Effects of Applied Interfacial Pressure on Li-Metal Cycling Performance and Morphology in 4 M LiFSI in DME

Abstract

Lithium-metal anodes can theoretically enable 10× higher gravimetric capacity than conventional graphite anodes. However, Li-metal anode cycling has proven difficult due to porous and dendritic morphologies, extensive parasitic solid electrolyte interphase reactions, and formation of dead Li. We systematically investigate the effects of applied interfacial pressure on Li-metal anode cycling performance and morphology in the recently developed and highly efficient 4 M lithium bis­(fluorosulfonyl)­imide in 1,2-dimethoxyethane electrolyte. We present cycling, morphology, and impedance data at a current density of 0.5 mA/cm2 and a capacity of 2 mAh/cm2 at applied interfacial pressures of 0, 0.01, 0.1, 1, and 10 MPa. Cryo-focused ion beam milling and cryo-scanning electron microscopy imaging in cross section reveal that increasing the applied pressure during Li deposition from 0 to 10 MPa leads to greater than a fivefold reduction in thickness (and therefore volume) of the deposited Li. This suggests that pressure during cycling can have a profound impact on the practical volumetric energy density for Li-metal anodes. A “goldilocks zone” of cell performance is observed at intermediate pressures of 0.1–1 MPa. Increasing pressure from 0 to 1 MPa generally improves cell-to-cell reproducibility, cycling stability, and Coulombic efficiency. However, the highest pressure (10 MPa) results in high cell overpotential and evidence of soft short circuits, which likely result from transport limitations associated with increased pressure causing local pore closure in the separator. All cells exhibit at least some signs of cycling instability after 50 cycles when cycled to 2 mAh/cm2 with thin 50 μm Li counter electrodes, though instability decreases with increasing pressure. In contrast, cells cycled to only 1 mAh/cm2 perform well for 50 cycles, indicating that capacity plays an important role in cycling stability.

Cryogenic electron microscopy reveals that applied pressure promotes short circuits in Li batteries

“…Such morphologies also exacerbate parasitic reactions due to their large surface areas, which enables significant parasitic SEI growth. Because of their theoretical promise, the research community has invested heavily in overcoming challenges ( Kim et al., 2013 ; Lin et al., 2017 ) with Li anodes including (1) electrolyte additives ( Ding et al., 2014 ; Zhang, 2006 ; Jozwiuk et al., 2016 ) and new electrolytes ( Qian et al., 2015 ; Han et al., 2011 ; Ding et al., 2013 ; Naoi et al., 1999 ), (2) artificial SEI coatings to better control morphological evolution ( Kozen et al., 2015 ; Aetukuri et al., 2015 ; Li et al., 2015 ; Zheng et al., 2014 ; Harrison et al., 2017 ), (3) periodic high rate cycling to promote self-heating and destruction of dendrites ( Li et al., 2018 ), (4) separator coatings to redirect dendrite growth ( Liu et al., 2017 ), (5) solid state electrolytes that provide a physical barrier to Li dendrites and are often less reactive with Li ( Takada, 2013 ; Sun et al., 2017 ), and (6) applied interfacial pressure in cells with liquid ( Mikhaylik et al., 2010 ; Hirai et al., 1994a , 1994b ; Wilkinson et al., 1991 ; Wilkinson and Wainwright, 1993 ; Ota et al., 2004 ; Weber et al., 2019 ; He et al., 2019 ; Harrison et al., 2017 , 2021 ; Louli et al., 2019 , 2020 ; Niu et al., 2019 ; Yin et al., 2018 ; Gireaud et al., 2006 ; Wang et al., 2021 ; Peabody and Arnold, 2011 ; Kanamori et al., 2020 ) and solid electrolytes ( Zhang et al., 2020 ; Wang et al., 2019 ; Krauskopf et al., 2019 ; Verma et al., 2021 ).…”

“…Although applied interfacial pressure is often unquantified, it can significantly impact performance in conventional Li-ion batteries ( Mussa et al., 2018 ; Cannarella and Arnold, 2014 ; Zhao et al., 2017 ; Bach et al., 2016 ; Rubino et al., 2001 ; Klett et al., 2014 ; Petzl et al., 2015 ; Nadimpalli et al., 2015 ) and Li metal anode batteries ( Mikhaylik et al., 2010 ; Hirai et al., 1994a , 1994b ; Wilkinson et al., 1991 ; Wilkinson and Wainwright, 1993 ; Ota et al., 2004 ; Weber et al., 2019 ; He et al., 2019 ; Harrison et al., 2017 , 2021 ; Louli et al., 2019 , 2020 ; Niu et al., 2019 ; Yin et al., 2018 ; Gireaud et al., 2006 ; Wang et al., 2019 , 2021 ; Zhang et al., 2020 ; Krauskopf et al., 2019 ; Verma et al., 2021 ; Peabody and Arnold, 2011 ; Kanamori et al., 2020 ). Pressure can have a particularly strong impact on Li metal anodes because Li is a soft metal ( Fincher et al., 2020 ) and deposits in compressible and high aspect ratio porous morphologies.…”

“…Pressure can have a particularly strong impact on Li metal anodes because Li is a soft metal ( Fincher et al., 2020 ) and deposits in compressible and high aspect ratio porous morphologies. Pressure impacts on Li anode cycling have been studied in a variety of liquid electrolytes ( Mikhaylik et al., 2010 ; Hirai et al., 1994a , 1994b ; Wilkinson et al., 1991 ; Wilkinson and Wainwright, 1993 ; Ota et al., 2004 ; Weber et al., 2019 ; He et al., 2019 ; Harrison et al., 2017 , 2021 ; Louli et al., 2019 , 2020 ; Niu et al., 2019 ; Yin et al., 2018 ; Gireaud et al., 2006 ; Wang et al., 2021 ; Peabody and Arnold, 2011 ; Kanamori et al., 2020 ) and the specific electrolyte formulation impacts the degree to which pressure affects performance ( Hirai et al., 1994a ; Louli et al., 2019 ; Weber et al., 2019 ; Niu et al., 2019 ; Wang et al., 2021 ). Pressure can enable dense morphology by constraining growth ( Mikhaylik et al., 2010 ; Hirai et al., 1994b ; Wilkinson and Wainwright, 1993 ) and promoting the reconnection of disconnected, mossy Li, which leads to improved Coulombic efficiency (CE) ( Wilkinson and Wainwright, 1993 ; Hirai et al., 1994b ).…”

“…Motivated to understand the effects of pressure in an electrolyte with high CE, long cycle life, and favorable deposition morphology, we have previously studied varied pressure in the promising electrolyte 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane (4 M LiFSI in DME) ( Harrison et al., 2017 , 2021 ). Despite its high viscosity which leads to lower ionic conductivity than conventional lower concentration electrolytes, 4 M LiFSI in DME cycles efficiently and has been previously shown to support high current density ( Qian et al., 2015 ).…”

“…Despite its high viscosity which leads to lower ionic conductivity than conventional lower concentration electrolytes, 4 M LiFSI in DME cycles efficiently and has been previously shown to support high current density ( Qian et al., 2015 ). After demonstrating proof-of-concept that pressure improves morphology in 4 M LiFSI in DME ( Harrison et al., 2017 ), we then systematically studied the effects of 0–10 MPa of pressure on Li cycling and morphology at a low current density of 0.5 mA/cm 2 ( Harrison et al., 2021 ). We generally found that increased pressure improved cell-to-cell repeatability, CE, and plating morphology from 0 to 1 MPa.…”