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Solid-state electrolytes with high room-temperature ionic conductivity and fast interfacial charge transport are a requirement for practical solid-state batteries. Here, we report that cationic aluminium species initiate ring-opening polymerization of molecular ethers inside an electrochemical cell to produce solid-state polymer electrolytes (SPEs), which retain conformal interfacial contact with all cell components. SPEs exhibit high ionic conductivity at room temperature (>1 mS cm −1 ), low interfacial resistances, uniform lithium deposition and high Li plating/striping efficiencies (>98% after 300 charge–discharge cycles). Applications of SPEs in Li–S, Li–LiFePO 4 and Li–LiNi 0.6 Mn 0.2 Co 0.2 O 2 batteries further demonstrate that high Coulombic efficiency (>99%) and long life (>700 cycles) can be achieved with an in situ SPE design. Our study therefore provides a promising direction for creating solid electrolytes that meet both the bulk and interfacial conductivity requirements for practical solid polymer batteries. High-performance polymer electrolytes are highly sought after in the development of solid-state batteries. Lynden Archer and co-workers report an in situ polymerization of liquid electrolytes in a lithium battery for creating promising polymer electrolytes with high ionic conductivity and low interfacial resistance.

Solid-state electrolytes (SSEs) have emerged as high-priority materials for safe, energy-dense and reversible storage of electrochemical energy in batteries. In this Review, we assess recent progress in the design, synthesis and analysis of SSEs, and identify key failure modes, performance limitations and design concepts for creating SSEs to meet requirements for practical applications. We provide an overview of the development and characteristics of SSEs, followed by analysis of ion transport in the bulk and at interfaces based on different single-valent (Li + , Na + , K + ) and multivalent (Mg 2+ , Zn 2+ , Ca 2+ , Al 3+ ) cation carriers of contemporary interest. We analyse the progress in overcoming issues associated with the poor ionic conductivity and high interfacial resistance of inorganic SSEs and the poor oxidative stability and cation transference numbers of polymer SSEs. Perspectives are provided on the design requirements for future generations of SSEs, with a focus on the chemical, geometric, mechanical, electrochemical and interfacial transport features required to accelerate progress towards practical solid-state batteries in which metals are paired with energetic cathode chemistries, including Ni-rich and Li-rich intercalating materials, sustainable organic materials, S 8 , O 2 and CO 2 . Solid-state batteries based on electrolytes with low or zero vapour pressure provide a promising path towards safe, energy-dense storage of electrical energy. In this Review, we consider the requirements and design rules for solid-state electrolytes based on inorganics, organic polymers and organic–inorganic hybrids.

Electrochemical cells based on alkali metal anodes are receiving intensive scientific interest as potentially transformative technology platforms for electrical energy storage. Chemical, morphological, mechanical and hydrodynamic instabilities at the metal anode produce uneven metal electrodeposition and poor anode reversibility, which, are among the many known challenges that limit progress. Here, we report that solid-state electrolytes based on crosslinked polymer networks can address all of these challenges in cells based on lithium metal anodes. By means of transport and electrochemical analyses, we show that manipulating thermodynamic interactions between polymer segments covalently anchored in the network and “free” segments belonging to an oligomeric electrolyte hosted in the network pores, one can facilely create hybrid electrolytes that simultaneously exhibit liquid-like barriers to ion transport and solid-like resistance to morphological and hydrodynamic instability. To address some critical issues facing Li metal batteries, the authors design cross-linked polymer networks to serve as either Li metal anode coatings or all solid-state electrolytes. Their favorable polymer chemistry is found responsible for the impressive performance of Li||NCM full cells.

Electrochemical cells that utilize lithium and sodium anodes are under active study for their potential to enable high-energy batteries. Liquid and solid polymer electrolytes based on ether chemistry are among the most promising choices for rechargeable lithium and sodium batteries. However, uncontrolled anionic polymerization of these electrolytes at low anode potentials and oxidative degradation at working potentials of the most interesting cathode chemistries have led to a quite concession in the field that solid-state or flexible batteries based on polymer electrolytes can only be achieved in cells based on low- or moderate-voltage cathodes. Here, we show that cationic chain transfer agents can prevent degradation of ether electrolytes by arresting uncontrolled polymer growth at the anode. We also report that cathode electrolyte interphases composed of preformed anionic polymers and supramolecules provide a fundamental strategy for extending the high voltage stability of ether-based electrolytes to potentials well above conventionally accepted limits. Here the authors use cationic chain transfer agents to prevent degradation of ether electrolytes by arresting uncontrolled polymer growth at the anode. This work provides a fundamental strategy for extending the high voltage stability of these electrolytes to potentials above conventionally accepted limits.

