Lithium battery

Lithium battery

Recently, Li-ion batteries (LIBs) have drawn significant attention due to the various applications to power portable electronics and electric vehicles (EV) since they have highest energy densities among commercialized secondary batteries. However, it is highly demanded to develop the next generation LIBs in terms of power density, rate capability, and electrochemical stability. In order to improve these performances, nanostructured energy materials as electrolyte and electrode of LIBs have been extensively studies.

Until now, polymer electrolytes are replacing carbonate based conventional liquid electrolyte owing to advanced safety and cycle life. However, they are insufficient to prevent lithium dendrite growth which induced cell short circuit. The realization of high mechanical strength from polymer electrolytes promote a wide variety of strategies to synthesize new polymer electrolytes comprising PEO chains and mechanically robust counterparts have been proposed. Herein, block copolymer electrolyte and composite polymer electrolyte have been studied as candidate materials possessing high mechanical strength and ionic conductivity simultaneously.

(1) Block copolymer electrolyte

Among solid electrolytes, block copolymer electrolyte in which hard polymers and PEO are covalently attached are attractive material having synergistic combination of phase separation behavior and ion transport property. The poly(styrene-b-ethylene oxide) (PS-b-PEO) block copolymer is one of representative block copolymer exhibiting strongly segregated PS and PEO phases in the nanometer scale, which allows the structural integrity to be maintained by effectively confining the lithium salt within the PEO domains. While such research efforts have opened new prospects in the era of high-conductivity solid-state electrolytes, defined as having a conductivity of ~10⁻⁵ S·cm⁻¹ at room temperature. In addition, coexisting non-conducting hard polymers influence lithium-ion diffusion across PEO domains due to interfacial mixing, which leads to lower lithium transport rates than predicted.

This has motivated researchers to study the relationship between the nanostructure and conductivity of lithium salt-doped PS-b-PEO block copolymers and a variety of observations concerning the morphology effects have been reported. However, the difficulties in correlating morphological effects on conductivity arises from the fact that the different molecular weights and compositions of block copolymers themselves also influence ion diffusion coefficients, which are tied to ion transport properties. Herein a method for the precise control over nanoscale morphology of polymer electrolytes was reported by introducing functional terminal group substitution.

A set of poly(styrene-b-ethylene oxide) (PS-b-PEO) block copolymers having dissimilar PEO end groups (−OH, −SO₃H, −SO₃Li, and −COOH) were successfully synthesized by means of anionic polymerization technique. These polymers were doped by bis(trifluoromethane) sulfonamide lithium salt (LiTFSI) with specific ratios. Neat and salt doped block copolymers exhibited various self-assembled morphologies including disordered, lamellar, gyroid, and hexagonal cylindrical phases.

Decreased conductivity was observed for PS-b-PEO tethered with acid groups when doped with lithium salts, which we ascribe to the slow segmental motion of acid group-terminated PEO chains. However, normalized conductivity of acid tethered samples are identical, or even exceeding hydroxyl terminated sample. Furthermore. organization of the PEO domains of PS-b-PEO with terminal acid units into self-assembled nanostructures having sharp interfaces proved beneficial for increasing ion transport efficiency by creating less tortuous ion conduction pathways. Our results suggest a unique methodology for controlling the morphology and ion transport properties of block copolymer electrolytes via attachment of a single end functional unit.

 

(2) Composite polymer electrolyte

Gel polymer electrolytes (GPEs) characterized by conducting matrix and cross-linked polymeric network generally have higher ionic conductivities than solid polymer electrolyte. However, GPEs still have low mechanical strength unable to block the dendrite growth. From this standpoint, the fabrication of composite gel polymer electrolytes (CPEs) by incorporating nanosized inorganic fillers into the GPEs is being explored.

Free-standing CPEs with ca. 80 mm thickness were prepared by mixing silica nanoparticles (SNPs), polyethylene glycol diacrylate (PEGDA), and lithium salt-doped tetraethylene glycol dimethyl ether (TEGDME), followed by UV curing for 3 min at room temperature. These CPEs impart improved mechanical properties without significant loss of ionic conductivity. During synthesized, it was found that while a simple UV curing procedure results in the homogeneous distribution of SNPs in cross-linked PEO matrices (referred to as hCPE), a thermal pre-treatment at 80 ℃ causes the CPE to maintain SNP density gradients in the thickness direction of the CPE (denser at air surfaces, attributed to low surface tension of the SNPs).

Synthesized CPEs were applied as lithium sulfur battery electrolyte offering the suppression of shuttling of lithium polysulfide intermediates between the electrodes, owing to the presence of physical barriers (high negative charge of −47 mV) when the polysulfides reach the liquid electrolytes. The lithium salt dissociation also promoted by incorporating high dielectric constant inorganic materials with a large surface area.

The exploitation of CPE enabled us to attain a stable discharge capacity of 970 mA·h·g⁻¹ after 100 cycles. The improved long-term cycle life is believed to be as a result of the strategic positioning of negatively charged SNPs near the cathode surfaces, in order to decelerate the dissolution of polysulfides into the CPE during battery cycling. To the best of our knowledge, this is the first work to unveil the important role played by the internal structure of the CPEs in determining the performance of Li–S cells.

 

(3) Cathode material

Elemental sulfur is one of the most attractive cathode active materials in lithium batteries because of its high theoretical specific capacity. Despite the positive aspect, lithium–sulfur batteries have suffered from severe capacity fading and limited rate capability. In this study, we develop advanced Li-S batteries by synthesizing a series of organic crystals based on trithiocyanuric acid having two functional groups of thiol and amine. Two crystal structures were obtained from different crystallization solvents; a unique square tube with hierarchical pores, i.e., micron-scale hole in the tube and abundant micropores at the surfaces, and a splice plate with only micropores at the surfaces. Our study demonstrated that the tubular crystals with hierarchical pores serve as a model system for confining sulfur and accommodating the volumetric expansion during cycling, as shown by long-life and excellent capacity retention. Under optimized conditions, our Li-S cell can deliver reversible discharge capacity of 1100 mAh/g at 0.2C with stable cycling performance over 300 cycles, corresponding to capacity retention of 90% from the initial discharge capacity. In addition to the good capacity retention property of our Li-S cell, improved rate performance was also determined. The Li-S cell based on the tubular organic crystal/sulfur composite cathode was found to deliver a reversible capacity of 900 mAh/g at 2C. After cycling at various rates, further cycling at a low rate of 0.2C brings it back to a reversible capacity of 1100 mAh/g. The markedly improved cycling performance of our Li-S cell is attributed to the increased Li⁺-ion transport of the organic crystal/sulfur composite cathode along amine moieties of crystal frameworks, as determined by Randles-Sevcik analysis.