Performance Loss Mechanisms in Lithium-Ion Cells with Nickel-Dominant Oxide Cathodes

Abstract

Development of low-cobalt oxide cathodes for lithium-ion batteries (LIBs) is a key focus area for the U.S. Department of Energy (DOE). These oxides have very high nickel content and high specific energies, which could increase the “range confidence” of EV owners. [1] The challenge is that these Ni-dominant oxides are susceptible to structural transformations, both in the bulk and at the surface, when repeatedly cycled to high states-of-charge. [2] Understanding the specific factors that can accelerate or negate such transformations, and how they are affected by changes in electrode and electrolyte composition, is essential to unleash the full potential of these cathode materials.Here, we present a systematic investigation of how advanced electrochemical analysis can help reveal the sources of performance fade in cells with Ni-dominant oxide cathodes. To this end, a series of oxides containing >90% of Ni were synthesized and their electrochemical performance was evaluated using test protocols that highlight the effect of repeated cycling and/or of exposure to high voltages. We show how differential voltage analysis (DVA) can be used to examine cathode stability in full-cells with these materials. DVA relies on using half-cell cycling profiles to approximate the instantaneous electrode potentials of the cathode and the anode in the full-cell. The mathematical operations required for this approximation can be related to specific modes of performance degradation, revealing details about how the cell ages. An example of the merits of this analytical framework applied to a full-cell containing a graphite-anode and LiNi0.9Mn0.05Co0.05O2-cathode is presented in Figure 1. The cell retained over 80% of its C/10 capacity for more than 500 cycles (Figure 1a) and exhibited limited impedance rise at the end of testing. The dV/dQ profile of the cell after 100 cycles is shown in black in Figure 1b, exhibiting features associated with both the cathode and the anode. Using DVA these features can be accurately reproduced (red curve, Figure 1b), revealing information about the stability of the cathode (Figure 1c) and its contribution to capacity fade (Figure 1d). Cathode capacity was observed to become increasingly inaccessible after 400 cycles (Figure 1c) and dominated the capacity fade of the cell upon extended testing (Figure 1d). These results indicate that the use of electrolyte additives that only act by improving the passivation of graphite anodes would bring limited benefits to the cell, as its longevity is ultimately limited by the stability of the cathode. The effect of cycling, exposure to high voltages, and electrolyte additives on cell capacity fade and impedance rise will be presented for a series of Ni-rich cathodes.Figure 1. Data for a graphite/LiNi0.9Mn0.05Co0.05O2 cell during constant-current cycling from 3.0 to 4.2 V. (a) Discharge capacity retention. (b) Differential voltage curves from the C/10 cycle 103 derived from experimental data (black) and from differential voltage analysis (DVA, red); main features in the data are captured by the model. (c) Accessible cathode capacity calculated using DVA; active material loss accelerates after ~300 cycles; (d) Total measured capacity loss (black) and the losses that can be ascribed to the SEI according to DVA (red); the decrease in cathode capacity outpaces losses of Li+ inventory after ~300 cycles. Electrolyte was 1.2 M LiPF6 in 3:7 (w/w) ethylene carbonate : ethyl methyl carbonate.