Spinel LiNi0.5Mn1.5O4 Cathode for High-Energy Aqueous Lithium-Ion Batteries

DOI: 10.1002/aenm.201600922 considering the overpotential during charge process. Recently, Yamada et al. reported that LiNi0.5Mn1.5O4 can only reversibly provide capacity of ≈75 mA h g−1 in the more concentrated hydrate melt electrolytes (≈30 mol kg−1), which is 50% of theoretical capacity.[14] The oxygen evolution side reaction also largely significantly reduce the coulombic efficiency. In addition to P4332 structure, LiNi0.5Mn1.5O4 also has another structure with the space groups of Fd-3m. In P4332 LiNi0.5Mn1.5O4, all Mn-ions exist as Mn4+, while in Fd-3m LiNi0.5Mn1.5O4, small amount of Mn3+ coexists along with Mn4+.[15] The larger ionic radius of Mn3+ compared to Mn4+ expands lattice, enhancing the Li+ diffusion.[16] The structure difference in LiNi0.5Mn1.5O4 also changes the lithiation/delithiation potentials.[17] Fd-3m LiNi0.5Mn1.5O4 has two distinguished plateaus at 4.6 V (Ni2+/3+) and 4.8 V(Ni3+/4+). The 4.6 V of redox Ni2+/3+ can be fully utilized since it is completely inside the electrolyte stable window even taking consideration of the potential shift. Although the 4.8 V plateau shifts to 5.0 V, which is beyond the 4.9 V window of water-in-salt electrolyte, part capacity of LiNi0.5Mn1.5O4 at 5.0 V can still be achieved due to the fast lithiation reaction in Fd-3m LiNi0.5Mn1.5O4 and slow oxygen evolution reaction. If the pH value of electrolyte can be reduced, all the capacity can potentially be utilized. In the present work, the electrochemical behaviors of two LiNi0.5Mn1.5O4 cathodes with Fd-3m and P4332 structures in the water-in-salt electrolytes were systematically investigated. After screening, LiNi0.5Mn1.5O4 with Fd-3m structure was selected and paired with Mo6S8 anode. A 2.9 V LiNi0.5Mn1.5O4/ Mo6S8 ALIB delivered 80 W h kg−1 energy density with capacity decay only 0.075% per cycle (5 C). After reducing the pH value of the water-in-salt electrolyte from 7 to 5, almost full capacity of LiNi0.5Mn1.5O4 (125 mA h kg−1) was achieved in the aqueous electrolyte for the first time, and 126 W h kg−1 energy density was provided for the LiNi0.5Mn1.5O4/Mo6S8 full cell, representing one of the highest voltage and energy density among all the aqueous batteries reported so far. LiNi0.5Mn1.5O4 with Fd-3m or P4332 structures were synthesized according to the previous literatures.[13,18,19] X-ray diffraction (XRD) Rietveld refinements confirm the two different structures (Figure 1a,b). By carefully comparing the XRD in Figure 1a,b, two small super lattice peaks at 15.3° and 39.7° are observed in P4332 LiNi0.5Mn1.5O4 in Figure 1b but they are absent in Fd-3m LiNi0.5Mn1.5O4 in Figure 1a. The structure difference between two LiNi0.5Mn1.5O4 was further enhanced by transferring the XRD patterns of the two structures into Log 10 intensity (Figure S1, Supporting Information). The structure difference between Fd-3m and P4332 LiNi0.5Mn1.5O4 is also captured by Raman spectrum, where more peaks are observed in the P4332 LiNi0.5Mn1.5O4 (Figure 1e) than that in Lithium ion batteries (LIBs) have been widely acknowledged as the high-energy battery system for grid storage and electric vehicles, but the safety concern due to the flammability of organic electrolytes still hinders their wide application.[1–3] To address the issue, aqueous lithium-ion batteries (ALIBs) using nonflammable and low-toxic aqueous electrolytes are receiving intense attention as the alternatives.[4–8] The aqueous electrolytes also make it possible to get rid of the rigorous moisturefree manufacturing environment and heavy reliance on the battery management systems at module or pack levels. Since the voltage of ALIBs is intrinsically limited by the narrow thermodynamic stability window of aqueous electrolyte, the ALIBs have a much lower energy density (40 W h kg−1) than that of LIBs (200 W h kg−1).[9,10] Despite of over two decades’ materials innovation, the battery community has not witnessed much progress in improving the capacity of ALIBs’ electrodes. The most effective method in increasing the energy density is to enhance cell voltage by enlarging the electrochemical stability window of aqueous electrolytes and identifying viable electrode materials. Recently, our group has made a significant breakthrough in doubling electrochemical stability window of aqueous electrolyte from 1.5 to 3.0 V (1.9–4.9 V)[11] using water-in-salt electrolytes. A 2.3 V LiMn2O4/Mo6S8 full cell using water-in-salt electrolytes was demonstrated to cycle up to 1000 times, with nearly 100% coulombic efficiency at both a low (0.15 C) and a high (4.5 C) discharge and charge rates.[11] However, LiMn2O4 with lithiation/delithiation potential of 4.2 V does not fully use the oxygen evolution potential of 4.9 V in water-in-salt electrolytes. Commercial spinel LiNi0.5Mn1.5O4 with P4332 structure has a higher operating voltage (a slope plateau from 4.6 to 4.8 V for a continuous redox reaction of Ni2+/3+/4+ in organic electrolyte) than LiMn2O4 (single plateau of 4.2 V).[12,13] LiNi0.5Mn1.5O4 should provide much high energy in water-in-salt electrolyte since it has similar capacity with LiMn2O4. However, due to the high salt concentration of the water-in-salt electrolytes, the redox lithiation/delithiation potential plateau of LiNi0.5Mn1.5O4 positively shifts by ≈0.2V[11] to 4.8–5.0 V, which is over the edge of the stable window of electrolyte. The single plateau of P4332 LiNi0.5Mn1.5O4 can only provide <50% of capacity if