Side Reactions/Changes in Lithium-Ion Batteries: Mechanisms and Strategies for Creating Safer and Better Batteries.

Lithium-ion batteries (LIBs), in which lithium ions function as charge carriers, are considered the most competitive energy storage devices due to their high energy and power density. However, battery materials, especially with high capacity undergo side reactions and changes that result in capacity decay and safety issues. A deep understanding of the reactions that cause changes in the battery's internal components and the mechanisms of those reactions is needed to build safer and better batteries. This review focuses on the processes of battery failures, with voltage and temperature as the underlying factors. Voltage-induced failures result from anode interfacial reactions, current collector corrosion, cathode interfacial reactions, overcharge, and over-discharge, while temperature-induced failure mechanisms include SEI decomposition, separator damage, and interfacial reactions between electrodes and electrolytes. The review also presents protective strategies for controlling these reactions. As a result, the reader is offered a comprehensive overview of the safety features and failure mechanisms of various LIB components.

Upgrading the Separators Integrated with Desolvation and Selective Deposition toward the Stable Lithium Metal Batteries.

A practical and effective approach to improve the cycle stability of high-energy density lithium metal batteries (LMBs) is to selectively regulate the growth of the lithium anode. The design of desolvation and lithiophilic structure have proved to be significant means to regulate the lithium deposition process. Here, a fluorinated polymer lithiophilic separator (LS) loaded with a metal-organic framework (MOF801) is designed, which facilitates the rapid transfer of Li+ within the separator owing to the MOF801-anchored PF6 - from the electrolyte, Li deposition is confined in the plane resulting from the polymer fiber layer rich in lithiophilic groups (C─F). The numerical simulation results confirm that LS induces a uniform electric field and Li+ concentration distribution. Visualization technology records the behavior of regular Li deposition in Li||Li and Li||Cu cells equipping LS. Therefore, LS exhibits an ultrahigh Li+ transference number (tLi + = 0.80) and a large exchange current density (j0 = 1.963 mA cm-2). LS guarantees the stable operation of Li||Li cells for over 1000 h. In addition, the LiNi0.8Co0.1Mn0.1O2||Li cell equipped with LS exhibits superior rate and cycle performances owing to the formation of LiF-rich robust SEI layers. This study provides a way forward for dendrite-free Li anodes from the perspective of separator engineering.

Breaking solvation dominance of ethylene carbonate via molecular charge engineering enables lower temperature battery.

Low temperatures severely impair the performance of lithium-ion batteries, which demand powerful electrolytes with wide liquidity ranges, facilitated ion diffusion, and lower desolvation energy. The keys lie in establishing mild interactions between Li+ and solvent molecules internally, which are hard to achieve in commercial ethylene-carbonate based electrolytes. Herein, we tailor the solvation structure with low-ε solvent-dominated coordination, and unlock ethylene-carbonate via electronegativity regulation of carbonyl oxygen. The modified electrolyte exhibits high ion conductivity (1.46 mS·cm-1) at -90 °C, and remains liquid at -110 °C. Consequently, 4.5 V graphite-based pouch cells achieve ~98% capacity over 200 cycles at -10 °C without lithium dendrite. These cells also retain ~60% of their room-temperature discharge capacity at -70 °C, and miraculously retain discharge functionality even at ~-100 °C after being fully charged at 25 °C. This strategy of disrupting solvation dominance of ethylene-carbonate through molecular charge engineering, opens new avenues for advanced electrolyte design.

Insights into the solvation chemistry in liquid electrolytes for lithium-based rechargeable batteries.

