Strategies to Avert Electrochemical Shock and Their Demonstration in Spinels

Abstract

We demonstrate that extensive electrochemical shock-electrochemical cycling induced fracture-occurs due to coherency stresses arising from first order cubic-to-cubic phase transformations in the spinels LiMn 2 O 4 and LiMn 1.5 Ni 0.5 O 4 . Electrochemical shock occurs despite the isotropy of the shape changes in these materials. This electrochemical shock mechanism is strongly sensitive to particle size; for LiMn 2 O 4 and LiMn 1.5 Ni 0.5 O 4 , fracture can be averted with particle sizes smaller than ∼1 μm. As a further critical test of the proposed mechanism, iron-doping was used to induce continuous solid solubility of lithium in LiMn Batteries based on ion-intercalation reactions-exemplified by lithium-ion batteries-enable high energy densities which are attractive for applications ranging from portable electronics to electrified transportation to grid-level storage. To achieve high storage capacities and energy density, ion-intercalation hosts must undergo large composition changes, which are often accompanied by large shape changes. These shape changes can result in electrochemical cycling-induced fracture, a phenomenon we have termed "electrochemical shock," which contributes to impedance growth and performance degradation. Electrochemical shock has been observed in a variety of ionintercalation materials with widely varying compositions and crystal structures 1-7 and it can cause severe deleterious effects on battery performance, manifested as impedance growth upon cycling. This has been clearly demonstrated during early cycling (first 500 cycles) of LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) cathodes, for example. 3 Previously, we and others showed that electrochemical shock in intercalation electrodes can be C-rate dependent, such as occurs when steep gradients in ion concentration generate diffusion-induced stresses sufficient to cause fracture. 8,9 Alternatively, electrochemical shock can occur by C-rate independent mechanisms, such as anisotropic shape changes in polycrystalline aggregates (e.g. secondary particles); this mechanism depends on crystal symmetry and state-of-charge, but not on cycling rate. Layered LiCoO 2 and isostructural derivative compounds such as Li(Ni,Co,Al)O 2 are a few examples amongst many in which electrochemical shock is dominated by the C-rate independent anisotropic shape change mechanism.10,11 (In distinguishing C-rate dependent vs. independent, we consider the rate dependence of a mechanism, not of a material. We have adopted a convention of calling C-rate independent any electrochemical shock mechanism that persists to arbitrarily small C-rates. Therefore, if electrochemical shock is observed during low C-rate cycling, this is clear evidence that a C-rate independent mechanism is active. At sufficiently high C-rates, the C-rate dependent concentration gradient mechanism will be simultaneously active and the observed damage accumulation in a given material may not be C-rate independent.) This classification scheme applies most directly to materials that undergo brittle fracture,...

Electrochemomechanical Fatigue: Decoupling Mechanisms of Fracture-Induced Performance Degradation in LiXMn2O4

“…In a previous study, 41 we demonstrated that we could induce fracture and associated acoustic emissions in LMO/Li cells by temporarily cycling at higher C-rates (current values denoted with respect to 1C, or the current required to discharge the cell in one hour) within the range 0 < X < 1 (4 V). That observation was consistent with the C-rate dependent "electrochemical shock" concept developed in detail by Woodford et al [42][43][44][45] However, their simulation results indicated that such diffusioninduced stress was small when compared with stresses caused by the cubic-cubic spinel phase transformation occurring in the 0 < X < 1 range; 44 the transformation stresses in the cubic-tetragonal regime are larger still. In the present work, we deliberately created controlled fracture events via the cubic-tetragonal transformation at 1 < X < 2, using LMO/Li half-cells.…”

Single-particle measurements of electrochemical kinetics in NMC and NCA cathodes for Li-ion batteries

“…[23][24][25][26][27] This phenomenon has been termed ''electrochemical shock'' in analogy to the thermal shock of brittle materials, which exhibits similar dependences on crystalline properties and microstructure. [28][29][30][31][32] Capacity fade and impedance growth in various lithium ion intercalation materials have been correlated to post-cycling observations of mechanical fracture. 24,26,27,33 Acoustic emission from electrochemical shock has also been directly recorded during charging and discharging.…”

“…24,26,27,33 Acoustic emission from electrochemical shock has also been directly recorded during charging and discharging. [28][29][30][31][32][34][35][36] Here, TXM was used to non-destructively generate 3D tomographs of the single particles at different states of charge, which we correlate with the electrochemical measurements. Individual NMC333 and NCA single particles having B10 mm diameter were charged at C/3 rate to 3.9 V, 4.1 V, and 4.5 V, respectively.…”

Electrochemical-acoustic time of flight: in operando correlation of physical dynamics with battery charge and health

“…[17][18][19][20][21] In some cases, the macroscopic strain can be exploited as an actuator. [29][30][31][32] While there have been efforts in ultrasonic imaging of full cells, they have focused on the examination of irreversible failure through delamination and cracking. [29][30][31][32] While there have been efforts in ultrasonic imaging of full cells, they have focused on the examination of irreversible failure through delamination and cracking.…”