The physiochemical nature of reactive metal electrodeposits during the early stages of electrodeposition is rarely studied but known to play an important role in determining the electrochemical stability and reversibility of electrochemical cells that utilize reactive metals as anodes. We investigated the early-stage growth dynamics and reversibility of electrodeposited lithium in liquid electrolytes infused with brominated additives. On the basis of equilibrium theories, we hypothesize that by regulating the surface energetics and surface ion/adatom transport characteristics of the interphases formed on Li, Br-rich electrolytes alter the morphology of early-stage Li electrodeposits; enabling late-stage control of growth and high electrode reversibility. A combination of scanning electron microscopy (SEM), image analysis, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and contact angle goniometry are employed to evaluate this hypothesis by examining the physical–chemical features of the material phases formed on Li. We report that it is possible to achieve fine control of the early-stage Li electrodeposit morphology through tuning of surface energetic and ion diffusion properties of interphases formed on Li. This control is shown further to translate to better control of Li electrodeposit morphology and high electrochemical reversibility during deep cycling of the Li metal anode. Our results show that understanding and eliminating morphological and chemical instabilities in the initial stages of Li electroplating via deliberately modifying energetics of the solid electrolyte interphase (SEI) is a feasible approach in realization of deeply cyclable reactive metal batteries.

The rapid rise of electric drive vehicles has accelerated research aimed at developing energy storage technologies with high gravimetric and volumetric energy densities. Lithium metal batteries (LMBs) are considered particularly important in this aspect but are not available today primarily because the lithium metal anode poses multiple challenges. Among them, the most difficult include the metal’s propensity to form an undesirable and dynamic corrosion layer known as the solid electrolyte interphase (SEI) that results in rough, non-planar electrodeposits at all current densities. These challenges arise from coupling of chemical reactivity of Li, highly reducing potentials during battery charge, and the out-of-equilibrium transport phenomena that drive morphological instabilities at the metal/electrolyte interface. In this thesis, crosslinked polymers are used as a powerful platform to develop design principles for electrolytes and electrode/electrolyte interphases that enable planar deposition in lithium metal anodes. The design principles are based on guidelines from a theoretical linear stability analysis of metal electrodeposition that captures chemical effects in the transport coefficients and their spatial variations at the electrolyte-electrode interphase. The aim is then to probe physical factors responsible for the nucleation and growth of morphologically unstable electrodeposits. During dendrite growth, it is revealed that the growing deposit front experiences a significant amount of compressive stress exerted by the bulk electrolyte, which if large enough can potentially slow down the growth rate. Development of structured electrolytes capable of increasing this compressive stress, while not yielding under compressive strain is proposed and demonstrated as an effective strategy to suppress dendrite growth. To address dendrite nucleation, artificial interphases and similar electrode engineering techniques are proposed as a potential drop-in solution. Careful design of the solid electrolyte interphase and tuning of its chemistry and physical properties are found to be crucial for driving stable nucleation of lithium electrodeposits. In particular, strategies to synthesize and fabricate electrochemically stable artificial interphases with precise thickness control are shown to be essential for achieving uniform ion transport, for enhancing surface tension forces, and for reducing the equilibrium reduction rate at the metal surface. Importantly, these methods enable planar lithium electrodeposition in both nucleation and growth stages.

Electrochemical cells based on alkali metal anodes are receiving intensive scientific interest as potentially transformative technology platforms for electrical energy storage. Chemical, morphological, mechanical and hydrodynamic instabilities at the metal anode produce uneven metal electrodeposition and poor anode reversibility, which, are among the many known challenges that limit progress. Here, we report that solid-state electrolytes based on crosslinked polymer networks can address all of these challenges in cells based on lithium metal anodes. By means of transport and electrochemical analyses, we show that manipulating thermodynamic interactions between polymer segments covalently anchored in the network and “free” segments belonging to an oligomeric electrolyte hosted in the network pores, one can facilely create hybrid electrolytes that simultaneously exhibit liquid-like barriers to ion transport and solid-like resistance to morphological and hydrodynamic instability.

Significance Spatial variations in chemical composition and transport properties of the material phases (interphases) formed on reactive metals in liquid electrolytes are thought to be responsible for the propensity of metal battery electrodes to electrodeposit in irregular, nonplanar morphologies. Equilibrium theoretical calculations using joint density functional analysis in vacuum and generic liquid media indicate that in-plane transport at such interphases is enhanced substantially if LiX (X = Br > Cl > F) species predominate. This study employs optical visualization experiments and nucleation theory to experimentally investigate nucleation and early-stage growth dynamics of metallic lithium in electrolytes enriched with LiBr. It is shown that the Li-Br–rich interphases formed profoundly alter the morphology of Li electrodeposits by enhancing Li-ion surface diffusion.