Trapped O2 and the origin of voltage fade in layered Li-rich cathodes

Oxygen redox cathodes, such as Li1.2Ni0.13Co0.13Mn0.54O2, 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 O2- ions forms O2 molecules trapped in nano-sized voids within the structure, which can be fully reduced to O2- 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 O2-/O2 redox process and O2 loss. The closed voids that trap O2 grow on cycling, rendering more of the trapped O2 electrochemically inactive. The size and density of voids leads to cracking of the particles and open voids at the surfaces, releasing O2. Our findings implicate the thermodynamic driving force to form O2 as the root cause of transition metal migration, void formation and consequently voltage fade in Li-rich cathodes.

Diagnosing the Electrostatic Shielding Mechanism for Dendrite Suppression in Aqueous Zinc Batteries

Aqueous zinc electrolytes offer the potential for cheaper rechargeable batteries due to their safe compatibility with the high capacity metal anode; yet, they are stymied by irregular zinc deposition and consequent dendrite growth. Suppressing dendrite formation by tailoring the electrolyte is a proven approach from lithium batteries; yet, the underlying mechanistic understanding that guides such tailoring does not necessarily directly translate from one system to the other. Here, it is shown that the electrostatic shielding mechanism, a fundamental concept in electrolyte engineering for stable metal anodes, has different consequences for the plating morphology in aqueous zinc batteries. Operando electrochemical transmission electron microscopy is used to directly observe the zinc nucleation and growth under different electrolyte compositions and reveal that electrostatic shielding additive suppresses dendrites by inhibiting secondary zinc nucleation along the (100) edges of existing primary deposits and encouraging preferential deposition on the (002) faces, leading to a dense and block-like zinc morphology. The strong influence of the crystallography of Zn on the electrostatic shielding mechanism is further confirmed with Zn||Ti cells and density functional theory modeling. This work demonstrates the importance of considering the unique aspects of the aqueous zinc battery system when using concepts from other battery chemistries.

Alternatives to fluorinated binders: recyclable copolyester/carbonate electrolytes for high-capacity solid composite cathodes

Optimising the composite cathode for next-generation, safe solid-state batteries with inorganic solid electrolytes remains a key challenge towards commercialisation and cell performance. Tackling this issue requires the design of suitable polymer binders for electrode processability and long-term solid-solid interfacial stability. Here, block-polyester/carbonates are systematically designed as Li-ion conducting, high-voltage stable binders for cathode composites comprising of single-crystal LiNi0.8Mn0.1Co0.1O2 cathodes, Li6PS5Cl solid electrolyte and carbon nanofibres. Compared to traditional fluorinated polymer binders, improved discharge capacities (186 mA h g-1) and capacity retention (96.7% over 200 cycles) are achieved. The nature of the new binder electrolytes also enables its separation and complete recycling after use. ABA- and AB-polymeric architectures are compared where the A-blocks are mechanical modifiers, and the B-block facilitates Li-ion transport. This reveals that the conductivity and mechanical properties of the ABA-type are more suited for binder application. Further, catalysed switching between CO2/epoxide A-polycarbonate (PC) synthesis and B-poly(carbonate-r-ester) formation employing caprolactone (CL) and trimethylene carbonate (TMC) identifies an optimal molar mass (50 kg mol-1) and composition (wPC 0.35). This polymer electrolyte binder shows impressive oxidative stability (5.2 V), suitable ionic conductivity (2.2 × 10-4 S cm-1 at 60 °C), and compliant viscoelastic properties for fabrication into high-performance solid composite cathodes. This work presents an attractive route to optimising polymer binder properties using controlled polymerisation strategies combining cyclic monomer (CL, TMC) ring-opening polymerisation and epoxide/CO2 ring-opening copolymerisation. It should also prompt further examination of polycarbonate/ester-based materials with today's most relevant yet demanding high-voltage cathodes and sensitive sulfide-based solid electrolytes.

