# Solid-state battery

A solid-state battery (SSB) is an electrical battery that uses a solid electrolyte (solectro) to conduct ions between the electrodes, instead of the liquid or gel polymer electrolytes found in conventional batteries. Solid-state batteries theoretically offer much higher energy density than the typical lithium-ion or lithium polymer batteries.
While solid electrolytes were first discovered in the 19th century, several problems prevented widespread application. Developments in the late 20th and early 21st century generated renewed interest in the technology, especially in the context of electric vehicles.
Solid-state batteries can use metallic lithium for the anode and oxides or sulfides for the cathode, increasing energy density. The solid electrolyte acts as an ideal separator that allows only lithium ions to pass through. For that reason, solid-state batteries can potentially solve many problems of currently used liquid electrolyte Li-ion batteries, such as flammability, limited voltage, unstable solid-electrolyte interface formation, poor cycling performance, and strength.
Materials proposed for use as electrolytes include ceramics (e.g., oxides, sulfides, phosphates), and solid polymers. Solid-state batteries are found in pacemakers, and in RFID and wearable devices. Solid-state batteries are potentially safer, with higher energy densities. Challenges to widespread adoption include energy and power density, durability, material costs, sensitivity, and stability.

## History
### Origin

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

### 1900s-2009

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. 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. Most immediately, molten sodium / β - alumina / sulfur cells were developed at Ford Motor Company in the US, and NGK in Japan. This excitement manifested in the discovery of new systems in both organics, i.e. poly(ethylene) oxide (PEO), and inorganics such as NASICON. However, many of these systems required operation at elevated temperatures, and/or were expensive to produce, limiting commercial deployment. A new class of solid-state electrolyte developed by Oak Ridge National Laboratory, lithium–phosphorus oxynitride (LiPON), emerged in the 1990s. LiPON was successfully used to make thin-film lithium-ion batteries, although applications were limited due to the cost associated with deposition of the thin-film electrolyte, along with the small capacities that could be accessed using the thin-film format.

### 2010-2019

In 2011, Kamaya et al. demonstrated the first solid-electrolyte, Li$_{10}$GeP$_2$S$_{12}$ (LGPS), capable of achieving a bulk ionic conductivity in excess of liquid electrolyte counterparts at room temperature. With this, bulk solid-ion conductors could finally compete technologically with Li-ion counterparts.
Researchers and companies in the transportation industry revitalized interest in solid-state battery technologies. In 2011, Bolloré launched a fleet of their BlueCar model cars. The demonstration was meant to showcase the company's cells, and featured a 30 kWh lithium metal polymer (LMP) battery with a polymeric electrolyte, created by dissolving lithium salt in polyoxyethylene co-polymer.
In 2012, Toyota began conducting research into automotive applications. At the same time, Volkswagen began partnering with small technology companies specializing in the technology.
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.
In 2017, John Goodenough, the co-inventor of Li-ion batteries, unveiled a solid-state glass battery, using a glass electrolyte and an alkali-metal anode consisting of lithium, sodium or potassium. Later that year, Toyota extended its decades-long partnership with Panasonic to include collaboration on solid-state batteries. As of 2019 Toyota held the most SSB-related patents. They were followed by BMW, Honda, Hyundai Motor Company., and Nissan.
In 2018, Solid Power, spun off from the University of Colorado Boulder, 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, with a predicted 10 megawatt hours of capacity per year.
Qing Tao started the first Chinese production line of solid-state batteries in 2018 to supply SSBs for "special equipment and high-end digital products".

