We propose a general solution to the problem of Li dendrite penetration through solid electrolytes such as LLZO. The solution is based on an analogy of Li dendrite penetration to stress corrosion cracking, a process in which a very soft material--water--penetrates through a very hard material--steel--generally at grain boundaries. The problem of stress corrosion cracking was solved many years ago by putting the surface into a state of residual compressive stress. A variety of materials have been strengthened this way, including metals, glasses (e.g., Gorilla glass), and ceramics. However, for this approach to be useful, it is critical, at a minimum, that the presence of a significant residual compressive stress not block ion transport. In this work, we used DFT to show that isotropic compressive residual stresses as high as -10 GPa—the maximum observed in strengthened ceramics—have only a minimal impact on the diffusion rate of Li+ ions through a near-surface region of LLZO. Figure 1

The use of lithium (Li) or sodium (Na) metal anodes together with highly ion-conductive solid electrolytes (SEs) could provide batteries with a step improvement in volumetric and gravimetric energy densities. Unfortunately, these SEs face significant technical challenges, in large part because Li and Na dendrites can penetrate through SEs, leading to short circuits. The ability of such a soft material (Li or Na metal) to penetrate through ceramic is surprising from the point of view of models widely used in the Li-battery field. We introduce a concept, new to the battery field, for preventing penetration of lithium dendrites through SEs by putting the SE surfaces into a state of residual compressive stress. For a sufficiently high compressive stress, cracks have difficulty forming, and cracks that do form are forced to close, inhibiting dendrite penetration. This approach is widely used to solve commercially important stress corrosion cracking problems in metals and static fatigue problems in ceramics and glasses (e.g., Gorilla Glass). However, the technique will not be useful for SEs if the Li-ion transport rate through a SE is substantially reduced when the SE is under compression. Our molecular dynamics calculations for Li-ion transport through a common SE demonstrate that the introduction of even very high residual compressive stresses (∼10 GPa) has only a modest effect on Li-ion transport kinetics, suggesting that this approach is viable and capable of providing a new paradigm for developing high-performance and mechanically stable SEs.

We evaluate the utilities of fluorescence-yield (FY) modes in soft X-ray absorption spectroscopy (XAS) of several cathode materials for Li-ion batteries. In the case of total-FY (TFY) XAS for LiNi0.5Mn1.5O4, the line shape of the Mn L3-edge XAS was largely distorted by the self-absorption and saturation effects, while the distortions were less pronounced at the Ni L3 edge. The distortions were suppressed for the inverse-partial-FY (IPFY) spectra. We found that, in the cathode materials, the IPFY XAS is highly effective for the Cr, Mn, and Fe L edges and the TFY and PFY modes are useful enough for the Ni L edge which is far from the O K edge.

The first-ever demonstration of stabilized Si/lithium-manganese-rich full cells, capable of retaining >90% energy over early cycling and >90% capacity over more than 750 cycles at the 1C rate (100% depth-of-discharge), is made through the utilization of a modified ionic-liquid electrolyte capable of forming a favorable cathode-electrolyte interface.