Direct Observation of Reductive Coupling Mechanism between Oxygen and Iron/Nickel in Cobalt-Free Li-Rich Cathode Material: An in Operando X-Ray Absorption Spectroscopy Study

Li-rich cathodes possess high capacity and are promising candidates in next-generation high-energy density Li-ion batteries. This high capacity is partly attributed to its poorly understood oxygen-redox activity. The present Li-rich cathodes contain expensive and environmentally-incompatible cobalt as a main transition metal. In this work, cobalt-free, iron-containing Li-rich cathode material (nominal composition Li$_{1.2}$Mn$_{0.56}$Ni$_{0.16}$Fe$_{0.08}$O$_{2}$) is synthesized, which exhibits excellent discharge capacity (≈250 mAh g$^{-1}$ and cycling stability. In operando, X-ray absorption spectroscopy at Mn, Fe, and Ni K edges reveals its electrochemical mechanism. X-ray absorption near edge structure (XANES) features of Fe and Ni K edges show unusual behavior: when an electrode is charged to 4.5 V, Fe and Ni K edges’ XANES features shift to higher energies, evidence for Fe$^{3+}$→Fe$^{4+}$ and Ni$^{2+}$→Ni$^{4+}$ oxidation. However, when charged above 4.5 V, XANES features of Fe and Ni K edges shift back to lower energies, indicating Fe$^{4+}$→Fe$^{3+}$ and Ni$^{4+}$→Ni$^{3+}$ reduction. This behavior can be linked to a reductive coupling mechanism between oxygen and Fe/Ni. Though this mechanism is observed in Fe-containing Li-rich materials, the only electrochemically active metal in such cases is Fe. Li$_{1.2}$Mn$_{0.56}$Ni$_{0.16}$Fe$_{0.08}$O$_{2}$ has multiple electrochemically active metal ions; Fe and Ni, which are investigated simultaneously and the obtained results will assist tailoring of cost-effective Li-rich materials

Thermal stability of Li1−ΔM0.5Mn1.5O4Li_{1-Δ}M_{0.5}Mn_{1.5}O_{4} (M = Fe, Co, Ni) cathodes in different states of delithiation Δ

The thermal stability of sol–gel synthesized Li12DM0.5Mn1.5O4 (M = Fe, Co, Ni) electrodes with differentdegrees of delithiation were analyzed with TG-DSC and in situ synchrotron diffraction under an Aratmosphere and compared. The onset temperatures for structural degradation are dependent on theamount of lithium 12D in the sample. The Li12DFe0.5Mn1.5O4 electrode exhibited the highest thermalstability among the three materials with different dopant M. The reason for this difference is discussedwith respect to the oxidation states of the transition metals. The mechanism of degradation for M = Fe, Cowas found to be through gas evolution, mainly CO2 and O2, and the carbon conductive additive was foundto play a major role in the thermal degradation process. For delithiated Li12DNi0.5Mn1.5O4 the temperatureinduced degradation includes phase separation into Mn3O4 with spinel structure and LixNi12xO with rocksaltstructure together with oxygen and carbon dioxide release

Influence of Iron on the Structural Evolution of LiNi0.4Fe0.2Mn1.4O4LiNi_{0.4}Fe_{0.2}Mn_{1.4}O_{4} during Electrochemical Cycling Investigated by in situ Powder Diffraction and Spectroscopic Methods

The cathode materials LiNi0.5Mn1.5O4 and LiNi0.4Fe0.2Mn1.4O4 were synthesized using a citric acid-assisted solgel method with a final calcination temperature of 1000 °C. An impurity phase exists in LiNi0.5Mn1.5O4 powders, which can be eliminated by substituting some of the Ni2+ and Mn4+ ions with Fe3+. The substitution of Fe into the spinel structure was confirmed by NMR and Mössbauer spectroscopy. The initial capacity of LiNi0.4Fe0.2Mn1.4O4 powder synthesized at 1000 °C (LNFMO) is slightly higher than that of LiNi0.5Mn1.5O4 powder synthesized at 1000 °C (LNMO). Additionally, its capacity retention of 92% at room temperature after 300 cycles at C/2 charging-discharging rate between 3.5–5.0 V is higher than that of the Fe-free sample (79.5%) under same conditions which could arise from the difference in their cycling mechanisms. In order to understand the structural evolution of these materials during electrochemical cycling, in situ studies under real operating conditions were performed. Measurements of initial powders in capillaries and in situ experiments during the first galvanostatic cycle were carried out by high resolution powder diffraction using synchrotron radiation

