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.

Pentafluoro(phenoxy)cyclotriphosphazene Stabilizes Electrode/Electrolyte Interfaces for Sodium-Ion Pouch Cells of 145 Wh Kg-1

Sodium-ion batteries are competitive candidates for large-scale energy storage batteries due to the abundant sodium resource. However, the electrode interface in the conventional electrolyte is unstable, deteriorating the cycle life of the cells. Introducing functional electrolyte additives can generate stable electrode interfaces. Here, pentafluoro(phenoxy)cyclotriphosphazene (FPPN) serves as a functional electrolyte additive to stabilize the interfaces of the layered oxide cathode and the hard carbon anode. The fluorine substituting groups and the π-π conjugated ─PN─ structure decrease the lowest unoccupied molecular orbital and increase the highest occupied molecular orbital of FPPN, respectively, realizing the preferential reduction and oxidization of FPPN on the anode and cathode simultaneously, which results in the formation of a uniform, ultrathin, and inorganic-rich solid electrolyte interlayer and cathode electrolyte interphase. The sodium-ion pouch cells of 5 Ah capacity rather than coin cells are assembled to evaluate the effect of FPPN. It can retain a high capacity of 4.46 Ah after 1000 cycles, corresponding to a low decay ratio of 0.01% per cycle. The pouch cell also achieves a high energy density of 145 Wh kg-1 and a wide operating temperature of -20-60 °C. This work can attract more attention to the rational electrolyte design for practical applications.

Enhanced high voltage performance of LiNi0.5Mn0.3Co0.2O2 cathode via the synergistic effect of LiPO2F2 and FEC in fluorinated electrolyte for lithium-ion batteries

LiNi_0.5Mn_0.3Co_0.2O₂ can achieve high energy density due to its merits of high theoretical capacity and a relatively high operating voltage, but the LiNi_0.5Mn_0.3Co_0.2O₂ battery suffers from capacity decay because of the unstable solid electrolyte interface on the cathode. Herein, we investigate the application of a fluorinated electrolyte composed of fluoroethylene carbonate (FEC) as a cosolvent and lithium difluorophosphate (LiPO₂F₂) as a salt-type additive extending the life span of the LiNi_0.5Mn_0.3Co_0.2O₂ cathode. LiNi_0.5Mn_0.3Co_0.2O₂ can achieve and maintain a capacity of 157.7 mA h g⁻¹ over 200 cycles at a 1C rate between 3.0 and 4.4 V, as well as a reversible capacity of 132.7 mA h g⁻¹ even at the high rate of 10C. The enhanced performance can be ascribed to the formation of the robust and protective fluorinated organic–inorganic film on the cathode, which derives from the FEC cosolvent and LiPO₂F₂ additive and ensures facile lithium-ion transport. The synergistic effect of the cosolvent and additive to boost the electrochemical performance of LiNi_0.5Mn_0.3Co_0.2O₂ cathode will pave a new pathway for high-voltage cathode materials.

LiNi_0.5Mn_0.3Co_0.2O₂ can achieve great electrochemical performances because of the robust and protective fluorinated organic–inorganic film on the cathode, which derives from the FEC cosolvent and LiPO₂F₂ additive.

Scavenging Materials to Stabilize LiPF6 -Containing Carbonate-Based Electrolytes for Li-Ion Batteries

In conjunction with electrolyte additives used for tuning the interfacial structures of electrodes, functional materials that eliminate or deactivate reactive substances generated by the degradation of LiPF₆ -containing electrolytes in lithium-ion batteries offer a wide range of electrolyte formulation opportunities. Herein, the recent advancements in the development of: (i) scavengers with high selectivity and affinity toward unwanted species and (ii) promoters of ion-paired LiPF₆ dissociation are highlighted, showing that the utilization of the above additives can effectively mitigate the problem of electrolyte instability that commonly results in battery performance degradation and lifetime shortening. A deep mechanistic understanding of LiPF₆ -containing electrolyte failure and the action of currently developed additives is demonstrated to enable the rational design of effective scavenging materials and thus allow the fabrication of highly reliable batteries.