Lithium dendrite (filament) propagation through ceramic electrolytes, leading to short circuits at high rates of charge, is one of the greatest barriers to realizing high-energy-density ...all-solid-state lithium-anode batteries. Utilizing in situ X-ray computed tomography coupled with spatially mapped X-ray diffraction, the propagation of cracks and the propagation of lithium dendrites through the solid electrolyte have been tracked in a Li/Li
PS
Cl/Li cell as a function of the charge passed. On plating, cracking initiates with spallation, conical 'pothole'-like cracks that form in the ceramic electrolyte near the surface with the plated electrode. The spallations form predominantly at the lithium electrode edges where local fields are high. Transverse cracks then propagate from the spallations across the electrolyte from the plated to the stripped electrode. Lithium ingress drives the propagation of the spallation and transverse cracks by widening the crack from the rear; that is, the crack front propagates ahead of the Li. As a result, cracks traverse the entire electrolyte before the Li arrives at the other electrode, and therefore before a short circuit occurs.
Void formation at the Li/ceramic electrolyte interface of an all-solid-state battery on discharge results in high local current densities, dendrites on charge, and cell failure. Here, we show that ...such voiding is reduced at the Li/Li6PS5Cl interface at elevated temperatures, sufficient to increase the critical current before voiding and cell failure from <0.25 mA cm–2 at 25 °C to 0.25 mA cm–2 at 60 °C and 0.5 mA cm–2 at 80 °C under a relatively low stack-pressure of 1 MPa. Increasing the stack-pressure to 5 MPa and temperature to 80 °C permits stable cycling at 2.5 mA cm–2. It is also shown that the charge-transfer resistance at the Li/Li6PS5Cl interface depends on pressure and temperature, with relatively high pressures required to maintain low charge-transfer resistance at −20 °C. These results are consistent with the plastic deformation of Li metal dominating the performance of the Li anode, posing challenges for the implementation of solid-state cells with Li anodes.
All-solid-state batteries with a Li anode and ceramic electrolyte have the potential to deliver a step change in performance compared with today's Li-ion batteries
. However, Li dendrites (filaments) ...form on charging at practical rates and penetrate the ceramic electrolyte, leading to short circuit and cell failure
. Previous models of dendrite penetration have generally focused on a single process for dendrite initiation and propagation, with Li driving the crack at its tip
. Here we show that initiation and propagation are separate processes. Initiation arises from Li deposition into subsurface pores, by means of microcracks that connect the pores to the surface. Once filled, further charging builds pressure in the pores owing to the slow extrusion of Li (viscoplastic flow) back to the surface, leading to cracking. By contrast, dendrite propagation occurs by wedge opening, with Li driving the dry crack from the rear, not the tip. Whereas initiation is determined by the local (microscopic) fracture strength at the grain boundaries, the pore size, pore population density and current density, propagation depends on the (macroscopic) fracture toughness of the ceramic, the length of the Li dendrite (filament) that partially occupies the dry crack, current density, stack pressure and the charge capacity accessed during each cycle. Lower stack pressures suppress propagation, markedly extending the number of cycles before short circuit in cells in which dendrites have initiated.
Ether solvent based electrolytes exhibit excellent performance with sodium battery anodes, outperforming the carbonate electrolytes that are routinely used with the analogous lithium-ion battery. ...Uncovering the mechanisms that facilitate this high performance for ether electrolytes, and conversely diagnosing the causes of the poor cycling with carbonate electrolytes, is crucial for informing the design of optimized electrolytes that promote fully reversible sodium cycling. An important contributor to the performance difference has been suggested to be the enhanced elasticity of the ether-derived solid-electrolyte interphase (SEI) layer, however experimental demonstration of exactly how this translates to improving the microscopic dynamics of a cycled anode remain less explored. Here, we reveal how this more elastic SEI prevents gas evolution at the interface of the metal anode by employing operando electrochemical transmission electron microscopy (TEM) to image the cycled electrode-electrolyte interface in real time. The high spatial resolution of TEM imaging reveals the rapid formation of gas bubbles at the interface during sodium electrostripping in carbonate electrolyte, a phenomenon not observed for the higher performance ether electrolyte, which impedes complete Na stripping and causes the SEI to delaminate from the electrode. This non-conformal and inflexible SEI must thus continuously reform, leading to increased Na loss to SEI formation, as supported by mass spectrometry measurements. The more elastic ether interphase is better able to maintain conformality with the electrode, preventing gas formation and facilitating flat electroplating. Our work shows why an elastic and flexible interphase is important for achieving high performance sodium anodes.
The role of ether electrolyte solvent in facilitating efficient Na metal anode cycling performance is identified to be preventing adverse gas formation and favouring a flexible conformal SEI.
Contouring or structuring of the lithium/ceramic electrolyte interface and therefore increasing its surface area has been considered as a possible strategy to increase the charging current in ...solid-state batteries without lithium dendrite formation and short-circuit. By coupling together lithium deposition kinetics and the me chanics of lithium creep within calculations of the current distribution at the interface, and leveraging a model for lithium dendrite growth, we show that efforts to avoid dendrites on charging by increasing the interfacial surface area come with significant limitations associated with the topography of rough surfaces. These limitations are sufficiently severe such that it is very unlikely contouring could increase charging currents while avoiding dendrites and short-circuit to the levels required. For example, we show a sinusoidal surface topography can only raise the charging current before dendrites occur by approx. 50% over a flat interface.
