Ambient and Elevated Temperatures. ChemRxiv. Preprint. download file view on ChemRxiv Manuscript.pdf (2.43 MiB) download file view on ChemRxiv Supporting Information.pdf (1.24 MiB)

In-Situ Neutron Diffraction and Performance in Li-Ion Full Cells. ChemRxiv. Preprint. Covers thorough study showing the possible strategies to decouple effect of oxygen deficiency, the presence of Mn 3+ and degree of cation ordering in the high power positive electrode material for lithium ion batteries, LiNi 0.5 Mn 1.5 O 4 . File list (2) download file view on ChemRxiv LNMO_ordering_vs_oxygen_October10.pdf (1.40 MiB) download file view on ChemRxiv Paper3_SI.pdf (277.95 KiB) Cation ordering and oxygen release in LiNi0.5-xMn1.5+xO4-y (LNMO):In-situ neutron diffraction and performance in Li-ion full cells

The crystal structure of LiNi0.5Mn1.5O4 (LNMO) can adopt either low-symmetry ordered (Fd3̅m) or high-symmetry disordered (P4332) space group depending on the synthesis conditions. A majority of published studies agree on superior electrochemical performance of disordered LNMO, but the underlying reasons for improvement remain unclear due to the fact that different thermal history of the samples affects other material properties such as oxygen content and particle morphology. In this study, ordered and disordered samples were prepared with a new strategy that renders samples with identical properties apart from their cation ordering degree. This was achieved by heat treatment of powders under pure oxygen atmosphere at high temperature with a final annealing step at 710 °C for both samples, followed by slow or fast cooling. Electrochemical testing showed that cation disordering improves the stability of material in charged (delithiated) state and mitigates the impedance rise in LNMO∥LTO (Li4Ti5O12) and LNMO∥Li cells. Through X-ray photoelectron spectroscopy (XPS), thicker surface films were observed on the ordered material, indicating more electrolyte side reactions. The ordered samples also showed significant changes in the Ni 2p XPS spectra, while the generation of ligand (oxygen) holes was observed in the Ni–O environment for both samples using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). Moreover, high-resolution transmission electron microscopy (HRTEM) images indicated that the ordered samples show a decrease in ordering near the particle surface after cycling and a tendency toward rock-salt-like phase transformations. These results show that the structural arrangement of Mn/Ni (alone) has an effect on the surface and “near-surface” properties of LNMO, particularly in delithiated state, which is likely connected to the bulk electronic properties of this electrode material.

The effect of the electrolyte additive fluoroethylene carbonate (FEC) for Li-ion batteries has been widely discussed in literature in recent years. Here, the additive is studied for the high-voltage cathode LiNi 0.5 Mn 1.5 O 4 (LNMO) coupled to Li 4 Ti 5 O 12 (LTO) to specifically study its effect on the cathode side. Electrochemical performance of full cells prepared by using a standard electrolyte (LP40) with different concentrations of FEC (0, 1 and 5 wt%) were compared and the surface of cycled positive electrodes were analyzed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The results show that addition of FEC is generally of limited use for this battery system. Addition of 5 wt% FEC results in relatively poor cycling performance, while the cells with 1 wt% FEC showed similar behavior compared to reference cells prepared without FEC. SEM and XPS analysis did not indicate the formation of thick surface layers on the LNMO cathode, however, an increase in layer thickness with increased FEC content in the electrolyte could be observed. XPS analysis on LTO electrodes showed that the electrode interactions between positive and negative electrodes occurred as Mn and Ni were detected on the surface of LTO already after 1 cycle. 1 In addition to the high voltage plateau, the intrinsic structure of the spinel phase allows fast lithiation and de-lithiation kinetics 2,3 which is attributed to the 3-D lithium transport among the available tunnels in the crystal. 4 The advantage of higher voltage (thus higher energy density) and good power capability are two main reasons behind the growing interest for this material, especially for electric vehicle applications. However, the electrode material is prone to side reactions with conventional electrolytes, not least electrolyte decomposition at high voltages and transition metal dissolution from the spinel structure, particularly observed at elevated temperatures. These obstacles need to be resolved before wide-scale commercial application of LNMO electrodes. 5,6 One possible approach to overcome these problems is to use electrolyte additives that could ideally form an in-situ passivating layer on the positive electrode surface, similarly to the solid electrolyte interphase (SEI) observed on negative LiB electrodes. 7,8 A number of electrolyte additives have been reported in recent years for the purpose of passivating the cathode/electrolyte interface at high operating voltages.7 Some of these additives include LiBOB, 9 succinic anhydride, 10 HFiP 11 and DMMP. 12 It is important to note that the use of such cathode interface additives should also be compatible with the anode interface in full cells, or vice versa for additives intended for the anode side. Therefore, it is essential to study the effect of anode additives on the cathode side as well, if new fullcell chemistries are to be realized. Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are common examples of such additives where the former has been used mostly to improve an...

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