An object of the present invention or a problem to be solved by the present invention is to provide, as a material for use as a positive electrode of a sodium secondary battery, a novel material that allows the resulting battery to have capacity characteristics superior to those of conventional batteries. The composite metal oxide of the present invention has a composition represented by the general formula Na.sub.xMe.sub.yO.sub.2, where Me is at least one selected from the group consisting of Fe, Mn, and Ni, x satisfies 0.8<x≦1.0, and y satisfies 0.95≦y<1.05, and consists of a P2 structure. The sodium secondary battery of the present invention includes: a positive electrode (13) containing the composite metal oxide of the present invention; a negative electrode (16) containing a material capable of absorbing and desorbing Na ions; and an electrolyte containing Na ions and anions.

A battery cell having an anode or cathode comprising an acidified metal oxide (“AMO”) material, preferably in monodisperse nanoparticulate form 20 nm or less in size, having a pH<7 when suspended in a 5 wt % aqueous solution and a Hammett function H.sub.0>−12, at least on its surface.

The present application discloses a positive electrode active material satisfying the chemical formula L.sub.xNa.sub.yM.sub.zCu.sub.αFe.sub.βMn.sub.γO.sub.2+δ−0.5ηX.sub.η and a preparation method therefor, a sodium ion battery and an apparatus including such battery, wherein L is a doping element at alkali metal site, M is a doping element at transition metal site, and X is a doping element at oxygen site, 0≤x<0.35, 0.65≤y≤1, 0<α≤0.3, 0<β≤0.5, 0<γ≤0.5, −0.03≤δ≤0.03, 0≤η≤0.1, z+α+β+γ=1, mx+y+nz+2α+3β+4γ=2(2+δ), m is the valence state of L, and n is the valence state of M; and the pH of the positive electrode active material is 10.5-13, wherein L is a doping element at alkali metal site, M is a doping element at transition metal site, and X is a doping element at oxygen site.

A mixed conductor represented by Formula 1:
A.sub.1±xM.sub.2±yO.sub.4−δ  Formula 1
wherein, in Formula 1, A is a monovalent cation, and M is at least one of a monovalent cation, a divalent cation, a trivalent cation, a tetravalent cation, a pentavalent cation, or a hexavalent cation, 0≤x≤1, 0≤y≤2, and 0≤δ≤1, with the proviso that when M includes vanadium, 0<δ≤1, and wherein the mixed conductor has an inverse spinel crystal structure.

According to one embodiment, an active material is provided. This active material includes active material particles containing orthorhombic Na-containing niobium titanium composite oxide, and satisfies the following formula (1):

1≦A5/A0  (1) where A5 is a mole content ratio of a Li mole content L5 to a total of a Ti mole content T5 and a Nb mole content N5, and A0 is a mole content ratio of a Li mole content L0 to a total of a Ti mole content T0 and a Nb mole content N0.

A method for efficiently preparing ferrate based on nascent state interfacial activity. The method is as follows: (a) preparing nascent iron solution; (b) adding an oxidizing agent to the iron solution of step (a); (c) adding alkali solution or alkali particles to the mixed solution of step (b), mixing by stirring, and carrying out solid-liquid separation; (d) adding a stabilizing agent to the liquid separated out in step (c), and thus obtaining ferrate solution. The yield is 78-98%. The prepared ferrate solution is stable and can be stored for 3-15 days.

The invention provides primary batteries that incorporate a desodiated sodium transition metal oxide into the positive electrode (a cathode). Batteries of the invention using a desodiated sodium transition metal oxide in the cathode exhibit discharge voltages, battery capacities, and energy densities higher than a traditional Zn—MnO.sub.2 dry cell battery, such as a commercially available AA battery. These batteries are also advantageous over comparable lithium ion batteries due to the high abundance and low cost of sodium precursor materials with similar electrical performance.

A battery comprising an acidified metal oxide (“AMO”) material, preferably in monodispersed nanoparticulate form 20 nm or less in size, having a pH<7 when suspended in a 5 wt % aqueous solution and a Hammett function H.sub.0>−12, at least on its surface.

A battery cell having an anode or cathode comprising an acidified metal oxide (“AMO”) material, preferably in monodisperse nanoparticulate form 20 nm or less in size, having a pH<7 when suspended in a 5 wt % aqueous solution and a Hammett function H.sub.0>−12, at least on its surface.

The present application discloses a positive electrode active material and its preparation method, a sodium ion battery and an apparatus containing the sodium ion battery. The positive electrode active material satisfies a chemical formula of Na.sub.1-xCu.sub.hFe.sub.kMn.sub.lM.sub.mO.sub.2-y wherein M is one or more selected from Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, and Ba, 0<x≤0.33, 0<h≤0.24, 0≤k≤0.32, 0<l≤0.68, 0≤m≤0.1, h+k+l+m=1, 0≤y≤0.2, and the positive electrode active material has a water content of 6000 ppm or less.