Design of porous calcium phosphate based gel polymer electrolyte for Quasi-solid state sodium ion battery

Highlights

• A hydroxyapatite based gel polymer electrolyte is prepared by a simple solution casting technique.

• The gel polymer electrolyte has been used as the separator cum electrolyte for room temperature sodium ion battery.

• The gel polymer electrolyte exhibits a superior cell performance with 23% enhancement in capacity and durability.

Abstract

The design of a suitable separator is an effective approach to enhance the performance as well as the safety of a rechargeable battery. The conventional glass fiber separator has electrolyte leakage due to the random distribution of pores in the structure. The design of a gel polymer electrolyte with phosphorus containing compound is considered to be safer for the operation of a rechargeable sodium ion battery. Hence, we have developed a gel polymer using hydroxyapatite, a calcium phosphate-based compound in poly (vinylidene fluoride-hexafluoropropylene)-poly (butyl methacrylate) blend membrane by a simple solution casting technique. The developed membrane has an ionic conductivity of 1.086 × 10⁻³ S cm⁻¹ with an electrochemical stability of up to 4.9 V, good porosity and electrolyte uptake, thereby making it a promising electrolyte to be used in a rechargeable sodium ion battery. To demonstrate its feasibility, the electrochemical properties of Na₃V₂(PO₄)₃/C are investigated using the prepared gel polymer electrolyte. The sodium ion cell using gel polymer electrolyte exhibits a specific capacity of 97 mAh g⁻¹ at 4 C which is about 33.5% enhancement in specific capacity when compared to the cell with the conventional glass fiber membrane. This study illustrates the feasibility of using gel polymer electrolyte as a replacement to the existing glass fiber separator.

Introduction

Sodium ion batteries are considered as low cost energy storage devices and have attracted great interest in the field of large scale energy storage applications [[1], [2], [3], [4]]. However, the larger size of the sodium ions when compared to lithium ions poses problems such as huge volume expansion leading to poor cycle life. The higher redox potential of sodium (−3.01 V for Li/Li⁺, −2.7 V for Na/Na⁺ vs. Standard hydrogen electrode (SHE)) reduces the energy density of the battery. This has inhibited the commercialisation of sodium ion batteries [5,6]. Recently, research interest in the development of electrode materials leads to the emergence of high capacity sodium ion batteries [7,8]. Other than the development of electrode materials, electrolytes also play a major role in improving the performance, cycle life as well as safety of the battery [[9], [10], [11], [12]]. So far, carbonate based liquid electrolytes are used in sodium-based batteries due to their higher ionic conductivity (10⁻³ S cm⁻¹). But the use of liquid electrolyte soaked in glass fiber separator poses safety concerns due to the electrolyte leakage. The liquid electrolyte possesses low boiling point and flash point that lead to high volatility and inflammability [13]. So, the battery with liquid electrolytes causes fire hazard during overcharging and also leads to the formation of a gas due to the decomposition of electrolyte. Other than safety, reaction of liquid electrolyte with the electrode surface also leads to degradation in electrochemical performance. Due to the above mentioned problems, focusing on other types of electrolytes as well as electrolyte additives can lead to the development of a high performance and safer sodium ion battery [14]. Recently, research and development (R&D) is focused on the development of efficient electrolytes for room temperature rechargeable metal ion batteries which can be used as separator cum electrolytes [15,16]. In this scenario, solid electrolytes are considered as an alternative to the liquid electrolytes in rechargeable batteries due to their leakage free nature and safety [17]. But the major drawback lies in the poor migration of metal ions at room temperature as well as its mechanical stability. Other than solid electrolyte, the use of polymer membrane is beneficial for obtaining higher ionic conductivity at room temperature as well as mechanical stability. In addition, polymer electrolytes have lower electrolyte leakage, low flammability and possess good flexibility that can be applied in practical batteries and are called as quasi-solid-state electrolytes. The liquid electrolyte absorbed in pores and cavities facilitates the migration of ions and leads to good electrochemical performance. The solid phase helps to enhance the mechanical, thermal and interfacial stability of the electrolyte with the electrodes. Thus, the gel polymer electrolyte has overall good physical and electrochemical properties than its contemporaries (solid and liquid electrolytes) [18]. The most investigated polymer hosts that are used as separator are polyethylene (PE), polypropylene (PP), polyethylene oxide (PEO), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polyvinylidene fluoride (PVDF) and its copolymers [[19], [20], [21], [22]]. Among the various polymers, copolymer poly(vinylidene fluoride-hexafluoro propylene) (PVDF-HFP) is widely studied as a polymer matrix for gel polymer electrolytes due to its high ionic conductivity, mechanical and electrochemical stability. Also, the high dielectric constant and lower crystallinity help to absorb large amount of liquid electrolyte when compared to other polymers [[23], [24], [25], [26], [27], [28], [29]]. Further enhancement in thermal stability and ionic conductivity of polymer membranes can be achieved by adding inorganic fillers into the polymer matrix. Different types of inorganic materials like SiO₂, Al₂O₃ and TiO₂ have been exploited for enhancing the ionic conductivity of polymer membrane in lithium ion battery [[30], [31], [32], [33]]. The conductivity of sodium ions in solid electrolyte like β-Al₂O₃ occurs at a temperature greater than 300 °C. So, designing a polymer electrolyte membrane that can conducts sodium ions at room temperature will be crucial for the normal operation of the battery. Till date, PVDF-HFP polymer is used as a coating layer on the surface of glass fiber membrane to control the porosity of glass fiber membrane [34,35]. So, the development of a separator with controlled porous structure, high mechanical stability, ionic conductivity, low electrolyte leakage and electrochemical stability is crucial for the further advancement of a safer sodium ion battery. Hence, we have attempted to utilise an inorganic filler with sufficient mechanical stability and to prevent the formation of dendrites.

Herein, we have used hydroxyapatite as an inorganic filler incorporated in PVDF-HFP/PBMA blend as a gel polymer electrolyte for sodium metal battery. Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) is an environmentally friendly inorganic biomaterial which is the major component present in bones. This has higher mechanical stability and the presence of functional groups in hydroxyapatite helps to form a composite structure within the polymer network. This aids in enhancing the safety as well as life time of the battery. To utilise these benefits, hydroxyapatite incorporated in poly (vinylidene fluoride-co-hexafluoropropylene) [PVDF-HFP] blend poly (butyl methacrylate) [PBMA] (HAP-GPE) is used as the gel polymer electrolyte. The polymer blends-based gel polymer electrolytes have been prepared by a simple solution casting technique. The operation of sodium ion battery using the gel polymer electrolyte is illustrated using carbon coated sodium vanadium phosphate (Na₃V₂(PO₄)₃)/C as the cathode and sodium metal as the anode. The cell exhibits a specific capacity of 97 mAh g⁻¹ at a higher current density of 4 C. This is ~ 33% enhancement in specific capacity when compared to the cell with the conventional glass fiber separator. The better performance with HAP-GPE membrane at higher current density is attributed to the better interfacial stability, low electrolyte leakage and good ionic conductivity. The developed gel polymer electrolyte will be a better replacement to the existing liquid electrolyte using glass fiber membrane as the separator and can lead to further advancement in sodium ion battery technology.