Abstract
The electrochemical activity of LiNiO2 at the initial cycle and factors affecting its activity were understood. Even though LiNiO2 can achieve almost theoretical charge capacity, it cannot deliver the theoretical discharge capacity that would result in low 1st Coulombic efficiency (CE). For different upper cut-off voltages at 4.3 and 4.1 V, the 1st CE barely increases. Given that the H2-H3 phase transition occurs at ∼4.2 V, the low 1st CE is not caused by this phase transition but is a result of the additional 3.5 V discharge reaction, which is kinetically limited and thereby not activated even at a reasonable current density. We found out that the several phase transitions during charge/discharge in LiNiO2 barely affect the 3.5 V reaction. Under galvanostatic intermittent titration technique (GITT) conditions, LiNiO2 can achieve ∼250 mAh/g of discharge capacity and 100% CE even with the 4.3 V cut-off voltage by fully activating the 3.5 V reaction. Using neutron diffraction and 6Li nuclear magnetic resonance (NMR) measurements, the sluggish kinetics of the 3.5 V reaction can be ascribed to difficult insertion of Li at the end of the discharge because this reaction can be accompanied by the rearrangement of cations or local structure change in the structure. To achieve high discharge capacity in LiNiO2 with the 4.3 V cut-off voltage, this 3.5 V sluggish reaction should be improved. The finding and understanding underlying the mechanism of the electrochemical activity will stimulate further research on high-capacity Ni-rich layered materials for high-performance Li-ion batteries.
Highlights
Many attempts to increase the energy density and reduce the production cost of the Li-ion battery (LIB) have been performed in both business and academic fields since LIB is commercialized
We found out that the low Coulombic efficiency (CE) in the 1st cycle in LiNiO2 is ascribed to the low discharge capacity that is caused by a sluggish additional discharge reaction at ∼3.5 V rather than detrimental phase transitions such as H2−H3 at ∼4.2 V, which leads to a severe volume change
This indicates that the low 1st Coulombic efficiency behavior can be caused by a limited discharge capacity that can be caused by a limited discharge reaction rather than the multiple phase transitions such as H2−H3 occurs at 4.3 V
Summary
Many attempts to increase the energy density and reduce the production cost of the Li-ion battery (LIB) have been performed in both business and academic fields since LIB is commercialized. As the demand for high energy density LIB increases, the conventional LCO material, with a lower practical capacity, has been replaced due to its structure instability after >50% delithiation and the substantial increase of the price of Co.. Ni-based layered materials such as LiNi1 −x−yMnxCoyO2 (NMC) and Li- Ni0.8Co0.15Al0.05O2 (NCA) have been of great interest because they can achieve high reversible capacity by only substituting Co with Ni. To further increase the energy density in Ni-based layered materials, the amount of Ni in the materials gets increased by reducing other transition metals such as Co and Mn because the Ni redox reaction determines the achievable capacity of the electrode materials.. As the Ni content increases in NMC and NCA materials, their electrochemical properties and structural changes via phase transformation behaviors during charge/discharge are increasingly similar to LiNiO2, an end member of Ni-rich electrode materials.. The understanding of the electrochemical activity of LiNiO2 can greatly help solving these problems of Ni-rich layered materials
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