Deciphering the Formation and Accumulation of Solid-Electrolyte Interphases in Na and K Carbonate-Based Batteries.

The continuous solid-electrolyte interphase (SEI) accumulation has been blamed for the rapid capacity loss of carbon anodes in Na and K ethylene carbonate (EC)/diethyl carbonate (DEC) electrolytes, but the understanding of the SEI composition and its formation chemistry remains incomplete. Here, we explain this SEI accumulation as the continuous production of organic species in solution-phase reactions. By comparing the NMR spectra of SEIs and model compounds we synthesized, alkali metal ethyl carbonate (MEC, M = Na or K), long-chain alkali metal ethylene carbonate (LCMEC, M = Na or K), and poly(ethylene oxide) (PEO) oligomers with ethyl carbonate ending groups are identified in Na and K SEIs. These components can be continuously generated in a series of solution-phase nucleophilic reactions triggered by ethoxides. Compared with the Li SEI formation chemistry, the enhancement of the nucleophilicity of an intermediate should be the cause of continuous nucleophilic reactions in the Na and K cases.

Investigating K+ Storage Behavior of Highly Graphitized Carbon Fibers as Anodes for a Potassium-Ion Battery.

Many problems of potassium-ion batteries (PIBs) are hidden under a low mass load of the active material. However, developing research based on areal capacity is challenging for PIBs, due to the lack of an anode capable of delivering a stable capacity of more than 1 mAh cm-2. This work investigates the K+ storage behavior of highly graphitized carbon fibers (HG-CF), which exhibit automatic structural adjustments to mitigate voltage polarization. The created defects and residual K+ in the structure favor the reversible insertion/deinsertion of K+. HG-GF after structural adjustment realizes a capacity of 2 mAh (1.13 cm-2) without K deposition and a stable cyclic stability (>500 h). In situ X-ray diffraction and in situ Raman spectra were used to detect defect formation and structural evolution during cycles. This work demonstrates the feasibility of HG-GF as an anode for PIBs and provides a suitable anode for further research of PIBs based on areal capacity.

Mechanistic Insight into Ultrafast Kinetics of Sodium Cointercalation in Few-Layer Graphitic Carbon.

Fast-charging sodium ion batteries remain deeply challenged by the lack of suitable carbonaceous anodes that exhibit intercalation plateau with fast kinetics. Here we develop a few-layer graphitic carbon with nanoscale architecture, which enables shortened Na+ ion diffusion path and fast formation of fully intercalated phase at the same time. Combined in situ Raman and electrochemical test reveal that this graphitic carbon with highly crystalline few layers follows surface-controlled intercalation rather than typical diffusion-controlled kinetics observed in natural graphite. As a result, a few-layer graphitic carbon anode maintains the reversible capacity of 106 mAh g-1 at 10 A g-1 and achieves 87% capacity retention even after 10 000 cycles at 1 A g-1. This work provides new insight on the Na storage mechanism in fast-charging graphitic carbon as well as the design of carbon anodes for high-rate sodium ion batteries.

Key Factor Determining the Cyclic Stability of the Graphite Anode in Potassium-Ion Batteries.

Graphite is the most commonly used anode material for not only commercialized lithium-ion batteries (LIBs) but also the emerging potassium-ion batteries (PIBs). However, the graphite anode in PIBs using traditional dilute ester-based electrolyte systems shows obvious capacity fading, which is in contrast with the extraordinary cyclic stability in LIBs. More interestingly, the graphite in concentrated electrolytes for PIBs exhibits outstanding cyclic stability. Unfortunately, this significant difference in cycling performance has not raised concern up to now. In this work, by comparing the cyclic stability and graphitization degree of the graphite anode upon cycling, we reveal that the underlying mechanism of the capacity fading of the graphite anode in PIBs is not the larger volume expansion of graphite caused by the intercalation of potassium ions but the continual accumulation of the solid electrolyte interphase (SEI) on the surface of graphite. By X-ray photoelectron and nuclear magnetic resonance spectroscopies combined with chemical synthesis, it is concluded that the accumulation of the SEI may mainly come from the continual deposition of a kind of oligomer component, which blocks intercalation and deintercalation of potassium ions in graphite anodes. The designed SEI-cleaning experiment further verifies the above conclusion. This finding clarifies the crucial factor determining the cyclic stability of graphite and provides scientific guidance for application of the graphite anode for PIBs.

