Mixed Metal Difluorides as High Capacity Conversion-Type Cathodes: Impact of Composition on Stability and Performance

With the most recent development of ultrahigh capacity anodes, such as Li- or Si-based anodes, metal fluorides hold promise as complementary high-capacity conversion cathode materials for next-generation energy storage devices. Despite their higher theoretical energy density compared to cells with sulfur cathodes, these materials have received dramatically less attention and little is understood about the origins of their electrochemical behavior. Here, the successful methodology to produce highly uniform size-controlled mixed metal difluoride nanocomposites is reported. It is discovered that such materials undergo reduction in a single step with a reduction potential intermediate to those for the corresponding single-metal difluorides and that a solid solution is reformed upon charging, which is advantageous for practical applications. For the first time the progressive formation of metal trifluorides upon repeated cycling of difluorides is reported. Systematic electrochemical measurements in combination with postmortem analyses lead to the conclusion that the cathode stability strongly depends on the ability to prevent formation and growth of a resistive cathode solid electrolyte interphase, which, in turn, strongly depends on the metal composition. This methodology and new findings will help to elucidate a path to developing metal fluoride-based commercial Li-ion batteries and provide guidelines for material selection.