Energy storage has become an important factor in both power electric networks and transport and mobility. Developments are resulting in rapid cost reductions as well as increase in capacity. Solid state batteries promise to both increase capacity and improve safety.
The current range of batteries, based on Li-ion technology, has limitations in both weight and volume. The general statement that battery development is aimed at increasing capacity actually means increasing energy density, which can have two metrics:
Current state of the art
Increasing demands for high-power and high-energy rechargeable batteries have developed battery technology which is led by the lithium-ion (Li-ion) battery. Li-ion batteries generally consist of a graphite negative electrode, organic liquid electrolyte, and lithium transition-metal oxide (LiCoO2) positive electrode. They were firstly commercialised in 1991 and since then have been widely used as a power source for mobile electronic devices such as cell phones, laptops and cameras.
Large-scale Li-ion batteries have been developed for applications from automotive propulsion and stationary load-leveling, to other standby and back-up applications. The Li-ion battery has found application from large grid-scale to small behind the meter storage applications. However, there are safety issues with Li-ion, due the flammability of the organic liquid electrolyte and increasing battery size makes safety issues more serious .
Batteries in use today have a liquid electrolyte which transfers electrons between the anode and cathode of the battery. The use of liquid electrolytes places limitations on the battery as well as affecting the safety of the unit as a whole. Solid state batteries under development offer increased energy density, longer life and improved safety.
The solid state battery (SSB)
The name, which is somewhat misleading as it is also used for electronic devices, describes a battery with a solid electrolyte. In the solid state battery, the liquid electrolyte is replaced with an ionic conductive solid material, which provides the path between the anode and cathode. The construction of a typical solid state battery is shown in Fig. 1.
Compared to Li-ion batteries with liquid electrolytes, SSBs offer an attractive option owing to their potential in improving the safety and achieving both higher power and high energy densities. Solid-state rechargeable Li-ion batteries have attracted much attention because the replacement of an organic liquid electrolyte with a safer and more reliable inorganic solid electrolyte (SE) simplifies the battery design and improves safety and durability of the battery.
Development of SE batteries is based mainly on existing Li-ion technologies, but is being extended to other materials such as magnesium as well. The first stage of development is the substitution of the liquid electrolyte with a solid one. This is not as simple as it seems as the layers of anode material, electrolyte and cathode material have to be deposited onto one another. The greatest challenge with the solid state battery development is finding a suitable electrolyte material. The most difficult property appears to be conductivity. Additional research into both anode and cathode material is also being undertaken.
SSB is an emerging technology and researchers are still coming to grips with the best types of solid-state electrolyte to use for different product categories. None have come out as clear leaders. Many of these types are still lithium (Li) based, because they are still using lithium electrodes. But new anode and cathode electrode materials are being considered to improve performance.
Solid state electrolytes
Solid electrolytes conduct ions, and the charge carriers could be cations, anions, or ion defects. Solid electrolyte electron conductivity is negligible. Solid electrolytes can be divided into polymer electrolyte (usually PEO and lithium bis-trifluoromethanesulfonimide (LiTFSI) and other mixtures for the electrolyte substrate) and inorganic electrolytes (such as oxides and sulfides). Solid materials as an electrolyte for battery application include a large variety of materials such as gel, organic polymer, organic–inorganic hybrids, and inorganic materials. A key material to develop all-solid-state batteries is a SE with high Li+ ion conductivity at room temperature. Inorganic SEs have been widely studied and, in recent years, several SEs having the same level of conductivity as organic liquid electrolytes have been discovered.
At present, SSBs fall into two categories according to the electrolyte: polymer-based with polymer solid electrolytes inorganic based SSBs with with inorganic solid electrolytes. The advantages of polymer-based SSBs include greater safety, easy preparation, and flexible shape. However, many problems remain to be solved, such as the instability of the electrolyte/electrode interface, the narrow temperature range of polymers, and poor mechanical stability.
Solid polymer electrolytes
Extensive investigations on solid polymer electrolytes (SPE) have been under progress in view of potential application in solid state batteries. By employing a thin film of SPE in place of a conventional liquid electrolyte, several advantages are anticipated. Of these, enhancement in energy density occupies primary importance. For the purpose of using in a solid state battery, a SPE film needs to possess several properties, among which a high ionic conductivity and a good mechanical stability are of essential importance. GPE essentially contains a liquid electrolyte retained in a polymer gel. Although the mechanical strength of a GPE is less than that of a SPE
Solid inorganic electrolytes
Inorganic solid electrolytes are non-flammable and highly stable mechanically. The replacement of liquid electrolytes with inorganic solid electrolytes is considered to be the ultimate solution for the safety issues. Also, inorganic SSB have a wide electrochemical window, which could be operated under high cut-off voltage .
