Effect of Particle Size and Polytypes on Redox Reversibility of Layered Na0.76Ni0.38Mn0.62O2 Electrode
Authors: Kim Eun Jeong , Tatara Ryoichi , Hosaka Tomooki , Kubota Kei , Kumakura Shinichi , Komaba Shinichi 
Published: February 2024

Interest in sodium ion batteries (SIBs) has considerably increased in recent years for low-cost and sustainable energy storage systems. Among positive electrode materials in SIBs, layered sodium nickel manganese oxides such as O3-type NaNi0.5Mn0.5O2 and P2-type Na0.67Ni0.33Mn0.67O2 and their derivatives have been extensively investigated. Despite their high capacity delivered by Ni2+/4+ redox processes, significant surface-electrolyte reactivity with Ni and inferior structural integrity due to phase transformations eventually lead to capacity fade over cycling. To suppress the problems, substitution for Ni and/or Mn, surface coating, and morphology modification have been applied. In lithium ion batteries, tunning morphology without altering chemical compositions has shown improved electrochemical properties for layered lithium cathode materials, LiNixMnyCoyO2 (NMC, x + y + z = 1). For example, single-crystal NMC of micron-scale particles limit the surface reactivity and particle cracking, which is beneficial for cycle life. Another way is spherical secondary particles in micron-size where primary nanoparticles are composed of. Due to the hierarchical morphology, detrimental surface reactions are suppressed while ionic and electronic conductivity are preserved. Those strategies are expected to be advantageous for the counterparts of SIBs, however less studies have been carried out.
In this study, we focus on the strategy of secondary spherical particles for layered sodium nickel manganese oxides. To understand the effect of secondary particle size, two different diameters of 4 µm and 10 µm were synthetized using (Ni0.38Mn0.62)OH2 and Na2CO3. In addition, different heating temperatures were used as polytypes are an interesting character for layered sodium oxides. Heating at 700 °C and 900 °C produces different polytypes of P3- and P2-type Na0.76Ni0.38Mn0.62O2, respectively. Increased temperature tends to enlarge primary particle size and yields NiO impurity, while no influence on the spherical morphology is found.
To examine the redox stability depending on voltage ranges as well as polytypes, 10 µm of P3- and P2-type Na0.76Ni0.38Mn0.62O2 were galvanostatically cycled at four diverse voltage ranges (1.6-4.4 V, 1.6-3.8 V, 2.5-4.4 V, and 2.5-3.8 V) in non-aqueous sodium cells. When the lower cut-off voltage is 2.5 V, charge/discharge profiles on the first cycle are almost identical regardless of polytypes. The difference occurs below 2.0 V where a sloping plateau is observed in P2-type Na0.76Ni0.38Mn0.62O2 while a flat plateau is found in P3-type Na0.76Ni0.38Mn0.62O2. Those dissimilarity originates from the phase transition of P2-P’2 and P3-O3 as confirmed by ex-situ XRD. The widest voltage range of 1.6-4.4 V results in capacity fade, which is more significant for P2-type Na0.76Ni0.38Mn0.62O2. The narrowest voltage range of 2.5-3.8 V permits the best cyclability where no apparent phase changes occur. Decreased lower cut-off voltage to 1.6 V, keeping the upper cut-off voltage at 3.8 V, deteriorates slightly the cycle life, which is again more pronounced for P2-type Na0.76Ni0.38Mn0.62O2. In the voltage range of 2.5-4.4 V, however, capacity notably decays, retaining only 57% and 51% of their initial capacity after 60 cycles for P3- and P2-type Na0.76Ni0.38Mn0.62O2, respectively. These results indicate that reactions beyond 3.8 V are detrimental to capacity retention for both polytypes, and P3-type Na0.76Ni0.38Mn0.62O2 shows enhanced cyclability.
4 µm of P3- and P2-type Na0.76Ni0.38Mn0.62O2, galvanostatically cycled at two voltage ranges (1.6-4.4 V and 1.6-3.8 V), show same trends as those of 10 µm: significant capacity fade in the voltage range of 1.6-4.4 V, and worse capacity retention for P2-type Na0.76Ni0.38Mn0.62O2. It is worth noting the capacity fade is more pronounced than that for 10 µm.
Given that no influence of secondary particle size on the phase changes, confirmed by ex-situ XRD, degradation of Na0.76Ni0.38Mn0.62O2 driven by surface reactions is suspected to play an important role for the capacity fade. To understand further, cross-sectional morphology for 4 µm and 10 µm of P3-type Na0.76Ni0.38Mn0.62O2 was examined. Pristine electrode of 4 µm sample exhibits some pores within the secondary particles, which is barely modified during the first cycle between 1.6 and 4.4 V. After 100 cycles, pulverized secondary particles are observed, accelerating electrolyte penetration into the particle interior. In the case of 10 µm, tightly packed primary particles are shown at pristine electrode and no significant microcracks appear on the secondary particles after charge to 3.8 V. Further charge to 4.4 V develops the microcracks and the prevalent microcracks are also observed after discharge to 1.6 V. After 100 cycles, the microcracks are more developed, however exposed internal surface is lower compared to that of 4 µm. This reduces the exposure of internal surface to the infiltrated electrolyte, retarding the degradation of active materials.

Figure 1