Understanding the Evolution of the Li-Rich Antifluorite Li5FeO4 upon Electrochemical Delithiation

In order to boost the capacity of the lithium ion battery, cathode materials that can cycle with more than one lithium (Li) ion per transition metal (LM) ion are intensively investigated. The antifluorite family of Li 8 O 4 (4Li 2 O unit cells) is an interesting structure to host high Li/TM ratio since the transition metal ions can substitute part of the tetrahedral Li ions the structure. A representative material is the Li 5 FeO 4 (LFO), with one Fe 3+ ion replacing one tetrahedral Li + ion and creating two vacancies. This material is capable of providing 5 Li/Fe or a capacity at about 867mAh/g in theory. Initial studies of LFO have yielded intriguing results that more than 4Li ions can be extracted at two plateaus at about 3.5 and 4.0V, with capacity higher than 700mAh/g. This super high delithiation capacity makes LFO a promising cathode material for the high energy Li-ion batteries. However, in the first discharge the plateaus decrease to 2.2V and 1.3V with a capacity of about 300mAh/g. In the following cycles, the capacity fades rapidly and the plateaus higher than 3V can’t be recovered. The cycle behavior of LFO shows that the deliathiation reaction of the LFO with the removal of over 4 Li ions is not reversible in conventional Li ion batteries. In order to fulfill the reversible charge-discharge of the LFO,better understanding of the delithation reaction of LFO is required. In this work a series of comprehensive in-situ characterizations of the LFO electrode during the first charge were performed to understand the crystal structure, valance and bonding as well as the electronic structure evolution of LFO upon the Li removal. These studies indicate that the first charging of LFO can be divided into two stages. Stage I is a two-phase reaction in the first charging plateau, where the antiflorite LFO converts to pseudo-cubic Li 3 FeO 4 (Li 5 FeO 4 →2Li + +2e+Li 3 FeO 4 ), and the oxidation state change of Fe and O in Stage I can be expressed as: Fe 3+ →Fe (3+δ)+ +δ e - and 4O 2- →4O (1.5+0.25δ)- +(2-δ)e - . Stage II is a one-phase reaction in the second plateau, with pseudo-cubic Li 3 FeO 4 converting into pseudo-cubic LiFeO 2 (Li 3 FeO 4 →2Li + +2e+O 2 +LiFeO 2 ) and the releasing of one O 2 from the Fe-O framework. The valance evolution Fe and O in Stage II can be expressed as: Fe (3+δ)+ + δ e - → Fe 3+ and 4O (1.5+0.25δ)- →2O 2- +O 2 +(2+δ)e - . Theoretical modeling confirms the formation of pseudo-cubic phase and O 2 releasing in the second stage. High energy barrier was found for conversion from the pseudo-cubic phase back to antiflorite phase with lithiation and the O 2 absorption, explaining the irreversibility of the LFO. Figure 1