Cell Chemistry of Sodium–Oxygen Batteries with Various Nonaqueous Electrolytes

Development of the nonaqueous Na–O₂ battery with a high electrical energy efficiency requires the electrolyte stable against attack of highly oxidative species such as nucleophilic anion O₂^•–. A combined evaluation method was used to investigate the Na–O₂ cell chemistry with various solvents, including ethylene carbonate/propylene carbonate (EC/PC)-, <i>N</i>-methyl-<i>N</i>-propylpiperidinium bis­(trifluoromethansulfonyl) imide (PP13TFSI)-, and tetraethylene glycol dimethyl ether (TEGDME)-based electrolytes. It is found that the TEGDME-based electrolytes have the best stability with the predominant yield of NaO₂ upon discharge and the largest electrical energy efficiency (approaching 90%). Both EC/PC- and PP13TFSI-based electrolytes severely decompose during discharge, forming a large amount of side products. Analysis of the acid dissociation constant (p<i>K</i>_a) of these electrolyte solvents reveals that the TEGDME has the relatively large value of p<i>K</i>_a, which correlates with good stability of the electrolyte and high round-trip energy efficiency of the battery

Tracking Formation and Decomposition of Abacus-Ball-Shaped Lithium Peroxides in Li–O₂ Cells

Study of formation and decomposition of Li₂O₂ during operations of Li–O₂ cells is essential for understanding the reaction mechanism and finding solutions to improve the cell performance. Using vertically aligned carbon nanotubes (VACNTs) directly grown on stainless steel meshes as the cathodes in the Li–O₂ cells with dimethoxyethane (DME) electrolytes, nucleation, growth, and decomposition processes of the Li₂O₂ in the first cycle are clearly visualized. Through cycles with the controlled discharge and charge capacities, the abacus-ball-shaped Li₂O₂ and the rust-like carbonates simultaneously formed around the VACNTs are further identified. It is indicated that the increasing coverage of carbonates on the cathode surface suppresses the formation of Li₂O₂, which maintains the shape of abacus ball. When the VACNT surfaces are predominantly covered by the carbonates, the cells tend to terminate

Sodium Storage and Pseudocapacitive Charge in Textured Li₄Ti₅O₁₂ Thin Films

Phase transformation reactions including alloying or conversion ones have often been utilized recently to improve the capacity performance of Na-ion battery anodes. However, they tend to induce larger volume change and more sluggish Na-ion transport at multiphase solid interfaces than for Li-ion batteries, leading to inefficiency of mixed conductive networks and thus degradation of reversibility, polarization, or rate performance. In this work, we use a structurally stable Li₄Ti₅O₁₂ spinel thin film as insertion-type model material to investigate its intrinsic Na-ion transport kinetics and coupled pseudocapacitive charging. It is found that the latter effect is remarkably activated by the nanocrystalline microstructure full of defect-rich surface, which can simultaneously promote Na-ion and electron accessibility to the surface/subsurface. It is proposed that the extra pseudocapacitive charge storage is a potential solution to the high-capacity and high-rate insertion anodes without trade-off of serious phase transformation or structural collapse. Therefore, a highly reversible charge capacity of 225 mAh g^–1 (exceeding the theoretical value 175 mAh g^–1 based on insertion reaction) at 1C is achievable

LiNi_0.6Co_0.2Mn_0.2O₂ Cathodes Coated with Dual-Conductive Polymers for High-Rate and Long-Life Solid-State Lithium Batteries

High-energy density and safe solid lithium batteries call for cathodes with high capacity and good kinetic properties. In this work, LiNi0.6Co0.2Mn0.2O2 (NCM622) cathodes are coated with the ionic–electronic dual-conductive polymers composed of poly­(ethylene glycol) (PEG)-doped polyaniline (PANI). Scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, and thermogravimetric analysis reveal that the dual-conductive polymers are homogeneously coated on the surfaces of NCM622 cathodes with a thickness of approximately 10 nm. The solid-state lithium batteries consisting of the NCM622 cathodes coated with PANI–PEG show a specific capacity of 158 mA h g–1 and a retention rate of 88% after 100 cycles at the rate of 0.1 C and room temperature, which are superior to the discharge capacity of 153 mA h g–1 and capacity retention of 59% after 100 cycles for the batteries with the pristine NCM622 cathodes. Moreover, the cells with the coated cathodes display a better rate performance of 84 mA h g–1 at 1 C than those with the uncoated ones which show a rate performance of 11 mA h g–1 at 1 C

