Interfacial challenges in solid-state Li ion batteries.

F over 2 decades, Li ion batteries have enabled the rise of portable electronics and dominated the battery market. The principal reason for this is that of all electrically rechargeable batteries with an adequate cycle life, the Li ion battery can store the most electrical energy, both in terms of weight (specific energy Wh/kg) and in terms of volume (energy density Wh/L). There has been steady but slow evolution of the important parameters describing Li ion battery performance since its commercial introduction in 1991 by Sony, for example, calendar and cycle lifetimes, the two energy densities (Wh/kg and Wh/L), power density, cost, and safety. Unfortunately, growth rates of the two energy densities have only been occurring at ∼7−8%/year. Today, cell-level-specific energies of ∼200 Wh/kg and energy density of ∼500 Wh/L are typical and are acceptable for most portable electronics applications. The electrification of light vehicle road transportation is generally considered the next important frontier for electrochemical energy storage, and it is currently debated whether Li ion batteries will ever be good enough for mass-market full electrification. At present, hybrids (HEVs) represent only ∼3% of new car sales in the U.S. market and plug-in hybrids (PHEVs) plus full electric vehicles (EVs) represent only ∼0.7% of U.S. new car sales. The principal issue inhibiting the massmarket electrification is simply a battery problem, that is, developing a cost-effective, safe, and long-lived battery with sufficient energy storage (both in terms of weight and volume) to give enough range for daily driving so that charging can be accomplished overnight at home. At present, the Li ion is the only practical battery for EVs and PHEVs, although this currently presents a difficult weight/volume−range−cost tradeoff in the design of the EV. It is projected that retail costs for Li ion batteries may decrease significantly in the future, from ∼$400/kWh at present to ∼$100/kWh in the 2030 time frame (especially with buildup of Tesla style battery “Gigafactories”). This would make EVs cost-competitive with traditional gasoline-powered cars. However, this still does not solve the weight/volume-range issue because of the projected limited increases in specific energy and energy density of conventional Li ion. In addition, because Li ion batteries use flammable nonaqueous liquid electrolytes, there is a serious safety issue in their use, especially for the large-format battery packs in EVs (e.g., Tesla fires). While some believe that these Li ion issues can all be tamed, for example, Elon Musk, others believe that mass-market electrification will require a totally different battery chemistry with significantly higher energy densities, so-called beyond Li ion (Li−S, Li−air, Mg ion, etc.). Of course these latter currently have many technical challenges; therefore, it is not at all clear if they will ever become practical batteries for use in EVs. It seems to us that a wave of optimism is also building in the battery community that most of the limitations of the conventional Li ion batteries for EVs can be addressed by using a solid-state electrolyte in place of the traditional liquid one. Ideally, the solid-state Li ion battery replaces the intercalated lithium graphite anode (LiC6) in the conventional Li ion battery with Li metal and the liquid electrolyte with a solid-state electrolyte (SSE) but keeps the conventional intercalation cathode (C), for example, LiCoO2. We therefore view the battery as a Li|SSE|C thin-film stack (or series of repeated stacks). Unfortunately, much of the current development of solid-state Li ion batteries is being done by startups, so that, although the claims are quite impressive for the solid-state Li ion batteries, their current status is unclear. For example, several companies quote an energy density (Wh/L) several times higher than conventional Li ion, but no mention is made of their power density or capacity. Nevertheless, optimism seems high enough that at least three major car-related companies have invested significantly in U.S. solid-state battery startups, VW in Quantumscape, GM in Sakti3, and most recently Bosch in Seeo. In addition, Toyota is also investing heavily in their own solid-state battery program. A solid-state Li ion battery could, in principle, yield many advantages relative to the current conventional Li ion battery. Perhaps most important is in terms of safety by removing the flammable liquid electrolyte. Second, using Li metal instead of LiC6 (because the SSE hopefully suppresses Li dendrite formation) and a higher-voltage cathode (because many of the SSEs have electrochemical windows of >5 V) could allow higher energy densities (however, see Table 1). Finally, because