Structural Evolution Of Orthorhombic LiMnO₂ During Electrochemical Processing And Micro-structural Optimization For High Performance Cathode Material

Layered lithium manganate (monoclinic and orthorhombic LiMnO₂) has attracted intense research interest due to its low cost, low toxicity, high theoretical capacity (285 mAh/g) and more than 200 mAh/g of its practical specific capacity. Many studies have shown that the poor cycleability and the low reversible capacity at high current density of LiMnO₂ could not fulfil the practical requirements for the cathode of lithium ion batteries. And, some studies found that there are static and dynamic instabilities in layered LiMnO₂. Up to now, the relation between instabilities and electrochemical performance is not clear. Orthorhombic layered LiMnO₂ (hereafter referred as to o-LiMnO₂) was selected as the study subject of this study. To disclose the transformation of crystal structure and microsturcture during electrochemical processing, ex situ X-ray diffraction measurements were taken and Rietveld method was adopted. Also, combined with SEM, TEM, galvanostatic charge and discharge etc., the relation between the structural evolution and electrochemical behaviors upon cycling was systematically studied. Base on the above results, better o-LiMnO₂ for rechargeable lithium batteries was obtained by means of improvement of preparation and microstructure optimization.Orthorhombic LiMnO₂, which used in this study, was prepared by a solid-state method at high temperature reported in literatures. The synthesis of the o-LiMnO₂ compounds was done by mixing Mn₂O₃ and LiOH·H₂O. The intimately ground powder with a molar ratio of Li/Mn=1.03 was heat-treated in tube furnace under a flow of Ar gas at 600～800℃. The results show that the final products are pure phase of o-LiMnO₂. The microstructures such as specific area, crystallite (or domain) and/or grain size, stacking faults that are caused by structure disorder between Li and Mn sites, and the Li-0 band evolve regularly by varying the heating temperature.Structural evolution of o-Li_1-xMnO₂ (0≤x≤0.95) during the first charge processing prestents as follow. At the initial charging stage (0≤x≤0.2), the layered structure is not changed while the structural reorganization involves only a redistribution of the cations in local region. Following this first stage, phase transits in o-Li_1-xMnO₂ and rock-salt type and spinel-like lithium manganese oxides successively emerges in the range of 0.3≤x≤0.5. Rietveld refinements were performed. The results show that cations of Li and Mn are distributed disorder of in the two new above-stated phases, respectively. As lithium is removed, the orthorhombic phase disappears and the content of rock-salt and spinel-like lithium manganese oxides increased in the range of 0.6≤x≤0.95.The relations between structural evolution and electrochemical characteristics (for example, activation performance and variation of voltage plateaus) of o-LiMnO₂ were further investigated. It is found that layered structure is a precursor, while the practical active materials which involve in charge/discharge cycles are rock-type and spinel-like lithium manganese oxides originated from the o-LiMnO₂. With the rock-salt phase transforming to spinel-like phase, the increase in 4V and 3V plateaus contributes to the increase in specific capacities. Like this way, the obtained capacities increase progressively with further cycling. The maximum capacity can be reached after activation finished. It can be stated that the rock-salt phase transform to a spinel-like phase on cycling which results in the former content decrease before activation finished. In spinel-like structure, both tetrahedral and octahedral sites are occupied by Li and Mn ions, and the ordered distribution improved upon cycling. When the activation is completed after several cycles at discharged state, the ratios of Li:Mn in 8a site and in 16d site are 0.86:0.14 and 0.07:0.93, respectively. Tetragonal Li₂Mn₂O₄ phase, which was confirmed in the flat 3V plateau in conventional LiMn₂O₄, was not appeared on cycling between 4.4 and 2.0V. The main reason is that the Jahn-Teller distortion be suppressed due to the disordering distribution of cation (Li and Mn) in spinel-like structure. Both the specific capacity and cycleability of the composite cathodes composed of rock-salt and spinel-like lithium manganese oxides are better than those of LiMn₂O₄ cathode. It is found that the two-phase coexistence of rock-salt type and spinel-like type lithium manganese oxides occurs within a single grain. The cycling-induced rock-salt and spinel transition concurrently creats antiphase domains about size of 20nm and 6nm, respectively. Such a nanostructure appears to be able to accommodate the strain induced on cycling, which prevent the mechanical failure accompanying the phase transitions.From the investigation on the relation between electrochemical properties and structures of various crystal state, it is found that o-LiMnO₂ with small grains, small crystallites and more stacking faults could be more easily transformed into the rock-salt and spinel-like structure, and the spinel-like phase content of the final material is high. Thus, the cathode could fleetly reached the maximum discharge capacity and the electrochemical properties at high current density is well. Unfortunately, o-LiMnO₂ cathode with small grains severe capacity fading on cyclying, compared with samples with large grains. The reason for the fading is the dissolution of Mn in the electrolyte.Based on the above-stated results, the relations between structures and electrochemical characteristics of o-LiMnO₂ can be stated as follow. (1) o-LiMnO₂ is only a precursor, while the practical active materials which involve in charge/discharge cycles are rock-type and spinel-like lithium manganese oxides. Both compose composite cathode with nano-domain as well as the unique crystal structure, which exhibit greatly improved specific capacity and cycling compared to that of conventional LiMn₂O₄. (2) The performance of o-LiMnO₂ depend on the bulk of grain. Small grain of o-LiMnO₂ cathodes are more easily reached high capacity, but poor cycleability. On the basis of the results which we found, high performance o-LiMnO₂ microstructure model was designed. In this microstructure, the particles should be large while the crystallites and grains which aggregated the products should be small in addition to poor-crystallized, more stacking faults.In order to obtained a microstructure as above designed, a series of measures were taken.Firstly, o-LiMnO₂ with high stacking faults and size of 20～30nm was synthesized by a rheological phase method. It is found that the o-LiMnO₂-phase has been completely transformed after first cycle. Specific capacity reached up to 209.7mAh/g and coulombic efficiency on the first electrochemical cycle was also improved greatly.Secondly, MnCO₃ precursors with various structures were obtained by a co-precipitation method and then o-LiMnO₂ were synthesized by solid-state method. How the microstructures of precursors affect the final products of o-LiMnO₂ were studied in detail. Results show that the shape, size and size distribution of precursors MnCO₃ were retained during the preparation process. Microstructures of o-LiMnO₂ designed in theory were optimized by rheological phase method using large particle MnCO₃. The electrochemical performances of the o-LiMnO₂-based cathodes were tested. The optimized o-LiMnO₂-based cathode had a. maximum discharge capacity of 205.1mAh/g on the 3^rd cycle at a current density of 15mA/g. The capacity retention is improved, discharge capacity was above 170mAh/g after 50 cycles. The discharge capacity at high currents is well retained during cycling. The maximum capacities are 149.9 and 122.7mAh/g at the current densities of 75 and 150mA/g, respectively.It can be concluded from this study that the microstructures of high performance o-LiMnO₂ are small crystallites and grains, high stacking faults and large particles. o-LiMnO₂-cathode with such microstructures could result in not only high capacity but also high capacity retention.