Described herein is a nanocrystalline ferrite having the formula Ni.sub.1-x-y M.sub.yCo.sub.xFe.sub.2+zO.sub.4, wherein M is at least one of Zn, Mg, Cu, or Mn, x is 0.01 to 0.8, y is 0.01 to 0.8, and z is −0.5 to 0.5, and wherein the nanocrystalline ferrite has an average grain size of 5 to 100 nm. A method of forming the nanocrystalline ferrite can comprise high energy ball milling.

An aqueous secondary battery according to an embodiment includes: a positive electrode; a negative electrode; a separator; and an aqueous electrolytic solution including water and a metal salt represented by Chemical Formula 1 and having molarity of about 5 m to about 40 m.

A.sub.xD.sub.y  [Chemical Formula 1]

In Chemical Formula 1, A is at least one metal ion selected from a sodium ion, a potassium ion, a magnesium ion, a calcium ion, a strontium ion, a zinc ion, or a barium ion, D is at least one type of atomic group ion selected from Cl.sup.−, SO.sub.4.sup.2−, NO.sub.3.sup.−, ClO.sub.4.sup.−, SCN.sup.−, CF.sub.3SO.sub.3.sup.−, C.sub.4F.sub.9SO.sub.3.sup.−, (CF.sub.3SO.sub.2).sub.2N.sup.−, AlO.sub.2.sup.−, AlCl.sub.4.sup.−, AsF.sub.6.sup.−, SbF.sub.6.sup.−, BF.sub.4.sup.−, and PO.sub.2F.sub.2.sup.−, and 0<x≤2, and 0<y≤2.

The present invention discloses an iron-based cathode material for a sodium-ion battery, which comprises a Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material and a carbon-based material embedded into the bulk structure of Na.sub.3Fe.sub.2(SO.sub.4).sub.3F material. The weight percentage of the carbon-based material is ranked between 1% and 10%. The present invention also provides a method for preparing the above-mentioned iron-based cathode material for a sodium-ion battery, and a corresponding sodium-ion full battery using the Na.sub.3Fe.sub.2(SO.sub.4).sub.3F-based cathode material. The Na.sub.3Fe.sub.2(SO.sub.4).sub.3F cathode material ensures desired electrochemical sodium storage performance, involving high specific sodium storage capacity, improved cycle stability and superior rate performance in comparison with that of various pristine Na.sub.xFe.sub.y(SO.sub.4).sub.z materials. The actual operating potential of the reported sodium-ion full battery in the present invention is significantly higher than the output potential of existing commercial sodium-ion full batteries, and the increase in battery energy density is also achieved.

The invention relates to O3/P2 mixed-phase sodium-containing doped layered oxide materials which comprise a mixture of a first phase with an O3-type structure and a second phase with a P2-type structure; wherein the O3:P2 mixed-phase sodium-containing doped layered oxide material has the general formula: Na.sub.aA.sub.bM.sup.1.sub.c M.sup.2 M.sup.3.sub.eM.sup.4.sub.f M.sup.5 O.sub.2±δ. The invention also provides a process for making such O3/P2 mixed-phase sodium-containing doped layered oxide materials, and use applications therefor.

A thermoelectric material, having a formula Tb.sub.xM1.sub.y-xM2.sub.zO.sub.w where M1 is one of Ca, Mg, Sr, Ba and Ra, M2 is at least one of Co, Fe, Ni, and Mn, x ranges from 0.01 to 5; y is 1, 2, 3, or 5; z is 1, 2, 3, or 4; and w is 1, 2, 3, 4, 5, 7, 8, 9, or 14. The thermoelectric material is chemically stable within 5% for one year and is also non-toxic. The thermoelectric material can also be incorporated into a thermoelectric system which can be used to generate electricity from waste heat sources or to cool an adjacent region.

A disordered rocksalt lithium metal oxide and oxyfluoride as in manganese-vanadium oxides and oxyfluorides well suited for use in high capacity lithium-ion battery electrodes such as those found in lithium-ion rechargeable batteries. A lithium metal oxide or oxyfluoride example is one having a general formula: Li.sub.xM′.sub.aM″.sub.bO.sub.2-yF.sub.y, with the lithium metal oxide or oxyfluoride having a cation-disordered rocksalt structure of one of (a) or (b), with (a) 1.09≤x≤1.35, 0.1≤a≤0.7, 0.1≤b≤0.7, and 0≤y≤0.7; M′ is a low valent transition metal and M″ is a high-valent transition metal; and (b) 1.1≤x≤1.33, 0.1≤a≤0.41, 0.39≤b≤0.67, and 0≤y≤0.3; M′ is Mn; and M″ is V or Mo. The oxides or oxyfluorides balance accessible Li capacity and transition metal capacity. An immediate application example is for high energy density Li-cathode battery materials, where the cathode energy is a key limiting factor to overall performance. The second structure (b) is optimized for maximal accessible Li capacity.

Embodiments of synthetic garnet materials having advantageous properties, especially for below resonance frequency applications, are disclosed herein. In particular, embodiments of the synthetic garnet materials can have high Curie temperatures and dielectric constants while maintaining low magnetization. These materials can be incorporated into isolators and circulators, such as for use in telecommunication base stations.

According to one embodiment, there is provided an active material. The active material includes secondary particles. The secondary particles include first primary particles and second primary particles. The first primary particles include an orthorhombic Na-containing niobium-titanium composite oxide. The second primary particles include at least one selected from the group consisting of a carbon black, a graphite, a titanium nitride, a titanium carbide, a lithium titanate having a spinel structure, a titanium dioxide having an anatase structure, and a titanium dioxide having a rutile structure.

The piezoelectric composition is represented by the following Chemical Formula (1):
x[Bi.sub.mFeO.sub.3]-y[Ba.sub.mTiO.sub.3]-z[Bi.sub.mAlO.sub.3](1)
wherein 0.5x0.7995, 0.2y0.4, 0.0005z0.1, x+y+z=1, 0.96m1.04.

Cathode materials for lithium ion batteries, lithium ion batteries incorporating the cathode materials, and methods of operating the lithium ion batteries are provided. The materials are composed of lithium metal oxides that include two different metals.