A solid-state battery is a battery technology that uses solid electrodes and a solid electrolyte, instead of the liquid or polymer gel electrolytes found in lithium-ion or lithium polymer batteries.
While solid electrolytes have been first discovered in the 19th century, several drawbacks, such as low energy densities, have prevented widespread application. Developments in the late 20th and early 21st century have caused renewed interest in solid-state battery technologies, especially in the context of electric vehicles, starting in the 2010s.
Materials proposed for use as solid electrolytes in solid-state batteries include ceramics (e.g., oxides, sulfides, phosphates), and solid polymers. Solid-state batteries have found use in pacemakers, RFID and wearable devices. They are potentially safer, with higher energy densities, but at a much higher cost. Challenges to widespread adoption include energy and power density, durability, material costs, sensitivity and stability.
By the late 1950s, several electrochemical systems employed solid electrolytes. They used a silver ion, but had some undesirable qualities, including low energy density and cell voltages, and high internal resistance. A new class of solid-state electrolyte, developed by the Oak Ridge National Laboratory, emerged in the 1990s, which was then used to make thin film lithium-ion batteries.
As technology advanced into the new millennium, researchers and companies operating in the automotive and transportation industries experienced revitalized interest in solid-state battery technologies. In 2011, Bolloré launched a fleet of their BlueCar model cars, first in cooperation with carsharing service Autolib, and later released to retail customers. The car was meant to showcase the company's diversity of electric-powered cells in application, and featured a 30 kWh lithium metal polymer (LMP) battery with a polymeric electrolyte, created by dissolving lithium salt in a co-polymer (polyoxyethylene).
In 2012, Toyota soon followed suit and began conducting experimental research into solid-state batteries for applications in the automotive industry in order to remain competitive in the EV market. At the same time, Volkswagen began partnering with small technology companies specializing in the technology.
A series of technological breakthroughs ensued. In 2013, researchers at the University of Colorado Boulder announced the development of a solid-state lithium battery, with a solid composite cathode based on an iron-sulfur chemistry, that promised higher energy capacity compared to already-existing SSBs. And then in 2014, researchers at Ann-Arbor, MI – based startup Sakti3 announced the construction of their own solid-state lithium-ion battery, claiming that it yielded even higher energy density for lower costs; in response, household appliance maker Dyson strategically acquired Sakti3 for $90 million.
In 2017, John Goodenough, the co-inventor of Li-ion batteries, unveiled a solid-state battery, using a glass electrolyte and an alkali-metal anode consisting of lithium, sodium or potassium. Later that year, Toyota announced the deepening of its decades-long partnership with Panasonic, including a collaboration on solid-state batteries. Due to its early intensive research and coordinated collaborations with other industry leaders, Toyota holds the most SSB-related patents. However, other car makers independently developing solid-state battery technologies quickly joined a growing list that includes BMW, Honda, Hyundai Motor Company and Nissan. Fisker Inc. has generated a proprietary in-house solution to SSB production for SSB-powered vehicle applications, but the aggressive initial target to bring the EMotion super-sedan to consumer markets for commercialization by 2020 was considerably delayed in favor of conventional EVs that could compete with the Tesla Model Y in 2020. Dyson announced and then abandoned a plan to build an SSB-powered electric car through its Sakti3 assets, though spokeswomen for Dyson reiterated the company's commitment to investing heavily into the technology. Other automotive-related companies, such as Spark plug maker NGK, have retrofitted their business expertise and models to cater to evolving demand for ceramic-based solid state batteries, in the face of perceived obsolescence of the conventional fossil-fuel paradigm.
Major developments continued to unfold into 2018, when Solid Power, spun off from the University of Colorado Boulder research team, received $20 million in funding from Samsung and Hyundai to establish a small manufacturing line that could produce copies of its all-solid-state, rechargeable lithium-metal battery prototype, with a predicted 10 megawatt hours of capacity per year. QuantumScape, another solid-state battery startup that spun out of a collegiate research group (in this case, Stanford University) drew attention that same year, when Volkswagen announced a $100 million investment into the team's research, becoming the largest stakeholder, joined by investor Bill Gates. With the goal to establish a joint production project for mass production of solid-state batteries, Volkswagen endowed QuantumScape with an additional $200 million in June 2020, and QuantumScape IPO'd on the NYSE on November 29th, 2020, as part of a merger with Kensington Capital Acquisition, to raise additional equity capital for the project.
