Introduction- The Li-ion battery technology has been attested as an efficient electrochemical energy storage system for the electronic devices, electric vehicles, and power grid. Currently, a state-of-the-art Li-ion battery delivers a volumetric and gravimetric energy density of ~750 WhL-1 and 250 Whkg-1, respectively at a cost of ~$125kWh-1. Scientists are expecting to surpass its energy density at a lower cost in the near future. Having such a high power and energy density with good recyclability, Li-ion battery is a hope for building a carbon footprint-free planet by effectively storing the energy harvested from renewable sources (solar and wind). But a few recent incidents with catch fire of Tesla car and Samsung Galaxy Note7 explosion raise a concern over the safety of Li-ion battery. The catch fire or explosion of battery is due to mainly –
(1) the short circuit by lithium dendrite formation.
(2) polymer separator damage.
(3) thermal runaway.
The increasing demand for safe and high energy density battery for the application of electric vehicle requires innovative technologies. Solid state batteries (SSB), made of solid electrolytes instead of the liquid one, are one of the promising technologies to improve the energy densities. The advantages of solid electrolyte are:- (1) Replacing graphite (372 mAh g-1) anode with high-capacity Li (3860 mAhg-1)
(2) Mechanical integrity of solid electrolyte improves the stability by preventing the Li dendrite formation
(3) Mitigating unwanted chemical cross-over of the electrode materials and inhibits self-discharge
(4) Solid inorganic/ceramic electrolytes deliver a high Li-ion conductivity at room temperatures and could be safe even during high temperature operation. Though solid electrolyte could be able to improve the energy density, but their poor charge transfer kinetics is a major concern.
Solid electrolytes, determine the performances of the solid-state battery, can be classified into two types:
(i) Organic solid polymeric and
(ii) Inorganic/ceramic electrolytes.
Polymer-based electrolyte are excellent choice as the elastic nature accommodate the volume changes of the electrode. But their poor ionic conductivity at room temperature is a major concern for the practical application. Polyethylene oxide (PEO), Polyvinylidene fluoride (PVDF)-based electrolytes have been successfully demonstrated as a potential candidate for solid-state Li-battery electrolyte. On the other hand, the inorganic/ceramic electrolyte exhibit a higher ionic conductivity. But the poor contact at the electrode-electrolyte interface reflected in a high interfacial resistance which is the major concern for employing inorganic electrolytes in solid-state battery. A few numbers of promising ceramic structures such as Garnet, NASICON, Argyrodite have been evaluated as potential solid electrolyte.
The soft, porous, and flexible nature of the polymer electrolyte is still a concern for the effective blocking of the Li dendrite and unwanted chemical crossover. So, scientists are more interested in the development of solid-state battery using ceramic electrolyte. Though ceramic electrolyte could mitigate the dendrite formation, but their slow kinetics are the main reason of poor power density. High-power density is required to develop a fast-charging battery. Researchers are trying to address the major issues of solid-state battery mainly focusing on the improvement of energy and power density, safety, and cell architectures.
The solid-state battery supposed to provide a higher volumetric energy density than the conventional Li-ion battery because of the thin Li metal anode. However, the solid electrolyte has a higher mass than the liquid electrolyte, but it could support a high voltage (> 5V) cathode. Successful employment of a high voltage cathode could deliver a much higher specific energy (or gravimetric energy density) than conventional Li-ion battery. Albeit solid-state batteries are one of the fastest growing research interests, still it is at an early stage.
The present challenges and associated research are focused on the-
(1) interfacial Li+ transfer properties between cathode particle and the solid electrolyte.
(2) utilizing Li metal anode with high current densities to achieve a high-power density.
(3) stable operation of the solid-state battery with an optimized mechanical pressure to maintain low charge transfer resistance, prevent the formation of interfacial voids and dendrite formation, and cell failure.
Samsung, Toyota, and a few solid-state battery start-up companies such as Quantum Scape, Solid Power are trying to develop solid-state batteries. However, it is only about time that our electric vehicles, electronic devices, and power grid will be transformed to more safe system by introducing solid-state battery.
Further Reading:- 1. Bag, S.; Thangadurai, V. 2020, Electrolyte Development for Solid State Lithium Batteries. In: Skinner, S (Ed.), Energy storage and Conversion Materials, Royal Society of Chemistry, pp. 100-135
2. Janek, J; Zeier, W. G. 2016, A solid future for battery development, Nat. Energy (1) 1-4.
Dr. Bag, Postdoctoral Research Assistant, Department of Materials, University of Oxford