Matter
ArticleInhibiting gas generation to achieve ultralong-lifespan lithium-ion batteries at low temperatures
Progress and potential
Low temperatures remain a huge challenge for safe operation of state-of-the-art lithium-ion batteries (LIBs) and greatly limit widespread employment of electric vehicles (EVs) around the world. Liquid electrolyte formulation strongly governs low-temperature operation of LIBs. Although usage of low-temperature solvents greatly broadens the operating range of LIBs, their side reactions with plated Li at low temperatures generate a large amount of gas, causing premature failure of LIBs. Here, we design a high-concentration ethyl acetate-based electrolyte for low-temperature LIBs. This electrolyte effectively passivates the plated Li and thus inhibits gas generation during low-temperature cycling. Consequently, the LiNi0.8Co0.1Mn0.1O2/graphite pouch cell maintains a record-breaking lifetime of more than 1 year at −20°C. This work opens an alternative path for developing low-temperature LIBs with an ultralong lifespan, as is increasingly desired for developing all-climate EVs.
Graphical abstract
Introduction
Because accelerated deployment of electric vehicles (EVs) around the world is key to eventual decarbonization of transportation, the importance of lithium-ion batteries (LIBs) has never been higher.1,2,3,4 The development of Ni-rich layered oxide (NLO) cathode/graphite anode lithium-ion chemistry with high energy density has greatly extended the driving range of EVs at room temperature.5,6 However, the global climate is diverse and variable, with temperature distribution differences over 100°C on temporal and spatial scales.7 State-of-the-art LIBs are highly temperature sensitive and only function acceptably between 0°C and 40°C, which is far from meeting the working requirements of all-climate EVs over a wide temperature range from −40°C to 60°C.8,9 Liquid electrolyte strongly governs the operating temperature range of LIBs.8,10 Because of heavy reliance on high-freezing-point (36.4°C) ethylene carbonate (EC) solvent in the electrolyte, ion transportation in conventional EC-based electrolytes almost shuts down at −20°C because of solidification of electrolyte, causing a devastating blow to the performance of LIBs.11,12,13,14 Hence, the rechargeability of LIBs at sub-zero temperatures, especially −20°C or lower, encounters more challenges than at high temperatures (≥45°C).
To improve the low-temperature performance of LIBs, many low-temperature solvents (LTSs) with a low freezing point and low viscosity have been proposed.15,16 In matters of low-temperature operation, the LTSs cut both ways. On one hand, the presence of high-content (≥75%) LTSs greatly broadens the operating range of the LIBs toward low temperature.17,18 On the other hand, most LTSs are unable to form a protective solid electrolyte interphase (SEI) on the graphite anode and are even destructive to bulk graphite structure.13 In the LTSs-rich electrolyte environment, a stable SEI cannot be formed because of insufficient EC to participate in the lithium (Li)+ solvation structure, resulting in rapid degradation of LIBs.19 The emerging requirements for LIBs to be charged at low temperatures will exacerbate battery life degradation because of the high propensity of Li plating on the graphite surface during the low-temperature charging process.20,21,22,23 Highly active LTSs react violently with the plated Li, resulting in significant gas evolution and Li dendrite growth, accelerating the failure of LIBs.24
Film-forming additives could form a protective SEI in LTSs.25,26,27 However, only relying on film-forming additives does not effectively passivate the plated Li. During the continuous SEI breakage and repair process, the additives will be exhausted prematurely, resulting in a limited low-temperature lifespan of LIBs. Use of a high-concentration electrolyte (HCE; ≥3.0 M) is another efficient strategy to improve the stability of the electrode/electrolyte interface.28,29,30,31 Carbon-based HCEs significantly improve the cycling stability of LIBs at high temperatures; however, low-temperature performance is inferior.32 In contrast, linear carboxylate esters (simply “esters”) with better cold tolerance properties are more promising for low-temperature HCEs.15,33 Taking ethyl acetate (EA) as an example, the freezing point (−83.6°C) and viscosity (0.45 mPa·s) of EA are lower than the lowest value in common carbonate solvents.33,34 Moreover, the suitable dielectric constant (6.02) and relatively high boiling point (77.2°C) of EA also ensure a wide liquid range and sufficient low-temperature ionic conductivity of the EA-based HCE. However, investigation of ester-based HCEs in low-temperature LIBs is rarely reported.
We propose that low-temperature NLO/graphite LIBs with a long lifespan and high capacity can be realized through rationally designing EA-based electrolytes via combining HCE and additive strategies. An EA-based HCE is achieved by dissolving 3.0 M LiPF6 in EA/fluoroethylene carbonate (FEC) (9:1 [v/v]) for low-temperature NLO/graphite LIBs. The high-content EA ensures Li+ transport in the bulk electrolyte even at −40°C. The synergistic effect of high-concentration LiPF6 and FEC constructs a robust LiF-rich SEI, effectively passivating the inevitable Li plating on graphite at −20°C or lower, suppressing the side reaction of EA with plated Li (Figure 1). Because of the sufficient Li+ transport kinetics and highly stable electrode/electrolyte interface, the proposed electrolyte enables NLO/graphite cells to operate well in a wide temperature range from −40°C to 60°C. Specially, the 1.0-Ah LiNi0.8Co0.1Mn0.1O2 (NCM811)/graphite pouch cell is cycled stably for more than 1 year (1,400 cycles) at −20°C and 0.2 C. At −40°C, the NCM811/graphite pouch cell still delivers 77.3% of its room-temperature discharge capacity.
Section snippets
Failure analysis of conventional EA-based electrolyte at low temperatures
The electrochemical performance of 1.0 M LiPF6 EA/FEC (9:1 [v/v]) under harsh conditions (−20°C and 45°C) were first evaluated in 1.0-Ah LiNi0.8Co0.15Al0.05O2 (NCA)/graphite pouch cells (Figure 2A; Table S1). At 45°C, the cell exhibits a discharge capacity of 0.92 Ah with a high capacity retention of 89.8% after 100 cycles at 0.5 C. However, when cycled at −20°C and 0.2 C, the pouch cell initially reveals high capacity, but the capacity declines rapidly. The capacity plunges seriously after 80
Lead contact
Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Qiang Zhang ([email protected]).
Materials availability
The materials in this study will be made available upon reasonable request.
Materials
Routine EC-based electrolyte (1 M LiPF6 in EC/dimethyl carbonate 3:7 [v/v]), LiPF6, and FEC were commercially available from Duoduo Chem. EA was purchased from Meryer (Shanghai) Chemical Technology. LiPF6, FEC, and EA were battery grade. Normal-loading (areal mass
Acknowledgments
This work was supported by the National Key Research and Development Program (2021YFB2500300), the National Natural Science Foundation of China (22005172, 22109083, and 21825501), the Shanxi Key Research and Development Program (202102060301011), the S&T Program of Hebei (22344402D), the Seed Fund of the Shanxi Research Institute for Clean Energy, and the Tsinghua University Initiative Scientific Research Program. Z.L. appreciates the Shuimu Tsinghua Scholar Program of Tsinghua University and
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