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On discharge, the Li–O 2 battery can form a Li 2 O 2 film on the cathode surface, leading to low capacities, low rates and early cell death, or it can form Li 2 O 2 particles in solution, leading to high capacities at relatively high rates and avoiding early cell death. Achieving discharge in solution is important and may be encouraged by the use of high donor or acceptor number solvents or salts that dissolve the LiO 2 intermediate involved in the formation of Li 2 O 2 . However, the characteristics that make high donor or acceptor number solvents good (for example, high polarity) result in them being unstable towards LiO 2 or Li 2 O 2 . Here we demonstrate that introduction of the additive 2,5-di- tert -butyl-1,4-benzoquinone (DBBQ) promotes solution phase formation of Li 2 O 2 in low-polarity and weakly solvating electrolyte solutions. Importantly, it does so while simultaneously suppressing direct reduction to Li 2 O 2 on the cathode surface, which would otherwise lead to Li 2 O 2 film growth and premature cell death. It also halves the overpotential during discharge, increases the capacity 80- to 100-fold and enables rates >1 mA cm areal −2 for cathodes with capacities of >4 mAh cm areal −2 . The DBBQ additive operates by a new mechanism that avoids the reactive LiO 2 intermediate in solution. Discharge in lithium–air batteries can be triggered by using solvents or salts that dissolve LiO 2 intermediate. Adding DBBQ to a weakly solvating electrolyte solution can help O 2 reduction to Li 2 O 2 in solution and enhance electrochemical performance.

During the charging and discharging of lithium-ion-battery cathodes through the de- and reintercalation of lithium ions, electroneutrality is maintained by transition-metal redox chemistry, which limits the charge that can be stored. However, for some transition-metal oxides this limit can be broken and oxygen loss and/or oxygen redox reactions have been proposed to explain the phenomenon. We present operando mass spectrometry of 18 O-labelled Li 1.2 [Ni 0.13 2+ Co 0.13 3+ Mn 0.54 4+ ]O 2 , which demonstrates that oxygen is extracted from the lattice on charging a Li 1.2 [Ni 0.13 2+ Co 0.13 3+ Mn 0.54 4+ ]O 2 cathode, although we detected no O 2 evolution. Combined soft X-ray absorption spectroscopy, resonant inelastic X-ray scattering spectroscopy, X-ray absorption near edge structure spectroscopy and Raman spectroscopy demonstrates that, in addition to oxygen loss, Li + removal is charge compensated by the formation of localized electron holes on O atoms coordinated by Mn 4+ and Li + ions, which serve to promote the localization, and not the formation, of true O 2 2− (peroxide, O–O ~1.45 Å) species. The quantity of charge compensated by oxygen removal and by the formation of electron holes on the O atoms is estimated, and for the case described here the latter dominates. The energy that can be stored in lithium-ion batteries is typically limited by the redox chemistry of the transition metals within the cathodes. Now it is shown that for Li 1.2 [Ni 2+ 0.13 Co 3+ 0.13 Mn 4+ 0.54 ]O 2 , a 3 d -transition-metal oxide that breaks this limit, Li-ion extraction is charge compensated not just by transition-metal oxidation but also through the generation of localized electron-holes on oxygen.

A critical current density on stripping is identified that results in dendrite formation on plating and cell failure. When the stripping current density removes Li from the interface faster than it can be replenished, voids form in the Li at the interface and accumulate on cycling, increasing the local current density at the interface and ultimately leading to dendrite formation on plating, short circuit and cell death. This occurs even when the overall current density is considerably below the threshold for dendrite formation on plating. For the Li/Li6PS5Cl/Li cell, this is 0.2 and 1.0 mA cm−2 at 3 and 7 MPa pressure, respectively, compared with a critical current for plating of 2.0 mA cm−2 at both 3 and 7 MPa. The pressure dependence on stripping indicates that creep rather than Li diffusion is the dominant mechanism transporting Li to the interface. The critical stripping current is a major factor limiting the power density of Li anode solid-state cells. Considerable pressure may be required to achieve even modest power densities in solid-state cells.