High-energy all-solid-state lithium batteries enabled by Co-free LiNiO2 cathodes with robust outside-in structures

A critical current challenge in the development of all-solid-state lithium batteries (ASSLBs) is reducing the cost of fabrication without compromising the performance. Here we report a sulfide ASSLB based on a high-energy, Co-free LiNiO2 cathode with a robust outside-in structure. This promising cathode is enabled by the high-pressure O2 synthesis and subsequent atomic layer deposition of a unique ultrathin LixAlyZnzOδ protective layer comprising a LixAlyZnzOδ surface coating region and an Al and Zn near-surface doping region. This high-quality artificial interphase enhances the structural stability and interfacial dynamics of the cathode as it mitigates the contact loss and continuous side reactions at the cathode/solid electrolyte interface. As a result, our ASSLBs exhibit a high areal capacity (4.65 mAh cm-2), a high specific cathode capacity (203 mAh g-1), superior cycling stability (92% capacity retention after 200 cycles) and a good rate capability (93 mAh g-1 at 2C). This work also offers mechanistic insights into how to break through the limitation of using expensive cathodes (for example, Co-based) and coatings (for example, Nb-, Ta-, La- or Zr-based) while still achieving a high-energy ASSLB performance.

The accumulation of Li2CO3 in a Li-O2 battery with dual mediators

One of the most important challenges facing long cycle life Li-O2 batteries is solvent degradation. Even the most stable ethers, such as CH3O(CH2CH2O)CH3, degrade to form products including Li2CO3, which accumulates in the pores of the gas diffusion electrode on cycling leading to polarisation and capacity fading. In this work, we examine the build-up and distribution of Li2CO3 within the porous gas diffusion electrode during cycling and its link to the cell failure. We also demonstrate that the removal of Li2CO3 by a redox mediator can partially recover the cell performance and extend the cycle life of a Li-O2 battery.

A lithium-air battery and gas handling system demonstrator

The lithium-air (Li-air) battery offers one of the highest practical specific energy densities of any battery system at >400 W h kgsystem-1. The practical cell is expected to operate in air, which is flowed into the positive porous electrode where it forms Li2O2 on discharge and is released as O2 on charge. The presence of CO2 and H2O in the gas stream leads to the formation of oxidatively robust side products, Li2CO3 and LiOH, respectively. Thus, a gas handling system is needed to control the flow and remove CO2 and H2O from the gas supply. Here we present the first example of an integrated Li-air battery with in-line gas handling, that allows control over the flow and composition of the gas supplied to a Li-air cell and simultaneous evaluation of the cell and scrubber performance. Our findings reveal that O2 flow can drastically impact the capacity of cells and confirm the need for redox mediators. However, we show that current air-electrode designs translated from fuel cell technology are not suitable for Li-air cells as they result in the need for higher gas flow rates than required theoretically. This puts the scrubber under a high load and increases the requirements for solvent saturation and recapture. Our results clarify the challenges that must be addressed to realise a practical Li-air system and will provide vital insight for future modelling and cell development.

Why charging Li-air batteries with current low-voltage mediators is slow and singlet oxygen does not explain degradation

Although Li-air rechargeable batteries offer higher energy densities than lithium-ion batteries, the insulating Li₂O₂ formed during discharge hinders rapid, efficient re-charging. Redox mediators are used to facilitate Li₂O₂ oxidation; however, fast kinetics at a low charging voltage are necessary for practical applications and are yet to be achieved. We investigate the mechanism of Li₂O₂ oxidation by redox mediators. The rate-limiting step is the outer-sphere one-electron oxidation of Li₂O₂ to LiO₂, which follows Marcus theory. The second step is dominated by LiO₂ disproportionation, forming mostly triplet-state O₂. The yield of singlet-state O₂ depends on the redox potential of the mediator in a way that does not correlate with electrolyte degradation, in contrast to earlier views. Our mechanistic understanding explains why current low-voltage mediators (<+3.3 V) fail to deliver high rates (the maximum rate is at +3.74 V) and suggests important mediator design strategies to deliver sufficiently high rates for fast charging at potentials closer to the thermodynamic potential of Li₂O₂ oxidation (+2.96 V).

