Design and Preparation of Phosphorus Based Electrolytes and Composite Separators for Safer Lithium-Ion Batteries


Student thesis: Doctoral Thesis

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Awarding Institution
  • Kim Meow LIEW (Supervisor)
  • Yuan Hu (External person) (External Supervisor)
Award date10 Jul 2019


In recent twenty years, lithium-ion batteries have obtained an extremely importance as the energy storage of portable devices such as phones, laptops computers and portable power sources due to its outstanding properties, for instance, high energy density, rechargeable properties, excellent cycling performances and controllable shape designs. Therefore, during the past five years, lithium ion batteries have been considered as the optimal power sources for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). However, accidents caused by the explosion and combustion of lithium ion batteries are frequently reported, such as the explosion of phone Samsung Note7 and the spontaneous combustion of HEVs buses in Xiamen in 2015. It is well-known that most lithium ion batteries comprising of separators such as polyethylene (PE) or/and polypropylene (PP) and liquid organic electrolytes containing ethylene carbonate, diethyl carbonate and dimethyl carbonate have potential safety problems due to their low thermal stabilities and flammability, which are usually regarded as the main reasons resulting the explosion and combustion of lithium ion batteries. Hence, it is of great significance to enhance the safety of lithium ion batteries. On one side, there a considerable reduction of flammability can be obtained by the incorporation of phosphorus-based liquid flame retardants, as well as higher electrochemical properties. It is well-recognized that phosphorus-oxygen radicals generated at high temperature could actively capture other free radicals emitted by the burning electrolytes to retard the combustion. So, electrolytes systems with phosphorus are expected to be a promising strategy to solve the issue of flammability of electrolytes. On the other side, inorganic based separators are extensively employed to improve the thermal stability and porosity of separators, as well as endowing lithium ion batteries with safety performance and outstanding electrochemical properties. Atom Layer Deposition technique is used to fabricate inorganic material on the framework surface of polyvinylidene fluoride hexa-fluoropropylene (PVDF-HFP) for excellent thermal shrinkage performance and outstanding compatibility with electrolytes. Furthermore, incorporating two-dimensional boron nitride sheets can efficiently improve the porosity and thermal stability of cellulose porous separator. In addition, for the purpose of obtaining separators with high mechanical strength, polyvinyl alcohol (PVA)/ hydroxyapatite (HAP) hybrid porous membrane are prepared to be used as separators for safer lithium ion batteries. Research work of this dissertation is composed of the following parts:

1.In chapter 2, trivalent and pentavalent phosphorus-based flame retardants are purposely selected to comparably investigate the influence on safety and electrochemical performance of Li-ion batteries. In order to eliminate the influencing factors of non-valence state, we selected two groups of phosphorus-containing flame retardants (triethyl phosphate, triethyl phosphite, and trimethyl phosphate, trimethyl phosphite) with similar structure but containing trivalent phosphorus and pentavalent phosphorus. The experimental results show that pentavalent phosphorus-based electrolytes display wider electrochemical windows and could endure higher voltage compared to trivalent phosphorus-based electrolytes, revealing that pentavalent phosphorus-based flame retardants are more suitable as efficient additives for cathode materials systems. Interestingly, trivalent phosphorus flame retardants may serve as solid electrolyte interphase layer formers, which can be decomposed and meanwhile promote the formation of stable solid electrolyte interphase layer on the surface of graphitic anode materials even with a low loading. Additionally, it is worth noting that both trivalent and pentavalent phosphorus flame retardants show satisfying high current rate performance indicating the promising application in the field of EV and electrical mobile devices. This work can provide help for scientific research or industry in the study of phosphorus-containing flame retardants for batteries.

Based on the conclusion above, to meet the requirement of electrolytes for lithium metal batteries, nonflammable pentavalent phosphorus-based flame retardants electrolytes are designed and used in LiFePO4|Li full cells. The corresponding results demonstrate that pentavalent phosphorus-based liquid flame retardants can endow lithium metal batteries with high safety, steady cycling stability and satisfying current rate capacity, thereby be promising candidates for lithium metal batteries.

2.Lithium metal batteries, due to its unique advantages such as light weight, the lowest anode potential and the highest theoretical specific capacity, have been regarded as the promising candidate for next-generation electrical energy storage. Nevertheless, the development and practical application of lithium metal batteries has been seriously hindered by the safety issue induced by lithium dendrites. In chapter 3, the atom layer deposition (ALD) technique is introduced to fabricate the Al2O3 on the surface of PVDF-HFP membrane to endow it with higher thermal stability, great affinity with electrolytes, improved ion conductivity and higher Young’s modulus. Employing LiFePO4|Li cells, ALD100/PH separator imparts batteries with the best cycling performances and rate capacity among various separators. The SEM images of morphology after cycled indicate that ALD100/PH separators with extremely high Young’s modulus and ion conductivity efficiently suppress the growth of lithium dendrites. Moreover, the ALD100/PH separator shows more than 1300 h of stable operation at current density of 0.5 mA cm-2, exhibiting the capability against metallic lithium and the potential for application in the field of lithium metal batteries. Thus, this interesting ALD technique capable of feasible fabrication procedure and desirable advantages maybe make commercial or as-prepared separators more advanced and potential candidates for the next-generation high-energy-density rechargeable battery systems such as Li-S and Li-O2 batteries, as well as other metal-based batteries.

3.As introduced in previous chapter, ALD technology mainly inhibits lithium dendrites by covering inorganic materials on the surface of the substrate. Different from this idea, inorganic materials embedded in polymer materix are also anticipated to suppress the dendritic lithium and enhance the Coulombic efficiency. In chapter 4, we report an environmentally friendly nanocomposites separator, comprising of polyvinyl alcohol and hydroxyapatite, which exhibits outstanding Young’s modulus and high mechanical flexibility. The results show that the as-prepared separator can efficiently suppress the growth of dendritic lithium and enable a homogeneous stable solid electrolyte interphase. Compared to lithium metal batteries using a commercial polyolefin-based separator, cells using the reported nanocomposite separator demonstrate a superior electrochemical performance with a much higher Coulombic efficiency during the charge/discharge cycling at a practical current density of 2 mA cm-2. This work indicates that nanocomposite engineering could be a promising strategy to develop stable, safe and sustainable Li-metal batteries.

    Research areas

  • Electrolytes, Separators, Safety, Lithium-ion batteries, Lithium dendrites