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 batteries1,2. However, Li dendrites (filaments) form on charging at practical rates and penetrate the ceramic electrolyte, leading to short circuit and cell failure3,4. Previous models of dendrite penetration have generally focused on a single process for dendrite initiation and propagation, with Li driving the crack at its tip5-9. 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.

Electron Localization in Rationally Designed Pt1Pd Single-Atom Alloy Catalyst Enables High-Performance Li-O2 Batteries.

Li-O2 batteries (LOBs) are considered as one of the most promising energy storage devices due to their ultrahigh theoretical energy density, yet they face the critical issues of sluggish cathode redox kinetics during the discharge and charge processes. Here we report a direct synthetic strategy to fabricate a single-atom alloy catalyst in which single-atom Pt is precisely dispersed in ultrathin Pd hexagonal nanoplates (Pt1Pd). The LOB with the Pt1Pd cathode demonstrates an ultralow overpotential of 0.69 V at 0.5 A g-1 and negligible activity loss over 600 h. Density functional theory calculations show that Pt1Pd can promote the activation of the O2/Li2O2 redox couple due to the electron localization caused by the single Pt atom, thereby lowering the energy barriers for the oxygen reduction and oxygen evolution reactions. Our strategy for designing single-atom alloy cathodic catalysts can address the sluggish oxygen redox kinetics in LOBs and other energy storage/conversion devices.

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.

High Power- and Energy-Density Supercapacitors through the Chlorine Respiration Mechanism.

Supercapacitor represents an important electrical energy storage technology with high-power performance and superior cyclability. However, currently commercialized supercapacitors still suffer limited energy densities. Here we report an unprecedentedly respiring supercapacitor with chlorine gas iteratively re-inspires in porous carbon materials, that improves the energy density by orders of magnitude. Both electrochemical results and theoretical calculations show that porous carbon with pore size around 3 nm delivers the best chlorine evolution and adsorption performance. The respiring supercapacitor with multi-wall carbon nanotube as the cathode and NaTi2 (PO4 )3 as the anode can store specific energy of 33 Wh kg-1 with negligible capacity loss over 30 000 cycles. The energy density can be further improved to 53 Wh kg-1 by replacing NaTi2 (PO4 )3 with zinc anode. Furthermore, thanks to the extraordinary reaction kinetics of chlorine gas, this respiring supercapacitor performs an extremely high-power density of 50 000 W kg-1 .

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 (tLi+ 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 LiNi0.8Mn0.1Co0.1O2 active material and Li6PS5Cl solid electrolyte-the resulting solid-state batteries demonstrate greater capacity retention than equivalent cells featuring no polymer or commercial polyelectrolytes.

Boosting Polysulfide Catalytic Conversion and Facilitating Li+ Transportation by Ion-Selective COFs Composite Nanowire for LiS Batteries.

The large-scale application of lithium-sulfur batteries (LSBs) has been impeded by the shuttle effect of lithium-polysulfides (LiPSs) and sluggish redox kinetics since which lead to irreversible capacity decay and low sulfur utilization. Herein, a hierarchical interlayer constructed by boroxine covalent organic frameworks (COFs) with high Li+ conductivity is fabricated via an in situ polymerization method on carbon nanotubes (CNTs) (C@COF). The as-prepared interlayer delivers a high Li+ ionic conductivity (1.85 mS cm-1 ) and Li+ transference number (0.78), which not only acts as a physical barrier, but also a bidirectional catalyst for LiPSs redox process owing to the abundant heterointerfaces between the inner conductive CNTs and the outer COFs. After coupling such a catalytic interlayer with sulfur cathode, the LSBs exhibit a low decay rate of 0.07% per cycle over 500 cycles at 1 C, and long cycle life at 3 C (over 1000 cycles). More importantly, a remarkable areal capacity of around 4.69 mAh cm-2 can still be maintained after 50 cycles even under a high sulfur loading condition (6.8 mg cm-2 ). This work paves a new way for the design of the interlayer with bidirectional catalytic behavior in LSBs.

Elevating Energy Density for Sodium-Ion Batteries through Multielectron Reactions.

It remains a great challenge to explore desirable cathodes for sodium-ion batteries to satisfy the ever-increasing demand for large-scale energy storage systems. In this Letter, we report a NASICON-structured Na4MnCr(PO4)3 cathode with high specific capacity and operation potential. The reversible access of the Mn2+/Mn3+ (3.75/3.4 V), Mn3+/Mn4+ (4.25/4.1 V), and Cr3+/Cr4+ (4.4/4.3 V vs Na/Na+) redox couples in a Na4MnCr(PO4)3 cathode endows a distinct three-electron redox reaction during the insertion/extraction process. The highly stable NASICON structure with a small volume variation upon cycling ensures long-time cycling stability (73.3% capacity retention after 500 cycles within the potential region of 2.5-4.6 V). The impedance analysis and interface characterization indicate that the evolution of a cathode electrolyte interphase at high potential is correlated with the capacity fading, while the robustness of the NASICON framework is redemonstrated.

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.

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.

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 .

Determinants of the Surface Film during the Discharging Process in Lithium-Oxygen Batteries.

Lithium-oxygen batteries have one of the highest theoretical capacities and specific energies, but several challenges remain. One of them is premature death caused by a passivation layer with poor conductivities (both electronic and ionic) on the electrode surface during the discharge process. Once this thin layer forms on the surface of the catalyst and substrate, the overpotential significantly increases and causes early cell death. Therefore, understanding this thin layer is crucial to achieving high specific energy lithium-oxygen batteries. Herein, we quantitatively compared the ratio of lithium carbonate to lithium peroxide during the discharge process in a flow cell at different potentials. We found that the ratio rapidly increased at low potential and high flow rates. The surface route led to significant byproducts on the Au electrodes, and consequently, a 3 nm thick discharge product film passivates the electrode surface in a flow cell.

Topological Engineering Electrodes with Ultrafast Oxygen Transport for Super-Power Sodium-Oxygen Batteries.

Sodium-oxygen battery has attracted tremendous interest due to its extraordinary theoretical specific energy (1605 Wh kg-1 NaO2) and appealing element abundance. However, definite mechanistic factors governing efficient oxygen diffusion and consumption inside electrolyte-flooded air cathodes remain elusive thus precluding a true gas diffusion electrode capable of high discharge current (i.e., several mA cm-2) and superior output power. Herein, 3D-printing technology is adopted to create gas channels with tailored channel size and structure to demystify the diffusion-limited oxygen delivery process. It is revealed that as the clogging discharging products increase, large channel size, and interconnected channel structure are essential to guaranteeing fast O2 diffusion. Moreover, to further encourage O2 diffusion, a bio-inspired breathable cathode with progressively branching channels that balances between O2 passage and reaction is 3D printed. This elaborated 3D electrode allows a sodium-oxygen cell to deliver an impressive discharging current density of up to 4 mA cm-2 and an output power of 8.4 mW cm-2, giving rise to an outstanding capacity of 18.4 mAh cm-2. The unraveled mystery of oxygen delivery enabled by 3D printing points to a valuable roadmap for the rational design of metal-air batteries toward practical applications.

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.

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.

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).