Editors' Choice—The Effectiveness of Multifunctional Li-Ion Battery Separators past Their Saturation with Transition Metal Ions

Abstract

Li-ion batteries (LIBs) for electric vehicles necessitate a 10-year useful life. The dissolution of manganese and other transition metal ions from positive electrodes, and the loss of Li + ions are two main causes for reduced LIB durability. Multifunctional separators (MFSs) designed to mitigate these degradation modes enable improvements in the electrochemical performance of LIBs at both room and above-ambient temperatures. We report herein on the benefits for the cycle life and rate performance of Li x Mn 2 O 4 ||graphite cells by composite MFSs that incorporate a crosslinked styrene-divinylbenzene copolymer functionalized with disodium iminodiacetate groups. Our study shows that: (1) forming effective passivating electrode surface films early in LIB life is essential for good performance and long life; (2) saturation of MFSs with manganese ions does not subsequently lead to an increased capacity loss rate during high-temperature (55 • C) cycling; (3) MFSs also improve rate performance. These benefits originate from a diminished crossover of dissolved Mn ions to the negative electrode, reduced losses of electroactive Li + ions, and smaller interfacial resistances at both positive and negative electrodes. Despite persistent problems with durability and abuse tolerance, rechargeable Li-ion batteries (LIBs) have captured the portable electronics market due to their superior energy density, low toxicity, and environmental friendliness.1 The lifetime criterion of 500 chargingdischarging cycles with 80% retention of initial capacity for portable electronics LIBs increases to 5,000 cycles for electric vehicle batteries. Thus, besides reduced cost, increased energy density and improved abuse tolerance, greater battery durability is also needed for electric vehicles to become the dominant automotive transportation mode worldwide.The dissolution of manganese and other transition metal ions from positive electrodes and the loss of (both electroactive and transport) Li + ions are two main causes for performance degradation and a shortened useful life of LIBs. Manganese dissolution initiates the so-called DMDCR (dissolution -migration -deposition -catalytic reactions) performance degradation mode in LIBs: dissolved Mn 2+ and Mn 3+ ions 2 migrate through the electrolyte solution and deposit at negative electrodes, where they catalyze electrolyte solution decomposition reactions, with consumption of Li + ions, gas generation, and thickening of passivating films, which inevitably results in reduced LIB power and a diminished battery life.3-5 None of the many proposed mitigation measures for the DMDCR performance degradation mode 5-12 proved effective to the point of making LMO-graphite batteries a commercial success. In recent years, several authors demonstrated that materials that can trap transition metal (TM) ions and/or scavenge HF provide a promising avenue for improving LIB durability. These materials can be used inside or on top of separators and electrodes, either as fillers, binders or coatings.13-23 So far, we focus...

Review—Multifunctional Separators: A Promising Approach for Improving the Durability and Performance of Li-Ion Batteries

“…There are two main reasons for investigating a variety of TM trapping materials. First, our data clearly show that Mn 3+ and not Mn 2+ is the dominant Mn cation in the electrolyte solutions from LMO//graphite 26,[60][61][62][63][64][65][66][67][68] (see Figure 2) as well as LNMO//graphite cells, 69 also that the Mn 2+ to Mn 3+ ratio is different in cells with LMO than in cells with LNMO. 69 Furthermore, the trapping capacities for various transition metal ions vary from material to material.…”

“…After having shown that separators functionalized with various TM trapping materials can prevent the deposition of Mn cations dissolved from LMO onto graphite during cycling at both room and above-ambient temperatures, [65][66][67] we focused recently on demonstrating that the chemically active separators concept may have practical relevance. Specifically, we showed that improvements in cycle life is still obtained with functional separators having realistic dimensions (10 to 25 μm thickness), 63 that the saturation of the functional separators with TM ions does not affect the retention capacity during high-temperature cycling 63,64 and that, in addition, functionalized separators can also improve rate performance. 63 We also investigated the microscopic underpinning of the observed improvements in electrochemical performance.…”

“…Specifically, we showed that improvements in cycle life is still obtained with functional separators having realistic dimensions (10 to 25 μm thickness), 63 that the saturation of the functional separators with TM ions does not affect the retention capacity during high-temperature cycling 63,64 and that, in addition, functionalized separators can also improve rate performance. 63 We also investigated the microscopic underpinning of the observed improvements in electrochemical performance. 64 Finally, we will review results obtained with a new class of separators that are capable of scavenging acid species, but do not trap TM ions.…”

Acid-scavenging separators promise long-term cycling stability of lithium-ion batteries

“…In the early stage of batteries operation with low HF content and the formation of passivation surface films, the acid-removing capability of the acid-scavenging separator is far from saturation, which can effectively remove acid substances and promote the formation of uniform and compact passivation film. Thereby it significantly improves the cycling performance of the battery, even with the generation of H 2 O in subsequent stages, which is similarly argued for in the study of Shilina et al and Banerjee et al 50,112 However, this effect may make a significant difference on the battery performance due to the variation of initial water (acid) content in the battery, which needs further in-depth exploration. Even for separators using ceramic oxide nanoparticles as acid removal materials, the hazardous species it clears may not be the HF in the electrolyte, but the H 2 O molecules.…”

Polymer‐Based Separators for Lithium‐Ion Batteries

“…The inclusion of different fillers into a PVDF matrix proved to increase the ionic conductivity of the membrane, its thermal and mechanical stability, as well as the rate performance of the batteries. Al2O3 [50], Poly(IDANa2) [51], SnO2 [52],…”