Insights into the interfacial chemistry and conversion mechanism of iron oxalate toward the reduction by lithium

[Display omitted] •“Organic” layer caused by electrolyte decomposition is reversible distinct with SEI.•Residue of Li2Fe(C2O4)2 is a part of initial irreversible capacity.•“Organic” layer provides more unobstructed and stable channels for Li+.•FeC2O4 suggests an excellent capacitive effect and interfacial lithium storage.•Design interface is a new way on achieving superior lithium storage. The origin of excellent lithium storage ability and high irreversible capacity is probably the least understood component for transition-metal oxalates as anode materials in lithium-ion batteries. Considerable efforts have been put into understanding their electrochemical reaction mechanisms, but these insights have mostly been unilateral and unsystematic. Herein, the interface characteristic between iron oxalate anode and electrolyte and detailed conversion process were investigated to explore the source of irreversible Li+ storage. In particular, a gelatinous “organic” layer identified oxygen, fluorine and phosphorus as the main chemical elements can be re-oxidized and exhibits an obviously reversible conversion between sedimentation and decomposition during its initial lithiation process, despite the general belief that it shows similar electrochemically inert to solid–electrolyte interphase (SEI). Meanwhile, this special interface layer leads to higher ability of Li+ ions diffusion and smaller charge-transfer resistance, which is the vital role for excellent rate capability. Furthermore, ex situ FTIR analysis confirms the formation and residue of new intermediate compound of Li2Fe(C2O4)2, thus making a part of initial irreversible capacity. It is also found that the iron oxalate electrode with larger capacitive contribution still has more widely application in energy storage of supercapacitors in future.