A Study of Relaxation Effects of the Negative Electrode Tisnsb Using 119sn Mössbauer and 7li MAS NMR Spectroscopies
Authors: Johnston Karen , Darwiche Ali , Dupré Nicolas , Sougrati Moulay-Tahar , Stievano Lorenzo , Monconduit Laure , Grey Clare P. 
Published: February 2020

Owing to their high energy density, lithium rechargeable batteries are now considered the technology of choice for electrical energy storage in isolated sites, portable electronic devices and zero emission vehicles. However, in such systems there are often limitations in the energy density of the electrode materials, most commonly caused by weak capacities and limited electrode cycling life. Hence, research is currently underway to find new electrode materials capable of higher performance.

Conversion type materials have recently been considered as a plausible alternative to conventional electrode materials, owing to their large gravimetric and volumetric energy densities. The ternary alloy TiSnSb was recently proposed as being a suitable negative electrode material in Li-ion batteries owing to its high electrochemical performance.1-4 TiSnSb has been shown to reversibly take up more than five lithium per formula unit, leading to reversible capacities of 540 mA h/g or 4070 mA h/cm3at a rate of 2C.

Using complementary in situ operando X-ray diffraction (XRD) and in situ operando 119Sn Mössbauer spectroscopy, it was determined that during the first discharge, TiSnSb undergoes a conversion process leading to the simultaneous formation of Li-Sb and Li-Sn intermetallic compounds and, as a result, the corresponding electrochemical equation was proposed for Li insertion:

TiSnSb + 6.5Li à Ti + Li3Sb + 0.5Li7Sn2

Some ambiguities however remain: A shifted, group of resonances appears on 7Li NMR spectra at approx. 20 ppm in addition to the expected contribution of Li3Sb at 3.5 ppm and a resonance at 8.5 ppm, tentatively assigned to Li7Sn2. The alloy Li7Sn3 has previously been reported at 18 ppm5, hence its presence cannot be ruled out. However, this phase has not been detected via 119Sn Mössbauer spectroscopy in this or any previous studies.

In addition, changes in the local environments of Sn and Li nuclei have been detected upon OCV relaxation after the lithiation process, using 119Sn Mössbauer and 7Li NMR spectroscopies, respectively (Fig. 1). These results suggest an intrinsic instability of the phases formed at the end of the lithiation process and/or the formation of non-stoichiometric phases. 119Sn Mössbauer spectroscopy and 7Li MAS NMR have been combined in order to better understand the phases formed upon discharge and subsequent relaxation of a TiSnSb electrode. Both "in situ" and "ex situ" type experiments have been completed using the two techniques in order to understand the evolution of lithiated alloys during this increase of potential on Mössbauer signal and 7Li NMR shifts. Clearly, an investigation of relaxation using both Mössbauer spectroscopy and NMR is crucial to understand its origin and particularly important if this material is to be used in a practical device.

Figure 1 : Relaxation profile after a C/2 lithiation of TiSnSb. Corresponding evolution of the 7Li NMR resonances and Mössbauer spectroscopy..

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