Hongahally Basappa Rajendra

The development of inherently safe energy devices is a key challenge, and aqueous Li-ion batteries draw large attention for this purpose. Due to the narrow electrochemical stable potential window of aqueous electrolytes, the energy density and the selection of negative electrode materials are significantly limited. For achieving durable and high-energy aqueous Li-ion batteries, the development of negative electrode materials exhibiting a large capacity and low potential without triggering decomposition of water is crucial. Herein, a type of a negative electrode material (i.e., Li_xNb_2/7Mo_3/7O₂) is proposed for high-energy aqueous Li-ion batteries. Li_xNb_2/7Mo_3/7O₂ delivers a large capacity of ∼170 mA ⋅ h ⋅ g⁻¹ with a low operating potential range of 1.9 to 2.8 versus Li/Li⁺ in 21 m lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) aqueous electrolyte. A full cell consisting of Li_1.05Mn_1.95O₄/Li_9/7Nb_2/7Mo_3/7O₂ presents high energy density of 107 W ⋅ h ⋅ kg⁻¹ as the maximum value in 21 m LiTFSA aqueous electrolyte, and 73% in capacity retention is achieved after 2,000 cycles. Furthermore, hard X-ray photoelectron spectroscopy study reveals that a protective surface layer is formed at the surface of the negative electrode, by which the high-energy and durable aqueous batteries are realized with Li_xNb_2/7Mo_3/7O₂. This work combines a high capacity with a safe negative electrode material through delivering the Mo-based oxide with unique nanosized and metastable characters. Significance This study describes a high-energy and durable aqueous battery system with metastable and nanosized Mo-based oxides used as high-capacity negative electrodes. A wider electrochemical window is achieved with concentrated aqueous electrolytes through which highly reversible Li storage without the decomposition of water molecules is achieved for the Mo-based oxides. A full cell with an Mn-based oxide shows good capacity retention over 2,000 cycles. X-ray absorption spectroscopy reveals that the solid-state redox reaction of Mo ions reversibly proceeds in aqueous electrolytes for the metastable Mo oxide. This study opens a way to develop high-energy, durable, and safe batteries on the basis of metastable and nanosized oxides with aqueous electrolyte solutions.

The development of an energy storage system with abundant elements is a key challenge for a sustainable society, and the interest of Na intercalation chemistry is extending throughout the research community. Herein, the impact of Ti integration into NaMnO ₂ in a binary system of x NaMnO ₂ –( 1 – x ) TiO ₂ ( 0.5 ≤ x ≤ 1 ) is systematically examined for rechargeable Na battery applications. Stoichiometric NaMnO ₂ , which is classified as an in-plane distorted O ′ 3-type layered structure, delivers a large initial discharge capacity of approximately 200 mAh g ^-1 , but insufficient capacity retention is observed, most probably associated with dissolution of Mn ions on electrochemical cycles. Ti-substituted samples show highly improved electrode performance as electrode materials. However, the appearance of a sodium-deficient phase, Na ₄ Mn ₄ Ti ₅ O ₁₈ with a tunnel-type structure, is observed for Ti-rich phases. Among the samples in this binary system, Na _0.8 Mn _0.8 Ti _0.2 O ₂ ( x = 0.8 ), which is a mixture of a partially Ti-substituted O ′ 3-type layered oxide (Na _0.88 Mn _0.88 Ti _0.12 O ₂ ) and tunnel-type Na ₄ Mn ₄ Ti ₅ O ₁₈ as a minor phase elucidated by Rietveld analysis on both neutron and X-ray diffraction patterns, shows good electrode performance on the basis of energy density and cyclability. Both phases are electrochemically active as evidenced by in situ X-ray diffraction study, and the improvement of reversibility originates from the suppression of Mn dissolution on electrochemical cycles. From these results, the feasibility of Mn-based electrode materials for high-energy rechargeable Na batteries made from only abundant elements is discussed in detail.