Solid-state batteries: Unlocking lithium’s potential with ceramic solid electrolytes Solid-state batteries: Unlocking lithium’s potential with ceramic solid electrolytes Induction coils heat a die for rapid densification of Li-ion Credit: Evan Dougherty/University of Michigan Engineering Communications and Marketing conducting Li7La3Zr2O12 ceramic solid electrolyte. By Nathan J. Taylor and Jeff Sakamoto that lithium deposits in dendritic structures upon battery cycling. These dendrites eventually grow through the separa- Recent progress indicates that ceramic materials may soon tor, connecting the anode and cathode and causing a dan- supplant liquid electrolytes in batteries, offering improved energy gerous short circuit of the cell. The solution was to replace the lithium anode with a graphite Li-ion host material, capacity and safety. thereby producing the modern Li-ion battery. First introduced by Sony in 1991, the graphite anode is paired with a LiCoO2 cathode and flooded with a liquid idespread adoption of electric organic electrolyte with dissolved lithium salt. The dissolved Wvehicles (EV) will require dra- lithium provides Li-ion transport within the cell. A thin and matic changes to the energy storage market. porous polymer separator prevents physical contact between the anode and cathode while allowing ionic transport Total worldwide lithium-ion (Li-ion) battery production between electrodes. was 221 GWh in 2018, while EV demand alone is projected This basic cell structure remains unchanged today, albeit to grow to more than 1,700 GWh by 2030.1 As economies with numerous energy-boosting innovations, including sili- of scale have been met in Li-ion battery production, price at con anode additions, electrolyte additives to increase cycle the pack level has fallen and is expected to break $100/kWh life, and high nickel-content cathodes. These innovations within the next few years. have led to an average of 8% annualized energy density Li-ion batteries are expected to address near-term energy improvement in Li-ion batteries.2 Despite this progress, the storage needs, with advances in cell chemistry providing steady volumetric energy density of Li-ion batteries can only reach a improvement in cell capacity. Yet Li-ion batteries will eventu- practical limit of about 900 Wh/L at the cell level. ally approach the practical limits of their energy storage capac- For Li-ion batteries, active cathode and anode powders ity, and the volatile flammable liquid electrolyte in Li-ion cells are mixed with binder and cast on a current collector using requires thermal management systems that add cost, mass, and doctor blade, reverse comma, or slot die coating. These complexity to EV battery packs. electrodes are slit into desired dimensions, interleaved with Recent progress demonstrates that Li-ion conducting solid a separator, and either wound—as is the case of an 18650 electrolytes have fundamental properties to supplant current (18 mm diameter; 65 mm length) cylindrical cell—or stacked Li-ion liquid electrolytes. Moreover, using solid electrolytes or folded to produce a prismatic pouch cell. Figure 1 shows enables all-solid-state batteries, a new class of lithium batteries 18650 cylindrical wound cells and 10-Ah pouch cells. that are expected to reach storage capacities well beyond that For EV applications, cells are arranged into modules, of today’s Li-ion batteries. The promise of a safer high-capacity which are placed into a battery pack. For example, a Tesla battery has attracted enormous attention from fundamental Model 3 contains more than 4,000 individual cylindrical research through start-up companies, with significant invest- cells, producing about 80 kWh of storage. Other manufac- ment from venture capitalists and automakers. turers, such as GM, use pouch-type cells, with 288 cells pro- ducing 60 kWh of storage in the Chevy Bolt. The Li-ion battery Li-ion battery packs contain significant battery management The 1970s marked development of the first Li-ion cathode systems to keep cells within a safe operating range. Heat gen- intercalation materials. Cells with a metallic lithium anode erated within the pack must be removed by cooling systems were commercialized in the 1980s, but it was soon discovered to protect both the performance and lifetime of Li-ion cells. 