Gas induced formation of inactive Li in rechargeable lithium metal batteries.

The formation of inactive lithium by side reactions with liquid electrolyte contributes to cell failure of lithium metal batteries. To inhibit the formation and growth of inactive lithium, further understanding of the formation mechanisms and composition of inactive lithium are needed. Here we study the impact of gas producing reactions on the formation of inactive lithium using ethylene carbonate as a case study. Ethylene carbonate is a common electrolyte component used with graphite-based anodes but is incompatible with Li metal anodes. Using mass spectrometry titrations combined with 13C and 2H isotopic labeling, we reveal that ethylene carbonate decomposition continuously releases ethylene gas, which further reacts with lithium metal to form the electrochemically inactive species LiH and Li2C2. In addition, phase-field simulations suggest the non-ionically conducting gaseous species could result in an uneven distribution of lithium ions, detrimentally enhancing the formation of dendrites and dead Li. By optimizing the electrolyte composition, we selectively suppress the formation of ethylene gas to limit the formation of LiH and Li2C2 for both Li metal and graphite-based anodes.

Enhancing the Reduction Kinetics of LiSF6 Batteries by Dispersed Cobalt Phthalocyanines on Porous Carbon.

Reducing SF6 (as gas cathode) in Li batteries is a promising concept for the double benefit of mildly converting greenhouse SF6 and providing a high theoretical energy density of 3922 Wh kg-1 . However, the reduction process is hampered by its sluggish kinetics. Here, cobalt phthalocyanine (CoPc) molecules immobilized on porous carbon matrix are, for the first time, introduced to the LiSF6 chemistry to deliver an enhanced energy density. It is revealed that the high redox potential of Co(II)Pc/[Co(I)Pc]- (≈2.85 V) facilitates the formation of Co(I)N4 sites to catalyze the SF6 electrochemical reduction. By using highly porous holey nitrogen-doped carbon nanocages as carbon matrix, the LiSF6 cells deliver a high discharge voltage of 2.82 V at 50 mA gC+CoPc -1 and an unprecedented areal capacity of 25 mAh cm-2 at 0.1 mA cm-2 , much superior to previous results. This work opens up new possibilities for high-efficiency conversion of SF6 in lithium batteries.

Insights of the Electrochemical Reversibility of P2-Type Sodium Manganese Oxide Cathodes via Modulation of Transition Metal Vacancies.

Among cathode materials for sodium-ion batteries, Mn-based layered oxides have attracted enormous attention owing to their high capacity, cost-effectiveness, and fast transport channels. However, their practical application is hindered by the unsatisfied structural stability and the deficient understanding of electrochemical reaction mechanisms. Among these issues, the research of transition metal (TM) vacancy remains highly active due to their modulation roles on the anionic redox reactions, but their effects on structural and electrochemical stability remain obscure. Herein, based on Al-substituted P2-type Na2/3MnO2, we comprehensively investigate the effects of TM vacancies on the corresponding layered oxides. With several characterization techniques such as neutron diffraction, superconducting quantum interferometry, in situ X-ray diffraction, ex situ solid-state nuclear magnetic resonance techniques, and X-ray photoelectron spectroscopy, we determined the TM vacancy content and further revealed that higher content of TM vacancies (7.8%) in the transition layer is beneficial to mitigate the structure evolutions and maintain the P2 structure during cycling in voltage range 1.5-4.5 V, while the oxides with lower content of TM vacancies (1.6%) deliver higher discharge capacity but experience complicated phase transition, including stacking faults and P2-P2' transitions. It is demonstrated that regulating the contents of TM vacancies can be utilized as an effective strategy to tune the structure stability and electrochemical performances of layered sodium oxide cathodes.

The stability of P2-layered sodium transition metal oxides in ambient atmospheres.

Air-stability is one of the most important considerations for the practical application of electrode materials in energy-harvesting/storage devices, ranging from solar cells to rechargeable batteries. The promising P2-layered sodium transition metal oxides (P2-NaxTmO2) often suffer from structural/chemical transformations when contacted with moist air. However, these elaborate transitions and the evaluation rules towards air-stable P2-NaxTmO2 have not yet been clearly elucidated. Herein, taking P2-Na0.67MnO2 and P2-Na0.67Ni0.33Mn0.67O2 as key examples, we unveil the comprehensive structural/chemical degradation mechanisms of P2-NaxTmO2 in different ambient atmospheres by using various microscopic/spectroscopic characterizations and first-principle calculations. The extent of bulk structural/chemical transformation of P2-NaxTmO2 is determined by the amount of extracted Na+, which is mainly compensated by Na+/H+ exchange. By expanding our study to a series of Mn-based oxides, we reveal that the air-stability of P2-NaxTmO2 is highly related to their oxidation features in the first charge process and further propose a practical evaluating rule associated with redox couples for air-stable NaxTmO2 cathodes.

Visualizing the growth process of sodium microstructures in sodium batteries by in-situ 23Na MRI and NMR spectroscopy.

The growth of sodium dendrites and the associated solid electrolyte interface (SEI) layer is a critical and fundamental issue influencing the safety and cycling lifespan of sodium batteries. In this work, we use in-situ 23Na magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) techniques, along with an innovative analytical approach, to provide space-resolved and quantitative insights into the formation and evolution of sodium metal microstructures (SMSs; that is, dendritic and mossy Na metal) during the deposition and stripping processes. Our results reveal that the growing SMSs give rise to a linear increase in the overpotential until a transition voltage of 0.15 V is reached, at which point violent electrochemical decomposition of the electrolyte is triggered, leading to the formation of mossy-type SMSs and rapid battery failure. In addition, we determined the existence of NaH in the SEI on sodium metal with ex-situ NMR results. The poor electronic conductivity of NaH is beneficial for the growth of a stable SEI on sodium metal.

High-Efficiency Lithium Metal Anode Enabled by a Concentrated/Fluorinated Ester Electrolyte.

Lithium (Li) metal anode (LMA) has received growing attention due to its highest theoretical capacity (3860 mA h g-1) and lowest redox potential (-3.04 V versus standard hydrogen electrode). However, practical application of LMA is obstructed by the detrimental side reactions between Li metal and organic electrolytes, especially when cycled in traditional carbonate ester electrolytes. Herein, we propose a novel fluorinated carbonate ester-based electrolyte by combining diethyl fluorocarbonate (ETFEC) solvent and 5 M LiFSI concentration (M = mol L-1). Using this electrolyte, an ultrahigh Li plating/stripping Coulombic efficiency (CE) of 99.1% can be obtained in Li||Cu cells and a stable cycle performance of Li||LiFePO4 is achieved under the conditions of limited Li metal (5 mA h cm-2), moderate loading LiFePO4 (7-8 mg cm-2), and lean electrolyte (40 uL). The fundamental functioning mechanism of this novel electrolyte has been carefully investigated by scanning electronic microscopy (SEM), operando optical microscopy (OM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and solid state nuclear magnetic resonance (SS-NMR). The results demonstrate that this optimized electrolyte facilitates formation of a high Li+ conductive SEI layer enriched with LiF and inorganic sulfur-containing species, which can effectively suppress the side reactions between electrolyte and Li metal and prevent formation of dead Li.