7 Li and 29 Si MAS NMR study of electrochemical performance of silicon nanoparticle electrodes

A. RUDERMAN; E. N. PRIMO; G. L. LUQUE; F. VACA CHAVEZ

Congreso; 7 th International Workshop on Lithium, Industrial Minerals and Energy; 2020

Electrodes with carbon coatings need to be cycled at low voltage regimes in order to achieve fullgraphite lithiation. During the lithiation stage amorphous Li-Si phases turn to crystalline phases [1-6], a process that is associated with a large overpotential on delithiation. This approach differs fromother practical strategies to improve capacity retention that limit the Si cycling regimes to 1,200?1,500 mAh/g, cycling at higher potentials between different a-Li x Si compositions [7]. To use Si toits full potential a deep understanding of the different structural processes that occur, the kinetics ofthe various transformations and how they correlate with capacity retention is required.In this work we carried out ex-situ 7 Li NMR spectroscopy to follow the lithiation and delithiation ofslurries prepared with acid and basic buffers. The approach allows us to follow the structuraltransformations that occur in the 2 nd cycle and beyond involving the nano-structured amorphous Siphase. The ability to control the voltage and current carefully enables an understanding of how thecharge and discharge processes are observed by NMR.Figure 1 shows the first charge-discharge cycle and its corresponding NMR spectra. The specificcapacity at the end of the first lithiation (discharge capacity) is 3950 mAhg −1 and after delithiation(charge capacity) is 3100 mAhg −1 . The irreversible capacity is about 850 mAhg −1 . From our results,it was possible to identify, for example, the inhomogeneous growth of crystalline Li 3.75 Si from theamorphous phase, the electrochemical and NMR signatures of the process involving the over-lithiation of c-Li 3.75 Si that occurs below 50 mV, and the formation of small clusters within theLi 3.75 Si phase on charge.