Solid State Batteries – A Promising Alternative to Conventional Batteries

INTRODUCTION

Lithium-ion (Li-ion) batteries, containing liquid/gel separators, provide a lightweight energy-storage solution for most of today’s high-end battery-powered gadgets, ranging from smartphones to electric vehicles. Despite their prevalence, Li-ion batteries have disadvantages such as low power density, short lifespan and high resistivity at electrode interface, leading to capacity loss, electrolytic decomposition at high voltages that limit the use of high voltage cathode materials, formation of HF at thermal runaway, and risk of leakage, resulting in battery fires and explosions. Safety issues assumed great importance, following incidents such as an explosion in a Japan Airlines 787 Dreamliner’s cargo hold in 2013 and Samsung’s Galaxy Note 7 catching fire in 2016. The leakage of the liquid electrolyte, or short-circuit of the electrodes due to failure of the polymer gel separator was identified as the main cause for the explosion. Replacing the liquid/gel components of the conventional Li-ion batteries with solid electrolyte was considered as an approach to solving the problem.

Solid-state batteries use both solid electrodes and electrolytes. The emerging all-solid-state battery (ASSB) technology offers high performance and safety at low cost. ASSBs have low flammability, high energy density, high electrochemical stability and high energy density, as compared to conventional liquid/gel electrolyte batteries and find application in various industries such as energy storage, consumer electronics, industrial, aerospace and automotive.^[1] Currently, different types of solid-state electrolytes based on oxides, sulfides, phosphates, nitrile or polymers such as polyethers, polyesters, polysiloxane, polyurethane, etc., exist. ^[2],[3],[4]

Though promising, ASSBs still face challenges with respect to solid interface stability, large scale production, sustainable manufacturing, recyclability, and so on. Although great potential exists, there is still plenty of efficiency to be gained by improving the chemistry of lithium-ion batteries. ^[5],[6]

RECENT DEVELOPMENTS

Researchers at the Samsung Advanced Institute of Technology (SAIT) and the Samsung R&D Institute Japan (SRJ) proposed a silver-carbon (Ag-C) composite layer as the anode to replace lithium metal anodes that are generally used in ASSBs. The 5µm ultrathin Ag-C nanocomposite layer incorporated within a prototype pouch cell enabled a larger battery capacity, a longer lifespan and enhanced safety. Such a single charge of the prototype pouch cell may enable an electric vehicle (EV) to travel up to 800 km (500 miles), while offering a lifecycle of more than 1,000 charges. The Ag-C nanocomposite layer also helped in the reduction of anode thickness while boosting energy density up to 900 Wh/L. ^[7]

Schematic of an All-solid-state lithium metal battery developed by SAIT and SRJ Lee et al. (Source)

Murata Manufacturing Co., Ltd. has developed a non-combustible, heat-resistant solid-state battery that employs a ceramic oxide electrolyte instead of the electrolytic solution used in conventional lithium-ion secondary batteries. The high degree of safety and durability of the new battery is ideal for wireless earphones and makes it far superior to lithium-ion secondary batteries. This solid-state battery also finds applications in internet of things (IoT) devices.^[8]

Solid-state battery developed by Murata Manufacturing (Photo: Murata Manufacturing) (Source)

To achieve satisfactory characteristics, Murata’s engineers worked towards the development of three aspects: 1) solid electrolyte material with high ion conductivity, 2) technology for forming precise, thin electrolyte layers, and 3) processes for improving the adhesion of electrode active materials and electrolytes. Of these approaches, the formation of precise, thin electrolyte layers proved to be a challenging issue, but it was resolved by drawing on Murata’s extensive technology and expertise for mass-producing multilayer ceramic capacitors (MLCCs). Like solid-state batteries, MLCCs have a structure where the area between the electrodes is filled by a dielectric body made of ceramic material. The technology for creating a clean, thin ceramic film and then fully hardening it enabled the mass production of microscopic pattern elements at high quality.

An energy density 10x to 100x more than that of any previous oxide-based solid-state battery was obtained. For instance, a compact battery with a size of 4 mm x 5 mm x 9 mm can provide an output of more than 10 mA, which is required for wireless transmission of data using Bluetooth LE. In Murata’s prototype, it was possible to obtain maximum capacities of 20 to 30 mAh, which could replace the existing lithium-ion secondary batteries being used as the power supply for wireless earphones. A design modification helped obtain the same 3.8 V output voltage as that of existing lithium-ion secondary batteries, which facilitated ease of use when incorporated into electronic devices.