Lithium-based rechargeable batteries have dominated the energy storage field and attracted considerable research interest due to their excellent electrochemical performance. As indispensable and ubiquitous components, electrolytes play a pivotal role in not only transporting lithium ions, but also expanding the electrochemical stable potential window, suppressing the side reactions, and manipulating the redox mechanism, all of which are closely associated with the behavior of solvation chemistry in electrolytes. Thus, comprehensively understanding the solvation chemistry in electrolytes is of significant importance. Here we critically reviewed the development of electrolytes in various lithium-based rechargeable batteries including lithium-metal batteries (LMBs), nonaqueous lithium-ion batteries (LIBs), lithium-sulfur batteries (LSBs), lithium-oxygen batteries (LOBs), and aqueous lithium-ion batteries (ALIBs), and emphasized the effects of interactions between cations, anions, and solvents on solvation chemistry, and functions of solvation chemistry in different types of electrolytes (strong solvating electrolytes, moderate solvating electrolytes, and weak solvating electrolytes) on the electrochemical performance and redox mechanism in the abovementioned rechargeable batteries. Specifically, the significant effects of solvation chemistry on the stability of electrode-electrolyte interphases, suppression of lithium dendrites in LMBs, inhibition of the co-intercalation of solvents in LIBs, improvement of anodic stability at high cut-off voltages in LMBs, LIBs and ALIBs, regulation of redox pathways in LSBs and LOBs, and inhibition of hydrogen/oxygen evolution reactions in LOBs are thoroughly summarized. Finally, the review concludes with a prospective outlook, where practical issues of electrolytes, advanced in situ/operando techniques to illustrate the mechanism of solvation chemistry, and advanced theoretical calculation and simulation techniques such as "material knowledge informed machine learning" and "artificial intelligence (AI) + big data" driven strategies for high-performance electrolytes have been proposed.

Constructing a Stable Interface Layer by Tailoring Solvation Chemistry in Carbonate Electrolytes for High-Performance Lithium-Metal Batteries.

Lithium-metal batteries (LMBs) are considered as promising next-generation batteries due to their high energy density. However, commercial carbonate electrolytes cannot be used in LMBs due to their poor compatibility with the lithium-metal anode and detrimental hydrogen fluoride (HF) generation by lithium hexafluorophosphate decomposition. By introducing lithium nitrate additive and a small amount of tetramethylurea as a multifunctional cosolvent to a commercial carbonate electrolyte, NO3 - , which is usually insoluble, can be introduced into the solvation structure of Li+ to form a conductive and stable solid electrolyte interface. At the same time, HF generation is suppressed by manipulating the solvation structure and a scavenging effect. As a result, the Coulombic efficiency (CE) of Li||Cu half cells using the designed carbonate electrolyte can reach 98.19% at room temperature and 96.14% at low temperature (-15 °C), and Li||LiFePO4 cells deliver a high capacity retention of 94.9% with a high CE of 99.6% after 550 cycles. This work provides a simple and effective way to extend the use of commercial carbonate electrolytes for next-generation battery systems.

Interface Engineering between the Metal-Organic Framework Nanocrystal and Graphene toward Ultrahigh Potassium-Ion Storage Performance.

The potassium-ion battery (PIB) has been recognized as a promising low-cost and high-energy battery; however, it suffers from a relatively low capacity and inferior cycling performance compared with current electrode materials. Herein, we report an effective interface engineering strategy to prepare metal-organic framework (MOF) nanocrystals tightly encapsulated by reduced graphene oxide (rGO) via strong chemical interaction as a free-standing anode for PIB. Based on experimental analysis and theoretical calculations, we systematically investigated the effect of the chemical-bonded interface between MOF nanocrystals and conductive rGO and revealed that the strong chemical interface can substantially enhance the adsorption energy and ion transport kinetics of the potassium ion within the MOF nanocrystals compared to the physical mixture of MOF and rGO with almost the same microscopic morphologies. As a result, such an MOF-rGO hybrid with strong interfacial chemical couplings delivered an ultrahigh reversible capacity of 422 mAh g-1 at 0.1 A g-1, superior rate performance (202 mAh g-1 at 5 A g-1), and outstanding long-term cycling performance (an ultralow decay rate of 0.013% per cycle after 2000 cycles at 2 A g-1), which are not only significantly better than those of the physical mixture of MOF/rGO but also among the best for anodes for PIB reported thus far.

Unexpected Effect of Electrode Architecture on High-Performance Lithium-Sulfur Batteries.