Influence of Gold Nanoparticles Anchored to Carbon Nanotubes on Formation and Decomposition of Li₂O₂ in Nonaqueous Li–O₂ Batteries

Gold nanoparticles (AuNPs) anchored to vertically aligned carbon nanotubes (VACNTs) act as additional nucleation sites for the Li₂O₂ growth, leading to the decreased size while increased density of Li₂O₂ particles in process of discharge. Correspondingly, at the deep discharge to 2.0 V the batteries show increased specific capacity. Upon charge, the AuNPs exhibit promotion effect on the Li₂O₂ decomposition by improving the conduction property of the discharge-formed particles, rather than by imposing the conventional electrocatalytic effect on the oxygen evolution reaction. Moreover, the AuNPs show promotion effect on decomposition of carbonate species arising from the side reactions. These effects consequently lead to the reduced charge overpotentials and extended cycle operation of the batteries. The results here provide a new as well as clear picture on the role of incorporated AuNPs in the Li₂O₂ formation and decomposition, which would be helpful for better understanding and constructing of high-performance air cathodes

Positive Role of Surface Defects on Carbon Nanotube Cathodes in Overpotential and Capacity Retention of Rechargeable Lithium–Oxygen Batteries

Surface defects on carbon nanotube cathodes have been artificially introduced by bombardment with argon plasma. Their roles in the electrochemical performance of rechargeable Li–O₂ batteries have been investigated. In batteries with tetraethylene glycol dimethyl ether (TEGDME)- and <i>N</i>-methyl-<i>N</i>-propylpiperidinium bis­(trifluoromethansulfonyl)­imide (PP13TFSI)-based electrolytes, the defects increase the number of nucleation sites for the growth of Li₂O₂ particles and reduce the size of the formed particles. This leads to increased discharge capacity and reduced cycle overpotential. However, in the former batteries, the hydrophilic surfaces induced by the defects promote carbonate formation, which imposes a deteriorating effect on the cycle performance of the Li–O₂ batteries. In contrast, in the latter case, the defective cathodes promote Li₂O₂ formation without enhancing formation of carbonates on the cathode surfaces, resulting in extended cycle life. This is most probably attributable to the passivation effect on the functional groups of the cathode surfaces imposed by the ionic liquid. These results indicate that defects on carbon surfaces may have a positive effect on the cycle performance of Li–O₂ batteries if they are combined with a helpful electrolyte solvent such as PP13TFSI

Charge Carrier Accumulation in Lithium Fluoride Thin Films due to Li-Ion Absorption by Titania (100) Subsurface

The thermodynamically required redistribution of ions at given interfaces is being paid increased attention. The present investigation of the contact LiF/TiO₂ offers a highly worthwhile example, as the redistribution processes can be predicted and verified. It consists in Li ion transfer from LiF into the space charge zones of TiO₂. We not only can measure the resulting increase of lithium vacancy conductivity in LiF, we also observe a transition from n- to p-type conductivity in TiO₂ in consistency with the generalized space charge model

Formation of Nanosized Defective Lithium Peroxides through Si-Coated Carbon Nanotube Cathodes for High Energy Efficiency Li–O₂ Batteries

The formation and decomposition of lithium peroxides (Li₂O₂) during cycling is the key process for the reversible operation of lithium–oxygen batteries. The manipulation of such products from the large toroidal particles about hundreds of nanometers to the ones in the scale of tens of nanometers can improve the energy efficiency and the cycle life of the batteries. In this work, we carry out an in situ morphology tuning of Li₂O₂ by virtue of the surface properties of the n-type Si-modified aligned carbon nanotube (CNT) cathodes. With the introduction of an n-type Si coating layer on the CNT surface, the morphology of Li₂O₂ formed by discharge changes from large toroidal particles (∼300 nm) deposited on the pristine CNT cathodes to nanoparticles (10–20 nm) with poor crystallinity and plenty of lithium vacancies. Beneficial from such changes, the charge overpotential dramatically decreases to 0.55 V, with the charge plateau lying at 3.5 V even in the case of a high discharge capacity (3450 mA h g^–1) being delivered, resulting in the high electrical energy efficiency approaching 80%. Such an improvement is attributed to the fact that the introduction of the n-type Si coating layer changes the surface properties of CNTs and guides the formation of nanosized amorphous-like lithium peroxides with plenty of defects. These results demonstrate that the cathode surface properties play an important role in the formation of products formed during the cycle, providing inspiration to design superior cathodes for the Li–O₂ cells