Paper EN+NS-ThA2
The Influence of Surface Chemistry as a Function of Salt Composition on the Rate Capability of LiNi_0.5Mn_0.5O₂ Composite Electrodes for Li-ion Rechargeable Batteries as Investigated using XPS

Thursday, November 3, 2011, 2:20 pm, Room 103

LiCoO₂ is currently the most commonly used cathode material in commercial Li-ion battery technology because of its high working voltage, structural stability and long cycle life. However, cobalt is expensive and there are safety and toxicity concerns. Therefore, there has been a considerable amount of work on developing cheaper alternatives for the positive electrode of large-scale lithium ion batteries. LiNi_0.5Mn_0.5O₂ (LNMO) has emerged as one of the best options due to its high specific capacity, thermal stability and low material costs. Previous studies have illustrated that decreasing the interlayer mixing can increase the rate capability and that increasing the heat-treatment temperature can also increase the rate capability. Recently, the influence that surface chemistry has on the rate capability of LNMO composite electrodes was investigated and it was shown that an additional annealing treatment after quenching the material during synthesis increased the device performance from 50 mAh/g to 180 mAh/g at 55°C and 8C. Via an inspection using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), it was shown that the annealing treatment did not result in significant changes in the lattice parameters or in the cation distributions of the layered structure. However, the annealing treatment was shown to be responsible for removing surface impurity phases such as lithium carbonate species and Mn³⁺-containing species, which was associated with the observed increase in performance.

In this study, we further investigate the role that surface chemistry, as developed during the operation of the cell, has on the device performance. LNMO electrodes were constructed using quenched and annealed LNMO starting material. Both the quenched and annealed electrodes were cycled in the range of 2.0 V – 4.6 V in 1M LiPF₆/EC:DMC (1:1) or 1M LiClO₄/EC:DMC (1:1). The composite electrodes (in the discharged state) were disassembled in an Ar environment glove box and transported to the XPS analysis chamber without exposure to ambient. Cycling performance shows enhanced capacity retention and device performance for annealed electrodes cycled in LiPF₆, with the little to no enhancement observed for annealed electrodes when cycled in LiClO₄. Our initial analysis indicates the formation of both Ni and Mn fluorides/oxyfluoride species on the surface of electrodes cycled in LiPF₆. Additional changes in the surface chemistry and the influence on the rate capability will be discussed.