A Brief Introduction to Solid-State Batteries
This story is contributed by Pranav Nagaveykar
Electric vehicles are taking the world by storm and unleashing the potential of lithium ion batteries, which were previously used primarily in mobile phones and laptops. While these batteries are now available on a broader scale for various energy storage applications, conventional lithium ion batteries will soon hit a plateau, and researchers are striving to break the conventional limits of energy density (volumetric and gravimetric), power, charging time, cycle life, temperature, safety and cost. Safety is one of the greatest concerns for electric vehicles, especially since improvements tend to come at the expense of power and charging time. The solid-state battery (SSB), often referred to as the holy grail of batteries, may have the potential to resolve these issues. Many companies, from startups, like Quantumscape, SES, Solid Power, Factorial Energy, and Cuberg, to the incumbent battery manufacturers and OEMs, like Toyota, Samsung SDI, Panasonic, and CATL, are working on different types of solid state batteries, as are many academic institutions and government labs. Some of these companies are claiming that the batteries will start rolling out in the second half of this decade. The market growth of solid-state batteries is projected to rise from $1.1 billion to $5.3 billion during this time, which would require at least a couple facilities with gigawatt-hour capacity to come online. To understand the appeal of solid-state batteries, it is important to first understand the challenges facing conventional batteries.
As we move towards higher energy densities in lithium batteries, new anodes, like lithium metal (specific capacity around 3500 mAh/g), are needed. The main strategy for enabling the use of pure lithium metal anodes is the development of solid-state electrolytes. Anodes used in current batteries are made primarily of graphite, which has good electrochemical stability, good lithium ion intercalation and decent specific capacity (around 350 mAh/g). In conventional batteries, a liquid electrolyte sandwiched between the cathode and anode serves as a medium for ion transfer, and a semipermeable separator is placed between the two electrodes to prevent direct contact. These liquid electrolytes have high ionic conductivity and maintain an excellent conductive interface with the electrodes. When lithium metal anodes are used, however, dendrite growth can occur. Dendrite formation is a phenomenon in which metallic microstructures can grow on the surface of the anode during the charging process if excess lithium ions are not quickly intercalated into the anode structure. This typically occurs at high charge current densities and can be exacerbated by factors like surface inhomogeneities and temperature. As these needle-like dendrites continue growing from the surface of the anode, they can puncture the separator and, over time, reach the cathode, triggering a short circuit in the battery. While this phenomenon can still occur to a lesser extent with graphite anodes, it is almost impossible to avoid when pure lithium metal anodes are paired with liquid electrolytes. As such, solid electrolytes are used to provide the mechanical strength needed to arrest the growth of dendrites. As an added benefit, solid electrolytes tend to be thermally stable, whereas organic liquid electrolytes are flammable and can act as a fuel source, posing an added fire hazard in the event of an accident. If dendrite formation can be eliminated or mitigated with solid state electrolytes, it may even be possible to increase the charging rate, which is currently limited by the rate of intercalation into the graphite anodes.
The general structure of solid state batteries is the same as that of conventional batteries, except that the liquid electrolyte and separator between the cathode and anode is replaced with a solid electrolyte, as shown in the figure below. The lithium ions move through the solid electrolyte by pathways between or through defects in the crystal lattice structure. Without the graphite intercalation anode, the cell is more compact, providing higher cell and module level energy density. With high limits of thermal operation for the solid electrolyte, solid state batteries may require less thermal management. In terms of the module and pack architecture, this allows them to be packed much more densely and saves space and cost on the system level, in addition to the density boost from removing the graphite anode. If dendrite growth can be suppressed, it may even be possible to fast charge these batteries at higher rates, with charge times of 15 minutes or less. Hence, solid state batteries have the potential to solve the three biggest challenges of EVs: safety, range anxiety and fast charging. While a commercially available solid state battery competitive with conventional lithium ion batteries has yet to be fully realized, there have been many developments in this exciting new technology.
Solid state electrolytes can be broadly classified as polymers (such as polyethers) and inorganic solids (such as oxides and sulfides). Properties of a good solid state electrolyte include high ionic conductivity, a good electrode-electrolyte interface, high thermal and electrochemical stability, the ability to suppress dendrites, high processability, and low manufacturing cost. Generally speaking, inorganic solid electrolytes have high ionic conductivity and mechanical strength but poor interfacial properties while organic polymers have good interfacial properties but poor ionic conductivity and mechanical strength.
