![]() ![]() 4), while 50% of Fe was reported in lithiated FeO xF 2 − x (0.4 < x < 0.7) 38. The first-principles calculations and comprehensive characterizations revealed that the less-reversible conversion reaction was entirely prohibited by reducing the potential f and the reversibility of the extrusion reaction II was significantly enhanced by formation of non-stoichiometric rocksalt phase with only <5% of Fe(Co) phase in lithiated Fe 0.9Co 0.1OF after 100 cycles (Eq. ![]() At a charge/discharge current of 500 mA g −1, the Fe 0.9Co 0.1OF can deliver a capacity of 350 mAh g − cycles. In this work, we report that such a high-performance Fe 0.9Co 0.1OF cathode with high energy density of ~1000 Wh kg −1 and long cycle life of 1000 cycles can be realized by a cost-effective and simple strategy of concerted doping Co/O in FeF 3. However, it will be plausible to suppress the conversion reaction by extending the capacity of highly reversible intercalation-extrusion reaction, thus achieving both high capacity and long cycle life. In recent years, the critical issues of high potential hysteresis (>1 V), poor rate capability, and limited cycling stability of conversion electrodes have been believed to be intrinsic nature of the conversion reaction chemistry, and the hope of using conversion reaction materials in the next-generation lithium batteries waned. However, the reaction kinetics, cycle life, and round-trip efficiency of FeO xF 2− x (0.4 < x < 0.7) still are far less satisfactory when compared to intercalation cathodes due to the existence of less-reversible conversion reaction. Since the less-reversible conversion reaction was partially replaced by a highly reversible extrusion reaction in FeO xF 2− x (0.4 < x < 0.7), electrochemical performance was significantly enhanced (Eq. The lithiation of FeO xF 2− x experiences intercalation-extrusion-conversion reaction pathway. Here the parent phase is the defected rocksalt phase. On recharge, these precipitated phases go into the parent phase with the Li + pumped out. During cell discharge, the incoming Li + enter the parent phase and prompt the precipitation of metal and LiF phase. Partial substitution of fluorine with oxygen (FeO xF 2− x (0.4 < x < 0.7)) enable the formation of an intermediate rocksalt phase through a reversible extrusion reaction before the conversion reaction of rocksalt phase. The co-substitution strategy to tune the thermodynamic features of the reactions could be extended to other high energy conversion materials for improved performance.Īlthough extensive researches have been conducted, only limited performance improvement was achieved due to several intrinsic issues of conversion reaction: (1) a severe voltage hysteresis (~1.3 V) between the lithiation and delithiation processes, thus a low round-trip energy efficiency of <60% was always observed 6, 7, 8, due to the slow phase separation and repeated breaking/reforming of Fe–F bonds in each conversion reaction cycle 9, 10 (2) sluggish conversion reaction kinetics, which is caused by both poor electronic conduction, low ion diffusivity in FeF 3 8, and the slow conversion phase transition, leads to extremely poor rate capabilities 7, 11, 12, 13 (3) the aggregation and continuous coarsening of Fe nanoparticles 13, 14 during repeated conversion reaction cycles, and sustained reactions of Fe with electrolytes 15, 16, result in the rapid capacity decay during cycling 4, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30. The anion’s and cation’s co-substitutions thermodynamically reduce conversion reaction potential and shift the reaction from less-reversible intercalation-conversion reaction in iron fluoride to a highly reversible intercalation-extrusion reaction in doped material. ![]() In the doped nanorods, an energy density of ~1000 Wh kg − 1 with a decay rate of 0.03% per cycle is achieved. Here we report that both a high reversibility over 1000 cycles and a high capacity of 420 mAh g −1 can be realized by concerted doping of cobalt and oxygen into iron fluoride. However, poor electrochemical reversibility due to repeated breaking/reformation of metal fluoride bonds poses a grand challenge for its practical application. Iron fluoride, an intercalation-conversion cathode for lithium ion batteries, promises a high theoretical energy density of 1922 Wh kg –1. ![]()
0 Comments
Leave a Reply. |