Ni-rich LiNi_xMn_yCo_zO₂ (NMCxyz, x > 0.6) cathode materials show high energy density and low material cost for lithium-ion batteries, but the Ni-rich NMC cathodes experience severe capacity degradation. Electrochemically-induced intergranular cracking within the polycrystalline (PC) particles of Ni-rich NMC has been proved as the main degradation cause. The intergranular cracking can be eliminated in the form of single-crystalline (SC) particles, and particle integrity can be preserved. Figure a shows that single-crystalline particles of NMC811 show superior capacity retention than polycrystalline particles of NMC811. Surprisingly, the larger ~10 µm polycrystalline particles have clearly higher C-rate capability than the smaller ~3 µm single-crystalline particles (Figure b); however, the rate-performance of single-crystalline and polycrystalline cathode particles is various and controversial in many reports. For the first time, the electrochemical kinetics of single-crystalline and polycrystalline cathode particles at the composite-electrode and single-particle scales are measured as a function of state-of-charge by using the three conventional techniques of electrochemical impedance spectroscopy (EIS), potentiostatic intermittent titration (PITT) and galvanostatic intermittent titration (GITT), and interpreted with the aid of concurrent focused ion beam and scanning electron microscope (FIB+SEM) of the evolving particle microstructure. Electrochemical kinetics of polycrystalline cathode particles are found to increase by two orders of magnitude caused by particle intergranular fracture. Using the measured bulk and interface kinetic parameters, C-rate capability is modeled for elucidating the rate-performance of single-crystalline and polycrystalline particles; furthermore, desired particle sizes are suggested for obtaining high durable and high rate-performance of single-crystalline Ni-rich NMC.

Figure. (a) cycling performance at 0.5 <i>C</i> and (b) <i>C</i>-rate capability of single-crystalline and polycrystalline particles of NMC811 cathodes.

Figure. (a) cycling performance at 0.5 C and (b) C-rate capability of single-crystalline and polycrystalline particles of NMC811 cathodes.

Chien Tseng

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