We report systematic studies of the microstructural changes of uncoated and AlF3-coated Li-rich Mn-rich (LMR) cathode materials (Li1.2Ni0.15Co0.10Mn0.55O2) before and after cycling using a ...combination of aberration-corrected scanning/transmission electron microscopy (S/TEM) and electron energy loss spectroscopy (EELS). TEM coupled with EELS provides detailed information about the crystallographic and electronic structure changes that occur after cycling, thus revealing the fundamental improvement mechanism of surface coating. The results demonstrate that the surface coating reduces oxidation of the electrolyte at high voltage, suppressing the accumulation of a thick solid electrolyte interface (SEI) layer on electrode particle surface. Surface coating significantly enhances the stability of the surface structure and protects the electrode from severe etching/corrosion by the acidic species in the electrolyte, reducing the formation of etched surfaces and corrosion pits. Moreover, surface coating alleviates the undesirable voltage fade by mitigating layered to spinel-like phase transformation in the bulk region of the material. These fundamental findings may also be widely applied to explain the functioning mechanisms of other surface coatings used in a broad range of electrode materials.
Developing advanced electrolytes is critical for stabilizing electrode/electrolyte interfacial reactions and thus extending cycling stability of sodium (Na) batteries, especially when a high‐voltage ...cathode (such as NaNi0.68Mn0.22Co0.10O2 (NaNMC)) is used to achieve high energy density in batteries. Here, an advanced electrolyte based on sodium bis(fluorosulfonyl)imide (NaFSI)–triethyl phosphate, which is highly stable against a high‐voltage cathode, enabling long‐term cycling of sodium batteries, is reported. Na||NaNMC cells with this electrolyte demonstrate 89% capacity retention after 500 cycles with a cutoff voltage of 4.2 V versus Na/Na+. A full cell of hard carbon||NaNMC also exhibits good capacity retention of 83.5% after 200 cycles. Postmortem analyses on the cycled electrodes reveal that stabilization of the high‐voltage cathode can be attributed to the formation of a stable electrode/electrolyte interphase layer. The interphase is generated mainly by salt decomposition, which suppresses transition metal dissolution and surface reconstruction on the cathode. The optimized electrolyte can minimize solid electrolyte interphase dissolution to avoid capacity loss. This study offers a feasible pathway to achieve extended cycling of high‐voltage sodium batteries and guide further improvements in cycling performance of batteries by manipulating the chemistry of the electrolytes and interphase properties.
A sodium bis(fluorosulfonyl)imide‐triethyl phosphate electrolyte is developed for high‐voltage sodium batteries. The uniform, inorganic‐rich electrode/electrolyte interphase layer can suppress transition metal dissolution and surface reconstruction of the NaNMC cathode and minimize solid electrolyte interphase dissolution for long‐term cycling of high‐energy‐density sodium batteries (>80% capacity retention after 500 cycles for a 4.2 V sodium metal battery).
Current density has been perceived to play a critical rule in controlling Li deposition morphology and solid electrolyte interphase (SEI). However, the atomic level mechanism of the effect of current ...density on Li deposition and the SEI remains unclear. Here based on cryogenic transmission electron microscopy (TEM) imaging combined with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) electronic structure analyses, we reveal the atomic level correlation of Li deposition morphology and SEI with current density. We discover that increasing current density leads to increased overpotential for Li nucleation and growth, leading to the transition from growth-limited to nucleation-limited mode for Li dendrites. Independent of current density, the electrochemically deposited Li metal (EDLi) exhibits crystalline whisker-like morphology. The SEI formed at low current density (0.1 mA cm–2) is monolithic amorphous; while, a current density of above 2 mA cm–2 leads to a mosaic structured SEI, featuring an amorphous matrix with Li2O and LiF dispersoids, and the thickness of the SEI increases with the increase of current density. Specifically, the Li2O particles are spatially located at the top surface of the SEI, while LiF is spatially adjacent to the Li–SEI interface. These results offer possible ways of regulating crucial microstructural and chemical features of EDLi and SEI through altering deposit conditions and consequently direct correlation with battery performance.
Lithium (Li) metal batteries (LMBs) have recently attracted extensive interest in the energy-storage field after silence from the public view for several decades. However, many challenges still need ...to be overcome before their practical application, especially those that are related to the interfacial instability of Li metal anodes. Here, we reveal for the first time that the thickness of the degradation layer on the metallic Li anode surface shows a linear relationship with Li areal capacity utilization up to 4.0 mAh cm−2 in a practical LMB system. The increase in Li capacity utilization in each cycle causes variations in the morphology and composition of the degradation layer on the Li anode. Under high Li capacity utilization, the current density for charge (i.e., Li deposition) is identified to be a key factor controlling the corrosion of the Li metal anode. These fundamental findings provide new perspectives for the development of rechargeable LMBs.
