Elettra | BACH Enhanced stability of Li ion batteries through surface modification and electron structure design Increasing the energy density is the key trend in the development of advanced Li ion batteries. This can be achieved via an increase of the capacity of the anode and cathode materials, or by using the cathode with a higher redox potential. The key challenge is the stability of the cathodes charged to a high voltage. In Li-rich layered transition metal (M=Co, Ni, Mn) oxides, the M3d and O2p states are strongly hybridized leading to the involvement of the lattice oxygen into redox reaction at a deep stage of delithiation, i.e. when cathodes are charged to a higher potential. However, the O2- ↔ O- redox reaction is generally irreversible process accompanied by a rapid battery degradation. In LiMPO4 olivine structure cathode materials, particularly in LiCoPO4, the O2p state of the lattice oxygen is energetically separated from Co3d state due to the inductive effect, thereby excluding the O2- → O- oxidation even at a charging potential above 5.1 V (vs. Li+/Li). Unfortunately, the upper threshold of the electrochemical stability window of most commercial electrolytes is limited by ~5 V that induces the electrolyte decomposition by charging the cathodes to high voltage. The accompanying side reactions at the cathode surface form poorly Li+ conducting interfaces that leads to sluggish kinetics across the interface and to fatigue of the cathodes. For instance, LiCoPO4 (redox ~4.8 V vs. Li+/Li) suffers from rapid capacity fade upon battery cycling that hinders its commercial application. Among strategies to mitigate degradation phenomena are surface modifications, e.g. coating of the cathodes. From the macroscopic point of view, such artificial interfaces protect the surface: a) against undesirable oxygen release, as in the case of the layered structure cathodes charged above 4.6 V (vs. Li+/ Li), and b) avoid the direct contact of the electrolyte with the cathode surface, thereby avoiding parasitic catalytic reactions with the involvement of the cathode. From the atomic level point of view, such surface modifications change the chemical (electrochemical) potential at the surface, thereby allowing or, on the contrary, prohibiting a charge transfer between both the intrinsic electronic levels of the cathode itself and the relevant energy levels of the electrolyte and the cathode. These factors emphasize the crucial role of the electron structure not only for redox activity, but also for governing the battery stability for a long-time electrochemical cycling. In the light of the arguments raised above, we are developing a new electron energy band diagram concept that includes the examination of the electrochemical- and ionization- potentials evolution upon cycling of various types of batteries. To confirm our concept, we perform in situ X-ray Photoelectron Spectroscopy (XPS), Soft x-ray Photoemission (SPES), X-ray Absorption (XAS) experiments, and operando Raman measurements to monitor the evolution of the electronic structure as a function of charging state/ discharging of various battery systems. Specifically, we have studied the effect of SO2 and NH3 double gas treatment of high-energy density lithium-rich 0.33Li2MnO3·0.67LiNi0.4Co0.2Mn0.4O2 (HE-NCM) cathodes on their stability at high voltages (≥ 4.8 V vs. Li+/Li) in comparison with the untreated HENCM counterpart. Our method reveals a specific modification of the surface electronic configuration in the treated HE-NCM that lifts up the electrochemical potential (i.e. the Fermi level, EF) towards the M3d and O2p (lattice) unoccupied electronic states, Fig. 1. (a, b). Such a EF shift is beneficial for the stability of the cathode. Namely, by charging the treated cathode, the EF is shifted downwards into the occupied O2p states to a less extent upon delithiation, as compared to the untreated counterpart charged to the same potential, Fig. 1 (c, d). As the result, the treated HE-NCM cathode exhibits reduced oxygen loss, reduced voltage hysteresis and smaller interfacial impedance, Fig. 1 (e). The concept of designing the artificial interfaces based on the experimentally derived electron energy band diagrams of the individual battery components, such as the electrolyte and the electrodes, was applied to the olivine structure cathode material with a higher redox potential. We consider a model 5 V battery system with the LiCoPO4 thin-film cathode not containing conducting carbon and binder, which are vital components of conventional commercial batteries. Figs. 1 (f, g) show cyclic voltammetry of LiCoPO4 thin-film cathodes. Despite a fast kinetics, the thin-film battery with the bare LiCoPO4 exhibits the expected fast degradation. Instead, coating of LiCoPO4 with LiPON (lithium phosphorus oxynitride) solid electrolyte drives the electrochemical potential into a favourable energy position at the LiCoPO4/LiPON interface (Adv. Mater. Interface 2020, 7, 2000276). Lifting up the EF to the unoccupied electronic state of as-prepared LiCoPO4 could also have a benefit via prohibiting unwanted charge transfer from the electronic levels of liquid electrolyte to the cathode upon a gradual increase of the electrochemical potential caused by delithiation of the cathode. This hypothesis is fully confirmed by electrochemical cycling demonstrating high stability and excellent performance of the electrolyte/LiPON/LiCoPO4 thinfilm battery, Fig. 1 (g). In conclusion, our approach enables to forecast the intrinsic stability of various cathodes, the cathode/electrolyte interfacial stability, and to discriminate the reaction pathways associated with interfacial electronic charge-transfer mechanisms. The explanation and forecasting catalytic activity towards energy conversion reactions makes possible to design artificial interfaces eliminating side reactions with electrolytes. Elettra Highlights 2024–2025 108
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