The allure of Solid-State Lithium Batteries

The quest for safer, higher energy density Lithium batteries continues, now spurred largely by the need for extended driving range in electric vehicles (1). New Lithium-ion battery anode and cathode chemistries continue to provide an incremental increase in energy density every year. However, the industry would prefer a leapfrog to Lithium Metal Batteries (LMB), a technology that has the potential to significantly increase energy densities by replacing the carbon (graphite) anode currently used in Lithium-ion batteries with a Lithium metal anode.

But the liquid-electrolytes and separator membranes used in Lithium-ion cells cannot be used as a drop-in replacement in Lithium-metal batteries. That is because repeated charge discharge cycling of a lithium metal anode can lead to the formation of needle- like dendrite structures on its surface, which can puncture through the separator layer, resulting in internal short circuits. At the very least, such a short-circuit could lead to performance issues such as high self-discharge or poor cycle life. The more serious consequence is a potentially catastrophic failure of the battery, sometimes resulting in smoke and fire, due to the presence of the combustible liquid electrolyte.

It is generally perceived that the dendritic growth on the Lithium anode can be thwarted by replacing the liquid-electrolyte & separator combination with a Solid-State Electrolyte (SSE). (2) Furthermore, the combustibility of the solid-state electrolytes is significantly lower than that of the liquid electrolyte, thereby incorporating an inherent safety benefit to batteries made with solid-state electrolytes.

The term Solid-State Lithium Batteries originates from the solid-state nature of the electrolytes used in Lithium metal batteries. While a solid-state lithium battery can utilize any anode material, there is an implicit assumption – based on current trends – that solid-state lithium batteries will utilize Lithium-metal anodes.

Selecting a Solid-state Lithium Electrolyte Chemistry

Lithium-ion conducting solid-state electrolytes are not a new concept. Lithium-Iodide inorganic solid electrolytes form spontaneously when a Lithium-metal anode comes in contact with an “Iodine” containing cathode during the fabrication of primary Lithium- Iodine batteries that are used in pacemakers. (3) Polyethylene oxide (PEO), a well- known conductor of Lithium ions particularly at temperatures above the polymer melting point (~60°C), has seen limited application in batteries. (2) Lithium Phosphorus Oxynitride (LiPON), (2) another inorganic Lithium-ion conductor deposited using sputtering processes, has been used in the fabrication of thin film batteries. To date, there have been no applications of solid-state Lithium batteries in mainstream products such as portable electronics or electric vehicles. But the perceived benefits of solid-state electrolytes for improving safety and for enabling the use of high energy- density Lithium metal anodes have always kept them on the technology roadmap for these products. For such products, the desired characteristics in an ideal solid- state electrolyte include:

  • High ionic conductivity over a wide temperature range for high-rate charge-discharge
  • Robust mechanical characteristics to withstand cycling and prevent dendrite formation
  • Stable electrochemical characteristics at anode and cathode
  • Scalable processing parameters for cost-effective manufacturing

With these in mind, three broad categories of Lithium solid-state electrolytes technologies are being pursued by solid-state Lithium battery developers:

Solid polymer electrolytes are a class of solid-state electrolytes composed of organic polymeric materials that contain dissolved Lithium salts. Polymeric electrolytes are expected to be easy to process and could enable thinner and wider form factors in cells. However, they tend to have low ionic conductivities, which will limit the charge-discharge rate capability of the battery. Like the separators in Lithium-ion batteries, polymer electrolytes may not have high enough puncture resistance to prevent dendrite growth. Polymer electrolytes based on polyethylene oxide (PEO) are believed to be more stable at the anode than the cathode.