A free-standing 3D porous all-ceramic cathode for high capacity, long cycle life Li-O2 batteries.

The all-ceramic RuO2@La0.7Ca0.3CuO3 membrane cathode contributes to an ultra-high capacity of 21 518 mA h g-1 over 110 cycles in Li-O2 batteries. A simple infiltration technique is effective for obtaining a highly active supported RuO2 catalyst, and a solvent with a high donor number should be preferentially chosen because it contributes to a much higher capacity.

Polyvinylpyrrolidone-Bridged Prussian Blue/rGO Composite as a High-Performance Cathode for K-Ion Batteries.

Prussian blue (PB) is a very promising cathode for K-ion batteries but its low electronic conductivity and deficiencies in the framework aggravate electrochemical performances. Compositing with conductive reduced graphene oxide (rGO) is an effective solution to address this problem. Nevertheless, little attention was paid to the loss of oxygen-containing functional groups on the rGO substrate during the compositing process, which weakens the interaction between PB and rGO and leads to poor electrochemical performance of PB/rGO. Herein, this interaction effect associated with surface functional groups is first openly debated. Two commonly used carbon substrates, graphene oxide (GO) and rGO, are investigated. A more stable interaction between PB and GO contributes to a higher capacity retention (91.8%) than that of PB/rGO (69.7%) after 300 cycles at a current density of 5 C. Meanwhile, polyvinylpyrrolidone (PVP) is employed to repair the weak interaction between PB and rGO substrates. PB is anchored to the rGO surface through the stable covalent linking of amide groups in PVP. A superior rate capability of 72 mA h g-1 at 10 C and an improved capacity retention of 96.5% over 800 cycles at 5 C are obtained by as-prepared PB/PVP-rGO. This study provides a deeper understanding of fabricating PB/carbon composites with a robust connection.

A highly concentrated electrolyte for high-efficiency potassium metal batteries.

We report a highly concentrated electrolyte consisting of 4 M potassium bis(fluorosulfonyl)imide (KFSI) in diethylene glycol dimethyl ether (DEGDME). This new electrolyte enables stable cycling of K metal anodes with a high CE (over 98% over 400 cycles), and excellent capacity retention (99.7% after 500 cycles) of K||potassium Prussian blue (KPB) batteries.

A Graphite Intercalation Composite as the Anode for the Potassium-Ion Oxygen Battery in a Concentrated Ether-Based Electrolyte.

Nowadays, alkali metal-oxygen batteries such as Li-, Na-, and K-O2 batteries have been investigated extensively because of their ultrahigh energy density. However, the oxygen crossover of oxygen batteries and the intrinsic drawbacks of the metal anodes (i.e., large volume changes and dendrite issues) have still been unsolved key problems. Here, we demonstrate a novel design of the K-ion oxygen battery using a graphite intercalation composite as the anode in a highly concentrated ether-based electrolyte. Instead of the metal K anode, the potassium graphite intercalation compound as the anode is depotassiated/potassiated in a binary form below 0.3 V (vs. K+/K); correspondingly, the discharged product KO2 is formed/decomposed at the carbon nanotube cathode, and an all-carbon full cell exhibits impressive cycling stability with a working voltage of 2.0 V. Furthermore, the utilization of graphite intercalation chemistry has been demonstrated to be applicable in Li-O2 batteries as well. Therefore, this study may provide a new strategy to resolve the key problems of the alkali metal-oxygen batteries.

Correlation between Microstructure and Potassium Storage Behavior in Reduced Graphene Oxide Materials.