Qing Tao started the first Chinese production line of solid-state batteries in 2018 as well, with the initial intention of supplying SSBs for “special equipment and high-end digital products”; however, the company has spoken with several car manufacturers with the intent to potentially expand into the automotive space.
In 2021, Toyota will unveil a prototype electric vehicle powered by solid-state batteries, with further plans to be the first automaker to sell an electric vehicle with solid state batteries. Solid Power anticipates "entering the formal automotive qualification process" in early 2022, and QuantumScape has "scheduled mass production to begin in the second half of 2024". Similarly, Fisker has claimed that its solid-state battery technology should be ready for "automotive-grade production" in 2023.
On Feb 26th, 2021, Fisker announced that based on unforeseen difficulties and perceived insurmountable obstacles, the company has decided to completely drop out of its SSB endeavor. 
Solid-state electrolytes candidate materials include ceramics such as lithium orthosilicate, glass and sulfides.The cathodes are lithium based. Variants include LiCoO2, LiNi1/3Co1/3Mn1/3O2, LiMn2O4, and LiNi0.8Co0.15Al0.05O2. The anodes vary more and are affected by the type of electrolyte. Examples include In, GexSi1−x, SnO–B2O3, SnS –P2S5, Li2FeS2, FeS, NiP2, and Li2SiS3.
One promising cathode material is Li-S, which (as part of a solid lithium anode/Li2S cell) has a theoretical specific capacity of 1670 mAh g−1, "ten times larger than the effective value of LiCoO2". Sulfur makes an unsuitable cathode in liquid electrolyte applications because it is soluble in most liquid electrolytes, dramatically decreasing the battery's lifetime. Sulfur is studied in solid state applications. Recently, a ceramic textile was developed that showed promise in a LI-S solid state battery. This textile facilitated ion transmission while also handling sulfur loading, although it did not reach the projected energy density. The result "with a 500-μm-thick electrolyte support and 63% utilization of electrolyte area" was "71 Wh/kg." while the projected energy density was 500 Wh/kg.
A Li/LiFePO4 battery shows promise as a solid state application for electric vehicles. A 2010 study presented this material as a safe alternative to rechargeable batteries for EV's that "surpass the USABC-DOE targets".
Hybrid and plug-in electric cars use a variety of battery technologies, including Li-ion, Nickel–metal hydride (NiMH), Lead–acid, and electric double-layer capacitor (or ultracapacitor), led by Li-ion.
Solid-state batteries are traditionally expensive to make and employ manufacturing processes thought to be difficult to scale, requiring expensive vacuum deposition equipment. As a result, costs become prohibitive in consumer-based applications. It was estimated in 2012 that, based on then-current technology, a 20 Ah solid-state battery cell would cost US$100,000, and a high-range electric car would require between 800 and 1,000 of such cells. Likewise, cost has impeded the adoption of solid-state batteries in other areas, such as smartphones.
Temperature and pressure sensitivityEdit
Solid lithium (Li) metal anodes in solid-state batteries are replacement candidates in lithium-ion batteries for higher energy densities, safety, and faster recharging times. Such anodes tend to suffer from the formation and the growth of Li dendrites.
Dendrites penetrate the separator between the anode and the cathode causing short circuits. This causes overheating, which may result in fires or explosions from thermal runaway. Li dendrites reduce coulombic efficiency.
Dendrites commonly form during electrodeposition during charge and discharge. Li ions combine with electrons at the anode surface as the battery charges - forming a layer of lithium metal. Ideally, the lithium deposition occurs evenly on the anode. However, if the growth is uneven, dendrites form.
Stable solid electrolyte interphase (SEI) was found to be the most effective strategy for inhibiting dendrite growth and increasing cycling performance. Solid-state electrolytes (SSEs) may prevent dendrite growth, although this remains speculative. A 2018 study identified nanoporous ceramic separators that block Li dendrite growth up to critical current densities.
They may avoid the use of dangerous or toxic materials found in commercial batteries, such as organic electrolytes.
Because most liquid electrolytes are flammable and solid electrolytes are nonflammable, solid-state batteries are believed to have lower risk of catching fire. Fewer safety systems are needed, further increasing energy density. Recent studies show that heat generation inside is only ~20-30% of conventional batteries with liquid electrolyte under thermal runaway.
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