Display omitted
•Li batteries show decent long cycle life with Li capacity usage up to 4.0 mAh cm−2•Increase in Li capacity usage causes variations in components of the surface layer•Slow charge rate leads to a similar Li metal expansion ratio with Li capacity usage•Charge current density has more of an effect on Li metal stability than Li capacity usage
Applications of high-energy-density rechargeable Li metal batteries have been hindered by safety concerns and short cycle life. So far, the research has been conducted with low to medium capacity utilization of the Li metal anode, which is inappropriate in practical battery systems. Here, we investigate the degradation of a Li metal anode in a liquid electrolyte and the long-term cycling stability of Li metal batteries by manipulating Li capacity utilization and charge current density up to practical levels. Higher Li capacity utilization causes more corrosion of the bulk Li metal anode, but the expansion rate of used Li metal anode stays the same at different Li capacity utilization. The increase in charge current density causes more severe corrosion of the Li metal anode, which greatly accelerates cell failure, indicating the charge current density plays a key role in Li metal batteries. These fundamental findings provide practical guidance for the design of next-generation Li metal batteries.
Effects of Li capacity utilization up to 4.0 mAh cm−2 and charge current density on the stability of Li metal anodes and Li metal batteries were investigated. The thickness of degradation layer on Li anode surface shows a linear relationship with Li capacity utilization. Increase in Li capacity utilization causes variations in the composition of the Li surface degradation layer. The charge current density is a key factor controlling the corrosion of Li metal anodes and the long-term cycling stability of Li metal batteries.
Lithium (Li)‐magnesium (Mg) alloy with limited Mg amount, which can also be called Mg‐doped Li (Li‐Mg), has been considered as a potential alternative anode for high energy density rechargeable Li ...metal batteries. However, the optimum doping‐content of Mg in Li‐Mg anode and the mechanism of the improved performance are not well understood. Herein, density functional theory (DFT) calculations are used to investigate the effect of Mg amount in Li‐Mg anode. The Li‐Mg with about 5 wt. % Mg (abbreviated as Li‐Mg5) has the lowest absorption energy of Li, thus all the surface area can be “controlled” by Mg atoms, leading to the smooth and continuous deposition of Li on the surface around the Mg center. A localized high concentration electrolyte enables Li‐Mg5 to exhibit the best cycling stability in Li metal batteries with high‐loading cathode and lean electrolyte under 4.4 V high‐voltage, which is approaching the demand of practical application. This electrolyte also helps generate an inorganic‐rich solid electrolyte interphase, which leads to smooth, compact and less corrosion layer on the Li‐Mg5 surface. Both theoretical simulations and experimental results prove that Li‐Mg5 has optimum Mg content and gives best battery cycling performance.
Mg‐doped Li with about 5 wt. % Mg (Li‐Mg5) has the lowest absorption‐energy of Li from density functional theory calculations. Experimental tests demonstrate that Li‐Mg5 exhibits superior cycling stability to pure Li and Li‐Mg10 anodes in Li metal batteries with high‐loading cathode and lean electrolyte under 4.4 V high‐voltage, leading to dense and less‐corrosive Li deposition thus validating the simulation results.
Rechargeable lithium metal batteries are considered the “Holy Grail” of energy storage systems. Unfortunately, uncontrollable dendritic lithium growth inherent in these batteries (upon repeated ...charge/discharge cycling) has prevented their practical application over the past 40 years. We show a novel mechanism that can fundamentally alter dendrite formation. At low concentrations, selected cations (such as cesium or rubidium ions) exhibit an effective reduction potential below the standard reduction potential of lithium ions. During lithium deposition, these additive cations form a positively charged electrostatic shield around the initial growth tip of the protuberances without reduction and deposition of the additives. This forces further deposition of lithium to adjacent regions of the anode and eliminates dendrite formation in lithium metal batteries. This strategy may also prevent dendrite growth in lithium-ion batteries as well as other metal batteries and transform the surface uniformity of coatings deposited in many general electrodeposition processes.
The energy storage density of Li‐ion batteries can be improved by replacing graphite anodes with high‐capacity Si‐based materials, though instabilities have limited their implementation. Performance ...degradation mechanisms that occur in Si anodes can be divided into cycling stability (capacity retention after repeated battery cycles) and calendar aging (shelf life). While cycling instabilities and improvement strategies have been researched intensively, there is little known about the underlying mechanisms that cause calendar aging. In this work, multiple electron microscope techniques are used to explore the mechanism that governs calendar aging from the sub‐nanometer‐to‐electrode scale. Plasma focused ion beam tomography is used to create 3D reconstructions of calendar aged electrodes and revealed the growth of a LiF‐rich layer at the interface between the copper current collector and the silicon material, which can lead to delamination and increased interfacial impendence. The LiF layer appeared to derive from the fluoro‐ethylene‐carbonate electrolyte additive, which is commonly used to improve cycling stability in Si‐based systems. The results reveal that additives necessary to improve cycling stability can cause performance degradation over the long‐term during calendar aging. The results show that high performing, stable systems require careful design to simultaneously mitigate both cycling and calendar aging instabilities.