Murata’s batteries may also be suitable for use in energy-harvesting technology for capturing light, temperature differences, vibrations, and other energy already found in the surrounding environment and converting it into power for use as an energy source.⁹

Interuniversity Microelectronics Centre (IMEC) of Belgium has developed a solid-state Li-metal battery cell with an energy density of 400 Wh/liter at a charging speed of 0.5C (two hours), using a solid nanocomposite electrolyte (SCE) in combination with a standard lithium-iron-phosphate (LFP) cathode and lithium metal anode. The electrolyte conductivity is 10 mS/cm with path to 100 mS/cm. The new material is applied as a liquid via wet chemical coating and is converted into a solid when in place in the electrodes. Ultrathin protective interface coatings have been provided for long cycle life.^{[10], [11]}

Toyota will unveil a car with solid-state batteries at the upcoming 2021 Olympics, to showcase its battery capability. This one-off prototype car, which is expected to be based on Toyota’s e-Palette platform, will form a part of the event’s opening or closing ceremony.^[12] The prototype, that will run on batteries capable of charging in 10 minutes, can provide a vehicle range of 500 kilometers. Toyota is the pioneer of Solid-state Battery Research and Development, with a lion’s share of patent ownership of over 1000 patents involving solid-state batteries, and is also expected to begin volume production of EVs in 2025.^[13]

Solid-state battery powered prototype car developed by Toyota based on e-Palette platform (Source)

A research team at Massachusetts Institute of Technology (MIT) has developed a three-dimensional honeycomb-shaped nanoarchitecture with an array of nanoscale tubes, made from ‘mixed ionic-electronic conductors’ (MIEC), infused with solid lithium metal to form the battery’s anode. The extra space left inside each tube allows the lithium metal to expand and shrink in the charging and discharging processes, moving like a liquid while retaining its solid crystalline structure throughout the process. This flow relieves the pressure from the expansion caused by charging, while maintaining the electrode’s outer dimensions.^[14]

Silicon Valley startup TeraWatt Technology’s 4.5 Ah prototype solid-state battery design branded as TERA3.0 achieved a record-breaking energy density of 432 Wh/kg (1122 Wh/L) in validation tests. This next-generation design will be available for select early adopters in 2021 and full release is expected in 2022.^[15] The company, a division of Chinese EV start-up Seres (formerly SF Motors, Inc), continues to further iterate the TERA3.0, and is developing additional designs, including different cell formats, sizes and energy capacities.

Prototype solid-state battery designed by TeraWatt Technology (Source)

Factorial Energy’s Factorial Electrolyte System Technology (FEST) is a novel solid electrolyte material that enables safe and reliable cell performance with high-voltage and high-energy density electrodes by suppressing lithium dendrite formation on anodes. FEST-based batteries yield 20-50 % improvements in driving range without compromising on pack longevity, energy density, cycle life and safety. FEST claims to be the first to reach the 40 Ah benchmark for a solid-state cell.^[16]

CHALLENGES

Planar solid-state thin-film batteries possess relatively low volumetric capacity as against large volume batteries occupied by inactive materials and packaging. Drawbacks such as inactive materials and packaging associated with large volumetric capacity are overcome by the three-dimensional (3D) designing of batteries.

3D geometries can be applied to increase the volumetric energy density of batteries so as to obtain higher storage capacities with similar amount of packaging and substrate material. Furthermore, internal surface area between cathode, electrolyte and anode is higher in 3D batteries, as a result of which, relatively high current and power capability is obtained. Various methods such as three-dimensional substrates based on templated deposition, arrays of interdigitated microrods, micellar templates, microchannel plate-based 3D batteries, etc., have been proposed for the development of 3D designs for batteries.^[17]

Schematic representation of various methods for obtaining three-dimensional solid-state batteries

However, most of these 3D designs are only conceptual and most published results have been focusing only on partial solid-state batteries.^{[18], [19], [20]}

START-UPS

Solid Power, a startup that spun out of a battery research program at the University of Colorado Boulder, is developing ASSBs by combining a state-of-the-art cathode with a metallic lithium anode that are inherently safer with a longer lifespan, and energy density greater than 50% as compared to the best available rechargeable batteries.^[21]

The solid electrolyte at the center of a Solid Power battery.
Image credit: Andy Cowell (Source)

Ionic Materials Inc., a materials technology company headquartered in Woburn, MA, has developed a novel solid polymer electrolyte material that conducts ions at room temperature. Ionic conductivities exceed those of traditional liquid systems over a wide range of temperatures, which results in significant improvements in battery safety, performance and cost. Improved cell-level safety may lead to performance improvements and cost reductions at the pack-level, which makes this polymer electrolyte material useful for electric vehicles. Ionic’s platform technology enables the use of a wide range of electrode chemistries, including lithium metal anodes, sulfur cathodes, etc.^[22]

