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

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 LixSn. 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, alpha-Sn is likely precipitated, which is transformed into beta-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.