Na-ion Batteries. Laure Monconduit

suppresses the voltage hysteresis, but the Na compensation and raising working voltage are challenging.

      1.4.4. P2-Na_2/3[Ni,Mn,M]O₂

Similar to P2-Na_x[Mn, Fe]O₂, a solid solution of P2-Na_2/3[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-Na_2/3Ni_1/3Mn_2/3O₂ 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-Na_2/3Ni_1/3Mn_2/3O₂ transforms into O2-type Ni_1/3Mn_2/3O₂ 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 Ni_1/3Mn_2/3O₂ slab. For example, our group synthesized P2-Na_2/3[Ni_1/3Mn_1/2Ti_1/6]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-Na_2/3[Ni_1/3Mn_2/3]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[Ni_1/2Mn_1/2]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).

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