Potassium-ion batteries (PIBs) are considered to be potential alternatives to the conventional lithium-ion batteries (LIBs) due to the similar working mechanism and abundant potassium (K) resource. However, it still remains challenging to directly apply commercial graphite anodes for PIBs owing to the large K ions, which may impede the electrochemical intercalation of K ions into the graphite interlayer and result in a poor cyclic stability and rate capability. Reduced graphene oxide (rGO) has shown remarkable electrochemical performance as an anode material for PIBs due to the fact that rGO possesses more active sites with an enlarged interlayer distance. Understanding the microstructure of rGO is crucial for optimizing its K-ion storage capabilities. Herein, it is revealed that the K-ion storage behavior of rGO is strongly dependent on the thermal treatment temperature on account of the difference in microstructure. rGO graphitized at 2500 °C exhibits a superior long-term cyclic stability for 2500 cycles due to the expanded interlayer distance and the unique graphite-like structure in a long range, enabling it to endure the huge volume change during uninterrupted K-ion intercalation/deintercalation processes.

Utilizing an autogenously protective atmosphere to synthesize a Prussian white cathode with ultrahigh capacity-retention for potassium-ion batteries.

A facile, low-cost precipitation method, utilizing an autogenously protective atmosphere without the assistance of an inert atmosphere, is proposed to synthesize nano-sized Prussian white K1.62Fe[Fe(CN)6]0.92·0.33H2O. The cathode delivers a high capacity of 120.9 mA h g-1 at 50 mA g-1 and an ultrahigh capacity retention of 98.2% after 100 cycles.

Room-temperature liquid metal-based anodes for high-energy potassium-based electrochemical devices.

A liquid Na-K alloy is adsorbed onto a super-aligned carbon nanotube membrane (denoted CM) at room temperature, which is driven by capillary force, fabricating a flexible CM@NaK membrane. The anode directly using the CM@NaK membrane exhibits a smooth electrode-electrolyte (liquid-liquid) interface, contributing to fast ion transport and a dendrite-free stripping/plating process.

Controllable Electrochemical Fabrication of KO2-Decorated Binder-Free Cathodes for Rechargeable Lithium-Oxygen Batteries.

Understanding the electrochemical property of superoxides in alkali metal oxygen batteries is critical for the design of a stable oxygen battery with high capacity and long cycle performance. In this work, a KO2-decorated binder-free cathode is fabricated by a simple and efficient electrochemical strategy. KO2 nanoparticles are uniformly coated on the carbon nanotube film (CNT-f) through a controllable discharge process in the K-O2 battery, and the KO2-decorated CNT-f is innovatively introduced into the Li-O2 battery as the O2 diffusion electrode. The Li-O2 battery based on the KO2-decorated CNT-f cathode can deliver enhanced discharge capacity, reduced charge overpotential, and more stable cycle performance compared with the battery in the absence of KO2. In situ formed KO2 particles on the surface of CNT-f cathode assist to form Li2O2 nanosheets in the Li-O2 battery, which contributes to the improvement of discharge capacity and cycle life. Interestingly, the analysis of KO2-decorated CNT-f cathodes, after discharge and cycle tests, reveals that the electrochemically synthesized KO2 seems not a conventional electrocatalyst but a partially dissolvable and decomposable promoter in Li-O2 batteries.

Molecular Sieve Induced Solution Growth of Li2O2 in the Li-O2 Battery with Largely Enhanced Discharge Capacity.

The formation of the insulated film-like discharge products (Li2O2) on the surface of the carbon cathode gradually hinders the oxygen reduction reaction (ORR) process, which usually leads to the premature death of the Li-O2 battery. In this work, by introducing the molecular sieve powder into the ether electrolyte, the Li-O2 battery exhibits a largely improved discharge capacity (63 times) compared with the one in the absence of this inorganic oxide additive. Meanwhile, XRD and SEM results qualitatively demonstrate the generation of the toroid Li2O2 as the dominated discharge products, and the chemical titration quantifies a higher yield of the Li2O2 with the presence of the molecular sieve additive. The addition of the molecular sieve controls the amount of the free water in the electrolyte, which distinguishes the effect of the molecular sieve and the free water on the discharge process. Hence, a possible mechanism has been proposed that the adsorption of the molecular sieves toward the soluble lithium superoxides improves the disproportionation of the lithium superoxides and consequently enhances the solution-growth of the lithium peroxides in the low donor number ether electrolyte. In general, the application of the molecular sieve triggers further studies concerning the improvement of the discharge performance in the Li-O2 battery by adding the inorganic additives.

