Solid-state batteries can use metallic lithium for the anode and oxides or sulfides for the cathode, increasing energy density.

Challenges to widespread adoption include energy and power density, durability, material costs, sensitivity, and stability.

[6] Between 1831 and 1834, Michael Faraday discovered the solid electrolytes silver sulfide and lead(II) fluoride, which laid the foundation for solid-state ionics.

[7][8] By the late 1950s, several silver-conducting electrochemical systems employed solid electrolytes, at the price of low energy density and cell voltages, and high internal resistance.

[9][10] In 1967, the discovery of fast ionic conduction β - alumina for a broad class of ions (Li+, Na+, K+, Ag+, and Rb+) kick-started the development of solid-state electrochemical devices with increased energy density.

[11][10][12] Most immediately, molten sodium / β - alumina / sulfur cells were developed at Ford Motor Company in the US,[13] and NGK in Japan.

[15][16] In 2011, Kamaya et al. demonstrated the first solid-electrolyte, Li10GeP2S12 (LGPS), capable of achieving a bulk ionic conductivity in excess of liquid electrolyte counterparts at room temperature.

In 2013, researchers at the University of Colorado Boulder announced the development of a solid-state lithium battery, with a solid iron–sulfur composite cathode that promised higher energy.

[26] In 2018, Solid Power, spun off from the University of Colorado Boulder,[27] received $20 million in funding from Samsung and Hyundai to establish a manufacturing line that could produce copies of its all-solid-state, rechargeable lithium-metal battery prototype,[28] with a predicted 10 megawatt hours of capacity per year.

[45] In October 2023, Toyota announced a partnership with Idemitsu Kosan to produce solid-state batteries for their electric vehicles starting in 2028.

[49] In January 2024, Volkswagen announced that test results of a prototype solid-state battery retained 95% of its capacity after 1000 charges (equivalent to driving 500,000 km).

[53][54] Mainstream oxide solid electrolytes include Li1.5Al0.5Ge1.5(PO4)3 (LAGP), Li1.4Al0.4Ti1.6(PO4)3 (LATP), perovskite-type Li3xLa2/3-xTiO3 (LLTO), and garnet-type Li6.4La3Zr1.4Ta0.6O12 (LLZO) with metallic Li.

[61] One promising cathode material is Li–S, which (as part of a solid lithium anode/Li2S cell) has a theoretical specific capacity of 1,670 mAh g−1, "ten times larger than the effective value of LiCoO2".

This kind of solid-state battery demonstrated a high current density up to 5 mA cm−2, a wide range of working temperature (-20 °C and 80 °C), and areal capacity (for the anode) of up to 11 mAh cm−2 (2,890 mAh/g).

In particular a lithium mixed-metal chloride family of solid electrolytes, Li2InxSc0.666-xCl4 developed by Zhou et al., show high ionic conductivity (2.0 mS cm−1) over a wide range of composition.

[71] Solid state batteries are desirable due to their lighter weight and higher energy density compared to batteries with liquid electrolytes, which can potentially increase a vehicle's range, reduce cost, and reduce curb weight, all of which are major challenges with current electric vehicles.

[88] In June 2023, Japanese research group of the Graduate School of Engineering at Osaka Metropolitan University announced that they have succeeded in stabilizing the high-temperature phase of Li3PS4 (α-Li3PS4) at room temperature.

In these systems, dendrites sometimes grow as a result of micro-crack extension due to the presence of plating-induced pressure at the sodium / solid electrolyte interface.

Aluminum-containing electronic rectifying interphases between the solid-state electrolyte and the lithium metal anode have also been shown to be effective in preventing dendrite growth.

[102] This volume change leads to the formation of interparticle voids which worsens contact between the cathode and SSE particles, resulting in a significant loss of capacity due to the restriction in ion transport.

Ideally a solid-state battery would use a pure lithium metal anode due to its high energy capacity.

[108] For lithium metal to be used as an anode, great care must be taken to minimize the cell pressure to relatively low values on the order of its yield stress of 0.8 MPa.

[111][112][113] While these alloys do expand quite a bit when lithiated, often to a greater degree than lithium metal, they also possess improved mechanical properties allowing them to operate at pressures around 50 MPa.

[116] This energy density boost is especially beneficial for applications requiring longer-lasting and more compact batteries such as electric vehicles.

Because most solid electrolytes are nonflammable, solid-state batteries have a much lower fire risk and do not require as many safety systems, which can further increase energy density at the cell pack level.

[2][118][117] Studies have shown that heat generation during thermal runaway is only about 20-30% of what is observed in conventional batteries with liquid electrolytes.

[119] Solid electrolytes enable a broader range of operating temperatures and voltages, which is crucial for high performance applications.

[117][122][123] The solid electrolyte and lithium metal anode combination enables faster ion transfer, which can reduce charging times compared to lithium-ion batteries.

[129] The patent landscape for solid-state batteries has been evolving since 2010, reflecting the global race to develop safer and more efficient energy storage solutions.

Major corporations, particularly in the automotive and electronics sectors, have been actively filing patents to secure the Intellectual property of their innovations in this field.