Common Battery Anode Testing Protocols Are Not Suitable for New Combined Alloying and Conversion Materials

Abstract

Here we report an interesting observation on anode materials for lithium ion batteries that undergo combined conversion and alloying lithiation processes during cycling (CAMs). These materials are generating interest as low cost and high capacity alternatives to graphite. We find that common testing protocols (CTPs) are unsuitable for assessment of CAMs due to their distinct multi‐step lithiation characteristics. CTPs involve reporting total gravimetric capacity in a half‐cell configuration alone (opposite Li foil), without individual analysis of each process; energy density and the problems associated with wide discharge voltages are not addressed. Through isolating the individual lithiation processes of a model system (Cu2ZnSnS4), we determine that the conversion processes are highly unstable, whereas the alloying processes exhibit remarkable capacity retention. We demonstrate that inclusion of the conversion processes in cycling actually reduced full cell energy density when compared with alloying alone. This indicates that CTPs may well underestimate the stability of CAMs. It is apparent that the true advantage of CAMs lies in the synergistic combination of the capacity of the alloying portion, and the stability provided by the uncycling Li2S buffer material. Finally, we prescribe a set of testing protocols for a meaningful assessment of new CAMs.

Electrophoretic Deposition of Tin Sulfide Nanocubes as High‐Performance Lithium‐Ion Battery Anodes

“…[39] However, most reports perform extended cycling of tin sulfide over a wide voltage range e. g. 0-2.5/3 V vs Li/Li + . [33,36,[43][44][45] This typically achieves the highest possible initial capacity by including both processes, [46,47,48] however given the irreversibility of the conversion process (Equation 1), the benefit of its inclusion is often short-lived or even limited to a single cycle. [36,49,50] To our knowledge, no examinations into how cycling the Li 2 S material affects its role as a buffer have been performed.…”