Carbon Aerocellulose as Cathode Material in Primary Batteries
Résumé
The Li/SOCl2 primary batteries are efficient devices but some drawbacks remain. Currently carbons used as cathode material are mainly carbon black powders based on thermal treatment of petroleum industry compounds. Their implementation as electrodes requires a relatively complex process involving solvent incorporation, mixing, drying and compressing steps. The porosity largely depends on the implementation process and the mechanical properties are low due to the use of powder. The volumic power needs to be increased and the cost needs to be decreased. With a sustainable development approach and promoting the use of cellulose as renewable and low cost material and a vector of high-technology materials, we developed a monolithic 3D opened porous mesostructured carbon based on pyrolysed cellulose as cathode material. The synthesis steps were the following. Cellulose/NaOH solutions, composed of X% cellulose (X=3-7wt%) dissolved in 8% NaOH-water, were prepared1,2. Cellulose/NaOH/water solutions were poured into cylindrical moulds and kept at room temperature for several hours, ensuring solution gelation. Cellulose was coagulated in water or alcohol. The samples were dried using CO2 under supercritical conditions (performed at 37°C, 85bar) in order to keep pores opened. White porous cellulose samples, aerocellulose, keeping the shape of the mould but presenting some shrinkage were obtained. Their porosity is higher than 90% with pore sizes from a few tens of nanometers to a few tens of micrometers. Finally, the samples were pyrolysed at 1000°C under nitrogen flow3. Same as for aerocellulose, the resultant carbon kept the cylindrical shape with a large shrinkage. The density ranged from 0.2 to 1g/cm3, the porous volume from 0.2 to 4cm3/g with mesoporous pore diameter between 40 and 210 nm. The advantages of this kind of materials are (i) fixed porosity adapted to the application (by controlling the operation condition of the synthesis), (ii) high, stable intrinsic conductivity (without binder due to the covalent link between the carbon particles) (iii) good mechanical properties and (iv) a monolithic carbon shape directly usable in industrial cells. In this study we analysed the impact of the elaboration (dissolution, gelation and regeneration), drying and pyrolysing conditions on the texture and electrochemical performances of the resultant carbon. In Li/SOCl2 batteries, the carbon electrode can be the limiting factor for capacity, through its capability to accommodate the insoluble product of the electrochemical reaction (LiCl) in its porosity. To make a first selection, the limiting capacity of different carbons was evaluated in Li/SOCl2 button cells with excess of Li and thionyl chloride. The carbon aerogel electrodes were tested as thin slices (1mm), at low rate of discharge (400 to 700h) and the capacity expressed in mAh/cm3 of carbon electrode. In special synthesis conditions, we obtained volumic capacities of 1157 mAh/cm3 better than those of the SAFT cathode reference (960 mAh/cm3). In cylindrical industrial cells, the higher performances were confirmed for aerogel cyclindrical monoliths at similar discharge rates but some technical problems of connection between the carbon and the cell have still to be solved.