Kinetics of lithium peroxide oxidation by redox mediators and consequences for the lithium–oxygen cell

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 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 here could sustain charging current of up to 1.9 A cm–2, based on a model for a porous electrode described here. Lithium-oxygen cells in which the cathode reaction of lithium peroxide formation takes place in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to oxidise it. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide particles in the pores of the cathode that are disconnected from the surface. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for operation of the lithium-oxygen cell. While molecules with fast electron transfer at the electrode surface are common, it is also essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain relatively fast charging. Here we investigate the kinetics of lithium peroxide oxidation by several classes of redox mediators, with varying electrochemical and structural properties (amines, nitroxy and thiol compounds). The rates range from 0.025 to 7.9 x10—3 cm s—1 with the nitroxy compounds exhibiting the highest rates. We show that the reaction is not a simple outer sphere electron transfer and that the nature of the mediator molecule plays an important role for example the steric hindrance around the active redox centre of the mediator. The fastest mediator studied here could sustain an areal current density on charging of up to 1.9 A cm—2, based on a model for a porous electrode described in the paper

A rechargeable lithium–oxygen battery with dual mediators stabilizing the carbon cathode

At the cathode of a Li–O2 battery, O2 is reduced to Li2O2 on discharge, the process being reversed on charge. Li2O2 is an insulating and insoluble solid, leading ultimately to low rates, low capacities and early cell death if formed on the cathode surface. Here we show that when using dual mediators, 2,5-Di-tert-butyl-1,4-benzoquinone [DBBQ] on discharge and 2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO] on charge, the electrochemistry at the cathode surface is decoupled from Li2O2 formation/decomposition in solution. Capacities of 2 mAh cmareal−2 at 1 mA cmareal−2 with low polarization on charge/discharge are demonstrated, and up to 40 mAh cmareal−2 at rates ≫1 mA cmareal−2 are anticipated if suitable gas diffusion electrodes can be devised. One of the major barriers to the progress of Li–O2 cells is decomposition of the carbon cathode. By forming/decomposing Li2O2 in solution and avoiding high charge potentials, the carbon instability is significantly mitigated ( < 0.008% decomposition per cycle compared with 0.12% without mediators)

Promoting solution phase discharge in Li-O-2 batteries containing weakly solvating electrolyte solutions

On discharge, the Li–O2 battery can form a Li2O2 film on the cathode surface, leading to low capacities, low rates and early cell death, or it can form Li2O2 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 LiO2 intermediate involved in the formation of Li2O2. However, the characteristics that make high donor or acceptor number solvents good (for example, high polarity) result in them being unstable towards LiO2 or Li2O2. Here we demonstrate that introduction of the additive 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ) promotes solution phase formation of Li2O2 in low-polarity and weakly solvating electrolyte solutions. Importantly, it does so while simultaneously suppressing direct reduction to Li2O2 on the cathode surface, which would otherwise lead to Li2O2 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 cmareal−2 for cathodes with capacities of >4 mAh cmareal−2. The DBBQ additive operates by a new mechanism that avoids the reactive LiO2 intermediate in solution

Kinetics of lithium peroxide oxidation by redox mediators and consequences for the lithium–oxygen cell

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 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 here could sustain charging current of up to 1.9 A cm–2, based on a model for a porous electrode described here.Lithium-oxygen cells in which the cathode reaction of lithium peroxide formation takes place in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to oxidise it. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide particles in the pores of the cathode that are disconnected from the surface. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for operation of the lithium-oxygen cell. While molecules with fast electron transfer at the electrode surface are common, it is also essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain relatively fast charging. Here we investigate the kinetics of lithium peroxide oxidation by several classes of redox mediators, with varying electrochemical and structural properties (amines, nitroxy and thiol compounds). The rates range from 0.025 to 7.9 x10—3 cm s—1 with the nitroxy compounds exhibiting the highest rates. We show that the reaction is not a simple outer sphere electron transfer and that the nature of the mediator molecule plays an important role for example the steric hindrance around the active redox centre of the mediator. The fastest mediator studied here could sustain an areal current density on charging of up to 1.9 A cm—2, based on a model for a porous electrode described in the paper

