Pure electric vehicles (EVs) have proposed energy density enhancements for lithium-ion batteries (LIBs). It is possible to increase the mass energy density of lithium-ion batteries to more than six times that of existing LIBs such as “Leaf” (Leaf). The new candidate positive electrode material has already been introduced. It is the Associate Professor of Department of Environmental Chemistry, Department of Engineering, Tokyo Denki University, Japan. The lithium-doped lithium niobate (Li1.3 Mn0.4 Nb0.3O2) successfully synthesized by the R&D team led by the Ming Dynasty (pictured).

Figure: New cathode material successfully synthesized by Tokyo Denki University, Japan

The left figure shows the synthesis of lithium-doped lithium niobate, whose structure is shown on the right and is a rock salt type (NaCl type) structural oxide.

From the chemical formula, it can also be seen that lithium doped lithium niobate is a material obtained by replacing part of niobium (Nb) and lithium (Li) of lithium niobate (Li1.5Nb0.5O2) with manganese (Mn). Similar to Lithium Cobaltate, Lithium Manganate, and Ternary Materials, which are commonly used as LIB cathode materials, they are rock salt type (NaCl type) structures (faceted cubic lattice). However, manganese-doped lithium niobate differs from these materials in that its lithium (Li) atoms are not arranged in layers. It is a structure in which an oxygen atom is inserted at the chlorine atom position of NaCl and a transition metal manganese atom, a transition metal hydrazine atom, or a lithium atom is randomly inserted at the sodium atom position.

In fact, oxides with this structure could not reversibly remove lithium atoms. However, a recent study by the Massachusetts Institute of Technology found that if the composition ratio of lithium to transition metal is increased, this problem can be solved. At this time, in the lithium-doped lithium niobate successfully synthesized by Shinai et al., the composition ratio of transition metal (Mn and Nb) to lithium was 0.7:1.3. By increasing the lithium ratio so high, lithium can be inserted or removed.

In this way, in the same 1 mole of oxide, the new material contains more lithium than the common layered rock salt type positive electrode material, and there is a three-dimensional skeleton in the crystal structure. Therefore, even if a large amount of lithium is inserted or removed, the structure is hardly collapsed. According to Yonechi's introduction, "The initial charge can remove about 90% of lithium. And, 80% of them can be reversibly recovered through discharge."

The average voltage of the new material is more than 3V on a lithium metal basis. Lithium metal for the negative electrode, Lithium hexafluorophosphate for the electrolyte, and Diethyl carbonate and dimethyl carbonate for the electrolyte The mass energy density at the current density of 10 mA/g is as high as 950 Wh/kg. This is more than six times the current LIB for EVs with a mass energy density of 100 to 150 Wh/kg. In terms of cycle characteristics, only 20 tests of charging and discharging have been carried out at present, but "there is no deterioration in capacity basically under the conditions of a voltage range of 1.5 to 4.2 V and a current density of 25 mA/g". Therefore, the cost "can be lower than lithium cobalt oxide."

The practical target time for this technology is five years later. The application that was originally aimed at was a portable electronic product. In the future, thermal stability will also be studied and the degree of completion as a battery material will be improved. The establishment of mass production technology for practical use will also be a major issue. (Reporter: Tomihiro Miooka)

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