Significant improvements in battery with ionic conductivities that exceed those of traditional liquid systems over a wide range of temperatures (Source)

Ion Storage Systems LLC, a US-based battery manufacturer, has developed a super-thin electrolyte that could deliver safe and fast-charging batteries. This ceramic electrolyte is only about 10 µm thick and has three layers. A thin, dense middle layer is made up of lithium-oxide ceramic with the chemical formula Li7La3Zr2O12. On either side of the middle layer is a slightly thicker porous layer of ceramic with a super-thin aluminum oxide coating. This electrolyte addresses problems such as high electrolyte resistance and low current capability, which have been associated with solid-state batteries. The company has developed prototype batteries that have an energy density of about 300 Wh/kg, higher than the maximum of 250 Wh/kg delivered by the known commercial lithium-ion devices.^[23]

Electron microscope photo of a thin, dense layer of a ceramic electrolyte that goes between two porous layers in a solid-state battery (Source)

Factorial Energy, a start-up solid-state battery technology company, emerged from stealth mode with a 40 Ah solid-state battery cell for electric vehicles (EVs) and other applications.^[16]

INVESTMENTS AND COLLABORATIONS

Volkswagen has invested $300 million in US-based QuantumScape, a key player in mass manufacturing of solid-state battery technology. The companies will select the location of their joint-venture solid-state battery pilot-line facility by the end of 2021. They are currently contemplating Salzgitter, Germany for the location. The pilot-line facility, QS-1, will initially be a 1-gigawatt hour (GWh) battery cell commercial production plant for electric vehicle batteries and there are plans to expand production capacity by a further 20 GWh at the same location.^{[24], [25]} With an “anode-free” lithium-metal battery, the company claims to deliver enhanced energy density as well as safety. This anode-free construct – where the deposition of lithium ions on the anode current collector forms a temporary anode that then disintegrates during the discharge process – reportedly delivers superior energy density and safety as compared to conventional Li-ion cells.

General Motors (GM) has invested a total of $139 million in US- based solid-state battery specialist SolidEnergy Systems (SES).^[26] The main purposes of this funding is to improve the key material, lithium metal electrolyte on the anode side and the cathode side, and, to improve the scale of the current cell from the iPhone battery size to the size that can be used in cars.^[27] Headquartered in Singapore and having operations in Boston, Shanghai and Seoul, SES that has made significant strides in developing breakthrough batteries and is now preparing for serial production. The first battery prototypes have already gone through 150,000 trial miles in simulations at GM’s R&D labs in Warren, Michigan.

Toyota Motor Corp. and Panasonic Corp. formed a joint venture named Prime Planet Energy & Solutions to develop solid-state batteries for cars. A new production line will be installed at the existing plant of the company’s Chinese subsidiary, Prime Planet Energy Dalian Co., in Dalian in the northeastern province of Liaoning to produce prismatic lithium-ion cells. The new facility, capable of turning out such batteries to be installed in some 400,000 hybrid electric vehicles a year, is scheduled to start production this year.

The Tokyo-based joint venture will also set up a new line at its plant in Himeji, Hyogo Prefecture, western Japan, to produce prismatic lithium-ion cells, with an annual production capacity to provide batteries for approximately 80,000 EVs. The company, set up in April 2020, is 51 percent owned by Toyota and the rest by Panasonic.^[28]

Musashi Seimitsu Industry Co., Ltd., a Japanese company, partnered with KeraCel Inc., a Silicon-Valley-based venture, to develop high energy solid-state batteries using 3D printing technology that provides power system solutions to the electric motorcycle market. This technology is used to 3D-print ceramic-based electrolytes that are essential to building high-energy density batteries. These batteries will achieve energy densities 2-3 times greater, at less than 50% cost of today’s lithium-ion cells. KeraCel Inc. recently changed its name to Sakuu Corp., and is strengthening its strategic partnership with Musashi through series of investments that will accelerate the commercialization of Sakuu’s solid-state battery technology (KeraCel™), Sakuu AM platform and other areas.^{[29], [30]}

Hydro-Québec, a Canadian battery material specialist, partnered with Mercedes-Benz AG to pursue research on solid-state battery technologies that can unleash new possibilities in vehicle electrification. The partnership will test new materials under field conditions to accelerate the development cycle. The joint research activities will be carried out at Hydro-Québec’s Center of Excellence in Transportation Electrification and Energy Storage in Canada as well as the SCE France laboratory, a Hydro-Québec subsidiary.^[31]

NIO, a China-based EV start-up and ProLogium, a Taipei-based developer of solid-state Li-ion batteries, signed a strategic cooperation agreement to create a prototype using a ProLogium solid-state battery pack. ProLogium’s new packaging approach offers the mechanism that is characteristic of solid-state battery along with high security and heat dissipation capability.^[32]

BMW and Ford Motor Company have partnered with Solid Power in developing ASSBS for next-generation electric vehicles. This partnership leverages Solid Power’s solid-state technology that combines a cathode, metallic lithium anode, and a safe, inorganic solid electrolyte layer.^[33] The company is currently producing 20 Ah multi-layer all solid-state batteries on its continuous roll-to-roll production line, utilizing industry standard lithium-ion production processes and equipment. Both Ford and the BMW Group will receive full-scale 100 Ah cells for automotive qualification testing and vehicle integration beginning in 2022.