In conventional intercalation cathodes, alkali metal ions can move in and out of a layered material with the charge being compensated for by reversible reduction and oxidation of the transition metal ions. If the cathode material used in a lithium-ion or sodium-ion battery is alkali-rich, this can increase the battery’s energy density by storing charge on the oxide and the transition metal ions, rather than on the transition metal alone 1 – 10 . There is a high voltage associated with oxidation of O 2− during the first charge, but this is not recovered on discharge, resulting in reduced energy density 11 . Displacement of transition metal ions into the alkali metal layers has been proposed to explain the first-cycle voltage loss (hysteresis) 9 , 12 – 16 . By comparing two closely related intercalation cathodes, Na 0.75 [Li 0.25 Mn 0.75 ]O 2 and Na 0.6 [Li 0.2 Mn 0.8 ]O 2 , here we show that the first-cycle voltage hysteresis is determined by the superstructure in the cathode, specifically the local ordering of lithium and transition metal ions in the transition metal layers. The honeycomb superstructure of Na 0.75 [Li 0.25 Mn 0.75 ]O 2 , present in almost all oxygen-redox compounds, is lost on charging, driven in part by formation of molecular O 2 inside the solid. The O 2 molecules are cleaved on discharge, reforming O 2− , but the manganese ions have migrated within the plane, changing the coordination around O 2− and lowering the voltage on discharge. The ribbon superstructure in Na 0.6 [Li 0.2 Mn 0.8 ]O 2 inhibits manganese disorder and hence O 2 formation, suppressing hysteresis and promoting stable electron holes on O 2− that are revealed by X-ray absorption spectroscopy. The results show that voltage hysteresis can be avoided in oxygen-redox cathodes by forming materials with a ribbon superstructure in the transition metal layers that suppresses migration of the transition metal. In oxygen-redox intercalation cathodes, voltage hysteresis can be avoided by forming cathode materials with a ‘ribbon’ superstructure in the transition metal layers that suppresses transition metal migration.

Oxygen redox cathodes, such as Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , deliver higher energy densities than those based on transition metal redox alone. However, they commonly exhibit voltage fade, a gradually diminishing discharge voltage on extended cycling. Recent research has shown that, on the first charge, oxidation of O 2− ions forms O 2 molecules trapped in nano-sized voids within the structure, which can be fully reduced to O 2− on the subsequent discharge. Here we show that the loss of O-redox capacity on cycling and therefore voltage fade arises from a combination of a reduction in the reversibility of the O 2− /O 2 redox process and O 2 loss. The closed voids that trap O 2 grow on cycling, rendering more of the trapped O 2 electrochemically inactive. The size and density of voids leads to cracking of the particles and open voids at the surfaces, releasing O 2 . Our findings implicate the thermodynamic driving force to form O 2 as the root cause of transition metal migration, void formation and consequently voltage fade in Li-rich cathodes. Oxygen redox cathodes deliver higher energy densities than those based on transition metal redox but commonly exhibit voltage fade on extended cycling. The loss of O-redox capacity and voltage fade is shown to arise from a reduction in O 2− /O 2 redox process reversibility and O 2 loss.

Lithium–oxygen cells, in which lithium peroxide forms in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to charge. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide stored in the cathode. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for the operation of the lithium–oxygen cell. It is essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain fast charging. Here, we investigate the kinetics of lithium peroxide oxidation by several different classes of redox mediators. We show that the reaction is not a simple outer-sphere electron transfer and that the steric structure of the mediator molecule plays an important role. The fastest mediator studied could sustain a charging current of up to 1.9 A cm –2 , based on a model for a porous electrode described here. The kinetics of Li 2 O 2 oxidation is of high importance to the operation of Li–O 2 batteries. Here the authors work on different types of mediators revealing the dependence of the kinetics on the nature of the redox active site and its steric hindrance.

