Ternary polymer electrolyte with enhanced ionic conductivity and thermo-mechanical properties for lithium-ion batteries
Introduction
In today's world, there is a high demand for electrical energy storage devices for stationary and mobile applications such as uninterrupted power supply, voltage regulations, electric vehicles (EVs) and hybrid electric vehicles (HEVs). Lithium-ion battery (LIB) technology has high energy density among other commercial secondary storage batteries with no memory effect and low self-discharge rate. The LIB has mainly three components namely, anode, cathode and electrolyte. The electrolyte separates the anode and the cathode to avoid short circuiting and to provide a conduction path for ions (Li+ ion in the case of LIBs) transportation during charge–discharge processes. Safety and performance of the LIBs depend significantly on the type of electrolyte used. Conventional electrolytes are prepared by using organic solvents such as ethylene carbonate (EC), propylene carbonate (PC) etc. These solvents are highly flammable in nature and responsible for safety concerns in LIBs [1], [2], [3], [4]. Based on the temperature sensitivity of these electrolytes, temperature monitoring and control during charge and discharge processes of the LIBs is typically needed. This raises LIBs manufacturing cost.
Solid polymer electrolytes have potential to overcome the safety concerns of the use of organic electrolytes and other drawbacks such as leakage, reactivity with the electrodes and poor electrochemical stability [5], [6]. Polyethylene oxide/lithium salt complexes are extensively studied polymer electrolytes for LIBs [4], [7], [8], [9], [10], [11], [12], [13], [14]. PEO can effectively solvate and conduct Li ions. However, the conductivity of PEO electrolytes containing lithium perchlorate (LiClO4) and lithium triflate (LiCF3SO3) [7], [8], [10], [15], [16], are in the range of 10−6 to 10−8 S cm−1 at room temperature and 10−3 to 10−4 S cm−1 at 100 °C. The low ionic conductivity is due to a semi-crystalline phase of PEO, above 60–65 °C the semi-crystalline phase melts and shows a 2–3 order of magnitude increase in conductivity. However, in the high conduction region, PEO/Li–salts complexes loose its mechanical strength [17], [18]. Other methods such as addition of low molecular weight and high dielectric constant plasticizers and inert fillers of micro/nanometer size particle in polymer electrolytes have been reported to improve the room temperature conductivity [18], [19], [20], [21]. The addition of plasticizers can improve the conductivity, but it deteriorates the mechanical properties of polymer electrolytes.
Cellulose is an abundant, biodegradable and low cost material. The cellulosic cell wall in plants provides structural integrity by encapsulating the cell components inside; this property has been explored to develop mechanically improved polymer electrolytes. Cellulosic constituents are gaining increasing importance due to their high strength and stiffness and high potential as a reinforcing agent [17], [22], [23], [24], [25], [26]. Cellulose has been used to prepare the separators for lithium-ion batteries and its nano-whiskers as reinforcement for polymer electrolytes [27], [28], [29]. In order to maintain structural stability of the polymer electrolyte and high conductivity in the battery operational temperature range, generally 25–60 °C, the authors have previously developed electrolytes based on PEG/NC [30] and PEO/NC [31]. The NC based electrolyte has proved NC capability in providing thermal and mechanical stability for the PEG and PEO without significantly affecting their ionic conductivity. For further improvement and development of NC based polymer electrolyte, a ternary polymer system is proposed.
In this work, quasi-solid polymer electrolyte with appropriate composition of PEG (or TEGDME), PEO and NC has been developed and studied in terms of its mechanical stability and ionic conductivity. The addition of 10–20 wt% of NC provides a sufficient structural support to PEO which helps to hold the 60–70 wt% liquid content (PEG or TEGDME) in the polymer matrix. Addition of NC assists to improve conductivity and mechanical integrity of the polymer electrolyte. Thermo-mechanical, structural and electrochemical properties have been investigated to determine the applicability of the developed material as an electrolyte for LIBs. In addition, Li-ion cells were devised using the developed electrolytes and their charge–discharge performance was tested.
Section snippets
Materials
Microcrystalline Cellulose (MCC) (Mw = 350,000) was provided by FMC Biopolymer (Philadelphia, PA). Sulfuric acid (99.9%), Polyethylene glycol (Mw = 200), Tetraethylene glycol dimethyl ether (TEGDME), Polyethylene Oxide (Mw = 100,000), Ethanol and Lithium perchlorate (LiClO4) were purchased from Aldrich (St. Louis, MO).
Preparation of NC suspension
Acid Hydrolysis of MCC was performed using a Varian® dissolution system following a procedure reported before_ENREF_26 [26]. The dissolution system bath was adjusted to 5 °C
Results and discussion
The schematic in Fig. 1 elaborates the formation of polymer electrolytes from the suspension of PEG/PEO/NC. When NC suspension is dried, it shrinks and forms a compact structure. Therefore, when NC suspension is added to PEG/PEO, the open structure of NC accommodates part of dissolved PEG/PEO. Simultaneously, upon drying, PEG/PEO forms hydrogen bonds with NC [32] and starts solidifying inside the NC network as described in Fig. 1. Consequently, the polymer electrolyte has entrapped PEG/PEO
Conclusions
Quasi-solid polymer electrolytes with entrapped mobile liquid phase PEG/TEGDME and solid phase PEO in networked cellulose (NC) were successfully prepared. The addition of 10 wt% NC helps to retain 70 wt% liquid phase (PEG or TEGDME) in the PEO/LiClO4 electrolyte. Ternary polymer electrolyte has higher strength than the pristine PEO at temperatures in excess of 60 °C and ionic conductivity of 10−4 S cm−1 at room temperature; which is almost three orders of magnitude higher than that of pristine
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