Transition metal speciation as a degradation mechanism with the formation of a solid-electrolyte interphase (SEI) in Ni-rich transition metal oxide cathodes

Abstract

An ever-growing demand for high-energy density and high-power Li-ion batteries has driven active research for electrode materials with superior capacity. Recent years have seen the development of Ni-rich transition metal oxide cathode materials due to their high reversible capacity and lower cost. To achieve full capacity from the charge compensation process, a high voltage (>4.4 V) charging is required. However, the battery operation at higher voltages eventually results in dramatic capacity fading and voltage decay with a rapid decomposition of the electrolyte upon further charge-discharge. While previous studies have reported the degradation mechanism within the electrode surface, there have been few empirical investigations into the solid-electrolyte interphase (SEI) formation on Ni-rich cathodes. In the current work, we visualize the different nature of the electrode-electrolyte interphase at various cut-off voltages (2.0-4.2 V, 2.0-4.5 V, and 2.0-4.8 V). We correlate the key properties of the SEI layer with the capacity fading mechanism in the high capacity battery system. The speciation of transition metal elements (Ni, Mn, and Co) into various oxidation and spin states has been identified as the dominant process of the capacity degradation. Fig. 7 (a) Proposed SEI structures/properties at different cut-off voltages with transition metal spin states, crystal structures, schematics of energy vs. density of states (N(E)) diagram, and visualizations of Li + trapping based on the SIMS profile in the Ni-rich transition metal oxide cathode. (b) Correlation between cut-off voltages, lithium diffusivity (D Li ), SEI impedance (R SEI ), and magnetic moment (m B ).

Surface/Interface Structure Degradation of Ni‐Rich Layered Oxide Cathodes toward Lithium‐Ion Batteries: Fundamental Mechanisms and Remedying Strategies

“…As a consequence, the LiPF 6 salt may undergo the following dissociation reaction: LiPF 6 ⇌ LiF + PF 5 , followed by the hydrolysis of PF 5 : PF 5 + H 2 O → POF 3 + 2HF, according to Aurbach and Heider's theory . Additionally, the electrolyte decomposition will be accelerated as the Ni‐rich cathodes operate at high cutoff voltages and operation temperatures . Accordingly, the detrimental side reactions are triggered, leading to the serious dissolution of TM ions, which follows a trend of Mn > Co > Ni, as reported by Sun and co‐workers .…”

Challenges and Strategies to Advance High‐Energy Nickel‐Rich Layered Lithium Transition Metal Oxide Cathodes for Harsh Operation

“…have reported that transition metal speciation into various oxidation and spin states during charged to higher voltage, and the deposition of the modified TM as SEI species cause the dramatic capacity fading and voltage decay of Ni‐rich cathodes. [ 25 ] The correlation between TM dissolution and the cut‐off voltage could be i) increasingly exposed surfaces due to particle cracking by high‐voltage charging; ii) the widening of the TMO band due to the oxygen loss by high‐voltage charging and iii) the phase transition induced by high‐voltage charging.…”

Microstructure controlled ZnCo 2 O 4 /C microhydrangea nanocomposites as highly reliable anodes for lithium‐ion batteries

“…It is observed that a reduction peak appeared at~0.71 V in the first discharge as a possible result of the typical redox features of transition metal oxides system. 25 Accordingly, two oxidation peaks appeared at~1.61 and 1.84 V during the first charge and can be attributed to the oxidation process. 26 However, the reduction peak during the second cycle is shifted to higher voltage of about 0.85 V, corresponding to the activation of active materials.…”