Functional Electrolytes for Advanced Lithium Ion Batteries

Abstract

Despite two decades of commercial history, enhanced lithium-ion batteries (LIBs) performance is required to satisfy the evolving electric vehicles (EVs) and energy storage system (ESS) market requirements which is essential for thermal stability, long-term cycle, safety, and fast charging. Here, we resolve these challenges using a mussel-inspired polydopamine (PD)-coating, novel additives, and analysis of superconcentrated electrolyte systems.

(1) it remains very difficult to simultaneously achieve both high rate capability and thermal stability in the graphite anodes of Li-ion batteries because the stable solid electrolyte interphase (SEI) layer, which is essential for thermal stability, impedes facile Li+ ion transport at the interface. The nanometer-thick PD coating layer allows the formation of a SEI layer on the coating surface without perturbing the intrinsic proper-ties of the SEI layer of the graphite anodes. PD-coated graphite exhibits far better performances in cycling test at 60 ℃ and storage test at 90 ℃ than bare graphite. The PD-coated graphite also displays superior rate capability during both lithiation and delithiation. The usefulness of the proposed PD coating can be expanded to various electrodes in rechargeable batteries that suffer from poor thermal stability and interfacial kinetics.

(2) Next study introduces 3-fluoro-1,3-propane sultone (FPS) as a novel SEI additive to replace VC and another popular SEI additive, 1,3-propane sultone (PS). Vinylene carbonate (VC) has been the best performing solid electrolyte interphase (SEI) additive for the current lithium-ion batteries (LIBs). However, it is also true that the current LIB technology is being stagnated by the limit set by VC. Electrochemical experiments confirm that the presence of an electron withdrawing fluorine group is favourable in terms of the anodic stability and the SEI forming ability. Considering the high anodic stability, the excellent cyclability, and the good thermal stability, FPS is an outstanding SEI additive that can expand the performance boundary of the current LIBs.

(3) The swelling issue by gas evolution at elevated temperatures (85-90 ℃) is one of the major challenges related to current Li-ion batteries (LIBs). We herein demonstrate that iodine (I2) as a redox shuttle additive, when its dose is properly determined, can suppress the swelling behavior of LiCoO2/graphite Al-pouch cells during storage at 90 ℃ without sacrificing other cell performances.

(4) Among the numerous additives, it remains very difficult to simultaneously adapt both spinel and layered cathode materials of Li-ion batteries. This study introduces a highly versatile new additive, sodium phosphate (P2), as a novel LIBs additive to improve the thermal stability in both spinel (LiMn2O4 and LiNi0.5Mn1.5O4) and layered (LiNi0.8Co0.1Mn0.1O2) cathode materials. Our investigation reveals that P2 additive scavenges harmful hydrofluoric acid (HF), effectively eliminates HF promoting metal dissolution from the cathodes, and forms a passivation layer on the cathode surface against electrolyte decomposition at high temperature. Considering the good thermal stability and the storage performance at high temperature, P2 additive is an outstanding additive that can be expand to regardless of the type of LIBs that suffer from poor thermal stability.

(5) Lastly, the superior rate capability of the superconcentrated LiFSI/AN electrolytes, claimed in recent reports, is scrutinized in relation to the active mass loading of the graphite electrodes. Compared to a conventional carbonate electrolyte, a superconcentrated (4.5 M) LiFSI/AN electrolyte exhibites enhanced rate capability in a low-loading (< 5 mg/cm2) graphite electrode. However, the superconcentrated electrolyte displays an inferior rate performance in a high-loading electrode (9 mg/cm2), which is commonly employed in commercial electrodes. The electrochemical impedance study reveals that the superconcetrated electrolyte enables the lower charge transfer resistance at the graphite/electrolyte interface (Rct), which is possibly associated with an unique solution structure in the concentrated electrolyte. However, as the graphite loading increases, the ion transport in the electrode pore (Rion) becomes dominant, which dilutes the merit of the low Rct in a superconcetrated electrolyte. This study indicates that the superior rate capability in superconcentrated solutions claimed in previous studies should be appreciated in conjunction with the electrode loading.