Unveiling the Electrochemical Mechanism of High-Capacity Negative Electrode Model-System BiFeO 3 in Sodium-Ion Batteries: An In Operando XAS Investigation

Careful development and optimization of negative electrode (anode) materials for Na-ion batteries (SIBs) are essential, for their widespread applications requiring a long-term cycling stability. BiFeO$_3$ (BFO) with a LiNbO$_3$-type structure (space group R3c) is an ideal negative electrode model system as it delivers a high specific capacity (770 mAh g$^{–1}$), which is proposed through a conversion and alloying mechanism. In this work, BFO is synthesized via a sol–gel method and investigated as a conversion-type anode model-system for sodium-ion half-cells. As there is a difference in the first and second cycle profiles in the cyclic voltammogram, the operating mechanism of charge–discharge is elucidated using in operando X-ray absorption spectroscopy. In the first discharge, Bi is found to contribute toward the electrochemical activity through a conversion mechanism (Bi$^{3+}$ → Bi$^0$), followed by the formation of Na–Bi intermetallic compounds. Evidence for involvement of Fe in the charge storage mechanism through conversion of the oxide (Fe$^{3+}$) form to metallic Fe and back during discharging/charging is also obtained, which is absent in previous literature reports. Reversible dealloying and subsequent oxidation of Bi and oxidation of Fe are observed in the following charge cycle. In the second discharge cycle, a reduction of Bi and Fe oxides is observed. Changes in the oxidation states of Bi and Fe, and the local coordination changes during electrochemical cycling are discussed in detail. Furthermore, the optimization of cycling stability of BFO is carried out by varying binders and electrolyte compositions. Based on that, electrodes prepared with the Na-carboxymethyl cellulose (CMC) binder are chosen for optimization of the electrolyte composition. BFO–CMC electrodes exhibit the best electrochemical performance in electrolytes containing fluoroethylene carbonate (FEC) as the additive. BFO–CMC electrodes deliver initial capacity values of 635 and 453 mAh g$^{–1}$ in the Na-insertion (discharge) and deinsertion (charge) processes, respectively, in the electrolyte composition of 1 M NaPF$_6$ in EC/DEC (1:1, v/v) with a 2% FEC additive. The capacity values stabilize around 10th cycle and capacity retention of 73% is observed after 60 cycles with respect to the 10th cycle charge capacity

Improving the rate capability of high voltage Lithium-ion battery cathode material LiNi0.5Mn1.5O4LiNi_{0.5}Mn_{1.5}O_{4} by ruthenium doping

The citric acid-assisted sol–gel method was used to produce the high-voltage cathodes LiNi0.5Mn1.5O4 and LiNi0.4Ru0.05Mn1.5O4 at 800 °C and 1000 °C final calcination temperatures. High resolution powder diffraction using synchrotron radiation, inductively coupled plasma – optical emission spectroscopy and scanning electron microscopy analyses were carried out to characterize the structure, chemical composition and morphology. X-ray absorption spectroscopy studies were conducted to confirm Ru doping inside the spinel as well as to compare the oxidation states of transition metals. The formation of an impurity LixNi1−xO in LiNi0.5Mn1.5O4 powders annealed at high temperatures (T ≥ 800 °C) can be suppressed by partial substitution of Ni2+ by Ru4+ ion. The LiNi0.4Ru0.05Mn1.5O4 powder synthesized at 1000 °C shows the highest performance regarding the rate capability and cycling stability. It has an initial capacity of ∼139 mAh g−1 and capacity retention of 84% after 300 cycles at C/2 charging–discharging rate between 3.5 and 5.0 V. The achievable discharge capacity at 20 C for a charging rate of C/2 is ∼136 mAh g−1 (∼98% of the capacity delivered at C/2). Since the electrode preparation plays a crucial role on parameters like the rate capability, the influence of the mass loading of active materials in the cathode mixture is discussed