Continuous operation of full cells with layered transition metal (TM) oxide positive electrodes (NCM523) leads to dissolution of TM ions and their migration and incorporation into the solid electrolyte interphase (SEI) of the graphite-based negative electrode. These processes correlate with cell capacity fade and accelerate markedly as the upper cutoff voltage (UCV) exceeds 4.30 V. At voltages ≥4.4 V there is enhanced fracture of the oxide during cycling that creates new surfaces and causes increased solvent oxidation and TM dissolution. Despite this deterioration, cell capacity fade still mainly results from lithium loss in the negative electrode SEI. Among TMs, Mn content in the SEI shows a better correlation with cell capacity loss than Co and Ni contents. As Mn ions become incorporated into the SEI, the kinetics of lithium trapping change from power to linear at the higher UCVs, indicating a large effect of these ions on SEI growth and implicating (electro)catalytic reactions. We estimate that each Mn II ion deposited in the SEI causes trapping of ∼10 2 additional Li + ions thereby hastening the depletion of cyclable lithium ions. Using these results, we sketch a mechanism for cell capacity fade, emphasizing the conceptual picture over the chemical detail. Increasing the energy and power density of Li-ion batteries (LIBs) for transportation applications is desirable for extending driving range and for providing the burst of power required for vehicle acceleration.1-3 Layered transition metal (TM) oxides containing Co, Mn, and Ni are promising high-energy electrode materials, as they exhibit theoretical oxide-specific capacities >200 mAh/g and can be cycled to potentials exceeding 4.50 V vs. Li/Li + . 4,5 However, the performance deterioration of full cells, in which these oxides are paired with a graphite (Gr) negative electrode, increases markedly at potentials exceeding 4.30 V, limiting their wider use as high-energy cathode materials.6,7 Therefore, it is imperative to examine the causes for their performance loss at high voltages. In this study we aim at understanding both the phenomenology and mechanism of capacity loss; mitigation of this loss will be the focus of future articles.Several possible causes for cell performance degradation have been identified over the years. These causes include, but are not limited to (i) TM oxide structure changes leading to voltage fade, 8 (ii) buildup of electrode surface films, especially the solid-electrolyte interphases (SEIs) 9,10 at the negative electrode, leading to Li + inventory depletion and/or impedance rise, 11-13 (iii) decomposition of the electrolyte, [14][15][16] (iv) pitting corrosion of aluminum current collectors, 17-19 (v) binder destabilization leading to delamination of electrode coatings, and (vi) dissolution of electrode-active materials. [20][21][22][23] This last source, especially the dissolution of Mn and its deposition in the negative electrode SEI, has been shown to be particularly detrimental to cell performance. [24][25][26] All steps o...

Energy density of full cells containing layered-oxide positive electrodes can be increased by raising the upper cutoff voltage above the present 4.2 V limit. In this article we examine aging behavior of cells, containing LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523)-based positive and graphite-based negative electrodes, which underwent up to ∼400 cycles in the 3-4.4 V range. Electrochemistry results from electrodes harvested from the cycled cells were obtained to identify causes of cell performance loss; these results were complemented with data from X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) measurements. Our experiments indicate that the full cell capacity fade increases linearly with cycle number and results from irreversible lithium loss in the negative electrode solid electrolyte interphase (SEI) layer. The accompanying electrode potential shift reduces utilization of active material in both electrodes and causes the positive electrode to cycle at higher states-of-charge. Full cell impedance rise on aging arises primarily at the positive electrode and results mainly from changes at the electrode-electrolyte interface; the small growth in negative electrode impedance reflects changes in the SEI layer. Our results indicate that cell performance loss could be mitigated by modifying the electrode-electrolyte interfaces through use of appropriate electrode coatings and/or electrolyte additives. The pursuit of high energy density lithium-ion batteries for transportation applications continues in order to increase the driving range of vehicles on a single charge. Because graphite (Gr) remains the active material of choice in the negative electrode, the search mainly revolves around layered oxides for the positive electrode. In recent years there has been immense interest in the lithium and manganese-rich layered oxides which are capable of delivering high energy density cells. [1][2][3][4][5][6][7][8][9][10] This interest has ebbed somewhat because the voltage fade exhibited by these oxides lowers the usable energy, and complicates state-ofcharge determination, of the battery cells. In this article we focus our attention on the NCM523 oxide which has been the subject of several recent articles. For example, Bak et al. conducted in situ X-ray diffraction studies on various (delithiated) Li x Ni a Co b Mn c O 2 materials and concluded that NCM523 is an optimized composition offering the thermal stability of lower-Ni oxides (such as NCM333) while displaying capacities closer to the higher-Ni oxides (such as NCM811).27 From AC impedance and DC polarization studies Amin and Chiang reported that the electronic conductivity of NCM523 increases with decreasing Li-content, from 10 −7 S/cm for a fully-lithiated oxide (Li 1.0 ) to 10 −2 S/cm for a significantly delithiated (Li 0.25 ) oxide.28 Dixit et al. confirmed from first * Electrochemical Society Member. z E-mail: abraham@anl.gov principle-based simulation studies that during oxide delithiation Ni oxidizes first, followed by Co, while Mn rem...

