Solid composite electrolytes (SCEs) that combine the advantages of solid polymer electrolytes (SPEs) and inorganic ceramic electrolytes (ICEs) present acceptable ionic conductivity, high mechanical strength, and favorable interfacial contact with electrodes, which greatly improve the electrochemical performance of all-solid-state batteries compared to single SPEs and ICEs. However, there are many challenges to overcome before the practical application of SCEs, including the low ionic conductivity less than 10-3 S cm-1 at ambient temperature, poor interfacial stability, and high interfacial resistance, which greatly restrict the room temperature performance. Herein, the advances of SCEs applied in all-solid-state lithium batteries are presented, including the Li ion migration mechanism of SCEs, the strategies to enhance the ionic conductivity of SCEs by various morphologies of ICEs, and construction methods of the low resistance and stable interfaces of SCEs with both cathode and anode. Finally, some typical applications of SCEs in lithium batteries are summarized and future development directions are prospected. This work presents how it is quite significant to further enhance the ionic conductivity of SCEs by developing the novel SPEs with the special morphology of ICEs for advanced all-solid-state lithium batteries.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201903088

Progress and Perspective of Ceramic/Polymer Composite Solid Electrolytes for Lithium Batteries

Towards better Li metal anodes: Challenges and strategies

Voltage fade is a major problem in battery applications for high-energy lithium- and manganese-rich (LMR) layered materials. As a result of the complexity of the LMR structure, the voltage fade mechanism is not well understood. Here we conduct both in situ and ex situ studies on a typical LMR material (Li1.2Ni0.15Co0.1Mn0.55O2) during charge–discharge cycling, using multi-length-scale X-ray spectroscopic and three-dimensional electron microscopic imaging techniques. Through probing from the surface to the bulk, and from individual to whole ensembles of particles, we show that the average valence state of each type of transition metal cation is continuously reduced, which is attributed to oxygen release from the LMR material. Such reductions activate the lower-voltage Mn3+/Mn4+ and Co2+/Co3+ redox couples in addition to the original redox couples including Ni2+/Ni3+, Ni3+/Ni4+ and O2−/O−, directly leading to the voltage fade. We also show that the oxygen release causes microstructural defects such as the formation of large pores within particles, which also contributes to the voltage fade. Surface coating and modification methods are suggested to be effective in suppressing the voltage fade through reducing the oxygen release.Voltage decay is a major problem in applications of high-energy Li- and Mn-rich layer-structured battery materials. Here, the authors report the evolution of redox couples as the origin of the voltage decay and discuss strategies to suppress the problem.

(Invited) Evolution of Redox Couples in Li- and Mn-Rich Cathode Materials and Mitigation of Voltage Fade by Reducing Oxygen Release

A review of composite polymer-ceramic electrolytes for lithium batteries

DOI: 10.1002/aenm.201903645 Li metal anodes (such as Li–sulfur batteries: 2600 Wh kg−1 and Li–O2 batteries: 3505 Wh kg−1) have demonstrated a huge improvement on energy densities in comparison with commercial LIBs.[3] However, the capacity loss, short life-span, and safety hazards caused by Li dendrite growth and unstable solid electrolyte interphase (SEI) layer in Li metal anodes have yet to be thoroughly solved after nearly four decades of research.[4] Tremendous efforts have been devoted to reveal the dendrite growth and SEI layer transformation mechanisms.[5] SEI plays the most essential role in the surface morphology evolution of working Li metal anodes during repeated electrodeposition due to its passivating feature.[6] Because of the high reactivity of Li metal, SEI primarily forms once Li is in contact with electrolytes, and grows continuously in subsequent electrochemical cycling. A stable SEI layer without rupture and regeneration is necessary for regulating the ionic flux and protecting Li from electrolytes, further achieving a uniform and stable electrodeposition. Previously, the regulation of SEI layer mainly focused on its physical and chemical properties, including structural uniformity, chemical homogenousness, mechanical strength, electric/ionic conductivity, etc. Such regulation in experimental methods is mainly achieved by electrolyte additives and/or artificial SEI layer designs, such as fluorine/nitrogen/sulfur-containing electrolyte additives (e.g., fluoroethylene carbonate, LiNO3, and lithium polysulfides).[7] Furthermore, X-ray photoelectron spectroscopy, atomic force microscope (AFM), and cryo-electron microscopy can visually reveal the component and structure evolutions of the particularly designed SEI.[8] Besides, a prolonged lifespan and enhanced Coulombic efficiency demonstrates the effectiveness of these experimental strategies.[9] Thanks to the development of interfacial chemistry, material technology, and upscale characterization, great progresses have been achieved on the modification of SEI.[10] However, a fundamental understanding of how SEI regulates Li dendrite growth and the key parameter of designing effective SEI is still not clear. If such an understanding can be achieved, a rational design of SEI can be used to effectively stabilize electrolyte–anode interface and to resist Li dendrite growth in practical applications of LIBs. The structural uniformity and mechanical strength of SEI can significantly and directly affect Li-ion transportation and Practical application of lithium (Li) metal anodes has been hindered by Li dendrite growth, which renders a low Coulombic efficiency and short lifespan of working Li metal batteries. A stable solid electrolyte interphase (SEI) is crucial in suppressing the formation of Li dendrites. Herein the local stress and deformation evolvement status of a SEI layer during Li electrodeposition are investigated through a quantitative electrochemical–mechanical model based on a finite element method. Furthermore, the impacts of structural uniformity and mechanical strength on the stability of the SEI under different working conditions are investigated. Improving the structural uniformity of SEI is the most effective way to enhance the stability of SEI, which regulates ion transportation. In addition, pursuing extremely high mechanical strength is shown to be pointless, and a moderate elastic modulus of 3.0 GPa is suggested. This work affords an insight into the rational design of stable SEI layers and sheds light on a possible pathway toward practical applications of Li metal anodes.

