Relaxation Effects of the Negative Electrode Tisnsb Using 119sn Mössbauer and 7li MAS NMR Spectroscopies
Authors: Dupre Nicolas , Johnston Karen , Darwiche Ali , Stievano Lorenzo , Guyomard Dominique , Grey Clare P. , Monconduit Laure 
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 strong 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. 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/cm3 at 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 --> 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 a 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 ppm, hence its presence cannot be ruled out. However, this phase has not been detected via 119Sn Mössbauer spectroscopy in this or any previous studies. Distinct differences in chemical shift have been observed for Li3Sb produced at the end of discharge of TiSnSb vs. samples of Li3Sb produced via solid state methods and during the discharge of other Sn and/or Sb alloys. It seems that the nature of the other elements present at the end of lithiation plays a key role in changing the 7Li chemical shift of Li3Sb. To confirm whether this is the case, additional model compounds have been studied via NMR, including TiSb2 and NbSb2. Positive NMR shifts are observed, confirming the influence of the“inactive” elements of ternary alloys such as Ti or Nb. The ternary alloy NbSnSb was also investigated as a direct comparison to TiSnSb and to establish the influence of the nature of the inactive metal on the 7Li NMR shift. The 7Li NMR spectra obtained for both materials at the end of electrochemical lithiation are very similar, with two groups of resonances at approximately 3 and 20 ppm, respectively. In the Ti-based material, both groups of resonances are clearly shifted towards lower frequencies. This result shows that 7Li NMR is sensitive to the chemical or electronic environment around the Li3Sb phase or clusters and not only to the direct local environment (Li3Sb).

The additional study of rough mixtures (not real alloys obtained through ball-milling syntheses) of Ti-Sn-Sb and Sn-Sb indicate that after electrochemical lithiation, the characteristic resonance of Li3Sb at −8 ppm is present . This result indicates that the intimate mixing of elements, achieved in the alloys, influences the overall electronic properties of the active material, therefore modifying the observed NMR shift of Li3Sb.

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. 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. Ex situ
7Li NMR indicates that this evolution is stopped or at least slowed down when the active material is in contact with the electrolyte. Both "in situ" and "ex situ" type experiments have been completed using the two techniques in order to understand the influence of small changes in composition on Mössbauer signal and 7Li NMR shifts. A systematic study using both Mössbauer spectroscopy and NMR the phases formed during discharge and subsequent relaxation will be presented and discussed.