Li-ion batteries (LIBs), commercialized in 1991, have the highest energy density among practical secondary batteries and are widely utilized in electronics, electric vehicles, and even stationary energy storage systems. Along with the expansion of their demand and application, concern about the resources of Li and Co is growing. Therefore, secondary batteries composed of earth-abundant elements are desired to complement LIBs. In recent years, K-ion batteries (KIBs) have attracted significant attention as potential alternatives to LIBs. Previous studies have developed positive and negative electrode materials for KIBs and demonstrated several unique advantages of KIBs over LIBs and Na-ion batteries (NIBs). Thus, besides being free from any scarce/toxic elements, the low standard electrode potentials of K/K+ electrodes lead to high operation voltages competitive to those observed in LIBs. Moreover, K+ ions exhibit faster ionic diffusion in electrolytes due to weaker interaction with solvents and anions than that of Li+ ions; this is essential to realize high-power KIBs. This review comprehensively covers the studies on electrochemical materials for KIBs, including electrode and electrolyte materials and a discussion on recent achievements and remaining/emerging issues. The review also includes insights into electrode reactions and solid-state ionics and nonaqueous solution chemistry as well as perspectives on the research-based development of KIBs compared to those of LIBs and NIBs.

Research Development on K-Ion Batteries.

As one of the most fascinating cathode candidates for Na-ion batteries (NIBs), P2-type Na layered oxides usually exhibit various single-phase domains accompanied by different Na+/vacancy-ordered superstructures, depending on the Na concentration when explored in a limited electrochemical window. Therefore, their Na+ kinetics and cycling stability at high rates are subjected to these superstructures, incurring obvious voltage plateaus in the electrochemical profiles and insufficient battery performance as cathode materials for NIBs. We show that this problem can be effectively diminished by reasonable structure modulation to construct a completely disordered arrangement of Na-vacancy within Na layers. The combined analysis of scanning transmission electron microscopy, ex situ x-ray absorption spectroscopy, and operando x-ray diffraction experiments, coupled with density functional theory calculations, reveals that Na+/vacancy disordering between the transition metal oxide slabs ensures both fast Na mobility (10-10 to 10-9 cm2 s-1) and a low Na diffusion barrier (170 meV) in P2-type compounds. As a consequence, the designed P2-Na2/3Ni1/3Mn1/3Ti1/3O2 displays extra-long cycle life (83.9% capacity retention after 500 cycles at 1 C) and unprecedented rate capability (77.5% of the initial capacity at a high rate of 20 C). These findings open up a new route to precisely design high-rate cathode materials for rechargeable NIBs.

Na+/vacancy disordering promises high-rate Na-ion batteries

Sodium-ion batteries (SIBs) are attracting increasing attention and considered to be a low-cost complement or an alternative to lithium-ion batteries (LIBs), especially for large-scale energy storage. Their application, however, is limited because of the lack of suitable host materials to reversibly intercalate Na+ ions. Layered transition metal oxides (NaxMO2, M = Fe, Mn, Ni, Co, Cr, Ti, V, and their combinations) appear to be promising cathode candidates for SIBs due to their simple structure, ease of synthesis, high operating potential, and feasibility for commercial production. In the present work, the structural evolution, electrochemical performance, and recent progress of NaxMO2 as cathode materials for SIBs are reviewed and summarized. Moreover, the existing drawbacks are discussed and several strategies are proposed to help alleviate these issues. In addition, the exploration of full cells based on NaxMO2 cathodes and future perspectives are discussed to provide guidance for the future commercialization of such systems.

Recent Progress of Layered Transition Metal Oxide Cathodes for Sodium-Ion Batteries

Material innovation on high-performance Na-ion cathodes and the corresponding understanding of structural chemistry still remain a challenge. Herein, we report a new concept of high-entropy strategy to design layered oxide cathodes for Na-ion batteries. An example of layered O3-type NaNi0.12 Cu0.12 Mg0.12 Fe0.15 Co0.15 Mn0.1 Ti0.1 Sn0.1 Sb0.04 O2 has been demonstrated, which exhibits the longer cycling stability (ca. 83 % of capacity retention after 500 cycles) and the outstanding rate capability (ca. 80 % of capacity retention at the rate of 5.0 C). A highly reversible phase-transition behavior between O3 and P3 structures occurs during the charge-discharge process, and importantly, this behavior is delayed with more than 60 % of the total capacity being stored in O3-type region. Possible mechanism can be attributed to the multiple transition-metal components in this high-entropy material which can accommodate the changes of local interactions during Na+ (de)intercalation. This strategy opens new insights into the development of advanced cathode materials.