Investigations of Si Thin Films as Anode of Lithium-Ion Batteries.

Amorphous silicon thin films having various thicknesses were investigated as a negative electrode material for lithium-ion batteries. Electrochemical characterization of the 20 nm thick thin silicon film revealed a very low first cycle Coulombic efficiency, which can be attributed to the silicon oxide layer formed on both the surface of the as-deposited Si thin film and the interface between the Si and the substrate. Among the investigated films, the 100 nm Si thin film demonstrated the best performance in terms of first cycle efficiency and cycle life. Observations from scanning electron microscopy demonstrated that the generation of cracks was inevitable in the cycled Si thin films, even as the thickness of the film was as little as 20 nm, which was not predicted by previous modeling work. However, the cycling performance of the 20 and 100 nm silicon thin films was not detrimentally affected by these cracks. The poor capacity retention of the 1 µm silicon thin film was attributed to the delamination.

Dendrite-Free Potassium-Oxygen Battery Based on a Liquid Alloy Anode.

The safety issue caused by the dendrite growth is not only a key research problem in lithium-ion batteries but also a critical concern in alkali metal (i.e., Li, Na, and K)-oxygen batteries where a solid metal is usually used as the anode. Herein, we demonstrate the first dendrite-free K-O2 battery at ambient temperature based on a liquid Na-K alloy anode. The unique liquid-liquid connection between the liquid alloy and the electrolyte in our alloy anode-based battery provides a homogeneous and robust anode-electrolyte interface. Meanwhile, we manage to show that the Na-K alloy is only compatible in K-O2 batteries but not in Na-O2 batteries, which is mainly attributed to the stronger reducibility of potassium and relatively more favorable thermodynamic formation of KO2 over NaO2 during the discharge process. It is observed that our K-O2 battery based on a liquid alloy anode shows a long cycle life (over 620 h) and a low discharge-charge overpotential (about 0.05 V at initial cycles). Moreover, the mechanism investigation into the K-O2 cell degradation shows that the O2 crossover effect and the ether-electrolyte instability are the critical problems for K-O2 batteries. In a word, this study provides a new route to solve the problems caused by the dendrite growth in alkali metal-oxygen batteries.

A lithium-oxygen battery based on lithium superoxide.

Batteries based on sodium superoxide and on potassium superoxide have recently been reported. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research into the lithium-oxygen (Li-O2) battery because of its potential high energy density. Several studies of Li-O2 batteries have found evidence of LiO2 being formed as one component of the discharge product along with lithium peroxide (Li2O2). In addition, theoretical calculations have indicated that some forms of LiO2 may have a long lifetime. These studies also suggest that it might be possible to form LiO2 alone for use in a battery. However, solid LiO2 has been difficult to synthesize in pure form because it is thermodynamically unstable with respect to disproportionation, giving Li2O2 (refs 19, 20). Here we show that crystalline LiO2 can be stabilized in a Li-O2 battery by using a suitable graphene-based cathode. Various characterization techniques reveal no evidence for the presence of Li2O2. A novel templating growth mechanism involving the use of iridium nanoparticles on the cathode surface may be responsible for the growth of crystalline LiO2. Our results demonstrate that the LiO2 formed in the Li-O2 battery is stable enough for the battery to be repeatedly charged and discharged with a very low charge potential (about 3.2 volts). We anticipate that this discovery will lead to methods of synthesizing and stabilizing LiO2, which could open the way to high-energy-density batteries based on LiO2 as well as to other possible uses of this compound, such as oxygen storage.