### 2020-present

QuantumScape is a solid-state battery startup that spun out of Stanford University. It went public on the NYSE on November 29, 2020, as part of a SPAC merger with Kensington Capital. In 2022 the company introduced its 24-layer A0 prototype cells. In Q1 2023, it introduced QSE-5, a 5 amp-hour lithium metal cell. Volkswagen's PowerCo stated that the A0 prototype had met the announced performance metrics. QuantumScape's FlexFrame design combines prismatic and pouch cell designs to accommodate the expansion and contraction of its cells during cycling.
In July 2021, Murata Manufacturing announced that it would begin mass production, targeting manufacturers of earphones and other wearables. Cell capacity is up to 25 mAh at 3.8 V, making it suitable for small mobile devices such as earbuds, but not for electric vehicles. Lithium-ion cells used in electric vehicles typically offer 2,000 to 5,000 mAh at a similar voltage: an EV would need at least 100 times as many of the Murata cells to provide equivalent power.
Ford Motor Company and BMW funded the startup Solid Power with $130 million, and as of 2022 the company had raised $540 million.
In September 2021, Toyota announced their plan to use a solid-state battery, starting with hybrid models in 2025.
In February 2021, Hitachi Zosen announced demonstration experiments on the International Space Station. The Cygnus No. 17, launched on February 19, 2022, confirmed that all-solid-state batteries would be tested on the ISS.
In January 2022, ProLogium signed a technical cooperation agreement with Mercedes-Benz. The investment will be used for solid-state battery development and production preparation.
In early 2022, Swiss Clean Battery (SCB) announced plans to open the world's first factory for sustainable solid-state batteries in Frauenfeld by 2024 with an initial annual production of 1.2 GWh.
In July 2022, Svolt announced the production of a 20 Ah electric battery with an energy density of 350-400 Wh/kg.
In June 2023, Maxell Corporation began mass production of large-capacity solid-state batteries. This battery has a long life and heat resistance. Production of 200 mAh cylindrical solid-state batteries was to begin in January 2024. Size: diameter 23 mm/height 27 mm.
In September 2023, Panasonic unveiled a solid-state battery for drones. It can be charged from 10% to 80% in 3 minutes and lasts for 10,000 to 100,000 cycles at 25 °C. The battery was expected to be available in the late 2020s.
In October 2023, Toyota announced a partnership with Idemitsu Kosan to produce solid-state batteries for their electric vehicles starting in 2028.
In October 2023 Factorial Energy opened a battery manufacturing facility in Methuen, Massachusetts, and began shipping 100 Ah A-samples to automotive partners totaling over 1,000 A-sample cells to Mercedes-Benz. Its technology uses a lithium-metal anode, quasi-solid electrolyte and high-capacity cathode. Its energy density is 391 Wh/kg.
In November 2023, Guangzhou Automobile Group announced that it would adopt solid-state batteries in 2026. The company also revealed that its battery has achieved 400 Wh/kg. Mass production was scheduled to begin in 2025.
On December 28, 2023, Hyundai published its patent for an "all-solid-state battery system provided with pressurizing device". The cell is a solid-state battery that maintains constant pressure regardless of charging and discharging rates. The system includes an iso-temperature element.
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). It also passed other performance tests.
In April 2024, Factorial signed a memorandum of understanding with LG Chem. In June it sent its first 106 Ah B-samples to Mercedes-Benz for testing.

