<i>In situ</i> observation of lithiation mechanism of SnO₂ nanoparticles

Tin oxide (SnO 2 ) has attracted a lot of attention among lithium ion battery anode materials due to its rich reserves, high theoretical capacity, and safe potential. However, the mechanism of the SnO 2 nano materials in the lithiation-delithiation reaction, especially whether the first-step conversion reaction is reversible, is still controversial. In this paper, SnO 2 nanoparticles with an average particle size of 4.4 nm are successfully prepared via a simple hydrothermal method. A nanosized lithium ion battery that enables the in situ electrochemical experiments of SnO 2 nanoparticles is constructed to investigate the electrochemical behavior of SnO 2 in lithiation-delithiation process. Briefly, the nanosized electrochemical cell consists of a SnO 2 working electrode, a metal lithium (Li) counter electrode on a sharp tungsten probe, and a solid electrolyte of lithium oxide (Li 2 O) layer naturally grown on the surface of metal Li. Then, the whole lithiation-delithiation process of SnO 2 nanocrystals is tracked in real time. When a constant potential of –2 V is applied to the SnO 2 with respect to lithium, lithium ions begin to diffuse from one side of the nanoparticles, which is in contact with the Li/Li 2 O layer, and gradually propagate to the other side. Upon the lithiation, a two-step conversion reaction mechanism is revealed: SnO 2 is first converted into intermediate phase of Sn with an average diameter of 4.2 nm which is then further converted into Li 22 Sn 5 . Upon the delithiation, a potential of 2 V is applied and Li 22 Sn 5 phase can be reconverted into SnO 2 phase when completely delithiated. It is because the interfaces and grain boundaries of nano-sized SnO 2 may impede the Sn diffusing from one grain into another during lithiation/delithiation and then suppress the coarsening of Sn, and enable the Li 2 O and Sn to be sufficiently contacted with each other and then converted into SnO 2 . This work provides a valuable insight into an understanding of phase evolution in the lithiation-delithiation process of SnO 2 and the results are of great significance for improving the reversible capacity and cycle performance of lithium ion batteries with SnO 2 electrodes.