The influence of point defects on the entropy profiles of Lithium Ion Battery cathodes: a lattice-gas Monte Carlo study

Abstract

In-situ diagnostic tools have become established to as a means to understanding the aging processes that occur during charge/discharge cycles in Li-ion batteries (LIBs). One electrochemical thermodynamic technique that can be applied to this problem is known as entropy profiling. Entropy profiles are obtained by monitoring the variation in the open circuit potential as a function of temperature. The peaks in these profiles are related to phase transitions, such as order/disorder transitions, in the lattice. In battery aging studies of cathode materials, the peaks become suppressed but the mechanism by which this occurs is currently poorly understood. One suggested mechanism is the formation of point defects. Intentional modifications of LIB electrodes may also lead to the introduction of point defects. To gain quantitative understanding of the entropy profile changes that could be caused by point defects, we have performed Monte Carlo simulations on lattices of variable defect content. As a model cathode, we have chosen manganese spinel, which has a well-described order-disorder transition when it is half filled with Li. We assume, in the case of trivalent defect substitution (M=Cr,Co) that each defect M permanently pins one Li atom. This assumption is supported by Density Functional Theory (DFT) calculations. Assuming that the distribution of the pinned Li sites is completely random, we observe the same trend in the change in partial molar entropy with defect content as observed in experiment: the peak amplitudes become increasing suppressed as the defect fraction is increased. We also examine changes in the configurational entropy itself, rather than the entropy change, as a function of the defect fraction and analyse these results with respect to the ones expected for an ideal solid solution. We discuss the implications of the quantitative differences between some of the results obtained from the model and the experimentally observed ones.

Quantifying structure dependent responses in Li-ion cells with excess Li spinel cathodes: matching voltage and entropy profiles through mean field models

“…This feature has been attributed to an order/disorder transition in the Li/vacancy structure. 25,29,34,36,37,57 This effect is also visible in the incremental capacity (dQ/dV) curves shown in Fig. 4.…”

“…We used a pairwise interaction model for the spinel lattice, similar to the one adopted elsewhere. 29,34,37,46 In the filled lattice, x = 1, each Li atom has N nn = 4 nearest neighbours on opposite sublattices and N nnn = 12 next nearest neighbours on like sublattices. The pairwise interaction model is shown in Fig.…”

“…The peaks arise from the formation of an ordered phase, Li 0.5 Mn 2 O 4 at intermediate state of charge, and a transition to a disordered solid solution phase either side. 29,[34][35][36][37][38] To better understand voltage and entropy profiles and enable more definitive structural assessments from them, we developed a Monte Carlo based model to quantify the changes in the EP peak amplitudes dependent on the proportion of point defects in Li x M y Mn 2Ày O 4 spinel (M = Co, Cr). 37 We showed that the experimental trends could be explained by pinned Li sites, required for charge balance, which occur in proportion to the defect fraction, y.…”

Transitions of lithium occupation in graphite: A physically informed model in the dilute lithium occupation limit supported by electrochemical and thermodynamic measurements

“…In addition to fundamental understanding, these phenomena are important for achieving a cell level picture for practical applications such as battery management and aging. In our recent and ongoing work, we described order disorder transitions, including the Stage I-Stage II transition, through Grand Canonical Monte Carlo simulations [1,[9][10][11]] and a two level, mean field approach [2,12] . This transition is responsible for the main step in voltage with respect to lithium occupation (state of charge) that occurs close to 50 % lithiation in graphite.…”

Electrosorption of a modified electrode in the vicinity of phase transition: A Monte Carlo study

“…Metal substitution (point defects) in spinel cathodes for lithium-ion batteries stabilize the structure, improving the cycle life [33]. The influence of this point defects in a spinel cathode was addressed with MC simulations [34].…”