beyond 4.1 V (Talaie et al.
2015). Despite extensive research, the mechanism of the voltage hysteresis has not been elucidated yet. Redox reaction of oxide ion (Talaie et al. 2015) and P2/OP4-like phase transition accompanied by formation of stacking faults (Mortemard de Boisse et al. 2014) and by migration of transition metals (Talaie et al. 2015) are estimated to cause the voltage hysteresis. Second, Na deficiency is a considerable issue in all P2-type materials showing capacity below 2.5 V and the large reversible capacity is achieved by Na compensation from external Na sources such as Na metal counter electrode in a half-cell and pretreatment with Na metal by ball milling (Zhang et al. 2016), but Na metal should be avoided in the Na-ion battery full cells and synthesis process due to safety issues. Recently, the addition of sacrificial Na sources such as NaN3 and Na2CO3 is proposed to compensate for Na in the full cells without Na metal (Singh et al. 2013; Martinez De Ilarduya et al. 2017; Sathiya et al. 2017). Third, as discussed in the hygroscopic property of O3-type materials above, P2-type materials are also generally hygroscopic except for P2-Na2/3Ni1/3-xCuxMn2/3O2 (0 ≤ x ≤ 1) (Lu and Dahn 2001b; Zheng et al. 2017). Air stable P2-Na2/3[Mn,Fe]O2 is achieved by Cu doping (Li et al. 2015; Talaie et al. 2017). Doping with Cu2+ and alkaline earth metals would be effective to enhance the waterand moisture-resistant properties of P2-type materials. Ternary and quaternary Fe–Mn-based P2-type materials such as P2-Nax[Fe,Mn,Ti]O2 (Han et al. 2016), P2-Nax[Fe,Mn,Co]O2 (Thorne et al. 2014b), P2-Nax[Fe,Mn,Ni]O2 (Talaie et al. 2015; Bai et al. 2016), P2-Nax[Fe,Mn,Co,Ni]O2 (Kaithwas and Kundu 2015; Li et al. 2017) and P2-Nax[Fe,Mn,Cu]O2 (Li et al. 2015; Talaie et al. 2017) have also been extensively studied and Fe in P2-type materials would also assist Na diffusion at the end of charge due to the flexibility of Jahn–Teller active Fe4+ (Liu et al. 2015) as observed in O3-type materials (Li et al. 2016). The metal doping in P2-Nax[Mn,Fe]O2 suppresses the voltage hysteresis, but the Na compensation and raising working voltage are challenging.
1.4.4. P2-Na2/3[Ni,Mn,M]O2
Similar to P2-Nax[Mn, Fe]O2, a solid solution of P2-Na2/3[Ni,Mn]O2 was also studied as a precursor to synthesize a positive electrode material in Li cells by means of Li+/Na+ exchange by Dahn’s group from the late 1990s to the early 2000s (Paulsen and Dahn 2000; Paulsen et al. 2000). Interestingly, the Na battery performance was examined with Na cells and phase transitions were investigated using in situ XRD in 2001 (Lu and Dahn 2001a). P2-Na2/3Ni1/3Mn2/3O2 delivers a reversible capacity of ca. 150 mAh g−1 with stepwise voltage profiles in the range of 2.0–4.5 V, as shown in Figure 1.14. The average working voltage is one of the highest (ca. 3.6 V) among O3- and P2-type materials. However, the reversible capacity rapidly deteriorates during cycles (Lu and Dahn 2001a; Lee et al. 2013). The capacity decay mechanism is explained by phase transition accompanied by significant shrinkage of interslab distance by Na extraction (Yoshida et al. 2014). P2-Na2/3Ni1/3Mn2/3O2 transforms into O2-type Ni1/3Mn2/3O2 by extraction of almost all Na on charge through the two-phase reaction above 4.1 V (Lu and Dahn 2001a) and the P2–O2 phase transition is accompanied by huge volume shrinkage of 23% mainly attributed to shrinkage of the interslab distance (Lee et al. 2013; Yoshida et al. 2014). The volume change significantly influences battery performances such as capacity retention and rate performance in Li and Na batteries and is recently reported to be suppressed by partial metal doping in the Ni1/3Mn2/3O2 slab. For example, our group synthesized P2-Na2/3[Ni1/3Mn1/2Ti1/6]O2 and the Ti-substituted phase delivers a reversible capacity of 127 mAh g−1 with smooth voltage curves as shown in Figure 1.14 (Yoshida et al. 2014). The volume change is successfully suppressed into 12–13% by Ti substitution. Our group systematically compared influence of inert-metal substitution in P2-Na2/3[Ni1/3Mn2/3]O2 and revealed that Al3+ or Ti4+ substitution successfully enhanced the cycling stability and rate performances (Kubota et al. 2018b). Multiple substitution by inert metals such as Cu2+-Ti4+ coupling (Mu et al. 2019) is expected to further improve the electrochemical performances as found for O3-Na[Ni1/2Mn1/2]O2.
Figure 1.14. Comparison of galvanostatic charge/discharge curves of layered P2 type binary and ternary 3d transition metal oxides (left). Morphology of particles for each sample is also compared (right)
1.5. Summary and prospects
Partial substitution of various metals for 3d transition metals in sodium layered transition metal oxides is efficient to modulate their electrochemical properties. An unique variety of sodium layered oxides is attractive unlike lithium layered transition metal oxides. Actually, selection of transition metals and dopant elements controls the crystal structure, electronic/ionic conductivity, moist air–resistant property, phase transitions during Na (de)intercalation and surface reactivity with the electrolyte.
This chapter’s role of mainly reviewing the 3d transition metal elements will help to further enhance electrode performance of layered sodium transition metal oxides as positive electrode materials for Na-ion batteries. From previous studies on Li-ion, Na-ion and K-ion batteries, not only the study on positive electrode materials but also comprehensive studies with negative electrode materials, current collectors, binders, surface coating and concentration gradient of active materials, and electrolyte salts and solvents, etc., are also required to enhance overall performances of Na-ion batteries. Even if a huge demand of lithium resources is actualized for the Li-ion batteries in transportation applications and there is a risk of undersupply of lithium resources, which are distributed unevenly in the Earth’s crust, further developments to enhance not only energy density but also cycle life and safety are, of course, required for the stationary use of Na-ion batteries. Furthermore, unique and featured advantages over Li-ion have been found and will be further desired for practical performance of future Na-ion batteries.
The advantages of lower Lewis acidity, weaker Coulombic interaction and smaller crystal ion size for Na+ than Li+ ions are the key to maximizing the characteristics of Na-ion batteries, whereas the hygroscopic property of sodium transition metal oxides is similar to that of LiNiO2-based materials. As development of high capacity LiNiO2-based materials is desired in next-generation Li-ion batteries, new findings and chemistry to control the hygroscopic issue are still being developed (Bianchini et al. 2019).
Similar progress is also sometimes expected in layered sodium transition metal oxides such as the hygroscopic property and coverage of alkali metal carbonate on the oxides disturbing phase transitions, possibly, at least until the 2040s. Therefore, interactive research and developments between Li-ion and Na-ion (possibly K-ion) batteries are believed to lead to a positive effect on the fundamental understanding and realization of sustainable energy technology.
1.6. Acknowledgments
This study is in part supported by Japan Science and Technology Agency (JST) through Adaptable and Seamless Technology Transfer Program (A-STEP) and by MEXT program “Elements Strategy Initiative to Form Core Research Center” (No. JPMXP0112101003).
