Depth-Dependent Redox Behavior of LiNi_0.6Mn_0.2Co_0.2O₂

Abstract

Nickel-rich layered materials are emerging as cathodes of choice for next-generation high energy density lithium ion batteries intended for electric vehicles. This is because of their higher practical capacities compared to compositions with lower Ni content, as well as the potential for lower raw materials cost. The higher practical capacity of these materials comes at the expense of shorter cycle life, however, due to undesirable structure and chemical transformations, especially at particle surfaces. To understand these changes more fully, the charge compensation mechanism and bulk and surface structural changes of LiNi 0.6 Mn 0.2 Co 0.2 O 2 were probed using synchrotron techniques and electron energy loss spectroscopy in this study. In the bulk, both the crystal and electronic structure changes are reversible upon cycling to high voltages, whereas particle surfaces undergo significant reduction and structural reconstruction. While Ni is the major contributor to charge compensation, Co and O (through transition metal-oxygen hybridization) are also redox active. An important finding from depth-dependent transition metal L-edge and O K-edge X-ray spectroscopy is that oxygen redox activity exhibits depth-dependent characteristics. This likely drives the structural and chemical transformations observed at particle surfaces in Ni-rich materials. The need for lithium-ion batteries with higher energy density and lower cost than currently available, particularly for transport applications, has led to intensified interest in Ni-rich NMC (LiNi x Mn y Co z O 2 ; x+y+z≈1, where x>y) cathode materials.1-5 These materials deliver higher practical capacities in a typically used voltage range than NMCs with lower Ni content (e.g., LiNi 1/3 Mn 1/3 Co 1/3 O 2 or NMC-333), and most formulations contain less of the expensive Co component, reducing raw material costs. The increase in practical capacity roughly scales with the Ni content, but comes at the expense of cycle life and thermal stability at high states-of-charge (SOC). 6 To circumvent these problems, several different strategies have been utilized to improve cycling, particularly to higher potentials. These include partial substitution with Ti 7-9 or Zr, 10 engineering the micro-or nano-structure to reduce surface Ni content using metal segregation, 11 surface pillared structures, 12 and concentration gradients, 13 coating particle surfaces, 14 and development of electrolyte additives. 15,16 While all of these approaches have resulted in improvements, further understanding of the factors that lead to capacity fading is clearly needed in order to meet the stringent performance requirements of traction applications.The formation of a resistive cathode/electrolyte interphase (CEI), such as an electrolyte decomposition layer, has been observed during cycling of LiNi 0. 20 In the study of NMC-532, it was found that surface reconstruction to a rock salt phase dominates when high voltage (4.8 V) cutoffs are used due to oxygen loss under the highly oxidizing conditions, while ...

Soft X-ray Absorption Spectroscopic Investigation of Li(Ni0.8Co0.1Mn0.1)O2 Cathode Materials

“…The intensity ratio is at its maximum for the fully charged cathode and reduces thereafter. Similar behavior is also observed by Tian et al [39]. This shows the modulation of metal-oxygen hybridization during charging and discharging.…”

“…This shows the modulation of metal-oxygen hybridization during charging and discharging. The ratio is different from that obtained from the Ni L-edge spectra (Figure 7a); this may be due to the influence of these symmetry states from other metal ions such as Mn and Co [39] as well as to the creation of oxygen vacancies during the charging and discharging process [18]. Figure 8 shows the O K-edge spectra of the NCM811 cathode during the charging and discharging process.…”

Surface Degradation and Chemical Electrolyte Oxidation Induced by the Oxygen Released from Layered Oxide Cathodes in Li−Ion Batteries

“…This behavior is identified by the strong Ni L 3 ‐edge feature evolution at 855 eV and the shift to higher photon energy of the Co L 3 ‐edge accompanied by a broadening of the feature at 782.4 eV. These variations correlate with the broadening of the O‐peaks at 528.8 eV and 531.9 eV, highlighted by the blue arrows, as a result of the progressive depopulation of the hybridized states O 2p ‐TM 3d . As reported recently,, the O K‐edge can be used also to track the “O‐redox” in the bulk of the particles, by monitoring the evolution of the component at 530.8 eV.…”

“…The Li-rich NCM in Region 1 is compared to NCM111 cycled at 4.11 V, a potential where both Co and Ni are supposed to be in a full oxidization state of + 4 (see NCM111 cyclic voltammetry curve in Figure S6, Supplementary Information) and where oxygen redox activity or gas evolution is not expected. [30,31] The local XAS on NCM111 confirm the presence of Co 4 + and Ni 4 + , as attested by the shift to higher photon energy of the Co L 3 -edge and the rapid increase of the high-energy peak in the Ni L 3edge at 855 eV ( Figure S7, Supplementary Information). The absorption feature at 855 eV, characteristic for the oxidized Ni 4 + , is considerably more pronounced in comparison to the Lirich NCM at 4.16 V. This behavior is not surprising since at 4.16 V the Li-rich NCM does not reach the full oxidation of the TMs as for NCM111.…”

Fracture behavior in battery materials

“…When the Ni concentration is low, such as for a LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC333) cathode, the H 3 phase only appears at potentials above 5 V vs. Li/Li + [82]. When the Ni concentration is intermediate, such as for a LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) cathode, the H 2 → H 3 transition occurs near 5 V during the charge process [83].…”