Pyrite based cathodes for lithium batteries: a systematic study about the effect of active materialâ?(TM)s concentration

LUQUE, GUILLERMINA L.; BERCOFF, PAULA G.; BRACAMONTE, M. VICTORIA; LEIVA, EZEQUIEL; PRIMO, EMILIANO N.; BARRACO, DANIEL E.

Bologna

Congreso; 69th Annual Meeting of the International Society of Electrochemistry; 2018

Pyrite (FeS2) is natural, abundant and non-toxic mineral which is promising candidate for cathode materials in lithium batteries. The theoretical capacity of pure pyrite is 894 mAh g−1, corresponding to the reaction of four equivalents of lithium per mole of FeS2 [1]: FeS2 + 2Li+ +2e- → Li2FeS2Li2FeS2 + 2Li+ + 2e- → Fe0 + 2Li2SAt room temperature, the two steps merge to the same voltage around 1.5 V. However, FeS2 suffers poor cycling stability due to the volume fluctuations, formation of soluble lithium polysulfides and poor ionic/electrical conductivity of lithiation products [2]. Most of the reported studies focus their attention in the influence of the pyrite?s grain size on the performance of the batteries and they demonstrate that large discharge capacities are obtained using submicron particles due to their relatively stable structure, dense powder packing and enhanced electrode reaction kinetics [3]. Nonetheless, this conversion type cathode with several solution-phase reactions has been poorly analyzed in terms of the effect of amount and source of active material in the electrochemical mechanism.In this work, we report the synthesis of pyrite (FeS2) by high-energy mechanical ball milling of a powder mixture of Fe and S powders with ratio in a 1:2 ratio. The formation of the FeS2 phase requires 72 h milling time. After that, a thermal treatment at 350 ºC in vacuum was performed to promote the complete formation of pyrite. The obtained material was characterized in terms of the milling time and temperature of the thermal treatment and further used as cathode materials for the implementation of lithium-FeS2 batteries. The electrodes used for electrochemical measurements were prepared by casting a slurry prepared with FeS2, PVdF as binder and carbon black (Timcal Super P) dispersed in N-methyl-2-pyrrolidone. The weight percentage composition of PVdF was constant at 10 % in every slurry, while the amount of active material was set on 30, 50, 70 and 80. The analysis of charge/discharge galvanostatic cycling as a function of FeS2 loading showed an improvement in the performance until a maximum from while the specific capacity and coulombic efficiency start to decrease. The phase segregation experienced during the formation of Li2S and Fe is another key feature that determines a major role in the overall performance in the discharge of the cell. This was accounted by measuring the amount of remaining Fe after heavy-cycling the batteries by Vibrating Sample Magnetometer. Natural FeS2, with high amount of polar inorganic minerals, did not showed any improvement in the specific capacity. Considering the polysulfide-scavenging properties of the minerals in the natural FeS2, it is clear that morphological changes of the active material during cycling are the phenomena that determine the general performance of these kind of cathodes.In conclusion, we hypothesize that the highest the amount of FeS2 present in the cathode, the more segregation of phases and, in consequence, the highest capacity fading.