26 www.ceramics.org | American Ceramic Society Bulletin, Vol. 98, No. 7 Capsule summary POWERING DOWN CHARGING UP ENERGIZING THE FUTURE The rise of electric vehicles is expected to de- Recent progress demonstrates that solid As solid-state battery technology builds mand the majority of lithium-ion battery capacity electrolytes have the properties to supplant cur- momentum toward commercialization, several in the coming years. While the cost of lithium-ion rent liquid electrolytes in lithium-ion batteries, challenges remain. Manufacturing techniques batteries is low, they are reaching their practical offering improved safety. Solid electrolytes also that leverage scalable processes are needed, as limits. This limit pushes innovators to offer new enable all-solid-state batteries, a class of batter- well as solutions to materials challenges such as kinds of batteries with higher energy density ies that could reach energy storage capacities preventing lithium filament growth and mitigat- storage with increased safety. well beyond that of lithium-ion. ing cathode volume change during cycling. Generally, temperatures must remain below 60°C to limit the rate of reactions between the electrolyte and electrodes. Finally, the pack and surrounding structures must be designed to prevent catastrophic failure of the pack in the event of a vehicle crash. These battery management systems lead to a pack cost of about 1.2–1.4 times the cell-level cost.3 While the graphite anode enabled the modern Li-ion battery, six atoms of car- bon are needed to intercalate one lithi- Credit: University of Michigan, Bruno Vanzieleghem Figure 1. (a) Typical formats for Li-ion cells: wound cylindrical 18650 (left) and pouch um ion, creating the compound C6Li in a fully charged battery. This requirement cells (right). (b) Cross section of 18650 battery shows electrode layers. limits the theoretical capacity of graphite sities of 1.8–5.0 g/cm3 are replacing organ- Solid electrolytes to 382 mAh/g. ic liquid electrolytes with densities close to Solid electrolytes have been investi- Silicon is a promising replacement 1.0 g/cm3. However, many consumer and gated for batteries since the discovery of with a capacity of 4,200 mAh/g, but automotive applications are increasingly fast sodium-ion conduction in ß-Al2O3 the significant (300X) mechanical strain driven by volumetric considerations. by Ford Motor Company in the 1960s. it experiences during cell cycling results In the mid-1990s, attention turned 4 Solid-state batteries also offer signifi- in capacity fade over time. Despite cant simplification over Li-ion batteries to lithium solid electrolytes when the these capacity losses, as understanding at the pack level, where individual cells first thin-film batteries using radio fre- of silicon lithiation and the effect of are connected. Solid-state batteries do quency magnetron sputtered lithium particle morphology have advanced, not require significant thermal manage- phosphorous oxynitride (LiPON) were manufacturers have created silicon/ ment systems, as battery performance introduced. Thin-film batteries consist graphite composite anodes with gradu- improves as temperature increases. of a LiPON solid electrolyte layer under ally increasing silicon levels. Ionic conductivity of solid electrolytes 10 µm with a thin layer of cathode and The most energy-dense anode is lithi- increases with increasing temperature, lithium metal anode.5 um metal with a capacity of 3,860 mAh/g. along with maximum charge and dis- The capacity of thin-film batteries is While lithium dendrite growth prevents charge rates. As a result, maximum limited by ionic transport in the cath- use in liquid electrolyte-based cells, physi- operating temperature of a solid-state cell ode. If cathode thickness is increased cally stabilizing the lithium metal with a is only limited by that of lithium, which beyond 10 µm, Li-ion diffusion rate Li-conducting solid electrolyte can prevent melts at 180°C. Additionally, elimination within the cathode limits the ability to dendrite growth and cell failure. Several of the flammable liquid electrolyte of access the full cathode capacity. Ideally, Li-ion-conducting solid electrolytes are Li-ion alleviates design considerations of the solid-state cathode emulates a Li-ion promising for this role. catastrophic cell or pack-level failure. In cathode that is a three-dimensional total, solid-state batteries offer significant blend of electrolyte and cathode particles Solid-state batteries mass and volume savings at the pack level, to increase areal loading. All solid-state batteries center around the translating to increased pack capacity. Nevertheless, thin-film batteries show approach of enabling a high-capacity metal- Because the graphite anode used in excellent capacity retention over tens of lic lithium anode, which greatly increases modern Li-ion batteries has a low poten- thousands of cycles. However, this technol- volumetric energy density at the cell level. tial compared to lithium (0.20 V), lithium ogy cannot fundamentally provide the stor- Figure 2 schematically illustrates both the metal-based solid-state batteries can be age of bulk-type Li-ion batteries needed for Li-ion and solid-state battery.