Understanding the Improvement Mechanism of Triethyl Borate as an Electrolyte Additive for 5 V Spinel/Graphite Lithium-Ion Batteries

In the past decade, cobalt-free LiNi_0.5Mn_1.5O₄ (LNMO) spinel has been considered as a next generation cathode because of its high operating voltage (~4.7 V vs. Li/Li⁺), low cost, and good power capability. However, the main challenge for a commercialization of LNMO cathode lies in parasitic reactions occurring at cathode-electrolyte interface (CEI) due to oxidative decomposition of electrolytes. Resulting side-reaction products migrate and attack solid-electrolyte interphase (SEI) layer on graphite anodes, which in turn leads to an unwanted consumption of active Li-ion and consequent capacity fading.<br/>Significant R&D efforts have been dedicated to finding strategies to overcome the electrolyte instability issues at high-voltage battery cells. One of the most effective pathways is adopting electrolyte additives. Among various additives, triethyl borate (TEB) has been reported as an effective additive that can passivate the LNMO CEI in half-cells (i.e., LNMO/Li). However, there is a lack of literature data proving that TEB is still useful in LNMO/graphite full-cells. In particular, the most critical barrier for the commercialization of LNMO is electrolyte oxidation and concurrent parasitic reactions at SEI on graphite anodes, which in turn leads to a poor cycle life of LNMO/graphite full-cells.<br/>In this work, we investigated the role of TEB additive on the interfacial stabilities of electrodes in full-cells. Electrochemical measurement results indicated that TEB additive greatly improve full-cells cycle life by lower active Li⁺ loss during cell formation cycles. By Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analyses, we confirmed that TEB participated in formations of both CEI and SEI layers. However, combinatorial study of the TEB-treated graphite and LNMO electrodes revealed that the performance improvement of the full-cells was mostly attributed to an improved SEI stability on graphite anodes. For example, dQ/dV profiles showed the reductive decomposition of TEB earlier than EC during the SEI formation on graphite anodes.<br/>In addition, TEB additives suppressed the formation of LiF on the cycle-aged graphite SEI as evidenced by XPS analysis. This result indicates that TEB inhibited electrolyte decomposition and suppress the Li-ion consumption by maintaining a thinner SEI on graphite. As a result, the full-cells with TEB additive delivered 10% improved cycle life compared with the baseline data (i.e., no additive). Among the various TEB compositions (0 – 4 wt%) investigated, 1 wt% TEB offered both improved capacity retention and low cell resistance (50% decrease from the baseline data at 50^th cycle) as evidenced by electrochemical impedance spectroscopy (EIS) analysis. Results from this work offered a fundamental understanding on the improvement mechanisms of TEB additive at electrode-electrolyte interphases.