Effective Lithium Transfer and Thermal Enhancement Based on Nitrile Functionality for Safer Lithium Ion Batteries

Abstract

As lithium ion batteries are scaled up for electric vehicles and large capacity storage, the concerns about safety become more and more crucial because fire or explosion on such a large scale results in more serious trouble. As an intuitive and primitive idea to solve safety problems just from the electrolyte side, it would be helpful to decrease the flammability of electrolytes. However, it has been very challenging to enhance safety without any sacrifice in the performance of cells by controlling the non-flammability of electrolytes. We focused on how to prevent the progress of exothermic reactions between cathode electrode and electrolyte which is the initial process during safety problems with thermal development. In this work, three types of effective molecules with nitrile (-C≡N) functional groups, aliphatic di-nitriles, mono-nitriles and cyano macromolecules, were adopted. First, among aliphatic di-nitriles, succinonitrile (SN, CN-[CH2]2-CN) was evaluated as an additive improving thermal stability in ethylene carbonate (EC)-based electrolyte for lithium ion batteries. Without any sacrifice of performances such as cyclability and capacity, introduction of SN into electrolyte with graphite anode and LixCoO2 cathode leads to (1) reducing the amount of gas emitted at high temperature, (2) shifting onset temperature of exothermic reactions and (3) decreasing the amount of exothermal heat. From the spectroscopic studies based on photoelectrons induced by X-rays, we reveals that nitrile functional groups (-C≡N) of SN forms strong complex with surface metal atoms of LixCoO2 (x≈0.5). On the basis of successful approach of SN, we additionally investigated whether other aliphatic nitrile molecules had a similar effect on thermal stability. Two different series of aliphatic nitriles were introduced as an additive into a carbonate-based electrolyte: di-nitriles (CN-[CH2]n-CN with n = 2, 5 and 10) and mono-nitriles (CH3-[CH2]m-CN with m = 2, 5 and 10). Here, we reported on molecular coverage of nitriles on surface of cathode active materials to block or suppress thermally-accelerated side reactions between electrode and electrolyte. Based on the strong interaction between the electro-negativity of nitrile groups and the electro-positivity of cobalt in LiCoO2 cathode, the surface coverage of nitrile molecules improved the thermal stability of lithium ion cells by efficiently protecting the surface of LiCoO2. Three factors, the surface coverage, the steric hindrance of aliphatic moiety within nitrile molecules and the polarity of ending group of adsorbed nitriles, affected cell performances at elevated temperature. These previous observations furthermore enable us to expand our concept to the high molecular weight (Mw) polymer with nitrile structure, cyanoethyl polyvinylalcohol (PVA) (abbreviated PVA-CN) and cyanoethyl pullulan (abbreviated Pullulan-CN). Distinctively, the electrolyte including PVA-CN transforms its phase to the gel-type electrolyte even in the condition without any initiators or cross-linking agents. Generally, high values of cationic transference number (t+) achieved by solid or gel electrolytes have resulted in low ionic conductivity leading to inferior cell performances. However, we present that this novel organogel polymer electrolyte characterizes a high liquid-electrolyte-level ionic conductivity (~10 mS cm-1) with high t+ of Li+ (> 0.8) for lithium ion batteries (LIB), working as a function of concentration and temperature. A conventional liquid electrolyte in presence of a cyano resin was physically and irreversibly gelated at 60oC without additional initiators and cross-linkers, showing the behavior of lower critical solution temperature. During gelation, ionic conductivity of the electrolyte followed a typical Arrhenius-type temperature dependency, even if its viscosity is increased dramatically with temperature. Based on the high Li+-driven ion conduction, LIBs using the organogel electrolyte delivered significantly enhanced cyclability and thermal stability. Meanwhile, besides safety of LIB induced by thermal stability of electrolyte, state-of-the-art LIBs also should consider overdischarge abuse, one of the problematic safety issues, because it engenders Cu corrosion on the negative electrode to increase capacity fading and higher resistance within the cell. On the basis of previous result relevant to the strong interaction of -CN functional group with metal ion, we investigated the activity of SN as copper corrosion inhibitor to provide overdischarge (OD) protection. The anodic Cu corrosion, occurring above 3.5 V (vs. Li/Li+) in conventional LIB electrolytes, is suppressed until 4.5 V in the presence of SN. The corrosion inhibition by SN is ascribed to formation of SN-induced passive layer, which is spontaneously developed on copper surface during the first anodic scan. The passive layer is composed mainly of Cu(SN)2PF6 unit, which is evidenced by Raman spectroscopy and electrochemical quartz crystal microbalance (EQCM) measurements. The effects of the SN additive on OD protection is confirmed by using 750mAh soft-packed full cells of LiCoO2 and graphite with lithium metal as a reference electrode. Addition of SN completely prevents corrosion of copper current collector without any decay in performance, thereby tuning the LIB chemistry to be inherently immune to the OD abuses.