Preparation And Modification Of LiMnPO₄ Cathode Material For Li-ion Batteries

Considering the threat of energy crises and air pollution, Li-ion batteries have attracted enormous attention in the past twenty years and are considered to be one of the most promising energy storage systems. Recently, LiMnPO₄, expected to have a large-scale application for high-energy devices, has been researched substantially as cathode materials for rechargeable Li-ion batteries, due to its high energy density, environmental benign nature, low cost.In this paper, we mainly adopt the solvothermal synthesis route to prepare the high-performance LiMnPO₄ nanomaterials. Many experimental variables, such as reaction time, additives, reaction temperature, solvents, have been studied in detail. In addition, the influence of metal cation substitution on the Li or Mn sites of LiMnPO₄ is also investigated.Plate-like LiMnPO₄ assembled in layer-by-layer has been synthesized by a facile solvothermal approach in mixed water-DEG?diethylene glycol? solvents with the mediation of CTAB?cetyltrimethylammonium bromide?. The influence of different reaction times and additives is investigated. It is found that the LiMnPO₄ prepared with the assistance of CTAB at 190 oC for 24 h exhibits the best electrochemical performance. On the basis of the optimum synthesis conditions, the effect of CTAB on improving the electrochemical performance of LiMnPO₄ is studied in detailed. The final obtained LiMnPO₄/C composites?named as LMP-CTAB/C? exhibits high discharge capacities of 148.6 mA h g^-1 at 0.1 C, 127.6 mA h g^-1 at 1 C and 93.8 mA h g^-1 at 5 C and it possesses excellent cycling performance with capacity retention ratios of 96.9% and 92.7% after 100 cycles and 500 cycles at 0.1 C and 1 C, respectively. Moreover, it is found that the coordinative effect of CTAB and DEG promotes the crystal orientation growth of LiMnPO₄ along ac plane.Based on the above mentioned synthesis route, experimental variables,including reaction time, H2 O solvent, reaction temperature and reactant mole ratio, are further discussed. As for the formation process of LiMnPO₄, clear phase and morphology transformations during the solvothermal reaction process are examined at short intervals. In addition, a tentative reaction mechanism is proposed on the basis of the time dependent trials. Moreover, it is found that an improved electrochemical performance can be obtained with the increase of reaction temperature and the decrease of reaction time. This result provides us thoughts and guidance to improve the electrochemical performance of LiMnPO₄ material by harmonizing the reaction temperature and reaction time, which is beneficial for the practical application.A solvothermal synthesis route is adopted to prepare LiMnPO₄ nanomaterial, in which mixed alcohols and water solvents?1:3, vol%? is used as reaction solvent. Keeping the volume ratio of alcohols and water unvaried, the influence of five different alcohols on the structure, morphology and electrochemical performance of LiMnPO₄ is studied in detail.The testing results indicate that the mixed DEG and water solvents are the optimum reaction solvents. On the basis of confirming the optimum solvents, Fe²⁺ ion substitution on the Mn site of LiMnPO₄ is applied to improve the electrochemical performance of LiMnPO₄. The final obtained LiMn0.6Fe0.4PO₄/C composite exhibits high discharge capacities of 147.3 mA h g^-1 at 0.1 C, 115.0 mA h g^-1 at 2 C and 102 mA h g^-1 at 5 C, indicating the sample possesses excellent rate performance.A solid-state synthesis route is applied to investigate the influence of Fe²⁺ ion substitution on the structure, morphology and electrochemical performance of LiMnPO₄. In addition, the effect of La³⁺ ion on the Li or Mn sites of LiMnPO₄ is also researched by using the solid-state method, hydrothermal method or solvothermal method.The testing results indicate that Fe²⁺ion can substitute the Mn of LiMnPO₄ and effectively improve the electrochemical performance of LiMnPO₄.