Protected High - Capacity Anodes for Li - Ion Battery Applications

The lithium-ion battery (LIB) is a widely used energy storage system with applications spanning from small electronics to automobiles. An important figure of merit for LIBs is specific capacity, typically reported in mAh/g. One way to improve the overall capacity of LIBs is to replace the ubiquitous carbon anode with silicon or tin, which promise specific capacity of 4,000 and 994 mAh/g, respectively, far surpassing graphite (372 mAh/g). The principal issue hindering commercial adoption for Si and Sn anodes is the necessity to expand their crystal structures up to 400% in volume to accommodate the added lithium. Repeated expansion and contraction causes problematic particle fracturing and exposes fresh anode surfaces to the electrolyte, which readily undergoes reduction on the conducting electrodes, producing a passivating film that irreversibly captures Li ions intended for reversible storage. For these reasons, this research effort is aimed at developing a smart dual-coating system featuring a conductive polymer on top of the electrode to capture pulverized anode particles, and a nonconductive polymer interface with the electrolyte to prevent electrolyte reduction and Li trapping. Although Si and Sn pulverization in non-nano-sized particles is practically unavoidable, we previously proposed that conducting polymers can mitigate the negative impact of anode pulverization by protectively coating the anode surface. Using Si as an example, the conducting polymers will behave as an elastic container that can retain the electrical connection between pieces of Si/Sn and the current collector even after the anode crumbles. The electrically wired anode pieces will continue to behave as “active” particles and the battery capacity will not degrade, thus resulting in a durable high capacity battery. Fortunately, large numbers of conducting polymers have been synthesized and characterized for applications in organic electronics. Two representative polymers from this field are poly(3hexylthiophene) (P3HT) and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3fluoro-2-[(2-ethylhexyl)-carbonyl]thi-eno[3,4-b]thiophenediyl]] (PTB7). P3HT and PTB7 are electron donor polymers rigorously characterized and successfully implemented in organic photovoltaics. They are known to have good charge mobility which is considered beneficial for this study. These polymers are also commercially available which enables fast implementation. Thus, P3HT and PTB7 were the focus of our first year’s efforts to discover and implement the conductive polymer segment of our dual-coating concept.