Dendrite initiation and propagation in lithium metal solid-state batteries

All-solid-state batteries with a Li anode and ceramic electrolyte have the potential to deliver a step change in performance compared with today's Li-ion batteries^1,2. However, Li dendrites (filaments) form on charging at practical rates and penetrate the ceramic electrolyte, leading to short circuit and cell failure^3,4. Previous models of dendrite penetration have generally focused on a single process for dendrite initiation and propagation, with Li driving the crack at its tip^5-9. Here we show that initiation and propagation are separate processes. Initiation arises from Li deposition into subsurface pores, by means of microcracks that connect the pores to the surface. Once filled, further charging builds pressure in the pores owing to the slow extrusion of Li (viscoplastic flow) back to the surface, leading to cracking. By contrast, dendrite propagation occurs by wedge opening, with Li driving the dry crack from the rear, not the tip. Whereas initiation is determined by the local (microscopic) fracture strength at the grain boundaries, the pore size, pore population density and current density, propagation depends on the (macroscopic) fracture toughness of the ceramic, the length of the Li dendrite (filament) that partially occupies the dry crack, current density, stack pressure and the charge capacity accessed during each cycle. Lower stack pressures suppress propagation, markedly extending the number of cycles before short circuit in cells in which dendrites have initiated.

Buffering Volume Change in Solid-State Battery Composite Cathodes with CO2-Derived Block Polycarbonate Ethers

Polymers designed with a specific combination of electrochemical, mechanical, and chemical properties could help overcome challenges limiting practical all-solid-state batteries for high-performance next-generation energy storage devices. In composite cathodes, comprising active cathode material, inorganic solid electrolyte, and carbon, battery longevity is limited by active particle volume changes occurring on charge/discharge. To overcome this, impractical high pressures are applied to maintain interfacial contact. Herein, block polymers designed to address these issues combine ionic conductivity, electrochemical stability, and suitable elastomeric mechanical properties, including adhesion. The block polymers have "hard-soft-hard", ABA, block structures, where the soft "B" block is poly(ethylene oxide) (PEO), known to promote ionic conductivity, and the hard "A" block is a CO₂-derived polycarbonate, poly(4-vinyl cyclohexene oxide carbonate), which provides mechanical rigidity and enhances oxidative stability. ABA block polymers featuring controllable PEO and polycarbonate lengths are straightforwardly prepared using hydroxyl telechelic PEO as a macroinitiator for CO₂/epoxide ring-opening copolymerization and a well-controlled Mg(II)Co(II) catalyst. The influence of block polymer composition upon electrochemical and mechanical properties is investigated, with phosphonic acid functionalities being installed in the polycarbonate domains for adhesive properties. Three lead polymer materials are identified; these materials show an ambient ionic conductivity of 10 ^-4 S cm^-1, lithium-ion transport (t_Li+ 0.3-0.62), oxidative stability (>4 V vs Li^+/Li), and elastomeric or plastomer properties (G' 0.1-67 MPa). The best block polymers are used in composite cathodes with LiNi_0.8Mn_0.1Co_0.1O₂ active material and Li₆PS₅Cl solid electrolyte-the resulting solid-state batteries demonstrate greater capacity retention than equivalent cells featuring no polymer or commercial polyelectrolytes.

Achieving Ultrahigh-Rate Planar and Dendrite-Free Zinc Electroplating for Aqueous Zinc Battery Anodes

Despite being one of the most promising candidates for grid-level energy storage, practical aqueous zinc batteries are limited by dendrite formation, which leads to significantly compromised safety and cycling performance. In this study, by using single-crystal Zn-metal anodes, reversible electrodeposition of planar Zn with a high capacity of 8 mAh cm^-2 can be achieved at an unprecedentedly high current density of 200 mA cm^-2 . This dendrite-free electrode is well maintained even after prolonged cycling (>1200 cycles at 50 mA cm^{- 2} ). Such excellent electrochemical performance is due to single-crystal Zn suppressing the major sources of defect generation during electroplating and heavily favoring planar deposition morphologies. As so few defect sites form, including those that would normally be found along grain boundaries or to accommodate lattice mismatch, there is little opportunity for dendritic structures to nucleate, even under extreme plating rates. This scarcity of defects is in part due to perfect atomic-stitching between merging Zn islands, ensuring no defective shallow-angle grain boundaries are formed and thus removing a significant source of non-planar Zn nucleation. It is demonstrated that an ideal high-rate Zn anode should offer perfect lattice matching as this facilitates planar epitaxial Zn growth and minimizes the formation of any defective regions.