Lithiated ternary oxides containing nickel, cobalt, and manganese are intercalation compounds that are used as positive electrodes in high-energy lithium-ion batteries. These oxides undergo changes, when they are stored in humid air or exposed to moisture, that adversely affect their electrochemical performance. There is a new urgency to better understanding of these "weathering" mechanisms as manufacturing moves toward a more environmentally benign aqueous processing of the positive electrode. Delithiation of the oxide and the formation of lithium salts (such as hydroxides and carbonates) coating the surface, are known to occur during moisture exposure. The redox reactions which follow this delithiation are believed to trigger all the other transformations. In this article we suggest another possibility: namely, the proton -lithium exchange. We argue that this hypothesis provides a simple, comprehensive rationale for our observations, which include contraction of the c-axis (unit cell) lattice parameter, rock salt phase formation in the subsurface regions, presence of amorphous surface films, and the partial recovery of oxide capacity during electrochemical relithiation. The detrimental effects of water exposure need to be mitigated before aqueous processing of the positive electrode can find widespread adoption during cell manufacturing. Improving the performance of lithium-ion batteries (LIBs) for electrically powered vehicles gives urgency to the development of high-capacity, high-voltage electrode materials, and layered Ni-rich oxides are presently among the most technological advanced materials that allow operation above +4.0 V vs Li + /Li. 1 Such materials (also known as NCMxyz materials) have the general composition of LiNi x/10 Co y/10 Mn z/10 O 2 , where x+y+z = 10. In these ternary oxides, the manganese stays in the Mn 4+ state during cell cycling, and the capacity mainly originates from the Ni 2+ /Ni 3+ and Ni 3+ /Ni 4+ redox couples. The monolayers of octahedra containing transition metal (TM) ions form a structural scaffold with the layers of mobile Li + ions placed in between. The crystalline material retains the structure of the progenitor material of the family, viz. α-LiCoO 2 , 2 with the unit cell belonging to the rhombohedral R(-3)m space group. During electrochemical cycling, the lithium ions intercalate into (and deintercalate from) the layered crystals as the redox state of (mainly the) nickel ions changes to provide the overall charge neutrality.In this study, we examine one such material, NCM523, which represents a particular trade-off between the cycling rate (that improves with Co content), safety (that improves with Mn content), and capacity (that increases with Ni content).1 The safety concerns mainly relate to the thermal and chemical stability, including oxidation of the organic electrolyte by catalytic centers at the oxide surface 3 and phase transitions in the delithiated material that occur when a significant fraction of nickel ions is reduced to Ni 2+ ions and molecular oxygen ...

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The capacity and power performance of lithium-ion battery cells evolve over time. The mechanisms leading to these changes can often be identified through knowledge of electrode potentials, which contain information about electrochemical processes at the electrodeelectrolyte interfaces. In this study we monitor electrode potentials within full cells containing a Li 1.03 (Ni 0.5 Co 0.2 Mn 0.3 ) 0.97 O 2 -based (NCM523) positive electrode, a silicon-graphite negative electrode, and an LiPF 6 -bearing electrolyte, with and without fluoroethylene carbonate (FEC) or vinylene carbonate (VC) additives. The electrode potentials are monitored with a Li-metal reference electrode (RE) positioned besides the electrode stack; changes in these potentials are used to examine electrode state-of-charge (SOC) shifts, material utilization, and loss of electrochemically active material. Electrode impedances are obtained with a Li x Sn RE located within the stack; the data display the effect of cell voltage and electrode SOC changes on the measured values after formation cycling and after aging. Our measurements confirm the beneficial effect of FEC and VC electrolyte additives in reducing full cell capacity loss and impedance rise after cycling in a 3.0-4.2 V range. Comparisons with data from a full cell containing a graphite-based negative highlight the consequences of including silicon in the electrode. Our observations on electrode potentials, capacity, and impedance changes on cycling are crucial to designing long-lasting, silicon-bearing, lithium-ion cells. As applications of lithium-ion systems expand beyond consumer electronics to the transportation and electricity storage markets, battery-cell longevity has become a primary barrier to widespread commercialization. While cells in smart phones are expected to function for about two years, energy storage units in electric vehicles are expected to maintain performance over a 10-15 year period. This performance is primarily defined by cell capacity and impedance characteristics, which change as electrode potentials and electrodeelectrolyte interfaces evolve over time. The knowledge of electrode potentials is very important for battery aging investigations because it can point to sources of performance degradation and, hence, lead to solutions that improve cell life. Such improvements are urgently needed for high-energy density cells, with silicon-containing-negative and layered-oxide-positive electrodes, being developed at Argonne National Laboratory as part of the U.S. DOE's Applied Battery Research (ABR) for Transportation program.The potentials of individual electrodes, such as that of a layeredoxide electrode, can be monitored in two-electrode cells by using a Li-metal counter electrode (CE) at low currents, or by using a CE with high surface areas, which reduce current densities and maintains a relatively stable potential. However, electrode potentials cannot be measured independently in commercial lithium-ion cells, which do not contain Li-metal. Yet aging investigations of ...