context: LiMn2O4 (LMO) was used as a cathode to evaluate the electrochemical performance of Al2O3-coated LTO in the WiSE (Fig. S5†). The mass ratio of LMO:LTO was set as 2.5:1 to accommodate the low CE of LTO during the initial several cycles. 1C was used instead of a high rate to demonstrate the stability of the electrolyte in the full cell. Such LTO/LMO full cells delivered a voltage plateau at ∼2.4 V during discharge. The discharging capacity based on LTO mass was 145 mA h g−1. In the first cycle, a coulombic efficiency (CE) of 84.5% was delivered, indicating that a relatively small amount of electrolyte was consumed to form an additional LiF-rich SEI on the Al2O3-coated LTO anode. In comparison, pairing the uncoated LTO and LMO delivered a CE of 50% (Fig. S6†), further confirming the effect of the Al2O3 coating in suppressing the side reaction. As we reported previously, the reduction of salt anions bis(trifluoromethane sulfonyl)imide (TFSI) occurs between 1.9 V and 2.9 V. Although reduction of TFSI anions is expected to occur if an Al2O3 coating is absent (Fig. S7†), formation of a complete SEI needs a long time (i.e., few cycles in galvanostatic charge/discharge cycles). The lithiation potential of pristine LTO resides beyond the cathodic limit of the WiSE, so a significant HER will occur before lithiation. The persistent evolution of gas undoubtedly prevents complete formation of the SEI. For the initial cycles, when a robust SEI has not been constructed, protection of the Al2O3 surface serves as a key barrier to ensure that SEI chemistry occurs, and that the SEI ingredient formed from the reduction of the TFSI anion adheres to the anode surface. After the most challenging period in the initial cycles, a dense and complete SEI will come into shape (Fig. S7†), eventually providing long-term protection and allowing LTO to deliver a reversible capacity.
answer: LiMn2O4 (LMO)