A123 Systems LLC, a developer and manufacturer of advanced lithium-ion batteries and systems, and Ionic Materials, an advanced materials company, have collaborated to develop an ASSB, by combining Ionic Materials’ advanced ionically conductive polymer with A123’s next-generation NMC/graphite lithium-ion chemistry, using high volume lithium-ion manufacturing equipment. This unique approach is a cost-effective way to make electric vehicles safer, lighter, and less complex, and a high-volume launch of this solid-state technology is expected to be launched in the market as early as 2022.^[34]

RECENT PATENT PUBLICATIONS

US20200014071A1 – Murata Manufacturing Co Ltd. has developed an all-solid-state battery, with a cathode layer surface roughness of Rz1 and an anode surface roughness of Rz2, such that Rz1+Rz2 ≤ 25 and the occurrence of short circuit between cathode and anode can be suppressed. Also, the porosity of the solid electrolyte layer is less than 10%. This battery can be used in an electric motor vehicle.

Schematic cross-sectional view for depicting surface roughness Rz1 and Rz2 of a cathode layer and an anode layer

US20200067128A1 – Fisker Inc. has developed a solid-state battery consisting of multiple composite electrodes with high loading of electrochemically-active materials, a dendrite-blocking separator placed between the anode and the cathode, and a secondary phase conducting interlayer at the electrode/electrolyte interface enabling cell area specific resistance lower than 100 Ohm-cm² and specific energy greater than 350 Wh/kg.

Exemplary cylindrical cell monoblock and an enlarged view of the stack prepared using solid-state electrolytes

US20200067068A1 – LG Chem Ltd. has developed an apparatus for manufacturing an electrode or a solid electrolyte for an all-solid-state battery, which can prevent a crack from being generated at the electrode or the solid electrolyte.

DE102018212448A1 – Robert Bosch GmbH has developed a lithium-ion solid-state electrolyte battery cell containing anode layer, cathode layer and a solid electrolyte layer in which a sensor layer is embedded such that the growth of the dendrites can be detected before the short circuit.

Schematic diagram of a lithium-ion solid electrolyte battery cell

US20200036044A1 – Toyota Motor has developed an all-solid-state battery with a cell laminate, and a resin layer covering a side surface of the cell laminate having a ratio of the compressive modulus of the resin layer to that of the cell laminate of 0.4 or less so that the cracking or deformation of the extending parts of the cell laminate and the resin layer can be suppressed.

Schematic cross-sectional view showing an example of the all-solid-state battery

US2021126281A1 – Solid Power Inc. has developed a solid electrolyte comprising glass ceramic and crystalline phases for the fabrication of lithium solid-state battery. Compositions provided conductivities of approximately 0.6-2 mS/cm at room temperature by employing electrolyte material in pellet form. Furthermore, the compositions including the novel phase improved resistance and capacity stability at elevated temperatures and charge cut-off voltages.

WO21044883A1 – Maxell Holdings Ltd. has developed a negative electrode material, for all solid-state battery, comprising lithium titanium oxide particles with a ratio between the primary particle diameter Dp and the particle diameter D50 of more than 0.6 and a specific surface area of 2 m²/g or more. It has been observed that battery with excellent output characteristics could be fabricated by modulating the ratio Dp/D50 larger than 0.8 μm.

CONCLUSION

ASSB technology offers attractive solutions to the problems encountered by the utilization of conventional rechargeable batteries in electric vehicles. ASSBs can provide better sustainable energy storage technologies in terms of energy density and safety features that are crucial requirements for electric vehicles. It is not the development of novel electrolyte materials, such as hybrid electrolytes, that is important, but also better understanding of their behavior at nano level. Also important are the fabrication techniques that are critical for large-scale production and commercialization of ASSBs in the near term. However, developing a unique solid-state electrolyte that is capable of meeting all required properties, prevention of dendrite formations at the solid electrolyte interface, recycling issues, etc., still remain challenging. The progress in the arena of 3D printing can serve as a promising aid in the development of compact ASSB designs with large packing density and low volume consumption.

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