The effect of volume change and stack pressure on solid‐state battery cathodes

Solid-state lithium batteries may provide increased energy density and improved safety compared with Li-ion technology. However, in a solid-state composite cathode, mechanical degradation due to repeated cathode volume changes during cycling may occur, which may be partially mitigated by applying a significant, but often impractical, uniaxial stack pressure. Herein, we compare the behavior of composite electrodes based on Li4Ti5O12 (LTO) (negligible volume change) and Nb2O5 (+4% expansion) cycled at different stack pressures. The initial LTO capacity and retention are not affected by pressure but for Nb2O5, they are significantly lower when a stack pressure of &lt;2 MPa is applied, due to inter-particle cracking and solid-solid contact loss because of cyclic volume changes. This work confirms the importance of cathode mechanical stability and the stack pressures for long-term cyclability for solid-state batteries. This suggests that low volume-change cathode materials or a proper buffer layer are required for solid-state batteries, especially at low stack pressures

Influence of contouring the lithium metal/solid electrolyte interface on the critical current for dendrites

Contouring or structuring of the lithium/ceramic electrolyte interface and therefore increasing its surface area has been considered as a possible strategy to increase the charging current in solid-state batteries without lithium dendrite formation and short-circuit. By coupling together lithium deposition kinetics and the me chanics of lithium creep within calculations of the current distribution at the interface, and leveraging a model for lithium dendrite growth, we show that efforts to avoid dendrites on charging by increasing the interfacial surface area come with significant limitations associated with the topography of rough surfaces. These limitations are sufficiently severe such that it is very unlikely contouring could increase charging currents while avoiding dendrites and short-circuit to the levels required. For example, we show a sinusoidal surface topography can only raise the charging current before dendrites occur by approx. 50% over a flat interface

Carbon-emcoating architecture boosts lithium storage of Nb2O5

Intercalation transition metal oxides (ITMO) have attracted great attention as lithium-ion battery negative electrodes due to high operation safety, high capacity and rapid ion intercalation. However, the intrinsic low electron conductivity plagues the lifetime and cell performance of the ITMO negative electrode. Here we design a new carbon-emcoating architecture through single CO2 activation treatment as demonstrated by the Nb2O5/C nanohybrid. Triple structure engineering of the carbon-emcoating Nb2O5/C nanohybrid is achieved in terms of porosity, composition, and crystallographic phase. The carbon-embedding Nb2O5/C nanohybrids show superior cycling and rate performance compared with the conventional carbon coating, with reversible capacity of 387 mA h g−1 at 0.2 C and 92% of capacity retained after 500 cycles at 1 C. Differential electrochemical mass spectrometry (DEMS) indicates that the carbon emcoated Nb2O5 nanohybrids present less gas evolution than commercial lithium titanate oxide during cycling. The unique carbon-emcoating technique can be universally applied to other ITMO negative electrodes to achieve high electrochemical performance

Competitive Oxygen Reduction Pathways to Superoxide and Peroxide during Sodium-Oxygen Battery Discharge

The sodium-air battery offers a sustainable, high-energy alternative to lithium-ion batteries. Discharge in the cell containing glyme-based electrolytes can lead to formation of large cubic NaO2 particles via a solution-precipitation mechanism. While promising, high rates result in sudden death. The exact nature of the discharge product has been a matter of contention, and Na2O2 has never been directly detected in a dry glyme Na−O2 cell. If Na2O2 were to form during discharge in the Na−O2 cell it would have a detrimental impact on cell performance. Here we show that Na2O2 forms during discharge at high overpotential in the glyme-based Na−O2 batteries. Na2O2 formation is confirmed by spectroscopic and electrochemical analysis and electron microscopy demonstrates that it results in thin insulating films at the electrode surface. The formation of these thin films results in rapid cell death during discharge, introducing an inherent chemical limitation to the Na−O2 battery

The Interface between Li6.5La3Zr1.5Ta0.5O12 and Liquid Electrolyte

An advantageous solid electrolyte/liquid electrolyte interface is crucial for the implementation of a protected lithium anode in liquid electrolyte cells. Li6.5La3Zr1.5Ta0.5O12 (LLZTO) garnet electrolytes are among the few solid electrolytes that are stable in contact with lithium metal. We show LLZTO is unstable in contact with the organic carbonate-based Li+ liquid electrolyte used in conventional Li-ion cells. The interfacial resistance between LLZTO and LiPF6 in (CH2O)2CO: OC(OCH3)2 (1:1 v/v) increases with time due to the growth of a lithium-ion-conducting solid electrolyte interphase (SEI) at the surface of the ceramic electrolyte. The interphase is composed of Li2CO3, LiF, Li2O, and organic carbonates. Even at a rate of 5 mA cm−2, a 3 V potential drop occurs across the LLZTO/liquid electrolyte interface. A practical LLZTO membrane (thickness ∼10 μm), in contact with a lithium anode, gives a potential loss of ∼16 mV, less than 1% of the resistance of the SEI