Lithium-rich manganese-based oxides (LRMOs; xLi2MnO3 (1 − x) LiMO2; M = transition metal, 0 < x < 1) with excellent specific capacity (> 300 mAh/g) and high operating voltage (≥4.8 Ⅴ) are the preferred cathode materials for high-specific-energy lithium metal batteries (LMBs) [1]. However, LRMOs face a series of serious problems such as irreversible lattice oxygen loss, transition metal (TM) migration, phase transfer, and interfacial side reactions at high voltages, resulting in rapid decay of capacity and voltage [2, 3]. In situ generating well-functional CEI through electrolyte engineering can effectively address these challenges [4]. The exploration of partially fluorinated electrolytes can improvethe high-voltage stability of electrolyte and the electrochemical performance of Li||LRMO batteries, and take into accounts the cost and ionic conductivity problems. Moreover, the performance of high-specific-energy LMBs can be enhanced by modifying the composition of CEI with a tiny amount of electrolyte additives.
Recently, Ma's group has explored partially fluorinated electrolyte with 0.5 wt% TMSB additive for achieving the stability of 4.9 Ⅴ Li||LRMO batteries [5]. Compared with EE electrolyte (1 mol/L LiPF6 in EC/EMC), the FE electrolyte (denoted as 1 mol/L LiPF6 in FEC/EMC) displayed better high-voltage stability, which could be seen from cycling performance at 4.8 Ⅴ and 4.9 Ⅴ. This is attributed to the advantage of partially fluorinated electrolyte from F-rich CEI for effectively suppressing the continuous decomposition of electrolyte. 0.5 wt% TMSB was added to FE electrolyte (TFE electrolyte) to further improve the cycling stability of Li||LRMO battery. In Fig. 1a, the capacity retention rates of Li||LRMO batteries with FE and TFE electrolytes after 300 cycles at 4.8 Ⅴ/0.5 C were 71.6% and 85.7%, respectively. Furthermore, the TFE electrolyte showed better performance at both 4.9 Ⅴ (Fig. 1b) and 5 C (Fig. 1d), which further indicated that TMSB inhibited the loss of Li during the cycling process. The EIS curves (Fig. 1f) of Li||LRMO batteries with additive had smaller impedance and little change with the cycling. In addition, Ma et al. designed a 9 Ah-class pouch cells with optimized components and obtained an energy density of 576 Wh/kg at 2.0–4.8 Ⅴ. Compared with other works, the pouch cell with TFE electrolyte showed higher energy density at relatively low N/P ratio (Fig. 1c) and operated steadily 38 cycles (Fig. 1e).
The mechanism of TMSB additive was revealed through theoretical calculations and experimental investigations. As showed in Figs. 2a–f, the LRMO cathode cycled in TFE electrolyte had a thinner and denser CEI and maintained well-defined layered structure for suppressing electrolyte decomposition and cathode phase transition. The etching X-ray photoelectron spectroscopy (XPS) indicated that TMSB induced an armor-like CEI (Figs. 2g and h) with LiBxOy and LixPOyFz outer layer and LiF- and Li3PO4-rich inner part. This CEI not only relieved the consumption of electrolyte and the dissolution of transition metal ions (Figs. 2i and j), but also decreased the loss of lattice oxygen and improved the conductivity of Li+. Moreover, the increased Li3PO4 and LixPOyFz, ~2% B-O and extremely tiny Si in CEI with TMSB confirmed the unique reaction behavior of the additive (Fig. 2k), which was also verified by theoretical calculations.
In conclusion, Ma's group successfully constructed a highly Li+conductive armor-like CEI by TFE electrolyte for guaranteeing the 4.8 Ⅴ and 4.9 Ⅴ Li||LRMO batteries operating stably and enabling the 9 Ah-class pouch cell to achieve a high energy density of 576 Wh/kg. This work provides guidance for exploring the next-generation lithium metal batteries and developing partially fluorinated high-voltage electrolytes.
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Baofeng Wang: Writing – review & editing, Validation, Supervision, Conceptualization. Yu Wang: Writing – original draft, Investigation, Data curation. Junxi Zhang: Writing – review & editing, Supervision. Qiang Wu: Writing – review & editing, Validation.
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Figure 1 The cycling performance of Li||LRMO batteries with FE and TFE electrolytes at (a) 4.8 Ⅴ/0.5 C and (b) 4.9 Ⅴ/0.5 C, respectively. (c) The comparison of energy density for pouch batteries in Liu's work. (d) The rate performance and (f) EIS curses of Li||LRMO batteries with FE and TFE electrolytes. (e) The cyclic performance of Li||LRMO pouch cells with EE and FE electrolytes. Reproduced with permission [5]. Copyright 2024, Wiley-VCH Verlag GmbH.
Figure 2 The TEM and HR-TEM images of LRMO cathodes after 200 cycles with (a, b) EE, (c, d) FE and (e, f) TFE electrolytes. Ⅰ − Ⅴ are Fast Fourier transform images of the chosen areas. The selected component ratio of CEI induced by (g) FE and (h) TFE electrolyte. (i, j) The comparison of C, F, P, Mn, B and Si atomic ratios in CEI with both electrolytes at different sputtering times. (k) The mechanism picture of partially fluorinated electrolyte with 0.5 wt% TMSB. Reproduced with permission [5]. Copyright 2024, Wiley-VCH Verlag GmbH.
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