In Operando Analysis of Cathode Reaction By Synchrotron X-Ray Absorption and Diffraction

1. Introduction

in operando analysis enables us to observe non-equilibrium behavior occurring in an operating battery condition without touching its components and so actual processes associated with charging/discharging can be elucidated. In this study, in operando techniques are applied to investigate two phenomena; (1) non-equilibrium phase transition behavior (2) stability at electrode / electrolyte interface.

2. Snapshot Observation of Phase Transition Dynamics in LiFePO₄ and LiNi_0.5Mn_1.5O₄ [1-4]

Phase transitions of solid state materials are widely utilized in lithium-ion batteries (LIBs).  Phase transitions are explained using a phase diagram with physicochemical parameters, e.g. concentration and temperature.  Phase diagrams, including the stability of phase transitions, are explained using the concept of Gibbs-energy, based on the equilibrium between stable phases on either side of a phase transition.  For phase transitions in practical devices such as LIBs, the actual phase transition takes place under non-equilibrium conditions, which can result in phenomena different to those expected from thermodynamics. The difficulty in in situ analysis has been low time resolutions. Recently the measuring probes have been much improved and observation at practical charging/discharging rate to 50 C is now available. In this study, in situ techniques by using synchrotron X-ray method and its application to lithium battery analysis using their measurement characteristics are demonstrated.

The phase transition between LiFePO₄ and FePO₄ under non-equilibrium battery operation was tracked in real-time using time-resolved X-ray diffraction. A metastable crystal phase appears in conjunction with increasing current density, in addition to the thermodynamically stable LiFePO₄ and FePO₄ phases. The metastable phase gradually diminishes under open circuit conditions following electrochemical cycling. We propose a phase transition path that passes through the metastable phase, and posit the new phase’s role in decreasing nucleation energy, accounting for the excellent rate capability of LiFePO₄.

We examined LiNi_0.5Mn_1.5O₄ as another example. It has been known that there are two phase transitions in its charging processes, namely LiNi_0.5Mn_1.5O₄ (Li1) to Li_0.5Ni_0.5Mn_1.5O₄ (Li0.5) and Li0.5 to Ni_0.5Mn_1.5O₄ (Li0). In the in situ measurement, it turned out that the Li0.5 phase is clearly observed during charging and is obscure during discharging, showing the thermodynamically reversible but kinetically asymmetric behavior. Using potential step experiments and kinetic analysis, this phenomenon is ascribed to the slow phase transition between Li0.5 and Li0, compared to that between Li1 and Li0.5. The high time resolution was effective in data acquisition during the potential step experiments.

3. in operando X-ray Absorption Spectroscopic Study on Stability at Electrode / Electrolyte Interface [5-7]

In the previously proposed deterioration mechanisms of LIBs, the electrode/electrolyte interface is thought to play a significant role. For example, the formation of a solid electrolyte interface, changes in the crystal structure on the surface of active materials during Li ion extraction/insertion, and improvement in the cyclic performance through  surface coating of the active materials have all been reported previously. However, determining the specific cause of the interface stability is quite difficult because the geometric and electronic structures must be tracked with a resolution of less than a few nanometers to reveal the phenomena at the electrode/electrolyte interface. To achieve further elucidation of the deterioration mechanism of LIBs, clarification of the electronic structure at the electrode/electrolyte interface under operation conditions is required.

We investigated the effects of the electronic structure at the electrode/electrolyte interface on the cyclic performance of cathode materials via in-situ total-reflection fluorescence X-ray absorption spectroscopy. In a LiCoO₂ thin-film electrode that exhibits gradual deterioration upon subsequent Li ion extractions and insertions (cycling), the reduction of Co ions at the electrode/electrolyte interface was observed upon immersion in an organic electrolyte followed by irreversible behavior after cycling. In contrast, in a LiFePO₄ thin-film electrode, the electronic structure at the electrode/electrolyte interface was stable and reversible upon electrolyte immersion with subsequent cycling. The increased stability of the electronic structure at the LiFePO₄/electrolyte interface impacts its cycling performance.

Acknowledgement

This work was supported by RISING battery project from NEDO, Japan.

References

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