The non-aqueous Li–air (O 2 ) battery is receiving intense interest because its theoretical specific energy exceeds that of Li-ion batteries. Recharging the Li–O 2 battery depends on oxidizing solid lithium peroxide (Li 2 O 2 ), which is formed on discharge within the porous cathode. However, transporting charge between Li 2 O 2 particles and the solid electrode surface is at best very difficult and leads to voltage polarization on charging, even at modest rates. This is a significant problem facing the non-aqueous Li–O 2 battery. Here we show that incorporation of a redox mediator, tetrathiafulvalene (TTF), enables recharging at rates that are impossible for the cell in the absence of the mediator. On charging, TTF is oxidized to TTF + at the cathode surface; TTF + in turn oxidizes the solid Li 2 O 2 , which results in the regeneration of TTF. The mediator acts as an electron–hole transfer agent that permits efficient oxidation of solid Li 2 O 2 . The cell with the mediator demonstrated 100 charge/discharge cycles. Recharging Li–O 2 batteries requires oxidation of the discharge product solid Li 2 O 2 . Now a redox-mediating molecule is shown to assist this process by transferring electron–holes between solid Li 2 O 2 and the positive electrode in a non-aqueous Li–O 2 cell. This allows the cell to be charged at rates that are otherwise impossible.

Lithium-rich disordered rocksalt cathodes display high capacities arising from redox chemistry on both transition-metal ions (TM-redox) and oxygen ions (O-redox), making them promising candidates for next-generation lithium-ion batteries. However, the atomic-scale mechanisms governing O-redox behaviour in disordered structures are not fully understood. Here we show that, at high states of charge in the disordered rocksalt Li 2 MnO 2 F, transition metal migration is necessary for the formation of molecular O 2 trapped in the bulk. Density functional theory calculations reveal that O 2 is thermodynamically favoured over other oxidised O species, which is confirmed by resonant inelastic X-ray scattering data showing only O 2 forms. When O-redox involves irreversible Mn migration, this mechanism results in a path-dependent voltage hysteresis between charge and discharge, commensurate with the hysteresis observed electrochemically. The implications are that irreversible transition metal migration should be suppressed to reduce the voltage hysteresis that afflicts O-redox disordered rocksalt cathodes. The oxygen-redox mechanism in lithium-rich disordered rocksalt cathode materials is still not well understood. Here, the authors show that in Li 2 MnO 2 F, molecular oxygen forms in the bulk during charge and is re-incorporated into the structure as oxygen anions on discharge, but this process is associated with irreversible Mn migration, causing voltage hysteresis.

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 6 PS 5 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. Lithium dendrite propagation through ceramic electrolytes can prevent the realization of high-energy-density all-solid-state lithium-anode batteries. The propagation of cracks and lithium dendrites through a solid electrolyte has now been tracked as a function of charge.

The energy density of Li-ion batteries can be improved by storing charge at high voltages through the oxidation of oxide ions in the cathode material. However, oxidation of O 2− triggers irreversible structural rearrangements in the bulk and an associated loss of the high voltage plateau, which is replaced by a lower discharge voltage, and a loss of O 2 accompanied by densification at the surface. Here we consider various models for oxygen redox that are proposed in the literature and then describe a single unified model involving O 2− oxidation to form O 2 , most of which is trapped in the bulk and the remainder of which evolves from the surface. The model extends the O 2 formation and evolution at the surface, which is well known and well characterized, into the electrode particle bulk as caged O 2 that can be reversibly reduced and oxidized. This converged understanding enables us to propose practical strategies to avoid oxygen-redox-induced instability and provide potential routes towards more reversible, high energy density Li-ion cathodes. Oxygen redox in Li-rich oxide cathodes is of both fundamental and practical interest in Li-ion battery development. Bruce and team examine the current understanding of oxygen-redox processes, especially those concerning O 2 formation, and discuss strategies that can harness oxygen redox with suppressed side effects.