Lithiation and Delithiation Mechanism of Zn 2 SnO 4 -Based Conversion-Type Negative Electrodes for Lithium-Ion Batteries

We unravel the lithiation/delithiation mechanism of Zn 2 SnO 4 as a conversion-type negative electrode material in a lithium-ion battery cell by rigorous phase identification via ex situ 7 Li magic-angle spinning (MAS) NMR, static 119 Sn WCPMG NMR, and operando X-ray diffraction (XRD) techniques. With ongoing lithiation/delithiation, a cascade of Li x Zn phases is observed, and the 7 Li shift for the formed Li x Zn phases is reported for the first time. Our results show that Zn undergoes an alloying/dealloying-type reaction during electrochemical lithiation/delithiation according to x Li + Zn ↔ Li x Zn (0 ≤ x ≤ 1). However, considering the Sn alloying/dealloying reaction, the 119 Sn WCPMG NMR results indicate that the formed Li x Sn species differ from those expected for the lithiation of metallic Sn; hence, the alloying/dealloying processes differ from those known for Sn: x Li + Sn ↔ Li x Sn (0 ≤ x ≤ 4.4), indicating a more complicated conversion mechanism for Zn 2 SnO 4 . Furthermore, the 119 Sn WCPMG NMR data of the delithiation reaction reveal the formation of amorphous SnO caused by partial oxidation of reformed Sn.