIBs, the O3-type sodium layered cathodes deliver relatively low reversible capacity with unstable cycling. The poor electrochemical performances are typically attributed to the parasitic surface reactions arising from oxidative electrolyte decomposition and subsequent HF attacks. [19] The comparatively large size of the Na + ion (Na, 1.02 Å vs Li, 0.76 Å) results in the cathodes undergoing severe phase transitions during the insertion/extraction of Na + ions, leading to poor cycling stability and low energy efficiency. [20] In addition, drastic volume changes in the deeply charged Na x MeO 2 can also contribute to structural degradation, possibly by inducing mechanical stress and the eventual disintegration of the cathode particles by the formation of microcracks. [21,22] Although internal microcracking is considered to be primarily responsible for the rapid capacity fading in LIBs, [23-25] an understanding of the relationship between capacity fading and the formation of microcracks is still unclear in O3-type cathodes for SIBs. In this study, we investigated the capacity fading mechanisms related to microcracks for the O3-type Na[Ni 0.5 Mn 0.5 ]O 2 cathode which is one of the most widely studied materials for SIBs. Microsized cathode particles with high sphericity were synthesized using the coprecipitation method to explore microcracks on the particles. In the regard, the electrochemical performances of the O3-type Na[Ni 0.5 Mn 0.5 ]O 2 cathode are correlated to the microstructural changes observed by crosssectional scanning electron microscopy (SEM) and in situ X-ray A spherical O3-type Na[Ni 0.5 Mn 0.5 ]O 2 cathode, composed of compactlypacked nanosized primary particles, is synthesized by the coprecipitation method to examine its capacity fading mechanism. The electrochemical performance cycled at different upper cutoff voltages demonstrate that the P3′ to O3′ phase transition above 3.6 V is primarily responsible for the loss of the structural stability of the O3-type Na[Ni 0.5 Mn 0.5 ]O 2 cathode. The capacity retention is great...

Impact of Mg and Ti doping in O3 type NaNi1/2Mn1/2O2 on reversibility and phase transition during electrochemical Na intercalation

“…The rened lattice parameters a ¼ 2.96277(4)Å and c ¼ 15.9132(2)Å for Non-sub are identical with those in previous reports. 7,26,27 In our Non-sub sample, the concerned mass fraction of the NiO phase is quite small (0. 9,28 but the amounts in our Mg-substituted samples are negligibly small compared to the reported values.…”

“…Such surface deterioration is similar to that reported by Yu et al and probably attributed to a side reaction with the electrolyte and dissolution of transition metals into the electrolyte. 27 In contrast, the TEM images of the cycled Mg-Tisub (Fig. 8i and j) display no signicant change in the outermost surface (Fig.…”

“…These values are consistent with those reported for Na x [Ni 1/2 Mn 1/2 ]O 2 . 27 The shrinkage of the interslab distance is known to generate microcracks between primary submicron particles, leading to electrical isolation of the primary particles. 27,52 In fact, as shown in Fig.…”

“…27 The shrinkage of the interslab distance is known to generate microcracks between primary submicron particles, leading to electrical isolation of the primary particles. 27,52 In fact, as shown in Fig. 8a, the ex situ XRD of the Nonsub electrode aer 10 cycles proves a residual P3 (or P 0 3) type phase, although the reversibly reverted O3 phase is the main phase.…”

“…Even without the oating process, surface deterioration and formation of the disordered rock-salt type cubic phase were proved by TEM images for the cycled Na[Ni 1/2 Mn 1/2 ]O 2 . 27 Herein, we conducted ex situ TEM observation for cycled Non-sub and Mg-Ti-sub electrodes.…”