Studies of the Effect of Triphenyl Phosphate on the Negative Electrode of Li-Ion Cells

The effects of the flame retardant, triphenyl phosphate (TPP), on the electrochemical properties of negative electrode materials for Li-ion batteries were studied. Petroleum coke and graphite were chosen for study. Coke/Li half cells with different concentrations of TPP were studied with a home-made automated storage system. The reactivity of intercalated lithium with TPP-containing electrolyte could be inferred from the potential-time graphs of the coke/Li cells during storage. Graphite/graphite symmetric cells with different concentrations of TPP were constructed and tested. Capacity loss in symmetric graphite/graphite cells is caused by reactions of intercalated lithium with electrolyte. The storage test results showed that coke/Li cells with TPP (10%–40% by volume) have similar rates of Li loss to electrolyte reactions at 30°C compared to control cells and show significantly lower rates at 60°C. Graphite/graphite symmetric cells with TPP (10%–40%) have capacities that decrease with TPP content, but show similar rates of% capacity loss per cycle. The impedance of the symmetric cells increases with TPP content. All the results together suggest that TPP forms a stable SEI but a SEI that impedes the transport of Li.