# Patent

## Title
MgMn2O4 with a crystal structure analogue to CaFe2O4, CaMn2O4, or CaTi2O4 as rechargeable magnesium battery cathode

## Abstract

A cathode active material for a battery includes a material of the formula MgxMn2O4 wherein 0≦x≦1 and the material has a crystal structure having an open channel formed in a single dimension or along a single dimensional axis. The crystal structure may be an analogue to CaFe2O4, CaMn2O4 or CaTi2O4.

## Background
FIELD OF THE INVENTION
The invention relates to cathode active materials for rechargeable batteries.
BACKGROUND OF THE INVENTION
Rechargeable batteries such as lithium ion and magnesium ion batteries have numerous commercial applications. Energy density is an important characteristic, and higher energy densities are desirable for a variety of applications.
A magnesium ion in a magnesium or magnesium ion battery carries two electrical charges, in contrast to the single charge of a lithium ion. Improved electrode materials would be useful in order to develop high energy density magnesium batteries.
SUMMARY OF THE INVENTION
In one aspect, there is disclosed a cathode active material for a battery that includes a material of the formula MgxMn2O4 wherein 0≦x≦1 and the material has a crystal structure including an open channel formed in a single dimension or along a single dimensional axis. The crystal structure may be an analogue to the CaFe2O4 phase or to the CaMn2O4 phase, or to the CaTi2O4 phase.
In another aspect, there is disclosed a cathode active material for a magnesium battery that includes a material of the formula MgxMn2O4 wherein 0≦x≦1 and the material has a crystal structure including an open channel formed in a single dimension or along a single dimensional axis. The crystal structure may be an analogue to the CaFe2O4 phase or to the CaMn2O4 phase, or to the CaTi2O4 phase.
In yet a further aspect, there is disclosed a magnesium ion battery that includes a cathode material of the formula MgxMn2O4 wherein 0≦x≦1 and the material has a crystal structure including an open channel formed in a single dimension or along a single dimensional axis, an anode material having a magnesium containing compound, an electrolyte disposed between the anode and cathode allowing diffusion of magnesium ions wherein magnesium cations are reversibly stripped and deposited between the anode and cathode. The crystal structure may be an analogue to the CaFe2O4 phase or to the CaMn2O4 phase, or to the CaTi2O4 phase.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the structure of a compound having the formula MgxMn2O4 with a crystal structure analogue to CaFe2O4, CaMn2O4 and CaTi2O4;
FIG. 2 is a plot of diffusion barriers for Mg2+ ions in MgMn2O4 with a crystal structure analogue to the CaFe2O4 or to the CaMn2O4, or to the CaTi2O4;
FIG. 3 is a chart showing the activation energy barriers for the diffusion of Li+ and Mg2+ in materials that are widely used as Li-ion battery cathodes;
FIG. 4 is a plot for electrochemical insertion of Mg2+ into CaFe2O4 phase MgxMn2O4;
FIG. 5 is a schematic representation of the one-dimensional diffusion of ions utilizing a distance between Mg2+ ions in units of angstroms in MgMn2O4 with crystal structure analogue to the CaFe2O4.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In one aspect, there is disclosed a cathode active material for a battery that has a material of the formula MgxMn2O4 wherein 0≦x≦1 and the crystal structure of the material has an open channel along a single dimension.
In one aspect, there is disclosed a cathode active material for a battery that has a material of the formula MgxMn2O4 wherein 0≦x≦1. In one aspect, the material has a crystal structure analogue to the CaFe2O4, or to the CaMn2O4, or to the CaTi2O4 phase. The crystal structures of the CaFe2O4, and the CaMn2O4, and the CaTi2O4 are shown in FIG. 1. In all these structures, the framework of MnO6 octahedrons form open channels along a single dimension. The ionic diffusion of magnesium ions happens through the channel and not through inter-channel diffusion.
The activation energy barriers for the diffusion of Mg2+ ions in MgMn2O4 with a crystal structure analogue to the CaFe2O4 or to the CaMn2O4, or to the CaTi2O4 phase are shown in FIG. 2. The activation energy barriers for the diffusion of Li+ or Mg2+ ions in other materials are plotted in FIG. 3. All the materials shown in FIG. 3, LiCoO2, LiFePO4 and spinel-LiMn2O4, are well-known cathode materials that have been widely applied in Li-ion batteries. As can be seen in the figure, the activation energy barriers differ widely for both magnesium and lithium materials for the spinel as well as the iron phosphate material. Additionally, as shown in the figure, the change from a lithium ion to a magnesium ion for various cathode materials results in differing properties and activation energy barriers between lithium and magnesium. Specifically, there appears to be little correspondence between the activation energy barriers between lithium and magnesium materials having similar type compounds. The diffusion barrier of Mg2+ in these materials is typically higher than 0.7 eV. It suggests that these materials have very sluggish Mg2+ mobility. It limits their application as possible cathode materials for Mg battery.
In one aspect, as shown in FIG. 2, the diffusion barrier of magnesium in MgMn2O4 with a crystal structure analogue to the CaFe2O4 or to the CaMn2O4 has a diffusion barrier that is comparable to that of lithium in a spinel type lithium ion battery cathode. As can be seen in the figure, the activation barrier is on the order of approximately 0.4-0.7 electron volts for the diffusion of Mg2+ ions.
The lower diffusion barrier in MgMn2O4 with the structure analogue to the CaFe2O4 or CaMn2O4 phase may be explained with reference to FIG. 5. The creation of a vacancy displaces two adjacent cations A1 and A2 in the figure towards the vacancy wherein cation A is Mg2+. If A1 hops from the equilibrium position to the vacancy, large Columbic repulsion is generated because of the short distance between A1 and A2. In this manner, A2 pushes along the hopping direction of A1. A3 is also attracted by the vacancy towards the direction of the diffusion of A1. Unlike the diffusion of individual ions in a spinel compound, the diffusion of Mg2+ in MgMn2O4 with the structure analogue to the CaFe2O4 or CaMn2O4 is a cooperative motion between A1, A2, and A3 along the open channel in a single dimension. Such a collective and collaborative hopping, as a characteristic for the hopping along the open channel in a single dimension, is energetically advantageous because it shortens the diffusion distance for each ion.
Cathode materials with high cationic mobility may be utilized as rechargeable battery electrodes. For the electrode materials, high ionic conductivity may be correlated to good rate capability. Slow diffusion may limit the rate capability of the electrodes and may prevent insertion and removal of cations.
As stated above, the low barrier for diffusion of Mg2+ ions in materials with a crystal structure analogue to CaFe2O4 or the CaMn2O4 with open channels in a single dimension indicates that it may be utilized for insertion and removal of Mg2+ ions in a cathode active material. Referring to FIG. 4, there is shown a plot of the voltage profiles for electrochemical insertion and removal of magnesium into the MgMn2O4 phase with a crystal structure analogue to CaFe2O4. The voltages for the magnesium insertion are between 2.84 and 1.68 volts (vs. Mg/Mg2+). In one aspect, if the full capacity for the MgMn2O4 phase with a crystal structure analogue to CaFe2O4 is achieved, the capacity would be 270.1 mAh per gram with half the magnesium per manganese. On the basis of this capacity and the voltage, the energy density of the MgMn2O4 phase with a crystal structure analogue to CaFe2O4 is about 1.3 times that of a lithium ion battery cathode which has a voltage  ̃3.5 volts vs. Li/Li+, and a capacity  ̃150 milliamp hours per gram.

