LiNi_0.5Mn_1.5O₄ Thin Films Grown by Magnetron Sputtering under Inert Gas Flow Mixtures as High‐Voltage Cathode Materials for Lithium‐Ion Batteries

Abstract

Delivering a commercial high‐voltage spinel LiNi0.5Mn1.5O4 (LNMO) cathode electrode for Li‐ion batteries would result in a significant step forward in terms of energy density. However, the structural ordering of the spinel and particle size have considerable effects on the cathode material's cyclability and rate capability, which are crucial challenges to address. Here, a novel mid‐frequency alternating current dual magnetron sputtering method was presented, using different Ar‐N2 gas mixtures ratios for the process gas to prepare various LNMO thin films with highly controlled morphology and particle size; as determined from X‐ray diffraction, Raman spectroscopy and electron microscopy. It resulted in enhanced cycling and rate performance. This processing method delivered N‐containing LNMO thin film electrodes with up to 15 % increased discharge capacity at 1 C (120 mAh g−1) with respect to standard LNMO (grown under only Ar gas flow) thin film electrodes, along with outstanding rate performance up to 10 C (99 mAh g−1) in the operating voltage window 3.5–4.85 V vs. Li+/Li. Besides, electrochemical impedance spectroscopy results showed that the intricate phase transitions present in standard LNMO electrodes were almost suppressed in N‐containing LNMO thin films grown under different Ar‐N2 gas flow mixtures.

Evaluation of Electronic–Ionic Transport Properties of a Mg/Zr-Modified LiNi0.5Mn1.5O4 Cathode for Li-Ion Batteries

“…Consistent with the Nyquist plots, for b-LNMO, the values of R elec continuously decrease during charge from 3.500 to 3.950 V; then, the values increase from 3.950 to 4.375 V of about 1 order of magnitude, to decrease again from 4.375 to 4.700 V. This behavior can be explained with the overlap of two concurrent and counteracting phenomena: an increase of material conductivity as Li + is progressively extracted from the structure, 60 which is partly counteracted in a limited potential region (3.950 ≤ E ≤ 4.375 V) by structural rearrangements associated with Mn 3+ to Mn 4+ oxidation, which leads to the coexistence of multiple cubic phases, resulting in lattice mismatch, structural stress, and, ultimately, entangled Li + and e – diffusion in the structure. 57 As the result of the two concurrent phenomena, the R elec reaches the minimum value at 4.700 V; then, the values significantly increase by about 2 orders of magnitude as a consequence of the rock salt-like phase formation, where some transition metal (TM) ions migrate from 16d to 16c sites. 9 , 10 The occupation of the 16c sites by TM ions at the end of the charging process may block Li + diffusion pathways through the active grains, resulting in limitations to the concurrent electron mobility.…”

“…56 In order to investigate the impact of reversible and irreversible structural transitions on the kinetics of the charge/discharge processes, EIS measurements at several states of charge and potentials have been All of the spectra show some common features, which may appear more or less pronounced depending on the state of charge and the sample investigated. Based on previous EIS investigations of LIB cathode materials, 36,57 the following contributions to overall impedance can be recognized in the Nyquist plots and assigned to specific steps in the electronic/ ionic transport and redox processes, from high to low frequencies: (i) an intercept with the real axis, which describes a pure resistive behavior and can be attributed to electrolyte resistance; (ii), (iii) two convoluted arcs in the high-to-middle frequencies, where the higher frequency one is much smaller, nevertheless, can be observed clearly, for instance, in the inset in panel 8g, and can be related, respectively, to migration/ accumulation of Li + at the CEI and charge transfer resistance/ charge accumulation at the electrical double layer; (iv) a lowfrequency arc, which is the most relevant feature at low potentials and can be assigned to bulk electronic resistance of the material 49 and intragrain charge accumulation at crystallite boundaries; 58 and (v) a 45 deg dispersion, bending toward a vertical line, which commonly describes diffusion toward a blocking electrode. The most relevant trend that can be observed through the impedance dispersions is variation of the diameter of the low-frequency semicircle, which undergoes contraction and expansion for both electrodes, even if with different paths, upon charge.…”

Oxide Coating Role on the Bulk Structural Stability of Active LiMn2O4 Cathodes

“…They are one of the first choices as a power supply for consumer electronics and electric vehicles . Today, with respect to the high capacity obtainable from anode materials, the limited capacity of the cathode materials represents the bottleneck of the technological advancement for these battery devices. , Recent studies highlighted the critical role of the active material–electrolyte interface in the prevention of the capacity fading of the cathode. , The repeated cycling of the battery electrodes outside the electrolyte’s voltage window of stability provokes the formation of a protective passivating layer on the electrodes, the so-called solid electrolyte interphase (SEI) on the anodes and cathode–electrolyte interphase (CEI) on the cathode. , Such protective layers prevent ulterior oxidation/reduction at the active material–electrolyte interface and stabilize the diffusion of Li + ions, essential for battery operation. It is known that in the presence of the electrolyte salt the redox process starts almost immediately, while the formation of passivation layers occurs primarily during the first cycle of battery operation. − However, a limited growth of the passivation layers on Li-ion cathode materials can result in a consequent aging of the electrodes and substantial deterioration of the active material.…”

“… 1 Today, with respect to the high capacity obtainable from anode materials, the limited capacity of the cathode materials represents the bottleneck of the technological advancement for these battery devices. 2 , 3 Recent studies highlighted the critical role of the active material–electrolyte interface in the prevention of the capacity fading of the cathode. 4 , 5 The repeated cycling of the battery electrodes outside the electrolyte’s voltage window of stability provokes the formation of a protective passivating layer on the electrodes, the so-called solid electrolyte interphase (SEI) on the anodes and cathode–electrolyte interphase (CEI) on the cathode.…”

Activation Energy of Ion Diffusion in an Electrode Material: Theoretical Calculation and Experimental Estimation with LiCoVO4 as an Example

“…The full realization of the theoretical possibilities corresponds to a specific energy of 622 W•h•kg −1 . This value is more than or comparable to commercially available cathode materials such as LiCoO 2 (518 W•h•kg −1 ), LiMn 2 O 4 (400 W•h•kg −1 ), LiFePO 4 (495 W•h•kg −1 ), LiCo 1/3 Ni 1/3 Mn 1/3 O 2 (576 W•h•kg −1 ), and LiNi 0.5 Mn 1.5 O 4 (610 W•h•kg −1 ) [4,5]. LiCoVO 4 has the potential to provide an alternative to the electrode materials currently in use, as well as complementing a set of materials to choose from for specific applications and conditions.…”