Abstract
Conversion and alloying type negative electrodes attracted huge attention in the present research on lithium/sodium-ion batteries (LIBs/SIBs) due to the high capacity delivered. Among these, SnO2 is investigated intensively in LIBs due to high cyclability, low reaction potential, cost-effectiveness, and environmental friendliness. Most of the LIB electrodes are explored in SIBs too due to expected similar electrochemical performance. Though several LIB negative electrode materials successfully worked in SIBs, bare SnO2 shows very poor electrochemical performance in SIB. The reason for this difference is investigated here through combined in operando and ex situ X-ray absorption spectroscopy (XAS). For this, the electrodes of SnO2 (space group P42/mnm synthesized via one-pot hydrothermal method) were cycled in Na-ion and Li-ion half-cells. The Na/SnO2 half-cell delivered a much lower discharge capacity than the Li/SnO2 half-cell. In addition, higher irreversibility was observed for Na/SnO2 half-cell during electrochemical investigations compared to that for Li/SnO2 half-cell. In operando XAS investigations on the Na/SnO2 half-cell confirms incomplete conversion and alloying reactions in the Na/SnO2 half-cell, resulting in poor electrochemical performance. The difference in the lithiation and sodiation mechanisms of SnO2 is discussed in detail.
Highlights
While intercalation-type materials are intensively used as positive electrodes in lithium-ion batteries, the search for highcapacity negative electrodes focuses on conversion-type materials
The investigated bare SnO2 was synthesized through a one-pot hydrothermal route and is thoroughly characterized using X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical methods
During Na extraction in the following charging, the XANES spectra were characterized by a slight increase in the white line intensity as well as a small shift to higher energy when compared with the end of discharge (EOD) state
Summary
While intercalation-type materials are intensively used as positive electrodes in lithium-ion batteries, the search for highcapacity negative electrodes focuses on conversion-type materials. The slow kinetics of the reaction from Na−Sn alloys into intermetallic Na15Sn4 and the formation of intermediate crystalline SnO during Na2O to SnO2 conversion that blocks the Na+ diffusion into metallic Sn are proposed as some of the reasons behind the low capacity obtained for the Na/SnO2 cell.[28] a systematic comparison of the sodiation- and lithiation-related phase changes reveals fundamental microstructural causes for the lower sodiation capacity than lithiation in SnO2.28 Different from the irreversible nature of sodiation, a fully reversible alloying and conversion reaction was observed for the Li/SnO2 cell using X-ray diffraction (XRD).[28] the presence of some amount of amorphous residual material, that did not react back to the initial state after discharging, below the detection limit of the XRD cannot be ruled out at the end of the cycle. The investigated bare SnO2 was synthesized through a one-pot hydrothermal route and is thoroughly characterized using X-ray diffraction, scanning electron microscopy (SEM), TEM, and electrochemical methods
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