High critical currents for dendrite penetration and voiding in potassium metal anode solid-state batteries

Potassium metal anode solid-state cells with a K-beta”-alumina ceramic electrolyte are found to have relatively high critical currents for dendrite penetration on charge of approximately 4.8 mA/cm2, and voiding on discharge of approximately 2.0 mA/cm2, at 20 °C under 2.5 MPa stack-pressure. These values are higher than generally reported in the literature under comparable conditions for Li and Na metal anode solid-state batteries. The higher values for potassium are attributed to its lower yield strength and its readiness to creep under relatively low stack-pressures. The high critical currents of potassium anode solid-state batteries help to confirm the importance of the metal anode mechanical properties in the mechanisms of dendrite penetration and voiding.

Batteries for wearables

This perspective article highlights the recent advances and future challenges of battery technologies for wearables.

Visualizing plating-induced cracking in lithium-anode solid-electrolyte cells

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/Li₆PS₅Cl/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.

Covalency does not suppress O2 formation in 4d and 5d Li-rich O-redox cathodes

Layered Li-rich transition metal oxides undergo O-redox, involving the oxidation of the O²⁻ ions charge compensated by extraction of Li⁺ ions. Recent results have shown that for 3d transition metal oxides the oxidized O²⁻ forms molecular O₂ trapped in the bulk particles. Other forms of oxidised O²⁻ such as O₂²⁻ or (O–O)ⁿ⁻ with long bonds have been proposed, based especially on work on 4 and 5d transition metal oxides, where TM–O bonding is more covalent. Here, we show, using high resolution RIXS that molecular O₂ is formed in the bulk particles on O^2‒ oxidation in the archetypal Li-rich ruthenates and iridate compounds, Li₂RuO₃, Li₂Ru_0.5Sn_0.5O₃ and Li₂Ir_0.5Sn_0.5O₃. The results indicate that O-redox occurs across 3, 4, and 5d transition metal oxides, forming O₂, i.e. the greater covalency of the 4d and 5d compounds still favours O₂. RIXS and XAS data for Li₂IrO₃ are consistent with a charge compensation mechanism associated primarily with Ir redox up to and beyond the 5+ oxidation state, with no evidence of O–O dimerization.

In this work, authors show that O-redox in 4d and 5d transition metal oxides involves the formation of molecular oxygen trapped in the particles. These results are in accord with observations in 3d oxides and show that the greater covalency of the 4d and 5d oxides does not stabilise peroxo-like species.

Temperature Dependence of Lithium Anode Voiding in Argyrodite Solid-State Batteries

Void formation at the Li/ceramic electrolyte interface of an all-solid-state battery on discharge results in high local current densities, dendrites on charge, and cell failure. Here, we show that such voiding is reduced at the Li/Li₆PS₅Cl interface at elevated temperatures, sufficient to increase the critical current before voiding and cell failure from <0.25 mA cm^-2 at 25 °C to 0.25 mA cm^-2 at 60 °C and 0.5 mA cm^-2 at 80 °C under a relatively low stack-pressure of 1 MPa. Increasing the stack-pressure to 5 MPa and temperature to 80 °C permits stable cycling at 2.5 mA cm^-2. It is also shown that the charge-transfer resistance at the Li/Li₆PS₅Cl interface depends on pressure and temperature, with relatively high pressures required to maintain low charge-transfer resistance at -20 °C. These results are consistent with the plastic deformation of Li metal dominating the performance of the Li anode, posing challenges for the implementation of solid-state cells with Li anodes.