This work investigates the underlying mechanisms that cause calendar aging effects in Si anode‐based Li ion batteries. Plasma focused ion beam tomography reveals the formation and growth of a LiF‐rich layer at the interface between the silicon and the copper current collector, and is caused by a commonly used electrolyte additive.
Display omitted
•Oxygen non-stoichiometry in different cathode materials is sensitive to the synthesis conditions.•Oxygen non-stoichiometry plays a critical role in manipulating the material crystal ...structure.•Oxygen non-stoichiometry affects the reaction pathway, electrochemical performances and thermal stability of cathode materials.•Oxygen non-stoichiometry should be carefully considered in the future development of high-energy-density cathode materials.
Cathode materials with both high energy density and high power capability are in great demand to improve the performance of lithium ion batteries and expand the driving range of electric vehicles. Of particular interest are transition metal oxide-based cathodes. However, electrochemical performance of these cathode materials is very sensitive to the synthetic conditions and, particularly, the oxygen non-stoichiometry that is induced during high temperature synthesis and post treatment. This review highlights the critical roles of oxygen non-stoichiometry in high-energy-density cathode materials including high voltage spinel LiNi0.5Mn1.5O4, Ni-rich layered LiNixMnyCozO2 (NMC, x>0.5), and the Li-rich, Mn-rich layered cathode, with the aim to provide a fundamental understanding on the effects of the oxygen non-stoichiometry governing the crystalline structure, electrochemical performance, and thermal stability of different cathode materials. This review also offers perspectives and directions on how to best utilize oxygen non-stoichiometry in the future development of high-energy-density cathode materials for lithium ion batteries.
Lithium (Li) metal is one of the most promising candidates for the anode in high‐energy‐density batteries. However, Li dendrite growth induces a significant safety concerns in these batteries. Here, ...a multifunctional separator through coating a thin electronic conductive film on one side of the conventional polymer separator facing the Li anode is proposed for the purpose of Li dendrite suppression and cycling stability improvement. The ultrathin Cu film on one side of the polyethylene support serves as an additional conducting agent to facilitate electrochemical stripping/deposition of Li metal with less accumulation of electrically isolated or “dead” Li. Furthermore, its electrically conductive nature guides the backside plating of Li metal and modulates the Li deposition morphology via dendrite merging. In addition, metallic Cu film coating can also improve thermal stability of the separator and enhance the safety of the batteries. Due to its unique beneficial features, this separator enables stable cycling of Li metal anode with enhanced Coulombic efficiency during extended cycles in Li metal batteries and increases the lifetime of Li metal anode by preventing short‐circuit failures even under extensive Li metal deposition.
Janus‐faced separator design with ultrathin copper (Cu) metal film coating onto one side surface of the conventional polyethylene separator is proposed for lithium (Li) dendrite suppression and cycling stability improvement. Enabling the separator to be electrically and ionically conductive is very effective to facilitate the electrochemical deposition/stripping of Li metal and modulate of the Li metal/electrolyte interface structure.
The stability of electrolytes against highly reactive, reduced oxygen species is crucial for the development of rechargeable Li–O2 batteries. In this work, the effect of lithium salt concentration in ...1,2‐dimethoxyethane (DME)‐based electrolytes on the cycling stability of Li–O2 batteries is investigated systematically. Cells with highly concentrated electrolyte demonstrate greatly enhanced cycling stability under both full discharge/charge (2.0–4.5 V vs Li/Li+) and the capacity‐limited (at 1000 mAh g−1) conditions. These cells also exhibit much less reaction residue on the charged air‐electrode surface and much less corrosion of the Li‐metal anode. Density functional theory calculations are used to calculate molecular orbital energies of the electrolyte components and Gibbs activation energy barriers for the superoxide radical anion in the DME solvent and Li+–(DME)
n solvates. In a highly concentrated electrolyte, all DME molecules are coordinated with salt cations, and the C–H bond scission of the DME molecule becomes more difficult. Therefore, the decomposition of the highly concentrated electrolyte can be mitigated, and both air cathodes and Li‐metal anodes exhibit much better reversibility, resulting in improved cyclability of Li–O2 batteries.
Superior performance of rechargeable Li–O2 batteries based on high‐concentration Li bis(trifluoromethanesulfonyl)imide (LiTFSI)–1,2‐dimethoxyethane (DME) electrolyte is related to enhanced stability of the Li‐metal anode and air electrode in this electrolyte. This finding provides a significant insight into the fundamental mechanism of high‐concentration electrolytes in enhancing the stability of air cathodes and Li‐metal anodes in rechargeable Li–O2 batteries.