Optimal annealing of Al foil anode for prelithiation and full-cell cycling in Li-ion battery: The role of grain boundaries in lithiation/delithiation ductility

While self-supporting Al foil anode has high theoretical capacity and appropriate potential for Li-ion batteries, the poor initial Coulombic efficiency (ICE) retards its wide industrial applications, making prelithiation necessary. Herein, we develop a facile and efficient mechanical prelithiation (MP) approach suitable for mass production. Even though ICE was improved from 70% to 90% after MP, the cycling performance is still deficient, which we attribute to the insufficient density of grain boundaries that leads to poor lithiation/delithiation ductility (LDD), characterized by the rapid loss in electronic percolation across the foil. As a classic strategy in metallurgy to tune the grain boundaries, thermal annealing and recrystallization is further exploited to optimize the LDD of Al foil. For standard Al foil, whose grain size is big with lots of dislocations inside, MP followed by electrochemical cycling causes cracking, electrolyte infiltration and SEI formation. Its electronic percolation (damage) is monitored in real time, and is found to decay rapidly as the insulating SEI blocked all the electron transport paths of one grain (or a cluster of grains). But if MP takes place after annealing standard Al foil at 300 degrees C for 15min, the dislocations recombined to achieve refined grains whose sharp GB network can slide effectively to relieve stress buildup due to phase transitions and suppress deep crack development, and the prelithiated foil achieved superior electrochemical performance, with significant life extension in the in-plane electronic conductance. However, interestingly, upon further increase in the annealing temperature, as the grain size turns bigger, the electrochemical performance of prelithiated foil deteriorates. Based on the comparison of the microstructures and electrochemical performance at different thermal annealing conditions, we found that the GB density before MP plays an essential role in the stress relief and keeping electronic percolation during cycling. Our optimized prelithiated Al foil maintains 80% capacity retention after 100 cycles in LiFePO4//LiAly full cell with 1.8 x excess lithium. It cycles significantly better than pure LiBcc foil of equal thickness (65 mu m), especially in lean-electrolyte condition, suggesting such safe, cheap and high energy density metallic foil anodes deserve comparable attention as pure lithium metal anode.