Li/graphite and Li/petroleum coke cells using a 1M LiAsF6 in a 50:50 mixture of propylene carbonate (PC) and ethylene carbonate (EC) electrolyte exhibit irreversible reactions only on the first discharge. These irreversible reactions are associated with electrolyte decomposition and cause the formation of a passivating film or solid electrolyte interphase on the surface of the carbon. The amount of electrolyte decomposition is proportional to the specific surface area of the carbon electrode. When all the available surface area is coated with the film of decomposition products, further decomposition reactions stop. In subsequent cycles, these cells exhibit excellent reversibility and can be cycled without capacity loss.

No abstract

Rechargeable cells can be made using two different intercalation compounds, in which the chemical potential of the intercalant differs by several eV, for the electrodes. We discuss the factors that play a role in the selection of appropriate lithium intercalation compounds for such cells. For ease of cell assembly the cathode should be stable in air when it is fully intercalated, like LiNiO2. For the anode, the chemical potential of the intercalated Li should be close to that of Li metal, like it is in Li=Cs. We discuss the intercalation of Li in LiNiO2 and then in petroleum coke. Then, we show that LiNiO2/coke cells have high energy density, long cycle life, excellent high-temperature performance, low self-discharge rates, can be repeatedly discharged to zero volts without damage, and are easily fabricated. In our opinion this type of cell shows far more promise for widespread applications than traditional secondary Li cells using metallic Li anodes.

We report studies of lithium insertion in a variety of organic precursors pyrolyzed below 1000~ These include two types of petroleum pitch, polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polypheny]ene sulfide (PPS), and epoxy novolac resin (ENR). Each of these materials shows reversible specific capacities for lithium of between 550 and 900 mAh/g when treated near 700~ The majority of this high capacity shows large hysteresis; that is the lithium is inserted near 0 V (vs. lithium metal) and removed near 1 V. Furthermore, the amount of capacity showing large hysteresis is well correlated to the hydrogen to carbon atomic ratio of the samples. As the pitch, PVC and PVDF samples are heated above 700~ the H/C ratio and the specific capacity decrease proportionally, suggesting that the hydrogen in these samples plays an important role. The PPS and ENR samples behave differently; as they are heated above 700~ the high capacity is maintained as H/C is reduced, although the voltage profile changes dramatically to one without significant hysteresis and with a plateau of several hundred mAh/g near 0 V. We believe that structural differences associated with the presence of single carbon layers in the ENR and PPS samples account for the difference between their behavior and that of the other samples. The materials made from ENR appear to be interesting candidates for high capacity anodes for lithium ion cells.