Understanding the electrochemical reaction mechanism to achieve excellent performance of the conversion-alloying Zn2SnO4 anode for Li-ion batteries

Abstract

Conversion-alloying-type compounds offer many new possibilities as anode materials for Li-ion cells, but also show some drawbacks related to their intrinsic characteristics. To overcome those limitations, the consecutive steps of electrochemical processes occurring in the selected Zn₂SnO₄ inverse spinel are studied in detail i.e. by operando X-ray diffraction, ex situ X-ray absorption spectroscopy, and transmission electron microscopy. Decomposition of the spinel phase upon the 1st lithiation proceeds with the precipitation of Li₂O and metallic particles, which further form LiZn and Li_xSn. A multicomponent matrix is created, which allows for reversible lithium storage. After dealloying upon the 1st delithiation, a Li₆ZnO₄/Li₄ZnO₃-type phase is formed in the conversion reaction, indicating reactivity between ZnO and Li₂O. Initially, α-Sn is likely precipitated, which is transformed into β-Sn during the 2nd cycle, indicating aggregation. Understanding the electrochemical reaction mechanism allowed identifying essential issues, important for the practical application of the Zn₂SnO₄ anode: too high reaction voltage vs. Li⁺/Li with large hysteresis during the conversion reaction; metallic particle aggregation; large volume changes during deep (de-)alloying; mechanical problems on prolonged cycling. While the full-range capacity of the developed anodes reaches 920 mA h g⁻¹ after 10 cycles at a current of 50 mA g⁻¹ and over 460 mA h g⁻¹ at 1000 mA g⁻¹, their operation range has to be limited in order to overcome the listed problems. This can be achieved by the controlled electrochemical prelithiation of Zn₂SnO₄ anodes before assembling full-cells. For the first time, excellent cycling stability is reached for micrometer-sized solid-state-synthesized Zn₂SnO₄, working in full Li-ion cells.