Niobium (Nb)-doped, Li 3 NbO 4 surface-modified LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) cathodes were prepared using solid-phase reactions. These modifications helped in improving the thermal stability and cycling performance of the cathodes. XRD and EDX measurements of the prepared samples confirmed the uniform distribution of Nb, whereas Li 3 NbO 4 was found to occur at the grain boundaries and on the surface of primary NCM622 particles. The thermal stability of the prepared samples was evaluated by measuring the amount of O 2 released from the cathode material during overcharging. This quantification was conducted using a gas chromatography-mass spectroscopy analysis. Decomposition of the NCM622 cathode material was suppressed by Nb-doping. Furthermore, electrochemical tests showed that the Nb-doped, Li 3 NbO 4 surface-modified NCM622 exhibited an excellent cycling performance over 500 cycles in the 3.0-4.1 V voltage range at a current rate of 2 C at 60 °C, during which the sample retained 91.4% of its initial capacity. This capacity retention was much higher than that for both the samples prepared using only Nb doping without Li 3 NbO 4 surface modification (36.8%) and that of undoped NCM622 (70.7%). Our results indicate that Nb doping and Li 3 NbO 4 surface modification are effective for improving the cathode's thermal stability and cycling performance, respectively.
To control the electrochemical properties of LiNi 0.35 Mn 0.30 Co 0.35 O 2 (NMC) acting as a positive electrode material, Ni 0.35 Mn 0.30 Co 0.35 (OH) 2 precursors with different morphologies were synthesized by controlling the dissolved oxygen concentration during coprecipitation. As the dissolved oxygen concentration increases, precursor particles become more porous and have higher specific surface area. X-ray absorption spectroscopy clearly shows that only the Mn valence in the precursors increased with increasing dissolved oxygen concentration. X-ray diffraction patterns of the precursor synthesized under a high dissolved oxygen concentration suggested the formation of oxyhidroxide. The morphology of NMC synthesized using the developed precursors resembled that of the precursors. NMC with dense morphology exhibited high volumetric energy density, while that with porous morphology exhibited a high discharge capacity and rate performance without any cycle performance drawbacks. We expect that this simple method of morphology control by control of precursor dissolved oxygen concentration can be applied to improve the electrochemical properties of positive electrode materials with a wide range of Mn-containing compositions.
Nano-scale Al-rich layers on the surface of LiNi0.92Al0.08O2 and substituted-Al in the crystal suppress both the surface degradation and bulk degradation, resulting in the excellent cycling performance Ni-rich electrode material.
Cobalt-free, nickel-rich positive electrode materials are attracting attention because of their high energy density and low cost, and the ultimate material is LiNiO 2 (LNO). One of the issues of LNO is its poor cycling performance, which needs to be improved. Referring to a current study to show the improved stability of single-crystal-like high-nickelate materials, we fabricated single-crystal-like (SC-) LNO and the counterpart polycrystalline (PC-) LNO samples and examined their electrochemical properties. SC-LNO was nearly single-crystal-like, as proved by electron backscattering diffraction, and had more cation mixing than PC-LNO. Cycle tests under 2.5−4.2 V, a 2C rate, and 45 °C conditions showed that the capacity retention of SC-LNO after 500 cycles (63.5%) was significantly better than that of PC-LNO (36.1%) under the same conditions and even better than that of PC-LNO cycled between 2.5 and 4.15 V (50.7%) with the same initial capacity as SC-LNO. The derivative dQ/dV profile of PC-LNO became featureless during a long cycling time, suggesting the progress of cation mixing in PC-LNO, whereas that of SC-LNO was better maintained, in accordance with the serious particle cracking in PC-LNO and no particle cracking found in SC-LNO as the result of post-mortem analysis after 500 cycles. The electrode impedance increase of PC-LNO was considerably larger than that of SC-LNO, corresponding to the formation of rock-salt phases at the surface and the cracked interface of the PC-LNO and the formation of scattered spinel-like phases with a thick cathode electrolyte interphase at the surface of SC-LNO. Accordingly, SC-LNO is shown to be less degraded in both the bulk nature (stable dQ/dV profile and no cracking) and the surface characteristics (high rate capacity maintenance and less impedance increase), suggesting the importance of single-crystal-like particles as durable electrode materials.
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