Electrochemical Li Insertion in Lamellar (Birnessite) and Tunnel Manganese Oxides (Todorokite)
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oxidizing agent (MnO 4 -BIR). Ion-exchange of the Na' for Mg 2, in both Na-birnessites was achieved by dispersing the solid in IL of IM MgCI2 solution, to give the resultant Mg-buserites (0 2-BUS and MnO 4-BUS). Li exchange of the interstitial cations was carried out by dispersing the as-synthesized material with a 50-fold molar excess of Li (as a 0.3M LiCl solution). This was repeated thrice, isolating the material each time. Todorokites were prepared by hydrothermal treatment of layered Mg-buserite. Approximately 0.5g of wet 0 2-BUS and MnO 4 BUS in their Mg-exchanged forms were resuspended in 16ml of DW in a Teflon liner of a 23mL capacity Parr bomb. These suspensions were autoclaved for 5 days at 170'C, and the black precipitates recovered and washed to yield 0 2-TDK and MnO 4 -TDK. Instrumentation. X-ray powder diffraction (XRD) patterns were obtained on random powder mounts in a Siemens D500 diffractometer equipped with a diffracted beam monochromator, using Cu-Kcx radiation. The transformation of the layered to tunnel framework was confirmed by high resolution TEM studies. A Varian Liberty 100-ICP-AES spectrometer was used to determine the atomic ratio of alkali (Na, Mg,) to Mn. Oxidation state measurements were performed following the method by Vetter and Jaeger.6 Thermogravimetric analysis was carried out on a PL Thermal Sciences STA 1500 thermal analysis system at a heating rate of 10°C/min from room temperature up to 700'C to obtain the full stoichiometry. Surface areas of -45m 2/g were measured for both the phyllomanganates and tektomanganates by the BET method. Samples were degassed at 80'C under vacuum for 24 hours prior to analysis. ElectrochemicalLi Insertion. Samples were ground through a 50%tm mesh to ensure a narrow distribution of particle sizes. The active material was then mixed with 20% acetylene black and 5% KYNAR FLEX® 2820-00 binder, and cyclopentanone was added to form a slurry. This mixture was coated onto an Al disc (1.44 cm2) and dried by allowing the solvent to evaporate. Typically, each cathode constituted 1-2 mg of active material. Li insertion was performed on active material pretreated at 180'C prior to battery assembly. To ensure there was no subsequent uptake of water during the preparation, the coated Al discs were then reheated at 180'C before being introduced into the glove box. Propylene carbonate (PC, Aldrich) was distilled under vacuum before being used to make up the electrolyte, 1M LiCIO 4 . The cell components were transferred into an Ar atmosphere-filled glove box and assembled in a Swagelock housing-type cell. The electrochemical testing was studied under galvanostatic control using a Mac-Pile TM system. The voltage limits set for galvanostatic cycling 2 were 4.2-2.OV, with a typical cycling rate of C/20 (15-20 ýtA/cm ). RESULTS AND DISCUSSION Table 1. Stoichiometries (reported per Mn) for MnO 4-and 02-180'C treated materials, Sample
Composition
MnO 4 -BIR Nao.I8Mg0.15MnO 2 .o6.0"35H2 0 MnO 4 -BUS Mgo.25MnO 2 .07 .0"59H 2 0 MnO 4 -TDK Mgo.24 MnO 2.o6 .0-52H 2 0 02 -
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