## Claims

1. A cathode active material for a battery comprising: a material of the formula: MgxMn2O4 wherein 0≦x≦1 and the material has a crystal structure analogue to the CaFe2O4, or to the CaMn2O4, or to the CaTi2O4 and a framework of MnO6 octahedrons form open channels along a single dimension.

2. The cathode active material of claim 1 wherein the material has a crystal structure analogue of CaFe2O4.

3. The cathode active material of claim 1 wherein the material has an activation barrier for Mg2+ of less than or equal to 0.4 eV.

4. A cathode active material for a magnesium battery comprising: a material of the formula: MgxMn2O4 wherein 0×1 and the material has a crystal structure analogue to the CaFe2O4, or to the CaMn2O4, or to the CaTi2O4 phase and a framework of MnO6 octahedrons form open channels along a single dimension.

5. The cathode active material of claim 4 wherein the material has a crystal structure analogue of CaFe2O4.

6. The cathode active material of claim 4 wherein the material has an activation barrier for Mg2+ of less than or equal to 0.4 eV.

7. A magnesium ion battery comprising: a cathode material of the formula MgxMn2O4 wherein 0≦x≦1 and the material has a crystal structure analogue to the CaFe2O4, or to the CaMn2O4, or to the CaTi2O4 and a framework of MnO6 octahedrons that form open channels along a single dimension; an anode material having a magnesium containing compound; an electrolyte disposed between the anode and cathode allowing diffusion of magnesium ions wherein magnesium cations are reversibly transported between the anode and cathode.

8. The cathode material of claim 7 wherein the material has a crystal structure analogue of CaFe2O4.

9. The cathode material of claim 7 wherein the material has an activation barrier for Mg2+ of less than or equal to 0.4 eV.