## Materials

Candidate materials for solid-state electrolytes (SSEs) include ceramics such as lithium orthosilicate, glass, sulfides and RbAg$_4$I$_5$. Mainstream oxide solid electrolytes include Li$_{1.5}$Al$_{0.5}$Ge$_{1.5}$(PO$_4$)$_3$ (LAGP), Li$_{1.4}$Al$_{0.4}$Ti$_{1.6}$(PO$_4$)$_3$ (LATP), perovskite-type Li$_{3x}$La$_{2/3-x}$TiO$_3$ (LLTO), and garnet-type Li$_{6.4}$La$_3$Zr$_{1.4}$Ta$_{0.6}$O$_{12}$ (LLZO) with metallic Li. The thermal stability versus Li of the four SSEs was in order of LAGP < LATP < LLTO < LLZO. Chloride superionic conductors have been proposed as another promising solid electrolyte. They are ionic conductive as well as deformable sulfides, but at the same time not troubled by the poor oxidation stability of sulfides. Other than that, their cost is considered lower than oxide and sulfide SSEs. The present chloride solid electrolyte systems can be divided into two types: Li$_3$MCl$_6$ and Li$_2$M$_{2/3}$Cl$_4$. M Elements include Y, Tb-Lu, Sc, and In. The cathodes are lithium-based. Variants include LiCoO$_2$, LiNi$_{1/3}$Co$_{1/3}$Mn$_{1/3}$O$_2$, LiMn$_2$O$_4$, and LiNi$_{0.8}$Co$_{0.15}$Al$_{0.05}$O$_2$. The anodes vary more and are affected by the type of electrolyte. Examples include In, Si, Ge$_x$Si$_{1−x}$, SnO–B$_2$O$_3$, SnS –P$_2$S$_5$, Li$_2$FeS$_2$, FeS, NiP$_2$, and Li$_2$SiS$_3$.
Lithium-ceramic batteries demonstrate potential improvements with the integration of single wall carbon nanotubes (SWCNTs). SWCNTs build durable, long-range conductive pathways between electrode particles, effectively reducing electrode resistance and enhancing energy density.
One promising cathode material is Li–S, which (as part of a solid lithium anode/Li$_2$S cell) has a theoretical specific capacity of 1,670 mAh/g, "ten times larger than the effective value of LiCoO$_2$". 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.
Another encouraging cathode is NCM662 (LiNi$_{0.6}$Co$_{0.2}$Mn$_{0.2}$O$_2$), especially when coated with NiCo$_2$S$_4$ in a resonant acoustic mixing process. This creates a material with a capacity retention of 60.6%, with minimal side reactions.
Li-O$_2$ also have high theoretical capacity. The main issue with these devices is that the anode must be sealed from ambient atmosphere, while the cathode must be in contact with it.
A Li/LiFePO$_4$ 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".
A cell with a pure silicon μSi||SSE||NCM811 anode was assembled by Darren H.S Tan et al. using μSi anode (purity of 99.9 wt %), solid-state electrolyte (SSE) and lithium–nickel–cobalt–manganese oxide (NCM811) cathode. 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). At the same time, after 500 cycles under 5 mA cm$^{−2}$, the batteries still provide 80% of capacity retention, which is the best performance of μSi all solid-state battery reported so far.
Chloride solid electrolytes also show promise over conventional oxide solid electrolytes owing to chloride solid electrolytes having theoretically higher ionic conductivity and better formability. In addition chloride solid electrolyte's exceptionally high oxidation stability and high ductility add to its performance. In particular a lithium mixed-metal chloride family of solid electrolytes, Li$_2$In$_x$Sc$_{0.666-x}$Cl$_4$ developed by Zhou et al., show high ionic conductivity (2.0 mS cm$^{−1}$) over a wide range of composition. This is owing to the chloride solid electrolyte being able to be used in conjunction with bare cathode active materials as opposed to coated cathode active materials and its low electronic conductivity. Alternative cheaper chloride solid electrolyte compositions with lower, but still impressive, ionic conductivity can be found with an Li$_2$ZrCl$_6$ solid electrolyte. This particular chloride solid electrolyte maintains a high room temperature ionic conductivity (0.81 mS cm$^{−1}$), deformability, and has a high humidity tolerance.
Perovskite-type
Meanwhile, Perovskite materials also have great potential for application in solid-state batteries. In order to improve the low efficiency and high pollution of traditional fossil-based energy sources, more and more researchers have put forward the idea of solid-state batteries, which will have a longer lifespan and higher efficiency. However, solid-state batteries still have a lot of safety concerns and drawbacks, so researchers are using a lot of new materials to solve this problem. One such material is perovskite materials.
Perovskite materials have excellent ionic conductivity, excellent charge storage capacity and good electrochemical activity, so this material has a very great potential for application in the field of electrochemical energy storage as well as energy conversion. This material is used in many new energy batteries, such as solid state batteries and solar cells. Its general formula is ABX3. In ABX3, the B ion is surrounded by the X ion octahedron and the A ion is located in the center of the cube. Transition metal perovskite fluoride as a perovskite-type electrode material, has high voltage window, specific capacity and stability, moreover, the structure of transition metal perovskite fluoride facilitates ion migration and its general pseudocapacitance-controlled kinetic features make it has a fast charge transport rate so this material has good electrochemical properties. Thus, more and more researchers focus on this material. Shan et al.'s research not only shows that lithium ions can be inserted into the lattice of perovskite oxides, but also demonstrates that perovskite oxides, with its high ionic conductivity, can be used as an electrode material. For the transition metal perovskite fluoride, it has a fast charge transport rate, high energy density and high stability because it has metal-fluorine bond and the strong electronegativity of fluorine. Jiao et al. used solvothermal method to make the perovskite-type fluoride with a hollow micrometer spherical structure, after testing, this material shows a good retention rate like it has capacity of 142 mAh/g after 1000 cycles at 0.1 A/g.

## Uses

Solid-state batteries are potentially useful in pacemakers, RFIDs, wearable devices, and electric vehicles.