Non-equilibrium metal oxides via reconversion chemistry in lithium-ion batteries

Binary metal oxides are attractive anode materials for lithium-ion batteries. Despite sustained effort into nanomaterials synthesis and understanding the initial discharge mechanism, the fundamental chemistry underpinning the charge and subsequent cycles-thus the reversible capacity-remains poorly understood. Here, we use in operando X-ray pair distribution function analysis combining with our recently developed analytical approach employing Metropolis Monte Carlo simulations and non-negative matrix factorisation to study the charge reaction thermodynamics of a series of Fe- and Mn-oxides. As opposed to the commonly believed conversion chemistry forming rocksalt FeO and MnO, we reveal the two oxide series topotactically transform into non-native body-centred cubic FeO and zincblende MnO via displacement-like reactions whose kinetics are governed by the mobility differences between displaced species. These renewed mechanistic insights suggest avenues for the future design of metal oxide materials as well as new material synthesis routes using electrochemically-assisted methods.

Imaging Sodium Dendrite Growth in All-Solid-State Sodium Batteries Using 23 Na T2 -Weighted Magnetic Resonance Imaging

Two-dimensional, Knight-shifted, T2 -contrasted 23 Na magnetic resonance imaging (MRI) of an all-solid-state cell with a Na electrode and a ceramic electrolyte is employed to directly observe Na microstructural growth. A spalling dendritic morphology is observed and confirmed by more conventional post-mortem analysis; X-ray tomography and scanning electron microscopy. A significantly larger 23 Na T2 for the dendritic growth, compared with the bulk metal electrode, is attributed to increased sodium ion mobility in the dendrite. 23 Na T2 -contrast MRI of metallic sodium offers a clear, routine method for observing and isolating microstructural growths and can supplement the current suite of techniques utilised to analyse dendritic growth in all-solid-state cells.

Imaging Sodium Dendrite Growth in All-Solid-State Sodium Batteries Using 23Na T2-Weighted Magnetic Resonance Imaging

Two-dimensional, Knight-shifted, T 2-contrasted 23Na magnetic resonance imaging (MRI) of an all-solid-state cell with a Na electrode and a ceramic electrolyte is employed to directly observe Na microstructural growth. A spalling dendritic morphology is observed and confirmed by more conventional post-mortem analysis; X-ray tomography and scanning electron microscopy. A significantly larger 23Na T 2 for the dendritic growth, compared with the bulk metal electrode, is attributed to increased sodium ion mobility in the dendrite. 23Na T 2-contrast MRI of metallic sodium offers a clear, routine method for observing and isolating microstructural growths and can supplement the current suite of techniques utilised to analyse dendritic growth in all-solid-state cells.

Redox Chemistry and the Role of Trapped Molecular O2 in Li-Rich Disordered Rocksalt Oxyfluoride Cathodes

In the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyfluorides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. However, a deep understanding of these phenomena and their redox chemistry remains incomplete. Using the archetypal oxyfluoride, Li₂MnO₂F, we show that the oxygen redox process in such materials involves the formation of molecular O₂ trapped in the bulk structure of the charged cathode, which is reduced on discharge. The molecular O₂ is trapped rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O₂, making it more characteristic of a solid-like environment. The Mn redox process occurs between octahedral Mn³⁺ and Mn⁴⁺ with no evidence of tetrahedral Mn⁵⁺ or Mn⁷⁺. We furthermore derive the relationship between local coordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li-O-Li configurations. This study advances our fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes.

Rational Design and Mechanical Understanding of Three-Dimensional Macro-/Mesoporous Silicon Lithium-Ion Battery Anodes with a Tunable Pore Size and Wall Thickness

Silicon is regarded as one of the most promising next generation lithium-ion battery anodes due to its exceptional theoretical capacity, appropriate voltage profile, and vast abundance. Nevertheless, huge volume expansion and drastic stress generated upon lithiation cause poor cyclic stability. It has been one of the central issues to improve cyclic performance of silicon-based lithium-ion battery anodes. Constructing hierarchical macro-/mesoporous silicon with a tunable pore size and wall thickness is developed to tackle this issue. Rational structure design, controllable synthesis, and theoretical mechanical simulation are combined together to reveal fundamental mechanisms responsible for an improved cyclic performance. A self-templating strategy is applied using Stöber silica particles as a templating agent and precursor coupled with a magnesiothermic reduction process. Systematic variation of the magnesiothermic reduction time allows good control over the structures of the porous silicon. Finite element mechanical simulations on the porous silicon show that an increased pore size and a reduced wall thickness generate less mechanical stress in average along with an extended lithiation state. Besides the mechanical stress, the evolution of strain and displacement of the porous silicon is also elaborated with the finite element simulation.

Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future

The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO₂ emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O₂ cells but include Na-O₂, K-O₂, and Mg-O₂ cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O₂ reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O₂ cells.

Sodium/Na β″ Alumina Interface: Effect of Pressure on Voids

Three-electrode studies coupled with tomographic imaging of the Na/Na-β″-alumina interface reveal that voids form in the Na metal at the interface on stripping and they accumulate on cycling, leading to increasing interfacial current density, dendrite formation on plating, short circuit, and cell failure. The process occurs above a critical current for stripping (CCS) for a given stack pressure, which sets the upper limit on current density that avoids cell failure, in line with results for the Li/solid-electrolyte interface. The pressure required to avoid cell failure varies linearly with current density, indicating that Na creep rather than diffusion per se dominates Na transport to the interface and that significant pressures are required to prevent cell death, >9 MPa at 2.5 mA·cm^-2.

Quantifying oxygen distortions in lithium-rich transition-metal-oxide cathodes using ABF STEM

Lithium-rich cathodes can store excess charge beyond the transition metal redox capacity by participation of oxygen in reversible anionic redox reactions. Although these processes are crucial for achieving high energy densities, their structural origins are not yet fully understood. Here, we explore the use of annular bright-field (ABF) imaging in scanning transmission electron microscopy (STEM) to measure oxygen distortions in charged Li_1.2Ni_0.2Mn_0.6O₂. We show that ABF STEM data can provide positional accuracies below 20 pm but this is restricted to cases where no specimen mistilt is present, and only for a range of thicknesses above 3.5 nm. The reliability of these measurements is compromised even when the experimental and post-processing designs are optimised for accuracy and precision, indicating that extreme care must be taken when attempting to quantify distortions in these materials.

Critical stripping current leads to dendrite formation on plating in lithium anode solid electrolyte cells

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.

7Li NMR Chemical Shift Imaging To Detect Microstructural Growth of Lithium in All-Solid-State Batteries

All-solid-state batteries potentially offer safe, high-energy-density electrochemical energy storage, yet are plagued with issues surrounding Li microstructural growth and subsequent cell death. We use ⁷Li NMR chemical shift imaging and electron microscopy to track Li microstructural growth in the garnet-type solid electrolyte, Li_6.5La₃Zr_1.5Ta_0.5O₁₂. Here, we follow the early stages of Li microstructural growth during galvanostatic cycling, from the formation of Li on the electrode surface to dendritic Li connecting both electrodes in symmetrical cells, and correlate these changes with alterations observed in the voltage profiles during cycling and impedance measurements. During these experiments, we observe transformations at both the stripping and plating interfaces, indicating heterogeneities in both Li removal and deposition. At low current densities, ⁷Li magnetic resonance imaging detects the formation of Li microstructures in cells before short-circuits are observed and allows changes in the electrochemical profiles to be rationalized.

Stabilizing Lithium into Cross-Stacked Nanotube Sheets with an Ultra-High Specific Capacity for Lithium Oxygen Batteries

Although lithium-oxygen batteries possess a high theoretical energy density and are considered as promising candidates for next-generation power systems, the enhancement of safety and cycling efficiency of the lithium anodes while maintaining the high energy storage capability remains difficult. Here, we overcome this challenge by cross-stacking aligned carbon nanotubes into porous networks for ultrahigh-capacity lithium anodes to achieve high-performance lithium-oxygen batteries. The novel anode shows a reversible specific capacity of 3656 mAh g^-1 , approaching the theoretical capacity of 3861 mAh g^-1 of pure lithium. When this anode is employed in lithium-oxygen full batteries, the cycling stability is significantly enhanced, owing to the dendrite-free morphology and stabilized solid-electrolyte interface. This work presents a new pathway to high performance lithium-oxygen batteries towards practical applications by designing cross-stacked and aligned structures for one-dimensional conducting nanomaterials.

