Layered lithium manganese oxide
LiMnO2 has attracted more and more attention because of its low price and no pollution to the environment, but it has poor thermal stability at high temperatures. Generally, it cannot be prepared by the method of preparing NaMnO2. Bruce and Delmas used ion exchange method to obtain LiMnO2 from sodium compounds, and studied the effect of stacking sequence on electrochemical performance. Low-temperature synthesis is also a preparation method, for example, hydrothermal synthesis or decomposition of alkali permanganate, in the presence of lithium to generate Li0.5MnO2·nH2O; after vigorously heating to remove water, layered LixMnO2 is obtained; continuing to heat to 150°C, spinel LiMn2O4 is formed. Birnessite-type crystal phase can also be obtained by acid treatment of manganese oxide.
LixMnO2 easily transforms into a spinel structure with good thermal stability during cycling. This transformation requires a ccp oxygen lattice in the structure and no oxygen diffusion. There is an oxygen ion layer in the ccp lattice, arranged as AcB| aCbA|cBaC|bA, and there are three stacked blocks-MnO2 blocks (the upper layer is oxygen, the lower layer is manganese, and the oblique lower layer is lithium). There are two ways to enhance the stability of the layered LiMnO2 structure.
(1) Geometrically stable mode. Converting a non-ccp structure into a tunnel structure, a two-piece structure or other non-ccp close-packed structure, or occupying the columnar space between two layers can improve its stability. KMnO2 and (VO)yMnO2 compounds are examples of such columnar structures. The former exists as a stable spinel configuration at low current density, while the latter has high stability but low specific capacity. The research group of Dahn and Doeff has studied the non-ccp structure by observing the Li0.44Mn2O4 tunnel structure, and obtained non-ccp-stacked oxygen flake compounds by ion exchange layered sodium manganese oxide. These flakes can no longer be reorganized after ion exchange, but become ccp accumulations. The use of ion exchange method will also produce a layered structure in the form of faults, preventing the irregular layered accumulation from transforming into a thermally stable 03 phase, but this kind of crystal phase can insert lithium in a wide voltage range, and there are more than two voltage steps on the electrochemical curve.
(2) Electronic stabilization method. The electronic properties of manganese are more similar to that of cobalt by replacing the manganese in it with a multi-electron element such as nickel. This kind of successful substitution of cobalt and nickel for manganese has been reported. Through the study of LiNi1-yMnyO2 (0<y≤0.5), it is found that this material has low capacity and poor cycle performance. Spahr et al. demonstrated a high capacity and high cycle performance LiNi0.5Mn0.5O2 (hereinafter referred to as 550 material), and has a discharge curve similar to the classic LiNiO2. LiNi1-y-zMnyCozO2 compound has excellent electrochemical properties and can be used instead of LiCoO2. In addition to having high electrochemical capacity and cycle performance, these compounds also show good thermal stability. A theoretical analysis of the transformation process from layered to spinel phase in LixMnO2 can be summarized as a two-step mechanism. In the first step, part of the lithium ions and manganese ions quickly move to the tetrahedral position; in the second step, the spinel cations are arranged regularly.
Composite lithium manganese cobalt oxide
The cobalt-substituted compound LiNiMn1-yCoyO2 can be synthesized from sodium congeners by ion exchange method LiNiMn1-yCoyO2. When the value of y reaches 0.5, this type of substituted material has a structure of α-NaFeO2. The non-substitution material LiMnO2 can be transformed into a spinel structure at a lower circulating current density of 0.1mA/cm2; when y=0.1, this structural transformation occurs in the first cycle; when y=0.3, no obvious changes occur until dozens of times, but the cycle performance of this material is good under the 3V voltage platform.
Substitution of cobalt, iron or nickel ions for part of manganese ions can significantly increase the conductivity of manganese oxide materials. In pure LiMnO2 or KMnO2, the conductivity is only about 10-5S/cm. The data in Figure 1 clearly illustrates the conductivity of the doped cobalt. At this time, the conductivity has increased by two orders of magnitude, and the nickel-doped manganese oxide material is less effective.
Figure 1 - Conductivity diagram of KMnO2 when 10% Co, Ni and Fe replace manganese at different temperatures
Cobalt-doped materials can also be prepared by hydrothermal method, and the cycle performance is greatly improved compared with cobalt-rich materials. In the first cycle, charging with a current of 1mA/cm2, even if cations with a larger radius such as potassium are supported and embedded in the structure, the configuration of the cobalt-doped compound still transforms to the spinel type.