### Electric vehicles

Hybrid and plug-in electric vehicles have used a variety of battery technologies, including lead–acid, nickel–metal hydride (NiMH), lithium ion (Li-ion) and electric double-layer capacitor (or ultracapacitor), with Li-ion batteries dominating the market due to their superior energy density. 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.
Honda stated in 2022 that it planned to start operation of a demonstration line for the production of all-solid-state batteries in early 2024, and Nissan announced that, by FY2028, it aims to launch an electric vehicle with all-solid-state batteries that are to be developed in-house.
In June 2023, Toyota updated its strategy for battery electric vehicles, announcing that it will not use commercial solid-state batteries until at least 2027.
In January 2022, Mercedes-Benz invested significantly in ProLogium to codevelop next gen ceramic solid-state battery cell. The company also collaborates on solid-state technology and plans to construct eight gigafactories with partners. By December 2023, Mercedes-Benz had invested in US-based Factorial Energy, advancing its solid-state battery initiatives.

### Wearables

The characteristics of high energy density and keeping high performance even in harsh environments are expected in realization of new wearable devices that are smaller and more reliable than ever.

### Equipment in space

In March 2021, industrial manufacturer Hitachi Zosen Corporation announced a solid-state battery they claimed has one of the highest capacities in the industry and has a wider operating temperature range, potentially suitable for harsh environments like space. A test mission was launched in February 2022, and in August, Japan Aerospace Exploration Agency (JAXA) announced the solid-state batteries had properly operated in space, powering camera equipment in the Japanese Experiment Module Kibō on the International Space Station (ISS).

### Drones

Solid-state batteries being lighter weight and more powerful than traditional lithium-ion batteries it is reasonable that commercial drones would benefit from them. Vayu Aerospace, a drone manufacturer and designer, noted an increased flight time after they incorporated them into their G1 long flight drone. Another advantage of drones is that all solid battery can be charged quickly. In September 2023, Panasonic announced a prototype all-solid-state battery that can be charged from 10% to 80% in 3 minutes.

### Industrial machinery

All-solid-state batteries have long lifespans and excellent heat resistance. Therefore, it is expected to be used in harsh environments. Production of Maxell's all-solid-state batteries for use in industrial machinery has already begun.

### Portable solar generators

In 2023, Yoshino become the first producer of solid-state portable solar generators, 2.5 times higher energy density, double rated and surge AC output wattage of non-solid state lithium (NMC, LFP) generators.

## Challenges
### Cost

Thin-film solid-state batteries are expensive to make and employ manufacturing processes thought to be difficult to scale, requiring expensive vacuum deposition equipment. As a result, costs for thin-film solid-state batteries 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 thin-film solid-state batteries in other areas, such as smartphones.

### Temperature and pressure sensitivity

Low temperature operations may be challenging. Solid-state batteries historically have had poor performance.
Solid-state batteries with ceramic electrolytes require high pressure to maintain contact with the electrodes. Solid-state batteries with ceramic separators may break from mechanical stress.
In November 2022, Japanese research group, consisting of Kyoto University, Tottori University and Sumitomo Chemical, announced that they have managed to operate solid-state batteries stably without applying pressure with 230 Wh/kg capacity by using copolymerized new materials for electrolyte.
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 Li$_3$PS$_4$ (α-Li$_3$PS$_4$) at room temperature. This was accomplished via rapid heating to crystallize the Li$_3$PS$_4$ glass.

### Interfacial resistance

High interfacial resistance between a cathode and solid electrolyte has been a long-standing problem for all-solid-state batteries.

### Interfacial instability

The interfacial instability of the electrode-electrolyte has always been a serious problem in solid-state batteries. After solid-state electrolyte contacts with the electrode, the chemical and/or electrochemical side reactions at the interface usually produce a passivated interface, which impedes the diffusion of Li$^+$ across the electrode-SSE interface. Upon high-voltage cycling, some SSEs may undergo oxidative degradation.

### Dendrites

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, non-uniform metal growths which penetrate the electrolyte leading to electrical short circuits. This shorting leads to energy discharge, overheating, and sometimes fires or explosions due to thermal runaway. Li dendrites reduce coulombic efficiency.
The exact mechanisms of dendrite growth remain a subject of research. Studies of metal dendrite growth in solid electrolytes began with research of molten sodium / sodium - β - alumina / sulfur cells at elevated 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. However, dendrite growth may also occur due to chemical degradation of the solid electrolyte.
In Li-ion solid electrolytes apparently stable to Li metal, as visualized and measured using photoelasticity experiments, dendrites propagate primarily due to pressure build up at the electrode / solid electrolyte interface, leading to crack extension. Meanwhile, for solid electrolytes which are chemically unstable against their respective metal, interphase growth and eventual cracking often prevents dendrites from forming.
Dendrite growth in solid-state Li-ion cells can be mitigated by operating the cells at elevated temperature, or by using residual stresses to fracture-toughen electrolytes, thereby deflecting dendrites and delaying dendrite induced short-circuiting. 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.