Low-Dose Aberration-Free Imaging of Li-Rich Cathode Materials at Various States of Charge Using Electron Ptychography

Imaging the complete atomic structure of materials, including light elements, with minimal beam-induced damage of the sample is a long-standing challenge in electron microscopy. Annular bright-field scanning transmission electron microscopy is often used to image elements with low atomic numbers, but due to its low efficiency and high sensitivity to precise imaging parameters it comes at the price of potentially significant beam damage. In this paper, we show that electron ptychography is a powerful technique to retrieve reconstructed phase images that provide the full structure of beam-sensitive materials containing light and heavy elements. Due to its much higher efficiency, we can reduce the beam currents used down to the subpicoampere range. Electron ptychography also allows residual lens aberrations to be corrected at the postprocessing stage, which avoids the need for fine-tuning of the probe that would result in further beam damage and provides aberration-free reconstructed phase images. We have used electron ptychography to obtain structural information from aberration-free reconstructed phase images in the technologically relevant lithium-rich transition metal oxides at different states of charge. We can unambiguously determine the position of the lithium and oxygen atomic columns while amorphization of the surface, formation of beam-induced surface reconstruction layers, or migration of transition metals to the alkali layers are drastically reduced.

Degradation Mechanisms at the Li10GeP2S12/LiCoO2 Cathode Interface in an All-Solid-State Lithium-Ion Battery

All-solid-state batteries (ASSBs) show great potential for providing high power and energy densities with enhanced battery safety. While new solid electrolytes (SEs) have been developed with high enough ionic conductivities, SSBs with long operational life are still rarely reported. Therefore, on the way to high-performance and long-life ASSBs, a better understanding of the complex degradation mechanisms, occurring at the electrode/electrolyte interfaces is pivotal. While the lithium metal/solid electrolyte interface is receiving considerable attention due to the quest for high energy density, the interface between the active material and solid electrolyte particles within the composite cathode is arguably the most difficult to solve and study. In this work, multiple characterization methods are combined to better understand the processes that occur at the LiCoO₂ cathode and the Li₁₀GeP₂S₁₂ solid electrolyte interface. Indium and Li₄Ti₅O₁₂ are used as anode materials to avoid the instability problems associated with Li-metal anodes. Capacity fading and increased impedances are observed during long-term cycling. Postmortem analysis with scanning transmission electron microscopy, electron energy loss spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy show that electrochemically driven mechanical failure and degradation at the cathode/solid electrolyte interface contribute to the increase in internal resistance and the resulting capacity fading. These results suggest that the development of electrochemically more stable SEs and the engineering of cathode/SE interfaces are crucial for achieving reliable SSB performance.

Operando Monitoring of the Solution-Mediated Discharge and Charge Processes in a Na-O2 Battery Using Liquid-Electrochemical Transmission Electron Microscopy

Although in sodium-oxygen (Na-O₂) batteries show promise as high-energy storage systems, this technology is still the subject of intense fundamental research, owing to the complex reaction by which it operates. To understand the formation mechanism of the discharge product, sodium superoxide (NaO₂), advanced experimental tools must be developed. Here we present for the first time the use of a Na-O₂ microbattery using a liquid aprotic electrolyte coupled with fast imaging transmission electron microscopy to visualize, in real time, the mechanism of NaO₂ nucleation/growth. We observe that the formation of NaO₂ cubes during reduction occurs by a solution-mediated nucleation process. Furthermore, we unambiguously demonstrate that the subsequent oxidation of NaO₂ of which little is known also proceeds via a solution mechanism. We also provide insight into the cell electrochemistry via the visualization of an outer shell of parasitic reaction product, formed through chemical reaction at the interface between the growing NaO₂ cubes and the electrolyte, and suggest that this process is responsible for the poor cyclability of Na-O₂ batteries. The assessment of the discharge-charge mechanistic in Na-O₂ batteries through operando electrochemical transmission electron microscopy visualization should facilitate the development of this battery technology.