Inorganic electrolytes refer to glassy or ceramic materials usually of Sulfide or Oxide compositions. Some of the materials being developed are known to have respectable ionic conductivities, even approaching numbers exhibited by liquid electrolytes. Processing inorganic electrolytes and integrating them into cells may be problematic, and an outside-the- box approach to cell design and fabrication may be required. Inorganic electrolytes are expected to be mechanically robust to prevent dendrites. Oxide electrolytes have a relatively high oxidation voltage and a wide- voltage stability window. Sulfide materials have low-oxidation stability and low- reduction stability at the cathode and anode, respectively.

Composite or hybrid electrolytes can be a combination of solid polymer electrolyte, and one or more inorganic electrolytes, designed to leverage the strengths of each of the above-mentioned solid-state electrolyte technologies, and balance out the weaknesses.

For example, depositing a thin layer of anode-stable solid electrolyte on the anode, and a thin cathode-stable inorganic layer on the cathode, and then laminating them with a very thin layer of a third solid electrolyte, could provide viable solution to electrochemical performance. Alternatively, a dispersed composite structure (inorganic particles in polymer matrix) with optimized percolation – to enhance ionic conductivity – may also provide a viable solid state cell structure. Some Lithium-metal battery developers have chosen to adopt yet another hybrid approach, using solid and liquid electrolytes. (4)

Electrode and Electrode Interface

In addition to optimizing the solid-state electrolyte, a viable solid state battery technology will need to select a suitable anode and cathode. The liquid electrolyte in Lithium-ion cells provides a very uniform solid-liquid electrode-electrolyte interface with both the anode and the cathode. However, with solid state electrolytes, the solid-solid electrode-electrolyte interface needs to be tailored to withstand repeated charge and discharge, and the associated expansion/contraction of the electrodes. Engineering the interfaces between inorganic electrolytes and the electrodes is expected to be particularly challenging, because of the hard inflexible nature of the ceramic and glassy electrolytes. Some solid-state battery developers have chosen to adopt a hybrid approach, using solid and liquid electrolytes, to address this issue. (4) This approach could sacrifice the safety benefits of an all solid-state electrolyte.

Anode: Most solid-state battery developers are using Lithium metal as the anode, in the hope of realizing the high-energy density potential of the material. Others have taken an incremental approach, starting with conventional anode like Graphite-Silicon, perhaps with plans to move to Lithium metal further down the road. (5) When using a conventional anode, one requires a plan to intersperse solid electrolyte material with the anode composite, to incorporate sufficient Lithium ion-conductivity within the anode. This allows for complete and homogenous utilization of the anode. It is also feasible in principle, to use an anode-free cell design in the as-manufactured cell. (6) In this case, Lithium metal is plated on the anode current collector at the very first charging event, thereby minimizing the amount of excess Lithium.

Cathode: Conventional cathodes (e.g. Lithium Nickel Manganese Cobalt Oxide (NMC), Lithium Nickel Cobalt Aluminium Oxide (NCA) like those being used in Lithium-ion cells, are being considered for many Lithium solid state battery designs. However, the potential of high energy density Lithium metal anodes may only be realized to a limited extent with conventional cathodes. Higher capacity Sulfur cathodes are also being considered by other developers. (2) While Lithium-Sulfur batteries have been under development for more than a decade now, it remains to be seen if there will be a viable mainstream battery commercialized anytime soon.

Beyond Sulfur cathodes, air (Oxygen) cathodes have also been demonstrated as viable candidates. (2) While in theory, these Lithium-air batteries can provide very high gravimetric energy densities, their practical applications have been limited at best. As with conventional anodes, Lithium ion-conductivity in the cathode layers also needs to be addressed. Composite cathodes with solid state electrolyte interspersed with the cathode particles would be required.

Cell Design & Manufacturing

For solid state batteries, cell designs may vary from those used in Lithium-ion batteries. Spirally wound designs may be feasible for cells with flexible polymer or composite electrolytes. However, cells with inorganic electrolytes will need to adopt cell designs, such as cut-and-stack designs, that can accommodate the inflexible nature of the inorganic solids. With regards to raw-materials cost and fabrication, polymer electrolytes are expected to be of lowest cost. Raw materials cost for the inorganic oxide and sulfide materials may be higher than polymers because some of the precursors required for production may be expensive. Furthermore, synthesis of the inorganic materials and their fabrication into electrolyte layer are expected to require more elaborate, high-temperature processes.

