Are Polymer‐Based Electrolytes Ready for High‐Voltage Lithium Battery Applications? An Overview of Degradation Mechanisms and Battery Performance

Abstract

High‐voltage lithium polymer cells are considered an attractive technology that could out‐perform commercial lithium‐ion batteries in terms of safety, processability, and energy density. Although significant progress has been achieved in the development of polymer electrolytes for high‐voltage applications (> 4 V), the cell performance containing these materials still encounters certain challenges. One of the major limitations is posed by poor cyclability, which is affected by the low oxidative stability of standard polyether‐based polymer electrolytes. In addition, the high reactivity and structural instability of certain common high‐voltage cathode chemistries further aggravate the challenges. In this review, the oxidative stability of polymer electrolytes is comprehensively discussed, along with the key sources of cell degradation, and provides an overview of the fundamental strategies adopted for enhancing their cyclability. In this regard, a statistical analysis of the cell performance is provided by analyzing 186 publications reported in the last 17 years, to demonstrate the gap between the state‐of‐the‐art and the requirements for high‐energy density cells. Furthermore, the essential characterization techniques employed in prior research investigating the degradation of these systems are discussed to highlight their prospects and limitations. Based on the derived conclusions, new targets and guidelines are proposed for further research.

Advances in studying interfacial reactions in rechargeable batteries by photoelectron spectroscopy

“…Such electrolytes can offer safety benefits and increased stability, allowing the use of high voltage cathodes or lithium metal anodes for greater energy density. 125,126 Just as for liquid electrolytes, electrochemical side-reactions occur between electrode and gel/polymer electrolyte during operation, often forming an interphase. This can be probed by XPS; however, sample preparation can be even more challenging for such investigations, especially when there is strong adhesion between the electrode and electrolyte.…”

Recent Progress of Polymer Electrolytes for Solid-State Lithium Batteries

“…This requires the electrolytes to be compatible with these high-voltage cathodes and possess sufficient high-voltage stability . Despite of the compatibility with lithium, traditional polyether-based SPEs have high HOMO values and are easily oxidized at high voltage, especially in the presence of oxide cathode materials. , Accordingly, the exploration of high-voltage stable SPEs is highly desired for high energy all-solid-state (ASS) batteries. As noted in the Introduction, the antioxidation capabilities of SPEs are theoretically determined by their HOMO energy levels.…”

“…In order to further improve the energy densities of SSLBs, high specific capacities and high-voltage cathodes (e.g., LiNi 1‑x‑y Co x Mn y O 2 , LiNi 0.5 Mn 1.5 O 4 ) have been widely utilized, which require the SPEs to be compatible with both high-voltage cathodes and low-voltage Li metal anodes. , However, low-voltage stable SPE (such as polyether) is easy to be oxidized and decomposed at high voltage, while SPE with good antioxidation capability (such as polyester) is easy to be reduced in contact with a Li metal anode, resulting in battery failure. , The compatibilities of SPEs with cathodes and anodes are decided by either their electrochemical windows or whether a stable SEI and cathode–electrolyte interphase (CEI) can be formed (Figure a). , When the highest occupied molecular orbital (HOMO) of SPE is lower than the cathode potential (μ C ) or a stable CEI is formed, the SPE has the capability to restrain oxidative decomposition and is stable with a cathode. Similarly, for the low-voltage anode, the lowest unoccupied molecular orbital (LUMO) should be higher than the anode potential (μ A ) or form a stable SEI. , However, it is extremely difficult for one electrolyte to satisfy both of these conditions at the same time (Figure b, c).…”

Multi‐Scale Characterization Techniques for Polymer‐Based Solid‐State Lithium Batteries

“…In the past decade, lithium batteries (LBs) [1] have gained tremendous interest owing to their high energy density [2] for practical DOI: 10.1002/macp.202200351 applications in portable electronics and electric vehicles (EV). [3] Especially, lithium metal batteries (LMBs) [4] equipped with lithium anode and exhibit high voltage cathodes possess superior energy density, which can meet the ever-increasing demand for high energy density. [5] However, conventional organic electrolytes [6] are flammable and possess low compatibility with lithium metal anode, bringing severe safety problems [7] and poor cycling performance for LMBs.…”

“…[17] Generally, the ionic conductivities of semicrystalline SPEs are below 10 −5 S cm −1 at room temperature, whereas amorphous SPEs can attain 10 −4 S cm −1 . [3,18] The low ionic conductivity leads to high internal resistance of solid-state battery, which severely limits the battery's rate performance and operating temperature range. [19] In addition, there are many complex processes and chemical reactions in interfaces of PSSLBs, including interfacial wettability, physical contact, chemical or electrochemical reaction, ionic transport, etc.…”