Evolution of the Electrode–Electrolyte Interface of LiNi_0.8Co_0.15Al_0.05O₂ Electrodes Due to Electrochemical and Thermal Stress

Abstract

For layered oxide cathodes, impedance growth and capacity fade related to reactions at the cathode-electrolyte interface (CEI) are particularly prevalent at high voltage and high temperatures. At a minimum, the CEI layer consists of Li2CO3, LiF, reduced (relative to the bulk) metal-ion species, and salt decomposition species but conflicting reports exist regarding their progression during (dis)charging. Utilizing transport measurements in combination with x-ray and nuclear magnetic resonance spectroscopy techniques, we study the evolution of these CEI species as a function of electrochemical and thermal stress for LiNi0.8Co0.15Al0.05O2 (NCA) particle electrodes using a LiPF6 ethylene carbonate: dimethyl carbonate (1:1 volume ratio) electrolyte. Although initial surface metal reduction does correlate with surface Li2CO3 and LiF, these species are found to decompose upon charging and are absent above 4.25 V. While there is trace LiPF6 breakdown at room temperature above 4.25 V, thermal aggravation is found to strongly promote salt breakdown and contributes to surface degradation even at lower voltages (4.1 V). An interesting finding of our work was the partial reformation of LiF upon discharge which warrants further consideration for understanding CEI stability during cycling.

Effects of Charge Cutoff Potential on an Electrolyte Additive for LiNi0.6Co0.2Mn0.2O2–Mesocarbon Microbead Full Cells

“…The peaks at 531.2 and 532.2 eV correspond to P═O (Li x PO y and Li x PO y F z ) and C═O (ROLi and LiO—C═O), respectively. In addition, the peaks at ≈533.2 eV belong to C—O—C (ether) and C—OH (alcohol), while the peaks at 534 eV are assigned to C—O (C—O—C═O, C—O—C[═O]O, O—P—F, and P—O—P═O) . However, after cycling at 4.25 and 4.45 V, the intensity of the peaks at ≈531.2 eV (P═O) is enhanced due to TMSPi decomposition.…”

Surface and Interfacial Chemistry in the Nickel‐Rich Cathode Materials

“…The decompositions of the electrolyte solvent were accompanied by the gas generation such as CO 2 , CO, and C x H y . Furthermore, it has been reported that the decompositions of the solvent were enhanced with increasing the temperature . Shikano et al.…”

“…Recently, Lebens‐Higgins et al. demonstrated that the decomposition of the LiPF 6 salt exhibited strong correlation with the battery operation temperature . They found that the by‐products formed by the LiPF 6 breakdown existed at the room temperature above 4.25 V, however, the thermal aggravation strongly promoted the salt decompositions at the lower voltage.…”

“…[103,[107][108][109][110][111] Furthermore, it has been reported that the decompositions of the solvent were enhanced with increasing the temperature. [105,112] Shikano et al analyzed the formation of organic SEI layer in the Li-Ni 0.73 Co 0.17 Al 0.10 O 2 cathode after the electrochemical test at different temperatures. [105] With increasing the temperatures, the intensity of the organic SEI layer components such as semi-carbonate and poly-carbonates type compounds significantly increased ( Figure 3b).…”

“…[17,[115][116] Such decompositions of the residual lithium compounds were highly facilitated as increasing the charge cut-off voltage in the cell (Figure 3c). [112] After first charge, the small amount of the Li 2 CO 3 was reformed during the discharge process. To date, the detailed mechanisms regarding the reformation of the Li 2 CO 3 have not been clearly revealed.…”