### Mechanical failure

A common failure mechanism in solid-state batteries is mechanical failure through volume changes in the anode and cathode during charge and discharge due to the addition and removal of Li-ions from the host structures.

#### Cathode

Cathodes will typically consist of active cathode particles mixed with SSE particles to assist with ion conduction. As the battery charges/discharges, the cathode particles change in volume typically on the order of a few percent. 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.
One proposed solution to this issue is to take advantage of the anisotropy of volume change in the cathode particles. As many cathode materials experience volume changes only along certain crystallographic directions, if the secondary cathode particles are grown along a crystallographic direction which does not expand greatly with charge/discharge, then the change in volume of the particles can be minimized. Another proposed solution is to mix different cathode materials which have opposite expansion trends in the proper ratio such that the net volume change of the cathode is zero. For instance, LiCoO$_2$ (LCO) and LiNi$_{0.9}$Mn$_{0.05}$Co$_{0.05}$O$_2$ (NMC) are two well-known cathode materials for Li-ion batteries. LCO has been shown to undergo volume expansion when discharged while NMC has been shown to undergo volume contraction when discharged. Thus, a composite cathode of LCO and NMC at the correct ratio could undergo minimal volume change under discharge as the contraction of NMC is compensated by the expansion of LCO.

#### Anode

Ideally a solid-state battery would use a pure lithium metal anode due to its high energy capacity. However, lithium undergoes a large increase of volume during charge at around 5 μm per 1 mAh/cm$^2$ of plated Li. For electrolytes with a porous microstructure, this expansion leads to an increase in pressure which can lead to creep of Li metal through the electrolyte pores and short of the cell. Lithium metal has a relatively low melting point of 453K and a low activation energy for self-diffusion of 50 kJ/mol, indicating its high propensity to significantly creep at room temperature. It has been shown that at room temperature lithium undergoes power-law creep where the temperature is high enough relative to the melting point that dislocations in the metal can climb out of their glide plane to avoid obstacles. The creep stress under power-law creep is given by:
$\sigma _{creep}=\left({\frac {\dot {\varepsilon }}{A_{c}}}\right)^{1/m}\exp {\left({\frac {Q_{c}}{mRT}}\right)}$
Where $R$ is the gas constant, $T$ is temperature, ${\dot {\varepsilon }}$ is the uniaxial strain rate, $\sigma _{creep}$ is the creep stress, and for lithium metal $m=6.6$, $Q_{c}=37\,\mathrm {kJ} \cdot \mathrm {mol} ^{-1}$, $A_{c}^{-1/m}=3\times 10^{5}\,\mathrm {Pa} \cdot \mathrm {s} ^{-1}$.
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. The normal operating cell pressure for lithium metal anode is anywhere from 1-7 MPa. Some possible strategies to minimize stress on the lithium metal are to use cells with springs of a chosen spring constant or controlled pressurization of the entire cell. Another strategy may be to sacrifice some energy capacity and use a lithium metal alloy anode which typically has a higher melting temperature than pure lithium metal, resulting in a lower propensity to creep. 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. This higher cell pressure also has the added benefit of possibly mitigating void formation in the cathode.

## Advantages
### Improved energy density

Solid state batteries offer the potential for significantly higher energy densities compared to traditional lithium-ion batteries. This is largely due to the use of lithium metal anodes, which have a much higher charge capacity than the graphite anodes used in lithium-ion batteries. At a cell level, lithium-ion energy densities are generally below 300Wh/kg while solid-state battery energy densities are able to exceed 350 Wh/kg. This energy density boost is especially beneficial for applications requiring longer-lasting and more compact batteries such as electric vehicles.

### Increase of safety and thermal stability

One significant advantage of solid-state batteries is their improved safety profile. Solid electrolytes greatly reduce the risk of thermal runaway—a primary cause of battery fires. 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. Studies have shown that heat generation during thermal runaway is only about 20-30% of what is observed in conventional batteries with liquid electrolytes.

### Expanded temperature and voltage operating ranges

Solid electrolytes enable a broader range of operating temperatures and voltages, which is crucial for high performance applications. SSBs can operate at temperatures above 60 °C, where traditional are generally only able to operate from -20 to 60 °C.
Solid state batteries also support high-voltage cathode chemistries such as lithium nickel manganese oxide, lithium nickel phosphate, and lithium cobalt phosphate. This allows voltages to potentially exceed 5 V (vs. a Li/Li$^+$ reference electrode) while traditional cathode chemistries in lithium-ion batteries are unable to exceed 4.5V (vs. a Li/Li$^+$ reference electrode).

