The physicochemical properties of lithium manganate.
Release time:
2021-08-03 14:54
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Industry News
Lithium manganese oxide is one of the more promising lithium-ion cathode materials. Compared to traditional cathode materials like lithium cobalt oxide, lithium manganese oxide has advantages such as abundant resources, low cost, no pollution, good safety, and excellent rate performance, making it an ideal cathode material for power batteries. However, its poor cycling performance and electrochemical stability greatly limit its industrialization. Lithium manganese oxide mainly includes spinel-type lithium manganese oxide and layered structure lithium manganese oxide, among which spinel-type lithium manganese oxide has a stable structure and is easy to achieve industrial production; currently, market products are all of this structure. Spinel-type lithium manganese oxide belongs to the cubic crystal system, with the Fd3m space group and a theoretical specific capacity of 148mAh/g. Due to its three-dimensional tunnel structure, lithium ions can reversibly de-intercalate from the spinel lattice without causing structural collapse, thus exhibiting excellent rate performance and stability.
Nowadays, the traditional view that lithium manganese oxide has low energy density and poor cycling performance has greatly improved (typical value from Wanli New Energy: 123mAh/g, 400 cycles; high cycling type typical value: 107mAh/g, 2000 cycles). Surface modification and doping can effectively improve its electrochemical performance. Surface modification can effectively suppress the dissolution of manganese and the decomposition of the electrolyte. Doping can effectively suppress the Jahn-Teller effect during the charge and discharge process. Combining surface modification with doping will undoubtedly further enhance the electrochemical performance of the material, and it is believed that this will become one of the directions for future modification research on spinel-type lithium manganese oxide.
LiMn2O4 is a typical ionic crystal and has both positive and negative configurations. XRD analysis shows that normal spinel LiMn2O4 is a cubic crystal with Fd3m symmetry, with a lattice constant a=0.8245nm and a unit cell volume V=0.5609nm3. The oxygen ions are densely packed in a face-centered cubic arrangement (ABCABC..., with adjacent oxygen octahedra sharing edges), lithium occupies the 1/8 tetrahedral interstitial site (V4) (in the Li0.5Mn2O4 structure, lithium is ordered and occupies 1/16 of the tetrahedral interstitial sites), and manganese occupies the 1/2 octahedral interstitial site (V8). The unit cell contains 56 atoms: 8 lithium atoms, 16 manganese atoms, and 32 oxygen atoms, with Mn3+ and Mn4+ each occupying 50%. Since the lattice constant of the spinel structure is twice that of a normal face-centered cubic structure (fcc), each unit cell actually consists of 8 cubic units. These eight cubic units can be divided into two types: A and B. Each pair of coplanar cubic units belongs to different structural types, while each pair of edge-sharing cubic units belongs to the same structural type. Each small cubic unit has four oxygen ions, all located at the center of the body diagonal from the midpoint to the vertex, specifically at 1/4 and 3/4 of the body diagonal. Its structure can be simply described as 8 tetrahedral 8a positions occupied by lithium ions, 16 octahedral positions (16d) occupied by manganese ions, with manganese in the 16d positions being occupied by Mn3+ and Mn4+ in a 1:1 ratio, and all positions in the octahedral 16c being vacant, with oxygen ions occupying the octahedral 32e positions. In this structure, the MnO6 oxygen octahedra are edge-sharing, forming a continuous three-dimensional cubic arrangement, which provides a three-dimensional pathway for lithium ion diffusion formed by the tetrahedral lattice 8a, 48f, and the octahedral lattice 16c. When lithium ions diffuse in this structure, they follow a linear diffusion path in the order of 8a-16c-8a (the energy barrier at the tetrahedral 8a position is lower than that at the oxygen octahedral 16c or 16d positions), with an angle of 107° in the diffusion path, which serves as the theoretical basis for its use as a cathode material in secondary lithium-ion batteries.
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