The Preparation Of λ-MnO₂ And The Study Of The Electrochemical Insertion/Deinsertion Of Na/Mg Ion Into/from λ-MnO₂

A simple H₂SO₄ leaching method is used to produce λ-Mn02 from LiMn₂O₄. λ-MnO₂,having the same spinel structure with LiMn₂O₄, shows high capacity as the positive electrode material for rechargeable batteries. In this thesis, we first optimize the best concentration of H₂SO₄, leaching temperature and leaching time for the extraction of lithium from LiMn₂O₄.Then we study the insertion/deinsertion of Na+ and Mg2+ in the different sodium salts and magnesium salts, respectively.When the temperature is 50 ℃, the concentration of H₂SO₄ is 0.56 mol·dm^-3, and the leaching time is 12 h,λ-MnO₂, produced through the acid leaching of LiMn₂O₄, matches well with the pure cubic spinel structure. ICP-AES analysis shows that the lithium in LiMn₂O₄ is nearly completely removed after the acid treatment. The SEM and TEM images demonstrate that λ-MnO₂ particles are distributed uniformly and are composed of nano-particles with the diameter of about 20 nm. When the current density is 13.6 mA·g-1, the initial discharge capacity reaches 390.7 mAh·g-1 with the best rate capacity and cycle performance.From the electrochemical testing results, we get that SO₄2- has the least influence on the insertion/deinsertion of sodium into/from λ-MnO₂. At the current density of 13.6 mA·g-1, the insertion capacity of sodium into A-MnO₂ is 409.7 mAh·g-1 in 0.5 mol·dm^-3 Na₂SO₄. After 500 cycles at 136 mA·g-1, the discharge capacity still remains 82.3 mAh·g-1. Through the study of the concentration influence of Na₂SO₄, we get that the optimal concentration is 0.5 mol·dm^-3 at the current density ranging from 13.6 to 680 mA-g-1. While λ-MnO₂ electrode has the better high rate performance in I mol·dm^-3 Na₂SO₄ at the current density ranging from 680 to 6800 mA·g-1. The capacitor battery λ-MnO₂/AC has the high energy density and high power density. It exhibits a high specific capacity of 82 mAh·g-1 and energy density of 71.7 Wh-Kg-1 at a low power density of 403 W·Kg-1. And it also delivers an energy density of 19.7 Wh·Kg-1 even at a high power density of 3500 W·Kg-1.The electrochemical performance of λ-MnO₂ in 1 mol-dm^-3 MgSO₄,Mg（NO3）2 and MgCl2 is investigated by cyclic voltammetry, constant current charge-discharge test and electrochemical impedance spectroscopy. We find that Cl- has the least influence on the insertion/deinsertion of magnesium into/from λ-MnO₂. When the testing current density is 13.6 mA·g-1 , the initial discharge capacity of λ-MnO₂ electrode in 1 mol·dm^-3 MgCl2 has reached 478.4 mAh·g-1. And the capacity still remains 62.1 mAh·g-1 when the current density is raised to 500 times, 6800 mAh·g-1. After 500 cycles at 136 mA·g-1, the discharge capacity still remains 130 mAh·g-1 . After the investigation of the concentration influence of MgCl2, We fimd that when the concentration of MgCl2 is 3 mol·dm^-3, the chlorine evolution reaction has exerted a tremendous influence on the deinsertion of magnesium ion from λ-MnO₂. λ-MnO₂ electrode has the best low rate performance in 0.5 mol·dm^-3 MgCl2, and the first discharge capacity reaches 545.6 mAh·g-1 at 13.6 mA·g-1. And >A-MnO₂ electrode has the best high rate performance and cycling stability in 1 mol·dm-1 MgCl2. The XRD and XPS studies support the formation of MgMn₂O₄ after the insertion of magnesium ion into λ-MnO₂.