Factors Affecting the Volumetric Energy Density of Lithium-Ion Battery Materials: Particle Density Measurements and Cross-Sectional Observations of Layered LiCo_1–<i>x</i>Ni_<i>x</i>O₂ with 0 ≤ <i>x</i> ≤ 1

Volumetric capacity <i>Q</i>_vol (mAh cm^–3), more correctly, volumetric energy density <i>W</i>_vol (mWh cm^–3), is a crucial property of lithium-ion battery (LIB) materials, because LIBs are devices that operate in a limited space. The actual value of <i>W</i>_vol (<i>W</i>_vol^act) is currently limited to 40–60% of the maximum (theoretical) value of <i>W</i>_vol (<i>W</i>_vol^max), for reasons that have not yet been fully clarified. Thus, to gain information that will enable an increase in <i>W</i>_vol^act such that it is closer to <i>W</i>_vol^max, systematic studies of the values for <i>Q</i>_vol, <i>W</i>_vol, true density (<i>d</i>_XRD), and particle density (<i>d</i>_p) obtained using gas pycnometry were undertaken for LiCo_1–<i>x</i>Ni_<i>x</i>O₂ samples with 0 ≤ <i>x</i> ≤ 1. Here, <i>d</i>_p is the density that includes the volume of the closed pores in the particles, and consequently is less than <i>d</i>_XRD, which is determined by X-ray diffraction (XRD) measurement. <i>D</i>_XRD monotonically decreased from 5.062(1) g cm^–3 for <i>x</i> = 0 to 4.779(1) g cm^–3 for <i>x</i> = 1, as expected. On the contrary, <i>d</i>_p decreased almost linearly from 4.98(2) g cm^–3 for <i>x</i> = 0 to 4.80(2) g cm^–3 for <i>x</i> = 0.5, then suddenly dropped to 4.63(2) g cm^–3 for <i>x</i> = 0.667, and finally leveled off to a constant value (∼4.6 g cm^–3) at larger values of <i>x</i>. The cross-sectional observations using a Focused Ion Beam system revealed that the significantly smaller values for <i>d</i>_p compared with those for <i>d</i>_XRD, particularly when <i>x</i> > 0.5, is due to the presence of closed pores in agglomerated secondary particles. This indicates that the closed pores in the secondary particles play an important role in determining the value of <i>W</i>_vol^act for LIBs. The formation of well-developed primary particles as a mean for increasing the value of <i>d</i>_p was also investigated.