Characterization of Electronic and Ionic Transport in Li_1-xNi_0.8Co_0.15Al_0.05O₂(NCA)

Abstract

Despite the extensive commercial use of Li 1-x Ni 0.8 Co 0.15 Al 0.05 O 2 (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. 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 LiNi 1-x Co x O 2 (NC) 2-10 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 also has lower cost than LiCoO 2 ; but despite extensive optimization, e.g., with respect to the Ni/Co ratio, 2-10 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.12-17 One of the most promising compositions that emerged is Li 1 Ni 0.8 Co 0.15 Al 0.05 O 2 (NCA), currently in widespread commercial use. This intercalation material exhibits solid solution behavior during the extraction of lithium 3,4,18 and is structurally stable upon cycling. 2 The majority of studies of NCA have focused on structural and electrochemical characterization. Surprisingly, there is limited, and conflicting, data on the basic transport properties of the NC/NCA family of compounds. Thus our objective in this work is to systematically characterize and interpret the transport properties of NCA. We use additive-free, single phase sintered samples in which the extrinsic effects that may be present in composite electrodes are avoided. Using electron blocking and ion blocking cell configurations, respectively...

Characterization of Electronic and Ionic Transport in Li1-xNi0.33Mn0.33Co0.33O2(NMC333) and Li1-xNi0.50Mn0.20Co0.30O2(NMC523) as a Function of Li Content

“…In our previous study of transport in NCA, we observed a sharp rise of conductivity beyond 60% delithiation. 33 This was explained as the oxidization of a small amount of cobalt from the trivalent to tetravalent state at this delithiation level. 33 It seems that there may also be a small degree of cobalt oxidation in NMC since the conductivity pattern is similar to NCA.…”

“…33 This was explained as the oxidization of a small amount of cobalt from the trivalent to tetravalent state at this delithiation level. 33 It seems that there may also be a small degree of cobalt oxidation in NMC since the conductivity pattern is similar to NCA. We were not able to measure the electronic conductivity beyond 75% delithiation in the NMC samples, as the samples became too fragile to assemble into cells, due to the intercalation-induced dimensional changes at the crystallite level.…”

“…Two of the most promising compositions that have emerged are LiNi 0. 33 (NMC 523 ), currently in widespread commercial use and development, respectively. This intercalation compound exhibits solid solution behavior during the extraction of lithium 3,4,18 and is structurally stable upon cycling.…”

Structural Relaxation of Lix(Ni0.874Co0.090Al0.036)O2 after Lithium Extraction down to x = 0.12

“…In this region, relatively larger differences in the c -axis between the H2 and H3 phases would restrict the reversible reactions to fail the high density and better cycle performance. On the other hand, NCA (Li(Ni,Co,Al)O 2 ; Co and Al doped LiNiO 2 ) exhibits good high-temperature stability and cycle performance [11,12,13,14,15,16,17] despite the of co-existence of H2 and H3 phases at the higher potential region as LiNiO 2 [15]. To achieve larger capacity, it is necessary to investigate the charge/discharge process and structural stability in this higher voltage region.…”

Nonlinear Conductivities and Electrochemical Performances of LiNi0.5Co0.2Mn0.3O2Electrodes

“…In lithium-ion batteries, the active materials of the cathodes typically exhibit relatively high resistivity due to their intrinsically poor electronic conductivity and the limited point contacts between their micro particles. [1][2][3][4][5][6] In addition, the active materials need to be mixed with binders to form the electrode, which insures the mechanical integrity and stability of the electrode. However, the binders are electric insulators that can further reduce the electrical conductivity of the electrode.…”