Elucidation of Li_xNi_0.8Co_0.15Al_0.05O₂ Redox Chemistry by Operando Raman Spectroscopy

Abstract

The local structure evolution of Li x Ni0.8Co0.15Al0.05O2 (NCA) is linked to its electrochemical response during cycling (and overcharge) by operando Raman spectroscopy with findings supported by complementary techniques, such as online electrochemical mass spectrometry (OEMS) and density functional theory (DFT) phonon calculations. The vibrational motion of lattice oxygens is observed to be highly dependent on the local Li x MO2 lattice environment, e.g. MO bonding strength/length and state of lithiation x. All vibrational modes generally harden upon delithiation due to MO bond character (ionic → covalent) evolution (disregarding an early bond softening due to Li+ vacancy formation) and evidence the important influence of the local structural lattice configuration on the electrochemical response of NCA. Although the intensities of all Raman active bands generally increase upon delithiation, a major inflection point at x = 0.2 marks the onset of a partly irreversible fundamental transition within NCA that is most likely related to electron removal from MO bonding states and partial oxidation of oxygen sublattice, which is also indicated by the observed concomitant O2 release from the particle surface. Operando Raman spectroscopy with higher time resolution provides unique possibilities for detailed studies of how chemical parameters (Li+ vacancy formation, transition metal cation concentration, and lattice doping, etc.) may govern the onset and nature of processes (such as bond character evolution and stability) that define the performance of the Li x MO2 class of oxides. The further insights thus gained can be exploited to guide the development of next-generation layered cathodes for Li-ion batteries operating stably at higher voltages and capacities.

Understanding of performance degradation of LiNi0.80Co0.10Mn0.10O2 cathode material operating at high potentials

“…2 . In this region, the capacity and reversibility of NCM811 are affected by both of the H2 → H3 phase transition and the lattice oxygen redox [35,[41][42][43][44] , and the similar conclusions have been well documented on the Li-rich layered cathode materials [22,45,46] . The above partition on the capacity region can be supported by the following facts: (1) oxygen evolution initiates at 4.3 V vs. Li/Li + and becomes significant above 4.55 V [12,24] , (2) oxygen evolution is simultaneously accompanied by the generation of CO 2 and CO as a result of the chemical reactions between the active oxygen evolution intermediates and electrolyte solvents [12,13,47] , (3) the ultimate delithiation products are thermodynamically instable so that up to date neither pure NiO 2 nor pure CoO 2 has been prepared/reported, and (4) there is a delay, even by an operando technique, in the time (and further the potential) between oxygen evolution and detection by the instrument due to the need for the diffusion of oxygen molecules from the lattice to the surface of NCM811 particles.…”

In situ and Operando Raman Spectroscopy of Layered Transition Metal Oxides for Li-ion Battery Cathodes

“…X-ray absorption spectroscopy further supports the hypothesis of bulk oxygen oxidation at high states of charge (Yoon et al, 2007;Kleiner et al, 2015). Our operando Raman spectroscopy measurements with high time resolution was demonstrated to provide further detailed insights into local structural evolution of NCA lattice upon electrochemical cycling (Flores et al, 2018).…”

“…Although NCA based electrodes provide outstanding specific charge, a performance fade commonly associated with Nimigration from the transition metal layer into the Li sites and the safety concerns due to highly exothermic reactions with electrolyte at high SOC motivate the further development of the LiMO 2 based oxides (Liu et al, 2015b). Partial substitution with Mn has been found to suppress Li/Ni exchange, improve FIGURE 7 | (A) Comparison of the in situ evolution of NCA Raman spectrum as recorded by Kostecki et al (Lei et al, 2005) and here adapted along with our results (Flores et al, 2018). (B) Differential charge plot of NCA together with the fitted peak positions and heights of the E g and A 1g bands as a function of lithium content during the first galvanostatic cycle between 3.0 and 4.3 V vs. Li + /Li (∼C/20 rate).…”

“…The crystallographic lattice parameters and symmetry might remain invariant to the occurrence of highly local phenomena, such as non-cooperative Jahn Teller distortion (Rougier et al, 1995;Chen et al, 2011) and cation ordering (Reimers, 1992;Peres et al, 1999;Zeng et al, 2007), which are thermodynamically favored but invisible when solely investigating the long-range order (Dompablo et al, 2001;Zhecheva et al, 2002). Our recent study of NCA make particular emphasis on the lack of correlation between the lattice parameters and Raman band positions (Flores et al, 2018). Alternative techniques probing the local lattice structure, such as x-ray absorption and nuclear magnetic resonance spectroscopy, are therefore most suitable and further validation studies in the field are desired.…”

Probing and Resolving the Heterogeneous Degradation of Nickel‐Rich Layered Oxide Cathodes across Multi‐Length Scales

“…Establishing fundamental understandings on the relationship between NRLO and its Raman spectra is necessary and needs further investigation by computational models and theoretic calculations. [ 97,100 ]…”