The Failure of Solid Electrolyte Interphase on Li Metal Anode: Structural Uniformity or Mechanical Strength?

Safety issues in lithium-ion batteries (LIBs) have aroused great interest owing to their wide applications, from miniaturized devices to large-scale storage plants. Separators are a vital component to ensure the safety of LIBs; they prevent direct electric contact between the cathode and anode while allowing ion transport. In this study, the first design is reported for a thermoregulating separator that responds to heat stimuli. The separator with a phase-change material (PCM) of paraffin wax encapsulated in hollow polyacrylonitrile nanofibers renders a wide range of enthalpy (0-135.3 J g-1 ), capable of alleviating the internal temperature rise of LIBs in a timely manner. Under abuse conditions, the generated heat in batteries stimulates the melting of the encapsulated PCM, which absorbs large amounts of heat without creating a significant rise in temperature. Experimental simulation of the inner short-circuit in prototype pouch cells through nail penetration demonstrates that the PCM-based separator can effectively suppress the temperature rise due to cell failure. Meanwhile, a cell penetrated by a nail promptly cools down to room temperature within 35 s, benefiting from the latent heat-storage of the unique PCM separator. The present design of separators featuring latent heat-storage provides effective strategies for overheat protection and enhanced safety of LIBs.

Thermoregulating Separators Based on Phase-Change Materials for Safe Lithium-Ion Batteries.

Highly Tough, Li-Metal Compatible Organic–Inorganic Double-Network Solvate Ionogel

Safer Lithium-Ion Batteries from the Separator Aspect: Development and Future Perspectives

Lithium-ion batteries are nowadays one of the ultimate energy-storage devices with high power and energy densities. Their thermal-abuse tolerance is needed urgently to meet the safety requirements for hybrid and electric vehicles, especially at elevated temperatures. Separators, while typically regarded as an inactive component, play a crucial role in battery safety and rate capability. Nowadays, in order to meet the safety demands of high-energy-density batteries, thermotolerant separators have become increasingly important for battery design and overall performance. In this review, the strategies and the recent advances in separators for high-safety batteries as well as their uses under extreme conditions are summarized. Several effective routes to thermotolerant separators for lithium-ion batteries are discussed, which include modified polyolefin separators, separators made of thermally stable materials, integration of electrolytes and separators, and smart separators as well. The review provides insights into the design principles of separators to enhance battery performance, and highlights the challenges of emerging energy-storage devices working under extreme conditions of high temperatures.

Le Yu

Papers

Thermoregulating Separators Based on Phase-Change Materials for Safe Lithium-Ion Batteries.

Highly Tough, Li-Metal Compatible Organic–Inorganic Double-Network Solvate Ionogel

Safer Lithium-Ion Batteries from the Separator Aspect: Development and Future Perspectives

Thermotolerant separators for safe lithium-ion batteries under extreme conditions

Morphosynthesis of 3D Macroporous Garnet Frameworks and Perfusion of Polymer-Stabilized Lithium Salts for Flexible Solid-State Hybrid Electrolytes