The Effect of Potassium Impurities Deliberately Introduced into Activated Carbon Cathodes on the Performance of Lithium-Oxygen Batteries.

Rechargeable lithium-air (Li-O2) batteries have drawn much interest owing to their high energy density. We report on the effect of deliberately introducing potassium impurities into the cathode material on the electrochemical performance of a Li-O2 battery. Small amounts of potassium introduced into the activated carbon (AC) cathode material in the synthesis process are found to have a dramatic effect on the performance of the Li-O2 cell. An increased amount of potassium significantly increases capacity, cycle life, and round-trip efficiency. This improved performance is probably due to a larger amount of LiO2 in the discharge product, which is a mixture of LiO2 and Li2O2, resulting from the increase in the amount of potassium present. No substantial correlation with porosity or surface area in an AC cathode is found. Experimental and computational studies indicate that potassium can act as an oxygen reduction catalyst, which can account for the dependence of performance on the amount of potassium.

Interfacial effects on lithium superoxide disproportionation in Li-O2 batteries.

During the cycling of Li-O2 batteries the discharge process gives rise to dynamically evolving agglomerates composed of lithium-oxygen nanostructures; however, little is known about their composition. In this paper, we present results for a Li-O2 battery based on an activated carbon cathode that indicate interfacial effects can suppress disproportionation of a LiO2 component in the discharge product. High-intensity X-ray diffraction and transmission electron microscopy measurements are first used to show that there is a LiO2 component along with Li2O2 in the discharge product. The stability of the discharge product was then probed by investigating the dependence of the charge potential and Raman intensity of the superoxide peak with time. The results indicate that the LiO2 component can be stable for possibly up to days when an electrolyte is left on the surface of the discharged cathode. Density functional calculations on amorphous LiO2 reveal that the disproportionation process will be slower at an electrolyte/LiO2 interface compared to a vacuum/LiO2 interface. The combined experimental and theoretical results provide new insight into how interfacial effects can stabilize LiO2 and suggest that these interfacial effects may play an important role in the charge and discharge chemistries of a Li-O2 battery.

Raman Evidence for Late Stage Disproportionation in a Li-O2 Battery.

Raman spectroscopy is used to characterize the composition of toroids formed in an aprotic Li-O2 cell based on an activated carbon cathode. The trends in the Raman data as a function of discharge current density and charging cutoff voltage provide evidence that the toroids are made up of outer LiO2-like and inner Li2O2 regions, consistent with a disproportionation reaction occurring in the solid phase. The LiO2-like component is found to be associated with a new Raman peak identified in the carbon stretching region at ∼1505 cm(-1), which appears only when the LiO2 peak at 1123 cm(-1) is present. The new peak is assigned to distortion of the graphitic ring stretching due to coupling with the LiO2-like component based on density functional calculations. These new results on the LiO2-like component from Raman spectroscopy provide evidence that a late stage disproportionation mechanism can occur during discharge and add new understanding to the complexities of possible processes occurring in Li-O2 batteries.

Disproportionation in Li-O2 batteries based on a large surface area carbon cathode.

In this paper we report on a kinetics study of the discharge process and its relationship to the charge overpotential in a Li-O2 cell for large surface area cathode material. The kinetics study reveals evidence for a first-order disproportionation reaction during discharge from an oxygen-rich Li2O2 component with superoxide-like character to a Li2O2 component. The oxygen-rich superoxide-like component has a much smaller potential during charge (3.2-3.5 V) than the Li2O2 component (∼4.2 V). The formation of the superoxide-like component is likely due to the porosity of the activated carbon used in the Li-O2 cell cathode that provides a good environment for growth during discharge. The discharge product containing these two components is characterized by toroids, which are assemblies of nanoparticles. The morphologic growth and decomposition process of the toroids during the reversible discharge/charge process was observed by scanning electron microscopy and is consistent with the presence of the two components in the discharge product. The results of this study provide new insight into how growth conditions control the nature of discharge product, which can be used to achieve improved performance in Li-O2 cell.