Efforts to avoid dendrites by increasing the interfacial surface area to lower local current densities are limited by significant local pressure accumulation associated with the topography of any surface contouring.
Hybrid solid-state batteries using a bilayer of ceramic and solid polymer electrolytes may offer advantages over using a single type of solid electrolyte alone. However, the impedance to Li+ ...transport across interfaces between different electrolytes can be high. It is important to determine the resistance to Li+ transport across these heteroionic interfaces, as well as to understand the underlying causes of these resistances; in particular, whether chemical interphase formation contributes to giving high resistances, as in the case of ceramic/liquid electrolyte interfaces. In this work, two ceramic electrolytes, Li3PS4 (LPS) and Li6.5La3Zr1.5Ta0.5O12 (LLZTO), were interfaced with the solid polymer electrolyte PEO10:LiTFSI and the interfacial resistances were determined by impedance spectroscopy. The LLZTO/polymer interfacial resistance was found to be prohibitively high but, in contrast, a low resistance was observed at the LPS/polymer interface that became negligible at a moderately elevated temperature of 50 °C. Chemical characterization of the two interfaces was carried out, using depth-profiled X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, to determine whether the interfacial resistance was correlated with the formation of an interphase. Interestingly, no interphase was observed at the higher resistance LLZTO/polymer interface, whereas LPS was observed to react with the polymer electrolyte to form an interphase.
Correction for 'The role of an elastic interphase in suppressing gas evolution and promoting uniform electroplating in sodium metal anodes' by Chen Gong
et al.
,
Energy Environ. Sci.
, 2023,
16
, ...535-545,
https://doi.org/10.1039/D2EE02606F
.
Abstract
Solid‐state lithium batteries may provide increased energy density and improved safety compared with Li‐ion technology. However, in a solid‐state composite cathode, mechanical degradation ...due to repeated cathode volume changes during cycling may occur, which may be partially mitigated by applying a significant, but often impractical, uniaxial stack pressure. Herein, we compare the behavior of composite electrodes based on Li
4
Ti
5
O
12
(LTO) (negligible volume change) and Nb
2
O
5
(+4% expansion) cycled at different stack pressures. The initial LTO capacity and retention are not affected by pressure but for Nb
2
O
5
, they are significantly lower when a stack pressure of <2 MPa is applied, due to inter‐particle cracking and solid‐solid contact loss because of cyclic volume changes. This work confirms the importance of cathode mechanical stability and the stack pressures for long‐term cyclability for solid‐state batteries. This suggests that low volume‐change cathode materials or a proper buffer layer are required for solid‐state batteries, especially at low stack pressures.
Potassium metal anode solid-state cells with a K-beta”-alumina ceramic electrolyte are found to have relatively high critical currents for dendrite penetration on charge of approximately 4.8 mA/cm
2
..., and voiding on discharge of approximately 2.0 mA/cm
2
, at 20 °C under 2.5 MPa stack-pressure. These values are higher than generally reported in the literature under comparable conditions for Li and Na metal anode solid-state batteries. The higher values for potassium are attributed to its lower yield strength and its readiness to creep under relatively low stack-pressures. The high critical currents of potassium anode solid-state batteries help to confirm the importance of the metal anode mechanical properties in the mechanisms of dendrite penetration and voiding.
Ag-carbon composite interlayers have been reported to enable Li-free (anodeless) cycling of solid-state batteries. Here, we report structural changes in the Ag-graphite interlayer, showing that on ...charge, Li intercalates electrochemically into graphite, subsequently reacting chemically with Ag to form Li-Ag alloys. Discharge is not the reverse of charge but rather passes through Li-deficient Li-Ag phases. At higher charging rates, Li intercalation into graphite outpaces the chemical reactions with Ag, delaying the formation of the Li-Ag phases and resulting in more Li metal deposition at the current collector. At and above 2.5 mA·cm−2, Li dendrites are not suppressed. Ag nanoparticles do not suppress dendrites more effectively than does an interlayer of graphite alone. Instead, Ag in the carbon interlayer results in more homogeneous Li and Li-Ag formation on the current collector during charge.
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•Operando XRD reveals structural changes in Ag-C interlayer during cycling•Ag nanoparticles do not increase critical current of graphite-based interlayer•The role of Ag nanoparticles is to facilitate homogeneous Li and Li-Ag deposition
Enabling all-solid-state batteries based on ceramic electrolytes and a Li anode could lead to increased energy storage and safer batteries. Dendrites (filaments) of Li penetrate the ceramic on charging at practical rates. Furthermore, plating Li where none is already present (anodeless cells) is desirable but difficult. Interlayers at the anode have been proposed to address these problems. Recently, promising performance was reported by Samsung Advanced Institute of Technology for a composite layer of carbon with Ag nanoparticles. Here, we explore the structural changes that occur in an Ag-graphite composite interlayer. Charging involves electrochemical Li intercalation into the graphite, then lithiated graphite reacts with Ag to form a series of Li-Ag alloys. Li-Ag and Li deposit between the interlayer and the current collector. Ag does not increase the maximum charging current before Li dendrites form, but it does lead to a more homogeneous Li-rich layer at the current collector.
Anodeless solid-state batteries have the potential to increase the energy density and safety of batteries, but they face challenges, including inhomogeneous plating of Li metal on the current collector and penetration of Li metal dendrites into the solid electrolyte. Introduction of an Ag-C composite interlayer between the solid electrolyte and current collector mitigates these problems. Here, we study the structural changes within the composite interlayer, the critical current density, and the morphology of the deposited Li/Ag.
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