Synthesis and Electrochemical Performances of Ni-Rich LiNixAl1 –XO2 (x=0.95, 0.92) Positive Electrode Materials for Li-Ion Batteries

Abstract

Lithium-ion batteries (LIBs) have been put to practical use as power sources for portable devices and environmentally benign electric vehicles (EVs) because of their excellent electrochemical properties such as high specific energy, high power, and long cycle life. For the widespread use of these devices, it is necessary to improve energy density and reduce the cost. To realize high-energy density and low-cost positive electrode materials, we focused on Ni-rich (Ni content more than 80 at.%) layered positive electrode materials that do not contain Cobalt (Co), an expensive element. We have synthesized NA-based positive electrode material LiNixAl1–xO2 (x=0.92, 0.95, NA92, 95) under conditions where Ni and Al are atomically mixed. LiNiO2 (LNO), LiNi0.95Co0.05O2 (NC95) and LiNi0.95Co0.03Al0.02O2 (NCA95) with uniform particle sizes were also synthesized to clarify the effects of Al and Co substitution in Ni-rich layered positive electrode materials. Figure. 1 shows the cycle performances of the samples measured in the full cells. After 500 charge-discharge cycles in full cells under conditions of 45°C, 2 C rate, and 2.5-4.2 V, NA95 and NC95 with the same Ni ratio showed the similar capacity retention (54.1%, 56.5%), whereas NA92 showed excellent capacity retention of 75%. Compared to LNO and NC95 that do not contain Al, the increase of charge transfer resistance (R ct) with cycling was remarkably suppressed in LiNixAl1–xO2. The chemical state of Al before and after cycling was analyzed by Hard X-ray Photoelectron Spectroscopy (HAXPES). The results suggested that Al existed either as a part of the LiNixAl1–xO2 solid solution or Al oxides before cycling while it existed as the lithium aluminate and Al-F compounds. This indicates that the trace HF in the electrolyte reacted with the pristine Al compound on the topmost of electrode materials during cycling to form a Li-conductive coating layer, which suppressed the degradation during cycling. Furthermore, though the Co substitution has some effects of preventing particle cracking, the least particle cracking was observed in NA92 of all samples, indicating that both the surface degradation and the bulk degradation were suppressed with the Al substitution to produce the excellent cycle performance. The results of this study indicated that LiNixAl1–xO2 (x=0.92, 0.95) synthesized by the optimum conditions could be a promising Co-free Ni-rich layered positive electrode material. Figure 1