Optimization of porosity

In the course of synthesis development we found that the heating regime (heating rate, the final temperature, etc.] had extreme influence on the final porosity of LiFeP04 particles (the size of the latter was typically several micrometers]. Of course, the heating rate affects the rate of gas evolution but also the rate of solidification of LiFeP04. Both processes combined will determine the final sample porosity. The porosity, in turn, will have important impact on the electrochemistry. The dependence of electrochemical 

LiFeP04 as discussed in detail in Ref. 27. (b) Pore volume and Brunauer-Emmett-Teller (BET) surface area for three samples with different porosities inset the corresponding distribution of pore sizes, (c) 2nd discharge curves for all three LiFeP04 samples at C/2 (85 mAh g ). (d) Voltage difference (proportional to electrode resistance) between charge and discharge as a function of pore volume. 

Given that electrode porosity is obviously very important for the kinetics, one might ask how the porosity could be optimized in the case of particulate materials. This problem becomes espedalfy relevant when the particle size becomes very small, say in the range below 20 nm. Namely, in such cases carbon black and other additives are too big to serve as spacers between individual active nanoparticles. Unfortunately, no experimental studies covering this topic are available for LiFeP04. Erjavec et al. have tackled this problem, however, on the example of anatase (Ti02) with typical particle size between 5 to 10 They have found that the 

Pad hi, A K., Nanjundaswamy, K. S. Goodenough, J. B. Phospho-olivines as positive-electrode materials for rechargeble lithium batteries. J. Electrochem. Soc., 144,1188-1194 (1997). 

Ohzuku, T. Brodd, R J. An overview of positive-electrode materials for advanced lithium-ion batteries./. Power Sources, 174,449 (2007). 

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Many heterogeneous catalysts need supports, which, from the economics viewpoints, is a means of spreading expensive materials and providing the necessary mechanical strength, heat sink/source, optimization of bulk density and dilution of an overactive phase. There are also geometric (increase of surface area, optimization of porosity, crystal and particle size) and chemical functions (improvement of activity, minimization of sintering, and poisoning, as well as beneficial effect of spillover) provided by the supports. 

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