### Faster charging and improved space efficiency

The solid electrolyte and lithium metal anode combination enables faster ion transfer, which can reduce charging times compared to lithium-ion batteries. Furthermore, bipolar stacking of cells can be incorporated, allowing for reduced cell size and more compact battery packs. This allows for improved overall energy efficiency and enables design flexibility for various applications.

## Thin-film solid-state batteries
### Background

The earliest thin-film solid-state batteries is found by Keiichi Kanehori in 1986, which is based on the Li electrolyte. However, at that time, the technology was insufficient to power larger electronic devices so it was not fully developed. During recent years, there has been much research in the field. Garbayo demonstrated that "polyamorphism" exists besides crystalline states for thin-film Li-garnet solid-state batteries in 2018, Moran demonstrated that ample can manufacture ceramic films with the desired size range of 1–20 μm in 2021.

### Structure

Anode materials: Li is favored because of its storage properties, alloys of Al, Si and Sn are also suitable as anodes.
Cathode materials: require having light weight, good cyclical capacity and high energy density. Usually include LiCoO2, LiFePO4, TiS2, V2O5and LiMnO2.

### Preparation techniques

Some methods are listed below.
* Physical methods:
** Magnetron sputtering (MS) is one of the most widely used processes for thin-film manufacturing, which is based on physical vapor deposition.
** Ion-beam deposition (IBD) is similar to the first method, however, bias is not applied and plasma doesn't occur between the target and the substrate in this process.
** Pulsed laser deposition (PLD), laser used in this method has a high power pulses up to about 10$^8$ W cm$^{−2}$.
** Vacuum evaporation (VE) is a method to prepare alpha-Si thin films. During this process, Si evaporates and deposits on a metallic substrate.
* Chemical methods:
** Electrodeposition (ED) is for manufacturing Si films, which is convenient and economically viable technique.
** Chemical vapor deposition (CVD) is a deposition technique allowing to make thin films with a high quality and purity.
** Glow discharge plasma deposition (GDPD) is a mixed physicochemical process. In this process, synthesis temperature has been increased to decrease the extra hydrogen content in the films.

### Development of thin-film system

* Lithium–oxygen and nitrogen-based polymer thin-film electrolytes has got fully used in solid-state batteries.
* Non-Li based thin-film solid-state batteries have been studied, such as Ag-doped germanium chalcogenide thin-film solid-state electrolyte system. Barium-doped thin-film system has also been studied, which thickness can be 2 μm at least. In addition, Ni can also be a component in thin film.
* There are also other methods to fabricate the electrolytes for thin-film solid-state batteries, which are 1.electrostatic-spray deposition technique, 2. DSM-Soulfill process and 3. Using MoO3 nanobelts to improve the performance of lithium-based thin-film solid-state batteries.

### Advantages

* Compared with other batteries, the thin-film batteries have both high gravimetric as well as volumetric energy densities. These are important indicators to measure battery performance of energy stored.
* In addition to high energy density, thin-film solid-state batteries have long lifetime, outstanding flexibility and low weight. These properties make thin-film solid-state batteries suitable for use in various fields such as electric vehicles, military facilities and medical devices.

### Challenges

* Its performance and efficiency are constrained by the nature of its geometry. The current drawn from a thin-film battery largely depends on the geometry and interface contacts of the electrolyte/cathode and the electrolyte/anode interfaces
* Low thickness of the electrolyte and the interfacial resistance at the electrode and electrolyte interface affect the output and integration of thin-film systems.
* During the charging-discharging process, considerable change of volumetric makes the loss of material.

## Makers

* CATL
* Cymbet
* Ilika
* Ionic Materials
* LG
* Panasonic
* Penghui Energy
* Sakti3
* Samsung
* Solid Power
* SolidEnergetics
* QuantumScape

## Innovation and IP protection

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. Toyota is the top company in terms of granted patent rights, followed by LG, Samsung, Murata and Panasonic. Japanese automaker Toyota was granted 8274 solid-battery patents between 2020 and 2023.
According to 2024 WIPO Technology trends future of transportation report, research and patenting activities in solid-state batteries have grown significantly between 2010 and 2023, but remain a niche within the broader field of battery technologies.