There are several major categories of solid-state batteries, which each use different materials for the electrolyte. Table 2 lists some of the solid state electrolytes under investigation. There are three categories of inorganic electrolytes: crystalline, glass and glass–ceramic.
|Name||Typical chemical formula||Ionic conductivity (Scm-1)||Type|
|Li-halide||Li1,3Al0,3Ti1,7(PO4)3||7 x 10-4||Crystal|
|Perovskite||La 0,5 i 0,34 TiO 2,94||1,4 x 10-3||Crystal|
|Li-hydride||Li2B12H12||2,9 x 10-4||Ceramic|
|Nasicon (Na Super Ionic Conductor)||Li1,5 Al 0,5 Ge 1,5(PO4)3||6,21 x 10-4.||Glass ceramic|
|Garnet||Li7La3Zr2O12||3 × 10-4||Crystal|
|Argyrodite||Li6PS5Cl||1,3 x 10–3||Crystal|
|LiPON ( Lithium Phosphorous Oxynitride)||Li2PO2N||2,3 x 10–6||Glass ceramic|
|Lisicon ( Lithium superionic conductor)||Li10GeP2S12.||1,2 x 10-2||Glass ceramic|
The specific capacity of cathode materials currently in use is less than half that of negative electrode materials. Therefore, high-capacity cathode materials are necessary for Li-ion batteries with higher energy densities. The most commonly used cathode materials in all-solid-state lithium batteries can be classified into two categories based on the cell potential. One is lithium transition-metal oxides, with a potential of 3,5 to 5 V, which are considered to be viable candidates for SSBs because of their highly reversible intercalation/deintercalation reaction and high operating potential .
Lithium metal In theory, SSBs with inorganic solid state electrolytes free from flammable components and having high mechanical strength can incorporate lithium metal as an anode and take advantage of maximizing its energy density. However, the internal short circuits caused by abnormal lithium dendritic growth and the electrochemical reaction of the SEs with lithium metal both limit the application of lithium metal anodes. 
Lithium alloys Lithium alloys (e.g., Li–In, Li–Si, Li– Al) anodes are good alternatives to replace lithium metal in SSBs, owing to their safety and high capacity. Lithium alloy Li–In alloy is the most commonly used anode in SSBs.
The use of a solid electrolyte allows for direct stacking of battery cells to produce higher voltages, as there is no electrolyte overflow. Conventional Li-ion cells have to be stacked together with cell casings. Direct stacking of solid-state cells in one package achieves a high operating voltage and reduces wasteful volume, and especially these features are favorable for vehicle applications.
An advantage of SSbs is a possibility to use large-capacity electrode materials, which are difficult to use in a conventional liquid electrolyte batteries. Lithium metal negative electrode and sulphur positive electrodes are typical examples for use in SSBs. Another challenge involves constructing a suitable solid–solid interface between electrode and electrolyte. Several techniques for increasing contact area at the interface have been developed to enhance utilisation of active materials and raise capacity.
Alternatives to the Li-ion solid state battery
Development of solid electrolyte materials has opened the door to the use of other technologies for solid state batteries.
The lithium sulphur solid state battery 
As a result of sulfur’s high electrochemical capacity lithium–sulphur batteries have received significant attention as a potential high-specific-energy alternative to current state-of-the-art rechargeable Li-ion batteries. The Li-S battery uses a Li based anode and a composite S based cathode, with an S based electrolyte For Li-S batteries to compete with commercially available Li-ion batteries, high-capacity anodes, such as those that use Li metal, will need to be developed fully exploit sulphur’s high capacity.
The development of Li metal anodes has focused on eliminating inefficient Li cycling, and , an interesting direction of research is to protect the Li metal by employing mechanically stiff solid-state Li+ conductors, such as garnet phase Li7La3Zr2O12 (LLZO), NASICON-type Li1+xAlxTi2–x(PO4)3 (LATP), and Li2S–P2S5 glasses (LPS), as electrode separators. Research is focused on quantifying useful targets for solid Li metal protective separator thickness and cost to enable Li-metal batteries in general and Li-S batteries specifically.
Controlling the complex polysulfide speciation chemistry in Li-S cells with realistic, minimal electrolyte loading presents a meaningful opportunity to develop Li-S batteries which are competitive on a specific energy basis with current state-of-the-art Li-ion batteries.
Solid state magnesium batteries 
In comparison to lithium as an anode material magnesium has a (theoretical) energy density per unit mass under half that of lithium ( 18,8 MJ/kg vs. 4,3 MJ/kg ), but a volumetric energy density around 50% higher (32,731 GJ/m3 vs. 22,569 GJ/m3 ) . Mg is an attractive anode material and it has been employed in aqueous primary and reserve batteries . Studies on solid state rechargeable magnesium batteries are interesting in comparison with lithium batteries on account of the following advantages:
Magnesium batteries have a higher energy density than lithium, but development has been hampered by the lack of a suitable liquid electrolyte. A possible solution has been found with the use of solid electrolytes . There is a dearth of good options for a liquid electrolyte, most of which tend to be corrosive against other parts of the battery. According to one researcher, “Magnesium is such a new technology, it doesn’t have any good liquid electrolytes”. The main research seems to be focused on inorganic electrolytes, with the recently developed, magnesium scandium selenide spinel which has magnesium mobility comparable to solid-state electrolytes for lithium batteries, being a main contender .
One of the potential electrolytes is a polymer gel, being investigated by several organisations. The electrolyte is claimed to have the same conductivity as the lithium solid electrolyte . The magnesium solid state battery is still at the laboratory stage, and much development work has to take place before a commercial version is available. Nonetheless it is regarded as a potential future replacement for Li-ion technology in both stationary and mobile applications.
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