Optimization of the Cathode Microstructure in Lithium Air Batteries through Multiscale Physical Modeling

Abstract

The performance of lithium air battery depends not only on the proper choice of cathode and electrolyte materials [1], but also on the geometric and microstructural optimization of the cathode. Physical modeling and numerical simulation afford straightforward ways to investigate possible schemes for performance improvement [2]. Here we report the galvanostatic discharge simulation results of several techniques: (i) a bi-layer cathode consisting of a high specific surface area porous material (e.g. Ketjen black carbon) and a large average pore radius porous material (e.g. Super P carbon); (ii) gradient of porosity across the cathode thickness, where the porosity varies linearly from the separator to the air inlet. In case (i), either the high specific surface material or the large pore radius material is arranged near the air inlet. The performance of the cell mainly depends on the cathode material near the air inlet. In case (ii), allocating larger porosity near the air inlet leads to both a higher cell voltage and a larger discharge capacity. Finally, we propose to insert a porous polymer "oxygen diffusion channel" into the active cathode materials. This channel is free of pore choking, since it is inactive in oxygen reduction reaction due to the absence of conduction electrons. Hence, a compromise may be reached between high surface area and smooth oxygen flow. The possible benefits and drawbacks of this scheme are discussed.[1] M. M. O. Thotiyl, S. A. Freunberger, Z. Peng, Y. Chen, Z. Liu, and P. G. Bruce, Nat. Mater. 12(11), 1050 (2013).[2] A. A. Franco and K.-H. Xue, ECS J. Solid State Sci. Technol. 2(10), M3084 (2013).