High-Entropy Layered Oxide Cathodes for Sodium-Ion Batteries

Electrochemistry and Solid-State Chemistry of NaMeO2 (Me = 3d Transition Metals)

Origin of Enhanced Capacity Retention of P2-Type Na2/3Ni1/3-xMn2/3CuxO2 for Na-Ion Batteries

P2-Na2/3Ni1/3Mn2/3O2 (P2-NiMn) is one of the promising positive electrode materials for high-energy Na-ion batteries because of large reversible capacity and high working voltage by charging up to 4.5 V versus Na+/Na. However, the capacity rapidly decays during charge/discharge cycles, which is caused by the large volume shrinkage of ca. 23% by sodium deintercalation and following electric isolation of P2-NiMn particles in the composite electrode. Serious electrolyte decomposition at the higher voltage region than 4.1 V also brings deterioration of the particle surface and capacity decay during cycles. To solve these drawbacks, we apply water-soluble sodium poly-γ-glutamate (PGluNa) as an efficient binder to P2-NiMn instead of conventional poly(vinylidene difluoride) (PVdF) and examined the electrode performances of P2-NiMn composite electrode with PGluNa binder for the first time. The PGluNa electrode shows Coulombic efficiency of 95% at the first cycle and capacity retention of 89% after 50 cycles, wher...

Poly-γ-glutamate Binder To Enhance Electrode Performances of P2-Na2/3Ni1/3Mn2/3O2 for Na-Ion Batteries

O3 type layered sodium nickel manganese oxide, NaNi1/2Mn1/2O2, which is isostructural with α-NaFeO2, has attracted attention as a promising positive electrode material for sodium-ion batteries owing to its large reversible capacity of ca. 200 mA h g−1. To improve the cycle stability for practical use, O3 type NaNi1/2Mn1/2O2 materials with Mg or/and Ti substitution are synthesized. The materials with Mg or Ti substitution exhibit better capacity capability, and Mg and Ti co-substituted material demonstrates even better capacity capability, with an initial discharge capacity of 200 mA h g−1 without any capacity loss due to substitution. Substitution of Mg2+ and Ti4+, which are larger ions than Ni2+ or Mn4+, results in a larger in-plane lattice of the O3 type structure, in contrast to the shrinkage during charging, and this has the potential to delay the phase transition during charging. In contrast to the non-substituted NaNi1/2Mn1/2O2, the Mg and Ti co-substituted material demonstrates more continuous phase transitions and lattice parameter changes, and no significant shrinkage of the interslab spacing in the layered structure, as evidenced by ex situ and operando X-ray diffraction. The coexistence of Mg and Ti enhances not only the reversibility of the structural change but also the structural stability at the surface, resulting in the excellent sodium battery performance.

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

Although O3-NaFe1/2 Mn1/2 O2 delivers a large capacity of over 150 mAh g-1 in an aprotic Na cell, its moist-air stability and cycle stability are unsatisfactory for practical use. Slightly Na-deficient O3-Na5/6 Fe1/2 Mn1/2 O2 (O3-Na5/6 FeMn) and O3-Na5/6 Fe1/3 Mn1/2 Me1/6 O2 (Me = Mg or Cu, O3-FeMnMe) are newly synthesized. The Cu and Mg doping provides higher moist-air stability. O3-Na5/6 FeMn, O3-FeMnCu, and O3-FeMnMg deliver first discharge capacities of 193, 176, and 196 mAh g-1 , respectively. Despite partial replacement of Fe with redox inactive Mg, oxide ions in O3-FeMnMg participate in the redox reaction more apparently than O3-Na5/6 FeMn. X-ray diffraction studies unveil the formation of a P-O intergrowth phase during charging up to >4.0 V.

Yusuke Yoda

Papers

Electrochemistry and Solid-State Chemistry of NaMeO2 (Me = 3d Transition Metals)

Origin of Enhanced Capacity Retention of P2-Type Na2/3Ni1/3-xMn2/3CuxO2 for Na-Ion Batteries

Poly-γ-glutamate Binder To Enhance Electrode Performances of P2-Na2/3Ni1/3Mn2/3O2 for Na-Ion Batteries

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

Elucidating Influence of Mg- and Cu-Doping on Electrochemical Properties of O3-Na x [Fe,Mn]O 2 for Na-Ion Batteries.