Effect of porosity on electrochemical and mechanical properties of composite Li-ion anodes

The electrochemical and mechanical performance of composite anodes for Li + batteries is greatly affected by the matrix porosity. The role of porosity in the retention of the electrochemical capacity and mechanical durability was investigated for composite anodes with polyvinylidene fluoride/acetylene black matrix and graphite or Sn microscale particles. Graphite anodes with porosities between 40% and 50% demonstrated reliable mechanical performance after electrochemical cycling and consistent electrochemical capacity above 45% porosity cycled at C/5 rate. However, graphite anodes with porosities larger than 50% had negligible mechanical strength. The results of the mechanical and electrochemical studies identified an optimum porosity of ∼45% at which the graphite anodes had the highest initial elastic modulus and good strength and extensibility, which also agreed with the properties of the polyvinylidene fluoride/acetylene black matrix for the same porosity. The mechanical performance of Sn anodes, however, was quite inferior to that of graphite, which was largely due to the large volumetric expansion of the Sn particles in the first lithiation cycle.