AMP_04_May_June_2021_Digital_Edition

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 1 2 2 T he Grand Challenge for next- generation energy storage tech- nologies is no longer the identifi- cation of electroactive cathode or anode materials thanks to extensive worldwide synthesis efforts along with theory and modeling like the Materials Project [1] . Instead, the challenge revolves around assembling materials in the right ar- chitecture to achieve maximum per- formance and cell life at reasonable temperatures and pressures. Nowhere is thismoreapparent than for all solid-state batteries. These batteries challenge sci- entists and engineers by requiring new ways to adheremultiple dissimilar mate- rials. This means that all interfaces must facilitate ion transport whilemaintaining the materials’ inherent structures during large volume changes due to shuttling of lithium, and they must survive the oxi- dation and reduction potentials of the electrodes. These criteria need to be maintained while facilitating ion motion at room temperature and minimal pres- sure. Addressing these needs requires insights and expertise from fields out- side the traditional lithium-ion battery community such as solid oxide fuel cells, synthesis science, barrier layers, het- erostructure design, ceramic sintering, and mechanicals. In present lithium-ion batteries, the electrolyte that shuttles the lithium ions is a salt-like LiPF 6 dissolved in an aprotic solvent mixture (e.g., ethylene carbonate, ethyl methyl carbonate). As the cell is charged, lithium is re- moved from the cathode and inter- calated within the graphite anode. During this initial charging, the elec- trolyte is decomposed forming the so- called solid electrolyte interphase (SEI), a protective layer that prevents further decomposition while maintaining Li ion shuttling [2] . A similar reaction occurs at the cathode, forming the cathode elec- trolyte interphase (CEI). There are sev- eral problems with these passivation layers. First, when the interphases fail, the cells rapidly lose capacity as addi- tional Li ions are used to reform the SEI/ CEI at the charged anode/cathode sur- faces. Second, a poor CEI leads to man- ganese dissolution from the cathode, resulting in Mn redeposition on the an- ode. At higher potentials oxygen and other gases are evolved from the cath- ode due to structural degradation and electrolyte oxidation at high electro- chemical potentials, Fig. 1. Together these reactions lead to premature bat- tery failure and are the subject of vast research efforts in industry, academic, and government laboratories. Howev- er, at the heart of all these reactions is the intrinsic instability of the flammable liquid electrolytes at the electrode po- tentials, which limits the use of higher voltage cathode materials (due to oxi- dation) and prevents incorporation of lithiummetal which is the highest ener- gy density anode. SOLID-STATE BATTERIES AND THE CRITICAL ROLE OF INTERFACES Energy storage solutions must overcome the challenges of synthesizing and processing dissimilar materials while maintaining high lithium transport through their interfaces. Gabriel M. Veith and Rebecca D. McAuliffe, Oak Ridge National Laboratory, Tennessee Fig. 1 — Schematic of degradation processes on layered LiNi x Mn y Co z O 2 cathodes.