Factors determining the packing-limitation of active materials in the composite electrode of lithium-ion batteries

The factors limiting the capacity of highly dense electrodes are elucidated by using composite electrodes for lithium-ion batteries, which consist of active materials LiNi_1/3Co_1/3Mn_1/3O₂ (NCM), conductive additives and binders. Electrochemical tests of such 100-μm-thick electrodes in 1 M LiPF₆ electrolyte indicate that a highly dense electrode (with 18% porosity) shows a capacity density significantly lower than the other sparse electrodes on 1C charging/discharging. Detailed analysis using position sensitive in situ X-ray diffraction indicates that, unlike the other sparse electrodes, NCM on the current-collector side barely functions for this dense electrode, due to the poor accessibility of Li⁺ to the inner part of the electrode. Interestingly, 2 M and 0.3 M electrolytes promote the discharge and charge reactions, respectively, over the entire area of the electrode, although they exhibit lower conductivity than the 1 M electrolyte, which indicates the importance of the initial amount of Li⁺ in the electrolyte impregnated in the electrode pores. Thus, for a high-energy-density cell, the initial amount of Li⁺ and the Li⁺ transport significantly affect the rate capability, which governs the practical capacity of the cell under constant-current operation.