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Abstract
Despite the extensive commercial use of Li1-xNi0.8Co0.15Al0.05O2 (NCA) as the positive electrode in Li-ion batteries, and its long research history, its fundamental transport properties are poorly understood. These properties are crucial for designing high energy density and high power Li-ion batteries. Here, the transport properties of NCA are investigated using impedance spectroscopy and dc polarization and depolarization techniques. The electronic conductivity is found to increase with decreasing Li-content from ∼10−4 Scm−1 to ∼10−2 Scm−1 over x = 0.0 to 0.6, while lithium ion conductivity is at least five orders of magnitude lower for x = 0.0 to 0.75. A surprising result is that the lithium ionic diffusivity vs. x shows a v-shaped curve with a minimum at x = 0.5, while the unit cell parameters show the opposite trend. This suggests that cation ordering has greater influence on the composition dependence than the Li layer separation, unlike other layered oxides. From temperature-dependent measurements in electron-blocking cells, the activation energy for lithium ion conductivity (diffusivity) is found to be 1.25 eV (1.20 eV). Chemical diffusion during electrochemical use is limited by lithium transport, but is fast enough over the entire state-of-charge range to allow charge/discharge of micron-scale particles at practical C-rates.
Highlights
The Massachusetts Institute of Technology (MIT) Faculty has made this article openly available
Ni0.8 Co0.15 Al0.05 O2 (NCA) powder of Li1-xNi0.8Co0.15Al0.05O2 composition was obtained from NEI Corporation Inc. (Somerset, NJ, USA)
Delithiation was performed in a Swagelok-type electrochemical cell using lithium metal foil as the counter electrode, the NCA pellet as the working electrode, and a liquid electrolyte mixture containing 1 M LiPF6 in 1:1 by mole of ethylene carbonate/diethyl carbonate (EC/DEC)
Summary
Creative Commons Attribution 4.0 International License http://creativecommons.org/licenses/by/4.0/. Cathodes having high energy and power density, adequate safety, excellent cycle life, and low cost are crucial for Li-ion batteries that can enable the commercialization of electric transportation.[1] Towards this end, much research has previously focused on the development of the LiNi1-xCoxO2 (NC)[2,3,4,5,6,7,8,9,10] cathode due to its high capacity (∼275 mAh/g) and favorable operating cell voltage (4.3 V vs Li/Li+), which is within the voltage stability window of current liquid electrolytes This compound has lower cost than LiCoO2; but despite extensive optimization, e.g., with respect to the Ni/Co ratio,[2,3,4,5,6,7,8,9,10] NC still suffers from poor structural stability during electrochemical cycling.[11] Significant efforts were subsequently focused on improving structural stability by doping with small amounts of electrochemically inactive elements such as Al and Mg.[12,13,14,15,16,17] One of the most promising compositions that emerged is Li1Ni0.8Co0.15Al0.05O2 (NCA), currently in widespread commercial use. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract)
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