Potential-Sensitive Separator With Overcharge Protection Mechanism For Li-ion Batteries

Safety concern has been a primary obstacle for commercial applications of large-size or high-rate lithium ion batteries (LIBs) in many high technology fields, such as electric vehicles and stationary energy storage systems. Among the inducing factors for safety hazards, overcharge seems to be the most dangerous abuse for LIBs. Because the non-aqueous electrolytes commonly used in LIBs are incapable of being reversibly oxidized or reduced, and therefore cannot provide a spontaneous intrinsic limitation for cell voltage runaway during overcharging. In pursuit of safety, great efforts have been focused in recent years on development of internal and self-actuating overcharge protection mechanisms for LIBs. Previous studies have demonstrated that use of electroactive polymer separator to clamp the charge voltage seems to be a more attractive means for overcharge protection of LIBs. The idea of using electroactive polymer separator for overcharge protection is based on the intrinsic property of the conducting polymer, which can be p-doped at high oxidation potentials into conductive state and de-doped at normal operating voltages into an electronically isolating state, functioning as a potential-sensitive switch for controlling the battery voltage at charging. This Ph.D work was oriented to develop several kinds of polymer-incorporated separators applicable for overcharge protection of 3.6V-class LiFePO4-based and 4.2V-class LiCoO2 or LiMn2O4-based LIBs, respectively. The main results are summarized as follows:1. To search for the potential-sensitive materials for overcharge protection, a number of electroactive polymers were synthesized by chemically oxidative polymerization, including polydiphenylamine, poly(2-nitro-diphenylamine), poly(diphenylamine-4-sodiumsulfonate), polytriphenylamine, polycarbazole, poly(3-decyl-thiophene), polyfluorene, poly(9,9-di-butyl-fluorene)(BPF), polypyrene, cyano-substituted poly(p-phenylene vinylene) (CN-PPV), and poly(p-phenylene) (PPP). Their electrochemical behaviors in organic electrolyte were also investigated by means of cyclic voltammetry. The results showed that the polymers, polydiphenylamine, polytriphenylamine and poly(3-decyl-thiophene), have appropriate oxidation potentials of～3.6 V, highly reversible p-doping/de-doping behaviors, and excellent cycling stabilities, suitable to use for overcharge protection of 3.6 V-class LIBs. Moreover, the electroactive polymers, polypyrene, BPF, CN-PPV and PPP, exhibited relatively high oxidation potentials of～4.2 V, showing possibilities for using as potential-sensitive materials to build up internal overcharge protection mechanisms for 4.2V-class Li-ion batteries.2. Based on the knowledge of that diphenylamine (DPAn) monomer can be electro-polymerized to form a conductive polymer PDPAn in battery electrolyte, DPAn was tested as an electrolyte additive for overcharge protection of LiFePO4-based LIBs. The experimental results demonstrated that the DPAn additive could polymerize to form a conductive polymer bridging between the cathode and anode of battery during overcharging, producing an internal current bypass to prevent the battery from voltage runaway. The charge-discharge tests of practical LiFePO4/C batteries indicated that the DPAn additive could clamp the cell's voltage at the safe value less than 3.7 V even at the high rate overcharge of 3 C current, showing an excellent capability for overcharge current shunting. Meanwhile, this monomer molecule has no significant impact on the charge-discharge performance of the batteries at normal charge-discharge condition. However, because of the poor stability of PDPAn polymer at high overcharging potentials, this polymer-incorporated separator can only work for limited overcharge cycles.3. A polytriphenylamine (PTPA)-modified separator was prepared by impregnating triphenylamine monomers into a commercial Celgard separator and in situ polymerizing the monomers within the separator into electroactive phase during overcharging. This type of polymer-incorporated separator can transform from an insulating state to a conductive state at overcharged voltage of～3.75V(vs. Li+/Li) and act as a self-actuating potential-switchable separator for overcharge protection of LiFePO4/C Li-ion batteries. The experimental results demonstrated that this electroactive separator can reversibly control the cell's voltage at the safe value less than 4.15 V at high rate overcharge of 3 C current without obvious negative impact on the normal charge-discharge performances of the commercial LiFePO4/C batteries even at prolonged overcharge cycling, showing a potential application in 3.6 V-class lithium-ion batteries.4. To simplify the manufacture process of polymer-incorporated separator, a simple processing method was proposed to prepare the electroactive polymer separator by directly impregnating a soluble polymer into a commercial Celgard separator. According to this ideas, a soluble polymer, poly(3-decyl-thiophene) (P3DT), was selected as an electroactive polymer for this type of separator. The influence of additional modifier and chemical doping on the structural properties and overcharge protection behaviors of modified separator was also studied. As a result, a co-modified separator with P3DT and microcrystalline graphite was successfully developed. Subsequently, the physicochemical properties of the.co-modified separator were characterized by means of SEM, air permeability tester, and DCS, etc. The experimental results from the charge-discharge tests of practical C/LiFePO4 batteries demonstrated that this type of co-modified separator could provide effective overcharge protection for LiFePO4 electrode, with a clamping voltage plateau of～3.75 V and a clamping current as high as 3 C rate. After 280 cycles for overcharge testing, the clamping voltage plateau can still be well controlled below 4.1 V. In addition, this membrane has no significant impact on the normal charge-discharge behaviors of the batteries, such as cycling performance, high current discharge capability, self-discharge rate, and low temperature performance, showing a great prospect for practical application in LiFePO4-based LIBs.5. A PPP/P3DT composite polymer separator with reversible overcharge protection mechanism for 4.2 V-class LIBs, was prepared by coating a porous PPP layer on the surface of P3DT-incorporated separator, and was tested as a self-activating overcharge protection separator for Li/LiMn2O4 coin cells. The experimental results demonstrated that PPP/P3DT composite separator could provide effective overcharge protection for LiMn2O4 cathode, with a clamping voltage plateau of about 4.3 V. Even at 3 C current, the overcharging voltage of the battery can still be stabilized at 4.35 V. In addition, this composite separator can reversibly work for more than 100 cycles at 1 C current and has no negative impact on the normal charge-discharge performances of the test batteries, such as cycling performance, high rate discharge as well as self-discharge rate.