Plating and stripping calcium in an organic electrolyte

There is considerable interest in multivalent cation batteries, such as those based on magnesium, calcium or aluminium. Most attention has focused on magnesium. In all cases the metal anode represents a significant challenge. Recent work has shown that calcium can be plated and stripped, but only at elevated temperatures, 75 to 100 °C, with small capacities, typically 0.165 mAh cm^-2, and accompanied by significant side reactions. Here we demonstrate that calcium can be plated and stripped at room temperature with capacities of 1 mAh cm^-2 at a rate of 1 mA cm^-2, with low polarization (∼100 mV) and in excess of 50 cycles. The dominant product is calcium, accompanied by a small amount of CaH₂ that forms by reaction between the deposited calcium and the electrolyte, Ca(BH₄)₂ in tetrahydrofuran (THF). This occurs in preference to the reactions which take place in most electrolyte solutions forming CaCO₃, Ca(OH)₂ and calcium alkoxides, and normally terminate the electrochemistry. The CaH₂ protects the calcium metal at open circuit. Although this work does not solve all the problems of calcium as an anode in calcium-ion batteries, it does demonstrate that significant quantities of calcium can be plated and stripped at room temperature with low polarization.

Phenol-Catalyzed Discharge in the Aprotic Lithium-Oxygen Battery

Discharge in the lithium-O2 battery is known to occur either by a solution mechanism, which enables high capacity and rates, or a surface mechanism, which passivates the electrode surface and limits performance. The development of strategies to promote solution-phase discharge in stable electrolyte solutions is a central challenge for development of the lithium-O2 battery. Here we show that the introduction of the protic additive phenol to ethers can promote a solution-phase discharge mechanism. Phenol acts as a phase-transfer catalyst, dissolving the product Li2 O2 , avoiding electrode passivation and forming large particles of Li2 O2 product-vital requirements for high performance. As a result, we demonstrate capacities of over 9 mAh cm-2areal , which is a 35-fold increase in capacity compared to without phenol. We show that the critical requirement is the strength of the conjugate base such that an equilibrium exists between protonation of the base and protonation of Li2 O2 .

LiO2: Cryosynthesis and Chemical/Electrochemical Reactivities

The reduction of O₂ to solid Li₂O₂, via the intermediates O₂^- and LiO₂, is the desired discharge reaction at the positive electrode of the aprotic Li-O₂ batteries. In practice, a plethora of byproducts are identified together with Li₂O₂ and have been assigned to the side reactions between the reduced oxygen species (O₂^-, LiO₂, and Li₂O₂) and the battery components (the cathode and electrolyte). Understanding the reactivity of these reduced oxygen species is critical for the development of stable battery components and thus high cycle life. O₂^- and Li₂O₂ are readily available, and their reactivities have been studied in depth both experimentally and theoretically. However, little is known about LiO₂, which readily decomposes to Li₂O₂ and is thus unavailable under usual laboratory conditions. Here we report the synthesis and reactivity of LiO₂ in liquid NH₃ at cryogenic temperatures and conclude that LiO₂ is the most reactive oxygen species in Li-O₂ batteries.

One-Pot Synthesis of Lithium-Rich Cathode Material with Hierarchical Morphology

Lithium-rich transition metal oxides, Li_1+xTM_1-xO₂ (TM, transition metal), have attracted much attention as potential candidate cathode materials for next generation lithium ion batteries because their high theoretical capacity. Here we present the synthesis of Li[Li_0.2Ni_0.2Mn_0.6]O₂ using a facile one-pot resorcinol-formaldehyde method. Structural characterization indicates that the material adopts a hierarchical porous morphology consisting of uniformly distributed small pores and disordered large pore structures. The material exhibits excellent electrochemical cycling stability and a good retention of capacity at high rates. The material has been shown to be both advantageous in terms of gravimetric and volumetric capacities over state of the art commercial cathode materials.

Promoting solution phase discharge in Li-O2 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.