(Invited) Discovery of New Capacity Fade Mechanism in Lixsn Batteries with Derivative Operando (dOp) NMR Spectroscopy

Abstract

Tin, silicon and other intermetallic forming electrodes have large volumetric lithium capacities—nearly 10 times that of graphite. Such a high capacity combined with their low cost make intermetallics promising materials for the next generation of rechargeable batteries. Unfortunately, severe capacity fade remains the greatest obstacle in their commercial use. For example, in 2005 Sony introduced their NEXELION battery technology using nanostructured SnCo electrodes but eventually had to pull these batteries from the market due to substantial capacity loss with cycling. While a major reason for capacity fade in these electrodes is thought to be cracking from large volume changes during transitions between intermetallic phases, little evidence for cracking in nanostructured electrodes has been found; and the mechanism for capacity fade in nanostructured intermetallic electrodes has remained a mystery. Using derivative operando (dOp) 7Li NMR we have discovered a mechanism for capacity fade in LixSn nanoparticle electrodes that may solve this mystery. In short, the capacity fade in nanoparticle electrodes is the result of nanopar- ticles losing contact with the carbon/PVDF binder. Yet, it occurs in a peculiar way which causes the cell to reach full capacity at the end of every cycle on lithiation. In other words, capacity fade only occurs during delithiation. We present clear NMR evidence showing that particle disconnection occurs during the delithiation (volume contraction of the electrode particles) process. During lithiation (volume expansion of the electrode particles) our NMR data further reveals that connected particles are surprisingly pushed back into contact with the disconnected particles which then rejoin the circuit and allow the cell to reach full capacity on lithiation. Our dOp 7Li NMR spectra also reveal the structural transformation pathways between Li-Sn intermetallics during lithiation and delithiation of Sn nanoparticles. The disconnecting particles are associated with LixSn phases undergoing large decreases in diameters on delithiation, i.e., Sn, Li2Sn5, LiSn, and Li7Sn3. The Li–Sn phases observed are somewhat consistent with the structural evolution expected from the equilibrium binary phase diagram, but there are some notable exceptions with the observation of a metastable phase Li2Sn3, and two vacancy rich metastable phases, Li7−ζSn3, and Li13−δSn5 during delithiation. Although this new capacity fade mechanism is simple, we know no in-situ microscopy method capable of detecting and following such a disconnection-connection mechanism. Our approach clearly show the power of operando NMR spectroscopy for revealing such fundamental processes. These results point the way for future efforts in both industrial and academic research labs to reduce capacity fade in a wider range of nano-structured intermetallic electrodes, involving Si, Ge and Sb. Figure 1