Effect of alkali-metal phosphate additives on the interphase structure of 5-V LiNi0.5Mn1.5O4-based lithium–ion batteries

High-voltage (5-V) lithium–ion batteries (LIBs) have garnered substantial interest due to their superior energy density. Nevertheless, the unstable interphase of high-voltage LIBs can cause electrolyte decomposition, electrode collapse, transition metal dissolution, and lithium dendrite growth during cycling, which impedes their widespread commercial use. To mitigate these challenges, various alkali-metal phosphates were incorporated into a carbonate-based electrolyte as film-forming additives, aiming to improve the electrochemical performance of LiNi _0.5Mn _1.5O ₄ (LNMO)-based LIBs. Following a detailed comparison of the electrochemical performances of various alkali-metal phosphates, KH ₂PO ₄ and NaH ₂PO ₄ were selected for an in-depth investigation in this research. The highly protective passivation film generated by KH ₂PO ₄ and NaH ₂PO ₄ on the electrode surface provided significant protection by effectively inhibiting continuous electrolyte decomposition. This finding was supported by density functional theory calculations and a variety of electrochemical and characterization tests. The H ₂PO ₄ ⁻ primarily contributed to the formation of the cathode electrolyte interphase film on the cathode surface. KH ₂PO ₄ (via the electrostatic shielding effect of K ⁺ions) and NaH ₂PO ₄ (through the co-precipitation of Na ⁺ and Li ⁺ ions) both effectively inhibited the growth of lithium dendrites on the lithium metal anode. Additionally, the KH ₂PO ₄ and NaH ₂PO ₄ additives regulated the amount of HF in the electrolyte through acid–base equilibrium reactions. The electrolytes containing additives significantly enhanced the electrochemical performances of Li||Li, LNMO/Li, LNMO/graphite, and LNMO/Li ₄Ti ₅O ₁₂ cells. LNMO/Li half-cells with standard electrolyte (STD) supplemented with KH ₂PO ₄ and NaH ₂PO ₄ additives demonstrated capacity retentions of 92.9 % and 90.0 % after 500 cycles at 5 C, respectively, in contrast to only 79.4 % for the STD electrolyte alone. Furthermore, the LNMO/graphite full cell exhibited an increased capacity retention, reaching 81.1 % (STD+KH ₂PO ₄) and 78.29 % (STD+NaH ₂PO ₄), up from 70.8 % with the STD, after 100 cycles at 0.5 C. Given their ease of use, affordability, and widespread availability, KH ₂PO ₄ and NaH ₂PO ₄ additives show significant potential to enhance the electrochemical properties of high-voltage LIBs.