Abstract
A family of mixed transition–metal oxides (MTMOs) has great potential for applications as anodes for lithium ion batteries (LIBs). However, the reaction mechanism of MTMOs anodes during lithiation/delithiation is remain unclear. Here, the lithiation/delithiation processes of ZnFe2O4 nanoparticles are observed dynamically using in situ transmission electron microscopy (TEM). Our results suggest that during the first lithiation process the ZnFe2O4 nanoparticles undergo a conversion process and generate a composite structure of 1–3 nm Fe and Zn nanograins within Li2O matrix. During the delithiation process, volume contraction and the conversion of Zn and Fe take place with the disappearance of Li2O, followed by the complete conversion to Fe2O3 and ZnO not the original phase ZnFe2O4. The following cycles are dominated by the full reversible phase conversion between Zn, Fe and ZnO, Fe2O3. The Fe valence evolution during cycles evidenced by electron energy–loss spectroscopy (EELS) techniques also exhibit the reversible conversion between Fe and Fe2O3 after the first lithiation, agreeing well with the in situ TEM results. Such in situ TEM observations provide valuable phenomenological insights into electrochemical reaction of MTMOs, which may help to optimize the composition of anode materials for further improved electrochemical performance.
Highlights
The ever–growing need for high energy density, power density and stable cyclability has prompted considerable attention to develop promising anode materials for lithium ion batteries (LIBs) to meet the rapid development of portable electronics
It is found that upon lithiation the ZnFe2O4 nanoparticle was converted into numerous Fe and Zn nanograins within Li2O matrix with a severe volume expansion
The electrochemical reaction mechanism of ZnFe2O4 for lithium ion battery anode is investigated by in situ transmission electron microscopy (TEM), and the results show that in the first lithiation process lithium-ion is intercalated into ZnFe2O4, generating ultrafine (1–3 nm) Fe and Zn nanocrystallites within Li2O matrix followed by obvious volume expansion
Summary
The ever–growing need for high energy density, power density and stable cyclability has prompted considerable attention to develop promising anode materials for lithium ion batteries (LIBs) to meet the rapid development of portable electronics. The Fe2O3 anode materials have been found to be irreversible in the first lithiation by in situ transmission electron microscopy (TEM); they undergo a reversible phase conversion between FeO and Fe/Li2O during the lithiation-delithiation cycles[13]. It is anticipated that they can effectively overcome the drawbacks of pure iron oxide anode; larger reversible capacity, better cyclability, and better rate performance can be achieved by the suitable combination of different metal species[19]. Chowdari B.V.R. et al.[25] have suggested that the reaction mechanism of ZnFe2O4 is reversible reactions of LiZn to ZnO and Fe to FeO after the first discharge process. Some successes have been achieved on understanding the electrochemical mechanism of SnO230, Si31,32, ZnO33, CeO234, Fe2O313, carbon nanotube (CNT)[35], graphene[36], and Co9S8/CNT37 in real time through the in situ TEM technique. Our in situ TEM results for provided the direct experimental evidence of the reaction mechanism of ZnFe2O4 during lithium-ion insertion and extraction
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