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MnO2 is shown for the first time to be electrochemically active as a conversion anode for Na-ion batteries (NIBs). Space-confined ultrafine (UF)-MnO2, with an average crystal size of 4 nm, synthesized using a porous silicon dioxide templated hydrothermal process exhibits a high reversible sodiation capacity of 567 mA h g(-1), in contrast to the negligible activity shown by the aggregates of larger (14 nm) MnO2 nanocrystallites. The remarkably enhanced sodiation activity of the UF-MnO2 is attributable to its greatly reduced crystal size, which facilitates diffusion of Na ions, along with high surface energy arising from extensive heterogeneous interfacial bonding with the SiO2 surrounding. The UF-MnO2 anode exhibits an exceptional rate and cycle performance, exhibiting >70% capacity retention after 500 cycles. In operando synchrotron X-ray absorption near-edge structural analysis reveals combined charge-storage mechanisms involving conversion reaction between Mn(III) and Mn(II) oxides, Mn(III)-O1.5 + Na(+) + e(-)- ↔ 1/2Na2O + Mn(II)-O, and non-Mn-centered redox reactions. The finding suggests a new strategy for "activating" the potential electrochemical electrode materials that appear inactive in the bulk form.

A series of nanocrystalline titanium (Ti) sub-oxides, including TiO, Ti2O3, Ti3O5, and Ti4O7, with high surface area and activity are successively synthesized using a facile synthesis method that combines the sol–gel and the energy-efficient vacuum-carbothermic (SG-VC) processes. The combination results in synergy in producing nanomaterials with high surface area (>100 m2 g–1), good conductivity, and rich intra-grain defect features, giving the oxides unique surface activities suitable for particular electrochemical applications. The phase compositions of the resulting powders are primarily determined by two process parameters, including the carbothermic carbon (C) content, expressed as the C-to-Ti molar ratio of the reactant powder, and the cooling protocol. Carbothermic C contents exceeding a threshold of C/Ti ∼ 3.7 exclusively produced non-Magnéli phase (MP) oxides including TiO and Ti2O3, while the MP oxides, Ti3O5 and Ti4O7, can be formed only with lower C contents combined with selected quenching protocols that kinetically limit oxygen replenishment during cooling. Examples of the resulting MP Ti4O7 powder exhibiting outstanding pseudocapacitive and oxygen evolution reaction catalytic behaviors are demonstrated.

Layered Li ternary transition metal oxides, LiNixCoyMn1-x-yO2 (denoted as NCM) is an important cathode material for lithium ion batteries because of its high capacity, good thermal stability, low cost, and low toxicity. However, its cycle life deteriorates rapidly with increasing operating temperature, because of strong interaction at the solid-electrolyte interface (SEI) causing rapid SEI formation and dissolution of the transitional metal ions. To solve these problems, some of researchers proposed to partially substituting the transitional metals of NCM with different metals, such as Mg2+, Al3+, etc., to stabilize a cation disordered structure after the transitional metals dissolve from the material. However, during long-term cycling, particularly at elevated temperatures, the LiPF6 based electrolyte can readily decompose and reacts with residual water in the electrolyte to generate hydrofluoric acid (HF). The acidic attacking from HF and other side reactions from electrolyte would still deteriorate the active material. On this account, surface coating with metal oxides such as MgO, TiO2, ZrO2, ZnO, Al2O3, etc., had been employed to prevent direct contact with the electrolyte solution. Unfortunately, these approaches also lead to additional drawbacks, such as cracking of the oxide layers, decrease of ion-conductivity and increase of production costs from additional complex steps. In this research, a simple approach using polymeric artificial SEIs to modify the surface of cathode materials to enhance the cyclic and rate performance of the Li-ion batteries at elevated temperatures is proposed. Various polymeric blends were employed for modification of the cathode surface. The results show that the polymeric SEIs can substantially suppress the deteriorating reactions between the electrolyte and electrode, improve the structural stability and decrease the polarization of the material during the electrochemical operations. Comparing to the pristine cell, the modified cells show better performances in both cycle life and rate capability, maintaining lower cell impedance due to the reduced electrolyte decomposition and enhanced structure stability. The underlying mechanism is discussed. Figure 1

Our investigation on manganese dioxide (MnO2) as an anode material for sodium (Na)-ion batteries (NIBs) reveals a remarkable size effect within the nanometer range on the electrochemical sodiation activity of the oxide. Space-confined ultrafine (UF)-MnO2, with an average crystal size of 4 nm, synthesized using a porous silicon dioxide templated hydrothermal process exhibits a high reversible sodiation capacity of 392 mAh g−1 compared with the negligible activity shown by the aggregates of larger (14 nm) MnO2 nanocrystallites. To our knowledge, the present study is the first to show that MnO2 is electrochemically active as an NIB anode. The UF-MnO2 anode exhibits exceptional rate and cycle performance, including a reversible sodiation capacity of >100 mAh g−1 at a current density of 7500 mA g−1 and >70% capacity retention after 500 cycles at 150 mA g−1. The enhanced cycle stability may be attributable to the solid-embedding architecture that enables the reduction of the dimensional variations in the active material. In operando synchrotron X-ray absorption near-edge structure analysis reveals combined charge–storage mechanisms involving a unique two-oxide conversion reaction between Mn(III) and Mn(II) oxides, Mn(III)–O1.5 + Na+ + e - « 1/2 Na2O + Mn(II)–O, and non-Mn-centered redox reactions.  Figure 1