Factors Affecting the Volumetric Energy Density of Lithium-Ion Battery Materials: Particle Density Measurements and Cross-Sectional Observations of Layered LiCo_1–xNi_xO₂ with 0 ≤ x ≤ 1

Volumetric capacity Q_vol (mAh cm^–3), more correctly, volumetric energy density W_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 W_vol (W_vol^act) is currently limited to 40–60% of the maximum (theoretical) value of W_vol (W_vol^max), for reasons that have not yet been fully clarified. Thus, to gain information that will enable an increase in W_vol^act such that it is closer to W_vol^max, systematic studies of the values for Q_vol, W_vol, true density (d_XRD), and particle density (d_p) obtained using gas pycnometry were undertaken for LiCo_1–xNi_xO₂ samples with 0 ≤ x ≤ 1. Here, d_p is the density that includes the volume of the closed pores in the particles, and consequently is less than d_XRD, which is determined by X-ray diffraction (XRD) measurement. D_XRD monotonically decreased from 5.062(1) g cm^–3 for x = 0 to 4.779(1) g cm^–3 for x = 1, as expected. On the contrary, d_p decreased almost linearly from 4.98(2) g cm^–3 for x = 0 to 4.80(2) g cm^–3 for x = 0.5, then suddenly dropped to 4.63(2) g cm^–3 for x = 0.667, and finally leveled off to a constant value (∼4.6 g cm^–3) at larger values of x. The cross-sectional observations using a Focused Ion Beam system revealed that the significantly smaller values for d_p compared with those for d_XRD, particularly when x > 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 W_vol^act for LIBs. The formation of well-developed primary particles as a mean for increasing the value of d_p was also investigated.