Young's modulus, hardness, and fracture toughness are measured by instrumented nanoindentation for amorphous Li 2 S -P 2 S 5 Li-ion solid electrolyte. Although low modulus suggests good ability to accommodate chemomechanical strain, highly brittle behavior can lead to disruptive crack formation.

Mechanical degradation of lithium-ion battery (LIB) electrodes has been correlated with capacity fade and impedance growth over repeated charging and discharging. Knowledge of how the mechanical properties of materials used in LIBs are affected by electrochemical lithiation and delithiation could provide insight into design choices that mitigate mechanical damage and extend device lifetime. Here, we measured Young's modulus E, hardness H, and fracture toughness K Ic via instrumented nanoindentation of the prototypical intercalation cathode, Li X CoO 2 , after varying durations of electrochemical charging. After a single charge cycle, E and H decreased by up to 60%, while K Ic decreased by up to 70%. Microstructural characterization using optical microscopy, Raman spectroscopy, X-ray diffraction, and further nanoindentation showed that this degradation in K Ic was attributable to Li depletion at the material surface and was also correlated with extensive microfracture at grain boundaries. These results indicate that K Ic reduction and irreversible microstructural damage occur during the first cycle of lithium deintercalation from polycrystalline aggregates of Li X CoO 2 , potentially facilitating further crack growth over repeated cycling. Such marked reduction in K Ic over a single charge cycle also yields important implications for the design of electrochemical shock-resistant cathode materials. Energy storage is an enabling technology for electrified transportation and for large-scale deployment of renewable energy resources such as solar and wind. For many applications, non-aqueous ionintercalation chemistries such as Li-ion are attractive for their high energy density and electrochemical reversibility. However, the electrode materials used in ion-intercalation batteries undergo significant composition changes-which correlate to high storage capacity-that can induce structural changes and mechanical stresses; these changes can degrade battery performance metrics such as power, achievable storage capacity, and lifetime.1-8 Microstructural damage has been observed directly in numerous electrode materials subjected to electrochemical cycling, both within single crystals (or grains) and among polycrystalline aggregates. 4,5,[7][8][9][10][11][12][13][14][15] While the relationships among electrode microstructure, electrochemical cycling conditions, crystallographic changes in the active materials, and resulting mechanical stresses have been elucidated, relatively little is known about the composition-dependency of the key physical properties. Numerous models have been developed to predict mechanical deformation in ion-storage materials during electrochemical cycling, as recently reviewed by Mukhopadhyay and Sheldon. 16 The quantitative utility of such models is dependent on measured elastoplastic properties, particularly the fracture toughness of these materials. To date, few experimental measurements of fracture toughness K Ic of battery materials have been reported; [17][18][19][20] similarly, few measureme...

Li-ion battery (LIB) electrodes subjected to repeated electrochemical cycling exhibit limited lifetime and gradual performance loss. Fracture of the active electrode particles, though one of the most widely discussed degradation mechanisms, is still not understood fully in even the most studied positive electrode systems. Here, we develop the connection between fracture and impedance in Li X Mn 2 O 4 composite electrodes via cycling schedules designed to produce discrete fracture events. We establish a correlation between these fracture events and acoustic emissions, as well as a parallel correlation between acoustic emissions and impedance growth. Through extensive impedance analysis, including conversion of impedance data to distributions of relaxation times, we identify the sources of impedance growth as electronic contact impedance and ionic surface layer impedance. Through measurements at multiple temperatures, we also estimate activation energies of ∼0.1 eV for electrolyte resistance, bulk contact resistance, and current collector resistance, ∼0.4 eV for charge-transfer resistance, and ∼0.3 eV for cathode surface layer resistance. We thus demonstrate a direct and correlative relationship between electrochemomechanical fatigue and performance loss, which can inform LIB design and characterization for improved longevity and late-life performance.

Decades of Li-ion battery (LIB) research have identified mechanical and chemical culprits that limit operational lifetime of LIB electrodes. For example, severe capacity fade of unmodified Li X Mn 2 O 4 electrodes has been linked historically with Mn dissolution and, more recently, fracture of the electrochemically active particles. Mitigation approaches targeting both effects have prolonged cycle and calendar life, but the fundamental mechanistic sequences linking fracture to capacity fade in Li X Mn 2 O 4 and many other cathode materials remain ambiguous. Here, we investigate specifically the temporal correlations of fracture, capacity fade, and impedance growth to gain understanding of the interplay between these phenomena and the time scales over which they occur. By conducting controlled excursions into the cubic-tetragonal phase transformation regime of Li X Mn 2 O 4 , we find that fracture contributes to impedance growth and capacity fade by two distinct mechanisms occurring over different time scales: (1) poorly conducting crack surfaces immediately hinder electronic conduction through the bulk of the electrode, and (2) capacity fades at a faster rate over multiple cycles, due plausibly to dissolution reactions occurring at newly formed electrode-electrolyte interfaces. The deconvolution of these effects in a well-studied cathode material such as Li X Mn 2 O 4 facilitates understanding of the complex relationship between mechanics and electrochemistry in LIB electrodes.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 18.82.0.142 Downloaded on 2018-09-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 18.82.0.142 Downloaded on 2018-09-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 18.82.0.142 Downloaded on 2018-09-13 to IP

LiMn 2 O 4 (LMO) derivatives partially substituted with transition metals (e.g., Ni) have received attention for their higher energy density achieved at higher charge voltage than pure LMO, and may be attractive cathode candidates for emerging all solid state batteries. Accurate mechanical properties of these high voltage spinels are required for prediction of electrode and electrolyte fracture that may compromise battery lifetime and performance. Here, we quantified the Young's elastic modulus E and hardness H for LMO, LiMn 1.5 Ni 0.5 O 4 (LMNO), and LiMn 1.5 Ni 0.42 Fe 0.08 O 4 (LMNFO) spinel microparticles via instrumented grid nanoindentation. Elastic modulus E and hardness H increased by more than 40% (up to 145 and 11 GPa, respectively) as a result of Ni or Ni/Fe substitution; such substitution also reduces the lattice parameter and increases the oxidization state of Mn. These results demonstrate how changes in transition metal occupancy can significantly affect the mechanical properties of LMO spinel, and provide critical parameters for designing against fracture in all solid state batteries.