Polymer-based solid electrolytes
Polymer-based solid electrolytes are physically flexible and have good wetting properties, which allow for a lower interfacial impedance, but have a low ionic conductivity (10−5 to 10−8 S cm−1). Common polymer electrolytes include PEO, PAN, and PVDF, which are mixed with lithium salts, like LTFSI, to allow for the conduction of Li ions. The greatest challenge with polymer solid electrolytes is that, being flexible and soft, they are not always able to prevent dendrite penetration. Another major issue is the low ionic conductivity. The mobility of Li ions is heavily influenced by the number of free Li ions and the mobility of the chain, with the quantity of free Li ions determined by the degree of dissociation of lithium salt in the polymer electrolyte. Conduction takes place via ion migration through polymer chains, and as such, the structure of the polymer affects the performance. There are two categories of mechanisms for ionic conductivity in polymers: Liquid-like mechanisms involve the movement of polymer segments and depend on factors like diffusion, viscosity and the rotation of monomers. Solid-like mechanisms are based on ion jumps over an energy barrier determined by electrostatic interactions and elastic forces. The ionic conductivity for polymer electrolytes is higher at higher temperatures, and only gel polymer electrolytes are currently capable of providing sufficient conductivity at room temperature, enabling their use in some commercial batteries. Dry polymer electrolytes are unable to take advantage of liquid-like mechanisms, so ionic conductivity is determined by the rate of ion jumps and limited by the low concentration or low mobility of the ions
Inorganic solid electrolytes
The ionic conductivity of Li ions in crystalline ceramic electrolytes is determined by the concentration and location of defects. The Li ion diffusion mechanism is governed by crystal lattice point defects due to dislocated and missing ions, known as Frenkel and Schottky point defects respectively. The collective motion of individual Li ions hopping from one vacancy to another governs macroscopic transport. The crystal defects are temperature dependent and transport is better at higher temperatures, although, given the small ionic radius of Li, they can exhibit significant ionic conductivities at low temperatures as well, giving them a wide operational temperature. Common types of ceramic solid electrolytes include garnets (lithium lanthanum zirconium oxide, or LLZO), sulfides (LiS-PS, LiGeS), perovskites (lithium lanthanum titanate, or LLTO), lithium superionic conductors (LISICONs) and lithium phosphorus oxynitride (LiPON). Inorganic solid electrolytes, however, have an interface problem. Solid surfaces have microscopic and nanoscopic irregularities and, when two surfaces come into contact, these tiny irregularities make it difficult to maintain a conformal interface. While liquid electrolytes are able to seep into the gaps, this is not the case for solids. A poor solid-solid interface can serve as a barrier to the ion diffusion process, increasing the impedance and affecting the electrochemical performance of the cell.
There are many different approaches for resolving the interface problem, but there is not yet a clear winner in the field. One approach is through manufacturing and processing techniques like dry polishing, wet polishing, and heat treatment. Others have tried to increase the wettability of solid electrolytes, with the addition of a lithophilic layer at the interface or even by adding liquid electrolyte to the solid electrolyte. Using molten liquid lithium metal has also been observed to reduce the surface resistance by an order of magnitude. On the system level, the application of high pressure, typically in the low MPa, has been observed to reduce the contact resistance between the electrodes and electrolyte. Oxide-based solid-state electrolytes need to be processed at very high temperatures for annealing. Sulfide electrolytes are softer than oxide electrolytes and have ionic conductivities comparable to liquid organic electrolytes, but they are extremely sensitive to moisture and unstable in the ambient atmosphere, necessitating stringent preparation and use conditions. They also have poor interfacial contact with electrodes. In addition, the cathode-sulfur electrolyte reaction leads to the formation of cobalt and lithium phosphates, sulfates, and sulfides as part of the SEI. This degradation of the sulfide solid electrolyte/cathode interface leads to a large interfacial resistance that appears upon cell fabrication and increases with cell cycling.
Many companies are working to develop solid-state technology. Quantumscape, a startup spun out of Stanford in 2010, is working on an oxide-based ceramic electrolyte with an organic liquid catholyte (an electrolyte used specifically on the cathode side) which is expected to provide an energy density of over 400 Wh/kg. With Quantumscape’s anode free configuration, the Li metal anode forms in-situ during the first charge process. The company has a joint venture with Volkswagen and is targeting 2025 for the launch of its battery pack. Solid Power, which spun out from Colorado Boulder in 2011 with funding from DARPA (Defense Advanced Research Projects Agency), is working on a sulfide-based solid electrolyte paired with a silicon anode and an NMC cathode. Solid Power is currently working on 20 Ah solid-state lithium-ion cells with an energy density of 320 Wh/kg. SES, a startup spun out from the Massachusetts Institute of Technology (MIT) in 2012, is working on a proprietary solid state battery, which also has a liquid catholyte coupled with a solid electrolyte. In 2021, SES demonstrated a solid state battery, Apollo, with 107 Ah capacity and 417 Wh/kg energy density. Toyota has filed 203 solid state battery patents in the United States through 2021, the most of any company. Samsung SDI, one of the world’s top lithium-ion battery producers, has begun construction on its solid-state battery pilot line.
Although there have been many developments at the lab scale, manufacturing a solid electrolyte with low impedance, good mechanical strength and high ionic conductivity at low cost remains a major challenge in the field. Nevertheless, with many companies from small startups to large manufacturers targeting the second half of this decade, the stage is set for a solid-state revolution in the battery industry.
Pranav Nagaveykar is currently working on the synthesis of solid state batteries at the Institute of Molecular Chemistry and Materials of Orsay (ICMMO), Université Paris Saclay. He completed his master’s degree in Heat & Power Engineering at Savitribai Phule Pune University and has 3 years of experience on battery design and testing at Caterpillar Inc, Ford Motor Company and TATA Motors Ltd. His hobbies include traveling, football, gaming and reading, and he also enjoys teaching students during his free time.