In the past years, considerable efforts have been devoted to the deliberate synthesis of nanosulfur in various hosts with sophisticated structures to improve the performance of lithium-sulfur batteries (LSBs) and reveal the structure-property relationship. It is taken for granted that these elaborate sulfur nanostructures are well maintained in the ultimate electrode after the traditional mixing and coating method. Herein, we, for the first time, reveal the unexpected sulfur structure deterioration in nanosulfur/graphene composites during the electrode preparation using the traditional method because of the long-term neglected dissolution-recrystallization effect of sulfur in solvents. Consequently, compared with binder-free three-dimensional graphene/sulfur electrodes, the milled graphene/sulfur electrodes exhibit much worse electrochemical performance. On the basis of this, we further propose a facile and universal graphene oxide-assisted assembly method to avoid the dissolution-recrystallization of sulfur, by which binder-free three-dimensional ethylenediamine-functionalized graphene/sulfur (3DEFGS) electrodes have been successfully prepared. The 3DEFGS electrodes with a high areal sulfur loading of ∼6 mg cm-2 exhibit an ultrahigh initial capacity of 1394 mA h g-1 at 0.1 C, an excellent rate performance with a capacity of 796 mA h g-1 at 4 C, and superior long-term cycling stability (885 mA h g-1 after 500 cycles at 1 C), which are among the best performances achieved by all reported LSB cathodes with high areal sulfur loadings.

Sub-5 nm Ultrasmall Metal-Organic Framework Nanocrystals for Highly Efficient Electrochemical Energy Storage.

Synthesis of ultrasmall metal-organic framework (MOF) nanoparticles has been widely recognized as a promising route to greatly enhance their properties but remains a considerable challenge. Herein, we report one facile and effective spatially confined thermal pulverization strategy to successfully transform bulk Co-MOF particles into sub-5 nm nanocrystals encapsulated within N-doped carbon/graphene (NC/G) by using conducting polymer coated Co-MOFs/graphene oxide as precursors. This strategy involves a feasible mechanism: calcination of Co-MOFs at proper temperature in air induces the partial thermal collapse/distortion of the framework, while the uniform coating of a conducting polymer can significantly improve the decomposition temperature and maintain the component stability of Co-MOFs, thus leading to the pulverization of bulk Co-MOF particles into ultrasmall nanocrystals without oxidation. The pulverization of Co-MOFs significantly increases the contact area between Co-MOFs with electrolyte and shortens the electron and ion transport pathway. Therefore, the sub-5 nm ultrasmall MOF nanocrystals-based composites deliver an ultrahigh reversible capacity (1301 mAh g-1 at 0.1 A g-1), extraordinary rate performance (494 mAh g-1 at 40 A g-1), and outstanding cycling stability (98.6% capacity retention at 10 A g-1 after 2000 cycles), which is the best performance achieved in all reported MOF-based anodes for lithium-ion batteries.

Integration of Graphene, Nano Sulfur, and Conducting Polymer into Compact, Flexible Lithium-Sulfur Battery Cathodes with Ultrahigh Volumetric Capacity and Superior Cycling Stability for Foldable Devices.

Lithium-sulfur batteries, as one of the most promising next-generation batteries, attract tremendous attentions due to their high energy density and low cost. However, their practical application is hindered by their short cycling life and low volumetric capacity. Herein, compact, flexible, and free-standing films with a sandwich structure are designed simply by vacuum filtration, in which nanosulfur is homogenously coated by graphene and poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). This unique hierarchical structure not only provides a highly conductive network and intimate contacts between nanosulfur and graphene/PEDOT:PSS for effective charge transportation, but also offers synergistic physical restriction and chemical confinement of dissoluble intermediate lithium polysulfides during electrochemical processes. Therefore, these conductive compact films, used directly as cathodes, show the highest reversible volumetric capacity of 1432 Ah L-1 at 0.1 C and 1038 Ah L-1 at 1 C, and excellent cycling stability with a minimal decay rate of 0.04% per cycle over 500 cycles at 1 C. Meanwhile, remarkable rate performance with a high capacity of 701 mAh g-1 at 4 C is also achieved. Soft-packaged batteries based on this flexible cathode are further fabricated and demonstrate excellent mechanical and electrochemical properties with little capacity decay under folded state, highlighting the practical application of our deliberately designed electrode in a flexible power system.