Supply Chain Factors

Introducing multiple new technologies, i.e. solid-state electrolyte, Lithium anode and a new cathode in a battery cell technology will not only have to leverage some of the logistics already in place for Lithium-ion, but also establish competitive new ones.

Much of the Lithium mined for batteries today is being used to produce Lithium Carbonate and Lithium Hydroxide for cathodes like NCM, NCA and Lithium Iron Phosphate (LFP). For Lithium anodes, a significant amount of the mined Lithium will need to be allocated to precursors for Lithium metal production.

This could be a daunting task for an application that requires materials at the scale of electric vehicle batteries.On the other hand, even a small slice of the enormous market that we see as imminent for electric vehicle batteries, would provide a profitable opportunity for solid-state battery developers.

Summary and Future Perspective

The potential to achieve high energy density using a Lithium-metal anode and enhanced safety enabled by the solid-state-electrolyte is a highly attractive proposition. A recent market report on solid-state batteries (7) lists more than fifty companies and institutions working on solid-state battery technologies. Several start-up companies have indicated that they have proprietary solid-state electrolyte technologies that are currently viable in Lithium-metal batteries. Some large battery and automotive manufacturers have also invested considerable internal R&D dollars for the development of solid-state Lithium batteries while simultaneously investing in the technology being developed by start-ups.

Considering the unique issues related to performance, manufacturability, or cell integration encountered in the solid-polymer and inorganic electrolytes, it is reasonable to infer that a composite or hybrid electrolyte would be the best near-term option for a viable solid-state Lithium-metal battery. For the longer term, improved materials with high ionic conductivities and optimum electrochemical stability, mechanical characteristics and processability are required. In addition, cell design will also be a key aspect to track on solid-state battery roadmaps.

The momentum garnered by conventional LIBs after decades of development and manufacturing will make them hard to displace unless a significant and compelling improvement in performance is guaranteed by any new technology. Over the last few years, major investments have been made by governments and industry to develop the Li-ion battery infrastructure. The entire supply chain, from metal mining to the OEM, has been energized to meet the challenge of electrifying the global transportation fleet. Any inroads made by solid-state technology into the Lithium-ion market may be minimal at best for the next decade. Beyond that timeframe, if the potential for the technology is realized and the supply chain and manufacturing learning curves are scaled, we can expect solid-state technology to gradually displace LIBs in mainstream products including electric vehicles.


Ganesh Venugopal: Ganesh is an EV battery industry observer, based in Atlanta, GA, USA. He has spent more than 20 years developing Lithium-ion battery technologies in MNC and start-up settings. At present, he is the Director of Business Development for a PhiChem America, a materials sciences and chemicals company serving the optic-fiber, display and electronics materials industries.

Doug Morris: Doug has over 30 years of experience in the telecommunications, components, battery, and energy storage industries. Prior to founding and managing Polaris Labs (a battery material evaluation company), he was Vice President of Operations at Enevate, a developer of advanced silicon anodes for consumer and electric vehicle applications. Doug also had a 21-year career with Motorola where he was Vice President for their Energy Systems Group.


References

  1. Electric Vehicle Outlook 2021: Bloomberg NEF Report (2021)
  2. Lithium battery chemistries enabled by solid state electrolytes, Nature Review Materials, (2017)
  3. Lithium-Iodine Battery US Patent No. US3874929 (1975)
  4. SES Battery Presentation (2021)
  5. Solid Power Battery Website (2021)
  6. QuantumScape Corporation Presentation (2021)
  7. Solid-State and Polymer Batteries 2021-2031, IDTechEx Market Report (2021)

This article was first published in EVreporter Jan 2022 magazine that can be accessed here.

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