In Operando Mechanistic Elucidation of Graphite-Silicon Anode Interfaced with Amorphous Carbon for Advanced Lithium-Ion Batteries
Authors: Parekh Mihit H. , Sediako Anton , Naseri Ali , Thomson Murray , Pol Vilas G. 
Published: December 2020

There has always been a huge thrust in developing, inventing, or fabricating new materials for the anode, having high capacity with long cycle stability. After the failure of the Holy Grail lithium metal anode, in the 1990s, graphite captured the lithium-ion batteries (LIBs) market and has been its mainstay for the past three decades. However, because of limited theoretical capacity (372 mAh g−1), alloy materials like Ge, Si, Sb and Sn garnered a great amount of attention owing to their high extravagant theoretical capacity. Amongst all, silicon is considered the likely promising candidate for the next generation LIBs, however significant challenges (e.g. electrode pulverization, volume expansion, particle fragmentation, excessive solid electrolyte interface formation etc.) need to be addressed for achieving long stable cycle life. Different approaches to overcome the aforementioned issues have been tried for better handling of stress built up on account of volume changes, which include designing nanostructures, or nanocomposites. Despite promising electrochemistry performance, critical problems need to be addressed viz., uneconomical and complex processes, accelerated side reactions due to the high surface area, low tap density, and poor scalability.

To tackle these complex issues, we designed a novel silicon-graphite anode interfaced with amorphous carbon derived from inexpensive starch. The presence of amorphous carbon helps accommodate the volume expansion along with enhanced electrical conductivity between Si nanoparticles and graphite particles. The synthesis process for silicon composite (GCSi) composite utilizes a pyrolysis technique, which can be easily scaled up, in an inert environment by homogeneously ball-milling Si-NPs, starch powder, and graphite. The fabricated composite material formation was mechanistically elucidated utilizing in-situ high-resolution environmental transmission electron microscopy (ETEM) as a function of temperature. Temperature ramping up was conducted from 40°C to 500°C at 0.2°C sec-1 with image-capturing every 5 minutes. The dehydrogenation and carbonization of the starch started around at 400℃ and finished about 500℃. With the homogenization of starch particles, they appeared to merge with the graphite. The inner structural homogenization and transition from coarser particles to the uniform continuum of surface further concurred from the BF images of the composite cross-section and the Secondary Electron recording, respectively.

The tailored GCSi composite architecture comprising of 25 wt% Si-NPs delivered a high initial discharge capacity of 1126 mAh g−1 with 83% initial coulombic efficiency, while retaining 448 mAh g−1 specific capacity after 100 cycles, cycled at 500 mA g-1. Safety aspects of the GCSi electrode were observed using Multiple Module Calorimetry (MMC), whose unique capabilities allow for the in situ measurement of heat changes during electrochemical reactions of the cells and the study of thermal runaway events. For the measurement, the full cells (with lithium cobalt oxide cathode) were charged from room temperature to 300°C at 0.5°C min-1 rate and the thermal signatures were analyzed. Comparing the energy released during thermal runaway, per specific capacity of the full cell, LCO – GCSi cell released 20.89 kJ Ah-1, which is slightly lesser than 21.56 kJ Ah-1 of LCO – graphite cell. It appears that the novel silicon composite may be slightly safer to use as anode rather than graphite.