Investigating LiNi1/3Mn1/3Co1/3O2 Polarization Resistances in Function of Temperature and Voltage at Different Stages of Lifetime
Authors: Varini Maria , Klett Matilda , Lindbergh Göran 
Published: February 2020

LiNi1-x-yMnxCoyO2 cathodes (NMCs) are currently employed or evaluated for EVs, due to their high theoretical capacities and operating voltage [1]. In view of EVs’ stringent requirements in terms of performance and lifetime, NMCs have been tested in several cycling protocols, simulating real-life applications, or moving towards more energy-dense devices (high voltage operation).
In such protocols, NMCs have displayed different failure mechanisms, often consisting in an overall increased resistance towards polarization. However, hindrances towards polarization are already present in NMCs at beginning-of-life (BOL). Such resistances are voltage (x Li+)-dependent [2], while the impact of temperature is less investigated. Although some of these resistances have been recently addressed in literature (namely, the discussion over the so-called “Irreversible Capacity Loss” [3], which is actually a kinetic limitation [4]), a comprehensive explanation of NMCs’ voltage characteristics at BOL is still missing. This furthermore complicates the understanding of NMCs’ electrochemical response after prolonged cycling, which is necessary for designing optimized cycling protocols.
LiNi1/3Mn1/3Co1/3O2 (NMC333) is chosen as representative of the NMCs series, as far as electrochemical [5], structural and electronic [1] properties are concerned. NMC333’s voltage characteristics at BOL and end-of-life (EOL) are investigated in terms of temperature, voltage (x Li+) and applied polarization, and compared with known structural and electronic transitions in the crystal, also x Li+-dependent.
Lab-scaled pouch cells in half-cell configuration (NMC333/Li) are tested between +3.6 V and +4.6 V vs. Li. NMC333 electrodes used are either pristine, or harvested from cells cycled at high voltage (+4.6 V vs. Li upper voltage cut-off) and different temperature (10, 25 and 40 °C). Through this protocol, it is possible to decouple temperature- and voltage- related effects on NMC333’s degradation. NMC333’s electrochemistry is investigated both at steady state, by means of OCP curves, and under moderate polarization, by means of Constant Current (CC) cycling at C/10 and Slow Rate Cyclic Voltammetry (SRCV).
In Figure 1, CC and SRCV of NMC333 at BOL are reported at different temperatures. Structural and electronic transitions in NMC333 are associated with a different electrochemical behavior below and above +3.8 V vs. Li. Specifically, while at higher voltages an apparent capacitive behavior is displayed, a resistance towards polarization can be identified below +3.8 V vs. Li for both CC and SRCV. Such resistance occurs during charge and discharge, although a T-dependent asymmetry emerges in SRCV between anodic and cathodic scan (namely, a shifting peak onset and an additional shoulder in the anodic peak). By the comparison between CC and SRCV, which are performed at different cycling rates, further insights into the causes of these hindrances can be gathered.
A discussion on the resistances towards polarization displayed by NMC333 at different stages of lifetime will bring further insight into its thermodynamic and dynamic voltage characteristics, and their evolution in function of different cycling conditions and operation over time. These notions will be helpful for modelling NMCs, by knowing which phenomena and variables must be taken into account when fitting/predicting performance and lifetime. Additionally, such information will allow for more conscious design of cycling protocols, apt to achieve optimized performance and lifetime of NMCs-containing Li ion batteries.
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Figure 1